No one wants dead animals in their cheese. At least no one I know. Not if they really think about it.

But virtually hidden in the very cheese that would be, in reasonable amounts, part of any vegetarian diet, is rennet, a "product" that comes from the stomachs of calves and other young animals such as goats and sheep. In simple terms, it is a substance that helps part of milk turn from its liquid state to a solid. Remember Little Miss Muffet? I don't think her curds and whey got that way without rennet. Cheese, and products that contain cheese, are grocery staples that often contain rennet, which is only obtained after the animal is killed.
But rennet does provide a prime example of the importance of reading the label on any food product that does not come to you in its natural - or nearly natural state. For the vegetarian, as for anyone concerned about the nutritional. caloric and chemical content of their food, labels provide a very important avenue toward learning exactly what we are eating. This can be an adventure,or simply a nuisance as we shop.
Take, for instance, my recent trip to Trader Joe's, a store that though imperfect, is one of my favorites because I believe it tries to provide a wide variety of choices to the consumer, including vegetarians and those seeking organic products. That said, however, I must point out that during that recent trip, I looked at multiple packages of cheese before finding one that was not made with animal rennet. Checking in with a store worker helped very little as she was able to point out only one cheese (a cheddar) that contained no animal rennet. I eventually settled on a Monterey Jack that contained vegetable rennet.
Vegetable rennet is available because, according to Wikipedia, "Many plants have coagulating properties. Some examples include fig tree bark, nettles, thistles, mallow, and Creeping Charlie. Enzymes from thistle or cynara is used in some traditional cheese production in the Mediterranean. These real vegetable rennets are also suitable for vegetarians. Vegetable rennet might be used in the production of kosher cheeses but nearly all kosher cheeses are produced with either microbial rennet or GM rennet. Worldwide, there is no industrial production for vegetable rennet. Commercial so-called vegetable rennets usually contain rennet from the mold Mucor miehei."
But rennet is not the only pitfall for those who have eliminated from their diets the products of dead animals.
Some are obvious, such as lard, the fat made from pigs. Among the others, the one I initially found surprising was gelatin. Derived from the bones, hooves and skin of animals, gelatin is used in marshmallows, some candy and frosting and even some yogurts. (Hooves in candy and yogurt?) Yuck, is all I can say to that. Some other ingredients of concern are pepsin - an enzyme made from pigs' stomachs and oleic acid (oleinic acid) - an animal fat.
While this is by no means a complete list of all the products out there that can be a problem for vegetarians - remember, rennet is in many products that contain cheese - think spaghetti sauce - it does serve as an example of the importance of staying informed, learning from where products are derived and making educated choices as consumers. And as for Trader Joe's, I hope the woman I spoke to there really did pass on my concern about the high number of non-vegetarian cheeses it carries and my wish for a mozzarella with no rennet.

For more information: http://www.ivu.org/faq/ingredients.html