milk, liquidsecreted by themammary glands of femalemammals to nourish their young for a period beginning immediately after birth. The milk ofdomesticated animals is also an importantfood source forhumans, either as a fresh fluid or processed into a number ofdairy products such asbutter andcheese.

A brief treatment of milk follows. For full treatment,seedairy product.

Almost all the milk now consumed in Western countries is from thecow, and milk and milk products have become important articles of commerce. Other important sources of milk are thesheep andgoat, which are especially important in southernEurope and the Mediterranean area; thewater buffalo, which is widely domesticated in Asia; and thecamel, which is important in theMiddle East andNorth Africa.

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Milk is essentially anemulsion offat andprotein inwater, along with dissolvedsugar (carbohydrate), minerals, and vitamins. Theseconstituents are present in the milk of all mammals, though their proportions differ from one species to another and within species. (SeeTable.) The milk of each species seems to be a complete food for its own young for a considerable time after birth. In the stomachs of the young, milk is converted to a soft curd that encloses globules of fat, enablingdigestion to proceed smoothly without the disturbance often caused by fatty food.Lactose, or milk sugar, is broken down into simpler digestible sugars by theenzymelactase, which is produced in the intestine of infants. Infants who do not produce lactase developlactose intolerance, a condition in which a variety of gastrointestinal problems arise. Lactose intolerance also commonly develops after weaning or with advancing age, when many individuals cease producing lactase.

Milkprotein is of high nutritional value because it contains all the essentialamino acids—i.e., those which infants cannot synthesize in the necessary quantities. Milk’s mineral content includescalcium andphosphorus in quantities sufficient for normal skeletal development, but littleiron. Milk contains B vitamins as well as small amounts of vitamins C and D. Commercialcow’s milk is commonly enriched with vitamins D and A before sale.

Microorganisms contained in raw (unheated) milk or picked up from theenvironment will quickly sour and curdle the milk. Cooling to slightly above itsfreezing point keeps milkpalatable for a longer time by reducing the multiplication of spoilagebacteria and the chemical changes that they induce.

Many countries have laws requiring that milk be pasteurized as a protection against pathogenic (disease-causing) organisms.Pasteurization is a partial sterilization accomplished by raising the milk to a temperature high enough to destroy pathogenic bacteria and a large proportion of those causing spoilage. Pasteurized milk that is kept refrigerated in closed containers will remain consumable for approximately 14 days.

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Milk fat, being less dense than other milk components, can be efficiently removed in acream separator bycentrifugation, yieldinglow-fat milk andskim milk. Low-fat milk contains 1–2 percent fat, whileskim milk contains less than 0.5 percent fat.

Much of the milk sold as a beverage has undergonehomogenization, a process in which the milk is forced under high pressure through small openings to distribute the fat evenly throughout the milk.