Dear Kosher Spirit,

I am often in a situation where I would like to kasher a microwave at work or in a hotel. What is the proper way to kasher a public microwave?


The OK responds:

A microwave works by transmitting micro radio waves through the water content of the food being heated. As the agitation increases, the food generates heat. Once the food gets hot, it heats up the container and the entire chamber fills up with steam. Therefore, it would seem to be relatively simple to kasher.

Just as the non-kosher food contaminated the microwave through its steam, the microwave can be kashered by filling the chamber with steam. The microwave must be clean of all food spills and splatters. One possible problem is that there can be food stuck in the vent (especially problematic if the vent is on the ceiling of the microwave where food can drip down), which is almost impossible to clean.

In addition, the microwave window is made of glass and some poskim do not permit kashering glass, but others are more lenient and permit kashering glass year round (but not for Pesach). In addition, the inner chamber is made from aluminum with a coating of plastic. Some poskim state that it cannot be kashered, but others do permit kashering.

In conclusion, it is preferable not to kasher a microwave from treif to kosher and food should be double wrapped before microwaving. If one must kasher a treif microwave, it should be done in the following manner:

  1. Clean the microwave and spray the vent with cleaning solution.
  2. It is preferable to clean the rubber gasket and outside as well.
  3. If possible, remove glass tray and kasher it with boiling water. Allow microwave to sit idle for 24 hours.
  4. Fill a Styrofoam cup with water and microwave for at least 10 minutes.
  5. Move the cup to a new spot and repeat step 4 to kashe the place where the cup stood.

L’chatchila, it is not a problem to kasher between kosher meat and kosher dairy, but it should not be done as a method to routinely use the microwave for both.