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Hotel Kashering

Rabbi Dovid Cohen
Administrative Rabbinic Coordinator of the cRc

February 2009

The following was prepared for presentation at the AKO Va’ad Convention held in West Palm Beach, Florida in February 2009. The body of the document contains the halachic background for kashering specific pieces of equipment and the text boxes (and footnote 9 & 27) contain practical methods of performing the kashering. These were written by and will be presented by Rabbi Dovid Cohen, of the cRc, and Rabbi Avrohom Stone, of the Va’ad HaKashrus of Metrowest respectively.

Twelve Pieces of Equipment
This document will discuss the three primary halachic issues which have to be considered when determining how to kasher equipment in a hotel. The three issues are cleanliness, choosing the method of kashering and aino ben yomo, and this document will focus how they apply to the following list of equipment which are commonly kashered at hotels:

  • Convection Oven
  • Dishwasher
  • Flat Top
  • Grill
  • Kettle (steam)
  • Oven
  • Sink
  • Stovetop
  • Table
  • Tilt Skillet
  • Vegetable Steamer
  • Warming Box

Disclaimer:  The procedures below were written intended solely for the use of very experienced, highly trained, competent Mashgichim.
Kashering is a very dangerous activity when done in a foodservice environment.  Many Mashgichim have been seriously injured by scalding water and have been badly burned by torches, coals, flames, latent heat from kashering and residual heat of kashered equipment.  Even when proper precautions have been taken, a simple slip on a floor when carrying a pot of boiling water, or opening foil covering a kashering stovetop too quickly can have lifelong consequences.

In addition, equipment in a foodservice kitchen is extremely expensive.  Putting this equipment through the regimen required by kashering, which often exceeds the normal limits of the equipment in a dramatic manner, can easily result in severe damage to the equipment and surfaces for which the Mashgiach and/or hashgachah can be held responsible.

For these reasons and many more, only a Mashgiach who is highly experienced in kashering and the use of kashering equipment should be entrusted to do this.

Safety First!  A few very basic rules of kashering:
1 – Never, under any circumstances, leave any coals or flames unwatched, even for a few moments.
2 – Never rush when kashering, no matter how late or hectic the situation.  All actions and movements should be deliberate and thought through.
3 – Never wear loose fitting clothing or shoes that slip on wet surfaces.
4 – Always exercise cautious judgement; err on the side of caution and safety.  If you are afraid something is too hot or exceeding its limits, it probably is.
5 – Always focus on, and pay complete attention to, the task at hand and do not be distracted by doing other things such as conversation, cell phone, etc.
6 – Always have a fire extinguisher and similar equipment nearby in case something goes wrong.

A – Cleanliness
The most obvious first step in kashering a piece of equipment is to make sure that it is cleansed of all residue of non-kosher food. For most pieces of equipment, this means that the hotel employees must scrub the equipment thoroughly, after which the Mashgiach will inspect it to be sure they have done a good job. Some of the common questions that arise in this regard are presented in the coming paragraphs.

How clean?
Although our natural inclination is to require that there not be even a slightest bit of residue on the equipment before kashering, in fact there is a bit of leeway in this halacha as per the following limitations:1

  • The person must clean the equipment to the “industry standard” of cleanliness and not – in Rav Belsky’s words – merely be “satisfied to use dirty equipment”.
  • There must be so little leftover food that it will be batel b’shishim regardless of how little kosher food is cooked in the equipment.
  • It does not apply when kashering for Pesach (for equipment which will be used on Pesach), because chametz is not batel b’shishim on Pesach.

There are three other noteworthy points regarding this halacha:

  • The letter of the law is that the leeway described above applies even when one cleans equipment which requires hot kashering.2
  • Libun kal (or libun gamur) with a direct flame or coal is assumed to remove residue from equipment,3 but experience has shown that libun kal which is performed by heating a chamber to a given temperature (as is commonly done with ovens) does not have the same effect. Therefore, the equipment must be thoroughly cleaned before kashering.
  • Rav Schachter suggested that when consumers kasher their own homes they do not leave even the most miniscule amount of residue on the utensils (i.e. they do not rely on the leniency outlined above), and they expect/assume that hashgachos hold themselves to a similar standard. If one subscribes to this position,4 there is no room for leniency in the cleaning of equipment.

