2.3 Extension of shelf-life
The section above describes the processing and packaging that is needed to preserve foods for their expected shelf-life. It also indicated that for different foods the importance of packaging can vary from simply keeping foods clean to being the main factor that controls the shelf-life. It was noted that for many foods the humidity of the climate in which the food is stored is one of the most important considerations.
In the following section the actual extension of shelf-life for different foods is shown for two types of climate. Readers should note that the data given is indicative only and that shelf-life will vary according to the local storage conditions used. Table 2-9 indicates the likely extension of shelf-life of the foods to be achieved by proper packaging.
2.3.1 Short shelf-life products
Breads and cakes are normally sold by small-scale bakers within a day of baking to retain the fresh baked flavour and odour and the correct texture. Staling takes place rapidly at tropical temperatures and leads to a toughening of the crumb and, in some breads a softening of the crust. The changes happen in both humid and dry climates but additionally there may be drying out of some products in dry climates. Because of the short shelf-life packaging is mainly used to keep these products clean and it is not used as a barrier to moisture or air.
Clean paper (for example tissue paper) is an adequate packaging material, but old newspapers should not be used because the ink is poisonous. Plain or coated cellulose is likely to be too expensive for most small bakers and is not needed if the products are to be consumed on the day of production or the following day. Polythene bags are used by some bakers to give a 'more attractive' or 'professional' image. If polythene is used to wrap baked goods, care should be taken to allow them to cool to room temperature after baking. This will prevent moisture condensing on the inside of the bag and wetting the surface of the food. If this is allowed to happen it will result in mould growth at the wet spots as well as a loss of the required texture in these places.
Many snackfoods are produced by small-scale entrepreneurs for immediate consumption as 'streetfoods' and these are rarely packaged except for a container to hold the pieces together and keep them clean. In particular some fried foods cannot be stored for more than a few hours because migration of oil softens the crisp crust of such foods and spoils their texture.
Others snackfoods are sold in packets with an expected shelf life of a few days to several weeks. These include puffed and toasted cereals and fried legumes/cereals in such products as 'Bombay mix'.
The main causes of spoilage are moisture pickup in humid climates, which leads to softening of the products within a few hours, and rancidity which develops over a few days or weeks depending on the type of oil used for frying and the storage conditions. Packaging should therefore be used to prevent moisture pickup in humid climates and also prevent air and light from reaching the product in both humid and dry climates to restrict the development of rancidity. As these products tend to be fragile the pack should also protect against crushing and other types of mechanical damage. It is usual to use a barrier film such as polypropylene for sealed bags of product and a cardboard box to protect against light and mechanical damage.
Pasteurized milk depends on both the heat treatment of pasteurization and post-processing cooling to maintain its shelf-life. Packaging should be sterilized to prevent contamination after processing but its main function is to contain the milk and keep it clean.
Spoilage is mostly due to micro-organisms that survive pasteurisation and these then cause the milk to sour after a few days. Spoilage may also be due to rancidity and loss of some vitamins, which is accelerated by sunlight. During distribution and storage the milk should therefore be kept cool and away from direct sunlight to achieve the expected shelf-life. Polythene bags are commonly used because they have lower costs than other packs. These are adequate if the milk is carried home by the customer and used straight away but they are less successful if the milk is distributed to retailers first. Reusable glass bottles with foil lids are used in some countries as a relatively low cost package. To be economical there should be a high rate of bottle return (usually above 90%) and particular care is needed to ensure that returned bottles are thoroughly cleaned and inspected (Section 3.1.1).
Yoghurt should be stored and distributed in a similar way to milk to give a longer shelf-life. Here however the higher acidity delays spoilage by micro-organisms and extends the shelf-life by several days compared to milk. As a result there is less need for cooling but the product should be protected from sunlight to prevent development of rancidity. Packaging is mostly to contain the yoghurt, keep it clean and insect free and keep out sunlight. Clay pots, and plastic pots are most commonly used. These may be unsealed or covered with a cloth or film. Plastic pots may also be heat-sealed with a foil lid.
2.3.2 Medium/long shelf-life products
The shelf-life of dried foods depends mostly on the relationship between the individual product and the humidity of the air during storage. Foods that are in equilibrium with the surrounding air will neither gain nor lose moisture and, provided that their moisture content is low enough to stop micro-organisms from growing, they will remain stable.
Foods that are traditionally dried in a particular area are suited to the local conditions and will remain stable for long periods with very simple packs to contain the food and keep it clean. However there are situations in which a dried food needs to have more elaborate protection if the humidity in an area changes (for example at the beginning or end of a rainy season), if the food is moved to another area which has a different air humidity, or finally if a new food is dried in a particular area. These are all reasons why packaging is needed to form a barrier to moisture.
Changes in humidity affect dried foods in one of two ways: if the humidity increases the food becomes more moist and this may then allow micro-organisms to grow and spoil the product. This also happens if the food is allowed to become wet from rain during storage. The food also becomes softer and the change in texture may also be seen as spoilage in some foods (for example snackfoods and biscuits). If the humidity falls this will cause the food to dry out further. For most foods this does not result in spoilage, but in some (for example some dried fruits) sugars may crystallize, the texture may harden excessively and the product may be seen as spoiled.
It can be seen from Table 2-9 that the benefits of packaging are substantial for some dried products, particularly in humid areas where the package should provide a barrier to moisture. Many dried foods are susceptible to rancidity and for these a barrier to air and light is needed. Many dried foods are also fragile and easily broken. They are therefore packed in cartons or boxes to give protection against mechanical damage. Products stored in dry areas do not benefit from an extension of shelf-life by packaging and the pack is simply used to contain the food and keep it clean.
