One of the thirty-nine melachah or prohibited categories of acts on Shabbat is that of cooking and baking. It also includes any form of heat treatment of non-foods as in the case of melting wax or firing ceramics. Even boiling water falls under this melachah.

Cooking is defined as the act of using heat to make a substance edible or to change its state. In order for food to be considered hot, the food must reach a temperature of yad soledes bo (120°F) or hot enough to cause one to withdraw his hand due to the heat. If the food will not reach a temperature of yad soledes bo there is no concern of cooking. In any event, lighting a flame on Shabbat is a prohibited act, as is extinguishing a flame. Acts that would result in this happening are likewise included.

Many Rabbinic laws were initiated to prevent one from breaking the rules by distancing people from what is prohibited and from preventing one from actions that may appear to be a transgression.

One is therefore prohibited to leave the food on or in the place where it will be cooking, even if it was placed there before Shabbat. The Rabbis are concerned that one may adjust the heat to enhance the flavor of the food. Unless the food was edible before Shabbat, it may not be left in the oven. Modern Jewish household make use of a blech over the flames to make sure that one does not adjust the flame. The blech should therefore also cover the temperature controls.

With an oven, there is also the concern that by opening the oven door, a light or switch may be triggered. In the case of convection ovens, the door opening may cause a circulating fan to go off. Even though one does not intend for this action to result, it is prohibited. If opening the oven door does not automatically set off an electrical reaction, then one may do so in order to remove food on Shabbat so long as all the food is removed at that one time.

Modern conveniences have both simplified our lives and made the rules a bit more complicated. There are Shabbat mode ovens with distinct rules for Shabbat and Yom Tov. Warming drawers may be used so long as food is not placed in the drawer during Shabbat and all the food in the drawer is removed at one time.

Rules differ whether the object is solid, liquid or a mixture of the two but it is clear that cooking by way of fire or red-glowing metal is prohibited. The use of solar heat is permitted so long as the heat is via direct sunlight rather than a solar powered device.

Adapted from the Shabbos Home by Rabbi Simcha Bunim Cohen