What Is Leavening?

The Rabbi’s Opening Post

What Is Leavening?

Sent on 5/4/2006

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What Is Leavening?

In the thread on overnight-rested water* the question was asked more than once: what is leavening? Is it a chemical phenomenon that can be measured, or merely an intuitive way of looking at things? Nisht referred me to an article by Dr. Monk in ‘Tehumin,’ vol. 1, but I do not have it before me at the moment. This morning I heard an interesting lecture by R. Aharon Katz, head of the Institute for Advanced Torah Studies (=the advanced full-time study program) at Bar-Ilan University, and I would like to present the substance of it here for the public benefit.

He showed there, quite convincingly (I understand that this is also the accepted approach among experts in the field; perhaps this is explained in Monk’s article?), that in dough there are two different and almost independent processes: rising and leavening. Rising is caused by yeast organisms, which are present in the glucose found in fruit and flour, and it consists in an increase in volume. This happens not only in the five grain species, but also in rice and millet. By contrast, leavening is caused by the gluten proteins in flour, and these exist only in the five grain species; therefore only there does the category of leaven apply (Jerusalem Talmud, Pesachim, ch. 2: ‘They examined and found that only the five species alone can become either unleavened bread or leavened bread’). During the period in which the dough is left standing, the gluten gives the dough a different texture, more convenient for working and for shaping forms (as opposed to flour and water immediately after mixing, which are not convenient for this), and it also gives it a grayer appearance (the dough tends toward gray). According to him, this very process is what leavening is. Chemists told him that when flour is mixed with fruit juice (without any added water), the gluten has no possibility of acting, and therefore there is no leavening at all in such a case. The entire concern regarding enriched dough is that water may have been mixed into it in addition to the fruit juice (as with natural juices in our day, which are mixed with water). In such a case, some opinions maintain that leavening occurs more quickly. The distinction between the two processes is important, since there are cases of rising without leavening and vice versa: 1. Dough that is submerged in water does not rise, because that requires contact with air, but it certainly does become leavened. Therefore, despite the appearance that it does not rise, the prohibition of leaven applies there. 2. Scalding in hot water prevents rising but not leavening. Rising is optimal and fastest at around 37 degrees Celsius, but leavening continues up to 98 degrees Celsius. Indeed, at higher temperature it acts more quickly. Below 10 degrees Celsius the gluten is inactive. At room temperature, leavening takes about 20 minutes. 3. Wheat kernels that became wet do not rise but do become leavened. 4. Fruit juice, as explained above. 5. ‘Deaf dough’ (Pesachim 46a) is dough with little water. According to the second formulation in Rashi there, it is not evident that it has leavened, because it does not rise. But leavening certainly does occur there. Therefore the concern that it may become leavened is measured by means of a parallel dough that has enough water; there, rising serves as an indication of leavening. From here comes the indicator of the time needed to walk a talmudic mile (18 minutes) when the dough is left unworked. And indeed, the leavening process takes about 20 minutes at room temperature. 6. When one is working the dough, it does not become leavened (the gluten is inactive).

One of my friends suggested a nice interpretation in light of these points: it says in Pesachim 35a that the five grain species can become leavened, whereas rice and millet come only to spoilage (R. Yoḥanan ben Nuri prohibits them as well because they are close to leavening—a source for the status of legumes). ‘Spoilage’ is related to the expression ‘the excess has spoiled,’ that is, going beyond the proper measure. Rice and millet do indeed rise in water, and therefore they fall under ‘spoilage,’ but they contain no gluten and therefore no leavening occurs in them.

If so, leavening is a distinct chemical process, and it can be measured by scientific means. The Sages apparently were impressively precise in their phenomenological descriptions (although of course they were not aware of gluten and its effects), and they distinguished well between these two processes.

I should note that in the morning there was a lecture there by R. Shabtai Rappaport, who made somewhat different claims (as is his way) regarding leavening, but I was not there.

* ‘Overnight-Rested Water Nowadays’ — Miky

(Corrected, and a link was added at the request of the esteemed author, Kelemer)

Source (the ‘Stop Here, Think’ forum): http://www.bhol.co.il/forums/topic.asp?topic_id=1869861