WHAT EVERY KOSHER COOK MUST KNOW
From the ancient kirah to our modern microwave, oven cooking requires specific kashrus knowledge. Rabbi Luban explains the halachos involved and provides a practical guideline.
A tasty food can be made inedible if not prepared properly. The same is true in the realm of kashruth. As we are meticulously careful to purchase only kosher food, the same degree of concern should be applied to the cooking process as well. This will insure that the food we eat is healthy for both body and soul.
As observant Jews we all maintain two sets of pots and pans for meat and dairy use. The one notable exception in most homes to this dual system of food preparation is the standard kitchen oven. While it is possible to use one oven for meat and dairy, certain halachic limitations and restrictions apply.
Perhaps you are wondering about two obvious questions. How does an oven become non-kosher, and how would a non-kosher oven affect the food prepared in it? A pot becomes non-kosher by absorbing ta’am issur (the taste of non-kosher food). Subsequently, otherwise kosher food prepared in a non-kosher pot will absorb the ta’am issur from the vessel wall. This is quite understandable for food that is cooked in a pot and makes direct contact with the vessel, but food cooked in an oven never touches the oven surface directly. How does the ta’am get in and out? Many people are perhaps unaware that there are three areas of concern regarding this question: zeiah, reicha and oven racks.
When liquids are heated, they evaporate and turn into steam and other vapors. If a kosher plate is suspended above the stream of steam rising from a pot of clam chowder, would the plate become non-kosher? Alternatively, if the pot contained kosher chicken soup, would the plate become fleishig ? These questions were addressed by the Rosh in a responsa written about 700 years ago. The Rosh rules that vapor retains the same status as the liquid from which it emanates, and he proves this from a Mishnah in Machshirim (2:1). The Mishnah deals with water that is tomai (ritually impure) that is heated in a bathhouse. The zeiah (vapor) that condenses on the walls of the bathhouse is considered tomai since the vapor evaporated from water which was tomai. From this halachah, the Rosh extrapolates that zeiah retains the same kosher status as the water from which it evaporates. Thus the zeiah of clam chowder and chicken soup are treif and fleishig respectively.
The Shulchan Aruch (Yorah Dayah 92:8) quotes this ruling of the Rosh. “If a pan of milk is placed under a meat pot and is heated in a kirah (an oven of sorts), the zeiah of the milk rises and is absorbed in the meat vessel, rendering it non-kosher.” This halachah provides one response to our two questions: How does an oven become non-kosher, and how does a non-kosher oven affect the food? Zeiah is the villain. If one bakes chicken in an oven, the zeiah rises and is absorbed into the oven wall. Subsequently, a cheese casserole baked in the oven will produce dairy vapor which will make contact with the oven surface. At this point, the oven has absorbed meat and dairy zeiah and is no longer kosher. Furthermore, the zeiah of the cheese casserole becomes treif since the dairy zeiah absorbs the ta’am of chicken from the oven surface. Eventually, the ta’am will circulate in the oven and make its way back down to the casserole rendering the casserole non-kosher as well.
The astute reader will detect a possible flaw in our analysis. The Shulchan Aruch posits that zeiah rises and is absorbed by the pot which is suspended overhead. This does not demonstrate that the zeiah of our hypothetical cheese casserole goes up and then comes back down to contaminate the casserole. While this distinction would seem plausible, the principle that zeiah circulates up and down in an oven is clearly established in a separate decision of the Rama (108:1). The Rama rules that food cooked in an oven previously used for non-kosher becomes treif whenever there is zeiah, even though the two foods were not in the oven at the same time. Thus, it is clear that zeiah circulates and thereby acts as a bridge through which ta’am is transferred from the oven surface to the food.
The case is not yet closed, and not all contemporary poskim agree with this conclusion. Our ovens are vented. Perhaps zeiah circulates only in an oven which is completely sealed but not in one that is vented. In 1954, Rabbi Chaim Shloss made this very argument to Rav Moshe Feinstein, zt”l. In a well-known response (Igros Moshe, Yorah Dayah I:40) Rav Moshe maintains that there is no halachic distinction between a vented and sealed oven. Rav Moshe demonstrates this from the Mishnah in Machshirim, which serves as the original source for the concept of zeiah in the responsa of the Rosh. The moisture on the walls
of a bathhouse are tomai, even though a typical bathhouse has open doors which would allow the zeiah to escape. Obviously, an opening is not adequate to allow all the steam to escape. Similarly, we cannot assume that all zeiah escapes through the vent in the oven ceiling or wall.
