Reasons for food being considered non-kosher include the presence of ingredients derived from non-kosher animals or from kosher animals that were not properly slaughtered, a mixture of meat and milk, wine or grape juice (and their derivatives) produced by gentiles, the use of produce from Israel that has not been tithed properly, or even the use of cooking utensils and machinery which had previously been used for non-kosher food.
In Judaism, most of the laws of Kashrut pertain to animals. The Torah explicitly states which animals are permitted or forbidden. In regard to birds, the Torah lists no general rule is given, and instead the Deuteronomic Code and Priestly Code explicitly list the prohibited birds, using names that have uncertain translations; the list seems to mainly consist of birds of prey, fish-eating water-birds, and the bat (which isn't a bird).
By contrast, for water creatures, Leviticus and Deuteronomy both give the general rule that anything residing in the waters (which Leviticus specifies as being the seas and rivers) is ritually clean if it has both fins and scales, in contrast to anything residing in the waters with neither fins nor scales, which Leviticus calls filth (Hebrew: sheqets). All flying creeping things were also to be considered ritually unclean, according to both Leviticus and Deuteronomy, but unlike Deuteronomy, Leviticus identifies four exceptions; the exceptions are of uncertain translation, but are clearly locusts and similar creatures, and there is a tradition upheld by Jews from Yemen about which animals constitute the kosher locusts.
With regard to land beasts (Hebrew:Behemoth), Deuteronomy and Leviticus both state that anything which chews the cud and has a cloven hoof would be ritually clean, but those animals which only chew the cud or only have cloven hooves would be unclean. The texts identify four animals in particular as being unclean for this reason - the hare, hyrax, camel, and pig - although the camel both ruminates and has two toes, while the hare and hyrax are coprophages rather than ruminants; the latter issues have been discussed by many, including the recent book on the subject by Rabbi Natan Slifkin. Leviticus, but not deuteronomy, also states that every creeping thing which creeps upon the earth should be considered filthy (Hebrew: sheqets).
One of the main biblical food laws is the forbidding of eating blood on account of the life [being] in the blood; this ban and reason are listed in the Noahide Laws, and twice in leviticus, as well as by Deuteronomy.
In order to comply with this prohibition, a number of preparation techniques became practiced within traditional Judaism. The main technique, known as melihah, involves the meat being soaked in water for about half an hour, which results in the pores being opened; after this, the meat is placed on a slanted board or in a wicker basket, and is thickly covered with salt on each side, and left for between 20 minutes and 1 hour. The salt covering draws blood from the meat by osmosis, and so the salt must be subsequently removed from the meat (usually by trying to shake most of it off, and then washing the meat twice) in order to complete the extraction of the blood.
Melihah is not sufficient to extract blood from the liver, lungs, heart, and certain other internal organs, since they naturally contain a high density of blood, and therefore these organs are usually removed before the rest of the meat is salted; roasting on the other hand will usually cause blood to be discharged, and it is therefore the usual treatment given to these organs (if they are to be eaten at all), and it is also an alternative cooking method for the rest of the meat.
The hindquarters of a mammal are not kosher unless the sciatic nerve and the fat surrounding it are removed (Genesis 32:32). This is a very time-consuming process demanding a great deal of special training, and is rarely done outside Israel where there is a greater demand for kosher meat. When it is not done the hindquarters of the animal are sold for non-kosher meat.
Bee honey is Kosher, even though bees are not, because the honey is made by the bee, not a secretion of the bee. One basis for this is that Israel is referred to in the Torah as the "Land of Milk and Honey," and it is accepted that this reference would not speak of a non-kosher entity.
The Shulchan Aruch (Yoreh De’ah 115:1) rules one may consume only "cholov yisroel" (חלב ישראל), or milk produced with a Torah-observant Jewish person present. Lacking proper supervision, one cannot be sure whether the milk came from a kosher animal. Some recent American rabbinical authorities, most notably Rabbi Moshe Feinstein, ruled that the protection provided by cholov yisroel is unnecessary because the regulations imposed on the US milk industry by the USDA are so focused and strict that the milk industry can be trusted to self-regulate themselves (i.e. when they label an item "cow's milk" to not include milk from any other animal). Some Haredi and Modern Orthodox rabbis hold that this leniency cannot be employed and only milk and dairy products with milk-to-bottle supervision may be consumed.
The situation of cheese is complicated by the fact that the production of hard cheese usually involves rennet, an enzyme which splits milk into curds and whey. Although rennet can be made from vegetable or microbial sources, most forms are derived from the stomach linings of animals, and therefore could potentially be non-kosher. Rennet made from the stomachs of kosher-animals, if they have been slaughtered according to the kosher rules, would itself be kosher, but mixing it with milk would violate the rule against mixing milk and meat, thereby making the resulting cheese non-kosher.
