Going Kosher – Koshering the Kitchen – Chapter 5

By: Rabbi Amiram Markel

Methods of Koshering

In the previous chapter we explained which substances and utensils may be koshered. We will now address the various methods of koshering and which items need which form of koshering. There are several different methods by which utensils may be koshered:

When koshering utensils, the general, overarching principle to keep in mind is that a vessel may be purged of a substance in a manner similar to how it absorbed it.[1]

A) Vessels that were used for cooking through fire without liquid, such as barbeque grills, roasting spits, broiler grills and pans, sheet pans or baking pans, must be koshered through fire. This is to say that they must be heated to very high temperatures. This method is called liboon kasheh – severe heating. The vessel must become red hot and emit sparks[2] or it must become so hot that its outer surface peels off.[3]

B) Vessels that were used for frying or for roasting with oil, such as frying pans, turkey roasters etc. should be koshered through fire of a lesser temperature. This is called liboon kal-light heating. Nonetheless, this is still very high heat. The vessel must be heated to the point that if a piece of straw or cord would be placed on the other side of the wall being applied with fire, the straw or cord would catch fire.[4] However, even though, optimally, they should be koshered through liboon kal-light heating, nonetheless, according to some opinions, if they were already koshered through immersion in boiling water, this is sufficient.[5] Furthermore, according to some opinions, an electric frying pan which is constructed in such a way that liboon kal would ruin it, may be koshered, in the first place[6], through being immersed in boiling water that is upon the fire.[7]

C) Vessels that absorbed a non-kosher substance or an admixture of meat and milk through boiling in water or any other liquid, such as soup or consommé pots, sauce and gravy pans, stewing pots etc. must be koshered through full immersion in water[8] that has reached a boil and which is still set upon the fire.[9] Optimally, the water should be at a rolling boil. However, as long as the vessels are koshered in water that is boiling, even if it is not at a rolling boil, this is sufficient.[10]

D) Vessels that absorbed a non-kosher substance or an admixture of meat and milk by being poured onto from a pot that had been on the fire, such as metal serving bowls for soups and stews, metal gravy serving cups, serving trays etc. may be koshered by pouring boiling water directly from a pot on the fire.[11]

E) Vessels that absorbed a non-kosher substance or an admixture of meat and milk by being inserted into a vessel that was not directly on the fire but that received from one that was, such as meat teaspoons inserted into a dairy coffee cup, dairy spoons inserted into a gravy serving cup, ladles inserted into a serving bowl for soup or stews etc. may be koshered by immersion in a secondary vessel that is not directly on the fire.[12]

However, the general principle is that anything that may be koshered through a lesser degree may be koshered through a greater degree. In other words, if something may be koshered through immersion in a secondary vessel that was not on the fire, it certainly may be koshered through being poured upon from a primary vessel that was on the fire. Likewise, if something may be koshered through being poured upon from a primary vessel, it certainly may be koshered by being immersed in a vessel that is boiling upon the fire. Furthermore, if something may be koshered through being immersed in a vessel that is boiling upon the fire, it may also be koshered through liboon kal-light heating (as defined above).[13]

Furthermore, in the matter of koshering utensils, there is a difference of opinion between Sephardic[14] and Ashkenazic[15] authorities as to whether we go according to the majority of how the vessel was used, as opposed to going according to the most stringent manner in which it was used. Sephardim generally follow the opinion that we go according to the majority use of the vessel, whereas Ashkenazim generally follow the opinion that we take into account the most stringent manner in which it was used.

For example, usually, hot liquid is poured into a metal mug from a teakettle or coffee urn. Only on rare occasion is it used as a ladle and inserted into a pot of boiling liquid. Sephardim would rule that, in such a case, it may be koshered by pouring boiling hot water on it from a primary pot on the fire. Ashkenazim, however, would rule that it must be immersed in a pot of boiling water which is on the fire.

This being the case, since it is difficult to ascertain the exact manner of how and how often each vessel was used, they should generally be koshered in the most optimal manner. Whenever possible, vessels that were used in fire should be koshered through fire and all other vessels should be koshered by full immersion in boiling water that is on the fire. Also, in practical terms, when koshering an entire kitchen, it is much quicker and more efficient to kosher all the utensils by these two methods than to ascertain the exact kosher status of each utensil individually. However, when in the course of usage, a vessel or utensil becomes non-kosher; it may be koshered according to how it was used, as explained above.

