PLASTICS BASED PACKAGE FORMS & SPECIALITY PACKAGING …icpe.in/Plastics in Food Packaging/pdf/4-Final.pmd.pdf · 75 plastics in food packaging plastics based package forms & speciality

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PLASTICS IN FOOD PACKAGING

PLASTICS BASED PACKAGE FORMS &SPECIALITY PACKAGING FOR FOOD PRODUCTS

Chapter 4PLASTICS BASED

PACKAGE FORMS &SPECIALITY PACKAGING

FOR FOOD PRODUCTS

H S Sathish

Food Packaging Technology DepartmentCentral Food Technological Research Institute

Mysore 570 020 (INDIA)

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PLASTICS BASED PACKAGE FORMSAND SPECIALITY PACKAGING

FOR FOOD PRODUCTS

Plastic materials are synthetics made fromoil, coal or natural gas and are now one of theprincipal packaging materials being used inthe form of wraps, pouches, bags, sacks,bottles, jars, tubes, trays, and boxes. Plasticshave applications in transport packagingtoo, where they have replaced steel, woodand glass. They are also used in the form ofstretch wrapping and shrink wrapping andshrink film, and as strapping for securing

palletized loads. There will be many otherpackaging openings in the future for thisgroup of materials. The properties of plasticslike pliability, flexibility, formability andease of handling will enable plastics toexpand their position in the market againstcompetition from other materials such asnatural fibers, glass and metal.

Speciality packages which possesscertain functional properties to enableprocessing and/or preservation of foodproducts include retort pouches/trays,aseptic packages, modified atmospherepackages, frozen and oven proof packagesand active and smart packaging.

New plastics, along with new combi-nations of natural and synthetic materials,will undoubtedly continue to be developedin the form of co-polymerized, laminatedand co-extruded packaging products to meeta wide range of needs.

FLEXIBLES

Flexible films are basically producedthrough extrusion, extrusion blowing and tosome extent through moulding. Flexible filmsare used to make wraps, pouches, bags, andtheir variations.Fig. 4.1. Flexible films and laminates

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WRAPS

Probably the simplest type of flexiblepackage ever devised is the intimate wrap,where a sheet material is used to enclose aquantity of product. The wraps are mostwidely used in bakery and confectioneryindustry. Metalised films and compositematerials in the form of twist wraps, Pleatedbunch wraps and folded wraps arecommonly used methods of wrapping.

Fig. 4.2. Bread wrapping

POUCHES

Pouches are totally enclosed packageswhich are intended to hold the product withor without assistance.

Flexible pouches can be made either frommonolayers or multilayered by laminates orco-extruded structures. Metalization onplastic films to improve the barrier propertiesand opacity is also common. Also composite

flexibles are widely used in food industriesparticularly for bulk packaging appli-cations.

Flexible pouches are used to pack foodssuch as: processed, frozen and fresh meats,poultry and seafood; dairy foods, frozenvegetables, dry bakery goods, coffee, tea,candy and snack foods; dry powders;prepared entrees and processed fruit andvegetables; condiments and liquids; andcereals and pet foods. Flexible pouchesbeing light weight and adaptable arecompeting favorably with other containers,such as glass and plastic bottles, metal andcomposite cans and paperboard boxes.Following are the commonly used pouchstyles.

Fin Pouch

Fin pouch is made by simply folding theweb material in half and sealing both sides.The bottom sometimes also has a sealdepending on web material and product.Fin pouches with an applied re-closeablezipper are also popular. Liquids andpowders are generally packed in thispackage style.

Gusset Pouch

Gusset pouch is formed by folding theweb material into a �W� shape at the bottomof the pouch. By adapting the fin pouch styleto have a gusset at its base, the fill volume ofthe pouch is increased.

Stand Up Pouch

Stand Up Pouch has proved to be one ofthe most successful means of packaging awide variety of products. The method ofproducing the Stand Up pouch is verysimilar to making a gusset pouch with the

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addition of sealing the edges and bottom ofthe gusset in order that the pouch will standupright on the store shelf. The style is idealfor snack foods, pet treats, delicate products,dried soup mixes, condensed soups andother liquids.

Membrane Pouch

The membrane pouch concept wasdesigned for separating a fin, gusset or standup pouch into two separate compartmentsby means of a center web. Membrane pouchesare used for separating any two ingredientsfrom each other prior to dispensing from thepouch.

Tandem Pouches

Tandem pouch configuration is ideal forincreasing output of a single product fill,increasing output of a multi-product fill orthe presentation of a two part product inseparate packages. Pouches may remain

attached with a perforation or serration. Theycan also be cut into individual pouches afterdischarge from the pouching machine.

Frangible Pouch

The frangible pouch is used for keepingtwo components separate from one anotherprior to being mixed them within the pouchitself. The two components are kept separateuntil one of the compartments is squeezedand the resulting pressure ruptures the sealbetween the compartments and allows thecomponents to mix. After mixing, the pouchis opened and the product is dispensed.

Bags

A plastic bag is defined here as a bagmanufactured from extensible film by heatsealing one or more edges. They are producedin different sizes for use in differentpackaging applications. The plastic bagsare identified by the type of heat seals

Fig. 4.3. Different pouch styles.

Fin pouch Gusset pouch Stand up pouch

Frangible pouch Membrane pouch Tandem pouch

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employed. There are basically three types ofseal methods in use and they are: side weld,bottom-seal, and twin seal. Plastic bags,available in virtually all shapes, sizes andcolours and configurations have replacedpaper in most light duty packagingapplications. Polyethylene such as LDPE,HDPE, HMHDPE and polypropylene arethe most commonly used materials for bags.Bags are generally classified as commercialbags and consumer bags. Commercial bagsare used as a packaging medium for anotherproduct, whereas consumer bags are used tocontain the purchased materials from themarket.

Fig. 4.4. Plastic bags

Bag-in-Box

Bag�in-box is a two package system usedfor commercial packaging of food and non-food products. It is used in bulk packagingof liquid and semi liquid products consistingof three main components. (1) a flexible,collapsible, fully sealed bag made from oneor more plies of plastic films; (2) a closureand a tubular spout through which contentsare filled and dispensed; and (3) a rigid outercorrugated or solid fibreboard box or

container, usually holding one but sometimesmore than one bag.

