Chapter 6 - The Laws of Shehiya

6:1 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

6:2 Which Fires Need Covering

a) Gas and electric ranges

Nowadays when most cooking is done on a gas or electric2 range, the common custom is to cover the flames with an aluminum sheet3 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

b) Electric hot plate

In general there are two different types of hot plates available on the market:

1. 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

2. 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

c) Crock Pots

Crock pots or slow cookers that have variable temperature ranges10 are subject to the laws of Shehiya and foil should be placed between the outer cooker and the inner pot.11

d) 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

6:3 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:

a) This avoids any problems with the prohibition of Shehiya.

b) 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.

c) 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

6:4 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.

6:5 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

6:6 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:

a) This avoids any problems with the prohibition of Shehiya.

b) 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.

c) As we shall soon learn, one of the conditions for Chazarah is that the food be fully cooked.

6:7 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