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OTHER TITLES OF INTEREST

B I R C H et al.

Food Science, 2nd edition

B R O W N

By Bread Alone

EARLE

Unit Operat ions in Food Processing, 2nd edition

GAMON & S H E R R I N G T O N

T h e Science of Food

K E N T

Technology of Cereals, 2nd edition

LAMB & HARDEN

T h e Meaning of H u m a n Nutri t ion

LAWRIE

Meat Science, 3rd edition

RHODES & FLETCHER

Principles of Industrial Microbiology

YEATESétftf/.

Animal Science: Reproduction, Climate, Meat, Wool

FRUIT A N D VEGETABLES

BY

R. B. DUCKWORTH, B.Sc, Ph.D. Senior Lecturer in Food Science, University ofStrathclyde, Glasgow

P E R G A M O N P R E S S OXFORD NEW YORK TORONTO SYDNEY PARIS FRANKFURT

U.K.

U.S.A.

CANADA

AUSTRALIA

FRANCE

FEDERAL REPUBLIC O F GERMANY

Copyright © 1966 Pergamon Press Ltd. All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers

First edition 1966

Reprinted 1979

Library of Congress Catalog Card No. 66-25308

Pergamon Press Ltd. , Headington Hill Hall, Oxford O X 3 OBW, England Pergamon Press Inc. , Maxwell House, Fairview Park, Elmsford, New York 10523, U.S.A.

Pergamon of Canada, Suite 104, 150 Consumers Road, Willowdale, Ontar io M2J 1P9, Canada

Pergamon Press (Aust.) Pty. Ltd. , P .O . Box 544, Potts Point, N .S .W. 2011, Australia

Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France

Pergamon Press GmbH, 6242 Kronberg-Taunus, Pferdstrasse 1, Federal Republic of Germany

Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter

ISBN 0 08 011973 5

Affectionately dedicated to

my wife Margaret

and to Robin and Susan

FOREWORD

I T IS probably true to say that during the past twenty years the growth of knowledge in food science and technology has over-topped in sheer volume the accumulation of the previous fifty. Furthermore, the field of activity at tr ibutable to the title of this book is large and diverse. The information is scattered through a wide variety of sources not all of which are easily accessible to the general reader or tc the student. T h e specialized research worker has effective means of access to the information sources relating to his own field of work, bu t the more general reader is faced with an almost impossible task if he wishes to see a topic in perspective without having to spend months of effort in searching the sources.

Fruits and vegetables are of such great importance nutritionally and in adding variety to diet in every par t of the world, that a book which attempts to see the state of knowledge of this subject in perspective is to be particularly welcomed as a primary source of information. I t will find its place in the food factory laboratory and as a reference source in food science libraries. However, the growth of university teaching departments in food science started in Europe shortly after the Second World War and the pace of activity has been rising. There has been and is a shortage of texts for the more advanced students, and here again is a purpose which will be fulfilled, not merely in terms of a price which the student can afford but also in terms of well-tailored content. As a teacher I have long felt the need for a series of such books.

Although these needs are important , the practising food scien-tist inevitably tends to see his problems not only in a local but also in an international context. T h e balance between population and food supply has never looked more precarious than now. In those parts of the world where food is scarce and where large

ix

X Foreword

populations are poised on a razor edge of existence, there is des-perate need for ready sources of information which can be turned to practical nutritional effect. It is for this reason that I hope that this work will find its way into many corners of the globe and make its own contribution to alleviating human need.

JOHN HAWTHORN

AUTHOR'S PREFACE

T H I S book is one of a series which is intended to cover the range of subject matter included in courses in Food Science and Food Technology at degree or associateship level. While it is therefore intended primarily for student use, it is hoped that the contents will be of interest to a wider readership, including persons en-gaged not only in the sphere of activity to which the name Food Science has been applied, but also to those concerned in other, older disciplines from which the subject of Food Science has evolved.

T h e arrangement of the book itself well illustrates the wide range of background material with which the food scientist or food technologist must make himself familiar. A primary require-ment is that he should thoroughly understand the nature of his raw materials. Accordingly, the first par t of the book is concerned with establishing this kind of foundation in respect of the com-modity group which forms the present subject. Part I I of the book then proceeds to deal with various aspects of the utilization of these raw food materials.

There is, of course, an enormous accumulation of published information relating to fruit and vegetables. This is scattered through the literature of many scientific disciplines and no at tempt has been made in the lists of suggested reading to do more than point the way towards a more detailed excursion into existing knowledge. The subject is so large that a book of this size can only begin to scratch the surface, and the author is only too well aware of the large gaps which have unavoidably been left.

I should like to take the opportunity at this point to acknow-ledge my indebtedness to the many people and organizations who have helped me during the preparation of the book. Professor

xi

xii Author's Preface

J. Hawthorn of this department was originally responsible for me taking on the task and has given me valuable editorial guidance at various stages. Others who have given me useful advice from time to time include Mr. W. B. Adam, Dr. R. J. L. Allen, Dr. V. L. S. Charley, Dr. J. G. Fidler, Mr. R. W. Graham, Mr. B. D. Hemmings, Dr. E. J. Miller, Mr. A. O. Roberts, Mr. R. K. Sanders, Mr. M. W. Spruzen and my various colleagues and friends in the Department of Food Science here. For the supply of photographic illustrations I should like to thank the following: Professor R. D. Preston; A.R.G. Ditton Laboratory; Batchelors Foods Ltd., Sheffield; Birds Eye Foods Ltd., Walton on Thames; William Brierley, Collier & Hartley Ltd., Rochdale; Elders & Fyffes Ltd., London; Erin Foods Ltd., Dublin; J. and E. Hall & Co. Ltd., Dartford; Hunter Associates Laboratory Inc., McLean, Va.; Mather & Platt Ltd., Manchester; Metal Box Co. Ltd., Acton; Petters Ltd., Southampton; the Shell Photo-graphic Library; the Unilever Research Laboratory, Sharnbrook; and Varley FMC Ltd., Brentford. Miss S. Shepherd rendered invaluable typing and secretarial services and Mr. K. A. Hamil-ton, Mr. K. Hollands and Miss C. Kelly helped from time to time in the checking of sections of the manuscript.

Finally, I should like to affectionately acknowledge the help of my wife who, apart from giving up many long days in the pre-paration and checking of the final typescript, provided constant encouragement and support without which this book would never have seen the light of day.

Department of Food Science, R. B. DUCKWORTH University of Strathclyde,

June 1965

INTRODUCTION

T H E classification of foods into commodity groups rests for the main par t on the natural and easily recognizable divisions of the living world and it would appear that in Fruit and Vegetables we have a class of foodstuffs that is quite sharply delimited from the various other groups of edible raw materials. However, the terms fruit and vegetable, when used in this kind of context, have often been applied rather loosely without adequate definition and it is well that we should establish a t the outset the extent of the range of commodities with which it is proposed to deal in the present book.

T h e word fruit has, of course, a precise botanical meaning, limited to the mature seed-bearing structures of the flowering plants. Such a connotation, however, covers a very wide and heterogeneous assortment of products, even among those members of the group which are utilized as foods. For example, the fruits of the Gramineae—the grass family—which are harvested dry and generally milled into a meal or flour, are conveniently placed in a special category—the Cereals. Again, there are other edible fruits which naturally mature in the dry state and can be stored in this condition for considerable periods of t ime—the pulses, the nuts and the spice fruits. Leaving these categories on one side, there remain the edible fleshy fruits and these, in spite of the variety which they exhibit, form a useful and reasonably well-defined class on their own.

These fleshy fruits have much in common from the culinary point of view with the soft edible structures developed in the main from other parts of the plant body and popularly referred to as vegetables. W e thus arrive at a composite class of materials— Fruit and Vegetables—the characteristic features of which are

xiii

XIV Introduction

that they are soft edible plant products which, because of their relatively high moisture content, are perishable in the freshly harvested state. There are, of course, borderline cases and indeed there are some examples, such as corn (maize) and the edible leguminous seeds, which may either be used directly in the moist fleshy condition or may be harvested dry, and which are therefore legitimately referable to more than one category. However, this present class as a whole is fairly well character-ized.

In so far as it may be necessary to subdivide the group into its two main constituent parts, greater difficulties arise in some cases when one attempts to draw a clear line between what is to be regarded as a fruit and what as a vegetable. Common usage here has generally been at variance with precise botanical nomenclature and considerations of the way in which a commodity is normally eaten have largely overshadowed those of botanical morphology. Popularly, especially in countries with sophisticated eating habits, the term fruit is restricted in its use to those botanical fruits which have fragrant aromatic flavours and either are naturally sweet or are normally sweetened with sugar before eating, i.e. which are essentially dessert items. The term vegetable, on the other hand, is applied to all the other soft edible plant products which are usually eaten with a meat, fish or other savoury dish and are commonly salted or at least are not sweet. Using such criteria, we must in-clude as vegetables many products which in the strict botanical sense are fruits while, at the same time, including under the head-ing of fruit some items, e.g. rhubarb, which morphologically are of quite a different nature.

The mode of utilization of a fruit or vegetable material may, of course, differ from one part of the world to another or even in some instances within a given community. A good example of the varying use to which a particular type of commodity may be put is provided by the banana and the plantain. These are the fruits of two very closely related plant species but, whereas the banana is the most important of our dessert fruits, the starchy plantain is usually cooked and eaten as a vegetable.

Introduction xv This brings us to a further distinction which is often conveniently

drawn between different members of this present class of food-stuffs, namely that between the various starchy commodities, which in many parts of the world assume the role of staple foods— particularly in those areas where the consumption of cereals is relatively low—and the remaining non-starchy fruit and vegetable commodities which individually are produced and consumed on a relatively much smaller scale. I t should not be concluded from this, however, that these non-starchy products are necessarily of a lesser importance. Nutritionally, they have a vital role to play. Moreover, these are the commodities which provide such a rich variety of flavour and of colour in the diet and which thereby help to raise the process of eating from the level of a mere satisfaction of hunger and of our nutritional requirements to that of a pleasur-able occupation, the delights of which can be further enhanced by the well-directed application of the culinary art. The world would indeed be a much duller and sadder place were it not for the wealth of fresh plant foods with which nature has endowed its inhabitants.

The history of the utilization of our present range of cultivated fruits and vegetables provides an extensive and fascinating field for study. Almost every individual species has undergone a long pro-cess of selection and improvement under cultivation, especially those which have become popular in the more highly-developed countries. In modern times, the increasing complexity of agri-cultural, marketing and processing methods has led to a pro-gressively rising demand for new varieties with a wider range of individual characteristics and the plant breeders have been kept in a state of constant activity. In spite of this, it is nevertheless true to say that practically every species which is used today as a source of food was already in existence in its original wild form when man first set out on his search for food.

Primitive man therefore inherited a vast array of possible plant foods and all the evidence suggests that from his first appearance he was omnivorous in his eating habits, taking such foods of either plant or animal origin as were available to him in his immediate

xvi Introduction

surroundings. The availability of plant foods was, of course, at first determined by the existing patterns of distribution of edible species and it appears in fact that the wild ancestors of all the important food plants of today were originally confined to one or other of four main centres of distribution (see Table 1 ).

T A B L E 1. T H E P R O B A B L E O R I G I N A L C E N T R E S OF DISTRIBUTION OF T H E ANCESTORS OF SOME OF OUR M O D E R N C U L T I V A T E D F R U I T AND

V E G E T A B L E SPECIES

Centre of distribution

Central Asia

Mediterranean

S.E. Asia

Central America

Species

■*

* s.

r Apple, broad bean, cherry, lentil, mulberry, olive, onion, pea, pear, plum, pomegranate, quince, radish, spinach, (barley, rye)

Carrot, celery, cucumber, date, egg plant, lettuce, melon, mustard, turnip, (wheat)

Artichoke, asparagus, cabbage, cauliflower, fig, horseradish, parsley, parsnip, (millet)

Banana, breadfruit, peach, persimmon, orange, yam, (rice, soya bean, sugar cane)

Avocado, cassava, cranberry, kidney and Lima bean, pineapple, potato, pumpkin, squash, sweet potato, tomato, (maize)

The first really big step forward in man's efforts to control his own environment took place between about 7000 and about 5000 B.C. when he first learnt to practise a primitive type of agri-culture. The inhabitants of western Asia during this period began to cultivate wild grasses—the forerunners of our modern cereals— and to domesticate animals. From having to travel in search of his food, man was thus enabled to live a sedentary existence and the earliest stable civilizations were able to develop. The gradual spread of this Neolithic culture over the rest of the Old World took several thousand years. Numerous factors were operative in determining the pattern of these changes. Glimatological and

Introduction xvii sociological considerations were important, but a necessary pre-requisite for the establishment of agricultural practices was the availability of edible plant species suitable for cultivation. Thus the important food plants of today were progressively dis-seminated from their original centres of distribution.

The earliest attempts to cultivate plant foods were on a garden scale but in Mesopotamia field cultivation was already practised as early as 3000 B.C. The predominant species grown were the cereals—varieties of wheat and barley—though the cultivation of fruit and vegetables appears to have started in this area at about the same time. The Babylonians, for example, planted date groves and grew grapes, figs, pomegranates, apples and mulberries be-tween the date palms. They also grew various vegetables on farms, including turnips, onions, beans, radishes, lettuces and cucumbers. Neolithic culture finally spread to Britain sometime during the second millennium B.C., when settlers from France, Spain and the Mediterranean region introduced the cultivation of cereal crops and of a few vegetable species suited to the climate, notably cabbage and parsnip.

The peoples of the ancient Greek and Roman civilizations were familiar with all the many edible plant species indigenous to Central and South-West Asia and the Mediterranean region. They cultivated a very wide range of fruit and vegetables and lived largely on a vegetarian diet, meat being expensive and therefore regularly available to only the richest members of the community. The sun-drying of fruits such as grapes and prunes was already widely practised during these times and a flourishing trade in the dried products was already taking place in the Mediterranean area. The Romans were particularly active in the agricultural sphere and they took with them a keen interest in fruit and vegetable culture to the farthest reaches of their exten-sive empire. Wherever they settled they established orchards and kitchen gardens of their own, growing such species as the local climate would allow.

As Roman influence declined, the tradition of fruit and vege-table growing was kept alive in Europe mainly by the inhabitants

XV111 Introduction

of the newly established manors and monasteries of the mediaeval period, but the common people were not slow to realize the value of vegetable growing as a means of providing a more varied and inexpensive diet, and more and more villagers came to cultivate a small kitchen garden of their own. The variety of species grown was restricted in the more northerly regions by the cooler climate and in Britain, for example, only apples, plums, cherries and the hardier vegetable species such as onion, leek, garlic, cabbage, turnip and parsnip were cultivated at all widely. Man is, of course, notably conservative in his eating habits and many superstitions arose during these times as to the possible harmful effects of eating individual commodities. Indeed, doubts were cast on the desir-ability of eating fruit and vegetables in general. According to Galen, a Greek physician of the second century A.D., the eating of fruit could lead to various diseases and this was a common attitude among medical men in Europe throughout the Middle Ages. Beliefs of this kind were encouraged by the observation that dis-orders of the alimentary tract ("summer diarrhoeas") were particularly prevalent during the fruit-eating season and this supposed causal relationship was not finally discarded until the end of the nineteenth century when the true nature of these diseases was finally demonstrated.

The cultivation of fruit and vegetables on a commercial scale for marketing reached a high level of development in Europe during the Middle Ages, particularly on the rich soils of the Low Countries. Market gardens were set up to supply the inhabitants of the larger cities and the gardens of Amsterdam were already famous at the beginning of the fifteenth century. By the sixteenth century, produce from these continental gardens was already being exported to England and this further stimulated interest in the culture of vegetable species in Britain. It was also during the sixteenth century that certain important species indigenous to the Americans were first introduced to the Old World. The most out-standing of these was the potato which, along with maize, had been used as a staple food by the peoples of South America for many centuries, but other important species which had also pre-

Introduction xix viously been confined to the New World include the tomato, the pineapple, cassava and sweet potato.

The potato itself was first brought to Spain in about 1580 and reached most other parts of Europe by the turn of the century. Sir Walter Raleigh is believed to have introduced this species both to England and to Ireland and in the latter country it quite quickly established itself as a major crop, eventually ousting the cereals as the staple food of the people. T h e resulting over-reliance on a single vegetable crop was later to result in the terrible famines in Ireland in 1845-7, when the potato crops were almost totally destroyed by potato blight—a disease caused by the fungus Phytophthera infestans. As a result, large numbers of Irish people were forced to emigrate and many found a home in the newly-emerging United States of America.

The great period of geographical exploration also saw a gradual spread of the major fruits of tropical and sub-tropical regions from their original centres of distribution to other areas where the climate was suitable for their cultivation. T h e banana, for ex-ample, which had been grown in Malaysia since the second millennium B.C., was introduced to tropical America at the begin-ning of the sixteenth century and the orange, another native of South-East Asia, is also thought to have reached the Americas at about the same time.

T h e seventeenth century was notable in Britain for a further burst of activity in the commercial cultivation of vegetables and fruit, particularly in the south, and in order to cope with the increased supplies of these commodities entering London a special market was established at Govent Garden. Another interesting development at this time was the introduction of glass-house cul-ture which made possible the small-scale cultivation, by the wealthy landowners, of exotic species such as vines, peaches and even citrus fruits. In the hundred years that followed, every large town came to have its own belt of market gardens and the problems of transporting and marketing fresh produce on a commercial scale began to make themselves widely felt.

X X Introduction

A prevailing medical opinion at this time was that the health of the body depended on maintaining a correct balance between acidity and alkalinity. Meat, which had traditionally been re-garded as the most desirable of foods, was said to be alkaline and therefore better eaten along with acid foods such as fruit and most vegetables (cabbage was considered an exception and was thought to be alkaline like meat). The disease, scurvy, for example, was thought to result from over-alkalinity and was regarded, there-fore, as being amenable to cure by eating acid foods. For the first time, medical authorities were positively affirming that the eating of fruit and vegetables was necessary for complete health.

In the Britain of the eighteenth century, fruit was for the most part beyond the reach of the poorer city-dwellers, but the cheaper vegetables were becoming everyday commodities. The quality of the produce from the market gardens was, however, very poor, largely because of the unhygienic way in which it was marketed. For example, the barges used to bring produce into the London markets were sent back full of £Cnight soil" from the city cesspools to fertilize the soil of the gardens. An interesting new introduction to Britain during this period was rhubarb, a species which is thought to have been a native of China. It was also during the eighteenth century that tropical fruits first began to make their appearance in quantity in the British markets with the importa-tion, in particular, of bananas and limes from the West Indies.

The increase in popularity of vegetables in Britain was hindered by the lack of imagination shown in methods of preparing and cooking these commodities. The Englishman still believed in eating a lot of meat and tended to look down on the more vege-tarian diet of his continental counterpart. The Europeans, on the other hand, were more thoroughly versed in the arts of vegetable cookery and succeeded in making their vegetable dishes much more appetizing. In spite of these factors, the consumption of fruit and vegetables in Britain showed a slow but steady increase during the nineteenth century, although there were still big differences between different sections of the population. Consumption was still generally higher, for example, in rural areas and, in the towns

Introduction xxi and cities, among the upper and middle classes. The potato, however, was by this time a staple food among the working people.

The nineteenth century witnessed a marked acceleration in the rate of growth of the population in the United Kingdom, the development of a wide range of new manufacturing techniques and the consequent further concentration of population into the large industrial conurbations. It was during this period that the foundations were laid for the subsequent exploitation of what might be called the modern methods of food preservation. For example, the pioneering work of Nicholas Appert, a French con-fectioner and chemist, in the first decade of the century led to the establishment of the canning industry, and the large-scale applica-tion of cold-storage techniques was made possible by the develop-ment of effective mechanical refrigeration machinery in the 1870's. Methods for the artificial drying of fruit and vegetable commodities, albeit not very satisfactory ones, were also developed during this period, especially in times of war, and the commercial production of jam, and of unfermented fruit juices (in Switzer-land), was also established by the end of the century.

The newly-developing United States of America was blessed with a range of climate suitable for growing a very wide variety of fruit and vegetable species. Her southern neighbours were coun-tries where the consumption of fruit and vegetables was tradi-tionally high and she quickly emerged as a leading producer and consumer of these commodities. The same was generally true of the British Dominions and other overseas territories, with the exception of India where consumption of fruit and vegetables among the mass of the people appears to have always been at a relatively low level. Largely because of the increasing availability of supplies from her overseas possessions, the consumption of fruit in Britain increased some 50% between 1900 and the start of the Second World War and a similar increase in the consumption of fresh vegetables resulted from improvements in home agriculture and horticulture and from an increase in trade with continental Europe.

XXII Introduction

An outstanding feature of the pattern of utilization of fruit and vegetable commodities during this present century has been the rapid expansion of the processing industry, particularly in the period since the end of the First World War. A novel development was the introduction, in 1929, of the process of "quick-freezing"— the brainchild of an American physicist and engineer called Clarence Birdseye. All branches of the processing industry have, however, joined in the general expansion. In recent years, the production of fruit and vegetable juices has shown a particularly rapid rate of increase, and dehydration, after a long period in the commercial wilderness, is now beginning to establish itself as a worthy rival to the other main methods of preservation. Finally, the last few years have seen the translation into commercial prac-tice of methods of processing foods by means of ionizing radiations —a completely new departure which has only become possible in the period since the end of the Second World War. This development is at present very much in its infancy and, although the application of these methods to the treatment of fruit and vegetable commodities is likely to be very limited within the next decade, no one can foresee precisely what the more distant future may hold.

Levels and trends in the consumption of fruit and vegetables in different parts of the world in recent times are summarized in Table 2, which was compiled from data contained in the 3rd World Food Survey of the Food and Agricultural Organization of the United Nations (1963). Per capita supplies of starchy vege-tables over the world as a whole appear to have increased by some 20% since the period immediately before the Second World War. However, the levels of consumption in different countries vary over a very wide range, from as little as 10 kg/head/yf in India and Pakistan to the exceptionally high figure of 320 kg/ head/y in West and Central Africa, an area where cassava and yam are the principal staple foods. The consumption of starchy vegetables in Europe (including Russia) and in the Americas has fallen slightly since the end of the Second World War, but in most

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87

xxiv Introduction

other regions there has been a modest increase, China being exceptional in having more than doubled her per capita consump-tion of these starchy products during this post-war period.

The overall output of non-starchy vegetables and fruit in modern times has been of a similar order to that of the major starchy com-modities, but the increase in per capita supplies since the pre-war years has been a more modest 10%. There is also a somewhat smaller variation in this case from region to region, although the discrepancy between the area having the lowest level of consump-tion—again India and Pakistan with 31 kg/head/y—and that with the highest—North America with 188 kg/head/y—is never-theless a very substantial one. Since the end of the Second World War, there has been a marked increase in per capita consumption of these non-starchy fruit and vegetable commodities in western Europe, South America, Japan and the Near East, but in North America and Australasia, regions with a traditionally high level of intake, a small but significant drop in consumption has taken place during the post-war era.

This then provides a general background against which we shall now proceed to a more detailed examination of various aspects of the nature and mode of utilization of the members of this important group of foodstuffs. The chapters immediately following are concerned with the nature of the commodities them-selves and with the way in which, as living structures, they behave when removed from the parent plant. Later, in the second part of the book, fruit and vegetables are considered more specifically as foodstuffs which, if they are to play their full part in helping to nourish and enrich the life- of the human race, must be suitably protected against the natural processes of decay to which all bio-logical materials are finally liable to succumb.

CHAPTER I

CHEMICAL CONSTITUTION

FRUIT and vegetables, like all living things, contain a very wide range of different chemical compounds and show considerable variations in composition and in structure. Apart from the obvious interspecific differences, no two individuals, whether animal or vegetable, are exactly the same, nor for that matter are any two like parts, e.g. two fruits from the same plant. Moreover, an individual fruit or vegetable, being largely composed of living tissues which are metabolically active, is constantly changing in composition, the rate and extent of such changes depending on the physiological role and stage of maturity of the organ con-cerned. This innate variability of biological structures must be constantly borne in mind when considering the details of com-position of such materials. The general features of composition of the more important fruit and vegetable commodities are described by the data summarized in Appendix A. In the present chapter, we shall consider in turn the various main groups of chemical constituent, starting here with that most ubiquitous of all bio-logical compounds—water.

WATER The most abundant single constituent of fruit and vegetables, as

defined in the introductory chapter, is water, which may represent up to about 96% of the total weight. Given an unlimited supply of available moisture, the water content of a living plant tissue assumes a characteristic maximum value which is associated with a state of complete turgor of the component cells. By turgor or

3

4 Concerning the Nature of Fruit and Vegetables

turgidity we mean the condition reached when the internal pres-sure (up to 9 or more atmospheres) developed in the vacuolar system of the cell due to osmotic forces is exactly balanced by the inward pressure of the fully extended cellulose cell-wall—the con-dition, in fact, in which the tissue is physically incapable of absorbing any further water. The maximum water-content varies somewhat between individuals because of fine structural differ-ences and it may also be markedly affected by cultural conditions which influence structural differentiation.

Interspecific differences in moisture content are generally smaller than differences between different types of tissue. For example, storage tissues containing starch grains have lower moisture contents when fully turgid than have normal mature cells with no such inclusions. Moreover, water is not always present in sufficient quantity to maintain full tissue turgor, especially after the material has been harvested. In the intact growing plant, the supply of water to the tissues depends on the balance between the amount of water absorbed through the roots and the amount lost by transpiration (evaporation) from the leaves and other aerial parts. An inadequate supply of water leads to wilting, a condition more common in harvested produce in which the nor-mal source of water has been cut off.

The susceptibility to wilting of harvested fruit and vegetables varies according to the extent to which their external surfaces are structurally modified to reduce water loss. Leaves, the normal sites of transpiration, are especially liable to post-harvest wilting. Thus, while the water content of a given material will normally lie within a characteristic and limited range, the ranges for indivi-dual products show considerable overlap and interspecific differ-ences may be overshadowed by the effects of environmental factors.

Most of the solid matter of fruit and vegetables is made up of carbohydrates along with smaller amounts of protein and of fat. Included in these groups are the constituents which build up the main structural features of vegetable tissues—the prominent cell-

Chemical Constitution 5

walls, the layers of living cytoplasm and, where present, the grains of storage starch. These substances, together with water, we may call the major constituents. Also represented, usually only in relatively small amounts, are many other classes of organic compound and a wide range of mineral elements drawn from the soil. Many of these "minor" constituents can have a most im-portant influence on the properties of fruit and vegetables—on their colour, flavour and nutritive value and in some cases on their texture. We shall be considering these in more detail later. Let us first look at those classes of compound which include the main structural constituents.

CARBOHYDRATES Simple sugars are the immediate products of the process of

photosynthesis and it is hardly surprising that the structural frame-work of plant tissues is largely composed of complex molecules built up from monosaccharides and closely related compounds such as the uronic acids. The total carbohydrate content of fruits and vegetables can range from as little as 2% of the fresh weight in some cucurbitaceous fruits j to over 30% in vegetables contain-ing storage starch. The total carbohydrate includes polysac-charides, which apart from starch are largely confined to the cell-walls, and sugars, mainly sucrose, glucose and fructose, which accumulate mainly in the cell sap. Starch, when present, is organ-ized into small grains of characteristic structure which are formed initially in the cytoplasm but which commonly come to occupy the greater part of the volume of the cell (see Chapter 2). The proportions of the different carbohydrate constituents can change due to the metabolic activity of the plant. For example, big changes take place during the ripening of fruit (see Chapter 3).

Apart from starchy products, fruits generally contain the high-est amounts of total carbohydrate (up to about 23% of the fresh weight) and at the ripe stage the greater part of this is usually present as sugar. Some non-starchy root vegetables, such as

•f Melons, cucumbers, squashes and related species.


parsnip, beetroot and carrot, which contain between about 8% and 18% of total carbohydrate are also relatively rich in sugars. Most other vegetables, however, contain smaller amounts of carbohydrate, usually less than about 9% of the fresh weight and the bulk of this is present as polysaccharide constituents of the cell-wall.

The relative proportions of sucrose and of the reducing sugars glucose and fructose, which are the monosaccharides combining together in the sucrose molecule, vary from material to material and in the same material from time to time. Most commonly, the reducing sugars are present in greater amounts than is sucrose, but in certain vegetables, e.g. parsnip, beetroot, carrot, onion, sweet corn, pea, sweet potato, and in some ripe fruits such as banana, pineapple, peach, melon and some varieties of date, the content of sucrose is the higher. Traces of other mono- and di-saccharide sugars, e.g. xylose, arabinose, mannose, galactose and maltose, may also be present in amounts readily detectable by modern Chromatographie techniques.

The cell-wall constituents are cellulose, hemicelluloses and pec-tic materials. Cellulose, which consists of long straight chains of ß-glucose residues, is the main cell-wall polysaccharide from the structural point of view (see the next chapter). It is largely in-soluble and indigestible by human beings and provides the bulk of the "unavailable carbohydrate" of food tables. The hemi-celluloses form a heterogeneous group of compounds, the mole-cules of which may contain any of a number of kinds of hexose and pentose monosaccharide residue, along in some cases with residues of glucuronic acid. Hemicelluloses can be classified ac-cording to the type(s) of sugar residue predominating in the mole-cule. Xylans, arabo-galactans and gluco-mannans occur most widely in higher plants, the first two'forming highly branched molecular chains, while the last is relatively unbranched. Hemi-celluloses have very low solubilities in water but are readily soluble in strongly alkaline solutions.

The pectic materials, which are not strictly carbohydrates but are conveniently mentioned at this point, are constructed from


chains of 1-4 linked D-galacturonic acid residues which are usually esterified to varying degrees with methyl alcohol. These chains can be cross-linked in various ways and pectic substances show a wide range of solubilities from the highly insoluble, extensively cross-linked molecules of native protopectin to readily soluble short unbranched chains of low molecular weight (see Fig. 1 ). The composition of pectic materials is discussed further in Chapter 3 in connection with the ripening of fruit.

The relative proportion of cellulose, hemicelluloses and pectic substances vary greatly from tissue to tissue, species to species and with the stage of maturity of the organ concerned. In soft vege-table tissues, each of these groups of substances can, in particular examples, constitute up to 50% of the solid content of the cell-wall. Cellulose, however, is always present in excess of about 25%, whereas the content of each of the other groups of constituents can fall below 5%. Pectins are, of course, used commercially, as for example in the manufacture of jam, and are extracted from certain waste vegetable tissues. The albedo (the whitish spongy layer) of the skin of citrus fruits, particularly grapefruit and lemon, is an especially rich source of pectin, which can represent up to 50% of the dry cell-wall material, while apple and beet pulps also contain considerable quantities of this substance.

Since we are dealing in this section with the main constituents of the cell-walls of vegetables and fruit, it is also convenient to mention here yet another substance—lignin—which, though not a carbohydrate, is always associated in plants with the cell-wall. Lignin, which has features of molecular structure in common with the flavonoid compounds described in a later section, is laid down in the walls of certain types of cells, notably those of xylem and of sclerenchymatous tissues (see the next chapter) as an encrusting substance which confers great rigidity and toughness on the wall. It occurs in wood to an extent of as much as 30% by weight, but in vegetables and fruit, lignified tissues are present only in small amounts. Nevertheless, lignification has a most important effect on texture, causing fibrousness, stringyness and grittiness accord-ing to the distribution of the tissues concerned, even when lignin

Concerning the Nature of Fruit and Vegetables

COOH

HOJTHH HVVOH

ÎT ÎH (a) D-galacturonic acid

OH COOH k—o

OH

-OVOHA / v - o - i A A

COOH OH COOH

(b). Polygalacturonic acid - PECTIC ACID

OH COOH OH

COO-CH3 COO-CH3

(c) PECTINIC ACID - PECTIN

♦ PECTIN .CHAINS \

■ - - HYDROGEN \ BONDING

(d) Suggested model for the structure of native protopectin

(Modified after Henglein in Handbuch der Pflanzenphysiologie (Ruhland, W . ed.)Springer-Verlag. Berlin, 1958.)

FIG. 1. The basic molecular structure of pectic substances.


represents less than 2% of the dry weight of the tissue. Lignin itself has a complex chemical structure, the details of which have not been fully elucidated. It is, however, known to be a three-dimensional polymer, the basic units of which are aromatic (syringyl and guaiacyl) residues which are probably linked to-gether in various ways through aliphatic three-carbon side-chains (see Fig. 2).

O C H ,

OC Ho

OH

O C H ,

FIG. 2. The basic molecular structure of lignin. Proposed building units and modes of linkage. Notice the similarity to the molecular structure of

flavonoid compounds such as the leuco-anthocyanins.

There is now reason to believe that lignification of cell-walls is commonly associated with the deposition of small amounts of encrusting leuco-anthocyanins, substances related chemically to the anthocyanin pigments (see later section). Indeed, in certain cases, e.g. in the testas of most varieties of broad beans, the wall encrustation is known to be largely of the nature of leuco-anthocyanin. F. & V.—B


PROTEINS Proteins, though commonly representing less than 1 % of the

fresh weight of fruit and vegetable tissues, must be considered as structural constituents since they are the major solid components of the cytoplasm of living cells. The composition of plant proteins and the overall contents of protein in different fruit and vegetable species are discussed more fully in Chapter 6 in relation to the nutritive value of these products. Briefly, leguminous seeds are richest in protein, containing up to about 8%. Some leafy vege-tables and sweet corn can contain over 4 % of protein, but in most other products the level is below 3 % . The protein content of fruits is usually particularly low, seldom rising above about 1 · 5 % and in many cases falling considerably below 1%. Enzyme systems, which are of such primary importance in the physiology and post-mortem behaviour of fruit and vegetables, always contain a pro-tein moeity, and traces of protein, probably enzymic, are found in parts of the cell other than the cytoplasmic layer, e.g. in the cell-wall.

LIPIDS The lipids of fruit and vegetables (with the notable exception

of the avocado and the olive), are, like the proteins, largely con-fined to the cytoplasmic layers in which they are especially associated with the surface membranes. The content of lipid materials in fruit and vegetables is generally below 1 %. Lipid and lipid-like materials are, however, particularly prominent in the protective tissues at the surfaces of plant organs—in the cuticle, epidermis and corky layers. These include wax-like substances which are soluble in fat solvents and contain mixtures of fatty acids, hydroxyacids, alcohols, esters, ketones, ethers and hydro-carbons, characteristically with long chains of between 18 and 22 carbon atoms, together with traces of complex aromatic sub-stances such as ursolic acid. Also present in the protective tissues of fruit and vegetables are two lipid-like substances, cutin (of cuticular and epidermal layers) and suberin (of corky layers),

Chemical Constitution 11 which are of very complex chemical structure and are not re-moved by fat solvents. Gutin and suberin, though not identical in structure, both appear to be built up from molecules of hydroxy-acids such as phloionolic acid [OH—(CH2)8—(CHOH)2— (GH2) GOOH], which readily react among themselves through both terminal and mid-chain hydroxyl groups to give polyesters of complex ramifying structure.

The foregoing sections have dealt with the classes of substances containing the macromolecular materials from which the struc-tural framework of fruit and vegetable tissues is largely built. Let us now consider various other groups of compounds, present in smaller amounts, which contribute in different ways to the general properties of these materials.

ORGANIC ACIDS Many organic acids are formed in plant tissues during the

course of normal metabolic processes. For example, the various acids of the Krebs' cycle are produced during the respiratory breakdown of carbohydrates, while the aromatic acids quinic and shikimic are now considered to be involved in the biosynthesis of aromatic amino acids. Some of these acids and various others such as oxalic and tartaric acids, which have not thus far been linked with particular metabolic cycles, can accumulate in the tissues in considerable amounts, while improved Chromato-graphie methods are showing the presence in smaller quanti-ties of an increasing number of additional members of this group.

As a result, fruits and vegetables are normally acid in reaction, the acid content ranging widely from very low levels in some vege-tables, such as sweet corn and leguminous seeds, up to about 50 m-equiv. of acid/100 g in certain fruits such as blackcurrant and loganberry. Among vegetables, spinach shows an unusually high level of acidity—up to about 40 m-equiv./100 g—due to an exceptionally high content of oxalic acid.


The most widely-occurring and abundant acids in edible plant tissues are citric and malic, each of which can, in particular examples, constitute over 2% of the fresh weight of the material. (Lemons generally contain over 3% of citric acid.) In most species either citric acid or malic acid is the predominant individual acid constituent but there are one or two notable exceptions. The blackberry, for example, produces mainly isocitric in place of citric acid. Grapes accumulate relatively large amounts of tar-taric acid, while the avocado is exceptional in being deficient in both of the major plant acids citric and malic.

Citric acid is the principal acid of citrus fruits, of black and red currants, raspberries, loganberries, strawberries, cranberries, bil-berries (blueberries), pineapples, pomegranates and pears. Malic acid predominates in apples, most drupe fruits (plums, cherries, apricots, etc.), and cucurbitaceous fruits, bananas and rhubarb. In peach and gooseberry, these two main acids appear to be present in about equal amounts.

Vegetables also differ in the relative abundance of citric and malic acids. In potato, sweet potato, leguminous seeds, many leafy vegetables, tomato and beetroot, citric is the main acid. Malic acid predominates in cucurbits, lettuce, artichoke, broccoli, cauli-flower, okra, onion, celery, carrot, parsnip, turnip and green beans. Asparagus contains similar amounts of each of the two.

It must not be forgotten, however, that the relative proportions of different acids in living vegetable tissues are by no means con-stant. In excised leaves for example, the levels of citric acid and malic acid have been shown to vary independently in a diurnal cycle. Again, fruits in general show a decrease in overall acidity during the ripening process. In clingstone peaches, citric acid has been shown to decrease more rapidly than malic acid, in apple and pear the reverse is the case, while quinic and shikimic acids actually increase appreciably in amount during the ripening of cherries and of strawberries. The proportions of different acids can also vary in different parts of the same structure. In the peel of oranges for example malic replaces citric as the major constituent.


CH2 'COOH C(OH>COOH CH^COOH

ÇOOH Ç H 2

HOCH COOH

ÇH 3 Ç(OH)COOH CH2COOH

Citramalic acid

ÇOOH COOH

Ç H 3

ÇO COOH

Pyruvic acid

H Hp

CH2-COOH ÇH-COOH CH(OH)-COOH

COOH HÇOH

HOÇH COOH

L-Tartaric acid

ÇOOH ÇO Ç H 2

COOH

Oxalo-acetic acid

ÇOOH ÇHOH COOH Mucic acid

H2OH OOH i

Glycolic acid

C H 3

ÇHOH COOH

ÇH-COOH Ç-COOH CH-COOH

cïs-Aconitic acid

COOH Ç H 2

Ç H 2

COOH Succinic acid

COOH ÇO Ç H 2

CH^COOH α-Ketogiutaric acid

CHpOH ÇHOH COOH

Glyceric acid

r*o COOH

Glyoxylic acid

:OOH

HIAHV C O O H

H C A W ^ H ' H 2 H

Quinte acid

Benzoic acid

4 H

,C " " C-COOH HO' \P_HC /

f4 H ,

FIG. 3. Organic acids which have been identified in extracts from fruit and vegetable materials.


In addition to citric and malic acids many other organic acids have been reported to occur, usually in much smaller amounts, in different fruit and vegetable species. Tartaric, oxalic, isocitric, quinic and shikimic acids have already been mentioned. Others are succinic, lactic, glyeerie, glycolic, glyoxylic, oxalo-acetic, benzoic, fumarie, citramalic, a-ketoglutaric, pyruvic, aconitic, mucic and lacto-isocitric. To these can be added various free uronic, amino and short-chain fatty acids which are often also present in small amounts. Quinic and shikimic acids are now thought to be very widely distributed in plant tissues, while the others mentioned, apart from oxalic and tartaric acids, have so far only been definitely identified in isolated cases. The molecular structure of a number of these plant acids is shown in Fig. 3.

Finally it should be mentioned that a particular acid can show a characteristic local distribution within an edible plant structure. Citramalic acid in apple, for example, appears to be confined to the peel and has not so far been isolated from the flesh.

NITROGENOUS CONSTITUENTS OF LOW MOLECULAR WEIGHT

The proteins of fruits and vegetables, like the polysaccharides, are built up from simpler substances—the amino acids—and, in the same way that the polysaccharides are always accompanied by their constituent monosaccharides, so also free amino acids and other related simple nitrogenous compounds always occur in association with proteins. It is normal practice in reporting the protein content of a foodstuff to simply multiply the total nitrogen content by a suitable factor. 6 · 25 has normally been used for fresh plant foods. This procedure is based on the fact that proteins norm-ally contain about 16% of nitrogen, the further assumption being made that all nitrogen is present as protein. This convention, while not without its usefulness, ignores the fact that appreciable amounts of simple nitrogenous substances can be present in an un-combined form. The actual proportion of non-protein nitrogen is very variable, but values of between one- and two-thirds of the

T A B L E 3 . T H E OCCURRENCE OF SOME LESS COMMON NITROGENOUS SUBSTANCES IN F R U I T AND VEGETABLE SPECIES

>. c

jo

a-Amino-butyric acid

α-Amino-adipic acid

ß-Amino-isobutyric acid

Citrulline

Ornithine

Djencolic acid

Pipecolic acid

Hydroxpipecolic acid

Trigonelline

Taur ine

γ-Methyl-prolinc

γ-Hydroxy-proline

γ-Methyl-hydroxy-proline

S-methyl-L-cysteine sulphoxide

3,4-Dehydroxyphenylalanine

1 -Amino-cyclopropane-1 -carboxylic acid

Homoserine

Choline

Putrescine

Acetylcholine

Ethanolamine

Imidazolylethylamine

Glutathione

Adenine

Stachydrine

Cadaverine

Hypoxanthine

Allantoin

Betaine

App

le

Pear

Che

rry

Plum

Peac

h

Nec

tari

ne

Ora

nge

Gra

pefr

uit

Lem

on

Lim

e

Gra

pe

Stra

wbe

rry

-------

Mel

on

Wat

er-m

elon

Pine

appl

e---_

._

--

Avo

cado

Man

go---------

Papa

ya

Fig

Dat

e

Rhu

barb

Pota

to

Egg

plan

t

Oni

on--------

Bee

troo

t

Bro

adbe

an

Cab

bage

Tur

nip

Chemical Constitution 15 total nitrogen are commonly quoted. For example, in potato 50-66% of the nitrogen has been found to be present in the form of simple soluble constituents, while for apple estimates range from 10% to 70%. Senescent tissues such as those of over-ripe fruits usually contain especially high proportions of non-protein nitrogen.

A large number of simple nitrogenous substances have been found to occur in the tissues of fruits and vegetables, but few accurately quantitative data are available and these indicate that the relative amounts of different constituents can show wide varia-tions within a given species. Free amino acids and related amines such as asparagine and glutamine, normally those which are also present in the proteins of the tissue, appear to make up the bulk (up to 80%) of this soluble fraction of the total nitrogen. The relative proportions of the various free acids usually show differ-ences from those in which they are present in the tissue proteins. Moreover, other soluble nitrogen—containing compounds such as purines, pirimidines, nucleosides, nucleotides, betaines, alka-loids, porphyrins and non-proteinogenic amino acids and amines can also be present.

Asparagine and glutamine and/or the related acids aspartic and glutamic appear to be especially abundant in many species, e.g. citrus fruits, potato, tomato, strawberry, gooseberry and black-berry, and together these compounds often represent more than half of the non-protein nitrogen. It appears that these substances have a special role as storage compounds for nitrogen. Asparagine is also by far the most abundant individual constituent of the non-protein nitrogen fraction in apple. Pears and oranges can be especially rich in proline, black- and redcurrants in alanine. All the common amino acids of proteins are, however, usually present, if only in trace amounts. Two relatively recently-discovered amino acids which appear to be very widely distributed in fruit and vegetable tissues are ß-alanine and γ-amino-butyric acid. Table 3 lists various other less common nitrogenous compounds which have been identified in extracts from fruit and vegetable materials.


PIGMENTS The natural colouring matters in fruit and vegetables include a

very large number of individual chemical compounds but these fall naturally into three main groups—the chlorophylls, the carot-enoids and the flavonoid pigments (anthocyanins).

Chlorophylls

These are the normal green pigments of plants which play such an important role in photosynthesis and are widely distributed in all green plant tissues. The chlorophylls, which occur to an extent of about 0 · 1 % of the fresh weight in green leaves, are localized in special plastids called chloroplasts which have a characteristic fine structure. Each chloroplast contains numerous small particles called grana which themselves consist of many proteinaceous laminae, between which lie the molecules of the chlorophyll pig-ments. There are two of these—chlorophyll a and chlorophyll b — and they always occur together in about the same ratio of 1:2-5, accompanied by smaller amounts of two carotenoid pig-ments, carotene and xanthophyll. The structural formula of chlorophyll a is shown in Fig. 4. Chlorophyll b is very similar, but has an aldehyde group on carbon atom 3 instead of the methyl group of chlorophyll a. Each therefore consists of a substituted tetrapyrrole skeleton (porphyrin) with a magnesium atom at the centre and they show close similarities with the haem pigments of animals, except that the latter contain iron in place of magnesium. The removal of the magnesium from chlorophyll molecules, as by heating in acid solution, yields brown to olive pigments called phaeophytins which are produced during the cooking of vegetables.

The chlorophylls are esters of dicarboxylic acids with methyl and phytyl alcohols, and the free acids, which are called chloro-phyllins and are especially bright green in colour, can also be produced under certain conditions. This change can be brought about by the action of an enzyme, chlorophyllase, which is norm-

Chemical Constitution 17 ally present in green plant tissue and can show high resistance to thermal inactivation (peas, green beans and asparagus appear not to contain chlorophyllase).

Carotenoid pigments

These are all yellow, orange or orange-red pigments, the mole-cular structure of which is based on the type of unsaturated hydrocarbon skeleton illustrated in Fig. 5. Some, such as ß-

COOC2QH39

FIG. 4. The molecular structure of chlorophyll a.

carotene and lycopene, are hydrocarbons soluble in fat solvents. Others, the xanthophylls, are oxygenated derivatives of these same hydrocarbons and are soluble in more polar solvents. In the tissues, however, the xanthophylls normally occur as esters of long-chain fatty acids such as oleic and palmitic. The colour is due to the large number of conjugated double bonds and more highly saturated compounds of similar structure such as phytoene and phytofluene, which also occur in some food plants, are colourless.

Small amounts of carotene and xanthophyll (about 0-005% and 0-008% respectively of the fresh weight of the tissue) are


always associated with chlorophyll in the chloroplasts and they commonly persist, after the chlorophylls have broken down, to colour senescent tissues and the skins of ripe fruits. Carotenoids may, however, also be present in non-green tissues where they

CH3CH3 CHj CH3 ^ Ύ Ί

KJc^ CH3 CH3 C^CH* β -Carotene

CH-aCH, CHo CH

CH3 CH3 CH3CH3

Lycopene

Carotenes

C H ^ x v j O H

CH3CH3

ß -Xanthophyll (lutein) Xanthophylls

Cryptoxanthin

FIG. 5. The molecular structure of carotenoid pigments,

occur in the form of small crystals in the cytoplasm or in small plastids called chromoplasts (see Fig. 13b, p . 60). These extra carotenoids, which in exceptional cases such as carrot can ac-cumulate to an extent of about 0 · 1 % of the fresh weight, are largely responsible for the yellow to orange-red colours of many vegetables and fruits. Pigmented root vegetables such as carrot

Chemical Constitution 19 and sweet potato accumulate mainly the hydrocarbon carotenoids (ß-carotene and its isomers), as does the squash. Citrus fruits con-tain the carotenes along with a wide range of oxygenated deriva-tives (notably, in orange, cryptoxanthine). Some products such as tomato, water-melon and apricot synthesize appreciable amounts of lycopene during ripening and this causes the pigmentation to have a more reddish tinge (lycopene can represent 90% of the total carotenoid fraction in tomatoes). Another decidedly reddish carotenoid is the xanthophyll capsanthin which is the main pig-ment in red peppers. The main carotenoid pigments of some com-mon fruit and vegetable species are listed in Table 4.

The molecular structure of carotenoids is of importance in con-nection with the nutritive value of different vegetables and fruits, since ß-carotene and those related compounds in which one-half of the biolaterally symmetrical molecule is identical with that of ß-carotene (i.e. contains a complete ionone ring) yield vitamin A when included in the diet (see Chapter 6).

A type of structural isomerism is also possible in the molecules of carotenoids. The structure illustrated in Fig. 5 is called the all-trans form, in which the carbon chain is more or less straight. Rotation is, however, possible on each carbon atom of this chain to produce a bending of the chain—a cis structure. For example, a eis rotation on a central carbon atom yields a V-shaped molecule. Such changes in configuration can alter both the colour and pro-vitamin A potency of the pigments. The al\-trans form is probably normal in the native pigments, possibly with some mono- and di-cis forms also present. Isomerization is encouraged by light, heat and acid, and undesirable changes can result from this during the cooking and processing of foods.

More important than this in relation to changes in the quality of foods is the susceptibility of carotenoid pigments to oxidative breakdown, which is a special problem in dehydrated products. Oxidation of ß-carotene yields ß-ionone, an aromatic ketone with a violet-like smell. Carotenoid oxidation can also be brought about by lipoxidase-type enzymes under conditions in which the latter are not inactivated.

TA

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T

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MEN

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(bl

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a a

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nd x

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(Car

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lyc

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a-

and

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C

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and

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(L

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(Lut

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and

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, ß-c

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("pi

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and

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(ski

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and

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n- a

nd d

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des

("bl

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ora

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) C

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(sk

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Flavonoid pigments—the anthocyanins

The anthocyanins form a group of naturally occurring red, blue or purple flavonoid substances which include the dominant pigments of many fruits and vegetables. For this reason they are introduced here rather than in the next section which deals with flavonoid substances in general.

Most substances classified as flavonoids, including the antho-cyanins, have the type of molecular skeleton illustrated in Fig. 6a. The anthocyanins occur naturally as glycosides dissolved in the cell sap, particularly of epidermal tissues. The characteristic features of anthocyanins, apar t from their colour, is the fact that they occur as oxonium salts, normally chlorides, (the oxygen atom of the ring being four-valent), and as glycosides in which the associated sugar molecule (s)—glucose, xylose, arabinose, galac-tose or rhamnose—are attached through hydroxyl groups at position 3 or at both 3 and 5 (see Fig. 6a). They may also be esterified with aromatic acids such as />-coumaric and sinapic acids through hydroxyl groups of the associated sugars.

The different anthocyanin pigments differ in the type and arrangement of the groups substituted on phenol ring B and in the type and number of associated sugar molecules. The structures of the anthocyanidins (aglycones) of the more important antho-cyanin pigments are illustrated in Fig. 6b-g.

An increase in the number of hydroxyl groups attached to ring G appears to deepen the blueness of the colour produced, while the introduction of methoxyl groups increases the redness of the pigments. The blue can also be changed by associations with metal ions and with other colourless flavonoid compounds. Finally, the shade of colour in anthocyanin pigments is also affected by pH, alkaline conditions accentuating the blue, while acid con-ditions bring out the red. At high pHs, the oxonium salt of the anthocyanin is readily converted into a pseudo-base form which is colourless. High pHs, therefore, result in a lower depth of colour as well as a change in hue. Commonly, several different antho-cyanin pigments are present in one and the same material.


In general, anthocyanin pigments based on the aglycone cyani-din appear to be especially prominent in plant groups with a pre-dominantly woody habit and therefore in tree and bush fruits.

8

5 4 (a) Basic flavonoid structure

Cl

(b) Cyanidin

R - / >OH R

(c) Pelargonidin

OCH

OH

OH (d) Delphinidin

, o c H 3

R - f >OH R - / \ θ Η

OH (0 Petunidin (e) Peonidin

n-( >OH

0CH3

(g) Malvidin

FIG. 6. The molecular structure of anthocyanins.

Those derived from the other anthocyanidins are characteristically found in herbaceous species. Of the more common plant foods, the product most prominantly and uniformly pigmented with anthocyanin is beetroot. T h e main pigment here, betanin, is a


complex nitrogen—containing anthocyanin, the molecular struc-ture of which is not yet known. The main anthocyanin pigments in some other fruits and vegetables are listed in Table 4, p . 20-21 .

The colour due to anthocyanin pigments is notably unstable, especially when hydrolysis of the molecule releases the free antho-cyanidin, a process occurring enzymically in harvested products. The anthocyanidins are also readily oxidized by tissue enzymes (phenoloxidases) to give brown oxidation products. Such oxida-tions are prevented in the presence of ascorbic acid and the development of the associated discoloration is a sign that the ascorbic acid in the material has already itself been destroyed. The formation of metal complexes, which causes changes in colour of the pigments, can also be a problem during processing (tin complexes with anthocyanins to give slaty-grey lakes). In canning, the anthocyanins also act as depolarizers, removing the hydrogen formed during the corrosion of tin-plate by fruit acids and thereby hastening corrosion. For these reasons specially lacquered cans must be used for the canning of anthocyanin-pigmented products.

Anthocyanin pigmentation is affected by growing conditions and is especially rich when the nitrogen supply in the soil is low.

O T H E R F L A V O N O I D SUBSTANCES

Several other groups of flavonoid substances, in addition to the anthocyanins described in the last section, occur widely in fruit and vegetables and can have important effects on the properties of these materials. These other flavonoids are either colourless or very slightly coloured, but, like the anthocyanins, they are readily oxidized to give brownish products and can form variously-coloured compounds with metals. Some, such as the leuco-anthocyanins, also develop reddish-brown colours when heated in acid solution and can therefore contribute to the colour of heat-processed products.

Chemically, three other main groups of flavonoids can be dis-tinguished: (a) the anthoxanthins, (b) the leuco-anthocyanins


and catechins, and (c) derivatives of coumarins and hydroxy-cinnamic acids, which, though not possessing the complete flavo-noid structure, are conveniently included under this general head-ing. Groups (b) and (c) together with derivatives of certain other aromatic acids such as gallic acid constitute the vegetable tannins.

The main chemical difference between the anthoxanthins and anthocyanins is that, in the former, a double-bonded oxygen atom is attached to the carbon atom at position 4 of the basic flavonoid skeleton (Fig. 6a, p. 23). Various subgroups of anthoxanthins have been distinguished according to the degree of saturation, the type of group attached at position 3 and the point of attachment (whether at 2 or 3) of the phenol ring B. The anthoxanthins, like the anthocyanins, normally occur as glycosides, but in this case the sugar residue is commonly attached at position 7. One of the most widely-occurring anthoxanthins is the flavonol quercetin (Fig. 7a), which has been identified in the form of different glyco-sides in such distantly related species as apple, orange, grape, corn, spinach, onion and asparagus.

Hesperitin (Fig. 7b) and naringenin, two flavanones, are also prominent constituents in citrus fruits. At very early stages of development, glycosides of hesperitin can represent up to 35% of the dry weight in orange, naringin (naringenin-7-rutinoside) as much as 75% of the dry weight in grapefruit. The total flavonoid content at maturity, however, is from 2% to 3% in both cases on a dry-weight basis. Naringin is responsible for the bitter taste of grapefruit. Anthoxanthins, though often varying in concentra-tion in different parts, are usually more generally distributed through the tissues of an organ than are the anthocyanins. Flavo-nols (with the extra hydroxyl group at position 3) are more common among species of woody habit, whereas flavones (with-out this hydroxyl group) and flavanones, which are saturated at positions 2 and 3, occur more commonly in herbaceous species.

The leuco-anthocyanins and catechins, like the flavonols, are widely distributed among woody plants, but are much less com-mon in non-woody dicotyledonous species. They are not normally


combined with sugars and generally show a discontinuous dis-tribution within the tissues, being largely confined to particular cells which are not otherwise differentiated from their neighbours. Leuco-anthocyanins, which appear to be more common than

HO(

pH

OH

OH o (a) Quercetin (a flavonol)

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OCH 3

y*2 OH o

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HO,

PH

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OH HO,,

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■°u.

Anthoxanthins

A leuco-anthocyanin

A derivative of hydroxycou marie (caffeic) acid

CH«CH-CO

HO OH

(d) Chlorogenic acid

FIG. 7. The molecular structure of flavonoids.

catechins have been definitely identified in apples, pears, plums, peaches, grapes and in runner and broad beans (exceptions to the above generalization), and there is every reason to expect that they will be identified in many other fruit and vegetable species. They occur both in the soluble form and in a polymerized in-soluble form as encrusting substances in the cell-walls in some


species, e.g. in the testa of broad beans. This wall-encrusting func-tion they share with lignin, with which they also have features of molecular structure in common, see p. 9. Leuco-anthocyanins can therefore affect the texture as well as the colour of fruit and vegetable tissues and in the soluble form they also, in common with the other vegetable tannins, can introduce a note of astrin-gency to the flavours of these materials.

Ghlorogenic acid (Fig. 7d), an ester of caffeic acid with quinic acid, is the most widely occurring member of our third and last group of flavonoid compounds. It is the main substrate involved in the enzymic oxidative discoloration of products such as apple, pear, peach, potato and sweet potato when the tissues are cut or bruised and exposed to the air. Chlorogenic and caffeic acids have also been implicated in the non-enzymic blackening of potato tissue after cooking (stem-end blackening) to which certain vari-eties are especially susceptible. In this case, the darkening is con-sidered to be due to the formation of complexes with ferrous iron. jb-Coumaric acid occurs in the form of esters of quinic acid in pear, while free caffeic and jfr-coumaric acids are commonly present in hydrolysates from leaves and other plant organs. Goumarins also are of wide occurrence.

These various flavonoid compounds (including the antho-cyanins) constitute, along with lignin, the simpler aromatic acids

such as gallic jHO <̂ y COOH) and the aromatic

^ HO ·

amino acids like tyrosine (HO<( )>CH2: GH (NH2) GOOH),

the great bulk of the phenolic constituents of fruit and vegetables. An increasing amount of evidence suggests that the biosynthesis of all these substances occurs by way of a common route through the simple aromatic acids quinic and shikimic. The degree of molecular elaboration subsequently reached and the character-istic associations of phenolic substances occurring in different


species and families of plants are genetically controlled and are proving of considerable taxonomic value.

MINERAL ELEMENTS

The total amount of mineral matter in a fruit or vegetable material is represented by the ash content (see Appendix A). This varies in different products from as little as 0 · 1 % in some vari-eties of yam to as much as 4 - 4 % in kohlrabi.

The mechanism of mineral uptake by plants is such that elements other than those which are necessary for normal de-velopment are absorbed from the soil. As a result, plants usually contain, in varying proportions, the full range of mineral elements which are present in the soil in which they are grown. However, while cultural treatments such as the application of fertilizers do under certain circumstances affect the contents of particular ele-ments, no general correlation has been found between the elemen-tary composition of the soil and that of the plant. The latter appears to be broadly characteristic of the species or variety, though considerable intra-varietal differences can occur. For example, the contents of the main mineral elements in different apples from a single tree have been found to vary by a factor of 2.

The most abundant mineral constituents of plants are the so-called macro-nutrients—potassium, calcium, magnesium, iron, phosphorus, sulphur and nitrogen, together with certain other elements such as sodium, aluminium and silicon which, though not essential to the plant, are often well-represented in the soil. Copper, manganese, zinc, boron, molybdenum and chlorine, all essential micro-nutrients (trace elements), are also consistently present, if only in minute amounts. The distribution of the various elements within the plant is not uniform, the amounts varying from organ to organ, tissue to tissue and even in different parts of the same cell.

The most abundant individual mineral element in fruit and vegetables is potassium, the content of which is usually between 60 and c. 600 mg/100 g of fresh material, but which can represent

Chemical Constitution 29 over 1 % of the fresh weight in some especially rich examples, e.g. parsley. The corresponding ranges for other important mineral elements are given in Table 4(a).

TABLE 4(a)

Element

Ca

Mg P

Na Gl S

Fe

Approximate normal range of concentration

(mg/100g fresh material)

3-300

2-90 7-230

0-124 1-180 2-170

0-1-4

Especially rich examples

Spinach is exceptional with up to c. 600 mg/100 g

Sweet corn Seeds and young growing

parts Celery Celery Materials of relatively high protein content

Parsley is exceptional with up to 8 mg/100g

Other individual mineral elements seldom occur to an extent of more than 1 mg/100 g of the fresh tissue and are usually present in much smaller amounts. Vegetables in general are richer in minerals than are fruits.

The potassium in fruit and vegetable tissues occurs mainly in combination with the various organic acids in the cell sap and there is little doubt that in fruit the pH of the tissue is controlled by the potassium/organic acid balance. Calcium is probably mainly associated with the pectic materials of the cell-wall. Mag-nesium is especially abundant in chloroplasts as a constituent of the chlorophyll molecule, while phosphorus is a constituent of cytoplasmic and nuclear proteins, phospholipids and nucleic acids, as well as taking an important part in the metabolism of carbohydrates. There is evidence that in some species (e.g. peas, potatoes) surplus phosphorus is stored as phytic acid (inositol hexa-phosphoric acid), a substance which, through its ability to bind calcium, can markedly influence the texture and nutritional value of these products.


The distribution of particular mineral elements between differ-ent tissues and cellular regions of fruit and vegetable structures has been studied in detail in very few examples. In the apple, the contents of calcium, potassium, magnesium and phosphorus can each increase several fold on passing from the outer cortex to the core. On the other hand, in the pea, calcium is many times richer in the testa than it is in the cotyledons, while, in the case of phos-phorus, this difference is substantially reversed.

The mineral elements can have an important influence on the quality of fruit and vegetable products. Apart from the effects of déficiences of essential elements during growth, which result in crops of initially poor quality, the levels of particular minerals can affect the post-harvest behaviour of materials such as pome fruits, which are often stored for long periods of time before being passed to the retail market. Calcium can have a marked effect on texture. Metallic constituents strongly influence colour by their combina-tion with organic components, while the trace metals are all constituents of prosthetic groups of tissue enzymes which control the metabolic activity of harvested products and can cause marked changes in quality during and subsequent to various processing procedures.

ENZYMES Most chemical changes in living tissues are mediated by

enzymes, and the number of enzyme systems which have been found to occur in fruit and vegetable tissues is very large. The importance of enzymes in these products is quite out of proportion to the extremely small quantities in which they are present. Apart from the predominant part which they play in controlling the composition of living tissues both before and after harvest, if they are not inactivated at some stage in a processing treatment, they can also cause marked changes in the quality of processed products.

Many enzymes are involved in major metabolic cycles and, since the latter are common to all species, the enzymes themselves

Chemical Constitution 31 must be equally widely distributed. Others, such as the proteo-lytic enzymes of the fig (ficin) and the pineapple (bromelin), appear to be confined to single species. Individual enzymes are generally highly specific in their action and, although a particular class of enzyme may be very widely represented, the precise nature of the system may vary from species to species according to minor differences in the nature of the substrate and in the conditions (of, for example, pH) under which the enzyme must carry out its function. There is only space here to mention a few examples of enzymes which are known to influence the quality of fruit and vegetables.

