Sabayon: Egg-yolk foam
Everyone knows you can make a foam out of egg whites; that’s what meringue is. But did you know that egg yolks can foam too? It’s a little trickier, though. Yolks won’t foam without a little help in the form of some added water. And unlike egg white proteins, which can be destabilized by mechanical action alone (i.e., whipping), egg yolk proteins need a little heat to encourage them to unfold.
Probably the most famous egg yolk foam is zabaglione, also known by its French name, sabayon. Traditionally a dessert sauce, or a dessert all on its own, zabaglione is made by adding sugar and wine, often Marsala, to egg yolks, and whisking it all together in a double boiler until it expands and thickens. The foam starts to become stable once the mixture reaches 120°F/49°C, and continues to thicken as the temperature rises, giving you some control over the final texture, from fairly fluid to nearly mousse-like. (And, of course, you can also make it in a whipped cream siphon.)
Egg yolk foams aren’t always sweet, though: I recently made two dishes involving savoury sabayons. For the “21st Century Tortilla,” the sabayon consisted of nothing more than egg yolks and water. (In typical el Bulli fashion.) In the case of “Oysters and Pearls,” the water is replaced by the liquid from freshly shucked oysters.
In fact, you can even look at hollandaise sauce as a savoury sabayon into which butter is emulsified. Personally, I’ve found that the thicker your egg yolk foam to begin with, the thicker your resulting hollandaise, though you have to be careful not to overcook it, which can break the emulsion by cooking off too much water or, as with any sabayon, curdle the egg yolks.