Equipment which is difficult to clean
Rema5 cites a custom to not use chametz strainers on Pesach because they are so difficult to clean that we do not rely on the person’s efforts and remain concerned that some residue remains. At first glance, this minhag would seem to preclude the kashering (and use) of commercial dishwashers and convections ovens, because they are notoriously difficult to clean. [In particular, the difficult areas to clean are the trap, curtain and belt of a dishwasher, and the fan assembly of a convection oven.]

However, in practice most hashgachos do allow the kashering of this equipment because:

  • The minhag appears not to apply in cases where the equipment will be cleaned with industrial solvents, scrubbers and other tools which make it more realistic to successfully accomplish the task. This is especially true if the equipment is broken-down as part of the cleaning.
  • Rav Belsky understood that the minhag is limited to utensils which will be used on Pesach, when chametz is not batel b’shishim.
  • Rav Schwartz further suggested that Rema only applies to cases where the residue might get into the actual food, and therefore to avoid the seriousness of eating actual chametz on Pesach, there is a minhag to not use such utensils. However, there is no basis for applying the minhag to the case of a dishwasher or microwave, where the residue cannot realistically do more than cause a b’liah to get into the dishes.

One must be careful to remove all food residue and rust6 before kashering, but there is no need to remove discoloration of the equipment.7 Thus, an oven or stovetop must be cleaned until the metal surfaces are smooth to the touch, but the equipment does not have to look brand new.8

B – Choosing the Method of Kashering
Different methods
For purposes of our discussion we can divide the methods of (hot) kashering into four groups:

  • Libun gamur – Using a torch or coals to heat all surfaces until they are red hot.
  • Libun kal – Either (a) applying a torch or coals to all parts of the inside of the equipment until the outside reaches yad soledes bo or (b) heating an oven chamber to 550° F and maintaining that temperature for an hour.9
  • Hag’alah – Filling the pot (or other utensil) with water, bringing the water to a rolling boil on the fire, and then overflowing the pot. Alternatively, the flatware or other item being kashered can be submerged into the pot of boiling water.
  • Irui kli rishon – Preparing a pot of boiling water as above, and methodically pouring water directly from the pot onto all surfaces of the counter or other area being kashered.

The following paragraphs will discuss how one chooses which method of kashering to use for the different pieces of equipment listed above.

Direct contact on the fire
Libun gamur is difficult, if not impossible, for most people and for most equipment, and therefore the first decision to be made is whether libun gamur is required. The principal rule for that determination is that libun gamur is only required for equipment which has direct contact with solid non-kosher10 food while on the fire. In this context, it is worth defining two terms:

Direct contact with solid food – means that libun gamur is only required if there was no air or liquid between the non-kosher food and the equipment, and the non-kosher food was a solid. For example, in a grill, the meat is broiled directly on the racks, and therefore it must be kashered with libun gamur. However, in an oven or warming box, where there is air/zei’ah between the chamber and the food, and the case of steam kettles which are used for cooking liquids, libun gamur is not required.

While on the fire – denotes that libun gamur is never necessary for utensils used off the fire, such as sinks and tables. It is also generally accepted that “fire” in this context refers to an actual flame or to an electric coil used for heating, but steam or water which is used as a heating media is not considered fire. Accordingly, pans used to heat (solid) food in an oven must be kashered with libun gamur, but pans used to heat that same food in a vegetable steamer could be kashered with less rigorous methods because the vegetable steamer is “off the fire”.