Oxido-reductases such as peroxidase, catalase, cytochromes, and dehydrogenases appear to be universally present in plant tissues. Peroxidase, which is particularly resistant to heat inacti-vation, has been implicated as a possible cause of the develop-ment of off-flavours in frozen fruit and vegetable products during storage. Phenolases are more limited in distribution but are known to occur in pome and drupe fruits, grape, banana, straw-berry, fig, potato and beetroot. They have been shown to be present in association with the solid structures of the tissues rather than in the cell sap and are responsible for the rapid discoloration of cut or bruised surfaces when these are exposed to the air. Phenolases contain copper in their prosthetic group (the other oxidases contain iron), a property which they share with the enzyme ascorbic acid oxidase, a common constituent of fruit and vegetables which causes the destruction of vitamin C.

Pectolytic enzymes can produce marked changes in texture, though their presence has so far only been definitely established in a surprisingly small number of species. Tomato and citrus fruits are relatively rich in pectin esterase (P.E.), which causes the removal of methoxyl groups from pectin molecules, while some varieties of apple, pear, banana, papaya and guava have also been shown to contain this same type of enzyme. Polygalacturonase ( P. G.), which splits the chains of galacturonic acid residues, has been found in tomato, avocado and in pear. Both these types of enzyme are probably involved in the changes which lead to the softening of


fruit during ripening and their distribution in fruits is probably much wider than the above examples would suggest. Native pectolytic enzymes have not been found in vegetables other than tomatoes. These enzymes, like the phenolases, appear to be bound to solid structures, probably the cell-walls.

Amylases are always present in tissues which contain starch and they control the changes in the balance between starch and sugars which occur during the storage of starchy vegetables and during the ripening of many fruits. Inulase instead of amylase occurs in artichokes (Jerusalem), which store inulin rather than starch.

Lypolytic and lipoxidase-type enzymes have been found in some vegetables such as peas and potatoes in association with the small amount of lipid material, and these can cause undesirable changes in flavour in dehydrated and in frozen products. Lipoxi-dase has also been implicated in the oxidation of carotenoid pig-ments which leads to loss of colour and of pro-vitamin A activity.

Chlorophyllase, an enzyme occurring widely in green leafy vegetables, converts chlorophylls to chlorophyllins (the corres-ponding water-soluble dicarboxylic acids which are bright green in colour) and therefore influences the changes in colour which occur during the processing of such materials.

Finally, it has been realized in recent years that the develop-ment of the characteristic flavours of cooked vegetable foods may depend in no small measure on the activity of native enzyme sys-tems. In dehydrated cabbage for example, the natural flavour can be enhanced by the introduction of an enzyme extract derived from mustard (a closely related species). This preparation hydro-lyses glucosidic flavour precursors in the cabbage so as to release various sulphur compounds which make an important contribu-tion to the flavour of this material.

These are but a few examples of plant enzymes which are known to influence the quality of fruit and vegetable products. Many other enzyme systems are present in plant tissues and it may well be that among these will be found the causative agents of other more subtle and hitherto unexplained changes in quality which these plant materials are sometimes found to undergo.


FLAVOUR CONSTITUENTS The flavours of different fruits and vegetables, like those of

other foodstuffs, are largely combinations of sensations of taste and of smell. Certain other phenomena such as that of astringency may also be included under the heading of flavour, but the sub-stances usually responsible for this type of sensation, viz. the flavonoids, have already been considered.

Many constituents which stimulate the sense of taste (gustation) have also been mentioned in earlier sections. The sugars, for example, produce sensations of sweetness, while organic acids are responsible for sour tastes. Bitter tastes are not common among fruit and vegetable products, but certain flavonoids and related compounds are intensely bitter, e.g. naringin of grapefruit and sour oranges, the cucurbitacins of cucurbitaceous fruits, and oleu-ropein of the olive. Limonin, a triterpenoid, is another bitter sub-stance which is widely distributed in citrus fruits, but which usu-ally disappears from the endocarp (flesh) as the fruit ripens. Salts are not usually present in sufficiently high concentrations in the fresh materials to produce marked sensations of saltiness. Each of these groups of substances, by stimulating the sense of taste, makes its own contribution to the overall flavour of these commodities.

However, it is the volatile odoriferous constituents stimulating the olfactory sense which are responsible for the more subtle features of flavour so characteristic of individual species. In some cases these are dissolved in terpenoid hydrocarbons to form the so-called essential oils, which may be secreted into special oil sacs, as in the skins of citrus fruits. More commonly, however, they are more or less generally distributed through the tissues. The odori-ferous compounds in fruits are largely oxygenated compounds— esters, alcohols, acids, aldehydes and ketones, many of which are derivatives of terpenoid hydrocarbons or of the lower aliphatic acids and alcohols.

The number of different constituents which have been identified in the volatile fractions from fruits, using modern Chromato-graphie methods, is very large and it would be quite impossible


in the space available to mention them all. Some of the lower fatty acids such as formic and acetic, and alcohols such as methyl and ethyl are very widely represented, as also are such compounds as acetaldehyde, diacetyl, acetylmethylcarbinol and geraniol. Characteristic differences in flavour between individual species appear to be due in the main to differences in composition of the higher boiling fractions.

The range of volatile constituents present in vegetables appears to be much more limited than in the case of fruits. Individual acids, alcohols, aldehydes and ketones have been found in particu-lar species, but esters are generally lacking. Members of the family Cruciferae (cabbage, Brussels sprout, cauliflower, turnip, radish, watercress, etc.) and species of Allium (onion, garlic, etc.) are especially rich in sulphur-containing compounds such as organic isothiocyanates, sulphides and mustard oils, which make a predominant contribution to the flavour of these materials. In cabbage, the isothiocyanates are present as glycosides, which can be hydrolysed by the action of an enzyme also present in other members of the family. Thus, an extract of white mustard will, when applied to dehydrated cabbage after reconstitution, produce a pronounced increase in the strength of the natural cabbage flavour.

The cooking of vegetables causes the formation of volatile com-pounds not present in the raw materials. In potato, for example, mercaptans, sulphides, aldehydes and ketones are produced dur-ing cooking, probably by decomposition of various amino acids, and these are largely responsible for the flavour of the cooked product. In cabbage, the unusual amino acid, s-methyl-L-cysteine sulphoxide, which is present in considerable amount (up to 4 % of the dry weight), yields dimethyl disulphide during cooking and this makes an important contribution to the flavour of cooked cabbage. L-Pyrrolidone carboxylic acid which is formed from glutamine on heating in aqueous solution is also probably a com-mon contributor to the flavour of cooked vegetable materials.


SUGGESTIONS FOR FURTHER READING AND FOR REFERENCE

General

BRAVERMAN, J . B. S., Introduction to the Biochemistry of Foods, Elsevier, London, 1963.

JACOBS, M. B., Food and Food Products, 4 vols., 2nd edn., Interscience, New York, 1951.

MCCANCE, R. H. and WIDDOWSON, E. M., The Composition of Foods,

M.R.G. Special Report No. 297, H.M.S.O., London, 1960. MEYER, LILLIAN H., Food Chemistry, Reinhold, New York, 1960. PLATT, B. S., Tables of Representative Values of Foods Commonly used in Tropical

Countries, M.R.C. Special Report No. 302, H.M.S.O., London, 1962.

Cell-wall materials and other carbohydrates

DEUEL, H. and STUTZ, E., Pectic substances and pectic enzymes, Advances in Enzymology, 20, 341 (1958).

JOSLYN, M. A., The chemistry of protopectin, Advances in Food Research, 11, 1 (1962).

KERTESZ, Z. I., The Pectic Substances, Interscience, New York, 1951. VARIOUS AUTHORS, Structural Elements of Vegetable Foods, Section 6 of

Recent Advances in Food Science, vol. 3 (Leitch, J . Muil and Rhodes, D.N. eds.), Butterworths, London, 1963.

PERCIVAL, E. G. V., Structural Carbohydrate Chemistry, Müller, London, 1960. PIGMAN, W. W., The Carbohydrates, Chemistry, Biochemistry, Physiology, Aca-

demic Press, New York, 1957. (See also the bibliography at the end of Chapter 2.)

Proteins

Fox, S. W. and FOSTER, J . F., Introduction to Protein Chemistry, Wiley, New York, 1957.

NEURATH, H. and BAILEY, K., The Proteins, 2 vols., Academic Press, New York, 1954.

Organic acids

BENNET CLARK, T. A., Organic acids in plants, Ann. Rev. Biochem., 18, 639 (1949).

BONNER, J., Plant Biochemistry, Academic Press, New York, 1950. HULME, A. C , Quinic and shikimic acids in fruits, Qualitas Plantarum

Materiae Vegetabilis, 3/4, 468 (1958).


Carotenoids

BOOTH, V. H., The stability of carotenoids in vegetable foods and forages, Qualitas Plantarum Materiae Vegetabilis, 3/4, 317 (1958).

GOODWIN, T. W., The Comparative Biochemistry of the Carotenoids, Chapman & Hall, London, 1952.

KARRER, P. and JUGKER, E., Carotenoids, Elsevier, London, 1950. LAND, D. G., Stability of plant pigments, in Recent Advances in Food Science,

vol. 2 (Hawthorn, J . and Leiten, J . Muil eds.), Butterworths, London, 1962, p. 50.

Flavonoids

BATE-SMITH, E. G., Flavonoid compounds in foods, Advances in Food Re-search, 5, 267 (1954).

BATE-SMITH, E. C , Leuco-anthocyanins 3. The nature and systematic distribution of tannins in dicotyledonous plants, J. Linnean Soc. (Bot.), 55, 669 (1957).

BATE-SMITH, E. C , The phenolic constituents of plants and their taxonomic significance, J. Linnean Soc. (Bot.), 58, 95 (1961).

GEISSMAN, T. A., (ed.), The Chemistry of Flavonoid Compounds, MacMillan, New York, 1962.

MAYER, F., The chemistry of natural colouring matters, Ann. Rev. Biochem., 21,472 (1952).

PRIDHAM, J . B., (ed.), Phenolics in Plants in Health and Disease, Pergamon Press, New York, 1960.

Enzymes

BALDWIN, E., Dynamic Aspects of Biochemistry, 4th edn., University Press, Cambridge, 1963.

BYER, P. D., LURDY, H. A. and MYRBÄGK, K., The Enzymes, 2 vols., Academic Press, New York, 1960.

DIXON, M. and WEBB, E. C , Enzymes, Academic Press, New York, 1958. JOSLYN, M. A. and PONTING, J . D., Enzyme catalysed oxidative browning

of fruit products, Advances in Food Research, 3, 1 (1951). SCHULTZ, H. W., (ed.), Food Enzymes, Avi, Westport, Conn., 1960.

Flavour constituents

GÜNTHER, E., The Essential Oils, 6 vols., Van Nostrand, New York, 1948-52.

KIRCHNER, J . G., The chemistry of fruit and vegetable flavours, Advances in Food Research, 2, 259 (1949).

VARIOUS AUTHORS, Chemistry of Natural Food Flavours Symposium, Quarter-master Food and Container Institute for the Armed Forces, Chicago, 1957.

Chemical Constitution 37 VARIOUS AUTHORS, Flavour and Odour, Section 4 of Recent Advances in Food

Science, vol. 3 (Leitch, J . Muil and Rhodes, D. N. eds.)," Butterworths, London, 1963.

VON SIDOW, E. and WESTBERG, M., Flavour Components in Food, Rapport No. 122, Svenska Institutet för Konserveringsforskning, Göteborg, 1962.

r. & V . —C

CHAPTER 2

STRUCTURE

T H E diversity of form shown by fruit and vegetable structures is extremely wide. Among the vegetables we have representatives of all the recognizable morphological divisions of the plant body— whole shoots, roots, stems, leaves, petioles, inflorescences, fruits, etc. Usually, the morphological nature of a particular commodity is obvious on first inspection but there are cases, particularly among the tuberous storage organs developed underground, which may defy immediate morphological categorization. Some underground storage organs, such as the common or garden potato, are modified stem structures, while others are simply swollen roots. The general appearance of two important tuberous root structures of tropical regions—cassava and yam—is illustrated in Plate 1.

Fruits, though falling naturally into a single large group, may also be classified on structural grounds into a number of distinct types. The associated botanical terminology can be most useful for descriptive purposes and is given in the simplified morpho-logical classification of fruits shown in Table 5. A few words of explanation of some of the terms used in this table should perhaps be included at this point.

The individual seed-bearing structures of the flower are called carpels and these may be separate from each other (apocarpous) or fused together (syncarpous). Collectively, they constitute the gynoecium. The seed-containing cavity of a carpel or syncarpous gynoecium is called the ovary > the wall of which develops into the pericarp of the fruit. The edible fleshy part of a fruit most com-monly develops from the ovary wall, but in some cases it may be

38

{Both by courtesy of the Shell Photographic Library.) (b)

PLATE 1. (a) Cassava. The picture, taken in Western Nigeria, shows the roots being separated immediately after lifting, (b) Yam. A hand-cart

load of yams being taken to market in Eastern Nigeria.

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derived partly or wholly from the tissues of the receptacle—the enlarged tip of the stem from which the floral organs arise. In yet other examples, additional organs such as bracts (leaf-like struc-tures protecting the flowers) may also enlarge and become fleshy as in the case of the pineapple.

So much for the general morphological features of fruits and vegetables. Let us move on now to examine in finer detail the intricate internal organization of these commodities. The various classes of chemical constituent discussed in the last chapter are distributed through a highly complex structural framework which is built up from the individual cellular units of a number of different kinds of tissue. Each tissue is structurally adapted in some degree to carry out a particular physiological function. Most of the normal metabolic activity of the plant is carried out in rela-tively unspecialized tissue called parenchyma, which generally makes up the bulk of the volume of all soft edible plant structures. The outermost cell-layer, the epidermis, which in some cases is replaced by a thin layer of corky tissue, is structurally modified to protect the surface of the organ. Mechanical support is pro-vided by highly specialized tissues called collenchyma and scler-enchyma. Water, minerals and organic products of metabolism are transported from one part of the plant to another through the so-called vascular tissues, xylem and phloem, which also show a high degree of structural specialization. Before considering the fine structure of the individual tissues, let us first examine briefly the way in which these tissues are distributed in some representative examples of vegetables and fruits.

The anatomical structure of some edible stem, root, petiole and leaf structures is illustrated in Fig. 8. The most prominent ana-tomical features in sections through plant organs are the strands of vascular tissue. The two main groups of flowering plants, the dicotyledons and the monocotyledons, show characteristic ana-tomical differences. In dicotyledonous stem structures such as the potato (Fig. 8a), the bundles of vascular tissue are characteristic-ally arranged in a single ring as seen in transverse section. The phloem is normally external to the xylem in each bundle. The

Structure

STEM STRUCTURES

(a) Potato (b) Asparagus ROOT STRUCTURES

(c) Carrot (d) Beetroot

A LEAF

(f) Lettuce

FIG. 8. The distribution of tissues in some edible plant organs, i, vascular bundles, ii, strands of phloem, iii, thin layer of cork, iv, ring of sclerenchy-matous tissue, v, chloroplast-containing parenchyma (chlorenchyma). vi, epidermis, vii, secondary xylem (mainly parenchymatous in the case of carrot), viii, cambium (producing xylem internally and phloem ex-ternally). ix, secondary phloem (mainly parenchymatous). x, additional concentric cambia formed successively to the outside, xi, proliferating parenchyma, xii, collenchyma. xiii, midrib of the leaf, xiv, spongy

chlorenchyma. xv, blade of the leaf.


potato is atypical in having small strands of internal phloem, a feature also found in a few other dicotyledonous families, notably the Gucurbitaceae (cucumber, melon, etc.). Oddly enough, the kohlrabi, the only well-known example of an edible fleshy aerial stem, though a dicotyledon (belonging to the genus Brassica), is anatomically anomalous for this group in having complete vascu-lar bundles scattered through the parenchymatous ground tissue, a feature otherwise characteristic of the stems of monocotyledons, represented by asparagus in Fig. 8b.

Fleshy roots are invariably formed by secondary growth due to the activity of usually cylindrical sheets of meristematic tissue called cambia. These cambia form tissues both to the outside (phloem) and to the inside (xylem).

The secondary vascular tissues in such cases consist of small groups of conducting elements scattered through a mass of paren-chyma-like tissue, which is responsible for the fleshy consistency of the organs concerned. A typical example is carrot (Fig. 8c). Beetroot shows an anomalous type of secondary growth in which a series of concentric cambia is formed, each individual cambium giving rise to phloem centrifugally and xylem centripetally, pro-ducing alternate layers of the two tissues (Fig. 8d). Multiple cambia are also formed in the tuber of the sweet potato, but in this case they arise around individual groups of cells in the original secondary xylem and are not arranged in concentric fashion.

The anatomical structure of the petiole of celery (shown in Fig. 8e) is fairly typical for this type of organ, while most edible leaf structures show an arrangement of tissues similar to that illustrated for lettuce in Fig. 8f.

Fruits in general show a very wide range of anatomical struc-ture. We are concerned, however, only with the soft edible vari-eties. The flesh, as we have seen, may be developed from carpellary tissue (the true pericarp), from the floral receptacle, or even from extra-floral structures such as bracts. Whatever its origin, it is generally largely composed of soft parenchymatous tissue. Con-ducting and supporting tissues are in most cases poorly developed, though there are exceptions such as the pineapple in which these

Structure 45

tissues are very prominently represented. The structural make-up of some common types of fruit is illustrated in Fig. 9.

Now these different tissues, the distribution of which we have been considering, differ from one another not only in structure

(c) A berry (tomato) (d) An hesperidium (orange)

FIG. 9. The general structure of some common types of fruit, i, the soft thin exocarp. ii, the fleshy mesocarpi iii, the stony (sclerenchymatous) endocarp. iv, fleshy receptacular tissue, v, vascular bundles originally supply-ing the floral organs, vi, fleshy carpellary exocarp and mesocarp. vii, sclerenchymatous endocarp. viii, carpellary vascular bundles, ix, seeds. x, fleshy pericarp, xi, vascular bundles, xii, original ovarian cavity filled with a parenchymatous pulp, xiii, collenchymatous exocarp (the flavedo). xiv, spongy parenchymatous mesocarp (the albedo), xv, endo-carp of juice sacks formed by the breakdown of groups of parenchyma-

like cells.

but in chemical composition and, most important of all, in physi-cal properties. They vary in their resistance to the effects of cook-ing and processing treatments and to the grinding and shearing action of the teeth during mastication. The textural quality of fruit and vegetables is therefore determined in no small part by


the fine structure of individual tissues and by the relative pro-portions in which they are present.

Let us then look at the structural features of the tissues them-selves, starting first of all with the most abundant of the tissues present—the parenchyma (Greek para =- beside; a tissue laid down beside and around the other tissues).

PARENCHYMATOUS TISSUES The ground tissue or parenchyma of plants is composed of

largely undifferentiated cells, which develop directly from the meristematic (actively dividing, formative) tissues of the growing points of roots or shoots. During development, each cell increases greatly in size, acquires a prominent cell-wall and the major part of the internal volume of the cell usually becomes occupied by a single large sap-filled vacuole. Mature parenchyma cells are generally more or less isodiametric and polyhedral in shape, the different faces—between about eleven and about twenty, depend-ing on the relative size and position—representing the areas of contact with individual adjacent cells or with the exterior. The final shape of the cell results largely from the interplay of forces (internal osmotic pressure and external pressure from neighbour-ing cells) during enlargement to its final size. Intercellular con-tact is not usually complete, air-filled spaces accounting for between less than 1% (e.g. in potato) and as much as 25% (e.g. in apple) of the total volume of the tissue. In apple, the biggest intercellular spaces are actually larger than the surrounding parenchyma cells. The cells of mature parenchymatous tissues are generally between about 50 μ and 300 μ in diameter, though exceptionally large cells up to 1 mm in diameter have been found in particular cases. The range of size and shape is broadly charac-teristic of the species or variety. The microscopic appearance of typical parenchyma tissues is illustrated in Plate 2.

The protein and lipid materials in such cells are largely con-tained in a thin layer of living cytoplasm lying between the cell-wall and the vacuole. The cytoplasm, which also contains the

Structure 47

PLATE 2. Photomicrographs from stained sections of the parenchymatous tissues of some common fruit and vegetable commodities. ( X 100.) (a) Brussels sprout, (b) Carrot, (c) Potato, (d) Apple, i, intercellular

spaces, ii, starch grains, iii, a xylem vessel.

nucleus of the cell, has the consistency of a viscous fluid and, while it is in the intact living state, shows properties of semi-permeability which are responsible for the osmotic behaviour of the cells.

The polysaccharide materials other than starch are largely con-fined to the cell-wall. These include cellulose, hemicelluloses (xylans, gluco-mannans, arabo-galactans, etc.) and pectic mater-ials. Between the walls of adjacent cells is a very thin layer of


material, traditionally referred to as the middle lamella. This layer, which is probably simply a cellulose-free extension of the amor-phous gel-like matrix of the cell-wall itself, is largely pectic in nature and appears to act as an intercellular cement. Its com-position largely determines the firmness with which adjacent cells adhere to each other, which in turn has important bearings on the textural quality of fruit and vegetables (see Chapter 8). In-solubilization of the pectic material of the middle lamella by cross-linking with divalent metal ions, notably calcium and mag-nesium, appears to be common, since easy cell separation can often be effected by the use of calcium-sequestering agents. This is not, however, always the case. Very small amounts of protein have also been claimed to be present in the middle lamella. If so, these are probably derived from plasmodesmata—fine strands of cytoplasm which are known to be present connecting the proto-plasts of adjoining cells. In older cells of other tissues, but not normally in parenchyma, the middle lamella may also contain lignin, leuco-anthocyanins and other encrusting substances.

The cell-wall itself also contains similar non-polysaccharide constituents. In parenchymatous tissue these are generally limited to small amounts of protein and lipid materials which again probably originate from plasmodesmata, though enzymes involved in cell-wall metabolism have been found in preparations free from cytoplasm and these may therefore contribute to the protein con-tent. The main constituents of the cell-wall are, however, the polysaccharides, the most important of which from the structural point of view is cellulose, the unbranched long-chain polymer of ß-glucose. The hydroxyl groups of cellulose protrude from the sides of the chain in regular order and the chains readily become hydrogen-bonded together to form crystalline structures. As much as 50-60% or, in exceptional cases 70%, of the cellulose in plant cell-walls is in the crystalline condition. The individual crystal-line regions or micelles in cellulose cell-walls are very small— between 25 Â and 60 Â in diameter and of varying length—but they are interconnected through non-crystalline regions and organized into fine unbranched threads called microfibrils 100-

Structure 49 300 Â in diameter and of indefinite length. The latter are readily visible in the electron microscope (see Plate 3). These micro-fibrils form the structural skeleton of the cell-wall and largely determine its physical properties, conferring very considerable tensile strength combined with moderate elasticity.

(By courtesy of Professor R. D. Preston.)

PLATE 3. An electronmicrograph of the surface of a parenchyma cell-wall from Jerusalem artichoke, showing the microfibrillar structure. (X7710) p, pit fields—thin areas of the wall, which coincide in the walls of adjacent cells and permit communication between the protoplasts

of the different cells.

Plant anatomists distinguish two parts to the cell-wall—the primary wall, which is formed during the enlargement of the growing cell, and the secondary wall, which is laid down after the cell has reached its full size. The primary wall has a relatively higher proportion of amorphous constituents and the cellulose


microfibrils show wide variations in orientation from pre-dominantly longitudinal in the first-formed (outer) par t to pre-dominantly transverse in the last-formed (innermost) layers. This gives what is referred to as a multinet structure. The secondary wall, which is commonly divided into a number of discrete layers, contains a higher proportion of cellulose and the microfibrils are more regularly arranged, lying almost parallel to each other within a given wall-layer. In parenchyma cells, little, if any, secondary thickening of the wall usually occurs and we can consider the latter as a single unit. The secondary wall is much more important in other kinds of cells, notably those of con-ducting and supporting tissues which we shall consider in later sections.

The cell-walls of parenchyma cells contain between 6 0 % and 90% of water. Cellulose constitutes 25-50% of the dry matter and hemicelluloses and pectic substances may each contribute be-tween about 5 % and about 50%, the relative proportions of these main constituents showing considerable variations between mater-ials. The molecules of hemicelluloses, which are more or less soluble in strongly alkaline solutions, may be straight chains or they may be branched. Straight-chain hemicelluloses can them-selves show a crystalline structure, either mixed with cellulose or by themselves, but it is likely that in the walls of parenchymatous cells the hemicelluloses are for the main par t in an amorphous state, forming, along with the pectic materials, a gel-like matrix of varying rigidity around the cellulose microfibrils. The pectic materials vary greatly in detailed molecular structure and in solubility. Their composition changes with the stage of maturity of the cell and they undergo especially characteristic changes during the ripening of fruit as will be described in the next chap-ter. Hemicelluloses and pectic materials together make up on average about 30% of the total volume of the cell-wall and their removal leads to a marked shrinkage of the wall and to a marked change in physical properties.

Structure 51

PROTECTIVE TISSUES Protective tissues are those developed at the surfaces of plant

organs, forming the skins of vegetables and fruit. The outermost layer of cells is called the epidermis. Epidermal cells are generally parenchyma-like in construction, but of characteristic shape be-cause of their surface position. T h e cells fit together compactly with no spaces between them and their outer tangential walls are commonly thickened and impregnated with lipid-like materials— wax and cutin. An extracellular water-impermeable surface layer, called the cuticle, is also usually present outside the epidermis (see Figs. 10a and 11a). Yet another common feature of the epidermis is the presence of hair-like protrusions called trichomes, the struc-ture of which is characteristic of the particular taxonomic group to which the species belongs. T h e inward transition to a normal parenchymatous structure is often quite sharp, but it is inter-rupted in some cases by the presence of collenchymatous support-ing tissue in the subepidermal region (Fig. 11a).

The surfaces of underground storage organs are usually pro-tected by a thin layer of cork (see Fig. 10b), which is formed as a result of the activity of a meristematic layer of cells—the cork cambium or phellogen—which generally arises in the hypodermal layer (the layer of cells immediately within the epidermis). Cork formation may also occur in fleshy aerial organs. I t is found, for example, in the skins of pome fruits where its presence removes the gloss due to the otherwise unbroken cuticle. Cork cells, like epidermal cells, are impregnated with lipid-like materials, in this case suberin.

The epidermal layers of leaves and fruit and sometimes also of stems (see Fig. 10a), are interrupted by small pores called stornata, each surrounded by usually two specialized guard cells, the struc-ture of which is again characteristic of the species or family. These stornata allow exchange of gases with the surrounding air and control the rate of transpiration or water loss, and they can also provide a means of ingress for various plant pathogens and spoil-age organisms. In cork-covered organs, stornata are replaced by structures known as lenticels, in which pores are formed by


(a) A simple epidermis - . asparagus stem

(b) Periderm (cork) - potato tuber

(c) A complex "skin" - testa (seed-coat) of a pea

FIG. 10. Protective tissues. ( X 240.) i, a stoma, ii, the cuticle, iii, the epidermis, iv, chloroplast-containing parenchymatous tissue, v, un-specialized parenchyma, vi, corky tissue, cut off to the outside by the activity of a cambial layer formed in the original hypodermis. vii, paren-chyma cells containing developing starch grains, viii, compact layer of elongated (palisade) epidermal cells with heavily thickened walls (wall-thickening mainly hemicellulosic in the pea), ix, hypodermal layer of hourglass-shaped sclereids with large intercellular spaces.

x, parenchymatous inner layer (not completely shown).

Structure 53

the breaking apart of the cork cells. Between such stoma tal and lenticular openings, epidermal and corky layers form continuous coverings which are largely water-impermeable and which protect the underlying tissues from mechanical injury, desiccation, and from inroads by insects, fungi and other micro-organisms.

More elaborate structural modification of the outer protective cell-layers is found in seeds such as peas and beans and in the pericarps of some fruits such as the citrus species and banana. In these cases, discrete and more or less readily separable "skins" several to many cell-layers in thickness are developed. The outer part of the peel—the flavedo—of citrus fruits is also the site of numerous oil sacs into which highly flavoured, essential oils are secreted. The high degree of structural differentiation in the testa of the pea is illustrated in Fig. 1 Oc.

SUPPORTING TISSUES Collenchyma (Greek colla = glue; sticky, glistening

cell-walls) This tissue, as mentioned above, is characteristically found im-

mediately under the epidermis, usually as a series of separate strands running longitudinally along the organ. It is found parti-cularly in petioles, stems and leaves, occupying the ridges which often occur at the surfaces of these organs. The individual cells of collenchyma are elongated parallel to the long axis of the organ, and the cell-walls show a characteristic type of thickening (Fig. 11a, b), especially at the corners of the cells and in some cases along the tangential walls. A number of different kinds of collen-chyma can be distinguished according to the precise arrangement of these thickened parts of the cell-wall. The thickenings, which are very prominent in the fresh (hydrated) condition, are especi-ally rich in pectic materials and hemicelluloses, while the content of cellulose itself is relatively low (c. 20% on a dry-weight basis). This, together with a relatively low degree of crystallinity, gives the walls unusual plasticity. However, the cellulose microfibrils,


COLLENCHYMA

(a) Celery (transverse section of (b) Rhubarb (transverse section of petiole) petiole)

(c) Sclerenchyma fibres - as seen (d) Sclereids (in optical section) in a transverse section of the from the flesh of a pear (these "s t r ing" of a runner bean pod cells normally occur adhering

together in clusters)

FIG. 11. Supporting tissues. (X 300.) i, the cuticle, ii, the epidermis. iii, parenchyma cells, iv, lignified sclerenchymatous fibres, v, fine tubular

pits (pores traversing the thickened cell-walls).

Structure 55

which are arranged in a number of distinct lamellae, lie pre-dominantly almost parallel to the long axis of the cell which there-fore has considerable longitudinal strength. Collenchyma, though soft, is in fact quite a resilient tissue and the strands remain intact under treatments such as cooking and mastication which can cause surrounding parenchymatous tissues to break down.

Sclerenchyma (Greek scleros = hard; a hard tissue) The term sclerenchyma covers those supporting tissues in which

the cells have uniformly thickened walls which are normally lignified. The cellulose content of the wall is usually high— 60-80% on a dry-weight basis—while that of lignin varies from less than 1% to about 30%. At maturity, the cells are normally devoid of cytoplasm and the wall takes up the major part of the volume of the cell. Lignification, though not contributing to the tensile strength of the cell-wall (the reverse has actually been found in some cases), imparts a greater rigidity and hardness. It is a progressive and irreversible process and appears to "lock in" the other wall-constituents.

There are two main types of sclerenchyma cell—the fibre (Fig. lie) which is long and pointed—in extreme cases fibres may be up to 10 cm long—and the sclereid, which, though variable in shape, is not usually much longer than it is wide and is not pointed at the ends (Fig. l id) . Fibres, like collenchyma cells, are generally found in closely knit longitudinal strands or sheets, commonly in associa-tion with bundles of vascular tissue. These groups of fibres persist more or less unchanged after cooking and give rise to fibrousness or stringyness of texture in products such as asparagus and green bean. Sclereids are especially abundant in hard structures such as the shells of nuts and other seeds. In certain other cases, e.g. pear and quince, they occur as isolated clusters scattered through the soft parenchymatous tissues of the fruit. This produces a grittiness of texture. Sclereids are also found in the testas of some seeds, including those of the Leguminosae (pea, bean, etc.), (see Fig. 10c).


VASCULAR TISSUES The vascular tissues, xylem and phloem, are both complex

tissues each composed of a number of different kinds of cells.

Xylem (Greek xylos = wood)

The characteristic cell-type of the xylem of flowering plants is the vessel element, a tubular-to barrel-shaped cell which, by the breakdown of its end walls and of its cytoplasmic layer, forms, along with the cell-walls above and below it, a long open tube (vessel), through which water is readily transported. The dia-meter of vessel elements and the thickness of the cell-walls varies considerably, though the latter is generally intermediate between that of parenchyma cells and fibres. The thickening of the walls is not uniform over the whole surface and vessel elements have a highly characteristic appearance under the microscope (see Fig. 12a). Unthickened areas of wall may be extensive between bars of thickening or confined to small oval areas called pits. The thickened parts of the wall normally become lignified. Intermixed with the conducting vessels are xylem parenchyma cells, which may or may not develop thick walls and become lignified. Fibres may also be present, but these are mainly confined to woody species undergoing secondary thickening. The toughness of strands or sheets of xylem and the extent to which they remain intact under the action of shearing forces, depend on the degree of and continuity of the lignification.

Phloem (Greek phleos = bark)

I t can be argued on developmental grounds that the scleren-chymatous fibres often associated with the outer part of a vascular bundle are really part of the first-formed phloem. If, as has been done here, all sclerenchymatous tissue is considered separately as supporting tissue, then the phloem in non-woody structures is essentially a soft tissue which has little significance in relation to

Structure 57

(a) XYLEM

FIG. 12. The microscopic structure of xylem and phloem, (a) Xylem. The appearance, in longitudinal view, of various types of xylem vessel occurring in fruit and vegetable species. Lignifìed and thickened areas of (secondary) cell-wall are stippled. The kinds of wall thickening illustrated are described as: 1, annular. 2, annular and helical. 3, helical. 4, reticulate. 5, scalariform. 6, pitted, (b) Phloem. Sieve tubes from selected commodities: 7 and 8 from carrot, 9 and 10 from squash. 9 shows a sieve plate in face view, 7, 8 and 10 are from longitudinal sections. (Modified after Esau, K., Plant Anatomy, Chapman & Hall, London, 1953.) i, companion cells, ii, sieve plates, iii, connecting strands (of cytoplasm) passing through the pores in the sieve plate which are lined with a carbohydrate deposit termed callose, iv, plastids. v, nucleus

of a companion cell.


the textural properties of fruit and vegetable structures. Three kinds of cell are present—sieve elements, companion cells and phloem parenchyma cells. The sieve elements, like the vessel ele-ments of the xylem, are arranged in longitudinal series, forming long conducting tubes, though in this case the cells, while func-tional, remain alive and the end walls of the cells persist as per-forated plates—the so-called sieve plates (see Fig. 12b).

The companion cells and phloem parenchyma cells show little structural modification from the normal parenchymatous type of cell, except that companion cells in particular show relatively little vacuolation. Companion cells are closely associated develop-mentally and functionally with the sieve tubes, while phloem parenchyma, apart from providing an alternative transport sys-tem for organic materials, commonly functions as a storage tissue.

The phloem, because of its physiological role in the conduction and storage of the organic products of metabolism, is usually especially rich in these latter materials. Though normally associ-ated with the xylem in vascular bundles, it may also occur as iso-lated strands, particularly in regions where the surrounding parenchyma is serving as a storage tissue.

CELLULAR INCLUSIONS The main structural features of the cells of plant tissues, apart

from the nucleus and cytoplasm, the fine structure of which is not relevant to our present discussions, have already been considered. There are however certain other kinds of organized structure, the occurrence of which can have an important bearing on the quality of fruit and vegetable products. A feature common to probably all living plant cells is the presence in the cytoplasm of small bodies of variable shape called plastids. These plastids can act as centres for the accumulation of various products of cellular metabolism. Some, for example, contain pigments and these are referred to as chromoplasts. The commonest type of chromoplast is the chloroplast in which the chlorophylls and associated pigments are contained (Fig. 10a). The fine structure of chloroplasts has been briefly

Structure 59 described in an earlier section. They are present in all green tissues.

The only other pigments which occur in special chromoplasts are the carotenoids (water-soluble pigments such as the antho-cyanins are dissolved in the cell sap). The carotenoids tend to crystallize out inside the chromoplasts causing the latter to assume various irregular shapes. Apparently free needle-shaped crystals of carotenoid pigments can also be present. (It is not clear whether these are formed independently of chromoplasts.) Examples of carotenoid-containing structures in pigmented vegetables are illustrated in Fig. 13b.

Colourless plastids are called leucoplasts and these can act as centres for the accumulation of storage materials such as starch. Starch occurs in fruit and vegetables in the form of grains, the structure of which is characteristic of the species. These grains are round, ovoid or rather irregular in shape, according to the source, and generally flattened in one plane. The microscopic appearance of starch grains from some common vegetables and fruit is illus-trated in Fig. 13a. Commonly the grains show a distinct con-centric layering which is thought to be due to diurnal periodicity in the activity of the plastid in which the grain is formed. The starch in these grains is partly crystalline (between 20% and 60%) and has been shown in some species to be organized into radially arranged microfibrils, not unlike the microfibrils of cellulose occur-ring in the cell-walls. The crystallinity is normally destroyed during blanching or cooking and the starch is gelatinized, going into solution in the cell sap. This is one of the main effects of heat processing in starch-containing products.

Small crystals of protein may also be present in some cells. In the peripheral region of the potato tuber, for example, cuboidal protein crystals occur in parenchymatous cells. Proteins also occur as so-called aleurone grains in certain seeds and fruits, a notable example being sweet corn.

Apart from these various organic cellular inclusions, inorganic crystals are commonly found in the tissues of fruits and vegetables. These generally consist of calcium salts, most commonly the


(a) Starch grains

Ä flb m M Bean (Phaseolus)

w w)

m ~ Corn (Zea-maize) Banana

(b) Chromoplasts and crystals of carotenoids

Tomato Carrot

FIG. 13

Structure 61

(c) Crystals

A druse in a cell of papaya Raphides in grape

FIG. 13. Cellular inclusions, (a) (all X 300). (b) (all X 450). (c) (all X 375). i, the hilum (developmental centre), ii, concentric lamellae of

starch thought to be deposited in a diurnal cycle, iii, the nucleus, around which chromoplasts are commonly clustered, iv, chromoplasts. v, crystals

of carotenoids.

oxalate which occurs in many crystalline forms. Certain types of crystal formation are so characteristic in appearance that they have been given special names. A spheroidal cluster of radiating crystals is called a druse, while long needle-like crystals, which generally occur in bundles, are referred to as raphides (see Fig. 13c). Isolated rhomboidal and prismatic crystals may also occur.

S U G G E S T I O N S F O R F U R T H E R R E A D I N G A N D F O R R E F E R E N C E

ESAU, K., Plant Anatomy, Wiley, New York, 1953. HAYWARD, H. E., The Structure of Economic Plants, MacMillan, New York,

1939. PRESTON, R. D., The Molecular Architecture of Plant Cell Walls, Chapman &

Hall, London, 1952. ROELOFSEN, P. A., The Plant Cell Wall, Gebrüder Borntraeger, Berlin, 1959. SETTERFIELD, G. and BAYLEY, S. T., Structure and physiology of cell walls,

Ann. Rev. Plant Physiol., 12, 35 (1961). STERLING, C , The effect of moisture and high temperature on cell walls in

plant tissues, Food Res., 20, 474 (1955).


STERLING, C. and PANGBORN, J., Fine structure of potato starch, Amer. J. Bot, 47, 577 (1960).

STERLING, C , Texture and cell-wall polysaccharides in foods, in Recent Advances in Food Science, vol. 3 (Leitch, J . Muil and Rhodes, D. N. eds.), Butterworths, London, 1963, p. 259.

WEIER, T. E. and STOCKING, C. R., Histological changes induced in fruit and vegetables by processing, Advances in Food Research, 2, 298 (1949).

WINTON, A. L. and WINTON, K. B., The Structure and Composition of Foods, Wiley, New York, 1935.

CHAPTER 3

PHYSIOLOGY

I T HAS already been emphasized in earlier chapters that fruits and vegetables are living structures, the composition and therefore the quality of which are liable to change due to the continuation of metabolic activity. We shall concern ourselves in this chapter with the nature of the physiological processes which take place after harvesting, with special reference to those changes which affect the quality of these products as foods.

O n removal from the parent plant, the tissues of fruits and vegetables are cut off from their normal supplies of water, minerals and, in some cases, of simple organic products of metabolism which would normally be translocated to them from other parts of the plant. With the possible exception of some short-lived photosynthetic activity in green leafy structures, the synthesis of new dry matter from carbon dioxide and water no longer takes place. However, the tissues remain capable of bringing about a wide range of metabolic transformations among the organic con-stituents which they already 'contain. They are also liable to lose water either, in aerial organs, by the continuation of the normal process of transpiration, or by evaporation from surfaces which, in the intact plant, are not normally exposed to drying conditions.

Physiological activity in harvested fruit and vegetables may lead in some cases to a deterioration in quality, while in other cases it may be essential for the at tainment of the desired degree of maturity. Water loss, for example, is almost invariably undesir-able, since it leads to drying out and wilting. The main metabolic process taking place in harvested plant products is respiration,

63


which involves the breakdown of organic substrates with a con-sequent progressive depletion of the accumulated reserves of dry matter. The metabolic activity in such materials is not, however, entirely catabolic. Some plant organs, utilizing energy released by respiration, can continue to synthesize pigments, enzymes and other materials of elaborate molecular structure long after they are removed from the parent plant. Such syntheses are an essen-tial part of the ripening process in many fruits.

The kind and intensity of post-harvest physiological activity, which depend, of course, on the natural functions of the plant parts concerned, determine to a large extent the longevity of the material during storage in the harvested state. Some organs such as seeds, fleshy roots, tubers, bulbs, etc., apart from their role in reproduction, are morphologically and physiologically adapted to tide the plant over periods during which environmental con-ditions are unfavourable for further development. Metabolic acti-vity in such organs is normally depressed but not completely halted during the dormant period. If harvested at a suitable stage, these products can therefore be stored for considerable periods with relatively little change in quality. The tissues of fleshy fruits and of the other soft aerial parts of plants are, however, not specialized in this way and they normally pass within a single growing season through a complete series of developmental stages in which maturity is followed by a period of senescence, ending finally in death. In such cases, harvesting can hasten the onset or progress of senescence and this is generally associated with a pro-gressive loss of quality.

Fleshy fruits present a rather special case in that maturation here normally culminates in a ripening process which is usually associated with the attainment of optimal eating quality. This ripening process involves certain well-defined physiological changes, the time relations of which vary from species to species. Some soft perishable fruits pass relatively rapidly through the stage of ripeness to senescence and the problem of marketing the fruit in good condition is a very serious one. Certain other fruits such as the banana and the pome fruits can be harvested and

Physiology 65 stored for variable periods in a pre-ripe condition and ripening can be induced more or less at will by suitable control of the con-ditions of storage. The ripening of citrus fruits is a slower and more gradual process which will only take place satisfactorily on the tree.

The range of physiological behaviour in fruit and vegetables is therefore quite wide and each individual product shows its own characteristic pattern of changes after it is harvested. Whatever this pattern and whatever the time relations of the changes, the useful life of all such materials is finally deter-mined, in the absence of any sudden lethal effect or of gross microbiological spoilage (which is in any case generally only possible in senescent tissues), by the progress of senescence, the running-down process to which all living tissues must finally succumb.

Senescence involves the progressive disorganization of the meta-bolic apparatus of the cell. In order to maintain the integrity of this apparatus, which has a physical basis in the fine structure of various cytological particles and membranes, a constant supply of energy is required. This energy is derived from respiration, which is therefore the key process in cellular metabolism. Respiratory activity is now known to be localized in certain small cytoplasmic bodies of variable shape called mitochondria. These mitochondria have a characteristic fine structure composed of elaborately folded layers of material called cristae. They also show a characteristic osmotic behaviour and are capable of accumulating salts by a process involving the expenditure of energy. As the cells mature, the layered structure of the mitochondria gradually disappears and this is associated with a loss of respiratory efficiency. During senescence, further disorganization of mitochondria takes place and it is the associated failure of respiratory function which appears to be the primary cause of the death of the cells. The cause or causes of the loss of function of mitochondria in senescent cells are at present unknown. Certainly there are ample supplies of respirable substrates still available in the tissues at the time that mitochondrial breakdown occurs and this "ageing" process


is therefore not simply due to a progressive depletion of the avail-able reserves of potential energy.

Since respiration plays such a central part in the metabolism of all harvested plant tissues, we shall give it pride of place, con-sidering first the nature of the process itself, then the normal patterns of respiratory activity in some representative examples, and finally the effects of the more important factors which in-fluence the rate at which the process takes place.

RESPIRATION

The nature of the process

Respiration involves the oxidation of energy-rich organic sub-strates to simpler compounds of much lower potential energy. The greatest yield of energy is obtained when the process takes place in the presence of molecular oxygen. Respiration is then said to be aerobic and the products of the reactions are carbon dioxide and water. Anaerobic respiration—respiration in the absence of oxygen —is much less efficient as a producer of energy and the chemical products are compounds of intermediate molecular size such as ethyl alcohol. Of the two processes, aerobic respiration is much more important in harvested fruit and vegetables, though anaerobic respiration does take place under certain conditions, especially in senescent tissues where structural breakdown may reduce the permeability of the material to atmospheric oxygen. Anaerobic respiration, which may also result from the inactiva-tion of the so-called terminal oxidase systems (see later, p . 69), is normally associated with marked deterioration in quality.

The usual substrates for respiration in plant tissues are the carbohydrates and organic acids which, apart from being relatively abundant , are generally utilized in preference to other possible energy sources such as fats, proteins, etc. One molecule of a monosaccharide sugar, on oxidation with six mole-cules of oxygen, yields six molecules of carbon dioxide and

Physiology 67 six molecules of water, according to the following summary equation :

C6H1206 + 6 0 2 = 6G0 2 + 6H 2 0 This transformation actually takes place in a large number of

individual stages, with the participation of many different enzyme systems. The commonest route of hexose degradation appears to

Glucose

Glucose-6-phosphate

CH20(ph)

CO I

PHOSPHOHEXOKINASE H Q C H _ ALDQLASE. 1 γ~\ HJÇÔH " / \ "HÇOH

ATP ADP || ATP ADP CH20(ph)

HEXQKINASE

COO H

CO C H 3

Pyruvic acid

Dihyd roxyocetone phosphate

CH20(ph) CO CH2OH

Fructose-6- SN

phosphate s Fructose -1 ,6-

phosphate

11 CHO ÇHOH CH20(ph)

3 Glyceraldehyde phosphate

DEHYOROGENASE (DPN)

ATP ADP ATP ADP

C O O H PHOSPHOGLYC- C O O H i ~ / i _ \ EROMUTASE ' I AU

CO(ph) ^ CHOH ^ I v

' ENOLASE C H 2

Phosphoenol pyruvic acid

-H20

+ H 3 P0 4

ÇOO(ph) CHOH CH 2 0 (ph) CH20(ph)

3-Phospho- 1,3-Glyceric acid glyceric acid diphosphate

FIG. 14. The Embden-Meyerhof-Parnas pathway of glucose degrada-tion. ADP, Adenosine diphosphate. ATP, Adenosine triphosphate.

DPN, Diphosphopyridine nucleotide.

be by way of the Embden-Meyerhof-Parnas (E.M.P.) Pathway, followed by the Krebs Tricarboxylic Acid Cycle. The E.M.P. path-way, which is common to both aerobic and anaerobic respiration, involves the breakdown of glucose to pyruvic acid (see Fig. 14). The glucose is first converted by way of hexosemonophosphates to fructose—1,6-diphosphate. This is then split by the enzyme


aldolase into two interconvertible three-carbon compounds, one of which—3-glyceraldehyde phosphate— is immediately oxidized in the presence of a dehydrogenase containing D P N (diphospho-pyridine nucleotide) as its prosthetic group, to 3-phosphoglyceric acid. This last conversion actually involves the formation of a diphosphate of glyceric acid, which yields its extra phosphate group to ADP [adenosine diphosphate) to form A T P (adenosine tri-phosphate), the main energy-carrying compound in living cells. The phosphoglyceric acid is next changed by isomerization, followed by dehydration, into the enol form of phosphopyruvic acid, which finally loses its phosphate group to ADP to form another molecule of A T P and a molecule of pyruvic acid.

In the absence of oxygen, the pyruvic acid formed in this way is decarboxylated to give aceteldehyde, which is then hydro-genated to ethyl alcohol by the action of reduced DPN. When oxygen is present, however, the pyruvic acid enters the Krebs tricarboxylic acid cycle, in the course of which much greater amounts of energy are released. The various stages of the Krebs cycle are illustrated in Fig. 15.

Some of the acids formed during the Krebs cycle, such as citric and malic, will be recognized as being among the more abundant acidic constituents of fruits and vegetables and the accumulation in the tissues of individual acids of this group may result from differences in the rates of particular steps in this respiratory cycle.

The energy released during the Krebs cycle is transferred through the agency of the phosphopyridine nucleotides (DPN and TPN) and of flavin adenine dinucleotide (FAD) to ADP which, by reforming ATP, is able to store this energy until it is required for various other energy-dependent metabolic reactions. The hydro-gen removed during the dehydrogenation stages is finally com-bined with molecular oxygen to form water. This process also occurs in a number of distinct stages. The reduced DPN or T P N first transfers hydrogen to a flavo-protein. The flavo-protein transfers electrons to molecules of cytochromes or other electron-transfer agents, which in turn are oxidized by molecular oxygen, water being formed in the process. This final oxidation is enzyme-

Physiology 69

Pyruvic acid COOH CO

Acetyl Coenzyme A

ÇO-COOH CH2COOH

Oxalo-acetic acid

CONDENSING Γ Η ^ Γ Ο Ο Η . LJ rs ENZYME \^2\

UUH

+ H2O ■ C:(OH)COOH CH2COOH Citric acid

-H00

C I S -

Aconitic acid CF^COOH

- C-COOH CHCOOH

COOH CH

HOÒH COOH

Malic acid

1 FUMARASE

+ H20

HOCOOH HOOOCH

Fumarie acid

CH2COOH CH-COOH CH(OH)COOH

Isocitric acid

ISOCITRIC DEHY DROGE NASE

ÇH2COOH CH2COOH

Succinic acid

GDP CH2COOH * I DPN CH^O-CoA v

v 2 H

Succinyl ^ - - ' Coenzyme A

co2

FIG. 15. The Krebs tricarboxylic acid cycle. TPN, Triphosphopyridine nucleotide. GDP, Guanosine diphosphate. GTP, Guanosine triphos-

phate. FAD, Flavin adenine dinucleotide.

catalysed and, in addition to cytochrome oxidases, a number of other enzymes such as phenoloxidases and ascorbic acid oxidase can also probably function as what are called terminal oxidase systems in plant respiration.

The scheme for carbohydrate oxidation outlined above, though probably the commonest pathway in plant tissues generally, is


not the only one which is known to occur. In some cases, the Krebs cycle may be short-circuited by the formation from isocitric acid of glyoxylic acid which, in the presence of acelyl coenzyme A, forms malic acid directly. (This is the so-called glyoxylate shunt.) A completely different route of carbohydrate oxidation known as the Pen tose Phosphate Cycle may also be followed. The stages of

Glucose

Fructose-1,6 diphosphate

+ Tri ose

+ °5Λ

Erythrose-4 phosphate

+ Hexose

Sedoheptulose 7 phosphate

+ Glyceraldehyde

3 phosphate

TRANSKETOLASE

Glucose-6 phosphate

6 phospho-gluconic acid

Ribulose-5 phosphate

I Xylulose-5 ^ phosphate

FIG. 16. The peritose phosphate oxidation cycle.

this cycle are shown in Fig. 16. In this case, only one hexose mole-cule out of every six entering the sequence is completely oxidized to carbon dioxide and water.

The relative importance of these different oxidative cycles appears to vary from species to species, organ to organ and with the stage of development of the part concerned. The E.M.P. pathway is particularly prominent in embryonic tissues such as those of seeds and growing points. Pentose oxidation appears to be relatively more important in older tissues and has been demon-

Physiology 71

strated in the mature fruits of several species. Changes in the mechanism of respiratory oxidation have been shown to occur during the ripening process in some fruits. In the banana, for example, there are indications that, as ripening proceeds, E.M.P. degradation takes over from pentose phosphate oxidation as the main route for hexose breakdown. During the ripening of apples, on the other hand, Krebs cycle activity, which is probably high during the stage of rapid growth, later appears to become less important than a mechanism capable of anaerobic decarboxy-lation of malic acid, which finally yields carbon dioxide and acetaldehyde (probably by way of pyruvic acid).

Patterns of respiratory activity in harvested fruit and vegetables

Generally speaking, the rate of respiration is indicative of the rapidity with which compositional changes are taking place within the material. If, as is commonly the case, the product is harvested at or near to the stage of optimal eating quality, a high rate of respiration is therefore usually associated with a rapid rate of deterioration, i.e. with a high degree of perishability. The levels of respiratory activity in some representative examples, as meas-ured by the rate of production of carbon dioxide, are shown in Table 6. Recent work has shown not only that there is consider-able intraspecific variation in rates of respiration, but that the rate can vary considerably in different parts of the same structure and even in different layers of cells. I t is generally true, however, as indicated by the results in Table 6, that the products with rela-tively low overall rates of respiration are those which can be stored for longer periods in the fresh condition without loss of acceptability. This relationship also holds within a given species, so that it is sometimes possible to make useful predictions of the marketable life of given batches of material from measurements of the rates of respiration. This has been done, for example, for some varieties of apples, in which it has been shown that, irrespec-tive of the rate of respiration, if the material is always harvested at the same stage of maturity, the limit of acceptibility is reached


TABLE 6. COMPARATIVE LEVELS OF RESPIRATION IN SOME HARVESTED

FRUITS AND VEGETABLES

Material

Avocado Banana Mango Apple Tomato

Orange (Valencia) Sweet corn (husked) Snap beans Peas (in pod) Peas (shelled) Asparagus Lettuce Spinach Potato

Temperature (°C)

20 20 20 23 22-2

21 22-2 22-2 22-2 22-2 22-2 25 20 22

Rate of respiration (mg G02/kg/hr)

Basic rate

70 40 44

12-20 51

26 267 183 306 418 271

55 60-70

7-14

Climacteric maximum

310 120 126

30-40 (also shows a climacteric, but a maximum value not available)

Source3

1 2 1 3 4

5 4 4 4 4 4 6 7 8

»I. Biale, J . B. et e/., Plant Physiol. 29, 168 (1954). 2. Gane, R., New Phytologist, 35, 383 (1936). 3. Smock, A. M., Botan. Rev. 10, 560 (1944). 4. Tewfik, S. and Scott, L. E., / . Agric. Food Chem. 2, 415 (1954). 5. Haller, M. H. et al, J. Agr. Res. 71, 327 (1945). 6. McKenzie, K. A., Proc. Am. Soc. Hort. Sci. 28, 244 (1931). 7. Platenius, H., Plant Physiol. 18, 671 (1943). 8. Appleman, C. O. and Miller, E. V., J. Agr. Res. 33, 569 (1926).

when a given total weight of carbon dioxide has been evolved (equivalent to a loss of from 16% to 20% of the reserve carbo-hydrate of the fruit).

The amounts of respirable substrates which can be lost from the material without adversely affecting its eating quality vary greatly from product to product. Winter squash, for example, can sustain

Physiology 73 a loss of as much as a half of its initial total carbohydrate content over six months of storage and remain acceptable. Peas, on the other hand, undergo a very rapid loss of sweetness when harvested, due to the swift depletion by respiratory oxidation of their con-tent of sucrose. The relatively rapid rate of respiration in leguminous seeds, and in other materials such as sweet corn and asparagus in which the synthesis of cellular and reserve food mater-ials is rapidly taking place, is illustrated by the data in Table 6.

Fruit and vegetables can be divided on the basis of their respiratory patterns into two main groups. Most fleshy fruits, and this includes fruit-vegetables such as the tomato, show a charac-teristic temporary rise in the rate of respiration, which normally coincides with the more obvious changes in colour, flavour and texture associated with ripening. This respiratory peak, which heralds the onset of senescence, is called the climacteric and fruits which show this phenomenon we can refer to as climacteric fruits. The climacteric rise in respiration, which is not dependent on any change in environmental conditions, has been shown in a number of cases to be associated with an increase in protein synthesis, and it appears likely that this new protein is largely enzyme protein concerned in the various changes which take place during the ripening of the fruit.

The causes of the climacteric rise in respiration are not fully understood. In the normal way, as pointed out before, phos-phorylation and oxidation are linked together in the respiratory cycles and the rate of respiration could be regulated by the amounts of ADP, the main phosphate acceptor, available to form energy-rich ATP. The rapid utilization of the terminal phosphate group of ATP during active synthesis would release extra ADP, thus lowering the ATP/ADP ratio and permitting respiratory oxidation to proceed at a faster rate. The rise in respiration could therefore result from the increase in protein synthesis. However, the results of recent work have generally not lent support to this explanation. The uncoupling of the processes of phosphorylation and of oxidation, such as appears to have occurred in post-climacteric tissue, would have a stimulatory effect on the rate of


respiration, and it may be that the climacteric rise is due to the formation in the fruit of some as yet unknown natural agent capable of effecting this separation.

Yet another possible explanation of the increased evolution of CO2 during the climacteric lies in the recently discovered malate effect. When post-climacteric apple tissue is treated with malate, a marked increase takes place in the production of carbon dioxide with no significant change in oxygen uptake. The anaerobic de-carboxylation of malate which occurs under these conditions could account for the climacteric rise in C 0 2 output, while at the same time explaining the increase in respiratory quotient

GO2 evolved \ 0 2 absorbed/

taking place during the course of the climacteric. The size of the climacteric peak in respiration, the ratio of the

maximum rate of respiration to the preclimacteric minimum rate, and the time relations of the phenomenon in relation to the time of harvesting of the fruit all vary from species to species and indeed from variety to variety. In general, tropical fruits show a larger peak, a greater maximum/minimum ratio and a more rapid onset of the climacteric. This is illustrated by the curves representing the course of post-harvest respiration in some important species of fruit shown in Fig. 17.

Certain fruits, notably the citrus species, but also apparently pineapple, grape, fig (and rhubarb) do not show a typical climacteric rise in respiration and these, along with all com-mon vegetables other than the fleshy fruit-vegetables, form our second main group of fruit and vegetable products. Pro-vided the environmental conditions remain unchanged, the members of this group normally respire at a fairly steady rate or show a slowly declining rate of respiration associated with the progress of senescence. Any sudden environmental dis-turbance, such as the process of harvesting itself, may result in a burst of respiratory activity, but there is no autogenous climacteric.

Physiology 75 Mechanical damage to a tissue usually stimulates respiration.

Increases, due to wounding, of up to five times the original rate have been reported in carrots, though the bruising of apples appears to have little effect. The shelling of leguminous seeds

60

50

40 h

30

20

10 h

[ Avocado

_1_ 15 20 25

Time (days)

30 35 40

FIG. 17. The course, at 15°G, of post-harvest respiration in some important species of fruit.

c-the climacteric peak.

causes a substantial increase in the rate of respiration. In this case, the effect is probably due in part to the removal of the peas or beans from the atmosphere of the pod, which accumulates car-bon dioxide especially during the night. Rather surprisingly, the cutting up of snap beans has been found to cause no significant


increase in respiratory activity. Storage organs juch as tubers, fleshy roots, bulbs, etc., if stored under conditions which permit sprouting to occur, show an increase in respiratory activity associated with this new growth.

The effects of environmental factors on the rate of respiration

Temperature, and the concentrations of oxygen and carbon dioxide in the storage atmosphere, are the main environmental factors which influence the rate of respiration in harvested pro-duce. These factors are all amenable to control and a knowledge of their effects can therefore be usefully applied to the practical problems of fruit and vegetable storage. We shall also deal in this section with the effects of ethylene, a volatile substance which is formed naturally during the ripening of most fruits and which can produce a marked effect on the course of respiration.

Temperature. Living plant tissues will only function normally over a limited range of temperature. Outside certain limits, which vary somewhat according to the natural environment of the species concerned, physiological injury occurs. The upper limit for harvested products generally lies between 30° and 35°G, but wider variations in susceptibility to injury are found at the lower end of the temperature range. Thus, some fruits of tropical origin, such as the banana, are injured by exposure to tempera-tures below about 11 °C, while certain especially hardy com-modities, such as onions and some varieties of apples and pears, can withstand long periods of storage at temperatures below 0°C (see Chapter 7). Within the "physiological" range for the species, the rate of respiration normally increases with rise in temperature, particularly sharply in many cases in the range between about 5° and about 20°C (see Fig. 18). Near the upper limit the rate again declines. Recorded values of the Q,iot for respiration in fruit and

j* The Q^Q is the ratio of the rate of reaction at a given temperature to that at a temperature 10°C lower.

Physiology 77 vegetable species vary from 7 to less than 1, though values of be-tween 1 and 2 are most common. In climacteric fruits, lowering the temperature delays the climacteric rise and reduces the size of the peak. In fact, near the lower limit of the physiological

o u

ouu

400

300

200

100

.

Peas / / ...'s,

/ / \ Avocado / / '·■

/ : '■■

/ : \ t ! \ ' \ \

/ · \ / / \

/ 1 / \ / / /Asparagus \ .

/ / / V

f / c / / /.·· bnap /

/ / ' ·' bean,· / Banana / . ; / / / / / / y

/ ' x s **'* / / / y / / / / /

; y y / Tomato^. Lettuce ^ >——*" ^ ' · , - - ; . Τ Τ ." ^ ^ -— . ■** ^. >,-?.·· ■ ·—

. ^ x T t r . : ^ ' ^ ^ ^ . ■ ' G r a p e f r u i t ι771

·'·*77

"'*.'!! - , · - "",* | . | ,

J

H

J

H

A

*' J

0 5 10 15 20 25 30 35 40

Temperature (°C)

FIG. 18. The effect of temperature on the rate of respiration in some harvested fruit and vegetable products.

range, the climacteric may disappear altogether. The value of cool storage in delaying deleterious changes associated with too high a rate of respiratory activity need hardly be pointed out.

The concentrations of oxygen and of carbon dioxide. Since oxygen is absorbed and carbon dioxide released during aerobic respiration, it is only to be expected that the concentrations of these gases in


the storage atmosphere will affect the rate of the respiratory pro-cess. Air normally contains 21% of oxygen and 0*3% of carbon dioxide. In general, either a reduction in oxygen tension or an increase in carbon dioxide concentration will slow down respira-tion, but if the oxygen content is reduced beyond a certain point, the process proceeds anaerobically and ethyl alcohol and acetalde-hyde accumulate, while too high a level of carbon dioxide produces tissue-damage. Both the depletion of oxygen and the accumulation of carbon dioxide are natural consequences of the progress of respiration when fruit and vegetables are stored in a confined space. Control of ventilation or the artificial modi-fication of the composition of the storage atmosphere, therefore, provide useful means of controlling the rate of respiration, though the limits within which a worthwhile effect can be ob-tained vary greatly from material to material and are further modified by the temperature at which storage is carried out (see Chapter 7).

