We often store food so that we can eat it later. However if food is kept for a long time before it is eaten, it may start to "go off". This may be because it is attacked by its own enzymes, oxidized by the air or decomposed by bacteria and fungi. Therefore food preservation is very important.
There are three main methods of preserving food. These are pasteurization and sterilization, refrigeration and freezing and drying. Heating foods and drinks that are sealed in tins or cartons, kills off organisms like bacteria. This process is called sterilization or pasteurization. For example, milk is heated to 72 ÃÂºC for fifteen seconds and then cooled rapidly. However this does not sterilize the milk. Sterilization occurs when milk is heated under pressure to 132]C for one second and sealed in cartons or plastic bottles.
The second method is refrigeration and freezing, that is, cooling the food or drink by placing it in the refrigerator.
This process slows down the activity of any enzymes present in the food and it also stops the growth and reproduction of bacteria in the food. The third method is drying, that is, the preservation of food by removing the water in them. This process stops the enzymes from working and prevents the growth of bacteria and fungi. The food is usually dried under low air pressure. This makes the water evaporates without raising the the temperature at which the food would start to cook. The dehydrated food is then sealed in packets and kept at room temperature until it is reconstituted by adding water. For example, milk is dried, to produce milk powder. Then, by adding water makes them ready to drink.
There are many other methods use for preserving food, for example, canning, salting, syrup, pickling and irradiation.