The Gemara learns out Matzah from Chametz, that only grains which can become Chametz are considered bread that they can be used for Matzah (and Challah, Chadash, etc.).
Yet one of the grains in the Mishnah is Shiboles Shu'al, which Rashi identifies as oat. One can just look at oat dough to see that it doesn't rise. How, then, can oats be considered a grain, when it doesn't fit the definition provided by the Gemara?
Doniel Filreis
Rabbi Levi Yitzchak Halperin of the Institute for Halachah and Science explained in a lecture that there are objective ways to define the five types of grains as Chametz as opposed to other types of grains. He described an experiment made by the institute to demonstrate why rice does not become Chametz. A chemist made dough from rice and from wheat, and then separated the gluten from the starch in each of the doughs. The chemist them took the starch from the wheat and mixed it with rice gluten, and the starch from the rice was mixed with the wheat gluten. The result was that the wheat gluten caused the rice to behave like wheat and to rise while the wheat starch did not rise.
This proves that the gluten is the main factor in leavening the compound. After investigating gluten levels in the five grain species as opposed to rice, oats were found to have an 11% gluten level (wheat had 12%), while rice had 4%. This shows that the gluten causes a chemical reaction.
Rabbi Mendel Eisenberg (Techumin 23) states that the criterion for Chametz is not the rising, since if a dough receives a small amount of water it will not rise but it is termed Batzek ha'Charash (Pesachim 46a) which is forbidden if a similar dough will rise. He concludes that the rising is not the Chametz factor, because, if so, how could one reverse the procedure as is suggested in the Mishnah (Pesachim 48a)? Rabbi Eisenberg therefore says that the main difference between the species is the gluten which makes the dough more flexible before it enters the oven, and not the rising of the dough.
Yoel Domb