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PLASTICS IN FOOD PACKAGING

PACKAGING ASPECTS OF MEAT, FISH AND POULTRY

Chapter 10PACKAGING ASPECTS OF

MEAT, FISH AND POULTRY

R S Matche

Food Packaging Technology DepartmentCentral Food Technological Research Institute

Mysore 570 020 (INDIA)

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PACKAGING ASPECTS OFMEAT, FISH AND POULTRY

MEAT

In the packaging of red meat, there aretwo factors of major importance: colourand microbiological. Since the wateractivity of chilled meat is very high,unprotected meat will lose weight due towater evaporation and its appearance willdeteriorate. Further, weight loss will occurwhen meat is cut, since the exposed sur-faces exude liquid which adversely affectsthe appearance of packaged meat. Thiscan be overcome by including an absor-bent pad in the base of the package.Although efficient chilling can reduce thequantity of exudates, a certain amount willalways be present when meat cuts are heldfor retailing. This unattractive bloodyexudate found in vacuum packaged meatis referred to as �purge�, �weep� and�drip�; 1-2% purge is consideredacceptable, while 4% is consideredexcessive. Values of 2-4% drip can havesubstantial economic implications if notcontrolled.

Meat and its products differ in physico-chemical properties including nature ofpigments, sensory attributes and microbialflora. The selection of packaging materialhas to be done very carefully to protect

these qualities of meat and its products.The purpose is to retard or prevent themain deteriorative changes and make theproducts available to the consumers in themost attractive form. However, initialquality of meat has to be very goodbecause packaging can at best maintainthe existing quality of meat. It cannotimprove it.

Packaging requirements depend uponthe type of meat, nature of processing anddistribution system.

Colour of Red Meat

Colour of meat is one of the mainattributes of meat quality. Consumersassociate this colour with good eatingquality, although there is little correlationbetween the two. This association ofcolour of red meat with freshness has beenthe dominant factor in retail meatmarketing. The loss of this bright redcolour is known as loss of �bloom�. It isaffected by many factors, although theconsumer usually relates the colour lossto bacterial growth. Due to the importanceof meat colour, various methods oftransportation, distribution and packagingof primals and subprimals have evolved

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which optimize the maintenance of adesirable meat colour. The colour of meatas perceived by the consumer is primarilydetermined by such factors as theconcentration and chemical form of themeat pigment myoglobin, the morphologyof the muscle structure and the ability ofthe muscle to absorb or scatter incidentlight.

The colour of fresh meat dependschiefly on the relative amounts of the threepigment derivatives of myoglobin presentat the surface: reduced myoglobin (Mb),oxymyoglobin (O

2 Mb) and metmyoglobin

(met Mb). Reduced myoglobin is purplein colour and predominates in the absenceof oxygen. Oxymyoglobin is bright redin colour and results when Mb is oxyge-nated or exposed to oxygen; this iscommonly known as �bloom�. Metmyo-globin is brown in colour and exists whenthe oxygen concentration is between 0.5%and 1% , or when meat is exposed to airfor long periods of time. The brownmetmyoglobin cannot bind oxygen eventhough it is oxidized by same oxygen thatconverts myoglobin to the bright redoxymyoglobin.

During storage, the rate ofmetmyoglobin accumulation on thesurface of red meat is related to manyintrinsic factors including pH, muscle typeand the age, breed, sex and diet of theanimal, as well as extrinsic factorsincluding pre-slaughter conditions and theprocessing conditions used. During retaildisplay, physical factors such astemperature, oxygen availability, type andintensity of lighting, microbial growth andgas atmosphere surrounding the meatsurface influence the shelf life of fresh redmeats.

PACKAGING MATERIALS AND

TECHNIQUES

Tray with Overwrap

The most common packages for retailfresh meat cuts in Western countries arepolystyrene foam or clear plastic traysover wrapped with a transparent film.These trays offer an aesthetically appea-ling background. The use of blotter unde-rneath eliminates the changes of excessivemeat juice accumulation. Meat thuswrapped may keep for approximately 10days at a temperature of 0°C before itbecomes microbiologically unacceptable.However, it can retain the desirable brightred colour only for about 5 days.

Cellophane coated with nitrocelluloseon one side was in use for wrapping freshmeat for a considerable period. Theuncoated side is kept in contact with meat.Moisture saturation on the film increasesits oxygen permeability, while nitro-cellulose coating on the outer sideprevents excessive moisture loss to theatmosphere. Another grade of cellophanewith polyethylene coating on one side canbe used for irregular shaped meat thoughit is not much in use.

The over wrapping film should haveexcellent optical properties. Among thesynthetic plastic films rubber hydro-chloride, low density polye-thylene (25

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µm), highly plasticised PVC films (18 µm),biaxially oriented polystyrene film can beused.

