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By Rabbi Dovid Cohen, Administrative Rabbinic Coordinator

Q. Is there any way to use a non-kosher microwave oven?

A. There are two choices: kasher it or use it while it is non-kosher, and we’ll consider both of them.

The main ways that a microwave becomes non-kosher are when (a) hot food spills onto the glass plate that is on the floor of the microwave, and (b) hot vapors from non-kosher food rise up and contact the ceiling (and walls) of the oven.  Glass cannot be kashered, so the plate must be removed and replaced.  The ceiling absorbed non-kosher ta’am through zei’ah/vapor, and, therefore, it can be kashered with water vapor.  [Although the general rule is that we do not kasher with vapor or steam, here it is acceptable to do so since that is how the ta’am was absorbed in the first place.]

To accomplish this, you should take the following steps:  Clean the interior surfaces thoroughly, and do not use the microwave for 24 hours.  Then, place a bowl of water in the microwave and bring it to a boil.  Allow the water to keep boiling for an extended period of time (e.g., 10-15 minutes).  This will cause the chamber to become filled with steam, and the steam’s contact with the surfaces serves as the kashering.

What about the possibility that bits of residue might be stuck in the small holes through which the steam vents? Doesn’t Rema rule that anytime an item has cracks or crevices which are difficult to clean, the custom is not to allow kashering?  Some understand that Rema is specific to Pesach (since chametz on Pesach is assur b’mashehu), and, therefore, do not allow kashering of microwaves for Pesach but will allow it year-round.  Others allow kashering even for Pesach, based on an assumption that any chametz that falls out of the holes on Pesach will drop to the floor of the microwave and not end up in the food.

If there is a microwave at your office or in a hotel room, and you cannot kasher it, it is possible to still use it to warm up food.  Basically, you have to block the absorbed non-kosher ta’am from getting to your food.  You do that by (a) wiping any food residue off the floor of the microwave, (b) place the kosher food on a plate, and (c) double-wrap the plate and food with a bag or container.  The wrapping does not have to be air-tight but should cover all sides.  These methods effectively prevent non-kosher (or dairy or meat) ta’am from getting into the kosher food.  However, the status of absorbed chametz taste is particularly strict, and, therefore, this method should not be relied upon on Pesach.

This article first appeared in the Let’s Talk Kashrus column, Yated Ne’eman, January 5, 2024.