Fruit and vegetable processing - Ch07 Packaging materials (2024)

Chapter7 Packaging materials

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7.1Introduction

7.1.1 Requirements and functionsof food containers

The following are among the moreimportant general requirements and functions of food packagingmaterials/ containers:

1. they must be non-toxic and compatible with the specific foods;
2. sanitary protection;
3. moisture and fat protection;
4. gas and odour protection;
5. light protection;
6. resistance to impact;
7. transparency;
8. tamperproofness;
9. ease of opening;
10. pouring features;
11. reseal features;
12. ease of disposal;
13. size, shape, weight limitations;
14. appearance, printability;
15. low cost;
16. special features.

7.1.2 Primary and secondary containers

The terms primary and secondarycontainers have been used. Some foods are provided with efficientprimary containers by nature, such as nuts, oranges, eggs and thelike. In packaging these, we generally need only a secondaryouter box, wrap, or drum to hold units together and give grossprotection.

Other foods such as milk, driedeggs and fruit concentrates often will be filled into primarycontainers such as plastic liners which are then packaged withinprotective cartons or drums. In this case the secondary containerprovided by the carton or drum greatly minimises the requirementsthat must be met by the primary container.

Except in special instances,secondary containers are not designed to be highly impervious towater vapour and other gases, especially at zones of sealing,dependence for this being placed upon the primary container.

Since primary containers bydefinition are those which come in direct contact with the food,we will be far more concerned with them than with secondarycontainers.

7.1.3 Hermetic closure

Two conditions of the greatestsignificance in packaging are hermetic and non-hermetic closure.

The term hermetic means acontainer which is absolutely impermeable to gases and vapoursthroughout its entirety, including its seams.

Such a container, as long as itremains intact, will automatically be impervious to bacteria,yeasts, moulds, and dirt from dust and other sources since all ofthese agents are considerably larger than gas or water vapourmolecules.

On the other hand, a containerwhich prevents entry of micro-organisms, in many instances willbe non-hermetic. A container that is hermetic not only willprotect the product from moisture gain or loss, and from oxygenpickup from the atmosphere, but is essential for strict vacuumand pressure packaging.

The most common hermeticcontainers are rigid metal cans and glass bottles, althoughfaulty closures can make them non-hermetic. With very rareexceptions flexible packages are not truly hermetic for one ormore of the following reasons.

First, the thin flexible films,even when they do not contain minute pinholes, generally are notcompletely gas and water-vapour impermeable although the rates ofgas and water vapour transfer may be exceptionally slow; second,the seals are generally good but imperfect; and third, even wherefilm materials may be gas- and water-vapour-tight, such ascertain gages of aluminium foil, flexing of packages and pouchesleads to minute pinholes and crease holes.

Hermetic rigid aluminiumcontainers can be readily formed without side seams or bottom endseams. The only seam then to make hermetic is the top end doubleseam, which may be closed on regular tin can sealing equipment.

Glass containers are hermeticprovided the lids are tight. Lids will have inside rings ofplastic or cork. Many glass containers are vacuum packed and thetightness of the cover will be augmented by the differential ofatmospheric pressure pushing down the cover.

Crimping of the covers, as in thecase of pop bottle caps which operate against positive internalpressure, also can make a gas-tight hermetic seal. But bottlesfail more often than cans in becoming non-hermetic.

7.2 Protection of food by packaging materials

Important factors in selecting apackaging unit for food storage are presented in Fig. 7.1.

Figure 7.1Factors for selection a packaging material for food storage

7.3Films and foils; plastics

Films and foils have differentvalues for moisture and gas permeability, strength, elasticity,inflammability and resistance to insect penetration and many ofthese characteristics depend upon the film's thickness.

Important characteristics of thetypes of films and foils commonly used in food packaging aregiven in Table 7.1. For the most part such films are used in theconstruction of inner containers. Since they are non-rigid, theirmain functions are to contain the product and protect it fromcontact with air or water vapour. Their capacity to protectagainst mechanical damage is limited, particularly when thinfilms are considered.

TABLE 7.1 Properties of packaging films

Source: FAD/WFP, (1970)

These materials can exist in manyforms, depending upon such variables as identity and mixture ofpolymers, degree of polymerisation and molecular weight, spatialpolymer orientation, use of plasticisers (softeners) and otherchemicals, methods of forming such as casting, extrusion orcalendering, etc.

One of the newer classes ofplastic materials referred to as copolymers illustrate what canbe done with mixtures of the basic units from which plastics arebuilt. The term copolymer refers to a mixture of chemical speciesin the resin from which films and other forms can be made. Themany variations possible make copolymers an important class ofplastics to extend the range of useful food packagingapplications.

