The question arose concerning a situation where the power went out on Shabbat morning, after the food for Shabbat lunch had been placed on the plata (electric hot plate) or blech (metal covering over the stove). In an effort to keep the food warm until the husband returned home from the synagogue for lunch, the wife wrapped the pots of food with towels to preserve the heat. Is this a permissible way of keeping food warm on Shabbat? Needless to say, towels are not Mukseh on Shabbat, and wrapping a pot of food with towels quite obviously does not involve fire or electricity, and does not have the effect of cooking the food. Seemingly, then, this should be entirely permissible on Shabbat.
The Gemara, however, in Masechet Shabbat (34), writes explicitly that Hatmana insulating food in a manner that preserves its heat is forbidden on Shabbat. Many people are unaware of this Halacha, and it often happens when there is a power failure, or if the food was taken off the plata and the woman then realizes that it will not be served for a while, that she will want to preserve the heat by wrapping it with towels or blankets. It is thus important to instruct members of the household that insulating food is forbidden on Shabbat.
If a woman mistakenly insulated food on Shabbat, may it be eaten, or is it considered forbidden, since it was kept warm in a prohibited manner?
The Halachic authorities rule that if the insulating was done mistakenly, out of ignorance of the Halacha, the food may be eaten on Shabbat. As long as one did not wrap the towels around the pot in intentional violation of the laws of Shabbat, the food remains permissible and may eaten on Shabbat.
Summary: If a pot of food was removed from the hot plate or blech, or if there was a power failure, one may not wrap the pot in towels, blankets and the like to preserve its heat. If, however, one did insulate a pot of food, not knowing that this is forbidden on Shabbat, the food remains permissible.
