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May the same microwave be used for meat and milk?[1]

Some Rabbanim[2] say that one may use the same microwave for both meat and dairy, one after the other, so long as it is cleaned in-between. The majority of Poskim[3], however, rule that a microwave cannot be used for both meat/dairy even one after the other.[4] This applies even if the microwave is cleaned, and has not been used for 24 hours[5], unless the food is hermetically sealed and the microwave is Koshered, as explained below. Practically, one must abide by the latter opinion, as the former opinion does not take all factors into account, and is evidently a mistaken ruling.[6] Thus, microwaves are to be designated as either meaty, dairy, or Pareve, and not be used for the opposite foods unless on occasion, as per the guidelines to be explained. According to all, a combination microwave oven, which is both an oven and a microwave, may not be alternated between meat and dairy use, and follows the same laws to be explained regarding an oven.

Cooking in a hermetically sealed container:[7] In all cases, it is permitted to cook in a microwave food [meat/dairy] that is contained within a hermetically sealed container which does not allow any vapor to escape or enter. This applies even if the microwave has not been Koshered, and certainly if it has been Koshered in the method mentioned below.[8] Some Poskim[9] however discourage using the microwave in this method as it occurs that the hermetic sealing tears or opens during the cooking, which would then pose a Kashrus issue. It is therefore best to only use the microwave in this method after first Koshering it.

How to Kosher it:[10] Some Poskim[11] rule that it is not possible to Kosher a microwave.[12] Other Poskim[13] rule it can be Koshered through steaming water inside it.[14] This is accomplished through the following steps: The microwave is to be cleaned, not used for 24 hours[15], and have water with bleach/soap placed in it and heated for about 10-20 minutes[16], until it steams out.[17] Practically, one is to do as stated above, and use a hermetically sealed container to cook the food after Kashering it.

Bedieved, if used meat microwave for uncovered dairy: If one heated a dairy food in a meat microwave, or vice versa, and the food was uncovered, then if the microwave has been used for meat within the past 24 hours, or if it was dirty with meat residue, the food is forbidden. If the microwave was clean and was not used for 24 hours with uncovered meat, then the food remains permitted, although the microwave must be Koshered through the method explained above. See the section regarding “Ovens” for the sources and full details of this matter!

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[1] See Piskeiy Teshuvos 451:22; Sefer Hakashrus [Fuchs] 1:47-50 [pp. 48-49]; Nitei Gavriel 80:16; See also Hadarom Choveret 6 Nissan 5722; Kovetz Beis Ahron Yisrael 4:3

How does a microwave cook? A microwave is a rapid cooking element, which can warm and cook food much quicker than traditional cooking methods. Now, how does the microwave achieve its rapid cooking? The microwave does not use the heat of a fire or electricity to cook but rather cooks the food using radiation, or radio electromagnetic waves, which is projected from a vacuum tube and bounced off the metal lined walls of the microwave which penetrate the food from all sides. These waves hasten the movement of the water molecules in the food to atomic levels hence generating heat. [Heat is generated from movement and friction.] This form of cooking cooks the food much quicker than fire or electricity, as the radioactive waves hits the food equally in all areas and furthermore, penetrates the inside of the food molecules hence making the entire mass of the food an equal recipient of the heat. This is unlike fire or electric cooking which heats the external part of the food, and that heat then must travel to the inner part of the food in order to cook it. Likewise, this form of cooking only heats the actual food, as it does not actually send heat to the food but causes the food to heat itself up. Accordingly, all other areas and items of the microwave might remain cold, including the walls and certain plastic or glass containers which cover the food. The only way these items will become hot is if they are in contact with the food itself. The radio waves harmlessly pass through these containers into the food and do not cause any heating within them being they do not contain water molecules or other polar charge component. [See Hakashrus ibid footnote 100; See here for an educational video on how a microwave works. https:::www.youtube.com:watch?v=kp33ZprO0Ck]

[2] Rav Yitzchak Yosef, as he related in his weekly Shiur, published in Beit Maran

[3] See all Gedolei HaPoskim mentioned in Hakashrus ibid and Piskeiy Teshuvos ibid, which include Rav Wozner, Rav Elyashiv, Rav Sheinberg, Rav Halbershtam; Nitei Gavriel ibid footnote 26 in name of Rav Neiman of Montreal;

[4] The reason: a) Although the walls of the microwave do not give heat, nevertheless the steam and spills of the food inside make the walls absorb the food and hence it must be Koshered. [Piskeiy Teshuvos ibid; See Admur 451:41, Michaber 451:14, and M”B 451:81 that a vessel which absorbed the steam of an Issur requires Hagala] b) As there is a vent duct in the microwave that contains actual steam of food, and that area is not Kosherable. [Rav Neiman ibid]

[5] As due to the steam it is similar to baking dairy in a modern-day meat oven, which is similar to cooking dairy food in a non-Ben Yomo meat pot, which prohibits the vessel, and is initially forbidden to be done even though the food remains permitted.

