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CLOSE THIS BOOKAppropriate Food Packaging (Tool)
3 Packaging materials
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VIEW THE DOCUMENT3.1 Rigid containers
VIEW THE DOCUMENT3.2 Flexible packaging

Appropriate Food Packaging (Tool)

3 Packaging materials

This chapter examines packaging materials that are commonly used by small and medium-scale food processors. It first covers rigid containers such as glass, pottery, tins and cans, plastic bottles, wood and cardboard. The second part deals with flexible packaging including paper, plastic films, aluminum foil and cloth.

3.1 Rigid containers

Rigid containers include glass and plastic bottles and jars, cans, pottery, wood, boxes, drums, tins, plastic pots and tubes. They all, to varying degrees, give physical protection to the food inside that is not provided by flexible packaging. While most rigid containers are strong, they are, because of the amount of material used in their production, more expensive than flexible packaging. Some types of rigid packaging have the advantage of providing a perfect airtight hermetic seal.

3.1.1 Glass

Glass is made by heating a mixture of sand, soda ash and lime usually with a proportion (up to 30%) of broken glass or gullet to about 1500 °C until it melts into thick liquid mass. The molten glass is blown into moulds, in two stages, to make bottles and jars which are then cooled under carefully controlled conditions to prevent weaknesses and breakage. As the raw materials for making glass are cheap and available in most countries glass factories are to be found worldwide. Glassmaking is however a very energy intensive industry.

Glass packaging manufacture is only economically possible at large scale. As the moulds are very expensive, only very large food companies can afford to have their own moulds in the glass factory to produce their own special bottles. Various standard colors are made including clear, green or brown depending upon the protection needed from light.

If a glass factory exists in a country then bottles and jars can be a good option for small scale food manufacturers. If the bottles have to be imported, however, they tend to be very expensive compared with alternatives such as plastic due to their extra weight. Many small manufacturers start packaging with second hand glass. As the enterprise expands however it is found that new bottles have to be bought. The high cost and poor availability of new bottles then becomes a major concern. Many producers finally turn to alternatives, such as plastic packaging, or accept total reliance on second hand containers. In many countries a sub-industry exists to collect, wash, sort and sell used bottles and jars.

As will be described later, use of the correct type of lid is vitally important. Once again however many small producers find obtaining lids a major problem as the minimum orders required by the suppliers are high and few glass manufacturers also supply lids. The names used for the various parts of a glass container are shown in Figure 3.1


Figure

Glass has several advantages and disadvantages as a packaging material as shown in Table 3.1.

Advantages

Disadvantages

Chemically inert (no reaction

Breaks with rapid

with any food)

temperature change

Strong, can resist internal

Fragile, poor shock resistance

pressure and weight




Can be re-used and re-cycled

is heavy


In-plant breakage carries


danger of splinters in food

Impermeable to gases,


aromas and moisture


Can give protection against


light


Barrier to micro-organisms,


insects etc


Can be heat-sterilised


Good product display in clear


glass


Long shelf-life possible


High customer appeal and


acceptability


Good protection against


physical damage


Table 3-1: Advantages and disadvantages of glass

Physical properties

The main physical advantage of glass is its inertness and impermeability. Processors do not need to worry about the type of glass needed as they do with cans and plastics which can react with certain types of foods. Glass has the additional major advantage of being re-usable, re-cyclable and not damaging to the environment.

Products that are affected by light or have a long shelf-life benefit from packing in coloured bottles. While glass is fragile to shock it is strong in terms of bearing weight so stacking on pallets is possible. Protection is needed against shock by the bottles knocking each other or being dropped. For this reason glass bottles are usually put into cardboard outer boxes with dividers or card layers.

By the nature of the way they are made, glass bottles can var, in wall thickness and also in weight. In many developing country glass factories such variations are greater than international accepted norms due to the reluctance of the producer to replace expensive worn moulds. This is very important as it can give false data on the true fill weight or net weight. This is discussed more fully later in the section on specific quality control aspects for glass.

Most glass containers are made with a wall thickness related to their size but carbonated dunks bottles, which have to withstand high internal pressures, are made of thicker glass. Table 3.2 shows typical data on the some common types of glassware used for foods selected from the very wide range available.

Container

Height

Diameter

Weight

Volume

Closure


mm

mm

mm

g

ml

Bottles






Round

188.7

55.3

280

192

Crown

mineral






Round

165.0

65.5

187

265

Crown

mineral






Round

254.4

75.5

454

550

Crown

mineral






Round

298.2

77.8

500

700

ROPP

liquor






Square

268.9

71.8

490

700

ROPP

liquor






Round

270.9

77.7

440

750

RO

cordial






Winw

285.0

81.1

440

750

Cork

Jars






Jam jar

120.9

70.6

180

1 lb/366*

Push-on

Jam jar

121.0

72.5

185

1 1b/375*

TO

*Jam jars are commonly measured in Ibs

Codes

RO Roll-on
TO Twist-off
ROPP Roll-on Pilfer Proof

Table 3-2: Common types of glassware

The shape of the bottle or jar is also important as some shapes are weaker than others and so need greater protection. A round bottle is about 4 times as strong as a square one with rounded comers and 10 times as strong as a square one with sharp corners. Unless there are important reasons related to marketing, the use of simple round bottles is thus recommended to reduce breakage and shipping container costs.

Preparation of glassware for filling

All glass packaging, whether new or second hand requires cleaning before use. In the case of new glass simple washing in clean water is all that is required. Much greater care is required when using secondhand or returnable bottles and the following steps are recommended;

- visual inspection for cracks, chips, etc.
- containers should be smelt to make sure they have not been used to hold a substance that might be poisonous or taint the food being packed,
- removing labels by soaking in 1% caustic soda and detergent,
- thorough washing, using bottle brushes,
- rinsing in clean water,
- if prepared re-used glass is to be held in store until required it must be re-washed prior to use.

As has been pointed out glass breaks if rapidly heated or cooled so bottles must be carefully heated before hot filling with product and then carefully cooled. It is usual to pre-sterilize glass by either pre-heating in water and holding at 100 °C for 10 minutes or steam sterilizing. Steam sterilizing has the advantage that any weak bottles are more likely to break at this time rather than when they have been filled with a hot product. This reduces the risk of contaminating the food or wasting the product. Steam sterilizing of bottles (Figure 3-3) must be carried out in an area that prevents any splinters from bottles that may break entering the product so injuring the consumer. This is discussed further in Chapters 4 and 5.


Figure

Food packaged in glass containers can have a very long shelf life provided that the food has been properly processed before packaging, no contamination occurs at the filling stage and that the container is properly closed with a lid or seal. It should be remembered that the pack is only as good as the closure. Recommended shelf-lives vary but are usually 6 to 12 months not because the product actually deteriorates, but because over time there is a gradual loss of colour and flavour. Some foods, wines and spirits for example actually improve during prolonged storage and it is not unusual for a bottle of wine to be drunk ten or more years after packing.

Filling and cooling glass containers

Foods packed in glass containers fall into two broad groups.

- Hot filled: Fruit juices, jams, some pickles and chutneys, some sauces.
- Cold filled: Wines, vinegars, milk, some pickles and sauces

In addition, in some countries vegetables and meats are packed in glass jars which, after closing, are heat treated under pressure in the same way as canned foods. The use of glass in this way cannot be recommended to small producers due to the health risk it carries.

When hot filling, the sterilization by hot water or steam described earlier ensures that the container is clean and 'sterile' when filled. In addition the hot filling operation at 80°C or above means that the product is also 'sterile'.

It has been found that when hot filling products such as fruit juice it is good practice to lay the capped filled bottles on their side for about ten minutes before cooling. This allows a vacuum to form in the bottle and the cap to 'tighten down' onto the bottle neck. Experience has shown that this laying on the side dramatically reduces post-filling contamination because it removes the possibility of small amounts of air being sucked into the bottle until the neck seal is perfectly formed.

After hot filling, careful cooling must take place as a hot bottle put in cold water will probably shatter. The packs can be laid on their side to cool, which takes time, may result in flavour changes and occupies valuable factory space. Alternatively a simple cooler shown in Figure 3-4 can be made which gives controlled cooling. In this cooler cold water enters at the deep end of the trough and overflows from the shallow end. The hot bottles enter at the shallow end and are taken out from the deep end. What happens in practice is that the temperature is cool at the deep end and becomes warmer and warmer towards the shallow end due to the heat being taken from the bottles as they cool down. At the start of the day the whole tank needs to be filled with hot water to prevent damage to the first few bottles placed in the cooler.


Figure

When cold filling there is much more risk of contamination and so cold filled glass must be thoroughly washed in water containing chlorine (about 5 to 10 drops of bleach per gallon of water). In some cases, particularly when bottling wines and vinegars, bleach can cause off-flavours and the use of sodium metabisulphite is recommended (one teaspoon per gallon of water).

Glass bottles may be filled by hand, from gravity fillers, piston fillers or vacuum fillers, either manual or automatic depending on the product and scale of operation. Suitable fillers are described in Chapter 4.

Sealing or closing jars and bottles

The type of cap or closure and the method of application depend on which of five main types of container is being used. Closures are mainly made from metal or plastic although corks still find wide application for wines. Whatever the type of closure used:

- no part of the closure should affect or be affected by the food in the container,
- it must seal properly and remain sealed for the shelf-life of the food,
- it should be convenient for the customer and if the product is one that is not all used at once, it should be able to be re-closed,
- it must meet the increasing demand of both customers and traders for being tamperproof.

When selecting caps for a particular combination of bottle and product it is very important to take advice from the supplier regarding the suitability of the closure for the intended use. The range of alternatives in terms of lacquers, finishes and linings is great. It appears that no written data is produced recommending a particular lacquer or coating for a particular food. In practice the best alternative is found by packing the food and testing for any interaction between closure and contents by visual inspection.

Metal caps are made from tinned steel or aluminium. Being strong they are very suitable when the bottle has a vacuum formed after hot filling or when it is under pressure. Steel caps are the strongest. Both types can be lacquered to give added resistance to reacting with the product.

Plastic caps are made mainly from polypropylene (OPP) and polythene, both low and high density (LDPE and HDPE). The gas barrier properties of LDPE and HDPE are lower than OPP. As they are supplied moulded with a pre-formed thread, plastic caps have to be very accurately matched to the type of bottle or jar being used. Variations in glass neck and thread sizes may cause sealing problems that will not occur when using crown, RO and ROPP caps.

Almost all caps contain a lining material which has two functions, first to provide a soft 'cushion' so that the cap will tighten down to the bottle neck and secondly to reduce contact of the food with the cap. Liners can be plastic (usually PVC), plastic coated paperboard, waxed paperboard etc.

Some caps such as screw-on and twist-on twist-off can be applied by hand. Crown caps, push-on type caps, ROPP caps, plastic hinge-open and snap-shut and corks need machines to put them onto the bottle. Small manual equipment is available for this. All caps can of course be applied by semi- and fully automatic machines but the small and medium-scale food producer is likely to use only manual methods.

Crown caps are applied by a combination of downwards pressure and crimping the skirt and small flutes over the lip on the bottle neck finish, so locking it on. They always have a liner. As crowns cannot be put back on by the customer they are only suitable for products that are opened and used at one time. For larger-scale operations semi- and fully automatic versions are available but beyond the scope of this publication.

Small manual machines are available for push-on caps commonly used on jars of jam - which crimp the rim of the cap around the bottle neck finish. The lowest-cost machine simply crimps the cap edge while the larger version is fitted with fingers that make small indentations in the cap edge, so giving a firmer seal.

Roll on caps (RO) are made of aluminium and supplied unthreaded as a small cup. The action of the capping machine forces the cap wall into the thread of the bottle, then forming a thread in the cap. RO closures are often supplied with a perforation along the bottom edge which breaks when the cap is unscrewed. Such caps are pilfer proof and thus called roll-on pilfer proof (ROPP). They are most commonly fitted to high-value food products where there is a risk of pilfering or adulteration.

As far as is known no commercially available very cheap manual RO or ROPP capping machines exist. Drawings are however available from ITDG, United Kingdom of such a machine that has been developed in Sri Lanka.

Corkers

Corks are mainly used to close wine bottles by pressing them into the neck under considerable pressure while at the same time squeezing them, to reduce the diameter so that they will enter the bottle neck. The corks are wetted before use so that they will slide more easily into the bottle neck. Once inside the neck the cork then expands to give a tight fit. As corks are natural materials and so may well be contaminated with micro-organisms it is recommended that they are soaked in a solution of sodium metabisulphite (approx 1 teaspoon to the gallon).

After corking it is widely recommended that bottles be stored laying on their sides. This prevents the corks drying out and shrinking which would increase the risk of external contamination.

Plastic hinge-open snap-shut closures

In some countries it may be possible to obtain closures of this type which are becoming increasingly common for liquid products that are opened and closed several times in use. Common applications include cooking oils, sauces and fruit toppings. Simple hand-operated presses are available to fit this type of closure.