Edible oils packed in bag-in-box are verypopular in the market. Other products beingpacked in this system are fruit juices andnon-carbonated beverages. Both hot fill andaseptic filling techniques can be used. Otherliquid products that are being used in bag-in-box include water, milk, wine, cider,vinegar, soya sauce, tomato ketchup, saladdressings, etc. Bag-in-box is not just confinedto small packs and packs up to 1,000 litercapacity are also in the market. The bag-in-box technique was originally used for liquidsbut has successfully diversified intopackaging of dry powdered or granularproducts. The typical materials used for theconstruction of bag are Kraft/PE/Foil/PE,Bleached Kraft/PE/Foil/PE and PET/PE/Foil/PE.

Fig. 4.5. Aseptic bag in a box.

Skin Packs

Skin packaging is another form of blisterpackaging. In skin packaging, the productitself is the mould over which the heatedplastic film or a �skin� is drawn by vacuumand heat sealed to paperboard card. Thereare three principal components associatedwith skin packaging: the plastic film, theheat seal coating and the paperboard card.

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There are three types of films used in skinpackaging: LDPE, PVC and Surlyn Ionomer.Normally, skin packaging films are heated,draped and formed, and bonded to thepaperboard card in one operation.

Fig. 4.6. Skin packaging of chickensausages.

Stretch Films

Stretch films are basically used to unitisesmaller individual items into a larger unitloads. The advantages of unitization are:

l Reduced handling costs

l Less manpower requirement

l Transportation savings

l Protection to the commodities.

In addition to the above advantages,stretch films offer further advantages:

l Low supply cost

l Protection from moisture, dirt andabrasion

l Reliable performance

l Automation

l Scan through optics

l Ease of removal

l Recycling

Stretch packaging operation consists ofstretching the film around the objects to bewrapped and then heat sealed. The residualtension in the film provides the tight contourwrap. Again, like shrink wrapping, it findsits use in collating, pallet overwrapping andretail wrapping.

The main films used in stretch wrappingare LDPE, EVA and PVC. The choice of stretchfilm depends on factors such as appearance,protection required and the susceptibility todamage by compression of the articles to bewrapped.

Stretch wrapping of pallet load is done byspiral winding so that a standard width offilm can be used, irrespective of the palletload dimensions. Pallet stretch wrapequipments are also available for thispurpose.

Shrink Films

Shrink films are basically used to wrapawkwardly shaped articles, whichotherwise are difficult to pack. Theavailability of tougher, cheaper films withhigher softening points such as LDPE, PVCalong with hot air shrink tunnels made shrinkpackaging a reality. The applications includecollation of cans, jars, bottles or cartons. Forcans, bottles and cartons, it is necessary touse fibreboard trays or plastic trays such asPVC or EPS. Cost-wise, shrink wrapping ischeaper compared to fibreboard boxes.

The use of shrink film at retail store levelhas a positive impact from the environmentalpoint of view. The amount of packaging to be

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disposed off at retail level is smallercompared to paperboard packaging, andthis is an important positive point from theenvironmental point of view.

Overwrapping of complete pallet loadsdisplacing the use of paper or boardcoverings held in place by metal or plasticstrappings by shrink wrapping ensures morestability than strapped loads and thisprovides extra protection against ambientconditions.

Fig. 4.7. Shrink wrapping of asepticfruit juice packages and HDPE container.

Plastic Woven Sacks

Woven sacks are manufactured byweaving of monoaxially oriented tapes ofHDPE, PP and LDPE for packaging offoodgrains, sugar, rice, salt, milk powder,onion, potato, etc.

For high moisture sensitive foods, wherehigher barrier properties are required, looseliners of LDPE, LLDPE or HMHDPE areused or LDPE lamination of the woven fabricis done before stitching into sacks.

Advantages of Circular WovenSacks

l Coverage is very much better because thetapes do not get twisted as in flat weaving.

l Savings up to 25% because of superiorcoverage.

l Much higher output compared to the

conventional flat looms.

l Improved strength of the weaving fabric.

l Savings in floor space, number ofoperations and labour.

l Higher output of sacks.

Advantages of Woven Sacksover Conventional Sacks ofJute, Paper, etc.

l Weighs only 20% of the jute sacks.

l Resistant to even most corrosiveproducts.

l Safe for direct contact with food products

and do not cause contamination as likelywith jute fibres.

l Are water repellent and can be madewater and moisture-proof by using a looseliner or lamination.

l Do not contain Jute Batching Oil (JBO).

l No corrosion even in presence of water orin corrosive atmosphere.

l Superior drop impact properties; as such

bursting losses are very low or almostnil.

l Resist fungal attack.

l Can be manufactured in any desired

colours for easy identification as well asfor the brand image.

l Attractive printing.

l Clean appearance.

l Easy disposability and reusability.

Possible Variations

l Leno bags for the packaging of potatoes,onions, apples, fruits, etc � fabric isopen mesh with average weight of less

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than 30 g/m2. For example, a 40 kg potatobag weighs only 28 g each.

l Fabric can be laminated to paper for

Plastic film.

l Medium size bulk containers up to 2 toncapacity are manufactured using a fabricweighing 200-250 g/m2 from UVstabilized PP Tapes.

Fig. 4.8. Plastic woven sacks.

Nettings

Plastic nettings are produced either byknitting or by extrusion. Netting is producedin the form of tube and sheet in a wide varietyof mesh sizes, diameters, width and colours.

Extruded netting is produced throughcounter rotating dies. As the inner and outerdies rotate, small strands of molten plasticoverlap each other, bonding themselvestogether where they overlap. The material isheated and stretched to the point just beforebreakage to get the orientation. Knittingmachines offer simple and sophisticatedstitch patterns. HDPE and PP are the mostcommonly used plastic materials fornettings.

Plastic netting is a valuable packagingmeans. It is resilient, strong, and flexible.Netting can be frozen and then heated or viceversa, keeping its strength and flexibility.Plastic netting is very cost effective comparedto other forms of packaging and is recyclable.Plastic netting is used in many packagingapplications. As a flexible material itconforms to irregular products. Nettingprovides an excellent packaging media as adecorative and protective overwrap. Nettinghas wide applications in the packaging offresh fruits and vegetables especially rootvegetables and in the poultry industry.