Vacuum Packaging

Vacuum packaging achieves itspreservative effect by maintaining theproduct in an oxygen-deficient environ-ment. In anaerobic conditions, potentspoilage bacteria are severely or totallyinhibited on low-pH (< 5.8) meat.However, their growth on high pH muscletissue or extensive fat cover of inevitableneutral pH, will spoil relatively rapidly ina vacuum pack. Vacuum packaging cantherefore extend the shelf life of primalcuts composed largely of low (normal) pHmuscle tissue such as beef and venison byabout five fold over that achieved in air.For other meats and small cuts, only a twofold extension of shelf life can be safelyanticipated.

Vacuum Packaging System

Buffalo meat and beef carcasses arebroken down into primal and sub-primalcuts, separated into boneless and bone-in-cuts, and then vacuum packaged. Sinceonly about two-thirds of beef carcass isusable meat, there are clear advantages inthe form of reduced refrigerated space fortransportation and storage and lesspackaging materials when boneless beefrather than bone-in beef or carcasses arestored and distributed. Another advan-tage is that evaporative weight lossincurred when carcasses are hung in thechill rooms is minimised.

Vacuum packaging involves enclosingboneless joints in flexible plastic containers(usually bags) to prevent moisture loss andexclude oxygen from the meat�s surface.

Packing under a vacuum reduces thevolume of air sealed in with the meat. Inpackaging fresh meat primals, many cutscontain bones, which are often sharp andabrasive and readily puncture the flexibleplastic materials used in vacuum packa-ging. To overcome this bone puncture, aBONEGUARD material consisting of awax-impregnated and coated cotton scrimis employed.

Suitable film for vacuum packagingmust combine a high resistance totransmission of gases and water vapourwith perfect seals and good mechanicalstrength.

Typical materials in use are :

1. Co-polymer - coated cellulose / PE filmlaminate.

Polyester / PE film laminate

Nylon / PE film laminate

These are usually with PE inside topromote efficient heat seals.

2. Laminates of plastic film withaluminium foil.

3. PVDC copolymer film

4. EAA/Saran/EAA laminate

5. Nylon / EAA laminate

6. PVDC / polyester / PVDC / PElaminate

7. LDPE / BA / Nylon / BA/ LDPE

Once a piece of meat has been vacuumpackaged in an oxygen barrier materialhaving a gas permeability (at 23°C and75% RH) below 50 ml m-2 day -1 atm -1 andadequately sealed to prevent air reentry,the shelf life of the meat is very much thesame regardless of the packaging material.Therefore, the significant differences

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between the packaging materials and /orsystems are not in the structure so muchas in the physical properties, theproduction speeds of the system and theabuse resistance of the package itself.

The oxygen in the small volume ofresidual air inside a vacuum package willbe quickly consumed by meat respirationso that within 2 days the oxygen partialpressure at the surface of the meat willhave dropped below 10 mm Hg. At thesevery low partial pressures the penetrationlimit of oxygen is very near the surfaceand the thin brown layer of MetMb whichdevelops cannot conceal the underlyingMb; therefore the visible colour of vacuumpackaged beef is purple. Vacuumpackaging for retail display packages ofred meat has not met with wide successbecause of consumers� negative perceptionof the purple colour.

Four basic methods are available forvacuum packaging of meats:

a. Shrink Bag

It involves placing the meat into a heatshrinkable barrier bag of EVA copolymer/ PVC /PVdC copolymer / EVA co-polymer / polyamide as the barrier layerand an ionomer as the inner or outer layerand then evacuating the bag prior tosealing. Formerly, sealing was being doneby applying a metal clip around thetwisted neck of the bag. But these days,it is done by using heated jaws. The bagis then heat shrunk by placing in water at90°C. After shrinking, the bag conformsclosely to the meat and produces a tightvacuum pack. However, if a high vacuumis not drawn, adequate shrink force ispresent in the film to cause bridging overconcave areas, and the voids are

eventually filled with free liquid andincrease the quantity of purge.

In the early days, a nozzle evacuationsystem (which offered low vacuum levels)and a clip closure was used. In typicalnozzle vacuumizing machine, the openend of the bag containing the product wasplaced over nozzle and air drawn byvacuum from inside the bag. With thismethod, it was difficult to consistentlyproduce packages with higher than 125mm Hg vacuum since the packagingmaterial began to collapse and block airremoval as soon as negative pressuredeveloped. Vacuum levels higher than250 mm Hg collapsed the bag and sealedthe end of the nozzle to air movement.

