Milk is an opaque white liquid produced by the mammary glands of mammals, most commonly cows, goats, sheep, water buffalo and yaks. It provides the primary source of nutrition for young mammals before they are able to digest other types of food. The early lactation milk is known as colostrum, and carries the mother's antibodies to the baby. It can reduce the risk of many diseases in the baby. The exact components of raw milk varies by species, but it contains significant amounts of saturated fat, protein and calcium as well as vitamin C. Cow's milk has a pH ranging from 6.4 to 6.8, making it slightly acidic.
Milk may be obtained in various forms:
- Fresh milk requires refrigeration and spoils quickly, though it flavour and texture are superior to other forms of milk. It is usually sold in waxed cardboard boxes with easy-open peaked tops (available in 1-cup, 1-pt or 1-qt sizes in the U.S. or 250 ml, 500 ml or 1 liter sizes elsewhere), plastic bottles/jugs (in 1, 2, 3 or 4 liter sizes, 1/4, 1/2 or 1 gallon sizes or a plethora of smaller container sizes both metric and imperial. In some areas it is still available in 1 qt or 1 lt glass bottles, though this is increasingly rare. It may be found "whole", "skim" or "fat-free", though texture and flavour often change across the various grades.
- UHT (Ultra High Temperature) milk is treated with high heat to kill all bacteria and active cultures, extending shelf life and precluding the need for refrigeration. It is available in 250 ml, 500 ml and 1 lt brick-shaped waxed cardboard boxes (leading to the popular nickname of "boxed milk"), often with an easy-open pull-top or pre-perforated hole for a drinking straw. It is available in whole, skim and fat-free versions. UHT milk's flavour is often considered deficient in comparison to fresh milk, though its longer shelf life and lack of refrigeration make it a more common choice for bars.
- Condensed milk is heated in order to evaporate much of the water content, resulting in a thicker, more concentrated product. Sometimes mixed with sugar to extend its shelf life, such as Eagle Brand milk. Commonly sold in metal cans, sugared condensed milk may also be found in easy-open pop-top plastic containers for consumption as a sweet treat or dessert snack. Several cocktail recipes specifically call for condensed milk; other forms of milk will result in an unsatisfactory drink or variant.
- Powdered milk simply removes all water content (and often fat) from milk, resulting in a milk protein powder that mat be mixed with water to reconstitute. It is never used in mixology due to its watery, grainy and inconsistent texture and poor flavour.