Sensory quality and chemical composition of culinary preparations ...
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SENSORY QUALITY AND CHEMICAL COMPOSITION OF CULINARY PREPARATIONS OF ROOT CROPS PhD thesis by Vibe Bach October 2012 Department of Food Science Aarhus University Research Centre Aarslev Faculty of Science and Technology Kirstinebjergvej 10 5792 Aarslev Denmark
Transcript of Sensory quality and chemical composition of culinary preparations ...
SENSORY QUALITY AND CHEMICAL COMPOSITION
OF CULINARY PREPARATIONS OF ROOT CROPS
PhD thesis by Vibe Bach
October 2012 Department of Food Science Aarhus University Research Centre Aarslev Faculty of Science and Technology Kirstinebjergvej 10 5792 Aarslev Denmark
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Main supervisor
Associate professor Merete Edelenbos
Department of Food Science, Aarhus University
Co-supervisors
Associate professor Ulla Kidmose
Department of Food Science, Aarhus University
Senior scientist Erik Larsen
Department of Food Science, Aarhus University
Assessment Committee
Senior scientist Marianne G. Bertelsen (chairman)
Department of Food Science, Aarhus Universtity
Head of Department, Professor Lars Porskjær Christensen
Institute of Chemical Engineering, Biotechnology and Environmental Technology,
University of Southern Denmark
Senior Researcher Randi Seljåsen
Bioforsk, Norwegian Institute for Agricultural and Environmental Research
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PREFACE
The work described in this Phd thesis was performed at the Department of Food
Science, Aarhus University in the research group Food, Metabolomics and Sensory Science
from October 2009 to September 2012. The PhD-project was part of the Gourmet roots
project, financed by the Danish Minestry of Food, Agriculture and Fisheries in the Food
Research Programme 2008 (RUFF, project No. 3304-FVFP-08-K-04-01).
First of all, I would like to acknowledge my supervisors Merete Edelenbos, Ulla
Kidmose and Erik Larsen. I would like to thank Merete Edelenbos for her support and
guidance throughout this PhD, and for helping me in the process of identifying my own
scientific interests. My gratitude goes to Ulla Kidmose for assisting me in all sensory
related questions I might have had during these three years, and Erik Larsen for the same
in regards to natural product chemistry.
The partners of the Gourmet root project (www.gourmetroots.dk) are thanked for
practical support and the contribution of raw material of Jerusalem artichoke tubers,
beetroots and carrots for laboratory studies.
A very special thanks goes to Birgitte Foged for her excellent technical help in regards
to the laboratory work performed. But most of all I would like to thank Birgitte for being a
great “buddy”, not only in the beginning of my time in Årslev, but also during the entire
duration of this PhD project.
I would also like to thank Caroline Nebel and Camilla Bjerg Kristensen for
introducing me to SPME analysis, and for helping me with my experiments when I was in
Foulum, and I would like to thank Jens M. Madsen for taking some great pictures.
A special thanks goes to my reviewers Sidsel Jensen and Sandie Mejer Møller for
constructive criticism, encouraging comments and helpful advice, and for quick feedback
when it was needed. I would also like to thank Aase Karin Sørensen for thorough
proofreading of this thesis.
I would like to thank all of my colleagues in the Department of Food Science for
making Årslev a very cheerful and pleasant workplace. A special thanks goes to every one
of you, who got up early in the morning and helped me cutting Jerusalem artichoke tubers.
Finally, I would like to send my love and gratitude to my family and friends, who have
supported me through both easy and tough times in this three-year period. Especially, I
thank Anders for scientific challenging discussions and for his enormous help the last
months of this PhD.
Vibe Bach, Odense, September 2012
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ABSTRACT
Root crops exist in many different varieties, colours and shapes, but consumers are
unaware of how to handle and prepare these varieties. This project has focused on the root
crops Jerusalem artichoke tubers and beetroots. Both root crops are available in many
varieties with many different qualities, and both are underutilised among Danish
consumers. Increased knowledge of the qualities of the different varieties can be used to
guide consumers and industry in the choice of the right product for their individual needs.
The main aim of the present PhD project was to provide a chemical approach to
understand the sensory variation in root crops as an effect of raw material diversity and
culinary preparation. This included an investigation of the aroma, flavour, taste, texture
and colour of root crops. These parameters were analysed by sensory and instrumental
analyses, by analysis of the chemical composition and by a consumer study on the
appropriateness of root crops for culinary preparation.
Overall there were only few differences in sensory quality between varieties of
Jerusalem artichoke tubers and beetroots regardless of culinary preparation. When
differences were found, they were related to texture and taste. Larger differences were
found for raw than for boiled, baked and pan-fried root crops. The appropriateness of
Jerusalem artichoke tubers and beetroots in all culinary preparations were related to
crispness, juiciness, sweetness and colour intensity.
The volatile profiles of raw, boiled and baked Jerusalem artichoke tubers and
beetroots consisted mainly of terpenes, but lipid oxidation and Maillard products were also
produced during heat treatment. The sweetness and carbohydrate content of Jerusalem
artichoke tubers were determined by the maturity of the tuber at the time of harvest.
Beetroots were evaluated as sweet and bitter, and large differences between raw varieties
were found in the sensory attribute sweetness. These differences were not reflected in the
content of sugars and may be influenced by the content of bitter compounds. Jerusalem
artichoke tubers softened during heat treatment and in some cases developed mealy
characteristics. The inulin content of Jerusalem artichoke tubers probably affected the
texture development of the tubers during boiling and baking, as inulin was thermally
degraded by heat treatment. In Jerusalem artichoke tubers, enzymatic browning of raw
tubers and after-cooking darkening of boiled tubers were identified and associated with
low appropriateness. However, it was not possible to identify the chemical background for
these colour changes.
The results of this thesis clearly show that texture, taste and colour are the most
important parameters of root crop quality. This novel information can be used by
producers and retailers when growing and promoting root crops, and by consumers when
they are preparing and handling root crops in the kitchen.
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RESUMÉ
Rodfrugter findes i mange forskellige sorter, farver og former, dog er forbrugerne
uvidende om hvordan disse nye varianter skal tilberedes. Fokus i dette projekt har været
på rodfrugterne jordskok og rødbede. Begge findes i mange sorter med forskellige
kvaliteter, som ikke udnyttes til fulde blandt danske forbrugere. Øget kendskab til
kvaliteterne af de enkelte sorter, kan bruges til at vejlede forbrugere og producenter i
valget af det rigtige produkt til deres behov.
Hovedformålet med dette Ph.d. projekt var at udvikle en kemisk tilgang til
forståelsen af sensorisk variation og kulinarisk tilberedning. Dette inkluderede en
undersøgelse af aroma, flavour, smag, tekstur og farve af rodfrugter. Disse parametre blev
analyseret ved sensoriske og instrumentelle analyser, ved analyser af kemisk
sammensætning og ved forbrugeranalyse af egnetheden af rodfrugter i kulinariske
tilberedninger.
Der var gennemgående kun få forskelle på sensorisk kvalitet mellem sorterne af
jordskokker og rødbeder, uanset hvordan de var tilberedt. Når forskelle fandtes, var de
relateret til tekstur og smag. Der blev fundet større forskelle mellem sorter i de rå end i de
kogte, bagte og stegte rodfrugter. Egnetheden af jordskokker og rødbeder i alle
tilberedninger var relateret til sprødhed, saftighed, sødhed og farveintensitet.
Indholdet af flygtige forbindelser i rå, kogte og bagte jordskokker og rødbeder bestod
hovedsageligt af terpener, men oxidationsprodukter af lipider og Maillard produkter blev
dannet ved varmebehandling. Sødheden og kulhydratindholdet i jordskokker afhang af
hvor modenheden på høsttidspunktet. Rødbeder blev bedømt til at være både søde og
bitre, og der blev fundet stor variation i den sensorisk egenskab sødhed, mellem de
forskellige rå sorter. Disse variationer kunne dog ikke forklares af indholdet af
sukkerstoffer og kan være påvirket af indholdet af bitterstoffer. Jordskokker blev bløde
under varmebehandling, og i nogle tilfælde, udviklede de en melet konsistens.
Udviklingen af teksturen i jordskokker ved kogning og bagning er sandsynligvis
påvirket af indholdet af inulin, da inulin blev termisk nedbrudt ved varmebehandling.
Enzymatisk brunfarvning af rå jordskokker, og mørkfarvning efter tilberedning af kogte,
blev identificeret og associeret med lav egnethed. Dog var det ikke muligt at identificere
det kemiske grundlag for disse misfarvninger. Resultaterne i denne afhandling
demonstrerer tydeligt at tekstur, smag og farve er de vigtigste parametre for kvaliteten af
rodfrugter. Disse nye informationer kan bruges af producenter og forhandlere når
rodfrugter skal dyrkes eller promoveres, og af forbrugere når rodfrugter skal håndteres og
tilberedes i køkkenet.
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LIST OF PUBLICATIONS
Paper 1 Effects of harvest time and variety on sensory quality and chemical
composition of Jerusalem artichoke (Helianthus tuberosus L.) tubers.
Vibe Bach, Ulla Kidmose, Gitte K. Bjørn and Merete Edelenbos.
Food Chemistry (2012) 133, 82-89.
Paper 2 Metabolomics reveals drastic compositional changes during overwintering
of Jerusalem artichoke (Helianthus tuberosus) tubers.
Morten R. Clausen, Vibe Bach, Merete Edelenbos and Hanne C. Bertram.
Journal of Agricultural and Food Chemistry (2012) 60, 9495-9501.
Paper 3 The effect of culinary preparation on chemical composition and sensory
quality of Jerusalem artichoke tubers (Helianthus tuberosus L.).
Vibe Bach, Sidsel Jensen, Ulla Kidmose, Jørn N. Sørensen and Merete
Edelenbos.
LWT – Food Science and Technology, submitted September 2012.
Paper 4 Characterization of enzymatic browning and after-cooking darkening of
Jerusalem artichoke (Helianthus tuberosus L.) tubers.
Vibe Bach, Sidsel Jensen, Morten R. Clausen and Merete Edelenbos.
Food Chemistry, submitted September 2012.
Paper 5 Sensory quality and appropriateness of raw and boiled Jerusalem
artichoke tubers (Helianthus tuberosus L.).
Vibe Bach, Ulla Kidmose, Anette K. Thybo and Merete Edelenbos.
Journal of the Science of Food and Agriculture, accepted, DOI
10.1003/jsfa.5878.
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ABBREVIATIONS
CAR/PDMS Carboxen/polydimethylsiloxane DH Dynamic headspace DM Dry matter DMAPP Dimethylallyl diphosphate FAO Food and Agriculture Organization of the United Nations FC Folin-Ciocalteu FOS Fructooligosaccharides FW Fresh weight GC Gas chromatography GC-MS GC-mass spectrometry GC-O GC-Olfactometry HPAEC High performance anion exchange chromatography HPLC High performance liquid chromatography HSPME Headspace SPME IPP Isopentenyl diphosphate LRI Linear retention index MEP Methylerythtritol phosphate NMR Nuclear magnetic resonance NNF New Nordic food PAL Phenylalanine ammonia-lyase PCA Principal component analysis PLS Partial least square POD Peroxidase PPO Polyphenol oxidase QDA Quantitative descriptive analysis Rt Retention time SPME Solid phase micro extraction TPA Texture profile analysis
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TABLE OF CONTENTS
Preface ........................................................................................................................... iii Abstract........................................................................................................................... iv
Resumé ............................................................................................................................ v
List of publications ......................................................................................................... vi Abbreviations ................................................................................................................ vii 1. Introduction ................................................................................................................. 1
2. Root crop ..................................................................................................................... 5
2.1 The plant root ......................................................................................................... 5
2.2 Root crop production............................................................................................ 6
2.3 Constituents in root crop ..................................................................................... 10
2.4 Raw material diversity ......................................................................................... 11
3. Sensory quality .......................................................................................................... 12
3.1 Perception ............................................................................................................ 12
3.2 Evaluating sensory quality .................................................................................. 13
3.3 Descriptive sensory analysis of root crops .......................................................... 14
3.4 Consumer evaluations of root crops .................................................................... 17
4. Aroma and flavour .................................................................................................... 22
4.1 Aroma and flavour compounds .......................................................................... 22
4.2 Isolation of volatile compounds ......................................................................... 24
4.3. relating volatile compounds and sensory analysis ............................................ 29
4.4 Volatile compounds in culinary preparations of root crops .............................. 30
5. Taste .......................................................................................................................... 36
5.1 Taste Compounds ................................................................................................ 36
5.1 Taste compounds in root crops ........................................................................... 36
6. Texture ...................................................................................................................... 42
6.1 Texture properties ............................................................................................... 42
6.2 Measuring root crop texture ............................................................................... 43
6.3 Texture of culinary prepared root crops ............................................................ 44
7. Colour ........................................................................................................................ 47
7.1 Pigments in root crops ......................................................................................... 47
7.2 Enzymatic browning ........................................................................................... 48
7.3 After-cooking darkening ..................................................................................... 50
7.4 Discolouration of Jerusalem artichoke tubers .................................................... 51
8. Conclusions and perspectives ................................................................................... 55
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1. INTRODUCTION
Root crops have several positive qualities, which make them ideal constituents of a
healthy diet, but their potential, as a food source is not fully exploited. In this thesis root
crops are defined as any underground part of a plant e.g. root or tuber, which can be eaten
cooked as part of a main meal. There are several advantages in increasing the intake of root
crops in the Danish population. First of all, root crops have great nutritional and health
beneficial qualities such as high fibre and mineral content, and they are rich sources of
secondary metabolites with possible biological activities (Saxholt et al. 2008; Brandt et al.
2004). Secondly, many root crops are suitable for growth in the temperate climate of
Northern Europe, and when used as a part of a locally produced diet, root crops can reduce
the carbon footprint. Furthermore, an increased intake will be an economical advantage
for the local producers, while root crops remains a cheap vegetable product for the
consumers.
Carrot and potatoes are the most prevalent root crops eaten in Denmark. The average
Danish consumption of root crops is 882 g/week of which 658 g/week are potatoes (Meyer
et al. 2010). Root crops have been an important part of the Northern diet for centuries, as
they could be eaten fresh in the summer and autumn, or stored and eaten over the winter
to provide nutrients and contribute to a varied diet all year (Haastrup 2003). During the
1970’s meat became the dominant part of the dinner, and although potatoes were still
important, they were gradually partially replaced by rice and pasta. During this period,
lettuces, tomatoes, cucumbers and other vegetables with high water and low fibre content,
which became available in the supermarkets all year round (Haastrup 2003), replaced
coarse vegetables like cabbage and root crops. In the last decade, the consumption of the
coarse vegetables including root crops has increased, with a simultaneous small decline in
the use of salad-vegetables (Fagt et al. 2008). This increase can probably be ascribed to the
focus on the concept New Nordic Food (NNF), which was introduced in 2004 by a group of
Danish chefs (Meyer et al. 2010). Root crops fit well in the context of NNF as the manifest
focuses on the use of products, which are suited for growth in the Nordic climate, and
which reflect the changing seasons. NNF recommends that the consumption of root crops
is increased to 1050 g/week for root vegetables and to 980 g/week for potatoes (Meyer et
al. 2010).
A large genetic diversity is found between and within the individual species of root
crops expressed as differences in colour, shape, aroma, taste, flavour and texture. The
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influence of this variation on the eating quality is well understood in raw carrots
(Kreutzmann et al. 2008b; Kreutzmann et al. 2007; Szymczak et al. 2007; Surles et al.
