Aluminum foil turned brown; should I be worried?

December 14, 2008 2:15 PM

I made (delicious!) mashed sweet potatoes in a conventional stove-top pot, covered the pot with Reynolds aluminum foil, then put it in a still warm but off-position oven (simply to save stove-top and counter space for the moment while wanting it to stay at room-temperature). Retrieving the pot a few hours later, I found that the inner surface of the foil had turned a light brown. It was also moist from the condensation, but had remained clean, unoiled, unburnt. I've cooked quite a bit and never seen this. What could be causing such discoloration, and is it a worrying symptom (of, say, toxins in the foil)?

Plus: if -- as FAQs on the subject seem to agree -- kitchen aluminum foil is 'the same' on both sides, even though one side is shiny and the other matte, does this discoloration phenomenon indicate otherwise?

posted by taramosalata to science & nature (6 comments total)

Acidic foods will discolor, and eventually eat through aluminum foil where there is contact.
posted by tomierna at 3:22 PM on December 14, 2008

I frequently see such discoloration on aluminum foil that has been exposed to hot, steamy foods (recent example: fresh-from-the-oven scones) and left to sit for a while. I don't know what it is, but think it's normal and not harmful. It looks a lot like oxidization on silverware, so I always assumed it was a similar kind of thing.
posted by Orinda at 3:25 PM on December 14, 2008

it could have made a battery, but when this has happened to me, a) the foil is in direct contact with the food, b) the oxidation is patchy, not uniform, and c) the oxide is black.
Even if it isn't a classic lasagna cell, my money's on some kind of oxidation/corrosion, not toxins. Eat it.
posted by pullayup at 3:29 PM on December 14, 2008

Normal. Common.

If you're not going to eat them I will.

Oh, and to answer your bonus question: At an educated guess, since the "back" side is rougher it has a much larger surface area and is therefore more susceptible to oxidation.
posted by Ookseer at 1:57 AM on December 15, 2008

Thanks for all answers so far. (Very impressed, pullayup, too, by your links to battery-making and the _Applied Spectroscopy_ article, and by Ookseer's stab at the bonus question.) I should clarify that no part of the foil came into direct contact with the food -- which I did eat, promptly -- hence the surprise. But it does sound like mere exposure to steamy heat can cause discoloration....
posted by taramosalata at 11:34 AM on December 15, 2008

pullayup, you linked to an awesome paper. If I read that right, aluminum foil is 0.4% iron by weight (or was, in 1997), and more iron comes off the discolored bits than the shiny bits. Probably in the factory the foil goes over a bunch of steel rollers, which wear down very slowly. I guess in warm, moist air the iron can leach out of the foil and rust? Rust is brown.

Kitchen metallurgy tonight!
posted by fantabulous timewaster at 11:24 AM on December 19, 2008