Preservation of food by any method is based on the following principles:
(A) Prevention or delay of microbial decomposition
(i). By keeping out micro-organisms (asepsis)
Nature provides protective coverings around the food in the form of shells of nuts, die skins of fruits and vegetables, the shells of eggs, and the skin or fat on meat or fish. These protective coverings act as a preservative factor, thereby preventing or delaying microbial decomposition.
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Even in the food industry several aseptic methods are adopted to prevent the contamination of foods during its processing. In the canning industry, the load of micro-organisms determines the heat process necessary for the preservation of food. This is better known as aseptic canning. In the dairy industry, the quality of milk is judged by its bacterial content.
Packaging of foods is also an application of asepsis. The coverings may range from simple wrappers to hermetically sealed containers of canned foods. Polythene bags and moisture proof wrappings including heavy foil, heavily mixed papers and cellophane are used.
(ii). By removal of micro-organisms
Filtration is a method used for the complete removal of microorganisms and is successfully applied only to clear liquids such as water, fruit juices, beer, soft drinks and wine. The filter used in this method is made of asbestos pads, unglazed porcelain and similar materials. This filter is sterilised and made “bacteria proof” before being used as a filtration device. The liquid is filtered by forcing it under pressure through the filter.
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(iii). By hindering the growth and activity of micro-organisms
This may be done by low temperature or drying or by providing anaerobic conditions.
When anaerobic (absence of oxygen) conditions are created, some aerobic organisms die while the spores of others may survive but are unable to multiply in the absence of oxygen. This principle is used as a preservative factor in canned and packaged foods.
Low temperature at which foods are preserved in cold storage slows down and sometimes prevents bacterial activity.
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Drying of foods is a very effective method of avoiding spoilage of food, since micro-organisms cannot flourish in the absence of moisture.
Certain chemicals like sodium benzoate and potassium metabisulphite may be used for preservation, but they should be used with great care as an excess of any of them may result in poisoning.
(iv). By killing the micro-organisms using heat or radiation
In this process gamma rays or high speed electrons are used to destroy the micro-organisms. Both types of radiations are termed as ionized radiations. This topic will be dealt with in detail under “Methods of Food Preservation”.
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(B) Prevention or delay of self decomposition of food
This is done by destruction or inactivation of food enzymes by blanching. The inactivation affects many plant enzymes which otherwise might cause toughness and change in colour. All plant and animal tissues contain enzymes which are highly active at room temperature and above. For each 10° C (19°F) raises in temperature the rate of the chemical change doubles. Rancidity of fats is an excellent example of undesirable oxidation and leads to the deterioration in flavour of foods that may contain only small quantities of fat. Oxidation also leads to a loss of ascorbic acid. Plant and animal tissue fiber is softened, the surfaces of cut non-acid fruits are oxidised and become darkened as a result of enzyme action, thereby changing the colour, texture and nutritive value.
Before freezing (to prevent the growth of bacteria) fruits and vegetables are blanched to inactivate the oxidative enzymes, Blanching is done with hot water or steam and the extent of treatment applied varies with the kind of food being treated. The brief heat treatment is supposed to accomplish reduction of the number of micro-organisms on the food, enhancement of the green colour of vegetables such as peas and spinach, and prevention of damage because of mechanical causes, insects and animals.
Items of food can be damaged either by insects and animals or by mishandling. Therefore, meticulous care should be exercised to minimize any damage to the foods. The entire operation of preserving foods is divided into three stages of careful handling:
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i. Proper packaging
ii. Quick and effective transportation
iii. Providing good storage facilities, like silos for grains and cold storages for fruits and vegetables.