2004; Alasalvar et al. 2001) and in raw and cooked potatoes (Seefeldt et al. 2011a), but
information on other Nordic root crops is lacking. The culinary possibilities arising from
product diversity is not exploited to its full extent. Consumers do often not know how to
handle unfamiliar and culinary diverse root crops, and the liking or preference of well-
known products is often higher than for new unknown products (Szymczak et al. 2007;
Surles et al. 2004; Sangketkit et al. 2000; Busch et al. 2000). Appearance is the key
attractant for consumers to buy novel products, but re-purchase is determined by the
actual experience of aroma, flavour, taste and texture (Barrett et al. 2010). The relationship
between expected and experienced quality is considered to be deciding for consumer
satisfaction, and the probability of repeated purchase (Espejel et al. 2008; Oliver 1993,
1980). An understanding of the sensory quality and of the chemical composition behind
quality differences, can be used to guide consumers and industry to choose the most
suitable raw material for a specific culinary preparation. This will increase the probability
of consumer satisfaction and eventually lead to a higher consumption of root crops.
Sensory analysis is the best descriptor of food quality perception (Martens & Martens
2001), but chemical analysis can provide an understanding of the underlying factors, and
an explanation for the sensory observation.
The work in this PhD project has focused on Jerusalem artichoke tubers and
beetroots. These two root crops have a large potential to expand their utilization in
Denmark. The Jerusalem artichoke tubers have increased in popularity within the last
decade, but the consumption is still limited outside restaurant settings. Knowledge on the
eating quality of Jerusalem artichoke tubers is sparse. Denmark is maintaining a gene
bank of 18 Jerusalem artichoke varieties placed at Aarhus University Aarslev, which
constitutes a solid basis for an investigation of their quality for culinary preparations.
Beetroots are traditionally eaten pickled, but have great potential for use in a range of
culinary preparations. Besides this, mainly the dark red varieties are employed at the
present, although beetroots exists in a large variation of colours, sizes and shapes, which
can add to the diversity of the Danish diet.
The overall objective of this PhD project is to investigate the aroma, flavour, taste,
texture and colour of root crops in relation to harvest time, variety and culinary
preparation. The quality parameters were chosen on the basis of sensory evaluation and
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investigated from a chemical perspective. A diagram of the flow of the work performed in
this PhD project is seen in Figure 1.
The hypothesis is that an understanding of the chemical and sensory mechanism for
quality development of culinary preparation of root crops, will provide new knowledge on
food quality and diversity, and increase preference and consumption of root crops. The
main aim is to provide a chemical approach to understand the sensory variation as an
effect of root crop diversity and culinary preparation, divided into the following sub-aims:
• To investigate the composition of aroma and flavour compounds in Jerusalem
artichoke tubers in relation to harvest time, variety and culinary preparation and
in beetroots in relation to variety and culinary preparation (papers 1,3).
• To investigate the composition of carbohydrates in Jerusalem artichoke in
relation to variety, harvest time and culinary preparation and in beetroots in
relation to variety and culinary preparation (papers 1,2,3,5).
• To elucidate the textural changes during culinary preparation of Jerusalem
artichoke tubers (paper 3).
• To investigate the chemical background responsible for the discolouration of
Jerusalem artichoke tubers (paper 4).
FIGURE 1. Diagram of the flow of work performed during this PhD-project with indication of investigated parameters, and the employed analyses.
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• To investigate which sensory and chemical characteristics are determinant for
consumer evaluation of appropriateness of beetroots and Jerusalem artichoke
tubers (paper 5).
In order to achieve these specific aims, several analytical methods were applied:
sensory profiling, consumer studies, aroma analysis by gas chromatography-mass
spectrometry (GC-MS), instrumental texture analysis, instrumental colour analysis,
analysis of phenolic acids, sugars and inulin by high performance liquid chromatography
(HPLC), analysis of total phenolics by the Folin-Ciocalteu (FC) method, as well as analysis
of organic acids and metabolomics by 1H nuclear magnetic resonance (NMR)
spectrometry.
In this thesis, the results of the project are outlined and discussed across the
individual sub-aims and in relation to established research on the eating quality of root
crops. Chapter 2 gives a general introduction to root crops. In chapter 3, results on sensory
studies of root crops are discussed. Chapter 4 evaluates the aroma and flavour quality of
root crops in relation to the contents of volatile compounds, and provides a critical
discussion of sampling methods and quantification. Chapter 5 provides a discussion of the
taste compounds of root crops, with focus on the influence of sugars and other
carbohydrates on sweet taste. In chapter 6, the textural quality is discussed and related to
chemical composition and structure of Jerusalem artichoke tubers. Chapter 7 discusses
colour of root crops in relation to chemical changes during culinary preparation. Finally,
conclusions and perspectives are given in chapter 8.
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2. ROOT CROP
A root crop is any food produced from the underground storage system of a plant. In
the context of this thesis, the root crop should also be used as a vegetable i.e. be eaten hot
as part of a main meal. This excludes storage organs, which are edible but normally used as
spices such as turmeric (Curcuma longa) and wasabi (Eutrema wasabi). Bulbs and corms,
which are swollen underground leaves and stems, are also excluded in this context. A root
crop can be an enlarged true root like a carrot (Daucus carota) or a tuber like a potato
(Solanum tuberosum). This chapter gives an introduction to the concept of root crops,
their spread, constituents and diversity.
2.1 THE PLANT ROOT
The two primary functions of a plant root system is anchorage to the surrounding
media and water and nutrient absorption. Most plant roots also function as storage organs,
and in some plants the roots are specialised for this purpose. This results in enlarged
underground organs, which in some cases are edible. A schematic diagram of the root
morphology is seen in Figure 2A.
A root consists of an inner core called the xylem surrounded by the phloem, and an
outer layer of epidermis (Rubatzky et al. 1999). Nutrients and water absorbed from the soil
are transported to the aerial part of the plant through the xylem. In addition to the root
system certain plants have underground stems called rhizomes. These can also function as
storage organs, either in their stem-like form as in the ginger root (Zingiber officinale) and
ginseng (Panax ginseng), or they can produce tubers at the end of the rhizome or at the
end of stolons, as seen in potato and Jerusalem artichoke (Helianthus tuberosus),
FIGURE 2. Schematic drawing of the morphology of a taproot (A) and a tuber (B).
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respectively. Figure 2B shows the morphology of a tuber. Tubers have the inner and outer
morphology of an aboveground stem with nodes seen as eyes on the surface of the tuber
and a pith in the core (Raven et al. 2005).
During photosynthesis sugars are formed in the aboveground part of the plant. These
are transported as sucrose through the phloem to the storage organs, where the excess
energy is stored in parenchyma as starch, fructans or simple sugars. The excess energy is
stored during adverse conditions such as overwintering. When the plant needs the stored
energy, it is transported back to the aerial part and e.g. used for production of flowers,
seeds and fruits. Some root crops only function as storage organs, whereas others are also
important in the propagation of the plant. This is seen in potatoes were a new plant can be
produced from tubers or parts of tubers containing at least one eye (Raven et al. 2005).
In Figure 3, examples of the large diversity in appearance of root crops are pictured.
The way in which the root crop is produced is dependent on the plant. In carrot the
parenchyma cells surrounds the vascular tissue, and a distinct core is formed from the
xylem in the middle (Figure 3A). In beetroot (Beta vulgaris), concentric circles of xylem
and phloem surrounded by parenchyma cells are produced resembling the growth rings of
a tree (Figure 3B) (Raven et al. 2005). Carrot and beetroot produces one root crop per
plant, whereas the Jerusalem artichoke produces several (Figure 3C).
2.2 ROOT CROP PRODUCTION
In Table 1, an overview of edible root crops, which meet the criterion of this thesis, is
seen. It is evident that root crops are produced from a large range of plant families, but
especially many roots of the Apiaceae (carrot) and Fabaceae (legume) families are edible.
The utilization of root crops is often regional, but expansions of the utilization areas
can provide consumers with increased access to diverse root crops. The plants in Table 1
FIGURE 3. Diversity of root crops. A, Carrot; B, Beetroot; C, Jerusalem artichoke.
A B C
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are adapted to different climates, and those belonging to a tropical or subtropical climate
are not suited for growth in Northern Europe. Others, like the Jerusalem artichoke tuber,
have the potential to be grown worldwide, although quality differences might occur as seen
for carrot grown at different latitudes (Rosenfeld et al. 1997). The normal cultivation area
of the individual root crops can be expanded if the right growing conditions and varieties
are chosen. The Hamburg parsley (Petroselinum crispum var. tuberosum) is a temperate
zone root crop suited for growth in the cold winters of Northern Europe, but can be grown
in the Mediterranean countries if sown in the autumn and harvested in the spring instead
of the reverse practise, which is normal in Northern Europe (Petropoulos et al. 2005).
Additionally, development of optimal post-harvest conditions will improve quality
retention and stimulate greater utilization (Rubatzky et al. 1999), both in the growth region
and if the roots are to be transported before consumption. The root crop with the highest
production in 2010 was the white potato – 425 million tonnes produced worldwide (FAO
2012). Potato, sweet potato (Ipomoea batatas) and cassava (Manihot esculenta) are staple
food for millions of people. In Denmark potatoes are also considered a staple food.
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TABLE 1. List of edible root crops.
Name Latin name Crop Regions grown Plant family
Amazonian yam bean Pachyrhizus tuberosus Root South and Central America23
Fabaceae Andean yam bean Pachyrhizus ahipa Root South America
23 Fabaceae
Beet Beta vulgaris Root Europe, North America1 Amaranthaceae
Black salsify Scorzonera hispanica Root Europe19
Asteraceae Breadroot Psoralea esculenta Tuber North America
6 Fabaceae
Carrot Daucus carota Root Temperate regions1 Apiaceae
Cassava Manihot esculenta Tuber Tropical regions1 Euphorbiaceae
Celariac Apium graveolens Root Europe, North America1 Apiaceae
Cheeky yam Amorphophallus galbra Tuber Australia, Papa New Guinea17
Araceae Chicory Cichorium intybus Root Europe, North America, West Asia
13 Asteraceae
Chinese artichoke Stachys affinis Tuber Subtropical regions, China, Japan1 Lamiaceae
Chinese potato Plectranthus rotundifolius Tuber Tropics20
Lamiaceae Chinese yam Dioscorea batatas Tuber Asia
1 Dioscoreaceae
Daikon Raphanus sativus var. longipinnatus Root East Asia11
Brassicaceae Desert yam Ipomoea costata Tuber Australia
21 Convolvulaceae
Earth chestnut Bunium bulbocastanum Root Europe22
Apiaceae Earthnut, pignut Conopodium majus Tuber Apiaceae Edible burdock Arctium lappa Root Japan
1 Asteraceae
Giant swamp taro Cyrtosperma chamissonis Tuber Tropic regions14
Araceae Greater yam Dioscorea alata Tuber South East Asia
1 Dioscoreaceae
Ground nut Apios americana Tuber North America, Japan4
Fabaceae Hamburg parsley Petroselinum crispum var. tuberosum Root Europe
3 Apiaceae
Hausa potato Solenostemon rotundifolius Tuber Sub-Saharan Africa10
Lamiaceae Italian turnip Brassica rapa var. ruvo Root Brassicaceae Jerusalem artichoke Helianthus tuberosus Tuber Europe, North America
1 Asteraceae
Livingstone potato Plectranthus esculentus Tuber Sub-Saharan Africa10
Lamiaceae Maca Lepidium meyenii Root South America
12 Brassicaceae
Marsh woundwort Stachys palustris Tuber Europe, North America8 Lamiaceae
Mashua Tropaeolum tuberosum Tuber South America12
Tropaeolum Mauka, chago Mirabilis expansa Tuber South America
12 Nyctaginaceae
Mexican yam bean Pachyrhizus erosus Root Central America, Caribbean23
Fabaceae Oca Oxalis tuberosa Tuber South America
12 Oxalidaceae
Parsnip Pastinaca sativa Root Europe, North America1 Apiaceae
Pencil yam Vigna lanceolata Root Australia16
Fabaceae Peruvian carrot Arracacia xanthorrhiza Root South America
1 Apiaceae
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Potato Solanum tuberosum Tuber Temperate regions1 Solanaceae
Radish Raphanus sativus var. esculentus Root Temperate regions1 Brassicaceae
Rutabaga, swede Brassica napus var. napobrassica Root Europe, North America18
Brassicaceae Salsify Tragopogon porrifolius Root Europe, North America
1 Asteraceae
Scotts ginger, jiddo Hornstedtia scottiana Tuber Australia7 Zingiberaceae
Skirret Sium sisarum Root Europe8
Apiaceae Sweet potato Ipomoea batatas Tuber Tropical, subtropical, temperate
1 Convolvulaceae
Tiger nut Cyperus esculentus Tuber Africa5 Cyperaceae
Tuberous pea Lathyrus tuberosus Tuber Europe, Western Asia Fabaceae Turnip Brassica rapa rapa Root Europe
3 Brassicaceae
Turnip-rooted chervil Chaerophyllum bulbosum Root Europe3 Apiaceae
Ulluco Ullucus tuberosus Tuber South America12
Basellaceae Welayta dinich Plectranthus edulis Tuber Africa and Asia
9 Lamiaceae
White yam Dioscorea rotundata Tuber West Central Africa1 Dioscoreaceae
Winged bean Psophocarpus tetragonolobus Root Soth East Asia, New Guinea1 Fabaceae
Yacón Smallanthus sonchifolius Tuber South America15
Asteraceae Yam daisy Microseris lanceolata Tuber Australia
16 Asteraceae
1 Yamaguchi (1983) 2 Petropoulos et al. (2005) 3 Rubatzky et al. (1999) 4 Nara et al. (2011) 5 Lasekan and Abdulkarim (2012) 6 Kaldy et al. (1980) 7 Ippolito and Armstrong (1993) 8 Łuczaj et al. (2011) 9 Taye et al. (2012) 10 Ukpabi et al. (2011) 11 Coogan et al. (2001) 12 Flores et al. (2003) 13 Koch et al. (1999) 14 Bradbury and Nixon (1998) 15 Ojansivu et al. (2011) 16 Incoll et al. (1989) 17 Punekar and Kumaran (2010) 18 Clariana et al. (2011) 19 Dolota et al. (2005) 20 Prathibha et al. (1998) 21 Thorburn et al. (1987) 22 Werger and Huber (2006) 23 Forsyth and Shewry (2002)
0
10
2.3 CONSTITUENTS IN ROOT CROP
The primary constituents of potato, beetroot and Jerusalem artichoke tubers are seen
in Table 2. Potatoes and other root crops classified as staple foods contain a large amount
of starch and have a high caloric value. A second class of root crops including carrot,
beetroot and radish (Raphanus sativus var. exculentus) has a high water content and a
low or no content of starch giving them a very low energy density. A third class of root
crops involves most of the root crops of the Asteraceae family including Jerusalem
artichoke and yacon (Smallanthus sonchifolius). They contain no starch and store energy
in the form of the fructose polymer inulin.
