On Food and Cooking

protein-rich, emulsifier-rich yolk is deficient
in water. Not only does it contain about half
the water that the white does, but nearly all of
it is tightly bound to all the other materials. In
one tablespoon/15 ml of yolk, the volume
typical of a large egg, there’s about a third of
a teaspoon/2 ml of free, foamable water. Add
two teaspoons to give it the same free water as
a white, and it foams enthusiastically.
Enthusiastically but fleetingly. Put your
ear to the foam and you’ll hear the bubbles
popping. The other deficiency of the egg yolk
is that its proteins are too stable. Neither the
physical abuse of whipping nor the presence
of air bubbles causes the yolk proteins to
unfold and bond with each other into a
reinforcing matrix. Of course heat will, as we
know from hard-boiled yolks and custards. So
supplement the yolk with liquid, and the
whipping with careful cooking, and the
mixture will rise to four or more times its
original volume. Exactly this procedure is the