The Mishnayot in this perek have been discussing the laws of ḥadash – the new grains that are permitted only after the second day of Passover. The Mishna on today’s daf enumerates the types of grain that fall into this category – wheat, rye, oats, barley and spelt – all of which are also obligated in the mitzva of ḥalla.
In the Gemara Reish Lakish explains that the Mishna specifically comes to exclude orez – rice (oryza sativa) – and doḥan– millet (panicum miliaceum). He derives this from the parallel between the commandment to separate ḥalla when eating leḥem (see Bamidbar 15:19-21), and the commandment to eat matza – leḥem oni – for it is specifically from these types of grains that matza can be made. The Gemara learns this from the passage (Devarim 16:3) that forbids the eating of ḥametz in the same context as the command to eat matza, 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 Mishna in Massekhet Pesaḥim (daf 35a), where we find that Rabbi Yoḥanan ben Nuri rules that rice is also a type of grain for which one would be held liable for eating if it became ḥametz, 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 ḥimutz – fermentation – but to sirḥon – decay. The Jerusalem Talmud explains that establishing which types of grains are those that can become ḥametz and matza 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 ḥimutz.