By Harold McGee
If we gently heat a piece of meat or fish alone in a pan, it releases flavorful juices. Normally we make the pan hot enough to evaporate the water the moment it comes out, so that the flavor molecules become concentrated on the meat and pan surfaces, and react with each other to form brown pigments and a host of new flavor molecules. But if the juices remain juices, they constitute a very basic sauce, a product of the meat that can be added back to moisten and flavor the mass of coagulated muscle protein from which they’ve been squeezed. The problem is that the meat or fish only gives up a small amount of juice compared to the solid mass. To satisfy fully our appetite for those juices, cooks have invented methods for making meat and fish sauces for their own sake, and in any quantity. The main thickening agent in these sauces is gelatin, an unusual protein that cooking releases from the meat and fish. Cooks also use other animal proteins to thicken sauces, but their behavior is very different and more problematic, as we’ll.