Chymosin /ˈkaɪməsɨn/ or rennin /ˈrɛnɨn/ is a protease found in rennet. It is produced by newborn ruminant animals in the lining of the fourth stomach to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption.It is widely used in the production of cheese.hymosin is produced by ruminant animals in the lining of the abomasum. Chymosin is produced by gastric chief cells in young ruminants and some other newborn animals to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption.