Shell eggs consist of about 9 5 percent shell, 63 percent albumen, and 27.5 percent yolk, according to Cotterill and Geiger (1977). The total solids of the albumen, yolk, and whole egg are about 12 percent, 52 percent, and 24 percent, respectively. Reviews of the chemistry of egg components were written by R. E. Feeney (1964), T. L. Parkinson (1966), Stadelman (1976), and W D. Powrie and S. Nakai (1990). Extensive analytical data on the nutrient composition of eggs are given in a publication by the U. S. Department of Agriculture (1989).
The shell membranes are composed of protein fibers, mostly keratin with some mucin, arranged in net fashion to form a semipermeable membrane. The shell membranes provide a significant barrier to bacterial invasion of the albumen.
The egg white consists of a number of proteins, a small amount (less than 1 percent) of carbohydrates and minerals, and no lipids. The composition of the albumen of a freshly laid egg is about 87 percent moisture, 11.5 percent protein, 0.8 percent ash, and 0.7 percent carbohydrates. The carbohydrates are the sugars in glycoproteins. The predominant protein is ovalbumin. The complete amino acid sequence with 385 residues has been determined by A. D. Nisbet and others (1981). The molecular weight of the polypeptide chain is 42,699.
The pH of the albumen in a freshly laid egg is about 7.6, which increases during storage to as high as pH 9.7. The rate of pH change is influenced by temperature, air movement, and shell quality. The pH increase is the result of carbon dioxide loss from the albumen. The pH of the albumen depends on the equilibrium between the dissolved carbon dioxide, bicarbonate ion, carbonate ion, and proteins.
The protein ovotransferrin is referred to as conal-bumin in older literature. Its unique characteristic is its ability to bind multivalent metallic ions. With aluminum salts a white precipitate is formed; with copper, the flocculant is yellow; and with ferric iron a red color results. The ovotransferrin is one of the most heat-labile proteins of the albumen. Powrie and Nakai (1990) discuss methods for the isolation of each of the proteins.
Egg yolk might be described as a complex system containing a variety of particles suspended in a protein solution. Another description is that it is an oil-inwater emulsion, with lecithin and the lysoproteins aiding in maintaining a stable emulsion. The yolk consists of a plasma with suspended granules. Fresh egg yolk contains about 52 percent solids and has a pH of 6.0, which rises during storage to 6.9.
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