Other tamperproof systems

Instead of fitting tamperproof caps it is possible to fit several types of sleeve over the bottle cap that will show if the bottle has been interfered with. Typical products are plastic shrink sleeves and aluminium foil capsules. Two types of plastic capsule are used. One type is supplied wet in tins and is simply slid over the bottle neck. As it dries it shrinks tightly around the neck. The other type is heat shrunk in a small electrically heated cylinder. At the small scale, aluminium capsules are applied with a simple push-down crimping machine.

All the above capping machines are easy to use and require little maintenance. They are suitable for small producers with production rates from a few hundred to several thousand packs per day. In developing countries it is unlikely that semi- or fully automatic capping ma
chines would be appropriate except in large plants. The use of several small hand-operated machines would be more economical.

Bulk transport of foods packed in glass

As has been mentioned glass is strong under compression so finished goods can be piled in boxes, onto pallets or stacked. Glass will break however if subjected to shock. Great care is needed not to drop full cases and avoid one bottle banging against the next by using cases with card dividers. The use and design of suitable boxes is described in detail in Chapter 3.1.6.

Quality control

One important quality control measure when using glass is the variation in weight of the empty packs which distorts the net weight when full packs are checked. General methods of controlling net weight are discussed in Chapter 6.4.3. In the special case of glass however, a random sample (approx 1 in 50 containers) should be taken from each delivery. Sampling should be scattered, not all from one case. The sample is then individually weighed and the net weight calculated using the heaviest container. Thus: required filled weight = Weight of heaviest bottle + net weight of product

In addition samples should be kept to make sure that cap corrosion does not occur and that good seals are maintained for the shelf-life. In hot filled packs this may involve checking for the maintenance of internal vacuum. If automatic fillers and cappers are to be used, then variations in height and diameter of glassware may become very important but production at this scale is outside the scope of this publication.

As described under Quality Control, (Chapter 6.4) defects are commonly divided into critical, major and minor defects. In the case of glass:

- Critical defects:

- broken bottles,
- cracks in bottle or neck finish,
- contaminated interior (bubbles, strings of glass).

- Major defects:

- glass weight below minimum,
- height or diameter outside tolerances.

- Minor defects:

- Uneven outer surface,
- slightly off-colour glass,
- rough mould lines.

3.1.2 Pottery

Pottery is one of the most ancient forms of traditional packaging. Pottery wine and oil jars have been used for thousands of years. Hundreds of yeas ago crude sugar was crystallised in pots, a stage known as potting. Potting later was used to describe the method of preserving 'potted meats' in clay containers. Although pottery containers have now been largely replaced by other materials for commercial food packaging they still are widely used in some countries for certain products, for example cooking oil, tomato paste and gun They also find application when packing high value, luxury foods. In Europe, for example, very expensive marmalades, meat pastes and cheeses may be bought in glazed pots. The use of pottery for the small producer will thus fall into two areas:

- as a low-cost, locally obtainable alternative to glass, etc.,
- to pack high-value foods for the richer customer or perhaps tourists.

Pottery containers are made from clays either by hand or with the use of moulds. Hand-made pots vary considerably in size and shape while moulded ware is far more standard and thus more suitable for routine food packaging. After production the pot has to be baked or fired in a kiln at high temperatures, between 600 and 1250°C. The appearance and properties of the final product depend upon the type of clay used, the firing temperature and whether or not the pot is glazed. Ordinary clays fired at low temperatures yield earthenware. Other clay types, fired at higher temperatures produces stoneware while the use of special clays and very high temperatures yields porcelain. The fired pot may, if required, be dipped in a glaze and returned to the kiln where the glaze melts to a glassy coating. Both external and internal glazing can be applied. When earthenware is glazed, the glaze does not bond into the pot but essentially sticks to the surface. As the pot cools such a glaze often 'crazes' and the tiny cracksso produced mean that an incomplete impervious glaze coating forms. When stoneware is glazed the glaze bonds into the clay and a far more perfect protection results.

As the moulds for pottery containers are cheap to make it is possible, unlike glass, to have special packaging made. An The basic properties of both unglazed and glazed pottery are shown in Table 3-3.


Earthenware

Glazed

Stoneware



earthenware


Chemical

Reasonably

Very resistant

Extremely

properties

resistant to

if glaze not

resistant


chemical

crazed



attack



Permeability

Very

Low if glaze

Impermeable

to moisture

permeable

not grazed


and gases




Pack product

Can react

Little if well

None

interaction

with very acid

glazed with



foods

correct glaze


Operating

Good resistance. Can break if not warmed before hot filling



temperature




Weight

Heavy, has to be made thicker than glass to give equal strength



Strength

Very easily

Stronger than earthenware but breaks if



broken if

knocked



knocked





All are strong under load


Table 3-3: Properties of pottery packaging

Glazes

Great care must be taken if considering the use of glazed pottery for food packaging that the glaze will not react with the food. This becomes even more crucial if even slightly acid foodstuffs such as honey or yoghurt are involved. Many glazes contain chemical salts of heavy metals which are toxic. The main problem lies with lead.

Lead glazes are widely used on pottery since they are cheap and easy to use. Other heavy metal glazes, which are generally highly colored are used for decoration and so unlikely to be encountered on the inside of pottery being used for food packaging. The food producer must make sure from the pottery supplier that lead glaze is not used. The seriousness of the problem has been highlighted from work in Central America where people traditionally use lead glazed bowls and cups for food. Changes in diet, particularly children drinking acid fizzy drinks, is showing up as increased lead in the body with resulting chances of impaired brain development. Simple chemical tests exist for checking for lead and any food manufacturer with doubts over the suitability of a glaze is advised to have a container tested in a local laboratory.

Packaging applications for pottery containers

Pottery is still widely used all over the world for the traditional packaging and storage of foods such as grains, pulses, wines, honey, pickles, yoghurts and dried foods. Such traditional uses are outside the scope of this publication which is concerned with the use of packaging materials for commercial production.

It is almost, if not totally, impossible to hermetically seal pottery containers due to the variations that occur in the neck diameter and shape (slight ovality). For this reason their use is limited to products that are either very stable, such as honey, or have a short shelf-life such as yoghurts and soft cheeses.

In cases where pottery is being used for products aimed at high-value markets, that is to say more for their visual appeal than barrier properties, dry foods that would tend to absorb moisture can be packed in plastic bags inside the outer pottery pot.

Product

Comments

Honey

Very stable, long shelf-life in


glazed pots, needs sealing to


keep out insects such as ants

Yoghurt and soft cheeses

Packed in shallow


earthenware bowls. Short


shelf-life. Needs covering ie


with paper tied round neck to


protect from dust and insects.


Must be very well cleaned if


re-used as food is absorbed


into the earthenware.

Solid block sugar (for

Very stable, pot needs to be

example gur)

broken to remove the product.

Spices, teas, herbs

Inner plastic liner needed in


humid climates, should be


sealed

Jams and jellies

Stable products with long


shelf life in glazed pots, must


be sealed

Table 3-4: Foods suitable for packaging in pottery containers

Using pottery containers

When using pottery packaging the same basic precautions apply as for glass. Incoming pots should be inspected and any showing damage rejected. Pottery pots are likely to be more dusty than glass due to the conditions they are made in and the rougher surface. Thorough washing in clean water is essential. The pots should be turned upside down and allowed to dry before filling.

If hot filling is planned, for example with jam, pottery pots may break during filling. Pre-heating in an oven is recommended. Pottery containers are not usually heat processed. In practically all cases the small producer wilt hand-fill the containers although small volumetric piston fillers, as described in Chapter 4, can be used. Indeed, the use of volumetric filling, even using a simple measuring jug, is recommended in view of the considerable weight and size variations that occur in pottery containers.

Sealing

As has been mentioned it is almost impossible to hermetically seal pottery containers. However, the following methods are commonly used to produce an acceptable seal:

- Use of a cork bung. The seal can be improved by running sealing wax around the bung edge.
- Use of a pottery insert and a disc of polythene.
- Waxed paper or polythene held on with a rubber band or string.

Examples are shown in Figure 3-12. It is essential to keep filled pots vertical as all the above seals are likely to leak if the pot is turned upside down.


Figure

Shipping containers

Pottery, like glass, is easily broken. Careful packing in outer boxes in the same way as glass jars is thus recommended. Larger pots, of the type used for solid sugars or bulk distribution, are often packed in hand-made wooden boxes lined with soft material like dry grass to absorb any shocks in transport

Skills required

No special skills are required when packing in pottery except perhaps learning to seal effectively with wax.

Quality control

There are several important quality control checks needed when using pottery. First considerable size (volume) and weight variations may exist from pot to pot or delivery to delivery. Samples should be taken and checked for volume and weight on delivery. Samples of filled containers need to be taken and checked for net weight more frequently than when using glass. If volumetric filling is not used, it may be necessary to fill each container on a scale so ensuring that the correct net weight is obtained.

The food producer must make sure that only safe, nonlead, glazes are used by the supplier. Re-check periodically as the potter may change his glaze.

If the pots are made of earthenware and re-used then great attention must be paid to thorough washing and cleaning as earthenware, being porous, will absorb food into the structure of the pot. This, due to microbiological grown, can make the food deteriorate.

- Critical faults:

- cracks,
- use of lead glaze,
- ovality of neck

- Major faults:

- large size and/or shape variations,
- incomplete layer of glaze

- Minor faults:

- minor variations in size and/or shape that allow declared net weight to be packed,
- variation in colour.

Pottery containers have certain advantages for small producers, particularly those living in very isolated areas where alternatives may be hard to obtain Indeed it is in such areas that the skills of the potter are most likely to have survived

3.1.3 Metal containers

Metal containers commonly used in the food industry include steel drums, tins with push-on or screw-on closures, sanitary cans (the 'tin' can), composite cans (usually a combination of paper board and steel), aerosols, aluminium cans and aluminium foil made into dishes, etc. The level of technology involved in filling into aluminium cans (used for beers and carbonated beverages) is high and as generally it is only applicable to large production units will be only briefly covered. Aerosols are beyond the scope of this publication.

Sanitary cans

Cans and glass are still perhaps the most common rigid containers used for packaging and preserving food. While almost any food, including dried goods, can be canned the most common applications are to fruit juices, fruit in syrup, tomatoes, meats, fish and vegetables.

The processing methods required to can acid foods, such as fruit, are very different from those needed to can low-acid foods safely, such as vegetables and meats. Acid food products only require heating to temperatures below 100°C in order to inactivate naturally occurring enzymes and destroy most micro-organisms present Low-acid foods on the other hand need to be heated in pressure vessels, called retorts, at 121°C for a pre-determined time based on the product and size of can being processed. Cooling, under pressure from compressed air, then has to take place in the retort

Equipment costs (steam boiler, retort, compressor) for low-acid food canning are high and considerable technical skills and knowledge are required to safely can low-acid products. Errors can cause severe food poisoning or even death. It is strongly recommended that small and medium-scale food manufacturers should not attempt to can low-acid foods such as vegetables, soups or meats unless they have, in house, the necessary expertise and laboratory facilities to make sure that production errors do not occur. The reasons for this, and the food poisoning dangers that exist have been described in Chapter 2. This chapter has been written with only the canning of acid foods in mind.

The can has distinct advantages over glass which include:

- good heat transmission,
- not subject to thermal shock so rapid heating and cooling are possible,
- lighter in weight,
- not subject to breaking,
- little or no interaction between the food and can occurs provided the correct type of can is selected,
- resistant to physical damage.

They are also totally impervious to light and air. The main disadvantage of cans is, of course, that the contents cannot be seen by the purchaser.

Canning may be a good option for small and medium-scale producers, particularly when a can-making facility exists in country. It should also be remembered that cans are considerably lighter than glass containers so transport costs can be lower. Cans are still an expensive option when compared to alternatives such as plastics.

Can manufacture

There are three main methods of making cans. The most common produces the traditional three-piece sanitary can which consists of a body and two end pieces that are joined together to provide a hermetic or perfect seal. While most commonly used for foods that are heat processed they also find application in packaging powders, syrups, etc. that are not heat-processed. The most common shape is a round cylinder but square and oval flat cans are used, particularly for fish processing. The other two methods which produce a two-piece can (integral body and base plus a lid) have become increasingly common in recent years. Two-piece cans require less metal and thus are lighter and cheaper.

Three-piece cans

Cans are most commonly made from thin sheets of steel that have been electrolytically coated with tin on both sides. The type of steel used depends on the corrosion resistance needed for the particular product to be canned. The most resistant grade is called Type Land is used for strongly corrosive foods such as apple juice, prunes, cherries and pickles.

Type MR steel is less resistant to attack and is used for mildly acid products such as apricots, peaches and grapefruit as well as low-acid foods like peas, corn, meats and fish Type MC is used for the low-acid foods mentioned previously.

The tin layer is 0.1 to 0.3 mm thick (2.8 to 11.2 g/m2). The layer thickness required depends on how corrosive the food being canned is. Thicker tin layers are needed for high-acid foods. The tin layer may be of equal thickness on both sides of the plate or thicker on one side. The sheets of tinned steel are coated with a lacquer on the 'inside' face.