Fig. 4.9. Potatoes and onionsin plastic netting

RIGID AND SEMI-RIGIDCONTAINERS

Bottles and Jars

When used to make jars or bottles, plasticshave some distinct advantages over glass ormetal including the following:

l Non breakability: At least most plastics inmany applications are less likely to break

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on impact than glass. With mostcommercial plastics, unbreakability is amajor advantage.

Fig. 4.10. Plastic bottles and jars invarious shapes and sizes.

l Lighter weight: As low as 1/3 the weightof metal and 1/8 the weight of glass incomparable applications.

l Lower noise level in packaging and inuse, again compared to glass and metalno corrosive as opposed to metal cans.

l Transparent: Some plastics�when compa-red to metal.

l Smaller size: When compared to glass,wall thickness for equivalent strengthsand volumes are less, and so equivalentcontents can be contained in packages ofsmaller exterior size.

l Flexibility of forming: Capital investmentrequired to produce rigid plasticcontainers is only a small fraction of thatrequired for glass or metal.

The major plastic materials used forbottles and jars are HDPE, LDPE, PP, PVC,PS, PC, PET, Multilayers of desiredcombinations.

Co-extruded Blow MouldedMultilayer Bottles

Co-extruded blow moulded multilayer

bottles offer new packaging possibilities forimproved barrier requirements with apotential to replace traditional glass andmetal containers. A wide range of differentpolymers can be combined in different ways,and also the wall thickness of the individuallayer can be optimized to achieve thenecessary functionality and offer the besteconomics. Manufacture of these bottlesinvolves use of flexible co-extrusionmachines suitable to rheological behaviourof the different polymers as well as qualitycontrol.

Co-extruded bottles are already in themarket for a wide range of foods such asketchup, sauces and jellies. These bottleshave replaced glass bottles and they areunbreakable, squeezable and light weight.

Fig. 4.11. Co-extruded multilayer plastic bottles.

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The most popular material combination inuse today is the six layer combination � PP orHDPE/regr ind/adhes ive/EVOH/adhesive/PP or HDPE. Another three layercombination is PC/PET/PC. Here, noadhesive material is required , since bothpolymers belong to the polyester family andhave excellent adhesion.

Stretch Blow Moulded Bottles

The hollow type of extrusion blowmoulded bottles made of various thermo-plastics like LDPE, HDPE, PVC, PS, PC, etc.have been in commercial use for over threedecades. However, the biggest breakthroughin bottle making was the invention of stretchblow moulding technique since it providesmaterial savings, improved clarity and goodmechanical strength. Following are theadvantages of stretch blown moulding:

l Raw material savings up to 25% sincethinner walls are possible because ofsuperior mechanical strength due tobiaxial orientation.

l Improved clarity particularly in case ofPET bottles.

l Improved barrier properties.

l Improved mechanical properties.

The major materials that are processed bythis process are PET and PVC followed byPP. Thermoforming is another techniqueused for making jars.

Cans

Cans are also made by plastics, byinjection moulding, blow moulding andthermoforming. Since plastics are not asrigid as metals, thicker walls are needed for

equivalent performance, and containerweight and cost can become excessive. Heatresistance is another important propertyrequired by the can to undergo pasteuri-zation and sterilization for most of the acidand low acid food products. A precise flangeis needed to guarantee a perfect seal, whichis absolutely critical for sterilized cans, andcertainly desired for contained liquids,especially under pressure. Plastic ends ormetal ends are used in these type of cans.Another design concern is the necked-in endnow customary for beverage cans, whichallows tighter six packer and cheaper ends.This can be done with plastics, but moulddesign is more complicated to permit theundercut needed.

These cans have huge applications insoftdrink and beer markets. Thermoformedand blow moulded PET bottles coated withPVDC are in the market in USA, Britain andItaly. For heat resistance PP is the mostsuitable material. However, PC can also beused but it is expensive. For better barrierproperties, a multilayered co-extruded PP/EVOH/PP can also be used. Injectionmoulded HDPE and PS cans are also in usefor some food products.

Fig. 4.12. Plastic can with metal lid.

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Collapsible Tubes

Many plastics can be used to make plastictubes, but LDPE/HDPE is the primarymaterial used today. It has high moisturebarrier properties, low cost and goodappearance. Its lack of oxygen and flavorbarrier have been improved with barriercoatings. HDPE and PP are also used, butthey are much stiffer than LDPE for tube sidewalls, and hence not as popular as LDPE.EVOH based co-extruded tubes offer excellentbarrier to oxygen and flavour.

These tubes are produced by Strahm andDowns method, basically an extrusionprocess. The collapsible tubes are coronatreated during extrusion for better inkadhesion during printing. HDPE and PPscrew caps are used as closure for collapsibletubes. At present, the all plastic barrier tubeis not economically viable with a laminatedstructure. Plastic collapsible tubes will finda major share in the market, in the event ofreduction of EVOH prices in the future.

In the open-head design configuration,the containers are usually supplied with aremovable lid designed to be used with eitherliquids or solids. The lids are available withspouts of various types. The most commonlyused pour fittings for liquids is the flexiblepolyethylene spout. This fitting snaps intoposition in a preformed hole or is a spoutthat incorporates metal collar that is crimpedonto a formed, ridged opening in the lid. Theopen head pails are generally non-verticaland are nested into each other for efficientstorage and transportation, prior to beingfilled and sealed. In the closed-headconfiguration, no nesting advantage exists.The size of pails varies from 3 L to 25 Lcapacity. Both open-head and closed-headpails are made from HDPE. These pails areuniversally injection moulded.

Fig. 4.14. Plastic pails for vanaspati

Blister Packs

Blister packs are the fastest packingmethod world over. It consists of combinationof plastic materials with paperboard toproduce visual, self vending packages. Therapid growth of self service retailing createda demand for innovative packaging thatprotects the product and also provides salesappeal in terms of product visibility andinstruction for use. The size and shape are

Fig. 4.13. Collapsible plastic tubes

Pails

Plastic pails are open head containerswith removable lids and containersproduced as a single unit called tight headcontainers. The container top or lid can bemanufactured with one or more openingsfor filling and dispensing. The openings aredesigned to be used with a variety of closures.

