Kosher Microwaves

This question was originally answered by Rabbi Baruch Rubanowitz

A Visitor asked:
April 6, 2006I am curious about the use of microwave ovens for both milchig and fleishig. Is it permissible to use a microwave for both as long as they are covered and the microwave is kept clean, or is it preferable to use it for only one or the other?

Rabbi Baruch Rubanowitz answered:

The preferred method is to keep the microwave clean, cover the food with plastic wrap or the like, and put a pot holder, cloth or plate under the food plate.

If the microwave is not clean, then when you heat up your food and moisture escapes (this may happen if the food is not covered, especially when liquids are being heated), it may draw the flavor of the residual food that is caked on the walls of the microwave into the food.

Technically, it is acceptable to cover either the milchig or fleishig food every time. The walls of the microwave will attain the status of the food type that is put in uncovered.[1] When the opposite type is put in covered, steam and moisture will not escape and there is no need to worry about drawing flavor of the opposite type into the covered food. Experience has shown, however, that the food is not always covered well and moisture escapes. Therefore, it is best to keep the microwave clean and attempt to cover the food every time. If the microwave is kept clean (i.e., there is no residual food caked on the walls), then even if moisture escapes, it can only draw out flavor that has been absorbed in the walls. That will only happen if the steam escaping the "covered" food reaches yad soledes bo. The kashrus of the food can only be affected if you are certain that the microwave has been used for the opposite food type within the past 24 hours. Even then, you should ask someone learned in the laws of kashrus before deciding the food is not kosher. There are many factors to consider before deeming the food cooked in the microwave treif.

Something should be placed under the plate of food every time in case of spillage. There is usually a treated glass plate (sometimes revolving) on the bottom of the microwave; if that becomes dirty with milchig food and you then place fleishig on it, you could potentially have a problem when the plate gets hot. That is why I recommend always placing the plate on a cloth or a plate that is pareve or that matches the type of food being put in (fleishig for fleishig or milchig for milchig). That way, when there is spillage, it will not contact the glass plate that will be in direct contact with the container you use afterwards for the opposite type of food. For example, if you heat up a bowl of onion soup that contains cheese and some of it spills, it will fall onto the milchig plate instead of onto the glass plate. This way the glass plate remains dry of any milchig liquid. You can then place a bowl of meatballs with sauce on the glass plate without being concerned that residual milchig soup may be absorbed by the bowl of meatballs. However, in order to make sure that no fleishig gravy gets on the glass bowl, you should place a fleishig plate underneath the bowl of meatballs. This way, if there is any spillage it will not land on the glass plate either. If you are putting macaroni and cottage cheese in the microwave, it may be sufficient to place a mat or cloth under the bowl, since any cottage cheese that spills will not pass through the cloth to the glass plate. The type of material to be used should be appropriate for the type of food that you are putting in. You have to consider what would happen if something spilled.

If you use a pareve separator between the food and the glass plate, you are taking a risk. If something spills, the separator may lose its pareve status, depending on the temperature of the food that spilled and the material the separator is made of. If nothing spills and no moisture escapes the covered food, the separator remains pareve. If you use a towel or cloth and food spills on it, the separator will remain milchig or fleishig until you throw it in the washing machine. Once it is washed in hot water with detergent it reverts to its pareve status.[2]

[1] Provided that the moisture escaping and reaching the walls of the microwave is yad soledes bo.

[2] Hot water can kasher any absorbed flavor in the cloth, since cloth, like most materials, is kasherable through hag’oloh. (Exceptions include ceramics, porcelain, china, earthenware and enamel; these items cannot be kashered at all). The general principle with regard to hag’oloh is that whatever method was used to introduce the non-kosher flavor into the vessel can be used to explete the flavor (ta’am). Once the flavor has been in the cloth for more than 24 hours, it cannot treif anything, since even if it gets into another food the flavor would be unfavorable. Nevertheless, there is a rabbinic prohibition on using a vessel even if its absorbed flavor is pagum (objectionable). Using a hand towel to dry hands and dishes would not be included in the prohibition on using a vessel that has ta’am pagum (gezeiroh kedeiroh she’eino ben yomo atu ben yomo). Even using a hand towel or a dish towel as a separator in a microwave between the glass and a bowl of food is not a violation of this gezeiroh, since normally there is no contact with the food itself. Thus, there is no serious cause for concern about the absorbed food in the towel.

However, hag’oloh can only kasher absorbed flavor, not food particles. Because some visible particles of food may remain enmeshed in the fibers, it is important to use detergent in the washing machine to neutralize the entangled food. Detergent will add an unpleasant, perhaps even objectionable, flavor. Once something becomes inedible for an animal, it loses the status of food and is not considered non-kosher.

The detergent can only kasher visible food, however, not the absorbed flavor. Should hot fleishig gravy spill onto a towel in the microwave, the fibers may absorb fleishig flavor. If the same towel is used within 24 hours under a milchig soup in the microwave and the soup spills onto the towel, the soup can potentially become treif, because the hot liquid can transfer the mixed flavor of milk and meat from the towel to the soup bowl. This is true even if the towel was clean of any visible food. It is the absorbed flavor in the cloth that is problematic, and this can only be removed using hag’oloh; detergent cannot affect the absorbed flavor.

Thus both hot water and detergent are important when kashering a towel. Many people do not pay particular attention to whether their dish towels that are used under food in the microwave are washed in boiling water or not. It will not cause a problem as long as 24 hours have elapsed from the time the gravy spilled until the milchig soup spilled. Thus, at worst ta’am pagum could be transferred, and nothing would become treif. Since spillage is hopefully not too frequent an occurrence and it usually takes more than 24 hours to wash, dry, fold and put away dish towels, there is usually no cause for concern about the absorbed flavor.

On Pesach, however, we are concerned about the transmission of ta’am pagum as well. That means that even if the towel absorbed chometz flavor more than 24 hours before being used on Pesach, it should not be used with hot utensils or food on Pesach. Thus,  it is important to stress that dish towels that you wish to use for Pesach should be washed with detergent in hot water. The temperature of the water must be at least as hot as the liquid was when the chometz flavor was absorbed.

Kashering a microwave for Pesach is impossible since only metal or wood can be kashered for Pesach. Although one can kasher plastic the rest of the year, one should not do so for Pesach. The inside of a microwave is generally not metal and is thus not kasherable.

However, there is a way to use a used microwave on Pesach. Once the interior has been cleaned well and there are no pieces of food inside, you can test the walls of the microwave to see if they reach yad soledes bo. Put in a raw potato or a cup of water and let it cook. Moisture will escape. After a few minutes it will reach the maximum temperature; then place your hand on the ceiling and interior wall. If it can stay there for at least 15 seconds you can be quite sure that nothing has ever been absorbed. The walls do not reach the level of heat required to change the halachic status of the food. Hence, you could use the same microwave for chometz, milk, meat or Pesach, provided it is perfectly clean inside the oven.

If your microwave will remain chometz and you have cleaned the interior well, you should still remove the glass plate on the bottom and replace with a new or Pesachdik one. A thick Styrofoam board can be temporarily used for Pesach instead of the glass tray.

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