Prof. (Dr.) Dharmakanta Kumbhakar
Milk is the best and most complete of all foods. It is secreted by the animals to serve as sole and wholesome food for their suckling young ones. It is a fine blend of all the nutrients necessary for growth and development of the young ones. The chief protein of milk is casein; it occurs in combination with calcium as calcium caseinogenate. The other proteins are lactalbumin and lactoglobulin. Animal milks contain nearly three times as much protein as human milk. Milk proteins contain all the essential amino acids. Human milk proteins contain greater amounts of tryptophan and sulphur-containing amino acids (especially cystein) than animal milk proteins. The fat content of milk varies from 3.4 percent in human milk to 8.8 percent in buffalo milk. Human milk contains a higher percentage of linoleic acid and oleic acid than animal milks. Milk fat is a good source of retinol and vitamin D. The carbohydrate in all milks is lactose or milksugar. It is found nowhere else in nature. It is less sweet than cane sugar and is readily fermented by lactic acid bacilli. Human milk contains more sugar than animal milk. Milk contains almost all known minerals needed by the body such as calcium, phosphorus, sodium, potassium, magnesium, cobalt, copper, iodine etc. Milk is particularly rich in calcium; it is however a poor source of iron. Milk is a good source of all vitamins except vitamin C. Thus milk is a good source of proteins, fats, carbohydrates, vitamins and minerals.
Though milk is the best and most complete of all foods, but it must be clean and safe. Unsafe milk is an efficient vehicle for a great variety of disease agents. The source of infection or contamination of milk may be the dairy animal, human handler or the environment (e.g. contaminated vessels, polluted water, flies, dust etc.). The infections of dairy animals that can be transmitted to humans through milk are: tuberculosis, brucellosis, streptococcal infections, staphylococcal enterotoxin poisoning, salmonellosis, Q fever, cow-pox, foot and mouth disease, anthrax, leptospirosis and tick-born encephalitis. The primary human infections that can be transmitted through milk are: typhoid and paratyphoid fever, shigellosis, cholera, enteropathogenic escherichia coli and nondiarrhoeal disease like streptococcal infections, staphylococcal food poisoning, diphtheria, tuberculosis, enterovirus, viral hepatitis.
The safety and the quality of milk are related to its microbial content. Methylene Blue Reduction Test is an indirect method for detection of microorganisms in milk. It is based on the observation that bacteriae growing in milk brings about a decrease in the colour imparted to milk. In conducting the test, definite quantities of methylene blue are added to 10 ml of milk and the sample is held at a uniform temperature of 37 deg. C. until the blue colour has disappeared. The milk, which remains blue the longest, is considered to be of the quality, and a scale of grading different milk samples, on the basis of the time to require reducing a definite quantity of Methylene Blue, has been worked out. The test thus serves a confirmation of heavy contamination and is compared with direct counts of bacteria.
The first essential in the production of clean and safe milk, therefore, is a healthy and clean diary animal. Milk from a healthy udder contains only a few organisms, and these are relatively unimportant. Secondly, the premises where the animal is housed and milked should be sanitary. The milk vessels must be sterile and kept covered. The water supply must be bacteriologically safe. Milkhandler must be free from communicable diseases, and before milking they must wash their hand and arms. Where possible, milking machines must be used. Milk should be cooled immediately to below 10 degrees centigrade after it is drawn, to retard bacterial growth. In the production of good quality milk, cleanliness of all containers and equipment in which milk is handled is very important.
Pasteurization is a preventive measure of public health importance and corresponds in all respects to the modern principles of supplying safe milk. Pasteurization may be defined as the heating of milk to such temperatures and for such periods of time, as are required, to destroy any pathogens that may be present, while causing minimal changes in the composition, flavour and nutritive value. There are several methods of pasteurization. Three are mostly used: (1) Holder (Vat) method: In this process milk is kept at 63–66 degree centigrade for at least 30 minutes, and then quickly cooled to 5 degree centigrade. VAT method is recommended for small and rural communities. (2) HTST method – In this ‚high temperature and short time‘ method milk is rapidly heated to a temperature of nearly 72 degree centigrade, is held at that temperature for not less than 15 seconds, and is then rapidly cooled to 4 degree centigrade. This is now the most widely used method, as very large quantities of milk per hour can be pasteurized by this method. (3) UHT-Method: In this ‘ultra-high-temperature’ method milk is rapidly heated, usually in two stages (the second stage usually being under pressure), to 125 degrees centigrade for a few seconds only. It is then rapidly cooled and bottled as quickly as possible. Pasteurization kills nearly 90 percent of the bacteria in the milk, including the more heat-resistant tubercle bacillus and the Q-fever-organisms. But it will not kill thermoduric bacteria nor the bacterial spores.
Therefore, despite pasteurization, with subsequent rise in temperature, the bacteria are bound to multiply. In order to check the growth of micro-organisms, pasteurized milk is rapidly cooled to 4 degrees centigrade. It should be kept cold until it reaches the consumer. Hygienically produced pasteurized milk has a keeping quality of not more than 8 to 12 hours at 18 degrees centigrade. A few tests can be done to check the efficiency of pasteurization and post-pasteurization contamination. Phosphatase test is based on the fact that raw milk contains an enzyme called phosphatase, which is totally destroyed upon heating at high temperature during pasteurization. Consequently, the test is used to detect inadequate pasteurization or the addition of raw milk. The bacteriological quality of pasteurized milk is determined by standard plate count. Most countries in the Western enforce a limit of 30,000 bacterial counts per ml of pasteurized milk. As coliform organisms are usually completely destroyed by pasteurization, and therefore their presence in pasteurized milk is an indication either of improper pasteurization or post-pasteurization contamination. The standard in most countries is that coliforms be absent in 1 ml of milk.