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numerous because of the flexibility it offers.The blister packs consists of a preformedplastic blister, paperboard and a heat sealingcoat on the paperboard. The selection ofpackaging material for blister in terms oftype, thickness and grade depends on manyfactors like height and weight of the product,sharp and pointed edges of the product, heatsealing properties, compatibility with theproduct and machinability. The mostcommonly used plastic materials for blisterpacks are PVC, PVDC coated PVC, OPS andrecently PET is also being used.

Heat seal coatings provide a bondbetween the plastic blister and the printedpaperboard card. These solvent or waterbased coatings can be applied to rolls orsheets of printed paperboard using rollcoaters, gravure or flexographic methods.Paperboard of thickness between 0.45 and

0.60 mm in generally used. The normalsequence of assembly involves loading theblister with product, placing the paperboardcard over the blister and heat-sealing the

package.

Thermoformed Containers

Tubs, trays and box inserts are thecommonest containers formed by this methodparticularly where very thin walls arerequired, such that it would be difficult for apolymer to flow between the mould walls inan injection moulding.

PS, Polypropylene, PVC, HDPE, and PEThave all been used for thermoformedcontainers. The latest materials in this fieldare foamed polystyrene andPP, which canbe thermoformed to give trays and containerswith built in cushioning properties. Thethermoformed articles are produced eitherby vacuum forming or by pressure forming.

Continuous on-line Thermo-Form-Fill-Seal (TFFS) machines are also available inthe market. This type of machine uses a reelfed web of plastics material into which isthermoformed a series of tray likedepressions. The depressions are indexedforward as a web and after filling a furtherweb is fed over the open top of the filled trayand sealed on to the flanges of the trays toform a closure. The web of filled and closedtrays is then punched to form individualpacks, or slit into partially jointed stripscontaining several units. Lidding materialscan be plastic film or film laminations,although metal foil reverse coated for heatsealing to the container is more widely used.Re-closable plastic lids can be provided forsome thermoformed containers, either by aseparate operation or by on-machinethermoforming at the lidding station.

Fig. 4.15. Artificial sweetener in ablister pack

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The products, which are commonlypackaged in thermoformed containers, arebutter, cream, yogurt, processed cheese,ready-to-eat foods, fruits, vegetables, andjams.

Use of thermoformed cups made ofpolypropylene are widely used. These havefunctional advantage of transparency andtoughness over HIPS cups. These can beused for packing Dairy products anddrinking water.

Fig. 4.16. Thermoformed trays ofvarious shapes and sizes.

Fig. 4.17. Food products in thermoformedcontainers

Drums

Open top plastic containers up to 25 Lcapacity are called jars or pails. The term�Drum� applies to containers larger than 25L capacity, and the container can be eitheropen or closed type. The standard drumsizes commonly used are 30, 60, 120 and 216L. The development of large plastic drumstook many years because they requiredspecial resins and processing equipment.These drums are used in food processingindustries for the shipment and storage ofproducts that include concentrated fruitjuice, vegetable pulps and condiments. Theyare made by extra high molecular weighthigh density polyethylene. These haverelatively high resistance to permeation,which can be further improved by usinghigher wall thickness. The service life ofHDPE drums cannot be predicted because itlargely depends on climatic conditions.Different colorings, especially black, blue,green, white and gray, increase resistance toweathering and protect the product fromlight. Depending on the climatic conditionsadditional UV stabilizers must be added.

Drums are manufactured by rotationalmoulding. In this process, a fine powder or

Fig. 4.18. Plastic drum and milk cans

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liquid thermoplastic takes on the shape of aheated mould subjected to tri-axial totalrotation. Once the receptacle has beenformed, the mould is cooled and opened. Theparticle size of the powder used also affectsthe surface appearance, the quality of whichimproves with the fineness of the powder.

Crates

Plastic crates are being used for manyyears in the food industries as returnableshipping containers. The more familiar useshave been as milk crates, fresh fruits andvegetable crates, beverage cases, as a storagecontainer in factory warehouses, etc.

Fig.14.9. HDPE crates.

There are five different methods employedto manufacture plastic crates: Injectionmoulding; Compression moulding; Blowmoulding; Rotational moulding; andThermoforming. Each process is suited tothe production of a range of geometries witha variety of materials at different costs. Butthe most often used method is injection

moulding. This method is most suited forreusable shipping container because it allowsintricate shapes to be moulded at high ratesof production. HDPE is the most commonlyused material for plastic crates. There arefour different types of crates available:

l Nest only

l Stack only

l Stack and nest

l Collapsible

Corrugated Sheets & Boxes

Plastic corrugated boxes are used insituations where the traditional paper basedCFB are inadequate in certain situations.Theoretically, it is possible to use all plasticmaterials for conversion into corrugatedsheets, but the costs can be prohibitive. Themost commonly used materials are HDPEand PP. PC is also used as sheets for somespecial applications. Following are theadvantages and disadvantages of plasticcorrugated boards, which designer shouldkeep in mind while selecting the material.

Table 4.1. Advantages anddisadvantages of plastic

corrugated boxes

Advantages Disadvantages

Long life Cost

Chemical resistance Formability

Insulation Temperature

resistance

Multiple colour UV degradation

choice

Strength:weight ratioWaterproofness

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Forming Methods

Corrugated plastic boards aremanufactured using standard box makingtechniques. Generally, flatbed presses usingcam action or single stroke are used to die-cut, score, crease, or fold the material. Three-point or four-point, single-side bevel-edgerule is used for cutting. Six-point creasing isused for creasing parallel with the flutes,three-point of creasing across the flutes, toobtain a 90° bend. The polyolefin board hasa �memory� and, unlike paperboard, willgenerally attempt to return to its previousshape. This characteristic calls for modifiedbending and creasing techniques.

High-frequency welding is the methodused for joining the material. Because of thenature of the polymer, glues are not generallysuccessful. But lap joints have beenaccomplished using corona-treated boardwith silicone-type or hot melt adhesives.Metal stitching can be used, but this createsa weak spot immediately surrounding the

staple.