Koshering an Oven

The best oven to own is the self-cleaning type because it is readily kosherable with great ease. Unless it is heavily caked with grease etc. all one must do is turn on the self-cleaning cycle, and it does the rest. It reaches temperatures well in excess of 1,000 degrees and is generally accepted as being liboon kasheh-severe heat. Any grease, sauces or foodstuffs that may have spilled or splattered on the floor or walls etc. become completely incinerated and turned into ash. Moreover, because it is so easy to kosher, if you will be using the same oven to alternately cook meat and dairy meals, this is the kind of oven you will want.[16] They tend to be more expensive than standard ovens but are well worth the extra cost. If the oven is heavily caked with grease etc. it is best to clean it first before setting the self-cleaning cycle. However, it is recommended that you wipe the cleaning chemicals off before turning on the oven, as they are toxic.  Because the walls of continuous cleaning ovens are generally made of ceramic, they cannot be koshered and are to be avoided.

However, if buying a new oven is out of the question, you may kosher your old one through a process of liboon kasheh-severe heating. As stated above, this entails heating the oven until it becomes red hot. Since your oven is not of the self-cleaning variety, this must be accomplished with a blowtorch. Unfortunately, most ovens made for home use are not made to withstand such high temperatures and may become warped or ruined due to overheating. Unless you are experienced, you will, most likely, not want to do so yourself. It is best to contact your local synagogue or Jewish organization and ask for expert assistance.

If you live in a remote area or cannot find help, abide by the following instructions and you will successfully kosher your oven:

1) Do not use your oven for a period of at least 24 hours:

(This is because only foodstuffs that have been absorbed in the walls of a vessel and are beneficial to the flavor of food subsequently cooked in it, render the food non-kosher. However, after a period of 24 hours, foodstuffs that have been absorbed into the walls of a vessel, including an oven, are considered to have been spoiled. Therefore, the taste emitted during subsequent cooking will be detrimental to the flavor of the food and, therefore, ineffectual in rendering it non-kosher. Nonetheless, out of concern that the vessel may inadvertently be used to cook food which is not harmed by the bad taste infused in it or that it will be usedwithinthe 24 hour period,[17] the sages decreed that the vessel be koshered anyway.[18] However, the requirements for koshering such a vessel are less stringent than the requirements for koshering a vessel which is still within 24 hours of use.)

2) Thoroughly clean your oven. It needs to be scrubbed clean with caustic chemicals (such as oven cleaner). This is especially true if the oven is being koshered before a period of 24 hours of disuse, because the caustic chemicals will ruin the flavor infused in the oven walls. Make sure to clean all the corners, cracks and crevices as well as the racks and shelves. Do not forget to thoroughly scrub the oven ceiling and door.

3) Turn the oven on to its highest temperature for approximately two hours but not less than one hour.


4) Using a heavy-duty blowtorch go slowly over all the surfaces of the oven, including the floor, ceiling, corners and walls. Blowtorch the racks and shelves too, preferably outside the oven. (It goes without saying that; of course, all the rules of fire safety should be kept with great caution.) It is greatly advised to take great care not to apply direct fire to the thermostat. This will surely destroy it, thus disabling your oven.

Koshering Oven-Another Method

Since the ovens we use today are very different from the ovens used in ancient times, many contemporary authorities rule that they may be koshered with libun kal-light heat (as defined above). The Tanoor-oven of our forefathers was very much like the Tandoori oven still in use in India, in which they make their Chapati bread. The dough is rolled into thin sheets and then stuck onto the inner walls of the oven. This being the case, since the food comes in direct contact with the oven walls it must be koshered by liboon kasheh-severe heat.

