Structure and composition. The shell forms about 11 per cent of the egg and is largely composed of calcium carbonate with some magnesium carbonate, calcium and magnesium phosphates, and organic matter.

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Fig. 32. - Showing standing up quality of yolk and white of a fresh and a deteriorated egg.

Within the shell is the shell membrane, a thin, semi-permeable membrane made up of two layers, the inner and outer. After the egg is laid, its contents cool and shrink. The air cell at the large end of the egg is formed during this shrinkage by separation of the two membranes.

For cookery purposes the white and yolk are the important parts of the egg. The white is clear, transparent, and jelly-like. It composes about 57 per cent of the weight of the whole egg. The layer next the shell is a thin soft white, its relative proportion varying somewhat at time of laying. Fig. 32. Next comes a layer of rather thick, viscous white, containing a larger proportion of mucin than the thin white. Beyond this is a small layer of thin white surrounding the yolk. Schaible, Moore, and Davidson say that the thick, "firm white is of laminated structure, composed of concentric layers containing mucin fibers." If a freshly laid egg is broken into a large quantity of distilled water (the thick white having been slit with scissors on one side to remove yolk) and allowed to stand, the mucin and probably some globulin are precipitated at the edge of the cut surface. Six or more sheets or layers, one over the other, can be distinctly seen. Fig. 33.

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Fig. 33. - Showing the laminated structure of firm egg white, yolk removed.

The chalazae are dense cord-like strands of white substance, one on each side of the yolk, which anchor the yolk near the center of the egg. They allow the yolk to revolve. Being dense the chalazae are not broken down readily when the egg is beaten, hence they are often caught on the blades or wires of the egg beater.

The yolk forms about 32 per cent of the whole egg. It is separated from the white by a yolk sac, called the vitelline membrane. The yolk is made up of layers. See Fig. 34. The fat is more concentrated around the germ spot; hence its specific gravity is less and when the egg is turned the yolk rotates so that the germ spot is always uppermost. In eggs of average size the white usually averages about 30 grams and the yolk about 18 grams.

Composition. The composition of the white is approximately 86.2 per cent water; 12.3 per cent protein; 0.2 per cent fat; and 0.6 per cent mineral. 1 he composition of the yolk is about 49.5 per cent water; 33.3 per cent fats, including lecithin and cholesterol; 15.7 per cent proteins; and 1.0 per cent minerals.

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Fig. 34. - Diagram showing internal structure of an egg.

Color of shell and yolk. The color of the shell is determined by inheritance, certain breeds laying white, others brown-colored shells. The Barnaveldters lay eggs with nearly chocolate-brown shells.

The coloring matter of the yolk, according to Palmer, Mattikow, and others is xanthophyll, with a small proportion of a carotene-like pigment. The intensity of the yolk color is determined by the amount of xanthophyll in the food of the hen. The color of the yolk may vary from a very pale, almost white yolk, through deeper yellows to orange and finally a deep-red-orange. Some feeds may produce red-colored yolks. Some attempts have been made to work out color charts, giving each shade a number; but most of these are only tentative. It is an advantage for bakers and mayonnaise manufacturers to obtain yolks of uniform color to insure uniformity of color of their products. A small number of manufacturers at present do specify the color of the egg yolks when they place their contract for them. Reaction of eggs. The egg white is alkaline. Sharp found that the pH of the white varies from 7.6 to 9.7, whereas the yolk of fresh eggs averages pH 6.0 or slightly lower. The pH of both the white and yolk increases with age because of the loss of carbon dioxide. Sharp and Powell state that since the loss of some carbon dioxide cannot be prevented before the pH is taken, that a pH of 7.6 is probably too high for freshly laid eggs. With further loss of carbon dioxide the pH may increase to 9.5. Then, probably owing to break down of some of the egg protein, the pH decreases. The pH of the yolk may reach 6.8, but changes more gradually than that of the white. When the whole egg is beaten so that the white and yolk are mixed, a pH intermediate between that of the white and yolk is obtained.