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PLASTICS IN FOOD PACKAGING

PACKAGING ASPECTS OF FRUITS AND VEGETABLES

Chapter 6PACKAGING ASPECTS OF

FRUITS AND VEGETABLES

R S Matche

Food Packaging Technology DepartmentCentral Food Technological Research Institute

Mysore 570 020 (INDIA)

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PACKAGING ASPECTS OFFRUITS AND VEGETABLES

PRODUCT CHARACTERISTICSFruits and vegetables are living organisms

even after harvesting; they can remain freshonly as long as normal metabolism continues.Metabolism involves absorption of oxygenwhich breaks down the carbohydrates in theproduct to water and carbon dioxide. If theavailability of oxygen is restricted, thechemical reaction changes and smallquantities of alcohol are produced. Thisresults in off-odours and flavours andbreakdown of plant cells. This series ofevents is called anaerobic decay and canspoil fruits or vegetables within a few hours.Fruits and vegetables have very high moisturecontent, ranging from 75-95%. Theirequilibrium relative humidites are as high as98%. Under any normal atmosphericcondition, they will dry rapidly. This causeswilting and shriveling due to loss of rigidityand shrinkage of the cells. Proper packagingcan prolong the storage life of fresh fruitsand vegetables by preventing moisture lossand thereby wilting. The rate of moisture lossvaries with the product and water vapourpermeability of the packaging film. The useof small perforations for oxygen permeationhas an insignificant effect upon moistureloss. A type of spoilage much prevalent in

fruits and vegetables is that caused bymicroorganisms such as yeasts, molds, andbacteria. These organisms can causedestruction by growing on the exterior ofthe product or they may invade the interiorthrough a surface bruise or cut and causeinternal decay. Therefore careful handlingand packaging are very important inpreservation of freshness and quality.

Normal ripening of fruits and vegetablescauses alterations in colour, texture, odourand flavour. At some point, for each type,ideal ripeness is achieved. Beyond thatpoint, the product becomes overripe andquality deteriorates. The primary goal offresh produce merchandising is to deliverthe product to the consumer at such a pointin the ripening scale, that it will achieveperfect ripeness at time of eating. This ofcourse is extremely difficult to do. Inpractice, the produce is delivered somewhatunderripe at the time of purchase and theconsumer delays consumption untilripening is completed. Since all theseprocesses are highly sensitive totemperature, they can be slowed down bystoring the produce under refrigeration.Each fruit or vegetable has an idealtemperature for storage. If this temperature

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is not used, deleterious result occurs—forexample, tomatoes will not ripen if chilledbelow 4°C, bananas will turn black below11°C and potatoes develop a sweet flavourbelow 4°C. An additional reason for thenecessity of refrigeration for fresh produceis the heat generated due to metabolism orrespiration. For example, green beans, sweetcorn, broccoli, green peas, spinach, andstrawberries generate from 15,000 to 50,000Btu of energy per ton per 24 hours of storageat 15°C. Even when chilled to 00 - 40C, theystill evolve 2,500-17,000 Btu per ton per 24hours. This heat must be taken intoconsideration in the design of refrigerationequipment. Some types of fresh producegive off volatile compounds during ripeningwhich will impart unacceptable odour andflavour if not allowed to escape, or theymay prematurely ripen the fruit.

PACKAGING REQUIREMENTS

For the most part, packaging cannotdelay or prevent fresh fruits and vegetablesfrom spoilage. Incorrect packaging canaccelerate spoilage. However, packagingcan serve to protect against contamination,damage and, most importantly, againstexcess moisture loss. Too much of a moisturebarrier will cause an excessively highrelative humidity in the package and resultin accelerated spoilage due to micro-organisms or in skin splitting on some fruits.

PACKING HOUSE OPERATIONS

The packaging line begins withunloading of harvested produce. Bulk binsof 200-500 kg capacity are used for bigpackaging houses but in India, reusableplastic crates are used. Freshly harvestedproduce is stacked up for interim storage.Interim storage may serve several purposes,depending on the type of produce. Latest

damage, sustained during harvesting, willappear as visual defects several hours laterand can be detected. Storing field-warmproduce for few hours in a cool place, suchas cold storage corridors, whereby producetemperature is reduced by several degreesbefore processing will help reducingsubsequent spoilage and damage in thepackaging line.

