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Milk most often means the nutrient fluid produced by the mammary glands of female mammals. It provides the primary source of nutrition for newborns, before they are able to digest more diverse foods. It can also be used to mean the white juice of a coconut. Coconut milk lacks the rich nutritional values of animal milk but is often used in place of animal milk in cookery, especially in varieties of Thai curry.

The composition of milk varies greatly among different mammals. Human milk, for example, is thin and high in lactose, its primary sugar. Cow's milk, in contrast, is lower in sugar and higher in fat. Cow's milk is composed of about 3.5% milkfat, 8.5% milk solids and 88% water. Its main protein (80%) is casein. The milk of some mammals, particularly cows, goats, water buffalo, horse and sheep, is collected for human consumption, either directly, usually after pasteurization, or is processed into dairy products such as cream, butter, yoghurt, ice cream, gelato, or cheese. In Russia and Sweden, small moose dairies exist[1].

When raw milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria turn the milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products.

Pasteurized cow's milk will also turn sour if kept unrefrigerated, and should be stored between 1° and 4° Celsius. The souring of milk can be forestalled by using ultra-high temperature (UHT) treatment; milk so treated can be stored unrefrigerated for several months until opened.

Prior to the widespread use of plastics, milk was usually commercially distributed to consumers in glass bottles. Glass is rare these days, and most people purchase milk in plastic jugs or bags, or in waxed paper cartons. Ultraviolet light emanating from fluorescent lighting can destroy some of the proteins in milk, so many companies that once distributed milk in transparent or highly transluscent vessels are now starting to use thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better.

Lactose in milk is digested with the help of the enzyme lactase produced by the bodies of infants. In humans, production of lactase falls off in adulthood, in many cases to the point where lactose becomes indigestible, leading to lactose intolerance, a gastrointestinal condition that afflicts many.

There is some controversy over whether consumption of cow's milk is good for adult humans. While milk is often touted as healthy for its significant amount of calcium, required for healthy bone growth and nerve function, there is some research to suggest that proteins in milk interfere with the use of its calcium to form bones. Milk has also been linked in a small number of studies to osteoporosis, cancer, heart disease, obesity, and high blood pressure.

Cow's milk is generally available in three varieties: full cream (or "whole" in North America), semi-skimmed ("reduced fat" or "low fat"), and skimmed. Full cream milk has the full milk fat content (about 4%). Semi-skimmed milk has about half the milk fat removed (1.5-1.8% fat content). Skimmed milk has almost all the milk fat removed (about 0.1% fat content). Milk in the U.S. is sold in skim, 1/2 per cent (low fat), 1 per cent (low fat), 2 per cent (reduced fat), and whole varieties. The best selling variety of milk is semi-skimmed; full cream is generally seen as less healthy and skimmed milk is often thought to lack taste.

Milk is the state drink of Minnesota.

Famous brands of milk

See also

For the American politician, see Harvey Milk.