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ANSWER: One may not insulate hot food before Shabbos if the food is on a fire or other heat source. Chazal were concerned that if it were permitted to do so, one might insulate the pot by surrounding it with ashes that contain hot coals, and this may lead to stoking the coals on Shabbos, which is forbidden. If one insulated food on the fire on Shabbos, even if done innocently without realizing that it is forbidden, the food may not be eaten on Shabbos if this caused any improvement to the food. If one insulated food on the fire before Shabbos, although it was forbidden to do so, the food would be permitted, as long as the food was already cooked at the beginning of Shabbos.
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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.