What is rennet? Rennet is a substance found in a mammal's stomach that aids in digestion. It is also an ingredient in the production of cheese. Rennet, or rennin, is an enzyme, actually a complex of enzymes, that are produced in a human or animal stomach. The complex of enzymes that form rennet includes the proteolytic enzyme known as protease that coagulates milk.
Historically cheese has been made using animal rennet. The old European process for obtaining rennet actually involves harvesting the linings of calves' stomachs. The mucosa is removed from the stomach chamber of a young calf. The stomachs used are retrieved from veal producers. The dried stomach of the calf is sliced into small pieces and put in salt water along with vinegar or wine and allowed to steep for several days. Then the solution is filtered. What remains is rennet and that can be used to coagulate milk into curds and whey. The curds are the basis for cheese.
Rennet can be purchased from commercial cheesemaking houses, but if you want to make rennet at home, here's how.
You will have to find a way to gain access to a freshly killed calf's stomach. Cut the stomach open so that you can clean it. Clean it inside and out by washing thoroughly with running water and salt. Sea salt works well for lightly scrubbing. Remove any food particles that may be stuck to the walls of the stomach. The rennet is contained in the lining so do not think you must remove the slimey substance that is the lining.
After the stomach is clean, tack it onto a wooden frame and dry it in direct sunlight. Drying requires a minimum of twenty-four hours. This drying will also kill any bacteria that may be present. When the stomach is thoroughly dried, cut it into small squares and pack it down in salt and store it in a clean covered container. A glass jar works well. In your jar you have rennet.
When you are ready to use the rennet to coagulate milk, or make cheese, choose a square and soak it overnight in cold water. Then rinse it off well and put it in the milk and wait for the curds. It is a good idea to tie the rennet up in a string so that it is easily retrieved after the curds have formed.
If one is interested in the purity of creating a homemade product, then making one's own rennet is the first step in creating cheese.