Aluminum foil has two sides: a matte one, and a shiny one.
My question is: which one should I put in contact with food?

Criteria are lowest risk of toxicity and best qualities for food preservation (e.g. do not favor oxidizing, or react with acid nor alkaline ingredients)


For information, I've come across these several explanations; they are however contradictory (and might not be accurate):

  • Both side have same properties. The difference in terms of appearance is only due to the manufacturing process. (Two foils are laminated at the same time, so each foil is in contact with the rolls on one side only — hence the asymmetrical look).
    In this case, we can use both side of the foil.
  • Aluminum foil is actually made of two layers: one of aluminum, another that is coating. The surface of the aluminum layer that is in contact with atmosphere oxidizes and turns into aluminum oxide — hence the matte side.
    In that case, we should use the shiny side, since aluminum oxide is not that healthy.
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    Aluminum oxide is very inert, so I'm not sure how one can imply that it is not healthy. Not that I'd suggest eating sapphires, but still... – Jon Custer Oct 17 '16 at 15:30
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    I'm mooting for the first argument. Even if the second argument is true, it still wouldn't matter which side you use; because whatever manufacturers use to coat the shiny bit it's gotta be safe (they wouldn't market it if it wasn't, aluminium foil like that has pretty much no other use than to wrap up food), also I wouldn't bother about the oxide layer either, it 'sticks' to the aluminium pretty fast. – paracetamol Oct 17 '16 at 15:31
  • @JonCuster : I'm not implying these statements are true: I actually don't know. I just listed them for highlighting I looked for answers by myself, and as potential explanations (that can be falsified because you explain why they are false as you did). || I've also found this question, but I am looking for an answer rather based on chemical reaction (hence here asked). – ebo Oct 17 '16 at 15:43
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    Both sides will be coated with aluminium oxide which is inert and safe. Shiny or matt has nothing to do with it. – matt_black Oct 17 '16 at 16:15
  • If i have a paper and I rub one side with sand paper it become rough, but that does not change its chemical properties, it still remains paper. The same way one side of the aluminum is optically reflective and the other is not. – Vedant Oct 17 '16 at 16:38

After some thought, I guess I'll go with the first argument, that seems plausible (though I don't think it's accidental). As for coating one side with a layer, that seems like a bit too much.

Assuming the second argument is correct, then it's most probable that it doesn't matter which side you use. Because if one side is a 'coated', that 'coat' has got to be safe. Since well, aluminium foil is used pretty much only for packaging food, I don't think people could go about selling Al foil coated with some toxic film for the past couple of decades or so without someone pointing out something very wrong.

Now if it's the first argument, then it still doesn't matter which side you use. Aluminium oxide isn't really as dangerous as out make it out to be, especially if you're dealing with an oxide film that's still on the foil (Hogging a tablespoon of powdered aluminium oxide is another story altogether...). We sometimes call that process (aluminum developing an oxide coat) Passivation, because the foil is rendered 'passive' or 'inert'. Why else do you think it takes so long to get aluminium to react with acids or alkalies?

In a nutshell, regardless of which argument is correct, it'd be perfectly safe to use either side of the foil.

The reflective surface will reflect heat and the matte side will absorb heat. If you're baking or defrosting, the matte side will absorb more radiant heat and reflect less infrared heat while the shiny side will reflect more of both, so it makes more sense to bake and defrost with the matte side facing up. Conversely, during freezing, it makes more sense to reverse the sides as the shiny side face up would reflect heat away from the food.

I've always felt that one of the more practical reasons for creating two visually dissimilar sides on the foil, is to make it easier for you to identify which side you had initially used to wrap some food in. Say, you had just hurriedly unwrapped a cheeseburger and you don't feel like finishing it off. If you re-wrapped using the wrong side, you just get greasy fingers, which is quite annoying.

EDIT: I've read @Vedant's answer and he seems to have provided a good explanation as to why the two surfaces are dissimilar. But I wouldn't entirely dismiss the reason I've mentioned (identification), since, from my personal experiences...I'm pretty sure my keyboard would be covered in grease by now.

But all the same, I've included @Vedant's explanation as well, to give you the complete package.

  • Thanks for citing my answer. If i have a paper and I rub one side with sand paper it become rough, but that does not change its chemical properties, it still remains paper. The same way one side of the aluminum is optically reflective and the other is not. – Vedant Oct 17 '16 at 16:38
  • Well, I didn't know about the advantage it holds when you're baking/freezing before I saw your answer, so kudos to you! – paracetamol Oct 18 '16 at 12:09

Surprisingly the comments did not yield the correct answer

The reflective surface will reflect heat and the matte side will reflect less heat . If you're baking or defrosting, the matte side will absorb more radiant heat and reflect less infrared heat while the shiny side will reflect more of both, so it makes more sense to bake and defrost with the matte side facing up. Conversely, during freezing, it makes more sense to reverse the sides as the shiny side face up would reflect heat away from the food.

Thought it is true that this happens because of the manufacturing process, it is also true that both have different uses.

EDIT:

Also note, that which side facing is out causes very little difference because aluminum is a very good conductor of heat.

EDIT 2:

From Aaron Abraham's answer

In a nutshell, regardless of which argument is correct, it'd be perfectly safe to use either side of the foil.

  • In a nutshell, regardless of which argument is correct, it'd be perfectly safe to use either side of the foil. – Vedant Oct 17 '16 at 16:41
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    I don't think it is accurate to say the matt side will absorb heat. It might be slightly less good at reflecting it, but that is a minor issue in practice. – matt_black Oct 17 '16 at 20:50

I have noticed in my years of use, that when you use aluminum foil to cover anything tomato based, if you use the matte side, the acid from the tomatoes eats through the foil. So, I have always put the shiny side down or inward, toward the food.

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    Neither side of the aluminum foil is coated. (Source.) Tomatoes are acidic. It's recommended not to let salty or acidic foods touch either side of aluminum foil. See here. – unforgettableid Mar 31 '17 at 17:55

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