Kosher in Depth
The Hebrew word "kosher" literally means "acceptable." Foods that are permitted by the Torah and prepared according to Jewish law are kosher. Below are some of the basic principles that make up the kosher dietary laws.
The Torah (Leviticus 11:3) lists the characteristics of permitted animals as those with fully split hooves, who also chew their cud (ruminants). Kosher animals are always mammals and herbivores. The kosher animals commonly eaten today are the cow, goat and sheep -- and sometimes deer and buffalo.
The Torah enumerates 24 forbidden species of birds, and the Talmud explains that, among other signs, all birds of prey (vulture, hawk, eagle) are forbidden. In practice today, we eat only those birds for which there is an established tradition that the bird is kosher -- e.g. chicken, turkey, duck and goose.
As for "kosher eggs," they must come from a species of kosher bird (e.g. chicken).
The Torah (Leviticus 11:9) teaches that a kosher fish must possess both fins and scales. (Fins help the fish swim, and scales are a covering over the body.) Even if the fish has only one scale or one fin, it is permitted. Tuna, for example, have very few scales, yet is kosher. Other popular kosher fish are bass, carp, cod, flounder, halibut, herring, mackerel, trout and salmon.
Crustaceans (such as lobster and crab) and other shellfish (such as clams) are not kosher, because they lack scales. Further, all aquatic mammals (e.g. whales and dolphins) are not kosher.
And yes, there are kosher varieties of sushi and caviar -- providing it's from a kosher species (fins and scales), and that it was prepared only with kosher utensils (knife, cutting board, etc.).
Many are surprised to discover that four species of grasshoppers are kosher (Leviticus 11:22). However, all other insects are not kosher. One might think that this has little practical application to our modern eating habits. But in truth, many leafy vegetables (lettuce, broccoli) often contain insects and must be carefully examined before they can be eaten. Some fruits like raspberries and strawberries are also problematic. Rabbis have developed specific methods to properly check these fruits and vegetables for insects.
Besides being from a kosher species, kosher meat requires that the animal/bird be slaughtered in the manner prescribed by the Torah (Shechita). (Fish do not have this requirement.) In this procedure, a trained kosher slaughterer (shochet) severs the trachea and esophagus of the animal with a special razor-sharp knife. This also severs the jugular vein, causing near-instantaneous death with minimal pain to the animal.
After the animal/bird has been properly slaughtered, its internal organs are inspected for any physiological abnormalities that may render the animal non-kosher (treif). The lungs, in particular, must be examined to determine that there are no adhesions (sirchot) which may be indicative of a puncture in the lungs.
Animals contain many veins (e.g. Gid HaNashe) and fats (chelev) that are forbidden by the Torah and must be removed. The procedure of removal is called "Nikkur," and it is quite complex. In practice today, the hind quarter of most kosher animals is simply removed and sold as non-kosher meat.
The Torah forbids eating of the blood of an animal or bird (Leviticus 7:26); fish do not have this requirement. Thus in order to extract the blood, the entire surface of meat must be covered with coarse salt. It is then left for an hour on an inclined or perforated surface to allow the blood to flow down freely. The meat is then thoroughly washed to remove all salt. Meat must be koshered within 72 hours after slaughter so as not to permit the blood to congeal. (An alternate means of removing the blood is through broiling on a perforated grate over an open fire.)
1. Meat and Milk
The Torah forbids eating meat and milk in combination, and even forbids the act of cooking them together (as well as deriving benefit from such a mixture). As a safeguard, the Sages disallow the eating of meat and dairy products at the same meal, or preparing them with the same utensils. Therefore, a kosher kitchen must have two separate sets of pots, pans, plates and silverware -- one for meat/poultry and the other for dairy foods.
One must wait up to six hours after eating meat products before eating dairy products. However, meat may be eaten following dairy products (with the exception of hard cheese, which also requires a six-hour interval). Prior to eating meat after dairy, one must eat a solid food and the mouth must be rinsed.
2. Chalav Yisrael
A Rabbinic law requires that there be supervision during the milking process to ensure that the milk comes from a kosher animal. In North America, many people rely on the Department of Agriculture's regulations and controls as sufficiently stringent to fulfill the rabbinic requirement for supervision. Some, however, do not rely on this, and will only eat dairy products that are designated as Chalav Yisrael (literally, "Jewish milk").
3. Bishul Akum
Bishul Akum is a Hebrew term meaning, "cooked by a non-Jew." As a rabbinic safeguard against assimilation, certain foods cooked by a non-Jew are considered not kosher. While the details of this law are many, the basic rule is that any cooked food which: 1) could not have been eaten raw, and 2) is important enough to be served at a fancy meal table, may not be eaten if cooked by a non-Jew.
If a Jew assists with lighting the fire or the cooking, the food may be eaten even if it was cooked by a non-Jew (assuming, of course, that the food itself was kosher in every other way).
In keeping kosher, there is a grain-related issue called Chadash and Yashan -- literally "new" and "old." The Torah (Leviticus 23:14) says that if a grain (such as wheat) was harvested prior to Passover, then we may not eat that grain until after (the second day of) Passover.
This means that we have two kinds of grain: grain that hasn't celebrated its first Passover is (temporarily) forbidden as Chadash, while grain that has been around long enough to already have a Passover under its belt is Yashan, and permitted to eat.
Another grain-related issue is Challah. (This is not to be confused with the braided bread that we eat on Shabbat.) When one kneads a significant amount of dough (over 2.5 pounds) for baking purposes, a small portion of the dough is removed and burned. (In the times of the Holy Temple, this portion was given to a Kohen.) Once challah has been separated from the larger dough, the dough is "kosher" for baking into bread or other items.
Fruit that grows during the first three years after a tree is planted is called Orlah and is not kosher to be eaten. This law applies to trees both in Israel and the Diaspora. If you plant a fruit tree in your backyard, you cannot eat the fruit for three years, and there is a special procedure to render the fruit permissible to eat in the fourth year. (Consult with a rabbi.)
3. Israeli Produce
Trumah and Maaser are terms for various tithes that apply to Israeli-grown produce, to be given to the Kohen and Levi. Untithed foods are called Tevel and are not kosher to be eaten. If you're visiting Israel, or even if you're buying Israeli oranges or tomatoes in your local supermarket, you should make sure that proper tithes have been taken from all grains, fruits and vegetables.
The Torah (Leviticus ch. 25) says that every seven years, agricultural work must cease in the Land of Israel. This is called Shmita -- the seventh, sabbatical year. Produce that grows on land that was "farmed and worked" during the seventh year is not kosher. Today, with the return of a Jewish agricultural industry to Israel, the laws related to Shmita are once again very relevant. So if you're buying Israeli produce, make sure the laws of Shmita were properly observed.
With thanks to aish.com