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's milk or animal's milk by-products, and so can be eaten by those who can't digest lactose or casein, as well as substituted for animal's milk in recipes. These milks should be treated much as animal's milk - that is, they should be refrigerated, especially after opening, and used within a few days.
When raw animal's milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria naturally present in raw animal's milk turn the animal's milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products.
Pasteurized animal's 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's milk can be forestalled by using UHT (ultra-high temperature) treatment; animal's milk so treated can be stored unrefrigerated for several months until opened.
Lactose in animal's 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.
The proteins in animal's milk can also cause intolerance. Casein is the main protein in animal's milk, and is particularly concentrated in cheese made with animal's milk. Whey is the other major protein in animal's milk, and is separated from casein in the manufacturing process of cheese made with animal's 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. There is some evidence that both casein and gluten contribute to some neurological disorders, thus requiring affected people to adhere to a gluten-free, casein-free diet.
Milks possess the curious ability to reduce the heat from spicy foods, occasionally exploited in chocolates or ice creams with spicy additions.