Fig. 4. 20. HDPE and PP corrugated sheets.

Pallets

Plastic pallets are primarily used insidethe plants or in closed-loop shipping systemdue to their higher costs. Plastic pallets arealmost always found in applications wherethe user can retrieve most of the pallets aftereach trip. Following are the advantages ofplastic pallets:

l Long pallet life

l Reduced load damage

l Easy cleanup

l Reduced worker injury

l Chemically inert

l Moisture proof

l No harbor for pests

l Nestability

l Interstackability

The commonly used materials are HDPE,PS and FRP. But HDPE is the most favouredmaterial because of low cost, uniformperformance, ready availability, wideacceptance, excellent resistance to impact,and good performance under wide range ofoperating conditions. The only negativepoint of HDPE is the poor creep resistance.Following are the methods employed for themanufacture of plastic pallets:

l Structural foam moulding

l Injection moulding

l Rotational moulding

l Thermoforming

l Reaction injection moulding

Foams

The mould comprises of two wallsbetween which steam is diffused through amass of plastic granules (EPS, PE,

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polyurethane), entering by a large number ofapertures or slitted plates uniformlydistributed over the inner wall.

Expanded Polystyrene Foaming

Conversion is carried out by raising thetemperature in two stages. In the first stage,pre-expansion or free expansion ofpolystyrene beads yields expandedpolystyrene flakes. These flakes are thenused to fill the mould, and they expand totake the form of the mould under heat andpressure. Two processes exit: heating by hotair followed by compression, and steamheating of the mould. Blocks and mouldedshapes may be produced.

Polyurethane Foaming

Conversion starts with dosing machinesthat feed the basic liquid ingredients to amixing head which deposits them on the belt(free foaming) or into a closed mould (forcedfoaming) where expansion begins. There aretwo kinds of machines: the low pressure�pre-polymer� machine (in which mixing isby agitator of compressed air and feeding bygear pump) and high pressure �one-shot�machine.

Polyethylene Foaming

Conversion starts with a mixture ofpolymer and blowing agent to whichcrosslinking agents (peroxides of di-cumulated butyl and tributyl) are added.Shaping is effected by calendaring. Chemicalor radiation crosslinking is accomplished attemperatures below the breakdowntemperatures of the blowing agent.

CLOSURES

Screw Caps

Screw caps are generally injectionmoulded from wide variety of plastics such

as polyethylene, polystyrene orpolypropylene or compression mouldedfrom phenol-formaldehyde and urea-formaldehyde. They are commonly used onplastic bottles or jars.

Polypropylene caps are particularlyvaluable as closures for cosmeticpreparations because of their designpossibilities. Polypropylene has goodresilience so that mouldings having slightundercuts can be �jumped-off� the mouldcore without damage to the moulding.Decorative inserts can be pressed into thesemoulded-in undercuts to give caps withhighly effective sales appeal. The resilienceof polypropylene also makes possible thedesign of linerless closures.

Plug Fitting

This type of closure is normally injectionmoulded from low-density polyethylenesince its softness and flexibility enables it togive a good seal, even against hard, smoothsurfaces such as walls of polystyrene tube.The plug itself is often ribbed to give evenbetter sealing.

An interesting example of the designpossibilities inherent in the use of plastic isthe plug closure which incorporates flexibleprongs on the underside of the plug. The useof this closure for tablet tubes eliminates thenecessity for a wad of cotton wool on top ofthe tablet to prevent their movement withconsequent risk of breakage during transport.Again no special equipment is necessary forclosing.

Push-on Covers

This type of closure is the normal one forinjection-moulded plastic pots or jars andfor some types of vacuum-formed containers.In addition to plastic push-on covers,

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paperboard ones are still used in someinstances. Before applying the push-oncover, a foil diaphragm is often crimped overthe top of the pot or jar. This gives extraprotection or a tamper-proof seal. Inthickness gauges, aluminium foil issometimes used as the only closure, as in thecase of yoghurt containers. Flexible push-on covers (in low density polyethylene) canalso be used for bottles.

Heat-sealed Covers

These are often used for closing vacuum-formed containers of the tray type, or thedeep drawn pyramid type used for fruitdrinks. The cover may be flat or recessed togive a shallow plug-type fitting to thecontainer with a consequent increase inrigidity and strength. Sealing is carried outwith a heated jig. With certain types ofequipment it is possible to vacuum formplastics sheet continuously from the reel, fillthe depressions so formed, then covers themwith another plastic sheet fed from a separatereel. After heat sealing, the cover on thecontainers are cut out and trimmed.

Miscellaneous Closures

The possibilities for design inherent inmoulding of plastics have led to many specialtypes of closures, such as combined plugand snap-on covers and plug or screw-capswhich incorporate means of dispensing thecontents in droplet form or as a jet or spray.Such closures are usually fitted to �squeeze�bottle designs. Another interesting designfeature which is often incorporated in plasticclosures is the integral moulding of a nozzleand cap to give a captive closure. Theassembly is fitted as a plug in the bottle neck.The same result has also been achieved bythe use of a snap-on action retaining ring.

Fig. 4.21. Closures.

Table 4.2. Some examples ofgas-flushed packaging atrefrigerated temperatures

Food Gases Shelf-life (days)

Red 70% Oxygenmeats 30% Carbon dioxide 7

Organ 40% Carbon dioxidemeats 50-60% Oxygen 6-10

Processed 20-50% Carbon dioxidemeats 50-80% Nitrogen 21

Fish 80% Carbon dioxide20% Oxygen 4-8

Pizzas 50% Carbon dioxide505 Nitrogen 21

SPECIALITY PACKAGING

Vacuum and Gas Packaging

Where foods are susceptible to oxygen,as frequently occurs in the presence of light,it is helpful to exclude air from the package.Vacuum packaging is more a means ofkeeping a food at better level of quality during

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respiration of the meat quickly consumes theresidual oxygen replacing it with carbondioxide which eventually increases to 10-25% within the package.

