Typically, it is said that Sephardic Jews are permitted to use their dishwashers for both milk and meat utensils and Ashkenazic Jews are not permitted to do so. Indeed, many Ashkenazic Posekim forbid using a dishwasher for both meat and milk utensils (Rav Feivel Cohen, Badei Hashulhan 95:77; Rav Binyamin Forst, The Laws of Kashrut page 261 and Rav Yisrael Rozen, Tehumin 11:130-136). Rav Yosef Adler reports that Rav Yosef Dov Soloveitchik also forbade using one dishwasher to clean both milk and meat utensils.
Moreover, it is fair to say that it is accepted in Ashkenazic families not to wash both milk and meat utensils in the same dishwasher. Although Rav Moshe Feinstein wrote no less than seven Teshuvot permitting the use of a dishwasher for both meat and milk, the fact that he required switching racks when switching from meat to milk (and vice versa) made the use of one dishwasher for both meat and milk quite inconvenient.
The premier Sephardic Poskim Rav Ovadia Yosef and Rav Yitzhak Yosef, on the other hand, permit using one's dishwasher for both meat and milk (Yalkut Yosef Otzar Dinim L'isha p. 618; Yalkut Yosef Isur Vheter 3:485 and Teshuvot Yabia Omer 10 Yoreh Dei’ah 4). They even (essentially) permit simultaneous washing of both meat and milk in the same dishwasher! Even Rav Eli Mansour who typically rules strictly, essentially endorses Rav Ovadia’s ruling on this matter, though he expresses a strict preference.
However, Rav Shlomo Amar told a group of Shaarei Orah congregants (during his visit to our congregation on Shabbat Nachamu 5777) that he believes that a dishwasher should not be used for both milk and meat unless one “kosherizes” the dishwashers between uses of the opposite food types. Rav Amar typically follows in the Halachic paths of Rav Ovadia Yosef. However, regarding this issue he is not comfortable doing so.
We learn from Rav Amar’s response, that this issue is not necessarily a debate that runs along classic differences between Sephardic and Ashkenazic Jews. In order to explain Rav Amar’s approach let us first present Rav Ovadia’s reasoning as set forth in Teshuvot Yabia Omer 10 Yoreh Dei’ah 4.
Rav Ovadia’s Reasoning
Chacham Ovadia first describes how a dishwasher works. First off, a person normally removes large residue or food particles from the dish before inserting into the dishwasher. After putting the dishes in the unit, one adds a cleansing agent into a designated compartment inside the machine. The process then begins, and first to occur is a rinsing of the dishes with cold water. After the cold cycle completes, the water is warmed and the cleaning agent is released. The hot water mixes with the cleaning agent and together washes the dishes. After this process, the water shuts off, and a heating element is turned on to dry the dishes. Once dry, the process is complete.
Rav Yosef argues that the dishwasher is perfectly analogous to an issue addressed by Maran Rav Yosef Karo (Shulchan Aruch Yoreh Dei’ah 95:4). He describes a situation where people wanted to use giant pots normally used for dairy cooking for the purpose of cleaning various tableware and cookware including meat dishes. Maran rules that it would be permissible to use this giant dairy pot to clean meat dishes so long as a distasting agent such as Eifer (ashes) or soap is added. Such an agent would render the milk in the walls of the pot Pagum (unsavory, unfit, spoiled), thus allowing the insertion of a meat vessel for cleansing. This leniency applies even if the meat pot is dipped in the giant milk pot together with boiling hot water.
Chacham Ovadia defends Maran’s ruling from its critics including the Shach (YD 95:21) and Taz (YD 95:15) and demonstrates that Maran’s ruling is accepted in practice. Indeed, both the Chachmat Adam (48:15) and the Aruch HaShulchan (Yoreh Dei’ah 95:24) accept Maran’s opinion.
Rav Yosef applies Maran’s reasoning regarding the giant milk pot to permit simultaneous washing of meat dishes and dairy dishes in a dishwasher. When released together with the hot water, the soap deposited into the designated compartment renders any food particles in the dishwasher as Pagum. Chacham Ovadia therefore concludes that not only is it permissible to use the same dishwasher for both meat and dairy dishes, but it is also permissible for one dishwasher to be used to wash dairy and meat dishes simultaneously.
Concerns with Rav Ovadia’s Ruling
Rav Ovadia’s ruling seems to be convincing and even airtight. The analogy between a dishwasher and the giant pot discussed in the Shulchan Aruch seems perfect. He cites those who disagree with his ruling and responds to their objections. Indeed, any objection can be rebutted by citing the precedent of the Shulchan Aruch’s giant pot.
