Normally if fresh milk is not boiled for sometime it becomes sour. But this does not happen with boiled milk for several hours. Do you know why it is so?
Fresh milk contains several types of bacteria. When milk comes in contact with air, the number of the bacteria multiplies very fast. These bacteria turn the milk sour. The bacteria come to the milk from three different sources. First, if the cow or the buffalo from which the milk comes is suffering from some disease it might be excreting bacteria in its milk. The tuberculosis germs are transmitted from cattle to man in this way. Secondly, the milkman could have certain infectious disease and might contaminate the milk while milking the cow. Thirdly, the water used to wash the milk pot or the teats of the cow or buffalo may have germs in it. The bacteria transmitted to the milk from any of these three sources grow very fast and spoil the milk.
Pasteurization is a process for sterilizing milk and other drinks invented by Louis Pasteur (1822-1895) – a French micro-biologist and chemist, for improving storage qualities and to protect them from spoiling. This method is named after him. The milk and butter sold in the market are normally first pasteurized, and then marketed.
In the process of pasteurization, milk, wine, butter etc. are heated up to a certain temperature and then quickly chilled. This kills the bacteria in them. If milk is heated up to 63° for 30 minutes in between 72°- 85° for 16 seconds and then chilled to 10° or less, the bacteria present in it are destroyed. Pasteurization not only protects the milk from being spoiled but also kills the bacteria of tuberculosis and other diseases. Pasteurized milk can be kept unspoiled for a longer time without boiling. The process does not affect the taste of the milk.
Nowadays gamma rays and beta rays are also being used for pasteurization. These rays kill the bacteria present in the milk and other drinks.