Using one oven for both meat and dairy demands vigilance lest one transgress
any of the laws pertaining to basar-b’chalav. Ideally, separate ovens for
meat and dairy are the solution for preventing basar-b’chalav mix ups, and
many people do have separate ovens for that very reason. Those who do not,
however, should familiarize themselves with the answers to the following
Question: May an oven be used for meat and dairy dishes at the same time?
Discussion: It is prohibited to bake uncovered meat and dairy dishes in
one oven for the following three reasons:
1. The meat may come into actual contact with the dairy or vice versa,
either through touching, or when particles from one dish splatter onto the
2. When two foods are baked or roasted simultaneously in one oven, they
absorb each other’s aromas (reicha).
3. When moist foods or liquid mixtures are baked in an oven, steam (zei’ah)
is emitted, carrying the taste of one food to the other.
If, b'diavad, one cooked meat and dairy dishes in the same oven
simultaneously, he must consult a rav to determine whether the food may be
eaten or not.
Question: May an oven be used for meat and dairy dishes that are baked
Discussion: The first two problems mentioned above—items touching or
splattering each other, and reicha—do not apply, since the meat and dairy
dishes will not be in the oven at the same time. We are, however, still
concerned with the issue of zei’ah. This is because whenever moist foods (as
opposed to “dry” foods like a potato) are baked in an oven, steam is
emitted. When, for instance, uncovered meat is cooked in an oven, the steam
emitted carries the taste of the meat and “deposits” it onto the walls and
roof of the oven. When moist dairy foods or liquid mixtures are subsequently
cooked uncovered in the same oven, their steam rises, absorbs the meat taste
from the walls and roof of the oven, condenses, and falls back into the
dairy dish. The dairy dish is now prohibited as basar b'chalav.
The poskim debate whether or not we need to be concerned with this problem.
Some hold that there is no issue of zei’ah when baking or roasting inside a
closed oven, since the oven’s heat is intense enough to completely dry out
and evaporate the steam which rises from the food even before it reaches the
walls or roof of the oven. Thus no steam is deposited onto the walls or
roof of the oven to later condense and fall back into the dairy food.
According to this opinion zei’ah is never a problem for foods baking inside
a hot oven, and it is, therefore, permitted to use the same oven for
meat and dairy consecutively, even l’chatchilah, as long as the surface of
the oven roof and walls is free of any meat spills or residue. Many people
follow this opinion.
But other poskim are more stringent. In their opinion we can not or do not
know for certain that all of the steam will be evaporated before being
deposited onto the oven walls or roof, and it is, therefore, still possible
that some meaty steam will enter the dairy food. [Note, however, that
even according to the strict opinion, this problem affects only most gas
ovens. With electric ovens or other ovens where the heating element is on
top, zei’ah would be no problem since the steam will definitely dry up and
vanish before it reaches the walls and roof of the oven ]. To avoid this
eventuality, one should adhere to the following procedure:
First, determine the primary use of the oven—is it going to be used mainly
for meat, or for dairy dishes? For the sake of this Discussion, let us
assume that the primary use of the oven will be for meat dishes. For
halachic purposes, this oven now becomes a “meat oven,” in which liquid or
solid meat dishes will be roasted and baked, covered and uncovered. [To
later use the oven for dairy dishes, one should line the racks with aluminum
foil. The foil should be changed when dairy dishes are placed in the oven.]
Before using this oven for dairy, one must first make sure that the oven is
completely clean from any meat spills or residue. Once that is ascertained,
a dairy dish (or a parve food which will be eaten with dairy) may be placed
into the oven as long as one of the following two conditions is met: 1) The
dairy dish is thoroughly covered; 2) The dairy dish is not moist. Since,
generally, only moist or liquid dishes produce steam, there will be no
way for the meat steam which found its way onto the walls and roof of the
oven to be released from the walls and roof and enter the dairy dish, as
occurs when moist dishes are cooked.
Question: According to the stringent opinions mentioned earlier, what can be
done so that an uncovered, moist dairy food or liquid mixture can be baked
l’chatchilah in a meat oven?
