In the Kollel recently, we learned the Halachos relating to the Kashrus issues of Zei’a, steam. One of the most practical applications of this issue is the propriety of using a single oven for meat and dairy.
The main potential problem would be the suspicion that vapors rose from the previous food cooked in the oven, carrying say Fleishig flavor, and deposited it on the roof of the oven. If this meaty taste would be likely to end up in a subsequent dairy food, a Kashrus issue would be present.
Additionally, if one is renting an apartment of visiting irreligious relatives, the issue would inevitably arise whether one may eat food cooked in the oven. The oven was previously utilized to cook non-Kosher food, must one be concerned that theTreif flavor will contaminate his Kosher meal?
As a disclaimer, I certainly highly recommend that for various practical considerations one should avoid using the same oven for Milchigs and Fleishigs without processing it in between. Also, an oven that becomes Treif should be Kashered in an appropriate manner before using. The question is, under exceptional circumstances where there is no acceptable alternative, are there adequate Halachic grounds for temporary leniency?
There are numerous factors at hand, let’s analyze them one at a time:
1) Is steam rising off of a food equivalent to the food itself? Not only is it forbidden to eat certain foods, but ingesting their flavor is forbidden as well. So, eating potatoes from a stew that contains non-Kosher meat is certainly forbidden. The Rishonim derive from a case mentioned regarding Taharos that the vapors emitted by a pot of liquid have the identical Halacha as the liquid itself. However, one rarely places a pot of soup inside the oven. The Pri Megadim quotes authorities that the above Halacha is limited to liquids and not relevant a solid food.
2) Is there any steam? Even more basic of a consideration, Reb Moshe Feinstein zatzal challenges the assumption that solid foods emit significant steam. He suggests that when baking or roasting a generally dry food, it can be assumed that no vapors were emitted unless it was directly observed to the contrary.
3) Does the steam reach the roof of the oven? An oven is an intrinsically hot and extremely dry environment. It is very logical to assume that any vapors that are expelled by the food being cooked will dissipate and dry up before reaching the ceiling of the oven. In fact, some Achronim observe that Zeia is exclusively mentioned regarding a pot on a stove; however, when the Gemara discusses potential problems inside an oven it limits the discussion to the much more lenient Reicha. They suggest that it can always be presumed that any steam that rises off of a roasted food burn up before causing any harm.
4) Even if we suspect that forbidden steam emanated from the non-Kosher food and reached the upper expanses of the oven, are they edible? There is a general principle in Hilchos Kashrus that a forbidden food that becomes inedible to the average human is no longer Assur to consume inadvertently. Even if it is edible, if it does not contribute any enhancement to the flavor of the food it enters, it cannot render the formerly Kosher food problematic. Any accumulation of grease on the roof of an oven is unlikely to be appetizing, if edible at all. Especially after any prolonged exposure to the intense heat inside the oven, the singed and charred residue is unlikely to remain Kashrus concern to your new recipe.
5) After all the above, we still need to explain how any delicious absorbed flavor is reaching the new, Kosher food being cooked. If the second dish is moist enough to fill the oven with a cloud of steam that circulates from the top back to the bottom, we have found a potential conduit for the Treif or meat/dairy to mingle. In lieu of this weather formation, any non-Kosher flavor on the ceiling of the oven is likely to remain there.
In conclusion, while I reiterate that I strongly suggest not using the same oven for uncovered Milchigs and Fleishig without running a cleaning cycle in between; when the need arises, it seems obvious to me that one may be lenient.
However, certain precautions should be taken. If one must use an oven for both milk and meat, the rack upon which the pan is placed should be cleaned well, and preferably a sheet of aluminum foil placed under the pan. Care should be taken that the pan and food avoids any contact with the possibly dirty surfaces of the oven. Ideally, the food should be covered, at least loosely.
All the above applies exclusively to a conventional oven. In a future article we will hopefully address the microwave.