On Food and Cooking

and twigs, shreds of dried fruits, and sponges
could deliver only a coarse froth at best (see
box, p. 101). Sometime around 1650, cooks
began to use more efficient whisks of bundled
straw, and meringues and soufflés start to
appear in cookbooks.
Like the head on a beer or a cappuccino, an
egg foam is a liquid — the white — filled
with a gas — air — in such a way that the
mixture of liquid and gas keeps its shape, like
a solid. It’s a mass of bubbles, with air inside
each bubble, and the white spread out into a
thin film to form the bubble walls. And the
makeup of those liquid walls determines how
long a foam can stand up. Pure water has such
a strong surface tension — such strong
attractive forces among its molecules — that
it immediately starts to pull itself together
into a compact puddle; and it’s so runny that
it puddles almost immediately. The many
nonwater molecules in egg white both reduce
the surface tension of the water they float in,