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Kitniyot (Hebrew: קִטְנִיּוֹת, qitniyyot) is a Hebrew word meaning legumes. During the Passover holiday, however, the word kitniyot takes on a broader meaning to include grains and seeds such as rice, corn, sunflower seeds, sesame seeds, soybeans, peas, and lentils, in addition to legumes.
According to Orthodox Ashkenazi and some Sephardic customs, Kitniyot may not be eaten during Passover. Although Reform and Conservative Ashkenazi Judaism currently allow for the consumption of Kitniyot during Passover, long-standing tradition in these and other communities is to abstain from their consumption. According to Torat Eretz Yisrael and Minhagei Eretz Yisrael, any Jew worldwide, regardless of origin, and despite the practice of their forefathers, may eat kitniyot on Passover, for it is a practice rejected as an unnecessary precaution by Halachic authorities as early as the time of its emergence.
The Torah only prohibits Jews from eating chametz during Passover. Chametz is leaven made from the "five grains": wheat, spelt, barley, shibbolet shu'al (two-rowed barley, according to Maimonides; oats according to Rashi) or rye. There are additional rabbinic prohibitions against eating these grains in any form other than matzo.
Among Orthodox Ashkenazi and some Sephardic Jews, the custom (minhag) during Passover is to refrain from not only products of the five grains but also other grains and legumes. Traditions of what is considered kitniyot vary from community to community but generally include maize (American corn), as well as rice, peas, lentils, and beans. Many also include other legumes, such as peanuts and soy, in this prohibition. The Chayei Adam considers potatoes not to be kitniyot because they were unknown in the time when the prohibition was created, an opinion followed today by nearly all Ashkenazi authorities.
Since wheat flour only becomes chametz after it is ground and then mixed with water, one might assume that the kitniyot custom does not forbid kitniyot that were never ground or never came in contact with water. By this logic, it might be permitted to eat fresh kitniyot (like whole beans), or processed kitniyot which never came in contact with water (like certain squeezed oils or toasted solids). In fact, Rabbi Mordechai Eliyahu states that Ashkenazim in Israel would eat fresh kitniyot on Pesach until the 1700s, when new Ashkenazi immigrants (the students of the Vilna Gaon and Baal Shem Tov) brought with them the custom not to eat fresh kitniyot. Conservative rabbis have ruled to permit fresh kitniyot.
The Halakhic argument (the argument according to Jewish law and tradition) against eating kitniyot during Passover originated in early medieval France and Provence and later flourished in high medieval Ashkenazi (Rhineland) Germany.
The original reasons behind the custom of not eating kitniyot during Passover are not clear, though two common theories are that these items are often made into products resembling chametz (e.g. cornbread), or that these items were normally stored in the same sacks as the five grains and people worried that they might become contaminated with chametz. It is also possible that crop rotation would result in the forbidden chametz grains growing in the same fields, and being mixed in with the kitniyot. Those authorities concerned with these three issues suggested that by avoiding eating kitniyot, people would be better able to avoid chametz. Since Jewish law is quite stringent about the prohibition against chametz in the house during Passover, even in small amounts, a tradition developed to avoid these products altogether.
Vilna Gaon (Hagaos HaGra, ibid.) proposes a different source for this custom. The Gemara in Pesachim (40b) notes that Rava objected to the workers of the Exilarch cooking a food called chasisi on Pesach, since it was wont to be confused with chametz. Tosafot understand that chasisi are lentils, and thus, argues Vilna Gaon, establishes the basis for the concern for kitniyot. Rabbi David Golinkin in the Responsa of the Masorati (Conservative) Movement cites Rabbenu Manoah (Provence, ca. 1265) who wrote an opinion in his commentary on Maimonides (Laws of Festivals and Holidays 5:1) that "It is not proper to eat kitniyot on holidays because it is written that ‘you shall rejoice in your festivals’ and there is no joy in eating dishes made from kitniyot". Lentils were a food of mourners.
Even where the prohibition against kitniyot was practiced, some poskim opposed it, among them Rabbi Yeruham of 14th century Provence. Others, including Rav Moshe Feinstein did not advocate abandoning the custom, but he opposed expanding the list of forbidden kitniyot 
Reform Jewish authorities, such as the Responsa Committee of the Reform Jewish Movement for the principal organization of Reform rabbis in the United States and Canada, have also ruled in favor of permitting kitniyot. Reform Judaism first formally permitted eating kitniyot during Passover in the 19th century.
While most Conservative Jews observe the tradition of avoiding kitniyot during Passover, the Committee on Jewish Law and Standards, an authoritative body in Conservative Judaism, issued two responsa in December 2015 that said it was now permissible to eat these previously prohibited foods throughout the world. These responsa were based on a 1989 responsa by the Responsa Committee of the Israeli Conservative Movement that permitted Conservative Jews in Israel to eat kitniyot. While eating kitniyot has become more common in Israel, due in large part to the influence of Sephardic Jewish food customs, it is not yet clear whether Conservative Jews in other parts of the world will embrace the new rulings or continue to refrain from kitniyot.
Some Orthodox rabbis, such as David Bar-Hayim at 'Beth HaWaad' beth din of Machon Shilo and Conservative Rabbi David Golinkin of the Schechter Institute of Jewish Studies, have argued that the prohibition of kitniyot, while appropriate in Eastern Europe where the Askenazi tradition began, should not apply to the United States or Israel. According to The Forward, some Israelis are choosing a more permissive rabbinical interpretation of kitniyot, which allows for the consumption of a wider range of formerly banned items, and some Ashkenazi Jews in Israel who are married to Sephardic Jews have adopted the Sephardic custom. However, the Union of Orthodox Jewish Congregations of America and other Orthodox organizations still maintain that the prohibition is binding on all Ashenazic Jews worldwide. The Orthodox Union maintains a kitniyot hechsher intended for non-Ashkenazic Jews who consume kitniyot on Passover.
In the 1930s, Maxwell House coffee hired the Joseph Jacobs advertising firm in the 1930s to market to a Jewish demographic. The agency hired a rabbi to research coffee, resulting in a determination that the coffee bean is more like a berry than a bean, thus making it kosher for Passover.