The most important process in freshproduce is respiration, a biochemicaloxidation of all living cells. Respiration rateis proportional to temperature, approxi-mately doubling every 10°C. Due to highrespiration, heat build up will be more,which in turn increases the temperatureand respiration of produce. This reducesshelf-life of the produce. The heat producedmay be calculated by

1 mg CO2/kg hr. = 61.2 k.cal/ton. day =220 BTU/ton.day.

The term, precooling, refers to severalpractices whereby the temperature of thefreshly harvested produce is quicklylowered to shorten the period of initial highrespiration rate as well as to reduce theloads on the long-term cold storage facilities.Success of efficient precooling dependsupon fast removal of field heat from allfruits, preferably within 2 to 3 hours.Regular cold storage room with about 150air changes per hour gives best result, butwith danger of excess moisture loss.Hydrocooling consist of drenching field-warm produce with a stream of cold watertaking care of chilling injury causedespecially to leafy vegetables. There arethree types of hydrocoolers: immersion,flooding and spraying. Another precoolingmethod is vacuum cooling. This systemuses hermetically sealed vacuum chamberswhereby the pressure is reduced until the

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vaporising temperature of water is achieved,which removes moisture uniformly from alltissues, not just from the surfaces.

The produce is subjected to a cleaningprocess, which begins with soaking tank,where dirt clods and pesticide residues aresoftened and diluted by warm or coldsolution of water detergents anddisinfectants. The fruit is thoroughlywashed by piece of cloth or soft brusheswith water spray. The produce is thentreated with fungicide treatment, if any.Before processing further, produce is driedby air streams from overhead fans.

In the next stage, the undersized producecalled as culls is eliminated, which can besent for converting, called as presizer stage.The grading process follows next tosegregate the produce into quality groups,such as ripe fruits which must be marketedimmediately, grade A,B,C, export grade orculls. Before or after grading, high qualityfresh produce processing may includewaxing operation, especially when longshelf-life is desired. Most produce has anatural wax layer on its rind or skin whichprotects it from excessive moisture losswhile allowing free metabolic gas exchange.This wax is largely removed by the cleaningoperation. By applying an artificial wax,the keeping quality of produce is restoredor even improved, by including chemicaladditives to inhibit spoilage or to add glossto colour for sales appeal.

Sizing is an additional sorting operationwhereby sorting is done according to size.Now uniformly sized and graded produceis ready to be packaged. Depending uponproduce type and grade, distance tomarkets, cost and availability of packagingmaterials, the produce may be packed in alarge variety of shipping containers.

The packaging operation usuallyincludes setting up of container, beforequantitizing of produce, filling andcontainer closure. Quantitizing may be bycount or by weight or a combination of thetwo. Accurate sizing provides proportionallink between count and weight wherebyonly ‘check-weighing’ is required afterfilling by count. Container fill may berandom or pattern packed. Pattern packingincreases protection to produce byminimising contact pressure by increasingnumber of contact points to a maximum of12 and maximises volume utilisation.Sometimes, the produce is pre-packed inpackaging house in consumer packs, mostlydifferent types of plastic bags oroverwrapped trays. The final packagingoperation is container closing which maybe performed by gluing, stapling andstrapping. Unitization and palletizationmay be done according to the containerrequirement and market need.

SPECIFIC APPLICATIONS

The particular type of package useddepends upon the shape and perishabilityof the product. There are five main classi-fications—soft fruit, hard fruit; stemproducts; root vegetables; and greenvegetables.

Soft fruits are highly perishable and easilysubject to anaerobic spoilage. They bruiseand crush easily which leads to rotting.They are packaged in semi-rigid containerswith a cover of cellophane, cellulose acetate,polystyrene or other suitable film cover.Sometimes, polyethylene bags with venti-lation holes are used. Adequate ventilationis a must to avoid fogging. Handling mustbe gentle and avoided as much as possible.Shelf-life is limited due to individual damage

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and decay. Some berries under idealconditions only retain top quality for 2 or 3days. Typical soft fruits are cherries, grapes,blueberries, strawberries, raspberries andplums.

Hard fruits are better able to resist damagefrom handling. They are also less perishableand have lower respiratory rates. Shelf-lifeis weeks rather than days. The most commonpackage is an open tray, a plastic filmoverwrap or sleeve. Hard fruits may also bebagged in perforated polyethylene film or innets. Examples of hard fruits are apples,bananas, citrus fruits, peaches, pears andtomatoes.