Rav Feivel Cohen and Rav Yisrael Rozen, for example, raise concern about food particles that remain in the dishwasher’s filter. They argue that these food particles remain edible, and that these particles are absorbed into utensils of the opposite type, rendering them an unkosher mixture of meat and milk.
However, the Shulchan Aruch was not concerned with this issue since the ashes render the food particles inedible. Similarly, Rav Ovadia argues that the detergent used in dishwashers, which are at least as caustic as the ashes mentioned in the Shulchan Aruch, also renders inedible any food particles remaining in the dishwasher’s filter. Why then is Rav Amar not comfortable with Rav Ovadia’s ruling?
There is one major chink in the armor of Rav Ovadia’s ruling. Rav Forst raises the very serious concern that “in the very beginning of the washing cycle, hot water may start spraying before it has been rendered Pagum by the detergents.” Rav Menahem Genack, Chief Executive Officer of OU Kosher, expressed a similar concern in his Shiurim to Yeshiva University Semicha students in 1988.
Rav Yosef’s description of the operation of a dishwasher shows that he assumed that the water released before the detergent is introduced is cold. However, this is not always the case. If so, if one simultaneously washes milk and meat utensils in the same dishwasher where hot water is released before the detergent, not only are all the utensils rendered non-kosher but one also runs afoul of the Torah level prohibition to cook milk and meat together! Even if one washes meat and milk utensils consecutively in such a dishwasher one renders the dishwasher itself a forbidden mixture of milk and meat, since its walls would absorb both meat and milk taste particles.
Rav Ovadia Yosef wrote a very persuasive responsum permitting washing meat and milk dishes in one dishwasher. However, this ruling presumes that hot water is introduced only after the detergent has been released into the dishwasher. This assumption does not apply to all dishwashers.
While one could try to determine that the dishwasher one purchases releases the water only after the detergent is introduced, this could lead to confusion and complications. If some dishwashers carry with them serious Halachic concerns it is sensible to adopt a policy of not permitting the use of any dishwasher for both meat and milk. Rav Amar’s atypical veering from Rav Ovadia’s ruling might stem from the concern that permitting the use of some dishwashers could lead to serious Halachic violations.
In practice, I advise Sephardic Jews to follow the ruling of Rav Amar and wash milk and meat in the same dishwasher only if one “kosherizes” the dishwasher between uses of the opposite food type by running it through an empty cycle with detergent.Indeed Rav Shmuel Khoshkermann told me that Rav Ovadia’s Teshuvah does not apply to today’s dishwashers due to concern that hot water is released into the dishwasher before the detergent is released.
 In my experience, Ashkenazic women typically are very uncomfortable using a dishwasher for both meat and milk.
 For example, Teshuvot Igrot Moshe Orach Chaim 1:104, Yoreh Dei’ah 2:28 – 29 and 3:10.
 Rav Amar requires running an empty cycle with detergent as a koshering process between uses of the opposite food type.
 Rav Ovadia and Rav Yitzhak both prefer that the dishwasher not be used to wash meat and milk items simultaneously.
 It is particularly touching to read the conclusion of this Teshuvah where Rav Ovadia, who wrote this responsum in his elder years, cites his son Rav Yitzhak, as support for his ruling. This is a beautiful example of the synergetic relationship between this outstanding father and son pair. In fact, Rav Aharon Soloveichik noted at the funeral of his brother Rav Yosef Dov Soloveitchik that Rav Yosef Dov and his father Rav Moshe rank among the great father son learning pairs in Jewish history. These include, according to Rav Aharon, Rabi Shimon Bar Yochai and his son Rabi Elazar and Rav Akiva Eiger and Rav Shlomo Eiger. We now may add Rav Ovadia and Yibadeil L’Chayim Arukim Rav Yitzhak alongside these outstanding father son learning teams.
 Obtaining reliable information about precisely how a particular dishwasher operates is not necessarily a straightforward and simple process.
 I asked Rav Khoshkermann if one is permitted to eat at a home that still relies on Rav Ovadia’s leniency and simultaneously washes meat and milk utensils in the dishwasher. He replied that he did not know how to respond. Rav Hershel Schachter agreed (personal communication) that this is a serious problem (unless one is convinced that there is sixty times the amount of water to nullify the meat and milk particles - which is subject to considerable debate as noted in Rav Rozen’s aforementioned article). There is, however, a defense for the use of a dishwasher for both meat and milk when done consecutively. Once the initial full cycle runs, let us say of meat dishes, the dishes experience the entire wash cycle, so the hot water with soap/detergent has already rendered any particles Pagum (unfit for human consumption), so the next wash should be 'ready' for a milk run without a koshering run in between.