Ethylene, The physiological activity of this simple unsaturated hydrocarbon has been known since the 1920's when it was first identified as the main active principle in the fumes given off by the kerosene stoves which were employed at that time in the "sweat rooms" used for degreening citrus fruits. Later it was dis-covered that ethylene is formed in small quantities during the ripening process in most fruits and that this is why the volatile emanations from ripe fruits do themselves have a stimulatory effect on the metabolic activity of other plant materials held in the same store. Ethylene formation appears to be closely linked with the process of respiration. Among the common fruits, only citrus species, pineapple and mango do not produce this substance in measurable amounts during ripening.

In addition to its more obvious effects on the colour of plant products—it causes the breakdown of the chlorophyll pigments, unmasking the underlying colours of leaves, stems and fruits— ethylene has a marked effect on the course of respiration, particu-

Physiology 79 larly in climacteric fruits in which it induces an early onset of the climacteric. The size of the peak in respiratory activity appears to be little affected, but the time scale is shifted forwards, and this is also true in most cases of the other changes associated with ripening of the fruit. In non-climacteric materials, the general level of respiration and of other metabolic activity is increased by ethyiene treatment. These effects, which can in many cases be produced by concentrations of less than 1 ppm of ethyiene in the storage atmosphere, are greatly reduced, if not entirely elimin-ated, at low storage temperatures (4°C or below). Ethyiene is used commercially to induce the ripening of products such as bananas and tomatoes, to produce full colour development in citrus fruits and to blanch celery (see Chapter 7).

TRANSPIRATION Transpiration is essentially a surface phenomenon and the rate

of water loss per unit weight of material depends directly on the area of surface exposed and on the extent to which the surface is structurally modified to reduce the rate of evaporation. Leafy vegetables are particularly prone to a rapid loss of moisture, but even products such as apples, which have a low surface volume ratio and a waxy skin, can lose appreciable amounts of water during storage, and this inevitably leads to a loss of quality. The immediate cause of water loss is the existence of a water-vapour pressure gradient betwreen the external atmosphere and the in-ternal atmosphere near to the surface of the material. Since the internal atmosphere is normally saturated, the main environ-mental factor determining the rate of transpiration is the rela-tive humidity of the surrounding air. Theoretically, transpiration can be prevented by holding the material in air saturated with water vapour, but because of other considerations, chiefly micro-biological, this is not usually practicable for commercial storage (see Chapter 7).

Temperature also affects the rate of water loss. Any increase in temperature causes an increase in the vapour pressure of water


but a lowering of the relative humidity of the surrounding atmo-sphere, and therefore produces an increased rate of transpiration. Temporary high rates of evaporation also result from the intro-duction of warm produce into a cold atmosphere, even though the air is initially saturated with water vapour. This is due to the difference between the vapour pressures of water at the initial temperatures of the produce and of the air, respectively. The rate of transpiration will diminish in such a case as the produce cools down towards the temperature of the surrounding atmosphere.

OTHER METABOLIC CHANGES TAKING PLACE IN HARVESTED FRUIT AND VEGETABLES

I t will be convenient here to deal separately with vegetables and with fruit, since only the latter undergo the relatively rapid and spectacular changes associated with the process of ripening.

Vegetables

Generally, there is no parallel in vegetables to the sudden up-surge of metabolic activity which occurs in climacteric fruit during ripening, unless one includes the burst of renewed activity in storage organs such as fleshy roots and tubers during sprouting after periods of dormancy in storage. Provided that the environ-mental conditions are not markedly altered, biochemical changes in harvested vegetables are generally gradual and progressive, the directions of change depending on the stage of maturity of the organs concerned. Growing parts, such as shoots of asparagus, may continue for a limited period to grow in length, provided they are kept moist, and to synthesize cell-wall material, including lignin, at the expense of their accumulated reserves of simple monomeric constituents. Similar changes occur in the pods of leguminous species such as green or snap beans if these are harvested at an immature stage. In this latter case, a hydrolysis of protein also occurs in the pod, the resulting amino acids being translocated to the beans where resynthesis of protein takes place.

Physiology 81 The continued synthesis of lignin in harvested vegetables, though only on a very small scale, is probably quite general and can have a highly significant effect on the textural quality of these products.

Quantitatively, the most important biochemical changes taking place in harvested vegetables are those among the carbohydrate constituents. Immature storage tissues, such as those of seeds (peas, beans, sweet corn) and underground storage organs (potatoes, sweet potatoes, etc.), can continue to synthesize small amounts of starch. The synthesis of starch competes with respiration for supplies of available sugars. The starch/sugar balance in storage tissues is markedly influenced by temperature. Sugars accumulate at low temperatures, while the equilibrium moves the other way if the temperature is raised. The critical range of temperature over which the change from starch hydrolysis to starch synthesis takes place varies from product to product. For the potato this range is from 1 · 7° to 4-4°C (35-40°F), for sweet potato, a species adapted to warmer conditions, it is from 12-8° to 15-6°C (55-60°F). The accumulation of sugars taking place in storage organs held at lower temperatures can be largely reversed if the material is subsequently held at a temperature above the critical range.

Sucrose, glucose and fructose are readily interconvertible in the plant, and changes in the relative proportions of these main sugars also occur daring post-harvest storage. When sugars ac-cumulate in potatoes, the reducing sugars do o more rapidly than does sucrose. Sweet potatoes in contrast progressively accumulate sucrose, while showing little change in the level of reducing sugars. In parsnips, the sucrose content normally increases during storage at the expense of starch. Carrots, which contain relatively very little starch, have been found to show a rapid inversion of sucrose to reducing sugars immediately after harvesting, a change which is reversed during subsequent storage. This cycle is prob-ably linked with the course of respiration, which is temporarily stimulated by the disturbance of harvesting.

A feature of the biochemical changes occurring in harvested vegetables, which is of importance in relation to their nutritive value, is that ascorbic acid almost invariably decreases in amount


during storage. The content of carotene and of other carotenoid pigments, on the other hand, either shows little change or may even increase significantly due to continued synthetic activity.

Fruit—changes during the ripening process

Carbohydrates. The changes in the carbohydrate constituents are among the more prominent biochemical changes occurring in ripening fruits. Sugars almost invariably increase in amount due to the hydrolysis of polysaccharides, though some of the sugar formed is used for respiration. In fruits which contain large amounts of starch at the time of harvesting, such as the banana and mango, the starch content falls drastically during the ripening process—in these examples from between 14% and 18% on a fresh-weight basis to less than 1% in the ripe fruit. The small amount of starch in pome fruits also disappears during ripening. Degradation of cell-wall polysaccharides can also make an im-portant contribution to the increase in sugar content. In the banana, for example, hemicelluloses fall from 8% to 10% of the fresh weight at the green stage to between 1% and 2% when the fruit is ripe. The cell-wall materials are in fact the only major available sources of sugars in products such as citrus and pome fruits, in which the content of starch in the harvested products is negligible. In pear, the sugars have been shown to be in dynamic equilibrium with the polysaccharides of the cell-walls, the equili-brium shifting during the course of ripening in the direction of sugar formation.

Characteristic changes in the proportions of the different sugars occur in particular commodities. In grapes, strawberries and redcurrants, for example, the sucrose content remains relatively low during ripening but reducing sugars progressively accumulate. The same is true of the pome fruits, though in apples small temp-orary increases in sucrose content have been found to occur im-mediately after harvesting and again during the climacteric. In both apples and pears, fructose invariably accumulates in greater amounts than does glucose. Certain other fruits such as the drupe

Physiology 83 fruits (peach, apricot, cherry) and pineapple accumulate mainly sucrose as ripening progresses, the content of reducing sugars remaining relatively low. Sucrose also predominates in the ripe mango, but reducing sugars increase rapidly at the expense of sucrose during the post-climacteric period. The avocado is excep-tional among fruits in that the content of sugars actually falls as the fruit becomes ripe.

Citrus fruits are normally harvested in the ripe condition and post-harvest changes in carbohydrate constituents are both small and slow to develop. Changes in sugar content depend on the balance between respiration and the breakdown of cell-wall poly-saccharides. The edible part of the fruit generally shows a small increase in sugar level during storage.

Organic acids. The overall content of organic acids in most fruits generally first increases during the early stages of development and later decreases slowly and progressively during and subsequent to the process of ripening. There is, therefore, most commonly a drop in acidity during ripening, though in some cases, e.g. banana and pineapple, the peak of acidity occurs at the fully ripe stage.

Organic acids, like carbohydrates, are respirable substrates and there is little doubt that changes in overall acidity and in the levels of particular acids are in some way linked to the functioning of the respiratory cycles. However, it has been shown that loss of acids in some species is not affected by placing the fruit in an oxygen-free atmosphere, and the actual mechanisms involved in the accumulation and removal of individual acids are at present obscure. There is also some evidence that, so long as the fruit remains attached to the tree, organic acids are translocated to it from other parts of the plant and this process no doubt affects the balance of acids in the fruit before harvest. In some cases, acids may be precipitated as insoluble salts. Tartaric acid, for example, is precipitated in grapes as cream of tartar and in other com-modities oxalic acid may form insoluble calcium oxalate.


Quinic acid is an important acidic constituent of the immature fruits of many species (apples, pears, peaches, etc.). As the fruit matures, this acid usually decreases rapidly in amount, until, at the time of harvesting, its content is small relative to that of either malic or citric acid. In apples and pears, the content of malic acid reaches a peak during the early stages of development while the level of quinic acid is falling, and thereafter shows a steady de-cline. Citric acid, though present in much smaller amounts than malic acid in these species, reaches its highest concentration at a later stage than malic and then also decreases steadily in amount. The progressive loss of acids in harvested pome fruits appears to be unaffected by the other changes during the climacteric, but is accelerated with advancing senescence. Relatively small increases in particular acids—citric, quinic and shikimic—have been found in stored apples, an interesting case being that of citramalic acid which makes its appearance in the peel and increases in amount there during post-harvest storage. The degradation of pectic sub-stances which is accelerated after ripening can lead to the ac-cumulation of appreciable amounts of galacturonic acid. Changes among the amino acids will be considered briefly in a later section.

Pectic substances. The basic chemistry of pectic materials has been described briefly in earlier chapters. One of the most obvious changes in ripening fruits is the softening of texture which is associated with the progressive solubilization and depolymeriza-tion of pectic substances. Protopectin, the insoluble native form of pectin, is rendered soluble, presumably by enzyme action, though the nature of the enzyme system (s) involved can only be surmised. The soluble pectins themselves are further modified and depolymerized by the action of two distinct types of enzyme —the pectinesterases (P.E.s) which de-esterify the methyl esters, freeing the carboxyl groups of the galacturonic acid residues, and the polygalacturonases (P.G.s) which split the polygalacturonide chains into smaller units and possibly finally to galacturonic acid.

Physiology 85 Probably a combination of enzymes of these two types is respon-sible for the initial solubilization of the protopectin.

The changes in solubility and the subsequent decrease in the overall content of pectic substances are both well documented for

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FIG. 19. Changes in pectic substances during the growth, storage and ripening of pears—variety Doyenne Boussoch (after Weurman). The unbroken lines represent changes during growth on the tree, the broken lines changes in the ripening room, and the dotted lines represent periods

of storage at 0°C.

a wide range of species, including pome fruits, drupe fruits, bananas, tomatoes and melons. Data for pear are illustrated in Fig. 19. Citrus fruits do not show the marked softening in texture during ripening which is so characteristic of most other species


and, though changes of a similar kind take place, these occur over a longer period and are less pronounced.

Less is known about the qualitative changes in pectic materials during ripening. The degree of esterification appears to increase slightly in apples, pears and peaches during maturation of the fruit and then falls again more drastically as ripening proceeds. The drop in methoxyl content is less marked in apples than in the other species; changes of a similar kind occur also in avocados and tomatoes. Extracted pectins show a progressive decrease in vis-cosity and this has been attributed to depolymerization. Diffi-culty has been encountered in demonstrating the widespread presence in fruits of P.E.- and P.G.-type enzymes responsible for these qualitative changes. This may be due to the relatively short-lived nature of the pectolytic activity, which in investigated cases only reaches measurable proportions for a relatively short period during ripening. Alternatively, natural inhibitors may be present in the fruit, such as have, for example, been found in pear juice.

The changes in pectic materials in fruit are closely linked with the progress of respiration. All known factors affecting the course of the respiratory climacteric have a similar effect on the trans-formations of pectic substances, and it appears likely that part of the increased protein synthesis occurring during the climacteric is due to the elaboration of pectic enzymes concerned in these changes.

Nitrogenous compounds. The main change among nitrogenous compounds during the ripening of fruit is in the balance between protein and non-protein nitrogen. The rise in protein synthesis associated with the climacteric has already been mentioned. This formation of new protein occurs at the expense of the free amino acids present, which simultaneously decrease in amount. During senescence, this process is reversed and there is a progressive break-down of tissue proteins. The proportions of the various soluble nitrogenous constituents change during maturation, ripening and

Physiology 87 senescence in a way which appears to be characteristic of the species but is of little importance in relation to the eating quality of these products.

Pigments. The alteration in the colour of fruits is usually the most obvious change taking place during ripening. This is almost invariably associated with some synthesis of pigments, although in most cases the breakdown of chlorophyll makes an important contribution to the colour change. Carotenoid pigmentation is easily masked by chlorophyll, and in yellow to orange fruits the bulk of the carotenoid pigments is usually already present before the chlorophyll disappears. In tomatoes, however, a rapid syn-thesis, particularly of lycopene, takes place during the later stages of ripening. Anthocyanin pigmentation is generally not masked by the presence of chlorophyll and the progress of anthocyanin synthesis can be followed visually as ripening proceeds.

Volatiles. Ripening is normally associated with the formation of the wide range of volatile compounds (esters, aldehydes, alco-hols, ketones, terpenes, etc.) which make such an important con-tribution to the characteristic flavour of the fruit. These volatile substances are produced in very small quantities, the loss of car-bon in this form probably never exceeding 1 % of the amount removed as carbon dioxide. Ethylene is usually by far the most abundant of the volatile compounds produced, contributing as much as 70-80% of the total carbon in this fraction, so that the amounts of the individual odoriferous flavour-active substances are extremely small and their presence can only be detected by the most sensitive of techniques. The production of volatiles normally begins during the climacteric stage and continues during the pro-gress of senescence. Apart from their importance in relation to flavour, these volatiles can cause various undesirable effects in stored fruits.


Flavonoid compounds. Some of these phenolic constituents are responsible for the astringent tastes of unripe fruit. During ripen-ing, the content of flavonoids, like that of acids, generally decreases and this contributes to the mellowing of flavour which is so com-mon a feature of the ripening process.

PHYSIOLOGICAL DISORDERS IN HARVESTED FRUIT AND VEGETABLES

Many functional disorders, attributable to a variety of causes such as deficiencies of essential minerals, imbalances in water relations, etc., can develop during the growth of the plant and affect the quality of the crop at the time of harvest. These, for lack of space, must remain outside our present terms of reference. There are in addition certain well-recognized disorders that are essentially post-harvest in origin and which cause deterioration during storage of products which are in perfectly good condition at the time of cropping. These diseases of storage, which are generally associated with the premature senescence and death of tissues in particular parts of the structures concerned, can be con-veniently classified into two main groups. Firstly, there are the physiological injuries arising from the exposure of the products to temperatures below the normal physiological range—chilling in-juries. The second group includes those disorders which result from the accumulation in the tissues of volatile toxic substances (or possibly toxic precursors of volatile substances) especially under conditions of poor ventilation.

Chilling injuries

Mention has already been made of the fact that different species and even different varieties of the same species show differences in the lower limit of temperature to which they can be subjected without showing signs of injury. Broadly speaking, temperate species can withstand lower temperatures than can those of tropi-cal and subtropical regions, though there are exceptions to this

(By courtesy of the A.R.C. Ditton Laboratory—Dr. J. C. Fidler.)

(b)

PLATE 4. Physiological disorders in stored fruit, (a) Low-temperature injury in Cox's Orange Pippin apples stored in air at below 3'3°C (38°F). The uninjured core and the ring of sound tissue immediately below the skin are both typical, (b) Superficial scald in Lane's Prince

Albert apples.


wide generalization. Injury, when it occurs, may take many forms. Obvious symptoms of chilling injury are pitting of surfaces and discolorations either at the surface or of deeper-lying tissues (see Plate 4a). Surface pitting is a characteristic chilling injury of fruit structures, notably of citrus fruits and of fruit-vegetables such as cucumber, squash, pepper and runner bean. It is due to the death of small groups of epidermal and associated cells, which dry up and become sunken below the general level of the fruit surface. Commonly, the pits later become discoloured (brown). More ex-tensive surface discoloration is found on cold-stored citrus fruits in the form of "brown stain" or "scald" of oranges and "red blotch" or "peteca" of lemons. The browning is sometimes limited to the albedo, the white spongy layer of the skin, in which case it is not easily seen from the outside. Internal discoloration is a common symptom of chilling injury in a wide range of fruit and vegetable products. It may show a rather irregular distribution, as in the low-temperature breakdown of drupe and pome fruits, or it may be localized in particular structural features, as in "membranous stain" of lemons. In the latter case, only the car-pellary walls between the juice segments become discoloured. The darkening of the tissues in all these cases appears to be due to the uncontrolled action of phenoloxidase enzyme systems, result-ing from the death of the cells and the consequent loss of integrity of cell-membranes.

In other cases, chilling injury may manifest itself in more subtle ways. Sweet potatoes, for example, may lose the ability to syn-thesize carotenoid pigments, while peaches develop an undesir-able wooliness of texture. Preclimacteric fruits in cold storage can become incapable of normal ripening when subsequently moved to a warmer environment. In yet other examples, such as the tomato, the only apparent effect of subjection to too low a tem-perature is an increased susceptibility to rotting during later periods of storage.

One rather surprising feature of chilling injury in some cases is the fact that, although it only occurs if the temperature falls below a critical level, the short-term effects are more marked at tern-

Physiology 91 peratures immediately below this point than if the temperature is reduced to even lower levels. This may be explained as a result of the depressing effect of further cooling on the rate of reactions which in these cases are only initiated when the material is ex-posed to temperatures below the critical point. In the longer term, the lower the temperature, the greater is the final extent of the chilling injury.

The general lethal effect of low temperatures can be attributed to the fact that different enzyme systems are affected to different extents and that these differential effects are such as to permit the accumulation of toxic intermediate products in sufficient quan-tities to kill the cells. This explanation is supported by the fact that if the temperature is temporarily raised, during a period of storage at a temperature below the critical value, the onset of chilling injury can be delayed if not completely avoided. Such intermediate periods of warmer storage would result in the re-newed activity of enzymes normally responsible for the removal of the accumulated toxins.

Disorders due to the accumulation of toxins at higher temperatures

It is not only at low temperatures that the normal physiological mechanisms of plant tissues can become disturbed. Substances normally formed during anaerobic respiration—ethyl alcohol and acetaldehyde—are powerful toxins which will kill the cells if not quickly removed. A lack of oxygen in the storage atmosphere can therefore cause the development of symptoms similar to those which result from chilling injury, though in the former case the discoloration is characteristically found in the deeper-lying tissues which are most remote from the atmosphere surrounding the pro-duct. Potatoes, for example, develop "black heart" and apples "brown heart" if the oxygen supply is reduced to too low a level.

The development of toxic symptoms may also be associated with the accumulation in the tissues of the volatile flavour-producing substances which begin to be formed in fruits during the climacteric rise in respiration. This appears to be the cause of


the common disease of stored apples called "superficial scald" or more simply "apple scald". In this case, the skin and the tissues immediately beneath become discoloured (see Plate 4b), and it appears that the skin itself forms a barrier which delays the out-ward passage of toxic volatile compounds and causes them to accumulate in the outer part of the fruit. The identity of the com-pound (s) actually responsible for scald has not been established, but there appears to be little doubt that at least one necessary factor is present among the volatiles emanating from the ripening fruit.

Scald can generally be reduced if not prevented by treatments which accelerate the removal of these volatile substances from the tissues and from the atmosphere surrounding the fruit. The use of oil-impregnated wrapping papers to absorb the volatiles, inter-mittent warming, and improved ventilation of the store, are use-ful practical measures of control. Some success in the control of apple scald has also been achieved in recent years by the use of sprays or dips with alcoholic solutions of diphenylamine (1000-2000 ppm) or ethoxyquin (1800-2700 ppm). High humidity of the storage atmosphere has been found to increase the incidence of the disorder.

THE EFFECTS OF SYNTHETIC GROWTH-REGULATING SUBSTANCES

Various growth-regulating substances such as 2,4-dichloro-phenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), naphthalene acetic acid (NAA) and parachloro-phenoxyacetic acid (CPA) have been used to modify the physio-logical behaviour of fruit and vegetables. These compounds were first employed as abscission-preventing agents to control the drop of fruit before harvest, but it soon became apparent that, when applied either before or after harvest, they can also affect the course of post-harvest physiological changes. In most fruits, the effect is generally to increase the rate of respiration and to stimu-late ripening, though in citrus fruits they have been found to have

Physiology 93 a depressing effect on post-harvest changes. In climacteric fruits, the size of the peak in respiration is increased, an effect different from that of ethylene which merely causes a shift in the time scale. GPA, in addition to its other applications, has been successfully used to reduce physiological breakdown during the low-tempera-ture storage of pineapple and has been shown to improve the retention of ascorbic acid in this commodity.

Growth substances also have useful applications in the treat-ment of stored vegetables. 2,4-D and the methyl ester of NAA (MENA), for example, prevent the abscission of leaves in stored cauliflowers, while 2,4-D and 2,4,5-T have been shown to retard deterioration in colour and loss of florets in broccoli. Excessive drying and deterioration in colour in snap beans can both be retarded by pre-harvest treatment of the material with GPA.

Another group of physiologically active compounds which have useful effects on post-harvest physiology are the sprout inhibitors now widely used on potatoes and other storage organs. These include 2,3,5,6-tetrachloronitrobenzene (TGNB), maleic hydra-zide, MENA, 3-chloro-isopropyl-JV-phenyl carbamate (GIPC) and amyl and nonyl alcohols. TGNB, MENA and GIPG have the disadvantage that they inhibit the formation of wound peri-derm (cork) and, if used with damaged potatoes, they may therefore increase the rate of rotting. Maleic hydrazide, which differs from most of the other growth-regulating substances in that it depresses the rate of respiration, has also been effectively used to inhibit the sprouting of onions.

The various growth-substances discussed in this section exert their effects at very low concentrations. 2,4-D and 2,4,5-T, for example, are effective when applied as sprays at levels of between 5 and 25 ppm, although concentrations of up to 1000 ppm have been used in washes. The sprout inhibitors also are only required in concentrations of the order of tens of parts per million based on the weight of the material being treated. Nevertheless, residues are present on the treated material and the use of these substances is subject to food additive legislation in various countries. The Food and Drug Administration in America, for example, has


fixed tolerance limits of 5 ppm for 2,4-D on citrus fruit. Of the sprout inhibitors mentioned above, only TGNB, MENA and maleic hydrazide have been given a general clearance in the United States. GIPC has been used in Europe and in Australia, while the commercial application of amyl and nonyl alcohols appears thus far to have been confined to Britain.


BALDWIN, E., Dynamic Aspects of Biochemistry, 4th edn., University Press, Cambridge, 1963.

BIALE, J . B., Post-harvest biochemistry of tropical and sub-tropical fruits, Advances in Food Research, 10, 293 (1960).

BONNER, J., Plant Biochemistry, Academic Press, New York, 1950. FIDLER, J . C , Fresh fruit and vegetables, in Recent Advances in Food Science,

vol. 1 (Hawthorn, J . and Leitch, J . Muil eds.), Butterworths, London, 1962, p. 269.

HULME, A. C , Some aspects of the biochemistry of apple and pear fruits, Advances in Food Research, 8, 297 (1958).

MILLER, E. V., The physiology of citrus fruits in storage. I I , Botan. Rev., 24, 43 (1958).

PENTZER, W. T. and HEINZE, P. H., Postharvest physiology of fruits and vegetables, Ann. Rev. Plant PhysioL, 5, 205 (1954).

ULRICH, R., Postharvest physiology of fruits, Ann. Rev. Plant PhysioL, 9, 385 (1958).

VARIOUS AUTHORS (in English), in Handbuch der Pflanzenphysiologie (Ruhland, W. ed.), Springer-Verlag, Berlin, 1960.

VARNER, J . E., Biochemistry of senescence, Ann. Rev. Plant PhysioL, 12, 245 (1961).

CHAPTER 4

MICROBIOLOGY

A L L fresh plant foods eventually become senescent and lose their acceptibility after removal from the parent plant, and examples have been given in the last chapter of physiological disorders which can hasten this loss of quality. Undoubtedly the most important immediate cause of spoilage in fruit and vegetables is, however, the activity of micro-organisms. Attack by micro-organisms can occur at any stage, from the early growth of the plant in field or orchard to the final period of storage in the home. Each crop is susceptible during growth to the ravages of certain disease-producing organisms which are generally highly specific to the particular species or variety of plant concerned. These agricul-tural diseases, if not effectively combated, commonly render the material unmarketable before the normal time of harvest and are therefore strictly outside the scope of the present volume. There are cases, however, in which, though the initial infection may occur during growth, the organism at first remains quiescent and only produces symptoms of disease when the host tissues reach a more advanced stage of maturi ty during the post-harvest period.

The organisms responsible for well-characterized plant diseases are true pathogens in that they are able to invade perfectly healthy tissues in order to develop at the expense of the host. As the tissues become senescent and the integrity of the cellular membranes is progressively lost, micro-organisms find it easier to gain access and to establish themselves by drawing necessary nutrients from the host until, on the death of the tissues, resistance to invasion finally breaks down. The activity of organisms during these later

95


stages is saprophytic rather than parasitic and, although differ-ences in the structure and composition of the material still limit the range of forms which is able to develop, this range is extended to include general spoilage organisms, some of which are considerably less specific with regard to the nature of their substrate.

Thus, throughout their life histories, fruits and vegetables are constantly liable to deterioration at the hands of these microscopic agents of spoilage. The full magnitude of the resulting wastage can only be surmised but is undoubtedly immense, and the dual problem of keeping infection to a minimum and of depressing the activity of potential spoilage organisms during post-harvest storage reaches enormous proportions.

An important factor determining the types of organism which can grow on the tissues of fruits and vegetables is pH. The low pH of most fruits, for example, ranging from about 2 · 4 for lemons to a little over 5 for banana, is an effective deterrent to the growth of most kinds of bacteria. In this case, spoilage is practically always due to the activity of moulds. The range of pH for vege-tables is considerably higher and, with the exception of some fruit-vegetables such as the tomato which fall between the limits quoted above for fruit, the tissues of most common vegetables have pH values of between 5 · 0 and 7-0. In spite of this, the spoilage of vegetables, like that of fruits, is most commonly due to the activity of moulds, though bacterial rots such as the "soft rots" of carrot, celery and many other vegetable species and "watery rot" of potato (generally following the fungal disease, blight) can be of very considerable importance. It has been estimated that, while rotting of fruit as a result of bacterial action is negligible, up to 36% of the total losses of vegetables due to microbiological spoilage is attributable to bacterial infections.

The agricultural environment is exceedingly rich in micro-organisms. The soil, for example, contains a veritable multitude of forms, while the vegetation itself and any dead or decaying plant material harbours a further wide range of species. The external surfaces of plant structures therefore readily become contamin-

Microbiology 97 ated with a rich and varied microflora, underground organs and those normally resting on or near to the surface of the soil being especially rich in soil-borne species, while tree-borne fruits are contaminated more readily with spores from infections on the surrounding vegetation. Given conditions suitable for their further development, these surface organisms may or may not be capable of gaining entry through the protective surface layers of the organ concerned. The most common route of entry into intact structures is through natural orifices such as stornata and lenti-cels, but some species appear to be capable of directly penetrating the cuticle, especially while the latter is still thin during its early stages of development. Some organisms, such as the species of Diplodia responsible for stem-end rots in various fruits, character-istically enter through the calyx end during the flowering stage.

The direct exposure of underlying tissues, either by cutting during harvesting or by unintentional mechanical damage, greatly facilitate the entry of micro-organisms. Even the most minute and indétectable of injuries such as might be caused by small pieces of grit provide a ready route for the invading organisms. The bac-teria responsible for the rotting of vegetables (mainly species of Erwinia and Pseudomonas) seem to enter most commonly through cuts or wounds, although related forms have been isolated from the internal tissues of intact organs. It appears in fact that motile rod-shaped bacteria belonging to the families Pseudomonodaceae and Enterobacteriaceae are by no means uncommon inhabitants of healthy plant tissues. The presence of these organisms, which appear to originate from the microflora of the soil, is usually not associated with the development of rots and it is clear that not all micro-organisms which are capable of effecting entry into the tissues of fruits and vegetables will necessarily cause spoilage, at least so long as the host tissues remain in a sound physiological condition.

A common feature of most spoilage organisms, both fungal and bacterial, is that they secrete pectolytic enzymes which, by soften-ing and breaking down the plant tissues, facilitate their own

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spread through the host. Some species, such as Pénicillium italicum and P. expansum, which produce blue mould of citrus and of pome fruits respectively, can invade sound healthy tissues and their spread is little affected by the stage of maturity of the fruit. In many other causes the rate of spread is highly dependent on the physiological state of the host tissues, and a considerable delay may occur between the initial infection and the appearance of visible rotting. The healthy tissues may be capable of metabolizing enzymes secreted by the invading organisms. A good example of this is provided by the species of Gloeosporium which cause bitter rot of apples (especially Cox's Orange Pippins) one of the most important diseases of English fruit.

A particular organism, once established, may, by sapping the vitality of the host tissues and causing disorganization, pave the way for the entry of other species. In the potato, for example, late blight, a disease of the growing crop caused by the fungus Phytophthora infestons', is commonly followed by watery bacterial rot of the tubers.

It would be quite impossible in the space of a short chapter like this to mention more than a few of the enormous number of individual species of micro-organisms which have been associated with the spoilage of fruits and vegetables. Members of as many as forty-two different genera of moulds have been isolated from apples alone and a complete list covering all fruit and vegetable commodities would be extremely lengthy. In fact, the respon-sibility for a very large part of the total wastage during storage and marketing can be attributed to the activity of a relatively small number of genera, the more important of which are listed in Table 7.

Probably the most widely destructive of all spoilage organisms are the species of Pénicillium. One American report estimates that as much as 30% of all fruit decay is caused by members of this genus. P. italicum (blue mould) and P. digitatum (green mould) are important spoilage organisms of citrus fruits, especially oranges and lemons, while P. expansum is common on pome fruits (see Plate 5a). Several other species of Pénicillium are also effective in

Microbiology 101

(By courtesy of the A.R.C. Dittoti Laboratory—Dr. J. C. Fidler.)

PLATE 5. Mould rots on fruit, (a) A rot caused by Pénicillium expansum (blue mould) on apples, (b) Λ "black rot" caused by a species of

Trichoderma on Australian navel oranges.

spoilage and, in addition to most common fruits, vegetables, in particular fruit-vegetables and those such as beetroot and sweet potato with a relatively high sugar content, are especially sus-ceptible to attack.

F. & v.—f-;


Botrytis cinerea (grey mould), which is the conidial stage of Sclerotinia fuckeliana, a species causing a disease of grape vines, is also an extremely widely-occurring agent of spoilage and has been associated with the rotting of practically all fruit and vegetable commodities. The related species S. sclerotiorum is a common cause of "watery soft rots" in vegetables, but also produces a "cottony" rot of lemons and limes, while yet other species of Sclerotinia cause "brown rots" of drupe and pome fruits.

"Soft rots" due to species of Rhizopus, in particular R. nigricans, are also very common on both fruits and vegetables, although citrus fruits are notably resistant to this type of decay. A similar kind of rotting is produced by species of Mucor, a related genus which is sometimes found growing on ripe fruits, especially strawberries.

Members of the genus Alternarla are responsible for several different types of decay. A. radicina causes a "black rot" of carrots, A. brassicae, a "brown rot" of Brassica spp. Alternaria citri is com-mon in citrus fruits, being especially troublesome in grapefruit where it causes a "soft rot". In oranges, on the other hand, this same species produces a dry type of spoilage.

Fusarium is another genus which is a common cause of spoilage, particularly among vegetables. Root vegetables, asparagus and cucurbitaceous fruits are especially susceptible to Fusarium rots. Attacks by this genus are not, however, completely confined to vegetable commodities. Species of Fusarium are also responsible for a serious "blossom-end rot" of oranges and for a "stalk-rot" of bananas. (F. oxysporum var. cubense is the cause of Panama disease, a devastating disease of banana plantations which is referred to in the next chapter.)

Phytophthora is perhaps best known as the causative agent ofthat most dreaded of potato diseases, late blight—due to P. infestans— but other species of this same genus produce spoilage during storage in the form of a "pink rot" of potatoes, "leathery rots" of lemons and of strawberries and "downy mildews" on leafy vege-tables. This last condition may also be caused by species of the related genus Peronospora and by Bremia lactucae, an organism

Microbiology 103

which normally confines its attacks to lettuce but which may occasionally be found on other members of the same family such as the globe artichoke.

Gloeosporium and Co l le to trie hum, two genera which are so closely related that they have been combined by some authorities into a single genus, produce a well-characterized disease of storage known as Anthracnose. In this disease, the rotting of internal tissues is associated with a brown spotting of the surface of the infected organ. The condition is especially common in fruits and in fruit-vegetables, including the legumes, but it is also found in onions and in leafy Brassica spp. Gloeosporium fructigenum and G. album are responsible for the important storage disease of apples called "bitter rot" . Gloeosporium has also been associated with a "stalk ro t" in banana.

Two other mould genera which are active in producing "stem-end rots" are Diplodia and Phomopsis. Diplodia natalensis (Phy~ salospora rhodina) and Phomopsis citri (Diaporthe citri) are responsible for important storage diseases of citrus fruits, while other species attack pome, drupe and cucurbitaceous fruits and yet others have been implicated in the spoilage of sweet potatoes.

Drupe fruits, bananas, figs, dates, grapes and onions are among the commodities which are most susceptible to the development of "black mould rots" caused by species of Aspergillus, notably A. niger. These same commodities, with the addition of the cucurbitaceous fruits and of raspberry, are also most commonly affected by "green mould rots" produced by species of Clado-sporium. These rots are characteristically different in appearance and in shade from those caused by the species of Pénicillium, and this is true also of the green rots which are occasionally produced on oranges by the growth of the mould Trichoderma viride. Another characteristically-pigmented spoilage organ-ism is Trichothecium roseum which sometimes causes a "pink mould ro t" in apples, pears, peaches and in cucurbitaceous fruits.

"Powdery mildews" on fruit are generally due to the growth of species of Sphaerotheca or of Podosphaera. Gooseberries and black-


and redcurrants are especially susceptible to this type of infection, the causative organism in these cases being Sphaerotheca mors-uvae. In apples, on the other hand, "powdery mildew" is normally due to the growth of Podosphaera leucotricha.

The genus Phoma is associated with a dry type of spoilage in a number of commodities. Phoma betae, for example, causes a "dry ro t" of beetroot, P.foveata a similar condition in potatoes, while P. destructiva can cause a troublesome leathery rot of ripening tomatoes.

An offensive "sour rot" of citrus fruits, which normally only develops after the product has been in storage for some time, is produced by the organism Oospora citri-aurantii. Another member of this genus, 0 . pustulans, affects potatoes, in which it produces a disease of storage characterized by the appearance of an unsightly spotting of the skin.

Finally, serious losses of harvested vegetables have been in-curred from time to time as a result of the activity of three other mould genera—Pythium, Rhizoctonia and Cerato stornella. Pythium in-duces a condition known as "cottony leak" in cucurbitaceous fruits and in legumes. Rhizoctonia a variety of spoilage conditions in root vegetables and Cerato stornella fimbriata has been implica-ted in a serious "black ro t" of stored sweet potatoes. Another species of this last genus—C. paradoxa—has commonly been associated with the post-harvest decay of pineapples in tropical regions.

These then are a few of the more important fungal agents of spoilage in harvested fruit and vegetables, f To complete this brief summary, mention should be made again of the bacterial genera,

■f Another remarkable fungal genus which should perhaps be mentioned at this point is Byssochlamys, two species of which, B. fulva and B. nivea, although not normally associated with the rotting of fresh fruit, have been implicated on a number of occasions in the breakdown of canned products such as berries and plums. These organisms produce unusually heat-resistant ascospores (sexual spores) which, if not destroyed by the heat-treatment during processing, are capable of germinating in and causing the spoilage of the contents of the can. Otherwise, the fungi as a class are relatively easily destroyed by heat and the microbiological spoilage of canned foods is almost always due to the growth of heat-resistant spore-forming bacteria (see Chapter 10).

Microbiology 105 Erminia and Pseudomonas, to which most of the forms causing bac-terial rots of vegetables belong. Most vegetable species are prone to attack by these more insidious agents of spoilage, but the greatest wastage appears to occur among the members of the family Umbelliferae, e.g. carrot and celery, the organism most commonly responsible for the rotting in these cases having been given the apt name of Erminia carotivora.

The appearance of a fungal rot, its distribution, colour and consistency, is generally quite characteristic of the organism causing it and, although the identification of the species is a specialized task which in some cases may be extremely difficult, the commoner genera can be fairly readily distinguished by the microscopic appearance of their spore-forming structures. A num-ber of examples from among the genera included in the above summary are illustrated in Fig. 20.

THE CONTROL OF MICROBIOLOGICAL SPOILAGE IN FRESH FRUIT AND VEGETABLES

The problem of controlling spoilage is naturally divisible into two distinct parts. In the first place, every endeavour should be made to keep the infection of the material with potential spoilage organisms to as low a level as is possible. Secondly, attempts may be made to remove or to destroy the infections once present, or otherwise to inhibit the development of the organ-isms so as to prevent them from producing their damaging effects.

Limitation of infection

Infection may occur either before or after harvest. In the field, contamination with soil-borne micro-organisms is unavoidable, but much can be done to remove other sources of infection such as established rots on old and discarded material and on material sticking to boxes, box-liners or other containers used for collecting

Alternar ia Rhizopus Fusarium

Phytophthera Gloeosporium Sclerotinia

FIG. 20. The characteristic microscopic appearance of the (asexual) spore-bearing structures of some of the more important mould genera causing spoilage of fruit and vegetables, i, conidia (asexual spores). ii, conidiophores. iii, sterigmata. iv, vesicle, v, sporangium, vi, sporangio-spores. vii, sporangiophore. viii, columella. ix, rhizoids (root-like hyphae). x, macroconidia. xi, microconidia. xii, compact stromatic mass of hyphae, giving rise to a closely packed group of conidiophores—

an acervulus.

Microbiology 107 the crop. Machinery used in the field for grading and other pur-poses should be kept as clean as possible. Despite all precautions, the surfaces of fruits and vegetables normally support a rich micro-flora, but the organisms present are usually mainly bacteria— lactic acid and coliform bacteria, micrococci and members of the Achromobacteriaceae. These organisms are not themselves cap-able of initiating spoilage in sound tissues and are therefore not of great importance in fresh products. They can, however, assume a greater importance in connection with methods of preservation, such as the pickling of vegetables.

The same precautions with regard to the avoidance of infection apply to all subsequent handling and storage operations. Rotting is seldom uniform and any individual fruit or vegetable develop-ing a rot should, where possible, be removed so as to prevent cross-contamination. Air-borne fungal spores readily contaminate all surfaces and care should be taken that containers, stores, etc. are kept scrupulously clean and that personnel handling the products take all reasonable precautions.

Under this heading, the importance of the avoidance of mech-anical damage to the surfaces of the products should also be re-emphasized. In the case of fruit, resistance to mechanical injury progressively decreases as the material softens during ripening, and this is an important factor in determining the stage at which the fruit should be picked. Common sense dictates the various precautions which can be taken to avoid damage during harvest-ing and subsequent handling, and failure to observe such pre-cautions inevitably results in a considerably greater incidence of rotting.

The removal, inhibition and destruction of spoilage organisms

The total numbers of micro-organisms present in freshly har-vested plant products vary greatly with the species, the degree of exposure of the part(s) concerned, the locality and the conditions under which the crop has been grown. Estimates have ranged from a few hundred to several million organisms per gram of


tissue. After harvesting, these numbers can increase very rapidly unless measures are taken to prevent this from happening. In one particular case for example, the numbers of organisms on black-currants were found to increase between about five- and eleven-fold during a period of not more than 24 hours involved in transporting the material from field to processing factory. Because of their simpler methods of reproduction, yeasts and bacteria multiply more consistently and uniformly than do moulds, in which the increase in numbers depends on the stage of develop-ment of the infection and, in particular, on the presence of spore-producing my celia. Complete exclusion of the latter is, however, extremely difficult.

Since the vast majority of the organisms present are situated on the external surfaces of the material, the most obvious and, not surprisingly, the longest standing method of reducing the load is simply to wash the material thoroughly with water. Washing, un-less combined with the application of germicidal or germistatic substances, with which we shall be dealing later, has however certain disadvantages. For example, infections can be spread from localized sites onto previously uninfected material, while the film of water left on the surface may encourage the growth of any remaining organisms. Moreover, the water itself may also be a source of additional forms capable of causing spoilage. In spite of this, the overall load of micro-organisms can be substantially reduced by such procedures, and this can be a contributing factor in the successful use of chemical washes, dips and sprays.

The rate of growth of contaminating micro-organisms is mark-edly affected by environmental factors such as humidity and temperature. High humidities and relatively high temperatures, within the normal physiological range, both favour growth. Cool-ing the material as soon as possible after harvest and storing it at as low a temperature as is consistent with the avoidance of physio-logical damage to the fruit or vegetable is therefore generally most effective in preventing a rapid build-up of infection. Cooling may be carried out using ice, solid carbon dioxide (which also

Microbiology 109

inhibits the growth of organisms by increasing the concentration of carbon dioxide in the atmosphere around the material), con-ventional refrigerated air-coolers, or by a process known as hydro-cooling, in which the heat is withdrawn from the material either by immersing it in circulating cold (usually iced) water in large tanks or tunnels, or by exposing it to continuous sprays of cold water. Hydro-cooling is used extensively in the United States, and increasingly in other countries, for the treatment of drupe fruits (peaches, apricots, cherries), and of vegetables such as celery, asparagus and sweet corn.

If the material is to be moved any considerable distance from the growing area, refrigerated transport is clearly desirable and the provision of the necessary facilities is progressively increasing. Ships used for transporting fruit and vegetables are generally equipped with conventional refrigeration machinery, while for carriage by rail and road, special trucks which can be kept cold with ice or solid carbon dioxide are now commonly provided. The continued maintenance of low temperatures during any sub-sequent periods of storage is obviously also desirable in order to further delay the onset of spoilage, though some causative organ-isms, such as Gloeosporium in English apples, are psychrophilic and low temperature in itself may be insufficient to prevent the development of an infection.

Apart from these purely physical methods of control, the organ-isms responsible for the spoilage of fruits and vegetables can in most cases be effectively inhibited, if not actually destroyed, by the use of various chemical treatments. Chemicals can be applied in solution as washes, sprays and dips, by the use of impregnated wrapping papers, box-liners, etc., in gaseous form by fumiga-tion, or even, for pre-harvest treatments, in a solid form as dusts.

The application of fungicides to growing crops in field or orchard has in many cases proved very effective in reducing the incidence of post-harvest spoilage. Pre-harvest treatments are especially useful with soft fruits, which are so easily injured during and after picking. Excellent results have been obtained, for


example, with strawberries, using compounds such as Captanj ( N- (trichloromethylthio) -4-cyclohexene-1,2-dicarboximide), Thi-ramf (bis (dimethylthiocarbamoyl) disulphide), Ferbamf (ferric dimethyldithiocarbamate) and dehydroacetic acid. However, not all fungicides are equally effective against all kinds of spoilage. Zinebf (zinc ethylene-l,2-bisdithiocarbamate), for example, has proved relatively ineffective for combating post-harvest decay in strawberries, but has been found to be useful in the control of certain storage rots on citrus fruits and of downy mildews on a range of fruit and vegetable species. In grapes, Cap tan is effective against "grey mould rot" but fails to prevent the growth of Alternarla and Cladosporium. Again, "brown rot" in peaches is more effectively combated by pre-harvest applications of sulphur or of Bioquin 1 (copper 8-hydroxyquinoleate) than by treatment with the dithiocarbamate-type fungicides. Gaptan, Thiram and Ziramf (zinc methyldithiocarbamate), however, have been shown to be highly effective in reducing the incidence of Gloeosporium rots in stored apples following treatment of the fruit in the orchard before picking. The Ministry of Agriculture, Fisheries and Food pub-lishes each year, through the Agricultural Chemicals Approval Scheme, a List of Approved Products for Farmers and Growers (H.M.S.O.) which gives information on the various fungicides which can be safely and effectively applied to the crops in the field.

Finally, the growth substances 2,4-D and 2,4,5-T, which were discussed in the last chapter in connection with the control of fruit drop, have also been shown to have an inhibitory action on the development of certain types of rot, especially those caused in citrus fruits by species of Alternarla, The effect in this case, how-ever, is probably due to an increase in the resistance of the plant tissues to infection, rather than to any direct effect on the organ-isms themselves.

f Unfortunately these particular fungicides, especially Captan and Thiram, have been found to produce taints in canned or frozen fruit or vegetable pro-ducts and are therefore not recommended for use on material which is to be processed (see p. 185).

Microbiology 111 Various chemical anti-spoilage agents have also been employed

for the treatment of fruits and vegetables after harvesting. The first compound to be widely used as a post-harvest dip appears to have been borax (sodium tetraborate) which, when employed as a warm 5-8% (preferably alkaline) solution for the dipping of citrus fruits, markedly reduces the incidence of rotting, particu-larly that due to species of Pénicillium. Thiourea has a similar effect, but the presence of either of these substances on the surfaces of fruit is now illegal in many countries, including the U.K. A substance which has largely replaced borax as a dip for citrus fruits is sodium ortho-phenylphenate which, in combination with hexamine (included to prevent an injurious effect of the phenyl-phenate on the rind of the fruit), is actually more effective than borax even at lower concentrations (2% or less) and without the need for warming. The phenylphenate appears to penetrate into the tissues of the fruit and is therefore probably effective against more deep-seated infections. This compound and the correspond-ing phenol (o-phenylphenol) have also been used to combat decay in several other products, including peaches, pears, mangoes and sweet potatoes. It has promising applications for use in the hydro-cooling of fruit and vegetables, though it can cause injury to delicate tissues if the surfaces are not rinsed after treatment.

Another substance which has been commonly used in washes and in hydro-cooling water is chlorine (50-125 ppm), usually in the form of the hypochlorite of either sodium or calcium. Chlorine has not always proved effective as an inhibitor of spoilage, but it reduces the population of micro-organisms in the wash water. It has been shown in some cases to reduce the bacterial rotting of vegetables, against which most other chemical treatments appear to be singularly ineffective.

The sodium salt of dehydroacetic acid, which has already been mentioned in connection with the pre-harvest treatment of fruit, has recently proved to be a very useful inhibitor of spoilage when applied as a post-harvest dip or in hydro-cooling water at con-centrations of 0*5-1-5%. Highly significant reductions in the


incidence of rotting have been obtained in peaches, strawberries, raspberries, cherries and blackberries.

Many other substances, including sorbic and peracetic acids, various fungicides normally used on growing crops in the field and the growth substances 2,4-D and 2,4,5-T, have been shown to reduce spoilage in particular commodities when applied in the form of post-harvest washes, dips and sprays. Bacterial rots of vegetables can also be effectively reduced by using sprays and dips containing various antibiotics, but the use of these substances has obvious dangers. (The question of the possible toxicity to the con-suming public of residues of these various anti-microbial agents is discussed briefly in the following section.)

Chemical anti-spoilage agents can also be used as imprégnants in wrapping papers, box-liners, etc. The most important com-pound used in this way is diphenyl, which has proved highly effec-tive against Pénicillium and stem-end rots in citrus fruits. Diphenyl, which volatilizes slowly from the impregnated paper, is fungistatic rather than fungicidal, and rotting can proceed if the fruit is removed from the source of the vapour. Small amounts of di-phenyl, which has a rather unpleasant pungent odour, are detect-able in the skins of treated fruit and the use of such fruit for the preparation of juices and squashes has been found in some cases to produce a taint in the finished products. Unfortunately, when used in the concentrations necessary to lower the incidence of spoilage, diphenyl generally gives rise to injurious effects in most other kinds of fruit. This is also true of o-phenylphenol, another compound which has been successfully used in wraps for citrus fruit.

Another imprégnant which has been used in wrapping papers for citrus fruits and also for grapes is iodine (with potassium iodide). However, the iodine vapour produces an unsightly dis-coloration of the packaging materials. Wraps impregnated with copper sulphate have also been employed, especially with pears and peaches. In this case, the paper merely forms a fungicidal barrier preventing the spread of organisms from infected to un-infected fruit. The use of oil-impregnated wrapping papers,

Microbiology 113

which can of course be combined with the presence of a volatile anti-fungal agent, was mentioned in the last chapter in connection with the control of physiological scald in pome fruits.

Treatment with gaseous rot-inhibiting substances is similar in principle to the use of volatile imprégnants such as diphenyl, but the material is normally only exposed to the gas for a relatively short period, although treatments may be repeated at intervals during storage. The most widely-used of such fumigants is sulphur dioxide, which has been used down the ages for treating grapes. Such treatments, which are now normally carried out using gas from cylinders, rather than by the traditional method of simply exposing the fruit to the fumes of burning sulphur, are not free from the danger of physiological injury to the fruit itself and they have to be carefully controlled. Concentrations of 0*25 -1% are usually employed for periods of 20-25 minutes, and an even dis-tribution of the gas through the atmosphere of the room is im-portant. These procedures are highly effective in reducing the incidence of Botrytis rot in grapes of the Vinifera varieties (Euro-pean grapes) and have also been used successfully with rasp-berries, but the concentrations required appear to be toxic to most other fruits and vegetables. Fumigation with nitrogen tri-chloride at concentrations of up to 25 mg per cubic foot has been used to reduce rotting in a wide range of crops, notably citrus fruits, melons and tomatoes, while ammonia has given good results when applied to citrus fruits and peaches. Treatment with gaseous compounds has the advantage that it can be applied to material which has already been packed, provided that the packaging is such that the penetration of the gas is not seriously hindered. Sulphur dioxide and ammonia can be generated gradu-ally during storage by the introduction of solid compounds such as bisulphites and ammonium compounds which decompose to release the respective gases.

No account of the use of gaseous anti-spoilage agents would be complete without some reference to the effects of carbon dioxide. Storage in atmospheres high in carbon dioxide depresses the rates


of metabolism both of micro-organisms and of the plant tissues themselves, and is a most useful means of retarding deterioration in fresh fruit and vegetables. In fruits, the ripening process is delayed by the use of increased concentrations of carbon dioxide and this in itself reduces the incidence of microbiological spoilage, but the gas also has a direct effect on the micro-organisms. Con-centrations of carbon dioxide of between 25% and 50% have been found to be highly effective in reducing the decay of soft fruits such as blackcurrants, especially at low temperature—4*4°C (40°F). High concentrations of carbon dioxide, however, can in-jure plant tissues, especially those of peaches, apricots, strawberries and raspberries, and anaerobic respiration, with the associated accumulation of ethyl alcohol and other toxins, is encouraged if the oxygen concentration is allowed to fall too far. Grapes, peas, carrots and sweet corn appear to be especially resistant to carbon dioxide injury. To obtain maximum benefit from the use of car-bon dioxide, low temperatures of storage are required, while at the same time moderate concentrations of the gas are helpful in reducing spoilage due to psychrophilic organisms such as Gloeo-sporium. The use of solid carbon dioxide in the transport of perish-able soft fruit results in a useful combination of low temperature and high carbon dioxide concentration.

TOXICOLOGICAL CONSIDERATIONS The commercial use of chemical anti-spoilage agents carries

with it the perennial problem of the possible toxicity to the con-sumer of residues remaining on the treated material. Some of the substances discussed in this chapter, such as carbon dioxide, are obviously free of any toxicological hazard. Others, such as sulphur dioxide, are long-established and well-tried food preservatives which are specifically permitted (with or without stated maxi-mum levels) by the food legislation in most countries. O-phenyl-phenol and the corresponding phenate, diphenyl and sorbic acid are more recently introduced anti-spoilage agents which are also widely included in permitted lists of preservatives, although in

Microbiology 115 Britain the use of sorbic acid is limited to certain foodstuffs other than fruit and vegetables.

The position with regard to the many other chemical agents discussed in this chapter is much more complicated. Some coun-tries have set up bodies whose concern it is to keep careful watch on the use of chemicals in the treatment of food crops and to recommend legislation where this is considered necessary. In the United Kingdom, pesticides applied to growing crops (even if they are intended to preserve those crops when in store) are con-trolled by the voluntary Pesticides Safety Precautions Scheme administered by the Ministry of Agriculture, Fisheries and Food. The scheme also applies, where it is not in conflict with the various regulations made under the Food and Drugs Act, to anti-spoilage agents applied to crops after harvest and when in store. At present no specific legislation exists with regard to the use of particular anti-spoilage agents other than those mentioned above. In the United States, fruit and vegetables treated after harvest with any chemical must, according to the Pesticide Residue Amendment of the Food, Drug and Cosmetic Act, be labelled as containing a preservative, and the Food and Drug Administration has estab-lished permissible residue tolerances for a large number of chemi-cal compounds in relation to specific commodities. The situation, however, is in a constant state of flux as new compounds are tested, improved methods of application devised and as further informa-tion is obtained relating to the degree of toxicological hazard associated with the use of particular substances.

Fruit and vegetables as possible sources of food-poisoning

So far in this chapter we have been concerned with the micro-organisms as agents of spoilage, but the presence of certain organisms in foodstuffs, including fruit and vegetables, can give rise to illness in human beings. True food-poisoning, as opposed to certain other conditions which result from the presence of purely chemical poisons, is almost invariably due to the activity of certain species of bacteria and two main kinds of effect can be


distinguished. In some cases, the bacteria produce toxins during their growth on the foodstuff and the ingestion of the toxin causes illness, even though the bacteria themselves may since have been destroyed. This is referred to as Intoxication. Other kinds of food-poisoning are due to pathogenic organisms which must be in-gested in the viable condition in order to produce the disease. Such disorders come under the heading of Infection. Causative agents of both these kinds of food-poisoning can be found on fruit and vegetables.

The most serious type of intoxication—botulism—which has re-sulted in a large number of fatalities, is caused by the organism Clostridium botulinum. This species is a common inhabitant of soils in all parts of the world and crops can readily become con-taminated with its spores. The deadly toxin of C. botulinum is only produced, however, during the growth of the organism and, in fruit and vegetables, this can only take place after the tissues have been killed, as by processing or cooking. Even then, growth is severely restricted or completely prevented if the p H of the tissue is below about 4 · 5. As a result, most fruits are relatively free of the danger of causing botulism, though a few cases of the disease have been attributed to the eating of processed olives, figs, apricots, pears, persimmons and peaches. Vegetables, however, provide more amenable substrates for the development of the organism.

Because of the considerable time necessary for growth and toxin production after the plant tissues have been killed, fresh fruit and vegetables have not been implicated in the causation of botulism, but under-processed canned and bottled products (and more especially home-canned or bottled products) have been involved in many outbreaks. In fact, of 462 cases of botulism occurring in the U.S.A. between 1899 and 1947, 305 were attributed to the eating of canned vegetables (mainly home-processed green beans, corn, beetroot and asparagus), and 37 to canned fruits (mainly home-processed olives and figs). I t should be stressed, however, that the heat treatments used in the commercial canning of vege-tables (and of other medium- and low-acid foods) have long been primarily determined by the necessity of ensuring destruction of

Microbiology 117

the spores C. botulinum and that since the modern methods of con-trol of processing were introduced in 1926, very few cases of botul-ism have been attributable to the consumption of commercially canned foods. Indeed, since the spores of some potential spoilage organisms are even more heat-resistant than those of C. botulinum, in order to ensure freedom from spoilage an additional safety margin must be introduced (see Chapter 10).

A much commoner and much less serious type of food intoxica-tion is that caused by species of Staphylococcus, notably S. aureus. These organisms reach food mainly from animal sources, and fruit and vegetables, either fresh or processed, are seldom impli-cated as immediate causes of the disease. Of 346 outbreaks of staphylococcal food-poisoning recorded for England and Wales between 1951 and 1955, only 6 were attributed to the consumption of vegetables (all canned) and only 2 to fruit (fresh).

The main organisms responsible for food infections are various species of Salmonella, notably S. typhimurium. These, like the staphy-lococci, are picked up originally either from infected human sub-jects or from animals (including poultry). The number of out-breaks of food-poisoning due to Salmonella in England and Wales between 1951 and 1955, which could be directly attributed to the eating of fruit and vegetables, was a mere 9 as compared with a total, for all foods, of 307. In 4 of the 9 cases, fresh fruit was impli-cated, while the other 5 were traced to vegetables—in one case fresh, in the other 4 processed. During the same period, out of a total of 503 outbreaks of food-poisoning for which the identity of the causal organism was not discovered (if indeed these outbreaks were of microbiological origin), only 28 were attributed to the consumption of either fresh or processed fruit or vegetables.

In conclusion, then, we can say that, provided adequate pre-cautions are taken against the development of Clostridium botu-linum in processed products, fruit and vegetables, compared with other foods (notably meat, poultry and eggs), are relatively un-important in the causation of food-poisoning. O n the other hand,


it should always be remembered that products such as salad vege-tables and fresh fruit, which are normally used in the raw state can be contaminated with other pathogenic organisms and should be thoroughly cleaned and washed before being eaten.


ALEXOPOULOS, C. J., Introductory Mycology, Chapman & Hall, London, 1952.

BERAHA, L., et al. y Control of decay of fruits and vegetables during market-ing, Rev. Appi. Mycol., 42, 94 (1963).

BROOKS, F. T., Plant Diseases, 2nd edn., Oxford University Press, London, 1953.

CHARLEY, V. L. S., The prevention of microbiological spoilage in fresh fruit, J. Sci. Food Agric, 10, 349 (1959).

DEWBERRY, E. B., Food Poisoning, 4th edn., Leonard Hill, London, 1959. FRAZIER, W. C , Food Microbiology, McGraw-Hill, New York, 1958. MINISTRY OF AGRICULTURE, FISHERIES AND FOOD, Agricultural Chemicals

Approval Scheme, Lists ofApprovedProductsforFarmers andGrowers,ii.M.S.O., London, published annually.

ROSE, D. H., et al., Market Diseases of Fruits and Vegetables: Citrus and Other Sub-tropical Fruits, U.S.D.A. Miscellaneous Publication No. 498, 1943.

SAMISH, Z. et al., The microflora within the tissues of fruits and vegetables, J. Food Sci., 28, 259 (1963).

SMITH, W. L. JR., Chemical treatments to reduce postharvest spoilage of fruits and vegetables, Botan. Rev., 28, 411 (1962).

TANNER, F. W. and TANNER, L. P., Food-borne Infections and Intoxications, 2nd edn., Garrard Press, Champaign, Illinois, 1953.

TOMKINS, R. G., The microbiological problems in the preservation of fresh fruits and vegetables, J. Sci. Food Agric, 2, 381 (1951).

VARIOUS AUTHORS, in a supplementary issue toy . Sci, Food Agric. 7 (1956), comprising papers read at the National Crop Protection Conference, Eastbourne, 1955.

VON SCHELHORN, M., Control of micro-organisms causing spoilage in fruit and vegetable products, Advances in Food Research, 3, 431 ( 1951 ).

CHAPTER 5

PATTERNS OF PRODUCTION AND TRADE

FRUIT and vegetables, much more so than the cereals, lend them-selves to cultivation on a c'cottage-garden" scale and the collec-tion of complete and accurate production data, therefore, presents a very difficult problem, especially in view of the large number of individual commodities which fall within the present group. In-formation is particularly lacking for many tropical species, even for some important starchy products, such as plantain and taro, which can make such a major contribution as staple articles of diet in the areas in which they are grown. However, there is a substantial international trade in some of the more widely used and popular items such as the major fruits, and for these much more complete and reliable data are available.

The patterns of production and of trade in these latter com-modities change little from year to year. Minor fluctuations do, of course, occur and for most individual items there is a slow but progressive increase in the general level of production, but the overall picture remains substantially unaltered. The main areas of cultivation of some important fruit and vegetable species are shown in Figs. 21 and 22, while recent data relating to the levels of production and of trade in some of the major commodities are summarized in Appendix B.

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124 Concerning the Utilization of Fruit and Vegetables

FRESH FRUIT AND VEGETABLES

Production

Total world production of the major fruits of commerce ap-proaches 90 million*]· tons per annum. This figure does not include the large amounts of fruit, e.g. wine-grapes, cider-apples, etc., which are used for the preparation of fermented drinks. Neither does it include the output of some important tropical species such as mango, guava, papaya, etc., which for various reasons do not enter substantially into international trade. The overall produc-tion of fruit must therefore be very considerably in excess of the above-quoted figure. I t has been estimated for example, that the production of mangoes in India alone is in the region of 5 | million tons per annum.

The pattern of production depends of course on climate. Thus, we have the tropical fruits such as the banana and the pineapple, the Mediterranean fruits of which the citrus fruits and grapes are the main examples and the deciduous fruits of temperate regions such as the apple. Predominant among fruit in terms of produc-tion is the banana, which together with its close relation, the plantain, probably gives a total yield of considerably more than 30 million tons per annum. More precise figures cannot be given, since these species are very widely grown in tropical regions and reasonably accurate information is only available in respect of bananas grown for export. The starchy plantains are commonly cooked and eaten as a vegetable in tropical countries and per capita consumption in some parts of Africa and America is as high as 2 lb per day. Bananas grown for export are of the sweet seedless varieties, the commonest being Gros Michel. Unfortunately, this variety is susceptible to a fungal disease called Panama disease (caused by a species of Fusarium) and this has resulted in very serious losses in some areas, particularly in the West Indies— traditionally the chief source of supply for the United Kingdom market. Large-scale replanting with disease-resistant varieties,

t Weights in this chapter will be given in long tons (2240 lb) unless otherwise stated.


such as Lacatan and Robusta, has, however, resulted in a recent recovery of production in this area. The country with by far the largest output of bananas is Brazil, with a crop representing more than a fifth of the world total.

The only other tropical fruit which figures prominently in world trade is the pineapple, a native of Brazil which, like the banana, is now grown to some extent in most tropical countries. T h e centre of pineapple production, however, is Hawaii which produces more than 4 0 % of the world crop. The greater par t of world supplies of pineapple are used for processing, the variety Cayenne being particularly favoured for this purpose.

