I.

Introduction

Milk, highly nutritious, versatile food. People enjoy drinking milk in its natural form and also use it to make a wide range of food products, including cream, butter, yogurt, cheese, and ice cream.

Female mammals produce milk to feed their newly born young. Milk is produced in the mother’s mammary glands, which are found, for example, in the breasts of humans or the udders of cows, sheep, or goats. Each species of mammal produces milk with a unique composition designed to meet the specific needs of its infants. For instance, the milk of animals that need to develop a thick layer of insulating fat, such as seals, has a high fat content. The milk of animals that grow rapidly, such as cows, which double their birth weight in 50 days, is rich in protein and minerals.

Humans drink the milk produced from a variety of domesticated mammals, including cows, goats, sheep, camels, reindeer, buffaloes, and llama. By far the vast majority of milk used for commercial production and consumption is from cows. This article focuses on the nutritional value and production of cow milk. For information on the branch of agriculture concerned with the raising of cows (or other domesticated mammals) for the purpose of collecting their milk, see also Dairy Farming.

II.

Nutritional Value of Milk

Most milk is composed of 80 to 90 percent water. The remaining 10 percent consists of an abundance of the major nutrients needed by the body for good health, including fats, carbohydrates, proteins, minerals, and vitamins.

Cow milk typically contains about 3.5 to 5 percent fat, which is dispersed throughout the milk in globules. In addition to providing milk’s characteristic taste and texture, fat supplies vitamins A, D, E, and K, as well as certain fatty acids that the body cannot produce on its own.

Lactose, a kind of sugar found only in milk, gives milk its sweet taste. Making up about 5 percent of milk’s content, lactose is a carbohydrate that is broken down by the body to supply energy. Infants digest lactose easily, but many adults, especially those of Asian and African ancestry, have lost some of their ability to digest this sugar. When these adults drink milk, they often suffer gastric distress and diarrhea.

The most important protein in milk is casein, accounting for 80 percent of milk protein. Casein is a complete protein, meaning that it contains all of the essential amino acids, which the body cannot manufacture on its own. Casein molecules and globules of fat deflect light rays passing through milk, giving milk its opalescent appearance. Other proteins present in milk include albumin and globulin.

Milk contains many minerals, the most abundant of which are calcium and phosphorus, as well as smaller amounts of potassium, sodium, sulfur, aluminum, copper, iodine, manganese, and zinc. Milk is perhaps the best dietary source of calcium—one liter (about 1 qt) of milk supplies as much calcium as 21 eggs, 12 kg (26 lb) of lean beef, or 2.2 kg (5 lb) of whole wheat bread. Milk is an excellent source of vitamins A and B₂ (see riboflavin). All other vitamins are present also, but in lower doses. Vitamin D is typically added to commercially sold milk. Vitamin A, which is found in the globules of fat, is removed when fat is skimmed away to make low-fat or skim milk. Generally, vitamin A is replaced during the production of commercially sold low-fat milk.

III.

Human Breast Milk

Human milk provides distinct advantages over formula or cow milk for human infants. Not only is breast milk’s nutritional composition uniquely designed for the needs of human babies, but it also contains antibodies from the mother’s immune system that help the infant fight off infections and diseases. During the first few days after giving birth, a mother releases colostrum, a yellowish liquid that contains less fat and lactose and more protein and antibodies than regular breast milk. After about three or four days, colostrum is replaced by a bluish-white milk that is higher in fats and carbohydrates, reflecting the energy needs of a growing baby (see Breast-feeding).

IV.

Milk Products

Milk in its natural form, directly from a cow, is called raw milk. It is an extremely versatile product from which a myriad of commercial products are derived.

A.