Jacob ben Meir, one of the most prominent medieval rabbis, championed the viewpoint that all cheese was kashrut, a standpoint which was practised in communities in Narbonne and Italy. Contemporary Orthodox authorities do not follow this ruling, and hold that cheese requires formal kashrut certification to be kosher, some even arguing that this is necessary for cheese made with non-animal rennet. In practice, Orthodox Jews, and some Conservative Jews who observe the kashrut laws, only eat cheese if they are certain that the rennet itself was kosher.
The status of gelatin is a controversial topic. True gelatin consists of denatured proteins, and comes from the processed hides or bones of animals, usually pigs or cows. This also affects the status of some brands of marshmallows. Most kosher products today use fish-based gelatin.
Another issue with gelatin is whether it is parve ('not dairy, nor meat'). A kosher parve 'gelatin' made from vegetable gums such as carrageenan combined with food starch from tapioca (which is also suitable for vegans) is commercially available in supermarkets which have substantial Kosher food sections. It does behave differently than protein-based gelatin, however, and cannot always be substituted directly for animal gelatin without modification of the recipe (mixing it with hot water instead of cold water). Other gelatin-like materials available include combinations of carrageen and other vegetable gums, such as guar gum, locust-bean gum, xanthan gum, gum acacia, and agar, chemically modified food starch, and chemically modified pectins. Recently, such products have been used in prepackaged gelled fruit products, replacing animal-based gelatin.
Although most gelatin is considered non-kosher, several 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 parve and kosher. This is the position adopted by some Orthodox rabbis, including Rabbi Ovadia Yosef, the former Sephardic Chief Rabbi of Israel.
A professional slaughterer, or shochet (שוחט), using a large razor-sharp knife with absolutely no irregularities, nicks or dents, and checked carefully between killing each animal, makes a single cut across the throat to a precise depth, severing both carotid arteries, both jugular veins, the vagus nerve, the trachea and the esophagus, no higher than the epiglottis and no lower than where the cilia begin inside the trachea, thus causing the animal to bleed to death. Any variation from this exact procedure invalidates the process; therefore, if the knife catches even for a split second or is found afterward to have developed any irregularities, or the depth of cut is too deep or shallow, the carcass is not kosher (nevela) and is sold as regular meat to the general public. The shochet must not only be rigorously trained in this procedure, but also a pious Jew of good character who observes the Sabbath, and who remains cognizant that these are God's creatures who are sacrificing their lives for the good of himself and his community and should not be allowed to suffer in any way. Traditionally in smaller communities, the shochet was often the town rabbi or the rabbi of one of the local synagogues. Large factories which produce kosher meat have professional full time shochtim on staff.
Once killed, the animal is opened to determine whether there are any of seventy different irregularities or growths on its internal organs, which would render the animal non-kosher. The term glatt kosher (although it is often used colloquially to mean "strictly kosher") literally means "smooth", and properly refers to meat where the lungs have absolutely no adhesions (i.e. scars from previous inflammation), thus there was never even a question of their not being kosher.
Compromises in countries with animal cruelty laws that prohibit such practices involve stunning the animal to lessen the suffering that occurs while the animal bleeds to death. However, the use of electric shocks to daze the animal is often not accepted by some markets as producing meat which is kosher.
The classical rabbis prohibited any item of food that had been consecrated to an idol, or had been used in the service of an idol; since the Talmud views all non-Jews as idolaters, and viewed intermarriage with appehension, it included within this prohibition any food which has been cooked/prepared completely by non-Jews. However, bread sold by a non-Jewish baker wasn't included in the prohibition; similarly a number of Jewish writers believed that food prepared on behalf of Jews, by non-Jewish servants, wouldn't count as idolatry, although this view was opposed by Jacob ben Asher.
Consequently, modern Orthodox Jews generally believe that wine, cheese, certain cooked foods, and sometimes even certain dairy products, should only be prepared by Jews. The prohibition against drinking non-Jewish wine, traditionally called yayin nesekh (literally meaning wine for offering [to a deity]), is not absolute. Cooked wine (Hebrew: yayin mevushal), meaning wine which has been heated, is regarded as drinkable on the basis that heated wine was not historically used as a religious libation; thus kosher wine includes mulled wine, and pasteurised wine, regardless of producer, but Orthodox Judaism only regards other forms of wine as kosher if prepared by a Jew.
Some Jews refer to these prohibited foods as akum, an acronym of Obhde Kokhabkim U Mazzaloth, meaning worshippers of stars and planets; akum is thus a reference to activities which these Jews view as idolatry, and in many significant works of post-classical Jewish literature, such as the Shulchan Aruch, it has been applied to Christians in particular. However, among the classical rabbis, there were a number who refused to treat Christians as idolaters, and consequently regarded food which had been manufactured by them as being kosher; this detail has been noted and upheld by a number of religious authorities in Conservative Judaism, such as Rabbi Israel Silverman, and Rabbi Elliot N. Dorff.