However, foods cooked in our ovens do not come in direct contact with the oven at all. Rather, only the steam emitted from the food touches the oven walls. This being the case, according to this opinion, our ovens may be koshered through liboon kal-light heat (as defined above). This has become common practice. To do this, just follow the first three instructions mentioned in the previous section, after which, you may use your oven.[19] Some authorities are of the opinion that, in addition, a pot of boiling water should be placed in the oven during the heating process.[20]


However, since some foodstuffs may have splattered or spilled on the racks, shelves and oven floor, it may be advisable to blowtorch them with libun Kasheh-severe heat, in addition to turning your oven on to the highest temperature. If you are in doubt as to which of these methods to use, inquire of your Rabbi for his opinion.

Koshering a Microwave Oven

A microwave oven cooks the food differently than a conventional oven in that only the food becomes hot, rather than the oven. This being the case, it becomes non-kosher due to vapor emitted by the hot food. Therefore, the method of koshering it is through steam[21]. You may kosher it as follows:

1) If possible, do not use your microwave for at least 24 hours[22].

2) Clean the oven thoroughly. Be sure to clean the walls, ceiling, floor, corners or any crevices. Do not forget the oven door.

3) Insert several water-filled (Styrofoam) cups into the oven. Turn on the microwave oven bringing the water to a boil so that the chamber becomes filled with steam[23].

4) If your microwave oven has a rotating glass plate, it may have become non-kosher through spillage of hot liquid, rather than through vapors. According to Sephardic authorities all that is necessary is a thorough cleaning[24]. According to Ashkenazic authorities the plate should be cleaned thoroughly and then boiling water poured on it[25]. Inquire of your rabbi.

Koshering a Dishwashing Machine

As stated above, dishwashing machines that have porcelain components may not be koshered.[26] On the other hand, if the interior of your dishwasher is made of stainless steel, it may be koshered.[27] Some opinions also allow it to be koshered if plastic components are included.[28] Ask your rabbi for his opinion. If he decides that you may kosher your dishwasher, you may do so in the following manner:

1) Clean the dishwasher thoroughly including the jets that spray the dishes with hot water. If there are filters or the like, make sure that they are perfectly clean too.

2) Do not use the dishwasher for a period of at least 24 hours.

3) Run the dishwasher for three cycles with the water set at the highest possible temperature. For extra precaution, it is best to operate it with detergent for at least the first cycle.

4) Drain the remaining water out. Your dishwasher is now kosher.

Koshering Utensils

As explained above, the method of koshering vessels that have absorbed non-kosher substance or an admixture of meat and dairy through being cooked in a hot liquid is by immersing them in water[29] that is on[30] the fire[31] and has reached a boil,[32] preferably a rolling boil. However, the water must come to a boil through being heated by fire,[33] hot coals[34] or electricity,[35] rather than heat from a solar panel,[36] microwave oven[37] or hot spring.[38] There is some controversy regarding the use of steam to heat the water. However, many authorities permit it[39] and it has become accepted practice. The koshering process is as follows:

1)  A large pot which has not been used for at least 24 hours should be used for the koshering process. However, the pot must be koshered first before proceeding to kosher other utensils in it. This is done in the following manner:

A) The pot must be thoroughly clean before koshering.

B) It must be filled to the brim with water so that when it is brought to a boil some of the boiling water will overflow upon its outer surface too.[40]

C) Once this has been done the pot is kosher. Empty out its contents, clean it, refill it with fresh clean water[41] and bring it back to a boil.[42] You may now begin koshering other utensils in it.

D) If it is impossible to wait the 24 hour period to kosher the utensils; use a pot that has a proportion of sixty to one[43] water compared to the utensils that will be immersed in it or add some detergent or Ajax to the boiling water.[44]

E) If possible, the water in the pot should be a proportion of sixty to one compared to the utensils immersed in it. However, if this is impossible and the pot has not been used in 24 hours or more, this does not obstruct the koshering process.

2) Under normal circumstances the utensils that are to be koshered should be set aside and not used for a period of at least 24 hours.[45]  If this is impossible, the procedure mentioned above should be used. That is, they should be immersed in a pot that contains water which is sixty times greater[46] than the utensils being immersed or detergent or Ajax should be added to the boiling water.[47] According to some, if the pot is not large enough and there is no detergent or Ajax available, the utensils may be koshered by immersing them in boiling water twice.[48] Consult your rabbi.

