You may have noticed that both of the meringue buttercream recipes I’ve put up have small amounts of cream of tartar in them. What’s it for? Oh, it’s just a little insurance policy, a bit of protective medicine in the event I get carried away with the whip while making my egg foam.
For you see the same action the creates a nice, light egg foam will transform it into a runny, clumpy pool of water and froth if applied with too much gusto. Again, the issue is proteins. As I mentioned yesterday, egg proteins are interesting and varied molecules. In addition to water-loving and water-hating regions along their length, they have bonding sites at various points. When the individual egg proteins are in their natural state (bunchy coils) many of these bonding sites are taken up, bonding the proteins to themselves (that’s what keeps them in a bunches). Whipping breaks these loose bonds, freeing up the bonding sites on the molecules so they can hook on to other molecules into those bubble-protecting networks I told you about.
The trouble is that if you keep the whipping up too long, the proteins start bonding to one another a little too eagerly. More and more “attachments” occur, causing the network to tighten, ultimately squeezing out the water molecules and popping bubbles. The result is an irretrievable, useless mess — and you can get there quicker than you think.
The good news is that the baker can protect against this (to a point) by plugging up some of those protein bonding sites with other types of molecules: copper (which is why copper pans are so popular among egg whippers), silver (which is why silver pans…oh wait, no they aren’t), or various types of acids like cream of tartar. None provide foolproof or lasting insurance against a ruined foam, but they will expand the margin of error should you be a little too free with the whip.