Milk is a white, nutrient-rich liquid food produced in the mammary glands of mammals. It is the primary source of nutrition for infant mammals (including humans who are breastfed) before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to its young and can reduce the risk of many diseases. It contains many other nutrients[ including protein and lactose. Interspecies consumption of milk is not uncommon, particularly among humans, many of whom consume the milk of other mammals.
A 2018 survey by the International Food Information Council Foundation suggests consumers in the United States do not typically confuse plant-based milk analogues with animal milk and dairy products. In the US, (mostly plant-based) milk alternatives now command 13% of the "milk" market, leading the US dairy industry to attempt, multiple times, to sue producers of dairy milk alternatives, to have the name "milk" limited to animal milk, so far without success. The Food and Drug Administration generally supports restricting the term "milk", while the US Department of Agriculture supports the continued use of terms such as "soymilk". In the European Union, words such as milk, butter, cheese, cream and yogurt are legally restricted to animal products, with exceptions such as coconut milk, almond milk, peanut butter, and ice cream.
For humans, the World Health Organization recommends exclusive breastfeeding for six months and breastfeeding in addition to other food for up to two years of age or more. In some cultures it is common to breastfeed children for three to five years, and the period may be longer.
The growth in urban population, coupled with the expansion of the railway network in the mid-19th century, brought about a revolution in milk production and supply. Individual railway firms began transporting milk from rural areas to London from the 1840s and 1850s. Possibly the first such instance was in 1846, when St Thomas's Hospital in Southwark contracted with milk suppliers outside London to ship milk by rail. The Great Western Railway was an early and enthusiastic adopter, and began to transport milk into London from Maidenhead in 1860, despite much criticism. By 1900, the company was transporting over 25 million gallons annually. The milk trade grew slowly through the 1860s, but went through a period of extensive, structural change in the 1870s and 1880s.
In the Western world, cow's milk is produced on an industrial scale and is by far the most commonly consumed form of milk. Commercial dairy farming using automated milking equipment produces the vast majority of milk in developed countries. Dairy cattle such as the Holstein have been bred selectively for increased milk production. About 90% of the dairy cows in the United States and 85% in Great Britain are Holsteins. Other dairy cows in the United States include Ayrshire, Brown Swiss, Guernsey, Jersey and Milking Shorthorn (Dairy Shorthorn).
Increasing affluence in developing countries, as well as increased promotion of milk and milk products, has led to a rise in milk consumption in developing countries in recent years. In turn, the opportunities presented by these growing markets have attracted investments by multinational dairy firms. Nevertheless, in many countries production remains on a small scale and presents significant opportunities for diversification of income sources by small farms. Local milk collection centers, where milk is collected and chilled prior to being transferred to urban dairies, are a good example of where farmers have been able to work on a cooperative basis, particularly in countries such as India.
A 2010 Food and Agriculture Organization report found that the global dairy sector contributes to four percent of the total global anthropogenic GHG emissions. This figure includes emissions allotted to milk production, processing and transportation, and the emissions from fattening and slaughtering dairy cows. The same report found that 52 percent of the GHGs produced by dairy cattle is methane, and nitrous oxide makes up for another 27 percent of dairy cattle's GHG emission. It is estimated that cows produce between 250 and 500 liters of methane a day. Methane has a heat-trapping potential nearly 100 times larger than carbon dioxide, and nitrous oxide has a global warming potential almost 300 times greater than carbon dioxide.
Like composition, fat globules vary in size from less than 0.2 to about 15 micrometers in diameter between different species. Diameter may also vary between animals within a species and at different times within a milking of a single animal. In unhomogenized cow's milk, the fat globules have an average diameter of two to four micrometers and with homogenization, average around 0.4 micrometers. The fat-soluble vitamins A, D, E, and K along with essential fatty acids such as linoleic and linolenic acid are found within the milk fat portion of the milk.
The first theory attributed to de Kruif and Holt, proposes that nanoclusters of calcium phosphate and the phosphopeptide fraction of beta-casein are the centerpiece to micellular structure. Specifically in this view, unstructured proteins organize around the calcium phosphate giving rise to their structure and thus no specific structure is formed.