All the methods of food preservation (household and commercial) can be broadly divided into two categories:
1. Bacteriostatic Methods are those in which the micro organisms (bacteria, moulds and yeast) present in the food are unable to grow e.g. in dehydration, freezing, salting, pickling and treatment with anti-biotics.
2. Bactericidal Methods in which the micro organisms present in the food are destroyed e.g. sterilization, cooking, canning and eradication.
HOUSEHOLD METHOD OF FOOD PRESERVATION
Dehydration is the process of removing moisture from the food stuffs for a long period and that they do not usually cause spoilage of foods. The moisture level is kept below a certain points.
Since time immemorial, Sun drying has been the oldest and most popular method of preserving food stuffs ranging from cereals, legumes, vegetables, fruits, fish, meat and shrimps products generally prepared at home include various types of pappads, dried vegetables, amchur, potato, banana and tapioca and sago wafers, amla, spiced vegetable, chilies, tamarind pulp, fried fish etc. In sun drying, there is no possibility of temperature and humidity control. So the hottest days of summer season are usually selected for sun drying of food products. Direct foods also bear the advantage of easy transportation because of light-weight and less storage space.
2. Addition of salt or sugar:
Salt and sugar solution of high concentration are very commonly used at homes to food stuffs as in preserving tamarind, raw mango, amla fish. In preparation of jams, jellies and pickles, also good amount of sugar is added.
Addition of such solutes dries up the moisture and prevents the growth of micro organisms, thus aiding in preserving food.
3. Use of oils and spices:
When oils and spices are used with salt and sugar to food stuffs, they prevent growth of micro organisms like moulds and yeasts. The layer of oil acts as a shield, thus preventing the food stuffs to the exposure of air. Moreover, spices like chilies, fenugreek, pepper, asafetida and mustard improve the flavor of the preserved food as in pickling.
4. Use of Acids and Chemical Preservatives:
Acid conditions in the food stuffs inhibit growth of any micro-organism. Acetic acid (Vinegar), Citric acid (lime juice) and lactic acid (dahi) are commonly used as preservatives at home. Vinegar is added to pickles, chutneys, sauces and ketchup preparations. Even onion is bottled in vinegar with a little salt. Citric acid is also used in the preparation of fruit squashes, jams and jellies. Lactic acid produced from lactose as in the preparation of ‘dahi’ increases the sell life of milk and in its nutritive values.
Certain chemical preservatives are also used at home in the preparation of jams, jellies, marmalade, squashes, and to juices of fruits in the preparation of squash. Sodium benzoate is an effective inhibiter of moulds and is used extensively for the preservations of all coloured fruits and vegetables. Potassium meta-bisulphate is used only for colorless fruits like apple, litchi and raw mango. These chemicals when added in very small hinder undesirable chemical reaction in food.
Pesticides are also used at home by spraying over fruits, vegetables and food grains to prevent their spoilage.
The quickest and simplest way of preserving food at home is through refrigeration or storing food in a refrigerator. Here, foods are frozen and kept at temperature much below the temperature of the surrounding air and thereby prevent the activity of the micro-organisms.
6. Sterilization of cooking:
In this method, food is preserved by preventing the growth of micro-organisms through the application of high temperature. While cooking food, if the boiling temperature (212°F or 100°C) is maintained for a long period of time, the bacteria apt to be present in the foods gets killed and it can preserved for a longer time. However, application of low heat in cooking as in preparation of custard will not make the food last longer.