The Key Catalyst: Rennet

The making and use of rennet was humankind’s first venture in biotechnology. At least 2,500 years ago, shepherds began to use pieces of the first stomach of a young calf, lamb, or goat to curdle milk for cheese; and sometime later they began to make a brine extract from the stomach. That extract was the world’s first semipurified enzyme. Now, by means of genetic engineering, modern biotechnology produces a pure version of the same calf enzyme, called chymosin, in a bacterium, a mold, and a yeast. Today, most cheese in the United States is made with these engineered “vegetable rennets,” and less than a quarter with traditional rennet from calf stomach (which is often required for traditional European cheeses).