7.3.1 Plastic sheets

• Cellophane paper can be used for packing of dried products, mainly for dried fruit leathers.
• Polyethylene sheets have a variety of uses. They are flexible, transparent and have a perfect resistance to low temperatures and impermeability to water vapour. An important advantage is that these sheets can be easily heat-sealed. Utilisation is in forms of sheets and bags. It is a good packing material for primary protection of dehydrated products. If a good protection is needed to prevent flavour and gas losses, it will be necessary to combine polyethylene with other materials.

7.3.2 Receptacles and packagings in plastic materials

In this class there are threecategories:

1. receptacles that can be heat treated: boxes, bottles and bags. Sterilisable bags used up to 120° C can be manufactured from same raw materials as described under plastic sheets and up to 100° C from cellophane. Polyethylene bags could be used to some extent for packing and pasteurization of sauerkraut.
2. receptacles that are not heat treated during processing of fruit and vegetables, also divided in bags and boxes. Bags are the most used type of packing from plastic materials and they are manufactured from polyethylene or cellophane; an important utilisation is for dried/dehydrated fruits and vegetables.
3. special packagings - which can be contacted (Criovac type) by action of heat once the finished product is already inside the pack and the air is evacuated.

7.3.3 Laminates

Various flexible materials suchas papers, plastic films, and thin metal foils have differentproperties with respect to water vapour transmission, oxygenpermeability, light transmission, burst strength, pin holes andcrease hole sensitivity, etc. and so multi-layers or laminates ofthese materials which combine the best features of each are used.

Commercial laminates containingup to as many as eight different layers are commonly customdesigned for a particular product.

Laminations of differentmaterials may be formed by various processes including bondingwith a wet adhesive, dry bonding of layers with a thermoplasticadhesive, hot melt laminating where one or both layers exhibitthermoplastic properties, and special extrusion techniques. Suchstructured plastic films may be complete in themselves or befurther bonded to papers or metal foils to produce more complexlaminates.

7.4Glass containers

7.4.1 Introduction

As far as food packaging isconcerned, glass is chemically inert, although the usual problemsof corrosion and reactivity of metal closures will of courseapply. The principal limitation of glass is its susceptibility tobreakage, which may be from internal pressure, impact, or thermalshock, all of which can be greatly minimised by proper matchingof the container to its intended use and intelligent handlingpractices. Here consultation with the manufacturer cannot beover-stressed.

The heavier a jar or bottle for agiven volume capacity the less likely it is to break frominternal pressure. The heavier jar, however, is more susceptibleto both thermal shock and impact breakage. Greater thermal shockbreakage of the heavier jar is due to wider temperaturedifferences which cause uneven stress between the outer and innersurfaces of the thicker glass. Greater impact breakagesusceptibility of the heavier jar is due to the lower resilienceof its thicker wall.

Coatings of various types canmarkedly reduce each of these types of breakage. These coatings,commonly of special waxes and silicones, lubricate the outside ofthe glass. As a result, impact breakage is lessened by bottlesand jars glancing off one another rather than sustaining directhits when they are in contact in high speed filling lines.

Surface coating after annealingprotects glass surfaces from many of the minute scratchesappearing in normal handling after annealing ovens; surfacecoating also improves the high gloss appearance of glasscontainers and is said to decrease the noise from glass to glasscontact at filling lines.

With regard to thermal shock, itis good practice to minimise temperature differences between theinside and outside of glass containers whenever possible. Somemanufacturers will recommend that a temperature difference of44° C (80° F) between the inside and the outside not beexceeded. This requires slow warming of bottles before use for ahot fill and partial cooling before such containers are placedunder refrigeration.

7.4.2 Classification

Glass used for receptacles infruit and vegetable processing is a carefully controlled mixtureof sand, soda ash, limestone and other materials made molten byheating to about 1500° C (2800° F).

Main classes of glass receptaclesare:

1. jars which are resistant to heat treatments,
2. jars, glasses, etc. for products not submitted to heat treatment (marmalades, acidified vegetables, etc.);
3. glass bottles for pasteurized products (tomato juice, fruit juices, etc.) or not pasteurized (syrups) and
4. receptacles with higher capacity (flasks, etc.)

7.4.2.1 Jars forsterilised/pasteurized canned products

These receptacles may replacemetal cans, taking into considering both the advantages anddisadvantages they present. Advantages are: they do not react tofood content; they are transparent and can be manufactured invarious shapes; they use cheap raw materials and are reusable.Disadvantages: heavier than metal can of same capacity; fragile;lower thermal conductance and a limited resistance to thermalshocks.

Receptacles in this category mustassure a perfect hermeticity after theirpasteurization/sterilisation and cooling and this has to beachieved by the use of metallic (or glass) caps and specificmaterials for tightness. Taking into account the receptacles'closure method, there are two categories of receptacles:

1. glass jars with mechanical closure;
2. glass jars with pneumatic closure;

7.4.2.2 Jars for products withoutheat treatment

For marmalades, jellies and jamsglass jars with non hermetic closures using metal, glass or rigidplastic caps are used; however for these products the receptaclesmentioned above may also be used.