Libun Gamur for a grill
1 – Clean grates thoroughly to avoid a grease fire.
2 – Make sure the grease-catcher is cleared and filled with charcoal.
3 – Cover grates with layer of charcoal, including a layer underneath the grates.
4 – Light coals.
5 – Allow coals to burn until grates turn red.

Libun Kal for a warming box
1 – Remove shelves and clean thoroughly.
2 – Box should be thoroughly cleaned from any residue.
3 – Replace shelves.
4 – Place 4-8 2-hour Sternos (depending on the size of the box) on the inside bottom of the box.
5 – Light Sternos, close door to box and allow Sternos to burn out.
6 – If there is a perforated electric heating box, unit must be cleaned and burned out, or else removed.

The following sections will discuss the more difficult questions which arise from this rule, namely, whether libun gamur is required for equipment which is used with just a bit of liquid or which is only occasionally used in a method that demands libun gamur.

Shulchan Aruch11 describes a machvas as a pan in which food is cooked with oil. He rules that a machvas can be kashered via hag’alah, and the accepted halacha is to follow this position regarding most issurim.12 Such a pan may be kashered with hag’alah even though the oil occasionally dries up and food burns onto the pan-walls.13

However, Mishnah Berurah14 adds that a frying pan15 that is merely greased with oil, fat or butter does not qualify as a machvas and instead we consider that the non-kosher food had direct contact with the pan such that libun gamur is required. There are two ways to interpret Mishnah Berurah’s statement, as follows:

  • Rav Belsky holds that Mishnah Berurah is referring to the amount of oil one might use when frying pancakes and the leniency of machvas only applies to deep fryers or pots used with considerable amounts of oil (or water). Accordingly, he holds that a flat top must be kashered with libun gamur because it is commonly used to fry eggs, hash browns and other foods that use a minimal amount of oil.
  • Rav Schwartz and others hold that Mishnah Berurah is only machmir if one uses an absolutely minimal amount of oil such as if one rubs a stick of butter across the pan before frying (as was once common) or sprays a Teflon pan with Pam (as is the current method). If however, one uses oil a bit more liberally (as described above), the pan is designated as a machvas and libun gamur is not required. [There are implications in the Acharonim to support this reading.]16 Rav Schwartz’s position regarding kashering a flat top will be discussed in more detail in the coming section.

Rov tashmisho
If a utensil is used to cook non-kosher food even one time, that utensil cannot be used for kosher food unless it is kashered. What if a utensil is primarily used in a manner which requires hag’alah but is occasionally used in a way which demands libun gamur? For example, in many facilities the primary function of the tilt skillet (brazier) is to cook foods with water (e.g. rice) but occasionally the tilt skillet will be used for frying without oil. Logically, we would assume that since occasionally the tilt skillet was used for solid non-kosher food directly on the fire,17 libun gamur should be required, regardless of the fact that most of the cooking is done with water or oil.

However, the truth is that the halacha is not as simple as this. Shulchan Aruch18 rules that in determining the method of kashering a utensil which is aino ben yomo,19 we only have to consider the primary use (rov tashmisho) of the utensil and may ignore the secondary uses. Rema20 says that the Ashkenazic custom is to be machmir and consider even the secondary uses (miut tashmisho). Accordingly, if a work table is generally used for cold food preparation but occasionally has hot food placed on it, Ashkenazim would require that the table be kashered with hot kashering and would not be satisfied with a mere cleaning of the table.21

In spite of Rema’s strict stand, he accepts the lenient position (a) in cases of b’dieved,22 (and there are those who suggest that in the appropriate situation one may also apply the principle of שעת הדחק כדיעבד ), and (b) in situations where following miut tashmisho will mean that the utensil cannot be kashered at all (for example, where libun gamur is required and the utensil cannot withstand that process).23 However, even in cases where the lenient position is justified, one may only follow rov tashmisho if the utensil is aino ben yomo24 and will not be used for (kosher) davar charif.25

