Generally, very little. There will be some potato chips, coffee in various forms (chocolate covered being best for nutrition, IIRC), a little candy once in a while, and human corpses, and usually, that’s it.
However, there’s one room type (the one with locked room of water that has leaked out) that has two refrigerators where actual useful food can spawn. It’s random, so it can be nothing, a beer, a gallon of milk, a frozen dinner… all the way up to several pizzas (most I ever found in one).
There is also the armory area of the barracks rooms, which can sometimes have MREs, but if you’re looting armories, you’re not worrying about food anymore, eh?