Assuming bread was prepared and baked by a Gentile using only kosher ingredients (e.g. kosher flour, water and kosher yeast) and only kosher equipment, what conditions would render such bread fit for consumption by a kosher-observant Jew Lekhatehhilah? Bedi'avad?
There's an entire chapter of Shulchan Aruch (Yore Dea 112) — sixteen subsections plus all the commentaries and later works — devoted to this, so obviously answers here, including this one, can't do the topic justice. But the Star-K and the OU summarize the main points, qq.v., and I'll briefly summarize the summaries:
- It's forbidden unless a Jew was involved in the baking.
- An exception is made for commercially baked goods, though for S'faradim only in case of need. (Many, even Ashk'nazim, don't rely on the exception at all, or at least not between Rosh Hashana and Yom Kipur inclusive.)
There's also a prohibition on eating bread that contains meat or milk in it unless it is markedly meaty or dairy. Again, there's a whole chapter of Shulchan Aruch (Yore Dea 97) on this, and the details are too many for this answer.
This article gives some of the conditions permitting bread cooked in a 'non-Jewish' bakery with kosher ingredients:
The Shulchan Aruch (37) says a heter to eat pas paltur is if it is higher quality (in taste orappearance) (38) than the Jewish bread since then it is considered you do not have Jewish bread (39). It would seem that the only time it would be permitted in this situation according to the Shulchan Aruch is if the pas paltur is better in taste etc, but if it costs less than the pas yisroel then one would not be able to buy the pas paltur (40). Others say that if there is a big price difference then one would be able to buy the pas paltur over the pas yisroel (41).
- Y.D. 112:5. See Kitzur Shulchan Aruch 38:1. This does not apply to bread baked by a non-Jew who is not baking for commercial use (Darchei Teshuva 112:57).
- Levush 112:7, Chelkes Binyomin 112:46.
- Aruch Hashulchan Y.D. 112:16.
- Chelkes Binyomin 112:51.
- Opinion of Harav Elyashiv Shlita quoted by Harav Shmuel Felder Shlita.
These conditions are not restricted to bread alone, but also to other products made in non-Jewish establishments - see the article source for details.