(Resources: Food Science and You, and The Epicurean Laboratory.)
Milk is an excellent source of protein. Because proteins have a large molecular structure, they do not dissolve in water. The proteins in milk are a colloidal dispersion. Colloidal dispersions are explained in the egg unit. There are two kinds of milk proteins: casein and whey or serum. Casein are approximately 80% of the milk proteins, and whey or serum proteins make up the other 20%.
The casein proteins in milk are combined with some of the minerals in milk and form micelles. Micelles are groups of molecules; the micelles in milk form a colloidal dispersion.
The light reflected from the micelles makes the milk white. Casein proteins are coagulated by acids. If an acid is added to milk, the casein proteins separate from the rest of the liquid and the milk curdles. Casein proteins are not effected very much by heat, however.
Casein proteins in milk cluster together and work like tiny sponges to hold water in the milk. They can contain and hold as much as 70% water in each protein cluster (see CASEIN PROTEIN CLUSTER ILLUSTRATION in Resources). Acids, salt, or high heat will cause the casein protein clusters to lose water.
The serum proteins, also called whey, are in colloidal dispersion in the water content of milk and are coagulated by heat, but acids and salt do not coagulate them. This is important in making yogurt and cheese. Commercially, the coagulation of milk to make cheese is accomplished with the use of rennin-an enzyme which has the capability to disable the stabilizing subunit of casein causing the normally separate casein micelles to clump together in the presence of dissolved calcium. Rennet (the commercial name for the enzyme rennin) has the power to join things that are dispersed and to disperse things that come together.
Milk is also a source of fat. Fat globules float in the water contained in milk. There are many different types of fat in milk. The fat in milk tends to be low in cholesterol. Higher levels of butter fat in milk usually mean higher prices for the milk product. As fresh milk sets, the fat droplets cluster together; eventually, the clusters get large enough that their lower density lets them float to the top. The milk seperates. To stop this from happening, most milk sold commercially is homogenized. When milk is homogenized, it is forced through very small holes under pressure. The fat globules are made small enough so they stay dispersed or separated in the milk so they do not cluster together.
Milk also contains some carbohydrate in the form of natural sugar. The sugar in milk is called lactose. It is a type of sugar found only in milk. Lactose gives milk a slightly sweet flavor. When milk is digested a special enzyme produced in the cells of the body burns the calories provided by lactose. This releases energy. Some people can not drink fresh milk because they are unable to digest the lactose in the milk. They lack the enzyme lactase in their digestive system. Without lactase, the lactose is not split into glucose and galactose so it can be absorbed in the body and burned for energy. As the lactose ferments in the digestive tract, it gives off gas and a variety of acids. (Lactose intolerant people lack lactose - the enzyme that lets the body use the energy in lactose). Lactose caramelizes when the milk is heated and turns the milk a tarnish color.
Milk is an excellent source of a variety of vitamins and minerals. Calcium and magnesium help keep the micelles in milk stable. Calcium helps strengthen bones and teeth. Milk is a good source of riboflavin, which is a vitamin that can be destroyed by light, so milk should be stored in light-proof containers. The butter fat in milk contains vitamin A. Many milk products are fortified with vitamin D.
Types of Milks Fresh milk is categorized by the amount of butter fat in it. The butter fat level is determined by federal standards. Skim milk has had all the fat removed. One percent milk is 1% butter fat, and two percent milk is 2% butter fat. Whole milk must contain at least 3.5% butter fat. As the fat is removed, milk appears more translucent because light does not have as many fat particles to reflect off. It is light reflecting off the fat that makes milk appear white.
Milk is marketed in other forms besides fresh:
Evaporated milk has been heated under a small amount of pressure until 60% of the water evaporates. Carrageenin (a vegetable gum) is added to the milk before it is processed to stabilize the casein proteins.
Condensed milk has 50% of the water removed and sugar added. Sugar is added so that it is 44% of the final product. The sugar content is high enough to inhibit bacterial growth in the condensed milk.
Dried milk or powdered milk is made by removing all the water from fresh milk.
There are several fermented milk products. Buttermilk, sour cream, and yogurt are made by adding a bacteria to fresh milk and allowing it to ferment for a period of time.
Ultrahigh-temperature (UHT) milk is flash-heated at higher temperatures than regular pasteurized milk; then it is flash-cooled, destroying additional bacteria and allowing longer shelf life. In addition, the processing plant seals the UHT milk (sometimes called parmalet milk) in several layers of aseptic packaging: polyethylene, paper, aluminum foil with a polyethylene lining. (This is the same packaging used for egg substitutes such as Egg Beaters, pancake mixes, some juices, and chopped tomatoes). Once opened, UHT milk keeps in a refrigerator for at least 10 days. It does taste a little sweeter than the milk most Americans are used to drinking. It has been used in Europe and Asian countries since the 1960s. UHT milk also comes in low-fat and chocolate versions.
Foams A foam is a mixture made by whipping or beating a liquid to trap air bubbles in it. The amount of fat in the milk has an effect on the stability of the foam. The higher the fat content, the more stable the foam. The viscosity, or thickness, of the cream increases in direct proportion to the amount of butter fat. Temperature also affects the viscosity of cream. The viscosity of cream increases as the temperature of the cream decreases. The more viscose the cream is when being beaten, the better the foam will be. The amount of cream being whipped also affects the formation of the foam. Smaller amounts produce more stable foams. Sugar makes the foam less stiff and smaller in volume. It also makes it take longer to form a foam when sugar is added too early. The rule of thumb is: produce foam then add sugar.
Cooking With Milk Cooking effects both types of milk proteins (casein and serum). When cooking with milk, one needs to be careful so that the milk does not scorch or curdle. Remember that the serum (whey) proteins are denatured by heat. These solid substances precipitate out of the milk solution. These serum (whey) proteins settle to the bottom of the pan and scorch easily. Because this happens when milk is heated at too high a temperature, milk must be cooked on low heat or in a double boiler which keeps the temperature in the top container below the boiling point of water.
Another problem when cooking milk is that a skin often forms over the top of the cooked milk. The casein and serum (whey) proteins clump together on the surface as the water in the milk evaporates when heated. Pulling the skin off the top of heated milk will remove nutrients that are in the skin. The skin on the top of the cooked milk can hold in steam and increase the chance that the milk will boil over while cooking. Creating a foamy surface reduces the chance that the skin will trap steam and the milk boil over. Stirring constantly will reduce skin formation. Adding fat also reduces skin formation, and covering the cooking pot keeps the surface from losing water (drying out) and forming the skin.
The third problem when milk is heated is curdling. Using high heat increases the chance that the casein proteins will curdle. A low pH increases curdling also. This happens because the calcium and magnesium ions which make the casein micelles in milk more stable are removed from the casein micelles when an acid is added to milk. When salt is added to hot milk, curdling is increased.