Thanks to an e-mail inquiry I received last week, I was forced to review my understanding of how to permissibly reheat food on Shabbat morning, and thought it would be worth sharing some of the results of my research. This topic is, however, fairly complicated— I will try to be clear, but please ask if I do not explain something fully.

It is prohibited to actually cook food on Shabbat, so we will only discuss how to handle food that has already been fully cooked, whether and how it can be reheated. Friday night questions of what foods we can allow to continue cooking are a separate issue for another time. Our question for now is how to handle fully cooked food that is now cold. First, to question the assumption for a second, one choice is to not let the food grow cold— many people allow moving food from the section of a blech that’s on the fire to the section that is not (although see below for another opinion). If we did not do that, however, how do we reheat the food?

A few basic principles: Halakhah assumes ein bishul ahar bishul, that once food has been fully cooked, it cannot be cooked again. According to this principle, it is impossible to re-cook a food, and therefore reheating it should not be a problem. However, there is a debate about whether that principle applies to liquids (davar lah), like soup. The general opinion is that liquids that get cool (room temperature) again are considered to have been azal leh bishulei, to have lost their cooked quality, so that reheating them violates the prohibition of cooking on Shabbat. For a davar yavesh, however, a dry food, we apply the rule of ein bishul ahar bishul, and it should therefore be permissible to reheat that food on Shabbat morning.

The two main questions about that statement are: how dry does a food have to be and are there limits to how we can reheat that food? In terms of dryness, the best option would be for the food to have only internal oil that might be exuded during reheating. So, for example, a kugel is not considered a davar lah even though in the course of reheating it will exude some oil (a liquid). Even in that case, however, there is a problem with that liquid in that we consider it nolad, an item that had not existed at the onset of Shabbat and therefore muqtsah and not to be used. Except for that problem, however, the kugel may be reheated (in the ways that I will mention in a moment).

Foods that have more sauce on them become a problem, because there the sauce is clearly a davar lah. While the Mishnah Berurah quotes an opinion that even a food with a sauce that was fully cooked is freed of the worries of bishul ahar bishul, the Shulhan Arukh rules stringently on that issue. So, in terms of types of foods, the question to consider is whether the food has internal oil only (or, perhaps, primarily) or whether it was cooked in a sauce that will reheat as well. In either case, the liquid that comes out in reheating should not be actively used, as it’s considered nolad and therefore muktsah, not to be used on that Shabbat.

Assuming that we are discussing a food that may be reheated without worries of bishul ahar bishul, we need to consider the heat sources we are allowed to use. Before coming to the crux of the issue, I would point out that there are still people, although their numbers are dwindling, who received the pesaq that any food (which is already completely cooked, and therefore does not have bishul problems) that was in the oven kol bein hashemashot, all of the time between sunset and when the stars come out, can be returned there at any time over Shabbat, even if it has become completely cold in the meanwhile. I mention that not to permit it—I believe the large majority of authorities today do not follow this opinion— but to explain why one might see that being done in a home that is fully shomer Shabbat.

The best heat source to use is one which is both nonadjustable and one where cooking is not ordinarily done (the prime example, the one Rabbi Willig always mentioned in his shiurim to semikha students, is a non-adjustable hot plate— people do not cook on such a utensil, nor are there worries that the people will adjust the level of the fire involved). Second to that are issues of a blech. Putting food directly onto the fire of the blech is rabbinically prohibited because it looks too much like cooking. Some authorities, however, permit putting food on that part of the blech that is not directly on the fire, since it is not on the fire. Those who follow that opinion, however, need to be careful not to move food from the "off the fire" section of the blech to the on the fire section. There are various other permutations of how to get around the problems associated with putting food back onto a fire— empty pots, a "qederah" blech, and so on. Each have their proponents, and a more detailed question would need to be asked.

Daf Yomi