The egg yolk is the yellow inside an egg. Its primary purpose is to serve as a food reserve for the developing embryo.
The egg yolk contains a small amount of fat, people on a low-cholesterol diet may feel the need to cut down on egg consumption, although most of the fat in egg is unsaturated fat and may not be harmful. The egg white consists primarily of water (seven eighths) and protein (one eighth) and contains no cholesterol and little if any fat.
All of the egg's vitamin A, D and E are in the yolk, they are one of the few foods naturally containing vitamin D. A large egg yolk contains approximately 60 calories, the egg white contains about 15 calories. A large yolk contains more than two-thirds of the recommended daily limit of 300mg of cholesterol.
The yolk makes up about 33% of the liquid weight of the egg. It contains all of the fat in the egg and almost half of the protein.
If an egg is overcooked a greenish ring sometimes appears around egg yolk, this is the result of iron and sulfur compounds in the egg. it can also occur when there is much iron in the cooking water. The green ring affects neither taste nor nutrition.
Yolk colour depends on the diet of the hen, if the diet contains yellow/orange plant pigments known as xanthophylls, then they are are deposited in the yolk, colouring it. A colourless diet, can produce an almost colourless yolk. Farmers may add natural pigments to enhance yolk colour, but artificial colours are banned.
Some hens will lay double-yolked eggs, this is a result of unsynchronised production cycles, although some hens have a higher propensity to lay double yolked eggs due to genetics. It is also possible for a young hen to produce an egg with no yolk at all.