Cookers: All burners and electric elements of the
cooker may be used indiscriminately for dairy and meat, since spills are
incinerated. Nevertheless, some keep separate burners for dairy and meat.
The areas between the burners are prone to both meat and dairy spills and
splatters, it is prudent to avoid putting food down or salvaging spills
from these areas.
Ovens: We must define two distinct processes
performed in an oven which are very different in the eyes of Halacha;
Cooking; Zeiah, a large amount of steam connects the food to and imbues its flavour in, the oven walls just as food cooked in a pot imbues its flavour into the pot.
Baking; Reycha, aroma, less intense than steam, no connection to the oven walls but may contaminate other food baking simultaneously in the oven.
Cooking: To be considered cooking, a great deal of steam must be generated. This means that there must be enough liquid added to the food so that some remains at the conclusion of the cooking. In these
matters, oil is not considered a liquid, neither are the liquids exuded from
the food being cooked.
Baking: Baking is a process that does not produce much steam. To be considered baking there must be insufficient added liquids so that at the conclusion of baking, no liquid remains. The liquid has either evaporated or been absorbed into the food. Cakes and lasagna are baked since at the conclusion of their baking all the added liquid has been absorbed.
Meaty or Dairy Ovens. Using an
oven to cook means that a great deal of steam is generated, and will render the oven itself either meaty or dairy, according to the food that was cooked in it. The heavy steam conducts the food flavour into the walls of the oven, just as a pot
absorbs the flavour of the foods cooked in it. If this steam is somewhat mitigated, for example by covering the food, the food's flavour will not be conducted into the oven walls. This cover need not prevent all the steam escaping.
Once the oven becomes dairy or meaty, it remains
so and foods of the opposite type must not be cooked but may be baked, in that oven. Cooking will connect the dairy food to the meaty oven walls. Covered food may even be cooked in that oven since there will be no connection between the food and the oven walls. For example, after cooking a meat roast, a covered milk pudding or an uncovered cheese cake may be baked in the same oven in spite of any
lingering meaty odour [GRA Sh”O Y”D 108:17], provided the oven is clear of actual meaty residue.
Simultaneous cooking or even baking of dairy
and meat products in the same oven is forbidden unless both are covered. Similarly, Pareve foods should not be cooked in the oven with meat or dairy where the Pareve foods are clearly intended to be eaten with the opposite type. When both are covered it is permitted,
notwithstanding the fact that some steam escapes from either or both. Without a covering
the aroma or steam of the foods taint one another.
These rules apply equally to electric and gas ovens. Based upon a variety of sources including Igros Mosher YD 40 and Star-K
Microwave Ovens It is not uncommon for foods to boil over in
microwave ovens. Assuming meat and milk boil over on separate occasions, even if the tray is cleaned between boilovers, the tray will become non-K. Any subsequent boilovers will connect the non-K tray with the vessel on the tray and render the vessel not Kosher. Therefore one must have separate trays for meat and
milk. A large (disposable if you wish) plastic platter sitting on top of the
turntable is perfectly adequate.
Steam is the enemy of the microwave. If it remains in the oven it will
seriously impair the oven’s efficiency. Therefore all microwave ovens are
designed with efficient fans to extract all steam. Consequently, there is no Zeiah in a microwave oven. Additionally, the internal
walls of a microwave oven do not become heated during its operation; there is no heating of the oven
cavity. Consequently, a microwave oven can not become Dairy or Meaty and one may consecutively boil a cup of beef soup followed by a cup of milk and need not cover either of them.
found on the walls of a microwave after prolonged boiling of liquids is simply
condensation, the same as one finds on the outside of cold bottles of juice. This in my experience only accumulates if the microwave is left closed after it has ended its cycle. During the running of the microwave all the steam is vented. The condensate is neither dairy nor meaty, ShO YD 92.
The Orthodox Union
However the OU has a
different perspective (HERE) as recorded below.
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 level of
zeiah might be significantly greater in a microwave than in a conventional
oven. A variety of factors must be considered, 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.
Thus, even dry foods may produce enough zeiah to be problematic.
When operated for prolonged periods of time, the oven walls will become heated,
providing the right circumstances for transfer of ta'am. 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. All foods microwave heated in
closed containers must be vented in order to permit the escape of steam which
may fill the oven chamber. Halachically, if there is sufficient steam on either
side of the container the ta'am (flavour) absorbed in the walls of the
microwave (meat for instance) will be imbued into the food (cheese for example)
via the steam. This connection is not made through the vent in the food
container but through the actual walls of that container.
Rabbi Yisroel Belsky, shlita, 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.) [This is an Urban Myth, check it out]
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.
Kashering a Microwave Heat water in the microwave until it is filled with
steam. Repeat procedure with water located in a different position 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 between meat and dairy can be
done immediately after the previous use, while kashering a non-kosher oven
requires a 24-hour downtime. In all instances, kashering must be preceded by a
thorough cleanup. As is true of a conventional oven, kashering can be bypassed
(even for a non-kosher microwave) by double wrapping the food.
BACKGROUND and DISPUTES
What is ReiCha? The Talmud (Pesachim 76b)
records a dispute between Rav and Levi: is aroma (ReyCha) emitted from heated food
capable of contaminating another food? Will Kosher food cooked simultaneously
in the one oven with non-kosher food become non-kosher because of the aroma?
