 In fact, rennet, which is the set of enzymes used to turn milk into curds and whey for cheese-making, 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 cheese-making came from a bioreactor instead of a calf.