Lacquer is used to:

- prevent taste changes that might occur from traces of metal that dissolve in the food,
- prevent discolouration of the inside of the can especially in foods rich in sulphur such as fish and meat,
- prevent discolouration of the product.

Lacquers are often described fruit juice, meat or fish grade. The actual detail of the composition, thickness, etc., of these lacquers is beyond the scope of this publication. The most common lacquers include:

- Oleoresin lacquer now being replaced by epoxyphenolics. These have poor resistance to attack by sulphur. R or fruit enamels have resistance to staining by fruit pigments of the type found in berry juices. C enamels are used when packing high-protein foods such as corn, peas and poultry.
- Vinyl lacquers have good adhesion and flexibility but do not resist high-temperature sterilization well. Often used as second layers for canned beer, wine and carbonated beverages as well as dry foods.
- Phenolic lacquers have very good chemical stability and low permeability especially against sulphide. Used for fish and meat products.
- Acrylic lacquers have good color retention and high heat resistance.
- Epoxy-phenolic lacquers, the most comonly used type. Resistant to acids, good flexibility and high heat resistance. A wide range is available to cover different uses such as fruits, vegetables, meats and fish

The choice of the correct lacquer is of great importance and readers considering canning are strongly recommended to consult specialists in the can supply industry regarding the best coating for the food to be processed.

Can lids have a ring of flexible sealing material around the rim which is compressed in the canning machine to give a perfect seal.


Figure

Three-piece cans are supplied to the user in two forms:

- with the base joined to the can body by the can manufacturer and sent to the food manufacturer together with the lid, or
- as a 'flattened' or 'collapsed' can with the body cylinder flattened into an oval shape and supplied with loose bases and lids

Flattened cans are cheaper to transport as more cans are packed into each cubic metre. The user however has to invest in can-reforming machines and incur additional labour costs in attaching both the can base and lid.

Two-piece aluminium cans are commonly used for beers and carbonated drinks. The technology used is high and costly. Recently, a small-scale beverage filling/carbonating/closing system has been developed. This unit can produce up to 5750 cans per day and can also accommodate bottles. It is of low cost compared to existing alternatives.

The production of cans involves high technology and large outputs. This is particularly true of the two-piece can where outputs of at least 150 million cans per year are needed for economical production.

Can sizes

While a wide range of can sizes exist with capacities between 71 ml and 10200 ml most foods are packed into cylindrical cans with a capacity of 140 to 900 ml. When ordering cans it should be remembered that in the United States and Imperial systems the first digit relates to the number of inches and the second digits the number of 1/16 of an inch Can sizes are always expressed as diameter x height. Thus a can 307 x 409 is 3 7/16" by 4 9/16". It should be noted that in the United Kingdom and Europe metric sizes are increasingly replacing imperial sizes.

When placing orders canners must ensure that the correct size of can for the chuck size of their can sealing machine is ordered.

Reforming and closing (or seaming) of sanitary cans

As has been mentioned previously cans will be delivered to the food processor either 'erected' (with the base fitted by the supplier) or 'Battened' (with the body as a flattened cylinder). If flattened cans are used the canner will need three machines in order to erect, flange and seal (or seam) the cans:

- A can-body reforming machine which re-forms the flattened body into a perfect cylinder.
- A body flanger which bends over the ends of the cylinder to form a flange into which the lid and base are sealed.
- A seaming machine which seals the bases and lids to the can body to make what is known as a double seam shown in Figure 3-14. Seamers go through two operations by means of two rollers. The first operation roller rolls the cover hook around the body hook and the second operation roller tightens the two hooks to provide a double seam.


Figure

If erected cans are bought, only one machine, the seamer, is required to seam the lid to the body.

The choice for the processor between using erected or flattened cans will greatly depend on local circumstances. Broadly, for the flattened can it can be said that:

- they are cheeper,
- transport costs are lower,
- equipment costs are higher,
- higher operator skills are needed, since three mechanical steps are involved,
- labour costs are higher.

The decision thus involves balancing the first two advantages above in financial terms against the latter three disadvantages largely on the basis of the level of production.

Washing cans

Cans received from the supplier must be washed prior to filling as shown in Figure 3-16. Hot water is sprayed into the can which is laying on its side. As the cans roll forward to the filling point they tip half upside down to allow any water to drain away.


Figure


Figure

Filling of cans

While automatic rotary or carousel fillers are used in large canneries hand-filling is the usual method for small and medium producers. If liquids, such as fruit juices are being packed normal filling systems involve jugs, piston fillers or simple gravity fillers. When products such as fruit in syrup are being produced the fruit should be packed into the can first and then topped up with hot syrup. In order to facilitate further processing, juices and top-up syrups are usually filled into the can at temperatures of about 80°C.

It is most important that the can is not filled to the top and that a 'headspace' of 0.3 to 0.5 cms is left. The simple device shown in Figure 3-17 will greatly assist in maintaining a standard headspace.

Exhausting

Before sealing the can, air present in the headspace is removed from the container by exhausting. This reduces any strain on the can that would result from the air expanding during further heat processing and reduces the possibility of the air oxidizing the inner can surface during storage. Exhausting is carried out by:

- filling very trot,
- cold filling and then putting the cans with the lids loosely fitted into a steam chest or exhaust box,
- blasting a jet of steam into the headspace immediately before seaming.

The cans are then closed using seamers of the type shown earlier. The next stage involves heat processing the sealed cans in boiling water or in steam retorts.

Cooling

After processing for the required time the cans should be cooled in clean chlorinated water. The cooler illustrated in Figure 3-4 in the section on glass packaging has been found equally applicable for can cooling. The cans are removed from the cooler while still warm as this allows them to dry quickly and prevent rusting. They then pass on for labelling.

Can quality control

The double seam is the potential weak point of a can and for proper hermetic seals it must be made to stringent tolerances. Routine inspection of cans by 'tearing down' is important. Table 3-5 shows typical tolerances for selected sizes of cans.


Table 3-5

Inspecting cans and adjusting the seamer is skilled work and operator training is essential. Such training is usually provided by the can supplier. It is not possible in a publication of this size to include full details of can seam inspection and machine adjustment, this can be found in special booklets, about 20 pages long, provided by can suppliers. The use of a special micrometer, called a can micrometer, is necessary to measure the seam width, seam depth and the cover hook and body hook. From these measurements the % overlap of the two hooks can be calculated. The % overlap is the main factor to maintain a hermetic seal. If necessary the canning machine is then adjusted by increasing or decreasing the tightness of the first and second rolls. As every change in roll tension made results in changes to all the other seam dimensions this is a highly skilled job. Common seam defects are shown in Figure 3-18.


Figure

After filling and seaming an internal vacuum will form as the hot contents cool. This internal vacuum is essential for preservation of the contents and regular samples need to be taken and tested with a special can vacuum gauge, again available from can suppliers.

Problems with poorly sealed or processed canned foods will usually show up in store as 'swells' or 'blown cans.' Swelling takes place as the food deteriorates and gives off gas. Instead of an internal vacuum the cans are under pressure and if punctured the contents will blow out. The lids will bow outwards due to the internal pressure; rather than inwards as in a can with normal internal vacuum. Samples of finished stock in store should be routinely checked for internal vacuum and any sign of blowing. If blowing occurs it is often necessary to reject the whole batch.

Better stockroom control and response to customer complaints are possible if the cans are date-coded. Small peddle-operated presses are available that indent a series of numbers or letters into the lid before it is joined to the can body.

The following critical, major and minor faults occur in cans:

- Critical faults:

- leaks in body seam or manufacturers end seam,
- seaming compound missing,
- seriously dented flanges,
- missing or incomplete interior lacquer,
- contaminated interior.

- Major faults:

- dents over 2.5 cm (1") long,
- out of round shape,
- too much or too little seaming compound in can end,
- loose solder in can.

- Minor faults:

- dents less than 2.5 cm (1") long,
- scratches on ends or exterior surface of the body.

Steel drums

Drums are large cylindrical metal containers with capacities between 10 and 240 litres, the most common size being 210 litres or 55 Gallons us. In the food industry, they find three main uses:

- for bulk safe,
- for bulk storage of ingredients,
- for safe storage of finished goods, particularly dried foods.

Drums are made of sheet steel 0.4 to 1.5 mm thick which may be galvanised and coated internally. They are strong and provide excellent protection against light, moisture and rodents etc. As many drums are made for use in the chemical industry it is important to check that any internal coating is of 'food grade' quality.

There are two main types of steel drums: closed head or open head as shown in Figure 3-20. Closed head drums are used for packaging liquids, in particular edible oils, while open head drums find use for packing solid products.


Figure

In many parts of the world second hand drums find use for bulk packaging and distribution. It is very important that the food manufacturer makes sure that these drums have not been used for dangerous chemicals.

The use of open-ended drums for storage of finished dried foods is important. Such storage can provide good protection against pests, light and moisture particularly if the drums are lined with plastic. Drums lined with heavy plastic bags also provide good packaging for semi-processed ingredients, for example fruit pulp preserved with sulphur dioxide or vegetables in brine.

Tins

Being totally impervious to light, air and moisture, tins provide excellent protection. A large range of shapes and sizes, round, square and cylindrical are available. Lids may be of the simple push-on type or hinged. In many cases tins used for food packaging are attractively printed and have great promotional and customer appeal. They also have appeal to the purchaser in that they can be used for food storage in the home after use. Tins, and in particular printed ones, are however an expensive form of packaging.

Two main types are of interest to the small to mediumscale food manufacturer. Round tins with push-on lids are excellent for packing high-value solids such as herbs. Round or square tins with a small pouring spout are commonly used for packing cooking oils.

As round tins for dry goods are expensive compared to alternatives such as plastic bags they are normally only used for packing high-value products, particularly those that may loose flavour, odour or colour if not well protected. Large manufacturers often pack such materials under an inert gas (carbon dioxide or nitrogen) in order to protect them from oxidation. Gas packing, as a technology, is generally considered to be beyond the means of the small to medium producer. A low-cost system shown in Figure 3-21 has however been successfully applied in trials for packing high-value herbs.


Figure

In use, the product is filled into the tin and the lid put on, a tiny hole is then punched through the can base. Vacuum is applied to this hole and the air drawn out. The vacuum valve is then closed and the gas valve opened. This fills the can with gas. Finally a small drop of solder is applied to the hole.

As in the case of drums, tins are often re-used. Again the food manufacturer must assure themself that they have not been used for any toxic or dangerous material.

Simple labour-intensive technologies for making cans of the screw-on-lid type for vegetables oils at rates between 20 and 1000 per hour have been developed and tested. The basic steps involved in making a round can with a pouring spout for packaging oil are shown in Figure 3-22. It is not known how much uptake there has been of this small-scale can-making technology.


Figure

In general smaller industries will fill tins by hand, possibly with the aid of funnels. However overhead gravity fillers, piston fillers and volumetric powder fillers, of the type described in Chapter 4 may be used.

Quality control for drums and tins

- Critical factors

- must not have been used for poisonous substances if second-hand,
- linings must be food grade.

- Major faults

- must not leak,
- lids must seal,
- no internal corrosion.

- Minor faults

- dented.

3.1.4 Plastic bottles, jars, tubes, cups and trayes

Largely for cost reasons rigid plastic bottles, jars, tubes, cups and trays are increasingly replacing glass and tin cans for food packaging. Unfortunately the widespread use of plastic is having a bad impact on the environment. Plastics do not rot or break down under the natural action of the environment. They cause visual pollution floating in water or laying on the ground and if burnt give off noxious and often toxic fumes. At the present time, biodegradable plastics are not commercially available. With time it is hoped, however, that safer, biodegradable plastics will be developed and, probably due to pressure from legislation, replace the existing range of plastic packaging.

The range of plastics and co-polymers used to make rigid plastic food containers is wide. In reality for most small food processors in developing countries the choice will be restricted to packaging made of polypropylene, polythene and polyvinylchloride (PVC). Polyethylene tetraphthalate (PET) is however rapidly becoming more common. For the food processor plastic containers have the great advantages of:

- lower cost,
- lightness,
- resistance to impact damage,
- availability both clear and colored,
- squeezability, useful for spreads and honey.

Plastic containers however give less protection than colored glass and cans against light and air. In addition they are not as strong, in terms of weight bearing and crushing, as glass or cans and are easily punctured by sharp objects. It should also be remembered that rigid plastic packaging has the considerable disadvantage of causing environmental since they are not biodegradable. In general they cannot be easily re-used or re-cycled.

As is described later most plastic packaging cannot be used at high temperatures so hot filling and heat processing are less common. If high-temperature resistant polypropylene packs are not available then the types of food that can be packaged at small scale into plastic are thus limited to:

- Foods that are naturally stable for the planned shelflife and can tee cool filled (such as dried goods, some pickles, cooking oils, fats, yoghurt, fruit juices containing preservative, beers, vinegar and honey).
- Jams and pasteurized pickles, such as chutney provided that the product is cooled to below about 60°C before filling. In the case of jams this means that a special recipe has to be used using a slow-setting pectin (a pectin that does not set until the jam has cooled).

The commonest uses for rigid plastic containers are shown in Table 3-6.

Container

Application

Plastic bottles

non alcoholic beverages,


cooking oils, ketchups,


sauces

Plastic jars

honey, spreads, peanut


butter, dry foods

Trays and tubs

butter, fats, spreads,ice


cream, jams, condiments

Cups

drinks, yoghurt

Tubes

honey, spreads

Table 3-6: Common uses for plastic containers

A wide range of different types and mixtures of plastics are used to make plastic containers many of which are not suitable for contact with food for they contain chemicals, known as plasticizers, that are toxic and can migrate from the plastic to the foods. Oily foods are particularly likely to dissolve plasticizers. The food manufacturer must make certain that the type of plastic being used to make the container is food grade. It should be noted that one particular type of plastic, PVC, is made in many grades only some of which are food grade. In many countries regulations state which types of plastic can be used and local Standards Offices can advise. In cases where no such standards exist the recommendations of countries with established standards should be consulted.

Production of plastic containers

Plastic bottles are made by several methods:

- Blow moulding is similar to glass bottle making and is used as a one or two stage process to make bottles, jars and pots
- Injection moulding. Here grains of plastic polymer are heated by a screw in a moulding machine and then injected under high pressure into a cool mould. The method is mainly used for wide necked containers and lids.
- Injection blow moulding. Polymer is injection moulded around a blowing stick and, while molten is transferred to the blowing mould. It is then blown into shape by compressed air (Figure 3-23).
- Extrusion blow moulding. In this method a continuously extruded tube of softened polymer is trapped between the two halves of a mould. It is then inflated by compressed air into the mould.
- Stretch blow moulding. A shape is prepared by either injection or extrusion moulding. It is then re-heated, which causes the molecules of plastic to 'line up'. This gives a glass clear container of greater strength which has good barrier properties to gases and moisture over a wide temperature range.

The costs of moulds for injection moulding are much higher than those for extrusion moulding, but the surface finish and size accuracy of the finished product is better. It is possible, by injecting two or more different types of softened plastic, one inside each other, to produce bottles with layers of different types of plastic. These are called co-extruded bottles and are used to give special properties such as improved gas permeability characteristics.

Tubs, trays and cups are made by heating sheets of thermoplastic material and then shaping the soft sheet into a mould by means of vacuum or pressure. While such packaging is normally made in large factories, smallscale semi-automatic vacuum thermoforming machines are available (Figure 3-23). Such small-scale local production of plastic containers could offer opportunities for entrepreneurs in developing countries.


Figure

The types of plastic commonly used for food packaging materials likely to be available in developing countries are shown in Table 3-7.


Table 3-7

In addition to the above, plastic packaging is made by combining different plastics or co-forming them to improve, for example, the packs water vapour or gas barrier properties. In this way the materials used in the coforming can be tailored to the product. In some cases up to six different layers of material may be used.

Selection of best material for a product

Ideally the type of plastic used for a particular packaging application should be selected with advice from the supplier. The reality for many small-scale food manufacturers however will be that only one or two types of bottle are available. The food producer should find out what type of plastic containers are available and then carefully consider aspects such as:

- Is the plastic suitable for contact with food.
- Is resistance to oils and fats important.
- Strength, particularly if gassy drinks are involved.
- Is permeability to gases (oxygen and carbon dioxide) important.
- Maximum filling temperature that can be used.
- Color, clarity and surface finish.
- If hand capping can be used or if special closing machines are involved.
- If heavier grade, stronger shipping containers will be needed to protect against crushing and impact damage.
- If plastic cups are to be used considerable transport savings can be made by selecting types that stack one inside the other. It is possible to pack 8700 conical stacking cups per cubic metre but only 1500 straight sided ones. Transport cost savings of over 80% could thus be made by using conical cups.

Processing, filling and sealing

Plastics, with the exception of polypropylene, have poor resistance to high temperatures; In general then packaging cannot be hot water or steam sterilized before filling. Thorough washing in clean water to remove any dust is thus essential followed by draining.

If OPP or HDPE are being used hot filling is possible. In the case of other plastics filling temperatures will need to be kept below 60°C. At a small scale, hand-filling will commonly be used but of course piston, vacuum and gravity fillers as described in Chapter 5 can be appropriate. Closing or sealing plastic packaging depends on the type being used.

Bottles and Jars.

The most common closures used are the same as those used for glass bottles. These include plastic screw-on caps that may if desired be pilferproof (ROPP) and hinge up/snap down plastic caps that are increasingly being used for products, such as oils, that are frequently opened and closed. In some cases these hinge up caps are fitted with a small pouring spout, very convenient to the customer for sauces and honey. Caps of these types and small equipment for applying them are described in Chapter 3.1.1 on glass packaging. Plastic shrink sleeves and aluminium foil capsules can be slipped over the cap to make it pilferproof and more attractive. Such sleeves are also described in Chapter 3.1.1 on glass.

Tubs and cups.

Containers of this type are closed in two ways: with snap-on or push-on plastic lids or heat sealed aluminium foil. At a small scale, hand-closing is invariably used. If heat sealed lids are to be used then the container must have the correct shaped rim to which the foil lid is sealed. This has an electrically operated sealing head operating at about 200 °C and can seal 10 pots/minute. As the thin foil closure can easily be damaged some manufacturers place a snap-on lid over it to give added protection

In operation the filled cup is placed onto the platform and a foil lid is laid onto the cup rim. The heat sealer head is brought down for a set time and the plastic on the back melts and heat seals onto the rim. It is possible, if heat sealers of the type above are unavailable or too expensive, to use a household domestic iron to heat seal foil to cups as shown in Figure 3-26. Several locally made sealers made this way have been seen. The main problems in using them are obtaining the correct sealing temperature and time. This must be found by trial and error.


Figure

Plastic tubes.

While plastic tubes are not commonly used for food packaging in developing countries they are an option for food manufacturers who wish to sell their products with greater customer convenience. Applications include honey, mustard and sauces. Tubes are purchased with the neck and cap complete, the bottom end being open. The product is filled into the open bottom, taking great care to keep the part that is later heat-sealed completely clean (Figure 3-27). Piston fillers are very useful for such filling. After filling the open end of the tube is heat-sealed in a jaw sealer of the type described in Chapter 3.2.2 on films.


Figure

After filling, sealing and labelling, plastic containers should be packed into outer cardboard shipping boxes. While it is best to use cardboard dividers in the boxes it is not as important as when using glass for plastic is not subject to impact damage. It should be remembered that plastic is not as strong as glass or cans and so care is needed not to stack boxes too high to avoid crushing. This is particularly true of products, such as yoghurt, packed in cups. Outer cases should then be sealed, preferably labelled and date-stamped to make stockroom control easier.

Quality control and special skills needed

The main quality control procedures needed when using rigid plastic packaging are those general methods described in Section 6.4 of this publication (net weight, shelf life etc). Because plastic packaging is light in weight variations will cause considerably less final net weight control problems than when using glass.

If heat-sealed foil is being used it is important to carry out frequent checks for proper sealing. Packs should be turned upside down to make sure they do not leak and checks made by filling with warm water and sealing so that an internal vacuum forms after cooling. This can be easily seen as the foil will bow inwards.

Likely faults in plastic containers include:

- Critical faults:

- split or punctured,
- badly formed neck or sealing area,
- incorrect non-food grade of plastic.

- Major faults:

- if printed poor printing quality,
- poor color or transparency,
- mishapen packs,
- denting.

- Minor faults:

- slight mix-alignment of print,
- surface scratching.

No specialized skills that cannot be learnt in the plant are needed for filling and sealing rigid plastic packaging at a small scale.

3.1.5 Wooden containers

While wood is widely used for packaging fresh produce its use is limited when dealing with processed foods. The most common applications are:

- barrels for wines, beers, spirits, salted fish and vegetables in brine,
- wooden crates, particularly for bottles that are returnable,
- tee chests,
- small fancy boxes for foods aimed at a tourist or gift market,
- to construct pellets.

Wood is strong and provides better protection against crushing and impact than cardboard boxes. It is however heavier and more expensive. Wood containers can be made lightproof and leakproof. As a material wood is porous and so does not form a perfect barrier to moisture and air. Depending on the method of construction wood containers can provide excellent protection against pests.

Barrels are very difficult to make and the training takes several years. They are also very expensive and so are re-used over and over again' being sent back to coopers to repair any damage. They are available in many sizes from less than 5 gallons up to huge barrels that contain a tonne or more of product. The common sizes however are light enough for a person to lift or move.

If barrels are to be considered as a packaging for foods the following points need to be borne in mind:

- they must be returnable, deposits should be charged,
- care should be taken if buying second-hand that no contamination, for example the odour of fish, is possible,
- the food producer should have space and facilities for thorough washing and cleaning and workers skilled in minor repairs, fitting the wood lids and tightening the metal barrel hoops.

Many small food manufacturers use wooden crates to distribute food in bottles to shops, particularly when the bottle is returnable. Such crates, which usually hold 24 packs, can easily be made by local carpenters and Figure 3-28 shows a simple jig being used to greatly speed up production.


Figure

If distribution of products that might suffer from crushing, for example yoghurt or foods in plastic bags, is considered then crates used should have 'stacking corners' as shown in Figure 3-29.


Figure

It is recommended that some form of permanent owners mark -painted or burned in -is made on delivery crates to make sure they are indeed returned.

Tea chests are a very special case where a wood packaging has become the accepted standard all over the world. They are made of thin plywood over a timber frame and corners and edges are bound with tin strips to give protection against dropping. Tea chests are lined internally with a paper/foil laminate which provides an excellent moisture and air barrier. The only real application of tea chests is for bulk distribution and export of tea.

The use of small wood boxes, for packaging goods for the tourist and gift market can, in certain cases, provide opportunities for small food manufacturers. Generally the containers will be supplied by a local craft group or carpenter. They are ideal for dry goods such as spices and herbal teas although an inner plastic bag would always be recommended to give better moisture protection and avoid the chance of wood splinters entering the food. Some producers market a range of local foods in an open-topped box over wrapped with cellophane.

Few small or medium-scale food producers are likely to distribute final products on pallets but their use is strongly recommended in order to hold finished products in store off the ground. They can easily be built by local carpenters, the simplest design being shown in Figure 3-31. It can easily be seen how such a pallet keeps the food off a possibly damp floor and also allows easy cleaning of the storeroom.


Figure

3.1.6 Paperboard

Paperboard is the general name given to a variety of different types of materials that are used to make boxes, cartons and trays to package foods. They can be used as shipping (outer) containers or as consumer packs, but only a few types of materials can tee used directly in contact with foods.

In this section the different boards are first described. Corrugated boxes are dealt with in more detail because these are among the most common types of shipping container available to small processors. It should be noted that paperboard packs can be designed, made up, printed and sealed by the processors and are therefore one of the few packaging systems that are within their control. The methods for doing this are described in some detail in this section and are also included in Chapter 5.

Paperboard is produced in the same way as paper (section 3.2.1) but it is made thicker and often in multiple layers, to protect foods from mechanical damage (crushing, puncturing, vibration). There is a large range of paperboard types for different applications as consumer packs or shipping containers. Cartons or boxes are printed (if necessary), cut out to the appropriate size and shape and creased. The flat carton (or 'blank') may then be glued and assembled by the board manufacturer or alternatively delivered to the food processor for assembly on site. Types of paperboard are discussed in the following sections.

Moulded Paper Packaging

A number of packaging materials are made from recycled waste paper. The most common of these are egg trays and egg boxes but others such as fruit trays, small shallow dishes and protective bottle cases are available.

Moulded paper packaging (MPP) is mainly produced at very large scale but technology has been developed to produce such items at medium scale, which is still too expensive for small or medium producers. Recently the technology has been further scaled down to a level that is within the reach of small entrepreneurs at a cost of approximately œ12,000 and with an output of 240 trays per hour .

The first step in making MPP is to prepare paper pulp by liquidizing paper in water. Printed paper should not be used if the packaging is to come into direct contact with food. If necessary colours may be added or, if a degree of waterproofing is necessary, waxes.

Moulding takes place on a two-part mould, a forming mould and a transfer mould. The forming mould is made of fine wire mesh and the transfer mould of plaster like material. Vacuum and compressed air are supplied to the moulds. The process involves dipping the forming mould into the paper slurry so sucking up a coating of fine fibres of paper. Compressed air is then used to blow the formed item off the transfer mould. After moulding the trays are very wet and have to be dried, usually in the sun.

Paperboard that is used for fibreboard (more commonly named cardboard) boxes has no coating and as a result the barrier properties to air and moisture are low. Cardboard boxes are widely used as shipping containers for almost all foods. The properties of cardboard can be improved by a coating of wax or by lamination with polythene for use in consumer packs (for example paperboard cartons). These are used alone for products such as salt, rice, pasta products and spices or to provide protection against mechanical damage, or for inner plastic or paper bags containing a wide range of foods such as cereal products, snackfoods, coffee and confectionery. The main types of paperboard that are used for foods are described below. It should be noted however that there are many variations on these basic types and a large number of tradenames, particularly for specialist boards that have specific properties. A comprehensive list of these boards is not included in this publication. Board thickness is one of the main considerations and the figures below are the weight of board per square metre which is a measure of the thickness (higher weight = thicker board).

