 In fact, rennet, which is the set of enzymes used to turn milk into curds and whey for cheesemaking, used to come from the stomach lining of the fourth stomach of calves, baby cows. And in 1990, a cell-cultured version hit the market, a version of the key enzyme chymosin. And today, only 30-ish years later, 90 percent of rennet used for cheesemaking came from a bioreactor instead of a calf.