Packaging material required for vacuumpackaging must possess high resistance togas permeability and water vapourtransmission with perfect seals and alsoshould have good mechanical strength.Typical materials used are: 1. K-Nylon/LLDPE, MXXT/LLDPE, K-PET/LLDPE 2.Laminates of plastic films with aluminiumfoil. Food products like bacon slices, ham,fish, cheese, coffee, tea, meat, etc. are vacuumpacked.

Table 4.3. Gases used in packaging and their applications

Gases Applications

Nitrogen As a pressure-relief agent to prevent external atmosphere from crushing theproduct. Nitrogen is inert gas. It does not react with foodstuffs. Commonpackaging applications: bulk-pack bacon and sauces, shredded and slicedcheese and beef jerky, snack foods.

Carbon Depending upon applications, 20-100% carbon dioxide may be used.dioxide Carbon dioxide lowers the pH of the food and can exert a powerful slowing

effect on the growth of microorganisms. Primary application is for bakedfoods, cookies, cakes, breads, dough and pasta products. Carbon dioxidetends to be absorbed into the actual body of the food product itself. Carbondioxide is frequently mixed with nitrogen to prevent the package fromclinging too tightly to the product. On the other hand, some products whichare not sensitive to strong pressure or tight cling, but which are susceptibleto spoilage by mould growth, are packaged in an atmosphere of 100%carbon dioxide.

Oxygen The applications using oxygen are primarily for red meat. The concept iswidely used in Europe for centrally packaged retail cuts. Oxygen is usedas an oxygenating agent at levels in the range of 40-80% to form the brightred meat colour. When oxygen is used, it is usually mixed with carbondioxide and nitrogen�carbon dioxide for its preservation effect, nitrogento provide a bulking agent. Oxygen tends to disappear inside the package.It can be metabolized by the meat to carbon dioxide which is absorbed inthe water phase of the meat as carbonic acid.

Fig. 4.22. Nitrogen filled snack

food packages.

its natural life than a means of increasing itsshelf life. Under good vacuum conditions,the oxygen level is reduced to less than 1%.In the case of vacuum packed meats,

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In gas flush packaging the gases usedand their applications are given in Table 4.2and 4.3.

Retort Packaging

Pouches

The materials used in making retortpouches should possess toughness andpuncture resistance normally required offlexible packaging, good barrier propertiesfor long shelf-life, and heat sealability over awide temperature range along with the abilityto survive retort temperatures. To get allthese desired properties in the pouch material,laminate structures or coextruded films areused.

The outer film of the composite structureis needed for strength and flex resistance. Itshould be resistant to heat-seal temperatures,printable, and be able to withstand retorttemperatures without bursting, shrinkingand delamination. The present material ofchoice is polyethylene terephthalate (PET).It has the added advantage of being reverseprinted so that ink is embedded between theouter layer and the next inner layer.

In order to achieve a shelf-life of one yearor more, aluminium foil layer as one of theinner layers for barrier properties is essential.The thickness range of aluminium foil variesfrom 9 to 25µm though 9 µm thicknesscurrently predominates. Thicker foils tendto result in more flexure failures and formmore permanent creases or peaks that act asloci for abrasion failures. In Japan, non-foilpouches are quite common since a muchshorter shelf life of 3-6 months is acceptable.Nylon is another material used as a barrierfilm in place of aluminium foil because of itslow gas transmission rate and toughness.Being transparent, nylon based laminatescannot provide protection from light unless

covered with a overcarton or wrap. The othertransparent materials used in place ofaluminium foil are EVOH, PVDC, aluminiumoxide and silicon oxide coatings on PET.

The current material of choice for innersealant layer is cast polypropylene, thoughhigh density polyethylene modified withisobutylene rubber is also used.

Since thermoprocessing imposes higherperformance criteria on pouch materials,developmental efforts centred around theformulation of adhesives and primerscapable of laminating the dissimilar pliestogether with adequate strength to withstandall the rigours of heat sealing, retorting anddistribution handling, while beingacceptable to regulatory agencies from thestandpoint of not contributing harmfulcomponents to the foods. Quality assuranceprocedures are strictly adhered to in view ofthe several critical factors involved in pouchperformance.

Semi-rigid Containers

Retortable semi-rigid containers are oftray or tub type with a structural supportingbody and a sealable flexible lid. The traypackage is a thin-profile package that offersall the advantages of the retortable pouchwith the added convenience of a servingdish. Improved techniques of thermo-forming or cold impact forming andcoextrusion have recently advanced thestate-of-art of this package. Thermoformingis relatively low-cost technique, wherein thesheet is heated to an optimum temperatureand it is then forced into a female cavitymould by compressed air, vacuum, or a maledie.

Advantages of semi-rigid packagesinclude ease of filling or top-loading ofproducts with or without in-line forming of

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the container, and potential increases in line

speeds due to multiple pocket forming on a

wide web-style operation.

Original semi-rigid packages were madeof aluminium foil laminates with cast

polypropylene as sealant layer. Oriented

polypropylene was used as exterior layer.

The 80�s have seen high barrier plastics

coming into use to serve as an effective

replacement for aluminium in order to makethe containers microovenable. Two resins

that are being used in these are

polyvinlyledene chloride (PVDC or saran)

and ethylene vinyl alcohol copolymer

(EVOH or EVAL).

Though these resins are prohibitivelyexpensive, multi co-extrusion permits the

burying of a thin layer of the barrier layer

between two or more thicker layers of less

expensive polymer. While PVDC provides

an excellent barrier to oxygen, its resistanceto re-melting and hence the inability of using

its regrind may prove to be a disadvantage.

In case of EVOH, the barrier to oxygen

transmission is adversely affected in presence

of high humidity. The use of a desiccant in

the tie layers to capture any errant moistureis believed to solve this problem.

In the rotary thermoforming process, a

multi-layer sheeting is directly coextruded

on to a rotary drum, where it is thermoformed

into multicavities while it is still above the

melt temperature of the polyethylene. Thehigher temperature is expected to result in

stress-free containers, thereby minimizing

warping during retorting.

