I wanted to know what the halacha is about l’chatchila using a fleishig oven for milchigs. I know it is recommended to cover up the milchigs with silver foil (either single or doubled over) because we are worried about zeyah. {let us say- it is an over without any dirt…} Should we all be machmir with zeyah or are you yourself machmir if you don’t see the zeyah?
Answer:
The problem of using a meaty over for milky foods is limited to foods that are moist, and therefore give off steam, which released the meaty steam absorbed by the walls of the oven. If the dish is completely dry, it is permitted to use a meaty over for milky foods, though as an additional precaution, some poskim prefer that different racks be used for meat and dairy, or that the rack be covered with new aluminum foil before the dairy dish is placed in the oven.
However, poskimwrite that because is it hard to determine which food is “dry,” and because any time we discern steam rising from the dish it is considered “moist,” it is generally preferable to have two ovens, one for meaty and one for milky. If the same oven is used for meaty and (moist) milky foods, the foods must either be covered up well — not advisable, because covers can come off or tear — or the oven kashered in between use for milky and meaty dishes — though some rule that this procedure should not be used on a regular basis (Chelkas Yaakov 2:136).
Sources: Concerning covered foods, see Shulchan Aruch, YD 108:1; Ksav Sofer 54; Igros Moshe YD 3:10; concerning dry foods, see Igros Moshe YD 1:40 (and 1:59, concerning baking a challah) based on Pischei Teshuvah 92:6.