Based on the above points regarding machvas and rov tashmisho, Rav Schwartz’s position is that preferably both flat tops and tilt skillets should be kashered with libun gamur, since they may occasionally be used for dry foods without liquid. However, where there is great need to use the equipment and libun gamur is not possible, and the primary use is in a manner which does not demand libun gamur they can be kashered with libun kal (flat top) or hag’alah (tilt skillet).26

Hag’alah for a tilt skillet
1 – Clean tilt skillet thoroughly.
2 – Fill with water until it comes out of pour spout.
3 – Heat water until boiling.
4 – Tilt the skillet backwards slightly (if possible) so water will boil over back.
5 – Tilt the skillet forwards so water will boil over front.
6 – See text below regarding areas where water does not reach (top of sides and cover).

Libun Gamur for a tilt skillet (or flat top)
1 – Clean tilt skillet thoroughly.
2 – Fill the skillet with coals.
3 – Light the coals.
4 – Turn on heat after coals are lit by torch.  When igniting by torch be careful not to set off the automated fire extinguisher (Ansel).
5 – Let burn in skillet for 20 minutes.
6 – Turn off heating element.
7 – Pour cold water onto the coals to extinguish them, pouring slowly.

K’bol’oh kach polto
A principle of hechsher keilim is that the kashering required to remove b’lios is commensurate with the way the b’lios were first absorbed. This principle, known as k’bol’oh kach polto, means that, for example, if a sink absorbs b’lios via irui kli rishon then it can also be kashered via irui kli rishon and a traditional hag’alah is not required.27

Contemporary Poskim28 extend this principle to even include temperature levels, such that if it is known that a dishwasher’s highest cycle is at 180° F, the dishwasher can be kashered at a few degrees above 180° F, and the water is not required to reach 212° F (although the minhag is to make every attempt to reach 212° F).29 In practice, kashering a dishwasher requires that (a) an engineer raise the temperature set point in the dishwasher so that the water is hotter during kashering than during operations and (b) the dishwasher be allowed to run at this higher temperature for an extended time (see the footnote).30 [This application of k’bol’oh kach polto is limited to hag’alah and does not apply to libun (i.e. libun for a utensil which had a b’liah at 350° F cannot be performed at a mere 350° F.)]

Maharsham31 suggests another application of k’bol’oh kach polto as relates to kashering with steam. As a rule, hag’alah cannot be performed with steam (but must rather be done with water) and therefore a combi-oven cannot be kashered with steam. However, if all of the b’lios were absorbed via steam then k’bol’oh kach polto teaches us that the hag’alah may also be performed with steam. A common example of this is a vegetable steamer, where all b’lios into the chamber are via steam, and therefore the chamber may be kashered via pumping steam into the chamber (for long enough that the chamber walls are saturated with heat). [If the pans used in the steamer are also used in the oven, then they must be kashered via a water-hag’alah since they had b’lios without steam.]

A related halacha is that although one may not kasher with steam, if the steam condenses into water on the surfaces of the equipment, the water will serve as an acceptable medium for hag’alah (assuming it reaches roschin temperatures and covers all surfaces of the equipment).32 In this case, steam is being used as a tool to facilitate hag’alah with pure water. This method of performing hag’alah is sometimes useful in kashering parts of a utensil (e.g. covers or the upper edges of a tilt skillet) which are too high to touch the water which is in the pot. Alternatively, those areas can be kashered via libun kal using a blowtorch.

It is worth noting that before using any equipment which is heated by steam, one must ascertain that the steam system is not shared between kosher and non-kosher equipment and that any residual (non-kosher) condensate is drained from the equipment.

C – Aino Ben Yomo
It is well known that utensils must be aino ben yomo before they are kashered via hag’alah. This ensures that b’lios expelled from the utensil during hag’alah cannot give a (positive) ta’am back into the item which was just kashered. Furthermore, Rema33 notes that even in cases where it is technically permitted to perform hag’alah as a ben yomo, the common custom is not to do so, so as to avoid questions and potential issues.

