The ĥametz prohibited by the Torah is produced from one of the five types of grain: wheat, barley, spelt, oats, and rye. Other species such as rice and millet, even if they rise, do not undergo the same fermentation process as the five cereal grains, and they may be eaten on Pesaĥ. Although one Tanna, R. Yoĥanan b. Nuri, maintains that rice is also a cereal grain and forbidden by the Torah in its leavened state, the rest of the Sages maintain that even if rice rises, it may be eaten on Pesaĥ (Pesaĥim 35a). This was the practice of all the great Tanna’im and Amora’im. In fact, Rava ate rice at the Seder (ibid. 114b).
During the medieval era of the Rishonim (c. seven centuries ago), the Jews of Ashkenaz (Germany, especially the Rhineland) began to refrain from eating kitniyot on Pesaĥ. Initially, only some communities observed this stringency, but within a few generations the custom had spread to all Ashkenazic communities.
Three principal reasons for this custom have been offered: A) Since kitniyot are cooked in the same manner as grains, in a pot, there is concern that if people cook rice on Pesaĥ they will end up mistakenly cooking forbidden types of grain. B) Since kitniyot, like cereal grains, are often made into flour, if the unlearned masses see pious Jews cooking and baking foods with kitniyot flour without concern for it becoming ĥametz, they are liable to do the same with grain flour as well. The rabbis of the Talmud were not concerned about this because, in their day, Jewish tradition was clear and established. However, the tribulations of the exile and the scattering of Jewish communities gave rise to a fear that some Jews would be cut off from tradition and come to forget what is forbidden and what is permitted. Eating kitniyot on Pesaĥ would cause them to err and eat forbidden cereal grains without taking care that they do not become ĥametz. C) Grain and kitniyot kernels are similar in appearance and are kept in the same storehouses for relatively long periods. It is therefore eminently possible that wheat or barley kernels would find their way into kitniyot, and when the kitniyot are cooked the grain will become ĥametz. This concern persists today, and indeed it is possible to find kernels of grain when checking kitniyot.
Another reason why kernels of kitniyot and cereal grain got mixed together was the common practice of crop rotation. Farmers often grow legumes for a year to replenish the soil of a field that had been used for growing grain for many years. However, kernels of the previous crop inevitably remain in the field. Thus, if a coriander or fenugreek crop is grown after a wheat crop, wheat will sprout among the coriander or fenugreek, and some kernels of wheat or barley will be found in the harvested crop of legumes. Experience shows that sometimes the quantity of cereal kernels in the kitniyot exceeds one sixtieth of the entire quantity. This problem applies to those species of kitniyot that physically resemble cereal grain.