The Laws Of Cooking On Shabbos: The Dinim Of Shehiya

The Definition of Shehiya
From the point of view of Bishul on Shabbos, there is no prohibition whatsoever in leaving a raw food or unboiled liquid on an open flame on Erev Shabbos so that it gradually cooks on Shabbos itself, since the Bishul occurs of its own accord. However, Chazal were concerned that on Shabbos a person may come to speed up the cooking process of raw or uncooked foods by stoking the fire - an Av Melachah of Mav'ir. Chazal therefore forbade leaving uncooked food on a cooker in the normal manner. The reason for this prohibition is called Gezeirah Shema Yechate, which means "a decree perhaps you may come to stoke the fire" and the prohibition itself is called Shehiya.
In order to leave uncooked food on the cooker, Chazal made a rule that the fire must be covered. Once the fire has been covered, there is no prohibition of Shehiya. The reason for this is that the covering of the fire indicates that the person does not wish to have a strong flame and this will remind the person not to increase the flame. A covered flame is called "Ketuma."[1]
Which Fires Need Covering
1. Gas and electric ranges
  Nowadays when most cooking is done on a gas or electric[2] range, the common custom is to cover the flames with an aluminum sheet[3] called a "blech."[4] Preferably the controls for the flames should also be covered,[5] either by the blech itself or with a piece of aluminum foil.[6]
2. Electric hot plate
  In general there are two different types of hot plates available on the market:
3. There are hot plates that do not have a temperature range, rather, once the hot plate is turned on, it maintains a constant temperature. With such a hot plate there is no worry that one will come to increase the fire since there is no possibility of increasing the fire and such a hot plate therefore requires no covering.[7]
4. A hot plate with a variable temperature range needs to be covered with a sheet of foil.[8] The same would apply to a warming cupboard with a variable temperature range.[9]
5. Crock Pots
  Crock pots or slow cookers that have variable temperature ranges[10] are subject to the laws of Shehiya and foil should be placed between the outer cooker and the inner pot.[11]
6. Oven
  The halachic status of an oven is rather different from that of a cooker in that the food in an oven is not placed directly on a flame, rather, the flame (in a gas oven, or the electric element in an electric oven) is at the rear of the oven (or in the roof of the oven as is the case of some electric ovens) and the food is cooked due to the heat of the air in the oven.
  There is a discussion among the Poskim as to how to achieve the state of Ketuma in an oven. Some Poskim write that the only way to achieve Ketuma is by using an oven insert, i.e., a metal box placed inside the oven that acts as a barrier from all sides (or alternatively covering all the sides and base of the oven with foil). Other Poskim write that Ketuma is achieved a) by covering one of the walls of the oven with foil and b) by covering the controls.
  Due to this difference of opinion as to how to achieve Ketuma in an oven, one should always place only fully cooked food in an oven thereby avoiding the need for the oven to be Ketuma.[12]
Cooked Foods
It was explained above that Chazal were worried that somebody may come to stoke the fire to hasten the cooking process. It was for this reason that Chazal said that when one places an uncooked food on the fire, the fire must be covered to act as a reminder not to increase the flame.
Would this rule also apply to a cooked food?
In the case of a cooked food, it would appear that there is no worry that one may come to stoke the fire. Hence it would seem that one need not cover the fire if the food has been cooked before Shabbos. Furthermore, which food is considered cooked?
There are three opinions in this matter:
1. A food is considered cooked when it is one-third cooked (Maachol Ben Drusoi). According to this opinion, once a food is one-third cooked there is no further worry that a person will hasten the cooking process, and the fire therefore need not be covered.
2. A food is considered cooked when it is half-cooked.
3. A food is only considered cooked when it is fully cooked and any further cooking would bring no advantage to the food.
In practice, the accepted custom is to make sure that all foods are fully cooked before Shabbos, but to still cover the flame with a blech.[13]
This is advisable for the following reasons:
1. This avoids any problems with the prohibition of Shehiya.
2. With an uncooked food, one may easily come to speed up the cooking process (as described above in Section 1:5) either by stirring or covering the food, or moving it directly onto a flame.
3. As we shall soon learn, one of the conditions for Chazarah - returning a pot to the fire - is that the food be fully cooked.
One should also make sure that water in a Shabbos kettle be boiled before Shabbos.[14]
In Cases of Difficulty
In case of difficulty when there was very little time to cook the food fully before Shabbos, then the partially cooked food should be placed on the blech - however, on Shabbos one may not move the pot from an area on the blech which is not directly on the flame to an area directly on the flame. To do so would speed up the cooking process, which is prohibited. Also one may not cover the pot with a lid (and therefore one may not lift the lid to "check" inside to see how the cooking is proceeding) nor stir the contents.
Where No Blech Is Available
When no blech is available, a fully cooked food may be left on an uncovered fire before Shabbos. In cases of difficulty, even if the food was only half cooked, or in great difficulty even one-third cooked, it may be left on an uncovered flame.
However in a case where the flame is not covered, the pot may not be returned to the flame once it has been removed due to the prohibition of Chazarah.
Under no circumstances may one leave a food that is less than one-third cooked or water that is less than Yad Soledes Bo on an uncovered flame before Shabbos.[15]
In Practice
Practically speaking, one should make sure that all foods are fully cooked before Shabbos. (This includes the boiling of the water in the Shabbos kettle.) This is advisable for the following reasons:
1. This avoids any problems with the prohibition of Shehiya.
2. With an uncooked food, one may easily come to speed up the cooking process (as described above in Section 1:5) either by stirring or covering the food, or moving it directly onto a flame, etc.
3. As we shall soon learn, one of the conditions for Chazarah is that the food be fully cooked.
Food Left by Mistake on an Uncovered Flame
If by mistake, one left a food that was less than one-third cooked on an uncovered flame, the food may not be used either by oneself or by others until some time after Motzaei Shabbos.[16]

