On Food and Cooking

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