Among citrus fruits, the orange is of outstanding importance, with a yield (of all types, including tangerines, bitter oranges, etc.) of over 15 million tons per annum. The output of oranges, which has almost doubled since before the Second World War, is about ten times that of either grapefruit or of lemons. T h e United States is the largest single producer of each of these three commodities. Over 80% of the world's grapefruit and around 30% of the oranges and lemons are grown in the U.S.A. Another outstand-ingly important producer of lemons is Italy who, with an output representing over a quarter of the world total, closely rivals the United States. Some two-thirds of the American crop of oranges and about a half of the crop of grapefruit is processed. The pro-cessing of citrus fruits, in particular of grapefruit, is also of im-portance in the West Indies and to a lesser extent in South Africa, Israel and J a p a n (mandarin oranges).

The other important fruit of Mediterranean regions is the grape, with a total annual production of some 40-45 million tons. However, about 8 5 % of this is used for making wine and about another 6% is dried, so that the amount consumed as fresh fruit is relatively small. Italy, Turkey, the U.S.S.R. and the U.S.A. are the main producers of table grapes. Raisins and sultanas account for about 8 5 % of the grapes dried, the United States, Turkey, Australia and Greece producing the greatest quantities, while Greece (about 90%) , Australia and South Africa dry al-most all the remainder as currants. The other dried fruits include


prunes (about 190,000 tons per annum) of which some 70% are prepared in the United States, figs (about 160,000 tons), over 50% of which are dried in Turkey and Italy, and dates (about 1 · 5 million tons). Dates provide a staple food over large areas of North Africa and South-West Asia as well as contributing signifi-cantly to international trade.

Finally, we come to the deciduous pome and drupe fruits of temperate regions, of which the apple is the most important repre-sentative with a production, excluding cider apples, of between 12 and 15 million tons per annum. World output of apples fluctu-ates somewhat from year to year largely because of variations in the European crop, which shows an alternation of good and bad seasons. Large-scale replantings carried out since the end of the Second World War, however, are now tending to reduce this variability. The United States and Italy grow the largest crops of apples. The bulk of European supplies is disposed of fresh, but about a third of the United States crop and about a quarter of the Canadian crop are taken by the processors.

The production of pears, like that of apples, tends to fluctuate as a result of the variability of crops in western Europe, which again shares with North America, and in this case also with China, the distinction of being one of the main centres of production. World output now averages about 5 million tons per annum, especially prominent European producers being Italy and West Germany. A large proportion of the pears grown in the U.S.A., in Commonwealth countries overseas and in South Africa is used for canning.

The U.S.A. and Italy together also produce about 60% of the world crop (over 4 million tons) of peaches, another commodity which is favoured by the processors. The clingstone varieties, which make up about 75% of the Californian crop and more than a half of each of the Australian and Canadian crops, are almost entirely canned. Production of plums, which appears to fluctuate more than that of any other common fruit (between less than 3 million and about 5 million tons per annum), is centred particu-

Patterns of Production and Trade 127 larly in eastern Europe, with Yugoslavia the chief producing country.

Cherries are of two main kinds—sweet and sour—the latter being used principally for canning or for brining prior to the pre-paration of glacé or maraschino cherries. Total world production of cherries is a little over 1J million tons per annum and more than 75% of the crop is grown in Europe. In spite of this, the U.S.A. again has the highest production of any individual coun-try. The United States also produces the largest crop of apricots— about 16% of the world total of around 1 million tons. A sub-stantial proportion of apricot production is processed, mainly by canning in the U.S.A., Australia and South Africa, but appreci-able quantities are also dried or pulped.

Turning now to vegetables, pride of place here must clearly go to the potato with an estimated annual production of over 250 million tons. This figure is actually higher than are the corresponding production figures for each of the major cereals, but it should of course be remembered that in the case of the potato a much higher proportion of the weight is water, so that the actual food value is therefore relatively smaller. Other starchy vegetables of major importance on a world scale are the sweet potato and the yam, for which the corresponding combined pro-duction total is about 115 million tons, and cassava with an output of some 75 million tons. For comparison, the combined total for the pulses—dried peas, beans, lentils, etc.—is about 29 million tons. These various figures are estimates of overall pro-duction and include large amounts of material which are used for stock-feeding and other purposes rather than for human con-sumption. For example, only about a third of the Russian potato crop is used directly as human food, and Russia with an output of over 70 million tons of potatoes accounts for more than a quarter of the total world production of this commodity. A similar pattern of utilization of potatoes is found in several other European countries.

Next in importance to the U.S.S.R. as a producer of potatoes is Poland, followed in order of decreasing output by China, West


Germany, France, the U.S.A., East Germany, the United King-dom and Czechoslovakia. Data for the major tropical starchy vege-tables are less complete and for many countries rough estimates rather than official returns have been used in arriving at the final totals. China (including Formosa), Japan , Indonesia and the countries of West Africa make especially important contributions to the overall production figure for sweet potatoes and yams, Brazil and Indonesia to that for cassava, while India is especially prominent as a producer of pulses, particularly of chick peas, haricot beans and lentils. The U.S.S.R., however, has the highest production of dried peas and Italy of dried broad beans.

Individual non-starchy vegetable commodities, by comparison, are grown on a much smaller scale and reasonably complete pro-duction data are only available for tomatoes and for onions. The tomato is of particular importance with a total output of about 17 million tons per annum. Over 30% of the total is grown in the U.S.A. and of this quantity about a half is used by the processing industry. Italy is second in importance, with Spain, Egypt, Bul-garia and Brazil following in order of decreasing production. The onion is another vegetable which is important on the world scene and one in which there is a considerable international trade. The United States and J a p a n each produce about 15% of the world crop of onions (almost 8 million tons), Spain, Egypt, Turkey, Italy, the Netherlands and France also growing large amounts.

Production data for other non-starchy vegetables are only avail-able for a limited number of countries. The total output j of fresh vegetables other than potatoes in the countries of the Organiza-tion for European Economic Co-operation (E.E.C, plus E.F.T.A.) in 1959-60 was about 30 million tons—a value little smaller than the corresponding one for potatoes. If one adds to this the North American figure for the same season we arrive at a combined total for western Europe and North America of about 50 million tons, a very substantial level of production in relation to those discussed above for the other major commodities. Second only to the tomato in importance in these areas is cabbage, of which the

| Output in this case is the amount actually marketed as human food.

Patterns of Production and Trade 129 United States produces the largest crops, followed by Italy, the U.K., France and Germany. The United States, not unnaturally, also has the highest output of lettuce, carrots, peas, celery and beetroot, but Italy grows the largest weights of cauliflower, the U.K. of Brussels sprouts and rhubarb, and France of asparagus. An appreciable proportion of the vegetable output in these coun-tries is used by the processing industries and, indeed, in many cases is specifically grown under contract for this purpose.

Trade

Fresh fruit and vegetables are not the easiest of materials to transport for long distances over land or sea and yet maintain in a marketable condition. Nevertheless, international trade takes place on a very considerable scale. The grand total of exports of fruit and vegetables runs at some 21 million tons per year, this total being made up of roughly equal weights of fruit and of vegetables.

In general, there is a constant drain of fruit from the poorer to the richer countries and from the warmer to the cooler parts of the world. A good deal of the trade in fruit is concerned with satis-fying the western European markets since, except for truly tropi-cal species, North America is more or less self-supporting. Traditional patterns, established when the introduction of fast shipping services with suitable storage facilities first made possible the large-scale overseas movement of fruit, have long dominated the picture. Thus, the United Kingdom has always drawn its im-ports of fresh fruit largely from the Commonwealth. Other Euro-pean countries have also tended where possible to obtain supplies of tropical and subtropical species from their own associated terri-tories. The country importing the largest amounts of fresh fruit in recent years has been West Germany with total imports in 1962 of almost 2*6 million tons. Rather surprisingly, the U.S.A. is next on the list, but her imports of nearly 2 million tons per annum are very largely made up of bananas. The United Kingdom and France lie some way behind, each with imports of between about


1 ·2 and 1*4 million tons per annum. Canada receives some 0 · 6 million tons, almost entirely from the United States, while the Netherlands (0-4) and the U.S.S.R. (0-3) are also notable fruit-importing countries.

The greatest volume of international trade in fresh fruit is naturally in those popular species, bananas and oranges being outstanding, which can only be satisfactorily grown in tropical or subtropical regions. Almost 4 million tons of bananas are shipped each year. Many tropical countries contribute to this total, but Ecuador is the main exporter. The United States takes over 4 0 % of the bananas entering international trade, and West Germany, the United Kingdom and France each also receive considerable amounts. Argentina imports appreciable quantities of bananas from Brazil to whom she supplies apples in return. Exports of fresh pineapples, by comparison, are a mere trickle, coming mainly from Brazil, Mexico and Cuba, with the United States and Argentina as chief importers. Canada, the United Kingdom, France and West Germany receive much smaller quantities. Pine-apple is second only to peach in the amount which is canned and most of the pineapple crossing international boundaries has al-ready been processed.

There is also a brisk international trade in the citrus fruits, especially in oranges. Spain supplies about a third of the world export total of some 3 million tons, sending large supplies to West Germany, the leading importer. France also receives large amounts from Morocco and Algeria, while the U.K. draws her supplies largely from South Africa and Israel. Over 50% of lemon exports come from Italy and, in addition to West Germany, France, the U.K. and Russia receive substantial supplies. The grapefruit entering international commerce comes mainly from the U.S.A., most of her exports going to Canada. After Canada, the U.K. is the main importer of grapefruit, receiving supplies chiefly from Israel, South Africa and the West Indies.

Although less than 10% of the world crop of grapes is used fresh as table grapes, exports of this commodity are only exceeded by those of the three main fruits—banana, orange and apple. Italy

Patterns of Production and Trade 131 and Bulgaria are important suppliers of table grapes to the Euro-pean markets, the United States meeting Canada's import require-ments. In Europe, Russian imports are second only to those of West Germany which generally represent between 35% and 40% of the total supplies crossing international boundaries.

This brings us to the range of deciduous pome and drupe fruits, in which the bulk of the trade is intra-European. Italy tops the list of exporters for all of these commodities except for the apricot, her exports of which are exceeded by those of Spain and of Hungary. The apple is outstanding as a trading commodity, with yearly exports running at between 1 · 4 and 1 · 7 million tons, over 40% of the total coming from Italy. The European deciduous fruit crop is of course seasonal and supplies have to be brought in from the southern hemisphere during the first half of the year, before the European and North American crops are ready. The United Kingdom receives such supplies mainly from South Africa and Australia, but improvements in storage techniques are help-ing to extend the season for European-grown pome fruits.

Exports of pears and of peaches each amount to around 0 · 3 mil-lion tons, almost 80% of the peaches and about 40% of the pears coming from Italy. West Germany easily tops the list of importers for both of these commodities, with other European countries notably the U.K. (for both), Switzerland (for peaches) and Sweden (for pears), also prominant. International trade in plums, apricots and cherries is of relatively minor importance in the northern hemisphere, although there are shipments of apricots from South Africa to the U.K. in the winter months.

The movement of fresh vegetable commodities across national boundaries occurs on a scale more or less equivalent to that of fresh fruit, but it is generally restricted to trade between neigh-bouring countries. The outstanding vegetable of commerce is the potato, and total exports of this commodity are about 2-6 million tons per annum. Most of the movement of potatoes is confined either within Europe or within North America, regions which are both largely self-supporting in this respect. Most European countries either produce a small surplus or have a small import


requirement for potatoes, except for Iceland where home supplies only meet about two-thirds of the demand and the Netherlands where production exceeds home requirements by about 3 5 % . The Netherlands is, in fact, the world's main exporter of potatoes. Other countries exporting smaller amounts include France, Po-land, the United States, Denmark and West Germany. Algeria, Czechoslovakia, the U.K., East Germany and, rather surprisingly, Ceylon, take most of the imports. Small amounts of new potatoes are exported from the Mediterranean region—Cyprus, Egypt and Israel—to northern Europe, notably to the United Kingdom, in the winter.

The Netherlands, in addition to being the world's chief ex-porter of potatoes, is also a major exporter of several other fresh vegetable commodities, in particular of onions and tomatoes which she also produces in considerable excess of her own require-ments. Bulgaria, Spain, the United States and Italy are the other main tomato-exporting countries, with West Germany, the U.K. and Canada as the principal international markets for this com-modity. The other main exporters of onions are Egypt, India, Spain, the United States and Poland and the supplies in this case go largely to the U.K. , West Germany, Ceylon, Malaya, France and Canada. Most European countries import small amounts of fresh vegetable products and only the Netherlands, Italy and Portugal are completely self-supporting in this respect. In North America, the United States produces a surplus which is used in the main to meet Canada's import requirements.

PROCESSED FRUIT AND VEGETABLES Canned

Production. The canning of fruit and vegetables is carried out on a very substantial and increasing scale, especially in North America, western Europe, the U.S.S.R., Australia, J a p a n and South Africa. Total world output, excluding juices, now runs at more than 8 million tons per annum, of which about 3-5 million tons are fruit. Vegetable canning is relatively more important in


temperate regions. For example, in the United Kingdom, the production of canned vegetables is about seven times that of canned fruit.

The United States produces almost two-thirds of the world total of canned fruit and is the principal producer of each of the main varieties, although most of the canned pineapple is actually packed in Hawaii. Australia, Canada, the U.K. and West Ger-many follow in order of decreasing output, with many other countries canning smaller amounts. The main product on a world scale is canned peach followed by pineapple, pear, apricot and apple, but the relative importance of varieties varies in different producing countries according to local preferences, export oppor-tunities and the availability of raw materials. For example, in the U.K. the largest packs, in most years, are fruit salad, plums and rhubarb . Hawaii specializes in canning pineapple, and J a p a n mandarin oranges.

The United States is also by far the leading producer of canned vegetables with a total output of between 3 million and 4 million tons per annum. Sweet corn, tomatoes, green beans, peas and asparagus are the main vegetable packs and, if tomato products such as juice, puree, catsup, etc. are taken into account, tomato is outstanding as easily the most important processing vegetable— over 3 \ million tons of tomatoes are grown in the United States for the processing industry each year. Compared with the United States, the United Kingdom produces about 0 · 6 million tons of canned vegetables, the main individual products here being beans in tomato sauce and processed peas. These together make up about 60% of the total, fresh peas and carrots accounting for the greater part of the remaining production.

Trade. Rather more than a quarter of the world production of canned fruit enters international trade. Most of this is fruit packed without sugar. The United States is the leading exporter as well as the main producer, followed by South Africa, whose exports of canned fruit account for as much as 2 % of all her exports. Australia, Spain and J a p a n also export canned fruit on a

F. & V.—F


considerable scale. The United Kingdom is easily the most impor-tant market for canned fruit, taking over 40% of world supplies. Most of the remainder goes to West Germany, with Canada and the U.S.A. next on the list.

International trade in canned vegetables, although much smaller than that in canned fruit, is nevertheless substantial. The principal American export is canned asparagus, which is imported mainly by West Germany, Switzerland and Sweden. Canned tomatoes and tomato products are also exported in appreciable quantities, mainly to Canada, as are smaller amounts of many other varieties. Rather surprisingly, the United States herself im-ports considerable quantities of canned tomatoes. The United Kingdom exports small amounts of canned vegetables particu-larly to the smaller Commonwealth countries but on balance she has large net imports—about 150,000 tons, as compared with about 390,000 tons of canned fruit. The large bulk of these im-ports of canned vegetables is of tomatoes and tomato products, mainly from Italy but with smaller quantities coming from Portugal and Bulgaria.

Frozen

Production. The freezing of fruit and vegetables is carried out on a much smaller scale than is canning but it has been of pro-gressively increasing importance since the end of the Second World War. World production of frozen fruit in recent years has been about 0-4 million tons per annum of which over 85% has been produced in the United States, with Canada and Mexico together accounting for about a further 10%. The main products in the Americas are strawberries and cherries. Many other countries freeze fruit on a relatively minor scale.

No accurate figure is available for the total production of frozen vegetables, but the United States, which again produces by far the largest quantities, has an output of frozen vegetables which, at about 0-9 million tons, is almost three times that of frozen fruit. Indeed, the weights of some vegetables grown for

Patterns of Production and Trade 135 freezing in that country are comparable to, or represent a sub-stantial proportion of, those which are grown for the canning industry. The main frozen vegetable packs in the U.S.A. are potato products, followed at some distance by peas and green beans. These last two commodities also provide the main frozen packs in the United Kingdom, which has a total production of frozen vegetables approaching a tenth ofthat of the United States.

Trade. Mexico is the leading exporter of frozen fruit, supplying over 40% of the grand total of about 44,000 tons which enters international trade. The main Mexican exports are of frozen strawberries. The United States, Poland and the Netherlands supply most of the remainder. The United States is also the lead-ing importer of frozen fruit, taking most of the Mexican supplies, while West Germany, Canada and the United Kingdom also have appreciable net imports of these commodities. British imports are mainly of unsweetened fruit for manufacturing purposes, in-cluding substantial quantities of frozen pineapple from South Africa.

Full information regarding the pattern of international trade in frozen vegetables is not available. However, the United States exports considerable quantities of these products, mainly to Canada and the United Kingdom. In Europe, the Netherlands and Sweden are the principal exporting countries.

FRUIT JUICES Juices are becoming such an important means of utilizing fresh

fruit that any account of production and trade which omitted to mention these products would be incomplete. The quantity of juice produced each year now amounts to the equivalent of about 10 million tons of fresh fruit. Considerably more than half of the total production is prepared in the United States, other important juice-producing countries being West Germany, Italy, France and Switzerland. About 60% of the American citrus crop is used for juice extraction, citrus, pineapple, grape, soft fruits and apple


being the main varieties used for the preparation of juices. The U.S.A., France, Italy and Israel are the main exporters of fruit juices and West Germany, Canada and the U.K. the main im-porters. Britain now imports citrus juices in quantities equivalent to some 0 · 2 million tons of fresh citrus fruit. This compares with actual citrus fruit imports of about 0-5 million tons.

A SUMMARY OF PRODUCTION AND TRADE FOR THE U.K.

The production of fruit and vegetables in the United Kingdom, although substantial, is far from being high enough to meet home requirements. Among the main commodities, self-sufficiency is most nearly approached for potatoes, the output of which varies between about 92% and 9 6 % of the total quantity needed. Only about 30% of the fresh fruit consumed is home grown, while the corresponding proportion for fresh vegetables other than potatoes is about 8 0 % . Practically all the soft fruits and plums for fresh consumption are grown in the United Kingdom, as also are about half the required amounts of apples and pears, but tropical fruits, citrus fruits and grapes, which together represent about half the total weight of fruit consumed, all, of course, have to be imported.

Apples and pears are imported both from Europe—mainly Italy—and America—both the U.S.A. and Canada—in the au tumn and winter, and from the southern hemisphere—South Africa, Australia and New Zealand—in spring and summer. Supplies of citrus come mainly from Spain, Israel, South Africa and Cyprus, with the West Indies sending much smaller but in-creasing quantities in recent years. The West Indies also send most of the bananas, the supplies of this commodity being aug-mented by smaller shipments from West Africa, the Canary Islands and Brazil. Grapes are imported mainly from Spain and South Africa, while peaches come largely from Italy and, in the winter months, from South Africa. Imports of other kinds of fresh fruit are relatively small—apricots from Spain and South Africa,


pineapples from South Africa and the Azores, cherries from Italy and France.

In addition to taking large imports of fresh fruit, Britain is also the world's most important market for canned fruit. Peach, pear, pineapple and orange are the main import packs in syrup; South Africa, Australia and the U.S.A. supplying most of the canned peach, South Africa and Malaya most of the pineapple, Australia the bulk of the pear and J a p a n most of the orange. Packs of un-sweetened fruit and pulp come mainly from Spain. Small amounts of frozen fruit are also imported, with the Netherlands providing most of the fruit packed in sugar. A trend in recent years has been towards the importation of proportionately greater amounts of fruit packed without sugar. In 1962, imports of unsweetened frozen fruit were over four times greater than those of fruit frozen in sugar, a major part of the unsweetened import packs being of frozen pineapple from South Africa. The United Kingdom also has large net imports of fruit juices, mainly citrus, the principal sources in this case being the West Indies, South Africa, Israel, Italy and the United States.

Dried fruits are, of course, all imported and the United King-dom figures very prominantly in world trade, taking about three-quarters of the currants and about a quarter of the raisins and sultanas entering international commerce. The currants come from Greece and Australia, the raisins and sultanas mainly from Australia, South Africa, the U.S.A. and Turkey. Smaller amounts of prunes, figs and dates are also imported, the principal sources for these commodities being respectively the United States, Tur-key and Iraq.

In common with most other countries, Britain produces a much higher proportion of her own requirements of fresh vegetables than she does of fruit. Nevertheless, she is second only to West Germany as an importer of vegetables. The main import com-modity here is tomatoes, of which Britain produces only about a half of her normal requirements. Tomatoes are imported mainly from Holland, but Spain, the Canary Islands and the Channel Islands also send substantial quantities. United Kingdom imports


of onions are the highest of those of any country, supplies in this case coming mainly from Spain and North Africa. New potatoes are brought in from Cyprus, Israel, North Africa and the Canary Islands, cauliflowers from Italy and France, and lettuce from the Netherlands and the season for many other fresh vegetables is extended by small imports from the Mediterranean region.

Compared with the production of canned fruit—about 100,000 tons per annum—home production of canned vegetables at almost 600,000 tons per annum is substantial. However, processed peas and beans in tomato sauce account for about two-thirds of the total so that the amounts of fresh vegetables used for canning are not so great as might at first appear. Fresh peas and carrots are the main fresh vegetables canned. While imports of canned fruit are about four times the level of home production, those of canned vegetables are less than a quarter of home production and about 87% of these latter imports are of tomatoes and tomato products, mainly from Italy. Frozen vegetable production now stands at about 85,000 tons per annum, more than a half of this being frozen peas. In comparison, about 25,000 tons of frozen vegetables are imported annually, mainly from the Netherlands, Sweden, Canada and the U.S.A.

The dehydration of vegetables is carried out on a relatively small scale and recent production figures are not available. It has been estimated that in 1960 about 35,000 tons of vegetables were dehydrated in the U.K. but, with the increasing popularity of some more recently introduced dehydrated products, it is likely that the above figure is now being substantially exceeded and the indications are that this method of preservation is likely to be of increasing importance in the future.

Britain then, as we have seen, has large net imports of both fresh and processed fruit and vegetables, although she does carry on a minor export trade in certain processed items. For example, small amounts of canned fruit and vegetables are sent to West Germany and to various Commonwealth countries overseas, and limited quantities of fruit juices—mainly lime—are also exported, notably to the United States. The broad picture, however, is

Patterns of Production and Trade 139 largely determined by climatic considerations and the pattern of production of fresh commodities is substantially fixed. In the future, we shall no doubt see a continuation of the recent trend towards an even greater expansion of the processing industry. This may be regretted by some, but the relative convenience of processed products is an important factor which is making them more and more an essential part of the organization of our com-plex modern society.

SOURCES OF STATISTICS O N P R O D U C T I O N

A N D T R A D E

Almanac of the Canning, Freezing and Preserving Industries, Edward E. Judge, Westminster, Maryland, published annually.

COMMONWEALTH ECONOMIC COMMITTEE, Fruit—A Review, H.M.S.O., London, published annually.

COMMONWEALTH ECONOMIC COMMITTEE, Fruit Intelligence, H.M.S.O., London, published monthly.

F.A.O. Yearbook of Production, F.A.O., Rome, published annually. F.A.O. Yearbook of Trade, F.A.O., Rome, published annually. FRUIT AND VEGETABLE CANNING AND QUICK-FREEZING RESEARCH ASSOCIA-

TION (now the FRUIT AND VEGETABLE PRESERVATION RESEARCH ASSOCIATION), Statistical Review, Chipping Campden, Glos., published annually.

MINISTRY OF AGRICULTURE, FISHERIES AND FOOD, Agricultural Statistics,

H.M.S.O., published annually. O.E.C.D. Bulletins of Agricultural Statistics, O.E.CD., Paris, published at

intervals. O.E.C.D. reports in the Documentation in Agriculture andFoodseries, O.E.C.D.,

Paris, published at intervals. 0.E.E.C. Agricultural and Food Statistics, O.E.E.C, Paris, published at

intervals. United Nations Yearbook of International Trade Statistics, U.N.O., New York,

published annually.

CHAPTER 6

CONTRIBUTIONS TO HUMAN NUTRITION

T H E R E are two primary requirements for good human nutrition. Firstly, there must be the provision of an adequate total amount of food, in terms of calories, to furnish the energy necessary to carry out the normal activities of a human being and to prevent actual hunger. In the second place, the food intake must be balanced, so that there is a sufficient intake of protein, which is necessary for the building and replacement of tissues, and of various accessory factors—the vitamins and minerals—without which the body cannot be maintained in a state of health. Fruit and vegetables as a class help to meet both of these requirements, but the contributions of individual members obviously depend on their specific composition and on the quantities in which they are eaten in different parts of the world.

This question of the level of consumption is obviously of primary importance. The major starchy vegetables, for example, are staple foods in the areas in which they are grown, and they therefore make a most important contribution not only in terms of calories but also with regard to certain other essential dietary factors of which they may actually, in comparison with other fresh plant foods, be relatively poor sources on a weight-for-weight basis. To take a specific example, the potato is a comparatively poor source of vitamin G, yet potatoes supply a considerable proportion—in Britain about one-third—of the total intake of this vitamin in the diets of most Europeans. Nutrition therefore must be considered in the light of the whole diet of a people, and any contribution made by fruit and vegetables must be examined in relation to that of the other foods which make up the remainder of the diet.

140

Contributions to Human Nutrition 141 The nutritional state of mankind over the world as a whole has

been very far from satisfactory in modern times. In the latest World Food Survey prepared by the Food and Agricultural Organization of the United Nationsf it is estimated that between 10% and 15% of the world's population is still undernourished, i.e. obtains insufficient calories, and that up to a half of the popu-lation still suffers from either hunger or malnutrition—the lack of a sufficient supply of particular essential nutrients. This appall-ing situation shows little, if any, general improvement on the con-ditions of half a century ago, and any slight amelioration which has taken place during this period has been largely confined to the so-called developed countries : the situation in the underdeveloped regions has hardly been maintained. The task of bringing about a more widespread improvement in nutritional standards creates problems of the greatest magnitude, especially in view of the alarming rate of increase in the world's population. It is estimated that, by 1975, the total output of food will have to be increased by about 35% (over 1963 levels) merely to maintain present stan-dards : any substantial improvement in standards would be depend-ent on an increase in output of the order of 50% during this same period. We must hope that progress in this direction in the future is more rapid than it has been in the past.

A useful general indicator of the nutritional quality of a diet which has been employed by the F.A.O. in their food surveys is the proportion of the total calorie intake which is derived from cereals, starchy roots and sugar. The use of this indicator directs attention to the quality of the protein in the diet. A high value indicates a relatively high proportion of plant protein, the bio-logical value of which is generally not so high as that of proteins from animal sources. A lack of good-quality protein, i.e. protein containing adequate and balanced amounts of the various essen-tial amino acids, is certainly a major factor in the incidence of malnutrition, particularly in the underdeveloped parts of the world. However, an increased supply of high-quality protein is by no means the only nutritional need at the present time.

f 3rd World Food Survey, F.A.O., Rome, 1963.


Deficiency diseases due to the lack of essential vitamins and min-erals are also widespread and are by no means completely confined to the less developed countries of the world. A list of the main nutri-tional deficiency diseases, together with notes on the parts of the world in which they are most prevalent, is given in Table 8. These diseases, which are particularly common among children, can, in their more severe forms, result in death. When developed to a less extreme stage, they cause incapacity and inevitably shorten the life span. There is also every reason to believe that relatively minor déficiences of these same factors have an adverse effect on the general health of large numbers of people in all parts of the world, even in those developed countries in which the diet is usually considered to be relatively adequate and well balanced.

The importance of fruit and vegetables as a class in helping to combat several of these disorders will be seen from the notes on the main dietary sources of the deficient nutrients included in the last column of Tabie 8. The F.A.O. indicator referred to in the last paragraph fails to take due account of the special nutritional-value of non-starchy vegetable foods because, in terms of calories, their contribution to a diet is often too small to have a significant effect on the calculated value. The actual levels of consumption of starchy and of non-starchy fruit and vegetable commodities in different parts of the world have already been briefly discussed in the introductory chapter and relevant data are contained in Table 2 (p. xxiii). From this table it will be seen that there are wide differences from region to region, and these differences are obvi-ously indicative of the varying contribution which fruit and vege-tables make to the nutritional well-being of peoples in different parts of the world.

The special nutritive value of fresh plant foods in a reasonably well-balanced diet is in supplying ascorbic acid (vitamin C), ß-carotene (pro-vitamin A), various B vitamins, especially folic acid, and the mineral elements, calcium and iron. This is illustrated by the data shown in Table 9, in which a comparison is made be-tween the relative contributions of this class of foodstuffs to the dietary supplies of various essential nutrients in the United

TABLE 8. T H E MAIN NUTRITIONAL DEFICIENCY DISEASES

Disease

Kwashiorkor

Vitamin A deficiency

Beri-beri

Pellagra

Riboflavin deficiency

Nutritional anaemias

Scurvy

Rickets 1

Deficient nutrient

High-quality protein 1

; Vitamin A

Thiamine

Niacin

Riboflavin

Iron, folic acid

Ascorbic acid (vitamin C)

Calcium (and/or vitamin D) |

_ i

Main symptoms

Lack of growth, wastage, apathy,

j oedema (excess body-water)

Blindness, particularly night-blindness, skin disorders

General malaise, circulatory disturbances, oedema

Dermatitis, diarrhoea, dementia

Sore lips and tongue

Anaemia (low blood haemoglobin)

Swollen, bleeding gums, external and internal haemorrhages

Deformities of the I skeleton, malfunction of muscles and nerves |

Regions of highest incidence

Africa (but incidence widespread in tropical and subtropical regions)

Far East (Indonesia, China, Burma), parts of S. America and

j S. Africa (drier regions)

Thailand, Burma, Vietnam, S. China, India, Pakistan

Parts of Africa, Asia and Latin America (especially among poor, maize-eating populations)

Underdeveloped areas generally, especially where starchy vegetables are the staple foods

Underdeveloped areas generally, especially where starchy vegetables are the staple foods

Arid regions

India, Burma, parts of the Near East and N. Africa

Important natural sources of the

deficient factor(s)

Meat, fish, dairy products, pulses

Green and yellow vegetables, red palm oil, dairy products, fish oils

Whole cereals, fruit and vegetables, animal foods

Whole cereals (except maize), meat, fish, vegetables and fruit

Cheese, milk, eggs, green vegetables

Vegetables and fruit, especially green leafy vegetables and pulses

Fruit and vegetables

Cheese, milk, vegetables and fruit

TABLE 9. THE APPROXIMATE CONTRIBUTION OF FRUIT AND VEGETABLES (EXCLUDING PULSES) TO THE SUPPLY OF VARIOUS NUTRIENTS IN THE DIET IN BRITAIN AND IN THE UNITED STATES

Nutrient % of total dietary

supplies contributed by fruit and vegetables

U.K. U.S.A.

Remarks

Calories

Protein Fat Vitamin A

Vitamins of the B group Thiamine

Riboflavin Niacin

Folic acid

Ascorbic acid

Calcium Iron

9-3

10-1 0-8

25-1

25-2a

13-8 21-6*

n.a.b

87-5

7.8a 21-2a

9-4

7-9 1-1

59-6

19-3

11-2 14-7

43-0

92-3

11-19·

Vegetables are more important than fruit, especially in the U.K., where potatoes supply relatively much higher proportions of the calories and protein

The outstanding source in the U.K. is carrot (13-6%). Leafy vegetables and citrus fruits make particularly large contributions in American diets, while the sweet potato is also a useful source

Vegetables are relatively more important in U.K. diets, with potatoes (15%) especially prominent

Potatoes again supply well over half the U.K. figure Potatoes contribute over f of the U.K. value, potatoes and sweet potatoes together almost half that for the U.S.A.

The figure for the U.K. would probably be somewhat lower, since potatoes are a relatively poor source of folic acid

Citrus fruits in the U.S.A. and potatoes in the U.K. contribute about | of the respective total dietary supplies

See footnoted potatoes supply almost half of the U.K. value for iron

a The U.K. values are artificially depressed as a result of the compulsory addition of these nutrients to wheat flours of low extraction.

b Figure not available. Sources.

Trends and Patterns in U.S. Food Consumption, Agricultural Handbook No. 214, U.S.D.A. Economic Research Service Washington D.C., 1961.

The Consumption of Fruit and Vegetables in O.E.E.C. Countries, O.E.C.D. Documentation in Agriculture and Food Report No. 22, O.E.C.D., Paris, 1960.

TA

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Oth

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0-56

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G

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erw

ise

< 0

-12

0-0-

11

0-0-

16

Nia

cin

(mg)

0-06

-5-3

A

rrac

acha

(-

+5-3

) Po

tato

(-

>5-l

)

Oth

erw

ise

< 1

-6

1*3-

3-3

0-2-

1-6

0-1-

1-8

Pars

ley

(-+

1 · 8

) R

adis

h (-

+1-7

)

0-1-

2-2

Pepp

ers

(-»2

-2)

Oth

erw

ise

< 1

-5

0-1-

1-1

0-15

-2-8

C

ape

goos

eber

ry

(->2

-8)

Avo

cado

(-

+2-6

) G

uava

(-+

1 ·

6)

age,

coo

king

and

pro

cess

ing

are

disc

usse

d in

the

text

Foli

c ac

id

(Mg)

6-10

8-46

14-1

29

Spin

ach

(-+1

29)

Bru

ssel

s sp

rout

(->8

6)

Cab

bage

< 2

0

6-54

L

ettu

ce

(-+5

4)

Wat

ercr

ess

(-*5

0)

4-15

6 A

spar

agus

(-

+156

) G

reen

bea

ns

(-+8

6)

Oth

erw

ise

< 5

0

1-18

1-10

Dat

a no

t av

aila

ble

for

man

y co

mm

odit

ies

Cal

cium

(m

g)

4-15

0 T

aro

(150

)

Oth

erw

ise

< 6

4 Po

tato

is

a re

lati

vely

po

or s

ourc

e (-

» 13

)

9-52

Iron

(m

g)

0-5-

2-1

0-7-

3-6

Con

tain

app

reci

able

qua

ntit

ies

of p

hyti

c ac

id w

hich

int

erfe

res

wit

h th

e ab

sorp

tion

of

Ca

and

Fe

(see

the

tex

t) 10

-595

Sp

inac

h (-

+595

)

The

hig

h ox

alic

aci

d co

nten

t of

Spi

nach

pr

obab

ly r

ende

rs

the

Ca

larg

ely

unav

aila

ble

9-32

5 Pa

rsle

y (-

»325

) C

eler

y (-

+307

)

Cuc

umbe

r an

d to

mat

o ar

e es

peci

ally

poo

r so

urce

s 9-16

9 O

kra

(-»1

69)

Mar

row

and

squ

ash

are

part

icul

arly

po

or s

ourc

es

1-5-

148

Rhu

barb

(-

>148

)'

Pres

ent

in r

huba

rb

mai

nly

as t

he

oxal

ate

and

ther

efor

e pr

obab

ly

larg

ely

unav

aila

ble

5-63

0-1-

4-5

Spin

ach

(-»4

-5)

0-3-

19-2

Pa

rsle

y (-

»19-

2)

Cel

ery

(-+9

-9)

Cuc

umbe

r is

a

part

icul

arly

'poo

r so

urce

0-15

-3-2

Gr.e

en b

eans

(-

»3-2

) O

nion

(-

+3-0

)

Tur

nip

and

swed

e ar

e es

peci

ally

de

fici

ent

wit

h <

0-5

0-

16-3

-8

Bla

ckbe

rry

(->3

-8)

0-2-

-3-6

Sw

eets

op

(~»3

4 6)

The

con

tent

s of

min

eral

ele

men

ts s

how

w

ide

intr

aspe

cifi

c va

riat

ions

aT

r =

tra

ce.

b Cel

ery,

chi

ve,

cucu

mbe

r, e

ndiv

e, l

ettu

ce,

mus

tard

and

cre

ss, p

arsl

ey,

radi

sh,

spri

ng o

nion

, to

mat

o, w

ater

cres

s.

c V

arie

ty H

al-b

erta

Gia

nt,

grow

n in

the

Leb

anon

(se

e Si

maa

n et

al.,

J.

Sci.

Foo

d A

gric

, 19

64).

Contributions to Human Nutrition 143 Kingdom and in the United States of America, respectively. The patterns in these two countries are in fact surprisingly similar. The only large discrepancy is that between the corresponding values for vitamin A, and this is readily accounted for by the exceptionally high consumption of citrus products in the U.S.A.

So far we have concerned ourselves with the broad picture, with the general contribution made by fruit and vegetables to the nutritional state of large populations. However, different groups of fresh plant products and, indeed, different individual species exhibit interesting differences in their contents of nutritionally important constituents. An attempt has been made in Table 10 to summarize available data on the nutrient contents of some im-portant groups of commodities, and where specific examples are especially rich, or alternatively are notably deficient in a particu-lar dietary factor, these are picked out for special mention. More detailed information for individual commodities will be found in the compositional tables given in Appendix A. In the sections that follow, the levels of various nutritional factors in fruit and vegetable commodities are considered in relation to what we know about man's daily requirements for these same nutrients. The first section is concerrted with that most basic of all nutritional requirements, namely energy.

CALORIES Man's average basic requirement for calories is variously esti-

mated at between 2300 and 2600 kcal per day, depending on the prevailing climate. Considerably higher intakes are necessary in men carrying out hard physical work and in women during preg-nancy and lactation.

The calorific values of different fresh plant foods range from about 5 kcal up to 194 kcal/100 g edible material. The corres-ponding range for cereals and cereal products is 114-557, for dairy products 66-813 and for meat and fish 84-612. Weight for weight, therefore, fruits and vegetables in general are obviously relatively poor sources of calories. However, the starchy vegetables


and legumes, which for the main part lie in the upper half of the above-quoted range, are staple foods in many areas and they can make a most substantial contribution to the total calorie intake— as much as 37-40% of the total in some parts of Africa. Other-wise, taking the broad view, only the banana among individual fruit and vegetable commodities can be said to have more than a purely local significance as a major source of calories, again because of its high level of consumption in many parts of the world. Fruits in general tend to have higher calorific values than non-starchy vegetables because of their higher contents of sugars, while the avocado and the olive are exceptional in containing appreciable amounts (8-16%) of fat and are therefore also especi-ally rich in calories.

The recognition of the special nutritional importance of fruit and vegetable products in providing essential vitamins and miner-als in the diet has prompted F.A.O. to recommend that the pro-portion of the total calorie intake contributed by fruit and non-starchy vegetables together should not be less than about 5%. At present this proportion varies greatly, from considerably less than 1 % in parts of the Far East to the exceptionally high value of 33% in some local areas where the diet is predominantly or entirely vegetarian. The corresponding values for Britain and America are about 3 ·5% and 6 · 3 % respectively, the higher American figure being largely due to the higher consumption of citrus and tomato products. Britons, though by eastern European standards not large eaters of potatoes, derive on average about 5-8% of their total calories from this particular source, while, in America, potatoes and sweet potatoes together account for about 3 · 1 % of the calories in the national diet.

PROTEIN An average intake of about 80 g protein per day is generally

considered desirable in a well-balanced diet. In the case of pro-tein, however, quality, i.e. a balanced content of essential amino acids, is of equal importance to quantity.

Contributions to Human Nutrition 145

The significance of fruit and vegetables as sources of protein is again generally small compared with that of cereals and foods of animal origin. Starchy vegetables contain little protein ( 0 - 5 -2 -9 g/100 g), cassava, which is so widely used in the tropics, being particularly deficient in this nutrient. However, even in the more highly developed countries where the consumption of animal pro-ducts is relatively high, these commodities cannot be completely ignored as sources of dietary protein. In Britain, for example, potatoes contribute about 4 - 9 % of the total protein in the diet, the corresponding American (U.S.) figure for potatoes and sweet potatoes together being 2 - 4 % . In many developing countries, particularly those in Africa, the contribution of starchy vegetables is very much higher. Legumes with between 2-9 and 8-2 g protein/100 g are better sources and they can make even more substantial contributions to dietary supplies of this nutrient, especially in the Far East where they are consumed in relatively large amounts.

Non-starchy vegetables are generally somewhat richer in pro-tein than are fruits, bu t the level in most cases is below about 3 g/100 g and no individual commodity can be singled out as an important dietary source of protein, except perhaps the banana which undoubtedly makes a highly significant contribution in some tropical areas. As a whole group, however, non-starchy vegetables and fruit together generally account for an appreciable, if rather modest, proportion of the total intake of protein. The figures for British and American diets, for example, are about 4 - 6 % and 5 - 5 % respectively. Notice the relatively greater con-tribution made by non-starchy as opposed to starchy commodities in the United States.

The value for the protein content of a fresh plant food is norm-ally obtained by multiplying the nitrogen content by the factor 6 ·25. In fact, appreciable amounts of free amino acids are usually present but, since protein is nutritionally valuable only as a source of certain essential amino acids, this does not affect the general picture. Indeed, the free amino acids can provide most useful supplements to those combined in the protein, as has been shown,


for example, in the case of the potato. It is therefore the overall balance of amino acids which is of special nutritional significance. Both the pattern of free acids and the composition of the protein itself is known to vary greatly in plants from one organ to another and even in the same type of structure at different stages of development, as, for example, between young and old leaves. It is generally true, however, that plant foods are of relatively low biological (nutritional) value in this respect as compared with those from animal sources because of deficiencies of particular essential amino acids. This is clearly illustrated by the data con-tained in Table 11, in which the contents, in a number of im-portant foodstuffs, of the eight essential amino acids are related to the F.A.O. reference pattern—an amino acid pattern which, in the light of available information on the daily human require-ments for individual acids, is considered to provide the ideal nutritional balance.

It will be seen from Table 11, that cereals, for example, repre-sented in this case by wheat flour, are especially deficient in lysine, methionine and tryptophan. Potatoes, on the other hand, are only consistently deficient in methionine. Sweet potatoes, although somewhat variable in amino acid composition, show a relatively well-balanced pattern, but cassava is notable for being deficient in most if not all of the individual essential acids. Yet another starchy vegetable included in the table is yam which, like the potato, appears to show a consistent deficiency of only one parti-cular acid, in this case isoleucine. However, the data for yam, as for many other vegetable commodities, are relatively few and generalizations of this kind must remain subject to possible modification in the light of subsequent work.

The composite data for three other important groups of vege-table foods—leguminous seeds (including pulses), leafy vegetables and other miscellaneous vegetable commodities—show that with-in each of these groups there are wide variations in amino acid patterns. The values in practically every case are spread both above and below the corresponding reference level, the only exception being the range for methionine in leafy vegetables

TA

BL

E 11

. T

HE

CO

NTE

NTS

OF

ESSE

NTI

AL

AM

INO

AC

IDS

IN

VA

RIO

US

FOO

DST

UFF

S IN

R

EL

AT

ION

TO

T

HE

"ID

EA

L"

AM

INO

AC

ID

PATT

ERN

PR

OPO

SED

BY

F

.A.O

.a

(Lig

ht f

igur

es i

ndic

ate

defic

ienc

y of

the

par

ticu

lar

acid

)

Foo

dstu

ff

F.A

.O.

"id

eal"

pa

tter

n E

gg (

hen'

s)

Milk

(co

w's

) B

eef

Whi

te (

whe

at)

flou

r P

otat

o Sw

eet

pota

to

Cas

sava

Y

am

Leg

umin

ous

seed

s (f

resh

an

d dr

y)

Lea

fy v

eget

able

s O

ther

ve

geta

bles

Am

ino

acid

s in

g/1

6 g

N (

100g

pro

tein

)

Lys

ine

4-3

5

-2-7

-5

6-3

-9-1

7

0-1

00

2-0

-2-9

5

-0-5

-5

2-9

-5-5

3

-9-8

-0

4-3

-7-0

1-4

-9-2

3

-1-7

-5

1-5

-5-8

Met

hio

nin

e

2-3

3

-1-5

-2

1-7

-3-3

1

-9-4

-1

0-7

-1-6

1

-4-1

-6

0-8

-2-5

0

-2-0

-8

0-8

-3-3

0-5

-3-9

0

-9-2

-0

0-5

-2-6

Try

pto

ph

an

1-4

1-4

-1-9

1

-1-2

-3

0-9

-2-0

0-7

-1-1

0

-1-1

-8

0-9

-2-7

0

-5-5

-3

1-5

-1-8

0-2

-1-6

0

-9-2

-1

0-6

-1-6

Leu

cin

e

4-9

8

-3-9

-0

8-4

-11

-1

7-2

-10

-0

5-5

-7-5

4

-6-1

1-3

4

-8-5

-6

2-7

-2-9

5

-1-5

-2

3-8

-13

-2

3-7

-9-3

2-7

-11

-9

Isol

euci

ne

4-3

5

-a-7

-l

4-8

-7-4

3

-0-6

-5

3-9

-4-7

3

-7-4

-5

3-6

-5-1

1

-7-1

-9

3-3

-3-9

0-3

-6-3

2

-4-6

-3

1-5

-5-1

Phe

nyla

lani

ne

2-9

4

-8-7

-6

4-6

-5-7

3

-5-4

-9

2-1

-5-6

3

-1-5

-4

4-3

-6-1

1-

9-2-

1 4

-7

2-4

-9-1

1

-9-6

-4

1-4

-4-5

Th

reon

ine

2-9

3

-9-4

-6

4-4

-7-8

3

-6-5

-8

2-0

-2-8

2

-5-3

-9

3-8

-5-7

2

-0-2

-2

3-4

1-9

-5-0

2

-2-5

-5

1-5

-5-0

Val

ine

4-3

6-

8^8-

3 6

-5-9

-0

3-5

-6-5

4-0

-5-1

4

-5-5

-8

5-6

-9-3

2

-1-2

-3

4-4

-4-7

1-9

-6-6

1

-8-7

-1

2-2

-6-4

The

com

posi

tiona

l da

ta f

or t

he v

ario

us f

oods

tuffs

hav

e be

en c

olle

cted

fro

m m

any

publ

ishe

d so

urce

s. T

he m

ajor

ity,

how

ever

, wer

e ta

ken

from

Har

vey'

s Ta

bles

of

the

Am

ino-

acid

s in

Foo

ds a

nd F

eedi

ng S

tuffs

(fo

r re

fere

nce

see

the

text

). a P

rote

in R

equi

rem

ents

, Nut

ritio

nal

Stud

y N

o. 1

6, F

.A.O

., R

ome,

195

7.


which indicates a general deficiency. For detailed information on individual commodities the reader is referred to Harvey, D., (1956), "Tables of the Amino-acids in Foods and Feeding Stuffs", Commonwealth Bureau of Animal Nutrition, Technical Com-munication No. 19, and to McCance, R. H. and Widdowson, E. M., (1960), The Composition of Foods, M.R.C. Special Report No. 297, H.M.S .O.

I t is a fortunate fact that deficiencies of individual essential amino acids in a particular food can be made good by supplies of the deficient acids from other constituents of the diet. In this way, fruit and vegetable commodities, although containing only rela-tively small quantities of protein, can nevertheless make a useful additional contribution by providing supplementary amounts of individual acids which may be deficient in the commodities supplying the bulk of the dietary protein.

VITAMINS AND MINERALS The provision of vitamins and minerals is the most important

contribution which fruit and vegetables make to human nutrition. The vitamin and mineral contents of these products show con-siderable difFerences not only between species and varieties but also between different batches of the same variety grown under different environmental conditions. Climate, soil, and fertilizer practices all have their effects on the levels of vitamins and miner-als in a crop. Local difFerences are also found within the plant. For example, mature dark-green leaves generally contain more ascorbic acid, carotene, calcium and iron than do younger paler leaves of the same variety, while the B vitamins and phosphorus are especially abundant immediately around the growing points of shoots and buds and in the embryos of seeds. An important environmental factor controlling the level of ascorbic acid is light. Generally, the greater the amount of sunlight during growth, the greater the ascorbic acid content. Thus, tropical fruit and vege-tables are usually better sources of this vitamin than are similar products grown in temperate regions. The content of certain


mineral elements in plants is probably directly related to their levels in the soil, though the relationship in other cases is more complex. The level of a particular element in the soil may affect the uptake of another mineral. For example, a high soil nitrogen content generally depresses the uptake of calcium. Thus, many factors contribute to differences in the contents of particular vita-mins and minerals in plant products and individual values, and even ranges of values, should be accepted only as general guides in the light of this known variability.

Ascorbic acid—vitamin C

Fruit and vegetable commodities are the only major food sources of this particular vitamin and they therefore contribute the bulk—in Britain and America over 87%—of the total dietary supplies. Fruit, particularly tropical species, and leafy vegetables are especially rich in ascorbic acid (see Table 10).

Estimates of man's daily requirement for this vitamin range from 30 mg to 75 mgf for a normal adult. These quantities are readily supplied by even small amounts of suitable fresh fruit and vegetable products. The maintenance of supplies of these com-modities throughout the year is therefore of considerable nutri-tional importance. Déficiences tend to occur in temperate regions during the winter months and in tropical regions during periods of drought.

The starchy vegetables, which figure so prominently in diets all over the world, contain only moderate amounts of ascorbic acid and their content of the vitamin progressively decreases during periods of storage. For example, potatoes can show losses of up to 75-80% of the original levels over nine months of storage. How-ever, these starchy products still make a most important contribu-tion to dietary supplies of vitamin G—about 3 3 % of the total in

"f Recent work suggests that adequate supplies of ascorbic acid can compen-sate to some extent for deficiencies of certain other vitamins such as thiamine, riboflavin, pantothenic acid, biotin, folic acid, vitamin B12, vitamin E and vitamin A (see Terroine (1960) in the bibliography at the end of the chapter).


British diets, about 19% in the U.S.A. The seeds of legumes are relatively deficient in ascorbic acid and the small amounts which they do contain are largely lost during the natural drying process, so that pulses are a negligible source of the vitamin, except when allowed to sprout—a common practice in the East.

Other vegetables, particularly the leafy vegetables, also lose ascorbic acid during storage in the raw state. These losses are accelerated by high storage temperatures and by high rates of wilting. For example, badly wilting kale stored at about 21°C (c. 70°F) can lose almost 50% of its initial ascorbic acid content within a single day. Bruising and mechanical damage also greatly increase the rate of loss because ascorbic acid is highly susceptible to oxidation, either directly or through the agency of an enzyme, ascorbic acid oxidase, which is widely distributed in plant tissues. The first step in oxidation is to dehydro-ascorbic acid which still maintains its vitamin potency but if oxidation proceeds beyond this stage the activity is lost.

Most vegetables are cooked before being eaten and further losses of ascorbic acid can occur during this process. Starchy vege-tables, for example, may lose between 4 0 % and 8 0 % of their ascorbic acid during cooking and other vegetables generally undergo losses within this same range. Two factors contribute to these cooking losses—leaching of vitamin into the cooking water, a process which can be largely eliminated by steaming, and oxida-tive destruction which again may be enzyme-catalysed during the warming-up period before the enzyme is inactivated. If boil-ing must be used, the introduction of the vegetable into already boiling water is to be recommended as a means of reducing losses of vitamin G during the cooking process.

The use of fresh fruit as a source of vitamin G has obvious advantages. In Britain and America about a third of the dietary ascorbic acid is derived from these products. Wide differences exist between the levels of the vitamin in different fruits. Among species indigenous to temperate regions the blackcurrant is especi-ally rich in ascorbic acid, the strawberry is also a relatively good source, while the common pome and drupe fruits—apple, pear,

Contributions to Human Nutrition 151 cherry, plum—usually contain relatively little. Of the main tropi-cal and subtropical species, the citrus fruits contain moderate amounts and winter supplies of citrus are most important for maintaining the intake of vitamin G in temperate regions. Guava, jujube, mango and papaya are comparatively rich sources which are widely eaten in the tropics, while the banana, although it only contains a modest amount of ascorbic acid, can also make a useful contribution because of its relatively high level of consumption. Finally, there are a few tropical species, notably the West Indian Cherry (Malpighia punici/olia) and the Myrobalan (Phyllanthus emblica), which can contain quite exceptionally high levels of this vitamin ( > 1800 mg/100g) but these are really only of local importance.

The stewing of fruit, like the cooking of vegetables, causes some destruction of ascorbic acid, but the presence of sugar probably aids in retention of the vitamin and McCance and Widdowsonf assume a loss of only 10% during this cooking operation.

Carotene—provitamin A

Vitamin A, as such, only occurs naturally in animal tissues, and the liver, which is the normal storage depot for this vitamin, usu-ally contains especially large amounts, but ultimately it is derived from plants in the form of ß-carotene or of certain closely related carotenoid pigments such as a- and γ-carotene and crypto-xanthine. These pigments are referred to as provitamins A, since in the body they are broken down in such a way as to release the vitamin. These provitamins in plant foods, especially fruit and vegetables, contribute substantially to the supplies of vitamin A in most diets. The percentages of the total dietary vitamin A derived from fruits and vegetables in British and American diets, for example, are about 25% and 60% respectively.

The average daily adult requirement for vitamin A is estimated at 5000 international units. This international unit is 0-3 /xg of

f The Composition of Foods, M.R.G. Special Report No. 297, H.M.S.O., 1960.


vitamin, which is equivalent to 0 · 6 /xg of ß-carotene, since each molecule of ß-carotene probably yields one molecule of vitamin of about a half of its own molecular weight. McCance and Widdowson suggest, however, that in estimating the vitamin potency of carotenoid pigments, the number of international units obtained from the above relationship should be divided by three to allow for the low efficiency with which the pigments are ab-sorbed by the walls of the small intestine. This makes the above daily requirement equivalent to about 9 mg of ß-carotene.

The chlorophyll pigments in plants are always associated with small amounts of carotene, and green tissues, therefore, always contain modest amounts of the provitamin. However, carotenoid pigments may also occur in other tissues to which they impart yellow to orange colours and the depth of colour in such materials can be taken as a useful general indicator of the likely provitamin A content. Unfortunately, this is not a valid guide in every case, since there are a number of less common carotenoid pigments, such as lycopene, which are not precursors of vitamin A.

Among the starchy vegetables, only the more highly pigmented varieties of sweet potato contain large amounts of carotene—up to 12 mg/100g. Plantain, cassava, yam and arracacha can, how-ever, each supply nutritionally important amounts when eaten in large quantities. Legumes are not rich in carotene, but leafy vege-tables may contain up to about 9 mg/100g, spinach being a particularly good source. Carrot is outstanding among the remain-ing vegetables with a carotene content of as much as 13-6 mg/ 100g. This commodity makes by far the largest contribution of any individual vegetable food in British diets. Other vegetables which can supply useful amounts of carotene include certain varieties of pumpkin and squash, peppers, and the tomato, al-though in this last example the principal pigment is not carotene but lycopene which does not yield vitamin A.

Compared with vegetables, fruits are generally not good sources of carotene. However, there are a few notable exceptions. Among tropical and subtropical species, for example, certain varieties of mango, persimmon, papaya, cape gooseberry and melon are


moderately rich in the provitamin, while in temperate regions the apricot is a consistent if relatively rather modest source. Most varieties of peach contain only small amounts of carotene, but there is one particular variety—Hal-berta Giant—which appears to be quite exceptional in this respect, containing as much as 16-4 mg/100g of edible material.

Garotenoid pigments, like ascorbic acid, can be oxidized under certain circumstances with a consequent loss of provitamin potency. For example, green leafy tissues contain a lipoxidase-like enzyme system which can destroy carotene rapidly if the tissues are damaged. Non-enzymic oxidation can also occur and one such mechanism is photosensitive, the reaction being acceler-ated in the presence of light. However, the extent of destruction of carotene in intact living tissues during storage is very small and in some products, e.g. carrots, tomatoes and peaches, the synthesis of carotene can continue after harvest, leading to an actual in-crease in the content of the provitamin. Normal cooking proced-ures cause little, if any, destruction of carotene and cooked vegetables are therefore equally good as sources of the vitamin as are the corresponding raw materials.

Vitamins of the B group

A whole complex of substances, each of them probably an essential constituent of the diet, makes up what was originally thought to be a single factor—vitamin B. The members of this group are all active prosthetic groups of tissue enzymes and are generally found together, particularly in tissues which are meta-bolically very active. Mature plant tissues do not normally show high rates of metabolic activity and, compared with most animal tissues, they are relatively poor sources of B vitamins. However, rather higher levels are found in the meristematic tissues of actively growing shoots and in the embryos of seeds. The cereals are the main plant sources of these vitamins, although legumes and other vegetables can supply useful quantities, particularly of folic acid, thiamine, niacin, riboflavin, pantothenic acid, biotin and vitamin


B6f. Information on the levels of the more important of these nutrients in fruits and vegetables is included in Table 10, and values for individual commodities will be found in Appendix A.

Thiamine—Vitamin Bx. The human requirement for this vitamin depends on the supply of calories and, more particularly, on the intake of carbohydrate, since thiamine is concerned in the meta-bolism of carbohydrates. A daily intake of between 1 mg and 2 mg is generally considered necessary for a normal adult. Among fresh plant foods, the legumes are especially rich in thiamine with levels—up to about 0-5 mg/100g—which are comparable to or in some cases even higher than those in whole cereal grains. Individually, other fruit and vegetable commodities are only moderate-to-poor sources of thiamine, the common fruits and the leafy vegetables lying in the lower part of the range. In spite of this, however, the contribution of fruit and vegetables as a class to the total dietary intake of thiamine can be quite substantial. In Britain the percentage contribution is about 25, 15% being pro-vided by potatoes, while in American diets potatoes and sweet potatoes together supply about 7% out of a combined total for all fruit and vegetable products of some 19%.

The situation varies considerably from one part of the world to another according to the pattern of cereal consumption and to the methods used in preparing cereal foods. In cereal grains the thia-mine is concentrated in the outer parts—the embryo and pericarp —which are removed to a greater or lesser extent during milling and polishing procedures. For this reason, white wheat flour, which is prepared mainly from the inner part of the grain—the endosperm—, is fortified in Britain, and in some other countries, by additions of thiamine. (In Britain, additions of niacin, calcium and iron are also mandatory.) The polishing of rice also removes most of the thiamine and this, coupled with the traditionally low levels of consumption of other food commodities in Far Eastern

•f Pyridoxine and certain closely related substances.

Contributions to Human Nutrition 155 countries, has been responsible for the especially high incidence of beri-beri, the disease caused by dietary deficiencies of thiamine, in the rice-eating areas.

Thiamine is readily soluble in water and may therefore be leached out of the material during cooking. Compared with ascorbic acid, it is relatively stable at cooking temperatures, especially in slightly acid solution—on the alkaline side it is destroyed more readily—and total losses of thiamine during the cooking of vegetables are generally between 25% and 40%.

Riboflavin, The average human requirement for this vitamin, like that for thiamine, is estimated at between 1 mg and 2 mg per day. Cereals are poorer sources of riboflavin than they are of thiamine and fresh plant foods therefore tend to assume a greater relative importance, but dairy products, especially cheese and eggs, are also excellent sources of riboflavin, containing between 0-1 mg and 0-8 mg/100g, and the pattern is therefore strongly influenced by the level of consumption of these last-mentioned products.

Among fruit and vegetable commodities, green leafy vegetables and young shoots such as those of spring onion and asparagus are especially rich in riboflavin (see Table 10). Leguminous seeds also contain useful amounts, but starchy vegetables and fruits are rela-tively poor sources of the vitamin. In western diets, fruit and vege-tables provide only a relatively modest proportion of the total dietary supplies of riboflavin—about 14% in Britain, about 11% in the United States—but in other parts of the world, notably in China, South-East Asia and parts of Africa, where the consump-tion of dairy products is comparatively very low, plant foods un-doubtedly make a much more important contribution to the dietary intake of this particular vitamin.

Riboflavin in plant tissues is chemically quite stable.f Indeed, cases have been reported of actual increases in the riboflavin con-

■f It can, however, be rapidly destroyed in solution in the presence of light, as, for example, in fresh milk exposed to sunlight.


tent of some vegetables during post-harvest storage. Being water-soluble, leaching losses occur during cooking and McCance and Widdowsonf assume losses of riboflavin of between 30% and 4 0 % during the boiling of vegetables and of 10% during the stewing of fruit.

Niacin {nicotinic acid, nicotinamide). The dietary requirement for niacin appears to be linked with that for the essential amino acid, trytophan, since there is evidence that niacin can be synthesized in the body from trytophan. These two are therefore regarded by some nutritionists as interchangeable in the diet. However, most diets are not sufficiently rich in the amino acid to meet this addi-tional requirement and a daily intake of 10-15 mg of niacin is recommended.

Fruit and vegetables are of roughly similar importance as sources of niacin as they are of thiamine. They contribute about 2 2 % of the total intake of niacin in British diets and about 15% of that in diets in the U.S.A. Starchy vegetables are relatively better sources of niacin than they are of the other vitamins of the B group. The tropical root, arracacha, and the potato, for ex-ample, can each contain over 5 mg of niacin per 100g of edible material and potatoes alone provide about 15% of the total British intake of this vitamin. The leguminous seeds, which con-tain up to 3 mg of niacin per 100g are also among the better, widely used sources. Otherwise, no individual fruit or vegetable species can be singled out as being especially rich in niacin, except perhaps two tropical fruits—the Cape gooseberry and the avocado —which, with reported levels of up to 2 · 8 mg and 2 · 6 mg/100g respectively, can contain substantially more of the vitamin than any other investigated commodity.

Losses of this vitamin during storage after harvest are negligible and, since niacin is also heat stable, losses during cooking are largely confined to those due to leaching into the cooking water.

t The Composition of Foods, M.R.C. Special Report No. 297, H.M.S.O., 1960.

Contributions to Human Nutrition 157 Other vitamins of the B group. Vegetables and fruit can also con-

tribute useful amounts of the other members of the B group of vitamins with the exception of vitamin B12 which is only present in foods of animal origin. They are particularly useful sources of folic acid which is necessary for the prevention of nutritional anaemias. It has been estimated that as much as 43% of the folic acid in American diets is derived from fruit and vegetable products. Asparagus, green leafy vegetables and legumes contain the largest amounts of this particular vitamin, lOOg-quantities of any of these commodities supplying more than enough to meet normal daily requirements.

The name folic acid is actually applied to a number of closely related compounds, all derivatives of pterin, the most important being pteroylglutamic acid. This is only sparingly soluble in water and is stable in slightly acid solution, so that losses during cooking are likely to be small. However, McCance and Widdowson suggest, on the contrary, that between 70% and 100% of the folic acid originally present may be lost during cooking.

