Dairy glasses mistakenly washed in dishwasher used for meaty dishes - Matan - The Sadie Rennert

Dairy glasses mistakenly washed in dishwasher used for meaty dishes Rabbanit Chanital Ofan

Cheshvan // October 2021

Topic : Kashrut ,


I have mistakenly washed dairy glasses in the meaty dishwasher, together with dirty meat dishes. Can I continue to use the glasses for dairy?


The answer to your question depends on the clarification of three different parameters:

The halakhic status of glassware
Whether the flavor ejected from a meat dish can render a dairy dish traif [unkosher] (the law of nat bar nat, explained below)
Whether the meat that remains stuck to the meat dishes can render a dairy dish traif (the rules of batel be-shishim and defiling food with soap)


The Halakhic Status of Glassware

While glass is an ancient material, it only appears as kitchenware in Jewish sources from the Second Temple Period. Chazal related to glass as susceptible to ritual impurity (BT Shabbat 14b), and stated that they had to undergo ritual immersion in a mikveh (tevilla) if purchased from a non-Jew, similar to metal vessels, due to a similar production process. Regardless, Avot de-Rabbi Natan (I, ch. 41) states: “Three things were stated regarding glassware … it does not absorb [the flavor of food], nor does it emit [that flavor].”

Three positions are found among the Rishonim regarding the absorption of glassware:

I.                    The Ran (Pesahim 8b on the Rif, s.v. כי), based on Avot de-Rabbi Natan, explains that glass does not absorb food due to its smoothness. This position was also adopted by Ra’avia (464), Semag (Lavin, 141), Rosh (Kol Sha’ah, 8), and Rashba (I, 233), who believe that glassware is non-absorbent, and can therefore be used interchangeably for meat and dairy.

The non-absorbance of smooth materials (referred to by the term שיע) is discussed in the Talmud (Pesahim 74b) regarding the case of a heart which is cooked along with the blood it contains [since it is forbidden to eat blood, the question is raised about the permissibility of consuming the meat]. The Talmud states that the heart does not absorb the blood, since it is smooth (there is an additional discussion on 30b regarding the absorbency of glazed ceramic dishes).

II.                 According to the Beit Yosef, Tosfot (Pesahim 74b s.v. שאני לב) distinguished between absorbing blood and absorbing fats, since the latter has more of a tendency to seep into a material, and cited Rabbeinu Tam’s position that this argument was rejected in the conclusion of the Talmudic sugya. This reading of Tosfot is arguable; does Tosfot’s statement that a heart will not absorb fat indicate a complete rejection of the concept of non-absorbance (שיע)? Contrary to the Beit Yosef, it is possible that Tosfot accept the concept of non-absorbance in the context of glassware, as indicated in the subsequent sugya regarding glazed ceramic. 

Moreover, Hagahot Maimoniyot (hilkhot hag’ala) and Mordechai (Pesahim 82:574) cite R. Yechiel, who argues that drinking glasses cannot be koshered for Pesach by pouring hot water (עירוי), since glass is likened to ceramic, which ‘absorbs but does not emit’ (Pesahim 30) – as they are both sand-based. (Incidentally, R. Yechiel is also stringent in the case of absorbing a cold drink, since soaking in water for 24 hours is considered similar to pickling, and ‘pickling is tantamount to cooking’). It is possible these Rishonim were unaware of the source in Avot de-Rabbi Natan; in fact, the Schechter edition of Avot de-Rabbi Natan cites two versions based on early manuscripts, and only one of the versions includes the statement that glassware is non-absorbent.

III.              Some Rishonim argue that glassware is absorbent, and it also emits that that which it absorbs. This is the position of Ba’al ha-Itur (Part I: hekhsher ha-basar, 14:3) who wrote that glassware can be koshered by immersion in boiling water (hag’ala) three times, like ceramic dishes.

Conversely, according to the Rambam (ma’akhalot asurot 17:3), glassware is comparable to metal, and could therefore be koshered by means of straightforward hag’ala – that is, immersion in boiling water once. This is also the position of the Or Zaruah (part II – hilkhot pesachim 276), Shibolei ha-Leket (seder Pesach 207) and Ra’ah (Avoda Zara 78b).


In practice, the Shulkhan Arukh ruled (Orah Haim Tanna 26) that glassware is not considered absorbent, and can be koshered for Pesach by simply washing them, while the Rema (ibid.) took a more stringent approach, and wrote that it is not customary to kosher them even through hag’ala. 