Table 2. Constituents in three representative root crops. Constituent1 Potato Beetroot Jerusalem artichoke Energy (kJ) 342 213 247 Water content (%) 80.5 85.9 82.1 Storage polymer Starch Sucrose Inulin Carbohydrate (g/100g) 16.9 11.4 11.5 Protein (g/100g) 1.9 1.7 2.1 Fat (g/100g) 0.3 0.3 0.6 Dietary fibre (g/100g) 1.4 2.3 2.6 1 all data from Saxholt et al. (2008)
After ingestion, starch is readily degraded to glucose in the intestinal tract and
absorbed by the body. Starch constitutes a major part of the diet of many people. The
Danish Veterinary and Food Administration recommends that consumed starch is
provided from starchy root crops instead of rice and pasta, as the roots in addition contain
fibres, minerals and vitamins. Root crops of the species Beta vulgaris include beetroots
and sugar beets, and they do not store energy as carbohydrates but as simple sucrose
(Levnedsmiddelstyrelsen 1991). The polysaccharide inulin is a soluble dietary fibre, which
is not degraded by enzymes in the human digestive system, but fermented selectively by
beneficial bacteria in the gut. Inulin and its degradation products are prebiotics, which are
compounds capable of stimulating and/or activating health-promoting bacterial growth in
the colon (Gibson & Roberfroid 1995). Moreover, inulin also increases blood glucose level
less than starch, and it is therefore suited as a constituent in an anti-diabetic diet
(Rumessen et al. 1990). Some of the root crops produced by leguminous plants in the
Fabeacea family have an extraordinarily high protein content. An example is the yam
beans (Pachyrhizus spp.), which have a crude protein content of up to 9 % (Zanklan et al.
2007).
Some root crops can be eaten raw, whereas others need heat treatment before
consumption. Cassava contains toxic cyanogenic compounds and needs to be prepared, as
11
it would otherwise be toxic (Falade & Akingbala 2011; Nyirenda et al. 2011). Potatoes and
sweet potatoes, which have high starch content, are normally not eaten raw as humans
digest uncooked starch poorly, whereas root crops with low starch content can be eaten
raw.
2.4 RAW MATERIAL DIVERSITY
A large range of different varieties is found within each individual species of root
crops. When comparing results obtained on different varieties one must keep in mind that
differences can be caused by several factors such as maturity (Akinwande et al. 2007),
growing conditions (Thybo et al. 2001; Hogstad et al. 1997) and climate (Seljåsen et al.
2012; Coogan et al. 2001), and not solely by differences between the varieties. The
beetroots and Jerusalem artichoke tubers investigated in this PhD project are grown either
at the Department of Food Science in Aarslev on Funen or at Tange Frilandsgartneri near
Bjerrringbro, Jutland, in sandy loam and coarse sandy loam soil respectively. This could
influence the quality of the root crops, and observed differences between varieties might be
caused by the different growing sites and not by true differences between varieties. As the
results did not show clear separations between the varieties from the two locations (paper
5), it can be assumed that such differences did not influence the results. The variety of
highest quality may change with different harvest times, as the root crops evolve during
growth and storage. In this PhD project the quality of Jerusalem artichoke tubers at
different harvest times was investigated (papers 1,2,3). The aim was to elucidate possible
quality differences between Jerusalem artichoke varieties grown under identical
conditions. We found that the early maturing variety Mari had a carbohydrate composition
in early harvests, which the later maturing varieties, Rema and Draga, did not obtain until
later harvests (papers 1, 3). This tendency was also seen in paper 2, where Mari already in
the first harvest had developed a metabolite profile characteristic of the profile seen in all
three varieties in the later harvests.
12
3. SENSORY QUALITY
Sensory analysis employs the human senses in the evaluation of product qualities.
Sensory analysis can be conducted by a sensory panel under controlled conditions, where
attributes are evaluated objectively, or it can be consumer studies, which often deals with
affective and subjective measurements of product quality. In this chapter the sensory
qualities of root crops are described and related to consumer-evaluated appropriateness.
3.1 PERCEPTION
The quality of a food product is assessed by the human senses of smell, vision, taste,
touch and hearing. The appearance, touch and smell of a food, are used to evaluate
whether the food is edible or spoiled, and to give an impression of what to expect from the
food. When the food is eaten, the sensory impression is composed of aroma, flavour, taste
and chemesthesis.
The aroma of a product is assessed by the sense of smell, when volatile compounds
released from the food are inhaled orthonasally. Aroma impressions are composed of
complex combinations of volatile compounds detected by the stimulation of the
approximately 300 different odorant receptors (Zarzo 2007) in the nasal cavity, all
encoded by the same multigene family (Buck & Axel 1991). Every odorant receptor can
recognise several odorants, and a single odorant can be recognised by several receptors,
leading to unique recognition patterns for each odorant (Malnic et al. 1999). This makes
humans capable of distinguishing among thousands of distinct odours (Buck & Axel 1991).
The flavour impression of food is the combination of retronasal odour, taste and
chemesthesis during eating (Rubini 1974). The odorant receptors are stimulated
retronasally during eating by volatile compounds released in the mouth and transported to
the nasal cavity. This impression is responsible for the major part of the distinctive flavour
of food.
Humans can taste five different basic tastes: sweet, salt, sour, bitter and umami.
These tastes are caused by non-volatile compounds detected by the taste receptors of the
oral cavity either by binding to these, or in the case of salt and sour, by changing the
electrical potential across the membranes (Kinnamon 2012). Each taste has a specialised
set of receptor cells, primarily arranged in taste buds on the tongue. Chemesthetic
sensations are caused by compounds reacting with tri-geminal nerves surrounding the
taste buds. This can give rise to sensations such as astringency, irritation, tickling, burning
13
and cooling. During eating, the sense of touch is used to assess the texture by how much
force is needed to manage the food with tongue, teeth and lips.
3.2 EVALUATING SENSORY QUALITY
The evaluation of eating quality can be conducted using various sensory methods
depending on the aim of the individual study. In this project, the purpose of the sensory
evaluations has been to obtain descriptions of the products in order to identify important
quality attributes and subsequently relate these to the chemical background. Descriptive
sensory analysis was employed, as descriptive methods can be used to describe sensory-
instrumental relationships (Lawless & Heymann 2010). In descriptive sensory analysis, a
panel of trained assessors agrees on a list of sensory attributes. Prior to the actual test, the
assessors are trained in the definitions of the attributes in order to obtain consensus
concerning definitions and use of scale. The selected attributes must be relevant,
discriminative and non-redundant (Lawless & Heymann 2010). Throughout the sensory
profiling performed during this PhD project, reference samples have been used to aid the
assessors in the generation and understanding of the attributes. Descriptive sensory
analysis can be performed with a few attributes of interest or it can be a complete sensory
profiling of the product.
One of the drawbacks of descriptive sensory analysis is the limited possibility of
comparing results obtained in different situations. In papers 1, 3 and 4, the development
in sensory quality of Jerusalem artichoke tubers over a time range was of interest. The
Jerusalem artichoke tubers were harvested and evaluated at different times during
growing season and the results compared. In papers 2, 3 and 5, culinary preparations of
Jerusalem artichoke tubers were evaluated by sensory profiling, but no comparison of
quantitative results between preparations were made. It was evident from the list of
attributes of the culinary prepared tubers that there were large differences in the sensory
quality between the culinary preparations, and they were to be considered as separate
products.
Descriptive sensory analysis is an objective measure of sensory attributes, and it is
not influenced by, or gives information concerning the affective responses that the product
evokes. If affective evaluations are desired, a consumer study must be conducted, as the
use of a sensory panel is not representative of the general consumer and thus not suitable
to evaluate terms like preference and acceptance (Shepherd et al. 1987). One must
however, be aware that several factors influence the consumer’s experience of quality of a
14
product: the product itself is one determinant, the preparation, situational factors like time
of day and type of meal, mood of the consumer, and the consumer’s previous experience
(Bech et al. 2001). In paper 5, consumers evaluated the appropriateness of Jerusalem
artichoke tubers for raw and boiled consumption. The term appropriateness is used to
assess the effects of context, such as preparation, associated with hedonic responses to
food (Schutz 1988). It has been shown that liking and appropriateness are not necessarily
correlated (Cardello & Schutz 1996), thus it can be assumed that consumers are able to
differentiate between their own preferences in the situation and the appropriateness of the
product. The consumer analyses were done as an in-home study, which is less controlled
than in laboratory surroundings or at a central location. On the other hand, the
surroundings of an in-home evaluation are more familiar and it is normally considered
more realistic (Hersleth et al. 2005). In paper 5, the consumers also performed a
descriptive sensory analysis of the culinary prepared Jerusalem artichoke tubers, in order
to see differences between the professional panel, and the consumers’ evaluation of
sensory attributes. It cannot be expected that consumers were capable of performing a
descriptive sensory analysis as discriminative as the sensory panel, as the consumers were
not as highly trained as the assessors, and the evaluations were not performed in an
analytically focused environment.
3.3 DESCRIPTIVE SENSORY ANALYSIS OF ROOT CROPS
To the knowledge of the author a complete descriptive sensory analysis of Jerusalem
artichoke tubers (papers 1,3,4,5) and beetroots in culinary preparations has in this PhD
project been performed for the first time.
Descriptive sensory analysis was performed on raw, boiled and baked beetroots on a
list of 19, 11 and 14 sensory attributes, respectively. The five beetroot varieties evaluated
was the elongate, red varieties Rocket and Taunus, the round red variety Pablo, the round
red and white-striped variety Chioggia, and the yellow round variety Touchstone Gold. The
varieties of beetroots are seen in Figure 4.
FIGURE 4. Pictures of analysed beetroot varieties. Taunus, A; Rocket, B; Pablo, C; Chioggia, D; Touchstone Gold, E.
15
The results of the sensory analysis are depicted in the spider plots in Figure 5. Only
few attributes showed differences between the investigated varieties. Raw beetroots had
low scores in aroma attributes, but when the roots were boiled or baked, they developed
aromas of raw beetroot, berry juice, and baked potato. When raw, the two varieties
Touchstone Gold and Chioggia separated from the red varieties by higher scores in
pungent flavour, soapy flavour, astringency and bitterness, but also by higher scores in
sweetness. It is interesting that the same varieties scored the highest in both bitterness and
sweetness, and this shows that the two attributes are not mutually exclusive.
Astringent and bitter attributes have also been identified in carrots (Seljåsen et al.
2012; Kreutzmann et al. 2007; Rosenfeld et al. 1997), but not in Jerusalem artichoke
tubers. Astringency is a complex chemesthetic sensation composed of drying of the mouth,
roughing of oral tissue, and a drawing sensation in the cheeks. Astringency is often
connected with wine, tea and beer (Lee & Lawless 1991). Bitterness and astringency can be
desired in some food types like coffee and also some vegetables, but only in moderate
amounts. The attributes of boiled potato flavour and aroma, which developed when the
beetroots were boiled, were also found in boiled Jerusalem artichoke tubers (paper 5).
Descriptive sensory analysis is an objective method, but in comparison to
instrumental analysis, where day-to-day fluctuations and drifting can be identified by the
analysis of standards, monitoring the sensory panel’s performance of root crop evaluation
is more problematic. Root crops are living materials changing in quality over time, which
excludes the possibility of presenting a standard in every situation; in this case at every
harvest and every year. Descriptive sensory analysis of raw, boiled and baked Jerusalem
artichoke tubers was performed in papers 3 and 4. During this evaluation, one of the
samples was assessed twice within every repetition without the panel knowing, in order to
test the reproducibility of the assessors, and the robustness of the results. There was good
agreement between the evaluations of the two identical samples for all attributes except for
whiteness of baked tubers, where a significant difference was found. This could be caused
by a large heterogeneity of the material, but it could also imply that one or more assessors
found the attribute difficult to assess or had misunderstood the attribute. In addition,
baking is a non-uniform process, which could produce variation in the data caused by
differences in the culinary preparation instead of differences between varieties. In this case
16
0
3
6
9
12
15Boiled potato a
Raw beetrootaroma
Cereal a
Berry juice a*
Earthy a
Sweetness*Boiled potato f
Earthy flavour*
Spiciness*
Bitterness
Moistness*
0
3
6
9
12
15Raw beetroot a
Pickled beetroot aFusty a
Corn a*
Boiled beetroot a
Sweet a
Cereal a
Sweetness*
Raw beetroot fFusty f*Green nut f
Carrot f*
Fruity f*
Soapy f*
Bitterness*
Astringency*
Pungent f*
JuicinessCrispness*
0
3
6
9
12
15Pickled beetroot a
Baked beetroot a
Fatty a
Berry juice a
Baked potato a
Earthy a
Sweet a
Baked beetroot f
Baked potato f*
Earthy f
Sweetness*
Bitterness
Moistness*
Hardness*
Figure 5. Results of sensory profiling of raw (top), boiled (middle) and baked (bottom) beetroots on a scale from 1 (low intensity) to 15 (high intensity). a, aroma; f, flavour. *, significant differences between varieties (p ≤ 0.05) (unpublished data)
Taunus, Rocket, Pablo, Chioggia, Touchstone gold
17
only one assessor was responsible for the disagreement and was consequently removed
from the dataset.
Texture and taste attributes are apparently the best attributes for discrimination
between varieties of root crops, whereas aroma attributes often receive low scores. In
Jerusalem artichoke tubers (papers 3, 5) and beetroots (Figure 5), the sensory differences
between varieties were evened out after boiling an baking, and for carrots, the scores for a
range of sensory attributes such as sweetness, raw carrot aroma and overall flavour have
been shown to decrease with increasing blanching time (Shamala et al. 1996) and boiling
time (De Belie et al. 2002). This implies that for heated preparation of root crops it is less
important to choose the right variety, than when the root crops are to be eaten raw.
3.4 CONSUMER EVALUATIONS OF ROOT CROPS
In paper 5, a semi-trained consumer panel evaluated the appropriateness of
Jerusalem artichoke tubers for raw and boiled consumption. During the experiment the
appropriateness for pan-fried consumption along with eight sensory attributes was also
evaluated by the same procedure as the raw and boiled tubers, but no descriptive sensory
analysis was made for this preparation. The largest differences between varieties were seen
for the attributes mealiness and crispness, with Draga and ´Tange´ scoring higher and
lower than the other varieties, respectively. The variety Dwarf, which differed clearly from
the other varieties in raw and boiled preparation, was characterised by a higher score in
browning and a lower degree of sweetness than the other varieties (unpublished results).
In figure 6, the results of the appropriateness evaluation for all three culinary preparations
of Jerusalem artichoke tubers are shown. There were no significant differences for the
appropriateness of Jerusalem artichoke tubers for any preparation.
18
A twin study of the one described in paper 5 was conducted on five varieties of
beetroots. The structure of the experiment was the same as performed on Jerusalem
artichoke tubers. Raw and boiled beetroots were evaluated by a trained sensory panel
using descriptive sensory analysis, and the same semi-trained consumer panel evaluated
sensory attributes and appropriateness for raw, boiled and pan-fried consumption. The
beetroots were the same varieties as described above for the sensory profiling of raw,
boiled and baked beetroots except that the yellow variety Touchstone Gold was replaced by
another yellow variety; Burpees Golden. In figure 7 the results of the appropriateness
evaluation of the five varieties of beetroots are shown.
FIGURE 6. Results of appropriateness analysis of raw, boiled and pan-fried Jerusalem artichoke tubers performed by a semi-trained consumer panel on a scale from 1 = not appropriate to 5 = very appropriate (paper 5 + unpublished results). No significant differences were found between varieties by Tukey’s honest significance test.
Draga Dwarf Mari Rema Tange
0
1
2
3
4
Raw Boiled Fried
Scor
e
0
1
2
3
4
Raw Boiled Fried
aabab
cbc
a
a
a
a
a
aa
ab
b
ab
FIGURE 7. Results of appropriateness analysis of raw, boiled and pan-fried beetroots performed by a semi-trained consumer panel on a scale from 1 = not appropriate to 5 = very appropriate (unpublished results). Letters indicate significant differences between varieties (p ≤ 0.05) as determined by Tukey’s honest significance difference test.
