Rennet

Rennet is a complex of enzymes, chiefly the protease chymosin along with pepsin and lipase, produced in the stomachs of ruminant mammals and used to curdle milk into curds and whey during cheesemaking by cleaving kappa casein so casein micelles can bond together. Traditionally sourced from calf stomachs, animal rennet now accounts for less than five percent of American cheese production; most cheese instead uses rennet from molds such as Rhizomucor miehei or fermentation-produced chymosin, made by inserting the animal rennet gene into bacteria, fungi or yeast, which supplied eighty to ninety percent of the US and UK market by 2021.