By: The OU Kosher Halacha Yomis Team
If a food was fully cooked before Shabbat and then cooled down, may it be recooked again on Shabbat? The Shulchan Aruch makes a distinction between recooking a dry food and a liquid. If a dry item was fully cooked before Shabbat, there is no prohibition to recook it again on Shabbat, but it is prohibited to recook a liquid that cooled down.
This does not mean that one may place a dry cooked food directly on the fire. Though there is no Biblical prohibition of bishul when reheating a dry food, there are nonetheless rabbinic injunctions which apply, either because one might adjust the flame or because it has the appearance of cooking.
One of the permissible methods of reheating a dry, fully cooked food on Shabbat is to place it into a boiling pot of water that has been removed from the fire. Once the pot is off the stove, there is no concern that one might adjust the flame, and since there is no fire, it does not appear as though raw food is being cooked.
Does this principle apply to sugar?
Granulated sugar is extracted via a cooking process. Since sugar is a dry food, one would assume that it should be permitted to add sugar to a pot of boiling water that is off the fire. However, the Mishnah Berurah (318:71) cites the Shaarei Teshuva that since sugar dissolves when placed in hot water, lechatchila we view sugar as a liquid. As such, sugar should not be added to a kli rishon (a pot that was on the fire), nor may one pour hot water onto sugar. Instead, one should first pour the hot water into a cup, and then it is permissible to add the sugar.