Our discussion up to this point has focused on gas and electric ovens. A microwave oven follows the same general principals, but the application of these rules is different because of the manner in which a microwave oven operates. A microwave oven heats the water molecules (every food has some moisture), and the heat of the water is then transferred to the rest of the food. The water is heated to a high temperature very quickly, and as a result the level of zeiah can be significantly greater in a microwave than in a conventional oven where the moisture escapes slowly. Thus, even dry foods may produce enough zeiah to be problematic. If operated for a short period of time, the microwave oven surface will remain cool, in which case no transfer of ta’am can occur. However, after some time the microwave walls are heated by the steam, though the rate at which this occurs varies greatly from food to food. In truth, a variety of factors impact on the level of zeiah in a microwave oven, such as the moisture content of the food, the microwave setting, the duration of the cooking time, the size of the oven and the size and location of the vent. To be on the safe side, one should cover all dairy foods cooked in a microwave designated for meat, or vice versa. (The cover will also obviate concerns about reicha, in the event the oven is not clean.) However, this solution is not always adequate. Because the stream of steam in a microwave is so significant, containers used in a microwave must be vented in order to prevent an explosion. The steam will eventually escape through the vent, and may fill the oven chamber. Halachically, if there is steam on either side of the container the ta’am may pass through the walls of the container and affect the inner contents. As such, Rabbi Yisroel Belsky, zt”l, recommends the use of a double wrap in a microwave to minimize the rate at which steam passes into the oven. The food can be covered by a paper towel, paper bag or plastic wrap. (Note: Plastic wrap may be carcinogenic when used in a microwave.) In addition, the food should be placed on a hard surface that will not leak through to the oven floor. This approach is adequate to solve the zeiah problem in all situations.
How is a microwave oven kashered to change the dairy or meat status, or to kasher from non-kosher use? A microwave can be kashered by placing a bowl of water in the oven. The oven is filled with steam by operating the microwave at the highest setting for approximately ten minutes. The bowl is refilled and moved to another location, and the above procedure is repeated in order to kasher the area where the bowl previously rested. If there is a glass plate on the oven floor, it is preferable to cover or change the plate since it is questionable how the halacha views glass. If the oven surface is plastic there are different opinions whether kashering is effective, but in case of necessity many poskim follow the lenient view. Kashering must be preceded by a thorough cleanup and a 24-hour downtime (see Mishna Berura 452:20). As is true of a conventional oven, kashering can be bypassed (even for a nonkosher microwave) by double wrapping the food.
By now, the reader has surely noticed that there are numerous differences of opinion among the poskim about many of the issues discussed in this article. I have tried to share with you the mainstream views of rabbinic authorities, but I encourage you to review specific questions with your local rabbi. It should also be obvious that from a halachic perspective, it is advantageous to design a kosher kitchen with two separate ovens for dairy and meat use. Nonetheless, this option is not always practical for financial or logistical reasons, and therefore the issues raised in this article must be addressed.
Initially, changing our oven procedures may require a bit of care and concentration. In time, however, using the proper kashruth safeguards will become second nature, as are the rest of our kosher kitchen habits. The effort is well worth it.
A. To use an oven for dairy and meat:
- Designate the oven for dairy or meat.
- A designated item can be cooked in any manner.
- A non-designated item can be baked without a cover if it is dry, and there is no edible residue of the designated category on the oven surface. It is preferable to change the rack or cover the surface under the pan with aluminum foil.
- A non-designated item which is moist can be cooked in the oven if it is covered. It is prefer- able to change the rack or cover the surface under the pan with aluminum foil. In a conventional oven, one cover is adequate, while in a microwave oven, a double wrap is preferable. Alternatively, the oven can be kashered. (See below.)
B. Pareve food prepared in a meat or dairy oven:
- Pareve food is unaffected by the cooking process in any of the following situations:
- The pareve food is dry and there is no edible meat or dairy residue in the oven, or
- The food is covered, or
- The oven is clean of meat and dairy residue and has not been used for meat or dairy products containing liquid for at least 24 hours.
- Pareve food is affected by the cooking process if:
- The pareve food contains liquid, and the food is cooked uncovered in an oven that was used for meat or dairy products containing liquid within the last 24 hours, or the pareve food is not covered and there is dairy or meat residue on the oven surface.
- Pareve food that is affected may be eaten before or after the opposite food category that affected it, but not together with the opposite food category.
C. To use a non-kosher oven without kashering: double wrap the food item.
D. To kasher a conventional oven:
- Clean the oven surface thoroughly with an oven cleaner.
- Allow a 24-hour downtime before kashering.
- Turn the oven on to the highest setting for one hour.
E. To kasher a microwave oven:
- Clean the oven thoroughly.
- Allow a 24 hour downtime before kashering.
- Place a bowl of water in the oven.
- Operate at the highest setting for ten minutes.
- Refill the bowl, move to another location and again operate the oven for ten minutes in order to kasher the area where the bowl previously rested.
- If there is a glass plate on the oven surface, it is preferable to cover or replace the plate.
by Rabbi Yaakov Luban, Executive Rabbinic Coordinator, OU Kosher
Reprinted with permission of Jewish Action Magazine (Winter 5756/1995 edition). Modifications have been made in the present version to clarify some issues.