The mammals and birds that may be eaten must be slaughtered in accordance with Jewish law. (Deut. 12:21). We may not eat animals that died of natural causes (Deut. 14:21) or that were killed by other animals. In addition, the animal must have no disease or flaws in the organs at the time of slaughter. These restrictions do not apply to fish; only to the flocks and herds (Num. 11:22).
Ritual slaughter is known as shechitah, and the person who performs the slaughter is called a shochet, both from the Hebrew root Shin-Cheit-Teit. The method of slaughter is a quick, deep stroke across the throat with a perfectly sharp blade with no nicks or unevenness. This method is painless, causes unconsciousness within two seconds, and is widely recognized as the most humane method of slaughter possible.
Another advantage of shechitah is that it ensures rapid, complete draining of the blood, which is also necessary to render the meat kosher.
The shochet is not simply a butcher; he must be a pious man, well-trained in Jewish law, particularly as it relates to kashrut. In smaller, more remote communities, the rabbi and the shochet were often the same person.
Draining of Blood
The Torah prohibits consumption of blood. Lev. 7:26-27; Lev. 17:10-14. This is the only dietary law that has a reason specified in Torah: we do not eat blood because the life of the animal (literally, the soul of the animal) is contained in the blood. This applies only to the blood of birds and mammals, not to fish blood. Thus, it is necessary to remove all blood from the flesh of kosher animals.
The first step in this process occurs at the time of slaughter. As discussed above, shechitah allows for rapid draining of most of the blood.
The remaining blood must be removed, either by broiling or soaking and salting. Liver may only be kashered by the broiling method, because it has so much blood in it and such complex blood vessels. This final process must be completed within 72 hours after slaughter, and before the meat is frozen or ground. Most butchers and all frozen food vendors take care of the soaking and salting for you, but you should always check this when you are buying someplace you are unfamiliar with.
An egg that contains a blood spot may not be eaten. This isn't very common, but I find them once in a while. It is a good idea to break an egg into a glass and check it before you put it into a heated pan, because if you put a blood-stained egg into a heated pan, the pan becomes non-kosher. If your recipe calls for multiple eggs, break each one into the glass separately, so you don't waste all of the eggs if the last one is not kosher!
Forbidden Fats and Nerves
The sciatic nerve and its adjoining blood vessels may not be eaten. The process of removing this nerve is time consuming and not cost-effective, so most American kosher slaughterers simply sell the hind quarters to non-kosher butchers.
A certain kind of fat, known as chelev, which surrounds the vital organs and the liver, may not be eaten. Kosher butchers remove this. Modern scientists have found biochemical differences between this type of fat and the permissible fat around the muscles and under the skin.