Today very high vacuum levels arebeing achieved on single chambermachines which offer heat sealing ofshrink bags instead of clipping. Due to

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the improved productivity and versatilityof this type of equipment, rotary singlechamber machines have become theindustry standard.

b. Non-shrink Bag

In this technique, meat is placed into apre-formed plastic bag which is then putin an enclosed chamber which isevacuated. When a predetermined lowpressure has been reached, heated jawsclose and seal. Typical bag constructionsconsist of laminates or co-extrusions whichinclude polyamide or PET as the outsidelayer to provide strength and a goodoxygen barrier, and inner layers of LDPE,ionomer or EVA copolymer which aregood moisture barriers and can be easilyheat sealed. A typical structure would beionomer-polyamide-EVA copolymer.

Because the bags used in this systemare not heat shrinkable, purge tends toaccumulate in corners during storage.Secondary sealing has been introduced toovercome this problem; it involves passingthe vacuum packs through a heatingtunnel. Because of the ease with whichthese films are heat sealed, the excess areaof film around the meat is sealed. Thisprevents purge from spreading into thefilm wrinkles, folds and corners as well asproviding a wider heat seal which, it isclaimed, reduces the incidence of leakingbags.

c. Thermoforming

In this method, deep trays arethermoformed in-line from a base web ofplastic. Meat is placed in the trays and anupper web of plastic is heat sealed undervacuum to form a lid. Generally, thematerials used for thermoforming arelaminates of polyamide, PET or PVC,sometimes with a PVC/PVDC copolymercoating and heat-sealing layers such asLDPE, EVA copolymer or ionomer.

This type of vacuum packaging isparticularly well suited to hot boned, pre-rigor meat which is difficult to package inbags, the tray providing a certain degreeof molding to the meat as it cools.Furthermore, with the correct selection ofplastic materials, the sealed package canbe processed in boiling water, if desired.

d. Vacuum Skin Packaging (VSP)

In this technique, meat portions areskin packed in a barrier film material, thetop web of which is softened by heatingbefore applying a vacuum and sealing.During this operation, the soft film moldsitself to the shape of the meat to give a

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skin-tight package, the meat thus beingheld under anaerobic conditions. It formsclosely around the meat and seals to thebase film. Although this type of vacuumpack is an excellent method of presen-tation, the meat remains in unoxygentatedstate which is not accepted by mostconsumers, even though it will stilloxygenate to the red colour when exposedto air.

VSP meat remains acceptable micro-biologically for up to at least 2 weeks afterpacking during which time the colour,although not red, is unchanged. However,the meat retains for at least 2 weeksstorage at 1°C its ability to develop abright red colour when exposed to air. Asit is typical for vacuum packaged meat,lactic acid bacteria predominate on VSPsamples on which spoilage bacteria growslowly, if at all, resulting in a long, odor-free shelf life. Off-odours develop much

more rapidly in MAP packs than in VSP.

Shelf Life of Vacuum PackagedMeats

Vacuum packaged beef of normal pHcan be stored at chiller temperatures forperiods in excess of 10 weeks. However,high pH, dark, firm, dry beef held underthe same conditions will generally spoilwithin about 6 weeks. There are alsoadvantages in aging beef in vacuumpackages as opposed to carcass aging,including less loss due to waterevaporation, less necessity for trimmingof exposed surfaces and more efficient useof refrigerated space.

Although for many years vacuumpackaging was only applied to chill beef,it has been applied in recent years to lamband pork. Because of their relatively small

size, pork and lamb carcasses are onlypartially boned before packaging, and thepresence of bone can lead to puncturingof package unless precautions are taken,like use of strong EVA copolymer film inthe package structure. The surface of lambcuts is adipose, rather than muscle, tissue.Adipose tissue has pH values close toneutrality and has no significantrespiratory activity. Packaged lamb cantherefore present a heterogeneousenvironment for microbial growth. Thisdifferent microbial environment of highpH and, possible relatively high oxygenconcentration, gives a shelf life of only 6to 8 weeks reported for vacuum packagedlamb whereas 11 to 22 weeks is routinelyattainable with beef. The longer shelf lifeof vacuum packaged beef is due to its florabeing dominated by lactobacilli whichovergrow other spoilage organisms at thelow temperature, low pH, low O

2, high

CO2 environment of the package.

Modified Atmosphere Packaging(MAP) of Fresh Meat

Vacuum packaging has the disadvan-tage that both package and meat aresubjected to mechanical strain. Mecha-nical pressure on the meat may increasedrip loss; if bone is present and notadequately covered with a suitable mate-rial, the pack may be ruptured. Atmos-pheric composition inside the package canalso be modified to extend the shelf lifeof meat while retaining the freshness,aroma, colour and weight. In this tech-nique, headspace air in the package isreplaced by gases usually nitrogen aloneor in combination with carbon dioxide oroxygen.

