30% of the whole egg is egg yolk. Egg yolk proteins serve mainly as a food reserve for young chick. The main components are lipovitellin, lipovitellinin, phosphovitin (a phosphoprotein) and livetin, a water soluble fraction. Lipovitellin is the major yolk protein and is rich in lysine and lucine. The ratio of lipovitellin to livetin in hen’s egg is 4:1.Egg yolk is used extensively as an emulsifier in mayonnaise, cake butter and confectionary. Its color is mainly due to the presence of xanthophylls pigments. An egg laid by a hen kept in deep litter tends to have a paler yolk than one from free-run poultry unless greens are fed or pigments added to the diet. A darker yolk does not necessarily mean a higher concentration of vitamin A or of carotene.
An egg yolk is rich in cholesterol, about 250 mg per egg. For this reason nutritionists recommend a reduction in egg intake to 2 or 3 per week for older children and adults. Egg yolk protein binds iron, forming an insoluble chelate, preventing iron absorption. The absorption of medicinal iron is also reduced if given with egg yolk. Therefore medicinal iron should not be taken with or immediately after a meal containing egg yolk.
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