Discussion: Before moist dairy foods can be baked in a meat oven, the
oven needs to be properly koshered. There are different opinions as to
whether our ovens can be koshered and what means may be used to kosher
them. Some poskim maintain that there is no easy, practical way to
kosher our ovens, since an oven cannot be koshered unless sparks are
actually seen during the koshering process. Other poskim are of the
opinion that our ovens can be koshered only through a self-cleaning cycle,
which heats the oven to approximately 900°F. But the majority of
poskim hold that heating the oven at its highest setting for one hour
is sufficient to kosher an oven for consecutive use of meat and dairy, and
many households follow this opinion.
To review: Whether or not zei’ah is a concern when cooking uncovered, moist
dairy inside a meat oven is debatable. There are many who follow the more
lenient opinion and use their oven for meat and dairy consecutively, as long
as they ascertain that there are no spills or residue on the surface of the
oven. Still, in deference to the more stringent opinions mentioned
earlier, it is appropriate, at the very least, to kosher the oven by heating
it at its highest temperature setting for one hour before using for moist,
uncovered dairy, or for parve food which will be eaten with dairy.
Obviously, having two separate ovens for meat and dairy, or self-cleaning an
oven between meat and dairy, avoids all of these halachic concerns.
Note: Challah may be baked in the same oven that was used to roast uncovered
meat, even though the challah may be eaten with dairy. Challah dough does
not produce enough steam to release the meat steam that was deposited onto
the walls and roof of the oven. The oven should first be thoroughly
cleaned from any visible meat residue. Preferably, the racks should be
changed or covered with foil.
Question: Do the halachos mentioned earlier concerning meat and dairy in the
same oven apply to microwave ovens as well?
Discussion: No, they do not. All poskim would agree that it is forbidden to
use the same microwave oven for meat and dairy consecutively. The air space,
roof and walls of a microwave oven do not become hot enough for us to assume
that the steam that is emitted from the foods will be “burned” before being
deposited onto the walls or roof of the oven. In addition, microwave ovens
are small, compact units, which quickly fill up with steam from the food
being warmed in them.
Koshering a microwave between meat and dairy or dairy and meat is
halachically problematic. Some poskim permit koshering a microwave by
first scrubbing it clean, waiting twenty-four hours, and then placing a
cup of water inside the microwave and heating it for 5-10 minutes, until
thick steam fills the oven. Other poskim, however, are wary of
permitting this procedure, and it is not recommended to rely on the lenient
Even if koshering a microwave would be allowed, the poskim discourage using
the same microwave for both meat and dairy, since it is a long-standing
custom that we do not kosher utensils from meat to dairy or vice versa.
The poskim also recommend not to use the same microwave for meat and dairy
even if one is careful to keep all of the food covered while being cooked or
warmed in the microwave.
It is clear, therefore, that those who need to use a microwave for both
meat and dairy foods should make every effort to get two separate microwave
ovens and designate one for meat and the other for dairy.
1. Technically, if one of the foods is tightly covered, they can both
bake in the oven at the same time; see Y.D. 108:1. Practically speaking,
however, this is not a good idea, for if the cover slips off or is lifted
off inadvertently, or if one of the foods spills over, the food might very
well be forbidden to eat, even b'diavad (see Kesav Sofer 54 and Igros Moshe,
2. Since, b'diavad, there are several factors that must be taken into
account, such as the type of oven, the proximity of the foods to each other,
the amounts of the foods in question, the type of foods (“sharp” or bland)
and other factors.
3. In addition, the walls and roof of the oven are hot enough so that
immediately upon impact any steam or condensation will burn and fizzle out
and will not remain in the walls or roof of the oven.
4. It must be stressed that these poskim surely reckon with zei’ah as a
factor rendering a food item dairy or meaty, but they maintain that inside a
closed oven there is no zei’ah as explained above.