The other vitamins of this group, while probably essential in small amounts for the maintenance of health, have not so far been shown to be deficient in human diets. They are very widely dis-tributed in biological materials and fresh plant foods probably make a roughly similar contribution to their supply as they do to the provision of the more important B vitamins discussed in earlier sections.

Minerals

Fruits and vegetables contain a very wide range of mineral ele-ments, the relative amounts of which vary greatly with the mineral composition of the soil, fertilizer practices and other agri-cultural factors. Man also has a dietary requirement for quite a wide range of mineral elements, the major mineral nutrients— potassium, sodium, chlorine, calcium, phosphorus, sulphur and iron—being required in appreciable quantities, while copper, co-balt, zinc, manganese, iodine and probably molybdenum, nickel


and fluorine are needed only in trace amounts. Deficiencies of potassium, sodium, chlorine and phosphorus, and of the trace ele-ments other than iodine and fluorine, are probably extremely rare. Sulphur-deficiency may arise where there is protein-deficiency, since sulphur-containing amino acids are probably the main dietary source. Fruit and vegetables undoubtedly make a signifi-cant and, indeed, an essential contribution to supplies of all these necessary mineral elements. They are, however, especially im-portant in supplying calcium and iron, deficiencies of which are not uncommon and result in the development of the well charac-terized deficiency diseases—rickets, osteomalacia and iron-deficiency anaemias.

Calcium. Estimates of the daily requirement for calcium range from 0 · 8g to 2 · Og, adolescents and pregnant and lactating women having particularly large requirements. Among common foods, leafy vegetables are second only to dairy products (cheese and milk) in their levels of calcium. The relative importance of fruit and vegetables as sources of calcium in the diet is modified by various factors; for example, in Britain, by the fortification of white flour with added calcium, but there is no doubt that they make a most important contribution to the dietary intake of this essential element in most, if not all, parts of the world. Leafy vege-tables, as mentioned above, are particularly good sources, spinach being exceptionally rich in calcium with up to 595 mg/100g. Un-fortunately, the calcium in spinach is probably largely unavailable because of the high content of oxalic acid, with which calcium forms an insoluble salt. Apart from spinach, only rhubarb among fruit and vegetable commodities contains sufficient of this acid to have an important effect on calcium absorption. Non-leafy vege-tables and fruit, although not as rich in calcium as leafy vege-tables, also make a highly significant contribution to the supplies of this mineral element.

The absorption of calcium by the body depends on it being present in a soluble form and there are various agents in foods,

Contributions to Human Nutrition 159 other than oxalic acid, which can render calcium insoluble. The most important of these is phytic acid—inositol hexaphosphoric acid. It was the discovery of this effect which lead to the original proposal to fortify cereals with calcium, because cereal grains con-tain appreciable amounts of phytic acid. Leguminous seeds also contain phytic acid in considerable quantities and the availability of the calcium from legumes is probably thereby reduced. Fortu-nately, while phytic acid occurs in other vegetable foods, it is only present in relatively small amounts and probably has little effect on calcium availability. Fatty acids form insoluble soaps with calcium but again are unlikely to interfere significantly with the absorption of calcium from vegetables and fruit.

The estimated percentages of the total dietary calcium derived from vegetables and fruit in British and American diets are about 8 and 11 respectively. Potatoes are relatively unimportant in this connection, because of their especially low content of calcium— recorded values for calcium in potato all fall below about 18 mg/100g.

Iron. Iron-deficiency anaemias are common not only in the underdeveloped parts of the world, where the general diet is poor, but also in more highly developed countries. The requirement for this element in an adult is variously estimated at between 12 mg and 30 mg per day. The absorption of iron, like that of calcium, is modified by various factors. For example, iron bound in a por-phyrin ring—e.g. blood iron—is but poorly absorbed. Ferrous iron is more readily absorbed than ferric iron and ascorbic acid is known to aid the absorption of iron, probably because of its re-ducing action in converting the iron to the ferrous form. Proteins may also assist the absorption of iron, while phytic acid forms an insoluble iron salt and thus reduces the availability of the element.

Fruit and vegetables generally make even more important con-tributions to dietary supplies of iron than they do to those of calcium. Leafy vegetables again provide the richest sources, most other vegetables and fruit containing relatively smaller but still


nutritionally significant amounts. The starchy vegetables as a group are poorest in iron, yet, in Britain, potatoes are estimated to provide about 9% of the total iron in the diet, almost a half of the proportion contributed by all fruit and vegetable com-modities together.

THE VALUE OF FRUIT AND VEGETABLES IN PROVIDING "BULK" IN THE DIET

No discussion of the contributions of fruit and vegetables to human nutrition would be complete without some reference to their valuable laxative effect. Because of their relatively high con-tent of unavailable carbohydrate or fibre—up to 16% of the fresh weight in some tropical fruits—they provide an indigestible matrix which stimulates the activity of the intestines and helps to keep the intestinal muscles in working order. Chronic constipation, from which about 30% of all women are said to suffer, is a con-dition which detracts from the general health of many millions of people the world over. This condition could probably be largely alleviated were it possible to increase the intake of fresh fruit and vegetables among its many sufferers. The problem is especially acute in the highly industrialized countries, where as much as 50% of the calorie intake may be derived from industrially re-fined, highly digestible foodstuffs which lack sufficient indigestible bulk.

THE EFFECTS OF METHODS OF PROCESSING AND PRESERVATION ON THE NUTRITIVE VALUE

OF FRUIT AND VEGETABLES The consumption of preserved fruit and vegetables is still small

over the world as a whole compared with that of the fresh pro-ducts, but it is progressively increasing and will undoubtedly con-tinue to do so within the foreseeable future.

In the United States, the consumption of processed fruit and vegetable products is actually higher than that of their fresh

Contributions to Human Nutrition 161 counterparts, while in other developed countries the trend is towards a progressively higher relative consumption of processed products. Processing industries are also beginning to emerge and grow in the less developed parts of the world. The effects of pro-cessing treatments on the levels of nutrients in fruit and vegetables is therefore of very considerable importance.

Drying is the most ancient method of preserving vegetable foods. The drying of fruits unfortunately results in the destruction of practically all of the original ascorbic acid. Treatment with sul-phur dioxide, a common practice, also destroys all the thiamine. Carotene, riboflavin, niacin and folic acid are, however, not lost during drying and dried fruits can be good sources of these vita-mins, as well as of calcium and iron. Modern methods of de-hydrating vegetables reduce losses of vitamins other than thiamine, and dehydrated leafy vegetables can be useful sources of vitamin G. Further losses of ascorbic acid and of carotene do, however, occur during storage of these products, particularly in the presence of oxygen and, in the case of carotene, of light.

The canning of fruit and vegetables causes some destruction of vitamins but in general this is no greater, and probably in many cases less, than that taking place during domestic cooking of the same materials. This statement has been substantiated by the results of numerous tests in which the vitamins have been deter-mined in fruit and vegetable commodities before and after can-ning. Losses of ascorbic acid have ranged from 0-4% to 76%, most commonly lying nearer to the lower figure. Losses of caro-tene have generally been insignificant. Thiamine has shown losses of between 0% and 78%, again more often on the lower side, while losses of riboflavin have ranged from 0% to 63%. When one re-members that the delay between harvesting and processing is probably in most cases considerably less than that between har-vesting and the sale of fresh produce on the retail market, no great anxiety need be entertained about the nutrient content of canned fruit and vegetable products. Moreover, as every food-processor knows, only first-class raw material can be satisfactorily processed to give a product of good quality. Further losses of vitamins,


notably of ascorbic acid and thiamine, take place during long-term storage of canned products. These changes are accelerated by high storage temperatures, but below about 18°G (c. 65°F) they are very slow to develop, and the losses do not generally exceed about 10% during a 12-month period of storage. A final point to remember about canned products is that the water-soluble vitamins dissolve out in the syrup or brine in which the material is packed and the liquid contents of a can may therefore be as rich in these nutrients as are the solid materials contained therein.

Vitamin losses during the quick-freezing of fruits and vegetables are small. Most of the destruction in vegetables occurs during the blanching operation, a preliminary heat-treatment which is also usually part of canning and dehydration processes. Further losses during frozen storage are very slow, provided that the temperature is not allowed to rise above — 17*8°C (0°F) and the materials retain the bulk of the vitamins originally present even after storage for 12 months at this temperature (see, however, Chapter 10).

Finally, some destruction of ascorbic acid also takes place dur-ing the preparation of jam and other sugar preserves. Differences occur according to the type of fruit being used and the details of the process, but available information suggests that losses of the vitamin are generally between about 30% and 45%.

We may summarize the contents of this chapter in the following way. Fruit and vegetables form an indispensible part of human diet. They are our only major source of ascorbic acid and make valuable contributions to our intakes of carotene, thiamine, ribo-flavin, niacin, folic acid and other vitamins of the B group. They also supply important amounts of calcium, iron and many other essential mineral elements. Apart from the starchy vegetables and legumes, which figure prominently in many diets, they are rela-tively unimportant as sources of calories and protein, but they do provide valuable supplies of indigestible carbohydrate which help to keep the intestinal muscles in trim. There is every reason to

Contributions to Human Nutrition 163 believe that the nutritional quality of diets in many parts of the world would be improved by increasing the consumption of fruit and vegetables. The greatest deficiencies at present occur in the Far East and in parts of East and South Africa. The Food and Agricultural Organization of the United Nations recommends that, for a balanced diet, fruit and vegetables other than starchy roots and pulses should contribute not less than 5% of calories to the total energy value. To achieve this uniformly over the whole world at the present time would necessitate an immediate increase in production of these commodities of some 100 million tons per annum. This is a measure of the further contribution which fruit and vegetables could make to human nutrition and to the allevi-ation of human suffering in this modern world.

SUGGESTIONS FOR F U R T H E R READING A N D FOR R E F E R E N C E

BEATON, G. H. (ed.), Nutrition, A Comprehensive Treatise, vols. 1 and 2, Academic Press, New York, 1964.

CLIFCORN, L. E., Factors influencing the vitamin content of canned foods, Advances in Food Research, 1, 39 (1948).

DAVIDSON, SIR F., MIEKLEJOHN, A. P. and PASSMORE R., Human Nutrition and

Dietetics, Livingstone, Edinburgh and London, 1959. FARRER, K. T. H., The thermal destruction of vitamin Βχ in foods, Advances

in Food Research, 6, 257 (1955). F.A.O. 3rd World Food Survey, F.A.O., Rome, 1963. HARVEY, D., Tables of the Amino-acids in Foods and Feeding Stuffs, Common-

wealth Bureau of Animal Nutrition, Technical Communication No. 19, University Press, Aberdeen, 1956.

MCCANCE, R. H. and WIDDOWSON, E. M., The Composition of Foods, M.R.C. Special Report No. 297, H.M.S.O., London, 1960.

MOTTRAM, V. H., Human Nutrition, 2nd edn., Arnold, London, 1963. PLATT, B. S., Tables of Representative Values of Foods Commonly Used in

Tropical Countries, M.R.C. Special Report No. 302, H.M.S.O., London, 1962.

SCHUPAN, W., Teneurs en amino-acides indispensables des végétaux ali-mentaires et leurs diverses organes, Qualitas Plantarum Materiae Vege-tabilis, 3/4, 19 (1958).

SOMERS, G. F. and BEESON, K. C , The influence of climate and fertilizer practices upon the vitamin and mineral content of vegetables, Advances in Food Research, 1, 291 (1948).

164 Concerning the Utilization of Fruit and Vegetables TERROINE, T., The vitamin interrelations of ascorbic acid, in World Review

of Nutrition, vol. 2, Pitman Medical, London, 1960. The Consumption of Fruit and Vegetables in 0.E.E.C. Countries, O.E.G.D.

Documentation in Agriculture and Food Report, No. 22, O.E.G.D., Paris, 1960.

Trends and Patterns in U.S. Food Consumption, Agricultural Handbook No. 214, U.S.D.A. Economic Research Service, Washington D.C., 1961.

CHAPTER 7

PROBLEMS OF TRANSPORT, STORAGE AND MARKETING

DELAYS between the harvesting and the utilization of crops are inevitable, and fresh fruit and vegetables, as we have seen, are especially prone to loss of quality during any intervening period. The various events and processes which can contribute towards this deterioration have been described in earlier chapters and methods have been outlined whereby the rate of loss of quality can be reduced. Individual products differ greatly in the length of time for which they can be maintained in a wholesome condi-tion and in their responses to various treatments and to adjust-ments in the conditions of the environment. Some procedures, such as the moderate lowering of temperature, are almost uni-versally effective in extending storage life, but each kind of material poses its own special problems and even very small changes in environmental conditions can have profound effects.

A most important prerequisite for the maintenance of good quality is the avoidance of mechanical injury. The latter causes structural and physiological disorganization of the tissues and greatly facilitates the entry of micro-organisms which cause spoil-age. Injury is most commonly caused during handling operations which are involved in harvesting, grading, washing and packing and again during unpacking and sale in shop or market. It can also occur during transport as a result of faulty packing, combined with the vibration and jerky movement of the vehicle. Everything possible should be done to reduce this damage to the bare mini-mum by dispensing with all unnecessary handling operations, using clean and well-designed containers and by conveying the

165 F. & V.—G


produce as gently as possible. The simplest and most obvious of precautions are often overlooked. Pickers and handlers with un-duly long and/or sharp fingernails can inflict untold damage. The use of wrapping papers and box-liners, which has been mentioned in earlier chapters in connection with the control of microbio-logical and physiological disorders, can also help in the reduction of mechanical injury.

Important features of the environment which influence the longevity of the material, and which are amenable to control are, temperature, humidity and the composition of the atmosphere surrounding the produce. Low temperatures depress both the physiological activity of the vegetable tissues themselves and of any micro-organisms capable of causing spoilage. High humidi-ties reduce loss of water from the tissues and therefore retard wilting or desiccation, but they may encourage the germination and growth of organisms on the surfaces. Increases in carbon dioxide concentration and reductions in oxygen concentration, whether arising naturally as a result of the respiratory activity of the products themselves or brought about by artificial means, both generally slow down the normal metabolic activity of the plant tissues and inhibit the growth of spoilage organisms. Finally, some constituents of the volatile emanations from ripening fruit— notably ethylene—are, as we have seen, physiologically active and, if these are not removed, they can initiate premature ripening and other unwanted changes in material exposed to the same storage atmosphere.

Changes leading to deterioration in the quality of fruit and vegetables are initiated at the time of harvest and, unless the material can be sold on the retail market or used for processing within a few hours of picking, which is seldom possible, immediate modification of the environmental conditions to delay the onset of spoilage is usually desirable. Modern procedures are aimed at cooling the produce to the most suitable temperature for holding, as soon as possible after removal from the plant. Account must of course be taken of the nature and intended use of the material concerned. If the product is intended for immediate sale on the

Problems of Transport, Storage and Marketing 167 retail market after a relatively short period in transit, then less stringent treatment is required than if longer journeys or extended periods of storage are involved. Again, some products, such as potatoes and other root vegetables, are by nature adapted to maintain their structural and physiological integrity for long periods under field conditions. In temperate regions, some such materials have traditionally been stored outdoors on the farm in pits or clamps and although in this case the temperature of storage must needs vary at the whim of the local climate, these procedures have more often than not proved adequate. However, such tradi-tional methods are now progressively giving way to more modern methods of indoor storage in which the conditions can be more accurately controlled.

The transport of fruit and vegetables from region of production to that of consumption can in modern circumstances involve con-siderable periods of time and it is highly desirable, and indeed in many cases essential, to control the conditions during transit so as to reduce wastage (see Plate 6a). The International Institute of Refrigeration (U.R.) had made recommendationsf with regard to the most suitable conditions for the land transport of perishable foodstuffs and their recommended temperatures for a range of fresh fruit and vegetable products are reproduced in Table 12.

The ranges of temperature given in Table 12 are suggested as suitable during relatively short periods in transit (or storage) and are not necessarily those which will give the longest possible useful storage life for the products concerned. Further reductions in temperature, which are limited by the need to avoid chilling in-jury, will in many cases permit the maintenance of good quality during varying periods of subsequent storage. Optimal tempera-ture ranges for the long-term storage of various fruit and vegetable commodities have also been suggested by U . R . J and these recom-mendations are summarized in Fig. 23, which also includes for

f Recommended Conditions for Land Transport of Perishable Foodstuffs, 2nd edn., International Institute of Refrigeration, Paris, 1963.

% Recommended Conditions for Cold Storage of Perishable Foodstuffs, International Institute of Refrigeration, Paris, 1959.

(By courtesy ofj. and E. Hall Ltd., Dartford.)

PLATE 6. (a) Modern refrigerated transport vehicles designed for the carriage of fruit and vegetable commodities. The mechanical refrigera-tion unit fitted in this case is the Thermo King PKW Unit, (b) The in-terior of a modern controlled-atmosphere fruit store. The unit in the background is an air-cooling unit, within which is an adjustable fresh-air inlet and an outlet piping system to compensate for pressure changes, ventilate the store and carry off excess carbon dioxide. The walls are insulated with 4 in. polystyrene and lined internally with gal-

vanized sheet steel vapour-sealed at the joins.

TEMP °C

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(GREEN)!

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AVOCADO!

2-4

GRAPEFRUIT (ISRAEL)

IO

EARLY POTATOESI SUMMER (SHORT-TERMj l SQUASH

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CURRANT h CURRANT h 2 ì FIG 1-2 GRAPE 3 - 2 4 J

BROAD, RUNNERl

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ART,CH?FKREENCH)3-4 BEETROOT Hi-12 CABBAGE 8 " 16 CAULIFLOWER 2 - 3 ENDIVE 2 - 3 GRAPEFRUIT (FLORIDA)

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cfc A R T ' fÄ ,LE^-20 BROCCOLI Mr - 3 _ CARROT (BUNCH) | - 2 CELERY 4 - 8 0

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LETTUCE 1 -3

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PIMENTO 4 - 5

RADISH 3 -16

REDCURRANT 2 - 3

STRAWBERRY %

TURNIP 1 6 - 2 0

TEMP

ORANGE 1(1 S RA EL)

24

APPLES (DIFFERING

WITH , VARIETY)

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ICASSAVA | P A R S L E Y 4 z 8

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VARIETY) "PERSIMMON;

4 - 2 4 IONION

24

FIG. 23. Optimum temperature ranges for the long-term storage of individual fruit and vegetable commodities. (The numbers following the names ofthe commodities are estimates (in weeks) of storage life under the specified conditions. These estimates are su bject to the

maintenance of a suitable humidity in the storage atmosphere.)

Problems of Transport, Storage and Marketing 169 each material an estimate of the storage life to be expected under the specified conditions. The marked variation in response of different products to the temperature of storage is well reflected in the wide spread of the recommended storage temperatures shown in this figure. Even different varieties of the same species may show characteristically different requirements.

The most detailed information about these varietal peculiarities has been obtained in the case of pome fruits. Thus, in the United Kingdom, Cox's Orange Pippin apples have been found to keep best at 3*5°C, while the optimum storage temperature for Laxton's Superb lies between —1° and 0°C. German authorities recommend storage at 3-4°G for the variety Belle de Boscoop, 2-2-5°C for the variety Jonathan and between —1° and 0°G for the variety Berlepsch Orange. In the United States, most kinds of pears are held at — 1 °G (a temperature now also favoured for cold storage of pears in the U.K.), but the French suggest an optimum storage temperature of 6°G for pears of the Lanscailler (Lanca-shire) variety. (Detailed recommendations for a fuller range of varieties of apples and pears are given in the above-mentioned report of the I.I.R.f and in various publications from the research organizations concerned with fruit storage in different countries.)

Not only the nature of the material, but also its previous history —geographical origin, climate during the growing season and stage of maturity at harvest—can affect its response to the tem-perature of storage. European-grown Golden Delicious apples, for example, store best at between 2° and 4°C, while apples of the same variety grown in the United States and in South Africa have an optimum storage temperature of between —1° and 0°C. Generally, the more mature a product, the greater its suscepti-bility to chilling injury. Pears which have begun to ripen are injured by exposure to the usual storage temperatures and sub-sequently fail to complete the normal ripening process.

The maintenance of a uniform temperature within the limits desirable for the satisfactory long-term cold storage of fruit and

f Recommended Conditions for Cold Storage of Perishable Foodstuffs, International Institute of Refrigeration, Paris, 1959.

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vegetables necessitates the use of well-designed storage rooms. Precooling of the produce in air (fruit) or by hydro-cooling (vege-tables) is desirable to reduce the load on the refrigeration plant of the store itself, but this is not always possible and the store should be capable of cooling the material to the desired storage temperature within a reasonably short period of time. Thereafter, the refrigeration in the store is required to remove heat generated by the respiratory activity of the material itself and by fan in-stallations introduced to maintain good air-circulation, and to remove any heat leaking into the room through the walls. The heat of respiration varies, of course, with the type and amount of materials in storage. Peas in the pod, for example, respire many times faster than apples, and, in the design of stores, account must be taken of these intrinsic differences. Efficient but gentle circula-tion of air around the produce is highly desirable to avoid the local development of atmospheric conditions which could be injurious to the material and this is normally obtained by using a suitable fan. Packaging and stacking of the material should also be such as not to interfere seriously with the circulation of air. All walls of the store should be suitably insulated to reduce the ingress of heat from the surroundings and unless this is efficiently done, direct contact of the produce with the inner surfaces of the store is to be avoided.

The presence of cooling coils at a temperature lower than that at the surfaces of the material inevitably leads not only to a trans-fer of heat but also to a transfer of water, i.e. to a desiccation of the produce. The greater the temperature difference between the cooling coils and the material and the more rapid the circulation of air, the greater the drying effect. Conversely, desiccation can be reduced by using a well-insulated store with a large cooler surface, which will permit operation with a minimal difference in temperature between cooling coils and material. Air circula-tion should be just sufficient to prevent the formation of stagnant pockets around the surfaces of the material.

Consideration of the drying effect in cold stores brings us to the question of the humidity factor. High humidities increase the

Problems of Transport, Storage and Marketing 173

danger of microbiological spoilage, but are necessary in most cases to prevent the material from shrivelling or wilting. Most fruits keep best at a relative humidity of about 90%. Some leafy vege-tables such as lettuce, spinach, endive, broccoli and celery are

TABLE 13. RECOMMENDED» RELATIVE HUMIDITIES DURING THE LONG-TERM STORAGE OF FRUIT AND VEGETABLE COMMODITIES

Recommended relative

humidity Commodity

70 70-75 80-85 85 85-90

85-95 90

90-95

Dates (cured) Garlic, onion, pumpkin Sweet potato Citrus fruits (Israel) Beans (green, runner, broad and Lima), bilberry (blueberry),

cabbage, cauliflower, cherry, citrus fruits, egg plant, grape, melon, nectarine, olive, parsley, pea, peach, persimmon, pimento, plum, potato, raspberry, strawberry, tomato, water-melon

Asparagus, cucumber, summer squash Apple, apricot, avocado, banana, blackberry, blackcurrant,

carrot (bunch), cranberry, fig, gooseberry, guava, litchi, loganberry, mango, papaya, pear, pineapple, pomegranate, quince, redcurrant

Artichoke (French), artichoke (Jerusalem), beetroot, broc-coli, Brussels sprouts, carrot (topped), celery, endive, horse-radish, kohlrabi, leek, lettuce, parsnip, radish, salsify, spinach, turnip, turnip-rooted celery. (Also apples and pears in controlled-atmosphere storage)

aFrom : Recommended Conditions for Cold Storage of Perishable Foodstuffs, Inter-national Institute of Refrigeration, Paris, 1959.

especially susceptible to wilting and are better stored at even higher humidities, while certain other products, notably onion, pumpkin and sweet potato, maintain their quality for longer periods when kept in a relatively dry atmosphere. Recommended humidities for the storage of individual commodities are given in Table 13.


Good insulation and a large area of cooling surface, as mentioned earlier, both help to keep the atmosphere moist, and a well-designed store with a reasonable load of produce will normally maintain a humidity of about 9 0 % . Higher humidities can be produced by introducing water into the store as a mist or spray, but this procedure is liable to reduce the heat-transfer efficiency of the cooling coils by causing the accumulation of larger amounts of ice on their surfaces. The water vapour in the atmosphere of the store comes very largely from the material itself and therefore smaller loads generally result in lower humidities. Reductions in humidity can also be obtained by ventilation with suitably dry air.

Some ventilation is usually necessary during the storage of fruit and vegetables to prevent the accumulation in the atmosphere around the material of physiologically active or toxic gaseous pro-ducts of metabolism. Ventilation is in fact a useful means of controlling the composition of the storage atmosphere which, in its turn, can have important repercussions on the storage life of the material. The normal respiratory activity of the tissues causes a progressive depletion of oxygen and a simultaneous build-up of carbon dioxide in the atmosphere of the store. Theoretically, therefore, if one assumes for the sake of the present argument that

. . . / G 0 2 evolved \ . , the respiratory quotient I 1 is close to unity, care-

\ 0 2 absorbed / fui control of ventilation can be used to maintain any desired combination of oxygen and carbon dioxide concentrations which gives a total for both gases of about 2 1 % . A respiratory quotient of less than one will obviously tend to lower the total concentra-tion of the two gases, while a quotient of more than one will tend to raise it by producing proportionately even more carbon dioxide.

Alternatively, by using external, usually alkaline, absorbents (scrubbers), such as milk of lime, caustic soda, ethanolamine, water or hydrated lime to remove carbon dioxide, particularly low levels of oxygen can be combined with low concentrations of

Problems of Transport, Storage and Marketing 175 or with the virtual absence of carbon dioxide, a gas which at higher concentrations can produce injurious effects in many pro-ducts. Such procedures, which have now been used commercially in the United Kingdom for the storage of pome fruits for between 30 and 35 years, are referred to as controlled-atmosphere storage (originally gas storage in the U.K.) (see Plate 6(b)). Fruits are even more sensitive to small differences in the composition of the storage atmosphere than they are to small differences in tempera-ture and, to obtain the best results in terms of the extension of storage life, the gas concentrations have to be very carefully con-trolled. Recommended concentrations of both oxygen and carbon dioxide for the storage of a number of varieties of apples and pears have been published by the International Institute of Refrigera-tion,! but improved procedures are constantly being developed and the use of low concentrations of oxygen in the absence of carbon dioxide, a relatively recent development, is showing particular promise, especially with varieties which are highly susceptible to carbon dioxide injury.

Controlled-atmosphere (c-a) storage must be regarded as an adjunct to rather than as an alternative to low-temperature storage. Reduced temperatures are still necessary for an extended storage life, although some relaxation of cooling may, and indeed in some cases must, be introduced because the injurious effects of high concentrations of carbon dioxide are more pronounced in certain materials at relatively low temperatures. For example, Laxton's Superb apples, which will keep for 4-5 months in cold storage at between — 1 °G and 0°C, may be stored for 6-7 months in 10% C 0 2 and 2-5% 0 2 , but at a temperature of 4-5°G.

The widespread commercial application of c-a storage techni-ques has so far been limited to apples and pears, but a greal deal of experimental work with many other materials, including cauli-flower, asparagus, peas, tomatoes, and various other fruits, has shown that the method could be more generally applied with use-ful results. Much time and expense are, however, involved in

ΐ Recommended Conditions for Cold Storage of Perishable Foodstuffs, International Institute of Refrigeration, Paris, 1959.


determining the optimum combination of conditions for a parti-cular product, and since the response may vary not only between varieties but also between different batches of a given variety grown in different areas, caution must obviously be exercised in the translation of experimental techniques into large-scale com-mercial procedures.

Useful effects have also been obtained by employing relatively short-term treatments with concentration of carbon dioxide con-siderably in excess of 10%, which is usually regarded as the upper safe limit for long-term exposure. The use of these higher con-centrations involves the artificial introduction of extra carbon dioxide, which is most conveniently added in the form of "dry ice" (solid C 0 2 ) . Such treatments are particularly effective in reducing the build-up of micro-organisms on soft fruits during the period immediately after harvest. For example, blackcurrants for juice production can be stored for 3-4 weeks by using an initial carbon dioxide concentration of 5 0 % (at 1 -5°C) falling to 2 5 % after the first week. This treatment effectively prevents micro-biological spoilage and maintains the appearance of the fruit, al-though some accumulation of alcohol and acetaldehyde takes place. Exposure to C 0 2 concentrations of between 20% and 60% has also been successfully used with cherries and bilberries (blue-berries), and, for yet shorter periods, as, for example, during trans-port by rail and road, with several other products including raspberries, blackberries, strawberries, apricots, peaches and plums.

The use of controlled atmospheres in the storage of fruit and vegetables requires, of course, that the walls of the store or con-tainer are effectively gas-tight, a feature which is not strictly neces-sary for ordinary cold storage. Various means of gas proofing c-a stores have been tested from time to time, but the method most commonly used has been to line the walls with sheets of metal sealed together at the joins with bituminous compounds. Recently, some success has also been achieved using plastic films such as polyethylene, pliofilm and cellophane, especially on a small scale as sealed box-liners, which, as a result of the respiratory activity

Problems of Transport, Storage and Marketing 177 of the fruit, can serve as miniature gas stores during limited periods of storage.

In this latter case the composition of the internal atmosphere is not, of course, amenable to proper control. It has long been the custom, whenever possible, to avoid keeping different kinds of produce together in the same store. This practice was based originally on the results of early tests in which it was found that ethylene produced by early-ripening varieties of apples adversely affected the keeping properties of later varieties held in the same store. It has since been shown that effects of this kind are mini-mized at the lower temperatures of storage which are now norm-ally employed, but the high specificity of requirements during cold and c-a storage still provides good grounds for the segregation of species and varieties, at least in those cases in which the aim is to store for the longest possible period. The case for segregating products such as onion and horseradish, which produce highly penetrating odoriferous compounds and are very liable to taint other commodities, needs no additional comment.

The maintenance of quality during the transport and storage of fresh fruit and vegetables therefore poses many problems and, in order to minimize wastage, very considerable care and atten-tion to detail is required at all stages. This is equally true of the final operation of marketing the produce. In the first place, it is most important that a constant check be kept on the material during storage, so that an adequate margin of time is allowed for the passage of the produce through the final stages of distribution and sale to reach the consumer in a wholesome and acceptable condition. The inadvertant accumulation of small amounts of ethylene for example, either from the plant tissues themselves, from moulds present on any rotting material, or due to accidental leakage of coal gas, can produce a dramatic and sudden end to the storage life of a whole roomful of material.

Special care must also be exercised during the removal of the material from a refrigerated store. The transfer of a product from cold storage directly into warm humid air results in condensation of water on the surfaces of the material, a process which will


encourage the rapid development of spoilage organisms. To avoid this, the material should be allowed to warm up slowly in rela-tively dry air. This is particularly important with soft-skinned fruits such as cherries and plums.

Many fruits are stored in the pre-ripe condition and the process of ripening may need to be carried more nearly to completion before the material is acceptable to the consumer. The rise in temperature on removal from cold storage accelerates ripening and a relatively short period at a higher temperature is in many cases sufficient to complete the process. In some cases, the most important example being the banana, the final stage of ripening may be accelerated by treatment with low concentrations of ethylene. Bananas for the United Kingdom market are normally cut at the " § full" green stage and shipped at a temperature of about 12°C, the voyage from the West Indies—the main source of supply—taking between 11 and 18 days. Ripening may be initi-ated on the ship, especially if the cargo is inadequately ventilated, but normally the fruit is ripened at its final destination in special ripening rooms held at temperatures of between 14° and 20°C. Concentrations of ethylene of as little as 1 ppm will produce rapid and even ripening, which is in all ways similar to the natural ripening process. Pears, melons and tomatoes can also be artifi-cially ripened in this way, but in some other cases the various changes associated with ripening are not all accelerated to the same extent. Plums for example change colour and appear ripe under the influence of ethylene, but the normal changes in sugar and acid concentrations do not take place and the flavour remains tart and unripe.

Another useful application of ethylene is in the degreening of citrus fruits which, for various reasons (genetic or cultural), may not have undergone the full development of colour associated with the particular variety. This is not strictly a ripening process, since the fruit is already " r ipe" in every respect except in colour. T h e effect of ethylene in this case is simply to cause the destruction of chlorophyll and so to allow the colour of the underlying yellow and orange carotenoid pigments to become fully evident. A

Problems of Transport, Storage and Marketing 179

; -m9mm ^%^^^^fcs

(By courtesy ofj. and E. Hall Ltd., Dartford.)

PLATE 7. (a) Stems of bananas hanging outside the special rooms in which they are ripened under controlled conditions and in the presence of low concentrations of ethylene (see the text), (b) Brussels sprouts in cold storage. Notice the netting sacks used to hold the sprouts and the two air-cooling and -circulating units in the background supplied with

refrigerant through the piping system on the back wall.


common treatment is to hold the material in circulating air con-taining 20 ppm of ethylene at about 27°C and 90% R.H. for between 24 and 72 hours, depending on the amount of de-greening required.

The very last stage in the movement of fresh fruit and vegetable products from field or orchard to the consumer is that in the shop or other retail selling outlet. Here, the material may be appreciably softer in texture and therefore even more susceptible to mechanical injury than at any earlier stage. Renewed dangers also arise of inadvertant exposure to injurious temperatures and conditions generally are likely to be more conducive to the development of spoilage organisms. The turnover of material at this stage should therefore obviously be as rapid as possible. A relatively new selling technique, the use of which is on the increase, is that of pre-packaging the material in transparent plastic bags or other con-tainers. This procedure, though increasingly popular, brings with it its own dangers. The plastic films used are relatively imperme-able to water vapour and to the permanent gases and, unless they are suitably perforated to allow adequate ventilation, excessively high humidities can develop leading to the condensation of water on the surfaces of the material and consequently to more rapid spoilage. Prepackaging of this kind is best combined with some prior fungicidal treatment. Injuries have also resulted, in this type of pack, from the accumulation of excessively high internal concentrations of carbon dioxide, but in other cases the modifica-tion of the internal atmosphere has, as in c-a storage, actually led to some improvement in storage life.

SUGGESTIONS FOR F U R T H E R READING A N D

FOR R E F E R E N C E FIDLER, J . C , Fresh fruit and vegetables, in Recent Advances in Food Science,

vol. 1, (Hawthorn, J . and Leitch, J . Muil eds.), Butterworths, London, 1962, p. 269.

HALES, K. C , Refrigerated transport on shipboard, Advances in Food Re-search, 12, 147 (1963).

Problems of Transport, Storage and Marketing 181 INTERNATIONAL INSTITUTE OF REFRIGERATION, Recommended Conditions for

Cold Storage of Perishable Foodstuffs, U . R . , Paris, 1959. INTERNATIONAL INSTITUTE OF REFRIGERATION, Recommended Conditions for

Land Transport of Perishable Foodstuffs, U . R . , Paris, 1963. MARTIN, D. and GERNY, J., LOW Oxygen Gas-storage Trials of Apples in

Tasmania, C.S.I.R.O. Division of Plant Industry, Technical Paper No. 6, 1956.

MONTGOMERY, H. B. S., Effect of storage conditions on the incidence of Gloeosporium rots of apple fruits, Nature, 182, 737 (1958).

PADFIELD, C. A. S., The Storage of Apples and Pears, New Zealand D.S.I.R., Bulletin No. I l l , 1954.

PENTZER, W. T., Handling and storage of fruits and vegetables for processing, Food TechnoL, 1, 565 (1947).

PENTZER, W. T., Temperatures required by fruits and vegetables after harvest, Food TechnoL, 5, 440 (1951).

SALUNKHE, D. K. et al., On storage of fruits: effects of pre- and post-harvest treatments, Food TechnoL, 16, No. 11, 123 (1962).

SCHOMER, H. A., Refrigeration of pre-packaged fresh fruits and vegetables, Refrig. Engng., 61, 742 (1953).

SMITH, W. H., The Commercial Storage of Vegetables, D.S.I.R. Food Investiga-tion, Leaflet No. 15, H.M.S.O., London, 1952.

SMITH, W. H., The use of carbon dioxide in the transport and storage of fruits and vegetables, Advances in Food Research, 12, 1 (1963).

VARIOUS AUTHORS, The refrigerated transport of perishable foods, Food Manu-facture, 39, No. 10, October, 1964.

CHAPTER 8

QUALITY

QUALITY is a very complex property which we can define in this context as the sum total of all those attributes which combine to make fruit and vegetables acceptable, desirable and nutritionally valuable as human foods. I t is therefore essentially a composite concept which can be broken down into a number of distinct yet related aspects. In the first place, appearance is obviously of great importance and much can be learnt about the general quality of the material by simple visual examination. Defects, due to a variety of causes, which detract from the acceptability of a pro-duct, can be readily picked out in this way. The size and shape of individual units is generally a factor in quality, while the develop-ment of other attributes is usually determined by the stage of maturi ty of the material, a feature which again can commonly be judged by eye. Especially important visual features are the colour and gloss of the external surfaces.

Appearance, however, although most important, is by no means the sole criterion of quality. Of at least equal importance are the properties appreciated through other human senses, those of taste, smell and touch, which can be grouped together under the headings of flavour and texture. The attainment of a satisfactory flavour and texture may often, as in ripening fruits, be associated with a certain development of colour, but appearance can be deceptive in this respect and the only completely satisfactory way of assessing flavour and texture is to taste the material.

Differences in growing conditions, in harvesting procedures and in post-harvest treatments combine with the innate variability of

182

Quality 183 the material itself to produce wide variations in quality, and many countries have evolved minimum standards and systems of grading in an attempt to maintain some sort of consistency in the general quality of produce reaching the consumer. The market in fruit and vegetables is, of course, an international one and the present trend is towards greater international standardization of grades of quality for these products.

F.A.O.j and W.H.O.J are at present co-operating in the ambi-tious task of preparing a Codex Alimentarius which will eventually include standards for a complete range of foodstuffs. As part of this project, various subcommittees, each chaired by a different member country of U.N.O., have been set up to deal with particu-lar groups of commodities, and the responsibility for preparing standards for fresh fruit and vegetables rests in the hands of the Economic Commission for Europe. E.C.E., in fact, pioneered the formulation of international standards for fresh fruit and vegetable commodities and a number of existing European standards have already been adopted by the Organization for Economic Co-operation and Development (O.E.CD.), a body which, in addi-tion to the countries of western Europe, also includes the U.S.A. and Canada. There is every reason to believe that, in due course, the standards laid down in this present O.E.CD. scheme will receive world-wide recognition and that, in this way, a complete uniformity of standards will finally be achieved.

The O.E.CD. scheme lays down certain minimum require-ments for material entering the international market and also institutes a number of quality classes. The minimum general requirements are, to quote the relevant publication,** that:

(a) they (the fruit or vegetables^) should be healthy and sound, that is to say free from any blemishes liable to affect their natural powers of resistance, such as traces of deterioration or decomposition, bruises or unhealed cracks :

f The Food and Agricultural Organization and % the World Health Organ-ization of the United Nations.

** O.E.CD. Documentation in Agriculture and Food No. 47, International Standardization of Fruit and Vegetables} Paris, 1961.

•f f The author's insertion.


(b) they should be whole, clean, practically free from extraneous matter, free from any foreign taste or smell and without abnormal surface mois-ture having regard to the nature of the produce :

(c) they should be of normal size and appearance having regard to the variety, season and production area: and

(d) they should have reached a degree of maturity which, having regard to the normal duration of the journey, will ensure the arrival of the produce in good condition, especially as concerns satisfactory taste, taking into account the variety.

Three distinct quality classes are designated in the scheme : an " E x t r a " Class to include "produce of superior quality, of the shape, appearance, colour and taste characteristic of the variety, virtually free from blemishes affecting their external appearance and particularly carefully packed": Glass I—"produce of good quality, 'commercially' free from blemishes and carefully packed" : and Glass I I—"produce which may have certain blemishes not impairing its intrinsic quality and which complies with the mini-m u m general requirements defined above".

Specific standards are already laid down for a number of individual commodi ties, f In these, the necessary requirements for inclusion in each quality class are described in considerable detail, with explanatory illustrations, so as to enable the scheme to be applied as uniformly as possible throughout the various member countries.

The above scheme and other systems of grading used in con-nection with trade between countries which are not members of O.E.G.D. are applied to material crossing international bound-aries, but government departments and trade associations in many countries also operate their own internal systems of quality grading. Minimum standards are generally required by law, but grade specifications are often voluntary such, for example, as those drawn up by the Agricultural Marketing Service of the United

Ί" O.E. C D . Documentation in Food and Agriculture, No. 47 (1961) Standard No. 1 — Apples and pears. No. 54 (1962) Standard No. 2 — Tomatoes.

Standard No. 3 — Cauliflowers. Standard No. 5 — Lettuces and endives. Standard No. 6a— Peaches.

Quality 185 States Department of Agriculture, in co-operation with various groups interested in the marketing and processing of fruit and vegetables. In the United States, and to a lesser extent in other industrialized countries, a substantial part of the fruit and vege-table crop is taken by the processors and the requirements here are not necessarily the same as those which apply to material destined for the fresh market. For example, green peas are only suitable for processing over a relatively narrow range of maturity. A good indication of the stage of maturity of peas is obtained from the content of alcohol-insoluble solids (A.I.S.), which is essentially a measure of the amount of starch present. In peas for canning, the A.I.S. should be between 13% and 14-5%, while for freezing an even younger stage is desirable—A.I.S. 10-5-11 -5%. Young peas are, of course, sweeter and more tender than more mature peas, but harvesting at a young stage involves the grower in a loss of potential yield, and material for the fresh market, therefore, tends to be harvested at a more advanced stage of maturity. Again, the canning process may result in drastic changes in the colour of the material, necessitating the use of artificial colouring matters and in such cases the requirements with regard to the colour of the raw material are much less stringent than is the case in material which is to be sold directly to the consumer in the fresh condition. This matter is discussed at greater length in Chapter 9.

A special problem which has been concerning the processing industry in recent times, but one which applies equally to the supply of material for the fresh market, is the production of flavour taints as a result of the increasingly extensive use of agri-cultural chemicals for crop protection. Several useful pesticides have in fact been shownf to produce such taints in canned and frozen fruit and vegetable products. Among these are the insecti-cides Aldrin, BHG, Ghlorobenside, Metaisosystox, Sevin and Formothion, the fungicides Gaptan, Dichloran, Griseofulvin,

| See Arthey, V. D. and Adam, W. B., The Tainting of Canned and Quick-frozen Fruit and Vegetables, Technical Bulletin No. 8, Fruit and Vegetable Canning and Quick Freezing Research Association, Chipping Campden, 1963.


Nabam, Thiram, Metiram, Zineb and Ziram, and the herbicide Simazine + prometryne.f An especially difficult problem is the control oïBotrytis rot on strawberries for processing, since the most effective fungicidal agents—Gaptan and Thiram—are both liable to produce taints. In recent tests,% however, a number of other efficient fungicides produced no such deleterious effects and there is reason to believe that this problem will eventually be satis-factorily solved.

In addition to having their own requirements with regard to the condition of their raw materials, processors must also conform to prescribed standards for their finished products. Some of these, such as the ones relating to the weight of the contents of a pack may involve statutory obligations, but quality grading is gener-ally self-imposed, either through the agency of trade associations or with the co-operation of advisory departments of government. In the United Kingdom for example, the Fruit and Vegetable Canning and Quick Freezing Research Association,** to which all the main processing firms belong, operates a grading scheme based on the results of tasting panels. Individual member firms can submit samples of their own products for testing by the central body as a check on their own grading procedure. A similar system is operated by the Agricultural Marketing Service of the U.S.D.A. There is, of course, a considerable international trade in pro-cessed fruit and vegetable commodities and it is intended that standards for these items shall eventually be included in the United Nations Codex Alimentarius. The subcommittee which is at present working on the formulation of standards for these particu-lar products is being presided over by the United States of America, a country with an unrivalled wealth of experience in the processing of this class of foodstuffs.

The drawing up of quality specifications for the grading of fruit •f The chemical nature of these various compounds is described in British

Standard No. 1831, The Recommended Common Names for Pesticides, British Standards Institution, London, 1965.

X Arthey, V. D., Technical Memorandum No. 59, Fruit and Vegetable Canning and Quick Freezing Research Association, Chipping Campden, 1965.

**Now the Fruit and Vegetable Preservation Research Association.

Quality 187

and vegetable products is no easy matter, since several of the attributes which go to make up quality are essentially subjective properties which do not lend themselves to precise measurement. This is particularly true of flavour, for which the only method of assessment available is that of tasting by human judges, a pro-cedure which is hardly to be recommended for the reproducibility of the results obtained. Quality specifications therefore inevitably lack precision and, even when written up in considerable detail, they still suffer from the absence of satisfactory objective methods of measuring some of the contributory factors.

A system used both in the United States and in the United Kingdom for assessing the quality of processed fruit and vegetables is that of scoring, in which a sample is marked for different quality attributes by reference to a special score-sheet drawn up for the purpose. In this way, by adding up the marks accorded for the different features, a total mark is obtained and this can be used as a measure of overall quality and therefore as a basis for quality grading. The use of such a system raises the problem of deciding what proportion of the total available marks should be allocated to each of the various properties which contribute to the general quality of the material. Both the American and British systems use a maximum score of 100, but while in the U.K. , for simplicity in operation, only three kinds of score-sheet are used, one for all kinds of fruit and the other two for groups of vegetable products (with slight modifications in a few special cases), the American system uses a separate score-sheet for each individual commodity, the allocation of marks for different quality attributes varying so as to place special emphasis on those features which are con-sidered of particular importance in a given case. The score-sheet developed by the British Fruit and Vegetable Canning and Quick Freezing Research Associationf for use with canned fruit is repro-duced in Table 13(a).

The desirability of operating systems of grading for fresh and processed fruit and vegetables, so as to ensure the maintenance of the highest possible standards of quality, can hardly be

t Now the Fruit and Vegetable Preservation Research Association.


questioned. The uniform implementation of such schemes, how-ever, as has already been pointed out, is certainly not without its problems, especially with regard to the actual assessment or measurement of the component attributes of quality. Let us now, therefore, look in somewhat more detail at the nature of these various attributes and at the methods which are presently avail-able for their measurement.

TABLE 13(a)

SCORE-SHEET FOR CANNED FRUIT*

Item

Colour Texture Absence of defects Size grading Flavour

Total

Maximum score

20 20 30 10 20

100

Minimum score for

Grade A* 16 16 24

8 16

90

Grade A 14 14 21

7 14

80

Grade B 12 12 18 6

12

70

Notice that, to be included kin a particular grade, a sample must score a minimum mark for each item, in addition to reaching or exceeding a prescribed total. The American system also usually recognizes three different grades, designated simply A, B, and C, with minimum scores respectively of 90, 80, and 70.

THE NATURE AND MEASUREMENT OF THE MAIN ATTRIBUTES OF QUALITY

We can classify the constituent aspects of quality as illustrated in Table 13(b).

The non-sensory aspects of quality, although they can obviously be of the greatest importance, usually require elaborate chemical or biological testing for their assessment and a satisfactory condi-tion in regard to these factors is normally taken for granted by the consumer. Factors affecting the nutritive value of fruit and vege-table commodities have been discussed in the last chapter. So far

U.K. Fruit and Vegetable Canning and Quick Freezing Research Association.

Quality 189 as the presence of adulterants or of toxic residues is concerned, this is subject to either general or specific legislation in most countries and the whole matter is carefully watched by the responsible government agencies.

It is the sensory aspects of quality which naturally are most im-portant in determining the acceptibility of these products. The nature of some of these factors, such as size, shape and defects, is sufficiently obvious to require no further explanation. These are

TABLE 13(b)

CLASSIFICATION OF QUALITY ATTRIBUTES

SENSORY ASPECTS

NON-SENSORY ASPECTS

Individual attribute

Appearance Size and shape Defects Colour Gloss

Flavour Odour Taste

Texture (kinesthetics) Hand feel Mouth feel

Nutritive value Presence of harmless

adulterants Toxicity (presence of

pesticide residues, etc.)

Sense involved in perception

Sight

Smell and taste

Touch

readily measured by simple everyday procedures. The problem here is simply one of deciding what limits of size and shape and what maximum number, type and size (or total area) of defects should be permitted for a particular grade or standard of quality. Size grading is especially important for the processor and is now usually carried out by mechanical means (see Chapter 9).

Colour, flavour and texture are more complex properties and, in order that the best possible use shall be made of available


methods of measurement in these cases, it is most important that a proper understanding should first be obtained of the nature of the particular attribute and of the mechanisms whereby it is appreciated by the individual consumer.

Colour and gloss

Our appreciation of the appearance of any object depends on the formation of an image on the retina of the eye by light which is reflected from the object itself. In order that an image shall be formed, a certain minimum amount of light must reach the eye from the object and, above this threshold, the brightness of the image depends on the extra light available.

The glossiness of the surface of an object depends on the man-ner in which light is reflected from that surface, which in turn depends on the smoothness and regularity of shape of the surface. Fine irregularity of the surface causes light to be reflected more or less evenly at all angles and the surface appears dull or flat. A perfectly smooth surface on the other hand causes directional or specular rather than diffuse reflectance, most of the rays being reflected at the same angle in relation to the incident beam, giving a shine or gloss.

Fortunately for us, however, visual sensations are not deter-mined simply by the quantity of light reflected towards the eye, but our sensory mechanisms are capable of distinguishing qualita-tive differences which give rise to sensations of colour. Within the visible range of the electromagnetic spectrum, radiations of different wavelength produce different colour sensations, e.g. 400-500 m/x—blue, 600-700 m^—red. Equal reflection of all wavelengths of light makes a surface appear white, complete absorption of all wavelengths makes it appear black but, if certain wavelengths are absorbed or reflected to a greater extent than others, then sensations of colour result. The nature of the sensation produced depends on three optical characteristics of the object. First of all, the overall amount of light reflected from it determines the brightness or value of the visual effect. Secondly, the dominant

Quality 191 wavelength of the reflected light determines its dominant colour or hue. Finally, the third important characteristic of the reflected light is its purity or chroma, i.e. the proportion of the total light having the dominant wavelength.

These characteristics of the light reflected from a food item can each be measured with a high degree of accuracy using modern physical instruments. By means of a spectrophotometer, for example, the light reflected from or transmitted through a speci-men can be measured at successive narrow bands of wavelength covering the whole visible spectrum. The resulting spectrophoto-metric curve can be reduced to three numbers by the use of a tristimulus system such as the one recommended by the Inter-national Committee on Illumination (G.I.E.). This system is based on a "standard observer" which consists essentially of three colour filters—X—amber, Y—green, and Z—blue—each with precisely specified light transmittances at each waveband of the spectrum. Using tabulated reflectance values of X, Y and Z for a standard illuminant, the spectrophotometric curve of reflectance for a test specimen can be integrated in terms of X, Y and Z to give three numerical values, which together accurately describe the colour of the specimen.

This procedure is, of course, highly elaborate, time consuming, expensive and quite out of the question as a means of routine colour measurement. There are instruments available, however, which combine relative simplicity and rapidity in operation with the production of results which are approximately convertible into the near-absolute, three-dimensional terms of the G.I.E. system. One such instrument, which has been widely used in America in recent years, is the Hunter Color and Color-difference Meter—a photo-electric tristimulus colorimeter using three separ-ate circuits with filters closely approximating X, Y and Z of the C.I.E. system (see Plate 8a). Considerably cheaper and simpler again are the various visual colorimeters such as the Munsell Disc Colorimeter, an instrument employing combinations of coloured discs which are carefully calibrated for hue, value and chroma. Finally, there are the many kinds of simple comparators which,

(By courtesy of Hunter Associates Laboratory, McLean, Virginia.)

(a)

(By courtesy of Unilever Ltd., and in particular Mr. R. W. Graham.)

(b)

PLATE 8. (a) The Hunter Color and Color-difference Meter being used to measure the colour of a sample of orange juice, (b) A close-up view of the "jaws" of the Tenderometer (from an article by R. W.

Graham and G. Evans in Food Manufacture, May 1957).

Quality 193

though of little use for the accurate measurement of colour, can serve as useful aids for grading purposes.

A constant difficulty in the instrumental measurement of colour, and this applies particularly in the case of fresh fruit and vegetable commodities, is the common lack of uniformity in the colour of the material itself. Instrumental methods are therefore especially useful in dealing with relatively homogeneous products such as juices, purees, sauces, etc. Where the colour of a material is largely determined by the concentration of a particular natural pigment, this may be measured by extracting the pigment in a suitable solvent, clearing if necessary, and measuring the trans-mi ttance of the extract at a suitable wavelength, generally that at which the pigment concerned gives the highest absorption.

The objective measurement of colour therefore presents no insuperable problems. However, fresh fruit and vegetable com-modities in the main do not lend themselves readily to the use of instrumental methods, because of their natural lack of uniformity. In any case, the high accuracy which is possible using elaborate and expensive equipment is generally quite unnecessary for the purposes of routine quality assessment and grading, operations which can usually be carried out quickly and satisfactorily by eye. A valuable aid in the visual assessment of colour, and one which has been extensively used by the Agricultural Marketing Service of the U.S.D.A., is to make available visual standards in the form of coloured plastic blocks, cards, etc., or even coloured reproduc-tions of the products themselves, which can be compared with the samples under examination. This permits a speedier and more objective assessment on the par t of the person making the inspec-tion and results in much greater consistency in the application of colour standards on a national scale.

Flavour

Flavour is a property which is largely due to the stimulation of the chemical senses of the consumer, i.e. those of gustation (taste) and olfaction (smell). Minor contributions to the overall sensation


of flavour may also be made by receptors concerned with tempera-ture, touch and pain, but taste and smell are the dominant aspects of this most subjective of all quality attributes. The structure, distribution and behaviour of the gustatory and olfactory recep-tors have been exhaustively studied and, although much useful information has been obtained, we are still woefully ignorant of the precise mechanisms whereby the chemical stimulation is brought about and the resulting sensation produced.

Taste is due to the presence of certain soluble constituents of the food which reach the sensitive taste buds through the film of saliva covering the tongue and other soft internal surfaces of the mouth. I t is a relatively simple sense producing only four types of sensa-tion—those of sweetness, saltiness, sourness and bitterness—the main chemical agents responsible for these sensations being, respectively, sugars, salt, titratable acid and a heterogeneous collection of bitter principles including the alkaloids. (Many other substances produce taste sensations, particularly sweet-ness and bitterness, but these are not normally encountered in foods.)

The sense of smell, which generally makes the major contribu-tion to the total flavour sensation, is considerably more complex. In order to stimulate this sense, a substance must be volatile so as to reach and be absorbed on the receptors in the olfactory epi-thelium, which is situated in the uppermost nasal cavities. Several attempts have been made in the past to produce a fundamental classification of odour types but, in the absence of definite know-ledge on the actual mechanism of odour perception, these have lacked a firm foundation. One modern theoryf provides a rational explanation for the differences between odours by postulating the existence at the surfaces of the olfactory cilia (sensitive projections from the receptor cells) of special receptor sites of molecular dimensions and of different shapes and sizes conforming to the different molecular structures of the substances causing the stimu-lation. According to this theory, which is based on a detailed survey of the sizes and shapes of the molecules of a very large

•f Amoore's Stereochemical Theory of Olfaction.

Quality 195 number of odoriferous compounds, there are seven such kinds of receptor site and therefore seven primary odour types,f but many substances can be accommodated at more than one kind of site, thus explaining the very wide range of odours found among individual chemical compounds.

The range of compounds which is capable of stimulating the sense of smell is very wide and, in view of the extreme sensitivity of the receptors—as little as 10~9 mg of a strongly smelling sub-stance like ethyl mercaptan will produce a sensation—the number of substances contributing to the flavour of an individual fruit or vegetable must also be very large. As a result of recent work using vapour-phase chromatography, infra-red and ultra-violet spectro-photometry and mass spectrometry, many such odoriferous vola-tiles have been isolated, identified and measured in extracts from a variety of plant foods. The accurate objective measurement of flavour is still, however, beyond the capabilities of existing scien-tific instruments.

Certain substances or groups of substances which contribute towards the flavour of these products can be determined by rela-tively simple physical and chemical methods. The concentration of sugars, for example, can be measured with a refractometer, acidity by means of a p H meter or by titration, and salt by simple chemical procedures. However, such determinations are of little value in relation to flavour quality except in a few isolated cases, and generally recourse must be had to subjective methods in-volving the use of human tasters. T h e procedures used and the many problems encountered in the use of tasting panels for the evaluation of quality in foods are outside the scope of this book. Suffice it to say that, with well-trained personnel, carefully selected methods and satisfactory statistical control, reasonably con-sistent and useful results can be obtained, although such methods inevitably lack precision, are commonly found want-ing in reproducibility, and are expensive in terms of the time for which panel members are distracted from their normal duties.

f Ethereal, camphoraceous, musky, floral, pepperminty, pungent and putrid.


Texture

T h e term texture, as applied to foods, has been interpreted very broadly by some authorities! so as to include certain features of appearance and of "hand feel" in addition to the textural charac-teristics experienced during the actual eating of the food. In the present context, the word will be used to cover only those pro-perties which are perceived by the sense of touch in the mouth. The organs responsible for the perception of this attribute are therefore the sensory nerve endings concerned with touch and pressure at the surfaces of the mouth, in the periodontal mem-branes surrounding the roots of the teeth and in the muscles and tendons concerned in mastication.

Texture is a complex property which manifests itself in many different ways. Among the more meaningful terms which have been used to describe textural characteristics in fruit and vege-table products are firmness, crispness, juiciness, fibrousness, gritti-ness and mealiness or flouriness. In the final analysis, these attri-butes are dependent on the physical properties and structural organization of the main tissue constituents and some of the more general relationships between structure and texture have already been pointed out in Chapter 2. The relative proportions and distributions of the various kinds of tissue, especially of thick-walled and lignified or leuco-anthocyanin-encrusted types of cells, are obviously of importance in this connection.

Firmness may be due either to the turgidity of thin-walled parenchymatous tissues or to the presence of a high proportion of thick-walled, possibly dead, mechanical tissue—notice that the simple measurement of resistance to a compressing force in such cases may fail to distinguish between samples which in other respects would produce entirely different textural sensations.

Crispness, a feature largely attributable in fresh tissues to the turgidity of the living cells, is also found in pickled vegetables, in which it is considered to arise as a result of a physical change in

f E.g. The Taste Testing and Consumer Preference Committee of the American Institute of Food Technologists.

Quality 197

the cellulose due to the effect of lactic acid. This feature is suffi-ciently similar in the two cases to war ran t the use of the same descriptive term, but the underlying causes are respectively quite different.

Juiciness must clearly be related in par t to the water content of the material, but since the sap is initially restricted in living tissues to the vacuoles of the individual cells, this property also depends on the extent to which the cells are burst open by the teeth during mastication.

The presence of discrete bundles of mechanical and/or conduct-ing tissues which resist the shearing forces applied by the teeth gives rise to fibrousness or stringyness of texture. Sheets of pro-tective tissue are often similarly resistant to mastication and such intact pieces of skin also generally detract from textural quality. Indeed, the persistence in the mouth of any cell aggregates of macroscopic dimensions after the sensations of flavour have died away is usually regarded as undesirable.

Grittiness may be caused by the presence of small particles of foreign matter such as sand or of small clusters of cells with highly thickened and rigid walls—the stone cells—which retain their integrity after the surrounding parenchyma breaks down.

The individual cells of plant tissues are for the main par t suffi-ciently large to be detected as separate particles by the h u m a n sense of touch. More subtle differences in texture, therefore, arise due to differences in the size and shape of the component cells and in the extent to which they become separated from each other while the food is being eaten. T h e separation of intact cells is a feature which is most commonly found in cooked vegetable materials, especially starchy products, and differences in this respect are related to differences in the composition of the middle lamella. Potatoes in which the cells readily separate after cooking are said to be "mealy" or "floury" in texture, while the failure of the cells to separate results in "waxiness" or "soapiness". T h e term mealy has an additional connotation relating to the dryness of the surfaces of food particles, as for example in cereal products, and it has also been applied to describe a textural characteristic

F. & V . —H


in some varieties of fresh fruit such as apples where again it is associated with a lack of juiciness.

From what has been said it will be appreciated that, although the texture of fruit and vegetable foods is related to the physical properties of the tissues, the mechanical model is a very complex and heterogeneous one and the measurement of no single physical attribute will adequately define this aspect of quality. This is not to say that physical measurements have no value in the assessment of texture, but merely to point out that the results of any such measurement should be interpreted with caution and related wherever possible to the opinions of human judges.

A variety of instruments has in fact been developed and used for the evaluation of texture in fruit and vegetable products. Some, such as the pressure testers used on fruit and on some vege-table products such as sweet corn, measure the pressure required to force a plunger for a given distance into the material. Others like the Tenderometer, Texturemeter and Maturometer, all originally designed and still mainly used for the assessment of maturity in peas for processing, measure the total force required to compress and shear through a given quantity of material. The Tenderometer (see Plate 8b) which, with peas, gives results show-ing good correlation with the content of alcohol-insoluble-solids, uses two sets of grids which are hinged together at one side so as to simulate the action of the jaws in biting. This latter principle has been carried even further in the design of one research instru-m e n t ! which actually employs a pair of human dentures as the test cell.

Other devices developed for the evaluation of particular tex-tural properties in fruit and vegetables include the Fibrometer, designed to distinguish between over-fibrous and acceptable asparagus stalks, and the Succulometer, which has been used to measure the quantity of juice extractable from products such as sweet corn and apples under carefully controlled conditions. A highly versatile texture-measuring device is the Shear-Press devel-

f The Recording Strain Gauge Denture Tenderometer—Proctor, B. E. et al, Food Technol, 10, 471 (1955).

Quality 199 oped by Professor Kramer and his associates at the University of Maryland. This incorporates a number of alternative test cells, each designed to measure a different kind of physical property, such as fibrousness, succulence or the resistance of the material to shearing forces. The instrument can therefore be used for the evaluation of quality in plant foods showing a wide range of textural characteristics.

Certain chemical and physicochemical procedures may also give results which correlate well with subjective assessments of textural quality. The determination of alcohol-insoluble solids, which indicates the stage of maturity of and therefore the tender-ness of certain starchy products such as peas, beans and sweet corn, has already been mentioned. Fibrousness can often be assessed by a direct determination of the fibre content of the material; grittiness by a determination of "grit". The physical properties of a tissue are also usually related to its moisture con-tent and the latter can therefore serve as an indirect measure of textural quality. Finally, in some cases, e.g. in leguminous seeds and potatoes, the density of the material, as determined by simple non-destructive flotation tests in brine, can provide a useful indi-cator of textural properties and one which will pick out the individual exception in a way that more elaborate methods of measurement would generally fail to do.

