Items Defined as Kitniyot 

Much debate has raged about which items to include in the minhag of kitniyot, with the outcome usually emerging in the stringent direction. Four issues are generally explored regarding this issue. Is the item defined as kitniyot (legumes, such as beans and lentils), or is it at least similar to kitniyot (see Rambam, Hilchot Kilayim 1:8-9)? Can the item be ground into flour in the same manner as grain? Was the item historically included in the custom of kitniyot? Does the item grow near grain fields (see Mishnah Berurah 453:13)? 

Historically, the Semak (thirteenth century) mentions rice, beans, lentils, chickpeas, sesame, and mustard. The Chok Yaakov (453:1) and Aruch Hashulchan (O.C. 453:3) include corn as kitniyot, most likely because it is ground into flour. While these foods have undoubtedly been included in the minhag of kitniyot, there are other foods whose status as kitniyot is more controversial. 

Potatoes 

Although surprising, some authorities include potatoes in the minhag of kitniyot, as they are ground into flour and can be confused with grain flour. The Chayei Adam mentions that some German Jewish communities include potatoes in the category of kitniyot. However, he notes that in 5531 (1771), rabbis permitted their use on Pesach due to famine (see Nishmat Adam 20). 

This ruling is not accepted (see Aruch Hashulchan, O.C. 453:5). Rav Moshe Feinstein (Teshuvot Igrot Moshe, O.C. 3:63) explains that since kitniyot were prohibited as a custom and not through rabbinical legislation, Rav Moshe asserts that we only define something as kitniyot if there is a custom to regard it as kitniyot. The early Ashkenazic rabbis did not forbid anything that could be ground into flour; rather, they forbade eating certain products. Thus, although there is a logical reason to include potatoes in the category of kitniyot, they were not historically included. Perhaps, Rav Moshe suggests, the great need for them kept them from being prohibited; perhaps the list of kitniyot was kept shorter because the reasons for banning any kitniyot are debated. Whatever the reason, Rav Moshe rules that we only must avoid products customarily avoided on Pesach. 

Peanuts 

Rav Tzvi Pesach Frank (Mikraei Kodesh 2:105) and Rav David Tzvi Hoffman (Teshuvot Melameid Leho'il 1:88) note the Jerusalem custom to regard peanuts as kitniyot. However, Rav Moshe arrives at the same conclusion regarding peanuts that he reached regarding potatoes: 

Peanuts were not regarded as kitniyot in many places. Thus, in an area where there does not exist a minhag to forbid peanuts, they should not be prohibited, for regarding these matters, one should not be excessively strict, as the Chok Yaakov writes [453:9]. Peanuts are forbidden for those whose minhag is to avoid them, although even they may be lenient in cases of doubt. One may give kashrut certification to peanut oil with no chametz, and those who do not follow the strict minhag may partake of it. 

Rav Moshe recorded in this responsum (written in 1966) the accepted practice of kashrut agencies. However, today, major kashrut organizations have changed, and they treat peanuts, and even peanut oil, as kitniyot (https://oukosher.org/passover/guidelines/food-items/kitniyot-list/). Once again, the general practice of being very strict about kashrut on Pesach has taken hold with kitniyot despite the rulings of such eminent authorities as Rav Feinstein. 

Soybeans 

Rav Yehudah Pris cites Rav Dov Lior as permitting soybeans for Pesach consumption (Techumin 13:176-178). Rav Pris explains that Rav Moshe's reasoning regarding peanuts applies equally to soybeans, as they were not known to the earlier generations that adopted the minhag prohibiting kitniyot. Despite the analogy to Rav Moshe’s Teshuva, the kashrut organizations view soybeans as kitniyot.

Quinoa  

The OU explains the reason for its policy change regarding quinoa:

Rabbi Menachem Genack, CEO of OU Kosher, announced today that quinoa, the grain-like seed grown in South America, is Kosher for Passover when processed with special OU Passover supervision and bearing the OU-P symbol. His statement is as follows: “It is only recently that quinoa has become popular outside of its high-altitude growing area in the Andean mountain region of South America. Known for its nutritional qualities, it has been referred to as a “superfood.” Due to conflicting reports regarding growing conditions and final usage of this new world, gluten-free pseudo-cereal plant, OU Kosher was hesitant to conclusively declare it Kosher for Passover and non-kitniyot. Following extensive research and on-site investigation of cross-contamination issues by OU Kosher personnel at all quinoa growing areas including: Puno, Cuzco, Arequipa, Ayacucho, Junin and Chiclayo in Peru; and Alto la Paz and Chayapata in Bolivia; as well as the collection, washing and milling stations of quinoa, OU Kosher is pleased to announce that, for the first time, it is recommending quinoa for Passover, when processed with special OU Passover supervision and bearing the OU-P symbol.

Rav Shoshan Ghoori describes his experiences as OU Kosher’s representative investigating quinoa processing in South America:

Since quinoa is a prized product both for export and for local consumption, it is generally grown in large fields that are focused on just quinoa. He added that as quinoa’s popularity and prices rise, this point is even stronger. He has found that the traditional use of quinoa is not the same as the use of grains. It is not generally made into breads or other similar grain type foods by the Andean communities, but rather is an all around ‘super food’ used for soups, and teas etc., just like maca, canihua and kiwicha. After visiting approximately fifteen quinoa processing plants he has yet to have found one (not a gathering nor a washing station) that produces or mixes problematic grains (that could be an issue for chometz or kitniyos) in the same plant as quinoa.

The Star-K reaches a similar conclusion:

It was determined that quinoa is Kosher l’Pesach. It is not related to millet, rice or the chameishes minei dagan, five types of grain products. Quinoa is a member of the amaranth family. STAR-K tested quinoa to see if it would rise. The result was sirchan, as termed by Chazal, which means the quinoa decayed and did not rise. Furthermore, quinoa’s growth does not resemble kitniyos and, as cited in Igros Moshe O.C. (3:63), we do not consider additional products beyond what was originally established. However, recent investigations have found that there is a possibility that quinoa grows in proximity to certain grains and/or is processed in facilities that compromise Kosher for Passover status. Therefore, quinoa may be used on Pesach only with reliable Kosher for Passover approval.

Conclusion 

It is surprising to discover a consensus among the major American kashrut organizations permitting quinoa for Ashkenazim on Pesach, especially since two major American Poskim, Rav Yisroel Belsky and Rav Mordechai Willig, rule strictly (and significant reason to forbid it remain). Moreover, a lenient consensus has not emerged regarding kitniyot oil, kitniyot shenishtanu, peanuts, and soybeans. Accordingly, although those who follow the lenient view may continue to do so, room to be strict remains.