Milk is a white liquid food secreted by female mammals from the mammary glands. It is the sole source of food for the young of most mammals at the start of life. Milk is a complete food in that it has most of the nutrients necessary for life: protein (see casein), carbohydrate, fat, minerals, and vitamins.
The composition of milk varies very much from mammal to mammal. Cows' milk contains nearly all the essential nutrients but is comparatively deficient in vitamin C and vitamin D. Human milk contains more sugar (lactose) and less protein that cows' milk. In any species the milky produced is a complete food for the young of that species until weaning.
Man has used the milk of other animals as food for at least 5,000 years. Milk for use by man is produced in the largest volume by cows and water buffalo (especially in India); goat milk is also produced in some areas, particularly the Middle East.
Milk is an extremely perishable liquid which must be cooled to 10°C within two hours of milking and maintained at that temperature until delivery. The storage life of milk is greatly improved by pasteurization. Because of the perishable nature of milk large quantities are processed to give a variety of products including butter, cheese, cream, evaporated, condensed, and dried milk, yoghurt, casein, and lactose.