Composition of Eggs

Structure. The egg has five major components: the yolk, albumen (egg white), shell membranes, air cell, and shell. Each of these plays an important role in the egg’s unique attributes that make it invaluable in food preparation.

Yolk. The sunny yellow yolk situated in the center of the egg constitutes about a third (30 percent) of the egg’s weight. Dense in nutrients, the yolk serves to nourish the chick. A white, pinhead-sized germinal disc sits on the surface of the yolk. This appears darker if the egg has been fertilized, but has no effect on the taste, functional properties, or nutritional value of the egg. Most eggs are screened to ensure they are not fertilized, but even if they do pass inspection, fertilized eggs do not develop once refrigerated. The color of the yolk, ranging from pale yellow to deep red, is caused by pigments in the chicken feed, such as beta-carotene.

Albumen. The albumen, or egg white, accounts for almost three-fifth (58 percent) of an egg’s weight and is made up largely of water and protein. Although it appears to be one mass, the egg white is actually constructed of layers differing in viscosity, alternating from thick to thin. Around the yolk is a layer of thick albumin. The chalazae (ka-lay-zee) anchor the egg yolk in the thick egg white surrounding it. They also secure the yolk to its vitelline membrane so it stays neatly centered in the middle of the egg. Differences in viscosity are determined by the type and amount of proteins in various parts of the egg white.

Shell Membranes. Between the egg white and the shell are two membranes, an inner and outer shell membrane. These press up against the shell and protect the egg against bacterial invasion.

Air Cell. Between the two shell membranes at the larger end of the egg is a pocket of air known as the air cell. As a freshly laid egg cools, its contents contract, causing the inner shell membrane to separate from the outer shell membrane, forming the air cell.

Egg Shell. Nature’s way of protecting the delicate internal contents of an egg is to surround it with a hard calcium carbonate shell (12 percent of an egg’s weight). Egg shells are not solid but contain thousands of small pores, allowing an exchange of gases between the inner egg and the surrounding air. Shell color indicates the breed of the hen, but has no bearing on the nutrient content or taste of the egg. Brown eggs tend to cost more because they usually come from larger hens, which require more food and produce fewer eggs.

The shell is protected by the cuticle or bloom. Commercially sold eggs are washed, which removes this protective cuticle. Producers compensate for this loss by applying a thin coat of edible oil on the shell.