At any rate, it must be food grade, so burning it out should be a sufficient precaution. I'm using the Asahi large beer cans to cut out aluminum sheeting from their sides, used them for wind screen with my pocket-rocket style burner many times (use sheets from two cans, joined by horizontal slits meeting at the edges), but didn't noticed any material burning on the inside anywhere, so, most likely, at least in these cans it's just an Aluminum oxide layer.
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