Rennin, or chymosin, is the active protease in rennet. Rennin splits the milk protein at a specific point, affecting the clotting. After splitting, the casein clumps together and separates from the whey, forming curds. Rennin is naturally found in the calf stomachs, where they use the enzyme to prolong the digestion of milk. In fact, it is believed that cheese was first discovered by accident when milk was kept in a bag made of the stomach of a calf or goat. However, now the more common practice is to either extract the rennin from the frozen stomach of a calf and incubate it before adding it to a vat of milk or by extracting rennin from thistles or cynara can also be used (Frankhauser, 2009).
However, there are environmental issues surrounding the production of rennet. For thousands of years, cheese makers have relied on traditional rennet sources. Over the last few decades the demand for cheese has increased at such an unprecedented rate, that supplies of rennin simply cannot keep up. Since the 1990s, cheese makers have begun using genetically modified rennin as an alternative to natural renin. There are many controversies that come with the use of GMOs and cheese makers are divided in this regard. France is one of only two countries that outlaw the use of genetically modified rennin, and due to not technically being an ingredient, is often not required to be listed as a GMO on the labels. Unlike the cheese makers in most countries, the French believe that natural renin is both better for the environment, but also provides better taste and more complex flavors (Chymosin, 2010).