Some contemporary poskim do not consider zeiah to be a concern in a vented oven, and the reader is advised to consult his rabbi on this matter. Nonetheless, the view of many of the preeminent poskim of our times (Rav Moshe Feinstein, zt”l,
ibid, Rabbi Yaakov Breisch, zt”l, in Chelkas Yaa-kov II:136 and Rabbi Yitzchok Yaakov Weiss, zt”l, in Minchas Yitzchok 5:20) is that the indiscriminate use of an oven for meat and dairy cooking is not permissible.
In the previously cited responsa, Rav Moshe Feinstein tempers his ruling on ovens with two important qualifications: Zeiah does not circulate in the oven if
a) the food is covered (see Rama 92:8), or
b) the food is a dry substance.
Although dry foods contain some moisture, it can be assumed that an insignificant amount is converted to vapor during the cooking process (unless we observe otherwise). This provides a practical means of preparing dairy and meat foods in a single oven. The oven is designated as either dairy or meat. Foods of the designated sort can be cooked in any manner. Food items of the non-designated group can then be baked in the oven provided they are dry or placed in a covered pan. (Preferably, the oven rack should be changed or the surface under the pot should be covered with aluminum foil. Furthermore, when baking a non-designated open dry item, the oven must be free of edible residue of the designated category. These issues are discussed further in this article.)
The $64,000 question is, what constitutes a dry food? How do we treat pizza, blintzes, and cheesecake? All of these foods have some moisture which evaporates into the air during the cooking process. Nonetheless, the level of zeiah is certainly less than that of liquids. How much zeiah is halachically significant? It may be argued that these foods do not produce a visible stream of vapor and therefore they should be considered dry foods. However, visibility may not necessarily be a criteria for halachic zeiah. Indeed, even the zeiah of a liquid is not necessarily visible in an oven. When water evaporates it becomes an invisible gas, and it only condenses when the air is oversaturated. Hot air can contain very high levels of moisture before becoming oversaturated. Rabbi Levi Yitzchok Halpern, shlita, of the Institute of Science and Halacha in Jerusalem, devotes an entire
chapter (Section I, Chapter 4) of his work Kashrus and Shabbos in the Modern Kitchen to exploring the status of “dry” steam. (He concludes that “dry” steam does have a status of zeiah.)
The author has discussed this issue of the definition of dry foods with various halachic authorities who have expressed divergent views. Some poskim with whom I spoke felt that pizza, blintzes, cheesecake and the like should be treated as liquids, while others took a more lenient position. The reader should discuss this matter with his rav or posek.
A related issue is the status of pareve food baked in a meat or dairy oven. Essentially, if the pareve food item has liquid content which produces zeiah, then it is as if the food was cooked in a meat or dairy pot. (Pareve food that was cooked in a pot used for meat within the past 24 hours may be eaten before or after dairy, but it is preferable not to eat the pareve and dairy items together. The reverse is true for a pareve food prepared in a dairy vessel.) However, pareve food is unaffected by the cooking process if any of the following conditions prevail:
a) the pareve food is dry and there is no edible meat or dairy residue in the oven, (the requirement that the oven be clean is because of reicha which is discussed shortly), or
b) the food is covered, or
c) 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.
(Although I indicated above that cheesecake is treated as a liquid by many poskim and may not be baked in a meat oven, there is more reason to be lenient with respect to a pareve cake batter. According to this view, one may drink a glass of milk while eating a slice of chocolate cake baked in a meat oven, provided there was no edible meat residue on the oven wall. The reason for this leniency is that we are not dealing with the potential of milk and meat being cooked together simultaneously. 
If an oven was designated for dairy or meat use, many authorities permit kashering the oven to change the status. The manner in which an oven may be kashered is also a matter of dispute. Many poskim recommend adjusting the oven to its highest setting for an hour to effect the kashering. The dissenting view has raised three primary objections:
a) The oven surface is generally coated with enamel, which some consider similar to earthenware substances that cannot be kashered without intense heat.
b) The heat source used for kashering must be in the oven and not under the oven floor, as is the case in a conventional oven.
c) The Magen Avrohom (Orach Chaim 509:11) writes that one should not kasher in order to change the dairy status.
These objections notwithstanding, the lenient opinion has found wide acceptance for a variety of reasons beyond the scope of this article. Nonetheless, many contemporary halachic works recommend waiting 24 hours after cooking meat or dairy before kashering as an added safeguard. Before kashering commences, the oven surface and racks must be thoroughly cleaned (preferably with a caustic oven cleaner) to remove all residual matter. After a 24-hour down time, the oven is set at its highest temperature for one hour and it is then considered kashered. Many poskim accept this same procedure to kasher a non-
kosher oven as well, while other rabbinic authorities require a more intense heat source  . Most poskim consider the cleaning
cycle of a self-cleaning oven to be the equivalent of libun chomur. The OU follows this view.