Rav rules that the food becomes non-kosher, and Levi rules that the food is
ReiCha is Different to Food The usual measure, tasting food to discern if non-Kosher contaminants are present, is
inapplicable here. Tasting determines if a non-Kosher ingredient is present in
significant proportions (Y”D 98:1) but taste itself is not what makes a
mixture prohibited. Rather Rav and Levi argue if this discernible taste [See Tur Y”D 108]
is something that the Torah or our Sages OBM prohibit or not. [If Zeiah is of
biblical or rabbinic origin - see Minchas Yitzchak
5:20; Yabia Omer 5: Y”D 7.]”
How We PasKen
Most Rishonim (see Tosafos) follow the ruling of Levi; aroma will not disqualify
another food; see RaMBaM, MaAchalot Asurot 15:33; Rif Chulin 32a and Rashi Pesachem 76b Amar LeCha. However, they understand that Levi’s rule
does not permit deliberate simultaneous uncovered cooking of kosher and
non-kosher food in the same oven if it is also small and closed. This is essentially the same
rule that prohibits using a non Kosher lid on a Kosher pot, Sh”O Y”D 92:2.
Accordingly the Shulchan Aruch (Yoreh Deah 108:1) permits foods that have been thus cooked
together but prohibits this arrangement in the first instance (LeChatChiLa).
The ReMa adds that these rules apply equally to cooking dairy and meat together
in the one oven. He also prohibits the following foods:
food cooked with Chamets;
Where either of the foods are spicy;
And also suggests that
the oven is large and well ventilated, they may not be cooked together in the first instance;
This is seen to include Parve foods which one intends to eat with dairy.
Rav Binyomin Forst (The Laws of Kashrus,
p.143) writes that “all household ovens are vented” which means that these rules will not apply since these restrictions apply only to small closed ovens.
Microwaves – Not Well Understood
However, Rabbi Forst suggests that microwave ovens might not be ventilated sufficiently
(this is very surprising since all microwave ovens are fan forced ventilated -
they must be so due to the way they function). There is no Halachic standard that
defines minimum degree of ventilation.
What Does ReiCha Require?
Both Taz 108:1 and Shach 108:1 explain that fat is the transfer medium
of aroma and therefore at least one of the foods must be fatty in order to
prompt any exchange between the two. The Aruch
HaShulchan (YD 108:9) however
rules that even lean Kosher and non-Kosher meats may not be baked in the same
A New Concept – ZeiAh
The Shulchan Aruch (Yoreh Deah 92:8), based on a teshuva from the Rosh(20:26, "if one placed a pot of meat on top of a [boiling] pan
of milk, the milk steam is absorbed into the pot [of meat] and prohibits it"), rules that steam (ZeiAh) produced in the
same or lesser circumstances will prohibit the foods. This appears to indicate
that steam emerging from milk is considered by Halacha to be milk and not
just evaporated water. However the Shulchan Aruch
Y”D 92 permits hanging
meat above a pot of boiling milk provided the steam rising to the meat is not
hot, suggesting that the steam is not milk but only water.
It seems therefore that the evaporated steam is NOT milk but if hot it will act
as an active link between the meat and the milk.
This still requires some explaining as we are all familiar with the build-up of
grime above cooking areas. Various compounds of the boiling milk will settle on
the meat suspended above it even though the steam reaching the meat is cool. As
foods cook they release water vapour and other volatile oils and compounds.
These all float up and condense on the relatively cool surfaces nearby forming
a greasy sticky mess, “Chef’s Purgatory”. So it would appear that this condensate
is not dairy or perhaps not deemed food at all.
There are two very important questions that must be
the difference between ReiCha and ZeiAh?
the Gemara of Pesachim ignore the issue of ZeiAh?
Three famous answers are provided.
The Aruch Hashulchan (YD 92:55) explains that ZeiAh exists only in very small ovens
that are not ventilated. Otherwise the vapours are classified as Reicha
only. Thus one may cook meat right after milk in our ovens that are both large
and ventilated. This was the custom in Eastern Europe and Rabbi Herschel
Schachter of Yeshiva
Moshe Feinstein (Iggros Moshe YD 40), quoting the Pri Megadim,
proposes that ZeiAh is generated only from liquids. Solids, it may be
assumed, do not produce significant steam (unless one clearly observes to
the contrary). Additionally, even when a liquid is boiling in an oven,
covering it will reduce the steam to levels that do not qualify as ZeiAh (ReMa
YD 92:8). Consequently, one may cook meat immediately after milk, provided
they are not both liquid or if one of them is covered. Rabbi Aharon
Lichtenstein of Yeshivat Har Etzion encourages people to follow this
Yitzchak Yakov Weiss (Minchas Yitzchak 2:50) maintains that the distinction between
aroma and steam is vague and whenever possible ovens should be dedicated
exclusively for meat or milk unless it is Kashered between uses or either
the meat or the dairy foods are double-wrapped. Rabbi Feivel Cohen, in Badia Hashulchan (YD 92:180) adopts this approach, and also requires
separate grates for the oven (a requirement which Rabbi Feinstein
explicitly—and the Aruch Hashulchan implicitly—rejects).