These products are stable for long periods without packaging due to their high acidity and high sugar content. Packaging is mostly used to contain the product, keep it clean and prevent contamination from insects that are attracted to the sugar. Care needs to be taken that the temperature of the preserve is at 80 - 85 °C when it is filled into containers. If it is higher, steam from the product may condense on the inside of the lid and moisten the surface of the preserve. This dilutes the product and may allow mould growth If the temperature of filling is too low it is more difficult to pour the preserve and a partial vacuum will not form in the container headspace (Section 3.1.1).
Acid and fermented preserves
Here preservation is achieved by the combined effects of salt, acid and sugar. In acid preserves, added vinegar (acetic acid) is used whereas in fermented preserves lactic acid is formed by the bacteria used in the fermentation. The proportion of salt, acid and sugar can be changed within limits to suit local tastes, but any decrease in one should be compensated for by an increase in the others. Acetic acid (in acid preserves) is volatile and the container should therefore be airtight to prevent it escaping and allowing mould growth Fermented preserves contain lactic acid which is not volatile. Here the packaging does not have a major effect on the shelf life of the preserve and is simply used to contain the food and protect it from contamination.
Beverages and fermented beverages
Fruit juices and nectars which are intended for immediate consumption after opening rely on pasteurisation for their preservation. The package is therefore used to protect the juice from re-contamination by micro-organisms and maintain its shelf life. For squashes and cordials that are diluted with water before drinking, preservation is achieved by a high sugar content, by pasteurization and by added chemical preservatives. Here the package is opened and the contents are used over a long period, a little at a time. The package therefore has no preservative effect and is simply used to contain the food and keep it clean and free of insects that are attracted by the sugar.
Wines and spirits rely on a high alcohol content and a high acidity for preservation. However in these drinks the flavour components and the alcohol itself are both volatile and also able to react with air to produce off-flavours. The packaging should therefore be airtight. In some products, particularly wine and beers, there may be chemical changes caused by sunlight and the packaging in this case should also be lightproof. Glass is the preferred packaging for each type of product because it is inert. If necessary coloured glass can be used to protect against light. Other containers, such as metal or some plastic pots and bottles, may react with the beverage and create off-flavours.
The main cause of spoilage for most types of confectionery is moisture pickup which softens the sweet, dilutes the sugar at the surface and leads to yeast or mould growth. This is related to the humidity of the storage air and the effects of this are similar to those described above for dried foods. A barrier film such as cellulose or polypropylene is used to form sealed bags and cardboard carton used to protect softer confectionery from mechanical damage. Chocolate is an exception in that the high fat content makes it more susceptible to spoilage by high temperatures or by rancidity. High temperatures (above 35°C) cause movement of the cocoa butter or vegetable fats to the surface of the chocolate where they appear as a white 'bloom'. This is harmless but is unacceptable to most consumers and effectively spoils the food. Aluminium foil or metallized film is commonly used to reflect heat from chocolate and cardboard boxes provide additional heat insulation as well as protection from mechanical damage.
Syrups, honey, pastes, purees and oils
In each case these foods rely for preservation on their low moisture content and in some cases, high acidity. They are mostly stable at a wide range of air humidities and packaging is therefore used to contain the products and keep them clean. Some products such as cooking oils, peanut butter and other nut pastes, have a high fat content and are therefore likely to go rancid if storage conditions are incorrect. In these foods the shelf life is increased by using an airtight and lightproof container. Care is needed when packaging fatty products in some types of plastic pots or bottles as there is a risk of migration of chemicals from the plastic into the fat. This produces off-flavours and is a potential health hazard (Section 3.1.4).
The main economic value of these food components is their volatile flavours and aromas. They rely entirely on the package to contain these flavours and aromas, and because of the high value of these products it is often worthwhile to use a more expensive package. These products are also susceptible to rancidity and the package should be lightproof and airtight. Metal cans and glass bottles are the only suitable containers for these products.
Cheese, butter, ghee and khoa rely for their preservation on a reduced moisture content. The shelf-life is also extended by cool storage. Packaging does not contribute significantly to the extended shelf-life and is mostly used to keep the foods clean and free of contamination. Simple packages such as plastic film, waxed or greaseproof paper are commonly used. Polythene is commonly used in developing countries but this should be discouraged for long-term storage because of migration of chemicals from the plastic into the fatly food. Some cheeses are dipped into edible wax which provides a protective coating and extends the shelf-life.
Biscuits and some types of cake are preserved by their low moisture content and for a long shelf-life, they should not gain nor lose moisture. They are therefore subject to the same factors that are described for dried foods above. When packaged for the humid tropics they require a moisture barrier and because of their often high fat content, they should also be protected from rancidity by a lightproof and airtight package. Plastic films such as polypropylene or coated cellulose together with a cardboard carton to resist mechanical damage are satisfactory. In dry climates glass jars, metal boxes or plastic tubs are suitable for protecting these products from breaking and for preventing contamination. Polythene should not be used for long term storage due to migration of chemicals into the fats contained in these foods.
In the context of this publication, the only frozen food being considered is ice cream and this should not be stored by small-scale producers for more than a day or so. It is usually kept in a freezing box and is sold directly to consumers. No packaging is therefore required other than to contain the product.
Smoked fish and to a lesser extent, meats and vegetables, are preserved by their low moisture content, the chemicals from the smoke and by added salt. They are essentially dried foods and as such are subject to the same factors that are described above for dried foods.
A summary of the packaging options for the groups of food described above is shown in Table 2-10.
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