The composition of milk varies greatly among different mammals. Human milk, for example, is thin and high in lactose, its primary sugar. Cow's milk, in contrast, is lower in sugar and higher in fat. The milk of some mammals, particularly cows, goats, water buffalo, horse and sheep, is collected for human consumption, either directly, usually after pasteurization, or is processed into dairy products such as cream, butter, yoghurt, ice cream, or cheese.
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° and 4° Celsius. 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.
There is some controversy over whether consumption of cow's milk is good for adult humans, although it is generally recognized as beneficial for children. While milk is often touted for its significant amount of calcium, required for healthy bone growth and nerve function, there is some research to suggest that proteins in milk interfere with the use of its calcium to form bones.