3) The utensils that are to be immersed in the boiling water should be clean and dry.[49]

4) If the vessels or utensils have been painted this does not obstruct the effectiveness of the koshering process.[50]

5) One should be careful not to immerse too many utensils at once even if they are placed in a perforated basket and then immersed. However, if the basket is shaken during the immersion, thus insuring that the boiling water will reach all their surfaces, it is permissible.[51] If tongs are used to hold the utensil while it is immersed, once the vessel is removed, it should be held with the tongs on a different part of the utensil and then re-immersed. An alternate method is to hold the utensil with two tongs and alternately loosen the grip with one tong while holding it with the other. Also, very heavy duty rubber gloves, that are unaffected by the heat of the boiling water, may be used.

6) The optimal method of koshering is to immerse the utensil in its entirety into the pot.[52] However, if this is impossible, it may be inserted partially, taken out and inserted again, until its entire surface, including the handles, has been immersed.[53]

7) Even if only part of a vessel was used with non-kosher foodstuffs, nonetheless, the entire vessel must be koshered.[54]

8) The utensils should not be immersed in the boiling water too long because this may cause them to re-absorb that which was purged from them. Rather, they should be inserted for a short time;[55] just enough for their pores to open and eject what was absorbed in their walls.[56]

9) It is not necessary to immerse the utensils deeply into the boiling water. Rather, it is sufficient that they be fully submerged.[57]

10) Optimally, the utensils should be rinsed or immersed in chilled water immediately after being taken out of the boiling water.[58] This causes their pores to close so that they do not re-absorb anything that may remain on their surface. However, if it is impossible to do so it does not obstruct the koshering process.[59]

11) If the water has cooled due to the immersion of previous utensils, so that it is no longer at a boil, no further utensils should be immersed in it until it again reaches a boil.[60] Optimally, it should be a rolling boil. However, as long as the water is boiling, even if is not a rolling boil, this is sufficient.[61]

12) If the water has become murky due to many vessels having been immersed in it, it should be replaced with fresh water and brought to a boil before continuing[62] to kosher further vessels. As stated above, optimally, it should be a rolling boil.

13) When koshering two vessels together, one of which has been used for dairy and the other of which has been used for meat, it is important that at least one of the vessels has not been used within the last 24 hour period[63] or for there to be a proportion of sixty to one[64] more water than at least one of the vessels.[65] If, however, both of them were used within the last 24 hours or there is insufficient water, not only do they not become kosher but one may be cooking an admixture of milk and meat, which is forbidden,[66] G-d forbid. In order to avoid this, whenever possible, it is advisable not to use the utensils for at least 24 hours before koshering them.[67]

After the vessels have been koshered they now have a status of being pareve (neither meat nor milk). They remain so until they are used specifically with meat or dairy. However, as soon as a vessel is used with meat or dairy it loses its pareve status and becomes either a meat utensil or a dairy utensil.

Koshering Countertops

Stainless steel or granite countertops may be koshered by pouring boiling hot water on them. Merely applying steam to them is insufficient. Before pouring, the counter tops should be clean and dry. Likewise, wooden countertops or cutting blocks may be koshered in a similar manner on condition that their surfaces are smooth and have no dents, grooves or cracks in them. However, if there are dents, grooves or cracks they must be sanded down to the virgin wood.

Countertops made of ceramic tiles or Formica may not be koshered. They should be replaced with stainless steel or granite. However, if this is not possible, great care should be taken never to place hot food or hot pots directly upon them. Rather, something that will act as a barrier, such as a trivet, should be placed on the counter and the food or pots should be placed on it instead.

Koshering Sinks

Similarly, stainless steel sinks may be koshered by pouring boiling hot water on them. However, as explained above in regard to countertops, they should be thoroughly clean and dry before doing so. If the sinks are made of ceramic or porcelain they may not be koshered. Optimally, they should be replaced for stainless steel. However, if this is impossible, great care must be taken never to place any dishes or utensils directly into them. Rather, the dishes and utensils should be placed on a sink rack or in a basin.

