Microwaves

The following discussion applies only to standard microwaves. Convection microwaves, which have the ability to cook by convection or by microwave, have different cooking methods and have a distinct set of cleaning and kashering rules.

A MICROWAVE IS a strong metal box. Inside, there is a microwave generator called a magnetron. When the microwave oven is turned on, the magnetron takes electricity from the power outlet and converts it into high-powered radio waves. The microwaves bounce back and forth off the reflective metal walls of the food compartment, just like light bounces off a mirror. As the microwaves travel through the food, they cause the molecules to vibrate more quickly. The faster the molecules vibrate, the hotter the food becomes.

Using the Same Microwave for Meat and Dairy
One may not use the same microwave for meat and dairy. Even if the microwave was cleaned in between uses, there would still be the issue of זיעה (vapors). Any person who uses a microwave can feel that the walls, top, and bottom of the cooking chamber become greasy over time. This is an indication that the flavors from the food can be absorbed into the walls and then discharged back into the food when the microwave is used again.

Kashering the microwave between meat and dairy consistently is not permitted, as this can lead to one becoming lax in the kashering or forgetting to do it all together.6 Ideally, one should have separate microwaves for meat and dairy. If one only has only one microwave, it should be designated for one primary use – either dairy or meat (usually the one you plan to use it for more). That food can be heated without any limits, in an open dish, etc. Any food of the other type may be heated only within a hermetically (airtight) sealed vessel. It is best to use a double covering (like 2 Ziploc bags). Standard microwave covers are not sufficient, because they have large holes.

Using a Non-Kosher Microwave
For the same reasons outlined above, one may not use a non-kosher microwave, even if it is clean. One may, however, warm kosher food in it inside of a hermetically sealed container. Again, one should use a double covering as mentioned above.

Kashering a Microwave
There are 2 types of kashering:
1. M’HETEIRA L’HETEIRA (from permissible to permissible) such as from meat to dairy or vice versa, when both the meat and the dairy are kosher.
2. M’ISURA L’HETEIRA (from prohibited to permissible) such as from non-kosher to kosher.

In general, the laws of kashering between two permissible categories are more lenient than kashering from non-kosher to kosher. Therefore, the Poskim make a distinction between kashering a microwave from meat to dairy or vice versa, and kashering from non-kosher to kosher.

The consensus seems to be that one may kasher a microwave from meat to dairy or vice versa. As mentioned, this can only be done as a one-time thing or on a very infrequent basis. One may not use the same microwave consistently for meat and dairy by kashering in between.

Kashering a microwave from non-kosher to kosher, however, can be problematic. Some of the concerns are food stuck in the vents that is impossible to reach and clean, and the plastic, rubber, or glass parts that some Poskim 7 maintain cannot be kashered. Due to these challenges, some Poskim8 do not permit kashering a microwave from non-kosher to kosher.

Since microwaves are inexpensive to purchase, it is certainly best to purchase a new microwave, rather than attempt to kasher a non-kosher one. However, in circumstances where this is not possible, the OK and other Poskim9 allow kashering from non-kosher to kosher if an experienced kashering rabbi is present. Please consult a competent Orthodox rav to discuss your personal circumstances because it is easy for the kashering to be done incorrectly.

When kashering a microwave, the following procedure must be followed:
THE INTERIOR of the microwave should be cleaned very well so there is no food residue left behind.
THE VENT area should be sprayed with soap or a chemical cleaning solution. This renders any food particles that are stuck inside the vents pogum (ruined).
AFTER CLEANING, the microwave should be left unused for 24 hours.
THE GLASS tray and the track wheels should be removed and kashered by hagolah.
TO KASHER the actual microwave, fill up a standard size cup of water and place it inside. Turn the microwave on and heat the water for about 20 minutes or until the cup empties.
REPEAT the previous step after moving the cup to a different spot to kasher the area where the cup sat during the first round of kashering.

*For Pesach, most Poskim agree that a microwave
should not be koshered.

7 Igros Moshe O”C 2:92 & 3:58.
8 Shoel Umeishiv Telisa 3/125; Sdei Chemed Mareches Hei 24; Chmaetz Umatzah 17/12.
9 Rav Wozner, Rav Elyashiv, Rav Sheinberg, Rav Halbershtam; Nitei Gavriel ibid footnote 26 in name of Rav Neiman of Montreal.