[6] As although the walls don’t heat up the microwave receives steam from the foods and hence must be Koshered.

[7] See Admur 447:10; Rama 92:8; Michaber 108:1; Hakashrus ibid; Nitei Gavriel ibid in name of Rav Neiman

[8] Pischeiy Halacha Kashrus p. 28; Piskeiy Teshuvos ibid footnote 106

[9] See Kovetz Mibeis Levi 3:22-9; Hadarom ibid; Beis Ahron Veyisrael ibid

[10] See Piskeiy Teshuvos 451:22; Sefer Hakashrus [Fuchs] 1:47-50 [pp. 48-49]; Nitei Gavriel 80:16; See also Hadarom Choveret 6 Nissan 5722; Kovetz Beis Ahron Yisrael 4:3

How does a microwave cook? A microwave is a rapid cooking element, which can warm and cook food much quicker than traditional cooking methods. Now, how does the microwave achieve its rapid cooking? The microwave does not use the heat of a fire or electricity to cook but rather cooks the food using radiation, or radio electromagnetic waves, which is projected from a vacuum tube and bounced off the metal lined walls of the microwave which penetrate the food from all sides. These waves hasten the movement of the water molecules in the food to atomic levels hence generating heat. [Heat is generated from movement and friction.] This form of cooking cooks the food much quicker than fire or electricity, as the radioactive waves hits the food equally in all areas and furthermore, penetrates the inside of the food molecules hence making the entire mass of the food an equal recipient of the heat. This is unlike fire or electric cooking which heats the external part of the food, and that heat then must travel to the inner part of the food in order to cook it. Likewise, this form of cooking only heats the actual food, as it does not actually send heat to the food but causes the food to heat itself up. Accordingly, all other areas and items of the microwave might remain cold, including the walls and certain plastic or glass containers which cover the food. The only way these items will become hot is if they are in contact with the food itself. The radio waves harmlessly pass through these containers into the food and do not cause any heating within them being they do not contain water molecules or other polar charge component. [See Hakashrus ibid footnote 100; See here for an educational video on how a microwave works. https:::www.youtube.com:watch?v=kp33ZprO0Ck]

[11] See Piskeiy Teshuvos ibid; Siddur Pesach Kehilchaso that 8:3 that it can only be Koshered through Iruiy Keli Rishon and Even Meluban

[12] The reason: a) Although the walls of the microwave do not heat, nevertheless the steam and spills of the food inside make the walls absorb the food and hence it must be Koshered. Now, it is not possible to Kosher the microwave as one cannot blow torch it, place it in boiling water, and many Poskim rule an item cannot be Koshered through steaming water inside it. [See Shoel Umeishiv Telisa 3:125; Sdei Chemed Mareches Hei 24; Chametz UMatzah 17:12 that it is not possible to Kosher through vapor] It therefore has no viable path for Koshering. A second reason is b) As there is a vent duct in the microwave that contains actual steam of food, and that area is not cleanable or Kosherable. [Rav Neiman ibid]

[13] Teshuvos Vehanhagos 2:212; Yalkut Yosef Pesach p. 360; Piskeiy Teshuvos ibid regarding Shaas Hadechak

[14] The reason: As we rule that it is possible to Kosher an item through steam, and just like the microwave absorbed the food through steam, so too it can be Koshered through steam. [See Peri Chadash 121; P”M  Y.D. 94 M”Z 1; Tevuos Hasadeh 3:3]

[15] See Hakashrus ibis footnote 105 that so ruled Rav Wozner, Rav Shternbuch and Rav Mordechai Eliyahu

[16] Piskeiy Teshuvos ibid writes to leave it on for one hour; Sefer Hakashrus ibid writes [based on Mitbach Kehalacha p. 58 and Techumin 8:21] to enter a half a liter of water and leave the microwave on until it steams out; Rav Yitzchak Yosef says to leave it on for six minutes

[17] Teshuvos Vehanhagos 2:212; Koveitz Mibeis Levi 3:22; 7:25; Yalkut Yosef Pesach p. 360; Piskeiy Teshuvos ibid; Hakashrus ibid