Q) We have a microwave that we use for dairy foods. The microwave also contains a conventional oven setting that has also been used to bake dairy foods. We would like to cook (or reheat) in the microwave oven parev foods that will be eaten during a meaty meal. Do we need to kasher the microwave first, and if so, how should we go about it?
A) Kashering microwave ovens is far from straightforward; however, since you are not intending to use the microwave for meat; rather for parev food that will be eaten with meat, we are dealing with a more lenient situation (referred to as nat bar nat).
[Nat bar nat stands for nosein ta’am bar nosein ta’am and refers to a secondary transfer of bliyos. This means that if a pot is first used to cook meat, and then it is cleaned and used to cook parev potatoes, the bliyos from the meat are first transferred to the pot (nat rishon) and then from the pot to the potatoes (nat bar nat). The Shulchan Aruch (YD 95:1) rules that these potatoes may be eaten together with dairy foods; and although the Remo (YD 95:2) argues and is stringent l’chatchilo, he agrees to the Shulchan Aruch in an ex post facto situation that if the potatoes are already mixed with dairy foods – they may be eaten.]
Since nat bar nat has a more lenient status (than bosor b’cholov) we can rely on the lenient views regarding kashering, and kasher the oven to the extent that it may now be used for parev (not milky) food specifically – even if this parev food is intended to be eaten with meaty food.
The microwave oven may be kashered in the following way:
- Clean the microwave thoroughly, then leave it for 24 hours (from cooking/heating dairy foods). This prepares the microwave oven for kashering by ensuring that there is no (tangible) food residue on the oven surface, and the 24 hour wait will render all bliyos – pogum.
- Place a container of water inside, and put the regular microwave setting on until some of the water has evaporated and the microwave is filled with steam (at least 5 minutes). This, at a basic level, kashers the microwave from bliyos absorbed while using the microwave setting[1].
- Turn the regular oven function on to the highest setting for half an hour. This, at a basic level, kashers the microwave from bliyos absorbed while using the oven setting[2].
The microwave oven may now be used (both microwave and conventional oven functions) to cook/heat parev foods even if they are intended to be eaten with meaty foods. However, one should not rely on the above kashering process to use the microwave for meaty foods.
In addition, the parev food should not be touching the glass plate directly – instead it should be placed on/in another receptacle (see footnote* ).
[1] כבולעו כך פולטו
[2] כבולעו כך פולטו, וגם ליבון קל
* שאם דנים הצלחת ככלי שבישלו בו חלב אז אסור לבשל בתוכו אוכל פרווה כדי לאכלו עם בשר (יו”ד צ”ה א’)