Shabbat: Cooking
Bishul B'Shabbat/Cooking on Shabbat
The Torah forbids cooking on Shabbat.   “Cooking” means making food edible by heating it to above 120° F (49° C).
Cooking includes:
  • You may not make a soft food hard (such as cooking an egg).
  • You may not make a hard food soft (such as cooking meat).
  • You may not, in any manner, heat (to 120° F or above ) liquids that you will drink or foods with liquids—such as sauces and gravies--whether fully cooked or not. 
Timing of Cooking
According to the Torah, you may eat food on Shabbat that had been placed on the heat source Friday afternoon but was not completely cooked by sunset.
ReasonNo action is being taken and the cooking will be completed by itself.
Kli Rishon and Kli Sheini
According to Torah law, food is only considered to be cooked if it has been directly heated from the heat source, such as a kettle on a fire or a pot on a flame (even if that utensil has been removed from its heat source). This is called a kli rishon.
Once you pour water from a kli rishon into a glass, the glass is a kli sheni. Some foods, such as an egg or tea, get cooked in a kli sheni (kalei bishul--easily cooked). These foods are forbidden by Torah law to be put into a hot kli sheni on Shabbat.
Exception Halacha allows spices and water to be “cooked” in a kli sheni.
NoteIf the water is less than 120° F, nothing gets halachically cooked in any kli, even in a kli rishon.
Reheating/Replacing to Heat Source
   1.    Do Not Reheat Food Unless It Is Halachically Dry.
This includes heating on a hotplate, stove, or oven and applies to even fully cooked food. Halachically dry means the food is solid at either the beginning or the end of the cooking, or both. To determine whether a food is liquid (and therefore may not be reheated on Shabbat), shake the container. If the food does not move around, it is considered to be solid. (For when solid food may be reheated, please see Shabbat: Reheating.)
   2.    To Replace Heated Food onto/into Its Heat Source (hachzara):
  • You must have taken it off with the intention of replacing it, and
  • You may not put the utensil down onto a surface; you must continue to hold the food (or the utensil) in your hand.
  • The heat source must be covered.
Reason Chazal were concerned that someone might see you put food on the heat, mistake it for actual cooking, and erroneously think that cooking is permitted on Shabbat.
Note You may not replace incompletely cooked food to a heat source.
Reason To do so would facilitate the cooking.
   3.    Do Not Put Food on a Heat Source that has Adjustable Controls.
This applies even to fully cooked food.
Reason You might adjust the heat and thereby violate a Torah law due to shehiya (stoking the fire or turning up the heat).
Note Shehiya is simple to avoid; just cover the flame (or electric heating element) and any temperature controls before Shabbat, as when using a blech (a metal sheet that covers the flames and controls). Then, on Shabbat, you may put fully cooked solid (but not liquid) food on top of other food (or utensils containing food) that were already on a blech from before sunset on Friday.
Reason Doing so does not look like you are cooking and the blech prevents you from adjusting the heat.
   4.    Do Not Insulate Food to which Heat Is Added.
You may not add insulation (which will help keep in the heat) during Shabbat to foods that are on a heat source, even to fully cooked foods, if they are “wet.” By rabbinic law, you may not apply heat to an insulated utensil—or apply any insulation that adds heat--even before Shabbat began and let it remain that way during Shabbat--even if the food was completely cooked before sunset on Friday.
Note Regarding food on a heat source, you may add insulation if there is at least one uncovered area at least the size of a quarter. Adding insulation on Shabbat is only a problem if the insulation completely surrounds the food or utensil on all surfaces and the top.