YESHIVAT HAR ETZION
ISRAEL KOSCHITZKY VIRTUAL BEIT MIDRASH (VBM)
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Hilkhot Kashrut

Shiur #:23: Transfer of Taste Through Smell and Steam:
Ovens and Microwaves (2)

By Rav David Brofsky

Introduction:

In last week's shiur, we discussed the impact of "reicha," i.e., smell, of a non-kosher substance which is roasted in the same oven as a kosher substance.  We concluded that the halakha is in accordance with the opinion of Levi, who maintains (Pesachim 76b) that "reicha lav milta."  Therefore, while one is NOT permitted to intentionally roast kosher and non-kosher meat in the same oven, be-diavad, the kosher piece of meat may be eaten.

Furthermore, we noted that the Gemara also cited the opinion of Rav Kahana and Rava, which is later brought by the Shulchan Arukh (YD 108:1), asserting that bread which was baked together with meat in the same oven, may not be eaten with milk.  

According to our conclusion regarding reicha, it would seem reasonable to permit cooking meat and milk in the same oven consecutively, as reicha only poses a problem during simultaneous roasting.  

However, the authorities discuss another potential obstacle to consecutive use of one oven for meat and milk: "zei'a" (steam).  What is the impact of zei'a which emerges from one pot and enters another? What impact does zei'a have on the walls of the oven? And can zei'a assist in the transfer of taste form the oven's wall to a substance cooked inside?

Zei'a:

Interestingly, zei'a isn't discussed by the Gemara, or even by the early Rishonim.  The Gemara in Pesachim (76b), for example, never raises the concern that the steam of the non-kosher substance may affect the kosher substance.  

However, the Rosh discusses the status of steam.  He questions (Shu"t Ha-Rosh 20:26) the status of a pot of meat that is cooked above an open pot of milk.  Are we concerned lest the steam from the pot of milk make contact with the meat pot and subsequently prohibits the pot and the food?

He answers, based on a Mishna (Makhshirin 2:2), that steam retains the status of the substance from which it emerges.  Therefore, he equates our case to a drop of milk which falls onto the side of a pot of meat (see shiur #14), which may potentially prohibit the pot and the food, if the milk isn't nullified by shishim.

The Shulchan Arukh (YD 92:8) accepts the position of the Rosh and rules that zei'a is considered the equivalent of the substance itself.   If so, one might ask, how could the Gemara completely ignore the "zei'a factor," which may potentially prohibit the kosher substance after all?! 

The Acharonim offer a number of explanations which may also be important for our understanding of when zei'a applies.  Some (Arukh Ha-shulchan), for example, claim that zei'a is only a problem in small ovens.  However, in larger, ventilated ovens, zei'a need not pose a problem.  

Alternatively, some (see Peri Megadim) suggest that only liquids produce zei'a, and not solids, especially not roasted meat.  

Still others (see Chavat Da'at) suggest that zei'a may pose a problem when pots are located one above the other.  However, pots placed side by side do not pose a problem, as the steam from one cannot enter the other, and we are NOT concerned with the zei'a which comes in contact with the oven walls and circulates throughout the oven.

Practically, we must ask, what is the scope of our concern for zei'a, and what role does it play in the kashrut of ovens?

Position of the Rema:

The Shulchan Arukh (YD 92) cites the position of the Rosh, who clearly maintains that steam attains the same status as the food itself.

The Rema seemingly agrees, and writes (YD 93:3) that if one takes a cold dairy pot cover and places it onto a pot containing hot meat, both the meat and the pot cover are prohibited if the food began to steam.  

The Rema is concerned that the dairy pot cover absorbed the steam, which has the status of the meat.  Subsequently, the hot steam extracts the non-kosher taste from the pot cover and "rains" onto the meat.  In such a case, the meat, as well as the pot cover, are prohibited.

Elsewhere, the Rema compares this case to a situation in which one cooks both meat and milk, even consecutively, under the same cover or oven roof.

He writes (YD 108:1): "If one were to cook prohibited and permitted meat under the same covering (machavat), uncovered, they are both prohibited, and similarly if one cooks milk and meat [under the same covering]. If, however, one were to cook them CONSECUTIVELY, the kosher meat is unaffected (ein la-chush) unless the covering absorbed "sweat" from both of them, in which case even if they are cooked one after another if they were both uncovered they are both prohibited, similar to the case of the pot cover..."

In other words, the Rema equates an oven ceiling to a pot cover, which has the potential to prohibit the food that it is placed upon.

Seemingly, there is room to ask if we should equate our ovens with the case cited by the Rema. How should we conduct ourselves regarding modern ovens in light of this Rema?
 
Regarding this issue, I recommend reading an excellent summary, written by Rabbi Howard Jachter (Journal of Halakha and Contemporary Society #XXXII), concerning the various practical approaches to oven usage.  I will present a survey of the different opinions in a similar fashion to the summary of Rabbi Jachter.

Modern Ovens and Zei'a:

As mentioned above, the question one must ask is how similar are our ovens to the Rema's scenario of a "machavat," which he equated to a pot cover.

The Arukh Ha-shulchan (108) writes that the Rema was concerned with thick steam.  However, our ovens, which are generally large and ventilated, do not produce such steam and therefore the zei'a should not pose a problem.  Rabbi Jachter records that this is also the position of Rabbi Herschel Shachter (a Rosh Yeshiva and Rosh Kollel at Yeshiva University).

Unlike microwaves, in which the wall actually becomes damp after a liquid is cooked, ovens simply do not contain enough steam to mimic the effect of the pot cover.

