The yolk is the rich, golden heart of the egg — the yellow-to-orange sphere that carries almost all of the egg's fat, its fat-soluble vitamins, its cholesterol and the emulsifier lecithin. Mild and faintly savoury raw, it thickens under heat into a dense, creamy paste, and it is the workhorse behind the kitchen's great emulsified and set sauces: mayonnaise, hollandaise, custard and ice cream all lean on the yolk to bind, thicken and enrich.