When thinking about hametz, we usually associate it with a fermentation process (leavening) that occurs when grain flour is mixed with water and baked. The Mishna on our daf enumerates the types of grain that undergo this process – wheat, rye, oats, barley and spelt – but in a different context. According to the Mishna, it is specifically these types of grains from which matza can be made. The Gemara learns this from the passage ( 16:3) that forbids the eating of hametz in the same context as the command to eat matza, connecting the two to one-another.
The Gemara understands this to be a clear rejection of the position put forward by Rabbi Yohanan ben Nuri, who rules that rice is also a type of grain for which one would be held liable for eating if it became hametz, and that one could fulfill the mitzva by baking it into matza. The accepted opinion understands that the process of mixing rice with water does not lead to himutz – leavening – but to sirahon – decay. The Jerusalem Talmud explains that establishing which types of grains are those that can become hametz and matza was based on extensive research done by the sages, who experimented with the baking process to ascertain whether the leavening process takes place. With regard to a small number of grain-type products, there remained differences of opinions as to whether the process that took place should be considered himutz.
Although the conclusion of our Gemara clearly rejects the opinion of Rabbi Yohanan ben Nuri, nevertheless over centuries of Jewish history traditions arose that limited the use of kitniyot (pulses, or the grains of a legume) on Pesaḥ due to a concern that kernels of grain may become mixed in with them. Generally speaking, Ashkenazi communities limit their use. Among the traditions:
• Some make full use of kitniyot;
• Some forbid the use of rice, but permit other types of pulses;
• Some forbid the use of all kitniyot.
As a rule, people follow the traditions of their parents and communities.