From two biblical passages (Leviticus 11 and Deuteronomy 14:3-21) the following rules are extracted regarding which animals, birds, and fishes are kosher and which terefah [non-kosher], or treyf.
Only animals that have cloven hooves and that chew the cud are permitted. The pig does have cloven hoofs but does not chew the cud and is, consequently, forbidden. In the course of time, Jews came to have an aversion to the pig in particular, especially after Jews, in the period of the Maccabees [second century BCE] were ready to give their lives rather than eat pig-meat when ordered by tyrants to do so as an expression of disloyalty to the Jewish religion as a whole. Many a Jew today, otherwise not too observant of the dietary laws, will still refuse steadfastly to eat swine-flesh. It might be remarked, however, it is only eating of the pig that is forbidden. Surprising though this may seem at first glance, there is no objection, in Jewish law, to a Jew having a pigskin wallet.
The passage in Deuteronomy (14:4-5) gives a list of the animals that chew the cud and have cloven hooves and are thus kosher: oxen, sheep, goats, deer, gazelles, roebuck, wild goats, ibex, antelopes, and mountain sheep. It is interesting to note that whale meat and whale oil are forbidden not because the whale is a forbidden fish but because the whale is a mammal that, obviously, does not have cloven hooves and does not chew the cud.
With regard to birds, the Bible gives a list of the forbidden birds, implying that all others are kosher. But since the exact identity of the birds mentioned is uncertain, it is the practice only to eat birds that are known by tradition to be kosher, such as chickens, turkeys, ducks, geese, and pigeons. The eggs of forbidden birds are terefah, but quails’ eggs are permitted since the quail is a kosher bird (see Numbers 12:31-2).
Nowhere in the whole of the Bible is there any reference to a particular fish, only to fish in general. In the two passages dealing with the dietary laws it is stated that only fish that have fins and scales are kosher. The Talmud lays down the rule that a fish that has scales also has fins, so that what actually determines which fishes are kosher is the existence of scales. A problem arises as to how “scales” are defined. [The medieval thinker] Nahmanides understands that only scales that are detachable from the skin of the fish qualify as scales. Where they cannot be detached they are not considered to be scales at all but part of the fish itself.
This is the reasoning behind a fierce debate that took place between rabbinic authorities in the eighteenth century regarding the permissibility of caviar, derived from the sturgeon, since the scales of the sturgeon cannot easily be detached from the skin of the fish, although they can be removed by the application of a lye solution. Some Orthodox Jews today do consider [such] caviar to be kosher, others do not.
There are similar problems regarding turbot and swordfish. Conservative Rabbis have ruled that swordfish is kosher, since the Talmud states explicitly that it is kosher. Most Orthodox Rabbis, however, are doubtful whether the fish mentioned in the Talmud as kosher is the swordfish. English rabbis in the nineteenth century ruled that the turbot is a kosher fish but their opinion is now generally rejected by British Orthodox Jews.
Worms, frogs, eels, and all shellfish such as crabs and prawns [shrimp] are not kosher. With regard to locusts, the Bible (Leviticus 11:21-2) does state that four species of locust are kosher, but it is difficult to know how these can actually be identified, so that nowadays very few observant Jews eat locusts (although in some oriental countries the kosher type of locusts are eaten, as they were in the biblical period).
As noted elsewhere, the Bible forbids the eating of the meat of an animal torn (terefah) by wild beasts and it also forbids (Deuteronomy 14:21) the meat of an animal that has died of its own accord, called nevelah, a carcass. The Rabbinic understanding of these two terms is that any animal that has not been killed in the manner known as shechitah [kosher slaughtering] istreated as nevelah, and any animal that has serious defects in its vital organs is treated as a terefah, so that its meat is forbidden even if it has been killed in the proper manner. This applies to birds as well as to animals.
There is a vast literature on how to determine which type of organic disease renders an animal or bird terefah. Observant Jews, for instance, will bring to a rabbi a chicken that seems to have some defect when it is opened up. After an examination, the rabbi will declare it to be either kosher or terefah. Similarly, after an animal has been killed, the shochet, the one who performs the act of shechitah, isrequired to carefully examine the lungs of the animal to see whether there are adhesions, some of which render the animal terefah.
Not all adhesions on the lungs render the animal terefah, and a rabbi is called upon to decide in doubtful cases. But the practice has developed among the more observant of permitting only animals the lungs of which have no adhesions at all. Such an animal is called glatt kosher, from the Yiddish “glatt” meaning smooth i.e., the lungs are smooth, without adhesions. A curious development from this in more recent years is to extend the term “glatt kosher” to all products, so that when a product is stated to be [“glatt”] the meaning is: free of any possible taint that can render it terefah. “Glatt kosher” has thus come to mean something like “very kosher” or “strictly kosher.”
Pronounced: KOH-sher, Origin: Hebrew, adhering to kashrut, the traditional Jewish dietary laws.
Pronounced: TALL-mud, Origin: Hebrew, the set of teachings and commentaries on the Torah that form the basis for Jewish law. Comprised of the Mishnah and the Gemara, it contains the opinions of thousands of rabbis from different periods in Jewish history.