Processing of milk:
There are three operations of milk processing and they are clarification, pasteurization and homogenisation.
Processing of milk:
- Clarification: Milk is passed through a centrifugal clarifier. The speed is adjusted in such a way that cream is not separated but dirt, filth and cells and some bacteria are removed. The clarified milk is ready for pasteurization.
- Pasteurization: Pasteurization derives at name from the French scientist Louis Pasteur, who found that heating of sodium liquid to high temperature improve their keeping quality. In general terms, it is the heating of milk to a temperature which destroys organisms responsible for tuberculosis and fever and nearly all the other microorganisms present in that product without seriously affecting the composition or properties of the product. Pasteurization should be followed by immediate cooling of the product to the temperature sufficiently low to check the growth of microorganism which are resistant to the temperature used. At present, pasteurization is considered as an essential feature in the manufacture of butter, ice cream and in cheese industries. Pasteurization also inactivates some of the natural enzymes like lipase. Three general methods are in use nowadays
(a)Holding or batch system: The holding system consists in bringing the milk or cream to a temperature usually 65֯ Celsius and holding at that point for at least 30 minutes followed rapid cooling. A higher temperature is sometimes used in which the time of holding may be shortened. For example, holding at 68.3֯ Celsius for 20 minutes. Careful control of both temperature and time of heating is important.
(b) High temperature short time method or the continuous system: The machines used are so constructed as to make possible containers for this reason the system is past continuous flow or flash pass riser. This depends upon the raising of the temperature of the milk at least 72֯ Celsius for 15 seconds as it passes through the machine. This is followed by quick cooling.
(c)Ultra-high temperature system: This Ultra-high temperature system results in a complete pasteurization of milk. In this system, milk is held for three seconds at 93.4֯ Celsius or for one second at 149.5֯ Celsius. The system is also used extensively for the treatment of milk or cream in dairy industry is. This product has a longer shelf life than milk pasteurised by other methods. After pasteurization, the milk is cold rapidly to 7 degrees Celsius or lower.
- Effect of pasteurization:
- Nutritive value:
Due to pasteurization, there is no important change in nutritive value, except for the slide decrease in the heat liable vitamins like thiamine and ascorbic acid.
- Flavour:
Pasteurization doesn’t produce objectional cooked flavour. the flavouring compounds present in milk are in no way destroyed by pasteurization. - Microorganisms:
Pasteurised products are not sterile. They contain vegetative organisms and sports still capable of growth. Pathogenic bacteria are destroyed but non-pathogenic remains in the pasteurised milk.
- Enzymes:
Apart from phosphatase, lipase also gets inactivated which would have affected adversely the quality of the milk. Pasteurised milk can be stored for a week or more in good condition. At room temperature it may be spoiled in a day.
- Homogenisation: The process of making a stable emulsion of milk fat and milk serum by mechanical treatment and rendering the mixture homogeneous is called homogenisation. This is achieved by passing warm milk or cream through a small aperture under high pressure.