A FEW FINAL GENERAL COMMENTS The foregoing brief discussion of the various attributes which

contribute to acceptability in fruit and vegetables will, it is hoped, serve to underline the essential complexity of the concept of quality in relation to this, or indeed to any other class, of food materials. The various factors of appearance such as size, shape, colour, etc., which primarily influence the consumer's assessment, can be measured with a high degree of accuracy. So also can cer-tain physical properties which contribute to textural quality. Flavour and the more subtle features of texture and appearance, on the other hand, are essentially subjective properties. Objective


methods of assessing quality in fruit and vegetable products are already widely used and will undoubtedly play an increasingly important par t in the future. Such methods provide factual in-formation about the product but can tell us nothing about the likes and dislikes of the man in the street. I t is therefore most im-portant to remember that the final judgement will always depend on the sensory mechanisms of the consumer, and that no reading on an instrument will compensate for the absence of some feature which he considers a necessary prerequisite for good quality in the product concerned.


General

ADAM, W, B., Standards of quality of canned fruit and vegetables, in Quality Control of Food, S.G.I. Monograph No. 8, S . d . , London, 1960, p. 45.

ADAM, W. B., The quality of fruit and vegetables for processing, in Recent Advances in Food Science, vol. 2, (Hawthorn, J . and Leitch, J . Muil eds.), Butterworths, London, 1962, p. 83.

DICKINSON, D., Relationship of chemical composition to quality in fruit and vegetables for canning, J. Sci. Food Agric, 10, 73 (1959).

KRAMER, A. and TWIGG, B. A., Principles and instrumentation for the physical measurement of food quality with special reference to fruit and vegetable products, Advances in Food Research, 9, 153 (1959).

KRAMER, A. and TWIGG, B. A., Fundamentals of Quality Control for the Food Industry, Avi, Westport, Conn., 1962.

O.E.C.D., International Standardization of Fruit and Vegetables, O.E.G.D. Documentation in Agriculture and Food No. 47 (1961) and No. 54 (1962), O.E.C.D., Paris.

SHIPTON, J., Characteristics required of vegetables for processing, Food Près. Quart., 20, 13 (1960).

TOMKINS, R. G., Unsolved problems in the preservation of food; the in-fluence of cultural conditions on the quality and preservation of fruits and vegetables, J. Sci. Food Agric, 5, 161 (1954)

Colour

MACKINNEY, G. and CHICHESTER, G. O., The color problem in foods, Advances in Food Research, 5, 262 (1954).

MACKINNEY, G. and LITTLE, A. C , Color of Foods, Avi, Westport, Conn., 1962.

Quality 201

Flavour CAUL, J . F., The profile method of flavor analysis, Advances in Food Research,

7, 1 (1957). CROCKER, E. C , Flavor, McGraw-Hill, New York, 1945. HARRIES, J . M., The quality control of food by sensory assessment, in

Quality Control of Food, S.C.I. Monograph No. 8, S.C.I., London, 1960. JELLINEK, G., Flavour testing with the profile method, in Recent Advances

in Food Science, vol. 2, (Hawthorn, J . and Leitch, J . Muil eds.), Butter-worths, London, 1962, p. 287.

JOSLYN, M. A. and GOLDSTEIN, J . L., Astringency in fruits and fruit pro-ducts, Advances in Food Research, 13, 179 (1964).

ARTHUR D. LITTLE, INC., Flavor Research and Food Acceptance, Reinhold, New York, 1958.

MONCRIEFF, R. W., The Chemical Senses, 2nd edn., Leonard Hill, London, 1951.

ZOTTERMAN, Y. (éd.), Olfaction and Taste, Pergamon Press, Oxford, 1963.

Texture ISHERWOOD, F. A., Texture in fruits and vegetables, Food Manufacture, 30,

399 (1955). MATZ, S. A., Food Texture, Avi, Westport, Conn., 1962. VARIOUS AUTHORS, in Texture in Foods, S.C.I. Monograph No. 7, S.C.I.,

London, 1960.

CHAPTER 9

PROCESSING I. PREPARATORY OPERATIONS

T H E processing of fruit and vegetable materials serves two main purposes. In the first place, processing methods are generally also methods of preservation which, by arresting the natural progress of deterioration, can be used to create outlets for and maintain supplies of perishable commodities during periods when, and in regions where, the fresh materials themselves would normally be unavailable. Secondly, processing provides a means of presenting the material to the consumer in a highly convenient form, requiring the minimum of preparation for the table, a feature which is becoming increasingly popular, especially in more highly developed countries. Certain simple procedures, such as the pre-peeling of material for the fresh market, contribute towards only the second of these objectives, but the major methods of process-ing—canning, freezing and dehydration—are aimed a t fulfilling both the above requirements.

The consumption of processed fruit and vegetable products over the world as a whole is still small compared with that of the fresh materials, but it is increasing year by year. In the United States, which has by far the largest processing industry, a major par t of the total crop of some commodities, for example over 80% of the apricots, over 6 5 % of the oranges (mainly for juice) and over 50% of the tomatoes, are used by the processors. The U.S.S.R. now also has a large canning, or more strictly bottling, industry with an output second only to that of the United States. Otherwise, the bulk of the fruit and vegetable processing is carried out in the countries of western Europe and of the Commonwealth (especially Canada and Australia) which, together with the

202

Processing 1. Preparatory Operations 203

United States, have always taken the lead in the development and exploitation of food-processing techniques. J a p a n and South Africa, however, also warrant special mention as producers of canned fruit.

The rate of growth of the processing industry in the United States and in the major countries of western Europe has steadied out at a relatively modest level in recent years, but in some other areas, notably in eastern Europe, South America, J a p a n and North Africa, productive capacity has been increasing since the early 1950's at a proportionately much faster rate. Many of the poorer agricultural countries also have a great bu t hitherto largely unexploited potential for this kind of development, and there is every reason to expect that the next few decades will see a further big upsurge of activity in the processing of these particular commodities.

Practically all fruit and vegetable materials can be processed to give highly acceptable products, bu t some species and varieties naturally lend themselves better to particular methods of preserva-tion than do others. Certain preparatory treatments, such as cleaning, grading, peeling and tr imming, slicing and dicing, and blanching, although they may not all be required for each indivi-dual product, are nevertheless generally common to each of the major methods of processing, and it will be convenient to deal with them separately in this chapter before proceeding to a description of the unique stages in each processing technique. First of all, however, so as to present the subject as nearly as possible in chronological order, we shall consider in more detail the im-portant question of the selection of raw materials for the processing industry.

THE SELECTION OF RAW MATERIAL FOR PROCESSING

I t need hardly be stated that, whenever possible, raw material for the processing industry should be in sound condition and of good overall quality. In addition, account must, of course, also be


taken of the likely performance of the material during the process itself. Changes during processing are inevitable and the end-product must conform to standards of quality which are neces-sarily different from those which would be applied to a fresh material.

One general feature which is highly desirable in material pass-ing through any mechanized process—and food processing in these days is becoming increasingly more highly mechanized—is uniformity, especially of size, shape and physical condition. Size grading is usually an integral part of the process and will be dis-cussed later. Irregularity of shape, though sometimes unavoidable, reduces the efficiency of mechanical operations such as peeling, and generally leads to a high rate of wastage. Certain kinds of shape are particularly undesirable. A good example is that of root vegetables such as carrots with a long gradually tapering form. The shorter stump-rooted varieties are much to be preferred for all kinds of processing. The ease with which unwanted parts such as stalks, cores, plugs, etc. can be removed is also a factor which helps to determine the suitability of material for a processing operation.

Apart from such purely mechanical considerations, the colour, flavour and texture of the material and the extent to which these may be modified by the process, are obviously factors of prime importance. Freezing, among the major methods of preservation, causes the least change in these quality attributes and the require-ments in material for freezing are therefore most similar to, al-though not necessarily identical with, those for the fresh market. The severe heat-treatments used in canning and the normal prac-tice of packing the material in syrup or brine result in much greater changes during this latter method of processing. Pigment changes, such as the conversion of chlorophyll to phaeophytin and the alteration and leaching of anthocyanin pigments, often necessitate the addition of artificial colouring matter. In such cases, the colour of the raw material is of relatively little im-portance. For example, many varieties of peas now used for can-ning are much paler in colour than would be desirable in material

Processing 1. Preparatory Operations 205 for the fresh market. In other cases the presence of leuco-antho-cyanins may lead to the formation of undesirable brown or reddish-purple compounds during heat processing. This occurs for example in most varieties of broad beans and in some varieties of gooseberries and pears. (Nearly all cooking varieties of goose-berries and most varieties of pears only discolour, however, if seriously overprocessed.)

Heat processing, apart from causing qualitative changes in flavour, also usually reduces the general intensity of flavour of the material. A full rich flavour is therefore generally desirable in material for canning. With some species such as apple and goose-berry, the sharply flavoured cooking varieties are more suitable for canning than the more pleasantly and delicately flavoured dessert varieties, sweetness being added with the canning syrup. In other cases, flavour may be sacrificed to obtain an improve-ment in some other quality attribute. For example, the rich-flavoured freestone varieties of peach, the segments of which have an untidy ragged appearance in the can, are less popular for can-ning than the poorer-flavoured neater-packing clingstone varieties.

Textural considerations are also most important. The main requirements here are that the material should be capable of withstanding the processing treatment without tissue breakdown, while at the same time being free of undesirably tough and heat-resistant skins or other tissue aggregates. Some varieties of plums and raspberries, for example, are especially susceptible to break-down during canning, while some kinds of peas and blackcurrants have undesirably tough skins. Another less common type of textural defect found in plums, notably in the popular variety Victoria, is the excessive secretion of gum around the stone of the fruit.

All the factors so far considered have a genetic basis and a great deal depends on the selection of suitable varieties. It would be quite impossible in the space available to deal fully with the suit-ability for processing of the exceedingly large number of different varieties of fruit and vegetables which are now grown. Even a


complete list of the varieties which are at present used by proces-sors would be extremely lengthy, f Moreover, the situation con-tinuously changes as new and improved varieties become available.

Growing conditions may also markedly influence the quality of a material for processing and different batches of a given variety can show appreciable differences in performance. Careful inspec-tion of the raw material is therefore always necessary and the processor must always be ready to modify his technique to suit the requirements of the particular batch of material passing through his plant at any given time.

Finally, a most important factor determining the suitability of a material for processing is the stage of maturity at which it is harvested. Fruit for processing should normally be used at the "firm r ipe" stage, when it is fully grown and well coloured, but before it has become soft. There are, however, a number of excep-tions to this broad generalization. For example, gooseberries are better to be under-ripe for canning: blackberries, on the other hand, should be fully ripe. The "soft r ipe" stage, at which the flavour has reached its full development, may also be preferred, as in the case of peaches, in fruit which is to be frozen.

Vegetables, as a group, comprise a more heterogeneous assort-ment of commodities but, in general, the best results are obtained when the tissues are young and tender. With many vegetable species, the stage of development of the material is less critical than it is with fruit and, since the normal cropping periods tend to overlap or coincide, the availability of processing plant may be the deciding factor. The most important fresh vegetable used for processing is, however, the pea, and in this case the stage of maturi ty is highly critical and the optima have been well defined in terms of the content of alcohol-insoluble solids (A.I.S.) and of the Tenderometer reading (T.R.) . In green peas for canning, the

■f For information on this point, the reader is referred to the larger texts on fruit and vegetable processing and, for British varieties, to the publications of the Fruit and Vegetable Canning and Quick Freezing Research Association (now renamed the Fruit and Vegetable Preservation Research Association).

Processing 1. Preparatory Operations 207 A.I.S. should be between 13-5% and 14-0% and the T.R. 115-120, while for freezing, an A.I.S. of 10-5-11-5%, corres-ponding to a T.R. of around 100, is desirable. The optimum stage of maturity for dehydration is similar to that for freezing.

Genetic, cultural and developmental factors are therefore all of importance to the processor in so far as they affect the quality of his raw material. In countries with a highly developed processing industry, large crops are grown under contract, specifically for processing. Detailed specifications can therefore be laid down and, for annual crops such as peas, the processing company normally supplies the seed, arranges a suitable planting schedule and generally supervises the growth of the crop, its harvesting at the desired stage of maturity and its transport to the factory with the minimum of post-harvest delay.

PREPARATORY TREATMENTS The preparation of material for processing is usually closely

similar whatever method of preservation is subsequently applied, although the sequence of operations varies considerably according to the individual requirements of particular commodities. Certain procedures, however, are very widely used and these will now be considered as unit operations.

Soaking

Prolonged soaking is only a necessary part of the preparation of the material when dried peas or beans are being used, as for example in the canning of "processed peas" or of beans in tomato sauce. The dry peas or beans need to be reconstituted before they can be used for the canning operation and to this end they are soaked in water, usually for periods of between 16 and 20 hours, preferably in stainless steel or monel-metal tanks or, failing this, in tanks which are heavily galvanized so as to prevent blackening of the material due to traces of iron. It is most important in the soaking of dried peas or beans that the temperature of the soaking


water should be kept as constant as possible from batch to batch or, if warm water is initially introduced to hasten reconstitution, that the rate of cooling of the tanks should be kept as similar as possible by maintaining a constant air temperature. Variations in temperature from batch to batch lead to differences in the degree of reconstitution and therefore to differences in perform-ance during the canning operation.

Cleaning and washing

Fruit and vegetables as received at the processing factory are very commonly contaminated with soil and other foreign materi-als, and these must be removed if a high-quality product is to be obtained. Dry cleaning by winnowing in an air blast and passing over screens is used in a few cases, notably with peas and goose-berries, to remove dust, leaves and other light-weight contami-nants and foreign bodies of markedly different size. Washing, on the other, is almost invariably introduced at some stage in prepara-tion, and several methods, each suited to a particular range of commodities, are employed. A simple soak in water is not usually sufficient in itself, but it does serve to loosen adhering soil which is then more easily removed during subsequent washing opera-tions. More effective are various methods in which the material is agitated while submerged in water or subjected to water sprays. For materials such as peas which can stand quite vigorous treat-ments, a special kind of rotary washer—the Duo or Olney Washer —is commonly used. In this, the product is first passed in a swiftly flowing stream of water over a riffle board, a device fitted with a number of small inclined metal plates which effectively trap stones and other heavy foreign bodies. Next, the material is carried through a trough where lighter contaminants which float are skimmed from the surface, and finally it is passed into a re-volving screen supplied with water sprays. A more recent develop-ment, which works on a similar principle to the Duo Washer but which is capable of a greater output and is more efficient in opera-tion is the Flotation Washer illustrated in Fig. 24.


More delicate products such as peaches can be passed through rotary drum washers in which the material is gently agitated and moved along through a trough of water by the revolution of a drum fitted on the inside with a spiral scroll. Alternatively, the material may be conveyed, submerged in water, on a perforated belt, the water being gently agitated by means of a paddle wheel —a system commonly used with plums and the other smaller soft

Feed hopper

Floating waste, leaves, thistle heads, pods etc.

o Water returned to pump

Pea boost-supply f rom pump

Stone -ejector

supplied from pump

FIG. 24. A diagram showing the construction and mode of action of a Flotation Washer. (By courtesy of Mather & Platt, Ltd.)

fruits. When material is washed by submersion, frequent replace-ment of water is desirable to avoid too great a build-up of con-taminants. Both of the above systems of conveyance can also be combined with the use of water sprays which, although more expensive in water, are generally more effective for washing than mere submersion, especially when high water-pressures are em-ployed. Sprays are used for example in the so-called rod-washers —drum-type washers in which the drum is constructed of parallel rods with intervening spaces which allow suspended soil and other contaminants of small size to be washed through (see Plate 9b).

(By courtesy of Mather & Platt Ltd., Manchester.)

(a)

(b) PLATE 9.

(By courtesy of F. Braby & Co. Ltd., London.)

Processing 1. Preparatory Operations 211 Rod-washers are suitable for a wide range of fruit and vegetable commodities.

Washing may be the first operation to be carried out after the initial sorting of the material, as for example with apples, rhu-barb, tomatoes, and root vegetables, or it may be delayed until after various unwanted parts have been removed. With yet other materials, such as peaches, apricots, citrus fruit and green beans, an early wash may be unnecessary. In any case, further washing is generally required later in the process, especially after peeling and trimming, to remove loose pieces of skin and residues of lye or brine which may have been used in the peeling operation. Washing, especially after blanching, can remove appreciable amounts of soluble constituents which may contribute towards the flavour and nutritive value of the material. For this reason, as well as for reasons of economy, it should not be continued beyond the point at which the necessary cleaning has been effected.

The removal of unwanted parts

Parts which are commonly removed in preparation for process-ing include vines, pods, husks, stalks, calyx remains, cores, peels, eyes and any parts of the material which are in any way damaged or otherwise unsuitable for inclusion. Many of the operations involved were originally, and in some cases still are, carried out by hand, but a wide range of machinery is now available and in

[Opposite page]

PLATE 9. (a) A load of newly shelled peas being tipped from the main collecting hopper of a mobile pea-viner into a transporting tank for immediate removal to the processing factory. The plants and pods are discharged by the viner back onto the field. The viner continues to operate during this procedure and further peas can be seen falling into a supplementary bin to the left of the main hopper, (b) Two rod-washers set in position at the ends of preparation belts on which the material is trimmed prior to passing through the washers. Notice the longitudinal rods forming the "wall'* of the drum and the spiral metal scroll which conveys the material along the length of the washer due to rotation of

the drum. The pipe along the centre conveys spray water.


most modern processing factories these operations are largely mechanized. The type of operation required is, of course, peculiar to the commodity being processed.

Vining, Vining and podding of peas is now almost entirely carried out on the farm by machines called viners designed specific-ally for the purpose. In recent years mobile viners have been developed which allow the process to be carried out in the field rather than at central vining stations (see Plate 9a). The complete vines are fed into the machine, the pods being burst and the peas removed by the action of beaters in a revolving cylinder. Riddle screens allow the peas and smaller pieces of vine to fall through, and a final separation is effected on rising belts which carry the flatter material upwards while the peas roll to the bottom. The separated peas should reach the factory with the minimum of delay, since this material is especially susceptible to rapid loss of quality after harvesting.

The other operations to be described in this section are norm-ally carried out in the processing factory.

Husking and silking. These procedures are peculiar to corn (maize) and are usually mechanized. Husking machines work on the principle of the clothes wringer, the husks being torn off by pairs of rubber rollers so spaced that the ears cannot pass between them. The silk is conveniently removed by pairs of revolving brushes.

Stemming, strigging, snibbing and snipping. The removal of stalks and calyx remains may be done by hand or by machine. A machine incorporating a series of inclined rubber or knurled rollers, which revolve towards each other in pairs and pull off the stalks as the product slowly rolls down the incline, is in use for cherries, plums, blackcurrants, etc. With currants, the process is referred to as strigging. Snibbing of gooseberries is normally

Processing 1. Preparatory Operations 213 effected in an abrasive peeling machine where it is combined with a spray wash. Machines are also available for the snipping of green beans. The beans are fed into a revolving drum with narrow perforations and as the ends of the tumbling pods project through the slots they are cut off by fixed knives set at a narrow clearance from the outside of the drum. The stemming and plug-ging—the removal of the harder central part of the receptacle— of strawberries is carried out by hand.

Pitting and coring. The pitting (the removal of the stone) of drupe fruits, where necessary, and the coring of pome fruits may again either be carried out manually or by machine. In the United States, peaches, especially the clingstone varieties, are normally halved and pitted mechanically and several kinds of machine are available for the purpose. This practice is now also common with apricots and with sour cherries. Plums and cherries, however (except sour cherries in the United States), are usually canned with the stone in place, a procedure which is also sometimes used with apricots.

Machines are widely used for the simultaneous coring and peel-ing of apples (see Plate 10a) and, in the United States, of pears, but elsewhere the coring of pears is normally carried out by hand. The hard central core and tough outer skin of the pineapple can be removed in a machine known as the Ginaca, but a single mach-ine of this kind can only take fruit of a narrow range of size, and several such units, each set for a different size of fruit, are there-fore usually necessary. Other products for which mechanical coring devices are available are tomatoes and cabbages.

Peeling

Many fruit and vegetable products require to be peeled in pre-paration for processing, and hand peeling, the original method, has now been largely replaced by other speedier labour-saving techniques.

{Both by courtesy of W. Brierley, Collier & Hartley Ltd., Rochdale.)

(b)

PLATE 10.

Processing 1. Preparatory Operations 215 Mechanically operated knives for peeling are incorporated in

the devices mentioned above for the coring of apples and pears. Carborundum-covered abrasive rollers or revolving discs, com-bined with water sprays, are employed in machines designed for the peeling of root vegetables (see Plate 1 Ob). Abrasive peeling is greatly facilitated or may even be rendered unnecessary if the peel is first softened and loosened by one of the methods described below.

A brief treatment ( | -3 min) in hot water or steam will some-times soften the skin sufficiently for it to be easily removed by hand. This is effective, for example, with tomatoes, peaches and citrus fruits. Tougher skins may be loosened by the use of pressure steam-peelers in which the material is subjected for a short period to steam at up to 100 lb/in2. The subsequent sudden release of pressure sufficiently disrupts the skin for it to be readily removed by hand or by water sprays (root vegetables, apples). Brief expo-sure to air at up to 900°C (thermo-peeling of tough-skinned plums and other fruit), to hot combustion gases (flame-peeling of onions and root vegetables) or to hot oil (oil-peeling of pimientos) can be used to produce a similar effect. Alternatively the skin may be softened and loosened by various chemical treatments.

Lye (caustic soda) solutions are almost universally employed in the peeling of clingstone peaches. A hot 1-2·|% solution of the lye is usually sprayed for 30-60 seconds onto the rounded outside

[Opposite page]

PLATE 10. (a) An apple-peeling and -coring machine. The apples are introduced to the machine by hand, but otherwise the operation is entirely automatic. This machine is capable of peeling and coring apples at the rate of between 25 and 30 per minute, (b) An abrasive peeling machine, viewed from above. Notice the carborundum-covered rollers forming the base of the peeling chamber. During operation these rollers revolve rapidly, causing the material to bounce about so that all parts of its surfaces at some time come in contact with the abrasive car-borundum. A longitudinal tilt causes the material to move progressively along the machine around a number of partitions with adjustable gaps, and water is continuously sprayed onto the material from the pipes running across the top of the peeling chamber. The output is between 1 ton and 2J tons per hour, depending on the commodity.


surfaces of the cut halves which are then sprayed with water to remove the loose skins. Submersion in lye has a similar effect but results in losses from the cut surfaces of the fruit. Higher or lower concentrations of lye and different periods and methods of appli-cation, depending on the toughness and depth of the skin to be removed, can be effectively used with many other products in-cluding apricots, prunes, citrus fruit (to remove the albedo after the main skin has been taken off), root vegetables and pome fruits. Boiling saturated brine may be used in place of lye for some materials and is less expensive and less hazardous to personnel. This last treatment has been found to be particularly useful in the peeling of pears.

Finally, a method of disrupting and loosening skins which is yet very much at the experimental stage, but which shows promise of useful future commercial application, is the use of ultrasonic radiations.

Trimming. However highly mechanized a process, the variability of the raw material is such that there are almost invariably minor trimming operations which need to be done by hand. Peeling methods may be less than 100% efficient in the removal of skin and eyes, blemishes cannot be detected and removed mechanic-ally and some batches of material may, because of their irregular or abnormal size and shape, be unsuitable for passing through the usual mechanical operations. Experienced personnel become highly adept at carrying out these necessary trimming operations and with good supervision a high degree of efficiency can be main-tained. These hand-trimming operations are normally carried out continuously, with the product moving along conveyor belts which are compartmented longitudinally to permit the separation of the trimmed from the un trimmed material. Generally, the trimmers are positioned at intervals along each side of the belt and after they have removed any remaining blemishes etc., they transfer the material to a central compartment.


Slicing and dicing

The cutting of material into pieces, whether they be halves, segments, whole slices, strips or dice, is a common feature in the processing of fruit and vegetables. Canned and frozen products prepared in the form of pieces of uniform size present an attrac-tive appearance to the consumer and are also highly convenient to use, while for dehydration in hot air a small uniform size of piece, not exceeding T% in. in its smallest dimension, is essential in order to obtain a commercially feasible and even rate of drying.

Gutting operations are almost invariably mechanized and a large range of machines is available, some designed for use with a specific product, others for more general application.

Grading

In order to obtain a product of uniformly good quality, grading is most important at one or more points during the preparation of the material. Grading for appearance factors other than size is normally done by eye, but size grading is readily mechanized. Delicate commodities, such as tomatoes and fruit which is becom-ing soft, and irregularly shaped materials, such as asparagus and green beans, may be best separated by hand into their various size grades. Some available machines are, however, quite gentle in their action and therefore are usable with a wide range of com-modities. Screens with perforations of different sizes in the form of vibrating sheets or revolving cylinders are utilized in many mechanical graders used for products such as peas, cherries and other near-spherical fruits. Other machines utilize diverging wires or rollers which permit the passage of increasingly larger units as the material moves slowly over the top. Root vegetables and the larger fruits are conveniently size graded in this way, while the roller-type of machine is especially useful with small soft fruits such as raspberries and strawberries. A grader of this latter type is illustrated in Plate 11a. Grading by weight using systems of counterpoised rods is an alternative method for the larger pome and citrus fruits.


Density separation is a useful means of grading for some pro-ducts. With peas, for example, a brine of specific gravity 1 · 065 (about 10% sodium chloride) can be used to separate younger and more tender peas which float, from their tougher denser neighbours. Flotation can also be used to pick out frost damaged, and therefore drier, citrus fruits. In this case, the sound fruit is heavier and sinks to the bottom.

Blanching or scalding

Blanching or scalding involves a short heat-treatment, the nature and purpose of which vary somewhat with the material and with the method of preservation to be employed. The use of blanching treatments for the softening of skins prior to peeling has already been mentioned. Apart from this particular application, the various reasons for blanching can be summarized as follows :

(a) It helps to clean the material and, in particular, it reduces the load of micro-organisms present on the surfaces.

(b) It removes intercellular gas, thus preventing the excessive build-up of pressure in the can during heat processing and in some cases improving the appearance of the product.

(c) It softens the tissues and causes some shrinkage, so allowing a greater volume of material to be introduced into a pack of given size.

(d) It inactivates enzyme systems which cause deterioration in quality. The discoloration of products such as apples and potatoes which contain active phenoloxidases is halted, and many other

[Opposite page]

PLATE 11. (a) The "Grovesend" Grader—a size grader suitable for use with a wide range of fruit and vegetable commodities. The grading bed consists of a continuous conveyer made up of revolving aluminium rollers, the gap between any two rollers being automatically widened as the conveyor moves away from the feed end. (b) A standard water-blancher. The blancher liquor is contained in the lower half of the outer casing, which is lined with stainless steel. The material is held in a revolving perforated stainless steel drum, the side of which is opened in the picture to show the spiral scroll which conveys the material along

the length of the blancher.

(Both by courtesy of Mather & Plait Ltd., Manchester.)

(b)

PLATE 11.


undesirable enzyme-mediated changes which would otherwise take place during and subsequent to the processes of drying and dehydration are likewise prevented. Enzyme (peroxidase) in-activation is in fact normally used as the criterion for the effective-ness of blanching in material which is to be frozen or dehydrated.

(e) It may help to "fix" the colour of the material. For example, the carotenoid pigments in carrot become dissolved in small intracellular oil-droplets during blanching, and in this way they are protected from oxidative breakdown during dehydration. The conversion of chlorophyll to phaeophytin during subsequent cooking is also claimed to be reduced in some materials by blanching.

(f) It is essential for maintaining a capability for adequate reconstitution in vegetable materials which are to be dehydrated and also has a beneficial effect on texture in many other cases.

(g) It affords a means of controlling the pH of the material which has an important effect on the colour and on the general performance of vegetable materials during dehydration.

(h) Finally, various other chemical treatments can be combined with blanching, such as the introduction of sulphur dioxide, a necessary preservative for many dehydrated vegetables, and of calcium salts which may sometimes be used to reduce the sus-ceptibility of material to tissue breakdown during processing or cooking.

Blanching, therefore, is a most important stage in many pro-cessing operations. In the freezing and dehydration of vegetables it is an essential preparatory treatment for practically all com-modities. (Onions are the only vegetable materials which are normally not blanched before drying.) Fruit which is susceptible to enzymic darkening is normally blanched before freezing or canning. Otherwise, the blanching of fruit is not essential, al-though its use may facilitate other operations such as peeling, slicing and filling (packing).

There are two main methods of blanching—water blanching and steam blanching. Each has its advantages and each its dis-

Processing L Preparatory Operations 221 advantages. For example, water blanching inevitably results in leaching of soluble constituents which contribute to flavour, and of water-soluble vitamins, notably ascorbic acid. Such leaching can also cause a significant loss of yield in a dehydrated product, although the leaching of sugars from potato which is to be de-hydrated results in a product which is less prone to non-enzymic browning during storage in the dry state. Steam blanching, on the other hand, causes much less loss by leaching, but a longer blanch is generally required for the effective inactivation of enzyme systems and additional problems arise with regard to the application of chemical treatments. In general, water blanching has been used much more extensively in the United Kingdom, while steam blanching has been favoured for several commodities (but not for peas) in the United States.

Most water-blanchers consist of a horizontal cylindrical tank holding the hot liquor through which the material is passed by the movement of a metal scroll, either open and moving at a very small clearance from the inner surface of the tank or enclosed in a perforated drum (see Plate l ib ) . The liquor is heated by direct steam injection, normally to between 190° and 210°Ff and the length of the blanch can be changed by altering the rate of move-ment of the scroll. Blanching times of between 1 | and 5 minutes are normally used, the precise time depending on the material and on the purpose of the blanch. The parts of the blancher which come in contact with the liquor are best constructed of stainless steel and the water used should be soft and clean. Hard water is particularly undesirable for peas since the calcium salts present can cause a toughening of texture by reacting with the pectic constituents of the tissues. When vegetables are being scalded prior to dehydration, controlled amounts of sulphites are introduced to the liquor and, with green vegetables, quantities also of sodium carbonate to give a slightly alkaline pH and so prevent the formation of phaeophytin. Phosphates may also be

f Temperatures in the remainder of this chapter and in Chapter 10 will be given in °F, since the Farenheit scale is normally used in industrial practice in English-speaking countries.


added for some products such as carrots in order to reduce a dis-coloration which results from the presence of traces of iron in the blancher liquor.

In steam blanching the material is passed through a steam chamber, either on a moving belt or by means of a turning screw conveyor which fits closely the inner contours of the base of the chamber (see Fig. 25). Blanching times again vary with the material over a range similar to that used in water blanching. If chemical treatment is required, the necessary solutions must be sprayed onto the material either before, during or after passage through the steam chamber.

Various other methods of blanching have been used experi-mentally, or to a limited extent in commercial operation. These include in-can blanching prior to canning, hydraulic hot-water blanching in steam-heated pipes, and blanching by dielectric heating or by infra-red irradiation. None of these methods has proved sufficiently advantageous for general use to supplant the established conventional methods of blanching in hot water or steam.

Material which is to be frozen or dehydrated requires to be cooled after blanching so as to facilitate subsequent handling and prevent overcooking of the material, and this is usually carried out in blasts of cold air.

THE PREPARATION OF JUICES More fresh fruit and vegetable material is now used for juice

production than for processing in any other form. The production of fruit juices alone amounts to the equivalent of about 10 million tons of fresh fruit per annum, while the output of tomato juice is over twice that of any other single-strength juice. Frozen orange concentrate, however, is the chief individual product. Once ex-tracted, fruit and vegetable juices need to be preserved by one of the methods described in the succeeding chapter. The extraction of the juice is therefore strictly a preparatory treatment and will be considered very briefly at this point. Unfortunately, space will

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not permit a fuller coverage of this increasingly important topic and for detailed information the reader is referred to one of the larger texts devoted specifically to the subject·!

Tomato juice and apricot, peach and pear nectars, as normally prepared, are really comminuted pulps. The fruit, after pitting, coring and peeling as necessary, and coarse crushing or chopping in the case of tomato, is heated and passed through a special juicing machine which consists of a tapered screw revolving inside a cylinder with many small perforations each about 0 · 02 in. in diameter (see Plate 12a). With tomato, the extruded juice can then be heat processed and canned directly, normally with the addition of 0-5-0-7% of salt. Homogenization may, however, be necessary to give a product of the desired uniform consistency. Tomato juice is often used as a base for the preparation of mixed juices containing up to 30% of juice derived from other vegetable commodities. Fruit nectars require the addition of sugar syrups, the mixing of the extruded pulp with an equal quantity of a 15-16° Brix syrup (a syrup containing 15-16% of sugar (see p. 232)), or the equivalent in terms of the final sugar concentration, being a satisfactory procedure. A flash pasteurization at between 170° and 205 °F in steam-heated tubular heat-exchangers is norm-ally used before packing into cans.

Citrus juices are extracted by passing the unpeeled fruit through special machines of which there are a number of kinds available.

1" E.g. Tressler, D. K. and Joslyn, M. A., Fruit and Vegetable Juice Processing Technology, Avi, Westport, Conn., 1961.

[Opposite page]

PLATE 12. (a) A juice extracter of the screw-and-screen type such as is widely used for the extraction of tomato juice. The cover is removed to show the perforated screen through which the juice is extruded, (b) The extraction of blackcurrent juice. The initial extraction in this case is normally carried out in "rack and cloth" presses similar to those used in the cider industry. The fruit is first milled and treated with a pectinase enzyme preparation for about 2 hours at 40°C. The treated pulp is then introduced into terylene cloths which are sandwiched between ash-wood racks, the finished "cheese" being pressed in an hydraulic press. The picture shows two "cheeses" being pressed while a further one is being

prepared in the background.

(By courtesy of Varly-FMC Ltd., Brentford.)

(a)


In one popular type of juice extractor the fruit is halved and reamed in a way similar to that in which the process is normally carried out in the home. With oranges, the essential oil from the peel may be extracted and collected separately during the same operation, but it is important that the amount of this peel oil which finds its way into the juice is kept to a minimum. Citrus juices, especially orange, are liable to undergo undesirable changes in flavour if subsequently held at above about 40°F and freezing is becoming very popular as a method of preserving these com-modities. The concentration of citrus juices has also become a common procedure and frozen concentrates are in fact now of outstanding importance in the United States. These are prepared by first concentrating the juice under vacuum at room tempera-ture, usually in falling-film evaporators, to between 50° and 60° Brix and subsequently adding back fresh juice to adjust the strength to about 42° Brix and to restore the fresh flavour.

The vacuum concentration of these juices aids in the removal of excess peel oil and of dissolved oxygen, both of which are in-volved in the processes of flavour deterioration and, even if the juice is not to be prepared in the form of a concentrate, a short iCde-oiling" and "de-aeration" treatment after extraction is gener-ally desirable. (De-aeration is also used to a lesser extent with other juices such as tomato.) Grapefruit juice usually needs to be sweetened by the addition of a little sugar, a procedure which is required only very occasionally in the preparation of orange juice. A short heat-pasteurization treatment is also normally required with citrus juices, whether they are to be preserved by canning or bottling, or by freezing. In addition to its effect on micro-organisms, this is necessary to inactivate pectin-degrading en-zymes which would otherwise cause a loss of "cloud"—a desirable natural feature in citrus juices caused by the presence of dissolved and suspended pectic materials.

Pineapple juice is obtained largely from the cores and peels dis-carded by the Ginaca machines (p. 213) during the preparation of the material for canning, from juice extracted incidentally during other precanning operations or from undersized fruit which is

Processing L Preparatory Operations 227 useless for other processing purposes. The solid pieces are shred-ded and comminuted and then passed through a suitable type of extractor, the expressed juice being thoroughly mixed with juice recovered from the other precanning operations. The mixed juice is then heated to 140-145°F and centrifuged to remove the larger particles of suspended solid matter. Homogenization may also be introduced to produce even greater uniformity. Finally, the juice is filled into cans either before or after the necessary heat-pasteurization treatment.

Other fruits such as apples, grapes, berries and currants are generally crushed or pulped, heated to precipitate proteins, in-activate enzyme systems and pasteurize the material, and pressed, usually on rack and cloth presses, to extract the juice (see Plate 12b). Procedures vary in detail from place to place and according to the type of product required. Apple juice is particularly sus-ceptible to rapid enzymic discoloration and some success in pre-venting this has been achieved by introducing ascorbic acid as an anti-oxidant during the initial pulping operation. In most cases, the object is to produce a clear rather than a naturally cloudy juice and, in contrast to the procedure used with citrus juices, preparations of pectolytic enzymes are commonly added to the extracted juices as clearing agents. With grape juice, an additional problem is the presence of tartrates which, if not removed during the process, will gradually precipitate out after the product is finally packed. A common procedure here is to first flash-pasteurize the material by heating to between about 175° and 190°F and to subsequently store the material in closed containers for an extended period—up to as long as twelve months—to allow the deposition of cream of tartar. Shorter periods of intermediate storage are also used with most other products of this kind.

Before the extracted juice is finally filled into bottles or cans, a further filtration is almost always necessary. This is normally carried out in filter presses, using diatomaceous earths as filter aids. In Europe, however, Seitz filters are sometimes used, as in the Seitz-Boehi Process, and in this case the filtration effectively sterilizes the product by removing micro-organisms and, if the


juice is directly filled into sterile containers, no subsequent heat-treatment is necessary. Berry and currant juices are usually sweetened by the addition of sugar syrups, while artificial colour-ing matters and ascorbic acid are also added in particular cases. The use of chemical preservatives in fruit juices is discussed in the next chapter.

Another process which is commonly applied to apple, berry and grape juices is carbonation. This may be carried out by subjecting the juice to carbon dioxide under pressure in a bulk tank or in a continuous carbonator as the product flows to the filling mach-ines. An alternative method where diluted juices are being pre-pared is simply to dilute with previously carbonated water.

Finally, a product which has gained considerable popularity in Australia in recent times and which is also produced in appreci-able quantities in Hawaii, is passion-fruit juice. Crushing, press-ing and screening of the pulp of this particular fruit yields a cloudy, strongly acid and highly aromatic juice which, when sweetened with sugar, produces a highly palatable beverage for which a wider market will undoubtedly be found in future years.


CRUESS, W. V., Commercial Fruit and Vegetable Products, 4th edn., McGraw-Hill, New York, 1958.

GOOSE, P. G. and BINSTED, R., Tomato Paste, Puree, Juice and Powder, Food Trade Press, London, 1964.

JOSLYN, M. A. and HEID, J . L. (eds.), Food Processing Operations, vol. 1, Avi, Westport, Conn., 1963.

LEE, F. A., The blanching process, Advances in Food Research, 8, 63 (1958). LOCK, A., Practical Canning, 2nd edn., Food Trade Press, London, 1960. MEYRATH, J., Problems in fruit juice pasteurization, in Recent Advances in

Food Science, vol. 2 (Hawthorn, J . and Leiten, J . Muileds.), Butterworths, London, 1962, p. 117.

PETERSON, M. S. and TRESSLER, D. K., Food Technology the World Over, vols. 1 and 2, Avi, Westport, Conn., 1963 and 1964.

TRESSLER, D. K. and JOSLYN, M. A. (fcds.), Fruit and Vegetable Juice Process-ing and Technology, Avi, Westport, Conn., 1961.

CHAPTER 10

PROCESSING 2. METHODS OF PRESERVATION

CANNING This method of preservation is aimed at the destruction by heat

of potential spoilage organisms after the material has been sealed in a gas-tight container.

The container

The primary requirements in any container to be used for the canning of fruit and vegetables are that it should be capable of being hermetically sealed by a rapid and efficient process, and that it should be strong enough to withstand the large internal pressures, in some cases up to 35 lb/in.2, which may be generated during the sterilizing treatment. Easily the most popular container now used for this purpose is the Open-top or Sanitary Can—made of tin-plated sheet steel. Glass containers, however, provide a satisfactory alternative and are still quite widely employed especi-ally for juices. In the U.S.S.R. in particular, bottling rather than canning is used for a wide range of fruit and vegetable com-modities.

Modern open-top cans are the result of a long process of re-search and development and several kinds have been evolved, each designed to meet the special requirements of a particular type of commodity. Particular problems associated with the pack-ing of fruit and vegetables are the corrosive and staining action of these largely acid materials on the inner lining of the can and the discoloring effect of traces of dissolved tin on anthocyanin

229 F. & V.—I


pigments. Special lacquers have been developed to protect the tin-plate and the can contents from these effects. For example, an-thocyanin-pigmented products, except when artificial colouring is added, are exclusively packed in cans which are coated internally with special acid-resistant lacquers. The latter should also be used when packing other products of particularly high acidity. Peaches, pineapples and apricots, on the other hand, can be packed in plain unlacquered cans, while for some products, e.g. grapefruit and apple, the presence of dissolved tin may actually retard un-desirable changes in colour and in flavour.

Some vegetables, such as peas and sweet corn, although less acid in reaction, contain sulphur compounds which break down under heat processing to release hydrogen sulphide. In plain cans, this hydrogen sulphide reacts with the metals of the tin-plate to produce black metallic sulphides, causing an unsightly staining of the can and sometimes also of the contents. For these low-acid sulphur-containing products, cans coated internally with special sulphur-resistant lacquers are used. These lacquers contain zinc oxide which reacts with the hydrogen sulphide to give white zinc sulphide, a substance which, in the quantities formed, produces no obvious discoloration of the can surface. Other vegetables such as asparagus, celery, spinach and root vegetables are better packed in plain unlacquered cans. Lacquers, when used, may be applied to the tin-plate before the can is made, but to ensure the presence of a continuous unbroken film of lacquer a process known as "flush lacquering" is to be preferred. In this case, the lacquer is introduced into the already fabricated can and is there-fore not subsequently subjected to the various mechanical strains and stresses which can cause the formation of cracks or other perforations.

Cans are available in a wide range of standard sizes, from the small 5 oz size to the large A 10 can which holds about 109-2 fluid oz and is used for institutional packs of fruit and vegetable products. Can sizes are designated by two three-figure numbers, the first giving the diameter in inches and sixteenths of an inch, and the second the height in the same units. Thus, the 5 oz can is

Processing 2. Methods of Preservation 231

211 X 202 (2H in. in diameter by 2γβ in. tall), while the A 10 can is 603 X 700 ( 6 Ä in. in diameter by 7 in. tall).

The cans are normally supplied to the factory with the cylin-drical body already formed and with one end sealed in place, ready for filling. All that remains to be done after filling is for the top to be sealed on at the required stage in the process, using special seaming equipment which is usually supplied by the can manufacturer (see later).

Syrups and brines

A few products such as apples and tomatoes are most com-monly canned in the form of a solid pack, but in most other cases the normal procedure is to can fruit in syrup and vegetables in brine. The composition of these filling fluids has an important effect on the quality, especially the flavour, of the product, and their preparation must therefore be in the hands of an experienced and responsible person.

The syrups used in fruit canning are usually prepared from suc-rose, but smaller amounts of liquid glucose (corn syrup) and in-vert sugar may also sometimes be included. In the United States, the canner often purchases his sucrose in the form of a heavy syrup (67° Brix), but in the United Kingdom canning syrups are usually prepared in the factory from commercial granulated sugar. T h e syrup is made up in large tanks of 100-150 gal capacity which should preferably be lined with glass or stainless steel. The tanks are heated by means of closed steam coils and usually fitted with mechanical mixers.

Different concentrations of sugar are used for different products and for different quality grades. Naturally sweet fruits require less sugar than do sour fruits, and a heavier syrup is used for high-quality dessert packs than for commercial packs. Fruit for some purposes such as pie-filling is in fact normally canned in water. Syrup strengths are measured in °Brix or Balling, using a special type of hydrometer, and may vary from 10° Brix for second-grade packs to as high as 70° Brix for top-quality packs of especially tart


commodities such as loganberries. 15° Brix is, however, the mini-mum allowed by the Code of Practice used in the United King-dom. The number of degrees Brix actually corresponds directly to the percentage of sugar in the syrup, the Brix terminology being used simply because of the method of measurement em-ployed. Hydrometrie measurement is influenced by temperature and tables are available by means of which a reading can be corrected to the standard measuring temperature of 20°C (68°F).

The salt used in the preparation of brines for vegetable canning should be pure and free of metallic contaminants. Traces of iron can cause discoloration and precipitation in the can, while cal-cium and magnesium may also adversely affect the texture of the product. Salt concentrations of between 1% and 2\% are norm-ally used and the concentration can again be measured using a special hydrometer called, in this case, a Salometer or Brino-meter—100° Salometer corresponds to saturated brine at 15-5°C (60°F) which contains 26-5% of salt, and therefore each degree Salometer is equivalent to 0-265% of salt. With some products, notably peas and corn, and sometimes also with green beans, broad beans and beetroot, between 2% and 5% of sugar is added to the brine to improve the flavour of the product. Normally the brine is made up before filling, but in some cases measured amounts of dry salt may be introduced directly into the can and covered with water.

Any artificial colouring matter or flavouring material which may be required is also normally introduced into the syrup or brine. Artificial colour is used for example with peas, green beans, gooseberries, strawberries and with some varieties of plums, while mint is often included as a flavouring agent for peas. Juice re-covered from earlier preparatory treatments may also be intro-duced into the syrup as, for example, in the canning of pineapple. Syrups and brines used in fruit and vegetable packs should, wherever possible, be filtered before they are finally introduced into the cans.


Filling

The minimum amount of a particular material to be packed in a given size of can, whether measured as filled weight (as in the U.K.) or as drained weight (as in the U.S.A.), is generally con-trolled by official or trade standards. It is also mandatory in most countries to declare on the label a minimum net weight for the can contents (solid plus added liquid). In addition to meeting these requirements, the canner is concerned to keep the filled weight for a given pack as constant as possible so as to ensure uni-formity of behaviour during heat processing. Many materials, including the smaller soft fruits and products such as grapefruit segments, peach and pear halves, pineapple slices, asparagus shoots and celery hearts, for which the arrangement of the pieces in the can is important, are packed by hand. In other cases, hand-filling can be considerably speeded up by using a machine called a hand-pack filler (see Plate 13a). A typical machine of this type consists of a circular revolving stainless steel table with a number of circular holes, each of a diameter slightly less than that of the can. Each can is positioned under a hole and is filled by scooping material into it from the table as it moves round. The cans are then passed automatically to the syruper or briner.

The filling of robust products of uniform size such as peas and beans is now completely automatic. The material falls through a hopper into the cups of a rotary filler, which introduces the same volume of product into each can. Modern machines (see Plate 13b) combine the filling of the material itself with that of the brine, the can passing directly from the solid-filling head to a liquid-filling head, where hot brine is introduced to a standard level. Modern machines are capable of filling up to 300 cans per minute in this way and, although the material is measured out by volume, an accuracy of ± τ$ οζ on the filled weight of solids has been obtained with products such as peas.

Where syruping or brining is not combined with the filling of the solid material, it must, of course, be carried out as a separate operation. In such cases, the syrup or brine is most commonly

(Both by courtesy of Mather & Platt Ltd., Manchester.)

(b)

Processing 2. Methods of Preservation 235 introduced after the can has been filled with material, but with some products such as grapefruit sections the syrup will not flow readily around the sections and is better introduced first. In the older type of liquid filler, the syrup or brine is simply run into the cans from a horizontal tube with a series of perforations on the under side, the cans being first filled to overflowing and sub-sequently tilted at a predetermined angle to obtain the desired headspace. The overflow is collected in a trough and pumped back into a storage tank above. More modern syruping machines, called pre-vacuum syrupers, first apply a vacuum to the can contents, then add the required amount of syrup and finally apply a further vacuum to remove any air which may have become trapped within or around the material. When used with steam flow or vacuum closing of the can (see later) this procedure obviates the need for exhausting. In any case, syrups and brines are normally introduced hot—up to 180°F or even higher—so as to facilitate exhausting where necessary and to allow sub-sequent heat-processing to be carried out in the shortest possible time.

Exhausting and can-closure

The object of exhausting is to remove air from the contents of the can and from the headspace immediately before closure. This prevents the build-up of undue pressure inside the can during heat processing, reduces internal corrosion which is accelerated by the presence of oxygen and ensures a good vacuum in the headspace when the can is finally cooled, so that the ends remain

[Opposite page]

PLATE 13. (a) A hand-pack filler. The material is scooped from the table through the holes into the cans, (b) A modern automatic can-filler. In this illustration the filler is arranged for double solid fill and liquid fill. The solid-filling head can be used with a range of commodities of small uniform size, such as peas, beans, diced vegetables, etc., while the liquid-filling head introduces the covering brine, syrup or sauce. An output of up to 280 cans per minute is obtainable, depending on the size

of can and on the product being filled.


flat or concave and do not bulge. Exhausting may be brought about by heat or by the application of vacuum.

Provided that entrapped air is effectively removed from the contents of the can—a condition which may be difficult to achieve with some products, especially with unpitted drupe fruits—the efficacy of heat exhausting depends on the temperature to which the material is raised, since this largely determines the amount of air remaining in the headspace of the can. Fruit and vegetable packs are normally heated with hot water or steam to between 180° and 210°F for a period—ranging from about 6 to about 15 minutes—sufficient to raise the centre of the pack to between 160° and 180°F. Vegetable commodities are generally subjected to more severe heat-treatments than are fruits. During heat ex-hausting, it is often useful to have the top of the can clinched— loosely caught bu t not sealed on—so that can closure can sub-sequently be effected more rapidly, at a higher temperature and with less ingress of air. Many kinds of heat exhausters are avail-able, most of them being undesirably bulky and space consuming.

Some homogeneous products such as cream-style corn and various purees and juices, which are effectively free of entrapped air, can be filled into cans at a sufficiently high temperature to obviate the need for exhausting, and in these cases, can closure can follow immediately on the filling operation. This is also possible with other products (if a sufficiently high filling tempera-ture is employed) using steam-flow seaming—a method in which the top of the can is sealed on immediately after the headspace has been flushed with a blast of steam. This last method is in fact becoming increasingly popular for fruit and vegetable packs, al-though it should be noted that the resulting vacuum in the head-space of the can is generally poorer than can be obtained using conventional exhausting and closing procedures. A change to steam-flow seaming without exhausting may therefore lead to an increase in the incidence of bulged (and therefore spoiled) cans, and a short vacuum-treatment prior to seaming is to be recom-mended in such cases. Vacuum-seaming, in which the can is closed while a vacuum is applied to the headspace, is another relatively


Curl of can lid, lined with sealing compound

Expansion ring

Body flange

(a) Section through a can-top with the lid clinched on to the flange of the body

(b) The action of the first operation roll

(c) The action of the second operation roll

FIG. 26. T h e process of double-seaming.

new technique, but one which has been little (if at all) used in the canning of fruit and vegetable products.

The sealing of the open-top can is a process which has reached a very high degree of refinement. Two distinct operations are in-volved and the machine which carries these out is known as the "double-seamer" (see Plate 14a). Recent models of this type of machine are capable of closing more than 400 cans per minute. The process of double-seaming is illustrated in Fig. 26. In the first


operation, a roller with a deep narrow groove folds the cover of the can-top over the flange of the can-body. The second operation roller, which has a broader, shallower groove, completes the seaming process by flattening the seam out against the side of the can. A rubber sealing compound present on the inside of the cover helps to make the seam air-tight and sufficiently strong to with-stand the rigours of heat processing.

Heat processing

The primary object of heat processing is to destroy all the micro-organisms which may initially be present inside the con-tainer and which may subsequently be able to grow under the conditions prevailing in the container. In particular, organisms which are capable of causing spoilage or which could give rise to food-poisoning must be eliminated. Heat processing also cooks the material, in some cases rendering it more palatable and attractive. In other cases, however, the cooking effect must be regarded as an unavoidable rather than desirable corollary to the preservative treatment.

The principles of heat sterilization are discussed at length in other texts"]" a n d cannot be dealt with fully in a short section such

f E.g. By Gillespy, T. G., in Recent Advances in Food Science, vol. 2, Butter-worths, London, 1962.

[Opposite page]

PLATE 14. (a) The Metal Box GRS 334/C Can Seamer. This type of machine, which can be adapted for steam-flow closing, is widely used in the United Kingdom by the fruit and vegetable canning industry. The seamer in this particular case is coupled by means of a split drive to the filling machine and can be used at speeds of up to 250 cans per minute. (b) A continuous rotary sterilizer. The two tanks to the right in the picture are sterilizing tanks operating under pressure at 265°F. To the left of these is a shorter pressure-reducing tank which is necessary to avoid deformation of the cans as they are passed to the cooling tank on the far left, which operates at atmospheric pressure. The installation illustrated has an output of 400 cans per minute at a cooking time of approximately 19 minutes. The cans are conveyed along the tanks by

the method illustrated in Fig. 27b.

(a) (By courtesy of Metal Box Co. Ltd., Acton.)

(By courtesy of Mather & Platt Ltd., Manchester.)

(b)


as this. Complete sterilization in the strict bacteriological sense is not normally practicable in commercial canning and the severe heat-treatment necessary would in any case usually overcook the material and render it unpalatable. In practice, something less than absolute sterility is perfectly acceptable, provided that Clostridium botulinum has been effectively destroyed or is incapable of growing in the pack, and the imprecise but useful term com-mercial sterility has been introduced to describe the desired condition.

The sterilizing effect of a process is primarily determined by the temperature used and by the period of exposure to this tempera-ture, but other factors have a marked effect on the severity of the heat treatment which is necessary to obtain commercial sterility. Of these, p H is perhaps the most important since it largely deter-mines whether any surviving organisms will be able to develop in the contents after heat processing. Bacterial spoilage*)* has in fact never been reported in canned foods with p H values below 3-7. Commodities of this kind, including most common fruits and a few vegetable species, such as tomato and turnip, which have p H values between 3-7 and 4 -5 , are collectively referred to as acid foods and can be effectively sterilized by relatively short treat-ments at 212°F. Highly acid fruit juices can usually be satis-factorily preserved by short pasteurization treatments at even lower temperatures—175-195°F. The p H 4-5 is the approximate lower limit for the development of the spores of C. botulinum and of many other spore-forming bacteria which are especially resistant to destruction by heat. Most of our common vegetables have p H values above this limiting level and, since they are also more likely to be contaminated with soil-borne organisms, they there-fore require more severe heat-treatments at higher temperatures in order to obtain a commercially sterile pack. The growth of a number of other spore-forming bacteria, which are thermophilic

f Microbiological spoilage of canned foods, when it does occur, is practically always caused (except in the case of "leaker spoilage" due to recontamination) by the growth of heat-resistant spore-forming bacteria, but see also the footnote on page 104.

Processing 2, Methods of Preservation 241 and therefore especially troublesome as agents of spoilage, is in-hibited at pH values below about 5 · 3 and this therefore provides the basis for a further subdivision of these latter commodities into medium-acid and low-acid groups. The members of this last group are naturally most susceptible of all to bacterial spoilage if for any reason an inadequate heat-treatment has been applied.

A great deal of information is available with regard to the thermal death rates of micro-organisms, in particular of the spores of C. botulinum, at different temperatures and, if the heat-transfer properties of a given type of pack are also known, it is possible to calculate the length of time for which a can must be subjected to a given temperature in order to obtain a given sterilizing effect. Such calculations, although useful, especially in the formulation of a process for a novel type of product, must, of course, be sup-ported by practical experience on a commercial scale, and for the major fruit and vegetable commodities a wealth of such experi-ence is already available to the canner.

The necessary process depends, of course, on the size of the can. For fruit packed in syrup in A 2 (307 X 409) cans, processing times at 212°F vary from about 7 to about 35 minutes according to the particular product and to the type of cooker in use, i.e. whether agitated or non-agitated. Larger cans, of course, require a longer period and smaller cans a shorter period at this same temperature. Vegetables are normally processed under pressure at between 240° and 252°F. Vegetables in brine in A 2 cans require between about 20 and 55 minutes at 240°F, low-acid products such as peas and corn requiring the longest periods. Spinach, a commodity which has always been particularly suspect as a carrier of C. botulinum, is normally given a long process—45-60 minutes— at a higher temperature (252°F), while tomatoes, on the other hand, because of their relatively low pH, can be sterilized at around 212°F in 12-45 minutes, the necessary period again depending on the type of cooker employed.

Many different kinds of heat-processing equipment are avail-able for use with canned foods. For fruit and tomato packs, which can be sterilized at 212°F, either batch or continuous cookers


operating at atmospheric pressure are normally used. Heating may be by hot water or by steam and the cans may remain still during processing or they may be agitated. One convenient method of agitation is to pass the cans in a rolling motion along a helical path at the outside of a revolving reel (see Fig. 27b). Agita-tion increases the rate of heat transfer into the can and thereby reduces the necessary processing time.

(a) End-over-end agitation in (b) More gentle 'side-over-side' a batch pressure cooker agitation in a continuous pressure

cooker - in this case all the longitudinal movement of the cans takes place in the lower half of the cylinder

FIG. 27. Methods of agitation employed in rotary pressure cookers.

The higher temperatures required in the processing of vege-tables necessitate the use of pressure cookers. These again may be of the batch type (vertical or horizontal retorts) or may be con-tinuous in operation (see Plate 14b) and the cans may be agitated or may remain still during the process. One type of batch pressure cooker induces an end-over-end agitation of the cans, which are attached to the outside of a revolving drum with their long axes along the radii of the cylindrical retort (see Fig. 27a). This type of movement is particularly useful with large cans of products

Processing 2. Methods of Preservation 243 such as cream-style corn which heat up relatively slowly unless vigorously agitated. Pure steam is normally employed in pressure cooking and with batch-type cookers it is most important, if a uniform rate of heating is to be obtained, that all the entrapped air is removed from the cooker during the warming-up period and that condensation of steam on the surfaces of the cans near the steam inlet is prevented.

A relatively recent departure in the design of pressure cookers was the development of the Hydrostatic Cooker, an example of which is illustrated in Fig. 28. In this equipment, the pressure required for a given steam temperature is maintained by a head of water and the cooker is therefore very tall—a 40 ft head of water corresponds to a steam temperature of 240°F, for 260°F a head of 61 ft is required—but a minimum amount of floor space is needed. The cans are passed on a single-chain conveyor, first down a column of hot water and thence into the pressure cham-ber. After passing through this steam chamber during the neces-sary processing time, they are carried into a column of cold water for final precooling. This type of cooker is especially suitable for use with processed peas, in which the relatively gradual heating up and cooling prevents undue turbulance of the contents and therefore causes relatively little damage to the peas.

Aseptic canning, another relatively recent technique in which the material is sterilized by a high-tempera tur e short-time treat-ment in a heat exchanger before filling into sterile cans, is not generally suitable for particulate materials, but it has been used successfully with products such as cream-style corn and banana pulp and is especially suitable for application to free-flowing liquid products such as juices.

Cooling

After heat processing, it is most important that the cans should be quickly cooled to around 100-110°F so as to avoid further cooking of the contents. This may be done by immersion in cold water or by subjection to cold-water sprays, and continuous


r^FQ, p<$CHARGC t DOIVC P L A t r g g H .

- TO f EED EUEVATQO

WATtft BALANCE TUBES

FIG. 28. The Garvallo Hydrostatic Pressure Cooker. (By courtesy of Mather & Platt, Ltd.)

Processing 2. Methods of Preservation 245 coolers, similar in design to the continuous cookers, are often employed. Rapid cooling to too low a temperature retards the subsequent evaporation of cooling water left on the surface of the can and may lead to external corrosion. The close stacking of cans after heat processing should be rigorously avoided, even if they have been cooled to the recommended final temperature.

Post-canning operations

These include can labelling, packing into cartons and quality testing and grading. Standards of quality, composition and fill have been mentioned in earlier sections and representative samples should be examined to ensure compliance with grade standards. In addition, it is necessary to carry out incubation tests to confirm the effectiveness of the sterilizing treatment. Bacterial spoilage during storage at 37°C and 55°C may sometimes be evidenced by the bulging of the can ends as a result of the production of gas by the organisms responsible. In other cases, as in the flat-sour spoilage of low-acid products such as peas, no gas is produced and spoilage is only discovered on examining the contents of the can after incubation. Gas production is not always the result of bacterial growth. It may also result from the corrosion of the tin-plate by the acid contents of a can, as in the formation of so-called hydro-gen swells. Although in such cases the contents may be in perfectly good condition, the bulging of the can raises doubts on this score and such cans would be justifiably rejected by a prospective purchaser.

FREEZING The effectiveness of freezing and frozen storage as a method of

food preservation rests on the fact that, although many micro-organisms survive the process, even the most psychrophilic of them are unable to grow and cause spoilage at temperatures below about 14°F (-—10°C). The tissues of higher plants, however, un-like the micro-organisms, are always killed by freezing and,

K


though the resulting alteration in quality may be small compared with that induced by heat sterilization, the firm, organized struc-ture of the living tissues is inevitably destroyed. As a result, when frozen products are ultimately thawed out, they are even more susceptible to spoilage than are the corresponding fresh materials and they must therefore be used with the minimum of delay.

Deterioration in the quality of frozen fruit and vegetables is commonly caused by the activity of enzymes, and with vegetables the effective inactivation of enzyme systems during blanching prior to freezing is especially important. Blanching of fruit is much less common and is confined to those commodities, e.g. peaches, apricots, apples and pears, in which the tissues undergo a rapid enzymic darkening when cut and exposed to the air. Alternative methods of controlling this type of deterioration are by dipping in brine or bisulphite solutions or, as described below, by the use of sugar and ascorbic acid. Otherwise, the treatment of material prior to freezing is very similar to that used in preparation for canning, except that in the case of strawberries—a most important frozen commodity—the berries are generally sliced or halved rather than packed whole.

Vegetables are normally frozen without any additions, bu t fruit, especially when prepared for the retail market, is com-monly mixed with sugar or covered with syrup before freezing. This procedure, apart from improving palatability, helps to reduce oxidative deterioration by forming a protective film over the surfaces of the material, and this anti-oxidant effect is greatly enhanced if ascorbic acid is introduced to an extent of between 0*05% and 0 - 2 % in the syrup. Another advantage of the use of sugar or syrup is that the rate of heat transfer from the pack during freezing is increased, though this is partly off-set by the fact that, for a given weight of fruit, a greater amount of heat must be withdrawn. When fruit is packed with sugar or syrup it is advisable to hold the mixture at a little above 0°C for 1-2 hours before freezing so as to allow the sugar to partially penetrate into the fruit. Dry sugar is normally used with strawberries, raspberries and other berries and with sour cherries, in proportions of be-

Processing J?. Methods of Preservation 247 tween 1 part in 3 and 1 part in 7 of the weight of the fruit. Syrups are used in particular with peaches and apricots, but also to a lesser extent with a range of other products. Syrup strengths vary from 20 to 50° Brix and the amount added is generally about 21 oz per 8 oz of fruit. It should, perhaps, be pointed out here that a high proportion of the fruit frozen—about two-thirds of the total product in the United States—is prepared for use in other manufactured products such as jam, ice-cream, bakery products, etc. and in this case the fruit is usually packed with-out sugar in large containers such as 50-gal drums or barrels or large cans or cartons each holding 20-30 lb of material. Large quantities of unsweetened frozen pineapple are also imported into the United Kingdom from South Africa for subsequent canning.

