A fundamental rule of the laws of Shabbat is that one may not create a new entity. During the other six days of the week, we emulate God, in whose image we were created, by busying ourselves with creating and developing. We take the raw materials of the world and develop and improve them, thus continuing the work of creation. Just as the Creator ceased creating on Shabbat, so too, we are commanded to emulate God’s attributes by establishing Shabbat as a day of holiness and rest. It is a day when we desist from creating new things, and thus enable ourselves to contemplate the world as God created it and increase our faith.
This principle is the principle behind Bishul as well. Cooking is forbidden because it changes a food from raw to cooked, thus giving it a new identity. But if the food is already cooked, there is no prohibition of reheating it, as reheating does not create a fundamentally new substance. Even when the reheating improves the taste, it is still not prohibited, following the accepted principle: ein bishul aĥar bishul (there is no prohibition of cooking something that has already been cooked). Therefore, the following question is critical to the laws of Bishul: When is a food deemed cooked? If it is considered cooked, then one may reheat it on Shabbat, and if it is not considered cooked, one may not do so.
Some of the greatest Rishonim disagree on this question. Some maintain that if the food can be eaten in a pinch, Bishul no longer pertains to it. However, in practice, it is agreed that a food is considered cooked only when it is fully cooked and one could serve it to guests without feeling compelled to apologize. Before that point, even if one could technically eat it in a pinch, it is not considered cooked (SA 318:4). Therefore, in practice, it is important to determine whether a food is mevushal kol tzorkho (fully cooked). As long as it is not mevushal kol tzorkho, any action undertaken to raise its temperature and speed up the cooking process is prohibited by Torah law.
For example, the heat of a plata is often uneven; there are hotter and cooler places on its surface. One may not move a food that is not yet fully cooked from its current place to a hotter place on the plata. Similarly, one may not put a towel over the food to contain the plata’s heat more effectively. Furthermore, if one removes the cover of a pot in order to check if the food is fully cooked, and it becomes clear that it is not, it is now forbidden to replace the cover, because doing so will make the food cook faster.
However, if the food is fully cooked, one may move it to a hotter place, and if he took off the cover he may replace it. Furthermore, one may cover the pot with a towel to preserve its heat. One may do so even if it improves the food’s taste, as it does with cholent. Similarly, one may remove fully-cooked meat from the freezer and heat it (in a way that does not resemble cooking; see section 18). For it is an accepted principle that after a food is classified as cooked, it is no longer prohibited to reheat it. (All of this refers to solid foods; liquids involve additional issues that we will expand upon in sections 5-6.)
Cholent is considered fully cooked even if the bones in it are not soft, and one may cover it to preserve its heat. However, if one is accustomed to eat the bones, then as long as they are not yet cooked the food is not deemed fully cooked, and one may not cover it and preserve the heat.
. According to Rambam, Teruma, Smag, Smak, Hagahot Maimoniyot, Or Zaru’a, Riva, and Tur, as long as the food is not fully cooked, cooking it is prohibited by Torah law. According to Ramban, Rabbeinu Yona, Rashba, Rosh, and Me’iri, once a food has reached the state of ma’akhal ben Derusa’i (see n. 14), the principle of ein bishul aĥar bishul applies. The bottom line is that, according to SA 318:4, only once a food is fully cooked does the principle apply. BHL 318:4 s.v. “afilu” states that those who prohibit the further cooking of a dish that has reached the state of ma’akhal ben Derusa’i maintain that it is a Torah prohibition. However, Eglei Tal, Ha-ofeh 7:16 maintains that most of those who forbid this see it as a rabbinic prohibition. Be-di’avad, if one cooked something on Shabbat that had reached the state of ma’akhal ben Derusa’i before Shabbat, it may be eaten on Shabbat. This is because when it comes to the question of benefiting from the result of a melakha, one may rely on the lenient opinion (below 26:5).Cholent with chicken bones: According to R. Shlomo Zalman Auerbach, if chicken bones in a cholent are not fully cooked, the whole cholent is considered not fully cooked, since most people eat the chicken bones that were softened by extended cooking. Therefore, even those who do not eat the bones must be stringent and not take any action that will heat cholent that contains partially cooked bones (Minĥat Shlomo §6; SSK 1:20). In contrast, R. Moshe Feinstein maintains that since most people do not eat these bones, one need not take them into account; as long as the cholent as a whole is fully cooked, one no longer has to worry about Bishul. Only in the case of an individual who generally eats the bones would the cholent be considered not fully cooked when the bones are not fully cooked (Igrot Moshe, OĤ 4:76-77; Yalkut Yosef 318:78). Nowadays, since most people do not eat the bones, the cholent is normally considered cooked even if the bones are not.