Kashrut: History and Development
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.
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 thus engendered debate, and the sturgeon. The properties of new materials used to produce cookware, such as Pyrex in the twentieth century, also raised questions about how to classify them, since different materials known earlier—metal, earthenware, fine porcelain, glass—are each subject to different rules.
Jewish law and practice are 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 like fruit from a tree or to grow from a tree by its beak. Various Jewish legal authorities declared it either permitted for consumption like a fruit, subject to the laws covering other fowl, or entirely forbidden for use as food.
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