The prepared fruit or vegetable may be packed into the final container before freezing or with some commodities, notably peas, corn, diced or sliced carrots, sliced green beans and berry fruits, the individual pieces may be frozen loose on trays or on moving wire-mesh belts in air-blast freezers before packing. Products which are conveniently frozen prior to packing also generally lend themselves to automatic filling. Otherwise, packing may be carried out using hand-pack fillers or, where careful arrangement in the container is required, as for example with asparagus shoots and corn on the cob, this is carried out completely by hand.

Containers for frozen products need not be hermetically seal-able, but they should be of low permeability to water vapour to reduce desiccation of the material during freezing and storage, and preferably also of low permeability to oxygen to lessen oxida-tive deterioration. When syrup is used, they must also, of course, be leak-proof. Finally, the container must be strong enough to withstand the stresses and strains developed during filling, freezing, storage, transport, etc., and must not impart any off-flavour to the contents.

The rectangular paperboard container is most widely used for retail packs, the board being waxed, lined with a plastic film or used in conjunction with a separate inner plastic bag. Overwraps


of waxed paper or cellophane are usually added. Composite con-tainers with metal (tin-plate) ends are also popular for some pro-ducts, such as strawberries. A useful additional property in packaging materials used in bags for this type of product is heat sealability. Plastics which have been used in containers for frozen foods include cellophane, polyethylene, vinyl and vinylidene poly-mers (e.g. Saran), polyesters (e.g. Mylar) , polyamides (e.g. Ril-san), pliofilm and polypropylene. Aluminium foil is also some-times used. No one material, however, combines all the properties which are desirable for a pack of this type and the ideal combina-tion can only be obtained by lamination. For some products, notably frozen juice concentrates, tin-plate cans are used, but the consumer is so used to regarding the contents of such cans as effectively sterile, that an extension of this practice could lead to unfortunate consequences. Aluminium cans and foil containers are free of this particular objection, but foil is not sufficiently rigid and aluminium cans are relatively expensive. The most popular sizes of container for the retail trade are those holding 5, 8 and 10 oz of material, the first two being usual in the United Kingdom, while the last is particularly common in the United States.

The actual freezing of the fruit or vegetable material may be brought about in various ways. Batch or continuous air-blast freezers, in which commodities may be frozen loose before packing have already been briefly mentioned. One such freezer is illus-trated in Plate 15b. These normally operate at between —20° and — 40°F and the product is quickly frozen, although in the absence of any barrier against the removal of water vapour, desiccation can be severe. Freezing in still cold air is a more pro-tracted procedure which has now largely been superseded by methods employing forced draughts of refrigerated air. Packaged materials may also be frozen in cold air, but with rectangular cartons the plate-type freezer is especially convenient and will produce the necessary cooling in less than 2 hours, as compared with the 3-4 hours which would be required in a cold air-blast. In a plate-freezer (Plate 15a) the cartons are sandwiched

(By courtesy of Birds Eye Foods Ltd.)

(b) (By courtesy ofj. & E. Hall Ltd., Dartford.)

PLATE 15. (a) Cartons of peas being introduced into a plate-type freezer, (b) An interior view of the "fluidizer" section of a modern air-blast freezer. The loose product is conveyed along a moving wire-mesh belt as refrigerated air is blown up through it. In a freezer of this type, loose peas can be cooled from an initial temperature of about 70° F

to a final outgoing temperature of 0°F in about 5 minutes.


between hollow cooling plates through which a refrigerated liquid (brine or ammonia) is circulated. The plates are normally mov-able so that a slight pressure can be applied top and bottom to improve heat transfer and prevent bulging during freezing. Plate-freezers can be made continuous by using moving refrigerated belts or by automating the loading and unloading of the cabinets.

A final method of freezing, but one which has been little used for fruit and vegetable products, is by direct immersion of the material or the filled pack in a liquid refrigerant. This gives the highest rates of heat transfer but it is difficult to satisfactorily control the condition of the coolant. Refrigerated brines and syrups (invert syrups or mixtures of sucrose and corn syrup) have been used for this purpose on a small scale. The most recently suggested coolant for immersion freezing is liquid nitrogen which theoretically has a number of advantages. For example, it gives an extremely rapid rate of cooling and is very effective in expelling oxygen from the tissues. I t remains to be seen, however, whether this method will prove commercially feasible. In the freezing of canned products such as juice concentrates, the cans are generally passed through refrigerated brine or alcohol in continuous freezers. These may simply take the form of cylindrical tubes filled with the refrigerant, but in one kind of can freezer the design is similar to that of the revolving drum-type of continuous cooker, the refrigerant in this case being confined to the space between the revolving drum and the outer cylindrical wall of the freezer.

The rate of cooling should be fast enough to bring the material to a suitable holding temperature—0°F or below—before its quality is adversely affected by enzymatic or microbiological pro-cesses. Considerable trouble has been experienced in the past when freezing in large containers such as barrels, because of the fact that the material in the centre of such packs may take as long as several days to reach the final holding temperature. Provided that such grossly excessive periods of initial cooling are avoided, however, the rate of freezing, in so far as this affects the size and position of the ice crystals formed within the tissues, is now re-garded as of less importance with fruit and vegetable commodities

Processing 2. Methods of Preservation 251 than it was in the earlier days of quick freezing. The formation of ice certainly causes considerable mechanical damage to the tissues and this alters the texture. The effect, however, is one of tenderi-zation and is not generally regarded as detrimental to the quality of the product. Indeed, relatively slow freezing may result in a better product in the case of fruit packed in sugar or syrup, since it permits a greater degree of penetration of the sugar into the fruit.

The freezing process itself may be divided into three distinct phases. In the first, the material is cooled from its initial tempera-ture to that at which freezing begins—usually a few degrees below 0°C. During phase two, most of the water in the tissue is frozen as the temperature falls more slowly to between —10° and — 15°C. The reduction in the rate of cooling at this stage is due to the need to withdraw the additional latent heat of fusion. The third phase involves the cooling of the frozen material to the final holding temperature. In fact, a small amount of ice continues to separate out as the temperature is lowered below — 15°C, and this process would continue on a progressively reduced scale until the material was cooled to — 30°C or even below. There is also a residual quan-tity of water in the tissues which fails to freeze however far the temperature is lowered. This water, estimated for fruit and vege-table material at some 20-30 g/100 g dry material, is in some way bound to the solid constituents and its influence on the behaviour of frozen products is a subject of continuing research.

The expression "quick freezing", although definable in terms of the time required for the material to pass through a given temperature range during ice formation, or in terms of the rate of movement of the ice-front into the material, has no fundamental significance. The effective rate of freezing ( Ve) for a product of given dimensions has recently been definedf in the following way:

ΐ Recommendations for the Processing and Handling of Frozen Foods, International Institute of Refrigeration, Paris, 1964.


where / is the half-thickness of the product (in cm) measured from the "thermic" centre and te is the time (in hr) required to lower the temperature at the "thermic" centre from the average initial temperature, 0a, to any stated temperature, 6e (e.g. — 10°C).

Once frozen, fruit and vegetable products should be held at — 18°G (0°F) or below, depending on the particular commodity and, in some cases, on the period for which it is intended to store the material. The International Institute of Refrigeration has recommended temperatures of storage for a number of the more important frozen fruit and vegetable products and these recom-mendations, together with estimates of the storage life which can be expected under the prescribed conditions, are given in Table 14. The performance of the material during storage depends, of course, on a number of factors, and the estimates of storage life given in this table must not be accepted too rigidly. They should be interpreted rather as average periods for which well-prepared packs of the commodities concerned can be stored at the recom-mended temperatures before the consumer will be able to detect any deterioration in quality. Changes in quality can be detected by trained tasters during considerably shorter periods in storage.

The storage life of frozen fruit and vegetable products is normally determined by changes in colour—chlorophyll degradation, oxi-dative breakdown of anthocyanin and carotenoid pigments—or in flavour. Flavour changes are also generally attributable to oxidative mechanisms, either direct or enzyme-induced. Peroxi-dase and lipoxidase systems, for example, have been implicated in the formation of off-flavours in peas and in a number of other products. "Freezer-burn", a condition resulting from the desicca-tion of the surface of the pack has now largely been controlled by the development of suitably moisture-vapour-proof packaging materials, but it may occur where the packaging is faulty. Desic-cation is encouraged by excessive fluctuations in the temper-ature of storage, and good temperature control, preferably within ± 1 °G of the nominal storage temperature, is highly desirable.

The need for maintaining a low and constant temperature extends, of course, from the time of freezing to the moment of


T A B L E 14. RECOMMENDED S T O R A G E T E M P E R A T U R E S AND ESTIMATED S T O R A G E LIVES FOR D E E P - F R O Z E N F R U I T AND V E G E T A B L E P R O D U C T S 3

Commodity

FRUIT Apricots in sugar Cherries (sour) in sugar Cherries (sweet) in sugar Peaches in sugar

Peaches in sugar + ascorbic acid

Raspberries without sugar

Raspberries in sugar

Strawberries in sugar Other frozen fruits Fruit juices (single strength

or concentrate)

VEGETABLES Asparagus Beans, snap Beans, Lima Broccoli Brussels sprouts Carrots Cauliflower Corn on the cob Cucumber, sliced

Peas Potatoes, french fried Potatoes, scalloped Spinach Other frozen vegetables

Recommended temperature of storage

°C °F

- 1 8 - 1 8 - 1 8 - 1 8 - 2 4 - 1 8 - 2 4 - 1 8 - 2 4 - 1 8 - 2 4 - 1 8

- 2 2 to - 1 8

- 2 0

- 1 8 - 1 8 - 1 8 - 1 8 - 1 8 - 1 8 - 1 8 - 1 8 - 1 8 - 2 4 - 2 9 - 1 8 - 1 8 - 1 8 - 1 8

- 2 2 to - 1 8

0 0 0 0

- 1 1 0

- 1 1 0

- 1 1 0

- 1 1 0

- 8 toO

- 4

0 0 0 0 0 0 0 0 0

- 1 0 - 2 0

0 0 0 0

- 8 toO

Expected storage life in months

12 12

8-10 8-10 12-14

12 18 12 18 18 24 12 12

9-12

8-10 8-10

12 12

8-12 12-15 10-12 8-12

5 8

12 8-12

6 1

10-12 12

a International Institute of Refrigeration


retail sale and, although in practice the ideal is not usually attain-able through all the stages of distribution and display, every attempt should be made to avoid any appreciable rise in tempera-ture at any particular point in the sequence. The results of recent surveys, both in the United States and in the United Kingdom, of the temperatures at which frozen products are held in retail display cabinets have been far from reassur-ing in this connection and constant re-emphasis of the impor-tance of low and uniform storage temperatures is obviously necessary.

DRYING AND DEHYDRATION The preservative effect of drying was one of man's earliest dis-

coveries, and even today more fruit is preserved in this way than by any other method. The effectiveness of drying rests on the simple fact that micro-organisms are unable to grow and thereby cause spoilage on materials in which the water activity has been reduced below 0-6. Water activity—the ratio of the water-vapour pressure exerted by the material to that exerted by pure water at the same temperature—is a more reliable indicator of the resist-ance of a dried material to spoilage than is the actual content of water, and the relationship between these two properties varies considerably from product to product, largely because of differ-ences in the content of soluble solid constituents.

The attainment of a water activity sufficiently low to prevent microbiological spoilage is relatively easy, but unless special pre-cautions are taken both before and during the drying operation, gross changes in quality are liable to occur. The products of the traditional methods of natural drying, for example, although acceptable as foods in their own right, bear little resemblance to the fresh materials from which they were prepared. The aim of more modern methods of dehydration, on the other hand, is to obtain products which, on rehydration, are as closely similar as possible in quality to the corresponding fresh materials as nor-mally prepared for eating.

Processing 2. Methods of Preservation 255 The disorganization of the tissues and the subsequent concentra-

tion of soluble constituents, which are inevitable results of the drying process, render the dried materials especially susceptible to certain kinds of chemical change and, with dehydration as with the other main methods of food preservation, the destruction or inhibition of micro-organisms is only the first of a number of obstacles which have to be overcome.

Dried and dehydrated foods have certain advantages over foods preserved by other methods in that they are relatively light in weight and less bulky, while at the same time they do not require refrigerated storage. As a result, they make possible considerable savings in shipping and storage space. It has recently been esti-mated that annual imports of food into the United Kingdom alone contain some 3 million tons of water. This figure gives some indi-cation of the enormous scope which exists for the further exploita-tion of these methods of food preservation, and of the magnitude of the economies which could be achieved in the utilization of transport and storage facilities.

Sun-drying

Some fruit and vegetable commodities such as the pulses dry out naturally during the normal course of development. In many other cases, attempts have been made to exploit the preservative effect of drying by exposing harvested materials to the desiccating action of sun and wind. The resulting dried commodities are often of very indifferent quality and many, such as the sliced tomato product, salsa, which is prepared in the Sudan and in other parts of Africa, are of purely local importance. Certain sun-dried fruits, however, have long been established as important articles of com-merce and these continue to be produced on a very considerable scale.

The sun-drying of fruit on a commercial scale is only possible in areas where the climate is hot, relatively dry, and free from rainfall during and immediately following the normal period of harvest. The regions mainly concerned in this industry are


California, the countries of the Mediterranean and Middle East, and parts of Australia. The process requires no elaborate equip-ment and is therefore the cheapest to carry out of all methods of long-term preservation. However, in comparison with dehydra-tion, labour costs tend to be high, more space is required, and lower yields are generally obtained because of losses of sugars by respiration and fermentation during the long drying periods involved. Infestation by insects, spoilage by rain, and contamin-ation with wind-borne debris are special additional hazards associated with this particular method of preservation.

Originally confined to fruits of high sugar-content which can be dried naturally after harvesting without additional treatment —products such as dates, grapes and figs—sun-drying methods were later extended to other commodities such as apricots, peaches, apples and pears, which need to be halved and sulphured before drying. Dates normally dry sufficiently on the tree before harvesting. A few days in the sun or in a warm room (90-95°F) will complete the "ripening" process, during which the sucrose is largely inverted and the initial astringency of the fruit is lost. Otherwise, fumigation (generally with methyl bromide), and cleaning are the only treatments which may be necessary. Figs also are partially dried when removed from the tree. Drying in this case may be completed in the sun within a few days, or in the shade on stacked trays which takes longer. In California, some varieties, notably Calimyrna, are normally dipped in a solution of 1 % salt and 1 % hydrated lime to remove surface hairs, while the variety Adriatic is usually given a sulphuring treatment— 3 hours or more in an atmosphere of burning sulphur—before drying. Fumigation is again a wise precaution.

Currants are dried in shade rather than in direct sunlight, to protect the delicate skins of the fruit. In Greece—the main pro-ducing country—the bunches are dried in the shade of the vines. Australian currants, on the other hand, are dried on wire-netting trays in tiers of 9-10 on racks with sheet-metal covers to protect the fruit from sun and rain. Drying usually takes from two to three weeks. Raisin and sultana grapes (except the larger dessert


raisins) have traditionally been dipped in or sprayed with emul-sions")" of olive oil and lye before drying. Such treatments increase the rate of drying and help to impart an attractive golden colour to the fruit. Either cold-dipping—the traditional method—or hot-dipping may be employed. In some cases, a simple lye dip without oil is used, while in the United States sultana grapes are sometimes also given a sulphuring treatment. Drying is effected partly in direct sunlight and partly in the shade. In Australia, the fruit is first partially dried on shaded wire-netting trays like those used for currants, and then finally exposed to the sun for a few hours in the case of sultanas or a few days in the case of raisins. American practice is to partially dry on wooden or paper trays exposed to the sun and to complete the drying with the trays stacked so as to shade the fruit. When drying is complete—this may take anything from about 10 to 40 days, depending on the variety, the preliminary t reatment employed and the weather— the fruit is placed in sweat-boxes or in heaps to equilibriate in moisture content, and is then finally packed. The moisture con-tent at this stage should be between 15% and 17%.$ Australian sultanas are generally washed and treated before packing with an emulsion of paraffin oil and oleic acid to prevent sticking of the fruit. An increasing proportion of raisins and sultanas, both in the United States and in Australia, are being dehydrated rather than sun-dried, so as to make the process independent of the weather. Prunes also are now mainly dehydrated, but they can be sun-dried after dipping in lye by methods similar to those described above for raisins.

Other tree-fruits such as apricots, peaches and pears are gener-ally too large for drying whole and these are normally cut in half and pitted as necessary. They are then placed on trays with the cut surface upwards and treated in an atmosphere of burning sulphur (about 1% to 2 % sulphur dioxide) for between 3 hours

f The composition of these emulsions varies considerably. The lye may be prepared from the hydroxides, carbonates or bicarbonates of potassium or sodium, either singly or in various combinations. Vegetable oils other than olive have also been used.

% Moisture contents in this section are given on a dry-weight basis.


(for small apricot halves) and 72 hours in some cases for pear halves, which require a particular long treatment in order to obtain the clear translucent product which is demanded by the consumer. After sulphuring, the fruit is dried in the sun, usually for several days (between 1 and 12, depending on the product), the usual practice in the United States being to complete the drying in shade with the trays stacked. At least 1 day's exposure to sun is necessary with apricots to obtain the desired reddish tinge. The final moisture content of the fruit should be between 12% and 15%. With the development of improved methods of dehydration, artificial drying is now gradually replacing the older-established methods for drying these various commodities.

Dehydration in hot air

The term dehydration is used to cover all methods of drying in which the removal of water is effected under controlled conditions of temperature, humidity, air speed, etc., in specially designed equipment. The greater control of drying conditions and the shorter drying times which are possible with these artificial methods produce less change in the material and permit accept-able dried products to be prepared from a wider range of raw materials.

Fruit. The development of fruit dehydration was a natural out-come of the need to speed up and improve upon the old-estab-lished methods of natural drying, and dehydration has largely been confined in its application to those commodities which have traditionally been dried by natural means. The preparation of fruit for dehydration is therefore generally similar to that described above for sun-drying. However, peaches and pears are normally prepeeled, and they benefit, as also do apricots, from a short steam-blanch immediately prior to the drying process, while prunes for dehydration are usually pretreated in hot water rather than in lye solutions. Because of the shorter drying times involved,


sulphuring treatments may also generally be reduced, and in some cases dipping in sulphite or bisulphite solutions has re-placed the traditional gaseous treatments.

Dehydration is normally carried out in heated air in forced-draught tunnel or cabinet driers, although apples are still com-monly dried in kilns or drying towers. The latter may be fitted with fans to increase the flow of hot air, bu t otherwise the move-ment of air depends on the natural convection currents created by a source of heat at the base of the kiln or stack. Ho t air per-meates upwards through the material which is present either as a single layer up to about 1 ft in thickness on the slatted floor of the kiln or, in the case of the drying tower, on a vertical series of more thinly loaded trays. The temperature around the lower material may vary between 145° and 180°F, and drying to a final moisture content of less than 24% usually takes from 7 to 12 hours. The thick layer of material in the kiln must be stirred at intervals to obtain a uniform rate of drying, while in the tower or stack drier the trays are progressively moved downwards in stages, the dried material being removed at the bottom and fresh trays introduced at the top.

In tunnel driers, which are most widely used for other fruits, the material is spread thinly on trays which are loaded onto trucks or trolleys and passed in stages through one or more tunnels, along which hot air is forced by means of powerful fans. The air can be heated by direct combustion (gas or oil), by steam-heated pipes, or by electrical heaters, but the last method is relatively expensive and little used. The flow of air in the tunnels may follow the direction of movement of the trucks (parallel flow) or may be the reverse (counter flow). A particularly satisfactory procedure which has been used more for vegetables than for fruit, is to dry in two stages, using a parallel-flow tunnel for the first stage of drying so that the initially wet material meets the hottest air, and a counter-flow tunnel for the final drying stage. This prevents material a t an intermediate level of moisture content, which is particularly susceptible to browning a t elevated temperatures, from being subjected to the higher temperatures obtaining at the


ends of the tunnels at which the heated air is introduced. It also permits the use of higher initial drying temperatures. The final drying temperature for fruit, which for a single counter-flow tunnel is also the maximum temperature in the tunnel, may be between 140° and 170°F, depending on the type of material being dried and on the conditions prevailing along the tunnel. The humidity of the air, of course, influences the rate of drying and this can be controlled by varying the proportion of the air in the tunnel which is recirculated through the heaters and fans. Drying times vary from between 5 and 8 hours for apples to as long as 36 hours for prunes, and final moisture contents of between about 12% and about 23% are obtained.

Vegetables. Dehydrated vegetables in the past have generally been of indifferent quality. They have been produced in largest quantities in times of war, when their relatively small bulk and weight has been especially valuable in the conservation of storage and shipping space. However, recent advances in technique have made possible considerable improvements in the quality of dried products, and the industry is now progressively increasing in size. In 1960, about 114,000 tons of vegetables, including 80,000 tons of potatoes (c. 3% of the total crop), were dried in the United States and an estimated 35,000 tons were dried in the United Kingdom. These figures are currently being exceeded, probably by a considerable margin. Unlike dehydrated fruits which are judged by comparison with their sun-dried counterparts, de-hydrated vegetables must compete with the corresponding canned and frozen products, and the avoidance of gross changes in quality during drying is therefore of the utmost importance.

The preparation of vegetables for dehydration is similar to that described earlier for freezing, except that, in order to obtain a sufficiently rapid rate of drying, the material must normally be prepared in the form of pieces—slices, strips or dice—which are no more than Ï3

6- in. thick in one dimension. This is not possible in all cases. For example, peas are dehydrated as complete units.

Processing 2. Methods of Preservation 261 Modern methods for dehydrating peas call for the pricking of each individual pea so as to make a hole through the testa and so increase the rate of drying. Brussels sprouts may also be de-hydrated whole, providing the effective thickness of the stalk is reduced by suitably positioned cuts.

The need for the effective inactivation of enzymes and the prac-tice of introducing sulphur dioxide into the material during blanching prior to dehydration have both been mentioned in the last chapter. Sulphur dioxide is a useful retardant of browning reactions—the principal cause of deterioration in dehydrated vege-tables—and it also increases the retention of ascorbic acid. In addition, sulphur dioxide has a useful antimicrobial effect during the initial stages of drying and, by varying the form in which it is introduced (sodium sulphite or metabisulphite), it can be used to control pH, which in turn influences the colour and subsequent handling and drying characteristics of the material. The amounts of sulphur dioxide introduced, which are subject in the United Kingdom to legally prescribed maxima (see Table 15, p. 274), normally range from between 200 and 500 ppm (in the dried product) in the case of potato to between 2000 and 2500 ppm for cabbage. With some materials, such as onion and carrot, sulphit-ing may be omitted. Onion, in fact, is the only major vegetable which is usually given no blanching treatment prior to drying, since the consequent loss of flavour is considered to outweigh any advantages resulting from the short heat-treatment. Sodium car-bonate is normally added during the blanching of green vegetables to raise the pH to a little over 7 and thus prevent the formation of phaeophytin, while with some other products phosphates may also be introduced to prevent discoloration due to traces of iron.

In order to obtain a satisfactory product, the severe limitation, if not the complete avoidance, of browning during dehydration is a primary requirement. The careful control of the drying condi-tions is therefore even more important than in the dehydration of fruit, and for satisfactory storage behaviour, lower final moisture contents must be achieved. Vegetable dehydration may be carried out in tunnel or cabinet driers such as those mentioned in the

F. & V . —K


previous section, and the two-stage process there described was widely, and for the main part successfully, used for the dehydration of vegetables during the Second World War. Higher inlet air tem-peratures—up to 210°F or even higher—are employed in the de-hydration of vegetables than is the case with fruit, and the use of more than two stages, enabling a better control of conditions during each successive phase of drying, is to be recommended. Conveyor driers, in which a thin layer of material is passed slowly through the drying chamber on a moving belt, are also available. These have the advantage of being continuous, but adequate control of the drying conditions is more difficult to obtain, and only a limited par t of the necessary drying can be effected by a single passage through such a chamber.

The types of drier mentioned above have the disadvantage that the material being dried remains in much the same position rela-tive to the drying air for extended periods, and turning of the product is only possible while it is removed from the drier in be-tween stages. This may lead to uneven drying and to the forma-tion of wet patches. A type of drier which overcomes this dis-advantage is the belt-trough drier, in which the belt holding the material is moved round slowly in a direction at right angles to the length of the trough. In this way, the pieces of material are con-tinuously turned by being carried upwards on the belt and sub-sequently dropped back into the trough, progressive longitudinal movement being obtained by inclining the trough lengthwise. The belt itself is constructed of wire mesh and the heated air is

[Opposite page]

PLATE 16. (a) Peas undergoing dehydration in a modern drying plant. The drying in this particular case is entirely through-draught, the heated air being blown up through the layers of material. The process is com-pleted in deep beds in bin-driers, which take up relatively little floor space and provide a satisfactory means of removing the last few per cent of

water to achieve the required final moisture content. (b) A general view of a commercial accelerated freeze-drying plant, showing the vacuum cabinets with heatable plates between which the foodstuff is sandwiched during the drying operation. This plant has been used for the freeze-drying of a wide range of food materials, including fruit

and vegetable commodities.

{By courtesy of Batchelors Foods Ltd., Sheffield.)

(a)


blown upwards through the material, thus causing further agita-tion of the pieces.

This latter means of effecting movement of the particles of material during drying, i.e. by the pressure of the drying air itself, is carried a stage further in the so-called fluidized-bed drier. In this case, the layer of material passes over a perforated diaphragm and the force of an upward-moving air-stream causes it to undergo a fluid-like movement from which the drier gets its name. Very rapid rates of drying can be obtained in this way using high air-temperatures, but the individual pieces must be light and there-fore relatively small.

Drying methods in which the heated air is passed through rather than simply over the material are referred to, aptly enough, as through-draught (as opposed to over-draught) methods, and yet another type of drier utilizing this type of air-flow is the bin-drier which can conveniently be used for removing the last few per cent of water from material which has previously been dried almost to the required moisture content. This final stage of drying normally requires an extended period in heated air and, since at this point in the process the velocity of the drying air has no appreciable effect on the rate of drying, the passage of a relatively slow stream of air up through deep beds of material in bins allows a considerable saving in floor space.

Through-draught drying methods have certainly been gaining favour in recent years for vegetable dehydration and in the most recently established plants in the United Kingdom the drying process is carried out entirely in this way. Plate 16a illustrates a stage during the dehydration of peas in a modern factory. The open through-draught driers shown arranged in cascade in this illustration hold an appreciable depth of material and are best described as "turbulent-bed" driers.

A variant on conventional dehydration processes which pro-duces a very light, porous and relatively quickly reconstituting product is puff-drying. In this process, the partially dried material (at c. 40% moisture) is subjected to pressure inside a closed con-tainer known as a "puffing gun". The latter consists of a rotatable

Processing 2. Methods of Preservation 265 drum heated externally by a gas flame and fitted with a quick-opening door. Heating of the semi-dry material in the rotating drum generates a steam pressure of between 30 lb and 60 lb/ina

and the sudden release of this pressure by opening the door pro-duces an explosive effect inside the individual piece. Following this procedure, the material can be dried more quickly to the desired final moisture content.

Whatever method of drying is employed, the temperature of the material itself should not be allowed to rise above about 145-150°F for any length of time or deterioration due to the progress of browning reactions is likely to occur. These reactions, which in dehydrated vegetables are largely initiated by Maillard-type interactions between sugars and amino acids, can also proceed during subsequent storage, especially rapidly if the temperature of storage rises above 25°G. The most important single factor affect-ing the rate of browning at a given temperature is the moisture content of the material, and specifications for the moisture content of dehydrated vegetables are fixed with this effect foremost in mind. A moisture content of about 5% is necessary with most products in order to obtain satisfactory stability during storage, although in some cases, notably with potato in dice or strip form, it may be difficult to achieve this level in commercial practice. Other factors which influence the rate of browning are the level of sulphur dioxide and the content of browning reactants, especi-ally of sugars. Particularly low moisture-contents, where these are feasible, may favour oxidative deterioration which can also be a problem, especially with carotenoid-pigmented products such as carrot, but even also with potato which is liable to the develop-ment of stale rancid flavours. Apart from peas, green beans, beet-root and onion, which appear to be little affected in quality by the presence of oxygen, dehydrated vegetables are therefore best packed in vacuum or in an inert gas such as nitrogen or carbon dioxide. Gas or vacuum-packing also reduces the rate of loss of ascorbic acid. Given attention to these various factors, de-hydrated vegetables may be expected to retain their acceptability for two years or more in temperate regions. Browning is much


more rapid, however, at tropical storage temperatures and may cause the material to become unacceptable within a few months. In this case, extensions of storage life can be obtained in oxygen-free packs by the inclusion of an in-package desiccant, such as calcium oxide, which removes further moisture from the material during storage.

Dehydrated vegetables may be packed in tin-plate cans or in flexible packages. For gas or vacuum-packing, a hermetically seal-able container is, of course, necessary. In any case, the material must be protected against rehydration by moisture vapour from the air. Flexible packaging materials, if used, should therefore be of low permeability to moisture vapour and preferably also to oxygen.

Liquid and semi-liquid products. Juices, purees and other com-minuted products, such as mashed potato, can be dehydrated by other procedures which are not applicable to particulate materials. Roller or drum-drying, spray-drying and the more recently developed foam-mat drying are methods which are used for this purpose. Vacuum- and freeze-drying methods can also be con-veniently applied to the drying of such liquid and semi-liquid materials.

Freeze-drying

Freeze-drying, which involves the sublimation of ice from the material at temperatures below its freezing point, has come to the fore in recent years as a promising method for the preservation of a wide range of foodstuffs. The products of freeze-drying are of excellent quality, comparable to those of freezing, and they re-constitute very rapidly compared with air-dried materials, but because of their open porous structure they are highly susceptible to oxidative deterioration. Until recently, this type of process has been far from economic because of the high cost of the necessary equipment and the long drying times required. However, im-provements in the design of equipment and in methods of opera-


tion have made possible a speeding-up of the process until the drying times now attainable are comparable to or little longer than those necessary in an air-drying operation.

The essential features of any freeze-drying plant are a vacuum cabinet which can be exhausted to a pressure of about 0 · 5 m m mercury or below, and a means of introducing heat to the material to replace the latent heat of sublimation of the ice. One such type of process, which was developed originally at the Research Establishment of the Ministry of Agriculture, Fisheries and Food at Aberdeenf and is now in limited commercial use for the freeze-drying of, among other materials, vegetable commodities, is referred to as accelerated freeze-drying (Plate 16b) (see also the bibliography at the end of the chapter) . I n this case the heat is introduced by hollow heating-plates between which the material is sandwiched during the process. Other designs incorporate radiant heating on the top-side of the material and experimental work is in progress on the possible use of dielectric heating—a method which has special theoretical advantages for this type of application.

A relatively novel design for this kind of equipment is used in the so-called tumbling freeze-drierj—an experimental American development in which the material is tumbled about inside a revolving drum fitted with a heated jacket and fins which serve to transfer heat to the material.

Freeze-drying methods have so far been little used for the pro-cessing of fruit and vegetables on a commercial scale and it re-mains to be seen whether they will indeed succeed in competition with existing methods for preserving these commodities.

THE PRESERVATION OF FRUIT WITH HIGH CONCENTRATIONS OF SUGAR

The main products which come under this heading are jams, marmalades, glacé and crystallized fruit, candied peels and mincemeat. Preservation with sugar is a traditional domestic

•f This establishment was closed down in 1961. t Kaufman, V. F. et «/., Food Eng., 1964, p. 58.


process which originated from the use of honey. The commercial production of jam in the United Kingdom began in the 1880's and has since developed into an industry of very considerable size. The principle of this method of preservation is that the addition of large quantities of sugar reduces the water activity of the product below the level at which microbiological spoilage can readily take place. In fact, the concentrations of sugar used—68-5% for non-vacuum packs and 65% for vacuum packs are statutory minima for jam in the United Kingdom—are insufficient to render the material completely free of the danger of yeast and mould growth, but in practice the methods of filling into glass jars and of sealing are generally effective in preventing spoilage.

Fresh fruit should be used for jam making at the stage of ripe-ness at which the content of undegraded but soluble pectic materials is at its maximum, since it is important that the product should set to a fairly firm gel. However, fresh fruit is only available for a relatively short season and much of the fruit used for jam making is preserved, either as pulp treated with sodium or calcium metabisulphite (the latter having a toughening as well as a pre-servative effect) or, more commonly in the United States, in the frozen state. The process of jam manufacture consists in boiling the fruit or pulp together with sufficient sugar to give slightly above the required statutory minimum concentration, usually with small additions of extra pectin, water, and citric acid, the last being required to adjust the pH to the optimum for gel formation and to help to cause partial (25-40%) inversion of the sucrose so as to prevent crystallization of sugar in the product. Boiling is carried out in steam-heated pans of copper, silver-plated copper, stainless steel, nickel, monel-metal or aluminium. The pans should be so constructed as to give high rates of heat transfer for rapid boiling and may be open or closed. Vacuum concentration causes less change in colour and flavour and is widely used in the United States. The boiling time should be kept to a minimum to avoid undue degradation of pectin and the point to discontinue boiling is judged by refractometer reading or by the boiling-out tempera-ture (around 222°F), although the latter varies slightly with


atmospheric pressure. Given a sufficient concentration of pectin to form a continuous gel, the rigidity of the gel depends primarily on p H , since the sugar concentration is fixed, at approximately the opt imum level, by statutory requirements. T h e opt imum p H varies between about 3 · 0 and about 3 · 7 according to the type of fruit. Standards are also laid down for the minimum fruit-content of j ams and marmalades. For jams this varies from 25 to 4 0 % , accord-ing to the variety, but for the more strongly flavoured marmalades is only 2 0 % . Glass jars are most commonly used for packing j am, but in the United Kingdom tinplate cans are also sometimes em-ployed. Fruit preserves, jellies, etc., are prepared by an essentially similar process except that in the latter case only the juice is used, the suspended solid matter being filtered off after boiling the fruit.

Glacé, candied and crystallized products are prepared by soak-ing the materials, after boiling in water, in progressively stronger sucrose-corn syrups contain roughly equal parts by weight of suc-rose and of corn sugar. The process is normally started in syrups of about 30° Brix and the concentration of sugar is worked up gradu-ally over a period of several weeks to a final level of about 72° Brix. T h e material is boiled at each concentration before being allowed to stand in the syrup for 1-2 days. After reaching equili-br ium at the final concentration, the product is drained for about 24 hours, or the surfaces are quickly washed and dried, preferably at a temperature of 120-140°F. Too rapid an increase in the con-centration of sugar causes shrinkage of the tissues, but it has been found possible to speed up the process by carrying out the infiltra-tion at a higher temperature (150°F), by cooking under vacuum or by removing water partially by drying in heated air. Glacé-ing—the formation of an attractively shiny surface—may be ac-complished by dipping in a heavy syrup before final drying.

THE PRESERVATION OF VEGETABLES WITH HIGH CONCENTRATIONS OF SALT

The brining or salting of vegetables is a very ancient process which has changed little over the centuries. Originally carried out


on a small domestic scale, it is now largely a commercial process. The main products are cucumbers, gherkins, sauerkraut (cab-bage), onions, cauliflower, beans and peppers, but the process is applicable to almost all vegetable products and is also used with some fruits, notably olives.

The procedure used is to first allow the material to ferment after mixing with brine or salt at a concentration sufficient to prevent the growth of normal spoilage organisms, but sufficiently low to permit the activity of lactic-acid bacteria. For brine fermenta-tions, the concentration should be between about 10% and about 16% of salt—40-60° Brinometer or Salometer—the level gener-ally being increased through this range during the fermentation. As lactic acid is produced, the pH drops and this, together with the anaerobic conditions which prevail in the fermentation vessel, fur-ther inhibits the growth of potential spoilage organisms. The length of the fermentation depends on temperature—the optimum being 65-70°F—and may vary from 4 to 8 weeks, during which period the lactic-acid concentration increases to between 1% and 1 ·5%. This lactic acid is responsible for the crisp texture of correctly fermented pickles. With some materials such as cucumbers, which are low in sugars, the addition of about 1 % of sugar encourages the development of the lactic-acid fermentation, while the addi-tion of small amounts of old pickle to act as starter cultures is also a common practice. Olives require a preliminary treatment in 0-5-2% lye (sodium hydroxide) solution to hydrolyse the bitter glucosides before fermentation in brine.

The fermented product in brine is referred to as salt stock and can be kept in this condition for extended periods provided the salt concentration is maintained at 16% or above. Some products such as olives and dill pickles—cucumbers fermented in brine flavoured with dill herb and spices—are normally packed for retail sale in brine, but in most other cases the material is finally packed in vinegar after thorough washing in running water. Concentrations of between 3% and 3-6% of acetic acid and of be-tween 3% and 7% of salt in the final pack are necessary for satis-factory storage, but there is a demand at present for less sour


pickles and, if lower concentrations of acetic acid are used, a short heat-pasteurization treatment, e.g. 30 min a t 160°F for a 10 oz jar , is generally necessary. In the United Kingdom, pickles are permitted to contain up to 100 ppm of sulphur dioxide or up to 250 ppm of the methyl or propyl esters of />-hydroxybenzoic acid (see Table 15). T h e spoilage of pickles, however, is most com-monly due to residual enzyme activity. Pectolytic enzymes origin-ating from moulds which infect the flower-end of cucumbers and gherkins have proved a common cause of softening during storage, while oxidase systems in the material can cause the development of off-flavours.

Easily the most important individual salt-preserved product, however, is sauerkraut, which is prepared by a dry-salting process and is used extensively in Europe and in America as a major vege-table, rather than simply as an adjunct to the main dish, which is the usual role of other vegetable "pickles". In the production of sauerkraut, finely-shredded cabbage of the pale, tight-headed Dutch variety is mixed with dry salt to an extent of between 2 % and 3 % of the weight of the cabbage. T h e opt imum salt concentra-tion is actually 2 - 2 5 % and the fermentation is carried out in large vats which are battened on top to help to express the plant sap and create anaerobic conditions near the surface of the mass. T h e fermentation is brought about by lactic-acid bacteria which are naturally present on the surfaces of the cabbage leaves and, if suitable conditions have been established, a well-characterized sequence of bacterial forms makes its appearance. Initially, the hetero-fermentative gas-producing coccus, Leuconostoc mesenteroides, dominates the fermentation. In addition to producing lactic acid, this species also yields carbon dioxide, which helps to maintain anaerobic conditions, and various other compounds such as alco-hol, acetic acid, etc., which contribute to the characteristic flavour of the final product. As the lactic-acid concentration builds up, homo-fermentative species of Lactobacillus, notably L· plantarum, gain dominance, the final member of the sequence being L. brevis, a species which is able to withstand especially high concentrations (2-3%) of lactic acid. T h e whole fermentation takes some 3-4


weeks at about 70°F, the lactic-acid concentration building up gradually during this period to about 1-5%. The lower concen-trations of salt used in this case are not in themselves inhibitory to spoilage organisms, but with careful technique the lactic-acid bac-teria normally gain ascendency and produce a product of the de-sired characteristics. Microbiological spoilage is more common in sauerkraut than in brine-fermented pickles and a considerable pro-portion of the sauerkraut produced is now heat sterilized in cans.

THE USE OF CHEMICAL PRESERVATIVES

The use of chemical additives in the preservation of food is in most countries stringently controlled by legislation because of possible dangers to public health. However, some chemical agents, which are either themselves effective in preventing spoilage or are useful adjuncts to other methods of preservation when added to the food in small amounts, have been convincingly shown to be substantially free of toxicity and these are now widely permitted as food additives.

The chemical preservatives most extensively used in fruit and vegetable products are sulphur dioxide (or sulphurous acid, or the sulphites or bisulphites of sodium, potassium or calcium) and benzoic acid (or the benzoates of sodium or potassium, or the methyl or propyl esters of />-hydroxybenzoic acid or the corres-ponding sodium or potassium phenolates).

Sulphur dioxide, as mentioned earlier in this chapter, is used for the preservation of fruit and fruit pulp for use in subsequent manufacturing operations, and for its valuable additional pre-servative effect in dried and dehydrated fruit and vegetable pro-ducts. I t is also commonly added to fruit juices and may be used in candied, crystallized and glacéed fruit, and in pickles. Benzoic acid and the benzoates, although in general less effective than sulphur dioxide, are also suitable for use in fruit juices and pickles. With both sulphurous and benzoic acids, the antimicrobial action is due to the undissociated acid and the preservative effect is therefore greater at lower pHs, while, in the case of benzoic acid,

Processing 2. Methods of Preservation 273 the parahydroxy derivatives are generally more effective than the parent acid itself.

The relevant legislation in the United Kingdom—The Pre-servatives in Food Regulations, 1962—lays down the maximum amounts of these substances which may be present in specified products (see Table 15). In the United States, on the other hand, a statutory maximum (0-1%) is only imposed for benzoic acid and for the above-mentioned derivatives of this acid. There is, however, in the U.S. a general requirement that preservatives declared as GRAS (Generally Recognized as Safe for use in Foods) should not be added in amounts exceeding those which are normal in good manufacturing practice.

Other chemical preservatives which may be present in specified fruit and vegetable products under the regulations currently")" in force in the U.K. are diphenyl and ö-phenyl phenol and the corresponding sodium phenate—in small amounts in citrus fruit, apples, pears, pineapples, melons and peaches (see Chapter 4), and the antibiotics nystatin—in banana skins—and nisin which may be present in canned foods where the pH is below 4-5 or which have been sufficiently heat-processed to destroy Clostridium botulinum.

Another substance which can exert a useful preservative action in fruit juices and which is permitted under the European (E.E.G.) and United States regulations is sorbic acid (or its sodium, potassium or calcium salts). In Britain, this compound is only permitted in cheese, marzipan, solutions of permitted food colours and silicone anti-foam emulsions.

Finally, a potentially most useful chemical preservative for fruit juices and similar products is diethyl pyrocarbonic acid. This sub-stance, the antimicrobial action of which was originally dis-covered in Germany, has the special advantage that it is broken down rapidly in the juice to ethyl alcohol and carbon dioxide— two natural constituents of the product—and it therefore leaves no potentially toxic residue. It is an effective preservative at con-centrations of less than 0 · 1 % and, although in most countries it

t May, 1965.

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has yet to be included in the permitted lists of preservatives which are currently! in force, there is little doubt that general and formal recognition of its value will not be long delayed.

PROCESSING WITH IONIZING RADIATIONS

T h e use of ionizing radiations for the processing of foods is a relatively new field which has opened up during the period since the end of the Second World W a r as a result of the availability of increasingly large quantities of long-lived radioisotopes produced as by-products of nuclear energy programmes, and of advances which have been made in the design of machines which are cap-able of producing intense beams of high-energy electrons. T h e B and γ rays emitted by radioisotopes and the machine-produced electrons can, if applied in sufficiently high doses, be used to sterilize foods without producing any significant increase in tem-perature, but many problems have been encountered in attempts to exploit the potential usefulness of such methods in food process-ing. Sterilizing doses of 2-5 M r a d j and above invariably produce deleterious side-effects on the quality of the food. Fruit and vege-table tissues subjected to such doses undergo a marked softening of texture due partly to the death of the cells, with a consequent loss of natural turgidity, and partly to actual chemical degrada-tion of pectic constituents. Of more importance, however, is the fact that colour and flavour are also usually adversely affected, particularly in the case of fruit.

I t is now fairly clear that the most likely immediate application of irradiation in the processing of foods, including fruit and vege-table commodities, will lie in the field of medium- and low-dose treatments, which, although insufficient to sterilize the material, can still bring about a highly worthwhile reduction in the numbers of viable micro-organisms on or in a product, and may also lead to certain desirable physiological effects, for example the

t May, 1965. X 1 rad is the dose of ionizing radiation causing an energy dissipation of

100 ergs/g of absorbing material, 1 M rad = 1 million rad.


inhibition of sprouting in stored products such as potatoes and onions and the retardation of ripening in certain fruits such as tomatoes and bananas.

So far as the reduction in the numbers of micro-organisms is concerned, mould spores are more susceptible to destruction by-radiation than are bacteria, and the spoilage of soft fruits, in particular strawberries, can be delayed for a few days, a highly significant period in commercial practice, by doses of 0 · 1 to 0 ·2 M rad. Unfortunately, treatments of this order cause appreci-able losses of ascorbic acid and the juice is rendered less viscuous due to a partial breakdown of pectic materials. Again, the spoilage of some products may in the longer term be increased rather than lessened as a result of physiological and structural disturbances which make it easier for remaining organisms to establish rots. However, the results of the experimental irradiation of soft fruit have been sufficiently promising to suggest that the commercial application of such procedures will not be long delayed.f

The only irradiation treatment of fruit or vegetable materials which has already been carried out on a large commercial scale is the treatment of potatoes to inhibit sprouting during storage. Such procedures have now been used for some time in the U.S.S.R. and to a lesser extent in North America. Metabolically active cells are especially susceptible to irradiation damage and the growing points of the sprouts are effectively destroyed by doses of between 5000 and 10,000 rad, which have little effect on the more mature tissues. Unfortunately, wound-periderm forma-tion and suberization also tend to be affected and damaged tubers do not heal as quickly. However, given suitable care in the hand-ling of the material, this type of treatment is highly effective in

f Permissive legislation is presently under consideration in the United States to allow the pasteurization (radurization) of oranges, strawberries, peaches, nectarines and carrots with moderate doses of ionizing radiations.

In the United Kingdom, a working party at the Ministry of Health has recommended that a body should be set up to consider the application of such treatments to particular groups of commodities and to recommend legislation where this is considered appropriate (see the bibliography at the end of the chapter).

Processing 2. Methods of Preservation 277 sprout inhibition and the treated material may be kept for up to two years in some cases without undue loss of quality.

Irradiation treatments, therefore, are now a thing of the present and, as further experience is gained of methods of avoiding or reducing undesirable side-effects of such treatments, it is likely that we shall see an increasing use of ionizing radiations in the processing of fruit and vegetable products in the coming years.

SUGGESTIONS F O R F U R T H E R READING A N D

FOR REFERENCE

General

GRUESS, W. V., Commercial Fruit and Vegetable Products, 4th edn., McGraw-Hill, New York, 1958.

DESROSIER, N. W., The Technology of Food Preservation, Avi, Westport, Conn, 1963.

JOSLYN, M. A. and HEID, J. L., Food Processing Operations, vols. 1, 2 and 3, Avi, Wesport, Conn., 1963 (1 and 2), 1964 (3).

LYNCH, L. J. et al., The chemistry and technology of the preservation of green peas, Advances in Food Research, 9, 61 (1959).

MORRIS, T. N., Principles of Fruit Preservation, 3rd edn., Chapman & Hall, London, 1951.

PETERSON, M. S. and TRESSLER, D. K., Food Technology the World Over, vols. 1 and 2, Avi, Westport, Conn., 1963 and 1964.

TALBURT, W. F. and SMITH, O., Potato Processing, Avi, Westport, Conn., 1959.

Various Scientific and Technical Bulletins and Memoranda issued by the Fruit and Vegetable Canning and Quick Freezing Research Association (now the Fruit and Vegetable Preservation Research Association), Chipping Campden, Glos.

Canning

BALL, C. O. and OLSON, F. C. W., Sterilization in Food Technology, McGraw-Hill, New York, 1957.

BAUMGARTNER, J. G. and HERSOM, A. C , Canned Foods, 4th edn., Churchill, London, 1956.

HARTWELL, R. R., Certain aspects of internal corrosion in tin plate con-tainers, Advances in Food Research, 3, 328 (1951).

HOWARD, A. J., Canning Technology, Churchill, London, 1949. F. & V.—L


JONES, O., Canning Practice and Control, 3rd edn., Chapman & Hall, London, 1949.

LOCK, A., Practical Canning, 2nd edn., Food Trade Press, London, 1960. VARIOUS AUTHORS, in Recent Advances in Food Science, vol. 2 (Hawthorn, J .

and Leitch, J . Muil eds.), Butterworths, London, 1962.

Freezing ADAM, W. B., Progress and problems in the quick-freezing of fruit and

vegetables, in Recent Advances in Food Science, vol. 2 (Hawthorn J . and Leitch, J . Muil eds.), Butterworths, London, 1962.

FENNEMA, O. and POWDRIE, W. D., Fundamentals of low-temperature food preservation, Advances in Food Research, 13, 220 (1964).

INTERNATIONAL INSTITUTE OF REFRIGERATION, Recommendations for the

Processing and Handling of Frozen Foods, I.I.R., Paris, 1964. TRESSLER, D. K. and EVERS, C. F., The Freezing Preservation of Foods, vol. 1,

3rd edn., Avi, Westport, Conn., 1957.

Drying and dehydration BURKE, R. F. and DEGAREAU, R. V., Recent advances in the freeze-drying

of food products, Advances in Food Research, 13, 1 (1964). COTSON, S. and SMITH, D. B., (eds.), Freeze-drying of Foodstuffs, Columbine

Press, Manchester, 1963. GOODING, E. G. B., The storage behaviour of dehydrated foods, in Recent

Advances in Food Science, vol. 2 (Hawthorn, J . and Leitch, J . Muil eds.), Butterworths, London, 1962, p. 22.

HALL, L. P., The Dehydration of Fruit and Vegetables, Technical Bulletin No. 9, Fruit and Vegetable Canning and Quick Freezing Research Association, Chipping Campden, 1965.

KILPATRICK, P. W. et al., Tunnel dehydrators for fruits and vegetables, Advances in Food Research, 6, 314 (1955).

MINISTRY OF AGRICULTURE, FISHERIES AND FOOD, The Accelerated Freeze-drying Method of Food Preservation, H.M.S.O., London, 1961.

MORRIS, T. N., Dehydration of Food, Chapman & Hall, London, 1947. SCOTT, W. J., Water relations of food spoilage micro-organisms, Advances

in Food Research, 7, 84 (1957). VAN ARSDEL, W. B. and COPLEY, M. J., (eds.), Food Dehydration, vols. 1

and 2, Avi, Westport, Conn., 1962 and 1964. WEST, C. A. et al., The Dehydration of English Fruit, D.S.I.R. Special Report

No. 56, H.M.S.O., London, 1952.

Preservation with sugar RAUCH, G. H., Jam manufacture, 2nd Edn., Leonard Hill, London, 1965. See also the general bibliography, especially Cruess (1958) and Morris

(1951).


Pickling

BINSTED, R., DEVEY, J. D. and DAKIN, J. C , Pickle and Sauce Making, Food Trade Press, London, 1962.

DAKIN, J. C , Pasteurization of acetic acid preserves, in Recent Advances in Food Science, vol. 2 (Hawthorn J. and Leitch, J. Muil eds.), Butterworths, London, 1962, p. 128.

PEDERSON, G. S., Sauerkraut, Advances in Food Research, 10, 233 (1960). POULTNEY, S. V., Vinegar Products, Chapman & Hall, London, 1949.

Chemical preservatives

JOSLYN, M. A. and BRAVERMAN, J. B. S., The chemistry and technology of the pretreatment and preservation of fruit and vegetable products with sulphur dioxide and sulphites, Advances in Food Research, 5, 97 (1954).

NICKERSON, J. T. R., Preservatives and antioxidants, in Food Processing Operations, vol. 2 (Joslyn, M. A. and Heid, J. L. eds.), Avi, Westport, Conn., 1963, p. 218.

The Preservatives in Food Regulations, 1962, H.M.S.O., London, 1962.

Irradiation treatments

DESROSIER, N. W. and ROSENSTOCK, H. M., Radiation Technology in Food, Agriculture and Biology, Avi, Westport, Conn., 1960.

GLEW, G., The current position in food irradiation, Food Trade Review (London), 34, No. 12, 46 (1964).

HANNAN, R. S., Scientific and Technological Problems Involved in Using Ionising Radiations for the Preservation of Food, D.S.I.R. Food Investigation, Special Report No. 61, H.M.S.O., London, 1955.

MINISTRY OF HEALTH, Committee on Medical and Nutritional Aspects of Food Policy, Report of the Working Party on the Irradiation of Food, H.M.S.O. London, 1964.

Enzymic and non-enzymic browning reactions

DANEHY, J. P. and PIGMAN, W. W., Reactions between sugars and nitro-genous compounds and their relationship to various food problems, Advances in Food Research, 3, 241 ( 1951 ).

HODGE, J. E., Dehydrated foods; Chemistry of browning reactions in model systems, J. Agric. Food Chetn., 1, 928 (1953).

JOSLYN, M. A. and PONTING, J. D., Enzyme-catalysed oxidative browning of fruit products, Advances in Food Research, 3, 1 (1951).

LEA, C. Η., Chemical changes in the preparation and storage of dehydrated foods, in Fundamental Aspects of the Dehydration of Foodstuffs, Papers read at a conference in Aberdeen, S.C.I., London, 1958.

REYNOLDS, T. M., Chemistry of non-enzymic browning, 1. The reaction between aldoses and amines, Advances in Food Research, 12, 1 (1963).

AP

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0-63

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0-6

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Tr

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0 —

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—

—

—

—

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Suga

r (g

)

5-7-

16-1

6-

7-10

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0-6-

2-5

15-1

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4 9-

7 6-

1-6-

4 5-

7-6-

6 9-

5-11

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2-2-

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18-1

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7-9-

6 3-

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2 11

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7-6

4-7-

7-0

11-7

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3-4-

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12-5

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9 4-

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T

r 6-

7-11

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5-9-

11-1

6-

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10-7

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12-1

6-

2-16

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11-6

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3 5-

6-7-

9 4-

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Tot

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2 1-

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2 0-

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2-12

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11-9

8

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3-10

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7-

7 8

5-24

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28-0

7

4-10

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1-3-

6-9

6-1-

7-5

6-0 —

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3-2-

6-7

—

3-6

48

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2-

2-3-

6 3-

8-7-

0 2

5-13

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3*5-

5-0

6-1-

15-9

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9-14

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13-0

—

2-2-

8-3

—

—

—

—

—

—

—

—

—

—

Ash

(g

)

0-17

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0-

5-0-

7 j

0-5-

1-4

0-6-

1-8

0-28

-0-2

9 0-

46-0

-56

0-61

0-

48-0

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I 0-

2 0-

66

0-67

0-

39

0-26

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3 0-

15-0

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0-3

0-23

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4 0-

3-0-

54

0-13

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0-

52

0-24

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28-0

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0-42

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7 0-

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0-2-

0-42

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3-0-

6 0-

5-0-

6 0-

47-0

-65

0-61

0-

72

0-6-

1-1

0-32

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0-

36-1

-42

0-27

—

—

—

—

—

—

—

Fat

(g)

Tr-

0-4

Tr-

0-5

8-0-

26-4

T

r-0-

4 0-

6-0-

8 T

r-1-

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r-0-

5 T

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7 T

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1-4

Tr-

0-3

Tr-

0-5

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1-31

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8 T

r-2-

82 1

T

r-0-

6 0-

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Tr-

1-4

0-09

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11

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0-05

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5 T

r-0-

4 T

r-0-

31

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0-6

Tr-

0-3

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1-6

Tr-

0-4

Tr

0-17

-0-9

7 T

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6 T

r-0-

66

Tr-

0-2

0 0-5

Tr

2-0

1 1-

0 0-

5

0-3

Prot

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(g)

(ΛΤ

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0-1-

0-4

0-5-

1-3

1-1-

2-9

1-1-

2-7

0-21

-1-5

4 1-

2-1-

3 0-

6-1-

6 0-

6-1-

3 0-

4-0-

45

1-0-

2-1

0-5

0-6-

0-8

0-4-

1-4

0-5-

0-8

0-6-

0-8

0-1-

1-5

0-6-

0-9

0-8

0-54

-1-5

0-

43-1

-2

0-5-

1-0

1-3-

1-5

—

0-9-

1-5

0-8-

0-9

0-3-

1-32

0-

6-2-

8 0-

3-0-

9 0-

2-0-

7 0-

36-0

-5

0-5-

0-9

0-2^

1-3

0-5-

1-5

0-6-

1-6

0-37

-0-8

2

—

0-33

-0-9

—

0-4-

0-5

1-8

0-9

0-5

1-0

0-4

1-5 —

0-

4

Cal

orie

s (k

cal)

37-7

4 28

-70

88-1

65

77-1

16

49-6

0 30

29

47

-91

15

77-9

3 38

37

60

-95

22-5

0 39

-48

39-5

8 15

36

17

39

-86

21-4

8 36

-74

—

106

35-5

3 39

35

-92

37-6

1 36

-81

46-5

7 38

-75

77

25

21

6 —

26-3

4

—

23-3

3 63

71

73

82

97

13

3 —

31

Asc

orbi

c ac

id

(mg)

4-49

6-

100

9-30

0-

31

7-10

20

-36

90-3

00

4-5 12

10

-50

—

40

3-11

30

-70

—

34-6

00

50

29-6

7 35

5-

180

| 24

-100

—

212-

1814

—

41-8

0 23

-180

13

-70

9-76

2-

13

18-1

65

3-99

4-

12

12-3

3 40

5-

11

30-3

5 37

-150

10

-51

; 3-

9 T

r-50

0 50

-72

9-15

3-

15

Tr-

35

—'

2 86

0-56

00

Car

oten

e (m

g)

j

0-03

-0-1

2 0-

04-1

-8

0-1-

0-55

0-

04-0

-66

0-03

-0-1

3 0-

1 0-

2 0-

12-0

-76

0-02

0-

01

—

0-18

T

r-0-

05

Tr-

0-12

—

0-

01-1

-25

0 0-

0-02

—

0-06

-6-0

0-

08-2

-0

—

0-01

-0-0

3 0-

15

0-01

-0-4

3 0-

12-2

-4

0-0-

93

0-3-

16-4

T

r-0-

05

0-01

-0-1

2 0-

1-0-

9 T

r 0-

01-0

-08

—

0-03

-0-0

7 0-

0-09

T

r-0-

04

Tr-

0-01

0-

02-0

-11

Tr —

0-

12-2

-4

Tr-

0-01

0-

03-0

-08

Tr-

1 · 1

4 0-

01

Tr-

0-3

Thi

amin

e (m

&)

0-02

-0-0

4 0-

01-0

-04

0-06

-0-1

4 0-

02-0

-06

0-01

-0-0

4 0-

03

0-03

0-

03-C

-05

—

0-09

-0-1

i

0-1 —

0-

04-0

-05

0-04

-0-0

7 —

0-

04-0

-09

0-02

0-

01-C

-06

—

0-03

-0-1

1 0-

01-0

-06

—

0-01

-0-0

3 T

r 0-

04-0

-12

0-02

-0-0

4 T

r-0-

04

0-02

-0-0

3 0-

02-0

-12

0-08

-0-1

2 0-

02-0

-05

0-02

-0-0

3 0-

01-0

-03

0-04

0-

01-0

-02

0-04

-0-1

;

0-01

-0-0

3 0-

11-0

-17

0-02

0-

02

0-04

|

0-02

0-

03-0

-08

Tr

Tr-

0-02

0-

37

0-02

-0-0

3

Rib

ofla

vin

(mg)

0-01

-0-0

6 0-

03-0

-07

0-07

-0-2

9 0-

02-0

-08

0-01

-0-0

7 0-

04-0

-07

0-06

0-

07

Tr

0-08

-0-1

0-

03

0-03

0-

02-0

-11

1 0-

01-0

-1

—

0-04

-0-1

1 T

r 0-

01-0

-03

0-03

0-

04-0

-1

0-02

-0-0

3 —

0-

05

Tr

0-02

-0-0

5 0-

02-0

-03

0-1-

0-18

0-

03-0

-06

0-02

-0-0

4 0-

02-0

-06

0-03

0-

03-0

-04

0-03

-0-1

—

0-03

-0-0

9 0-

05

0-01

-0-0

6 0-

08-0

-16

0-02

-0-0

3 ;

0-04

0-

04

0-02

0-

03-0

-05

0-03

-0-0

5 0-

03-0

-11

0-19

0-

03-0

-05

Nia

cin

(mg)

0-1-

0-7

0-5-

1-05

0-

76-2

-6

0-4-

0-8

0-38

-0-7

8 0-

4-0-

64

0-25

0-

27-0

-32

0-1

! 0-

42-0

-8

0-25

0-

3 0-

2-0-

3 0-

13-0

-3

—

0-26

-1-6

0-

1 0-

05-0

-27

—

0-3-

1-4

0-3-

0-63

—

0-

18-0

-28

—

0-2-

0-3

0-18

-0-3

9 1-

5-1-

9 0-

3-1-

0 0-

2-0-

52

0-1-

0-59

0-

3-0-

5 0-

2 0-

4-0-

78

0-1

0-18

-0-3

0-

72-1

-28

0-3-

0-84

0-

7-1-

0 0-

2 0-

9 0-

3 0-

2 0-

3-0-

6 0-

17-0

-2

2-0-

2-4

2-14

0-

4-0-

55

:

Foli

e ac

id

(fg)

1-4

1-4 10

5-10

12

j

5-7 —

—

—

6 3 __

—

7 —

__

—

. 6 —

—

1 5 —

—

1-

18

2 3-8 2 —

1-

14

—

2-8 —

2-

10

—

—

—

—

—,

—

—

—

—

—

Cal

cium

(m

g)

2-11

8-

47

8-32

7-

22

8-20

63

60

8-

16

15

12-2

6 24

19

11

-42

9-31

!

15-1

7 11

-49

107

5-17

35

5-

12

8-27

36

-61

13-1

6 61

30

-62

12-2

8 10

-16

2-63

7-

29

12-3

2 3-

39

3 8-41

36

57

-148

10

-41

10-4

2 21

-32

5-6

25

5 10

11-1

8 12

-22

30-4

0 31

12

-16

Iron

(m

g)

0-3-

0-9

0-4-

1-2

0-3-

1-2

0-4-

1-6

0-5-

1-3

0-8-

3-8

1-3

0-4-

1-3

1-1

0-5-

1-0

0-4

0-6

0-3-

1-0

0-3-

0-8

0-4-

0-6

0-5-

1-3

0-35

0-

3-1-

4 1-

4 0-

2-1-

0 0-

2-0-

6 1-

6-3-

0 0-

5-0-

6 1-

0 0-

3-0-

6 0-

5-2-

2 1-

0-1-

1 0-

2-1-

2 0-

2-0-

8 0-

3-0-

6 0-

4-1-

7 0-

7 0-

4-1-

2 1-

2 0-

4-1-

0 0-

4-0-

6 0-

4-1-

5 0-

4-3-

6 0-

2-0-

4 0-

8 0-

5 0-

4 0-

5-0-

7 0-

8 1-

0-1-

1 2-

3 :

0-5-

0-7

a The

ran

ges

give

n in

the

se t

able

s ar

e ba

sed

on r

esul

ts d

raw

n fr

om m

any

publ

ishe

d so

urce

s. T

he w

iden

ess

of t

he r

ange

in

som

e ca

ses

is p

roba

bly

due

in

part

to

diff

eren

ces

in t

he m

etho

ds u

sed

as w

ell

as t

o va

riat

ions

in

the

mat

eria

ls.

