What would a tour of thickeners be without a quick look at humanity’s first functional food ingredient/hydrocolloid? Rennet is of course the stuff responsible for thickening milk into the gels we call curds, the basis of cheese. A protein-digesting enzyme is what it is, found in the stomach of calves less than 30 days old. Why less than 30 days? Because that’s the point at which the biology of the calf changes and the critical enzyme, chymosin, fades from the animal’s system and is replaced by others. Today chymosin can be made artificially, by genetically engineering vegetables to produce it, and indeed most cheese in the U.S. is made with vegetable rennet these days. Most European cheese is still made with the traditional calf-derived enzyme. Some people don’t like that, but then some people don’t like GMO products either, which is why cheese is problematic for food ethicists of a certain stripe.
How does chymosin work? It performs not unlike acid, which also causes milk proteins to clump together and form gels (see yogurt). The problem with using only acid for cheese making, however, is that it doesn’t work on the main milk protein that’s required for creating tight, dense protein gels: kappa casein. Chymosin is miraculous in that it works on kappa casein but in a fascinating way: by digesting only the parts of the casein molecules that cause them to repel each other, leaving the bulk of the molecules undigested, intact and free to bond into large delicious masses. Even stranger, it leaves all the other proteins in the milk alone as well. Chymosin’s behavior is so specific and odd that, like yeast, some take it as evidence of a benevolent Creator. I’ve heard worse arguments.