Oxygen at concentrations of 75%penetrates almost twice as far into the

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surface of meat as it does at the 21% levelin air. This thick layer of oxygenatedtissue allows red meat to retain its brightred colour for up to 5-6 days in normalretail display. The relative volumes of gasand meat are important in determining thechanges in concentration of gases duringstorage. The high solubility of CO

2

compared to the relatively low solubilityof O

2 and N

2 in meat must be taken into

account.

1. High Oxygen MAP

High oxygen MAP systems which have30% CO

2 and up to 70 % O

2 are used to

extend the colour stability and delaymicrobial spoilage. Although both thecolour stability and the time to spoilageare approximately doubled by high O

2

MAP, this extension in shelf life is notadequate. So the use of high O

2 MAP is

not widespread for prolonged storage ofmeat for bulk packaging.

2. Low Oxygen MAP

a. Carbon Dioxide Gas Flush

In these packages, the air is largelydisplaced by CO

2, either by itself or mixed

with N2 or air. The shelf life extension is

similar to that of vacuum packaging,higher concentration of CO

2 leading to a

longer shelf life. Low oxygen concen-trations lead to formation of oxymyoglobinon the meat surface which prevent theformation of metmyo-globin which leadsto discolouration. Low O

2 MAP system is

not suitable for retail packs.

b. Nitrogen Gas Flush

As an inert gas, nitrogen is convenientfor gas packaging, it generally being

considered neutral filler. If the air in thepackage is removed prior to the additionof nitrogen, the effect on meat is similarto that of vacuum packaging, except thatresidual oxygen is diluted andmetmyoglobin formation on the surfaceshould be less pronounced than with avacuum. However, although theformation of metmyoglobin on the surfaceis reduced, the nitrogen also dilutes theCO

2 produced by tissue respiration,

prolonging the time required for theconcentration to accumulate to levelssufficient to inhibit growth of spoilagebacteria. Although there is littlecommercial use of nitrogen flushing withfresh meat, several studies have confirmedthat 100% nitrogen is as effective asvacuum for storing fresh meat joints, theonly advantage being reduced exudatesdue to less mechanical pressure on meatcompared to vacuum package.

3. High Carbon Dioxide MAP

Carbon dioxide is highly soluble inboth water and oils. Therefore, when CO

2

is applied to meat in a rigid pack, the gaswill be absorbed by the muscle and fattissue until equilibrium is attained. Atequilibrium, the partial pressure of CO

2

will be less than that of the original gasmixture, and the total gas pressure willalso be less than at which the gas mixture

was initially applied. Similar conside-rations apply in flexible packagingsystems. If CO

2 alone is added, the pack

will collapse around the meat as gas isabsorbed unless CO

2 is added in excess of

the quantity required to saturate the meatat atmospheric pressure. When mixturesof gases are used, the less soluble nitrogenand unrespired components of the gas

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mixture maintain a volume of gas atatmospheric pressure around the meatthat is less than the volume addedinitially.

The solubility of CO2 in muscle tissue

of pH 5.5 at 0°C is approximately 960 mlper kg of tissue at STP. As thetemperature increases, the solubilitydecreases by 19 ml per kg for each °Crise. As tissue pH increases, the solubilitydecreases by 360 ml per kg for each pHunit. With raw meat, CO

2 at ∼ 1.5 L kg-1

meat is required to saturate the productwithout the package collapsing tightlyaround the contents. The necessary initiallarge gas volume can result in pouches atfirst overfilling the cartons in which theyare contained. However, the gas dissolvessufficiently rapidly for the pack volumeto fall to its final value during overnightstorage.

It has been reported that wholesaleprimal pork loins stored in 100% CO

2 at

0°C had a shelf life of approximately 3months. Red meat colour is not adverselyaffected by high CO

2 atmosphere if

oxygen is rigorously excluded from thepackage. Trials have also demonstratedthat it is suitable for the prolonged storageof pork, poultry, venison, offal, fish,cooked meats, blanched vegetables andcomplete meals.

Beef needs a high oxygen content tomaintain a bright red colour which isdesired by the consumer. Chicken incomparison needs little oxygen andreduced amount of carbon dioxide. Porkwith its high fat content needs less oxygenand some nitrogen for glow. Thereplacement of air already in the pack bya mixture of 70 percent oxygen, 20 percentcarbon dioxide and 10 percent nitrogen at

0°C slows bacterial growth so greatly thatmicrobially caused discolouration,sliminess and aroma changes areeliminated. Beef and pork portionsgenerally have a shelf life of seven to eightdays. Higher oxygen concentration canresult in accelerated development ofrancidity. A mixture of 80 percent carbondioxide is even better bacteriologicallyalthough some colour loss may occur.Bulk packs of fresh pork in 10 percentcarbon dioxide had a microbiologicalstorage life of 40 days at 4°C.

PACKAGING OF FROZENMEATS

Frozen meat is stored and displayedbetween -10 and -30°C, at whichtemperatures microbiological growth isarrested. Therefore, the changes in meatinfluenced by packaging are thoseassociated with appearance, i.e., colourand the absence of frost inside thepackage.

