Animal rennet was obtained from the stomach (abomasum) of young cattle, goats and sheep.
The stomach was emptied and washed and then inflated (tying one end) and hung in a cool, ventilated place to dry. After about three months, the dried stomach was ready to be cut and immersed in a solution of water, boric acid and hydrochloric acid so that it could ferment for a period of about 25-30 hours. The fermented product was then filtered and the filtrate added to the milk during cooking to allow coagulation.