It is the preservation of food in sealed containers, usually after the application of heat, through steam under pressure. During the process of canning part of the micro-organisms are destroyed and the rest are rendered inactive. The enzymes are also inactivated. The containers are then sealed, since sea1ing prevents recontamination of foods. During aseptic canning high temperature is applied for a very short time. The product is first sterilized at 135° -I72°C (275°-350°F) in a few seconds, is cooked and is filled aseptically into containers which have been previously sterilized.
This method has been successfully used for fluid products such as fruit juices, syrups and sauces and with fruit products such as pineapples, apricots, mangoes and peaches. In this process, heat-stable vitamins are retained and the colour and flavour are better. Canning is an industry with a great economic importance. The following factors should be carefully looked into during canning:
i. Selection of the food
ii. Sterilization of cans
iii. Sealing of cans
Milk is an essential item of food, but is also most susceptible to contamination. Thus, to safe guard milk against bacterial infection, resulting in milk-borne disease, the process of pasteurization has been introduced on a large scale. The milk used for the preparation of milk products like cheese, butter and ice-cream is also pasteurized. Pasteurization may be brought about by the holding process in which milk is heated to at least 62° C(143° F) and kept at that temperature for at least 30 minutes; or by the high temperature short-time method in which milk is heated to 71° C (160° F) and kept at that temperature for at least 15 seconds.
Milk may be sterilized either by boiling for a period for time or by the application of heat as in the preparation of evaporated milk. Sterilization deepens the colour of milk and gives it a slightly caramelized flavour, while pasteurization does not change the colour or flavour of milk.
3) Cold Storage
The principle of refrigeration has been utilised in establishing commercial cold storage rooms. This system of large-scale refrigeration has been responsible for t the availability of a variety of foods all over the country in season and out. Foods can be kept in cold storage for long periods of time, and this facility is not being utilised with canned and dehydrated foods so as to retain optimum colour, flavour and the nutritive value. Most fruits, vegetables and meats are stored and preserved in cold storages.
Bacterial cells have a unique resistance to low temperatures, and they regain their survival power once they return to normal temperature. Freezing does not sterilize the food, although it inactivates the enzymes and reduces bacterial activity. Freezing is of two types: quick freezing and slow freezing. The effect of freezing on bacteria depends upon the speed with which the organisms are frozen. In the quick-freezing of foods, bacteria are unable to grow and enzymes are inactivated; but rapid freezing is much to grow and enzymes are inactivated; but rapid freezing is much less harmful to the microorganisms than slow freezing.
In quick-freezing the ice crystals formed are very small. The destruction of cells is less this prevents the loss of nutrients in the foods. Therefore, a variety of frozen foods can be made available fruits, vegetables, meats, poultry, fish. On the other hand when foods are subjected to slow-freezing, the ice crystals are larger and their shearing action on the substance of the cell is much greater. This principle is of great importance to the food industry where frozen foods are concerned. For example, slow-frozen strawberries would thaw into shapeless mush because of the breakdown of the tissue cells caused by large sharp ice crystals.
It must also be borne in mind that the frozen product is never better than the raw materials from which it was frozen. Foods to be frozen must be carefully selected for quality and maturity and before freezing the, vegetables are blanched to inactivate the oxidative enzymes. Fruits are immersed in sugar syrup or ascorbic acid to prevent darkening.
If frozen foods are kept at room temperature after thawing, micro-organisms multiply rapidly. Once the food has been thawed, it should not be frozen again, to prevent further deterioration.
This process is a combination of partial dehydration and freezing. The vegetables and fruits which are subjected to dehydrofreezing are first heated in order to lose about half their water content, and then frozen. Later when the food is cooked water is added to reconstitute it.
During this process the frozen food is placed under vaccum to remove the water, and it is then packaged in the presence of an inert gas such as nitrogen. The advantage of this method is that the product retains its original volume and shape and rehydrates easily. Products which have been subjected to freeze-drying are coffee, beef, pork, chicken, soups and several food mixtures. Products of freeze-drying have a long shelf life, possess a light weight and can be stored and refrigerated, but they are considered inferior to frozen or canned foods.
5) Preservation of foods by irradiation
In this process gamma rays or high speed electrons are used to destroy micro-organisms. These radiations are termed as ionizing radiations. The unit of radiation absorbed is the rad, which is the energy absorption of 100 ergs per gram. One million rad will increase the temperature of a food by about 2°C. This process is termed as cold sterilization.
Low level irradiation presents the germination in potatoes, onions and carrots. Metabolic processes can be controlled by this treatment in foods thus delaying their ripening. Pork and beef can be made free of tapeworms and trichenella by irradiation. Heavy losses in the granary due to insect infestation can be checked by irradiation.
In spite of so many apparent advantages of irradiation over the conventional methods, its application to foods for human consumption needs several clearances regarding wholesomeness.
Irradiation does not render the food radioactive but such foods are not for sale in the market, nor have they been approved by the Food and Drug Administrations.
6) Antibiotics in Food Preservation
Very often antibiotics are used in the feed of animals or in treating them. Sometimes if cows and poultry are fed on antibiotics, residues may be found in milk from such cows which have been treated for mastitis. The Food and Drug Administration, specifies that milk from such cows should not be used for human consumption for at least three days following the treatment of the animal with the antibiotic.
Crops are sometimes sprayed with antibiotics. Since no residues of antibiotics are present in the foods eaten, there is no hazard to the health of the consumer.
When fish and poultry are treated with antibiotics, their shelf life increases two to three times, because of the reduced growth of the micro-organisms. Chlorotetracycline (Aureomycin) and Oxytetracycline (Terramycin) have been used in the cooling water for dressed poultry. Similarly, antibiotics are added in the crushed tee used for packing raw fish and shell fish. The cooking of fish and poultry destroys the antibiotic residues, rendering them harmless for human consumption.