Milk prepared for sale is often homogenized; in this process it is pumped under pressure through small openings to break up the milk-fat globules, thus ensuring an equal distribution of fat throughout the milk rather than permitting it to rise to the top as cream. In most countries where milk is a commercial product, it is subject to regulations concerning its composition (i.e., the proportion of butterfat and other solids) and its purity, with sanitary measures in force that cover milk handlers, herds, plants, and equipment. Pasteurization (partial sterilization by heating) checks bacterial growth, thereby making milk safer to drink and increasing its keeping qualities and range of transport.
Milk, an almost complete food, consists of proteins (mainly casein
), fat, salts, and milk sugar, or lactose
, as well as vitamins A, C, D, certain B vitamins, and lesser amounts of others. (Many people are unable to digest milk after childhood because they stop producing an enzyme needed to break down lactose, but usually they still can digest yogurt, hard cheeses, and lactose-reduced milk products.) Commercial dairies often supplement natural vitamin D with a concentrate. Milk is a major source of calcium and a good source of phosphorus. Low-fat and skim milk fortified with vitamins A and D have the same nutritional value as whole milk, but with fewer calories and less cholesterol
. Whole milk has 3.5% milkfat, low-fat milk 1% to 2%, and skim, 0.5%. Heavy cream has a minimum of 36%