EGG

the content of the hard-shelled reproductive body produced by a bird, considered as food. While the primary role of the egg obviously is to reproduce the species, most eggs laid by domestic fowl, except those specifically set aside for hatching, are not fertilized but are sold mainly for human consumption. Eggs produced in quantity come from chickens, ducks, geese, turkeys, guinea fowl, pigeons, pheasants, and quail. This article describes the processing of chicken eggs, which represent the bulk of egg production in the United States and Europe. Duck eggs are consumed as food in parts of Europe and Asia, and goose eggs are also a food in many European countries. Commercial production of turkey and pigeon eggs is almost entirely confined to those used for producing turkey poults and young pigeons (squabs). Pheasant and quail eggs provide birds for hobby or sport use. the content of the hard-shelled reproductive body produced by a bird or reptile, considered as food. The egg has been a protein source for man since earliest times but was only locally important and seasonal until the domestication of fowls became widespread. In the 2nd millennium BC, Indian jungle fowls, the ancestors of the modern hen, were dispersed throughout Europe, the Middle East, and China. The birds were brought to the New World with Columbus's second voyage in 1493. These fowls, which laid year-round, were more valuable for their eggs than for their meat. By far the preponderance of eggs eaten today are hen's eggs. The eggs of ducks, both domestic and migratory, are also widely eaten; salted and preserved duck eggs are a specialty of Chinese cuisine. Other eggs often eaten are those of geese, quail, pigeons, gulls, lapwings, plovers, pheasants, and ostriches. Local regulations govern the gathering and sale of the eggs of wild birds. The eggs of some reptiles are edible, but of these only turtle's eggs are notable. A two-ounce (56.6-gram) egg provides 6 grams of complete protein, about 15 percent of an adult's daily requirement, and 12 grams of fat, for about 80 calories. In addition, the egg contains significant amounts of iron, vitamins A and D, thiamine, and riboflavin. The mineral and vitamin content varies with the diet of the chicken. Brown and white hen's eggs are nutritionally identical, although one is sometimes preferred over the other for aesthetic reasons. Because the protein in the liquid albumen of the egg coagulates as it is heated, eggs are used in many foods as a structural component. Sponge and angel food cakes are leavened by the expansion with heat of the air trapped in stiffly beaten egg whites. Souffls and meringues also exploit this characteristic. In cake batters, eggs provide leavening, moisture, and texture. Foods to be coated in bread crumbs or flour are first dipped in beaten egg, which makes the coating adhere and binds it. Soups and sauces are thickened by the addition of egg yolks. In terrines, croquettes, and other mixtures, egg contracts as the dish is cooked, helping maintain the shape. In ice cream and sherbet, egg white acts as an interfering agent that prevents the formation of large ice crystals. In both hot and cold sauces, eggs enable a stable emulsion to be formed of substances that would otherwise separate. When beaten into hot stock or coffee, egg whites trap solid particles and render the liquid clear. In cooking eggs, a major consideration is the use of low to moderate heat. At 165 F (74 C) both the yolk and the white of the egg coagulate. Above this temperature the protein becomes tough and rubbery. In custards and meringues, high heat will cause weeping, the loss of water. Eggs can be boiled, fried, poached, scrambled, and baked. They can be a component of any course of a meal and combine well with virtually any sweet or savoury food. in biology, the female sex cell, or gamete. In botany the egg is sometimes called a macrogamete. In zoology the Latin term for egg, ovum, is frequently used to refer to the single cell, while the word egg may be applied to the entire specialized structure or capsule that consists of the ovum, its various protective membranes, and any accompanying nutritive materials. The human female reproductive cell is also usually called an ovum. The egg, like the male gamete, bears only a single (haploid) set of chromosomes. The egg, however, is usually larger than its male counterpart because it contains material to nourish the embryo during its early stages of development. In many animal species a large quantity of nutritive material, or yolk, is deposited in the egg, the amount depending on the length of time before the young animal can feed itself or, in the case of mammals, begins to receive nourishment from the maternal circulation. The plant egg is never so disproportionately large, because the developing sporophyte embryo is nourished until self-supporting by the plant on which it is formed (in liverworts, mosses, and ferns by the gametophyte; in seed plants by the sporophyte on which the gametophyte is parasitic). With the exception of those of some cnidarians (coelenterates), all animal eggs are enclosed by membranes, the innermost of which is called the vitelline membrane. The vitelline membrane is the only membrane in the eggs of various invertebratesctenophores, many worms, and echinodermsand of certain lower chordates. All higher vertebrates and many invertebrates have one or more additional membranes. Insect eggs, for example, are covered by a thick, hard chorion, and the amphibian egg is surrounded by a jelly layer. The bird egg includes the vitelline membrane, the white of the egg, two egg shell membranes, and the outermost membrane, the shell. As pointed out above, this entire structure is commonly referred to as an egg. Mature eggs remain functional for a relatively short period of time, after which fertilization cannot occur. The eggs of most invertebrates, fish, and amphibians must be fertilized a few minutes after they are shed into the water; an exception is sea urchin eggs, which are viable for about 40 hours after their release. Most other animal eggs have life spans similar to that of the human eggi.e., 12 to 24 hours. See also ovum. Additional reading R. MacRae, R.K. Robinson, and M.J. Sadler (eds.), Encyclopaedia of Food Science, Food Technology, and Nutrition, 8 vol. (1993); and Y.H. Hui (ed.), Encyclopedia of Food Science and Technology, 4 vol. (1992), are general works that cover all aspects of the science of food. P. Fellows, Food Processing Technology: Principles and Practices (1988), is an introductory text. R. Paul SinghWilliam J. Stadelman and Owen J. Cotterill, Egg Science and Technology, 3rd ed. (1986, reissued 1990), provides in-depth information on egg chemistry, composition, specialized processes, functional properties, quality measurements, and new uses for eggs and egg products. G.W. Froning, New Product Innovations from Eggs, chapter 4 in B.J.F. Hudson (ed.), New and Developing Sources of Food Proteins (1994), pp. 7194, provides information on new processing technologies and new egg products. William J. Stadelman et al., Egg and Poultry-Meat Processing (1988), deals with nutritional aspects and contains a full listing of USDA-approved poultry products. Glenn W. Froning