The phospholipids, lipoproteins and proteins found in egg yolks are surface active agents that enable the formation of emulsions between immiscible liquids such as oil and water. Egg yolk emulsions improve food mouthfeel in two ways. First, the egg yolk’s natural emulsifiers can coat liquids with fat to create smooth, creamy textures in products from custards to chocolate truffles. Second, these emulsifiers also thicken. For example, in yolk-emulsified mayonnaise, an entire cup of tiny oil droplets is packed into about two tablespoons of liquid to produce a thick spread. An egg yolk, whisked into a sauce after it is removed from the heat, binds fats and liquids for slight thickening and improved texture. In ice cream, egg yolk helps disperse fat throughout the mix and prevent it from clumping. Yolk also helps improve whipping properties for desired overrun and makes ice cream dry and stiff. Many salad dressings and sauces also depend on eggs’ ability to produce a stable emulsion.

— American Egg Board,