Egg white is composed of thin and thick portions. Some hens secrete a higher ratio of thick to thin white than do others. Storage conditions also affect the thickness of the albumin and the ratio of thick to thin egg white. Twenty to twenty-five percent of the total white of fresh eggs (1-5 days old) is thin white. Egg whites consist of water largely with no fat or carbohydrate but contain 8-12 percent protein. Different types of proteins are present in egg white.
Ovalbumin: This constitutes 55 percent of the proteins of egg white. This is a phosphor glycoprotein and is composed of three components A1, A2 and A3 which differ only in phosphorous content. The relative proportion of A1, A2, A3 component is about 35:12:3. The carbohydrate component of ovalbumin is mannose and glucosamine in the ratio of 5:3. Ovalbumin in solution is readily denatured simply by mechanical agitation (whipping) but is resistant to thermal denaturation. At pH 9 and at 62o C, only 3-5 percent of ovalbumin is denatured.
Conalbumin: This constitutes 13 percent protein of the egg albumin. It consists of two forms neither of which contains phosphorous nor sulphur. Conalbumin is more easily heat coagulated and less susceptible to surface denaturaion than ovalbumin. The protein easily binds metallic ions such as iron, copper, aluminium, zinc-forming heat stable complexes.
Ovamucoid: It is a glycoprotein. This constitutes about 10 percent of the egg white proteins. It exists in three forms and all of which are trypsin inhibitors. It is resistant to heat denaturation in acid media but is rapidly denatured in alkali solution.
Ovomucin: This protein is responsible for the jelly-like character of egg white and the thickness of the albumin. It contains 2 percent of the egg white. Its content in the thick layers of albumin is about 4 times more than in thin layers. It is insoluble in water but soluble in dilute salt solution. Purified ovomucin in solution is resistant to heat denaturation. It is a very large molecule with a filamentous or fibre-like nature.
Lysozyme: 3.5 percent of the egg white protein is lysozyme. This is an enzyme capable of lysing or dissolving the cell wall of bacteria. It is heat sensitive. Hen’s egg white has three to four times more lysozyme than does duck egg white.
Avidin:This protein is 0.05 percent of the egg white protein. It is composed of 3 components A, B and C. It binds biotin and makes the vitamin unavailable. Avidin is denatured by heat and cooked eggs do not affect the availability of biotin.
Ovoglobulin: It is a protein consisting of two components G1 and G2 and both are excellent foaming agents.
Ovoinhibitor: 0.1 percent of egg protein is made up of ovoinhibitor. It is another protein capable of inhibiting trypsin and chymotrypsin. Summary of egg white proteins is given in Table.