Rennet: (n) An extract from the stomachs of calves and lambs that contains the enzyme rennin, which is used to curdle milk in foods such as cheese. Or more explicitly ... after butchering, the fourth stomach of a young calf is removed and freed of its food content. Next, the stomach goes through several steps including being dry-salted, washed, scraped to remove surface fat, stretched onto racks where moisture is removed, then finally ground and mixed with a salt solution until the rennet is extracted. Many vegetarians don't consider that some of the cheeses they are eating could actually contain unfamiliar animal ingredients. That's right. Cheese, a common staple in many vegetarian diets, is often made with rennet or rennin, which is used to coagulate the dairy product.
Because of the limited availability of proper stomachs for rennet production (a by-product of veal production), cheese makers have looked for other ways to coagulate the milk. There are many sources of enzymes, ranging from plants and fungi to microbial that will substitute for animal rennet. Cheeses produced from any of these varieties of rennet are suitable for lacto-vegetarians to consume. Microbial rennet is used more often in industrial cheesemaking in North America today because it is less expensive than animal rennet, whereas cheese from Europe and artisnal cheeses are more likely to be made from animal rennet due to tradition.
At Trader Joes you will find an extensive list of cheeses offered, with explanations for each as to what type of rennet was used to make it. Most of the cheese labels will also tell you if it is made with animal, microbial or vegetable rennet enzymes. Whole Foods also employs numerous "cheese whizes" who have a plethora of knowledge in this area. No matter where you shop, ask questions, look at labels and think about what you are eating!