The milk of some mammals, particularly cows, goats, sheep, and buffalo, is collected for human consumption, either directly, usually after pasteurization, or is processed into dairy products such as cream, butter, yogurt, ice cream, or cheese. When used alone, milk almost always refers to cow's milk.
Another use is to refer to plant-based milks, usually soy milk or rice milk; typical variations are almond milk, oat milk, hemp milk, and banana milk. These can either be made at home or bought commercially; they rarely include any animal milk or animal milk by-products, and so can be eaten by those who can't digest lactose or casein. They can be substituted for animal milk in recipes in some situations, although they may impact the flavour or texture of the resulting dish. These milks should be treated much as animal milk - that is, they should be refrigerated, especially after opening, and used within a few days.
When raw animal milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria naturally present in raw animal milk turn the animal milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products.
Pasteurized animal milk does not have these bacteria, so instead of souring it will putrefy if kept unrefrigerated, and should be stored between 1 °C and 4 °C. The putrefaction of animal milk can be forestalled by using ultra-high temperature (UHT, also known as ultra-pasteurization) treatment; animal milk so treated can be stored unrefrigerated for several months until opened.
Lactose in animal 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.
Casein is the main protein in animal milk, and is particularly concentrated in cheese made with animal milk. Whey is the other major protein in animal milk, and is separated from casein in the manufacturing process of cheese made with animal milk. Both proteins can provoke undesirable symptoms in some people, although casein intolerance is more widely recognised and is often connected with gluten intolerance and coeliac disease.
Milk possesses the ability to reduce the perceived spiciness level of some spicy foods thanks to the casein which binds with capsaicin oil and then carries it away.