The above applies to hag’alah and irui kli rishon, but libun gamur incinerates all b’lios and may therefore be performed even if a utensil is ben yomo.34 Pri Megadim35 is unsure as to whether libun kal may be performed as a ben yomo or is essentially similar to hag’alah and must be an aino ben yomo. From a halachic perspective, most hashgachos take a lenient approach to this question, although as a matter of policy many do not perform any kashering whatsoever on equipment which is ben yomo.

As noted earlier, in cases where one relies on the lenient opinion and follows miut tashmisho in determining the method to kasher a piece of equipment, all opinions agree that the equipment must be aino ben yomo regardless of the method of kashering.


1 A zar (non-kohen) may not eat terumah, and when someone cleans out a container of terumah in preparation for using it with chullin, the Mishnah (Terumos 11:6 & 8) rules that he is allowed to leave some residue on the container, if he follows the generally accepted practice for cleaning out such containers. Rash (on Mishnah #8) explains that this leniency, which the Mishnah refers to as k’derech hamichabdim, is based on the assumption that the leftover residue will be so minute as to be batel b’shishim in the chullin. Rash clarifies that this does not raise concerns of bitul issur l’chatchilah because the fact that the person cleaned the equipment thoroughly shows that he has no interest in having the residue mixed into his kosher food (i.e. ain kavonoso l’vatel). This Mishnah is the basis for Shulchan Aruch YD 138:11.

2 Rav Schachter, based on the logic presented in the previous footnote.

3 Mishnah Berurah 451:33.

4 The reasons to question it are that (a) consumers’ overzealousness in cleaning may be based on ignorance of this leniency rather than on an informed choice, (b) most consumers understand that hashgachos cannot do as good of a job overseeing kashrus as the average housewife, and (c) even consumers are lenient in cases of sha’as hadchak and/or if a given piece of dirt stubbornly refuses to come off.

5 Rema 451:18 as per Mishnah Berurah 451:100.

6 Shulchan Aruch 451:3.

7 Mishnah Berurah 451:22.

8 The simple reading of a number of halachos (including Shulchan Aruch 451:3 & 13) would indicate that calcium deposits and polymerized oil must be removed from equipment before it is kashered. The rationale for possibly not requiring this, is beyond the scope of this document.

9 Although there is a chance that food in a pan on one rack touches the rack above it directly, we will see below (in the section on rov tashmisho) that that use does not require that the racks be kashered via libun gamur. Nonetheless, some do perform libun gamur, and use the following procedure (written by Rabbi Stone): (1) Clean racks thoroughly (as incomplete cleaning can result in a grease fire); (2) pile racks onto stovetop; (3) completely cover stovetop, including edges, with foil; (4) turn fire on low; (5) seal foil around stovetop; (6) turn fires to high, and leave them at that temperature for no more than 20 minutes; (7) use a pliers to carefully and slowly lift foil to verify that racks are glowing red.

10 [Unless the utensil is cheress,] libun gamur is not required for a utensil which had been used for heter, such as kosher meat or kosher milk, assuming they were not used in a manner which created basar b’chalav (Magen Avraham 451:11 based on Shulchan Aruch 509:5, and R’ Akiva Eiger on Shach YD 121:8).

11 Shulchan Aruch 451:11.

12 In fact, Rema ad loc. says that for Pesach one should l’chatchilah perform libun, and even Shulchan Aruch YD 121:4 himself rules that libun gamur is required when kashering a machvas which had been used for other (non-chametz)issurim. Nonetheless, Iggeros Moshe YD III:14:b rules that for non-chametz one can kasher an aino ben yomo machvas with hag’alah. [He gives no explanation for this posit13 and may merely be reporting the common custom to follow the Rishonim who take a len14 t stance (and not accept Shulchan Aruch).]

13 up>13 See Mishnah Berurah 451:63.