Conservative Judaism is more lenient; in the 1960s, Rabbi Israel Silverman issued a responsum, officially approved by the Committee on Jewish Law and Standards, in which he argued that wine manufactured by an automated process was not manufactured by gentiles, and therefore would be kosher. A later responsum of Conservative Judaism was issued by Rabbi Elliott Dorff, who argued, based on precedents in 15th-19th century responsa, that many foods, such as wheat and oil products, which had once been forbidden when produced by non-Jews, were eventually declared kosher; on this basis he concluded that wine and grape products produced by non-Jews would be permissable.
For obvious reasons, the Talmud adds to the biblical regulations a prohibition against consuming animals which have been poisoned. Similarly the Yoreh De'ah prohibits the drinking of water, if the water had been left overnight and uncovered in an area where there might be serpents, on the basis that a serpent might have left its venom in the water; it has since been discovered that snake venom is generally non-toxic if drunk, rather than injected via a bite.
A concern for the health of the eater is also behind the instigation, by the Talmud and Yoreh Deah, to never eat or cook fish with meat, and instead ensure that the mouth is washed between consuming fish and consuming meat; these texts explain that the prohibition is for the purpose of avoiding leprosy, a disease which the texts suggest would be caused by eating meat and fish together, although it is now known that leprosy is caused by a parasitic species of bacterium. Some rabbis of modern Orthodox Judaism continue to follow this ban
There are some restrictions on consumption of produce grown in Israel. The fruit of a tree for the first three years is not consumed (in keeping with the law of orlah, meaning foreskin). For crops grown in Israel, tithes must be taken and allocated according to the precepts of the Bible, otherwise the entire crop will not be considered kosher. In Israel, stores that sell fruits and vegetables will usually display kosher certification. The certificate ("teudah") must be from the current month.
Outside of Israel, it is generally accepted by kosher consumers that all fresh produce is considered kosher and may be purchased from any store without restriction. However, laws apply to inspection for insects.
All fresh fruits and vegetables are kosher in principle. 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 (except certain grasshoppers and crickets according to the Jews of Yemen only, 21.2C_29-30.2C_42-43.29). The Orthodox community is particular not to consume produce which may have insect infestation, and check and wash certain forms of produce very carefully. Many Orthodox Jews avoid certain vegetables, such as broccoli, because they may be infested and exceedingly hard to clean. Some kashrut certifying organizations completely recommend against consumption of certain vegetables they deem impossible to clean.
According to the U.S. Dept. of Agriculture, commercially it is not possible to remove all insects, and a sizeable amount remain. Responding to this issue, some companies now sell thoroughly washed and inspected produce for those who do not wish to do it themselves, even going to the trouble of filtering the wash water to ensure that it carries no microscopic creatures [see discussion of such animals in tap water, above]. These may or may not meet rabbinical standards for being insect-free.
Processed items (e.g. dry cereals, baked goods, canned fruits and vegetables, frozen vegetables, and dried fruit such as raisins) can also include small quantities of non-kosher ingredients. This is because these items are often cooked and processed in factories using equipment that is also used for non-kosher foods, may involve containers used for processing that have been greased with animal fats. Sometimes additives are introduced, and fruits or vegetables may have been prepared with milk products or with ingredients such as non-kosher meat broths.
For these reasons, Orthodox rabbis advise against consuming such products without a hechsher (mark of rabbinical certification of kashrut) being on the product. By contrast, some Conservative rabbis regard a careful reading of the ingredients to be a sufficient precaution. However, certain processed foods are usually regarded (by most Jews) as being an exception: plain tea, salt, 100% cocoa, carbonated water, some frozen fruits, including berries, and coffee, have only very basic processing from their natural state; these fruits are often frozen in their natural form and then bagged, while carbonated water is generally just the addition of carbon dioxide to natural water.
During Passover, there are additional food restrictions in Orthodox Judaism; in this branch of Judaism, leavened products are prohibited during the festival. Jews who are concerned about accidentally consuming leavened food, during passover, typically maintain an entirely separate set of crockery and cutlery for Passover; it is also common for those concerned about such things to rigorously clean their homes, to ensure that even the tiniest of remains of leavened products are removed. Some Jews even have a separate kitchen exclusively for use during Passover.
Any products made from the traditional five species of grain, which might have been inadvertently moistened after harvest, and thus begun to ferment (an aspect of the leavening process), are regarded by Orthodox Jews as prohibited during Passover; the five species are conventionally viewed to be wheat, rye, barley, emmer (sometimes confused with spelt, which did not historically grow in the Middle East), and either two-rowed barley or oats, with Maimonides and the Jerusalem Talmud supporting two-rowed barley, and Rashi including oats instead.
Among the Ashkenazi Jews there is an additional customary practice of avoiding the consumption of kitniyot (literally meaning little things) during Passover; the list of items regarded as kitniyot varies between communities, and can include things such as rice, legumes (including peas, peanuts, and beans), and corn. Due to the prevalence of corn syrup in certain well known processed foods, such as Coca-Cola, many items common in western countries are regarded as impermissible by Ashkenazic Jews, during Passover; to take account of this cultural issue, Coca-Cola produces and distributes kosher for Passover Coke, made without corn syrup, in the United States of America during Passover.
The following rule of kashrut is not universally observed: