Which foods may be reheated on Shabbat, and which may not? For those which one may reheat, how should one do it?
When we deal with reheating food on Shabbat, there are several halakhic problems.:
1) Cooking. This is one of the 39 categories of Torah-prohibited activities on Shabbat.
2) “Lest one stir the coals.” This is a rabbinic enactment aimed at preventing someone from stoking the fire in order to increase the flame. It is considered lighting a fire, which is prohibited. (The modern equivalent is using electricity, which is prohibited because it is considered either lighting a fire or building.)
3) “It looks like cooking.” This is a rabbinic prohibition to engage in an action which looks like one is cooking on Shabbat.
The Mishnah states (Shabbat 22b), “Anything which had been placed in hot water before Shabbat can be placed in hot water during Shabbat.” This is the source of the principle, “There is no prohibition of cooking something which has previously been cooked (Ein bishul achar bishul).” One does not transgress the prohibition of cooking when reheating something that had been cooked before Shabbat. This rule applies to solid foods. However, for liquids we follow the opinion that there is a prohibition of reheating what had previously been cooked (Yesh bishul achar bishul). Therefore, it is prohibited to reheat liquids on Shabbat. However, one may leave hot liquids on the plattah (warming tray) before Shabbat.
In order to avoid the problem of “stirring the coals,” we reheat food at a pre-set temperature. (The standard plattah has no settings. It is either on or off.) One who is using a gas or electric burner to reheat food should cover the knobs which control the temperature. This ensures that one will not adjust the temperature out of habit or by accident.
One can avoid the problem of “looking like cooking,” either by covering the fire (with a metal sheet called a blekh) or by putting an empty pot or other item on the plattah, since nobody cooks that way.
To summarize: When it comes to reheating food on Shabbat there are three principles which must be followed.
1) One may not reheat liquids on Shabbat.
2) Foods should be reheated at an unadjustable temperature.
3) Foods should not be reheated over an open flame.
What is the halakhah of food which is composed of both solid and liquid — chicken in its juices, potatoes in oil, and the like? Are they to be considered solids or liquids?
According to the Pri Megadim,it depends which item is in the majority. If the food is primarily solid, it is permitted to reheat it. If it is primarily liquid, it is forbidden.
The Iggrot Moshe finds this difficult. He asks why one should be permitted to reheat something just because the liquid is only a small part of it. That small part is still being cooked! Therefore, he rules that it is proper to be stringent regarding this case.
The Shulchan Arukh (O.C. 318:16) rules that it is permitted to reheat food even though the fat in the food will melt and become liquid. The Magen Avraham explains that whether a food is deemed as liquid or solid depends on its state when the reheating began. Therefore, even though the fat will liquefy, it is still considered solid, so it is permitted to reheat it.
To summarize: One may not reheat liquids on Shabbat. However, one may reheat coagulated gravy even though after it is heated it will become liquid gravy.
There are additional details about reheating food on Shabbat, and it is proper to learn them in an in-depth fashion. This column was limited to basic principles only.