White board

This is the only type of paperboard that is recommended for direct contact with foods. It is made from several thin layers of bleached chemical pulp and it is usually used as the inner layer of a carton combined with other types of board which form the outer layers of the carton. It may be coated with wax or laminated with polythene to enable it to be heat sealed.

Triplex board (or foodboard)

This is widely used for food packaging. It normally has three layers, the inner and outer layers being made from white board (bleached chemical pulp). The outer layer may be machine glazed and/or coated to enable a better print quality to be achieved and it is supplied as 200 400 g/m2. Another board named Duplex board (or boxboard) is similar but the inner layer is made from grey (ie unbleached) chemical pulp.

Chipboard

This is made from re-cycled paper and is used to make the outer cartons for packets of foods such as tea and cereals. It is not suitable for direct contact with foods. It is less strong than Triplex or Duplex board for an equivalent thickness but it is cheaper than these materials. It is usually supplied as 300 g/m2 and it may be lined with white board to improve its appearance and strength

Solid board

This is a multiple layer of bleached sulphate board that is white, strong and durable. It is usually supplied as 150 - 400 g/m2 and when laminated with polythene it is used for liquid cartons (sometimes named Liquid Packaging Board or Milk Board). Examples of typical products packaged in liquid packaging board include fruit juices and soft drinks.

Fibreboard

This can be either solid or corrugated board The solid type has an outer kraft layer and an inner white board It provides good protection against impact and compression and is often spirally wound into cylinders or small tubs which are fitted with a plastic or metal cap at the top and a board inserted at the base. A composite can is a can-like package with the body and ends made of different materials. The body is usually made of paper and then ends of metal and increasingly plastic. The body is made from spirally wound paper in a tube shape. Better barrier properties are obtained if the paper is laminated with plastic or aluminium foil. Composite cans may, rather like sanitary cans, be bought plain or printed with the base fitted or for total assembly in the factory. They mainly find use for packing dry goods including coffee, cocoa, ilk powder and mustard powder. They are cheaper than metal tins and can be made from re-cycled paper. Small containers are used to package spices or confectionery for retail sale and larger drums are used to package a variety of powders and dried foods for shipping and distribution.

It is important that these containers are kept dry at all times and not stored in a humid environment to avoid delamination and loss of integrity of the drum.

Corrugated cardboard (or fibreboard)

This is made with two layers of kraft paper and between them, a central corrugating (or fluting) material. The corrugations are made by softening the fluting paper with steam and then passing it over corrugating rollers. The kraft paper is then glued to each side. Thicker boards may have several layers of corrugated board glued together, although these are not widely used in food
applications. The best quality board has unbleached kraft paper with equal thickness either side of the fluting and uses no re-cycled material. Both bleaching and the use of re-cycled paper reduces the strength of the cardboard.

The degree of protection from mechanical damage that is provided by cardboard depends on the size and number of corrugations or 'flutes'. Smaller more numerous corrugations give rigidity to resist compression from stacking, whereas larger corrugations give a cushioning effect that resists impacts and puncturing.

Corrugated boards resist impact, abrasion and crushing damage and are therefore widely used for shipping containers for bulk foods such as dried fruit, nuts, etc., and for containers such as glass jars and plastic films that require protection. They are also used to contain cans and plastic tubs or bottles for convenient handling. An alternative to boxes is shrinkwrapping or stretchwrapping (Section 3.2.2) which, if available, gives a more sophisticated image. In many countries however these remain more expensive than cardboard.

Supply of boards

Dimensions of boxes and cartons are always quoted in the order: length x width x height and the dimensions are always the inside measurements of the box taken from the centre of a crease to the centre of the next crease (Figure 3-34). Designs of cardboard box are shown in Figure (3-35).


Figure


Figure

The board can be supplied in a number of ways to smallscale processors. The most convenient, but also the most expensive, is to receive the board already formed into blanks. Here the board is cut to the correct shape and scored along the folding lines so that it can be easily assembled in the production area (Figure 3-36). It can also be supplied pre-printed.


Figure

Alternatively the board can be supplied as plain sheets which must be cut to shape, scored and folded by the processor. This is more time-consuming and requires a separate preparation area away from the food production to prevent contamination of the food with dust and fragments of card. Additionaly the processor is paying for the waste card that is not used (Figure 3-37). However in many countries where large packing boxes are available, perhaps from imported equipment, the supply of corrugated cardboard sheets is a suitable way for smallscale processors to solve their packaging problems by producing their own boxes. Methods for doing this are therefore described below.


Figure

All cardboard boxes should be carefully stored, especially in humid conditions to prevent deterioration of the material and separation of the corrugations or delamination of the layers of a package. This depends on both the type of adhesive that is used to seal the board and the conditions under which the containers are stored and handled. In general they should be kept dry, cool and off the ground on pallets or shelves.

Calculation of box size

The factors to consider when deciding on a cardboard box for packaging foods are the box size required to adequately contain the contents and the most economical box shape. The size of box required can be found by placing together the containers to be packed (not forgetting dividers if these are to be used between jars or bottles) and measuring the size of the stacked food (Figure 3-38). These sizes are then the minimum internal dimensions of the box. In practice it is usual to then choose the nearest standard size of box that is supplied, rather than pay the extra cost of a specially made box. Care is needed that there is not too much free space inside when it is full. The containers should be firmly held in place to prevent them from moving and being damaged during transport.


Figure

The most economical design which minimises the amount of board used to make a box of a given volume is found when the ratio of length:width:height equals 2:1:2. This is because less card is used to form overlapping flaps compared to other designs where the ratio is different (Figure 3-39).


Figure

Sealing

The most common type of adhesive used for gluing cardboard boxes is based on starch (usually cornstarch) which is specially treated for hot, humid conditions to make it more resistant to moisture pickup and consequent weakening of the bond. Boxes may also be stapled (Figure 3-40) or occasionally stitched. After filling the boxes may be sealed using glue, staples or stitches as above. Glues should be fast-setting (for example polyvinyl acetate glues) to ensure that the cardboard flaps stay in place. Alternatively they may be tied with string/rope or taped or strapped with tape (Figure 3-41). Simple tape applicators are available which make sealing faster and more economical.


Figure


Figure

Quality control tests for paperboard

In practice most small-scale producers have only one supplier of boxes or cartons and in some countries the only supply is recycled used boxes. In these situations it is unlikely that any action can be taken if the quality of boxes falls below specification. However, for those producers that have a choice of supply it is worth monitoring the quality of boxes and cartons and ensuring that the supplier understands the needs of the processor. Specific tests for paperboard are described below and more general quality control considerations are described in Chapter 6.4.

The main characteristics of paperboard are as follows:

- thickness,
- stiffness,
- ability to crease without cracking,
- whiteness,
- surface properties,
- suitability for priming.

The main faults found in paperboards are as follows:

- Critical faults:

- dimensions outside limits,
- bursting strength too low so that box/carton splits,
- score lines cut right through,
- tears or holes in the box,
- contamination with odours or foreign materials (especially reused boxes),

- Major faults:

- incomplete gluing,
- joints not square,
- incomplete, illegible or incorrect printing,
- flaps do not fold along score lines,
- gap between flaps greater than 5 mm (when boxes made up)

- Minor faults:

- printing faults,
- stains or scratches on box

The operators in a food processing unit can check the appearance, print quality, etc., of cartons and boxes by looking for these faults on a routine basis. If a problem arises then the dimensions of the boxes can easily be measured with a rule and the position and depth of score lines can be checked. No other special quality control equipment is needed.

3.2 Flexible packaging

Flexible packaging is a major group of materials that includes plastic films, papers, foil, some types of vegetable fibres and cloths that can be used to make wrappings, sacks and sealed or unsealed bags. The wide variety of bags, wrappings and sacks that are available makes this group of packaging materials very important for small scale producers and for this reason they are dealt with in detail in this chapter.

Paper is often made locally in large-scale factories and where this is the case the cost may be low enough to make it a feasible option for food processors. In general small-scale papermaking is not able to produce the quality of paper required for direct contact with foods but it may be suitable for outer cartons.

Flexible plastic films are not often made in developing countries but are imported from industrialized regions. However in some countries plastic film manufacture from imported granules is becoming established and the cost of packaging may be substantially reduced as a result.

Woven sacks and bags made from sisal, jute or cotton are highly suited to small-scale manufacture and can form a viable rural industry. Details of each of these materials are given in the following section and the re-use of some of these materials is described in Chapter 5.

Leather packaging and some traditional vegetable fibre packs are not included because they are not normally used in commercial processing but are restricted to household storage.

Many of the sophisticated plastic films that are used in industrialized countries are also not included because they are not widely available or they are very expensive in developing countries.

3.2.1 Papers

There are many different types of paper used for food packaging and this section describes the main types that are likely to be found in developing countries. Treated papers are used for dry foods (flours, dried fruits), fats, baked goods and confectionery. However plain paper is not heat-sealable and has poorer barrier properties and therefore finds fewer applications. Paper is produced by beating wood chips to break them down to a pulp which contains the wood fibres and then treating the fibres with alkali or acid. After treatment the fibres are pressed through a series of rollers to form paper. Sizing is the term given to chemicals that are added to the pulp during preparation to give particular properties to the final paper.

There are two basic types of paper which result from the alkaline or acid treatment: alkaline treatment produces a 'sulphate' pulp which is used to make Kraft papers and vegetable parchment whereas acid treatment produces a 'sulphite' pulp which is used to make sulphite papers. The differences in the properties of these two types of paper are described in more detail below.

Papers and boards are fully biodegradable in the environment because their component chemicals (mostly cellulose) are broken down by moulds, bacteria and animals.

Types of papers

The properties and applications of different types of paper that are used for foods are shown in Table 3-8.

Type of paper

Weight(g/m2)

Notes

Applications

Kraft

70 -180A

strong paper that can be bleached white and printed or

25 - 50 kg sacks for



unbleached and brown. Usually used in multiple layers or 'plies'

(3 flour, sugar, dried fruits



or 4 ply are most common) to give the necessary strength. Can also

and vegetables



be laminated to polythene or wax treated to give greater moisture




protection. The different plies need not be the same weight of paper.




Sack material is described from the outer ply inwards according to




the number and weight of the layers (for example 2/90 1/80 kraft




means that there are three plies, the two outer ones having a weight




of 90 g/m2 and the inner having a weight of 80 g/m2).


Vegetable

40-75

Kraft paper that has been further treated with acid during its

Fats such as butter or

parchment


preparation to make the surface smoother and more resistant to

lard fresh/smoked fish



penetration by oils or water (more greaseproof and greater wet




strength than kraft paper). Negligible barrier properties to air or




moisture and not heat sealable. Not therefore used to package




foods that require protection against air or moisture pickup over a




long storage period.


Sulphite

30 - 50

A lighter and weaker paper than Kraft or parchment, usually made

Used as small bags or

paper


with a glazed surface to improve the appearance and to increase

wrapper for biscuits or



wet strength and oil resistance. (When glazed it is known as MG

confectionery



Sulphite paper- MG = machine glazed.) The glazed surface can be




printed using flexographic methods (section 4.2.3) but for higher




print quality the paper should be coated. It is also used in laminates




of paper and plastics or foil (Sections 3.2.2. and 3.2.3).


Greaseproof

40 - 60

Made by beating fibres more thoroughly during the manufacture of

Fresh fish or meat,

paper


sulphite pulp. The smaller fibres make a more dense surface which

liner for shipping



is more resistant to oils. However this resistance is lost when the

containers for



paper becomes wet.

butter/cheese, liner for




packs of biscuits, fats




and other greasy foods

Glassine

20 - 40

A translucent sulphite paper that is given a high gloss surface by the

Liner for biscuits,



heated rollers used in its manufacture. The gloss makes it more

cooking fats, fast foods



resistant to water when it is dry, but if the paper does become wet it

and baked goods



loses this resistance.


Tissue paper

25

A thin, weak sulphite paper. It is often machine glazed on one side

Wrapping fresh fruit to



(known as MG tissue). A special type of tissue paper with small

prevent bruising



regular perforations is used to make tea bags.


Newspapers

-

Commonly available in most developing countries and are often




used for food packaging. However newsprint should not be used in




direct contact with foods (especially fatty foods) as the ink is




carcinogenic (causes cancer). It is also an unattractive outer wrap




and does not give a professional image to the processor. However it




is cheap and widely available and is therefore a source of material




for making into paperpulp for the production of moulded trays




(Section 3.1.6).


Re-cycled

-

Recycled paper from Government forms and school exercise books


paper


is also widely used for packaging in many countries. There is a




flourishing small industry in some countries which converts this type




of paper into pre-formed bags which are used to contain foods and




other items for short periods of time. Again care should be taken to




avoid direct contact with foods, especially fatty foods as any ink on




the paper is likely to contaminate the food.