If a bright red colour is required, itmust be produced by oxygenation of themeat surface before freezing followed bypackaging in a material, which is relativelypermeable to oxygen. It has been claimedthat ionomer film will keep the bright red

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colour for at least a year if the meat isstored in the dark at -20°C. However,when exposed to light, the red colourbegins to darken after about a week. Thisis caused by light activated oxidation ofthe pigment in the meat surface and isinevitable in meat, which has been frozenin the bright red state and subsequentlyexposed to light. When frozen meat andmeat products are stored without anadequate moisture vapor barrier anopaque dehydrated surface known asfreezer burn is formed. Freezer burn iscaused by the sublimation of ice on thesurface of the product when the watervapor pressure of the ice is higher thanthe vapour pressure in the environmentalair. The key to avoiding freezer burn andlessening oxidative deterioration duringfrozen storage is to eliminate or reducethe headspace in the package whichshould also serve as an effective barrier tooxygen and water vapour. Freezer burncan occur even when using a packagingmaterial, which is an excellent barrier tomoisture vapor if the package headspacehas not been essentially eliminated.

Oxidative changes are even moreeffectively reduced through exclusion ofair by means of vacuum packaging.Because there is no space between themeat and the packaging material, frostcannot develop to mask the attractiveappearance.

Frozen, restructured meat products havetraditionally been packaged in bags,pouches, trays, overwraps and plastic-coated paperboard, with polyolefins beingthe most common material used. Theymust, of course, contain appropriateplasticizers so that their mechanicalproperties are not impaired at sub-zerotemperatures. Vacuum skin packaging isalso used for frozen, restructured meatproducts. Typically, a heat-softenedionomer film is draped over the productwhich is supported on a lower web of thesame material. Air is withdrawn frombetween the two webs and the webs heatsealed together. This results in a packagethat is sealed skin tight to the edge of theproduct, regardless of its contour or size.There are no empty spaces for moisturecondensation to occur, freezer burn isvirtually eliminated during frozen storage.

CURED AND COOKED MEATS

During storage, cured meats deterioratein the first instance because of discolo-ration, secondly because of oxidativerancidity in the fat, and thirdly on accountof microbial changes. Although the pig-ment of cured meat (nitrosylmyoglobinor nitrosylhemochrome) is stable in theabsence of oxygen or under vacuum, itsoxidation to metmyoglobin is very rapidwhen oxygen is present. The rate ofnitrosylmyoglobin oxidation increasesdirectly with increasing oxygen tension

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unlike myoglobin itself where the rate ofoxidation is maximal at 4 mm Hg oxygenpartial pressure. The most common andeffective antioxidants used are ascorbateor erythrobate which are either incorpo-rated into the curing brine or sprayed ontothe surface of the product after maturing.

Nitrosylmyoglobin and nitrosylhemo-chrome are much more susceptible to lightthan myoglobin; cured meats fade in 1hour under retail display lighting condi-tions. Since light accelerates oxidativechanges only in the presence of oxygen,vacuum or inert gas packaging caneliminate the effect. Holding vacuumpackaged meats in the dark for 1-2 daysbefore exposing them to display lightsallows residual surface oxygen to bedepleted by microorganisms and tissueactivity, thus reducing subsequent colourdeterioration.

Smoke, traditionally produced by theslow combustion of sawdust derived from

hard woods, inhibits microbial growth,retards fat oxidation and imparts flavourto cured meats. However, these dayssmoke essences are sometimes usedinstead of actual smoke, and they contributeonly flavour to cured meats.

Cured hams undergo a different typeof spoilage from that of fresh or smokedhams, due primarily to the fact that curingsolutions pumped into hams containsugars which are fermented by the naturalflora of the ham and also by thoseorganisms such as lactobacilli pumpedinto the product in the curing solution.

THERMO-PROCESSED MEATS

Thermal processing at above 100°C,usually accomplished under pressure, isdone to prepare commercially sterile meatproducts.

1. Short-term storage: Meat productslike patties, sausages, nuggets, meat balls,etc, can be packaged in pouches made upof polyethylene, polypropylene, PVDC,rubber hydrochloride, etc, for short termstorage lasting for 10-12 days at 4°C.

