Which side of an aluminum foil should I put in contact with food?

Question

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

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

• Both sides have the 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 sides 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 the 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.

Answer 1

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.

Answer 2

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.

Answer 3

It makes no difference; but, if the food is wet and has salt or is acidic it will corrode through in a matter of hours (at ambient temperature, not if immediately frozen). The different appearance is because the final rolling is done with 2 layers of foil. One side of each sheet contacts the polished steel roller, the other side faces the second sheet of foil.

Answer 4

Back when I was in my final year of high school, the subject of our major chemistry assignment was Aluminium. I remember the class being taken on a tour of an enormous aluminium factory, where we stood overlooking the massive rolls of aluminium and the successive stages it went through during production. I distinctly recall the foreman/guide telling us that one side of the aluminum foil was coated during the rolling process with an extremely thin layer of another substance, and this is why it had a duller appearance. I don't remember what the substance was (I believe he used the words 'plastic substance' but I doubt he was referring to the usual concept of 'plastic'), or any reason given for this. This was in the late 80's. I never forgot it, & from that day forward I always used the shiny side against any food.
Some people don't believe me when I tell them this, but it's absolutely true, and I see no reason why the man would have made something like that up.

Answer 5

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.