I hope you dont find this too offensive bobby, but I think this is really interesting.
A good chef friend of mine (shes a member here but declines to post, I think shes shy

) was telling me that one of the first things they are taught is to not use disimilar metals (eg and aluminum pot and a stainless steel spoon).
She mentioned that if you do this with, say, bechamel sauce or custard, you quite likely end up with a disgusting grey colour.
Presumably its because an electolytic cell (battery) is set up between the two disimilar metals. I was suprised that the reaction rate is so high but I guess its to do with the elevated temperatures.
Just goes to show huh? Maybe its not just onions that can cause bitterness in the base?
