A reader writes:

I read a book recently that asserts that the rennet used in curdling milk for cheese production comes the stomachs of calves, lambs, kids or pigs. I have not been able to verify the use of pigs for rennet. I found one source (only one, and I do not whether it is reliable) that says that since cheese consumption has outstripped the availability of stomachs of ruminant animals (calves, lambs and kids), the enzyme pepsin is now used as well to much the same effect. Is is true that pepsin is obtained from pigs?

Jeanne Yacoubou, MS, VRG Research Director, responds:

No source whom I have contacted, (and I have spoken with many major enzyme manufacturers in the world), has ever claimed that pepsin, by itself, is used to make cheese. Although animal rennet market supply does fluctuate, and this is one reason why animal rennet is not commonly used today to make cheese, the largest source of the enzyme used in cheese making in the United States today is microbial.

Sometimes, cheesemakers, especially those in Europe and Japan seeking a "natural" non-GMO product, use a combination of enzymes, (proteases and lipases), to make their product. But in these cases, the enzymes are generally derived from calves, lambs and kids (i.e., baby goats), not pigs.