What if one completely forgot to separate the Challa? Is it still possible to do it after baking? If not, that means the bread is not eligible to be used for Shabbat? Thank you!
Shulchan Aruch Yore Dea 327, 5:
אם לא הפריש החלה בצק אלא אפה הכל הרי זה מפריש מן הפת.
If he did not separate the challa from the dough, but already baked the whole, he has to separate it from the bread.
Note, it's not allowed to separate the challa on Shabbat (see OC 339, 4).
Note in the Diaspora you can eat on Shabbat and separate Challah after Shabbat. Apr 20, 2017 at 15:46
@DoubleAA Why is the rule different in Diaspora? Because there challah is derabanan?– DanFApr 20, 2017 at 17:33
@DanF Yup. (more precisely it's a double derabanan, since all challah nowadays is derabanan anyway as most Jews don't live in Israel) Apr 20, 2017 at 17:35
@Double AA see demai 4, 4, for this I wrote the additional fact that challa is not eaten bizman haze bechuts laarets– koutyApr 20, 2017 at 20:37
@kouty I don't see what oyu refer to in the mishna there, but a kohein can eat Challat Chu"l even bizman hazeh if he goes to the mikva. Apr 20, 2017 at 20:41