Yolk

Yolk is the yellow or orange part located in the center of an egg. It is a food store that provides nourishment (embryo) to the developing chick while it is in the shell. The other part of the egg, which is white and surrounds the yolk, is called egg white (albumen); it consists mainly of water and protein.

In bird eggs, particularly chicken eggs, the yolk is the part with the most nutrients and fat. It contains fat, protein, fat-soluble vitamins, and minerals. The white is composed mainly of protein and water, with very little fat.

Structure

The yolk is enclosed in a thin membrane that protects and separates it from the white. Within the white are two cord-like structures (chalaza) that hold the yolk at the center of the egg so it does not touch the shell. On the top of the yolk is a small pale spot; this is where the chick begins to develop if the egg is incubated.

Color

The color of the yolk depends on what the chicken eats. Foods containing green vegetables, red pepper, or pigmented substances (such as carotenoids) produce yolks with deep color. A dark yellow color does not necessarily mean the food is always abundant; it first reflects the type of feed.

Culinary use

Yolk is used to blend oil and water, because it contains an emulsifier, such as lecithin. For this reason it is used in mayonnaise, creams, and sauces. It also thickens or enriches food when cooked, because its protein hardens at a lower temperature than that of the white.