The most famous prohibitions in kashrut, the body of laws regarding what is kosher, may be against eating pig, and consuming meat and milk together.
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Like the other Jewish dietary laws, these restrictions have their origins in a few laconic lines in the Bible, in the books of Leviticus and Deuteronomy. But it is only in later Hebrew texts – in the Talmud and later rabbinical works – that the full range of laws are fully developed and delineated, and they range far from mere "meat" and "milk."
The word "kosher" comes from the Hebrew “kasher,” literally meaning “fit” – in this case, for consumption. Those foods that are not kosher, called tref or trefah, are ritually unclean or unfit according to Jewish law. (Tref is Yiddish for “unkosher,” from the Hebrew word terefah, meaning “torn,” referring to an animal found dead or injured in the field, but used today to describe any forbidden animal, including one that has died of natural causes.)
So what is kosher and Jewish food?
The laws of kashrut apply throughout the entire year and detail the raw foods that one is permitted to eat, the manner in which animals are to be slaughtered, and restrictions on how foods can be cooked or served. During the festival of Passover, additional restrictions apply.
Leviticus 11:3-8 and Deuteronomy 14:3-21 outline the actual animals whose flesh is allowed for consumption, or is forbidden. All invertebrates, with the exception of certain types of locust – though not all rabbis are agreed on this – are forbidden. To be kosher, mammals must have split (cloven) hooves and be ruminants; that is, they must chew and then redigest their cuds.
The Torah (the Hebrew Bible’s first five books) specifically notes that the pig, camel, hyrax and rabbit are forbidden because they each lack one of these two requirements.
The milk of non-kosher animals is also forbidden. Marine animals must have scales on their exterior and fins, thus disqualifying all shellfish and some other creatures, such as catfish.
Contrary to popular conception, food does not need to receive a rabbinical blessing to be considered kosher. Similarly, it is not people who are kosher – it’s the food they eat.
All fruits and vegetables are permitted, but they must be clean of insects.
Living animals must be slaughtered according to the laws of kashrut. Thus even a potentially kosher animal not killed in accordance with regulations is rendered tref.
The method of slaughter, known as shehitah, uses a sharpened blade to cut the animal's throat. It is designed to cause the least amount of pain to the animal and some evidence has shown the animals lose consciousness within two seconds of being slaughtered.
In the contemporary context, meat is permitted only if the animal has been slaughtered with rabbinical supervision in a manner considered to be quick and relatively painless. Animal flesh must have the blood drained from it, and certain cuts of meat are forbidden in most circumstances.
Mixing meat and milk
The biblical line warning against “seeth[ing] a kid in its mother’s milk” (appearing first in Exodus 23:9) is the basis for the body of kashrut laws requiring separation of meat and milk.
Meat products are not to be cooked or served, or even stored, together with dairy products, and traditionally, one must wait some hours (the amount varies from one tradition to another) after eating meat before one can consume a milk product. (No waiting period is required for eating meat after dairy products.)
Fowl was not included in the Biblical prohibition against mixing meat and milk, but a rabbinical decree extended the prohibition to include it.
The separation of meat and milk consumption has led to separation of cooking utensils and tableware for the two types of foods – "meat dishes" also called fleishig in Yiddish and "dairy dishes" known as milchig in Yiddish.
Eggs, fish and all produce are considered pareve (neither meat nor milk, hence neutral), and can be eaten with both dairy or meat products.
It is not cuisine or particular dishes – for example, bagels or cholent or gefilte fish – that determines if food is kosher, nor does the term “kosher-style” have meaning in Jewish law. Any style of food can be kosher if its ingredients and manner of preparation are in accord with the rules of kashrut.
Why keep kosher?
There are many theories regarding the rationale for the laws of kashrut, but the truth is that the Bible does not provide a moral justification for them: They are to be followed because they are divinely commanded.
In effect, though, the laws have served to keep Jews separate from non-Jews over the centuries. Until recently, the restrictions made it very difficult for Jews, most of whom kept kosher, to break bread with non-Jews, so that social interaction was very limited. This was one factor helping to keep intermarriage to a minimum, and may in part explain the long survival of the Jewish people.
In modern times, the Reform movement has made kashrut a matter of personal choice. In Conservative, or Masorti, Judaism, all the laws of kashrut are considered binding, although the movement is more lenient than Orthodoxy on certain issues, such as consumption of non-kosher wine. (Roughly, wine is kosher if it has not been handled by non-Jews during production.)
In practice, it has been estimated that one-sixth of American Jews, and three-quarters of Israeli Jews, maintain kosher homes. But even among people who consider themselves to keep “strictly” kosher, there has always been significant variation in the way the dietary laws are observed.
Glatt kosher and mehadrin
In particular, as levels of Orthodoxy and ultra-Orthodoxy have grown, differences between communities (for example, different Hasidic sects) are often manifested by the stringency of kashrut observance.
Terms such as “glatt” and “mehadrin” refer to the strictness by which the laws are interpreted and applied; in practice, they mean that different Orthodox communities will rely only on food whose preparation and packaging has been supervised by the kashrut authority their rabbinical leaders trust.
Within Israel, for example, for some decades, different ultra-Orthodox communities have been unwilling to depend on the kashrut supervision provided by the Chief Rabbinate, and have established their own kashrut authorities, generally called “badatz” organizations, a Hebrew acronym referring to the particular rabbinical court that oversees the supervision.
In the United States, meanwhile, there are a number of kosher symbols on food packages, but the OU symbol of the Union of Orthodox Jewish Congregations of America is the most recognized.