Rennet derived from animal slaughter, and thus cheese made with animal-derived rennet, is not vegetarian. Chymosin or rennin is a protease found in rennet. A study found bovine calf rennet could be used to coagulate dromedary milk. First, acidified milk is mixed with rennet taken from milk calves. The two key components of natural rennet are chymosin and bovine pepsin. In Portugal, traditional coagulation of the curd relies entirely on this vegetable rennet. Vegetarians also avoid food containing by-products of animal slaughter, such as animal-derived rennet and gelatin. To produce cheese, rennet or an edible acid is added to heated milk. The only permitted additives are natural rennet or another approved coagulating enzyme and salt. The milk is first heated and curdled by the addition of rennet. It is produced from cow's milk, rennet, and cream. Usually this is done by acidifying (souring) the milk and adding rennet. "Serra da Estrela" is handmade from fresh sheep's milk and thistle-derived rennet. He moves on to the blood, bone marrow, milk including rennet and cheese, and semen. Cheeses produced from any of these varieties of rennet are suitable for lactovegetarians.