Reheating Microwaved Food

Q. I cooked potatoes in a microwave oven before Shabbos. Is there any issue with heating the potatoes on Shabbos on top of my cholent pot?

A. There is a disagreement among poskim whether cooking on Shabbos in a microwave is a Torah or Rabbinic prohibition. Rav Moshe Feinstein (Igros Moshe OC 3:52) writes that this is a Torah prohibition, while Rav Shlomo Zalman Auerbach and others disagree. According to this second opinion, we would assume that reheating a microwaved potato is prohibited, because the potato was never cooked on a Torah level. However, that conclusion is incorrect for the following reason:

The Pri Megadim (318:6) writes that foods cooked in the sun (which is a permitted method of cooking on Shabbos) may be reheated on Shabbos. Why should that be? If cooking in the sun is not considered cooking, why may such food be reheated? The answer is that, sunbaked food is cooked. Nonetheless, it is permissible to bake food in the sun on Shabbos because that was not the manner of cooking that was used when building the Mishkan. Since sunbaked food is cooked, ain bishul achar bishul applies, as reheating will not change the food other than to make it hot. Similarly, according to Rav Shlomo Zalman Auerbach, though there is no Torah violation involved in cooking with a microwave, because it is not a standard method of cooking, nonetheless, food prepared in a microwave oven is considered cooked, and for that reason it may be rewarmed on Shabbos. However, Shevet Halevi (10:62) writes that some microwaved foods have a distinct taste which is different than food prepared in a traditional manner, and one may not reheat a microwaved food if the taste will change.

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The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.