Stem products are highly perishable asthey rapidly lose moisture. They should bebagged or wrapped in moisture proofcellophane or polyethylene with ventilation,or they should be banded or sleeved withshrink film. Typical stem products are celery,rhubarb, and asparagus.

Root vegetables are not highly peri-shable.They can be stored for long periods; however,

it is desirable to protect them against moisturelosses. They are washed, graded and sizedprior to packaging, which is usually indurable polyethylene bags with perforations.Typical root vegetables include carrots,turnips, radishes, onions, beets, yams andpotatoes.

Types of packaging: Packaging can beclassified in a number of ways; the mostimportant one is by stages of distributionsystem for which it is primarily intended.

• Consumer or unit packaging,

• Transport packaging;

• Unit load packaging.

Fig. 6.3. Root vegetables in pouch

Fig. 6.1. Transparent trays

Fig. 6.2. Apple with individualcushioning of expanded PS net

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CONSUMER PACKAGINGThe package in which consumer receives

the produce is called consumer packaging.The term prepackaging of produce refers toconsumer units prior to its presentation tothe final consumer. Prepackaging may beundertaken at any stage throughout thedistribution chain from the field to theretailer's premises, depending upon theneed of produce for protection, expectedtransport and storage time, required shelf-life, packaging material costs and costs ofpackaging and sorting at different points,transport and storage cost and latestknowledge of the market requirements.

Types of Consumer Package

Bags

Fig. 6.4. Frozen fresh peas in bags

Fig. 6.6. Net bags; perforated bags

Bags are the most common and favouredretail packs because of their low materialand packaging cost. In terms of cost tostrength ratio, 25-40 mm low densitypolyethylene or 12.5 mm high densityFig. 6.5. Grapes in ventilated pouch

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Tray with stretch wrap

Carton withtransparent

window

Fig. 6.7. Trays and cartons

Tray with shrink wrap

Sleeve Packs

These combine the low cost of bags andprotective qualities and sales appeal of traypacks. Wraps of plastic film such aspolyethylene or PVC, in the form of shrink-wrap, stretch film or cling film and regularnet stocking or expanded plastic nettingcan also be used. The traditional fruits andvegetable retail trader packs the produce inthe presence of consumers in the qualitiesand quantities required by them. Thepackage normally used is a simple wrap ofpaper or a paper or polyethylene bag. Sleevepacks can be fabricated to contain from oneto as many as ten fruits. The main advantagein sleeve packs is that they immobilize theproduce at a fraction of cost of tray packsand the produce can be seen from all sideswithout damage to the fruit.

TRANSPORT PACKAGINGTransport packaging for fresh produce

may be divided into two size groups:

i) The predominant size group, suitablefor carrying by man, is in the range of 15to 25 kg.

ii) The other group, recently becomingincreasingly popular in 200-500 kg rangesuitable for fork lift handling is referredas pallet container.

Wooden BoxesIncludes natural wood and industrially

manufactured wood-based sheet materials.Timber used must be inexpensive and easilyworked. All wood that is used for theproduction of the packaging should be welldried in order to prevent cracks and mouldgrowth later. Manufactured wood basedsheet materials include ply wood, hard boardand particle board. Plywood is usually made

polyethylene bags are most suited. Net bagsare used to provide desired ventilation andallow free air movement for the producesuch as citrus fruits, onions, potatoes, etc.The bags can be made of paper, perforatedpolyethylene or polypropylene film, plasticor cotton nets.Tray

Tray packs made of foamed polystyreneor PVC or PP are overwapped with heatshrinkable or stretch films. A tight wrapimmobilizes the fruits and keeps them apart.Trays of moulded pulp, card board,thermoformed plastic or expandedpolystyrene are also used.

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from birch. It is rigid and strong, thoughperhaps somewhat less resistant tosplintering than poplar, but is smoother andflatter to be suitable for direct printing. Hardboard is dark in colour but its appearancecan be improved with decorative printing,but deforms after long storage in high relativehumidity. Particle board is thicker and rigidbut relatively brittle.

