The yolk accounts for over a third of the eggs weight and its purpose is almost exclusively nutritive. It carries three-quarters of the calories of the egg and most of the iron, thiamin, and vitamin A. It obtains its color from the plant pigments xanthophylls found in the hen's feed. The yolk also contains amylase, a starch-digesting enzyme.
The yolk consists of spherical compartments and tightly packed. These chambers are filled with water that suspend tiny subspheres that contain water and phosphorus-rich proteins where the iron in an egg is bound. Suspended in the water inside these subspheres are low density lipoproteins that contain aggregates of fat, protein, cholesterol and phospholipids, mainly lecithin.
Cholesterol is an essential component of animal cell membranes, therefore, it is a substantial component in the egg. One large egg contains around 215 mg of cholesterol, while an equivalent portion of meat has about 50 mg. With that said, concerns of high blood cholesterol and increased risk of heart disease have placed dietary restrictions on yolk consumption. However, recent studies suggest that egg consumption has little effect on blood cholesterol which is more susceptible to saturated fats than cholesterol itself. Most of the fat in egg yolks is unsaturated.
Egg yolks are far less allergenic than whites and can be safely consumed by infants.
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