2. Long-term Storage: Meat productslike corned beef, corned pork, meatgravies, meat soups, liver sausages,chicken curry, boneless chicken, etc., arehermetically sealed and cooked to makecommercially sterile for long term storageat room temperature. Two types of con-tainers are suited for this purpose.

a. Metal cans: Canned meat products areshelf stable for a number of years atroom temperature. Tinplate cans havemaintained their lead in this field.These are coated on the inner side withsulphur-resistant lacquer. Shallowdrawn aluminum cans with internal

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lacquer have been successfully used forcertain meat products.

b. Retort pouches: Thermoprocessed meatin retort pouches are shelf stable for aminimum period of one year. Compositefilm especially film/foil/film laminatesare used in making the retort pouchesas follows:

i) Outer plastic film made up ofpolyester, polyamide or orientedpolypropylene provides supportand physical strength to thecomposite.

ii) Middle layer of aluminum foilcontributes excellent barrierproperties.

iii) Inner layer consisting of poly-propylene provides head seala-bility.

PACKAGING OF DEHYDRATEDMEATS

Dehydration is a successful means ofpreserving many meats with properpackaging. There is a marked suscepti-bility of dehydrated meats to oxidationresulting in rancid odour.

Packaging Materials

1. Tinplate cans

2. Metal / foil / plastic film laminatescompressed bars of dehydrated mincedmeat with inner cellophane and outerpaper/Aluminium foil/PE laminatewrap are reported to be shelf-stable forone year.

Flexible pouches most suitable forvacuum and modified atmospherepackaging consist of polyester / PE /

Aluminium foil / PE or cellophane / PE /Aluminium foil / PE laminates.

POULTRY AND EGGS

Poultry

Raw poultry meat is a perishablecommodity of relatively high pH (5.7 - 6.7),which readily supports the growth ofmicroorganisms when stored under chillor ambient conditions. The shelf life ofsuch meat depends on the combinedeffects of certain intrinsic and extrinsicfactors, including the numbers and typesof spoilage organisms present initially, thestorage temperature, muscle pH and type(red or white), as well as the kind ofpackaging material used and the gaseousenvironment of the product.

The vacuum packaging of poultrycarcasses, cuts and other manufacturedproducts can extend shelf life providedthat the product is stable under chillconditions. During storage at 1°C in eitherO

2 permeable film or vacuum packs,

extension in shelf life from 16 to 23 days(pH 6.1 - 6.3) was observed for the vacuumpackaged products. The maximum usableCO

2 concentration was 25% since above

this, the meat became discoloured; evenat 15%, a loss of bloom was sometimesnoted.

While the number and the type ofmicroorganisms found on stored poultryare important factors when determiningthe shelf life, the real determinant is thesensory quality of the raw and cookedproduct. One study which evaluated thequality of raw and cooked poultry thathad been stored under MA andrefrigeration for up to 5 weeks, foundthat MAP (80% CO

2) poultry would be

quite acceptable to consumers for up to 4-

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6 weeks depending on the temperature ofstorage. It was noted that commercialpoultry processors may not get a longershelf-life because of difficulties incontrolling the packaging process andtemperature under production conditions.

In a study of broiler carcasses packa-ged under vacuum in film of low O

2

permeability, or under CO2 in gas-

impermeable packages, shelf life wasfound to be a function of storage tempe-rature, packaging and O

2 availability.

Putrid spoilage in gas-impermeablepackages after 7 weeks storage at 3°C or14 weeks storage at -1.5°C was attributedto enterobacteria. In vacuum packages

with oxygen transmission rates of 30-40ml m-2 day-1, putrid odors were detectedafter 2 weeks storage at 3°C and 3 weeksstorage at -1.5°C.

Packaging of Dressed and Cut-upPoultry

By nature, poultry fat is unsaturatedand is very prone to the development ofoxidative rancidity. Dressed poultry hasa shelf life of 5-7 days during refrigeratedstorage. Packaging of poultry meat shouldbe undertaken immediately after thedressing operations are over. Unpackedrefrigerated storage may result in surfacedehydration, whereas frozen storage may

give rise to freezer burn, characterized bysurface discoloration, tough texture anddiminished juiciness as well as flavour.

Packaging Materials andTechniques

Over-wraps: Packaging of dressed wholebirds, halves or cut up parts for retailingand early use can be done in plastic films

such as polyethylene, polypropylene,PVDC, rubber hydro-chloride or Nylon-6films of 38 to 50 µm. Polyethylene is themost widely used packaging material inour country because of its low cost andeasy availability. These thermoplastic filmsheets can be fabricated into bags. Eachdressed eviscerated bird is inserted into abag. Giblet of individual bird is wrappedin waxed paper or parchment paper andplaced into the body cavity before bagging.The problem of body fluid accumulationis avoided by putting an absorbent pad orblotter on the back of each bird to soak upthe liquid. The bag can now be heat-sealedor twist-tied or clipped shut.

Tray with over-wraps: The commonpackages for retail dressed small wholefryers, broilers, roasting chicken or evencut up parts can be placed in polystyrenefoam trays over-wrapped with a trans-parent plastic film. A blotter underneathabsorbs the excessive meat juice accumu-lation. Chickens thus wrapped have ashelf life of 7 days at 4°C in refrigerator.

