Milk can be preserved from spoilage only in conditions that impede the vital activity of microorganisms. Milk is very active lactic acid bacteria, splitting milk sugar and forming lactic acid. Under the influence of lactic acid, the protein in milk coagulates, turning it into curdled milk. To protect milk from souring, it is necessary to create conditions unfavorable for the activity of lactic acid bacteria.
Sterile milk is excreted from the mammary glands of the animals, but during the milking and storage, various microorganisms enter the environment from the environment. The temperature of fresh milk is optimal for the development of microorganisms.
The activity of bacteria can be temporarily suppressed if the freshly milked milk is sharply cooled to 8 degrees and stored at this temperature. However, transporting and storing milk at this temperature, especially in summer, is difficult, therefore lactic acid bacteria are destroyed by heating.
There are a number of ways to heat up. When pasteurized milk it is heated to 63-85 degrees. Under the influence of this temperature, the vegetative forms of bacteria die, and the composition of milk changes insignificantly, but the enzymes lose their activity. Pasteurized milk, if stored in a sealed, tight container, does not get sour, but after a while it becomes bitter, as spores of putrid bacteria do not die during sterilization, they develop and decompose proteins contained in milk. To use such milk in food is not recommended. Keep the pasteurized milk only in the refrigerator, the shelf life is usually 3-5 days.
When sterilized, the milk is heated in autoclaves to a temperature of 100 degrees. Under the influence of this temperature, spores of microorganisms also perish, the protein substances of milk change greatly, some of the protein coagulates. Sterilized milk in closed tight containers can be stored for a fairly long time.
For long-term storage, the milk is condensed with sugar and dried in powder, thus obtaining, respectively, condensed and dried milk. They can be stored for a long time at room temperature.
Milk easily absorbs foreign smells and tastes, so it should be stored in clean dishes. Milk can not be kept in close proximity to herring, kerosene, sour cabbage and other sharply smelling substances.
The best milk storage can be aluminum or glass.