- Weight = g/m2 = the weight of one square metre of paper.

Table 3 - 8: Properties and use of different types of paper

Improving the properties of papers

The lack of heat sealing properties and poor barrier properties to air and moisture are disadvantages of paper that have been addressed in a number of ways:

Wax treatment

Papers can be treated with wax to improve their barrier properties and make them heat-sealable. These papers are used to package cereal products, bread and spices. The three methods of applying wax to paper are as follows:

- Coating: this is applied after the paper has been made. However the coating is easily damaged by folding the paper or by abrasive foods (eg dried foods). Damage can be avoided by laminating the layer of wax between two layers of paper or between a layer of paper and a layer of polythene.
- Dry waxing: during manufacture, the hot paper is treated with melted wax so that it penetrates into the fibrous structure of the paper. This improves the durability of the wax barrier.
- Wax sizing: here wax is added to the pulp during the initial stages of preparation and becomes fully integrated into the structure of the paper. Both this method and dry waxing enable the wax to become deeply ingrained into the paper and therefore it is not easily damaged by folding the paper or by abrasion.

Laminates

Paper can be laminated to low-density polythene to make it heat sealable and improve its barrier properties to air and moisture. Other methods include lamination to aluminium foil or to other types of plastic. However in each case the cost is increased and many of these paper laminates are not widely available in developing countries. Where laminates are available they are used to package coffee, dried soup, herbs and spices and other dried foods that require a barrier to moisture and air during a long shelf-life.

Wrapping

Wrapping is a type of packing in which a solid food is enveloped in a sheet of flexible material, usually paper, cellulose, cloth or foil. Wrappings of paper cloth or foil are not usually sealed and therefore do not provide a substantial barrier to moisture, air or micro-organisms. They are used to keep food clean and hold items together. Examples include wrapping spices in paper, wrapping confectionery in cellulose film, and wrapping chocolate in foil.

Sealing

Plain paper is not heat-sealable and as the barrier properties of papers are insufficient to protect most foods for long storage periods, the seal on paper packages is designed to simply contain the contents. Paper wraps for confectionery and small amounts of flour, spices, sugar, salt, soils etc are very common for containing the food to carry home and for short term storage. They are made by twist-wrapping, folding the paper by hand or by tying with string or cotton.

Paper bags can be folded, stapled, taped or glued. Larger paper sacks can be stitched using an electric bag-stitcher or glued with adhesive. Wire twisting tools (Figure 3-43) are also used for larger sacks. Details of equipment are given in Section 3.1.6. Locally-made adhesives using starch or gelatin are suitable provided that the humidity is not too high during storage as this would cause them to fail. One special type of sealing involves heating of special perforated tissue paper to make tea bags. This is described further in Chapter 5. Heat-sealing of waxed paper requires equipment as described in Section 3.2.2.


Figure

Specific quality control tests for papers

The main tests that are important for papers are described below. Other more general requirements for adequate quality control are described in Section 6.4.

- Weight (or substance): this is the weight of one square metre of paper measured in grams - ie g/m2. (in USA in lbs per 3000 sq. ft).
- Yield: is the area of paper and hence the number of packs that can be made from a unit weight of paper or film and is expressed as m2/kg. As paper is usually sold by weight, especially in larger quantities or on rolls, the yield is important in ensuring that the most economical use is made of available supplies.
- Surface formation: should be smooth and even without loose fibres or other faults. This is particularly important if machine-wrapping is to be used.
- Folding endurance: this is particularly important for papers that are to be used for twist wrapping.

There are no critical faults that are likely to be found when new paper is used. Newsprint and reused paper may be contaminated by inks or other materials and as has been mentioned above, these are not recommended for contact with foods. Major faults include tears or stains in the paper, incorrect surface preparation and excessively high or low yield. Each fault can be checked for by a visual examination of the paper and for yield, by carefully weighing a sample of paper (Section 3.2.2). Minor faults include creases, minor surface imperfections and minor color variations

Storage

All papers are sensitive to changes in the humidity and temperature of storage. Under humid conditions they may curl or stick together and any adhesives may lose their strength. In general they should be stored for the shortest time possible under constant low temperatures and moderate humidities (for example 20 °C and 50% relative humidity). However in many developing countries packaging cannot be bought in small quantities and if supplies are irregular a food manufacturer may wish to buy as much as possible to guarantee continuity of production.

Under these circumstances great care should be taken to store the packaging materials properly. All papers should be stored without opening the outer wrapping, rolls should be stored uptight, flat sheets should be stored on a firm, flat surface so that they stay absolutely flat. All materials should be kept off the ground, and especially off concrete floor&which can make papers damp very quickly.

Ideally papers should be stored on shelves or pallets in a well ventilated room where the temperature does not vary much throughout the day (eg a dry underground cellar). Rats and cockroaches eat papers and measures should be taken to prevent them entering the paper store. Similarly birds and animals should be excluded and papers should be covered to prevent them getting dirty.

If the humidity in the production area is different from that in the store mom the papers should be moved to the processing room the day before they are to be used to allow them to acclimatize slowly to the new humidity.

3.2.2 Films

Plastic films are becoming increasingly important in most developing countries because they have several advantages over other forms of packaging used in food processing. Briefly:

- they are better able to protect foods and extend the shelf-life,
- they are tough and durable to withstand rough handling during transport and distribution,
- they are convenient to handle by both processors and customers,
- they add very little weight to the product which reduces transport costs,
- they can be easily printed to inform customers about the product (eg the type of food, its optimum storage conditions or any special preparation needed),
- they fit closely around the product which takes up little extra space for transport,
- they have an attractive appearance to most customers which helps the processor to increase sales,
- they are mostly inert (they do not react with foods or taint them),
- they have good barrier properties to moisture and air.

The importance of plastic films is reflected in the more detailed descriptions found in this chapter. The plastic films described below are those that are becoming increasingly available in developing countries and include polythene, polypropylene and cellulose. Many large scale plastics manufacturers also make an extensive range of films that are a combination of these plastics or laminates of these films with paper, foil, or other plastics. These may be available through local agents, but are often only available in large quantities and at a relatively high cost. They are not generally suitable for small-scale processors for these reasons (and in some countries because of restrictions on foreign exchange) and they are therefore only briefly described at the end of this section.

Barrier properties

Barrier properties are the resistance that a package has to moisture, air, light, micro-organisms, puncturing, etc. Measurement of the properties gives an indication of the amount of protection that is given to a food by a particular packaging material.

Flexible films have large variations in their barrier properties in contrast to other materials such as cans and glass jars. The processor does not have to specify the degree of protection required when ordering cans or bottles because they are all a complete barrier. However because of the wide variations in film barrier properties, due to differences in types of film or even differences in the thickness of the same film, it is necessary for the processor to carefully specify the degree of protection required for a given product.

Alternatively if there is only a limited range of films available it is more difficult for the processor to know whether they are suitable for the intended use. In these cases the producer should ask the film supplier whether the intended use will be suitable for the available film.

The barrier properties of films and other packaging materials are described by two main factors: the Water Vapour Transmission Rate (WVTR) and the Oxygen Transmission Rate (OTR). These are a measure of how much water vapour or oxygen is able to pass through a known area of packaging material in a given time (by convention this is usually the amount passing through one square metre of material in 24 hours). The units of WVTR and OTR are therefore: g or ml/m2/24 h.

The higher the value of WVTR or OTR the more permeable the material is to moisture or air (or to put it another way, the lower the value the better the barrier to moisture or air). Because the permeability of most materials varies with the temperature and humidity of the surrounding air it is usual to measure WVTR and OTR under known air conditions (eg 25°C and 65% relative humidity).

It is important for a food processor to know the conditions under which the food is likely to be stored and then get data on the barrier properties of the proposed packaging which have been measured under similar conditions (available from the packaging supplier). This will enable the processor to assess the likely shelf-life of the food under these conditions. An example of the importance of this is shown in Figure 3-44 where the amount of moisture taken up by the crisps during storage is measured by their gain in weight. When this reaches 2% the crisps are spoiled.


Figure

Figure 3-44 shows that different films control the WVTR to different extents so that the gain in moisture is slower for some films than for others. As a result the shelf-life of the crisps varies from a few days in a hot humid climate using plain polypropylene to more than 50 days using a coated polypropylene film. Also the barrier properties of all films are much better in cooler climates and the shelf-life of the crisps is extended in all cases, often doubling the time before spoilage compared to the shelf-life in a hot/humid climate.

An important general implication of this is that food processors may need to put different 'best before' dates on packages of the same food that is intended to be sold in areas which have different climates.

Types of plastic films are discussed in the following sections.

Low Density Polythene

Polythene (full name: polyethylene) is the cheapest and most widespread plastic films used for food packaging in developing countries. It is available in a wide range of thicknesses and grades, all of which are flexible, relatively tough and transparent and heat-sealable. In general thicker films are stronger and have better barrier properties to moisture and air, but thicker films are also less transparent and less flexible. All thicknesses are susceptible to damage by sunlight over a period of time which leads to them becoming more brittle and more opaque.

Polythene is widely used as a single bag to protect almost any food from dust or dirt over a short period. It is also widely used in combination with other flexible packaging such as paper or cellulose to make these materials heat sealable.

Compared to some other films Polythene has a relatively poor resistance to oils and also allows moisture and air to pass through at a higher rate than many films. It is not therefore recommended for the long term storage of foods that are affected by air or moisture (eg fatty foods where the products are susceptible to spoilage by rancidity, or those that should be crisp or dry).

The thin film is known as low-density polyethylene (LDPE) and is transparent and glossy. The barrier properties of LDPE to moisture and air are relatively poor and the film has little strength to resist puncturing, although it does not tear easily. Because LDPE, like other films, does not protect foods against mechanical damage, these packages require outer cartons or boxes for transport and distribution.

The film also has a relatively low melting point which makes it easily heat sealable. Details of heat sealing are given below. The properties of LDPE in comparison to other films of a similar thickness are shown in the Figures 3-45 and 3-46.


Figure

LDPE is relatively inert in that it does not react with foods. However, recently research indicated that the plasticizers used to make the film flexible can be absorbed by fats in foods and may be linked to nerve damage to eyes and development of cancers. LDPE should not therefore be used to package fatty foods (including cooking oils, butter, cheese or biscuits) for long periods of time.

Medium and High Density Polyethylene

Increasing the thickness of polythene (to a gauge sometimes named Medium Density Polyethylene or MDPE) improves the barrier properties to moisture but it remains a relatively poor barrier to air and odours. Thicker grades of Polythene become progressively less transparent.

Thick Polythene (0.03 -0.15 mm often expressed as 200 - 500 gauge film) is known as High Density Polyethylene (HDPE) and this is a relatively good barrier against moisture, air and odours (Figures 3-45 and 346). It is stronger, less flexible and more brittle than LDPE or MDPE and has a higher softening temperature
(121°C).

HDPE is a strong film that gives a strong heat seal and will withstand puncturing, tearing and stretching. This makes it suitable for use as sacks where it withstands the rough handling that they often receive. However it is more slippery than jute, paper or other natural fibres and this makes it more difficult to stack piles of more than four or five sacks.

All grades of polythene have relatively poor resistance to sunlight and become less flexible and more brittle after approximately six months' exposure to light under tropical conditions. This is more noticeable with the thicker films that have less plasticizer than LDPE.

Polypropylene

This film (full name: oriented polypropylene or OPP) is a clear, glossy film that is fully transparent and sparkling. It is strong, heat sealable and it withstands puncturing and tearing. It does not stretch as much as Polythene and has good barrier properties to moisture, air and odours (Figures 3-45 and 3-46) which make it more suitable for foods that have a long expected shelf-life (eg biscuits, snackfoods and confectionery).

Unlike polythene it is not damaged by sunlight and unlike cellophane it is not affected by drying out or by low temperatures. Thicker films (above 50 microns) have greater barrier properties and seal strengths than thinner films and are therefore more suitable for larger heavy duty packs or as stronger packages (for example for foods which have sharp pointed particles). Thicker grades are used for pasta, pulses, dried fruits and cereal products.

Polypropylene does not have the same problem of movement of plasticizers into fatty foods that is found with polythene, but it has a higher sealing temperature than polythene which requires an electric heat sealer to seal it effectively. It is becoming more widely available in developing countries where, because of its attractive, glossy appearance and better barrier properties, it is replacing polythene in many applications. It is however usually more expensive than polythene.

Polypropylene is also woven into sacks for bulk transport of both fresh and processed foods. Until recently the production of sacks was confined to industrialized countries but they are now made on continuous equipment in a number of developing countries. These sacks are very tough and resist puncturing, tearing and stretching. They allow moisture and air to pass through the weave (in contrast to HDPE sacks) and they are therefore more useful for fresh produce or for foods that do not require protection against these factors.

In some countries there is a viable small industry which converts used polypropylene sacks into shopping bags and other domestic containers.

Cellulose (cellophane)

Cellulose is one of the very few plastic films that is made from renewable materials instead of from petroleum products. It is made from wood pulp (mostly eucalyptus) by a complex chemical process, to produce a clear glossy transparent film that is biodegradable within approximately 100 days under tropical conditions.

