Methods of Milk Preservation

Preservation of Milk:

Milk preservation may be defined as the process of keeping milk safe as drinking food for a definite period of time free from spoilage. Milk has to be screened and chilled immediately after milking. Milk preservation system is not so easy. Because, the chemical structure of milk is easily changed. Generally, raw milk is sold everywhere in Bangladesh. If milk is kept for long time in raw condition, milk quality is deteriorated. In normal temperature (room temperature), various microorganisms in milk cause sour taste by producing lactic acid. Streptococci, a microorganism, mainly produce acid in milk. In normal temperature, microorganisms cause deterioration of milk by their multiplication. 

Method of Milk Preservation

A) Old method for milk preservation:

Milk preservation by boiling: This method is most easy in household level. If boiling once, it will keep milk fresh up to 4 hours. So, if boiling milk for 20 minutes in every 4 hours, all kinds of disease causing organisms are destroyed, although their nutritional value may be decreased. Because, some vitamins and amino acids are destroyed by high temperature.

B) Modern method of milk preservation:

1. Milk can be preserved by keeping milk in refrigerator for only 40 seconds.
2. Milk can be preserved in deep freezer. Although micro-organisms are not multiplied here, but they break the chemical bond of milk. So, quality of milk is slightly decreased.

Milk is one of the standard foods in the world. It is a standard food for calf as well as human being and at the same time it is similar standard for micro-organisms. After milking, milk quality is starting to be deteriorated by time and it is fully deteriorated if it is kept for long time in normal temperature. The deterioration may be caused mainly for microorganisms. The microorganisms cannot grow and multiply in high and low temperature. Using this temperature, disease causing microorganisms could be controlled and the process is called pasteurization. The aim of milk pasteurization is to destroy disease causing microorganisms. To preserve milk long time, undesirable organisms should be destroyed and enzyme presented in milk should be neutralized.

Louis Pasteur (1860-1865), a French Chemist first realized that deterioration process had been caused by one kind of microorganism. Although pasteurization process had been invented by Louis Pasteur, but it was first applied by a German scientist, Dr. Soxhlet. Pasteurization is the process of heating the every particle of milk at 145�F (62.8�C) for 30 minutes or 162�F (72.2�C) for 15 seconds to destroy disease causing microorganisms and enzymes presented in milk. Milk has to be cooled down at 4�C immediately after pasteurization.

Advantages of Pasteurization

1. Pasteurized milk is safe, because all disease causing microorganisms are destroyed.
2. Pasteurization increases the storage life of milk because it decreases lactic acid producing micro-organisms.
3. Enzymes in milk are destroyed by pasteurization and that�s why milk can be safely stored long time.
4. Most of the pathogenic (harmful) bacteria are destroyed by pasteurization.
5. Pasteurization does not alter nutritional value and taste of milk.

Disadvantages of Pasteurization:

1. Fat globules in milk may be emulsified by heavy stirring if pasteurization is not performed properly.
2. Heat sensitive vitamins may be deteriorated.
3. The taste of milk may be decreased by high temperature.

Types of Pasteurization:

1. Pasteurization with low temperature and long holding time: 62.80�C temperature for 30 minutes.
2. Pasteurization with high temperature and short holding time: 72.20�C temperature for 15 seconds.
3. Pasteurization with high pressure: 137.80�C temperature for 2 seconds.