A Question of Jewish Law

July 28, 2010

Heating and Reheating

Filed under: Shabbat — chaimweiner @ 9:35 am

Question: Is it permissible to reheat food on Shabbat? If the reheating of food is permitted, does it matter how it is heated, or how hot it gets?

Answer: The prohibition against working is the central mitzvah of Shabbat. Cooking is one of the 39 master categories of work. From a halachic point of view, the question is whether reheating food is considered to be a form of cooking.

In the Mishnah [Shabbat 22:2] we read: Whatever was put into hot water before  Shabbat may be steeped again in hot water on  Shabbat; but whatever was not put into hot water before  Shabbat may only be rinsed with hot water on  Shabbat.

This means that any food which was cooked before Shabbat may be placed in boiling water and reheated on Shabbat, but food which had not been cooked before Shabbat may only be placed in warm water to prevent cooking from taking place. From this Mishnah we learn an important halachic principle – ain bishul achar bishul – there is no cooking after cooking. Once something is fully cooked it cannot be cooked again.  Therefore cooked food can be reheated to any temperature without risk of transgressing the prohibition against cooking. This ruling was codified in the Shulchan Aruch [OH 318:15].

However, many important commentators, including Rashi and the Rosh, rule that this principle only applies to dry food.  They maintain that it is forbidden to reheat liquids. The reasoning behind this is quite simple. If you take a slice of bread and put it in the toaster it will become toast. If after you toasted it you let it cool down it will be cold toast. This is because dry food does not ‘uncook’ itself when it cools. However, if you boil a kettle of water it becomes boiling or ‘cooked’ water.  When you let the boiling water cool it returns to being cold water – just as it was before it was ‘cooked’. In this case, cooling returns the water to its previous state and if it is then reheated, it cooks again. Thus cooked dry food can be reheated without fear that it will cook again, but liquids cannot be reheated on Shabbat.

Whenever there is a difference of opinion among the main halachic authorities concerning a Torah Law (as in this case) the stricter view is adopted. Therefore, one is allowed to reheat dry food on Shabbat, but not liquid food.

But what about combinations of dry and liquid foods – such as meat with gravy or a ratatouille sauce with lots of vegetables? Dry food is rarely completely dry – so when the Mishnah permitted reheating food it must have meant food with some liquid element. When determining whether something is a liquid or a solid – we calculate the proportions of each. If more than 50% of the dish is dry it may be reheated. If more than 50% is liquid it may not be reheated on Shabbat.

The question of whether reheating is considered to be cooking, is not the only consideration when reheating food. There are other concerns, such as whether it is acceptable to use fire or other sources of heat on Shabbat. These will be considered in a forthcoming post.

Rabbi Chaim Weiner

Based on R. Ovadia Yossef, Yachve Da’at,  4, 9.

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