Corrugated Fibreboard Boxes

Corrugated fibreboard (CFB) boxes arethe most commonly used shippingcontainers where cartons, glass, cans andpouches are the unit containers. Thepopularity of CFB box as a container in foodindustry as well as in other industrialpackaging is for the following reasons:

1. Low cost to strength and weight ratio.

2. Smooth and non-abrasive surface.

3. Good cushioning characteristics.

4. Excellent printability.

5. Easy to set up and collapsible for storage,

6. Reusable and recyclable market.

Fig. 6.8. Corrugated fibreboard boxes

Plastic Corrugated BoxesThe most commonly used material for

plastic corrugated box is polypropylene andHDPE. Its advantage over CFB is low weightto strength ratio and its reusability. Theprintability is also excellent when comparedto CFB boxes. But CFB box has an edge overplastic fibreboard boxes when cushioningproperties are taken into consideration. Thedisadvantages are ultraviolet degradationand temperature resistant, which can betaken care by use of additives.

Plastic CratesPlastic crates, usually made up of HDPE

or polypropylene by injection mouldinghave been replacing wooden and wirecrates. These crates must have goodresistant properties to ultraviolet degra-dation and shock damages.

SacksThese are flexible shipping containers

which are generally used in food industriesto transport raw materials viz. fruits andvegetables from the field. If the weight ofcontent is more than 10 kg then it is calledsack otherwise bag. The commonly usedmaterials for sacks are cotton, jute, flan,

Fig. 6.9. Plastic crates

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woven plastics (HDPE, Polypropylene).These sacks are advantageous to use as theycost less, have high strength, reusable andrequire little space for the empties.Disadvantage of plastic woven sack is poorstackability due to low coefficient of friction,whcih can be overcome by making anti-slip bags.

Palletization

Pallets have been standardised keepingin view of the standard package sizes andsea containers. The sizes of the pallets are ofstrategic importance since they corresponddirectly to the sizes of various types ofcontainers, ship cargo compartments,trucks, fork trucks, etc. Most commonlyused pallet sizes are 120x80 cm (Euro pallet)and 120x100 cm (Sea pallet). Sea pallets aremost commonly used outside Europe.Reusable plastic pallets are in use. They aremade up of HDPE or polypropylene. Theyare easy to clean and light in weight ascompared to wooden pallets.

Palletized loads are used in order toreduce handling costs by allowingsubstitution of mechanical handling formanual methods with the followingadvantages:

• A decrease in sorting operations.

• Reduced labelling requirement.

• Better utilization of storage space.

Fig. 6.10. Pallet types

• A reduction in mechanical strains anddamages.

• A reduction in total distribution time.

• Better maintenance of product quality.

Two principles are used in the assemblyof pallet loads.

1. The modular principle, in which allpackages are oriented in the samedirection.

2. The two-way principle, in which thepackages in each tier form a patternsuch that some packages are orientedlengthwise and others cross-wise on thepallet.

Unitization

Corner posts made of plastic or wood ormoulded paper boards are generally used ascolumns for unitization. The boxes are heldtogether by means of strapping or stretchwrapping around the boxes as shown in Fig.10. The strapping is of polypropylene.

70-80 cm

50-70 cm

10-815 cm

Europallet Fig.6.11. Unitization of pallets using cornerposts and straps

Sea pallet

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NEW TRENDS

Several new technologies offer thepacker opportunities to modify the atmos-phere inside the shipping package duringdistribution even though such controltraditionally finishes with the sealing ofthe package.

Packaging is termed as “active” when itperforms some desired role other than toprovide an inert barrier to the externalenvironment. The goal of developing suchpackaging is the achievement of a more idealmatch of the properties of the package to therequirements of the food. Hence, it addressesone or more specific needs of the food withoutnecessarily having any impact on other foodproperties.

SOME TECHNOLOGICALINNOVATIONS

Most applications of active packaginginvolve the use of polymers. The role playedby the polymer may be that of a conventionalpackaging material as in the fabrication ofsachets that are used commercially toscavenge oxygen or carbon dioxide, or torelease ethanol or carbon dioxide. When theactive agent is dispersed in a packaging filmor sheet, as in OXYGUARD™ thermoformedtrays (Toyo Seikan Kaisha, Japan), thepolymer acts as a somewhat permeablecarrier. The polymer can play an even moreactive role when it interacts physically orchemically with the components of theheadspace of the package. Such interactionsinclude absorption as in the case of humiditybuffering linings for produce cartons, orreactions as in the case of oxygen scavengingplastics.

