Shechita

In Judaism, shechita is ritual slaughtering of certain mammals and birds for food according to kashrut. One who practices this, a kosher butcher, is called a shochet.

Biblical sources

Deuteronomy 12:21 states that sheep and cattle should be slaughtered "as I have instructed you", but nowhere in the Torah are any of the practices of shechita described. Instead, they have been handed down in Rabbinic Judaism's Oral Torah, and codified in halakha.

Species

The animal must be of a permitted species. For mammals, this is restricted to ruminants which have split hooves. For birds, although biblically any species of bird not specifically excluded in Deuteronomy 14:12–18 would be permitted, doubts as to the identity and scope of the species on the biblical list led to
rabbinical law permitting only birds with a tradition of being permissible.
Fish and grasshoppers do not require kosher slaughter to be considered kosher, but are subject to other laws found in Leviticus 11:9-12, which determine whether or not they are kosher.

''Shochet''

A shochet is a person who performs shechita. To become a shochet, one must study which slaughtered animals are kosher, what disqualifies some animals from being kosher, and how to prepare animals according to the laws of shechita. Subjects of study include the preparation of slaughtering tools, ways to interpret which foods follow the laws of shechita, and types of terefot.
In the Talmudic era, rabbis started to debate and define kosher laws. As the laws increased in number and complexity, following ritual slaughter laws became difficult for Jews who were not trained in those laws. This resulted in the need for a shochet to perform the slaughtering in the communities. Shochtim studied under rabbis to learn the laws of shechita. Rabbis acted as the academics who, among themselves, debated how to apply laws from the Torah to the preparation of animals. Rabbis also conducted experiments to determine under which terefot animals were no longer kosher. Shochtim studied under these rabbis, as rabbis were the officials who first interpret, debate, and determine the laws of shechita.
Shochtim are essential to every Jewish community, so they earn elevated social status. In the Middle Ages, the shochtim were a close second to rabbis in social status. Shochtim were respected for committing their time to studying and for their importance to their communities.
An inspection of the animal is required for it to be declared kosher, and a shochet has a double title: Shochet u'bodek, for which qualification considerable study as well as practical training is required.

Procedure

The shechita procedure, which must be performed by a shochet, is described in the Yoreh De'ah section of the Shulchan Aruch only as severing the wind pipe and food pipe. Nothing is mentioned about veins or arteries.
However, in practice, as a very long sharp knife is used, in cattle the soft tissues in the neck are sliced through without the knife touching the spinal cord, in the course of which four major blood vessels, two of which transport oxygenated blood to the brain the other two transporting blood back to the heart are severed. The vagus nerve is also cut in this operation. With fowl, the same procedure is followed, but a smaller knife is used.
A [|special knife] of considerable length is used; no undue pressure may be applied to the knife, which must be very sharp. The procedure may be performed with the animal either lying on its back or standing.
In the case of fowl the bird is held in the non-dominant hand in such a way that the head is pulled back and the neck exposed, while the cut made with the dominant hand.
The procedure is done with the intention of causing a rapid drop in blood pressure in the brain and loss of consciousness, to render the animal insensitive to pain and to exsanguinate in a prompt and precise action.
It has been suggested that eliminating blood flow through the carotid arteries does not cut blood flow to the brain of a bovine because the brain is also supplied with blood by vertebral arteries, but other authorities note the distinction between severing the carotid versus merely blocking it.
If the entirety of both the trachea and esophagus are not severed, then an animal may still be considered kosher as long as the majority of the trachea and esophagus were severed of a mammal, or the majority of either one of these in the case of birds. The cut must be incised with a back-and-forth motion without employing any of the five major prohibited techniques, or various other detailed rules.