Info

rmat

ion

has

been

dr

awn

from

nu

mer

ous

rese

arch

rep

orts

on

indi

vidu

al

com

mod

itie

s bu

t th

e fo

llow

ing

gene

ral

publ

icat

ions

hav

e pr

ovid

ed

the

bulk

of

the

data

use

d.

McC

ance

, R

. H

. an

d W

iddo

wso

n, E

. M

., M

.R.C

. Sp

ecia

l R

epor

t N

o. 2

97,

H.M

.S.O

., L

ondo

n, 1

960.

Pl

att,

B.

S.,

M.R

.C.

Spec

ial

Rep

ort

No.

302

, H.M

.S.O

., L

ondo

n, 1

962.

Ja

cobs

, M

. B

., F

ood

and

Foo

d P

rodu

cts,

vol

. 2,

Inte

rsci

ence

, N

ew Y

ork,

195

1.

Mor

ris,

T.

N.,

Pri

ncip

les

of F

ruit

Pre

serv

atio

n, 3

rd e

dn.,

Cha

pman

& H

all,

Lon

don,

195

1.

Ase

njo,

C.

F. e

t al

, F

ood

Res

earc

h, 1

7, 1

33 (

1952

). A

senj

o, C

. F.

and

Mun

iz,

A.

I., F

ood

Res

earc

h, 2

0, 9

7 (1

955)

. Fi

sher

, K

. H

. an

d D

odds

, M

. L

., F

ood

Res

earc

h, 2

0, 2

47 (

1955

). M

unse

ll,

H.

E.

et a

l., F

ood

Res

earc

h, 1

8, 3

91 (

1953

). M

usta

rd,

M. J

., F

ood

Res

earc

h, 1

7, 3

1 (1

952)

. N

avia

, J.

M.

et a

l, F

ood

Res

earc

h, 2

0, 9

7 (1

955)

. N

avia

, J.

M.

et a

l, F

ood

Res

earc

h, 2

2, 1

31 (

1957

). Pe

ters

, F.

E.

and

Will

s, P

. A

., F

ood

Res

earc

h, 2

5, 2

11 (

1960

). Si

maa

n, F

. S,

et

al.

J.

Sci.

Foo

d A

gric

, 15

, 799

(19

64).

b Phy

llant

hus

embl

ica

e A

nnon

a m

uric

ata

d Am

wna

squ

amos

a e Z

izyp

hus

juju

ba

f Li

tchi

chi

nens

is

i C

aloc

arpu

m s

apot

a 8

Dio

spyr

os k

aki

k

Tam

arin

dus

indi

ca

h Chr

ysop

hyllu

m c

aini

to

* M

alph

igia

pu

nici

/olia

»

Ach

ras

zapo

ta

App

endi

x A

Con

tinu

ed o

n R

ever

se

AP

PE

ND

IX A

TH

E C

OM

PO

SIT

ION

O

F F

RU

IT A

ND

VEG

ETA

BLE

S

2. V

EG

ET

AB

LE

S (A

LL

VA

LU

ES

GIV

EN

AR

E P

ER

100

g E

DIB

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MA

TE

RIA

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OR

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PR

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FO

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Com

mod

ity

Arr

acac

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(Sn)

Art

icho

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glob

e)

Arti

chok

e (J

erus

alem

)0

Asp

arag

us

Bea

ns (

broa

d)

Bea

ns (

gree

n)

Bea

ns (

Lim

a)

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troot

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rocc

oli

Brus

sels

spro

uts

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bage

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arro

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assa

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S)

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eler

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hard

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hayo

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umbe

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g pl

ant

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ve

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and

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ss

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on

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on (

Sprin

g)

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ley

Pars

nip

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Pepp

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tain

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mpk

in

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to (

S)

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ish

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ach

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sh

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t co

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t po

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nip

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t Y

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»

Wat

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(g)

69-7

6 80

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80

93

64-8

9 89

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67

83-8

9 89

-91

84-8

9 86

-93

84-9

5 50

-74

84-9

2 89

-96

91-9

4 74

-95

91-9

7 89

-94

93-9

4 80

-96

69-9

0 71

-92

92-9

7 59

-74

98

92-9

3 81

-93

87-9

3 79

-89

79-8

3 65

-81

70-9

3 58

-74

80-9

6 76

-85

92-9

5 91

-93

86-9

5 57

-80

60-8

0 89

-91

54-8

3 90

-96

87-9

3 90

-94

54-8

4 58

-78

Fibr

e (g

)

0-4-

0-7

0-8-

3-2

0-8

0-7

0 1 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 1 0 2 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0

6-0

0-3

5-2

6-3

3-1

3-3

6-3

6-2

6-1

8-1

7-2

6-1

4-0

3-0

9-2

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•3

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1-1

0-3

3-1

4-1

0-6

8-3

5-1

0-3

9-9

2-4

8-5

5-2

3-1

5-1

3-2

5-1

6-0

3-1

6-1

5-1

3-2

4-3

4-1

7-2

5-3

4-1

9 0 6 1 5 6 4 9 3 5 7 6 6 5 5 •2 8 •1

•4

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1-0

Star

ch

(g) —

—

0-

4 —

0-2-

2-2

9-9-

14

0 0 0-

1-0

0 0 31

0-4

0-1

0-1 —

0 0-2 0 —

—

0 0

34-5

0-

1 0 0-

0-5

0-0-

5 0

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2-5

3-9-

12-3

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2 18

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2-6

10-6

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3 0 —

0-

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0 8-

7-21

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10-8

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5 0-

0-1

33-0

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5 0-

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0 0-1

14-0

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0 —

Suga

r (g

) —.

7-2-

14

1-3-

2-3

—

0-4-

2-7

1-1-

2-0

6-0

0-4-

1-9

3-6

3-3-

3-8

5-4-

7-5

—

2-4-

2-6

1-0-

1-2

0-8 —

1-

8-2-

6 2-

1-3-

4 1-

0 —

2-2

6-0

1-1-

2-2

—

1-3

0-4-

0-9

5-2-

6-7

3-7-

8-5

Tr

8-8-

9-5

2-3-

7-4

1-7-

13-9

—

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5-2-

7 0-

3-1-

1 2-

8-3-

4 0-

3 1-

0-3-

9 3-

2-5-

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4-6-

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7 —

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4 3-

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6 0-

6 —

—

Tot

al

Aci

dity

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-equ

iv.

—

7-6 —

0-

8 —

5-4-

7-7

28-4

10

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3-2-

9-3

11-0

2-

6-5-

6 9-

0-10

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—

5-3

7-8-

8-4

15-8

1-

0-13

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3-2-

7-9

4-5-

6-3

5-4-

7-0

—

8-0

7-0

3-&

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15

-0

1-9

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0-

5-1-

5 8-

4 —

7-5-

12-0

1-

2-1-

3 1-

7 —

1-5-

7-8

7-0-

10-3

2-

9-7-

2 27

-0-3

9-6

1-0-

2-6

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6-

7 4-

9-8-

5 18

-1

5-6-

13-4

2-

7-6-

5 7-

5-12

-0

—

—

Ash

(g

)

1-0-

1-I

1-1-

1-2

1-2

0-7

0-8-

1-1

0-5-

1-7

1-7

0-8-

1-1

1-1

1-1-

1-3

0-3-

3-4

0-6-

2-6

0-5-

1-0

0-7-

0-9

1-1-

1-6

1-0-

1-5

—

0-3-

0-5

0-4-

0-6

0-9

0-3

1-1-

4-4

0-5-

0-9

0-2-

1-0

1-0-

1-5

—

1-2

0-4-

0-9

0-6

1-4-

2-4

1-15

0-

75-1

-05

0-3-

0-7

0-6-

0-9

0-8-

1-4

0-6-

1-3

0-4-

1-0

0-5-

1-5

0-2-

0-9

0-6-

1-0

0-5-

1-1

0-4-

0-8

0-9-

1-3

0-4-

0-7

0-5^

0-7

0-7-

1-6

0-1-

0-9

—

Fat

(g)

0-1-

0-3

0-3-

0-4

0-1

0-2

0-1-

1-9

Tr-

0-4

0-8-

1-9

Tr-

0-7

Tr-

0-3

Tr-

0-5

Tr-

0-7

Tr-

0-7

Tr-

0-7

Tr-

0-3

Tr-

0-5

0-2-

0-4

0-1

Tr-

0-2

Tr-

0-7

Tr-

0-2

0-1

0-1

Tr-

0 T

r-0

0-2-

0 T

r T

r-0

Tr-O

T

r-0

Tr-

1 T

r-0

Tr-

0 0-

1-0

0-05

-0

Tr-

0 T

r-0

Tr-

1 0-

3 0-

03-0

0-

8-2

0-04

-0

Tr-

0 0-

1-0

Tr-

1 T

r-0

Tr-

O

0-03

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21

0 5 8 7 78

2 1 1 23

1 7 1 5 26

2 6 0

0-2-

0-3

Prot

ein

(g)

(N

X 6

-25)

1-0-

1-3

0-5-

4-5

2-2

2-2-

3-9

2-9-

4-1

1-1-

2-4

2-9-

7-5

1-3-

1-8

3-1-

4-0

2-4-

4-4

1-4-

3-3

0-6-

2-0

0-7

1-8-

3-4

0-7-

2-0

1-5-

2-6

0-8

0-6-

1-4

0-7-

2-4

1-6-

1-8

0-7

2-1

1-3-

2-5

0-8-

1-6

—

0-4

1-6-

2-4

0-9-

2-2

0-9-

1-0

3-7-

5-2

1-5-

1-7

4-6-

8-2

1-2-

2-0

1-0

0-6-

1-8

1-6-

2-3

0-7-

1-2

2-3-

5-1

0-6-

1-5

2-9-

4-5

1-4-

2-8

1-1

0-5-

2-9

0-7-

1-2

0-6-

1-1

1-7-

3-1

1-5-

2-4

2-0

Cal

orie

s (k

cal)

99

17-7

0 19

-78

9-27

43

-48

7-42

73

-138

44

-58

—

16-5

8 8-

36

19-4

7 15

3 11

-34

5-22

16

29

9-

16

15-3

8 11

-24

28

3-6

25-5

2 11

-27

—

7 10

-28

13-4

9 37

-69

21-6

0 56

-83

49-1

38

29-3

7 12

8 15

-36

75-1

02

15-2

2 16

-35

19-4

4 10

8-14

2 98

-125

18

-41

111-

129

14-2

3 11

-35

15-2

9 10

4 10

7-13

3

Asc

orbi

c ac

id

(mg)

8-36

5-

33

5 (r

aw)

13-4

1 15

-37

5-28

—

T

r-6 40

35-1

28

20-2

20

4-58

9-

30

8-11

4 5-

15

63-7

2 16

-20

&-1

9 3-

5 12 ♦

15

-27

—

15-3

2 3-

33

10-1

5 2

30-1

02

6-10

7-

50

110-

200

10-1

8 12

-35

73-3

42

6-54

4-

20

8-64

6-

43

1-59

3-

46

10

7-68

17

-25

3-8

19-4

8 17

-37

37-1

53

Tr-

13

7-20

Car

oten

e (m

g)

0^1-

0-25

0-

06

—

0-38

-0-5

5 0-

02-0

-23

0-02

-0-6

—

T

r-0-

1 2-

5 0-

1&-0

-7

Tr-

4-8

6-13

-6

0-1-

0-85

T

r-0-

04

0-0-

8 1-

4-2-

0 T

r-0-

01

0-0-

04

Tr-

0-07

2-

0 0-

02

O-iU

-1-4

6 0-

15-7

-8

0-03

-0-0

4 —

35

-5-0

0-

0-06

T

r-0-

87

4-4-

8-8

Tr

0-18

-0-5

0-

15-2

-7

0-06

-2-0

0-

17-5

-9

Tr-

0-03

T

r-0-

04

2-88

-7-3

5 T

r-4-

3 0-

0-06

T

r-12

T

r T

r-0-

02

0-19

-1-4

5 T

r-0-

01

1-64

-6-8

4 0-

01-0

-72

·—

Thi

amin

e (m

g)

0-05

-0-0

9 0-

2 0-

2 (r

aw)

0-1-

0-23

0-

28-0

-36

0-04

-0-2

4 0-

2-0-

6 0-

01-0

-03

0-06

0-

06-0

-13

0-03

-0-1

7 0·

04μυ

·07

0-04

-0-1

1 0-

04-0

-13

0-02

-0-5

0-

0&-0

-07

0-01

-0-0

2 0-

02-0

-1

0-05

0-

06

0-0-

04

—

0-06

-0-8

0-

04-0

-14

0-11

-0-1

2 T

r 0-

04-0

-09

0-02

-0-0

3 0-

04-0

-06

0-09

-0-2

0-

07-0

-11

0-25

-0-5

2 0-

03-0

-1

0-02

-0-1

0-

04-0

-05

0-04

-0-1

6 0-

0-04

0-

05-0

-15

0-02

-0-1

0-

15

0-1-

0-15

0-

04-0

-06

0-03

-0-2

7 0-

04-0

-11

0-03

-0-0

7 0-

05-0

-2

0-05

-0-1

5 —

Rib

ofla

vin

(mg)

0-02

-0-0

5 0-

01-0

-17

Tr

(raw

) 0-

08-0

-3

0-04

-0-2

4 0-

05-0

-14

0-15

-0-3

0-

01-0

-06

0-2

0-09

-0-1

9 0-

03-0

-21

0-03

-0-0

5 0-

02-0

-05

0-04

-0-1

0-

02-0

-4

0-06

-0-1

4 0-

02-0

-03

0-02

-0-1

1 0-

02-0

-04

0-1

0-01

-0-0

3 —

0-

06-0

-1

0-03

-0-1

0-

02-0

-03

—

0-09

-0-2

0-

02-0

-04

0-03

-0-9

0-

18-0

-6

0-09

0-

06-0

-14

0-02

-0-1

8 0-

01-0

-1

0-03

-0-0

8 0-

02-0

-04

0-01

-0-0

8 0-

08-0

-24

0-01

-0-1

0-

1 0-

03-0

-06

0-03

-0-0

4 0-

03-0

-1

0-02

-0-1

2 0-

03-0

-06

0-09

-0-3

0-

01-0

-03

—

Nia

cin

(mg)

0-6-

5-3

0-1 __

0-8-

1-5

2-0-

4-0

0-2-

1-14

1-

5-3-

0 0-

06-0

-4

0-6

0-4-

1-04

0-

15-1

-55

0-2-

1-16

0-

28-1

-23

0-25

-0-8

9 0-

2-0-

4 0-

61-1

-14

0-4-

0-44

0-

1-0-

6 0-

5-0-

8 —

0-

3-0-

6 —

0-

39-0

-5

0-2-

0-5

0-44

0-

2 0-

36-0

-8

0-1-

0-2

0-2-

0-6

0-53

-1-8

0-

2-0-

7 1-

3-3-

3 0-

3-2-

17

0-16

-1-4

0-

4-0-

9 0-

8-5-

1 0-

2-0-

65

0-35

-0-7

5 0-

2-1-

4 1-

7 0-

41-1

-56

0-8-

1-2

0-06

-1-1

6 0-

45-0

-91

0-4-

0-94

0-

36-1

-38

0-1-

0-46

—

Folie

ac

id

(Mg)

—

—

25-1

56

—

12-4

8 —

20

(ra

w)

50 (

raw

) 14

-86

20 (

raw

) 10

(ra

w)

—

30 (

raw

) 7

(raw

) —

—

6 —

—

—

—

—

20

—

—

29

-43

—

10

40

20 (

raw

) 8-

46

—

—

—

6 (r

aw)

10

53-1

29

8-23

—

10

(ra

w)

5 (r

aw)

—

5 4

(raw

) 50

—

—

Cal

cium

(m

g)

15-2

3 19

-74

30

13-2

8 21

-47

30-6

5 —

15

-32

160

10-5

3 30

-204

29

-57

23-6

4 13

-43

31-5

3 10

5-17

6 12

-2

15-2

3 10

-36

44

21

—

50-8

5 17

-107

15

13

-6

65-2

20

24-5

2 28

-135

13

9-32

5 36

-57

13-5

2 10

-29

7-23

20

-66

4-13

25

-52

60-5

95

9-40

9

14-4

5 42

-56

25-1

50

5-14

30

-65

63-2

22

8-37

—

Iron

(m

g)

1-0-

2-0

0-2-

1-0

0-4

0-5-

2-0

1-0-

3-0

0-5-

3-2

—

0-4-

2-8

1-5

0-1-

2-5

0-5-

1-9

0-2-

1-2

0-6-

1-5

0-2-

1-9

0-5-

9-9

1-4-

4-0

0-35

0-

3-0-

8 0-

4-1-

0 2-

8 0-

48

—

1-0-

2-0

0-5-

4-0

0-6-

1-4

0-22

1-

3-4-

5 0-

2-0-

5 0-

6-3-

0 2-

3-19

-2

0-5-

0-7

1-2-

3-6

0-7-

1-5

0-1-

2-1

0-3-

0-8

0-5-

1-2

0-3-

1-9

0-8-

4-5

0-2-

2-4

0-7

0-6-

1-3

0-3-

0-4

1-0-

2-0

0-4-

1-2

0-1-

0-5

1-3-

5-1

0-5-

1-2

—

m A

rrac

acia

xan

thor

rhiz

a n

(S)

deno

tes

a st

arch

y tu

ber

(or

plan

tain

). °

A s

tem

-tub

er (

deve

lope

d un

derg

roun

d) w

hich

acc

umul

ates

inu

lin n

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a.

COMMODITY INDEX Bold type indicates references to Figui

Apple xvi, xvii, xviii, 7, 12, 15, 20, 25, 26, 27, 30, 31, 40, 45, 46, 47, 71, 72, 75, 76, 79, 82, 84, 86, 89, 91, 99, 101, 103, 104, 122, 124, 126, 130, 131, 136, 150, 169, Fig. 23, 170, 172, 173, 175, 177, 184, 198, 205, 211, 213, 214,215, 218, 227, 230, 231, 246, 256, 273, App. A, App. B

canned 138 juice 135, 227, 228 var. Belle de Boscoop 169 var. Berlepsch Orange 169 var. Cox's Orange Pippin 100,

169 var. Golden Delicious 169 var. Jonathan 169 var. Laxton's Superb 169, 175

Apricot 12, 20, 40, 83, 109, 114, 116, 122, 126, 131, 136, 152, Fig. 23, Table 10,170, 173, 175, 202, 211, 213, 216, 230, 246, 247, 253, 256, 257, 258, App. A, App. B

canned 133 nectar, 224

Arracacha 152, 156, Table 10, App. A

Artichoke (Globe) 103, Fig. 23, Table 10, 173, App. A

Artichoke (Jerusalem) xvi, 12, 49, Fig. 23, Table 10, 170, 173, App. A

Asparagus xvi, 12, 25, 43, 44, 52, 72, 73, 81, 98, 102, 109, 129, 155, 157, Fig. 23, Table 10,170,173, 175, 198, 230, 247, 253, App. A

canned 133, 134, 217, 230, 233

or Tables

Avocado xvi, 10, 12, 20, 31, 72, 75, 83, 86, 98, 144, 156, Fig. 23, Table 10, 173, App. A

Banana xiv, xvi, xix, xx, 6, 12, 20, 31, 40, 60, 64, 71, 72, 75, 76, 79, 82, 83, 85, 96, 98, 99, 102, 103, 122, 124, 125, 129, 130, 136, 144, 145, 151, Table 10, 170, 173, 178, 273, 275, App. A, App. B

pulp 243 Beans (Broad) xvi, 9, 26, 27, 41, 75,

98, 128, Fig. 23, 173, 205, 232, App. A

Beans (Green) 12, 41, 81, Fig. 23, Table 10, 170, 173, 211, 213, 217, 232, 265, App. A

canned 133 frozen 135, 247

Beans in tomato sauce 133, 137, 207

Beans (Kidney) xvi, xviii, 41, 60, 75, 98, Fig. 23, 233, 270

Beans (Lima) xvi, Fig. 23, 173, 253, App. A

Beans (Runner) 26, 41, 54, 75, 90, 98, Fig. 23, 170, 173

Beans (Snap) 72, 75, 81, 93, 98, 253

Beetroot 6, 7, 12, 21, 23, 31, 43, 44, 99, 101, 104, 129, Fig. 23, 173, 232, 265, App. A

Bilberry (Blueberry) 12, 20, 40, Fig. 23, 173, 176, App. A

Blackberry 12, 15, 41, 112, Fig. 23, Table 10,173, 176, 206, App. A

290 Commodity Index

Blackcurrant 11, 12, 15, 40, 99, 104, 108, 114, 150, Fig. 23, Table 10, App. A

juice 224, 225 Brassica spp. 98, 99, 102, 103,

Fig. 23 Broccoli 12, 93, Fig. 23, 173, 253,

App. A Brussels sprout 34, 47, 129, Fig. 23,

Table 10, 170, 173, 253, 261, App. A

Cabbage xvi, xvii, xviii, xx, 21, 32, 34, 128, Fig. 23, Table 10, 170, 173, 213, 261, 271, App. A

dehydrated 274 Cape Gooseberry 152, 156, Table

10 Carrot xvi, 6, 12, 18, 21, 43, 44, 47,

57, 60, 75, 81, 96, 98, 99, 102, 105, 114, 129, 138, 152, 153, Fig. 23, Table 9, Table 10, 171, 173, 220, 222, 247, 253, 261 265, 276, App. A

canned 133 Cashew Apple Table 10 Cassava xvi, xix, xxii, 38, 39, 123,

127, 128, 145, 146, 147, 152, Fig. 23, Table 10, App. A, App. B

protein 147 Cauliflower xvi, 12, 34, 93, 129,

138, Fig. 23, Table 10,170, 173, 175, 184, 253, 270, App. A

Celery xvi, 12, 29, 43, 44, 54, 79, 96, 105, 109, 129, Fig. 23, Table 10, 173, 230, App. A

hearts (canned) 233 turnip-rooted Fig. 23, 173

Chard Table 10, App. A Chayote App. A Cherry xvi, xviii, 12,20,40,83, 109,

112, 122, 127, 131, 137, 151, Fig. 23, 170, 173, 176, 178, 212, 213, 217, App. A, App. B

frozen 134

Cherry (cont.) glacé 127 sour 127, 213, 246, 253 sweet 127, 253

Chicory 170 Citrus Fruit xix, 12, 15, 25, 31, 33,

40, 65, 74, 78, 79, 82, 83, 85, 90, 92, 93, 98, 99, 100, 102, 103, 104, 110,111,112,113,122,124,125, 135, 136, 143, 144, 151, Fig. 23, Table 9,170, 173, 178, 211, 215, 216,217,218,273

juice concentrates 226 juices 135, 136, 137,224,226

Corn on the cob (frozen) 247, 253 Corn (maize) see Sweet Corn Cranberry xvi, 12, 20, 173, App. A Cucumber xvi, xvii, 90, Table 10,

171, 173, 253, 270, 271, App. A Cucurbitaceous fruit 5, 12, 33, 40,

98, 99, 102, 103, 104, Fig. 23 Currants (dried) 125, 137, 256 Custard Apple App. A

Damson App. A Date xvi, xvii, 6, 40, 99, 103, 126,

137, 173, 256 Drupe fruits 12, 31, 40, 45, 82, 85,

90, 98, 99, 102, 103, 126, 130, 150, Fig. 23, 213, 236

Egg plant xvi, 21, 99, Fig. 23, 173, App. A

Endive 98, Fig. 23, Table 10, 170, 173, 184, App. A

Fig xvi, xvii, 20, 31, 41, 74, 99, 103, 116, 126, 137, Fig. 23, 173, 256

var. Adriatic 256 var. Calimyrna 256

Gherkins 171, 270, 271 Gooseberry 12, 15, 40, 99, 104,

Fig. 23, 173, 204, 206, 207, 212, App. A

Commodity Index 291

Grape xvii, xix, 12, 20, 25, 26, 31, 40, 61, 74, 82, 83, 98, 99, 102, 103, 110, 112, 113, 114, 122, 124, 125, 130, 136, Fig. 23, Table 10, 171, 173, 227, 256, App. A, App. B

juice 135, 227, 228, 274 Grapefruit 7, 20, 25, 33, 40, 75,102,

122, 125, 130, Fig. 23, 170, 230, 235, App. A, App. B

juice 226 segments (canned) 233

Greengage App. A Gourd (Wax) App. A Guava 31, 40, 151, Fig. 23, Table

10, 173, App. A

Horseradish Fig. 23, 173, 177, 274

Jujube 151, Table 10, App. A

Kale 150, Table 10 Kohlrabi 28, 44, 171, 173, App. A

Leek xviii, Fig. 23, 173, App. A Legumes 41, 98, 99, 103, 104, 144,

145, 146,147,152, 153, 154, 157, Table 10

protein 147 Lemon 7, 12, 40, 96, 98, 100, 102,

122, 125, Fig. 23, 170, App. A, App. B

Lettuce xvi, xvii, 12, 43, 44, 72, 98, 103, 129, 137, Fig. 23, Table 10, 171, 173, 184, App. A

Lime xx, 40, 98, 102, Fig. 23, App. A

juice 138 Litchi Fig. 23, 173, App. A Loganberry 11, 12, 41, Fig. 23, 173,

232, App. A

Mandarins see Orange Mango 72, 78, 82, 83, 98, 111, 124,

151, 152, Table 10, 173, App. A Marrow 10, App. A Melon xvi, 6, 40, 85, 98, 113, 152,

Fig. 23, Table 10,171, 173, 178, 273, App. A

Mulberry xvi, xvii, 20, 41, App. A Mustard xvi, 32, 34, App. A Myrobalan 151, Table 10, App. A

Nectarine Fig. 23, 173, 276 Nuts 41

Okra 12, Table 10 Olive xvi, 10, 33, 40, 116, 144,

Fig. 23, Table 10, 173, 270, App. A

Onion xvi, xvii, xviii, 6, 12, 21, 25, 34, 76, 93, 98, 99, 103, 128, 131, 138, Fig. 23, Table 10,171,173, 177, 215, 220, 261, 265, 270, 275, App. A, App. B

spring 155, Table 10, App. A Orange xvi, xix, 12, 15, 20, 25, 33,

40, 45, 72, 98, 99, 100, 102, 103 122, 125, 130, Fig. 23, 170, 202, 226, 275, App. A, App. B

frozen concentrate 222, 226 juice 226 mandarin 125, 133, 137, Fig. 23,

170

Papaya 31, 61, 151, 152, Table 10, 173, App. A

Parsley xvi, 29, Fig. 23, Table 10, 173, App. A

Parsnip xvi, xvii, xviii, 6, 12, 81, Fig. 23, Table 10, 173, App. A

Passion fruit juice 228 Peas xvi, 6, 29, 30, 32, 52,72,73, 75,

81, 114, 128, 129, 138, Fig. 23, 171,172, 173, 175, 185, 198, 204, 205, 206, 207, 208, 217, 218, 221,

292 Commodity Index

Peas (cont.) 230, 232, 233, 241, 247, 249,252, 253, 260, 261, 265, App. A

canned 133, 230, 232, 241, 245 frozen 135, 138

Peach xvi, xix, 6, 12, 20, 26, 27, 40, 45, 83, 84, 86, 90, 98, 103, 109, 110,111,112,113,114,116,122, 126, 130, 131, 136, 153, Fig. 23, Table 10, 171, 173, 176, 184, 209, 211, 213, 215, 246, 247, 253, 256, 257, 258, 273, 276, App. A, App. B

canned 133, 137 clingstone 12, 126, 205, 206, 215,

230 freestone 205 halves (canned) 233 nectar 224 var. Hal-berta Giant 153, Table

10 Pear xvi, 12, 15, 26, 27, 31, 40, 54,

75,76, 82, 84, 85,86, 103, 111, 112,116,122,126,131,136,150, 169, Fig. 23, Table 10,171,173, 175, 178, 184, 205, 213, 216, 246, 256, 257, 258, 273, App. A, App. B

canned 133, 137, 233 nectar 224 var. Lanscailler 169

Peppers 21, 90, 152, Table 10,171, 270, App. A

Persimmon xvi, 116, 152, Fig. 23, Table 10, 173, App. A

Pimento Fig. 23, 173 Pimiento 215 Pineapple xvi, xix, 6, 12, 20, 31, 41,

44, 74, 78, 82, 83, 93, 99, 104, 122, 124, 125, 130, 137, 171, 173, 213, 230, 273, App. A, App. B

canned 133, 137, 233 frozen 135, 137, 247 juice 135, 226

Plantain xiv, 121, 124, 152, Table 10, App. A

Plum xvi, xviii, 12, 20, 26, 40, 104, 122,126,131,136,151,171,173, 176, Fig. 23, 178, 205, 209, 212, 213, 215, 232, App. A, App. B

canned 133 Pome fruits 30, 31,40,45,64,82,84,

85, 90, 98, 99, 100, 102, 103,126, 131, 150, 109, 175,213,216,217

Pomegranate xvi, xvii, 12, Fig. 23, 173, App. A

Potato xvi, xviii, xx, 12, 15, 21, 27, 29, 31, 32, 34, 42, 43, 46, 47, 52, 59, 60, 72, 81, 91, 93, 96, 98, 99, 100,104,123,127,131,132,136, 140, 144, 145, 146,147,149, 154, 156, 159, 167, Fig. 23, Table 9, Table 10, 171, 173, 197, 199, 218, 253, 261, 265, 275, App. A, App. B

dehydrated 274 new 132, 138 peeled 274 products (frozen) 135 protein 147

Prunes xvii, 126, 137, 216, 257, 258 Pumpkin xvi, 40, 152, Fig. 23,

Table 10, 173, App. A

Quince xvi, Fig. 23, 173

Radish xvi, xvii, 21, 34, 99, Fig. 23, Table 10,173, App. A

Raisins 125, 137, 256, 257 Raspberry 12, 20, 41, 99, 103, 112,

113, 114, Fig. 23,171, 173, 176, 205, 217,246, 253, App. A

Redcurrant 12, 15, 40, 82, 99, 104, Fig. 23, 173, App. A

Rhubarb xx, 12, 54, 74, 129, 158, Fig. 23, Table 10, 211, App. A

canned 133 Rutabaga see Swede

Salsa (sun-dried tomato) 255 Salsify Fig. 23, 173

Commodity Index 293 Sapodilla App. A Sapote App. A Sauerkraut 270, 271 Soursop App. A Spinach xvi, 11, 25, 29, 72,152,158,

Fig. 23, Table 10,171, 173, 230, 241, 253, App. A

Spring onion see under Onion Squash xvi, 21, 40, 57, 90, 152,

Fig. 23, Table 10, 173, App. A winter 72

Star Apple App. A Strawberry 12, 15, 20, 31, 41, 82,

98, 99, 102, 110, 112, 114, 150, Fig. 23,171, 173, 176, 186, 213, 217, 232, 246, 248, 253, 276, App. A

frozen 134, 135, 246 Sultanas 125, 137, 256, 257 Swede 99, Table 10, App. A Sweet corn 6, 9, 11, 21, 25, 29, 59,

60, 72, 73, 81, 109, 114, Table 10, 198,212,230,232,241,247, App. A

canned 116, 133 Sweet potato xvi, xix, 6, 12, 21, 27,

44, 81, 90, 98, 99, 101, 103, 104, 111, 123,127,128, 144, 145,146, 147, 152, 154, Table 10, 173, App. A, App. B

protein 147 Sweetsop Table 10, App. A

Tamarind App. A Taro 121, Table 10, App. A

Tomato xvi, xix, 12, 15, 21, 31, 32, 40, 45, 60, 72, 73, 79, 85, 86, 87, 90, 96, 98, 99, 104, 113, 123, 128, 131, 137, 144, 152, 153, Fig. 23, Table 10,171, 175, 178, 184, 202, 211, 213, 215, 217, 231, 241, 275, App. A, App. B

canned 133, 134, 138, 231 juice 222, 224 pulp 274

Turnip xvi, xvii, xviii, 12, 34, 99, Fig. 23, Table 10, 171, 173, App. A

Vegetables (leafy) 9, 12, 79, 98, 102, 146,147, 149, 152, 154, 155, 157, 158, 159, Table 10

Vegetables (starchy) xxii, xxiii, 5, 123, 127, 128, 143, 145, 149, 150, 152, 155, 156, 160, Table 10, 197

Watercress 34, Table 10, App. A Watermelon 173, App. A West Indian Cherry 151, Table 10,

App. A

Yam xvi, xxii, 28, 38, 39, 123, 127, 128, 146, 147, 152, App. A, App. B

protein 147

SUBJECT INDEX Bold type indicates references to Figui

Abcission-preventing agents 92 Abrasive peelers 214, 215 Accelerated Freeze-Drying 263, 267 Acetaldehyde 34, 68, 71, 78, 91, 176 Acetic acid 34, 270, 271 Acetyl co-enzyme A 69, 70 Acetyl methyl carbinol 34 Achromobacteriaceae 107 Acid content 11, 83, 194 Acid foods 240 Acid-resistant lacquers 230 Aconitase 69 Aconitic acid 13, 14, 69 ADP (Adenosine diphosphate) 67,

68,73 Aerobic respiration 66 Aggregate fruits 41 Agitated cookers 241,242,243-4 Air-blast freezers 247, 249 Alanine 15 Albedo 7 ,45 ,90 ,216 Alcohol-insoluble solids 185, 198-

9,206 Aldolase 67, 68 Aleurone grains 59 Alkaloids 194 Alternarla 98, 102, 106, 110

A. brassicae 102 A. ciiri 102 A. radicina 102

Aluminium 28 Aluminium foil 248 Amino acids 14, 144-6 y-Amino-butyric acid 15 Ammonia 113 Ammonium compounds 113 Amyl alcohol 93-94 Amylases 32

; or Tables

Anaerobic respiration 66, 78, 114 Anthocyanins 22, 23, 24, 87, 204,

229 Anthoxanthins 24-25 Anthracnose 98, 103 Antibiotics 112 APPERT, NICHOLAS xxi Artificial colouring matters 185,

204, 228, 232 Ascorbic acid 24, 31, 81, 93, 140,

142, 148, 149-51, 159, 228, 246, 261, 265, 275

losses on cooking 150 losses on drying 161 losses on storage 149-50 use as an antioxidant 227

Ascorbic acid oxidase 31, 69, 150 Aseptic canning 243 Ash 28 (see also minerals) Asparagine 15 Aspartic acid 15 Aspergillus 99, 103, 106

A. niger 103 ATP (Adenosine triphosphate) 67,

68, 70, 73 Automatic can filling 233, 234

Bacteria 96-97,104,107,115 Bacterial soft rots 96, 98, 112 Bacterial watery rots 96, 98, 112 Batch cookers 241, 242, 243 Batch freezers 248, 249 Beef protein 147 Belt-trough drier 262 Benzoates 271-2, 274 Benzoic acid 13, 14, 272-4 Beri-beri Table 8, 155

296 Subject Index

Berry 40, 45 Betanin 23 Bin driers 264 Biological value of proteins 146,147 Bioquin 1, 110 Biotin 153 BIRDSEYE, CLARENCE xxii Bisulphites 113, 246, 259, 272 Bitter rot 98, 100, 103 Bitter tastes 33, 194 Black heart of potatoes 91 Black mould rot 99, 103 Black rot 98-99, 101, 104 Blanching 218-22

times 221 Blossom-end rot 102 Blue mould of citrus fruits 98, 100 Blue mould of pome fruits 98, 100,

101 Borax 111 Boron 28 Botrytis 98, 102, 106, 113, 186

B. cinerea 102, 113, 186 Botulism 115 Bremia lactucae 103 Brine flotation tests 199 Brine peeling 216 Brines for canning 232 Brines for density grading 218 Brining of vegetables 269-71 Brinometer 232, 270 Brix scale of sugar concentration 232 Bromelin 31 Brown heart of apples 91 Brown rot 98-99, 102, 110 Brown stain of oranges 90 Bulk (in the diet) 160 Byssochlamys 104

B. fulva 104 B. nivea 104

Cabinet driers 259, 261 Caifeic acid 27 Calcium 28-30, 48, 59, 142, 148-9,

154, 157-9, 220, 221, 232 Calcium hypochlorite 111

Calcium metabisulphite 268 Calories 140, 143-4 Calyx remains 211-13 Cambia 44 Can-closure 237, 238 Candied fruit 269,272 Can lacquers 230 Canned fruit 132-4, 138 Canned vegetables 132, 134, 138 Canning xxi, 229-45 Canning brines 232 Canning syrups 231 Can sizes 230-1 Captan 110 Carbohydrates 5-9, 66, 81-83 Carbonation 228 Carbon dioxide 113-14, 174-5

injury 78, 175, 180 a-Carotene 151 j3-Carotene 16-17, 82, 142, 148,

151-3 y-Carotene 151 Carotenoid pigments 17, 18, 19, 59,

60, 82, 87, 90, 220 Caryopsis 41 Catalase 31 Catechins 25-26 Cellophane 176, 248 Cell separation in relation to texture

197 Cellular inclusions 58-59, 60, 61 Cellulose 6, 48, 197 Cell wall 48,49 Ceratostomella 99, 104

a fimbriata 104 C. paradoxa 104

Cereal proteins 146, 147 Charcoal rot 99 Chemical preservatives 272-5 Chemical spoilage inhibitors 109-14 Chilling injury 88, 89 Chlorine 28, 111, 157 Chlorogenic acid 26, 27 Chlorophyllase 16, 32 Chlorophyllins 16 Chlorophylls 16,17,78,87,204,220,

252

Subject Index 297 Chloroplasts 16, 58 Chromoplasts 18, 58, 60 CIPC 93,94 Citramalic acid 12,13,14, 84 Citric acid 12, 13, 14, 68-69, 84,

268 Cladosporium 99, 103, 110 Cleaning and washing 208-11 Climacteric 72, 73, 75, 79, 82, 86 Climacteric fruits 73, 93 Clinching of can top 236 Clostridium botulinum 116, 117, 240,

241 "Cloud" in citrus juices 226 Coal gas 177 Cobalt 157 Codex Alimentarius 183, 186 Co-enzyme A 69 Cold storage xxi, 77, 80, 108, 114,

165, 167-77 Coliform bacteria 107 Collenchyma 42, 53, 54 Colletotrichum 98, 103 Colorimeters 191 Colour 182, 190-3 Colouring matters (artificial) 185,

204, 228, 232 Commercial sterility 240 Companion cells 57, 58 Comparators (colour) 191 Condensing enzyme 69 Containers for canning 227 Containers for dehydrated products

266 Containers for frozen products 248 Continuous cookers 239, 242, 243,

244 Continuous coolers 245 Continuous freezers 248, 249, 250 ControUed-atmosphere storage 78,

113-14, 174^7 Control of microbiological spoilage

105-14 Conveyor driers 262 Cooling 166

after canning 243, 245 Cookers 241-4

Copper 28, 157 Copper sulphate 112 Coring 211,213,214 Cork 10, 51 Cork cambium 51 Corrosion of tin-plate 229, 245 Cottony leak 99, 104 Cottony rot 98, 102 Coumaric acid 27 Coumarins 25, 27 Counterflow driers 259 CPA 92,93 Cream of tartar 83, 227 Cream style corn 236, 243 Crispness 196, 197 Crown rots 99 Cryptoxanthin 18, 151 Crystalline inclusions 59, 60, 61 Crystallinity of cellulose 48 Crystallized fruit 267, 269, 272 Cucurbitaceae 44 Cucurbitacins 33 Cuticle 10 ,51 ,52 ,97 Cutin 10,11,51 Cytochrome oxidases 69 Cytochromes 31

2 , 4 - D 92,93,110,112 Deaeration of juices 226 Defects 189 Deficiencies of ascorbic acid 149 Degreening of citrus fruits 178 Dehydration xxi, 217, 254, 258-67 Dehydroacetic acid 110,111 Dehydroascorbic acid 150 Dehydrogenases 31, 67, 68 Density as a measure of texture 199 Density grading 218 Deoiling of citrus juices 226 Desiccation of cold stored and frozen

products 172, 247, 248, 252 Design of storage rooms 172 Diaporthe 99

£>. atri 103 Diatomaceous earths (as filter aids for

juices) 227

298 Subject Index

Dicing 217 Dielectric heating 222, 267 Diethyl pyrocarbonic acid 273 Dill pickles 270 Diphenyl 112, 114,273 Diphenylamine 92 Diplodia 97, 98, 103

D. natalenis 103 Discoloration due to chilling injury

89,90 Dormancy 64 Double (can) seaming 237, 238,

239 Downy mildews 98, 99, 102, 110 DPN (Diphosphopyridine nucleotide)

67, 68, 69 Dried fruit 125, 137, 254-8, 274 Drum driers 266 Drum washers 209, 210 Drying (sun) xvii, 254, 255-8

effect on vitamins 161 Drying towers 259 Dry rot 98, 99, 104 Dry salting 271 Duo washer 208

Egg protein 147 Electron-generating machines 275 Embden Meyerhof Parnas pathway

of hexose degradation 67 Endocarp 40, 45 Enolase 67 Enzyme inactivation 218, 219, 220,

246, 261 Enzymes 30-32, 246, 271 Epicarp 40, 45 Epidermis 10, 42, 51, 52 Erwinia 97, 98, 105

E. carotivora 105 Essential amino-acids 141, 146, 147 Essential oils 33, 226 Ethoxyquin 92 Ethyl alcohol 67, 68, 91, 114, 176,

271 Ethylene 78, 87, 166, 177, 178 Exhausting 235-7

FAD (Flavin adenine dinucleotide) 68, 69

Falling-film evaporators 226 FAO amino acid reference pattern

146, 147 FAO indicator (of nutritional status

of a national diet) 141, 142 Fatty acids 14,159 F.D.A. (U.S. Food and Drug Ad-

ministration) 93, 115 Ferbam 110 Fermentation in brine 270 Fibre content 199 Fibres 54, 55 Fibrometer 198 Fibrousness 196, 197, 199 Ficin 31 Filling (can) 232-5 Filter presses 227 Filtration of juices 227 Firmness 196 Flame peeling 215 Flat-sour spoilage 245 Flavedo 45,53 Flavonoid substances 24-28, 33, 88 Flavonols 25,26 Flavonones 25, 26 Flavour 182, 187, 193-5, 205 Flavour constituents 33, 87, 88 Flotation washers 208, 209 Flouriness 196, 197 Fluidized-bed driers 264 Fluorine 157 Flush lacquering 230 Foam-mat drying 266 Folic acid 142, 153, 157

loss on cooking 157 Food and Drugs Act 115 Food poisoning 115-18 Formic acid 34 Free amino acids 145 Freeze-drying 263, 266, 267 Freezer burn 252 Freezing 245-54

rate of 250-2 Frozen fruit and vegetables 134,

135, 137, 138

Subject Index 299 Frozen juice concentrates 226, 248,

250 Fructose 5, 6, 82 Fructose 1:6 diphosphate 6, 7 Fruit xiii, xiv, xxi, xxii, xxiii, 82-88,

122, 124-7, 149, 155, Table 10 Fruit for manufacturing purposes

274 Fruit juices 135, 136, 137, 138, 222-

8, 274 Fruit nectars 224 Fruit salad (canned) 133 Fumarase 69 Fumarie acid 14, 14, 69 Fumigants 109, 113 Fumigation 109, 113, 256 Fungicides 109, 110, 111, 112

producing flavour taints 186 Fusarium 98, 102, 106, 124

F. oxysporum var. cubense 102, 124

Galacturonic acid 8, 84 Gallic acid 27 Garlic xviii, 34, 173 Gas-packing of dehydrated vegetables

265 Gas storage see Controlled-atmos-

phere storage GDP (Guanosine diphosphate) 69 Geraniol 33 Germicidal washes sprays and dips

108-9 Ginaca machine 213, 226 Glaced fruits 267, 269, 272 Gloeosporìum 98, 100, 103, 106, 109,

110,114 G. album 103 G. fructigenum 103

Gloss 182, 190 Glucose 5, 6, 67, 70 Glutamic acid 15 Glutamine 15, 34 Glyceric acid 13, 14 Glycolic acid 13, 14 Glyoxylate shunt 70 Glyoxylic acid 13, 14, 70

Green mould rot 99, 103 Grey mould rot 98, 102, 110 Grittiness 196, 197, 199 GTP (Guanosine triphosphate) 69 Gummosis 99 Gustation 33, 193

Hand filling 233 Hand-pack fillers 233, 234, 247 Head-space vacuum 235 Heat exchangers 224, 243

exhausting 236 of respiration 172 processing 238, 239, 240-3, 244 sterilization 240

Hemicelluloses 6, 50, 82 Herbicides producing flavour taints

186 Hesperetin 25, 26 Hesperidium 40, 45 Hexamine 111 Hexokinase 67-68 High temperature short time pro-

cessing 243 Home-canned fruit and vegetables

116 Homogenization 224, 227 Honey 268 Hot-air dehydration 258-66 Humidity 79, 92, 166, 172, 174, 260 Hunter Color and Color-difference

Meter 191, 192 Husks 211,212 Hydraulic hot-water blanching 222 Hydrogen swells 255 Hydrostatic pressure cookers 243,

244

Ice crystal size in frozen products 250

Impregnated wrapping papers and box liners 109, 112, 166

Immersion freezing 250 In-can blanching 222 Incubation tests 245

300 Subject Index

Infection (human) 116 Infra-red blanching 222 In-package desiccants 266 Insecticides producing flavour taints

185-6 Intercellular spaces 46 Intoxication 116 Inulase 32 Inversion of sucrose 268 Invert sugar 231 Iodine 112, 157 Ionizing radiations xxii, 275 Iron 28,142,148,154,157,159-60,

207, 222, 232, 261 Iron-deficiency anaemias 159 Isocitric acid 12, 13, 14, 69, 70 Isocitric dehydrogenase 69 Isoleucine 146, 147 Isothiocyanates 34

Jam 274 manufacture xxi, 267-8 optimum pH 269 pans 268

Juice concentrates 226, 250 Juice extraction 224, 225 Juices xxi, xxii, 135-6, 138, 193,

222-8, 236, 243, 253, 266, 272, 274

Juiciness 196, 197

a-Ketoglutaric acid 14, 69 Kilns 259 Kreb's tricarboxylic acid cycle 67-

68,69 Kwashiorkor Table 8

Lacquered cans 24, 230 Lactic acid 13, 14, 197, 270

bacteria 107, 270-2 fermentations 270-2

Lactobacillus L. brevL· 271 L. plantarum 271

Lactoisocitric acid 13 Latent heat of fusion of ice 257 Laxative effect of fruit and vegetables

160 Leaching during blanching 221 Leaf structure 43, 44 Leathery rots 98, 102, 104 Legislation governing the use of

preservatives 272 Leguminous seeds 10, 11-12, 73,

75, 146, 147, 150, 153, 155-6, 159, 199

Lenticels 51, 97 Leucine 147 Leuco-anthocyanins 9, 24-25, 26,

27, 196, 205 Leucocyanidin 26 Leuconostoc mesenteroides 271 Leucoplasti 59 Lignin 7, 9, 55-56, 80-81, 196 Limitation of infection 105-7 Limonin 33 Lipoxidase 32, 153, 252 Liquid glucose (corn syrup) 231 Liquid nitrogen (for immersion freez-

ing) 250 Losses of vitamins

on blanching 221 on canning 161 on cooking 150 on drying 161 on freezing 162 on jam-making 162

Low-acid foods 241 Lycopene 17, 18, 87, 152 Lye 257,270

peeling 215-16 Lypolytic enzymes 32 Lysine 146, 147

Magnesium 28-30, 157, 232 Malate effect (on respiration) 74 Maleic hydrazide 93-94 Malic acid 12,13,14, 68, 69, 70-71,

84 Malic acid dehydrogenase 69

Subject Index 301

Malnutrition 141 Manganese 28 Marketing 177-80 Marmalade 269 Maturometer 198 Mealiness 196-7 Mechanical damage 97, 107, 150,

165, 251 Medium-acid foods 241 Membranous stain of lemons 90 MENA 93-94 Mercaptans 34 Mesocarp 40, 45 Methionine 146, 147 Methyl bromide 257 s-Methyl-L-cysteine sulphoxide 34 Methyl />-hydroxybenzoate 271-2,

274 Micelles 48 Microbiology 95-118 Micrococci 107 Microfibrils. 48, 49, 59 Micro-organisms

build-up of numbers after harvest 108

on harvested produce 107 Middle lamella 48, 197 Milk protein 147 Mineral elements 28-30, 140, 142,

148, 157-60 Ministry of Agriculture, Fisheries and

Food 115 Mint flavouring for canned peas 232 Mitochondria 65 Moisture content 199, 254, 257-61,

265 Molybdenum 28, 157 Mucic acid 13, 14 Mucor 99, 102 Multinet structure (of cell walls) 50 Munsell Disc Colorimeter 191 Mustard oils 34

NAA 92 Naringenin 25-26 Naringin 25

Niacin 153-4, 156 Nickel 157 Nisin 273 Nitrogen 28, 149 Nitrogenous constituents of low mole-

cular weight 14-15, Table 3, 86-87

Nitrogen trichloride 113 Non-enzymic browning 221, 259,

261,265 Non-protein nitrogen 14, 15, Table

3, 86 Nonyl alcohol 93-94 Nutrition 140-64 Nutritional anaemias Table 8 Nutritional deficiency diseases 142,

Table 8 Nystatin 273

Objective measurement of colour 191-3

Objective measurement of texture 198

Off-flavours in frozen products 3 1 -32

Oil-impregnated wraps 92,112 Oil-peeling 215 Oleic acid 257 Oleuropein 33 Olfaction 193 Olfactory cilia 194 Olfactory epithelium 194 Olive oil 257 Oospora 99, 104

O. citri-aurantii 104 O.pustulans 104

Open-top can 229-31 Optimal storage temperatures 169,

Fig. 23 Organic acids 11-13, 66, 83-84 Organic sulphides 34 Orthophenylphenate (sodium) 111,

114,279 Orthophenylphenol 111-12, 114,

273 Osteomalacia 158

302 Subject Index

Overdraught driers 264 Oxalate crystals 61 Oxalic acid 11, 13, 14, 83, 158 Oxalo-acetic acid 13, 14, 69 Oxalo-succinic acid 69 Oxalo-succinic decarboxylase 69 Oxidation of anthocyanins 252 Oxidation of ascorbic acid 150 Oxidation of carotenoid pigments

153, 252, 265 Oxidative deterioration of frozen

products 246-7

Panama disease 102, 124 Pantothenic acid 153 Paraffin oil 257 Parallel-flow driers 259 Parenchyma 42, 46-50, 196 Pasteurization of

citrus juices 226, 240 fruit nectars 224 fruit pulps 227 grape juice 227 pickles 271 pineapple juice 227

Pathogenic micro-organisms 116, 118

Pectic acid 8 Pectic materials 6-7, 8, 48, 84, 85,

86, 226, 268, 275 Pectin esterase 31, 84, 86 Pectinic acid 8, 85 Pectolytic enzymes 97, 226-7,

271 Peeling 213-16 Peels 211 Pellagra Table 8 Pénicillium 97-98, 100-1, 111-12

P. digitatum 98, 100 P. expansum 97-98, 100, 101 P. italicum 97-98, 100

Pentose phosphate cycle 70 Pepo 40 Perace tic acid 114 Pericarp 38 Periderm 52 see also Cork

Permitted preservatives, lists of 114 Peronospora 99, 103 Peroxidase 31, 220, 252 Pesticide Residue Ammendment,

Food, Drug and Cosmetic Act 115

Pesticide residue tolerances 115 Pesticide Safety Precautions Scheme

115 Peteca of lemons 90 Petiole, structure of 43, 44 pH xx, 22, 29, 96, 116, 220, 221,

240,261,268-9,272 optimum for jam 269

Phaeophytins 16, 204, 220, 221, 261 Phellogen see Cork cambium Phenoloxidases 24, 31, 69, 90, 218 Phenylalanine 147 Phloem 42 ,56 ,57 Phloionolic acid 11 Phoma 99, 104

P. betae 104 P. destructives 104 P.foveata 104

Phomopsis 99, 103 P. citri 103

Phosphates 221, 261 Phosphoglyceromutase 67 Phosphohexokinase 67-68 Phosphorus 28-30, 148, 157 Physalospora 98, 103

P. rhodine 103 Physiological injury 76, 88-92, 113 Physiology 63-94 Phytic acid 29, 159 Phy toene 17 Phytofluene 17 Phytophthera infestons xix, 98, 100,

102, 106 Pickled vegetables 196, 272, 274 Pigments 16-24, 87 Pink mould rot 99, 102, 103 Pits 49 ,54 ,56 ,57 Pitting 213 Plant diseases 95 Plasmodesmata 48 Plastids 58

Subject Index 303

Plate-type freezers 248, 249 Pliofilm 176,248 Podding 211 Podosphaera 99, 104

P. leucotricha 104 Pods 211 Polyamides 248 Polyesters 248 Polyethylene 176, 248 Polygalacturonase 31, 84, 86 Polypropylene 248 Polysaccharides 6, 48 Post-canning operations 245 Potassium 28, 29, 30, 157 Potato blight xix, 96, 100 Powdery mildews 99, 104 Precooling 172 Prepackaging 180 Preparatory treatments 207-28 Prepeeling 202 Preservation with salt 269-72 Preservation with sugar 267-9 Preservative in Food Regulations

273-4 Pressure cookers 242, 243, 244 Pressure steam peelers 215 Prevacuum syrupers 235 Pricking of peas for dehydration 261 Primary odour types 195 Process calculations 241 Processed peas 133, 137, 207, 243 Processed products

production of 132-5 trade in 132-5

Processing 202-77 Processing times

for fruit packs 241 for vegetable packs 241

Production and trade 121-39 Protective tissues 51-53 Proteins 9,48,86,140-1,144,147,159 Protein synthesis 73, 86 Protopectin 7, 8, 84, 85 Provitamin A—see j8-Carotene Pseudomonas 97, 98, 104 Psychrophilic spoilage organisms

109, 245

Puff drying 264 Puffing gun 264 L-Pyrrolidone carboxylic acid 34 Pyruvic acid 13, 14, 67-69, 71 Pythium 99, 104

Q 1 0 for respiration 76-77 Quality 182-200

attributes 189 grades for processed products 186 grading 183-4, 186-7, 193, 217,

245 of fruit and vegetables for pro-

cessing 185 scoring 187 standards 183-4, 269 testing 245

Quercetin 25, 26 Quick freezing xxii, 251 Quinic acid 11-12, 13, 14, 27, 84

Rack and cloth presses (for juice extraction) 224, 225, 227

Radioisotopes 275 Raphides 61 Rates of respiration 71, 72, 76-79,

93 effect of ethylene 78 effect of 0 2 and C 0 2 concentra-

tions 76-77 effect of temperature 76

Receptacle 42 Recommended storage temperatures

for fresh produce Fig. 23, 170, 171

frozen produce 252 Reconstitution of dehydrated pro-

ducts 220 "Red blotch" of lemons 90 Reducing sugars 5-6, 81-83 Refractometer 195,268 Refrigerated transport 109, 167,

168 Relative humidity 79,92,166,172-4 Removal of unwanted parts 211-16

304 Subject Index

Respiration 63, 66-79, 92 Respiratory patterns 73-74, 75, 76 Respiratory quotient 74, 174 Rhizoctonia 99, 104 Rhizopus 98, 102, 106

R. nigricans 102 Riboflavin 153, Table 8,155-6

losses on cooking 156 Rickets Table 8, 158 Ripening of fruit 82-88, 114 Ripening rooms 178, 179 Rod washers 209, 210 Root structure 43, 44

Safety of commercially canned foods 117

Salmonella 117 S. typhimurium 117

Salometer (Salinometer) 232, 270 Salt 194 Saltiness 194 Salt stock 270 Saran 248 Savings in bulk and weight by

dehydration 255 Scald of apples 89, 90, 92 Scald of oranges 90 Scalding see Blanching Sclereids 52, 54, 55, 197 Sclerenchyma 7, 42, 54, 55 Sclerotinia 98, 102, 106

S. fuckeliana 102 S. sclerotiorum 102

Score sheets 187 Scrubbers (CO,) 174 Scurvy xx, Table 8 Seitz niters for fruit juices 227 Selection of raw material for proces-

sing 203-7 Senescence 65 Sensitivity of olfactory receptors 195 Shear press for texture measurement

198-9 Shikimic acid 11-12, 13, 14, 84 Sieve elements 57, 58 Sieve plates 57, 58

Silicon 28 Size and shape 182, 189, 204 Size graders 217, 219 Size grading 189, 217 Sizing screens 217 Skins (structure of) 51, 52 Skin spot 99 Slicing 217 Snibbing 212-13 Snipping 212-13 Soaking 207-8 Soapiness of texture 197 Sodium 28, 157 Sodium carbonate 221, 261 Sodium dehydroacetate 111 Sodium hypochlorite 111 Sodium metabisulphite 261, 268 Sodium o-phenylphenate 111, 114 Sodium sulphite 261, 272 Soft rots 98-99, 102 Solid C 0 2 108-9, 176 Solid packs (canned) 231 Sorbicacid 112,114-15,273 Sorosis 41 Sourness 194 Sour rot 99, 104 Spectrophotometry 191 Sphaerotheca 99, 104

S. mors uvae 104 Spoilage 95-105 Spray driers 266 Sprouting 76 Sprout inhibition 276-7 Sprout inhibitors 93 Stage of maturity for processing

206-7 Stalk rots 102-3 Stalks 211-13 Staphylococcal food poisoning 117 Starch 5, 59, 82

crystallinity of 59 grains 52, 59, 60

Starch/sugar balance 81 Steam blanching 220-2, 223, 258 Steam-flow seaming 236 Stem-end blackening 27 Stem-end rots 97-99, 103, 112

Subject Index 305

Stemming 212-13 Stem structure 42, 43, 44 Stewing of fruit, effect on vitamins

156 Stornata 51, 52, 97 Stone cells see Sclereids Storage 165-80 Storage atmosphere, composition of

166 Storage life of frozen products 252,

253 Storage rooms, air circulation in 172 Strengths of canning syrups 231,

232 Strigging 212-13 Stringiness 197 String of runner bean 54 Structure 38-61 Stump-rooted varieties of root vege-

table 204 Suberin 10-11,51 Succinic acid 13, 14, 27, 69 Succinyl co-enzyme A 69 Succulometer 198 Sucrose 5, 6, 81, 82, 83, 231, 232,

246, 267-9, 270 Sugars 5, 6, 81-83, 194-5, 265 Sugar syrups 224, 228, 231, 246

for canning 231-2 Suitability of varieties for processing

204-6 Sulphites 221,259 Sulphur 28,110,157,230 Sulphur dioxide 113, 114, 220, 261,

265, 271-2, 274 Sulphuring of fruit before drying

256, 257, 259 Sulphur-resistant lacquers 230 Sulphur staining 230 Sultanas 125, 137, 256, 257 Sun-drying of fruit 255-8 Supporting tissues 53-55 Surface pitting 90 Sweat boxes 257 Syconium 41 Synthetic growth regulating sub-

stances 92-94

2, 4, 5-T 92, 93, 110, 112 Tainting 110, 112, 177, 185-6 Tannins 25, 27 Tartaric acid 11-12, 13, 14, 83 Tartrates 227 Taste (gustation) 33, 193 Taste buds 194 Tasting panels 186-7, 195 TGNB 93-94 Temperature 166 Temperatures

during long-term storage 167, Fig. 23

during transport 167, Table 12 of retail display cabinets for frozen

foods 254 Tenderometer 192, 198, 206 Terminal oxidase systems 68, 69 Terpenoid hydrocarbons 33 Testa (structure of pea) 52 Texture 45-46, 55-56, 81, 84-86,

182, 196-9, 205, 251, 270, 275 Texturemeter 198 Thermo-peeling 215 Thermophilic spore forming bacteria

240 Thiamine 153-5

losses on cooking 155 Thiourea 111 Thiram 110 Threonine 147 Through-draught driers 263, 264 Toxicity of fungicides 114-15 Toxins, accumulation of at normal

temperatures 191-2 TPN (Triphosphopyridine nucleo-

tide) 68, 69 Trade (patterns of) 129-32, 133-4 Transketolase 70 Transpiration 4, 51, 79, 80 Transport 165-71 Trichoderma 99, 101, 103

T. viride 103 Trichomes 51 Trichothecium 99, 103

T. roseum 103 Trimming 216

306 Subject Index

Tristimulus system of colour measure-ment 191

Tryptophan 146, 147, 156 Tumbling freeze-drier 267 Tunnel driers 259, 261 Turbulent-bed driers 263, 264 Turgor 3, 196, 275 Two-stage tunnel driers 259, 261 Tyrosine 27

U.K. exports 136-9 U.K. imports 136-9 Ultrasonic radiations, use in peeling

216 Umbelliferae 105 Unfreezable water in fruit and

vegetables 251 Uronic acids 5, 13 Ursolic acid 10 U.S. Quality standards 184, 185

Vacuum concentration of citrus juices 226

Vacuum packing of dehydrated products 265

Vacuum seaming 236 Valine 147 Vascular tissues 56 Vegetable dehydration 138, 260-7 Vegetable proteins 147 Vegetables xiv, xxi, xxii, xxiii, 96,

123, 127-9, 131-2 canned 132-3 pickled 196, 272, 274

Ventilation of stores 174,178 Vessel elements 56, 57 Vinegar, 270 Viners 210,212 Vines 211 Vining 212

Visual standards for quality grading 193

Vitamin A 151 (see also j3-Carotene) deficiency of Table 8

Vitamins B 140, 142, 148, 153-7 Vitamin Bx see Thiamine Vitamin B6 154 Vitamin G see Ascorbic acid Vitamins 140, 142, 148, 149-57 Volatile constituents 87

changes during the ripening of fruit 91

Washing 208-11 Water 3-4 Water activity 254-68 Water blanching 219, 220, 221 Water content 254, 257-61, 265 Water loss 51,63 Watery soft rot 98, 102 Wavelength of light in relation to

colour 190 Wax 51 Waxiness of texture 197 Wheat protein 147 Wilting 4, 63, 166, 173 Winnowing 208 Wooliness of texture in peaches 90 Wound periderm 93

Xanthophylls 16-17, 18 Xylem 7, 42, 56, 57

Zinc 28, 157 Zinc oxide 230 Zineb 110 Ziram 110

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