However, this stringency by the Rema is not unequivocal; in the Darkei Moshe (ibid.) the same author attests to the custom not to kosher glassware, but also cites more lenient positions, and writes that they can be relied upon post-factum. Achronim argued about the practical ramifications of this position: according to the Magen Avraham (Orah Haim, 451:49), the post-factum allowance relies on the assumption that glassware can be koshered through hag’ala (Rambam), and in this case, if they were koshered and used on Pesach, the food would not be prohibited post-factum. Conversely, the Taz (30) understood the post-factum allowance as relevant even without hag’ala (as long as the dishes were clean), in accordance with the position of the Shulhan Arukh and the Spanish Rishonim. The Mishna Berura (155; Sha’ar ha-Tziyun 196) ruled according to the Rema.

It is noteworthy that the Ashkenazi custom to avoid koshering glassware for Pesach is not based on the post-factum status of the dish, since all dishes are automatically returned to their previous ‘kosher’ status the moment Pesach is over. Therefore, the debate between the Magen Avraham and the Taz refers not to the status of the dish – but rather to the permissibility of eating the food; the discussion therefore centers on the post-factum case in which the food has already been cooked in the non-Pesach glassware, and whether it can now be consumed. The Mishna Berura (156) ruled based on the Hayei Adam that if one owns only one set of dishes, they may be koshered through immersion in water for three days, since this too is considered a post-factum (בדיעבד) case.

In light of all this, let us return to the question of dairy glasses washed in a dishwasher with meat dishes (which is comparable to cooking meat in a dairy dish). If these cannot be koshered through hag’ala, they will be prohibited for normal use altogether, and will never serve their purpose; therefore it seems that they can clearly be koshered through hag’ala. [u1] The Arukh ha-Shulhan (Yore De’ah 121:27) ruled, in accordance with Ba’al ha-Itur, that traif glassware may be koshered by hag’ala three times. Many Achronim indicate that one hag’ala is sufficient; this is the position of the Minhat Yitzhak (Orah Haim I:86), the Mishne Halakhot (IX 168), Resp. Teshuvot ve-Hanhagot (I: 432), and Sde Hemed (Asefat Dinim 5:29) citing Kehal Yehuda. This was also the ruling of the Ben Ish Hai (shana rishona 96:14), and the recorded custom in Baghdadi and Moroccan communities, as well as the ruling of R. Mordechai Eliyahu (hilkhot chagim 5:52-58).

Accordingly, R. Melamed wrote in Peninei Halakhah (Kashrut II, ch. 32, kelalei hakhshara) that the custom of the majority of Jewish communities is to kosher glassware through hag’ala. (Moreover, today the Rema’s concern that hag’ala will not be done properly [with appropriate heat] due to the concern about glass breaking is irrelevant, since glassware today is generally made to withstand heat).

In light of the approach that allows koshering glassware through hag’ala, it is important to note that Duralex and Pyrex, which contain metals as well as glass, may have to be koshered through hag’ala even according to the Shulhan Arukh (who holds that glass can be koshered by washing alone, as explained above; see Tzitz Eliezer VIII:20; IX:26). R. Ovadia Yosef does not distinguish between the halakhic status of Pyrex and standard glass, and permits using both for meat and dairy interchangeably (Yabia Omer, Orah Haim IV 41; Yore De’ah IV 5; Yehave Da’at I 6).

(Clean) Meat and Dairy Dishes ‘Cooked’ in Water Together

The dishwasher cleaning process involves spraying boiling water on all the dishes in the dishwasher. In order to address the status of glassware in the dishwasher, we first need to understand the status of the metal dishes therein. For the purpose of this discussion we will ignore the question of food scraps that stuck to the plate, which will be addressed later.

The Talmud (Hulin 111b) argues that fish cooked in a meat pot may be eaten with dairy. The term used in the context of this discussion is נותן טעם בר נותן טעם (“noten ta’am bar noten ta’am”), indicating ‘secondary taste’ – food that absorbed the taste indirectly from the pot, and not directly from the meat. This indirect flavor does not affect the parve status of the fish, so it can be eaten with dairy. Halakhic literature uses the shorthand abbreviated acronym for this term – nat bar nat (נ”ט בר נ”ט).

However, the case of glassware in a dishwasher is different, since the flavor emitted is not sequential, but rather simultaneous, from the meat and dairy dishes that are being washed (‘cooked’) together, into the parve ‘food’ (the water).

The Beit Yosef cites a debate among Rishonim on the halakhic ramifications of food cooked with meat and dairy dishes simultaneously.

