The practice of having separate dishes for meat and dairy products (respectively called fleisch and milch in Yiddish, which is close to German)—indeed, of having the kitchen itself divided between meat and dairy use—is based on a rabbinic interpretation of the prohibition in the Torah that forbids boiling a young goat in its mother’s milk. This prohibition is found three times (see Exodus 23:19; 34:26; Deuteronomy 14:21).
What are the origins of the prohibition? Jewish biblical scholar Nahum Sarna outlines the history of Jewish interpretation of this prohibition, the explanation of which “largely remains an enigma.” Citing the opinions of different Jewish scholars, he writes:
Its importance may be measured by its being repeated twice more in the Torah, in Exodus 34:26 and Deuteronomy 14:21. In this latter source the prohibition appears in the context of the dietary laws, but the other two sources indicate that its origin lies in the overall context of the festivals. The juxtaposition of this rule with the law of the first fruits led Menahem ibn Saruq (10th cent.) to interpret gedi not as a kid of the goats but as “berries.” This eccentric explanation was taken up by Menahem ben Solomon (first half 12th cent.), who took “mother’s milk” to be figurative for the juice of the bud that contains the berry. The entire passage conveyed to him a proscription on bringing the first fruits before they are ripe. Many scholars, medieval and modern, follow the suggestion of Maimonides that this law prohibits some pagan rite—although no such rite is presently known.
Rashbam, Bekhor Shor, Ibn Ezra, and Abravanel all, in various ways, adduce a humanitarian motivation akin [see also Exodus 22:29]. Rashbam further suggests that because festivals were celebrated with feasts of meat, and because goats are generally multiparous and have a high yield of milk, it was customary to slaughter one of the kids of a fresh litter and to cook it in its mother’s milk. The Torah looks upon such a practice as exhibiting insensitivity to the animal’s feelings. The explanation of Rashbam has been buttressed by the modern observation that in biblical times goats were far more plentiful than sheep in the Land of Israel and were the main source of milk. The flesh of the young kid is more tender and more delicate in flavor than lamb. Also, since the estrous cycle of goats occurs during the summer months and parturition takes place in the rainy season, the earliest litter would be produced just around the time of Sukkot. This injunction, therefore, regulates the festivities at the Festival of the Ingathering of the Harvest.[46]
There is debate, then, about the rationale behind the law, but its application seems fairly straightforward: Do not boil a kid in its mother’s milk. The Talmudic rabbis, however, expanded greatly on this, as explained by biblical and Semitic scholar Jeffrey H. Tigay:
The text specifies only boiling the flesh of a kid in its own mother’s milk. Halakhic [Jewish legal] exegesis interpreted the rule more broadly, prohibiting the cooking or eating of any domestic cattle with the milk or milk products of any domestic cattle, or deriving any benefit from such mixtures. Supplementary regulations also prohibited eating fowl or game with milk and required the use of separate utensils for milk and meat, including their products. This broad interpretation is presumably based on the desire to prevent inadvertent violation of the original prohibition. In a society of small settlements where dairy and cattle farming were not kept separate, there was considerable likelihood that if a young animal was boiled in milk, the milk would come from its own mother. Furthermore, milk and meat from different types of animals are similar in appearance, and small pieces of food can adhere to most types of utensils. The halakhah minimizes the possibility of errors due to these factors.[47]
Thus, as summarized by Sarna,
The interdiction of boiling a kid in its mother’s milk was generalized to outlaw the mixing of all meat and milk (meaning all dairy products). Its threefold repetition in the Torah was explained by Rabbi Simeon bar Yohai as indicative of three aspects of the prohibition: cooking such a mixture, eating it, and deriving any benefit from it. [b. Hullin 115b.][48]
Put another way, the threefold repetition of this law in the Torah meant that Jews were prohibited from eating, tasting or enjoying milk and meat together, prompting a sarcastic rejoinder from a nineteenth-century nonreligious Jew who argued that, because the explicit prohibition against adultery is found but twice in the Torah, that means that Jews can eat it and taste it but not enjoy it!
For religious Jews, however, this is anything but a laughing matter. As explained on the Judaism 101 website:
This separation [of milk and meat] includes not only the foods themselves, but the utensils, pots and pans with which they are cooked, the plates and flatware from which they are eaten, the dishwashers or dishpans in which they are cleaned, and the towels on which they are dried. A kosher household will have at least two sets of pots, pans and dishes: one for meat and one for dairy. . . .
One must wait a significant amount of time between eating meat and dairy. Opinions differ, and vary from three to six hours. This is because fatty residues and meat particles tend to cling to the mouth. From dairy to meat, however, one need only rinse one’s mouth and eat a neutral solid like bread, unless the dairy product in question is also of a type that tends to stick in the mouth.
Note that even the smallest quantity of dairy (or meat) in something renders it entirely dairy (or meat) for purposes of kashrut. For example, most margarines are dairy for kosher purposes, because they contain a small quantity of whey or other dairy products to give it a dairy-like taste.[49]
So, the simple prohibition not to boil a kid in its own mother’s milk—either based on humanitarian grounds or in reaction to pagan practices—became the basis for an elaborate system of dietary laws, requiring different sets of kitchen utensils—indeed, a dual kitchen—not to mention minutely detailed rabbinic specifications of exactly what can be eaten and when.
Not surprisingly, verses such as Genesis 18:8 have caused considerable difficulty for the rabbinic interpreters who claim that Abraham kept both the oral and written Torahs. The verse states that Abraham “brought some curds and milk and the calf that had been prepared, and set these before [his guests]. While they ate, he stood near them under a tree.” So then, did Abraham violate the rabbinic law by serving milk and meat together? One rabbinic explanation is that, being angels, the guests did not really eat but only appeared to. (Presumably, that is why Abraham felt free to violate the rabbinic laws, since he knew they would not eat!) Another explanation is that, contrary to what the text seems to say, Abraham followed the rabbinic law and waited several hours to serve the meat after serving the dairy products. (After all, he had to give the guests something to eat, and it took a long time to prepare and cook the goat!)
Reform Jewish scholar Gunther Plaut, who, as a liberal rabbi, does not accept these Orthodox interpretations that project the later, rabbinic customs back on to the biblical text, notes:
Traditional [Jewish] interpreters experience great difficulties here [meaning Genesis 18]. If the three are divine messengers, why do they eat? According to the Midrash, they merely appeared to eat. According to Rashi, they pretended out of courtesy. The text of course is oblivious of later Jewish dietary laws which forbade serving milk and meat at the same meal.[50]
Certainly, Plaut is correct, and Abraham never heard of the rabbinic rules calling for the separation of milk and meat products. But for an ultra-Orthodox Jew, these are sacred commandments, going back to patriarchal times and clearly legislated at Sinai. This is also part of what rabbinic Judaism refers to as “making a fence [seyag] around the Torah” (see m. Avot 1:1), the rough equivalent of enforcing a 70-mile-per-hour speed limit by making a new, 55-mile-per-hour rule—just to make it easier for people to keep the law. This is similar to a host of rabbinic laws, such as the rule that Sabbath candles must be lit at least eighteen minutes before sundown, a custom discussed in the next answer.