This brief survey of how the meat of kosher animals is prepared, including its separation from milk products, is not an exhaustive guide. Those wishing to observe these regulations should seek guidance from a rabbi or someone else familiar with the laws and practice of kashrut. Reprinted with permission from The Jewish Religion: A Companion, published by Oxford University Press.
The strict prohibition of blood in the Bible (Leviticus 7:26-27; 17:10-14) is the basis for the laws governing the preparation of the meat of animals and birds for food. The usual practice is first to soak the meat in cold water for half an hour and then to salt it thoroughly and leave it covered in salt on a draining-board so that as much as possible of the blood is drained off. An alternative method is to roast the meat over a naked flame. In many communities, nowadays, the butcher attends to this process, relieving his customers from having to do it themselves. The blood of fishes is permitted so there is no process of "salting" for fish.
The biblical text repeats three times the prohibition of "seething a kid in its mother's milk" (Exodus 23:19; 34:26; Deuteronomy 14:21). This might originally have been intended to prohibit an act of such callousness or to ban any attempt at influencing nature by some kind of sympathetic magic, but in the rabbinic tradition, the prohibition means that no meat of any animal may be cooked in any milk (the "kid" and the milk of the "mother" referring only to the type of the forbidden mixture, i.e. the milk and meat of animals like a "kid" and its "mother"). As for the threefold repetition, this is said to forbid the initial cooking-together of meat and milk; to forbid the eating of meat and milk cooked together; and to forbid any benefit from the mixture (by selling it, for example, to a non-Jew to whom these laws do not apply).
According to one opinion in the Talmud, it is permitted to cook the meat of birds in milk but the accepted opinion is that this, too, is forbidden by rabbinic law on the grounds that if such is permitted, people may conclude that it is also permitted to cook in milk the meat of animals. This is an example of the rabbinic principle of "making a fence around the Torah." A further rabbinic extension is to forbid the eating of meat and milk together even when they have not been cooked or boiled together; for example, to have a glass of milk together with meat, or to eat meat together with buttered bread.