Notes:

(Back to text) If the flame was not covered, the food must be removed from the fire as soon as Shabbos comes in. The fire must be Ketuma in all circumstances, i.e., even if the intention is only to eat the food on Shabbos morning, e.g. the cholent, and therefore there is no worry that one will come to increase the flame, even so, the flame must be covered.

(Back to text) Although some cookers today have ceramic coverings over their elements, they still require a blech to act as a reminder not to turn up the flame (Shabbos KeHalachah, p. 300, Biurim 5, based on the Alter Rebbe's Shulchan Aruch 253:26).

(Back to text) If only silver/aluminum foil is available, it is correct to fold the foil a few times to create a thicker sheet as a blech (Shabbos KeHalachah, p. 304). Heavy-duty foils are available on the market.

(Back to text) The Poskim write that once the flame has been covered with the blech before Shabbos, the flame should not be adjusted. This is because the whole reason one covers the flame is to indicate that the flame no longer will be adjusted. Any further adjustment of the flame is a contradiction to the purpose of covering the flame.
However if one notices that there is an extra flame alight that will not be needed for Shabbos, one may extinguish that flame (ibid., p. 310).

(Back to text) Ibid., p. 306. If the blech was not large enough to cover the controls, or if no foil is available, as long as the area under the pot is covered by a blech it is still considered Ketuma. It is only an additional precaution that the controls be covered.
However all agree that just covering the controls either with foil or tape without covering the flame would not be enough to be considered Ketuma (ibid., p. 306).

(Back to text) It should be noted that even if the flames were on the highest setting so that it is impossible to turn the flame higher, the fire still needs covering (ibid., p. 300).

(Back to text) The same rule would apply to a warming cupboard. However many cupboards have variable thermostats and a Rav should be consulted as to their usage.

(Back to text) However it is not necessary to cover the entire plate; it is sufficient only to cover the place upon which the pot rests (ibid., p. 305).

(Back to text) Ibid., p. 345.

(Back to text) A crock pot with only one setting would have the same rule as a hot plate with only one setting and does not need to be covered (ibid., p. 301).

(Back to text) Ibid., p. 300. The foil should not be folded, rather a thin sheet used. Otherwise this would involve a question of Hatmanah (see ibid., fn. 23).

(Back to text) Ibid., p. 312.

(Back to text) Ibid., p. 320. However it is a widespread custom to leave fully cooked meat, cholent, etc., in an oven although the flame is not covered.
Furthermore, one may use an electric Shabbos kettle with variable temperature settings without being Ketuma as long as the water is boiled before Shabbos.
The reason for these leniencies is that there is a difference of opinion among the Poskim if a cooked food requires to be Ketuma - the most stringent opinion being that of the Beis Yosef. He maintains that only a food that is fully cooked, and any further cooking would be disadvantageous, does not need to be Ketuma. The most lenient opinion states that once a food is one-third cooked, it does not require Ketuma.
Practically, in order to cater to the stringent opinion, all foods should be cooked and still have the flame covered just in case any further cooking to the food would be advantageous. However this is only a stringency. Therefore, as regards a blech - where there are other reasons for being stringent as enumerated in the text - we are stringent to require a blech. However regarding an oven and an electric kettle, the widespread custom is to be lenient and as long as the food is fully cooked, no Ketuma is required.

(Back to text) Shabbos KeHalachah, p. 322. One may also be lenient even if the water is only Yad Soledes Bo (ibid., p. 317, Biurim 14 and also p. 338).

(Back to text) The Shulchan Aruch does rule that raw meat may be cooked on an uncovered flame if it was placed in the pot immediately before Shabbos, providing that: a) it would take the entire night to cook without touching the flame; and b) there is no intention to eat the food before the following day. In practice, however, there are many conditions for this halachah (see Shabbos KeHalachah, p. 326). There is one practical case in which one may rely on this halachah, and that is with a crock pot. If the meat was inserted immediately before Shabbos and the crock pot would cook it slowly over Shabbos, then it would be allowed even if the pot was not covered with foil (Ketuma).

(Back to text) I.e., until after the amount of time that it would take to cook the food after Shabbos (Alter Rebbe's Shulchan Aruch 253:12).
If one realized before Shabbos, or on Shabbos, that the food was not cooked, the food must be removed from the uncovered flame (Shabbos KeHalachah, p. 297).