Whole Milk, Low-fat Milk, and Skim Milk

Since the fat in raw milk is lighter in weight than the rest of the milk, it will naturally rise to form a layer of cream if allowed to stand. Spinning the milk in a large machine, called a centrifuge, accelerates the formation of a cream layer, or the separation of fat, from raw milk. Varying amounts of fat are removed from the raw milk, resulting in different kinds of fresh milk. If the fat content is lowered to 3.25 percent, the milk is sold as whole milk. Low-fat milk typically has 1 percent or 2 percent fat. Skim milk, or nonfat milk, is the liquid that remains after removing all the cream; it contains about half a percent milk fat. About half of the milk produced in the United States is consumed as one of these types of fresh milk; the rest is processed into other products such as cream, butter, cheese, yogurt, and ice cream.

Once the fat level has been reduced to the desired level, most fresh milk is homogenized to prevent the further separation of a cream layer. Homogenization is accomplished by forcing hot milk under high pressure through small nozzles. The fat globules become so small that they remain evenly dispersed throughout the milk. In order to insure its safety for human use, almost all milk undergoes pasteurization, in which milk is heated to a high temperature for a specified length of time to destroy pathogenic bacteria. Pasteurized milk can be stored in a refrigerator for a week or longer. Ultrapasteurized milk, common in Europe and Canada, is heated to an even higher temperature and can be stored at room temperature for several weeks.

Condensed, evaporated, and powdered milk are produced by evaporating some or all of the water in milk. Sweetened condensed milk is sweetened with the addition of sugar. These products have very long shelf lives.

B.

Cream, Butter, and Ice Cream

In order for a product to be labeled as cream, it must contain 18 percent milk fat or more. Light whipping cream must contain at least 30 percent fat, and 36 percent fat is the minimum fat content for heavy whipping cream. Half-and-half, an equal mixture of both milk and cream, must have at least 10.5 percent fat. The various grades of cream are valued for their smooth, thick texture. They are often used in cooking instead of milk, because they are less likely to form a skin when heated and can be whipped into a stiff foam.

When chilled cream is churned gently, the fat globules gather together to form butter, leaving buttermilk as the by-product. Butter contains at least 82.5 percent fat. In the past, before the advent of widespread refrigeration, salt was added to butter to retard bacterial spoilage; now salt is added mainly for flavor.

Ice cream, a popular frozen dessert, is made of milk, cream, sugar, and flavoring. The mixture is slowly beaten while chilled until it is partially frozen, then packed into containers, and chilled until firm. Variants of ice cream include ice milk, which contains less fat; sherbet, which has even less fat; and French ice cream, which is enriched with egg yolks.

C.

Yogurt, Cheese, and Other Fermented Milk Products

Fermented milk products, including yogurt and cheese, are formed when bacteria break down lactose to produce lactic acid, which sours the milk. Yogurt is usually made from milk that has been fortified by the addition of nonfat powdered milk to improve its texture and taste. The fortified milk is pasteurized at a very high temperature and homogenized. A culture containing the bacteria Streptococcus thermophilus and Lactobacillus bulgaricus, and sometimes Lactobacillus acidophilus, is then added. The two most popular types of yogurt sold are Swiss-style, with added fruit mixed throughout, and sundae-style, with fruit on the top or bottom.

Cultured buttermilk and sour cream, like yogurt, are produced by the breakdown of lactose by bacteria. Cultured buttermilk used to be made from buttermilk, but is now more commonly made from skim milk. Sour cream is made from cream. Both have a tangy, sour taste imparted by lactic acid.

Cheese making is a complex process, with each of the more than 400 varieties of cheeses differing in the details of the process. However, most cheese making follows the same basic steps. Bacteria are introduced into milk that consume lactose and produce lactic acid. Cheeses are allowed to ferment longer than yogurt, buttermilk, and sour cream, which gives the bacteria time to also digest some of the fat and protein present. The waste products produced by bacteria from digesting fats and proteins provide cheeses with their distinctive and stronger flavors. Acidification of milk or the addition of the enzyme rennin precipitates moist lumps of protein called curds. The curds contain all of the milk’s protein and most of the fat, minerals, and vitamins. The residual thin liquid is known as whey. The curds are salted (for taste) and pressed into shape, which eliminates more of the whey. The pressed curd is then cured under controlled temperature and humidity for varying lengths of time to produce cheese.