What’s a foam but a bunch of bubbles? Masses of air surrounded by thin walls of liquid. The shape of a bubble is a factor of surface tension, i.e. the pull that the water molecules that make up the wall exert upon one another. That pull causes the wall to reduce itself into the most efficient possible shape, the one with the least surface area: a sphere.
There is of course a still more efficient shape that a mass of water molecules will assume if the forces of surface tension have their way: a droplet. Thus what all bubbles really want in the end is to pop. If the bubbles are to remain, some other something needs to be present in the water to reduce its surface tension. For little girls blowing bubbles on a sunny summer afternoon, that something is soap. For cooks whipping egg whites, that something is protein.
Proteins make up just under ten percent of the volume of an egg white. Not terribly much, yet enough to keep the surface tension of the remaining 90% of the white (which is virtually all water) low enough so that once it’s is whipped up into a foam the bubbles hang around for a while.
How does this work? Proteins, as some of you may recall from other posts, are long string-like molecules. Egg whites contain many different kinds of them, yet there are two that are of particular interest to foam makers: globulin and ovotransferrin, which are unusually good surface tension reducers. In their natural state these two proteins are found wadded up in clumps, bonded to themselves with weak side-to-side chemical bonds. All it takes to get them to un-wad themselves is a little elbow grease.
Whisking “denatures” (science-speak for “messes up”) the proteins, breaks their bonds and frees them to uncoil into long and languid shapes. Yet all those bonding sites really want, once they’re free, is to re-attach to something. We all know people like that. But being uncoiled it’s difficult for them to re-attach to themselves. So, most of them end up attaching to other globulin and ovotransferrin molecules. The result is a mesh that reinforces the bubble wall, keeping it from popping.