5. See She’alas Ya’avets 1:193, quoted by Yad Efrayim, Y.D. 97:2,
Maharsham 3:208. See Sefer Zikaron Yad Moshe Tzvi ha-Levi, pg. 201, where
Rav B.Y. Wosner proves from several sources that this is the opinion of most
poskim and the prevalent custom.
8. Igros Moshe, Y.D. 1:40 based on Pischei Teshuvah 92:6. This is a
generality, since most solid foods do not produce much steam. If, in fact, a
fair amount of steam was detected rising from a solid food, then we need to
be concerned with it as we would be with a liquid food.
9. The following procedure is for using the oven l'chatchilah. B'diavad,
there are several possible heterim that allow dairy food that was baked in a
meat oven (or vice versa) to be eaten. See Maharsham 3:26; Igros Moshe, Y.D.
1:40; Yabia Omer, Y.D. 5:7. A rav should be consulted.
10. The poskim discuss the following issues: 1. Are our ovens—which are
made from metal but coated with porcelain—considered klei cheres which
cannot be koshered without libun chamur? 2. Is it sufficient to kosher an
oven by heating it with fire from an external source, or does the fire have
to originate inside the oven? 3. Since most of our baking is done in pans,
may we rely on libun kal since the food does not (usually) touch the actual
oven surface? 4. Even if libun chamur is required, must sparks actually be
seen during the libun?
11. Although an oven can be koshered by using a blowtorch, this is a
dangerous and cumbersome process which should be undertaken only by a
professional who is also well-versed in the halachah.
12. Sha'ali Tziyon 2:20; She'arim Metzuyanim B'halachah 116:2; Badei
ha-Shulchan 92:8, s.v. l’chatchilah. See also Igros Moshe, Y.D. 1:60.
13. Rav M. Feinstein (oral ruling, quoted in Sefer Hilchos Pesach, vol. 1,
pg. 181, and in Ohalei Yeshurun, vol. 1, pg. 72 and 77); Minchas Chein, pg.
29. When koshering the oven from meat or dairy to parve, half of the
self-cleaning cycle (about an hour and a half) is sufficient; Rav M. Stern
(quoted in Pischei Halachah, Kashruth, pg. 114).
14. Rav Y.E. Henkin (written responsum published in ha-Darom, vol. 15);
Rav A. Kotler (oral ruling, quoted in Sefer Hilchos Pesach 1, pg. 180);
Yesodei Yeshurun, vol. 6, pg. 157; Minchas Yitzchak 3:66 (see, however,
5:20, where he is hesitant); Chazon Ovadyah, pg. 73:4; Rav M. Stern, Pischei
Halachah, Kashruth, pg. 114).
15. See Seder Pesach K'hilchaso, pg. 64, who quotes many poskim as
allowing this leniency.
16. Another practical suggestion is to place some foil under the meat item
being baked so that whatever spills over will not land directly on the racks
or the oven floor but on the foil. The foil is then removed before the dairy
item is inserted.
17. Some poskim additionally recommend that twenty-four hours elapse after
using the oven for meat before the oven is used for dairy.
18. Igros Moshe, Y.D. 1:40; 1:59.
19. Rav M. Feinstein (oral ruling, quoted in Sefer Hilchos Pesach, pg.
182); Rav S. Wosner (mi-Beis Levi, Nissan 5753); Teshuvos v'Hanhagos 2:212;
Yalkut Yosef (O.C. 451), pg. 360.
20. Scrubbing the microwave and waiting 24 hours before koshering a
microwave is required, since the koshering process of a microwave is
hagalah, not libun kal, and a through cleaning and a wait of 24 hours are
prerequisites of hagalah; see Mishnah Berurah 451:22, 452:20 and Rama, Y.D.
21. This leniency does not apply to microwave ovens with a browning
element or to convection microwave ovens.
22. See Shevus Yitzchak, Mikrogal, pg. 57, quoting Rav Y.S. Elyashiv; The
Laws of Kashrus, pg. 234.
23. Quoted by Mishnah Berurah 509:25. See The Daily Halachah Discussion to
29 Cheshvan for the details concerning this custom.