By Harold McGee
Mayonnaise is an emulsion of oil droplets suspended in a base composed of egg yolk, lemon juice or vinegar, water, and often mustard, which provides both flavor and stabilizing particles and carbohydrates. It’s the sauce most tightly packed with oil droplets—as much as 80% of its volume is oil—and is usually dense and too stiff to pour. It can be thinned and flavored with various water-based liquids, including purees and stocks, or it can enrich such liquids the way cream does; it can also be aerated with the addition of whipped cream or egg whites. As a room-temperature preparation, mayonnaise is generally served with cold dishes of various sorts. But thanks to the yolk proteins, it also reacts usefully to heat. It lends body and richness when added to thin broths and briefly cooked; and when layered onto fish or vegetables and broiled, it moderates the heat, puffs up and sets into a rich coating.