Taunus Rocket Pablo Chioggia Burpees Golden
Scor
e
19
In contrast to the appropriateness results on Jerusalem artichoke tubers (Figure 6),
differences were found between varieties for the appropriateness for raw and pan-fried
preparation of beetroots. Chioggia and Burpees Golden were the least appropriate for raw
consumption. The differences were aligned when the roots were boiled and they became
equally appropriate. Chioggia was evaluated to be the least appropriate variety for fried
preparation. A partial least square (PLS) regression analysis was made in order to predict
the appropriateness (Y) from the sensory data (X) of the trained and the semi-trained
consumer panel. The produced PLS biplots of raw and boiled beetroots are shown in
Figure 8.
In Figure 8A, it is seen that appropriateness of raw beetroot was related to high
scores in crispness, beetroot flavour and juiciness. Especially Pablo was associated to these
attributes, whereas Chioggia and Burpees Golden were not. The appropriateness of boiled
beetroots were not as determined by texture, but primarily related to high scores in raw
and baked beetroot flavour and earthy flavour (Figure 8B). Both Taunus and Pablo were
associated with these attributes. Where Chioggia and Burpees Golden were described by
the same attributes when raw, this was not the case when they were boiled. Boiled Burpees
Golden was related to mealiness and earthy aroma, whereas boiled Chioggia was related to
sweet and bitter aroma. Generally, a good agreement between the evaluations by
-1.2
-0.9
-0.6
-0.3
0
0.3
0.6
0.9
1.2
-1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2
Sweet Bitternes
Chioggia
Waterines
Sweetness
Boiled potato Sweetness
Fusty f Pungent f
Rocket Mealiness
Earthy
Mealiness Burpees
Boiled potato
Raw beetroot Cereal a
Colour
Raw beetroot Appropriatenes
Cereal a Earthy f
Liquorice Raw beetroot
Taunus
Fusty f
Bitterness
Baked beetroot
Pablo
Wateriness Elder a
-1.2
-0.9
-0.6
-0.3
0
0.3
0.6
0.9
1.2
-1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2
Fusty a Sweetnes
A B
Raw carrot a
Sweet a
Bitternes
Dryness Acidity
Astringency Chioggia Soapy f
Acidic a
Cereal a
Crispness Raw beetroot a Raw beetroot f Juiciness
Crispness
Juciness Pabl
Swetness Taunus Raw beetroot f
Green nut f Raw carrot f
Colour intensity Rocket
Fruity f
Fusty f
Appropriateness
PC1 (65%)
PC2 (15%
)
PC1 (27%)
PC2 (2
5%)
Burpees golden Fusty f
Bitternes Pungent f
FIGURE 8. PLS regression biplots of raw (A) and boiled (B) beetroots, showing prediction of appropriateness from sensory attributes (unpublished results). Scores, Attributes evaluated by professional panel, Attributes evaluated by semi-trained consumer panel, Appropriateness.
20
professional panel and the semi-trained consumer panel was observed, except for the
attribute sweetness in the raw tubers and bitterness in the boiled. The sensory panel is
highly trained in separating the individual attribute impressions, whereas the consumers
might find it problematic to identify and separate sweet and bitter taste, when they are
present in the same product. There were larger discrepancies between the semi-trained
consumer panel’s and the professional panel’s evaluation of Jerusalem artichoke tubers,
especially after boiling (paper 5). It cannot be expected that consumers have the same
vocabulary and the same discrimination abilities as the professional panel, but knowing
which attributes are challenging for the consumers may be useful in interpretation of
future consumer evaluations of sensory attributes. The pan-fried beetroot attributes were
only evaluated by the semi-trained consumer panel, and appropriateness was related to
beetroot flavour, colour intensity and crispness (unpublished results).
The attribute earthy flavour is considered undesirable in carrots as it is negatively
correlated to liking (Varming et al. 2004), and negatively correlated to appropriateness of
raw Jerusalem artichoke tubers (paper 5). But these results show that earthy flavour is a
desirable attribute in boiled beetroots, which is probably because it is a characteristic and
expected flavour often associated with beetroots.
Sweetness is not the primary attribute that comes to mind when a root crop is to be
described, but these results (paper 5) (Figure 8) have shown it to be an important attribute
for the quality of beetroots and Jerusalem artichoke tubers. Sweetness has also been
shown to be related to appropriateness of boiled, mashed and oven-fried potatoes (Seefeldt
et al. 2011b), liking of raw carrots (Varming et al. 2004; Surles et al. 2004) preference for
hydroponic carrots (Gichuhi et al. 2009), liking of steamed and baked oca (Oxalis
tuberosa) (Sangketkit et al. 2000), acceptability and liking of sweet potato (Ali et al. 2012;
Leksrisompong et al. 2012) boiled white yam (Dioscorea rotunda) (Akinwande et al.
2007), and overall quality perception of baked potatoes (Jansky 2008). Besides this,
texture attributes are important for the quality in all culinary preparations. The
importance of texture for preference of all preparations of potatoes (Montouto-Graña et al.
2012; Thygesen et al. 2001) as well as in raw carrots (Szymczak et al. 2007; Surles et al.
2004) is well known.
Besides sweetness, texture and colour attributes it were not the same attributes,
which were related to the appropriateness for raw and boiled preparations of Jerusalem
artichoke tubers, although it was the same varieties, which were evaluated as appropriate.
In beetroots it was also different varieties which were appropriate for the culinary
21
preparations. Seefeldt et al. (2011b) found that the same sensory attributes were found to
characterise the appropriateness of potatoes for boiled, oven-fried and mashed
preparation; yellow, creamy, moist, sweet, butter and potato taste. They also found that the
same varieties were appropriate for all three preparations, and a negative relationship
between dry matter (DM) content and appropriateness. There were no significant positive
correlations between DM content and appropriateness for any of the preparations of
beetroot and for raw Jerusalem artichoke tubers, but there was a negative correlation
between DM content and appropriateness of boiled Jerusalem artichoke tubers (r = -0.91,
p = 0.032) (unpublished results).
Sweetness, texture and colour attributes were the best sensory attributes for
discrimination between varieties of raw and culinary prepared root crops, and they had the
largest influence on consumers affective evaluations of appropriateness. The complex
attributes of Jerusalem artichoke tuber aroma and flavour and beetroot aroma and flavour
were also associated with appropriateness.
22
4. AROMA AND FLAVOUR
Aroma and flavour impressions of root crops are caused by volatile compounds
emitted from the food matrix. Raw root crops have very subtle aromas and unlike fruits,
root crops do not emit volatile compounds before the tissue is wounded, cut, heated or
chewed. When the tissue is disrupted, volatile compounds are released from the surface.
They are either present in the intact tissue, formed by the reaction between enzymes and
metabolites, which are separated in the intact tissue, or formed during cooking by heat-
induced reactions. In Chapter 3 it was reported that only few aroma and flavour attributes
were significant in the description of Jerusalem artichoke tubers and beetroots, but it was
not the same attributes, which described the different culinary preparations. Identification
of the volatile compounds responsible for the aroma impressions of beetroot and
Jerusalem artichoke tubers is relevant in the description of their quality, and in the
mapping of the processes occurring during culinary preparation.
4.1 AROMA AND FLAVOUR COMPOUNDS
Terpenes are the most abundant group of volatile compounds found in many root
crops including Jerusalem artichoke tubers (paper 1) and carrots (Radulovic et al. 2011;
Kreutzmann et al. 2008b; Varming et al. 2004; Kjeldsen et al. 2003, 2001; Alasalvar et al.
1999; Macleod & Ames 1991; Van Wassenhove et al. 1990; MacLeod & Ames 1989). The
terpenes are normally either mono- or sesquiterpenes. The biosynthesis of mono- and
sesquiterpenes is seen in Figure 9. Terpenes are build from the five-carbon building blocks
isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP): monoterpenes
from two units, and sesquiterpenes from three units.
23
The precursors for IPP and DMAPP are methylerythtritol phosphate (MEP) in the
case of monoterpenes, or either mevalonic acid or MEP in the case of sesquiterpenes
(Hampel et al. 2005).
Figure 10A-D shows representative volatile terpenes identified in root crops. The
sesquiterpene β-bisabolene (10A) and the monoterpene α-pinene (10B) are the two most
abundant volatile compounds in raw Jerusalem artichoke tubers (paper 1). α-Pinene is
along with β-caryophyllene (10C), limonene, sabinene and terpinolene key aroma and
flavour compounds in raw carrots (Kreutzmann et al. 2008b; Varming et al. 2004). The
flavour and aroma impression of root crops is normally caused by a combination of several
volatile compounds, but in some rare cases a single compound is responsible. The
terpenoid alcohol geosmin (10D) has been found to be almost solely responsible for the
earthy aroma and flavour of beetroot (Lu et al. 2003; Tyler et al. 1979; Murray et al. 1975).
FIGURE 9. Biosynthesis pathway of mono- and sesquiterpenes from DMAPP and IPP. Revised from Dewick (2009).
24
Volatile compounds originating from lipid oxidation have also been identified in
several root crops (Soria et al. 2008; Varming et al. 2004; Petersen et al. 1998; Macleod &
Ames 1991; MacLeod & Ames 1989). Autooxidation of lipids is a radical chain reaction,
starting with the reaction between unsaturated lipids and oxygen or free radicals. The
primary oxidation products are lipid hydroperoxides, which do not contribute to the
flavour or aroma. Lipid hydroperoxides are unstable and decompose to secondary
oxidation products such as hydrocarbons, alcohols, aldehydes, acids, ketones and furans.
The autooxidation of lipids is catalysed by heat (Frankel 1983).
During heat treatment volatile compounds are produced by the Maillard reaction.
The Maillard reaction starts by the reaction of the carbonyl group of a reducing sugar and
an amino group of a amino acid or protein forming N-substituted glycosylamine or
fructosylamine depending on the sugar. These are rearranged to Amadori or Heyns
rearrangement products, respectively. Subsequent fragmentation and release of the amino
group results in deoxyosones, which can react in countless ways to produce Maillard
reaction products, such as furans, pyridines and pyrazines. Which volatile compounds are
produced depends on the precursors present in the root crop, and on reaction conditions
such as temperature and pH (Van Boekel 2006).
4.2 ISOLATION OF VOLATILE COMPOUNDS
The volatile compounds isolated from food products are highly dependent on the
extraction method (Aceña et al. 2010; Prosen et al. 2010; Majcher & Jelen 2009; Richter &
Schellenberg 2007; Kanavouras et al. 2005; Mallia et al. 2005). The ideal method should
FIGURE 10. Chemical structures of volatile compounds identified in root crops. β-bisabolene (A), α-pinene (B), β-caryophyllene (C), geosmin (D).
C
B
CH3
CH3
CH3
CH2
CH3
CH3
CH3
CH3
CH2
CH3
CH3
CH3
CH3
OH
A
D
25
extract all compounds, which contribute to aroma and flavour, it should not alter the
composition, and it should not cause formation of artefacts.
Different extraction techniques have been employed in the sampling of volatile
compounds from root crops: among others static headspace extraction, dynamic
headspace (DH) extraction, solid phase micro extraction (SPME), solvent extraction and
distillation methods. In this PhD project, a comparative study on the extraction of volatile
compounds from carrots was performed. Volatile compounds were extracted from three
varieties of carrots by DH, solvent extraction with hexane and headspace SPME (HSPME).
Dynamic headspace was conducted using Tenax TA traps and thermal desorption by a
method slightly modified from the one described in paper 1. Hexane was used for solvent
extraction, and the extracts directly injected to the gas chromatograph (GC). HSPME was
done using carboxen/polydimethylsiloxane (CAR/PDMS) fibre coating and thermal
desorption. HSPME has previously been performed on dehydrated carrot samples, and the
SPME fibre CAR/PDMS showed higher recoveries of total volatiles than other tested fibre
types (Soria et al. 2008). All collected volatiles were separated and identified by GC-MS. A
representative chromatogram for each extraction method is shown in Figure 11.
26
FIGURE 11. GC-chromatograms of analysis of volatile compounds from the raw carrot Nipomo. Volatiles were extracted by Dynamic headspace with Tenax TA (A), solvent extraction with hexane (B) and solid phase micro extraction using CAR/PDMS fibre (C). Representative peaks important for carrot aroma and flavour are indicated (unpublished results).
minutes
27
The chromatograms obtained by the three extraction methods did not show the same
pattern. DH extracted both high and low volatility compounds, whereas solvent extraction
and HSPME extracted mostly high volatility and low volatility compounds respectively.
The distribution of collected volatile compounds is shown in Figure 12.
DH using Tenax TA and HSPME with CAR/PDMS fibre were more suitable for
extraction of terpenes than solvent extraction with hexane, as 99% and 98% respectively,
of the extracted volatiles were terpenes. The hexane extract contained 64% terpenes. The
remainder of the compounds extracted by hexane were alkanes - which were easily
extracted into hexane - and compounds normally categorised as non-volatile, such as
falcarinol. HSPME extracted more very volatile compounds and thus more monoterpenes
than sesquiterpenes. The DH extraction on the other hand extracted nearly the same
amount of mono- and sesquiterpenes. Previously, HSPME has been found unsuited for
extraction of compounds with high molecular weight and with affinity for the fibre
(Majcher & Jelen 2009), but better suited to extract more compounds with high volatility
than DH (Mallia et al. 2005).
In HSPME, equilibrium arises between volatile compounds in the headspace and
volatile compounds adsorbed to the SPME-fibre. During DH extraction the equilibrium
between headspace and product is constantly displaced, as a gas flow is purged over the
sample, and volatiles transported to the trap material. An inert gas is often used to avoid
oxidation of constituents in the food, but this creates an anaerobic environment, which can
FIGURE 12. Composition of volatile compounds extracted from the raw carrot Nipomo by dynamic headspace (DH), solvent extraction with hexane and headspace solid phase microextraction (HSPME). Data are presented as mean across replicates (n =3) in percent of total extracted volatiles. (unpublished data). DH, Hexane, HSPME.
0
10
20
30
40
50
60
70
80
90
100
Total Terpenes Monoterpenes Sesquiterpenes
% of t
otal ext
racted
volat
iles
28
cause formation of artefacts, when used on raw plant material (Hansen et al. 2001). During
DH there is also a risk of breakthrough, where volatile compounds are purged through the
trap.
When sampling by DH or HSPME, volatiles are adsorbed onto a solid trap or
absorbed by a liquid trap. The choice of trapping material is essential, as the material will
show higher affinity for some volatiles than for others. This results in discrimination
during adsorption; high-affinity volatiles can displace the lower affinity-volatiles from the
trap, and in some cases be adsorbed irreversibly (Majcher & Jelen 2009; Câmara et al.
2007; Pillonel et al. 2002).
The differences in extraction of volatiles between DH and HSPME in this study may
be influenced by the difference in trap material. Tenax TA is a weak sorbent material,
meaning that it does not hold on strongly to very volatile compounds. On the other hand,
Tenax TA is highly recommended for overall volatile analyses, whereas CAR/PDMS is
normally recommended for highly volatile and polar compounds. The solvent extraction
method was ruled out for use in further analyses, as it extracted to many non-volatile
compounds. Headspace analysis techniques extract the volatile compounds released from
the food matrix and not the total volatile content of the food as the solvent extraction does.
