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.

Whole Milk, Low-fat Milk, and Skim Milk

American microbiologist, Alice Evans, discovered that certain bacteria in raw milk could cause disease in humans. Her pioneering research led the dairy industry to begin pasteurizing all milk in the 1930s.

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 world 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 is 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.

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 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.

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.