Some of the most important household methods of food preservation are:
The process of dehydration is based upon the fact that if bacteria are deprived of water for a long period they do not usually cause spoilage of foods. The moisture level has to be kept below a certain point. Dried foods can also be transported easily because of being light in weight and small in volume.
Meat and fish have been dried for centuries. Foods such as dried figs, dried apples, prunes (dried plums) and raisins (dried grapes) keep well because of a combination of physical changes. As moisture is removed, the relative concentration of sugar goes up and this increased sugar concentration is unfavourable for bacterial growth. A variety of dehydrated products are available in the market. These include dried milk, dehydrated soups, instant coffee, citrus juice powder, pre-cooked peas and cereals.
Sun-drying is a very common method of food preservation. In this process the direct rays of the sun are used for dehydration. More generally, fruits are partially cooked or otherwise treated before dehydration. Dehydrated foods include papdi, papads, macaroni, bari, potato wafers and so on. Foods such as vegetables and fish are often salted before sun-drying.
The process of refrigeration works on the principle that bacterial and enzyme activity are kept to the minimum at low temperatures. By utilising this method, foods can be kept for long period of time and can also be made available for longer periods of time, thereby reducing the number of trips of the homemaker, to the market. Fruits, except bananas, and vegetables should be properly wrapped in cellophane paper and stored in the refrigerator just above the temperature at which they will freeze. Butter and meats must be kept at much lower temperatures. Besides refrigerators, ice-boxes are also used as cooling appliance. In the rural parts of India, perishable foods are stored in cool earthenware pots covered with wet sand and gunny cloth, wire baskets, meat safe. Grameen Sheetal is an indigenous device for keeping seeds at lower temperature.
3. Chemical Preservation
This method may employ high concentrations of sugar, salt and acids.
Salting is a good method of preserving vegetables and fruits like tamarind, raw mango, amla, etc. and also fish and meat. Salt is used in dry and brine form. The presence of a high concentration of salt prevents the water from being available for bacterial growth. This is because the concentration of salt in the water is higher than that in the bacterial cells. Thus the water cannot be absorbed by the cellular membrane of the bacteria.
The principle of preservation with sugar is also the same as that for salting. However, salting is generally a cold process while in case of sugar the mixture is heated. Sugar acts as a preservative because the high concentration of sugar solution exerts a high osmotic pressure and withdraws water from the micro-organisms, thereby preventing their growth. However, moulds will grow on the surface of jams, jellies and other preserves, if proper sterility is not maintained.
When the medium in which the food is preserved is strongly acidic, most organisms cannot survive. The use of vinegar (acetic acid) and lemon juice (citric acid) is common in home methods of Pickling.
Chemical Preservatives like benzoic acid (or sodium benzoate) and sulphur dioxide (or sulphites) should be used with meticulous care. In higher concentrations they are injurious to health. In fact government regulations have specified the quantities of certain commonly used chemicals. Sodium benzoate-which is used for all coloured fruits and vegetables, is generally used upto a concentration of 0.1 %. Potassium metabisulphite, which is used as a source of sulphur dioxide is used only for colourless fruits like apple, lichi and raw mango. The potassium radicle reacts with the acid of the juice forming the corresponding potassium salt and sulphur dioxide. Glacial Acetic acid and its milder household form vineger, are also used in preservation. Meats may be cured with smoke that contains phenols. Pectin powder is used to make jams and jellies with foods poor in pectin content (pineapple).
Pesticides are sprayed over fruits, vegetables, and food-grains to prevent spoilage. If these pesticides are used beyond safe levels they are highly toxic. Thus an indiscriminate use of these chemicals should be avoided.
4. Use of oil and spices
The principle is the same as that for salting. The oils and the spices along with salt and sugar provide a medium that resists the activity of the mico-organisms in food. Moreover, they improve the flavour of the food being preserved. Spices, such as chillies, fenugreek, mustard and pepper are used in pickling.
Similarly when oil is used in pickling, a top layer of oil prevents the micro-organisms in the air from coming into contact with the food.
5. Sterilization by cooking
Sterilization by cooking is a method of preserving foods by preventing the growth of micro-organisms through the application of high temperatures. Boiling temperature (212° F or 100° C) if maintained for a sufficiently long period of time, helps the heat to completely penetrate the foods and kill the bacteria. However, if low heat is used, as in the preparation of custards or when the heat penetrates the food very slowly as in casseroles or stuffed poultry all kinds of bacteria may not be killed. Thus, such foods may cause food poisoning.
Cooked foods which have been subjected to sufficient heat and have been improperly handled are rapidly spoilt. In such foods, spores of Clostridium perfringes and Clostridium botulinum, being highly resistant to heat, are present. Clostridium botulinum, an organism present in non-acid and semi-acid foods such as peas, corn, green beans, as well as meat produces injurious toxins in food. The spores of these bacteria are destroyed only if food is cooked under pressure. The enterotoxin produced by staphylococci is not rendered inactive by boiling, but botulin is inactivated by boiling foods for at least 10 minutes.