One Microwave for Dairy and Meat

A microwave oven uses electro-magnetic radiation to heat food. The answer to this question depends on whether the scent and steam from the heated food are halakhic factors that require consideration in microwave heating, as they would be in a regular baking oven.

1. Mixing the scents that emerge from the meat and dairy foods: From a halakhic standpoint the scents emitted from the heated food can mix even without moisture. There is a halakhic debate whether scent should be considered tangible (‘ריחא מילתא או ריחא לאו מילתא’). The accepted conclusion is that scent is not tangible.

2. Steam: In halakhic terminology steam is referred to as זיעה – literally, ‘perspiration’, referring to moisture. The steam that emanates from cooked food can transfer flavor from one dish to another (whether wafting onto the food directly, or the dish, or the walls of the oven). Steam can affect dishes cooked one after another, since the vapor remains in the oven even after the food has been removed.

Scent

The question of scent is discussed in two Talmudic texts: In Avoda Zara 66b, Abaye and Rava debate whether one may smell yayin Nesech (wine used for idolatrous libations, which is prohibited for drinking); Abaye prohibits actively enjoying the scent, since he argues that scent is tangible (ריחא מילתא), while Rava permits, based on his position that scent is not tangible (ריחא לאו מילתא). The Talmud in Pesahim 76b debates the question of kosher meat which was roasted together with nevela meat (meat which did not undergo proper shechita). Rav prohibits eating the kosher meat, since he holds that scent is tangible, whereas Shmuel permits the kosher meat, based on his position that scent is not tangible.

The majority of poskim ruled according to Rava’s position that scent is not tangible (e.g. Rif, Hullin 31b in Alfasi; Rambam, The Laws of Prohibited Foods, 15:33). However, a difficulty emerges from the end of the sugya in Pesahim, where the Talmud explicitly prohibits eating bread with dairy if the bread was baked together with meat; furthermore, the Talmud prohibits eating fish with dairy if it was roasted with meat. The Rif (ibid.) and Ran (Hullin 32a in Alfasi, s.v. דתני) debated the difference between these cases in the Gemara, and suggested two different understandings of the tangibility of scent. According to the Rif, the allowance is only post-factum (בדיעבד), in correlation with the laws of nullification (ביטול), which state that a prohibition cannot be nullified from the outset (לכתחילה). Conversely, according to the Ran, since scent is not tangible, it is permitted even from the outset.

The Shulhan Arukh (108:1) rules according to the Rif, and states that the allowance is only post-factum.

When one of the foods is covered, or when they are cooked one after another and not simultaneously, there is no need to be concerned about the transfer of flavor through scent; therefore, in such a case, this would be permitted even from the outset.

Steam (Moisture - זיעה)

The Mishna (Machshirin 2:1-2) implies that moisture (in this case, steam) carries the status of the liquid from which it was produced, both regarding its kosher status, and its ritual purity status. The Rosh (Teshuvot ha-Rosh, 20:26) writes that steam from a frying pan with milk, cooking under a pot with meat, will transfer from the dairy to the meat dish:

“Regarding your question about a dairy frying pan, and whether it can be placed under a stewpot of meat. I believe this is prohibited, and even post-factum, if this was the case I would prohibit the stew, since the steam produced by the frying pan is comparable to milk. As it is written in Machshirin, ‘if the liquid is impure, the moisture is impure.’ From here we see that moisture produced by something is the equivalent of that same thing.”

According to the Rosh, the steam is equivalent to the milk; therefore, dairy steam will prohibit a meat dish.

However, the Rosh continues:

“It should be considered whether a boiling stewpot will absorb the moisture, or whether the boiling heat delays the moisture, and in this case it is possible that the frying pan prevents the boiling of the pot on top.”

The Rosh qualifies his statement by questioning whether the heat emanating from the frying pan may prevent the transfer of moisture.

The Shulhan Arukh (92) rules like the former text in the Rosh: “A frying pan of milk placed on the burner under a stewpot of meat: the moisture rises and is absorbed by the stewpot, and prohibits it.”

The Rema adds: “This is all true when the frying pan is uncovered, and the steam rises from the food directly to the stewpot above it. Moreover, it has to be close enough that a hand would recoil from the [heat of the] moisture where it touches the stewpot. But if the hand would not recoil from the moisture, it is all permitted. Likewise, if the frying pan is covered, it is all permitted, just like the case of two pots touching each other, which do not prohibit one another by contact; all the more so with moisture.” The Rema concludes: “But from the outset, caution should be practiced in all these cases.”

Here we see that there is an advantage to covering food to prevent the transfer of moisture or steam to the food; however, all of these leniencies apply post-factum, and from the outset, care should be taken to avoid cooking meat and dairy in the same oven even when they are covered.

The Rema ruled in hilkhot ta’arovot (Yore De’ah 108): “If kosher and unkosher food were baked or roasted together under one pan, uncovered, these are prohibited even post-factum. But [if baked or roasted] one after the other – there is no concern, unless the pan produced moisture from both, and then they are prohibited even when one after the other if they were both uncovered, since in this case the pan served as the cover of the pot.”

Here we see that when two dishes are cooked, one after the other, as long as one is covered, the problem of moisture is not a concern.

This concept was the foundation of the discussion regarding the halakhic status of the microwave oven.

The Shevut Yitzchak (ch. 5) cited R. Shlomo Zalman Auerbach and R. Shalom Elyashiv, who prohibited using the microwave oven for meat and milk one after the other, even if the dishes are covered, since “there is concern that one might forget to cover the dish, or that the dish will not be covered properly throughout the cooking time, and the moisture will come out.” In Sefer ha-Kashrut he cited R. Shmuel Vazner as ruling the same way.

Conversely, R. Bar Shaul concluded in his book Maarchei Lev (ch. 11) that one may cook meat and dairy one after the other in a microwave as long as the dishes are covered, for the following reasons: [a] There is nearly generally little or no moisture on the sides of the microwave, or on its top; [b] The sides are cold and distant from the pot; [c] The fan efficiently removes the steam from the microwave, and the little moisture present is not very hot, since it spreads throughout the microwave; [d] According to many Achronim the prohibition of moisture is Rabbinic in nature (מדרבנן), and this allows for more leniency; [e] The top of the microwave oven is not comparable to the cover of a pot.

R. Nachum Rabinovitch (Siach Nachum 48) writes:

“the walls of the microwave oven are not heated while heating food, and therefore do not absorb the moisture of the food. Even if some of the moisture was absorbed, that which is absorbed is not released into the food heated subsequently, since there is no contact between the food and the walls of the oven – and there is no concern about the scent. A microwave oven may be used to heat meat food as well as dairy, as long as they are separate, but there is no need to wait between heating meat and dairy, as long as the microwave is kept clean, with no obvious remnants of food. Care should be taken to cover one of the two consistently, but there is no need for the cover to be hermetic. Some cover both meat and milk food; this is a stringency from the outset (לכתחילה), but post-factum, if the food was heated uncovered – the food is permitted.”

In conclusion, while care should be taken to cover at least one of the types of food (meat or dairy) when heated one after the other, if you forgot to use a cover, as long as the food was heated one after the other and not simultaneously, both the food and the microwave remain kosher.