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,
yogurt, ice cream, or cheese. When used alone, milk
almost always refers to cow's milk.
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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.