The Mishnayot in this perek (=chapter) have been discussing the laws of hadash – the new grains that are permitted only after the second day of Passover. The Mishnah on today’s daf (=page) enumerates the types of grain that fall into this category – wheat, rye, oats, barley and spelt – all of which are also obligated in the mitzvah of hallah.
In the Gemara Resh Lakish explains that the Mishnah specifically comes to exclude orez – rice (oryza sative) – and duhen– millet (panicum miliaceum). He derives this from the parallel between the commandment to separate hallah when eating lehem (see Bamidbar 15:19-21), and the commandment to eat matzah – lehem oni – for it is specifically from these types of grains that matzah can be made. The Gemara learns this from the passage (Devarim 16:3) that forbids the eating of hametz in the same context as the command to eat matzah, connecting the two to one-another.
Although our Gemara takes for granted that rice is not considered a type of grain, this is subject to a dispute between the Sages of the Mishnah in Masechet Pesachim (daf 35a), where we find that Rabbi Yohanan ben Nuri 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 mitzvah by baking it into matzah.
The accepted opinion understands that the process of mixing rice with water does not lead to himutz – fermentation – but to sirhon – spoilage. The Jerusalem Talmud explains that establishing which types of grains are those that can becomehametz and matzah was based on extensive research done by the sages, who experimented with the baking process to ascertain whether the fermentation 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.