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The leavening of the dough:
The Biblical term of Chametz refers only to dough [of the five grains] that has been kneaded with water and has leavened. The term leaven means to rise. There are various ways of achieving leavening of dough, some which include a fermenting agent, such as yeast or bacteria, while others do not include a fermenting agent but rather a leavening agent. The following will discuss the exact definition of leavened dough that is defined as Chametz.
The cause of the leaven: Leavened dough is considered Biblical Chametz whether the dough leavened due to fermentation of a yeast or has leavened on its own after being left alone for a while without being worked on, or even if it was leavened through an external fermenting agent, such as oven dried wine dregs. In all these cases, the dough is considered absolute Chametz, and one who eats it is liable for Kareis. [Thus, whether the dough leavened due to a fermenting agent, or a non-fermenting agent, or even on its own, it is Chametz.]
How long does it take dough to become leavened and become Chametz? If dough remained a Shiur Mil without being worked on, it becomes Chametz. A Mil is a form of measurement of distance, similar to miles and kilometers. A Shiur Mil is the amount of time that it takes to walk the distance of a Mil. There is a dispute regarding the exact length of this distance. Some Poskim rule it is 18 minutes worth of walking. Other Poskim rule it is 24 minutes worth of walking. This draws us to a great dispute regarding the amount of time it takes unworked dough to become Chametz. According to the former opinion, it takes 18 minutes, while according to the later opinion, it takes 24 minutes. Practically, the main Halachic ruling follows the latter opinion, that there are 24 minutes in a Mil, and it takes 24 minutes for dough to become Chametz. Nonetheless, one is to be stringent like the first opinion [that a Mil is 18 minutes and it thus takes dough 18 minutes to become Chametz] unless this involves a great loss. The above measurement of 18/24 minutes applies in general circumstances, however, in certain circumstances, dough can become Chametz in even less than 18/24 minutes, and in certain circumstances, it will not become Chametz even after 18/24 minutes. The following are the cases of exception:
Things that prevent dough from leavening even past 18/24 minutes:
There are a number of matters that prevent dough from becoming Chametz even if it were to delay 18:24 minutes prior to being baked. These are:
1) Working on the dough: Working on the dough prevents it from becoming Chametz. This applies even if one were to work on the dough for the entire day. In order for the dough to become Chametz, it must remain a full 18/24 minutes without being worked on at all in that time. If one worked on the dough even one time within the 18/24 minutes, it undoes the partial leavening that occurred prior to the kneading, and the dough then requires another full 18/24 minutes, without being worked on, in order to leaven and become Chametz. Thus, even if in total the dough remained many hours without being worked on, if it was worked on every 17/23 minutes, and never remained a consecutive 18/24 minutes without being kneaded, it does not become Chametz.
The definition of working on the dough: Working on the dough only prevents it from becoming Chametz if one did a complete form of kneading to the dough in the interim, such as kneading it, punching the dough with one’s fists, or rolling the dough into Matzos using a roller. However, puncturing the holes in the dough to prevent it from rising is not considered a complete form of working to nullify the leavening that occurred prior to the puncturing, although it does prevent it from leavening while it is being punctured. Certainly, merely rubbing on the dough does not nullify its previous leavening, and on the contrary, it heats the dough and hastens its leavening.
2) Drops of water which fall on flour: If drops of water fall onto flour immediately one after the other, without any break at all in between, it prevents the flour from leavening and becoming Chametz, and thus even if the flour were to remain under the drips for the entire day, it is valid. Nevertheless, as soon as the drips stop, one must knead the flour immediately and bake it.
Things that cause dough to leaven even within 18:24 minutes:
There are a number of matters that cause the dough to leaven and become Chametz in even less than 18:24 minutes. These are:
1) Warm flour/dough: Dough only requires a Shiur Mil to become Chametz if the dough was not warmed up. However, if the dough was warmed, such as by being left in the sun, or near an oven, or through rubbing one’s hands on it, then it can become Chametz instantly.
2) Liquids defined as fruit juice: If liquids defined as fruit juice are kneaded into the flour together with water, it causes the dough to become Chametz in even less than 18:24 minutes, as brought above in B.
