(This should not be relied upon for practical halacha. When a question arises a Rabbi should be consulted.)
Cooking Fish In A Meat Pot-
1. The Torah requires us to use separate utensils for meat and milk because we assume that the taste of the food becomes absorbed in the utensil and would then impart taste into future foods cooked in the pot. The poskim discuss whether one muse have separate pots for fish and meat.
2. The Tur (Yoreh Deah 116:2) writes the following, “One must be careful not to eat fish and meat together as it may cause tza’raas. Some are stringent to have separate utensils for fish.” The Sefer Shulchan Chai (1:7) also writes that one should have separate pots for fish and meat. The policy of the Star-K is also not to allow its caterers to cook fish in a meat pot (based upon an article from the Star-K website).
3. However, most poskim are not convinced that we must concern ourselves with the infusion of taste into the utensils and they allow for fish to be cooked in a clean meat pot. Indeed, this is the view of the Issur V’Heter, Maharshal, Taz, Knesses Hagedolah, Chochmas Adam, Edus B’Yehusef and Kaf Hachaim (see Yoreh Deah 116:20). The Chasam Sofer also writes that the common custom is to eat fish on meat utensils and that many great leaders also maintained that one need not have a separate set of pots for fish.
4. The Sefer Dalsei Teshuva (cited by Darkei Teshuva 27) writes that most people commonly cook fish in clean meat pots. He adds that if onions or garlic (or any other sharp foods) are being cooked with the fish, one should not cook them in the meat pot. He basis this on the fact that generally, in the laws of kashrus, sharp foods are treated more stringently. According to this opinion one should not slice an onion with a meat knife and cook those onions with fish.
However, others are lenient and permit one to slice a sharp food with a meat knife and cook it with fish. For normative halacha, a rav should be consulted. (see Shemiras Haguf V’Hanefesh page 8)
5. If the fish and meat were cooked in the same pot simultaneously, the Pischei Teshuvah (Yoreh Deah 116:3) cites Tiferet L’Moshe that the utensil must be kashered. He reasons that while mere infusion of taste is not strong enough to create a prohibition, if fish and meat were cooked together there is “poison” in the walls of the pot that needs to be removed.
6. The Divrei Malkiel (cited by Darkei Teshuva 28), however, disagrees and feels that one need not kasher the pot. Rather, one may just wait twenty four hours and continue to use the pot without concern. The Kaf Hachaim (Yoreh Deah 116:13) rules in accordance with the Divrei Malkiel.
7. Harav Shmuel Wosner shlit”a (Shevet Halevi 6:111) cites the above argument and concludes, “We commonly rule in accordance with the Pischei Teshuva and require for the pots to be kashered. The Sefer Mishmeres Shalom does, however, rules leniently if the pot is made of earthenware or porcelain (which cannot become kashered).”
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