Q) How should one dispose of (separated) challah? Is it supposed to be burned in an oven?
A) Challah (i.e. separated or designated challah) has the same halachic status as teruma. Consequently, it may only be eaten by a Kohen who is tohor, and not by a Levi or Yisroel. Not only is the challah itself prohibited to be eaten by a non-Kohen, also taste that it imparts has the same status as the challah itself.
Because Kohanim today do not eat challah for various technical reasons (i.e. doubts in authentic lineage and tumah[1]), challah is supposed to be burnt (because it is ossur to eat or to benefit from it)[2]. We find in the earlier Poskim that one should burn the challah in the oven; however, this was only an option in the past when ovens contained an actual fire, and therefore were effectively “self-kashering”. Nowadays, where conventional ovens contain heating element and not actual fire, they are generally not “self-kashering” (with the possible exception of pyrolytic). This means that if one were to burn the challah in a present day oven, the oven would absorb b’liyos of challah which could subsequently prohibit other food cooked inside the oven.
Experience has shown that it does not suffice to wrap the challah in foil and burn it in the oven; dough expands when heated and all too often the challah bursts through the foil and poses a serious halachic issue – of challah bliyos inside an oven. Scorching the challah on the gas hob is also practically unadvisable – since the hot challah will come into contact with one of the surfaces and the surface will similarly absorb b’liyos of challah.
Since burning the challah is not (generally) a practical possibility, it is recommended to wrap the separated challah in a plastic bag (ideally immediately upon being designated – so as to avoid accidental mix ups) and dispose of it by placing it in the bin[3].
[1] ע’ ש”ך יו”ד שכ”ב ס”ק ט’, וע’ רמ”א או”ח סי’ תנ”ז א’ ואכמ”ל
[2] רמ”א יו”ד שכ”ב ה’
[3] שו”ת מנחת יצחק ח”ד י”ג