Gemara

(Reish Lakish): If one buys brine from an Am ha'Aretz, he may do Hashakah (connect it to a Mikveh). Even if the majority is brine, brine is not Mekabel Tum'ah, and the water is Batel in the majority.

(R. Yirmeyah): This does not help if he will (add water, e.g.) cook it in a pot, and altogether there will be more water than brine. Then, the Tum'ah is revived.

Question (Abaye): After Tum'ah was Batel, does it really return?!

Answer (Rav Dimi): Surely, you agree that it does!

(Mishnah - R. Eliezer): If a Se'ah of Tamei Terumah fell into 100 Sa'im of Chulin, we take one Se'ah from the mixture and let it rot. We consider it to be the Se'ah that fell in.

Chachamim say, the Se'ah removed must be eaten in a way that avoids Tum'ah, e.g. dry...

(Beraisa): R. Eliezer requires the Chulin (the remainder) to be eaten dry...

(Ula): Even though they hold that it is (Tahor) Chulin, they decree to eat it dry, lest one join it to a Tamei food to be Mevatel it, and with the Se'ah the majority is Tamei, and the Tum'ah will be aroused.

Pesachim 30a (Rava): The Halachah is, in its time, Chametz forbids b'Mashehu, whether b'Mino or Lo b'Mino. This is like Rav;

Not in its time it is permitted even b'Mino. This is like R. Shimon.

44a (Beraisa): If Terumah and Chulin fell into boxes of Terumah and Chulin, and we are unsure which fell into which, both boxes are permitted. We say that the Terumah fell into the Terumah, and the Chulin into the Chulin.

Inference (Abaye): The Torah permits k'Zayis bi'Chdei Achilas Pras. If not, we would not be lenient about a Safek mid'Oraisa!

Rishonim

Rambam (Hilchos Chametz 4:12): If Chametz became mixed with something that is not a food, or which most people do not eat, such as Tri'ak, one may keep it, but he may not eat it until after Pesach. This is even if contains only a drop of Chametz.

Note: Tri'ak is a medicine containing crumbs of dry Chametz. Minchas Shlomo 1:17:1 was unsure how it is used.

Rosh (Pesachim 2:5): Chametz not in its time (after Pesach) is permitted even b'Mino. R. Shimon fines only intact Chametz, but not a mixture. This is even if he left over pure Chametz, and it became mixed after Pesach. After six hours on Erev Pesach, even though one is lashed mid'Oraisa for Chametz, it is like other Isurim and it is Batel in 60 and it is permitted even during Pesach, since it became Batel before Pesach.

Terumas ha'Deshen (114): Semak says that if we find in a stack wheat that sprouted from moisture of the ground, it is not Batel even if the wheat was ground before Pesach. Since it is dry, it gives taste at the time of baking on Erev Pesach to 60 parts (of Matzah and forbids them), and during Pesach b'Mashehu. The Tur says that it is Batel before Pesach. This is not considered being Mevatel an Isur l'Chatchilah, since he does not intend for Bitul. Semag says that flour mixes well; it is considered a wet mixture. He brought an absolute proof from Menachos. Also Pesachim 44a connotes like this. (If it did not mix uniformly, in any case there is a Safek Torah, for in some places it is mostly Terumah! - PF) There is a clear custom to permit a wet mixture that became Batel before Pesach. We do not say that the Isur is revived during Pesach.

Rashba (1:485): If some wheat became wet and dried and was mixed with much wheat, one may not add more wheat to be Mevatel it, in order to eat it during Pesach. This is like being Mevatel an Isur l'Chatchilah. A case occurred, and Rava obligated that the wet wheat be finished before Pesach (Pesachim 40b). He did not permit to be Mevatel it in 60! If even one part was mixed in 1000, the Ge'onim forbid eating it during Pesach. Rav Hai Gaon forbade wine due to two wheat kernels found inside. The Tosafists permit, but we are stringent like the Ge'onim regarding cheese made in a Chametz Kli, and all the more so here. Regarding (a dry mixture of) stringent Isurim that are not Batel, one may not eat each piece by itself. One may not even leave the mixture until after Pesach. Also, perhaps one can recognize the wheat that became wet, so it cannot be Batel.

Poskim

Shulchan Aruch (OC 447:4): If Chametz became mixed before Pesach and it was Batel in 60, it is not revived during Pesach to forbid b'Mashehu.

Beis Yosef (DH uv'Pesach): The Rosh, Mordechai, Hagahos Maimoniyos citing Semak, and Semag all permit Chametz that became Batel before Pesach. Hagahos Maimoniyos said that some forbid, but it seems that he holds like those who permit.

Gra (DH Im): If one mixed camel's wool (with sheep's wool and the majority is camel's wool), one may mix in linen, and the sheep's wool is not revived (Kil'ayim 9:1). The Mechaber wrote only this opinion in Sa'if 5.

Kaf ha'Chayim (76): Our custom is like the Rema; we do not say that Chametz is revived if the Isur is wet, or a mere taste. If the taste was detrimental, we are not stringent even if there was not 60. This is b'Di'eved. L'Chatchilah we are concerned even for taste. Many Sefardic communities accepted all stringencies of the Rema regarding Pesach, except for the Isur of rice (and other legumes - PF).

