The Forbidden Work of Cooking
The Mishnah in Masechet Shabbat (73a) states that one of the works forbidden on Shabbat is that of baking. We have previously explained that all types of work forbidden on Shabbat were works performed in the Mishkan (Tabernacle). The Gemara explains that the primary way that baking manifested itself in the Mishkan was through cooking the ingredients for the Ketoret (incense). The reason why the Mishnah calls this the forbidden work of “baking” and not “cooking” is because baking is included in cooking, as are roasting and frying which are equally forbidden on Shabbat just like cooking; the Tanna nevertheless chose the term “baking” because the forbidden works preceding this one in the Mishnah had dealt with the preparation of bread, such as, gathering, threshing, winnowing, grinding, selecting, sifting, and kneading. Thus, the Mishnah continues by listing another forbidden work associated with the preparation of bread although the main aspect of this forbidden work is indeed cooking.
Cooking a Food which was not Fully-Cooked Before Shabbat
Included in this prohibition is taking a food which was not fully-cooked before Shabbat, for instance, if it was only half-cooked, and placing it on the fire or on an electric hotplate on Shabbat; this constitutes a transgression of the Torah prohibition of cooking on Shabbat (even if the food was fully-cooked before Shabbat, it is not always permissible to place it on the hotplate on Shabbat as we shall discuss, G-d-willing).
The Prohibition to Cook on an Electric Hotplate
The forbidden work of cooking on Shabbat has absolutely nothing to do with the forbidden work of kindling a fire on Shabbat, for kindling is a completely separate forbidden work. Thus, one is liable for cooking on Shabbat even if one cooks on a flame that existed before the onset of Shabbat. It is likewise forbidden to cook on an electric hotplate which was turned on before Shabbat. This must be publicly emphasized, for unfortunately there are individuals who believe that it is permissible to cook on an electric hotplate on Shabbat and by doing so they desecrate the Shabbat as a result of simply not being aware of the correct Halacha.
The Temperature at which “One’s Hand Recoils”
The prohibition of cooking on Shabbat applies when one heats a food to the temperature that “one’s hand recoils” when coming in contact with such a hot substance. Several luminaries of our generation estimated this temperature to be approximately 45˚ Celsius or 113˚ Fahrenheit. Hagaon Harav Shlomo Zalman Auerbach zt”l rules likewise and supports his view with sources.
Nevertheless, Maran Rabbeinu Ovadia Yosef zt”l writes in his Chazon Ovadia-Shabbat (Part 4, page 371) writes that practically speaking, an “infant’s stomach” can be burned at a temperature of 40˚ Celsius or 104˚ Fahrenheit and this temperature is halachically considered the temperature at which “one’s hand recoils.” Hagaon Harav Ben Zion Abba Shaul zt”l and others rule likewise.
One may not place a food on a fire on Shabbat although one intends to remove it before it becomes very hot, for as long as if the food were to stay on the fire all day it would reach the temperature that “one’s hand would recoil,” placing such on the food on the fire on Shabbat constitutes the forbidden work of cooking. However, placing water opposite a fire in a way that it is not close to it and cannot reach such a temperature is permissible on Shabbat, for this is not considered cooking.