Following are the typical laminate

structures used for retort pouches and trays:

Pouches

CPP

PET/CPP

CPP/Nylon/CPP

PET/Al.foil/CPP

PET/Al.foil/Nylon/CPP

PET/SiO2 on PET/CPP

PET/Al2O

3 on PET/CPP

PET/PVDC on PET/CPP

PET/EVOH/CPP

Trays

CPP

PET/CPP

PET/Regrind/PVDC on PET/CPP

PET/Regrind/EVOH/CPPCPP/Regrind/EVOH/Regrind/CPP

Fig. 4.23. Retortable pouches and trays.

Aseptic Packaging

Aseptic processing and packagingbasically consists of filling of thecommercially sterilized product intopresterilized package under asepticconditions and sealing with a presterilizedclosure in an atmosphere free ofmicroorganisms. Aseptic packaging consistsof the following steps:

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l Heating the product to sterilization

temperature and holding it at thattemperature to achieve commercialsterility,

l Cooling of the product,

l Filling into sterile containers in sterileatmosphere and sealing aseptically.

The versatility of aseptic technology hasgiven rise to the use of a variety of plastic andpolyolefin materials for packaging. Sinceaseptic applications require both productpreservation, and utility from the packages,there are several basic requirements thatthese relatively new packaging materialsmust meet for successful application in themarketplace, and the most of them areproduct-or usage-dependent.

1. The packaging materials must beacceptable for use in contact with theintended product, and must comply withmaterial migration requirementsapplicable.

2. Physical integrity of the package isnecessary to assure containment of theproduct and maintenance of sterility. Theterm integrity applies to the structuralintegrity of the container itself as well asthat of the closures and seals to assurepackage soundness and hermeticityduring handling and distribution.

3. The package material must be able to besterilized and be compatible with themethod of sterilization used (heat,chemical or radiation).

4. The package must provide the barrierprotection necessary to maintain productquality until it is used. Barrier protectionmeans control over the transmission ofoxygen, moisture, light, and aroma

through the package as required by theproduct.

The package forms that are used in asepticpackaging are: Rectangular and tetrahedronshaped cartons, Cups, Bags, bottles and bagin box/drums. The packaging materials aresterilized by using either H

2O

2 in combi-

nation with heat or gamma irradiation.

The typical packaging structures used

are:

LDPE/Paperboard/PE/Al.foil/PE/LDPE/ LLDPE.

LLDPE/EVOH/LLDPE

PET/Al.foil/Paperboard/PP or PE

Fig. 4.24. Aseptic consumer packsand bulk bags.

Frozen Food and Oven-Proof Trays

The continuing growth of microwaveoven sales and increasing consumer use ofmicrowave ovens for cooking processes aswell as other conventional ovens has madedevelopment of dual-oven-proof trays animportant proposition in packaging. One ofthe largest markets for microwave oven-proof trays is restaurants equipped withmicrowave ovens for quick service. Mostplastic trays and paperboard trays withgrease-proof coatings are available formicrowave oven use only, since temperaturesto which trays are exposed are lower than100°C(boiling point of water).

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Social trends and changing life styles,that have made dual-oven-proof paperboardtrays dominant in the market, include:

1. Growing popularity of conveniencefoods,

2. Increasing institutional catering,

3. Development of the microwave oven,

4. Development of dual-oven-proof PET-coated paperboard,

5. The higher production cost of metal andplastic containers,

6. Energy factors,

7. Environmental factors.

Requirements for Dual-Oven-ProofTrays

The Primary requirements of dual-oven-proof tray for food packaging are as follows:

1. Protect the food contained,

2. Aesthetic appeal to the user,

3. Ease of heating or cooking the food forserving,

4. Operating efficiency in manufacturing,and

5. Cost efficiency.

Each requirement must be satisfied by thephysicochemical functionalities orproperties of materials used in the traymanufacture.

Requirements for Dual-Oven-ProofPaperboard

Dual�oven-proof paperboard has beendeveloped primarily in conformity with therequirements for dual-oven-proof traysalready described. Detailed requirements or

advantages of dual-oven-proof paperboardfor oven-proof trays are as follows :

1. Dual-oven-proof quality : Paper itselfhas a high heat resistance, which isreinforced by the PET coatings. An oventemperature in cooking could be as highas 240°C, and heat resistance against200-220°C for 30 min is sufficient for mostapplications.

2. High temperature resistance : Oven-proof trays used for frozen foodpackaging must have a high heatresistance, going from deep freezing at �40°C to reheating at 200-220°C for 30 min.

3. Browning and odor developmentresistance : Browning of paperboard is amajor problem to be overcome with PETcoating on paperboard as a material ofdual-oven-proof trays. Heat isaccumulated along the tray flange, whilethe walls are protected from risingtemperature by the heat-sink effect fromthe foods contained. Coatings shouldnot be degraded to generate odor at oventemperatures.

4. Good performance under deep freezeconditions : Frozen foods are handledand distributed under deep-frozenconditions at lower than �15°C, typically�40°C. Oven-proof trays must havesufficient properties as packaging forfrozen foods at that temperature.

5. Grease and water resistance : These areother important roles of coating, and alsoare primary requirements for a packagingof frozen foods that will be used to heatand serve.

6. High in productivity : This refers to notonly productivity of tray itself but alsoproductivity at the packaging operation,

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Chapter 4

which are very important in the costperformance.

7. Printability : Ease of printing and aestheticqualities are distinctive properties ofpaperboard.

8. Heat sealability and gluability : Theseare primary requirements for foodpackaging to protect the contents. Coatingaffects these requirements strongly,especially heat sealability, which is notoriginal property of paperboard.

9. Heat tolerance in physical properties : Atray with food, reheated for serving, musthave sufficient physical properties forholding food and serving on the tablewithout any troubles. Rigidity and greaseand water resistance are importantproperties.

10. Safety as food packaging materials : Allthe materials used should be inconformity with regulatory requirementsand be approved.

11. Microwave safe : Molecules oftengenerate heat from electric oscillation ofmicrowaves which may result in theirdegradation and hazardous substances.

PET Tray

Dual-oven-proof PET trays have beendeveloped for more than 10 years, and a fewproducts have been introduced. PET hasappropriate properties as a material for dualoven-proof trays, including cost of resin,heat resistance, mechanical properties, easeof processability, and compatibility withfood products as well as availability of anamorphous and a crystalline state. PET canbe extruded to sheeting and can bethermoformed at its amorphous state, andhigh heat resistance can be achieved in thecrystalline state, which is the primaryproperty of the tray for conventional oven

use.