Permeability Regulation viaThermally Sensitive PackagingMaterials

A significant development is that of side-chain-crystallizable (SCC) polymers with theability to effectively and reversibly melt asthe temperature increases and thus fosterincreased gas transmission through them.SCC polymers are acrylics with side-chainsindependent of the main chain. By varyingthe side-chain length, the melting point canbe altered. By making the appropriatecopolymers, it is possible to produce anymelting point from 0 to 68°C., well within theextreme distribution temperature range ofminimally processed foods. When elevatedto the switch temperature, SCC polymersbecome molten fluids which are inherentlyhigh in gas permeability. The permeationproperties may be modified by inclusion ofother polymers to change the carbon dioxideto oxygen permeability ratios. The resultingmaterials can permit the packagingtechnologist to achieve the lowest oxygenconcentration without going anaerobicwithin the package. Thus, optimum gasconcentration may be employed withminimum concern for elevated distributiontemperatures. In addition to the reversibletemperature sensitivities, the materials aregenerally capable of 100 times greater oxygenpermeability than mainstream polyethylenefilms without compromising the carbondioxide to oxygen permeability ratio. This isaccomplished by coating a porous substratewith a proprietary SCC polymer and applyingthe membrane as a package label over anaperture on an otherwise reasonably wellsealed package. These SCC materials aremanufactured by Landec Corp., Menlo Park,California.

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Ethylene ScavengersEthylene (C2H4) acts as a plant hormone

that has different physiological effects onfresh fruit and vegetables. It acceleratesrespiration, leading to maturity andsenescence, and also softening and ripeningof many kinds of fruits. Furthermore, ethyleneaccumulation can cause yellowing of greenvegetables and may be responsible for anumber of specific postharvest disorders infresh fruits and vegetables. Although someeffects of ethylene are positive such asdegreening of citrus fruit, ethylene is oftendetrimental to the quality and shelf life offruits and vegetables. To prolong shelf lifeand maintain an acceptable visual andorganoleptic quality, accumulation ofethylene in the packaging should be avoided.Most of these ethylene absorbers are suppliedas sachets or integrated into films. Potassiumpermanganate (KMnO4), which oxidizesethylene to acetate and ethanol and in thisprocess, colour changes from purple tobrown indicating the remaining C2H4scavenging capacity. Products based onKMnO4 cannot be integrated into food-contact materials, but are only supplied inthe form of sachets because KMnO4 is toxicand has a purple colour. Rengo Co. (Japan)developed ‘Green Pack’, a sachet of KMnO4embedded in silica. The silica adsorbs theethylene and the permanganate oxidizes itto acetate and ethanol.

Another type of C2H4 scavenging conceptis based on the adsorption and subsequentbreakdown of ethylene on activated carbon.Charcoal containing PdCl as a metal catalystwas effective at 200C in preventing theaccumulation of ethylene, in reducing therate of softening in mini-mally processedkiwifruits and bananas and in reducingchlorophyll loss in spinach leaves but not in

broccoli. Other C2H4 adsorbing technologiesare based on inclusion of finely dispersedminerals such as zeolites, clays and Japaneseoya into packaging films. Most of thesepackaging films, however, are opaque andnot capable of adsorbing C2H4 sufficiently.Although the incorporated minerals mayadsorb ethylene, they also alter thepermeability of the films: C2H4 and CO2 willdiffuse much more rapidly and O2 will entermore readily than through pure PE. Theseeffects can improve shelf life and reduceheadspace C2H4 concentrationsindependently of any C2H4 adsorption. Infact, any powdered material can be used toreach such effects.

Furthermore, the adsorbing capacity isoften lost when incorporating these mineralsinto a polymer matrix. Commer-ciallyavailable examples of these mineralcontaining materials are the Orega plasticfilm (Cho Yang Heung San Co., Korea), Evert-Fresh (Evert-Fresh Co., USA), Peak-fresh TM(Peakfresh Products, Australia), BO film(Odja Shoji Co., Japan) and ProfreshTM

(Europe). C2H4 scavengers are not yet verysuccessful, probably because of insufficientadsorbing capacity. A large proportion ofthe fresh fruits and vegetables harvestedeach year are lost due to fungalcontamination and physiological damage.The C2H4 adsorbing packaging conceptscould possibly contribute to an increase inthe internal trade as well as export of freshproduce.

