Answer to Question #186263 in Material Science Engineering for lily

Question

Aluminium is used in food containers for different reasons: it provides a very good barrier to

protect food against light and air and preserves food quality; it is corrosion resistance; it can be

recycled and designed individually according to customers’ requirements; it can be produced

in different sizes and shapes due to its atomic bonding and its FCC crystalline structure.

However, in order to produce aluminium cans, manufacturers must strip-mine bauxite ore to

gain alumina. This method leaves behind liquid waste that can pollute soil and groundwater.

1. Suggest one other type of other crystalline solid (it can be polymer, ceramics, or other

metal), can replace aluminium as food packaging but with less impact on our soil and

groundwater. Justify your answer (cost of the material is not a factor)

2.suggestion,explain the mechanical properties of the material and relate

these properties to the atomic bonding and the crystalline structure of the material.

Compare these properties to aluminium.

Answer 1

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.