Sefer ha-Teruma (61), Semag (lavin 140; 141:53b), Semak (end of 213 in haga n. 9) and Sha’arei Dura (57) prohibit, while the Ran (41a, s.v. ראשון) citing Ramban (Hullin 111b, Reichman ed.), Rashba (Torat ha-Adam, Bayit III, Sha’ar IV 86) and Rosh (25:29) allow.

The Shulhan Arukh (Yore De’ah 95:3) follows those who permit (as long as there are no visible food scraps), but Rema is stringent when both the meat and dairy dishes were used in the last 24 hours.

Noten Ta’am lifgam (Emitting Detrimental Flavor)

The Shulhan Arukh (Yore De’ah 95:4) also permits the consumption of the food when fat was stuck to the dishes, as long as there was dirt in the water. The dirt enables the principle of נותן טעם לפגם (noten ta’am lifgam), which nullifies the prohibited mixture by infusing it with a detrimental, defiling flavor. The Shakh (21) and Taz (15) disagreed with the Shulkhan Arukh’s ruling; but it seems in a post-factum situation (as the case in the question before us) his lenient ruling may be relied upon. Moreover, the Darkei Teshuva (95:4) cites the Yad Yehuda (peirush ha-arokh 33; ha-katzar 37), who argued that soap (בורית) certainly defiles the flavor of the dish, and the Hazon Ish (33) ruled accordingly. Since dishwashing detergent is a powerful cleaning substance, it certainly can be relied upon to defile the flavor of the remnant food on the dishes or flavors absorbed in the dishes. R. Ovadia Yosef writes accordingly that the purpose of the soap is to get rid of the flavor altogether (Yabia Omer, Yore De’ah 10:4).

Pouring Boiling Water for the Purpose of Koshering (irui)

In the case of pouring boiling water from a parve dish onto meat and dairy dishes simultaneously, instead of cooking them together in a large pot (as in the cases described above), according to the Rema the food cooked in these dishes may be permitted post-factum, since pouring boiling water can potentially cook no more than the outer layer, and nat bar nat does not apply in that case.

The water in a dishwasher is boiled in a water tank under the machine, and injected through a pump into the machine. This may be viewed as pouring from a כלי ראשון (kli rishon – a vessel heated directly); this was the understanding of R. Ovadia Yosef, as cited by his son, R. Yitzchak Yosef, in his book, Laws for the Mother and Daughter, 30:15). Conversely, the entire dishwasher may be viewed as one large cooking pot, and in this case the action taking place is tantamount to cooking, not to pouring boiling water.

The Rema extends the allowance even when there is fat on the dish; but the Shakh (20), Pri Megadim (7:20), and others wrote that this allowance relies on Rishonim who hold that pouring boiling water is not considered cooking at all; however, since the ruling is that pouring hot water cooks the outer layer, this leniency should not be used unless the dishes were clean.

Moreover, if the dairy glasses were not used with a boiling dairy drink in the 24 hours before they were placed in the dishwasher – even the Rema admits that the dishes are kosher, since this is nat bar nat which defiles the food (note that hot coffee that had milk added to it, is not considered a boiling dairy drink, since the milk cools the beverage).

Food Scraps in the Dishwasher

Generally people tend to scrape off large pieces of food before placing dishes in the dishwasher, but they still contain some remnants. Are these remnants defiled by the soap, as mentioned above in the flavor emitted from the meaty pot? Furthermore, can these remnants render the dishes traif, or are the remnants nullified in sixty parts (ביטול בשישים) in the water (the law of ביטול בשישים – nullifying in sixty parts – is mentioned by the Shulhan Arukh, Yore De’ah 92a, and the laws of noten ta’am lifgam (emitting detrimental flavor) is mentioned in Yore De’ah 103:1-2; see below regarding the latter).

Since dishwashing detergent is a powerful cleaning substance, and according to his position above the dishwasher is considered pouring hot water and not cooking in hot water, R. Ovadia permitted also Ashkenazi Jews who follow the Rema to use the same dishwasher for meat and dairy simultaneously, as long as no large pieces of food were left on the dishes (R. Yitzchak Yosef, Issur ve-Heter, 152-153). He also argued that this is not considered ‘nullifying a prohibition from the outset’ (ביטול איסור לכתחילה), since the intention was never to mix the meat and dairy food remnants, or to benefit from the prohibition.