CO2 Scavengers

CO2 is formed in some foods due todeterioration and respiration reactions. TheCO2 produced has to be removed from thepackage to avoid food deterioration and/or

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package destruction. CO2 absorbers mighttherefore be useful. The active compoundCa(OH)2 of FreshLock® reacts at sufficientlyhigh humidity with the CO2 to produceCaCO3. Multiform Desiccants have patented(US 5322701) a CO2 absorbent sachetincluding a porous envelope containing CaOand a hydrating agent such as silica gel onwhich water is adsorbed.

Humidity Regulators

This approach allows the food packer toreduce the surface concentration of water ina food by reducing the in-pack relativehumidity. This can be done by placing one ormore humectants between two layers of aplastic film which is highly permeable towater vapour. An example of this type ofproduct is “Pitchit” manufactured by ShowaDenko in Japan. The film duplex is describedas containing an alcohol, as propylene glycoland a carbohydrate, both of which arehumectants.

A different approach to humiditybuffering is being developed for use in thedistribution of horticultural produce whichis normally distributed in fibreboard cartons,usually with a polyethylene liner or madefrom very expensive waxed fibreboardwithout a liner. A recent development hasbeen the water-barrier coating of the inside offibreboard cartons to allow moist produce tobe placed directly into the carton. Besides theintroduction of liquid water with the produce,packing into closed spaces allows the build-up of water vapour. Since temperature cyclingis very difficult to avoid during handlingthere is every likelihood of condensationand with this the growth of microorganismson fruits and vegetables.

Two widely different approaches havebeen used to buffer the humidity in the cartonsin order to prevent condensation while notconcurrently causing desiccation of theproduce. One is to include micro-porousbags or pads of inorganic salts and the otheris to line the carton with a protected layer ofa solid polymeric humectant.

The main purpose of moisture control isto lower water activity (aw), thereby reducingthe growth of moulds, yeast and spoilagebacteria on foods with high aw. The shelf lifeof packaged tomatoes at 200C was extendedfrom 5 to 15-17 days with a pouch containingNaCl, mainly by retardation of surface mouldgrowth. Another application is in the removalof melting water from frozen foods. A thirdreason for moisture control is to preventcondensation when fresh horticulturalproduce respires. Drip-absorbent sheets suchas Thermarite® (Australia), ToppanTM

(Japan) or Peaksorb® (Peakfresh Products,Australia) for liquid water control in high aw

foods such as meat, fish, poultry, fruits andvegetables, basically consist of a superabsorbent polymer in between two layers.The preferred polymers for absorbing waterare polyacrylate salts and graft copolymersof starch.

Oxygen Scavenging

Oxygen is such a broadly effective agentof deterioration in foods that a substantialindustry has been established to provide awide range of alternative means of oxygenremoval from package headspace to reducechemical deterioration.

Technologies for thin films typically usedin MAP systems need an additional featureto prevent premature reaction, if they are toprovide maximum scavenging capacity. Thetransition-metal-catalyzed (optionally light-

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activated) process patented by W.R. Grace,Inc. approaches this by pre-plannedactivation involving generation of fullcapacity by consumption of antioxidants.This type of film, involving side-chainoxidation of a polydiene, appears to bedesigned as a permeation barrier for chilled,short shelf life processed meats. AmocoChemicals have reported someperformance data for their Amosorb®,water-activated masterbatch for blendinginto a variety of plastics.

Examples of light-activated scavengers,incorporated in the packaging film, areZero2 TM developed by CSIRO and marketedby Southcorp Packaging (Australia) andOS1000 developed by Cryovac Sealed Air.

Oxygen scavengers can be used aloneor in combination with MAP. Their usealone eliminates the need for MAP machi-nery and can increase packaging speeds.However, it is usually more common incommercial applications to remove most ofthe atmospheric O2 by MAP and then use arelatively small and inexpensive scavengerto remove the residual O2 within the foodpackage. For an O2 absorber to be effec-tive, the packaging material needs to havean O2 barrier of intermediate performance(20 ml/m2.d.atm), otherwise the scavengerwill rapidly become saturated and lose itsability to trap O2.

