I think many discussions of the gemara about rice not producing chameitz head in the wrong direction.

The definition of chameitz isn’t entirely scientific. After all, if you mix wheat flour with 100% apple juice, it also does not produce chameitz.

(Ashkenazim have a minhag for healthy people to avoid “egg matzah“, but that’s a tangent I found too long to go into. Sticking to halakhah…)

Second, there is a scriptural basis for the choice of the 5 grains that can be used to make matzah and whose leavened dough qualifies as chameitz. Israel is praised for its ability to grow 7 kinds of crops, the first two of which are grains (Devarim 8:8):

אֶ֤רֶץ חִטָּה֙ וּשְׂעֹרָ֔ה וְגֶ֥פֶן וּתְאֵנָ֖ה וְרִמּ֑וֹן אֶֽרֶץ־זֵ֥ית שֶׁ֖מֶן וּדְבָֽשׁ׃

a land of wheat and barley, of vines, figs, and pomegranates, a land of olive trees and honey;

And the beraisa (Manachos 70a, bottom) says:

כוסמין מין חיטים שיבולת שועל ושיפון מין שעורין כוסמין

Spelt is a type of wheat, oats (?) and rye are [each] a type of barley.

The two species of grain in the pasuq comprise a total of 5 strains. The list of grains that can yield chameitz, like the list of liquids, isn’t entirely scientifically defined. Not everything called “leavening” is called “chimutz” or fits the prohibition.

And yet, the Yerushalmi (Challah 1b; Pesachim 17a) records a dispute between the Chakhamim, who said that chameitz cannot be made from rice flour, and Rabbi Yochanan ben Nuri who said that it could. So, the gemara suggests, “So check it!” A scientific determination! But it turns out they did check it, but the two sides disputed about “iqar bediqasah — the essence of the check.” The argument was over how to determine the result.

It is science’s job to define the possible physical states reality could be in. It is the poseiq‘s job to tell us which of those states are included in a halachic statement, and which not.

To the Rabanan, the physical state they saw when they looked at what happened to rice dough was in the halachic set of states we call sirchon , to Rabbi Yochanan ben Nuri, the result is chimutz. Knowing the science is not enough to know how to categorize it halachically.

And we cannot confuse the same word, such as “leaven”, when used as a scientific term and when used as an English translation of a halachic term. And it must be even more confusing in Israel, where the terms coincide without any translation. But this is a case of homonyms.

We have a similar and more severe issue when it comes to defining death. The AMA adopted the definition of brain death. This isn’t a “we know what death is better now” situation. It’s a group of doctors and medical ethicists deciding what moral value to put on certain states of a human body, and not on others.

Halakhah may still retain a different definition. Or it may not; a poseiq may decide that Chazal used approximations for things we can now measure. Such as, perhaps halakhah‘s definition of death should distinguish between someone whose heart is running because their brain functions well enough to keep it running, and someone whose heart is running artificially. But that is a halachic question that has to be answered by posqim.

Just as a discourse on the biochemistry of fermentation and leavening could help inform someone about how the halakhah should be decided, how its definitions should be applied. But not what halakhah‘s criterion is for the definition itself.