Cabinets and Refrigerators

All that needs to be done to kosher a refrigerator is to thoroughly clean it. Moreover, since all the foodstuffs in it are cold, there is no concern about storing meat and dairy in it simultaneously. Nonetheless, care should be taken to avoid the accidental spillage of one food onto another. Therefore, it is recommended that all foods be sealed, stored in containers or wrapped in plastic or foil. If possible, containers holding liquids, such as soup, should be stored on the bottom shelves to avoid spillage onto other foods. Cabinets and pantries, likewise, need only be cleaned to ready them for kosher use. Here too, meat and dairy items may be stored in the same pantry as long as they are in sealed packages.

Separation of Meat and Dairy Utensils

1) Generally, in order to avoid error and confusion, it is important that the meat and dairy utensils be recognizably different from each other. This is accomplished by buying different shaped, colored or patterned plates, cups, saucers, pans and pots etc. Also, there are special adhesive labels available for indicating meat and dairy. These are made to withstand high temperatures. They may be placed on pots and pans etc. without fear of burning. It is also advisable to label the cabinets in which meat and dairy utensils, dishes etc. are stored.

2) There should be two separate dishwashing machines, one for meat dishes and one for dairy dishes. However, if there is only one dishwasher, it is advisable to designate it only for whichever type of food is eaten most by the members of the household.

3) Generally, there should be two different ovens for meat or dairy, especially in regard to conventional ovens, as opposed to self-cleaning ones. However, there are Halachic authorities that permit koshering a conventional oven from meat to milk and vice versa on a regular basis if acquiring two ovens is not possible.[68] Consult with your rabbi for his opinion.

[1]  כבולעו כך פולטו, גמרא פסחים ל ע”ב.

[2] שו”ע תנא:ד

[3] משנ”ב תנא:כט, כה”ח סז.

[4] שו”ע יו”ד קכא:ד, וראה שו”ע הרב תנא:טז.

[5] אג”מ יו”ד ח”ג, יד אות י”ב.

[6] לכתחילה.

[7] שו”ת באר משה ח”ז קנב.

[8] רמ”א או”ח תנב:ה, וראה שו”ע הרב תנא:יג, ואם הגעיל בנוזלים אחרים צריך לחזור ולהגעיל שוב במים, פמ”ג תנב:ה ופר”ח שם ומטה יהודה שם, וראה משנ”ב תנא:כו, ולענין שעת הדחק ראה רמ”א או”ח תנב:ה, שו”ת אג”מ יו”ד א:ס, ויו”ד ח”ב מא, וראה שו”ת מלמד להועיל או”ח צו.

[9] ר”ן סוף ע”ז לח ע”ב, שו”ע הרב תנא:כה

[10] ספר מועדים וזמנים רפא.

[11] שו”ע או”ח תנא:ה-ו, שו”ע יו”ד קכא:א.

[12] שו”ע או”ח שם.

[13] רמ”א או”ח תנא:ד.

[14] דעת המחבר, שו’ע שם.

[15] דעת הרמ”א שם.

[16] ראה שו”ת מנחת יצחק ה:כ.

[17]  גמרא ע”ז עה ע”ב, וראה ספתי דעה קג:יז, יד יהודה קג:כג ע”א.

[18] ראה הגרע”א על יו”ד קג:ה.

[19] פיסקי תשובות תנא:י

[20] אג”מ יו”ד א:נט, מנחת יצחק ג:סו.

[21] כבולעו כך פולטו.

[22] שו”ת תשובות והנהגות ב:ריב, קובץ מבית לוי ז עמ’ כה.

[23] כבולעו כך פולטו.

[24] שו”ע או”ח תנא:כו, שו”ע יו”ד קלה:ח, שו”ת יבי”א ח”ד יו”ד, חזון עובדיה הל’ הגעלה ח.

[25] ט”ז יו”ד תנא:ל, ש”ך יו”ד תנא:מט, שו”ע הרב או”ח תנא:עג, משנ”ב או”ח תנא:קנה, חיי אדם קכה:כב ועוד.

[26] ויקרא ו:כא, גמ’ פסחים ל:ב, שו”ע או”ח תנא:א.

[27] שו”י או”ח תנא:ח.

[28] מנח”י ג:סז, חלק”י ב:קסג, אג”מ ב:צב.

[29] ראה שו”ת שבט הלוי ו:קטו אות ד על הכשרת כלים בנוזלים אחרים חוץ ממים.

