Gmara

35a (Reish Lakish): One is not Chayav Kares for a dough kneaded with wine, oil or honey.

(Rav Idi bar Avin): This is because Mei Peiros do not ferment.

36a (Beraisa): During Pesach, one may not knead a dough with wine, oil or honey;

R. Shimon ben Gamliel says, if one did so, one must burn it immediately. (It ferments very quickly);

Chachamim say, one may eat it.

R. Akiva says, I once made a dough for R. Eliezer and R. Yehoshua [during Pesach] with wine, oil and honey, and they did not comment.

Chachamim say, one may smear only with things with which one may knead.

40b (Mishnah): One may not put flour into Charoses (a dip containing vinegar. This is not the dip used at the Seder) or mustard. If one did so, he should eat it immediately;

R. Meir forbids eating it.

(Rav Kahana): They argue about flour put into mustard, but if flour was put into Charoses, all agree that it must be burned immediately.

Support (Beraisa): One may not put flour into Charoses. If it was put, it must be burned immediately;

R. Meir says, if flour was put into mustard, it must be burned immediately;

Chachamim say, he should eat it immediately.

(Rav Huna brei d'Rav Yehudah): The Halachah follows Chachamim.

Rav Nachman bar Yitzchak: Do you refer to Charoses or mustard?

Rav Huna: What difference does it make? (No one distinguishes between them!)

Rav Nachman: Rav Kahana distinguishes!

(Rav Kahana): They argue about flour put into mustard. All agree that if flour was put into Charoses, it must be buried immediately.

Rav Huna: I disagree with him.

Support (for Rav Kahana - Rav Ashi): Shmuel taught that the Halachah does not follow R. Yosi [who says that if barley is inflating, one soaks it in vinegar to impede Chimutz].

Suggestion: Vinegar does not impede Chimutz. Rather, it is Mechametz!

Rejection: Perhaps it does not impede Chimutz, nor is it Mechametz.

Rishonim

Rosh (3:1): The Mishnah forbids beer of Madai and vinegar of Edom, i.e. when it has water. We hold that [pure] Mei Peiros do not ferment. Also the Yerushalmi says so. The Rif rules like R. Akiva that Mei Peiros do not ferment even with water, i.e. if the dough was kneaded with Mei Peros it does not ferment quickly, and one can guard it from Chimutz like a regular dough. If one puts barley in beer or vinegar for a long time, it becomes Chametz if there is water.

Rosh (2:13): R. Tam explains that Mei Peros without water do not ferment. When there is also water, it becomes Chametz quickly. If one kneaded with Mei Peros and water, the Ritz Gei'us rules like Chachamim, that one bakes it immediately. Bahag says that Rabanan at the end of the Beraisa, who forbid smearing with things with which one may not knead, hold like R. Gamliel, that it must be burned immediately. I do not understand. They discuss only l'Chatchilah! R. Gamliel and Chachamim argue about b'Di'eved, and the Halachah follows Chachamim.

Poskim

Shulchan Aruch (OC 462:2): Mei Peiros with water ferment more quickly than a regular dough. Therefore, one should not knead with them. If he did, he bakes it immediately.

Mishnah Berurah (7): It can become Chametz in less than the time to walk a Mil (18 minutes).

Mishnah Berurah (8): If one did not bake it immediately, one may not eat it, even if he did not delay 18 minutes. If this will cause a big loss, Eliyahu Rabah is lenient if it was within 18 minutes and we do not see any signs of Chimutz. If this happened b'Shogeg, surely one may rely on this in pressed circumstances, like we say about kneading with warm water (455:3).

Shulchan Aruch (464:1): One may not put flour in mustard or other dips that have water.

Gra (DH Oh): The Tur wrote "other dips" in place of Charoses, for they have the same law, since they are not [so sharp].

Mishnah Berurah (1): These are dips for meat. They have vinegar and others acidic matters. The custom is to put flour in them. It becomes Chametz through the acidity.

Mishnah Berurah (2): The Rema (462:4) wrote that we are stringent about pure Mei Peiros. If so, we forbid even dips of vinegar, wine and flour without water.

Shulchan Aruch (ibid.): If one put it in mustard, he should eat it immediately, and not leave it to become Chametz.

Beis Yosef (DH b'Nosen): Rashi says that they argue about mustard, for it is sharp and does not let [the flour] become Chametz. Even though we reject Rav Ashi's proof, since Rav Ashi holds like Rav Kahana, we rule like him. Also the Poskim rule like him.

Mishnah Berurah (3): The strength of the mustard does not let [the flour] become Chametz quickly.

Kaf ha'Chayim (4): He may not keep it [even] less than 18 minutes. Surely, after 18 minutes it becomes Chametz.

Shulchan Aruch (ibid.): Other dips are not as sharp as mustard, so surely it became Chametz, so one must burn it if it is after Chametz became forbidden.

Mishnah Berurah (4): One may eradicate it in ways other than burning, like above (445:1).

Rema: The custom is not to eat mustard at all during Pesach, even if it became mixed before Pesach, for it is like Kitniyos, which our custom is to forbid.

Taz (1): This is l'Chatchilah. b'Di'eved, we do not forbid the Tavshil. Similarly, one may keep it until after Pesach if the mustard does not have flour. There is no Chimutz at all. It is a mere stringency. The Roke'ach explicitly said that we eat mustard during Pesach, as long as it was not ground in a Chametz bowl.

Magen Avraham (1): I.e. they make it with wine and grind it. It is not recognizable like Kitniyos. Even so, it is forbidden.

Kaf ha'Chayim (7): Shulchan Gavoha says that the Rema discusses his country, but here we eat mustard, even though we are stringent about other Kitniyos. Ru'ach Chayim says that Sefardi communities accepted the stringency [of mustard], except for the last day. Sedei Chemdah says that we Sefardim eat mustard. Either he did not see Ru'ach Chayim, or he discussed the custom in his place.

Mishnah Berurah (5): This is because mustard is piled up, therefore it is like Kitniyos, which our custom is to forbid.

Kaf ha'Chayim (9): Likewise, one may keep mustard with flour without water. It is good to sell it to Nochrim. However, our wheat flour is Lesusah (mixed with water before it is ground). It is Chametz even without [adding more] water.

Mishnah Berurah (6): This does not mean that it became mixed with flour. Surely we forbid this, whether it became mixed before Pesach or during Pesach! Rather, it became mixed with wine. The Chidush is that even though at the time of mixing we crush it, and it does not appear like Kitniyos when Pesach enters, even so it is forbidden. In any case, b'Di'eved, if mustard fell into a Tavshil, it does not forbid it. If mustard was put into wine while it was bubbling (fermenting), so it will remain sweet and not spoil, one may keep it, but not drink it [during Pesach].

Kaf ha'Chayim (10): Sedei Chemdah says that it is an excessive stringency not to drink the wine. In any case one may drink it through straining it.