(Today’s Halacha was presented by Rav Shemuel Pinhasi of Jerusalem)
The Talmud in Masechet Aboda Zara records a debate concerning the issue of "Reha Milta," meaning, the status of the fragrance emitted by non-kosher foods. The practical implication of this question is whether a person may walk in the street near a store where non-kosher food is prepared, and thus from which the smell of non-kosher food emanates. Halacha accepts the view that "Reha Lav Milta," meaning, we do not afford significance to the smell of non-kosher food. Hence, although one should not have specific intention to avoid the smell of non-kosher food, he may walk past a facility that emits the smell of non-kosher food without concern, since smelling the food does not constitute any violation.
The exception to this rule is forbidden foods that are specifically used for fragrance. For example, the Rashba (Rabbi Shemuel Ben Aderet of Barcelona, 1235=1310) ruled that one may not smell roses of a tree that is Orla (that had been planted within the previous three years), or wine used for idolatry if it contains special spices for fragrance. Since these items are specifically intended for fragrance, one may not smell them. Generally, however, as mentioned, it is permissible to smell forbidden foods.
A separate issue concerns the status of "Ze’a," the "sweat" or steam that a food produces when it is heated. Unlike fragrance, the steam of forbidden food is deemed equivalent to the food itself, and is thus forbidden. Likewise, steam produced by meat that comes in contact with a dairy food renders that dairy food forbidden for consumption (and vice versa). Since the steam produced by the meat is deemed equivalent to the meat itself, a dairy food that absorbs this steam is no different from dairy food that absorbed actual meat.
For this reason, it is forbidden to use a microwave oven for both meat and dairy foods. The steam produced by meat as it is cooked is absorbed by the interior walls of the microwave, which are no different than the walls of a pot. Therefore, if one would then cook a dairy food in the microwave, that food would absorb the taste of the meat food and would thus be forbidden. Of course, the same would apply to cooking meat in a microwave that had been used for dairy foods.
One may not use a microwave oven for dairy foods even after twenty-four hours have passed since it was used it for meat (or vice versa). Even though the cooking was done in violation of Halacha, after the fact the food is permissible since the cooking took place more than twenty-four hours after the microwave oven was used for meat. But if one cooked dairy foods in a microwave oven within twenty-fours after it was used for meat, the food must be discarded.
One may cook meat in a dairy microwave oven (or vice versa) only if he ensures to cover the food, with either the special microwave covers sold in stores, or with saran wrap. The food must be completely sealed so that no steam from the microwave penetrates the food.
If one heated sambusak in a meat microwave, or kibbeh in a dairy microwave, the food is nevertheless permissible. The cheese in the sambusak and the meat in the kibbeh are covered by dough, and are thus not directly exposed to the steam in the microwave. Therefore, although this certainly should not be done, if one mistakenly warmed sambusak in a meat microwave, or kibbeh in a dairy microwave, he may eat the food.
Finally, Hacham Ovadia Yosef permits cooking parve foods in a meat or dairy microwave oven and then eating those foods with meat or dairy foods. This ruling is based on the Halachic principle known as "Noten Ta’am Bar Noten Ta’am," or "second-degree taste." The meat taste absorbed by the parve food in the microwave oven does not have the capacity to then render a dairy food forbidden when the parve food comes in contact with it, and there is thus no problem at all with warming a parve food in a meat or dairy microwave oven.
Summary: One may smell foods that are forbidden for consumption, with the rare exception of food items that are specifically intended for fragrance. It is forbidden to prepare meat foods in a dairy microwave oven (or vice versa) unless the meat food is thoroughly covered. If one did cook meat food in a dairy microwave (or vice versa), the food is forbidden for consumption, unless the microwave had not been used for dairy foods in twenty-four or more hours. One may cook parve foods in a microwave oven. It is forbidden to prepare sambusak in a meat microwave oven or kibbeh in a dairy microwave oven, but if one did so the food is permissible for consumption.