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 is also processed into dairy products such as cream, butter, yoghurt, ice-cream, gelato, cheese, casein, whey protein, lactose, dried milk, and many other food-additive and industrial products.
It can also be used to mean
Human milk is often fed to infants through breastfeeding, either directly or by the female expressing her milk to be saved and fed later. As colostrum, it carries the mother's antibodies and intestinal bacteria to the baby.
Composition and nutrition
The composition of milk varies greatly among different mammals.
- Human breast milk is thin and high in lactose, its primary sugar.
- Cow's milk, in contrast, is lower in sugar and higher in protein, and is composed of about 3.5% to 6.5% milkfat, 4% to 8.5% milk solids and about 88% water. Its main protein (80%) is casein, while whey proteins make up most of the rest.
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 towards adulthood (depending on the person's ethnic origin), 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 disputed research to suggest that proteins in milk interfere with the use of its calcium to form bones by increasing the acidity level of the blood and triggering a response which balances that acidity level by leeching calcium that is presently in bones. However breeds of cattle produce milk that is significantly different from that of others as do different mammals' from others. Such factors as the lactose content, the proportion of and size of the butterfat globule and the strength of the curd, formed by the human enzymes digesting the milk, can differ from breed to breed and mammal to mammal.
Milk has also been linked in a small number of studies to osteoporosis, cancer, heart disease, obesity and high blood pressure. Because of milk's high protein content, and inability of the body to digest it, it is unable to fully absorb the calcium. In some countries where dairy is rarely used, such as China, these diseases are rare, although it is unclear whether dairy consumption is a cause.
Cow's milk is produced on an industrial scale for human consumption.
Varieties and brands
Cow's milk is generally available in several varieties. In some countries these are:
- full cream (or "whole" in North America)
- semi-skimmed ("reduced fat" or "low fat", about 1.5-1.8% fat)
- skimmed (about 0.1% fat)
Milk in the U.S. and Canada is sold as
- "whole" varieties
- "2 percent" (reduced fat)
- "1 percent" (low fat)
- "1/2 percent" (low fat)
- "skim" (very low fat)
Full cream, or whole milk, has the full milk fat content (about 3-4% if Friesian- or Holstein-breed are the source). For skimmed or semi-skimmed milk, all of the fat content is removed and then some (in the case of semi-skimmed milk) is returned.
The best-selling variety of milk is semi-skimmed; in some countries full-cream (whole) milk is generally seen as less healthy and skimmed milk is often thought to lack taste.
Whole milk is recommended to provide sufficient fat for developing toddlers who have graduated from breast milk or infant formula.
There are many brands of milk currently, most milk brands vary little from each other. These brands include:
Milk is the state drink of Minnesota, Pennsylvania,Maryland and South Carolina.
Other milk animals
In addition to cows, the following animals provide milk for dairy products:
In Russia and Sweden, small moose dairies exist . Donkey and horse milk have the lowest fat content, while the milk of seals contains more than 50% fat. 
When raw milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria turning the milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products such as cheese and yogurt.
Pasteurized cow's milk, on the other hand, spoils in a way that makes it unsuitable for consumption, causing it to assume a disgusting odor, which alone may induce vomiting in sensitive persons, and pose a high danger of food poisoning if ingested. The naturally-occurring lactic acid bacteria in raw milk, under suitable conditions, quickly produce large amounts of lactic acid. The ensuing acidity in turn prevents other germs from growing, or slows their growth significantly. Through pasteurization, however, these lactic acid bacteria are mostly destroyed, which means that other germs can grow unfettered and thus cause decomposition.
In order to prevent spoilage, milk can be kept refrigerated and stored between 1 and 4 degrees Celsius. Most milk is Pasteurized by heating briefly and then refrigerated to allow transport from Factory Farms to local markets. The spoilage of milk can be forestalled by using ultra-high temperature (UHT) treatment; milk so treated can be stored unrefrigerated for several months until opened. Sterilized milk, which is heated for a much longer period of time, will last even longer, but also lose more nutrients and assume a still different taste. Condensed Milk, made by removing most of the water, can be stored for many months, unrefrigerated. The most durable form of milk is milk powder which is produced from milk by removing almost all water.
UHT milk is very popular in Europe, whereas in North America, most of the milk sold is pasteurized.
Prior to the widespread use of plastics, milk was usually commercially distributed to consumers in glass bottles. In the UK, milk can be delivered daily by a milk man who travels his local milk round (route) on an electric milk float, although this is becoming less popular as a result of supermarkets selling milk at cheaper prices. In New Zealand in some urban areas milk is still delivered to customers' homes.
Glass containers are rare these days. Most people purchase milk in plastic jugs or bags or in waxed-paper cartons. Ultraviolet light from fluorescent lighting can destroy some of the proteins in milk, so many companies that once distributed milk in transparent or highly translucent vessels are starting to use thicker materials that block the harmful rays. Many people feel that such "UV protected" milk tastes better. (However, few people have ever tasted fresh, unprocessed, milk straight from the cow.)
In the United States, milk is commonly sold in gallon, half-gallon and quart containers (U.S. customary units) of rigid plastic or waxed cardboard. The U.S. single serving size is usually the half-pint. In much of Canada, a 1 1/3 litre plastic bag (sold as 4 litres in 3 bags) is the most common, while 2 litre, 1 litre and 500 millilitre cartons are also available. In Europe, metric sizes of 500 millilitres, 1 litre (the most common), 2 litres and 3 litres are commonplace (in the UK, some stores instead still stock the equivalents of old Imperial sizes: 568 ml (1 pint), 1.136 l (2 pints), 2.273 l (4 pints)).
Condensed milk is distributed in metal cans and powdered milk is distributed in boxes or bags.
Many people concerned about animal welfare (especially vegans) do not drink milk. An increasing number of dairy cows are being raised on factory farms, which some people consider cruel. On many farms, the calves are separated from their mothers within days of birth to prevent the calf from drinking the milk so that humans can drink it instead. Some of the calves born by dairy cows are raised in crates for veal and are slaughtered three to eighteen weeks later. On many farms, once a dairy cow's milk production decreases, she is also slaughtered at an age that is a fraction of her natural lifespan. Some people also believe that the use of bovine growth hormone to increase milk production in cows is unethical. For these reasons, either in an attempt to reduce animal suffering or to prevent animals from being killed, some people choose to not consume milk. Some also object to drinking milk for environmental reasons.