An egg yolk is the part of an egg which serves as the food source for the developing embryo inside. Prior to fertilization the yolk together with the germinal disc is a single cell. Mammalian embryos live off their yolk until they implant on the wall of the uterus. The egg yolk is suspended in the egg white (known more formally as albumen or ovalbumin) by one or two spiral bands of tissue called the chalazae.

As a food, yolks are a major source of vitamins and minerals. They contain all of the egg's fat and cholesterol, and almost half of the protein.

If left intact while cooking fried eggs, the yellow yolk surrounded by a flat blob of egg white creates the distinctive sunny-side up form of the food. Mixing the two components together before frying results in the pale yellow form found in omelettes and scrambled eggs.

Nutritional Value

The yolk makes up about 33% of the liquid weight of the egg; it contains approximately 60 calories, four times the caloric content of the egg white. All of the fat soluble vitamins, (A, D, E and K) are found in the egg yolk. Egg yolks are one of the few foods naturally containing vitamin D.

Using

It is sometimes separated from the egg white and used in cooking (for mayonnaise, custard, hollandaise sauce, creme brulee, avgolemono, and ovos-moles).
It is used in painting as a component of traditional egg-tempera.
It is used in the production of egg-yolk agar plate medium, useful in testing for the presence of Clostridium perfringens.
Egg yolk also contains an antibody called Immunoglobulin yolk or (IgY). The antibody transfers from the laying hen to the egg yolk by passive immunity to protect both embryo and hatchling from microorganism invasion.
Egg yolk can be used to make liqueurs such as Advocaat.