1. The two major milk proteins are casein and whey proteins. These proteins can be coagulated and separated from the rest of the milk by three different techniques: heat, acid, and enzymes.a. Heat: whey proteins are heat sensitive, and begin to coagulate at ~150°F (66°C). Coagulated whey proteins are responsible for the "skin" that forms on the surface of heated milk, and also precipitate along the sides and bottom of the pan. Casein is relatively unaffected by heat.b. Acid: milk contains more casein than whey proteins, and casein can be coagulated by reducing the pH of milk (~6.6) to 5.2 or lower. Casein coagulates and forms high moisture, soft curds, but the curds are low in calcium because most of the calcium remains in liquid portion of the milk.Cottage cheese is formed by acid precipitation of casein.c. Enzymes: the enzyme most commonly used to precipitate milk proteins for the manufacture of cheese is rennin. Rennin was originally extracted from the digestive tracks of cattle, but now is synthesized and sold under the brand name Rennet. Rennin has optimal activity at a temperature of 104°F (40°C), and forms curds that are lower in moisture and more rubbery than acid formed curds.
2. Milk proteins, along with egg proteins, are the only proteins with the necessary emulsification properties to form foams. A foam is a colloidal dispersion with a gas (usually air) as the dispersed phase, and a liquid as the continuous phase. Milk proteins are able to function like emulsifiers at the gas ¾ liquid interface to stabilize the foam. Considerable force in the form of beating must be applied in to facilitate the formation of a foam.
3. Milk foams are typically unstable, but the stability of a milk foam can be increased by 1) increasing the viscosity of the solution, 2) lowering the temperature of the solution, and 3)increasing the fat content of the solution. The viscosity of a foam may be increased by addingprotein in the form of milk proteins (non-fat dry milk/NFDM) or gelatin.
4. The stability of a milk foam can also be stabilized by adding sugar after the foam has formed.
5. Many varieties of cheese are available depending on 1) type of milk used (cow, goat, sheep); 2)amount of fat (whole, nonfat/skim, etc.); 3) type of curd (acid vs. rennin); 4) type of bacteria or mold (if any) added, and 5) length of aging/ripening.