The Torah repeats three times that it is forbidden to cook a kid in its mother’s milk (Ex. 23:19,34:26; Deut. 13:21). The repetition tells us that not only is the cooking forbidden, but so is eating or deriving benefit from the cooked mixture (87:1). These Biblical prohibitions apply to the meat or milk of any kosher BEHEMAH (87:2-3). The sages prohibited eating mixtures of the meat and milk of any kosher animal, or of meat and human milk, or of milk and the flesh of kosher birds, but mixtures of milk with kosher fish or locusts are permitted (87:3-4). Cooking any type of meat in any type of milk is forbidden for appearance’s sake (see 87:4).

Biblically, “cooking” must involve a fire, but the sages extended it to include other types of cooking (87:6;91:8). “Meat” includes edible body parts (87:7,11) and unlaid eggs unless they are fully developed (87:5). “Milk” includes edible milk products (87:6,8-9). An animal’s udder should not be cooked in a pot or roasted with other meat, or even roasted by itself unless the milk has been squeezed out of it, but once this has been done it is like other meat (90:1-4).

Meat and milk products must not both be put on a table where people are eating if there is a chance that someone may eat from both of them (88:1-2). Storing them together, or storing either of them near food that may be eaten in combination with the other, should be avoided (see 88:1; 91:1;95:5-6). A person has eaten or chewed meat must not eat milk products at the same meal; it is customary to wait six hours before doing so, and traces of meat remaining on the hands or mouth must also be removed (89:1). According to most authorities a person who has eaten milk products may eat meat immediately provided there are no traces on his hands or mouth or on the table (89:2-4). It is customary to extend these prohibitions to foods that contain meat or milk, but not to foods that have been cooked in utensils in which meat or milk has been cooked (89:3;95:1). Bread should not be baked with ingredients derived from meat or milk unless there is no danger that it will be eaten with milk or meat (97:1).

Shulchan Aruch, Copyright (c) 1999 Project Genesis, Inc.