The question is frequently asked if it is permissible or not to put meat dishes and dairy dishes into the same dishwasher. For that matter, would it be permissible to put meat and dairy dishes in the same dishwasher at the same time?
Before we analyze the answer to the question, we need to review 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. Following this process the water shuts off, and a heating element is turned on to dry the dishes. Once dry, the process is complete.
Based on the above process, it would seem that the procedure would render dairy and meat dishes washed together un-Kosher. It appears there is potential during the hot cycle for remnant food particles to become cooked. This poses a big problem, for cooking dairy foods on meat plates renders the meat plates un-Kosher and vice versa. So seemingly putting dairy dishes together in the dishwasher with meat dishes is a problem.
However, Chacham Ovadia Yoseph has a very important Teshuba (answer) granting permission for this in his book ‘Yabia Omer’ in Helek 10. There, he writes an opinion based on what Maran wrote in the Shulchan Aruch, in Yore De’Ah siman 95:4. Maran writes of the case 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 says that it would be permissible to use this giant dairy pot to clean meat dishes so long as a distasting agent such as ‘Efer’ (Ashes) or soap is added. Such an agent would render the milk in the walls of the pot ‘pagum’ (bitter, unsavory, unfit, spoiled), thus allowing the insertion of a meat vessel for cleansing. This leniency even applies if the meat pot is dipped in the giant milk pot together with boiling hot water.
Chacham Ovadia Yoseph applies Maran’s reasoning by the giant milk pot to permit simultaneous washing of meat dishes and dairy dishes in a dishwasher. The soap deposited into that designated compartment when released together with the hot water renders the dishwasher cavity ‘pagum’. Chacham Ovadia Yoseph therefore comes out 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. This is also the opinion of Chacham Matloub Abadi in his Sefer ‘Magen Ba'adi’.
It should be pointed out that Rabbi Feinstein was against this opinion and said in his Teshuvot that it would only be permissible to wash meat and dairy dishes simultaneously if there is no other option. He prefers that one utilize the dishwasher for a dairy cycle and then afterwards for a meat cycle (or vice versa.) No one really argues on this preference but from the letter of the law, it would be permissible to wash meat and dairy dishes together in the dishwasher. It is permissible so long as a cleaning agent is used in the process. (Any type of dishwasher is permitted, and it does not need to be of a stainless steel interior.) But for those who are strict and separate their meat washes from their dairy washes, or for those who use separate dishwashers altogether, upon them will come blessing.