Shrink Film Over-wraps: Many thermo-plastic films such as polyethylene, poly-propylene, polyvinylidene can be biaxci-ally-oriented to stay stretched at ambienttemperatures. Dressed birds are over-wrapped with such films and passedthrough hot air tunnel or dipped in watertub maintained at 90°C for few seconds toeffect shrinkage of the film.

Vacuum Packaging: The ideal materialsare Polyester/Polyethylene(PE), Polya-mide/Polyethylene, PVDC co-polymerfilm, Nylon/EVA.

Modified Atmosphere Packaging: In thistechnique, the atmosphere surrounding theperishable commodity is modified. The

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manipulation of package atmosphere isdone by flushing carbon dioxide, nitro-genand oxygen alone or in combination.Studies have shown that a mixture of 60%nitrogen and 40% carbon-dioxide or 50%nitrogen and 50% carbon dioxide is idealfor modified atmosphere packaging ofchicken meat.

Bulk Packaging: Plastic crates havebecome popular as bulk containers due totheir toughness, light weight, dent resis-tance and ease of stacking and handling.

Packaging of MechanicallyDeboned Poultry Meat (MDPM)

To stabilize the colour, to check thelipid oxidation, to inhibit microbialdeterioration and to maintain the texture,vacuum packaging can be useful. Modi-fied atmosphere packaging (MAP) can bedone by providing 20-30% carbon dioxide,0-10% oxygen and 70-75% nitrogen.

Eggs

Infertile eggs are used almost exclu-sively for human consumption. The poresin the shell permit gaseous diffusion aswell as moisture loss and the entry routefor microorganisms which might infect theegg. The factors associated with the lossof shell egg quality are time, temperatureand humidity. During the storage of shelleggs, the pH of albumen increases at atemperature-dependent rate from about7.6 to a maximum value of about 9.7, therise in pH of albumen causes a breakdownin the gel structure of the thick white.

Several methods of altering theenvironmental conditions around eggshave been used to prolong their shelf life,the major one being refrigeration. Coatingof the shell with mineral oil has also come

into common usage to increase the shelflife by reducing the rate of CO

2 and

moisture loss. Spray oiling has come intousage for the shells of eggs, light mineraloils of food quality normally being used. Itis essential that the treatment be applied tothe eggshell within a few hours, sinceweight loss during the first few days canbe significant. Clearly, the rate of CO

2 loss

at any particular temperature will be afunction of the partial pressure of CO

2 in

the external environment. Thus theattempts to extend the shelf life of shelleggs have involved storing the eggs in anatmosphere containing CO

2 in an

impermeable package. A comparative studyevaluating the quality and shelf life of freshshell eggs stored at room temperature withfour different treatments (not packaged;packaged in air; packaged with 15% CO

2;

and oil coated) concluded that controlled

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atmosphere was the most efficient methodof preserving egg quality at roomtemperature for a period of 7 weeks.Although such procedures increase thestorage life of eggs, the economics andsuccess of spray oiling have preventedsuch procedures from becoming generallyadopted.

Packaging of Shell Eggs

1. Moulded pulp filler flats or plastic fillerflats each containing 30 eggs are used.

2. Paperboard cartons with dividers.

3. Folded paperboard cartons with shrinkfilm overwrap of PE, PVC or PVDC.

4. Expanded polystyrene foam eggcartons or trays provide excellentcushioning and strength besides beinglight weight

5. Bulk packaging, under Indianconditions, it is advisable to keep 210eggs per carton (7 trays of eggs each)during shipment. For export purposes,all white paperboard shipping cartonseach containing 360 eggs are used.

SEAFOOD

Spoilage of flesh foods such as fish andshellfish results from changes broughtabout by chemical reactions such asoxidation reactions due to the inherentenzymes, and the metabolic activities ofmicroorganisms. The chemical composi-tion and microbial flora and seafood varyconsiderably between species, differentfishing grounds and seasons.

Both salt water and fresh water fishcontain comparatively high levels ofproteins (about 18%) and other nitrogenous

constituents. Non-fatty fish such as codand haddock have a lipid content of lessthan 1% in contrast to fatty fish such asherring and mackerel which can have lipidcontents of up to 30%. The spoilage of saltand fresh water fish appears to occur inessentially the same manner.

It is generally accepted that the internalflesh of healthy, live fish is sterile,microorganisms that exist on fresh fish aregenerally found in the gills, the outerslime, and the intestines. The post mortemchanges leading to spoilage dependprincipally on the chemical compositionof the fish, its microbial flora andsubsequent handling, processing andstorage.