It is a strong, puncture-resistant film that can be 'dead folded' (ie a crease or fold made in the film will stay in place). This is particularly useful for twist-wrapping small foods such as confectionery. It also has excellent clarity, high gloss and a crisp feel which is attractive to most customers. Unlike polythene it is not damaged by sunlight.

However cellulose tears easily and more importantly, it is not heat sealable in its plain form. In addition it is a relatively poor barrier to moisture (Figure 3-45 and 3-46) and the dimensions and barrier properties of the film can change if the humidity of the surrounding air changes. If the air is very dry the film becomes brittle and tears very easily. As a result, plain cellulose is mostly used for foods that do not require full protection against moisture or air, or where an exchange of moisture is required (eg fresh baked goods where moisture is 'breathed out' at a controlled rate to maintain a crisp crust, or for dried foods where moisture should not collect inside the package where it could cause mould growth).

The barrier properties of plain cellulose film are improved and made more constant if it is coated with nitrocellulose or PVdC (polyvinylidene chloride, Figure 3-47). Nitrocellulose coating improves the barrier to air and odours but it does not improve the barrier to moisture. PVdC coated films vary depending on whether the coating is applied using water as a solvent or using an organic solvent. The aqueous solvent has a much lower risk of odour remaining in the film and it is therefore used for bland foods that have a particular risk from odour pickup.


Figure

PVdC coatings also make the film heat sealable and resistant to oils, moisture, air and odours. They are used typically for foods such as cereal products, dates, currants, sultanas, pasta, pulses, snackfoods, nuts and biscuits. An international code is used to identify the various forms of cellulose as follows:

Code

Meaning

P

plain

T

transparent

M

moistureproof

C

coloured

S

heat sealable

A

anchored

X

PVdC coated

W

winter quality (withstands


low temperatures)

F

freezing quality (withstands


freezing)

One of the most common types of cellulose is MSAT which is moistureproof, heat sealable, anchored and transparent.

Other films

In some countries films such as metallized film, and laminated films that have very good barrier properties are becoming more widely available. However, because these are not yet widespread and are in general considerably more expensive than polythene, polypropylene or cellulose, they are only mentioned briefly here. These films offer considerably greater protection to foods over a long shelf-life than do the films described above.

Metallized films are plastics such as cellulose or polypropylene on which a very thin layer of aluminium metal is deposited. This not only makes the film highly reflective, like a mirror, which is attractive to many consumers, but also greatly improves the barrier properties to moisture, oils, air, odours and light. Other advantages are that the film is less expensive and more flexible than laminated films which have similar properties.

Laminated films are those in which two or more films are bonded together or bonded to paper or to aluminium foil. The most common method is to apply adhesive to one film and then the two films are passed between rollers to pressure bond them together. Examples include cellulose/LDPE/cellulose for coffee, cellulose/paper/foil/LDPE for dried soups, LDPE/foilpaper for dried vegetables and polypropylene (coated with PVdC)/LDPE for confectionery and dried fruit.

In general laminated films are only used in developing countries where special protection is required for high value foods. They are expensive and generally not widely available. Similarly nylon and nylon laminates are very effective barrier films but are expensive and not widely available.

PVC (polyvinylchloride) is also used for shrinkwrapping and stretchwrapping (described below) but is more expensive and less easily available than polythene in most developing countries. Additionally there may be restrictions on its use in some countries because of residues from the vinyl starting material and plasticizers in the film.

Filling methods

When filling food into plastic bags the most important consideration is to prevent any food from contaminating the inside of the bag where the seal will be formed. If food is trapped between the two layers of film it will make the seal ineffective and the barrier properties of the film will have no effect. This is a particular problem with fine powders that can cover the inside of a bag very easily. Some simple techniques to fill plastic film containers are described in more detail in Chapter 4.

Wrapping

To wrap food is to envelop it in a sheet of flexible material. It may then be tied, taped, glued or heat sealed depending on the material. Cellulose films are used for twistwrapping and overwrapping of cartons such as tea and confectionery cartons. Small overwrappers for plastic films are available and heated plates or bars (Figure 3-49) are suitable for the manual sealing of overwraps.


Figure

Shrinkwrapping, stretchwrapping and clingfilm

Special types of LDPE are also available in forms known as shrinkwrapping film and stretchwrapping (or cling) film. For shrinkwrapping, the property of LDPE to shrink when it is heated is used to make a pack that holds the contents together tightly. The film (45 - 75 microns thick) is placed over the items to be wrapped and then heated with hot air either in a tunnel or using a hot-air gun to make the film shrink (Figures 3-50 and 3-51).


Figure

Note: it is not possible to shrinkwrap cartons that have a wax or polythene coating because this will fuse with the shrink film when it is heated. In this case stretchwrapping is a suitable alternative.

The film is also made to have low-slip properties which allows the wrapped packages to be safely stacked. The most common use is to replace cardboard boxes as shipping containers for smaller packages of foods. Cans, bottles or packets of food are placed on a card tray and the shrinkwrapping film holds them together.

Different films are available which shrink by a known amount from 10 - 35% across the film (known as the transverse direction) and by 20 - 60% along the length of the film (known as the machine direction). For bag type shrinkwrapping or 'full wrap' the required shrinkage is usually the same in each direction whereas in sleeve type shrinkwrapping it is usually specified as 60% in the machine direction and 20% in the transverse direction (Figure 3-53).


Figure

There are two types of shrinkwraps:

- a full wrap (or bag type) which completely encloses the product to be wrapped except for a small hole to allow air to escape during the shrinking process,
- a sleeve wrap in which a sleeve of polythene is longer that the items to be wrapped.

The sleeve is formed around the product and when heated, the ends contract over the ends of the product to securely grip it. This type is often used for trays of bottles, jars, cans, etc.

Shrinkfilms are available in different thicknesses and this can be used to calculate the amount of film that is needed for each pack as follows. For sleevewrap packing (Figure 3-54):

Width of film =A +3/4C
Length of wrap = 2(B + C) + 10% shrink allowance
Total film used per pack (kg) = (width of film x length of wrap)/yield


Figure

A simple 'rule' is that the thickness of shrinkwrapping film should be increased by 10 microns for every kilogram of product wrapped. For example if a shipping load of 24 cartons, each weighing 250g is to be shrinkwrapped the film should be 24 x 0.25 x 10 = 60 microns.

Details of the yield of a film and its relation to thickness are given in the section on quality control below.

LDPE is also produced in a thinner (25-38 micron), more stretchable form known as linear low density polythene (LLDPE) or 'stretch film' (in domestic use it is known as 'clingfilm'). This is made so that it can stretch by up to 60% without breaking. One side of the film has greater 'cling' properties than the other and this makes it stick to other film when wound around a stack of boxes or other items (Figure 3-55). This keeps the items together during transport and also keeps the load clean. Because the film only has cling properties on one side the wrapped loads do not stick to each other. The film also has the property of not tearing easily if punctured so that the load remains together even if the film is damaged during handling. Both shrinkwrapping and stretchwrapping help to prevent pilferage.


Figure

In domestic use or during processing stretchfilm can be used for covering containers or for wrapping small amounts of food for short-term storage. The film should not be used for long-term storage or for fatty foods because of the risks from migration of plasticizer into the food as described above.

Sealing

The simplest sealing for plastic bags involves tying a knot in a pre-formed bag. Other methods include stapling folded layers at the top of a bag or using adhesive tape (Figure 3-56). In each of these methods the bag is not completely sealed and air or moisture can enter and leave, although more slowly than if it is not sealed at all. These methods should therefore be used only for foods that do not require much protection during storage or for those that are expected to have a shelf life of only a few days.


Figure

Heat sealing

To form a moistureproof and airtight seal it is necessary to heat seal the film by melting the plastic on either side of the bag opening, fusing the two films together. This provides a more effective barrier than folding or tying the film. This type of seal can only be made using materials that are thermoplastic (ie they melt on heating and then solidify on cooling) or by using thermoplastic coatings on a base film Waxed paper is also a heat sealable material which is used to wrap bread whereas cellulose is used as an overwrap for cartons of tea or confectionery.

Heat sealing of polythene can be done simply by folding the top of the bag over an old hacksaw blade and heating the film with a flame (e.g. from a candle or spirit burner). This produces a thin seal which is adequate for short term storage. However there are likely to be small faults in the seal which allow air and moisture to enter over a period of time. In addition the seal is weak and easily torn and, unless produced carefully, is less attractive to consumers than machine-made heat seals. This method is not suitable for cellulose or polypropylene because of their higher sealing temperatures.

A broader seal can be made using an electric or fuel heated sealer . This type of seal has similar barrier properties to the film being used. It is stronger than other seals and more attractive to many consumers. In operation two films are pressed together between coated metal bars (the coating stops the film from sticking to the metal). The bars are heated by an electric element or burning fuel. The heat melts the plastic and the pressure fuses the two films together. The heat is then stopped and the plastic cools and solidifies. Full seal strength is obtained when the plastic has cooled to room temperature.

The strength of a seal formed in this way is determined by the temperature of the bars (controlled by an adjustable thermostat in electric equipment), the pressure applied by the operator and the time of heating and cooling. Each type of plastic film has its own range of sealing temperatures, pressures and times over which it will form a good seal.

The following sections discuss different types of heat sealers.

Jaw sealer

This type has two coated metal bars and the films to be sealed are placed between them. One or both of the bars are heated and one bar moves to press the films together. The heating time can vary from 1/20th second to several seconds depending on the film, and it is controlled by a switch that is activated when the bars are pressed together.

Roller sealer

This type of sealer has a heated metal wheel that is pressed along the film to be sealed (Figure 3-59).


Figure

The seal thickness is determined by the width of the interchangeable wheel and the seal strength is determined by the speed and pressure used by the operator. The wheel is not usually coated and it is necessary to use a sheet of paper between the sealer and the film to prevent the film from sticking to the wheel. With practice it is possible for an operator to obtain perfectly straight seals. An advantage of this type of sealer is that curved seals can be made which may have a decorative appeal for consumers. There is no limit to the length of seal that can tee made.

Hot wire sealing

Here a metal wire, heated to red heat, is used to form a bead seal while cutting the film at the same time (Figure 3-60).


Figure

Impulse sealer

This is similar in appearance to the jaw sealer but operates in a different way. Initially both bars are cold, but when they are closed together on the films one bar is heated electrically for a pre-set time. After heating the pressure is maintained for a few seconds to hold the seal in place while it cools and sets. Each of these sealers is relatively cheap and simple to make by a local engineering company.

Types of bags

Plastic films can be bought as pre-formed bags, as tubing or as 'flat' film (either in sheets or on a roll). Preformed bags am filled and sealed by the packer and they are commonly used in small and medium-scale enterprises where production volumes are too small for formfill-seal machines (below) or where the products have an irregular shape or size. Typically they are used for flours, confectionery, sugar, salt, root crops, bread and processed fruits.

Pre-formed tubing is used to form bags by heat sealing the base. Flat film can be used to make bags or other types of pack described below by sealing each side separately. These bags are often made with only side seals although bottom seals are also common. They can be made flat or with gussets on the sides and/or the base. Although there are no international standards for bag manufacture most bag makers have a range of standard products and it is always cheaper to use a manufacturer's standard product than asking for a special design, especially if small quantities are ordered.

Different types of bag designs are shown in Figure 3-61. The gusseted bags (or wedge bag) are normally used for packing solid, bulky foods such as bread. They are also used as an inner liner for cartons containing such products as cereals or biscuits.


Figure

Small bagging machines in which a product is filled and sealed into pre-formed bags are also available. They can be manually operated or semi-automatic and are ideally suited to small items such as confectionery, dried fruit pieces, nuts, etc.

Vacuum packing

Vacuum packaging is a development of the impulse sealer, but here most of the air is first removed from a bag of food which is then heat sealed. For foods that are susceptible to deterioration due to air, vacuum packing can extend their shelf life. The tight fitting package around the food is also more attractive to some consumers. In general a strong film such as polypropylene is needed to avoid puncturing and thus to retain the vacuum in the pack.

There are, however, two important constraints on the use of vacuum packaging which should be considered seriously before it is used: first the removal of most of the air from a bag creates an 'anaerobic' environment inside the bag. If certain types of bacteria are present on the food, this oxygen-free environment will encourage them to grow rapidly. Many types of food poisoning bacteria are of this anaerobic type and vacuum packaging could therefore cause good food to become dangerous. In particular low-acid, moist foods such as meat, fish, dairy and vegetable products should not be vacuum packed at the small scale.

Secondly, vacuum packing is not suitable for many dried foods unless a strong film is used. The sharp points on pieces of dried food can easily puncture a film as it is drawn onto the food by a vacuum. This destroys the purpose of vacuum packing and dramatically reduces the shelf-life of the food.

Until recently vacuum packing was beyond the reach of many small-scale processors of the high cost of the equipment and difficulties and cost in maintaining the vacuum pump. Cheaper, locally produced and maintained vacuum packing equipment is available in some developing countries and as a result this method is becoming increasingly common.