Forbidden techniques

Shehiyah Pausing during the incision and then starting to cut again makes the animal's flesh unkosher. The knife must be moved across the neck in an uninterrupted motion until the trachea and esophagus are sufficiently severed to avoid this. There is some disagreement among legal sources as to the exact length of time needed to constitute shehiyah, but today the normative practice is to disqualify a kosher cut as a result of any length of pausing.
Derasah The knife must be drawn across the throat by a back and forth movement, not by chopping, hacking, or pressing without moving the knife back and forth. There are those who assert that it is forbidden to have the animal in an upright position during shechita due to the prohibition of derasah. They maintain that the animal must be on its back or lying on its side, and some also allow for the animal to be suspended upside down. However, the Rambam explicitly permits upright slaughter, and the Orthodox Union as well as all other major kosher certifiers in the United States accept upright slaughter.
Haladah The knife must be drawn over the throat so that the back of the knife is at all times visible while shechita is being performed. It must not be stabbed into the neck or buried by fur, hide, feathers, the wound itself, or a foreign object which may cover the knife.
Hagramah ''Hagramah refers to the location on the neck on which a kosher cut may be performed; cutting outside this location will in most cases disqualify a kosher cut. According to today's normative Orthodox practice, any cutting outside this area will in all cases disqualify a kosher cut. The limits within which the knife may be applied are from the large ring in the windpipe to the top of the upper lobe of the lung when it is inflated, and corresponding to the length of the pharynx. Slaughtering above or below these limits renders the meat non-kosher.
Iqqur If either the esophagus or the trachea is torn during the shechita incision, the carcass is rendered non-kosher. Iqqur can occur if the esophagus or trachea is torn out while handling an animal's neck or if the esophagus or trachea is torn by a knife with imperfections on the blade, such as nicks or serration. In order to avoid tearing, the kosher slaughter knife is expertly maintained and regularly checked with the shochet's fingernail to ensure that no nicks are present.

Breaching any of these five rules renders the animal nevelah''; the animal is regarded in Jewish law as if it were carrion.
Temple Grandin has observed that "if the rules are disobeyed, the animal will struggle. If these rules are obeyed, the animal has little reaction."

The knife

The knife used for shechita is called a sakin, or alternatively a chalaf by Ashkenazi Jews. By biblical law the knife may be made from anything not attached directly or indirectly to the ground and capable of being sharpened and polished to the necessary level of sharpness and smoothness required for shechita. The tradition nowadays is to use a very sharp metal knife.
The knife must be at least slightly longer than the neck width but preferably at least twice as long as the animal's neck is wide, but not so long that the weight of the knife is deemed excessive. If the knife is too large, it is assumed to cause derasah, excessive pressing. Kosher knife makers sell knives of differing sizes depending on the animal. Shorter blades may technically be used depending on the number of strokes employed to slaughter the animal, but the normative practice today is that shorter blades are not used. The knife must not have a point. It is feared a point may slip into the wound during slaughter and cause haladah, covering, of the blade. The blade may also not be serrated, as serrations cause iqqur, tearing.
The blade cannot have imperfections in it. All blades are assumed by Jewish law to be imperfect, so the knife must be checked before each session. In the past the knife was checked through a variety of means. Today the common practice is for the shochet to run their fingernail up and down both sides of the blade and on the cutting edge to determine if they can feel any imperfections. They then use a number of increasingly fine abrasive stones to sharpen and polish the blade until it is perfectly sharp and smooth.
After the slaughter, the shochet must check the knife again in the same way to be certain the first inspection was properly done, and to ensure the blade was not damaged during shechita. If the blade is found to be damaged, the meat may not be eaten by Jews. If the blade falls or is lost before the second check is done, the first inspection is relied on and the meat is permitted.
In previous centuries, the chalaf was made of forged steel, which was not reflective and was difficult to make both smooth and sharp. Shneur Zalman of Liadi, fearing that Sabbateans were scratching the knives in a way not detectable by normal people, introduced the Hasidic hallaf. It differs from the previously used knife design because it is made of molten steel and polished to a mirror gloss in which scratches could be seen as well as felt. The new knife was controversial and one of the reasons for the 1772 excommunication of the Hasidim. As of present time, the "Hassidic hallef" is universally accepted and is the only permitted blade allowed in religious communities.