Antimicrobial Packaging

Substantial recent research has beendirected at determining how the surfacesof plastics can be made not only sterile butalso capable of having an antimicrobialeffect on the packaged food or beverage.This type of effect has already been achieved

in outer layers of laminates by use of modifiedprinting presses.

Release of antimicrobial agents will berestricted by regulatory approval asintended food additives. The concept hasbeen advanced in the case of an ediblecoating. An alternative approach whichhas been developed in Japan by Mitsubishiis based on the inclusion of zeolite particleson the surface of the food-contact layer inlaminates. The zeolite has some of its surfaceatoms replaced by silver which appears torelease silver ions as aqueous solution fromthe food enters the exposed cavities of theporous zeolite structure. The water thenappears to leach traces of silver from theparticles giving the highly efficientantimicrobial activity of this ion. The use ofedible coatings to apply relatively constantconcentrations of permitted antimicrobialagents at the food surface has beendeveloped. The relative rates of diffusionwithin the coating and within the fooditself are very important. Such a processcould be important where antimicrobials,such as sorbate, are lost by degradation.Application of antimicrobial via ediblecoatings is likely to gain importance withminimally processed foods distributedunder MAP conditions.

To control undesirable microorganismson foods, antimicrobial substances can beincorporated in or coated onto foodpackaging materials. The principle action ofantimicrobial films is based on the release ofantimicrobial entities, some of which couldpose a safety risk to consumers if the releaseis not tightly controlled by some mechanismswithin the packaging material. The majorpotential food applications for antimicrobial

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films include meat, fish, poultry, bread,cheese, fruits and vegetables.

Several other compounds have beenproposed and/or tested for antimicrobialactivity in food packaging including organicacids such as sorbate, propionate andbenzoate or their respective acid anhydri-des, bacteriocins e.g. nisin and pediocin,enzymes such as lysozyme, metals andfungicides such as benomyl and imazalil. Afilm contains a natural antimicrobialcompound derived from grape-fruit seed.Many of the incorporated antimicrobials arenot yet permitted for food use. The choice ofthe antimicrobial is often limited by theincompatibility of the component with thepackaging material or by the heat unstabilityof the component during extrusion. Twocommercial biocidal films are currentlymarketed. One is composed of a chlorinatedphenoxy compound and the other consistsof chlorine dioxide. A commercial antifungalcoating containing chitosan is also sold as ashelf-life extender for fresh fruit.

CSIRO (Australia) is developing systemswhich gradually release SO2 to control mouldgrowth in some fruits. However, excessiverelease of SO2 from pads of sodiummetabisulphite incorporated microporousmaterial has been shown to cause partialbleaching problems in grapes. Theaccumulation or absorption of the excessSO2 by foods could cause toxicologicalproblems. Knowledge about the diffusionrate of SO2 into the food is essential toaddress toxicological issues and ensuresafety in SO2 releasing active packagingsystems. In contrast to conventionalantimicrobial films, some functionalgroups that have anti-microbial activity havebeen immobilized on the surface of polymerfilms.

CONCLUSION

In recent days, entrepreneurs in India areshowing greater interest in internalmarketing as well as export of fresh produce.The trade is attractive but is not an easyenterprise; a high degree of organisation andprofessionalism is necessary to export freshproduce successfully, especially to thesophisticated markets of Europe. It can makegood use of indigenous horticultural skills.The combined requirements of fresh produceand of its transport environment often imposeunusually severe conditions on thepackaging employed. As a result, higherpackage quantity is usually needed for freshfruits and vegetables when compared tomanufactured goods of the same weight. Fordesigning of a package for a specific productand particular target market, a clear pictureof distribution system should be drawn up,as hazards involved in transportation aredifferent for different modes (i.e., packagingrequirements for ship transportation aretotally different than that by air transpor-tation). Models may be used dependingupon the characteristics of the produce andthe market.

Active packaging is an emerging andexciting area of food preservation technique,which can confer many preservation benefitson a wide range of foods. The aim of activepackaging is to match the properties of thepackage to the more critical requirements ofthe food. Adoption of some of these methodswill require changes in attitude to packagingand a willingness to address regulatoryissues where chemical effects are used.Application of these and other emergingtechnologies offers the prospect of greatersatisfaction in India as these are relativelynew concepts that maximise the benefits

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from some of our traditional agriculturalindustries.

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