The use of jars with pneumaticclosure presents the advantage that some products (e.g.marmalades, jams) can be filled hot and therefore sterile inreceptacles. Pneumatic closing generally protects againstnegative air action which is in this case eliminated fromreceptacles.

7.4.2.3 Glass bottles

These receptacles are widely usedboth for

a) finished products which needpasteurization (ea. tomato juice, Knit juices, etc.) and for

b) those which are preserved assuch (ea. fruit syrups).

Glass bottles in category a) areclosed hermetically with metallic caps, provided with specialmaterials for tightness.

For glass bottles in category b)various corks, and aluminium caps with tightness materials may beused.

7.4.2.4 Glass receptacles withhigh capacity

In this category there are glassflasks with 3 and 10 litre capacity which can be hermeticallyclosed by a SKO caps system and are resistant to productpasteurization (ea. tomato juice).

As bigger receptacles it ispossible to use glass demijohns with usual capacity of 25 and 50litre; these receptacles are used for preservation of fruitjuices by warm process. Closing is performed with flexible rubberhoods.

7.5 Paperpackaging

As primary containers few paperproducts are not treated, coated or laminated to improve theirprotective properties. Paper from wood pulp and reprocessed wastepaper will be bleached and coated or impregnated with suchmaterials as waxes, resins, lacquers, plastics, and laminationsof aluminium to improve water vapour and gas impermeability,flexibility, tear resistance, burst strength, wet strength,grease resistance, sealability, appearance, printability, etc.

7.5.1 Paper sheets

• Kraft paper is the brown unbleached heavy duty paper commonly used for bags and for wrapping; it is seldom used as a primary container;
• parchment paper: acid treatment of paper pulp modifies the cellulose and gives water and oil resistance and considerable wet strength to this type of packaging material;
• glassine-type papers are characterised by long wood pulp fibres which impart increased physical strength;
• paper with plastic material sheets.

7.5.2 Receptacles from paper or cardboard

(paper = 8 to 150 g/m²;cardboard = 150 to 450 g/m²).

7.6 "Tincan"/tinplate

The "tin can" is acontainer made of tinplate.

Tinplate, a rigid and imperviousmaterial, consists of a thin sheet of low carbon steel coated onboth sides with a very thin layer of tin. It can be produced bydipping sheets of mild steel in molten tin (hot-dipped tinplate)or by the electro-deposition of tin on the steel sheet(electrolytic tinplate). With the latter process it is possibleto produce tinplate with a heavier coating of tin on one surfacethan the other (differentially coated).

Tin is not completely resistantto corrosion but its rate of reaction with many food materials isconsiderably slower than that of steel. The effectiveness of atin coating depends on:

1. its thickness which may vary from about 0.5 to 2.0 µm (20 to 80 x 10(-6) in.);
2. the uniformity of this thickness;
3. the method of applying the tin which today primarily involves electrolytic plating;
4. the composition of the underlying steel base plate;
5. the type of food, and
6. other factors.

Some canned vegetables includingtomato products actually owe their characteristics flavours to asmall amount of dissolved tin, without which these products wouldhave an unfamiliar taste. On the other hand, where tin reactsunfavourably with a particular food the tin itself may be lacquercoated.

The classes of foods requiringdifferent steels are seen in Table 7.2.

The thickness of tinplate sheetsmay vary from 0.14 mm to 0.49 mm and is determined by weighing asheet of known area and calculating the average thickness.

Tinplate sheets may be lacqueredafter fabrication to provide an internal or external coating toprotect the metal surface from corrosion by the atmosphere orthrough reaction with the can contents. They may also be printedby lithography to provide suitable instructions or information oncontainers fabricated from tinplate sheets (otherwise paperlabels can be attached to the outer tinplate surface).

Under normal conditions thepresence of the tin coating provides a considerable degree ofelectrochemical protection against corrosion, despite the factthat in both types of tinplate the tin coating is discontinuousand minute areas of steel base plate are exposed. With prolongedexposure to humid conditions, however, corrosion may become aserious problem.

Common organic coatings of FDAapproved materials and their uses are indicated in Table 7.3.

The coatings not only protect themetal from corrosion by food constituents but also protect thefoods from metal contamination, which can produce a host colourand flavour reactions depending upon the specific food.Particularly common are dark coloured sulphides of iron and tinproduced in low acid foods that liberate sulphur compounds whenheat processed, and bleaching of red plant pigments in contactwith unprotected steel, tin, and aluminium.

TABLE 7.3 General types of can coatings

Source: Ellis (1963)

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