Headspace techniques are therefore expected to give a more representative picture of the
volatile compounds perceived during eating. The studies on volatile compounds in this
PhD project focused on the entire volatile profile of root crops, including important high-
boiling volatiles, such as geosmin in beetroot and β-bisabolene in Jerusalem artichoke
tubers. It was evident from the results on HSPME that these compounds, which are known
to influence the sensory quality of root crops, might not be extracted, and that the obtained
volatile profile therefore not representative of the sensory impression. For these reasons
DH extraction was used in further analyses of volatile compounds in this project.
When performing analyses of volatile compounds, one must always bear in mind
that the obtained results are only representative of the chosen method, and not necessarily
representative of the true sensory impression of the food product. The release of volatile
compounds from the food matrix is influenced by odorant polarity, volatilities, properties
of the matrix, and the partitioning of the volatile compound between food matrix and
air/saliva (Buettner & Beauchamp 2010). These aspects are seldom taken into
consideration during extraction of volatile compounds from food. Often the method that
gives the largest amount of extracted volatile compounds is chosen, but this method might
not be the most representative of the sensory impression. In relation to the study described
29
above, a descriptive sensory analysis of the carrots was also performed. It was attempted to
elucidate which aroma extraction technique was the most suitable in expressing the
sensory perceived aroma and flavour composition of the carrots. Interestingly, none of the
profiles were directly linked to the results of the sensory analysis when relating significant
sensory attributes and extracted aroma compounds. Overall, the results were not clear with
regards to relations to sensory quality because the sensory variation between samples was
too small.
4.3. RELATING VOLATILE COMPOUNDS AND SENSORY ANALYSIS
The biggest challenges concerning the analysis of aroma and flavour compounds are
the translation between identified volatile compounds in the food, and the real sensory
impression the food gives.
Volatile compounds identified in food are not necessarily contributing to the aroma
or flavour, and the most abundant compounds may not be the most important
contributors. Further, not all volatile compounds have aromatic properties, and
compounds can be present in concentrations below their human olfactory threshold. The
human olfactory threshold is the lowest concentration needed to produce an olfactory
response. This value is highly dependent on the individual person, the matrix and the
temperature (Czerny et al. 2008). Olfactory thresholds are determined on individual
compounds and do not take into account additive effects or interaction with other
compounds.
In order to relate an obtained volatile profile of a food to the sensory evaluated
quality, GC-Olfactometry (GC-O) can be performed. GC-O uses human assessors as a
sensitive GC-detector and provides odour descriptions of the individual aroma active
compounds present in detectable concentrations (Delahunty et al. 2006). Alternatively,
odour descriptions and olfactory thresholds of single compounds or combinations of
compounds can be determined by dilution of standard compounds in air, solvent or water
and analysed by GC-O. From the combination of odour thresholds and concentrations in
the analysed food, the contributions to the overall aroma impression can be deduced. The
aroma impression can be reconstituted by combination of standards, and the most
influential compounds identified by omission tests (Liu et al. 2012). Aroma reconstitution
and omission tests require that standard compounds are available, and that combinations
of these can be made in a matrix representative of the food matrix. Finally, it is possible to
30
attempt to explain the relations between sensory analysis and volatile composition by the
use of statistical methods such as multivariate data analysis, as in this PhD project.
There is no direct link between the structure of a volatile compound and its aroma.
Small changes in molecular structure of odorants can lead to profound changes in the
perceived odour (Buck & Axel 1991). Even different enantiomers of the same volatile
compound can have different odour descriptions or different odour potencies. An example
of this is carvone, which in the form of (S)-(+)-carvone has an olfactory threshold of 85-
130 ppb and an odour of caraway, whereas the enantiomer (R)-(-)-carvone has an olfactory
threshold of 2 ppb and an odour described as spearmint (Friedman & Miller 1971). The
aroma and flavour impression of a volatile compound can also depend on the
concentration. At concentrations below 5.8 µg/L in beetroot juice geosmin (Figure 10D) is
described as having beet flavour. But at higher concentrations it is described as earthy
(Tyler et al. 1979). When relating the extracted volatile compounds with odour
descriptions, which have not been made in the same experiment, one must be attentive to
whether enantiomers exist and have different odours. Besides this, bio-transformation of
volatile compounds in the mouth and nasal cavity can change the composition towards
more or less odorant structures, which will not be the same as those detected
instrumentally (Buettner & Beauchamp 2010).
4.4 VOLATILE COMPOUNDS IN CULINARY PREPARATIONS OF ROOT CROPS
Jerusalem artichoke tubers
As described in Chapter 3, the sensory quality of root crops changes with culinary
preparation. For Jerusalem artichoke tubers, the differences in perceived aroma and
flavour between varieties are evened out during boiling and baking (papers 3, 5). The
volatile compounds of the raw, boiled and baked Jerusalem artichoke tubers investigated
in paper 3 were sampled by DH and analysed by GC-MS at two harvest times. The aim was
to elucidate the changes in the volatile profile resulting from boiling and baking. The
volatile compounds were sampled as described in paper 1 on Tenax TA traps at 25°C. The
tubers were cut in 1 cm x 1 cm x 1 cm cubes for the sensory analysis. These cubes were
blended before extraction of volatile compounds to increase surface area. The identified
volatile compounds are shown in Table 3. Twenty-seven volatile compounds were isolated
from the headspace of the Jerusalem artichoke tubers, 18 of these have previously been
identified in raw Jerusalem artichoke tubers either in paper 1 or by Macleod et al. (1982).
31
TABLE 3. Identified volatile compounds isolated by dynamic headspace sampling of raw, boiled and baked Jerusalem artichoke tubers (unpublished results). Rt = retention time. Rt Volatile compounda Aroma description
11.3 α-Pinene Resin, pinec,d 13.5 Camphene Terpene, camphorousd 15.2 Hexanal Green, grassyc,d 15.9 β-Pinene Carrot top, fresh green pined
16.1 Undecane Alkanef
17.6 Ethylbenzene Etheral, floral, sweet 18.2 m- and/or p-Xylene m-Xylene: plasticf 18.6 o-Xylene Geraniumf 21.9 Limonene Citrus, fruityd 22.8 Dodecane Alkanef
24.8 γ-Terpinene Herbaceous, citrus, fruitye 25.8 2-Pentylfuran Fruity, green bean, butterf 27.3 ρ-Cymene Citrusd 27.9 Terpinolen Sweet-piney, citrusd 30.6 Tridecane Alkanef
35.7 Allo-ocimene 37.8 Nonanal Citrus-like, soapy,c fattyd 38.9 Unknown sesquiterpene 1
(m/z 175, 204, 121, 133, 119)
40.4 p,α-dimethylstyrene Spicyd 41.4 A Pyrazine 43.8 α-Copaeneb Wood, spicef 45.1 Decanal Sweet, waxy, florald 46.5 Benzaldehyde Bitter almondd 54.6 α-Cedrene 55.6 β-Sesquiphellandreneb Woodf 57.1 β-Bisabolene Sweet, balsamicd,e
68.7 Benzoic acid Odourlessd aMass spectra and linear retention indices are consistent with those of authentic standard compounds unless noted. bTentatively identified. No standard available but identified from MS and LRI. c Czerny et al. (2008) d Burdock (2010) e Kjeldsen et al. (2003) f Flavornet (2012)
Very few of the isolated compounds were present in a concentration above the limit of
quantification. The low content of volatile compounds is somewhat expected from the
sensory results (paper 3), which in the evaluated aroma and flavour attributes showed
relatively low scores. The concentrations of quantified volatile compounds are shown in
Table 4.
32
TABLE 4. Concentrations of volatile compounds (ng/g tuber as eaten) isolated by dynamic headspace sampling of raw, boiled and baked Jerusalem artichoke tubers, quantified in at least one harvest time (unpublished results).
Harvest 1 Harvest 2
Mari Rema Draga Mari Rema Draga
Raw α-Pinene 0.3±0.2 5.7±0.9 1.3±0.7 1.6±1.2 10.4±7.3 8.0±4.9
Limonene nqa 0.1±0.01 nq nq nq nq β-Bisabolene 0.2±0.04 0.1±0.02 0.2±0.03 0.2±0.1 0.1±0.01 0.2±0.02 β-Pinene nq nq nq 0.3±0.06 0.2±0.07 0.3±0.2
Undecane ndb nd nd 0.2±0.06 0.2±0.1 0.2±0.01
Boiled α-Pinene nq 1.6±0.3 0.2±0.2 0.7±0.5 0.1±0.01 0.2±0.1 β-Bisabolene 0.1±0.04 0.1±0.01 0.2±0.03 0.5±0.4 0.1±0.06 0.3±0.2
Nonanal nq nq nq 0.1±0.02 nq nq
Hexanal nq nq nq 0.2±0.2 0.2±0.2 0.2±0.1
Baked α-Pinene 0.1±0.05 2.4±0.8 0.4±0.6 0.9±0.4 0.5±0.4 0.9±0.6 β-Bisabolene 0.1±0.07 0.1±0.01 0.1±0.1 0.2±0.1 0.1±0.1 0.2±0.05
Hexanal nq nq nq 0.4±0.1 0.5±0.1 0.4±0.1
Undecane nd nd nd 0.5±0.2 0.3±0.1 0.3±0.1
Tridecane nq nq nq 0.1±0.01 0.1±0.03 0.1±0.02
Dodecane nd nd nd 0.1±0.06 0.1±0.08 nq
Nonanal nq nq nq 0.1±0.04 0.1±0.02 0.1±0.02 anq = not quantified. A signal to noise ratio of 5 was set as the limit of quantification. bnd = not detected
The Jerusalem artichoke tubers had an average total content of volatile compounds of
5.0, 1.1 and 1.8 ng/100 g tuber as eaten for raw, boiled and baked respectively, when
averaged across harvest times. In paper 1, by far the largest portion of the volatiles
extracted from raw Jerusalem artichoke tubers consisted of α-pinene and β-bisabolene.
These two compounds were also the most abundant in all three preparations in the results
presented in Table 4. None of the other identified compounds were present in a
concentration higher than 0.5 ng/g tuber as eaten under any conditions. The total
concentrations of volatile compounds in the raw tubers were considerably lower than the
concentrations obtained in paper 1. This might be caused by the difference in cutting of the
tuber. During blending of the tubers, as in the present experiment, a large amount of
volatiles might have been released to the surroundings before the sampling was initiated.
The total content of volatile compounds decreased during heat treatment. Volatile
compounds are lost because of evaporation during baking and leaching out to the cooking
water during boiling. In carrots the total content of volatile compounds decreases with
increasing boiling and blanching time, with as much as 95% of the total volatile
33
compounds being lost. Besides this, the sesquiterpenes are retained better in the carrot
than monoterpenes (Alasalvar et al. 1999; Shamala et al. 1996). This tendency is also seen
in the Jerusalem artichoke tubers, as the sesquiterpene β-bisabolene was retained better
during boiling and baking than the monoterpene α-pinene (Table 4). During boiling and
baking of Jerusalem artichoke tubers aldehydes and alkanes are formed by thermally
induced lipid oxidation. These are only produced in quantifiable amount in the second
harvest, which suggests that the content of lipids in the tubers was higher in the second
than in the first harvest.
In Table 3 odour descriptions of the identified volatile compounds are noted, but
these are not necessarily describing the impressions the volatile compounds give in
Jerusalem artichoke tubers. As an example, the boiled and baked Jerusalem artichoke
tubers have a higher content of hexanal and nonanal than raw. Hexanal are described as
green and grassy, and nonanal are described as fruity and soapy (Table 3), but according to
the chosen sensory attributes these characteristics are not developed during the cooking
process. On the contrary, a tendency towards less fresh aroma and flavour attributes, were
chosen for the boiled and baked tubers in comparison to the raw (data not shown). In
paper 1 PCA was performed on sensory data as well as volatile profile of raw Jerusalem
artichoke tubers, and the plots were subsequently compared. No simple relation between
sensory attributes and volatile compounds was observed and it was concluded that the
aroma and flavour of Jerusalem artichoke tubers was caused by a combination of volatile
compounds, some of which may have been present below the limit of detection.
Beetroots
In section 3.3 the results of descriptive sensory analysis of five varieties of raw, boiled
and baked beetroots were described. In connection with that study, the volatile compounds
of the beetroots were collected by DH on Tenax TA traps and analysed by GC-MS. A total
of 37, 45 and 55 volatile compounds were extracted from raw, boiled and baked beetroots
respectively. The identified compounds and their relative concentration are shown in Table
5.
34
TABLE 5. Volatile compounds identified in raw, boiled and baked beetroots. Concentrations are stated as relative peak areas in percent of total peak area of each preparation. Data is given as mean of five varieties (unpublished results). Volatile compounda
Raw Boiled Baked Volatile compound
Raw Boiled Baked
Decane 0.2 2-Heptenal 0.3 α-pinene 1.1 0.6 2,6-Dimethylpyrazine 0.8
Toluene 2.6 0.5 0.5 6-Methyl-5-heptene-2-one
1.8 2.3 1.2
2,3-Pentadiene 0.4 2-Isopropyl-5-methyl-2-hexenal
1.8
Dimethyldisulfide 1.6 1.2 Nonanal 3.9 17.5 8.4
Hexanal 15.6 4.3 3-Octen-2-oneb 1.5
2-Methyl-2-butenal 3.2 Tetradecane 1.3 0.6 β-pinene 5.5 0.9 2.3 Durene 0.2
Undecane 0.5 0.1 0.3 3,4-Dimethylstyrene 2.0 1.1 0.3
Ethylbenzene 1.2 2-(1-Methylvinyl) thiophene
2.2
p-Xylene 1.0 1-Octen-3-ol 0.5 0.3
m-Xylene 3.3 0.4 0.7 Acetic acid 4.3 1.5 0.6
o-Xylene 2.8 0.3 0.8 Fufural 4.4
Heptenal 0.5 2-Ethyl-1-hexanol 1.2 2.1
Limonene 0.3 0.3 Decanal 3.5 2.6
Eucalyptol 4.1 6.7 0.1 Benzaldehyde 18.9 8.5 5.8
Dodecane 2.4 0.5 0.8 Hexadecane 1.9
2-Pentylfuran 2.6 Butanoic acid 0.2
Styrene 1.8 0.3 0.3 Acetophenone 8.2 3.6 1.1
1-Pentanol 1.0 0.6 2-Methyl hexanoic acidb
12.1
4-Methylpyrazine 0.7 Naphthalene 1.3
p-Cymene 0.5 0.3 Octadecane 3.9 1.4 0.4
Terpinolen 1.7 0.7 Geosmin 2.6 7.2 0.6
Octanal 1.1 1.6 2.8 Geranyl acetoneb 2.8
3-Methylpyridine 0.7 1.0 2.6 Phenol 6.0 2.8 1.1
Tridecane 2.4 0.6 0.7 aMass spectra and linear retention indices (LRI) are consistent with those of authentic standard compounds unless noted. bTentatively identified. No standard available but identified from MS and LRI.
Besides the compounds listed in Table 5, many unidentified compounds were
extracted from the beetroots. These were especially pyridines and other Maillard
compounds produced during boiling and baking. To the knowledge of the author geosmin
was until now the only volatile compound extracted from raw beetroots (Murray et al.
1975). As shown in Table 5, the volatile composition of beetroots is in line with that of
other root crops containing mainly mono- and sesquiterpenes. Hexane, 2-pentylfuran, 4-
methylpyridine, benzaldehyde and geosmin have previously been identified in boiled
35
beetroot (Parliment et al. 1977). As mentioned in section 4.3 geosmin has a beetroot
flavour at low concentration and an earthy flavour at higher concentrations. The sensory
attributes earthy flavour and aroma and beetroot aroma and flavour were evaluated in the
sensory descriptive analysis of beetroots, however no correlations was found between these
sensory attributes and the content of geosmin (data not shown).
