Eggs Basics

The egg is an organic reproductive body laid by the female of bird and animal species. It contains the germ of an embryo and nutritional reserves, but when left unfertilized, eggs can be eaten as a delicious and highly nutritious food.

For millemium, the egg has been a symbol of fertility and an object of pagan and religious worship. Some of these traditions are still alive today; for example, the coloring and painting of eggshells is an ancient custom that was particularly common among the Egyptians, Chinese, Persians, and Greeks and is still practiced today, particularly in Ukraine. The association of eggs with Easter goes back to a time when hens laid very few eggs during the winter. The return of spring, which coincided with the return of the laying season, was celebrated around Easter time. In addition, because eggs were considered to be a rich food, they were forbidden during Lent. Christians thus had to wait until Easter to eat eggs.

Though often taken for granted as a ubiquitous food staple, eggs have not always been consumed as widely as they are today. The development of industrial-scale poultry farming and breeding techniques have made it possible to meet rising consumer demand for eggs. After eggs have been laid, they are dispatched to a packing station for assessment of their inner and outer quality. Those that are cracked are set aside, while the rest are placed in front of an intense light to allow inspectors to assess the internal contents of the egg. Eggs that pass the inspection are washed, graded, and packaged for sale.

Eggs come in various sizes and types, which vary depending on the species that produce them. The eggs most commonly eaten are hen’s eggs, the familiar white or brown eggs sold in supermarkets. Though far less popular, duck, goose, quail, and pheasant eggs are also eaten. Unless otherwise specified, the word “egg” generally refers to a hen’s egg.

The egg is composed of six main parts: the shell, the albumen (the “egg white”), two chalazae, the blastodisc, the air chamber, and the yolk.

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 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.

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, 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 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 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 yolk of unfertilized eggs contains a small, irregularly shaped, pale spot called the embryonic disk. The blastodisc is not present in fertilized eggs.