Custards run the gamut from sauces and flans to cheesecakes and flourless chocolate cakes. What all these preparations have in common is that they are semi-solid gels that use egg proteins to set what would otherwise be rather runny liquids (this is of course less true of something like a flourless, whose ingredients are solids and semi-solids to begin with, though the batter obeys the same basic set of rules).
How do they work? Essentially by applying gentle heat to a mixture of beaten eggs and liquid (usually dairy of some sort). The heat coaxes the long, languid protein molecules in both the egg white and the egg yolk to uncoil by breaking the chemical bonds that ordinarily keep them wadded up like lengths of yarn. Once the egg proteins are “unwound” they’re free to re-bond with one another, which they readily do, trapping other types of molecules like water and fat between them. The result is, well, you get it.
What’s especially interesting about this process is that it only occurs in the presence of trace minerals. Trace mineral ions, you see, are positively charged, which causes them to collect around the negatively charged egg proteins. This action prevents the proteins from repelling each other and entices them to bond. And while that might sound like an off-the-cuff nugget of geek science, it’s actually quite important. Ever ask yourself why custards are made with things like milk and cream and not water? The answer is that dairy products contain minerals. Heat a mixture of beaten eggs and water and the result will egg drop soup, but add a little salt (a mineral) and you get — yes — a custard. Not one I’d like to eat of course, but a custard still. Pretty cool.
Custards come into two basic varieties, firm baked gels (so-called “still” custards) and pourable creams (known as “stirred” custards). Stirred custards include soupy sauces like crème anglaise, ice cream mix and pastry cream. They’re made (usually on the stove top) by stirring the egg-liquid mixture as it sets, and action which breaks up the continuous lattice of bonded proteins, but which nevertheless yields a thickened liquid. “Still” custards are baked in the oven (usually in a water bath) until they form a single, continuous gel.
One other interesting thing about egg proteins is that the degree to which they are diluted significantly effects the temperature at which they coagulate or “set”. Thus a very light custard mix (like, say, a quiche) must be heated to about 180 degrees before it sets, while a flourless chocolate cake, in which the eggs are essentially undiluted by water, will set at 140.