Lipid oxidation products are formed during heat treatment of beetroots, along with
Maillard reaction products like pyrazines, pyridines and decanal. Decanal is important in
the flavour of cooked potatoes (Duckham et al. 2002). The sensory attributes boiled potato
flavour and aroma were identified in the sensory descriptive analysis of the boiled
beetroots (Figure 5). Even though there were no significant differences in the evaluation of
the attributes, decanal might be the responsible volatile compound. Pentanal, hexanal and
nonanal have been shown to contribute to a cardboard-like off-flavour of boiled potatoes
(Petersen et al. 1999). Boiled potatoes have been found to contain more lipid oxidation
products than baked potatoes, which in turn contain more Maillard reaction products
(Oruna-Concha et al. 2002). This also seems to be the case for boiled and baked beetroots.
The major part of the volatiles extracted from boiled beetroots is composed of hexanal and
nonanal, whereas no specific compounds are dominating the extract from baked beetroots.
The volatile compounds in root crops are mainly constituted of terpenes. The
influence of the individual compounds on the sensory aroma and flavour impression of the
root crops, have only been attempted elucidated by correlations in this project. However,
GC-O analyses could provide further knowledge on the volatile compounds responsible for
the sensory impression.
36
5. TASTE
Taste is caused by non-volatile compounds, and the sense of taste is more simple than the
sense of odour, as taste receptor cells detects stimuli of only one taste quality each. As
described in chapter 3, sweetness is a desirable attribute in root crops, whereas bitter taste
often is undesirable and can cause consumer rejection (Alasalvar et al. 2001). In this
project the background for sweetness of Jerusalem artichoke tubers and beetroots was
investigated by analysis of the carbohydrate content of the root crops.
5.1 TASTE COMPOUNDS
The perception of sweetness is activated by sugars and to a lesser extent certain
carbohydrate polymers, terpenes and sweet tasting proteins (Wintjens et al. 2011; Behrens
et al. 2011). Sweetness of root crops is determined by the type and composition of the
present sugars, which normally are variety-dependent (Nookaraju et al. 2010). Many
diverse compounds have bitter tastes, but the most common bitter tasting compounds in
root crops are phenolics, polyacetylenes, glucosinolates and alkaloids (Schreiner et al.
2011; Kreutzmann et al. 2008a; Schmiech et al. 2008; Czepa & Hofmann 2003). Bitter
compounds are often produced during stress conditions such as tissue damage, cold or
infection. The taste of umami is important in some preparations of root crops such as
boiled potatoes (Morris et al. 2007), where it is caused by production of 5´-nucleotides
during boiling. Umami is described as a meaty and savory taste (Behrens et al. 2011). Sour
taste is caused by organic acids, and even though sourness is rarely associated with the
taste of root crops, sourness was chosen as a sensory attribute in the description of raw
Jerusalem artichoke tubers in paper 1, and significant differences between varieties were
found.
5.1 TASTE COMPOUNDS IN ROOT CROPS
Jerusalem artichoke tubers
In Jerusalem artichoke tubers the taste attribute sweetness was identified in all
culinary preparations of tubers (papers 1,3,5), and the degree of sweetness was highly
associated with appropriateness for raw, boiled and pan-fried consumption (paper 5).
37
The content of sugars in Jerusalem artichoke tubers was analysed by high
performance anion exchange chromatography (HPAEC). The concentration of total sugars
in raw tubers was between 0.5 and 2.15 g/100 g fresh weight (FW) (papers 1, 3, 5),
between 0.5 and 1.89 g/100 g tuber as eaten in boiled tubers (papers 3, 5), and between
0.7 and 1.6 g/100 g tuber as eaten in baked tubers (paper 5). The identified sugars in
Jerusalem artichoke tubers were fructose, glucose and sucrose (Figure 13A-C), with
sucrose constituting the major part.
Two studies with Jerusalem artichoke tubers, harvested at different times across the
season, were performed in two consecutive years (papers 1, 3). In both studies raw Mari
had a higher content of total sugars than the varieties Rema and Draga at early harvest. In
Table 6 is seen the content of individual sugars identified in raw Jerusalem artichoke
tubers from the results of the first year presented in paper 1. As shown in Table 6, Mari
actually had a higher content of sucrose, but a lower content of fructose and glucose in the
first harvest than the other varieties. Mari is an early maturing variety, and the results
throughout this project indicated that Mari had already gone into dormancy at the early
harvests (papers 1, 2, 3). Sucrose is synthesised during tuber dormancy (Noël & Pontis
2000), and Mari may therefore have had higher activity of sucrose synthesising enzymes at
the first harvest than the other varieties.
FIGURE 13. The chemical structure of glucose (A), fructose (B), sucrose (C) and inulin (D).
38
Table 6. Content of glucose, fructose and sucrose in three varieties of raw Jerusalem artichoke tubers harvested at three different times (g/100g FW). Data are presented as mean (n =3). Harvest 1a Harvest 2 Harvest 3
Glucose Mari 0.1 ab 0.1 a 0.1 a
Rema 0.4 a 0.10 b 0.1 b Draga 0.7 a 0.1 b 0.2 b Fructose
Mari 1.4 a 0.7 b 0.5 b Rema 1.4 a 0.5 b 0.7 b Draga 2.3 a 0.7 b 0.8 b Sucrose
Mari 19 b 20.3 a 19.3 ab
Rema 13.7 b 19.7 a 19.8 a
Draga 14.2 b 20.7 a 20.1 a a Harvest 1, November; Harvest 2, February; Harvest 3, March. b Different letters indicate significant differences (p ≤ 0.05) between harvest times for the individual varieties (unpublished data).
Rema and Draga showed a decrease in glucose and fructose content and an increase
in sucrose content across the season. In the late harvests the sugar contents between
varieties were aligned. The same tendency was observed from the results of the analysis of
sugars in the same varieties in the second year (data not shown). An increased sucrose
content and a decrease in glucose and fructose content have previously been observed in
Jerusalem artichoke tubers, and other inulin-containing vegetables during storage and
dormancy (Imahori et al. 2010; Kocsis et al. 2007; Saengthongpinit & Saijaanantakul
2005; Koch et al. 1999).
In paper 3, a positive correlation between total sugar content and sensory evaluated
sweetness was found. Fructose, glucose and sucrose do not have the same sweetness value,
and therefore not the same impact on overall sweetness of the tubers. Glucose has a
sweetness value of 69% of that of sucrose, whereas fructose has a sweetness value of 114%
of that of sucrose (Belitz et al. 2009). None of the three individual sugars were significantly
correlated to the sweetness of the tubers (data not shown).
Jerusalem artichoke tubers contain the carbohydrate polymer inulin, which might
influence the perceived sweetness of the tubers. Inulin is a fructose polymer build from D-
fructose units connected through β(2→1) glycosidic linkages with a terminal α(1→2)
bonded glucose (Panchev et al. 2011). The structure of inulin is shown in Figure 13D. The
inulin in Jerusalem artichoke tubers is a mixture of polysaccharides with different degree
39
of polymerisation. Inulin polymers with less than 12 fructose units are normally referred to
as fructooligosaccharides (FOS) (Kocsis et al. 2007). The degree of polymerisation of inulin
from Jerusalem artichoke tubers varies from 3-50 fructose units depending on harvest
time and growing conditions (Yildiz 2011). As seen in paper 2, inulin is degraded to FOS
and sucrose during overwintering of tubers in the soil, which is also seen during cold
storage and dormancy in other studies (Kocsis et al. 2007; Saengthongpinit &
Saijaanantakul 2005; Cabezas et al. 2002; Schorr-Galindo & Guiraud 1997). FOS and
sucrose are most likely used for osmotic adjustment, and for protection of cell structure by
stabilisation of cell membranes during water deficit caused by low temperatures (Portes et
al. 2008). Sucrose from the degradation of inulin contributes to the higher sucrose content
in Mari and the increase in sucrose content of Rema and Draga across the season.
In this project the content of total inulin was quantified by HPAEC, but the method
gives no information on the polymerisation of the inulin in the tubers. In order to evaluate
contribution of inulin to the sweetness of tubers, the individual polymers of inulin must be
quantified. Saengthongpinit and Saijaanantakul (2005) and Böhm et al. (2005) have
separated the inulin polymers in Jerusalem artichoke tubers by HPAEC with a gradient
programme designed for this. They were however unable to quantify the single polymers
due to the lack of individual standards. Inulin standards normally contain an unknown
mixture of polymers, and only the total inulin content is known. During storage above
freezing point inulin can moreover rearrange to inulo-n-oses (n = number of fructose
units), which are inulin polymers without end glucose (Saengthongpinit & Saijaanantakul
2005; Ernst et al. 1996). These components will additionally complicate the analysis and
quantification of individual inulin polymers. In paper 2, the inulin and sugar content in
Jerusalem artichoke tubers were analysed by 1H NMR metabolomics. The samples were
the same as those presented in Table 6. The analysis showed an increase in sucrose, and a
decrease in glucose content during dormancy, along with an increase in inulin terminal
glucose, because of degradation of inulin to shorter polymers. Opposed to the HPAEC
results, 1H NMR results showed a decrease in total inulin and no change in fructose
content. The changes in fructose content may have been too small for the NMR to detect.
The degree of polymerisation of inulin determines its sweetness. A mixture of
polymers with 2 to 60 fructose units are 10% as sweet as sucrose, whereas inulin polymers
with 2 to 7 fructose units are 35% as sweet as sucrose (Franck 2002). Besides this,
branched inulin chains can also be present. As inulin polymers are degraded during the
dormancy, the early variety Mari may contain more short inulin polymers in the first
40
harvest than the other varieties, which could contribute to its higher sweetness along with
the higher sucrose content. Inulin is also degraded during heat treatment to FOS, sucrose
and di-D-fructose anhydrides (Böhm et al. 2005; Blize et al. 1994; Ponder & Richards
1983), and increased sweetness could be expected after heat treatment. During boiling, the
Jerusalem artichoke tubers lost sugar and inulin to the water by leaching. After boiling and
baking sugar content decreased because of Maillard reactions and caramelisation (paper
3).
Beetroots
The sensory quality of raw, boiled and baked beetroots were evaluated for the taste
attributes sweetness and bitterness. Sweetness was significantly different between the five
varieties in all preparations, whereas bitterness was only significantly different between
raw varietes of beetroot (Figure 5). The total sugar content in raw, boiled and baked
beetroots is seen in Table 7. The sugar content of beetroots is higher than the sugar content
of Jerusalem artichoke tubers, but followed the same trends in relation to culinary
preparation. As mentioned earlier, beetroot store its excess energy as sugar and not as
carbohydrate polymers. The sugar is primarily stored in the form of sucrose, and in all
preparations sucrose constituted more than 99% of the total sugar.
TABLE 7. Content of total sugar in five varieties of raw, boiled and baked beetroot (g/100 g tuber as eaten). Data are given as mean (n = 3) (unpublished results).
Taunus Rocket Pablo Chioggia
Touchstone Gold
Raw 7.8 6.7 7.2 8.2 7.4 Boiled 5.4 4.9 4.2 5.6 5.4 Baked 10.1 9.5 10.4 12.3 10.8
As beetroots stores simple sugars and not carbohydrate polymers it could be expected
that a simpler relationships between sugar content and sensory evaluated sweetness could
be found than in Jerusalem artichoke tubers. This was however not the case, as significant
correlation between sensory evaluated sweetness and total sugar content was only found
for baked beetroots (r = 0.95, p = 0.014). The varieties Chioggia and Touchstone gold were
evaluated substantially sweeter than the other varieties in the sensory descriptive analysis
(Figure 5), but this tendency was not reflected in the content of total sugars. The perceived
sweetness of beetroots may be influenced by bitter tasting compounds in the root by
mixture interactions (McBurney & Bartoshuk 1973). In relation to perceived sweetness of
beetroots the presence of bitter tasting compounds might suppress the perceived
sweetness. In general, bitter tasting compounds are well known for their ability to supress
the perceived sweetness in mixtures (Lawless 1979). In carrots, a positive correlation
41
between sugar content and sweetness has been found at some occasions, even though
carrots also contain bitter tasting compounds (Kreutzmann et al. 2008a; Varming et al.
2004; Seljasen et al. 2001). The content of bitter tasting compounds in beetroots was not
analysed in this PhD project, but phenolic compounds and flavonoids have previously been
identified in beetroots (Kujala et al. 2002; Kujala et al. 2000), thus these compounds
might be responsible for the bitter taste. If the above were true, it would then be expected
that the sweet varieties Chioggia and Touchstone gold had a lower content of bitter
compounds than the other varieties when raw. However, the raw varieties Chioggia and
Touchstone gold were also evaluated higher than the other varieties in the attribute
bitterness, indicating a higher content of bitter compounds.
Sweetness in root crops is caused by the content of the sugars glucose, fructose and
sucrose. The sweetness of Jerusalem artichoke tubers is also influenced by the content of
inulin and its degradation products, the content of which is determined by the season and
the maturity of the tubers. If the precise chemical background for the sweetness of inulin is
to be deduced, the chain lengths of inulin polymers must be identified, quantified and the
sweetness assessed. In beetroots there seems to be an interaction between bitterness and
sweetness, and the total sugar content could not alone describe the perceived sweetness.
Obviously, the relations between bitterness and sweetness in beetroots are complicated,
and analyses of the bitter compounds present in these varieties are needed before further
conclusions can be drawn.
42
6. TEXTURE
The definition of texture is stated as “the sensory and functional manifestation of the
structural, mechanical and surface properties of foods detected through the senses of
vision, hearing, touch and kinaesthetic” (Szczesniak 2002). It includes mechanical
characteristics such as mealiness, geometrical characteristics like graininess, and
compositional characteristics such as wateriness (Szczesniak 1963). Jerusalem artichoke
tubers and beetroots were evaluated on several texture attributes (papers 1, 3, 5), and
especially crispness and mealiness discriminated between varieties and influenced the
appropriateness. The basis for the textural quality can be measured instrumentally or by
analysis of responsible constituents. Both methods were used in an attempt to explain the
texture of culinary preparations of Jerusalem artichoke tubers.
6.1 TEXTURE PROPERTIES
Plants have complex structures, comprising of parts with very different textural
qualities. Root crops are however mainly composed of thin-walled storage parenchyma
cells. The texture of plant material is determined by cell wall characteristics and the size
and distribution of vacuoles and intercellular air-spaces. During mastication, the cell wall
undergoes deformation or breaking depending on the cell wall properties (Waldron et al.
1997b). The texture properties of root crops are influenced by the cell wall polymer
composition and the turgor pressure of the cells. Root crop cell walls are composed of 90%
polysaccharides and 10% phenolic compounds and glycosylated proteins Smith et al.
(2003). The polysaccharides are normally cellulose, hemicellulose and pectic substances.
Adhesion between cells is caused by crosslinking of phenolic compounds attached to
polysaccharides of the cell wall by ester bonds. If cell walls are weak, and cell adhesion
strong, the cells walls will rupture during mastication by crack propagation. The food will
be perceived as hard and crisp, and if liquid inside the cell is released, also juicy. If cell
walls are strong and the adhesion between cells weak, the cells will separate instead of
rupture during mastication, resulting in a mealy or gritty sensation (Smith et al. 2003).
