May I keep food hot for Shabbos with sous vide?
By Rabbi Chaim Hillel Raskin, Rov of Anash in Petach Tikvah
In the sous vide method of cooking, food in a sealed bag, placed in a water bath kept at a precise temperature by a heat element submerged in the water. Two potential issues must be addressed before setting up hot food for Shabbos in this manner.
As a rule, Chazal prohibit leaving food on a source of heat at the start of Shabbos (shehiya) out of concern that you may increase the heat to quicken the cooking. Three permissible options are: (1) Placing a blech over the source of heat, demonstrating that you don’t need so much heat; (2) Adding raw meat right before Shabbos which can’t be expected to be ready for the night meal, precluding concern for adjusting the heat; (3) If the food is already at least halfway ready (maachal ben drusai) before Shabbos, thus it will be ready in any case for the meal without any adjustment. The Alter Rebbe rules that this is sufficient to permit shehiya, but leaves room for stringency like the opinion that the food must be fully ready to the point that adjusting the heat will ruin it.
Though a blech is not an option for sous vide, the other two options would theoretically be applicable. An additional Shabbos issue, however, must be addressed.
Chazal forbade insulating food (hatmana) in a heat increasing substance (mosif hevel) even if you set it up before Shabbos, as a safeguard to prevent you from insulating the food with burning embers (remetz). This prohibition is stricter and none of the leniencies of shehiya apply. Since food is generally insulated to keep it hot for the next day, there is greater need to increase the heat, and even a slight addition of heat would be effective due to the insulation which preserves the temperature.
A classic example of prohibited insulation is wrapping pots that are on the blech with a towel before Shabbos. Although the towel itself doesn’t increase heat, the pots are on a source of heat and insulating them is prohibited, even if set up before Shabbos. (If the majority of the pot’s walls are exposed, it is permitted to cover them on top with a towel.)
By sous vide, the bag of food is completely surrounded by water that is being heated by an element. The food in the bag is a distinct entity in the water, and is considered to be insulated with a heat increasing substance. This is analogous to the Talmud’s discussion of having cold water run through a channel of hot water from the hot springs of Tverya. It would therefore be prohibited to keep food hot for Shabbos in this manner, unless the heater is set to shut off before Shabbos starts, and the food is just kept hot by the previously heated water.
The implication of the poskim is that a sous vide should not be set up on Friday even to keep food hot for motzei Shabbos.
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