Milk
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.
A rarer use is to refer to plant-based ‘milks’, usually soy milk or rice milk; unusual variations are almond milk [usually heavily sweetened and dessert-like] and banana milk. These can either be made at home or bought commercially; they rarely include any animal milk or by-products, and so can be eaten by those who can’t digest lactose, as well as substituted for cow’s milk in recipes. These ‘milks’ should be treated much as cow’s milk - that is, they should be refrigerated, especially after opening, and used within a few days.
When raw milk is left standing for a while, it turns sour. This is the result of fermentation: lactic acid bacteria turn the milk sugar into lactic acid. This fermentation process is exploited in the production of various dairy products.
Pasteurized cow’s milk will also turn sour if kept unrefrigerated, and should be stored between 1 °C and 4 °C. The souring of milk can be forestalled by using UHT (ultra-high temperature) treatment; milk so treated can be stored unrefrigerated for several months until opened.
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 in adulthood, in many cases to the point where lactose becomes indigestible, leading to lactose intolerance, a gastrointestinal condition that afflicts many.