The Shell
The egg’s shell provides an effective barrier against germs and preserves the humidity within the egg, which helps the embryo develop. The shell of a medium-sized egg has between 6,000–8,000 pores and represents 9–12% of the total weight of the egg. It is composed mainly of calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%), and organic matter (4%). The membranes that make up the shell contain 2–3 layers of protein fibers that cling to the shell and provide additional protection against harmful elements such as mold and bacteria.
The Color and Thickness of an Egg’s Shell
The color of an egg’s shell is a genetic trait that depends on the breed of hen—it is not an indication of the nutritional value or flavor of the egg. The shell’s thickness depends both on hereditary factors and on the hen’s diet—the more eggs a hen lays, the thinner the shells. Since hens that lay white eggs tend to be more fertile than hens that lay brown eggs, the shells of white eggs tend to be thinner and more fragile.
The Air Chamber
When an egg is first laid, the egg’s contents fill the shell completely. However, as the temperature within the egg drops after it has been laid, the contents shrink, causing an air chamber to form at the rounder end. The size of this air chamber varies with the temperature and the degree of humidity. A loss of humidity or dehydration causes the air chamber to increase in volume. The size of this chamber is thus an indication of freshness—the larger it is, the older the egg.
The Albumen (Egg White)
The albumen, or “white,” consists of 87% water and 12.5% albumin, a protein substance. The egg white makes up two-thirds of the total weight of the egg and coagulates at 144–149°F during cooking. The fresher the egg, the denser and firmer the white around the yolk.
The Chalazae
The chalazae are two spiral membranous cords of albumin located at either end of the yolk. Each chalaza helps hold the yolk in position in the center of the albumen.
The Yolk
The egg yolk is formed of several layers of a substance called vitellus, alternating between a pale yellow and a dark yellow. It’s protected by a transparent membrane (the vitelline membrane). The color of the yolk varies according to the animal’s diet. For instance, in hens, a diet rich in wheat produces eggs with pale yellow yolks, while a diet of corn produces yolks of a deeper yellow. The yolk is made up of approximately 50% solids, 16% protein, and 30% fat. It coagulates during cooking at 150–158°F.
The Blastodisc
The yolk of unfertilized eggs contains a small, irregularly shaped, pale spot called the embryonic disk. The blastodisc is not present in fertilized eggs.
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