We have already studied that spoilage of perishable food materials takes place due to their high moisture content, attack by insects or micro-organisms (such as yeast, mould and bacteria) and by the action of enzymes. The various methods of food preservation are based on either of the following two basic approaches:
(a) According to the first approach, the microorganisms responsible for spoilage are eliminated by some suitable technique. The methods of food preservation, which adopt this approach, are called bactericidal methods. For example, cooking, canning etc.
(b) According to the second approach, conditions are created which are unsuitable for the growth and action of microorganisms and enzymes. These methods of food preservation are known as bacteriostatic methods.
Some examples of the methods are salting, pickling and deep-freezing.
Let us now study some common methods of food preservation.
1. Dehydration and Sun-drying
Drying in sun is an old method of preserving food materials such as vegetables, fish and meat. Removal of water from fruits and vegetables is called dehydration. Dehydration results in decrease in their moisture content. Due to reduced moisture content the growth of microorganisms as well enzyme action is retarded and thus they can retain their nutritive value for longer periods.
Dehydration can be carried either by drying fruits and vegetables in the sun or by removing their water under controlled conditions of temperature and humidity. Preserving food materials by drying in the sun is one of the oldest methods. For example, vegetables such as methi leaves, spinach, and cauliflower are generally preserved by sun drying. Peas are preserved by carrying out dehydration under controlled conditions. Dehydration of fish, meat and their products is carried out by a process called smoking. In this process, small pieces of meat are first rubbed with salt and then hung from the ceiling or kept on wooden rafters (sloping beams). A fire is lit below. The pieces of meat get dehydrated with the heat and smoke produced by the fire.
2. Preservation by salt and sugar
Preservation of food by adding sufficient amount of salt to it is called salting. It is extensively used in the preservation of raw mango, amla, tamrind (imli), beans, fish and meat. When salt is used in high concentration between 15% to 18%, it drains out water from the food material through osmosis. As a result of removal of water, the bacterial growth in the food material is inhibited and thus the food can be preserved. For carrying out preservation salt is used in dry fro as well as in solution from.
Sugar can also be used for the preservation of food. Bacteria cannot grow in a sugar syrup containing more than 68% sugar because it has very little free moisture available. Thus, food materials such as fruits and vegetables can be prevented from being spoiled by adding large quantities of sugar. The microorganisms already present in the food materials gradually die due to lack of available moisture in the sugar syrup. Many fruits such as apple, pineapple, mango, strawberry and guava are preserved in this way in the form of jams, jellies and murabbas.
In addition to salt and sugar some other substances which are used as preservatives are vinegar, oils, spices and critic acid. These substances are used in pickles, ketchups, jams, squashes etc.
3. Deep Freezing
This is a direct method for inactivating enzymes and preventing the bacterial growth. In this method, the food material is cooled below 0`C. At low temperature, enzymes become inactive and bacterial growth is also inhibited. As a result, food material can retain its nutritive value for a longer period. This method of food preservation is used for storing fresh fruits, vegetables, and meat and fish products.
4. Chemical Preservation
Growth of microorganisms in a food material can also inhibited by adding certain chemical substances. However, the chemical substance should not be harmful to the human beings. Such chemical substances, which are added to food materials to prevent their spoilage, are known as chemical preservatives. In our country, two chemical preservatives which are permitted for use are:
1. Benzoic acid (or sodium benzoate)
2. Sulphur dioxide (or potassium metabisulphate).
Benzoic acid or its sodium salt, sodium benzoate is commonly used for the preservation of food materials. For the preservation of fruits, fruits juices, squashes and jams sodium benzoate is used as preservative because it is soluble in water and hence easily mixes with the food product. 0.06% to 0.1% concentration of sodium benzoate is sufficient for the preservation of fruit juices and squashes.
Potassium metabisulphite or sodium metasulphite is used for the preservation of colourless food materials such as fruit juices, squashes, apples, liches and raw mango chuntney. These are not used for preserving coloured food materials because sulphur dioxide produced from these chemicals is a bleaching agent. These preservations on reaction with acid of the juice liberate sulphur dioxide which is very effective in killing the harmful micro-organisms present in the food and thus prevents it from getting spoiled.
Some other Modern Methods of food Preservation
In addition to the various methods of food preservation described above, there are some modern methods of food preservation such as canning, irradiation, bottling and vacuum drying. All these methods are bacteriasidal methods.
1. By irradiation:
Irradiation means exposing to high energy radiations. In this technique of food preservation, the food material is exposed to high energy of radiations such as gamma rays or X-rays. These radiations kill the harmful microorganisms present in the food and thus prevent the food spoilage. Irradiated food materials can be preserved for quite long periods.
2. By Canning
In the canning process, the foodstuff is cooked and then stored in sterilized tin containers, and then the containers are sealed and made airtight. Canned food materials can be safely stored for long periods.
3. By Bottling
Bottling technique is applied mainly for the preservation of milk. Milk is first pasteurised by heating to a temperature of nearly 70`C and then suddenly cooling it to about 20C. Pasteurisation kills most of the disease causing microorganisms present in the milk. Therefore, the milk is stored in bottles or polythene pouches. The process of pasteurisation is also used to disinfect some other food products such as fruit-juices, squashes, ketchups, etc.
4. By Vacuum Drying
Milk can be preserved in the form of milk powder. Milk is converted into milk powder by vacuum drying. Due to the absence of moisture, milk powder can be stored without spoilage for quite long periods.