Rennet is an enzyme produced in an animals stomach which enables the young to digest its mothers milk. It is usually taken from the fourth stomach chamber of a calf when used on cows milk or can be taken from the stomachs of lambs and goats for production of cheese from these animals milk.

When added to milk it causes coagulation enabling the production of milk curds.

Vegetable Rennet can be taken from certain plants such as nettles and thistles and is suitable for vegetarians.

Some molds are used for vegetarian microbial rennet as well as genetically modified enzymes.