What would happen if you are staying in a motel and wish to use the oven to prepare your meals? The oven is dirty and you are not inclined to spend your vacation cleaning the oven. Based on our previous discussion, it follows that one may use a non-kosher oven simply by covering the food. The cover eliminates the circulating zeiah, and therefore the non-kosher oven has no impact on the food. However, because the oven is treif, it is best to use a double wrap to insure against any zeiah leakage. It is precisely this logic that is utilized with kosher airline meals. The meals are double-wrapped and may therefore be heated
in non-kosher ovens without compromising the kosher integrity of the product.
There is a second concern regarding using an oven for dairy and meat. The Talmud (Avodah Zorah 66b) raises the following question: Let us suppose that a rib steak and pork chop are roasted in the same oven. Even if the two pieces rest on separate pans and there is no gravy in the pans which will be transformed into zeiah, the rib steak will absorb some of the aroma of the pork chop. What halachic status does the aroma have? The Talmud formulates this question: “Reicha milsa” (is aroma significant?) or, “ reicha lav milsa” (is aroma insignificant?). The halachah, as recorded in Shulchan Oruch (Yorah Dayah 108:1) is that lichatchilah (before the fact) we are concerned that perhaps reicha milsa – aroma is significant (unless the food is covered), but bidieved (after the fact) we generally  assume reicha lav milsa – aroma is insignificant. What this means is that
one should not bake dairy and meat foods simul-taneously in the same oven, but if one did so the food may be consumed, provided they are both dry and there is no zeiah factor. Furthermore, a pareve product baked in an oven simultaneously with meat cannot be eaten with dairy, since we are dealing on the level of lichatchilah (unless no other substitute to the pareve item is available).
There is a practical application of reicha which is often overlooked. If an oven is not clean, it may produce reicha even if there is no liquid in the oven. (If the residue is not charred, it maintains its halachic status.) Therefore, before using a meat oven for dry dairy food or vice versa, and before baking bread and cakes (or other pareve foods) which may be eaten with milk and meat, the oven should be inspected and found free of residual material which is in an edible state. Alternatively, if the food is covered, the concerns of reicha are obviated. (Yorah Dayah 108:1).
The final issue that must be addressed is the use of one oven rack for both meat and dairy use. It was previously explained that the oven surface absorbs the ta’am (taste) of whatever liquids are cooked in the oven. Can one place a dairy pot which produces no zeiah (i.e. the pot is covered or contains a dry food) on a clean rack which has previously absorbed the ta’am of meat through
zeiah? Some rabbinic authorities allow this because it is axiomatic in halachah that ta’am does not pass between vessels without liquid. Nonetheless, Rabbi Yitzchok Yaakov Weiss (Minchas Yitzchok V: 20-14) maintains that it is preferable to use a separate rack for dairy and meat, or to cover the surface under the pan with aluminum foil for the non-designated use. (Note: Covering the entire rack may cause a fire.)
-to be continued-
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.
 For a lengthy analysis of this topic, see Rabbi Feivel Cohen’s Badei Hashulchan (92:166 and Biurim pages 213-214).
 A pareve cake baked in a meat oven is a nat bar nat dihetaira through aphia. The rama (Yoreh Dayah 95:2) rules that lichatchila, such foods should not be eaten with dairy, but bidieved, if the foods were mixed, they may be consumed (although the Maharshal is in disagreement, see Shach 95:4). since it is unclear whether baking a cake produces zeiah, one can be more lenient when dealing with a situation that is permissible bedieved.
 The issue is as follows: when non-kosher food is cooked in a pot with liquid, hagolah or libun kal is sufficient for koshering, while libun chomur is necessary for a dry cooking situation. Many poskim maintain that a non-kosher oven can be kashered with libun kal because the ta’am of issur was absorbed through zeiah, which has the status of a liquid. Accordingly, operating the oven at the maximum setting for an hour constitutes libun kal and would affect kashering. even this view acknowledges that this procedure is not sufficient to kasher a non-kosher broiling pan which is used in direct contact with food. There is another view which requires the use of a blowtorch to kasher a non-kosher oven because dry residue on the oven wall is baked into the oven. Therefore, libun chomur is necessary. These considerations do not relate to kashering an oven after meat or dairy use since hagola or libun kal is adequate to kasher hetaira even after a dry cooking situation. (shach 121:8 and gloss of rabbi Akiva eger.)
 There are two main exceptions to the rule that reicha is permitted bidieved: If the baked item has a sharp taste or the oven is completely sealed, reicha is significant even bidieved. In the latter instance, one can be lenient in cases of significant financial loss.