Is Your Oven Kosher? What Every Kosher Cook Must Know

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01 Jul 2010

A tasty food can be made inedible if not prepared properly. The same is true in the realm of kosher. 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. If not used properly, it is possible for a kosher oven to become non-kosher.

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 zeiah 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 can not assume that all zeiah escapes through the vent in the oven ceiling or wall.

A noted posek, Rav Faivel Cohen, shlita, notes in his work Badei Hashulchan (92:166 and Biurim pages 213?214) that the issue of vented ovens was addressed hundreds of years ago, and a number of significant halachic authorities, such as the Bach and Yad Yehuda ruled leniently in such situations. Nonetheless, Rabbi Cohen concludes a lengthy analysis of modern ovens by acknowledging his own uncertainty about the proper size and location of a vent that would adequately ensure the escape of all zeiah from an oven. 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 pre?eminent poskim of our times (Rav Moshe Feinstein, zt“l, ibid, Rabbi Yaakov Breisch, zt“l, in Chelkas Yaakov II:136 and Rabbi Yitzchok Yaakov Weiss, zt“l, in Minchas Yitzchok V: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

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 cheese cake? 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 over?saturated. Hot air can contain very high levels of moisture before becoming over-saturated. 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 Kashruth 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. Most poskim with whom I spoke felt that pizza, blintzes, cheese cake and the like should be treated as liquids.

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 meat 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:

  1. 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
  2. the food is covered, or
  3. 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 cheese cake is treated as a liquid and may not be baked in a meat oven, many poskim are more 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 two 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. 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. (This method of kashering, known as libun kal, is adequate because the ta’am was absorbed through zeiah, which has the same status as a liquid. Even this view acknowledges that this procedure is not sufficient to kasher a non-kosher broiling pan which is used in direct contact with the food. This can be kashered only with intense heat known as libun chomur. There is another view which requires the use of a blowtorch or hot coals to kasher a non-kosher oven because dry residue on the oven wall is baked into the oven. Therefore libun chomur is necessary.)

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 (Yoreh 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 assume reicha lav milsa — aroma is insignificant. What this means is that one should not bake dairy and meat foods simultaneously 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 (before the fact -? see Yoreh Dayah 97:3)

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. (Yoreh Dayah 108:1).

Oven Racks

The final issue that must be addressed when using an oven for dairy and meat is the oven rack. It is axiomatic that ta’am (taste) does not pass between vessels without liquid. For this reason, if a dry hot meat pot came into contact with a dry dairy pot , both remain kosher. (Rama, Yoreh Dayah 92:8). As such, Rav Moshe Feinstein (ibid and Igros Moshe Yoreh Dayah III:10) permits the use of the same oven rack for dairy and meat pots. Rav Moshe explains that even though pots occasionally overflow onto the oven rack, the liquid dries immediately because of the intense heat, and the spill is considered irrelevant. Rabbi Yitzchok Yaakov Weiss (Minchas Yitzchok V:20?14) disagrees and maintains that the halachah does not permit the intentional use (lichatchilah) of one rack for dairy and meat, although if one did so (bidieved), the pot would be unaffected. Rabbi Faivel Cohen (Badei Hashulchan, page 216) also notes that pots are often placed in ovens at low temperatures, and spills do not necessarily dry up. As such, it is preferable to use a separate rack for dairy and meat, or at the very least, one should cover the surface under the pan with aluminum foil for the non-designated use. (Note: covering the entire oven rack may cause a fire.)

Microwave Ovens

Our discussion up to this point has focused on gas and electric ovens. A microwave oven follows the same general principals, but the application of these rules is different because of the manner in which a microwave oven operates. A 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 water is heated to a high temperature very quickly, and as a result the level of zeiah can be significantly greater in a microwave than in a conventional oven where the moisture escapes slowly. Thus, even dry foods may produce enough zeiah to be problematic. If operated for a short period of time, the microwave oven surface will remain cool, in which case no transfer of ta’am can occur. However, after some time the microwave walls are heated by the steam, though the rate at which this occurs varies greatly from food to food. In truth, a variety of factors impact on the level of zeiah in a microwave oven, 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. 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. Because the stream of steam in a microwave is so significant, containers used in a microwave must be vented in order to prevent an explosion. The steam will eventually escape through the vent, and may fill the oven chamber. Halachically, if there is steam on either side of the container the ta’am may pass through the walls of the container and affect the inner contents. As such, 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.) 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.

How is a microwave oven kashered to change the dairy or meat status, or to kasher from non-kosher use? A microwave can be kashered by placing a bowl of water in the oven. The oven is filled with steam by operating the microwave at the highest setting for approximately ten minutes. The bowl is refilled and moved to another location, and the above procedure is repeated 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 halachah 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 clean?up. As is true of a conventional oven, kashering can be bypassed (even for a non-kosher microwave) by double wrapping the food.

By now, the reader has surely noticed that there are numerous differences of opinion among the poskim about many of the issues discussed in this article. I have tried to share with you the mainstream views of rabbinic authorities, but I encourage you to review specific questions with your local rabbi. It should also be obvious that from a halachic perspective, it is advantageous to design a kosher kitchen with two separate ovens for dairy and meat use. Nonetheless, this option is not always practical for financial or logistical reasons, and therefore the issues raised in this article must be addressed.

Initially, changing our oven procedures may require a bit of care and concentration. In time, however, using the proper kosher safeguards will become second nature, as are the rest of our kosher kitchen habits. The effort is well worth it.

In Brief

A. To use an oven for dairy and meat:

  1. Designate the oven for dairy or meat.
  2. A designated item can be cooked in any manner.
  3. A non-designated item can be baked without a cover if it is dry, and there is no edible residue of the designated category on the oven . It is preferable to change the rack or cover the surface under the pan with aluminum foil.
  4. A non-designated item which is moist can be cooked in the oven if it is covered. It is preferable to change the rack or cover the surface under the pan with aluminum foil. In a conventional oven, one cover is adequate, while in a microwave oven, a double wrap is preferable. Alternatively, the oven can be kashered. (See below.)

B. Pareve food prepared in a meat or dairy oven:

  1. Pareve food is unaffected by the cooking process if: a) the pareve food is dry and there is no edible meat or dairy residue in the oven, 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.
  2. Pareve food is affected by the cooking process if:
    1. the pareve food contains liquid, and the food is cooked uncovered in an oven that was used for meat or dairy products containing liquid within the last 24 hours, or
    2. the pareve food is not covered and there is dairy or meat residue on the oven surface.
  3. Pareve food that is affected may be eaten before or after the opposite food category that affected it, but not together with the opposite food category.

C. To use a non-kosher oven without kashering: Double wrap the food item.

D. To kasher a conventional oven between dairy and meat use:

  1. Clean the oven surface thoroughly with an oven cleaner.
  2. Allow a 24-hour downtime before kashering a non-kosher oven. After meat and dairy use the oven can be kashered immediately.
  3. Turn the oven on to the highest setting for one hour.

E. To kasher a microwave oven:

  1. 1. Clean the oven thoroughly.
  2. Allow a 24-hour downtime before kashering a non-kosher oven. After meat and dairy use the oven can be kashered immediately.
  3. Place a bowl of water in the oven.
  4. Operate at the highest setting for ten minutes.
  5. Refill the bowl, move to another location and again operate the oven for ten minutes in order to kasher the area where the bowl previously rested.
  6. If there is a glass plate on the oven surface, it is preferable to cover or replace the plate

The words of this author reflect his/her own opinions and do not necessarily represent the official position of the Orthodox Union.