Turgor pressure is responsible for keeping the cells rigid, as water flows down an osmotic
gradient in to the vacuole of the cell filling out the cell wall compartment. If turgor
pressure is lost the cells becomes flaccid. Cells with high turgor pressure are perceived as
stiff or hard, whereas flaccid cells are perceived as rubbery. During heat treatment of root
crops, the cell membranes are disrupted, and turgor pressure is lost as water leaches from
the cells. During boiling, uptake or adsorption of water reduce the cohesiveness and soften
43
the cell walls (Taherian & Ramaswamy 2009). Besides this, pectic polymers involved in cell
adhesion are degraded by β-elimination at increased temperatures (Ng & Waldron 1997;
Keijbets & Pilnik 1974), and the content of divalent cations, especially Ca2+ and Mg2+ can
reduce softening during heat treatment, as the ions cross-link the pectic polysaccharides
involved in cell adhesion (Favaro et al. 2008). The tissue firmness of root crops is lost
during heat treatment in a two-step process (Taherian & Ramaswamy 2009). The first step
is a rapid loss of firmness, within the first few minutes of heat treatment, caused by loss of
turgor pressure (Greve et al. 1994b). The second step is slow persisting for the rest of the
process, and is related to loss of cell wall integrity caused by loss of pectic compounds
(Greve et al. 1994a).
6.2 MEASURING ROOT CROP TEXTURE
The multi-parameter perceptions of texture render them very difficult to relate to
instrumental measurements or analyses of food composition. In paper 3, instrumental
texture analysis was performed on raw, boiled and baked Jerusalem artichoke tubers, with
the aim of describing the sensory variation. Previously, relations between sensory
perceived texture and analytically measured texture, have been found in several root crops
(Goldner et al. 2012; Beleia et al. 2004; Thygesen et al. 2001; Thybo & Martens 2000;
Truong et al. 1997; Van Marle et al. 1997). Instrumental texture analysis includes among
others compression tests, puncture tests or penetration tests. Compression tests are easy to
perform and also the most widespread texture analysis technique in food science. During a
compression test, the sample is compressed by a given load until it cracks or until a
predetermined level of compression is reached. The result of the analysis is a force-
deformation curve. The first compression can be followed by another cycle of compression
on the same sample, in which case the test is called a texture profile analysis (TPA). In
paper 3, a compression test with one cycle was performed on cylinders of raw, boiled and
baked Jerusalem artichoke tubers. A representative graph from the instrumental texture
analysis is seen in Figure 14. From the force-deformation curve in Figure 14 the hardness
and modulus of the sample was calculated. The hardness is the maximum force applied to
the sample i.e. the peak of the curve. During compression of the raw and boiled tubers the
cylinders cracked, whereas the boiled tubers were mashed in the point of maximum force.
44
The rise of the force-deformation curve is ideally linear in the part from 20-80%
compression, were the modulus of deformation is calculated as the slope. In Figure 14 it is
seen that the curves for the boiled and the baked tubers were linear in this area, but the
curve for the raw tubers were not and the calculated modulus might be misleading. The
modulus of a sample can often be related to the elastic properties of the sample and is an
expression of the stiffness (Taherian & Ramaswamy 2009). If the analysis had been
performed with two compressions, a second peak of the curve would have been seen in the
plot, and more texture parameters such as adhesiveness, cohesiveness, gumminess and
chewiness could have been derived (Thybo & Martens 1999).
Crispness is defined as the degree to which a food product suddenly breaks when
chewed, and is assessed by our hearing as well as the sense of touch. Sound emitted during
chewing has been measured to assess the degree of crispness of food samples, and positive
correlations have been made between the volume of the sound and the sensory evaluated
crispness (Zampini & Spence 2010; Vickers 1985). Juiciness can be measured as the
amount of liquid released from the food during compression (Smith et al. 2003).
6.3 TEXTURE OF CULINARY PREPARED ROOT CROPS
In paper 3, the texture of raw, boiled and baked Jerusalem artichoke tubers was
assessed by sensory evaluation and instrumental analysis. The tubers decreased in
hardness and modulus when they were boiled and baked. In the sensory analysis Rema
was evaluated most crisp and least mealy. Draga on the other hand was the most mealy
and least crisp (paper 3). In the mealy Draga, the cell adhesion strength is low and the cells
had separated during boiling. In the crisp Rema, the adhesion strength was high and
0
10
20
30
Time(s)
Loa
d(k
g)
Figure 14. Compression curve obtained from instrumental texture analysis of raw, boiled and baked Jerusalem artichoke tubers. Top curve, raw; middle curve, boiled; bottom curve, baked.
45
during eating the cells were still joined, but the cell walls disintegrated (Smith et al. 2003).
From the appropriateness results on Jerusalem artichoke tubers (paper 5) and beetroots
(Figure 8) mealiness was considered an inappropriate attribute. However, this is not the
case for all root crops. In boiled white yams, cassava and sweet potatoes mealiness is
considered a positive attribute (Franck et al. 2011; Akinwande et al. 2007; Kulembeka et al.
2004).
In paper 3, it is shown that Jerusalem artichoke tubers loose weight after boiling and
baking. Beetroots increase in weight after boiling and decrease in weight after baking (data
not shown). This latter tendency is also seen in starchy root crops such as cassava (Beleia
et al. 2004) and sweet potato (Leighton et al. 2008). The weight loss during baking is
expected as water evaporates from the root crops. During boiling the sugar-containing
beetroot absorbs surrounding water. To test whether the decrease in weight after boiling of
Jerusalem artichoke tubers was caused by loss of inulin and sugars to the cooking water,
the glucose, fructose, sucrose and inulin content was determined in the water by the same
HPAEC method used for sugar and inulin analysis of the tubers. The cooking water
contained no inulin, but had a high content of sucrose (data not shown). The content of
sucrose in the cooking water was too high to only originate from the sucrose content of the
tubers. This shows that inulin was degraded either before leaching into the cooking water
or in the water. Previously, different results of the thermal stability of pure inulin have
been reported (Panchev et al. 2011; Böhm et al. 2005; Kim et al. 2001). In this case the
thermal degradation of inulin in a real food system was shown to occur already at 100°C, in
line with Scher et al. (2009), who found that inulin in yacon tubers started degrading at
70°C.
The DM of potatoes is a determinant of the texture of both raw and cooked potatoes
(Seefeldt et al. 2011a; Van Dijk et al. 2002; Thybo & Martens 1999). Studies have shown
that, raw potatoes with low DM content are soft, and potatoes with high DM are more firm
and hard (Gilsenan et al. 2010). Besides this, potatoes with high dry matter are more mealy
after boiling (Ukpabi et al. 2011; Kaur et al. 2002; Thybo & Martens 2000). The DM of
potatoes is mainly composed of starch. Starch in root crops is present in amorphous and
crystalline forms in starch granules. During heat treatment, the crystalline regions are
disrupted, water absorbed and the starch gelatinised (Adams 2004). In potatoes, the
gelatinised starch can in some cases fill up the entire cell, in which case the potato will be
considered mealy (Adams 2004; Martens & Thybo 2000).
46
In raw, boiled and baked Jerusalem artichoke tubers we found no correlations
between DM and any sensory texture attributes (paper 3), and neither in raw and boiled
beetroots (unpublished results). In Jerusalem artichoke tubers, the major part of the DM is
composed of inulin (paper 1), which also gelatinises with heat treatment, although the
underlying mechanisms are not as well understood as in starch. Inulin gels are formed
from tri-dimensional networks of insoluble crystalline inulin particles in water, which
provides a smooth texture and mouth feel. The network can immobilise large amounts of
water (Franck 2002). Fructose and FOS are incapable of forming gels. Formation, strength
and rheological properties of carbohydrate gels are affected by concentration of
carbohydrate and salts as well as temperature and pH (Kim et al. 2001). Kim et al. (2001)
showed that heat induced gel formation of inulin started at a concentration of 15% at 40°C,
but also that increasing the temperature above 80°C inhibited gel formation, because of
inulin hydrolysis. Longer chains of inulin are less soluble in water than short chains and
will therefore have higher crystallisation temperature and less water-binding capacity (De
Gennaro et al. 2000; Hébette et al. 1998). The sensory attributes smoothness and
creaminess were not evaluated in these studies. They could have been indicators of the
amount of gelatinisation occurring in the tubers during heat treatment.
Beetroots are more thermally stable in relation to texture than Jerusalem artichoke
tubers. Beetroots contain substantial amounts of ferulic acid dimers, which are involved in
cross-linking of pectic polysaccharides between cells, leading to a strong cell adhesion even
after heat treatment (Waldron et al. 1997a). Softening of beetroots after boiling is only
caused by fracture of cell walls and loss of turgor pressure.
Texture is one of the most important quality attributes of root crops, but also one of
the most challenging characteristics to measure instrumentally. No direct connection
between the instrumentally measured texture and the sensory evaluated texture attributes
was found for Jerusalem artichoke tubers. Neither was a clear relation between total sugar,
total inulin content and texture. It is highly possible that the balance between degradation
and gelatinisation of inulin during heat treatment influences the texture development,
along with the turgor pressure and cell wall characteristics.
47
7. COLOUR
Root crops exist in a diversity of colours, both between species but in some cases also
within species. The colour of root crops stems from the contents of carotenoids, betalains
and anthocyanins. Discolouration may occur during cutting and processing of raw root
crops or during heat treatment. The colour intensity and colour changes of root crops can
be assessed visually by sensory analysis or measured spectrophotometrically. In addition
colour potential can be estimated by analysing the content of constituents responsible for
colour and discolouration. In Chapter 1, the importance of colour in relation to consumer
preference and acceptance was described. This applies both to the intensity of the original
colour and to the degree of possible discolouration. To understand the biochemical
background for the discolouration, constituents involved in the enzymatic browning and
after-cooking darkening were determined, related to the sensory evaluated colour
attributes, and instrumentally measured colour.
7.1 PIGMENTS IN ROOT CROPS
The colour of beetroots is caused by betalains. Betalains are nitrogenous compounds
and can be divided into red-violet betacyanins and yellow betaxanthins. Betalains have
only been identified in plants of the Caryophyllales order, and they are mutually exclusive
with anthocyanins in their natural occurrence (Svenson et al. 2008; Kujala et al. 2000).
Betalains are water-soluble and thus colour pigments leaches into the water when
beetroots are boiled or washed. Carotenoids are the most widespread pigment in root
crops. They have yellow, orange and red colour notes, and are classified into carotenes and
oxygen-containing xanthophyll’s (Lesellier et al. 1993). Carotenoids are responsible for the
colour of red, yellow and orange carrots, sweet potatoes and potatoes. Carotenoids are not
water-soluble, but can thermally degrade during heat treatment (Mazzeo et al. 2011; Miglio
et al. 2008). The colours of red and purple carrots and potatoes are caused by
anthocyanins (Li et al. 2012). Anthocyanins are water-soluble flavonoids with red, purple
and blue colour notes depending on pH. Jerusalem artichoke tubers have white flesh,
which do not contain pigment compounds, but as shown in Table 1, paper 5, Jerusalem
artichoke tubers can have white, red and purple skin colours as a result of differences in
anthocyanin content. The biggest concern relating to colour of white root crops is
discolouration reactions resulting in darkening, whereas for colourful root crops the loss
and degradation of pigments is the largest concern.
48
During heat treatment, the colour of root crops may change as a consequence of
Maillard reactions and caramelisation. The Maillard reaction products can be unsaturated,
brown nitrogenous compounds. Caramelisation produces the same reaction products in
the absence of amines, but at higher temperatures, as the amines function as a catalyst
(Van Boekel 2006).
7.2 ENZYMATIC BROWNING
Some root crops undergo enzymatic browning after cell damage, such as cutting and
bruising, when the enzyme polyphenol oxidase (PPO) is mixed with its substrates in the
presence of oxygen. The substrate for PPO are oxidisable hydroxy groups. PPO converts
monophenols to o-dihydroxyphenols, and o-dihydroxyphenols to o-benzoquinones. Both
reactions requires oxygen as a co-substrate (Martinez & Whitaker 1995). The o-
benzoquinone subsequently polymerises non-enzymatically to the pigment melanin
(Figure 15). Melanins can be brown, black or red (Cantos et al. 2002).
Enzymatic browning is clearest in light-coloured root crops such as Jerusalem
artichoke tubers and potatoes. However, coloured vegetables like red beetroots also
undergo enzymatic browning, although the dark discoloration is masked by the betalain
pigments. Factors determining the degree of enzymatic browning are PPO activity, content
of phenolic compounds, pH, temperature and oxygen availability (Martinez & Whitaker
1995). Enzymatic browning of fruits and vegetables can be prevented or slowed by
lowering the pH, by immersing the product in water thereby reducing the available oxygen,
by cold storage or by addition of ascorbic acid. Ascorbic acid can reduce o-quinones back to
o-phenols.
In this project the content of total phenolics in raw and boiled Jerusalem artichoke
tubers was assessed by the Folin-Ciocalteu method (FC) in order to investigate whether
any relations between the content of phenolics and the degree of enzymatic browning
could be identified (paper 4). The Folin-Ciocalteu reagent is a yellow phosphomolybdic/-
FIGURE 15. The mechanism of enzymatic browning.
OH
R
OH
OH
R
O
O
R
OH2+½ O2 ½ O2
PPO PPO
monophenol o-dihydroxyphenol o-dihydroxyquionone
melanin
49
phosphotungstic acid reagent, which is reduced by phenolic and non-phenolic reducing
compounds to a blue complex that can be measured spectrophotometrically at 765 nm
(Singleton et al. 1999). FC is a non-specific method, as it does not only measure phenolics.
Some of the non-phenolic compounds oxidized by FC reagents are aromatic amines,
sulphur dioxide, Fe2+, tryptophan, tyrosine, guanine, xanthine, hydrogen sulphide,
reducing sugars and ascorbic acid (Gülçin 2012). The result of FC are normally designated
as total phenolics, but in reality FC is an expression of the total reducing capacity of the
sample. In agreement with this, linear correlations have been shown between FC and other
electron transfer based antioxidant assays (Bavec et al. 2010).
Ascorbic acid reacts readily with FC and interactions with the assay results are
considerable at concentrations above 1.0 mg/L (Magalhães et al. 2010). In paper 4 the
content of ascorbic acid in raw and boiled Jerusalem artichoke tubers was analysed using
1H NMR. No ascorbic acid was found in raw Jerusalem artichoke tubers (paper 4), but it
was detected in boiled tubers. The ascorbic acid content was not quantified in this project,
but the vitamin C content of raw tubers has previously been determined to be 5-8 mg/100g
(Saxholt et al. 2008). No data on pure ascorbic acid content in Jerusalem artichoke tubers
has been found. As seen in the results of paper 4, the content of total phenolics in root
crops decrease with maturity and storage, as phenolics are consumed in protection of the
plant from injuries and environmental attacks.
Attempts have been made in order to relate the activity of PPO and the enzymatic
browning of fruits and vegetables. Some studies have shown no correlation between PPO
activity, phenol content and enzymatic browning in potatoes (Cantos et al. 2002), whereas
other studies have (Cabezas-Serrano et al. 2009; Thybo et al. 2006). It has been suggested
that PPO is not the only enzyme involved in enzymatic browning. The enzyme peroxidase
(POD) has been suggested to play a role in the process of enzymatic browning. POD is
normally associated with wound-healing processes in fruit and vegetables. It performs
single-electron oxidation of phenolic compounds in the presence of hydrogen peroxides.
