Can one warm a baby bottle in a parev pot that is a kli rishon? Do we say there are so many nat-bar-nats occuring that the pot will still remain parev or is this all at one time and therefore there is no nat-bar-nat in this case?
If warmed, the pot would not become milky, and remains parev.
However, it is preferable to warm the bottle in a milkly pot, out of concern for possible cracks, and due to the issue of nat-bar-nat during bishul (see sources).
The question is based on Yoreh De’ah 94:5, whereby placing a milky spoon into a new pot, and then a meaty spoon into the same pot, is considered a problem. This would indicate that placing a milky bottle into a parev pot might also be problematic, in making the pot milky. However, see Shach (94:15, 21, expressing wonder at the pesak of Shulchan Aruch), whose conclusion would appear to apply specifically to a pot that a person is in any case going to speficy for meaty or milky use, but not for a pot that a person wishes to keep parev. See Vayizra Yitzchak (p. 282) who also gives this ruling.
There is an added issue, however, of nat-bar-nat besha’as bishul. According to some authorities, the transfer of nat-bar-nat “at the time of cooking itself” is more stringent than the general transfer of nat-bar-nat. This is ruled by Chavas Daas (95:1), based on Taz (92:24, quoting from Issur Veheiter), and is also stated by Chamudei Daniel (Basar Bechalav 44). The ruling is further implied by Taz (95:2) and Shach (97:2), and Be’er Yitzchak (9) and Yad Yehudah (97:7) write that the question is a matter of doubt, and one should be stringent. Yet, it is clear that a number of rishonim did not follow this chumra (see Yevakesh Torah, Basar Bechalav 16, anaf 2), and many later authorities are also lenient, such as Peri Megadim (M.Z. 2), Beis Efraim (YD 37), Peleisi (97:2), Yeshuas Yaakov, Beis Meir, Kanfei Yonah, and others.
Because the question is a matter of doubt, and it can be relevant to de’oraisa issues, one needs to be concerned for the stringent opinion.
The question is whether this chumra would apply to the case of the baby bottle, the reason for this being that in this case three transfers of ta’am are involved: the ta’am of milk into the bottle, the bottle into the water, and the water into the pot. In this case, we do not have a clear precedent to prohibit, and I found that Yevakesh Torah (Yoreh De’ah 31:13) writes that one may be lenient.
Rav Moshe Feinstein (Iggros Moshe, Yoreh De’ah 14, sec. 1), when discussing a plastic container, is concerned that the container may be cracked, which is an added reason not to place the milk bottle into a parev pot.