I promised a follow-up to my previous article addressing ovens, that would deal with the similar issues as they relate to a microwave oven.
It is very common that one works in an office where there is a communal microwave that all the employees use to heat their lunch. Can one place covered food in the microwave that his non-Jewish or not observant co-workers reheated their presumably non-Kosher food in? Can one use a single microwave for Milchigs and Fleishigs? Can a microwave oven be Kashered?
After reading my previous article, one might presume that I am equally lenient regarding microwaves as well. However, nothing could be farther from the truth. We described a number of considerations leading to the leniency of conventional ovens, but practically speaking almost none of them are relevant to microwaves.
First a little background information. A conventional oven works by means of convective heat transfer, which means that the gas flame or electric coil heats the air in the oven chamber, which then heats the food placed in the oven. However, a microwave oven works by means of dielectric heating generated by radiation. This means that the air is not directly heated by the oven’s actions, only potentially from the subsequently hot food. Rather, the microwaves emitted by the magnetron directly heat the water and fat molecules in the targeted food.
Therefore, since the electromagnetic radiation heats the water molecules directly, and the air is not nearly as hot and dry as in a conventional oven, and the likelihood of steam rising even off of an apparently dry item is much greater. This abundance of vapor is also significantly more likely to reach, and even condense upon, the internal surfaces of the microwave. It also tends to be quite concentrated and does not easily dissipate, especially since unlike a gas oven there is no need for ventilation holes, as no combustion occurs.
The occurring residue is also much less likely to char than in a conventional oven, due to the absence of a direct heat source. While no one would scrape off the splatters from inside his microwave to eat them, it is conceivable that they are theoretically edible.
An additional problem presented by microwaves is that their propensity for generating steam makes it very difficult to seal food in them. While theoretically, there should be no problem placing sealed Kosher food in a non-Kosher microwave or Milchig food in a Fleishig one (provided something is placed under the plate to prevent direct contact with the floor or the microwave oven), practically speaking this is difficult to achieve. The cloud of steam they generate applies pressure on the bag enclosing the food, and can often cause it to burst or open. Once this happens and the steam is released, a serious Shaila has been created. While there are grounds, under certain circumstances, to rule leniently b’Dieved; I strongly advise avoiding such situations.
Koshering a microwave, though often touted a simply boiling a cup of water inside, is also fraught with problems.
First of all, a prerequisite for Kashering is that the subject must be spotlessly clean. This is almost impossible to accomplish in a microwave oven. The seams around the corners are impenetrable, as are the holes covering the fan.
Secondly, Rav Moshe Feinstein zatzal ruled that steam alone is inadequate for Kashering and liquid water must be used. This is virtually impossible in a microwave.
In conclusion, in contrast to a conventional oven, microwave ones present unique and significant challenges. Unless one has devised a method of containing the food that is highly unlikely to rupture and leak steam, it is strongly advised not to reheat food in a non-Kosher microwave or one of the opposing Milchig/Fleishig side.