Kashrut: History and Development

Rabbinic Judaism elaborated a series of practices for putting the biblical restrictions into practice.

Jewish women preparing a family meal in Minnesota, circa 1940. (Jewish Historical Society of the Upper Midwest/Wikimedia Commons)

Judaism is known for its unique system of kashrut, distinguishing between foods that are permissible and those that are not. The system is rooted in Torah, but significantly indebted to later rabbinic developments.

Dietary Requirements in the Torah

The consumption of food and drink, considered one of the great joys of life in the Bible, is subject to a number of restrictions in the Torah. In large measure, the concern is eating only approved species, which the Torah describes extensively (though not exhaustively). Among land animals, reptiles and most insects are forbidden (with the exception of certain types of locusts). Mammals are permitted provided that they chew their cud and have split hooves, which includes sheep, cows and goats, but excludes many others, including rabbits and pigs. Only certain types of fowl are similarly acceptable, and only their eggs may be eaten. Among sea creatures, only fish with fins and scales may be consumed.

In addition, even among species that are permitted, certain organs are not. The flesh of acceptable mammals and fowl could be consumed, but not their blood. The sciatic nerve is also forbidden.

One additional restriction is the ban on cooking a kid [young goat] in its own mother’s milk.

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Rabbinic Development

The rabbis of Judaism’s formative period laid out complex rules for the slaughtering of kosher animals and for the removal of blood from meat by salting it and soaking it in water. In addition, from the ban on cooking a kid in its mother’s milk, repeated three times in the Torah, the rabbis made a tripartite ban on combinations of the taste from the meat of a kosher animal (even kosher fowl) with the taste of the milk of a kosher mammal. One is forbidden by rabbinic law to cook such a combination, consume it or derive economic benefit from it in any way.

Over centuries of development, with popular practice influenced by the rulings of rabbinic authorities, these last restrictions developed into a system of separation articulated in great detail: different sets of utensils for milk and meat, for example, and, in many homes, color-coded tablecloths, placemats and dishtowels. Elaborate systems were developed for undoing (or letting pass) near-infractions of these rules — such as the accidental inclusion of a very small amount of milk in a meat dish, or vice-versa.

In most cases, rabbinic rulings on the dietary laws developed in the direction of increasing stringency. The Babylonian Talmud, for example, bans the consumption of milk during a meal after meat has been consumed at that meal. The post-talmudic practices in various communities, though, range from waiting one hour to waiting as long as six hours after consuming food containing meat before consuming food containing milk.

Ongoing Questions

New discoveries and new technologies have occasioned questions about the application of the principles of kashrut in medieval and modern times. The rabbis had extrapolated rules from the biblical lists of kosher fowl, but the turkey and the pheasant, once Jews were exposed to them, were the subject of debate and disagreement. The same is true of the swordfish, which shed their scales in adulthood, and the sturgeon. The properties of new materials used to produce cookware, such as Pyrex in the 20th century, also raised questions about how to classify them, since different materials — metal, earthenware, fine porcelain, glass — are each subject to different rules. The development of lab-grown meat also raised questions about consuming meat that has not come from a slaughtered animal.

Jewish law and practice are always dependent on the state of knowledge in the larger society at any given time. This is illustrated in a most piquant fashion by the debates in medieval European Jewry about the barnacle goose, which was widely believed to grow from a tree by its beak. Accordingly, some Jewish legal authorities at the time declared it permissible to consume, like a fruit.