In the previous Halacha, we have discussed that just as it is forbidden to cook on top of an open flame on Shabbat, it is likewise forbidden to cook on top of an electric hotplate. The only distinction between an open flame and an electric hotplate IS regarding a food which was already cooked before Shabbat: Whereas it is forbidden to even warm up a pre-cooked food on an open flame on Shabbat, it is permissible to warm up such a food on an electric hotplate on Shabbat as long as the food is considered “dry,” such as bread or Challah and not “liquidy,” such as a soup. We have likewise explained the reasons for this.
A Dish Mostly Comprised Mostly of Sauce
A dish which contains mostly liquids and only a minority of “dry” or solid food, such as a vegetable soup, may not be placed on an electric hotplate on Shabbat since the prohibition of “cooking after cooking” on Shabbat applies to liquid foods. However, a food which is completely dry, such as bourekas or bread, may be placed on an electric hotplate or a stovetop covered with a layer of metal, for the prohibition of “cooking after cooking” does not apply to “dry” foods. Thus, since this food was fully-cooked before Shabbat, there is no prohibition to warm it up on Shabbat.
A Dish Which Contains a Minimal Amount of Sauce
A food which is mostly dry but contains a little bit of sauce, such as a dish of rice which has some moisture to it, is considered like a completely dry food which is permitted to be warmed up on an electric hotplate on Shabbat, for “cooking after cooking” does not apply to dry foods. Even if a dish contains sauce but cooking it causes the sauce or liquid to diminish in quality, it is indeed permissible to warm up such a dish on an electric hotplate or a stovetop covered with a metal sheet on Shabbat.
The Permissible Way to Warm up a Liquid Dish on Shabbat
If one would like to warm up a dish containing sauce or a completely liquid dish, such as a vegetable soup and the like (besides for water, which we shall not discuss presently), there is a halachically acceptable way to do so: One should set a timer to turn the hotplate off at a certain hour. When the hotplate is off, one may place anything one likes, even a soup, on top of it so that when it turns back on, whatever is on it will become warmed. In this manner, there is room for leniency without any concern.
Thus, if one wishes to heat up liquid foods or foods whose dry/liquid status is questionable for Shabbat day, one may set a timer to turn of the hotplate between eight and nine o’clock in the morning, for instance, and once the hotplate is off and is no longer hot, one may place any fully-cooked food on it that one wishes and avoid any concern.
Summary: One may not heat up any dish or food on an open flame on Shabbat. However, it is permissible to warm up a dry food on a stovetop covered with a layer of metal or an electric hotplate on Shabbat. Nevertheless, one may not warm up a liquid dish, such as a soup, on Shabbat. If one places the dishes on the hotplate when it is turned off by a timer, this is completely permissible; in this way, one may place even a soup or anything else one wishes on the hotplate and when the hotplate turns on again as a result of the timer, the foods will be permissibly reheated in honor of Shabbat.