By Harold McGee
Some very useful chemical relatives of the true fats, the triglycerides, are the diglycerides and monoglycerides. These molecules act as emulsifiers to make fine, cream-like mixtures of fat and water—such sauces as mayonnaise and hollandaise— even though fat and water don’t normally mix with each other. The most prominent natural emulsifiers are the diglyceride phospholipids in egg yolks, the most abundant of which is lecithin (it makes up about a third of the yolk lipids). Diglycerides have only two fatty-acid chains attached to the glycerol frame, and monoglycerides just one, with the remaining positions on the frame being occupied by small polar groups of atoms. These molecules are thus water-soluble at the head, and fat-soluble at the tail. In cell membranes, the phospholipids assemble themselves in two layers, with one set of polar heads facing the watery interior, the other set the watery exterior, and the tails of both sets mingling in between. When the cook whisks some fat into a water-based liquid that contains emulsifiers—oil into egg yolks, for example—the fat forms tiny droplets that would normally coalesce and separate again. But the emulsifier tails become dissolved in the droplets, and the electrically charged heads project from the droplets and shield the droplets from each other. The emulsion of fat droplets is now stable.