Heating up Food in a Non-Kosher Oven
Is there a way I can use the toaster oven at work to heat up food?
The Aish Rabbi Replies
Yes, the typical way of doing so is to fully double-wrap your food, such as with aluminum foil. In addition, when you afterwards eat, you should discard the outer layer of foil.
The brief background is as follows. Old non-kosher food or grease might remain in the oven. There are a few ways they could potentially render your food non-kosher. One is by direct contact. If you would put your pan on a rack which is actually dirty from non-kosher residue, it might cause your pan to absorb non-kosher which will then be transferred to the food.
A second issue is the steam or vapor. In some scenarios, non-kosher residue, even at the bottom of the oven, might cause either of these to spread throughout the oven and render your food non-kosher (if it wasn’t covered). Alternatively, if your food would not be covered, it might emit steam which will spread throughout the oven, absorb non-kosher taste, and land back on your food.
How likely such scenarios are and the conditions required for them to actually render your food non-kosher are more complex. But again, the simple solution is to avoid all possible questions by double-wrapping. Once you do this, at most the outer cover would become non-kosher. So long as the two layers are dry, there is no concern that the non-kosher absorbed in the outer layer will spread to the inner one, since absorbed taste does not spread from one utensil to another without the medium of liquid. Thus, although the outermost foil must be discarded, at that point you can go ahead and enjoy your leftover cholent!
(Sources: Rema to Shulchan Aruch Y.D. 92:8, 108:1.)