Egg Yolks Egg yolks are the most efficient protein thickeners, in part because they are so concentrated: egg yolks are only 50% water and 16% protein. They are also the handiest, being a common, inexpensive ingredient, and their proteins are already finely dispersed in a rich, creamy fluid. They’re mainly used to thicken light-colored white sauces, blan quettes, and fricassees. Yolk-thickened sauces can be brought to the boil as long as they’re also partly thickened by starch.
Sabayon sauces are also partly thickened by the coagulation of yolk proteins
(p. 639).