The role of POD in enzymatic browning of fruits and vegetables is however, debatable
because the content of hydrogen peroxide in fruit and vegetables is normally low. But as
hydrogen peroxide is generated in PPO-catalysed reactions, a synergistic effect between
PPO and POD could be expected (Cantos et al. 2002). If there are plenty of phenolics in the
root crop, enzymatic browning could be expected to be limited by PPO activity. On the
other hand, if polyphenols are scarce the browning have been shown to be correlated to the
50
activity of the enzyme phenylalanine ammonia-lyase (PAL), which is a key enzyme in the
synthesis of phenolics activated by tissue damage (Cabezas-Serrano et al. 2009).
Cantos et al. (2002) investigated the activity of PPO in fresh potato tissue. They
added the substrate catechol and exogenous PPO to raw potato material, in order to see the
browning potential, and not just evaluate the correlations of extracted compounds. They
found that neither PPO activity nor the content of phenolics were rate limiting in the
browning of potatoes. In the future, the employment of this method to Jerusalem artichoke
tubers could relate the correlations investigated in paper 4 to true reactions occurring in
the tissue. Not all substrates have the same affinity for PPO, and knowledge of the exact
phenolic composition and their individual affinities could give a much more precise
impression of the influence of phenolic compounds and PPO on enzymatic browning.
7.3 AFTER-COOKING DARKENING
After-cooking darkening is a phenomenon, which is primarily known from potatoes,
where the industry invests much effort in the prevention of after-cooking darkening of
processed potato products, such as French fries. After-cooking darkening of potatoes
occurs when the potatoes are exposed to oxygen after heat treatment. During cooking, a
colourless Fe2+-chlorogenic acid complex is formed, which oxidises to a bluish-black Fe3+-
chlorogenic compound on exposure to oxygen (Figure 16) (Wang-Prusky & Nowak 2004).
The after-cooking darkening is non-enzymatic and occurs after all heat treatments,
including boiling, baking and frying. In some cases the darkening can be disguised by the
simultaneous browning caused by Maillard reactions and caramelisation. During this PhD
project it was observed that Jerusalem artichoke tubers turned grey upon boiling, and in
paper 4, the potential and degree of after-cooking darkening was investigated. In after-
cooking darkening of potatoes only chlorogenic acid is considered to contribute, as it
OH
OH
O
O
OH
OH
OH
O
OH
Chlorogenic acid
C16H18O9-Fe2+ �(C16H18O9)2-Fe3+
Colourless Dark coloured
A B
FIGURE 16. Chemical structure of chlorogenic acid (A) and the chemical reaction of after-cooking darkening between chlorogenic acid and iron (B).
51
constitutes 90% of the total phenolic acid content of potatoes (Friedman 1997). But
actually several phenolic acids are capable of producing dark-coloured complexes with Fe3+
(Hughes & Swain 1962). In foods other than potatoes it would be interesting to unfold the
colours, stabilities and the kinetics of iron-phenolic acid complexes. In paper 4, the
content of iron and the content of phenolic acids were analysed in order to investigate the
potential for complex formation.
The tendency to after-cooking darkening in potatoes is related to the size of the
tubers; the bigger the tuber the more after-cooking darkening takes place (Siciliano et al.
1969). This can be ascribed to the higher content of ascorbic acid of the smaller tubers. In
the study in paper 4, the sizes of the tubers were the same in the first and second harvest
(unpublished data), and there where no differences in content of ascorbic acid.
7.4 DISCOLOURATION OF JERUSALEM ARTICHOKE TUBERS
In paper 4, the colour changes of raw and boiled Jerusalem artichoke tubers is
described. In order to elucidate the time dependency of the enzymatic browning of the
tubers in paper 4, ten whole tubers of each variety were cut transversally, and the internal
colour measured immediately, and every 20 seconds for 10 minutes. The results of the
instrumental colour analysis was expressed as L*, a* and b*-values. L* expresses the
lightness of the sample, the a*-value expresses the red-green colour an b* the yellow-blue
colour of the sample. A diagram of the Lab colour space is seen in Figure 17.
FIGURE 17. Representation of L*,a*,b* colour space.
52
The results of the time-dependent enzymatic browning of raw Jerusalem artichoke
tubers are shown in Figure 18. Mari had a lower starting point than the other varieties,
with an L*-value of approximately 70 as opposed to the other varieties starting points
around an L*-value of 72. All three curves showed a rapid initial decline, followed by a
decrease in the slope of the curve. The curve of Rema showed the fastest initial decline and
reached a plateau after approximately 200 seconds, whereas the other two varieties did not
reach a plateau within the 600 seconds measured. The a* and b*- values increased as a
function of time, towards a less green and more yellow colour. The b*-values showed the
opposite tendency of L*, with Mari having the highest values and Rema the lowest,
whereas Draga had the lowest a* values. Clearly, the process of enzymatic browning in
Jerusalem artichoke tubers was rapid. To avoid browning when raw tubers are to be used,
consumers should take precautions immediately after cutting, such as soaking in water or
adding acid.
Paper 4 describes the colour changes of raw and boiled Jerusalem artichoke tubers,
but instrumental colour analysis and descriptive sensory analysis were also performed on
baked tubers. The assessors in the sensory panel were instructed to evaluate the browning
on the bottom side of the cubed tubers, in order to avoid influence of darkening because of
Maillard reactions, and the instrumental analysis was performed on that side as well. The
whiteness was evaluated on the inside of tubers cut in half. The results of the instrumental
and sensory analysis of colour are shown in Table 8.
FIGURE 18. Instrumental colour analysis of raw Jerusalem artichoke tubers as a function of time after cutting (unpublished data). Data are presented as mean (n= 10). Rema, Mari, Draga.
53
TABLE 8. Instrumental colour analysis and sensory evaluation (italics) of baked Jerusalem artichoke tubers. Sensory was evaluated on a scale from 0 (low intensity) to 15 (high intensity), data presented as mean of repetitions and assessors. Instrumental data are presented as mean (n = 10).
Harvest 1 Harvest 2
Mari Draga Rema Mari Draga Rema
L* 60.5 aa 52.4 c 57.9 b 57.9 a 54.0 c 55.8 b
a* -1.5 c -1.2 a -1.8 b -1.0 b 0.7 a 0.0 c
b* 12.4 a 10.5 ab 11.6 a 10.8 a 11.8 b 12.7 a
Browning 5.3 b 12.3 a 8.6 b 7.0 a 7.4 a 8.8 a
Whiteness 9.9 a 3.4 b 7.3 a 8.5 a 6.6 a 6.5 a aDifferent letters indicate significant differences (P ≤ 0.05) between varieties within harvest times.
The baked Jerusalem artichoke tubers all had significantly lower L* values than the
raw tubers, and in both harvest times tubers of Draga had a lower L* value than the other
two varieties. In the first harvest Draga was evaluated more brown and less white
compared to the other two varieties by the sensory panel. In the second harvest there were
no differences between varieties with concern to the sensory attributes. This separation of
Draga from the other varieties was not seen in boiled and raw tubers (paper 4). Actually, in
boiled tubers, the variety Mari separated from the others by higher scores in whiteness and
L*. Both boiled and baked tubers were expected to undergo after-cooking darkening by
reactions between iron and phenolic acids, and it was therefore expected to see the same
pattern of discolouration in the boiled and the baked tubers. As this was not the case, it
could indicate that it is not the same process, which is responsible for the discolourations
during boiling and baking of Jerusalem artichoke tubers. A possible explanation could be
that PPO is not fully inactivated during the culinary preparation. The boiling time of the
Jerusalem artichoke tubers in this study was 90 seconds. PPO activity in extracts of whole
Jerusalem artichoke tubers after 2 min boiling has been investigated with different results:
Tchoné et al. (2005) found the activity to decrease to 1% of its original value, whereas
Takeuchi and Nagashima (2011) found the activity to decrease to only 50% of its original
value. It is hard to say whether these data can be directly translated to the activity in real
tubers, but if PPO activity remained in the tubers after boiling, enzymatic browning could
be contributing to the discolouration of boiled tubers. The baked tubers were heat treated
for 20 min at 180°C, and thus PPO activity is expected to be completely abolished.
In paper 5, consumers evaluated the colour intensity of raw Jerusalem artichoke
tubers on a scale from 1-5. This was an expression of the degree of enzymatic browning and
54
was found to be highly influential on the appropriateness of the tubers for raw
consumption.
Results of a consumer study on five varieties of beetroot in raw, boiled and pan-fried
preparation was described in section 3.4, and Figure 8 showed that colour intensity of raw
and boiled beetroots was positively related to appropriateness. The assessed colour
attribute was defined as the colour intensity of the original colour of the root crop. In the
study, the pink and white striped variety Chioggia scored significantly lower in colour
intensity, than the other varieties after boiling and pan-frying. During boiling the pink
colour of the striped Chioggia were lost, as pigments leached into the cooking water and
during both boiling and pan-frying the white stripes turned grey. This rendered the
beetroot less appetizing. The grey colour of the stripes suggests the presence of after-
cooking darkening.
Discolouration in the form of enzymatic browning and after-cooking darkening of
Jerusalem artichoke tubers, as well as loss of colour pigments in beetroots are undesirable
quality characteristics. The translations between sensory evaluated colour attributes,
instrumentally measured colour and chemical composition are far from simple and no
satisfying replacement for the sensory analysis was identified in this project.
55
8. CONCLUSIONS AND PERSPECTIVES
This thesis focus on elucidating the chemical background and on describing the
sensory quality changes of root crops in relation to harvest time, variety and culinary
preparation. Texture, colour and taste were found to be the primary descriptors
responsible for the quality variation and consumer acceptance of root crops.
Sensory differences between varieties of both Jerusalem artichoke tubers and
beetroots were primarily related to taste and texture. Raw Jerusalem artichoke tubers had
very low scores in sensory evaluation of aroma and flavour attributes. Sensory differences,
between varieties of Jerusalem artichoke tubers, were evened out when tubers were boiled,
baked or pan-fried (papers 3, 5). There were large sensory differences between varieties of
raw beetroots, but these were also evened out when the roots were heat treated. Raw
beetroots were both bitter and sweet.
There were no differences between varieties of Jerusalem artichoke tubers in the
appropriateness for raw, boiled and pan-fried consumption (paper 5). Appropriate
Jerusalem artichoke tubers should in all preparations be sweet, have positive texture
attributes – such as being crisp when raw and juicy when boiled, and they should have a
high colour intensity and a low degree of enzymatic browning (paper 5). Appropriateness
of beetroots was associated with sweetness, beetroot flavour, juiciness, crispness and
colour intensity. The studies on appropriateness and reviewing of literature on consumer
data, revealed that for different root crops to be accepted and used by the consumers, it is
the same quality parameters which should be fulfilled, i.e. the root crops should be sweet,
crisp and have bright colours, but should not be mealy, bitter and discolour during
preparation. These results can probably be transferred to other root crops as well, and be
used when new root crops are introduced on the market.
The content of volatile compounds in Jerusalem artichoke tubers (paper 1) and
beetroots was low, and the content decreased even further during culinary preparations.
The isolated volatile compounds were mainly terpenes, but during culinary preparation,
Maillard and lipid oxidation products were formed. The low scores in sensory evaluation of
aroma and flavour attributes, and the low concentrations of collected volatiles fits well
together, but it renders the volatile profile problematic to use as a quality descriptor of
Jerusalem artichoke tubers and beetroots. A method to extract more volatiles from root
crops should be developed and the extracts further investigated by GC-O or optimally
aroma-recombination experiments, before better correlations can be developed between
the sensory impression and the volatile profiles of the root crops.
56
The choice of method for extraction of volatile compounds has a high influence on the
final volatile profile. In this thesis extraction of volatiles from carrots by SPME, DH and
solvent extraction was compared. The three extraction techniques resulted in three distinct
GC chromatograms, but it could not be concluded which method was the most
representative of the sensory quality of the carrots. If the number of samples was larger
and the sensory variation was bigger, multivariate data analysis could be used to
investigate data obtained from several aroma extraction techniques and to identify the one
resulting in the chromatogram, which best represented the sensory data.
Sweetness was identified as one of the most important descriptors of root crop
quality, but both in Jerusalem artichoke tubers and in beetroots, simple correlations to
sensory sweetness were rarely seen. In Jerusalem artichoke tubers the sweetness and sugar
content was determined by the maturity of the tuber at the time of harvest (papers 1, 2, 3).
The inulin in the tubers degraded to shorter and sweeter polymers during overwintering in
the soil (paper 2), but the impact of inulin content and inulin polymer lengths on the
sensory sweetness was not clear. Isolation and quantification of the individual polymers in
Jerusalem artichoke tubers, and not only the total content, could be used to deduce their
precise impact on sensory sweetness. The content of sugars and inulin in Jerusalem
artichoke tubers decreased during heat treatment because of degradation, leaching out,
and participation in Maillard reactions (paper 3, 5). The sensory attributes bitterness and
astringency were identified in beetroots, but the chemical background for these
compounds was not investigated. It is likely that a high content of bitter and astringent
compounds could influence sensory sweetness. It would be relevant to identify the bitter
and astringent compounds in beetroots, and to elucidate their contribution to bitter taste
and astringency in beetroots. Bitter tasting compounds often have positive health-related
properties, but too high contents can result in decreased consumer acceptance. Beetroots
would be an excellent real food system for further investigations of bitterness, sweetness
and astringency and their interactions in root crops.
Sensory analysis and instrumental texture analysis showed that Jerusalem artichoke
tubers softened during heat treatment. A positive correlation was found between mealiness
and instrumental hardness of boiled tubers (paper 3). The season influenced variation in
texture, probably because of the different polymer lengths of inulin. Further conclusions
on Jerusalem artichoke texture require characterisation of the exact inulin polymer
composition, and more knowledge on the technological aspects of inulin during heat
treatment of tubers. Inulin behaviour and composition could be monitored by scanning
57
electron microscopy or fluorescence microscopy to investigate degradations, water binding
properties and gelatinisation during culinary preparation.
Jerusalem artichoke tubers showed enzymatic browning when raw and after-cooking
darkening when boiled (paper 4). Both discolourations were considered inappropriate by
consumers (paper 5). It was not possible to identify the responsible chemical components
in the tubers by correlation studies between phenolic compounds, iron and organic acids
and sensory assessed and instrumentally measured colour changes. Many parameters can
play a role in enzymatic browning and after-cooking darkening reactions, and this PhD-
study only considered a part of them. Further studies could include analysis of relevant
enzyme activities, characterisation of phenolics, along with their affinity for the enzymes. It
could also be attempted to isolate the dark complexes formed during enzymatic browning
and after-cooking darkening of Jerusalem artichoke tubers. Finally the analysis of the
enzymatic browning reactions could be investigated in real live tubers instead of extracts.
This project has contributed to an understanding of the chemical background of root
crop diversity, and of the processes occurring during culinary preparation in relation to
aroma, flavour, taste, texture and colour changes. The choice of the right root crop variety
is highly relevant when the root crops are to be eaten raw, but as differences are less
pronounced after culinary preparations the choice of variety becomes less important. The
results showed that maturity and variety of root crops were determinant for the quality.
This knowledge can be employed by growers, which can exploit the natural variation of
root crops in raw material production and product development. There are economical,
environmental and health benefits from increasing the Danish consumers intake of root
crops. These results can be used to guide consumers in the choice of the right root crops, in
turn leading to increased consumer satisfaction and acceptance. Therefore, the results can
indirectly aid in increasing the consumption of root crops and thus also in increasing
public health.
58
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