Since aluminum foil has a shiny side and a dull side, many cooking resources say that when cooking foods wrapped or covered with aluminum foil, the shiny side should be down, facing the food, and the dull side up. This is because the shiny side is more reflective and so will reflect more of the radiant heat than the duller side.
Is this true?
Although most resources still say it is true, the shiny side should be down, some newer sources say that it makes no difference which side of the aluminum foil faces up.
For example, Robert L. Wolke in What Einstein Told His Cook and America’s Test Kitchen say that it makes no difference at all in cooking. You can place either side in either direction whether cooking or freezing food with aluminum foil.
However, their explanations make little sense. They are in effect pseudo-explanations.
Both sources provide the following, similar explanation (paraphrased):
Aluminum foil has a shiny side and a dull side. Many people believe that it matters which side is used up or down. The truth is that it makes no difference at all. The reason the two sides look different is due to the manufacturing process. When the sheets of aluminum are rolled out, the side that comes in contact with the rollers come out shiny..
This explanation does not explain why the shiny side makes no difference. It simply reiterates, with a bit more explanation, that foil does, in fact, have a shiny side and a dull side. Not very informative!
Regardless of why one side of aluminum foil is shiny and one is dull, it stands to reason that the shinier surface will be more reflective than the duller surface. The explanations given seem to indicate that since the shiny and dull sides are simply a by-product of the manufacturing process and are not purposely put there, they make no difference. The explanation does not support the claim. So what’s the truth?
The truth is that the shiny side of aluminum foil is only a little bit shinier than the dull side. While some small amount of additional energy will be reflected by the shiny side, the difference is so small that it will make no practical difference in cooking. To say that there is no effect whatsoever is inaccurate and it probably still is a little more efficient to cook with the dull side out. However, when measured over time in high temperatures, the difference is so slight that there should be little discernable change in cooking times. This may seem like a lot of explanation to come to the same conclusion, but it is not my purpose to give inaccurate explanations!
Why Doesn’t the Shiny Side of Aluminum Foil Make A Difference?
Yes, the shiny side is more reflective but reflective of what?
There are three basic ways heat can be transferred: conduction, convection, and radiation. Conduction is when heat is transferred through contact of one object with another hot object. This is what happens when we cook on the stove.
Convection is the transfer of heat by the physical movement of surrounding fluid (liquid or gas). Radiation is light waves, radio waves, microwaves, X-rays, etc. carrying heat energy from one surface to another.
Any object with a temperature above absolute zero emits infrared radiation. This means that the heated coils, sides, and racks in your oven are emitting infrared energy. Even the heated vessel and the heated food itself emits this energy.
However, when you cook food in an oven, the major source of cooking heat is convection. The hot air of the oven transfers heat to the food being cooked. Don’t be confused by “convection ovens.” All ovens use convection, convection ovens just use a fan to make the convection more efficient. Only a little of the heat transfer in an oven is through infrared radiation, which is invisible light rays.
A shiny surface should make no difference to convection but it will make a difference to radiation. Shiny surfaces reflect waves more than dull surfaces. The reason one side of the foil is more shiny than the other is because it is smoother and has less little imperfections: hills and valleys. So, the shiny surface of the foil should reflect more radiation than the dull surface, which will be better able to trap incoming waves rather than reflect them back. But it should not affect convection, the major source of heat transfer.
Consider a baked potato. When wrapping potatoes in aluminum foil for baking, you probably tend to place the dull side out. In reality, in the long length of time it takes to bake a potato, either side will work basically the same. The foil will be heated through convection and this energy will be transferred to the potato, and as the moisture in the potato heats up, the potato is cooked through steam.
Rather than which side is out, a bigger difference may be made by how tightly the potato is wrapped. Any air trapped in the aluminum foil pouch and surrounding the potato may act as an insulating barrier, slowing the transfer of heat. So, wrap your potatoes tightly before baking.
So, it is up to you which side of the aluminum foil goes up.
How is Aluminum Foil Made?
Since I already mentioned the manufacture of foil thought the explanations given above, I may as well write a bit more about how foil is made.
The manufacture of aluminum foil is similar to making pasta at home. A large block of almost pure aluminum is rolled through giant steel rollers, several times, reducing the thickness of the aluminum block and spreading it out to make it longer. Lubricants are added to facilitate the operation. At each successive pass through the rollers, the thickness is reduced. The process is repeated until foil thickness is reached, and the large flat sheet is then split into the desired widths.
This may seem simple enough, but the actual process can be tricky. For example, as the aluminum is rolled out, it heats up. If it heats up too much, it can stick to the rollers, so the roller pressure must be carefully controlled.
Once the sheet of aluminum is 5mm thick, it must be rolled again in the cold-rolling stage. First, the sheet spooled into a roll and then it is fed into the cold-rolling mill for the final stage of milling. It is at this point that the shiny and dull sides of the aluminum are created. Since the aluminum is now so thin, the tension needed to feed it through the cold rollers could easily break it. So, the sheet is doubled. The sides of the aluminum that come in contact with the steel rollers become more polished and shiny, and the sides of the aluminum that come in contact with itself become duller.