Milk

Milk is a liquid food that nourishes young mammals. Humans also obtain milk from domesticated animals, including cattle, goats, sheep, water buffalo and yaks. Its composition varies between animals and with factors such as feeding, breed and season.

Composition and structure

Cow's milk is approximately 87 per cent water. Its other components include milk fat, proteins, the sugar lactose, minerals and vitamins. These substances are arranged in different ways: fat forms droplets dispersed through water, casein proteins form suspended particles called micelles, and lactose is dissolved. Milk therefore combines the properties of an emulsion, a colloid and a solution.

The arrangement of these components influences milk's behaviour during processing and storage. Heating and homogenisation alter the size or organisation of fat droplets and protein particles. Microscopy reveals changes underlying separation of cream, sediment formation and gel formation in processed milk. In condensed milk, the growth of lactose crystals can produce a gritty texture.

Milk also provides the basis for dairy products such as yoghurt, cheese, butter, ice cream and powdered milk. Their differing structures help determine texture, melting behaviour and stability.

Heat treatment

Pasteurisation uses heat to reduce harmful microorganisms in milk. Its application followed Louis Pasteur's work on preserving wine; the agricultural chemist Franz von Soxhlet proposed applying the process to bottled milk in 1886. Adoption involved disputes over cost, flavour and the relative merits of heating and stricter hygiene. Chicago introduced compulsory milk pasteurisation in 1909, and New York enforced its ordinance in 1914.

Heating does not remove the need for hygienic production and handling. Pooling milk from several farms increased the importance of controlling contamination as distribution expanded. Heat treatment also changes flavour, particularly when milk is heated to higher temperatures.

Human use and adaptation

Archaeological evidence shows that milk use preceded widespread adult lactase persistence in Europe. A 2022 study, reported by Smithsonian Magazine, compared fats preserved in pottery with ancient human DNA. Its evidence traced European milk use across about 9,000 years, while the genetic variant enabling continued adult digestion of lactose appeared much later. The researchers found no simple correspondence between increasing milk use and the spread of that variant.

Their models suggested that famine and disease could have intensified selection for lactase persistence. This explanation concerns a particular European historical investigation; the same report notes that the trait evolved on several occasions in different regions.

Milk's place in the diet has also been shaped by distribution systems and public attitudes. In the United States, closed containers, refrigerated rail transport and professional dairies changed urban supplies during the nineteenth century. Twentieth-century industry and government campaigns increasingly promoted milk through claims about growth and health. Historical scholarship reviewed by Catherine Long describes these changing messages as part of milk's cultural image, rather than an unchanging nutritional consensus. American consumption of fluid milk peaked in the mid-1940s and later declined despite successive promotional campaigns.

Attribution

Adapted from English Wikipedia's Milk, revision 1379624351 (contributors and history), under CC BY-SA 4.0. The prose has been rewritten and condensed, and the references independently checked.