The yolk, or yellow portion, of an egg makes up about 34% of the liquid weight of the egg. It contains all of the fat in the egg and a little less than half of the protein. The yolk of a large egg contains about 55 calories.
With the exception of niacin and riboflavin, the yolk contains a higher proportion of the egg’s vitamins than the white, including vitamins B6and B12, folic acid, pantothenic acid and thiamin. All of the egg’s vitamins A, D, E and K are in the yolk. Egg yolks are one of the few foods naturally containing vitamin D. The yolk also contains more calcium, copper, iron, manganese, phosphorus, selenium and zinc than the white.
Double-yolked eggs are often produced by young hens whose egg production cycles are not yet completely synchronized. They’re often produced too, by hens which are old enough to produce extra large-sized eggs. Genetics is a factor, also. Occasionally a hen will produce double-yolked eggs throughout her egg-laying career. It’s rare, but not unusual, for a young hen to produce an egg with no yolk at all.
In fertilized eggs, the yolk is the site of embryo formation.
It’s the yolk which is responsible for the egg’s emulsifying properties.