Rennet is a substance that is used to curdle milk. It is a complex of enzymes that are usually derived from the stomachs of young mammals, such as calves, lambs, and kids. Rennet is used in the production of cheese, yogurt, and other dairy products. It is also used in some types of bread and other food products. Rennet is an important ingredient in the production of cheese, as it helps to separate the solids from the liquid whey.