Form-fill-seal

There are three types of form-fill-seal equipment; the vertical (VFFS) and horizontal (HFFS) types which are distinguished by the way in which the food and film pass through the machine, and a pouch former which seals the pouch simultaneously on four sides. At present there are no simple, low-cost, horizontal or pouch machines so these are not described further. There are a limited number of low-cost vertical machines and interest is increasing in further developments to these.

This equipment (VFFS) makes bags from a wide range of packaging materials in a continuous operation and then fills food into them and seals the bags. In operation a film is formed into a tube and the long seal is made by a heated roller. An impulse sealer then makes a seal across the film - to make the base of a bag - and the bag is filled with food. A second seal is then made above the food to seal the bag and simultaneously form the bottom seal on the next bag (Figure 3-63).


Figure

This type of equipment is suitable for powders and flours, granular foods such as beans, nuts, confectionery or liquid foods. As in all heat sealing care is needed to prevent food from sticking to the inside of the film and contaminating the seal. The film must also be strong enough to form a bottom seal that will withstand the weight of food while the seal is still warm. Commonly used films include heat sealable cellophane, polypropylene, and coated or laminated papers. Polythene can be used but filling speeds are slower to prevent the film from stretching when placed under tension.

Specific quality control procedures for flexible films

The reader is advised to read Section 6.4 in conjunction with this section to find additional, more general quality control procedures that are needed when packaging foods

There are a number of faults that can occur in rolls of plastic film which can result in an inability to use the film, a poor appearance after packaging or a reduction in the barrier properties of the film and seat There are no critical faults in films (that could injure operators or customers) but a number of major faults are possible and these are described below, as are the routine tests for checking films before they are used.

It must be remembered that if film is bought on a roll, there is no simple way of finding out whether there are faults inside the roll. The only checks that can be done are on the film at the outside of the roll and it is then assumed that the remainder of the roll has a similar quality. It is therefore essential to find a reliable supplier of film and if possible to agree the quality checks with the supplier.

The faults that can occur are as follows (the classification assumes that manual and not automatic packaging equipment is used. A different classification is needed for automatic packaging):

-Major faults

- Incorrect yield: the barrier properties of a film depend in part upon its thickness.

Yield is the area per unit weight of film (m2/kg) and is a measure of the thickness of a film (In the USA the yield is measured as square inches per pound - sq. in./Ib.)

A film thicker than specified is unlikely to be important technically in small-scale processing (although it will be important financially) but a high yield (thinner film) could result in inferior barrier properties and the risk of incorrect sealing, jamming in a sealing machine or tearing.

Thickness can be measured directly and is usually expressed in microns (= 0.001 mm) or in the USA as gauge (0.00001 inches = 0.254 microns). Normally a variation of 10% on the specification for the film is acceptable. A comparison of yield and thickness is essential when film is being ordered because it is usually sold by weight especially when it is sold as a roll.

For example cellophane and polypropylene can both be used to wrap a product. The cellophane has a yield of 22.7 m2/kg and a thickness of 30 microns, whereas the equivalent polypropylene film has a yield of 44.0 m2/kg and a thickness of 25 microns. If one package requires 0.05 m2 of film then 1000 packages would use 50 m2 of film. This means that 2.2 kg of cellophane would be needed but only 1.1 kg of polypropylene. In many countries the price of cellophane is about 1.5 times the price of polypropylene which means that the cost of packaging in cellophane would be 2 to 3 times the cost of packaging in polypropylene, other factors being equal. It is necessary to use a micrometer to accurately measure the thickness of a film but for most processors the high cost of this equipment is not usually justified.

- Incorrect printing: this can be a fault in the design, the wrong design, incorrect colours, smudged, blurred or incorrectly positioned print.
- Odour: in some films a coating is applied using organic solvents and some printing inks may be solvent based. If the film is not properly prepared, residual solvents may be present and these will contaminate the product. The only simple method of checking this is to smell the film.
- Blocking: this fault causes layers of film on a roll to stick together and not unwind smoothly. In extreme cases it may cause the film to tear. For manual methods minor blocking is not a problem but for machine sealing it can cause the film to become misaligned (so that any printing is not positioned correctly on the pack) or cause the machine to jam Serious blocking can make a film unusable even with manual methods.

- Minor faults

- Marks: the presence of blemishes on the film.
- Dimensions: the width of the film should be correct for the intended package. Oversized or undersized film is more difficult to handle in the sealing machine although this is unlikely to be important for manual sealing methods.
- Curl: this is a fault which causes the film to curl up instead of lying flat. It is due to incorrect storage conditions (especially humidity), a build-up of static electricity, incorrect film thickness or variation in the coating on a film. In martial sealing this is a nuisance which will slow down the operation but in machine sealing the film may jam the machine and be unusable.
- Winding: this is where a film is wound too loosely on a roll. In extreme cases it may cause the film to slip off the roll and become damaged. In manual sealing this fault is unlikely to be a problem but in machine sealing the lack of correct tension in the film may cause it to feed through the machine incorrectly and cause jamming.
- Slip: is the property of a film to slide over machine parts or other film. It is important when automatic filling machines like form-fill-seal equipment are used. If slip is too low the film will not rim through the machine properly but if it is too high it will slip away from the sealer before a seal can be formed.
- Register mark position: these marks are used in machine sealing to correctly position the film so that the printing appears in the proper place on the package. If they are misplaced or not clear the packs will be improperly labelled. These marks are not used for manual sealing.

Film testing

To inspect a roll of film for these faults the following procedures are used:

- Remove any outer covering and check the roll for looseness (winding).
- Measure the width of the roll to check that it is within +5 mm or -5 mm of the expected width.
Check the diameter or weight of the roll to ensure that the correct quantity has been supplied.
- Remove two layers from the roll and either discard them (in machine sealing) or used them for packing (in manual methods) if the subsequent checks show that the film is satisfactory.
- Use the next two layers for testing, check for any blocking as they are unwound.
- Lay the sample on a flat surface and note if there is any curling. Examine it closely for print quality and position, correct colours, register marks and the presence of any blemishes.
- Cut out five standard sized squares of film using a 10cm x 10 cm template (Figure 3-64) and weigh them carefully (eg using scales reading 0- 50 g in 0.2 g divisions). Convert this weight to a yield value using a calibration curve such as the one shown in Figure 3-65.


Figure


Figure

- Crumple some of the film and smell it for any solvent odours.

A final test is to take part of the sample and heat seal it under the conditions that are used in production. When the seal is cold test it by gently pulling the two films apart at right angles. A faulty seal is one that:

- does not form at all,
- comes apart with little force, or
- tears unevenly when pulled.

If required the permeability of a film to moisture can be measured to predict the shelf-life of a product under known conditions. Although it is possible to buy special permeability testers they are very expensive. In practice similar results can be obtained by using a sealed box (an old refrigerator cabinet is ideal) in which a tray of saturated salt solution is used to control the humidity of the air in the cabinet. If necessary the temperature can be changed by a small electric heater or a light bulb in the cabinet. Both temperature and humidity should be similar to those expected during the shelf-life of the food in the area in which it will be sold.

A weighed amount of food is packaged, placed in the cabinet and then re-weighed at regular intervals. If the weight falls or increases too much before the end of the expected shelf-life it is then known that the film is not a sufficient barrier to moisture.

The gas barrier properties of a film can also be measured, but this requires more expensive and sophisticated equipment and would not usually be undertaken by a small-scale producer.

Need for shipping containers

Although plastic films provide a good barrier to moisture, air, sometimes light, micro-organisms, etc., they do not protect the food against mechanical damage such as crushing, vibration and puncturing. In addition few films can prevent tats, birds and some insects from attacking the processed food during storage. It is therefore necessary to protect the plastic bags or packs during transport, distribution and storage using shipping containers. The most commonly used in developing countries are cardboard or wooden boxes, baskets and crates. Shrinkwrapped or stretchwrapped containers are also now being used in some countries.

Special skills needed

In manual and sealing methods, a few simple skills need to be acquired by production staff. Similarly quality control procedures for packaging materials also need a certain amount of experience and expertise. The operation of automatic packaging equipment such as form-fill-seal equipment requires training and this would normally be given by the equipment supplier. However in general, packaging in flexible materials requires little formal or intensive training.

3.2.3 Foil

Aluminium foil is generally expensive and thus not widely used by small and medium-scale producers. However for some applications where very good protection of a food is needed or where local aluminium production makes foil cheaper, this can be an important material. It is therefore included in this publication but the level of detail is less than for some other materials.

Aluminium foil is made by rolling out pure aluminium metal into very thin sheets and then annealing it to give dead-folding properties. It is available in a range of thicknesses from 7 - 20 microns when it is used as wrapping for foods and 50 - 100 microns when it is used as trays for streetfoods. In this section the use of foil for wrapping foods is described.

Foil is an excellent barrier to moisture, air, odours, light and micro-organisms and it is therefore used to wrap foods that are sensitive to off-flavours or odours, light or air. Its properties are described in Table 3-9. In addition it is reflective, which is attractive to most consumers and helps to reflect heat from the wrapped food. It has no reaction with foods and is therefore entirely inert with acidic foods, oily foods or others that may react with some types of packaging. As a result foil does not need to be lacquered or protected in any way from contact with foods.

It is either used alone or as a component of laminated packaging where it is bonded to paper or a plastic film to improve the barrier properties of these packs. Foil is not heat sealable unless laminated to a plastic, but the deadfolding properties allow it to be folded tightly. The resulting seal is not a total barrier to moisture and air, but it is adequate for short/medium term storage. The major disadvantage of foil is the relatively high cost.

Quality control

There are normally no critical faults (causing harm to operators or customers) in the use of foil. Major faults include excessive numbers of pinholes, incorrect thickness and tearing or creasing.

Foil thickness (micron)

7

9

12

15

20

Yield (m2/kg)

52.9

41.2

30.8

24.7

18.5

Number pinholes/m2

<800

<200

<150

<75

<10

Water Vapour



almost zero



Transmission Rate






(WVTR)






Oxygen Transmission



almost zero



Rate (OTR)






Table 3-9: Properties of foils

The most common fault with foil is the presence of tiny holes (named 'pinholes') that are formed during its manufacture. Most foil manufacturers conform to a voluntary standard on the number of pinholes per square metre of foil which is set at a level that does not affect the performance of the foil (for example in a commonly used foil that is 9 microns thick the average number of pinholes should be less than 200/m2). If the number is substantially higher than the agreed standards the foil will tear more easily and the barrier properties to moisture and air are reduced. Although it is not a routine quality control procedure the number of pinholes in foil can be checked by the following method:

- Cut a length of 30 cm of foil from the width of a roll and hold it up against the sun or a bright light.
- Check which part of the foil seems to have the most pinholes and then carefully cut a 10 x 20 cm sample.
- Count the number of pinholes in the sample and multiply the result by 50 to find the number of pinholes per m2.

Other useful checks on a roll of foil are first that the roll is wound tightly which can be checked by seeing that the roll can stand vertically without falling over, and that when the vertical roll is lifted with both hands the layers of foil do not slip. Secondly that there are no wrinkles or creases in the foil.

Resistance to handling

Foil is easily damaged by handling and it should therefore be handled as little as possible. It is almost impossible to remove creases from foil once they are made and these will not only spoil the appearance of the pack but may also damage the foil and lower its barrier properties.

Foil is usually bought in roll care should be taken that scratches dents and cuts are not made in the roll by careful handling and storage. When the roll is being used a simple dispenser with a serrated metal cutting edge allows the foil to be unwound without wrinkling. With practice, food can then be wrapped by hand without causing creases.

3.2.4 Cloth and vegetable fibres

The main types of material that are used for food packaging are cotton, jute, linen and sisal. With a few exceptions these materials are not used for small retail or consumer containers but are more commonly used to transport larger quantities of food as shipping containers. For this reason the level of detail in this section is less than for some other materials. One particular use for cloth packs is for foods that are sold in specialist markets such as tourist souvenirs. Here a decorative package made from a locally produced jute or cotton material may have good promotion potential.

Textile containers have no significant barrier properties to moisture, odours and air. In addition they do not protect foods from mechanical damage such as crushing or puncturing or from micro-organisms, insects, rodents or birds. They are therefore used for foods that are not susceptible to odour pickup or changes in humidity and foods that are not easily damaged by crushing. They are mostly intended as a lightweight container to hold the food together in a package that can easily be bandied and transported. They are used for free-flowing foods such as flours, sugar, salt, spices, cereals, tea and coffee beans. They are also widely used for short-term transport of a wide variety of other foods including fresh fruits and vegetables and dried fish, although the protection offered to foods carried in this way is minimal.

The main advantages of textiles are that they can be manufactured locally from available materials and they can be easily repaired by sewing with a suitable sack needle and thread. They are lightweight and have good non-slip properties which means that sacks can be safely stacked. They are re-usable when cleaned and they are biodegradable when discarded

Textile packages can be closed by sewing with a bag stitcher or by tying with wire or rope (Figure3-43).

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