Rennet is substance containing rennin, an enzyme having the property of clotting, or curdling, milk. It is used in the making of cheese and junket.
Animal Rennet is obtained from the fourth, or true, stomach (abomasum) of milk-fed calves.
The preparation of rennet was formerly a part of the domestic function of making cheese; the inner membrane was kept in salt, dried, and, when rennet was needed, soaked in water.
Now extract of rennet is made and sold commercially. It is usually prepared by soaking the tissues in warm, slightly salted water and straining and preserving the resulting liquid.
Non- animal rennet is an alternative substance that does the same thing to milk as the animal product.
This can be made form plants such as fig tree bark, nettles, thistles, mallow, and Creeping Charlie. Rennet from thistle or 'cynara' is used in some traditional cheese production in the Mediterranean.
Alternatively some microbes or molds produce enzymes that will curdle milk and these too can be used, in purified form, as a rennet substitute.