Q. Is it permissible to use a microwave oven for both milchigs (dairy) and fleishigs (meat), if one kashers it back and forth?
A. The Magen Avraham (509:11) writes that the accepted minhag is not to use the same utensil for both milchigs and fleishigs even if one kashers in between. The concern is that one might forget the current status of the utensil and inadvertently use it for fleishigs when it is milchigs or vice versa. The Aruch Hashulchan (509:10) disagrees with the Magen Avraham and argues that we do not currently have the proper level of Rabbbinic authority to institute new prohibitions.
Rav Moshe Feinstein (Igros Moshe, Yoreh De’ah I:43:6) supports the Magen Avraham and maintains that establishing a minhag does not require Rabbinic authority. Furthermore, Rav Moshe (Igros Moshe, Yoreh De’ah III:25) suggests that it may be an actual prohibition to have only one utensil and use it for dairy and meat, although kosherized between uses. This is based on the Gemara in Chulin 8b that a butcher must have separate knives for cutting kosher meat and forbidden fats. Since these knives are used regularly, we cannot assume that the butcher will remember to clean them in between uses.
Based on the above, Rav Hershel Schachter ruled that it is not appropriate to kasher a microwave oven back and forth between milk and meat.
This column comes from OU Kosher’s Halacha Yomis dedicated in memory of Rav Chaim Yisroel ben Reb Dov HaLevy Belsky, zt’l, Senior OU Kosher Halachic Consultant (1987-2016). Subscribers can also ask their own questions on Kashrus issues and send them to firstname.lastname@example.org. These questions and their answers may be selected to become one of the Q and A’s on OU Kosher Halacha Yomis.