By Harold McGee
Gelatin is a protein, but it’s unlike the other proteins that the cook works with. Nearly all food proteins respond to the heat of cooking by unfolding, bonding permanently to each other, and coagulating into a firm, solid mass. It turns out that gelatin molecules can’t easily form permanent bonds with each other, due to their particular chemical makeup. So heat simply causes them to shake loose from the weak, temporary bonds that hold them together, and disperses them in water. Because gelatin molecules are very long and get tangled up with each other, they give the mixture a definite body, and can even set it into a solid gel. However, gelatin is relatively inefficient at thickening. Its molecules are very flexible, while those of starch and other carbohydrates are rigid and better at interfering with the movement of water and each other. This is one reason why gelatin-thickened sauces are usually augmented with starch. A sauce that contains only gelatin requires a large concentration, 10% or more, to have real weight. But at that concentration, the sauce is quick to congeal on a cooling plate, and it can also cause the teeth to stick together (gelatin makes an excellent glue!).