rennin

This term is a specialized biochemical designation used primarily in food science, chemistry, and veterinary medicine. It refers specifically to the enzyme responsible for the initial stage of cheese making, where it triggers the precipitation of milk proteins. In a professional laboratory or industrial setting, it is often discussed alongside chymosin, as the two are functionally identical in their effect on casein.

Because it describes a specific chemical substance, the word is treated as a mass noun. It does not have a plural form in standard scientific usage, as the focus is on the substance itself rather than individual units of the enzyme.