Is there a Halachically acceptable way of reheating soup or other liquids on Shabbat? For example, many people enjoy kibbehamda a soupy food on Friday night, and the leftovers are put in the refrigerator after dinner. Under what circumstances, if any, would it be permissible to reheat the pot on Shabbat morning so it can be served at lunch?
The Shulhan Aruch rules explicitly that reheating liquid on Shabbat is tantamount to cooking and thus constitutes a Torah prohibition. This ruling is based upon the view of several Rishonim (Medieval Halachic scholars), including Rashi and the Rosh, that liquid that has cooled is treated by Halacha as raw food with respect to the prohibition of cooking on Shabbat. Once a soup has cooled, it is no different than a piece of raw steak, for example, and reheating it would thus be Halachically equivalent to cooking raw steak, which is strictly forbidden on the level of Torah prohibition, and in ancient times would be treated as a capital offense. Clearly, then, it is strictly forbidden to place a pot of cold liquid on a fire or hotplate on Shabbat.
There are, however, two acceptable ways to reheat cold liquid on Shabbat. The Hid"a (Rav Haim Yosef David Azulai, 1724-1806), in his Birkeh Yosef, writes that it is permissible to ask a non-Jew to place a pot of cold liquid on a covered flame on Shabbat. The reason, the Hid"a explains, is because the aforementioned ruling of the Shulhan Aruch is not the only view on the subject. The Rambam (Rav Moshe Maimonides, Spain-Egypt, 1135-1204) was of the opinion that even when it comes to liquids, reheating is distinct from cooking. In his view, once a solid or liquid food has been cooked, it can never be "cooked" again in the Halachic sense, even after it has cooled. Although the Shulhan Aruch does not accept this view, we may nevertheless rely on it to permit asking a gentile, such as a non-Jewish housekeeper, to reheat a pot of hot food, as long as the fire is covered or if the pot is placed on an electric hotplate. Hacham Ovadia Yosef, in several contexts, accepts this ruling of the Hid"a.
The second option is to set an electric hotplate on a timer. Although the Torah forbids performing certain activities (Melacha) on Shabbat, indirect Melacha is permissible. Thus, if one places a pot of cold soup on an electric hotplate when the timer is turned off, he has not performed a forbidden act; placing it on the hotplate is no different from placing it on a countertop. Although he knows the timer will soon turn on and the food will be reheated, nevertheless, this is indirect Melacha and is thus permissible. The solution, then, would be to set ones hotplate before Shabbat on a timer so it goes on an hour or so before lunch. On Shabbat morning, before the timer goes on, he can take the pot of soup out of the refrigerator and put it on the hotplate, and then the soup will be warm by lunchtime. This is the ruling of Hacham Ovadia Yosef, as well as Rav Shlomo Zalman Auerbach (Israel, 1910-1995). (Of course, after the pot is removed from the hotplate, it may not be put back on.)
It must be emphasized that one may not draw conclusions from one situation to another in regard to these Halachot, or other areas of Halacha. Just because one is allowed to place a pot of cold soup on a hotplate when the timer is off, this does not mean one can put his barbeque on a timer and place raw meat on it on Shabbat. Each case must be addressed independently, and therefore a Rabbi must be consulted for guidance with regard to every situation.
Summary: It is strictly forbidden to reheat cold liquid on Shabbat. However, one may ask a non-Jew such as a non-Jewish housekeeper to place a pot of cold ALREADY COOKED liquid on a covered flame or on a hotplate on Shabbat. Alternatively, one may set an electric hotplate on a timer before Shabbat, and on Shabbat place a pot of cold liquid on the hotplate before it turns on.
