Bishul is the melakha that prepares or improves food, whether through seething, baking, or roasting. What defines this melakha is that food is prepared by means of fire’s heat. The food is softened by the heat, and the tastes within it merge to create a new taste, which is intense and contains a variety of flavors.
Cooking initially softens food, and most foods remain soft after being cooked. However, there are some foods that harden in the course of cooking, like eggs. Similarly, the process of roasting softens the food at first, but eventually makes it tougher.
Torah law prohibits cooking, and this prohibition even includes foods that can be eaten raw, if cooking improves them in some way. For example, although one can certainly drink cold water, nevertheless, since cooking improves the water, doing so is prohibited by Torah law. Cooking foods that are not improved by this process is not prohibited by Torah law, but it is still prohibited rabbinically (MT 9:3; SHT 318:114).
One who cooks using fire or something that was heated by fire (toldot ha-esh) transgresses a Torah prohibition. Therefore, if one heats a frying pan on the fire, then turns off the fire and fries an egg in the hot frying pan, he has transgressed a Torah prohibition. This is the case even though the pan was no longer on the fire during the actual frying (SA 318:3; MB ad loc. 17; concerning cooking in the sun, see section 25 below).
Any accepted form of cooking is prohibited by Torah law. Therefore, one who cooks in a microwave transgresses a Torah prohibition, because nowadays this is one of the normal ways of cooking (Igrot Moshe, OĤ 3:52).
Before we explain the laws of Bishul in detail, we will preface them by saying that there are three types of prohibitions relevant to Bishul:
- Bishul by Torah law;
- Rabbinic enactments that forbid doing anything that might cause someone to turn up a flame (laĥatot ba-geĥalim, literally, “to stir the coals”);
- Rabbinic enactments that forbid doing anything that resembles cooking.
To determine whether one may do a particular action, he must examine it from these three perspectives. To elaborate:
- One must clarify if this action is forbidden because it qualifies as Bishul. Any action that leads to food’s transformation from a raw state to a cooked state is prohibited by Torah law. (This prohibition and those deriving from it will be explained in sections 3-13 below.)
- From the perspective of the Torah prohibition of Bishul, one may leave food on the fire when Shabbat begins, with the intention that it will continue cooking on Shabbat. However, the Sages prohibit doing so when it may be necessary to adjust the heat, due to the concern that one may forget it is Shabbat and turn up the flame, thus transgressing the Torah prohibitions of Mav’ir and Bishul. But if the fire is covered, as it is in the case of a plata (warming tray), one may place food on it, because there is no chance that he will forget it is Shabbat and turn up the flame.
- Even if a food is fully cooked, and thus there is no concern that one will turn up the flame, the Sages still forbid doing anything that looks like cooking. Therefore, one may not put fully-cooked cold food directly on the fire. However, one may put such food far from the heat source, in such a way that it does not look like cooking. (As we will explain in sections 18-21, there is disagreement whether placing food on a plata looks like cooking and is thus forbidden.)