Modified Atmosphere Packaging(MAP)

It is a food preservation technique, inwhich the composition of atmospheresurrounding the food is changed from thenormal composition of air. Unlike controlledatmosphere storage, in MAP, there is no wayof controlling the atmosphere componentsat specific concentrations once a packagehas been hermetically sealed. There are twodifferent methods of MAP: Activemodification and Passive modification.

Table 4.4. Dual-oven-proof traysBasic Material Coating Forming

Paperboard (pulp) PET Pressed formingAcrylic Folded formingSilicone Molded formingPolybutylene terephthalate (PBT)

Plastic (PET) - Vacuum formingMolding (thermoset)

Aluminium Plastics Pressed formingDrawn forming

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The typical packaging forms used inMAP are pillow pouches, bag-in-boxes andtrays with overwrapping. MAP has wideapplications in the area of fresh fruits andvegetables and in meat preservation.

The packaging materials used are asfollows:

Pouches

PET/PVDC/LDPE/LLDPE

PA/PVDC/LDPE/LLDPE

PC/EVOH/EVA

MPET

MOPP

OPP/PVDC

Trays

UPVC/LDPE

HDPE

EPS/EVOH/LDPE

Bag-In-Box

PA/LDPE

PA/EVOH/LDPE

Active and Smart Packaging

Active packaging can be defined as

packaging that performs a role other than aninert barrier to the external environment.

The active functions can be achieved byany one of the methods given below:

1. Scavenging undesirable substances,

2. Release of active substances,

3. Using other new functional packagedesigns/materials.

Oxygen present in the headspace in foodpackage results in oxidation anddevelopment of moulds, fungi and bacteria,resulting in spoilage of food. The oxygencan be removed from the package by usingthe following types of oxygen scavengers:

1. Iron-based,

2. Metal/acid-based,

3. Metal (e.g. platinum) catalyst based,

4. Ascorbate/metallic salts based,

5. Enzyme-based.

Ageless® is a very popular sachet typeoxygen scavenging system being used worldover. Similarly ethylene scavengers andantimicrobial agents are also incorporatedin films to perform specific functions toincrease the shelf life of the commodities.

Smart packaging is an advanced activepackaging system that contains additional

Fig. 4.25. Packaging for frozen foods.

Fig. 4.26. Baby corns and ladies fingers inPS tray wrapped with transparent films.

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Chapter 4

Table 4.5. Common plastic materials and forms and theirapplications in food industries.

Types of Plastics Applications

Low density polyethylene Foods, milk, frozen foods, agricultural products, shrinkwrapping, heavy duty sacks, coating, lamination, multi-layer composites, coextrusion, containers and liners.

Linear low density Trash bags, ice bags, produce bags, shrink, asepticpolyethylene bags films, heavy duty shipping sacks, liquid milk

packs, edible oil packs and lamination film, etc.

High density polyethylene Woven sacks, films, drums, barrels, milk can bottlecrates, fish crates, transport containers, pallets, mouldedcontainers, bags, tapes, strips, blown films, extrusioncoating, corrugated sheets, etc.

High molecular high Films, rolls, bags, pouches, drum liners, garbage bags,density polyethylene lamination for fertilizer packages, milled wheat

products, wrapping films, etc.

Polypropylene Woven sacks, strappings, films and laminates, pouches.Packages for textiles, apparels, bakery products (biscuits,bread, cakes etc.), metallized films, shrink films closures,corrugated sheets, blown moulded bottles, extrusionand stretch blow moulded items, heavy crates, boxes,bucket containers, extruded nets, knitted sacks,corrugated boxes, etc.

Expanded polypropylene Insulation, packaging medium for electrical andelectronics items as a cushioning material, wrapping,liner, etc.

Polyvinyl chloride Films, sheets, bags, liners, shrink films, shrinkable tubes,skin/blister packs, laminates, adhesive tapes, bottles,etc

Nylon-6 Oxygen and odour sensitive foods, edible oils, spices

Polyester Spices, instant foods, bakery and confectionery, bottles,laminates etc.

Polyurethane Coatings, foams, sheets, etc.

Polystyrene Thermoformed containers for ice cream, condiments,jam, etc.

Expanded polystyrene Insulating and cushioning material, beverages, frozenfoods, fans, motors, typewriters, computers, electricalsand electronics, etc.

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functions beyond barrier and protectingfunctions. It also exhibits sensing andinteractive and responsive mechanisms. Thefunction of smart packaging are:

l To improve product quality and productvalue,

l To provide more convenience,

l To change gas permeability,

l To provide protection against theft,counterfeiting and tampering.

There are different techniques used insmart packaging: Time temperaturetechnique; Oxygen indicators; Microbialgrowth indicators; Moisture indicators, etc.

BIBLIOGRAPHY

Athalye AS (1992). Plastics in Packaging.Tata McGraw Hill, New Delhi.

Brody A (1997). The Encyclopedia ofPackaging Technology. John Wiley andSons, USA

Brown WE (1992). Plastics in FoodPackaging. Marcel Dekker Inc, New York.

Jenkins WA and Harrington JP (1991).Packaging Foods with Plastics. Techno-mic Publication, Lancaster, Pennsyl-vania.

Paine FA (1962). Fundamentals of Packa-ging. John Wiley and Sons, London.

Paine FA (1977). Packaging Media. JohnWiley & Sons, London.

Paine FA and Paine HY (1983). Handbookof Food Packaging. Leonard Hill,Glasgow, UK.

Parry RT (1993). Principles and Appli-cations of MAP of Foods.

Peleg K (1985). Produce Handling, Packa-ging and Distribution. AVI Publi-cations, West Port, Connecticut.

Robertson GL (1993). Food Packaging:Principles and Practice. Marcell Decker,New York.

Sacharow S and Griffin RG (1980). Princi-ples of Food Packaging. AVI Publica-tions, West Port, Connecticut.

Sathish HS (1996). Profile on FoodPackaging. CFTRI, Mysore.

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