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.
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.
- 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:
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.
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 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.
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.
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).
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
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.
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,
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 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 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.
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
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.
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 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.
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:
One of the most common types of cellulose is MSAT which is moistureproof, heat sealable, anchored and transparent.
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.
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.
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.
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).
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).
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,
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
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.
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.
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.
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.
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.
This type of sealer has a heated metal wheel that is pressed along the film to be sealed (Figure 3-59).
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).
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.
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 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.
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).
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):
- 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.
- Minor faults
- Marks: the presence of blemishes on the film.
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).
- 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,
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.
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.
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.
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.
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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