My sunflower oil foams like crazy. I've been reusing this oil for a while now and was going to discard it, however a friend of mine told me this oil was still looking good. Relatively clean (I filter it regularly), no off smell and it doesn't smoke (in my fryer, that is). I only fry potatoes in this batch of oil to avoid flavor transfer. These are cut into French fries, chips or straw potatoes. Specially the chips and straws retain a lot of water, and I guess that has something to do with it.
Also, I noticed the frying takes longer with the foaming oil.
Edit: I've tried poaching(?) the French fries, but they foam during poaching. Then when frying, they really foam. I have to keep an eye on it or the oil spills over the top. Anything I can do about it, or should I just go ahead and discard the oil altogether?
Edit: Serious Eats has an article about frying oil.