[30] שו”ע או”ח תנא:ג ורמ”א שם.

[31] לענין אם חשמל גם נקרא אש ומה דינו ראה שו”ת אבני נזר יו”ד דיא, וראה שו”ת שבט הלוי ו:נו, כה”ח או”ח תנב:פ, שדי חמד, אסיפת דינים מערכת ה:כד.

[32] שו”ע או”ח תנב:א ורמ”א שם. ט”ז יו”ד צד:ג.

[33] שו”ע או”ח תנא:ג ורמ”א שם

[34] תבואות שמש מ:ד, וראה פסקי תשובה יו”ד סז.

[35] שו”ת אבני נזר יו”ד קיא, שו”ת פאת שדך עז, יסודי ישורון ח”ו עמ’ קצב, פסקי תשובה סי’ קכא.

[36] שו”ע או”ח תנב:ה.

[37] קובץ הדרום חוברת טו עמ’ עז.

[38] שו”ע או”ח תנב:ה, וראה שו”ע הרב שם כז, חיי אדם קכה:כט, ערוה”ש שם יט.

[39] שו’ת אבני נזר יו”ד קכא:טז, דרכ”ת יו”ד קכא:טז, שו”ת מהרש”ם א:צב, הגהות מהרש”ם לשו”ע תנא:יד.

[40] שו”ע הרב או”ח תנב:א.

[41] Specifically water rather than any other liquid should be used for koshering.

[42] שו”ע הרב או”ח תנב:ב.

[43] This means 59 parts water to one part utensil in volume. In other words the volume of the utensil should be 1/60th  of the volume of water.

[44] שו”ע יו”ד צה:ד, וראה שו”ת חת”ס או”ח קכ, שו”ת חכם צבי קא, יסודי ישורון ח”ו עמ’ קפו, חזו”א יו”ד כג:א ואו”ח קכב:ו ועוד.

[45] שו”ע או”ח תנב:ב ורמ”א שם, וראה שו”ע יו”ד קכא:ב

[46] 59 parts water to one part utensil.

[47] שו”ע יו”ד צה:ד, וראה ט”ז שם וש”ך שם. וכן ראה שו”ת צמח צדק צא, שו”ת חכם צבי קא ושו”ת חת”ס או”ח קכ

[48] שו”ת אג”מ יו”ד ב:לד.

[49] מג”א תנב:ט, שו”ע הרב או”ח תנב:ז.

[50] ערוה”ש תנא:מז, וראה תבואות שמש פא.

[51] שו”ע או”ח תנב:ג, ועין משנ”ב פב.

[52] ט”ז או”ח תנב:ה, וראה ביה”ל שם, וראה שו”ע הרב כו.

[53] שו”ע או”ח תנא:יד.

[54] שו”ע יו”ד קכא:ו, וראה ט”ז וש”ך שם.

[55] טור תנב.

[56] עיין ט”ז שם ב, שו”ע הרב או”ח תנב:ד.

[57] רמב”ם הל’ חמץ ומצה ה:כו.

[58] שו”ע או”ח תנב:ז, שו”ע יו”ד קכא:א, שו”ע הרב או”ח תנב:ה.

[59] כה”ח תנב:עה, וראה משנ”ב שם לד, וחיי אדם קכה:לז, ועיין שו”ת שבט הלוי ו:קטו.

[60] שו”ע או”ח תנב:א ורמ”א שם, ט”ז יו”ד צד:ג, שו”ע הרב או”ח תנב:ג.

[61] ספר מועדים וזמנים רפא.

[62] רמ”א תנב:ה, ועין כה”ח שם סא, ואהל מועד שער או”ח דרך ט נתיב ו, ומשנ”ב תנב:כז.

[63] שו”ע או”ח תנב:ב, וראה משנ”ב שם.

[64] 59 parts water to one part utensil.

[65] מג”א תנב:ח, וראה משנ”ב יח, כה”ח לז.

[66] רמ”א יו”ד צה:ג.

[67] שו”ע הרב או”ח תנב:יד.

[68] מנח”י ה סי’ כ:יד, יבי”א ה:ז.

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