Conversely, R. Moshe Feinstein (Igrot Moshet, Orah Haim I 54; Yore De’ah II 29) permitted use of the same dishwasher for meat and dairy – but not simultaneously, since he believed the dishwasher is tantamount to cooking, not to pouring boiling water (irui). He added a demand to replace the racks since they come in direct contact with the dishes and with food remnants (Yore De’ah II 28). However, he is not concerned with food remnants, which are nullified in sixty parts (בטל בשישים) and defiled with soap.

It is important to clarify that different dishwashers may have different mechanisms. Moreover, dishwashers in the time of R. Ovadia Yosef and R. Moshe Feinstein also may have had different mechanisms than more modern dishwashers. Many dishwashers have a special compartment for the detergent, and this may delay the injection of the detergent until the washing cycle is underway.

R. Moshe Feinstein (Yore De’ah III 10) argues that while the dishwasher is comparable to a kli rishon (a vessel heated directly), it is still not similar to a pot on a fire, and should be treated with leniency appropriately. This ruling, coupled with the nullification in sixty parts, resolves another problem that comes up in the context of some dishwashers, which run a first cycle without detergent. In this case one cannot rely on the principle of defiling the flavor by soap, but the nullification in sixty parts and the fact that the process is not fully comparable to a pot on the fire can be relied upon sufficiently for nat bar nat (Shulhan Arukh, Yore De’ah 95:3).

One other factor for leniency is that the hot water in a dishwasher only reaches 70-80 degrees Celsius. The washing process includes several cycles. Therefore, after only one cycle, the dishes are essentially clean, the food scraps are defiled and nullified, and the next cycle may be treated as hag’ala, since this is clearly a process of כבולעו כך פולטו (‘it absorbs and omits in the same manner’). If the dishes were to become traif by the heated water in the dishwasher, they would require hag’ala by water in the same level of heat – which occurs in the final washing cycle (in any dishwasher).

Conversely, R. Yisrael Rosen (Tehumin 11, p. 130) contends the reality of this approach, and argues that the food remnants do not transition into noten ta’am lifgam (emitting detrimental flavor). He is also takes a stringent approach with regard to the nullification in sixty; it is his understanding that some cycles use too little water to warrant nullification.

However, R. Moshe Feinstein would contest the premise of these arguments; moreover, R. Rosen’s article relates to old dishwashers, and in newer dishwashers the amount of food that sticks to the dishes is smaller and considered inedible.  [u2] 

The Status of Cups as a Kli Sheni (a vessel heated indirectly)

Cups are generally used as a kli sheni (a vessel heated indirectly), which allows greater leniency, since they are not considered to emit the flavor of the food they contain (this is disputed among Rishonim, but the Shulhan Arukh and Rema ruled with leniency post-factum; Yore De’ah 68:11; 105:3).



In light of the discussion above, several halakhic principles assemble to indicate a lenient approach when it comes to placing dairy drinking glasses in a meat dishwasher, even for those who hold by the Rema, and even for those who demand hag’ala for glassware:

1.      The position of the Taz according to which even the Rema was only stringent with glassware from the outset (לכתחילה) but not post-factum (בדיעבד).

2.      The position of R. Ovadia Yosef according to which the dishwasher is tantamount to pouring boiling water on dishes (עירוי) and not to cooking dishes in a pot – therefore, even the Rema would tend toward leniency. [u3] 

3.      As long as the cups were not used for boiling milk in the 24 hours before they were places in the dishwasher, since they are only absorbing nat bar nat (the dishwasher water), even according to the Rema they would not become traif (even if washed with meat dishes that were used in the last 24 hours).

4.      If there were scraps of food in the water, they would be nullified in sixty parts due to the amount of water in the dishwasher, and furthermore, defiled by the detergent.

5.      Since the heat of the dishwasher is short of actually boiling, there is no concern for the comparison of meat and dairy dishes being cooked together in one large pot. Moreover, as explained above, the dishwasher cycles are tantamount to hag’ala; the mechanism includes several cycles which resemble כבולעו כך פולטו (‘it absorbs and omits in the same manner’), since they use the same degree of heat.


According to Ashkenazi custom and some Sephardic halakhists, dairy cups should not be places in a meat dishwasher along with meat dishes; however, post-factum, there is good reason to allow their use without koshering them.

Rabbanit Chanital Ofan graduated from Matan’s  Advanced Talmudic Institute.  She has an M.A. in Talmud from Bar Ilan University and is a Certified Halakhic Advisor (Yoetzet Halakha) by Nishmat.  For the last 18 years she has taught Talmud and Midrash in a number of women’s Batei Midrash. She is in the first cohort of Hilkhata Matan’s Advanced Halakha Institute and a lecturer in Matan’s Metivta program.

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