On Food and Cooking

the foaming wands on espresso machines
shoot steam, a mixture of water vapor and air;
and foaming devices for whipped cream and
seltzer water mix a stream of pressurized
carbon dioxide or nitrous oxide with the
liquid. Any dissolved or suspended molecules
in the liquid collect at the interface of the air
and liquid and give the bubble wall some solid
reinforcement.
However, the reinforcement will be
momentary and the bubbles short-lived unless
the molecules can form a stable layer at the
interface. This is exactly what emulsifiers like
lecithin and proteins do, and for the same
reason that they stabilize oil droplets in
emulsions: they have a water-soluble portion
that rests in the bubble wall, and a water-
insoluble portion that rests in the air. Because
the bubbles in a typical foam are between 0.1
and 1 millimeter across, much larger than
most emulsion droplets, they require very
little emulsifier to cover their surface area,