Mishnah Berurah (28): This is even after six hours on Erev Pesach.

Mishnah Berurah (29): A case occurred in which Matzos removed from the oven were unknowingly put on a Chametz dough. It was not known which touched it. Poskim ruled that the Matzos are Batel in a simple majority, for they were mixed before Pesach. One may warm the Matzos during Pesach, since it is a mere (absorbed) taste. However, if we know that a Matzah touched, one must peel off from it, since a known Isur is never Batel.

Shulchan Aruch (ibid.): Some disagree.

Gra (DH v'Yesh): The stringent opinion learns from Bechoros. We say that Tum'ah is revived, and the same applies to Isur.

Mishnah Berurah (30): This opinion holds that Bitul in 60 before Pesach permits to eat it on Erev Pesach, or if it was a mere taste.

Kaf ha'Chayim (78,81): The Rambam is stringent. The Magid Mishneh says that the Ritz Gei'us and Rashba agree; one should be stringent. Many are lenient, for the Mechaber favors the lenient opinion, and the Isur b'Mashehu is only mid'Rabanan. Others say to be stringent. One must rule like the local custom.

Rema: The custom is like the first opinion regarding all wet mixtures.

Taz (5): If before Pesach a wet mixture does not have 60 parts to be Mevatel the Chametz, one may add to it before Pesach to make 60. This is not being Mevatel an Isur l'Chatchilah. I brought a proof in 626:1. The Rema does not say 'wet' to exclude flour, for also it is considered wet, like the Beis Yosef brought. Also regarding a spit, we do not say that the Isur is revived (451:27).

Kaf ha'Chayim (84): If either is wet, this is called a wet mixture. Some consider flour to be dry, therefore it is good to bake the Matzos before Pesach, for usually some of the wheat sprouted. Olas Shabbos says that likewise, one should not cook with Matzah (meal) during Pesach, but others are stringent only for baking.

Mishnah Berurah (32): A wet mixture is Batel even if one did not know that it became mixed until Pesach.

Rema: However, we forbid if dry matters mixed, or there is concern for a mixture. E.g. if bread fell into wine, even though it was removed, we are concerned lest crumbs remained in the wine and they give taste during Pesach.

Taz (6): It would not help to strain out the crumbs, for perhaps some are very fine and pass through the holes of the strainer.

Magen Avraham (11): (All agree that) if Chametz became Batel in a dry mixture, one must eat it before Pesach. Even though all agree that one may keep a wet mixture, a dry mixture is forbidden, for in any case Chametz is in his house. However, according to the Rosh (brought in Tur YD 109), who says that Isur that was Batel in a dry mixture becomes total Heter, and afterwards one may cook them together, if so similarly one may keep it, for it is total Heter. However, we follow the Rosh only to avoid a loss. However, it seems that if one can cook it until it dissolves and mixes before Pesach in 60, one may eat it during Pesach, and this is not being Mevatel an Isur l'Chatchilah.

Magen Avraham (12): Even so, one may keep the wine.

Shach (Gilyon Orach Chayim, brought in Magen Avraham 13): The Rashba is consistent with his opinion that the Isur is revived during Pesach, but we permit in 60, like the Bach wrote in 442:5. Poskim rule like this.

Rebuttal (Magen Avraham 13): When he strained it and only a little remained, all agree that the Isur is not revived, like it says in Sa'if 3. Rather, the Rema discusses one who removed the bread, but did not strain it until Pesach. Therefore it is forbidden. If he strained it before Pesach, all permit. Regarding mead, sometimes it is thick, and it cannot be strained. The Rashba says that if after the bread was removed, he mixed wine from the barrel with other wine, it is totally forbidden.

Magen Avraham (14): All forbid, even if it was cold, for since the Chametz was soaking for 24 hours, this is pickling, which is like cooking.

Gra (DH v'Nohagim): Even the first opinion, which learns from Kil'ayim, admits about a garment totally of camel's wool, that if a thread of (sheep's) wool was inserted on one side and a thread of linen was inserted on the other side, it is forbidden, for it is evident and stands by itself, so it is not Batel. The same applies to crumbs. In YD 99:6, the Rema rules (regarding Stam Isurim) that the Isur is revived. This is only to forbid a mixture less than 60, but not to forbid b'Mashehu.

Mishnah Berurah (33): A dry mixture must be eaten before Pesach. If there were not 60 times as much as Heter, one may not keep it during Pesach. If there was 60, one may cook it before Pesach to dissolve it, and then it is like a wet mixture. Some permit eating even a dry mixture during Pesach, just one may not cook it, for then the Chametz gives taste to the rest.

Mishnah Berurah (35): One may not drink the wine, but one may keep it and benefit from it, since it is a mere stringency to be concerned.

Mishnah Berurah (36): If one strained it before Pesach with a fine strainer that does not let crumbs pass through, it is permitted, even if it soaked for 24 hours. There must be 60 times as much wine as bread.

Mishnah Berurah (38): Even though many are lenient about cold mixtures, here the bread soaked for 24 hours. The same applies if bread fell into water.