 Admur 462:1
 Difference between leavening and fermenting: Leavening is rising by any means, so baking soda and baking powder (chemical leaveners) as well as yeast (a fermenter) is all included in the leavening process. Chemical leaveners like baking soda and powder work by mixing an acid and base to produce carbon dioxide gas. Fermentation is the process of yeast converting sugars to carbon dioxide and alcohol. In bread making, the carbon dioxide is the desired product. In beer or wine making, the desired product is the alcohol. There are other types of fermentation by bacteria [such as those used in yogurt-making, pickles, or sauerkraut] that produce lactic acid and is responsible for the taste of sourdough breads. Yeast breads undergo leavening through fermentation, while quick breads, like banana bread, use chemical leaveners, which are not fermentation.
 Admur 462:1; Mishneh Menachos 52b; Pesachim 28b and 43a
 The reason: As the verse states “Ki Chol Ochel Machmetzes Venichrisa etc” and not “Kol Ochel Chametz”. The term Machmetzes is coming to teach us that even if the dough leavened due to an external element, it is considered Biblical Chametz. [ibid]
 Admur 459:9-10
 Admur 459:9; Michaber 459:2; Pesachim 46a
 1st opinion in Admur 459:10; Michaber ibid and in Yoreh Deah 69:6; Rama 261:1; Maryu 193; Terumos Hadeshen 123
 The reason: As an average person can walk 40 Mil during the daylight walking hours of an average month, such as Nissan and Tishrei, in which there is an equal amount of day and night hours. [Admur ibid; Pesachim 93b] Now, this opinion rules that there are exactly 12 [60 minute] daylight walking hours available in Nissan and Tishrei, [from sunrise to sunset]. Accordingly, if it takes an average person 12 hour to walk 40 mil from sunrise to sunset, it turns out that each mil subdivides to 18 minutes [12×60 divided by 40=18]. [Admur ibid]
 2nd opinion in Admur 459:10; Rambam in Pirush Hamishnayos Pesachim 3:2
 The reason: As an average person can walk 40 Mil during the daylight walking hours of an average month, such as Nissan and Tishrei, in which there is an equal amount of day and night hours. [Admur ibid; Pesachim 93b] Now, there are opinions who rule that there are more than 12 [60 minute] daylight walking hours available in Nissan and Tishreiy, as one is able to walk from daybreak: Alos until nightfall: Tzeis. [Admur ibid; Chok Yaakov 459:10] Based on this, it takes an average person 12 hours, from sunrise to sunset, to walk 30 mil, and not 40 Mil, as the extra 10 Mil spoken of by the Sages is walked prior to sunrise and after sunset, which are also walking hours. Accordingly, it turns out that each Mil subdivides to 24 minutes [12×60 divided by 30=24]. [Admur ibid] In essence the dispute boils down to how many walking hours exist in an average day in Nissan: Tishrei, in which one can walk 40 Mil, and based on that they calculate the amount of Mil one can walk from sunrise to sunset. The first opinion rules an average person can walk 40 mil in 12 hours, which thus subdivides each Mil to 18 minutes. The second holds that an average person can only walk 30 Mil in 12 hours, thus each mil subdivides to 24 minutes.
 Admur ibid; See also Siddur Admur [24 minutes]; Admur 249:3 [Mil is 24 minutes regarding traveling Erev Shabbos]; 184:3 [Mil is 18 minutes regarding Shiur Ikkul of blessings]; So also rules: Peri Chadash Y.D. 69:26; Kitzur SHU”A 36:11
 Admur ibid
 Admur 459:9; Michaber 459:2; Pesachim 48b; See Aruch Hashulchan 459:7
Other opinions: Some Poskim rule that the working on the Matzah does not interrupt the 18 minutes, and hence all Matzahs must be baked within 18 minutes of the start of kneading. [Bach 459 in name of Riaz[
 Admur ibid; M”A 459:4; Terumos Hadeshen 123
Other opinions: Some Poskim suspect that perhaps the total amount of 18/24 minutes without working on the dough does make the dough become Chametz, even it never remained a consecutive 18/24 minutes without being worked on. [Rama 459:2; Maharil 51; Aruch Hashulchan 459:7; See M”B 459:16]
 Admur 466:16; Michaber 466:6; Pesachim 39b
 Regarding under the sun: Admur 459:1-6; Michaber 459:1; Pesachim 42a; Regarding near an oven: Admur 459:7 and 11; Michaber 459:1; Rama 459:2; Hagahos Maimanis; Mordechai; Regarding warming with hands: Admur 459:9 and 11; Rosh Kelal 14
 Admur 462:3 and 466:3; Michaber 462:2; Rambam 5:2 based on Pesachim ibid