The laws of Kashrut are the Jewish dietary laws, based on the book of Leviticus. Food that is in accord with Jewish law is said to be "kosher" (ritually clean); food that is not in accord with Jewish law is said to be "treif" (ritually unclean, also spelled "trayf").
The basic laws of kashrut are in the Torah and their details are explicated in the oral law, contained in the Mishnah and the Talmud. A common misunderstanding is that the Jewish dietary laws have to do with cleanliness; this is incorrect. The laws have to do with Levitical laws of ritual purity and holiness, sanctification.
The term "kosher" is frequently used in a metaphorical sense to mean "acceptable" or "approved". It is also part of some common product names. For example, "kosher salt", which is a particular form of salt in large crystals that makes it particularly suitable for preparing meat in accordance with Kashrut law, and is not meant to imply that the salt itself is kosher in the original sense (although it is, as is normal table salt, sea salt, and just about any other form of salt).
Kosher animals must have both cloven hooves and chew their cud. All kosher animals are herbivores that can be domesticated, such as cows, goats, deep and sheep. Jewish law states that kosher animals must be slaughtered in accordance with Jewish law, large blood vessels must be removed, and all blood must be removed from the meat; This is most commonly done by soaking and salting, but also can be done by broiling. The internal organs must be free of major defects.
Meat products are never mixed with dairy products in the same meal.
Milk, and milk-derived products, from kosher animals, is always kosher. In theory, then, all milk from cows is kosher. In practice, many Orthodox Jews use only "Cholov Yisroel" milk and dairy products; this label means that the milk that has been under constant rabbinical supervision from milking to bottling, to make sure that it is not adulterated with the milk of a non-kosher animal. In the past this was a serious issue; today this is not a practical concern in the USA or in most western countries; As such, most Modern and Centrist Orthodox rabbis, and all Conservative rabbis, have ruled that FDA supervision is sufficient for milk and dairy products to be considered automatically kosher.
No mixing of meat and dairy
One of the primary rules in kashrut is that milk products and meat products may not be eaten together in the same meal, much less cooked together. Jewish law thus mandates a set of fence laws that prevent this from ever happening: Jewish homes have two complete sets of silverware cookware, cups, and dishes. One is for milchik (milk) dishes, and one is for fleishing (meat) dishes. Foods that contain neither milk nor meat are called pareve (The Hebrew term), parve (the Yiddish term); both of these terms translate as "neutral".
Jewish law considers glass (and some say Pyrex) to be unabsorbent; thus, one theoretically could use one set of glass plates and dishes. In practice, this is never done because both of the cost and impracticality, but also because it would weaken the traditional system of kashrut observance. However, it is common even within the most religiously observant households to allow drinking glasses to be used for both dairy and meat meals, as long as they are throughly washed.
All fresh fruits and vegetables are kosher. Jewish law requires that they be carefully checked and cleaned to make sure that there are no insects on them, as insects are not kosher.
Canned and frozen foods
Most such goods are usually permissible since manufacturers add only water and spices during the packaging process. Sometimes, however, fruits or vegetables are prepared with milk products or with nonkosher ingredients such as nonkosher meat broth. Orthodox Judaism thus holds that canned and frozen goods should generally not be consumed unless there is a heksher (mark of rabbinical certification of kashrut) on the product. Conservative Judaism often is more lenient, and holds that a careful reading of the ingredients is sufficient.
Grains and cereals
Unprocessed grains and cereals are kosher. Processed items (e.g. dry cereals, baked goods) often contain small quantities of non-kosher ingredients. As such Orthodox Judaism holds that these goods should generally not be consumed unless there is a heksher (mark of rabbinical certification of kashrut) on the product. Conservative Judaism often is more lenient, and holds that a careful reading of the ingredients is sufficient.
Eggs from kosher birds are kosher; they are also considered pareve (neutral; neither milk nor meat.) Eggs that contain blood may not be used.
Kosher birds include: capon, duck (domestic), goose (domestic), chicken, turkey, guinea fowl and many others.
With one exceptions, all bugs and insects are forbidden as trief (un-kosher). The only exception is a type of locust from the Arabian peninsula; this type of locust encompasses four distinct species of locust. The tradition for identifying which species of locust were and were not ksoher has been lost among all Jews except the Jews of Yemen.
Cheese made from milk and non-animal enzymes is kosher. But much cheese is made from milk and rennet, and the kashrut of such cheeses is a matter of debate in the religious Jewish community.
Rennet is the enzyme used to turn milk into curds and whey; most forms of rennet derive from the lining of the stomach of an animal, and thus is classified by most religious Jews as a meat product. Thus, when making cheese, rennet must come from a halakhically slain animal, and the process must be supervised by Jews. A vegetable substitute for rennet can be used, in which case none of these restrictions apply. Other Jewish authorities maintain another long standing Jewish legal tradition: rennet is held to be a secretion of the stomach wall, and thus does not have the status of meat. Further, in its normal processing, rennet undergoes a chemical change and becomes inedible, this halakhic ally becoming a non-food; All foods in this category automatically lose any kashrut restrictions; They are considered to have changed so much from their original state that they are a d'var chadash, a new substance with properties significantly different from that of their original form. All such substances are considered pareve (neutral and kosher).
Fish and Seafood
To be kosher, a fish must have both fins and scales. The lack of either characteristic renders that species of fish unclean. Examples of unkosher fish include shark and catfish.
All shellfish, such as crabs, lobster, and shrimp are not kosher. All sea mammals, such as dolphins, whales and seals are not kosher. All other sea animals, such as octopus, squid, jellyfish and eels are unkosher.
Seaweed and other sea plant life are all kosher.
There are two fish that are controversial: Swordfish and sturgeon. Both of these have scales as young fish, but lose them later in life. Most Orthodox rabbis rule that these fish are unkosher; many Conservative rabbis rule that they may be kosher.
Another controversial topic is the status of gelatin. This substance comes from the processed bones of animals. If the source of gelatin is a kosher animal that was properly slaughtered according to Jewish law, than such gelatin is considered kosher by all Jews. All other gelatin is usually considered treif (non-kosher).
However, a number of prominent rabbinic authorities have noted that gelatin undergoes such extensive processing and chemical changes that it no longer has the status of meat, and as such may be considered pareve and kosher. Most Conservative Jews, and a significant minority of Israeli Orthodox Jews, accept that all gelatin is kosher.
The Reform and Reconstructionist Jewish movements do not mandate observance of the laws of Kashrut. Instead, they advise the study of Kashrut, and suggest that people follow those particular rules that the individual feels increases the sanctity of their life. As a result, there are some Reform and Reconstructionist Jews who do keep kosher.
James M. Lebeau "The Jewish Dietary Laws: Sanctify Life" United Synagogue of Conservative Judaism, NY, 1983
Samuel Dresner, Seymour Siegel and David Pollock "The Jewish Dietary Laws" United Synagogue, New York, 1982
Isidore Grunfeld "The Jewish Dietary Laws" London: Soncino, 1972
Isaac Klein "A Guide to Jewish Religious Practice", JTSA, 1992