milk, liquid secreted by the mammary glands of female mammals as food for their young. The milk of the cow is most widely used by humans, but the milk of the mare, goat, ewe, buffalo, camel, ass, zebra, reindeer, llama, and yak is also used. The composition of milk varies with the species, breed, feed, and condition of the animal. Jersey and Guernsey cows produce milk of high butterfat content; Holsteins produce larger quantities of milk but with a lower butterfat content.
Milk prepared for sale is often homogenized; in this process it is pumped under pressure through small openings to break up the milk-fat globules, thus ensuring an equal distribution of fat throughout the milk rather than permitting it to rise to the top as cream. In most countries where milk is a commercial product, it is subject to regulations concerning its composition (i.e., the proportion of butterfat and other solids) and its purity, with sanitary measures in force that cover milk handlers, herds, plants, and equipment. Pasteurization (partial sterilization by heating) checks bacterial growth, thereby making milk safer to drink and increasing its keeping qualities and range of transport.
Milk, an almost complete food, consists of proteins (mainly casein), fat, salts, and milk sugar, or lactose, as well as vitamins A, C, D, certain B vitamins, and lesser amounts of others. (Many people are unable to digest milk after childhood because they stop producing an enzyme needed to break down lactose, but usually they still can digest yogurt, hard cheeses, and lactose-reduced milk products.) Commercial dairies often supplement natural vitamin D with a concentrate. Milk is a major source of calcium and a good source of phosphorus. Low-fat and skim milk fortified with vitamins A and D have the same nutritional value as whole milk, but with fewer calories and less cholesterol. Whole milk has 3.5% milkfat, low-fat milk 1% to 2%, and skim, 0.5%. Heavy cream has a minimum of 36% milkfat, half-and-half not less than 10.5% nor more than 18%.
A patent was issued for the production of dried milk in Great Britain in 1855, and for concentrated milk in the United States to Gail Borden in 1856. The two types of concentrated milk are condensed and evaporated; condensed milk is a sweetened product (over 40% sugar), and evaporated is unsweetened. Dried, or powdered, milk is made by passing a film of partially evaporated milk over a heated drum or by spraying it into a heated chamber in which the particles dry. Malted milk is a dried mixture made of milk and the liquid from a mash of barley malt and wheat flour.
See S. K. Kon, Milk and Milk Products in Human Nutrition (1972); T. Quinn, Dairy Farm Management (1980); D. Carrick, Milk (1985).