Talmud - Pesahim 40a פסחים מ׳ א׳

two wheat grains in boiling water at the same time, lest one move and rest in the slit [tzirya] of the other, thereby preventing the column [dikkula] of boiling water from entering all four sides of the grain, and the grain will come to a state of leavening.

And Abaye said: A person may not roast two stalks together, lest water leave one and be absorbed by the other, and it come to be leavened. Rava said to him: If so, one should not even roast one stalk, lest the liquid leave one end of the stalk and be absorbed by its other end. Rather, Rava said: One need not be concerned about this possibility, as it is considered fruit juice, and fruit juice does not leaven grain.

The Gemara adds: And Abaye retracted his opinion with regard to that halakha of stalks, as he maintains that anything that is absorbing liquid will not become leavened unless it is fully soaked in water. As Abaye said: With regard to this jug used for drying the stalks by means of roasting, if it is inverted it is permitted, as the liquid that is discharged from one stalk will not be absorbed by the other stalks. However, if the vessel is upright it is prohibited, as the liquid retained in the vessel might be absorbed by the other stalks and leaven them. Rava said: Even if the jug is upright, it is also permitted. This liquid is considered fruit juice, and fruit juice does not leaven grain.

The Sages taught: One may not soak barley in water on Passover to make it easier to remove the chaff from the grain. And if one did soak barley grain and it split, it is prohibited. If it did not split, it is permitted.

Rabbi Yosei says: If one sees that the grain is expanding, he should soak it in vinegar, and the vinegar will cause the grain to contract, thereby preventing leavening. However, Shmuel said: The halakha in not in accordance with the opinion of Rabbi Yosei.

Rav Ḥisda said that Mar Ukva said: When the Sages were referring to a case where the barley grain split, the prohibition does not apply only if it actually split and a crack is visible. Rather, this is referring even to a case where if the barley grains were placed on a barrel of wine they would split by themselves, due to the effect of the wine. And by contrast, Shmuel said: This halakha applies only if it actually split. The Gemara relates: Shmuel took action in accordance with his ruling, when he was in the village of bar Ḥashu’s house. He prohibited only barley grains that had actually split, but permitted those that were about to split.

Rabba said: A pious person will not soak wheat at all during Passover. The Gemara asks: Why discuss particularly a pious person? This halakha should apply to everyone as well, as it was expressly taught in a baraita: One may not soak barley on Passover. The Gemara answers that this is what Rabba is saying: A pious person will not even soak wheat, which is firmer than barley and less likely to split, due to the concern that it might become leavened.

Rav Naḥman said to Rabba: Anyone who listens to Abba, Rabba’s first name, will eat moldy bread during Passover, as flour cannot be properly prepared without soaking, and therefore matza should not be prepared from this flour. As in Rav Huna’s house they would soak the grain, and in Rava bar Avin’s house they would likewise soak their grain. But Rava said: Soaking grain should not be avoided merely for reasons of piety; rather, it is absolutely prohibited to soak grain.

The Gemara asks: Rather, that which is taught in a baraita: One may not soak barley on Passover, indicates that it is barley that one may not soak; however, it is permitted to soak wheat, how will Rava explain this baraita? The Gemara explains: The baraita is speaking employing the didactic style of: Needless to say. It should be understood as follows: Needless to say, wheat may not be soaked, since wheat grains have a slit through which water will enter, and the wheat will therefore expand quickly when left to soak. However, in the case of barley, which is smooth and no water will enter the grain, one might say that this seems well, i.e., it is permitted, to soak barley. Therefore, the baraita teaches us that it is prohibited to soak even barley.

Rava reconsidered and then said: This is not the case. Rather, it is permitted to soak grain, as it was taught in a baraita: One can fulfill his obligation with a matza loaf prepared from refined flour and with matza baked from coarse flour. And it is impossible to produce refined matza without soaking the grain, as this is the only way to remove the chaff completely from the grain.

Rav Pappa raised an objection to the opinion of Rava from a baraita: With regard to flour and refined flour that belong to gentiles, in villages they are ritually pure, and in cities they are ritually impure. It is assumed that grain in cities is soaked before it is ground into flour. Once water comes in contact with this grain, it becomes susceptible to ritual impurity, and it subsequently becomes impure when touched by gentiles.

As for flour in villages, what is the reason that it is ritually pure? Is it not due to the fact that they do not soak the flour, and therefore it does not become liable to ritual impurity? And yet their flour is nonetheless called refined flour. It can be inferred from this that it is possible to prepare refined flour without soaking the grain.

Rava answered: You should interpret the distinction of the baraita as referring only to ordinary flour, not refined flour, which is invariably susceptible to ritual impurity due to the soaking. After Rava left, Rav Pappa said: What is the reason that he did not say a proof from this statement that Rabbi Zeira said that Rav Yirmeya said that Shmuel said: With regard to wheat that will be used for meal-offerings, one may not soak it, and yet it is nonetheless called refined flour? The Torah insists that the flour used for meal-offerings be prepared from refined flour.

Rava reconsidered and then said: It is not only permitted to soak the grains; it is actually a mitzva to soak them, as it is stated: “And you shall guard the matzot” (Exodus 12:17). The Gemara explains this statement: If it is not the case that grain requires soaking, for what purpose is guarding necessary? If you claim that this verse is referring to guarding when kneading, that cannot be the case, as guarding grain while kneading is not considered guarding. If one failed to protect the wheat from becoming leavened up to that point, it is of no use to be careful while kneading it. Consequently, this mitzva to guard the dough cannot be referring to the kneading stage.

As Rav Huna said: In the case of dough prepared by gentiles, if one knows that it has not become leavened, a person may fill his stomach with them on Passover night, provided that he eats an olive-bulk of matza in the end, to fulfill the obligation to eat matza. The Gemara infers from this statement: With regard to the matza that he eats in the end, yes, he fulfills his obligation with this matza. But with regard to the matza he ate in the beginning, no, he does not fulfill the mitzva with dough prepared by gentiles.

What is the reason that one cannot fulfill his obligation to eat matza with dough prepared by gentiles? It is because he did not perform his duty to guard this dough. But one can perform his duty to guard it from the time of baking and onward. Rather, isn’t it correct to conclude from this baraita that the grain must be guarded from the beginning, i.e., from the time it is soaked, which proves that soaking the grain is part of the process of preparing matza?

The Gemara rejects this: And from where do we know that this conclusion is correct? Perhaps it is different there, as at the time when guarding is required, i.e., when water was added to the flour, he did not perform his duty to guard it properly. However, in a case where at the time when guarding is required, when water is added the flour, he properly performed his duty to guard it, so too, it is possible that guarding flour at the time of kneading is considered proper guarding. Therefore, this question cannot be resolved based on the case of dough prepared by gentiles.

And although this proof was rejected, Rava did not retract his statement that guarding grain must begin before kneading. For he would say to those who cut and tied the stalks [kifei] of grain in the field: When you cut the grain, cut it for the purpose of the mitzva. Apparently, Rava maintains that it is necessary to guard the grain from the outset, i.e., from the beginning of its preparation until its end.

The Gemara relates with regard to Mar, son of Ravina,

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Pesachim 40a examines Passover leavening risks when roasting or soaking grain, weighing whether absorbed liquid or fruit juice causes fermentation.