Immediately after post mortem, awhole series of tissue enzyme reactionsbegin the process of autolysis (basicallyself digestion of the fish muscle) whichleads eventually to spoilage. The autolyticenzyme reactions predominate for 4-6days at 0°C after which the products ofbacterial activity become increasinglyevident with the appearance of undesi-rable odors and flavours. The rate of theautolytic changes are determined by manyfactors, the most important being tempe-rature, pH, availability of O

2 and the

physiological condition of the fish beforedeath.

The third type of spoilage is chemicalspoilage, primarily oxidation of the fattycompounds leading to the developmentof rancid flavours. Because seafood has amuch higher content of polyunsaturatedfat than meat, it is more prone to thedevelopment of oxidative rancid flavours.The rate of rancidity development isclosely related to the temperature of storageand the reactions can still occur at freezer

185



temperatures as low as -30°C. Somesubstances such as salt and some processes

such as drying and smoking can aggravatethe oxidation problem, and therefore frozen

smoked fish has a shorter shelf life thanunsmoked fish of the same species.

Shellfish are divided into two main

groups; crustaceans which include shrimpLobster, crabs and crayfish, and mollusks

which includes oysters, clams, squid andscallops. The microbial flora of shellfish

reflects the waters from which they arecaught, contaminants from the deck and

handlers, and the quality of the washingwaters used. Mollusks differ from

crustacean material with a lower totalquantity of nitrogen in their flesh. Because

the carbohydrate is largely in the form ofglycogen, the spoilage of molluscan

shellfish is largely fermentative. Thisresults in a progressive fall in tissue pH as

spoilage develops, from pH 5.9-6.2 in freshmollusks to less than about pH 5.5 in

spoiled mollusks.

Modified Atmosphere Packaging

The normal spoilage bacteria, which

cause off-odours and flavours, are inhibitedand microorganisms not usually involved

in aerobic spoilage eventually predominate.These are microorganisms such as

Streptococci and Lactobacilli, which are lessaffected by the elevated CO

2 atmosphere

and which grow more slowly than thenormal aerobic spoilage bacteria. Because

the microorganisms which predominateunder MAP also cause less noticeable and

less offensive organoleptic changes, the netresult is a significant extension of shelf life

under MAP at refrigeration temperatures.

However, there are significant diffe-rences in the application of MAP to meats,including poultry, and to seafoods.Because seafoods contain much lowerlevels of myoglobin than do meats, higherlevels of CO

2 can be used before discolo-

ration becomes a problem. Vacuum andmodified atmosphere packaging includingN

2 and CO

2 flushing suppress the normal

spoilage flora and thereby extend the shelflife of seafoods. Whereas, the presence ofCO

2 does not lead to an increase in growth

of C. botulinum, replacing air with nitrogenproduces anaerobic conditions and anincreased susceptibility to the growth ofC. botulinum in advance of spoilage signalsdue to spoilage microflora. Vacuum

packaging in high barrier films producesthe same growth conditions as replacingair with nitrogen. However, if an oxygen-permeable film is used for vacuumpackaging, distinct spoilage signals will beproduced if the pack is held at abusetemperatures because of the ingress ofoxygen into the package.

Effective gas compositions vary

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Chapter 10

according to fish species, with low oxygenconcentrations being used with fatty fishwhich are susceptible to oxidationrancidity. Generally, gas mixtures fornon-fatty fish would be 30% O

2, 40% CO

2

and 30% N2, and for smoked and fatty

fish 40% CO2 and 60% N

2. Problems

caused by too high a CO2 include taint,

an acid flavour to certain species of fish,and clouding of the eyes.

It is evident from the publishedliterature that MAP can extend the shelflife of a variety of fish and fish products.However, MAP is not equally effective forextending the shelf life of all fish products.Although some claims have been madeof a shelf life of up to 3-4 weeks for therefrigerated storage of MAP fish. Generaltarget shelf lives of these products are inthe range of 10-14 days, but may reach18-20 days if the temperature is controlledvery tightly.

High temperature abuse (21-27°C) forperiods of 12-24 hours is a major concernsince MAP fish generally do not becomeovertly spoiled under these conditions yetmay be toxic, whereas fish held at thesame temperatures under similar aerobicconditions begin to become putrid beforetoxin production occurs.

Safety Aspects

The only effective way to assure thesafety of refrigerated vacuum packaged orMAP fish products would be to either (a)keep the product below 3°C at all times;(b) heat the product sufficiently to destroyspores of all strains, or (c) heat the productsufficiently to inactivate the nonproteolyticspores and then keep the product wellbelow 4°C. The latter two points may be

effective form a theoretical standpoint, butin practice, it may be difficult in a fishprocessing environment to avoid post-processing contamination with spores ofC. botulinum. Inactivation of botulinumtoxins by heating the food prior to consum-ption requires an internal temperature of80°C for 10 min or 86°C for 1 min, suchtemperatures are not being achieved in astandard 700 W microwave oven in 5 min.

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