For a few cheeses, the milk is curdled by adding acids such as vinegar or lemon juice. Most cheeses are acidified to a lesser degree by bacteria, which turn milk sugars into lactic acid, then the addition of rennet completes the curdling.See Cheese and kashrut .) As cheese is a dairy food, under kosher rules it cannot be eaten in the same meal with any meat. Rennet derived from animal slaughter, and thus cheese made with animal-derived rennet, is not vegetarian.