The methods of preserving food include:
CHILLING AND FREEZING
The growth of micro-organisms is slowed when food is cooled. Food kept in a domestic refrigerator at 0oC may keep for several days. Food deep frozen at -18oC will keep for several months, provided it is not allowed to thaw and then be refrozen. The growth of all micro-organisms ceases and many are killed. Food that is to be deep-frozen must be frozen rapidly to prevent the formation of ice crystals which damage the food. Meat, fish and vegetables are generally preserved by this method and can then be transported.
Micro-organisms grow and multiply in the presence of moisture: by removing water these organisms are unable to grow and decomposition is prevented. This is done by:
1. Desiccation (dried vegetables and fruit)
2. Evaporation (condensed milk)
3. Smoking (smoked fish and meat)
STERILIZATION AND EXCLUSION OF AIR
Heat destroys yeast, moulds, and bacteria. Heating to the appropriate temperatures is a convenient method of preserving fruit and vegetables, as in tinning and bottling. This is the basis of the household processes of making jams, jellies, bottling fruit and vegetables.
Sugar, salt, spices, vinegar, oil are harmless preservatives; their action I purely chemical in nature. They make the foods unsuitable for use by micro-organisms without making them unfit for use as food for human beings.