As we have learned, the difference between bread and matza is that the dough used for making bread has undergone a leavening process resulting from the fermentation of ingredients within the flour that have come into contact with water. In order to augment the leavening process, bakers customarily mix se’or into the dough, causing the dough to ferment more thoroughly and quickly. However, even without the leavening agent, if dough were left without kneading, it would ferment and rise. Therefore, when preparing matzot one must work quickly to ensure that the leavening process within the dough does not begin.
As long as the dough is in motion, being kneaded, it does not become ĥametz. Even if the kneading were to continue an entire day, the dough would not become ĥametz, since kneading inhibits the leavening process. However, if the dough sat motionless for eighteen minutes, the leavening process has begun and all the prohibitions concerning ĥametz apply to it. This applies to normal conditions, but where it is hotter, the leavening process is accelerated, and the dough becomes ĥametz in even less than eighteen minutes.
Cracks appearing in the dough are a physical indication that the dough has become ĥametz. Even if eighteen minutes without kneading had not yet passed, since there are cracks in the dough, it has certainly become ĥametz; apparently conditions were warm and so it took less time to become ĥametz. Moreover, the kneading may have been inadequate, so that certain parts of the dough were neglected, causing those areas to become ĥametz. Even if there are only a few cracks, and they appeared only in part of the dough, the entire dough is ĥametz. If no cracks appeared but the dough blanched, it is ĥametz nuksheh (hardened ĥametz; see next section), which is rabbinically forbidden (SA 459:2).
. The rising of the dough indicates that it has become ĥametz (Me’iri), and other symptoms, namely, cracks in the dough, a blanched appearance, and the time that has elapsed, are only relevant in a case where the dough did not begin to rise. However, there are situations in which the dough rises but this does not indicate that it has become ĥametz; rather, it has undergone what the Sages call “sirĥon” (spoilage). This applies to the case of rice, or of flour mixed with fruit juice, according to most poskim. However, when wheat flour mixes with water and begins to rise, this is in fact a sign that the mixture has become ĥametz. If the dough sat and was not kneaded for the time that it takes to walk one mil, it has become ĥametz even if there is no visible indication, as explained in the Mishna and Gemara in Pesaĥim 46a. SA 459:2 explains that this amount of time is eighteen minutes, although Rambam and R. Ovadia of Bertinoro maintain that it is twenty-four minutes. BHL ad loc. rules that these lenient positions may be relied upon to prevent a significant loss of money, and that the authorities rule according to the Shulĥan Arukh without even mentioning the more lenient opinion because of the strict nature of the prohibition of ĥametz.
According to Rashi and Me’iri, however, one must check the amount of time that the dough has been sitting only when it is unclear whether or not the leavening process has begun: if more than the time it takes to walk one mil has elapsed, the dough is ĥametz, and vice versa. If one is certain that the dough is not ĥametz, even if it has been sitting for longer than the time that it takes to walk one mil, it is not considered ĥametz. Nonetheless, according to most Rishonim in any situation where the dough has sat for longer than it takes to walk one mil, the dough is ĥametz. See, for example, MT, Laws of Ĥametz and Matza 5:13; SA 459:2. Pri Megadim’s introduction to §467 contends that such dough is complete ĥametz; eating it on Pesaĥ incurs the punishment of karet. Rashbatz writes that one must suspect that such dough has become ĥametz. In a warm place, dough becomes ĥametz in less than the time it takes to walk a mil, as written in Aguda in the name of the Ge’onim. This is the halakhic consensus, as it is also the ruling of Yerei’im, Mordechai, and Hagahot Maimoniyot. Additionally, Rosh writes that even if the dough became warm in one’s hands, the dough will become ĥametz more quickly. This is cited in SA and Rema 459:2 (see also Birur Halakha 46a). Although there is dispute as to whether or not the “sitting times” of the dough are combined, Terumat Ha-deshen rules that a full kneading of the dough cancels the previous sitting time, though merely poking the dough would not be effective. This is cited in MB 459:16.
Pesaĥim222222 48b states: “as long as the dough is being worked it cannot become ĥametz.” The vast majority of Rishonim, including Rambam in Laws of Ĥametz and Matza 5:3 and Tur and SA 459:2, explain that as long as one keeps kneading the dough it will not become ĥametz, even if he does so for the entire day. Yet it appears that the Yerushalmi disagrees with the Bavli and says that if one kneaded the dough for the amount of time that it takes to walk four mil, the dough is considered ĥametz. Baĥ cites Ri’az that ideally we should act in accordance with the Yerushalmi. There is an even more stringent opinion – Ritva’s – according to which as long as one kneads the dough at the appropriate pace so that he will complete the kneading in less time than it would take to walk one mil, the dough does not become ĥametz. However, if the kneading goes on for longer than this time, the dough is considered ĥametz (see Birur Halakha on Pesaĥim 48b; Encyclopedia Talmudit, s.v.“ĥametz,” §5, pp. 74-75). Even though the vast majority of poskim disagree with Ritva, they rule that it is still preferable to be stringent and complete the kneading process within eighteen minutes, as explained in AHS 459:7.