If so, then one is certainly permitted to consecutively cook meat and dairy uncovered in the same oven, as long as the oven is clean (to prevent reicha issues). Needless to say, a pareve food cooked in a dairy oven should be considered pareve, not "be-chezkat chalavi," as one would even be permitted to cook meat in the same oven.

One might question whether this leniency should apply even to smaller European ovens as well, or even to toaster ovens.  It would seem that those who adopt this lenient approach should at least be stringent regarding toaster ovens, in which zei'a might actually pose a problem.

Another approach is that of Rav Moshe Feinstein (Igrot Moshe YD 1:40), who IS concerned about zei'a in one's oven.  Therefore, he insists that one should not use the same oven for UNCOVERED dairy and meat foods.  If one chooses to cook meat uncovered in the oven, then dairy food must be cooked COVERED in the oven, and vice versa.  The cover prevents the food from emitting steam, which prevents the extraction of taste from the oven walls and its subsequent return to the food.

However, Rav Moshe does note that one need not assume that every food produces zei'a.  Therefore, one may cook or reheat a dry dairy food, uncovered, such a cheese burekas, in a meat oven.

Seemingly, a pareve food baked in such an oven should at least be considered "be-chezkat besari" and not be eaten with milk, unless the food was dry or the oven was "eino ben yomo" (see YD 95:1), in which case if the oven was clean, the food should be considered pareve. 

A number of Acharonim suggest a middle approach, offering two other solutions.

Rav Ovadia Yosef (Yabia Omer 5:7) discusses the possibility of heating the oven to its highest temperature for about 20 minutes, thereby kashering the oven through libun kal, and permitting cooking of milk in the same oven.  This suggestion was adopted by a number of contemporary authorities, including Rabbi Moshe Stern (Pit'chei Halakha p. 157), Rabbi Shlomo Wosner, and Rabbi Mordechai Eliyahu (see Halikhot Ve-Hanhagot Kashrut p. 46).  Rav Aharon Lichtenstein also adopts this view.

While it is not clear how these authorities would view a pareve cake baked in a ben yomo oven, some Acharonim suggest that one should at least be permitted to cook a pareve food, le-chat'chila, in this oven when it is eino ben yomo and continue to treat it as a pareve food, not "be-chezkat besari." Incidentally, Rav Lichtenstein maintains that the food may be treated as "pareve" even if cooked in a clean ben yomo oven.  In any case, many foods are not moist enough to produce zei'a, rendering them pareve even according to Rav Moshe Feinstein!

Rav Lichtenstein (as cited by Rabbi Jachter), however, suggests that one may also wait twenty-four hours from the last time it was used for meat or dairy, thereby rendering the oven "eino ben yomo," and then cook meat or dairy uncovered.  After twenty-four hours, the entire question is reduced to a rabbinic question of "noten ta'am lifgam," at which point one may seemingly be lenient in light of the doubts raised above.  

It is important to stress that the above leniencies apply only if the oven is clean. Some ovens, especially “turbo” ovens, in which the oven walls do not actually heat up, are often rather dirty. In that case, not only does reicha pose a problem, but also the oven may not be considered “eino ben yomo”, if there is edible food still attached to the walls. 

Furthermore, since wet foods often “spill over”, the grates that support the baking pans absorb actual ta'am.  Thus, regardless of the issue of zei'a, if one cooks a wet food on an exposed grate, a spill may serve as a medium to transfer taste from the grate to the pan, especially if the oven (or grate) is “ben yomo”. Therefore, one might recommend covering the grate with a sheet of aluminum foil, as a precaution, especially when cooking wet dairy foods in a “ben yomo” meat oven. 

In summary, we have presented three approaches to modern ovens.  Some maintain that zei'a does not pose a problem in our larger, ventilated ovens.  Others disagree and recommend using an oven for only one type of food uncovered.  And others offer alternative ways of avoiding the potential zei'a problem, such as waiting twenty-four hours or kashering the oven between meat and dairy using "libun kal," i.e., heating the oven to its highest temperature for a minimum of 15-30 minutes.

Microwaves:

Microwaves seem to present the following kashrut problems:

Microwaves clearly produce much more steam than a standard oven.  Therefore, we may suspect that the walls of the microwave have absorbed the steam of meat or dairy foods, which may be subsequently extracted and returned to the food each time the microwave is used.  While one may claim that since the walls of the microwave do not get hot, they never reach "yad soledet bo" and therefore they never absorb or emit ta'am, it does seem that if the microwave is on for a significant time, the walls may actually reach yad soledet bo.

Furthermore, the walls of the microwave may have a thin film of dairy or meat "be-ein," i.e., actual food substance, not to mention the splashes that are common in a microwave.  If so, the circulation of steam may allow the meat or dairy residue or spills to be transferred throughout the microwave.  

Moreover, it seems that many are not meticulous in cleaning their microwaves, which may lead to a scenario in which a dairy plate placed over a meat spill or residue may actually absorb directly from meat be-ein!

Therefore, it is clear the one must take great care to protect one's food from both the steam and the residue left in a microwave.  This entails two steps.

Firstly, any food or utensil should preferably be placed on a special tray on top of the glass bottom of the microwave that has been thoroughly cleaned.  (This also is an important precaution for spillover).

Secondly, one should either completely (although not hermetically) cover all food cooked in microwave, or adopt a policy similar to Rav Moshe Feinstein's policy regarding ovens, i.e., to only cook one "type" of food uncovered, or to cover all food placed in a microwave.

I should note that while a posek might find it very difficult to actually prohibit food heated improperly in a microwave, one should preferably take precautions to protect one's food from steam and residue.

Next week we will begin our discussion of knives.