One may leave food on an open fire on Friday afternoon if there would be no reason for anyone to raise the flame under the food. Poskim disagree what constitutes “no reason.” According to Rif and Rambam, if the food is fully cooked, and further cooking will be detrimental to it, then one may do so. But if additional cooking will improve the flavor of the food, then one may not leave it on the fire, since there is motivation to turn up the flame.
In contrast, the Ge’onim and Tosafot maintain that if the food has reached the point where it could be considered a ma’akhal ben Derusa’i, we are no longer concerned that one would desecrate Shabbat to turn up the flame. Only if the food is totally inedible are we worried that, due to concern that the food will not be ready for Friday night dinner, one might turn up the flame.
The bottom line is that one may leave food on an open flame if it can be eaten in a pinch, as this is the opinion of the majority of Rishonim as well as the custom of most Jewish communities. However, le-khatĥila one should take into account the stringent opinion and refrain from leaving food on an open fire if continued cooking will improve its flavor. This problem can be avoided by covering the flame, as we will now explain.
. Tanna’im disagree about this (Shabbat 36b). The majority of Sages maintain that only food that is fully cooked and that further cooking will damage may be left on an open fire. This is the ruling of Rif and Rambam. However, according to Ĥananya, one may leave food that has reached the state of ma’akhal ben Derusa’i on the fire as well. This is the ruling of the Ge’onim, Tosafot, and many Rishonim. SA 253:1 presents the stringent opinion as the primary one, and the lenient position as the secondary one. Rema, based on Rosh, writes that the custom is to be lenient even though le-khatĥila it is preferable to be stringent. BHL s.v. “ve-nahagu” reaches this conclusion as well. Some Sephardic communities are customarily lenient (Yalkut Yosef 253:1). Nowadays, since it is easy to cover the flame with a blekh, one can satisfy all opinions, as explained in the next paragraph. Ma’akhal ben Derusa’i is named for a robber who was on the run from the law, and became accustomed to making do with minimally cooked food so that he could eat quickly and then continue to flee. Rambam understood that this refers to half-cooked food, while Rashi maintains that it refers to food that is a third cooked. MB 253:38 states that in extenuating circumstances, one can be lenient once the food is a third cooked. (Although SA 254:1 explains that one may leave a pot with raw meat on an open flame because it cooks slowly, nowadays since we cook with gas whose flame is extremely hot, this is no longer permitted, as explained in Harĥavot.)It is important to note that the rabbinic rules pertaining to the concern that one will raise the flame relate specifically to cooking food; when using heaters or radiators to heat the house this is not an issue. (However, in the case of a wood-burning stove, one must make certain that the fire has indeed taken hold before Shabbat begins, as explained in SA §255.)