By Harold McGee
The yolk accounts for just over a third of a shelled egg’s weight, and its biological purpose is almost exclusively nutritive. It carries three-quarters of the calories and most of the iron, thiamin, and vitamin A of the egg as a whole. The yolk’s yellow color comes not from the vitamin-A precursor beta-carotene, the orange pigment in carrots and other plant foods, but from plant pigments called xanthophylls, which the hen obtains mainly from alfalfa and corn feeds. Producers may supplement the feeds with marigold petals and other additives to deepen the color. Duck yolks owe their deeper orange color both to beta-carotene and to the reddish pigment canthaxanthin, which wild ducks obtain from small water insects and crustaceans, egg-laying ducks from feed supplements. One minor component of the yolk that can cause a major culinary disaster is the starch-digesting enzyme amylase, which has liquefied many a normal-looking pie filling from within.