typically just 0.1% of the liquid weight (1
gram per quart or liter).
Stabilizing Foams A liquid that is even
modestly supplied with proteins or yolk
phospholipids will form an impressive mass
of bubbles, solid enough to stand up without
flowing or even slumping. However, the foam
may still collapse within a minute or two. Air
and water have very different densities, so
when the foam is left to stand on its own, the
air bubbles rise while gravity pulls the liquid
in their walls in the opposite direction. This
means that liquid drains from the bubble
walls, which also lose water to evaporation.
Eventually, the foam at the surface becomes
dry, around 95% air and just 5% liquid, the
bubble walls become too thin, fail, and the
bubbles pop.
This instability of the foam as a whole can
be prevented by the same materials that
stabilize the emulsified sauces: namely