Sometimes I want to make a big pareve vegetable soup in a meat pot (my largest) and later put some of it in milchig or pareve pots or bowls. Is this permissible? Answer:
Questions of nat bar nat (twice removed taste, i.e., food into pot and then pot into food) are often complex due to the multiple permutations of l’chatchila (proper action) and b’dieved (after the fact). Let us proceed from rules to details.
Amoraim dispute whether pareve food that was placed while hot on a fleishig utensil can be eaten with milk, and we rule leniently (Chulin 111a). Therefore, the Shulchan Aruch (Yoreh Deah 95:1) rules that one may mix pareve food cooked in a fleishig pot (nat bar nat of fleishig) intomilchig food. However, the Rama (whom Ashkenazim follow) rules that cooking food in a fleishig pot is more severe than simply placing hot food in a utensil. He says that in the former case, the originally parevefood may not be mixed in with milchig food (ad loc. 2).
However, the Rama incorporates a few leniencies. If the food cooked in the fleishig pot was subsequently mixed into milchig food, it may be eaten, b’dieved. Also, the pareve food may l’chatchila be placed hot into a milchig utensil without affecting the status of the pot or the food (ibid.). Thus, the soup you describe may be placed in a milchig pot or bowl.
However, there is a complicating factor – a further level ofl’chatchila. The Beit Yosef cites several Rishonim who say that one may not set up l’chatchila a situation of nat bar nat. While his final opinion is unclear, most prominent Sephardi poskim (see Kaf Hachayaim, YD 95:1) say that one should not put hot pareve food in afleishig pot if he intends to subsequently mix it in with milchig. The question is whether there are other cases where a food would be treated as pareve, b’dieved, but should not be "created" in that way.
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One case in point is when a fleishig pot has not been used forfleishig within the 24 hours before the pareve use. The Rama says that in such a case, the resulting food is pareve enough to mix in withmilchig. The Gra (95:10) says that in such a case it is even permitted to l’chatchila cook the pareve in that fleishig pot with intention to mix it in with milchig. However, the Chochmat Adam (48:2) says that one should not cook it in the fleishig pot with that intention, and this is the more accepted position.
Regarding your first specific question, making the soup in a fleishigpot with intention to put it into a milchig pot, there is a machloket among the Acharonim. Among the earlier authorities, the Bach allows it, and the Pri Megadim (Mishbetzot Zahav 95:4) forbids it. Amongst contemporary authorities, Rav Moshe Feinstein (Igrot Moshe, YD III:10) leans toward leniency, whereas several less prominent authorities lean toward stringency (see differences of degree in Badei Hashulchan 95:30, Ma’adanei Hashulchan (95:23), and Laws of Kashrus (Artscroll), p. 242). (Realize that there are serious opinions that even b’dieved, it should not be put into a milchig pot (see discussion in Darchei Teshuva 95:23).) It seems to be better policy to have a large pareve pot for big soups to avoid this issue. However, in cases where this is not readily feasible, leniency is legitimate.
In cases where there is an additional reason for leniency, one can be lenient freely. One is the second case you ask about – where the second utensil is itself pareve, not milchig. Since nothing can go wrong to the food in this utensil, and it is just a question of making the utensilfleishig, we do not have to go so far in our concern. It also makes sense that if the fleishig pot has not been used in 24 hours, it is permissible to cook in it with the intention of putting the food in a milchig pot.
It is important to realize in questions such as these that "all bets are off" if one is dealing with onions or other sharp vegetables that were sautéed in the fleishig pot or cut with a fleishig knife (Rama, ibid.). The details are beyond our present scope.