One issue that the foremost Aĥaronim dealt with is the status of milk which came from a cow that ate ĥametz. Clearly the milk itself does not contain ĥametz, for it was digested and completely transformed to the point that it is no longer considered ĥametz at all. However, the cow was able to produce milk by virtue of the ĥametz, and since it is forbidden to derive benefit from ĥametz, perhaps it is forbidden to benefit from milk produced by the virtue of ĥametz.
The poskim agree that milk obtained from a cow before the onset of the prohibition of ĥametz is kosher for Pesaĥ, because one is allowed to derive benefit from ĥametz before Pesaĥ. Just as it is permissible to sell ĥametz before Pesaĥ and use the money to buy food for Pesaĥ, so too it is permissible to feed a cow ĥametz before Pesaĥ in order to produce milk that will be consumed on Pesaĥ.
The disagreement concerns milk from a gentile’s cow that ate ĥametz after the onset of the prohibition. Some poskim take a lenient stance, contending that since the ĥametz prohibition does not apply to the animal of a gentile, its milk is not considered produced in a forbidden manner. Furthermore, ĥametz alone does not cause milk to be produced. Rather, it must be combined with other foods and processed by the animal’s body. Since the ĥametz is only one component, it is not prohibited. On the other hand, some poskim rule stringently that as long as ĥametz is a factor causing the production of the milk, the milk is forbidden. Others say that if twenty-four hours have passed since the cow ate ĥametz, the milk is kosher.
If the animal owned by a Jew was fed ĥametz in violation of halakha, one may not drink its milk, firstly because it is forbidden for the animal’s owner to derive benefit from ĥametz, and secondly because one may not assist those who violate the Torah. The same applies to eggs and meat.
During Pesaĥ, Tnuva, a major Israeli dairy producer (and perhaps others) only accepts milk from dairy farms that have been made kosher for Pesaĥ and whose cows are not fed ĥametz. In this case, it is unnecessary to be scrupulous and buy milk products before Pesaĥ, because even dairy products manufactured on Pesaĥ are completely kosher for the duration of the holiday.
. A cow’s milk is the product of two factors: the cow’s body and the food it eats. If the cow ate ĥametz, the status of the milk is subject to the tannaitic dispute (AZ 48b) about something that is produced by multiple factors (“zeh ve-zeh gorem”). One disputant forbids and the other permits. SA YD 142:11 states that something that is produced by a combination of two factors is permissible. Accordingly, the milk of a cow that ate ĥametz is permissible.However, MA 445:5 (as well as Taz) states that since we are stringent about ĥametz, maintaining that even a drop renders a mixture forbidden, it follows that something produced by a combination of two factors is forbidden if one of them is ĥametz. Most poskim, including SA, Shakh, and Gra, maintain even in the case of ĥametz “zeh ve-zeh gorem” is permitted. SAH (445:10 and Kuntrus Aĥaron) concludes that the consensus of most poskim is to be lenient, and one may certainly rely on this consensus in a situation of significant loss or an extenuating circumstance. This is also the view recorded in BHL 445:2.
Some, however, argue that regardless of one’s position on zeh ve-zeh gorem, milk from a gentile’s cow that ate ĥametz is permitted. As explained in Beit Ephraim §35 (cited in Sha’arei Teshuva at the end of §448), since ĥametz is permissible for a gentile on Pesaĥ, we do not view the milk from his cow as having been produced by something from which one is forbidden to benefit. Nishmat Adam §9 permits on different grounds: MA’s stringency only applies when the ĥametz is intact, unlike in the case of the milk. Responsa Mahari Aszod §127 and Responsa Maharam Schick §§212 and 222 echo the same idea and state that the milk of the gentile’s animal that ate ĥametz is kosher on Pesaĥ. Igrot Moshe OĤ 1:147 states at length that even if the gentile’s cow ate only ĥametz, its milk is permissible even according to the strict opinions.
On the other hand, Pri Megadim (in Eshel Avraham on §448) is concerned about causing benefit that derives from ĥametz. Thus, there is still uncertainty if less than twenty-four hours passed between when the cow ate ĥametz and the milking (if more than twenty-four hours passed, the milk is permissible). Yeshu’ot Yaakov also states that one should preferably use milk that was extracted from the cow more than twenty-four hours after the cow ate ĥametz, but if less than twenty-four hours passed, the milk is still permissible as long as the cow ate permissible foods in addition to the ĥametz (because of zeh ve-zeh gorem; see MB 448:33). Some authorities ruled stringently: Kitzur SA 117:13 cites both opinions and concludes: “One who guards his soul should be strict, and especially in places where the prevalent custom is to be stringent, God forbid one should be lenient.” Arugot Ha-bosem §138 states that even according to those who permit the milk, a righteous person (“ba’al nefesh”) should refrain from drinking it, since it has negative spiritual effects. Ben Ish Ĥai (Year One, Tzav 42) states that one should not drink milk from a gentile’s cow out of concern that the gentile fed it ĥametz. R. Ĥayim Palachi writes similarly in Ru’aĥ Ĥayim 448:1.
If the cow ate ĥametz before the ĥametz became prohibited and was milked after the ĥametz became prohibited, the vast majority of poskim maintain that the milk is kosher for Pesaĥ. Sdei Ĥemed mentions the opinion of Rinun Yitzĥak forbidding the milk of a cow that ate ĥametz before Pesaĥ and was milked after the ĥametz became forbidden, and states that this goes too far, since all other poskim say that this milk would be permissible. Nonetheless, because of this opinion some act stringently and only purchase dairy products before Pesaĥ. Sdei Ĥemed expands this topic in Ma’arekhet Ĥametz U-matza 2:4, and see Kaf Ha-ĥayim 448:113 as well.
. According to Nishmat Adam and Igrot Moshe OĤ 1:147, if a Jew feeds his animal ĥametz on Pesaĥ, the animal’s milk is kosher for Pesaĥ. Nonetheless, many others are stringent, not only because of zeh ve-zeh gorem, but because the Jew assists in the violation of a prohibition (“mesayei’a”). In fact, I heard from R. Weitman, the rabbi of Tnuva, that all dairy products produced on Pesaĥ are made from the milk of cows that did not eat ĥametz, so that the milk will be acceptable to everyone and can be bought on Pesaĥ. Another potential problem was that straw and possibly some grain might stick to the cows’ bodies as they wallow in mud, and these grains might accidentally get mixed into the milk. If the milk was produced before Pesaĥ, the taste of the grain is batel be-shishim even if it found its way into the milk, and since this is a liquid mixture even Rema (447:4) would agree that the taste of the ĥametz is not ĥozer ve-ne’or. If the grain fell into the milk on Pesaĥ, however, it is not batel. Although in the present case it is uncertain that any grain fell in, it would nevertheless be commendable to buy dairy products before Pesaĥ. However, I heard from R. Weitman that Tnuva recently introduced the practice of filtering all the milk very thoroughly right after the milking, so that no grain that falls in would have enough time to flavor the milk. Thus, one may purchase dairy products on Pesaĥ even according to the strictest opinions.