Kashrut
Among the numerous laws that form "kashrut" are prohibitions on the consumption of certain animals (such as pork and shellfish), mixtures of meat and milk, and the commandment to slaughter mammals and birds according to a process known as "shechita". There are also laws regarding agricultural produce that might impact the suitability of food for consumption.
Foie gras
The Judaic dietary law, Kashrut, forbade lard as a cooking medium, and butter, too, was proscribed as an alternative since Kashrut also prohibited mixing meat and dairy products. Jewish cuisine used olive oil in the Mediterranean, and sesame oil in Babylonia, but neither cooking medium was readily available in Western and Central Europe, so poultry fat (known in Yiddish as "schmaltz"), which could be abundantly produced by overfeeding geese, was substituted in their stead. The delicate taste of the goose's liver was soon appreciated; Hans Wilhelm Kirchhof of Kassel wrote in 1562 that the Jews raise fat geese and particularly love their livers. Some Rabbis were concerned that eating forcibly overfed geese violated Jewish food restrictions. The Chatam Sofer, Rabbi Moses Sofer, contended that it is not a forbidden food (treyf) as none of its limbs are damaged. This matter remained a debated topic in Jewish dietary law until the Jewish taste for goose liver declined in the 19th century. Another kashrut matter, still a problem today, is that even properly slaughtered and inspected meat must be drained of blood before being considered fit to eat. Usually, salting achieves that; however, as liver is regarded as "(almost) wholly blood", broiling is the only way of kashering. Properly broiling a foie gras while preserving its delicate taste is difficult, and therefore rarely practiced. Even so, there are restaurants in Israel that offer grilled goose foie gras. Foie Gras also bears resemblance to the Jewish food staple, chopped liver.
Appreciation of fattened goose liver spread to gastronomes outside the Jewish community, who could buy in the local Jewish ghetto of their cities. In 1570, Bartolomeo Scappi, chef de cuisine to Pope Pius V, published his cookbook "Opera", wherein he describes that "the liver of [a] domestic goose raised by the Jews is of extreme size and weighs [between] two and three pounds." In 1581, Marx Rumpolt of Mainz, chef to several German nobles, published the massive cookbook "Ein Neu Kochbuch", describing that the Jews of Bohemia produced livers weighing more than three pounds; he lists recipes for it—including one for goose liver mousse. János Keszei, chef to the court of Michael Apafi, the prince of Transylvania, included foie gras recipes in his 1680 cookbook "A New Book About Cooking", instructing cooks to "envelop the goose liver in a calf's thin skin, bake it and prepare [a] green or [a] brown sauce to accompany it. I used goose liver fattened by Bohemian Jews, its weight was more than three pounds. You may also prepare a mush of it."