Foaming Principles of Ingredients: Egg Yolks

Yolks, because of their high fat content, take longer to whip than whites and will not gain a significant increase in volume. They might whip up to four times their original volume, but only if there is a liquid, such as water, added or if they are whipped over a hot water bath. It is almost impossible to over-whip egg yolks, because, although they have a large amount of proteins, egg yolks lack the water content that egg whites have (by about half) and their proteins are very tightly bound to each other. Whipping alone will not make enough of the proteins unfold (as happens when whipping whites) and trap air bubbles efficiently. When a liquid is added, it makes the proteins lighter and helps untangle them, so they will trap air bubbles easily. However, if heat is not applied, this foam will collapse quickly. Therefore, whip the egg yolks over a hot water bath, and as the air is trapped in the water by the yolk’s proteins, the heat will help coagulate these proteins around the bubbles, creating a stable foam. When yolks are whipped without the addition of a liquid and indirect heat (hot water bath), their volume will increase by only two to three times their original volume. Be sure to never stop whisking your egg yolks when they are cooking over a hot water bath. Otherwise they will coagulate in large clusters around the rim of the bowl and will not result in a very smooth finished product. Having one of those chunky clusters in your mouth is very unpleasant. Think hard-boiled eggs, which are fine when you know what to expect, but are definitely not what you want when you eat a semifreddo.