14 Mishnah Berurah 451:65.

15 One of Mishnah Berurah’s examples is a סקאווראדעס , which Rav Schwartz told me is a frying pan.

16 See Pri Megadim MZ 451:16 and Gra”z 451:36.

17 We have seen above that, as relates to this discussion, a utensil heated by electric coils is considered to be “on the fire”. Therefore, food cooked in a tilt skillet heated by electric coils (or a traditional flame) will potentially require libun gamur.

18 Shulchan Aruch 451:6.

19 Rashba (Responsa I:372) explains that the basis for relying on rov tashmisho is that since the utensil is aino ben yomo and the kashering requirement is merely d’rabannan, Chazal allowed the person to only consider the primary use of the utensil when deciding how it should be kashered. Accordingly, if the utensil is ben yomo from the secondary use, all opinions would agree that one must consider even the miut tashmisho (Mishnah Berurah 451:46).

20 Rema 451:6 and YD 121:5 (as per Gr”a 121:13).

21 See Rema (and Mishnah Berurah 451:45 explaining Shulchan Aruch’s position in this situation).

22 Rema 451:6 (and Mishnah Berurah 451:27).

23 Sha’ar HaTziun 451:51 citing Beis Meir, who in turn is based on Rema YD 121:5 (see Gr”a 121:14).

24 See Mishnah Berurah 451:46 cited above in footnote 19.

25 See Chazon Ish OC 119:15.

26 In weighing the “need”, we are more lenient regarding flat tops, as our experience has been that it is very rare for them to be used in a manner which does not qualify as a machvas (as per Rav Schwartz’s understanding given in the text above). According to Rav Belsky’s understanding, both pieces of equipment are of reasonably equal status, where miut tashmisho is in a manner which demands libun gamur, and a lower level of kashering is only justified in cases of שעת הדחק or where the equipment cannot withstand libun gamur.
Most tilt skillets are heated by flames or electric coils. However, some are heated by steam coils, and as noted above, items heated by steam coils are not considered to be “on the fire”; therefore, all would agree that libun gamur is not required, regardless of how the tilt skillet was used.

27 A sink might also have b’lios from a hot davar gush, and there are those who hold that the b’liah of a davar gush is considered to be a kli rishon even after it leaves the fire. A strict interpretation of this situation would require that the sink therefore be kashered with irui kli rishon using an even m’lubenes, as per Mishnah Berurah 451:114. [Rabbi Stone wrote the following procedure to perform libun kal to a sink: (1) Place 2 Sternos on sink bottom; (2) light the Sternos; (3) cover sink with foil, and leave it covered until the Sternos burn out.] However, most people kasher sinks without an even m’lubenes, and they likely rely on the combination of the fact that many hold davar gush does not have the status of kli rishon, in this case the b’liah of davar gush is merely miut tashmisho, and we have seen that one need not be concerned for miut tashmisho in certain situations.

28 See for example Iggeros Moshe YD I:60 and Minchas Yitzchok III:67:7-13.

29 See for example Iggeros Moshe YD I:60.

30 When a commercial dishwasher runs for an extended amount of time, the surfaces of the dishwasher get hotter than if the dishwasher just runs through a quick cycle. Accordingly, in order to mimic the temperature reached during non-kosher use, the kashering cycle has to last for long enough to reach similar temperatures. Another reason to extend the kashering is so that any walls which are heated to the point of no longer being דפנות מקררות such that they are considered a kli rishon (see, for example, Taz YD 92:30) will attain a similar status during the kashering. Kashering is invariably done using the dishwasher’s own water, and therefore it seems to be of no significance whether the dishwasher’s water is heated by an internal element (which makes it more similar to a kli rishon) or not, because the kashering will be based on k’bol’oh kach polto.

31 Maharsham I:92; this position is mirrored in other Poskim.

32 Iggeros Moshe YD I:60.

33 Rema 452:2.

34 See Darchei Moshe YD 121:15.

35 Pri Megadim MZ 452:4.