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I assume that you are talking about doing something like wrapping something if foil and placing it in the oven. If so, which side you have facing in and which side you have facing out makes very little difference at all.
If you are baking something wrapped in foil, you should probably have the dull side of the foil out. The dull side reflects heat a bit less than the shiny side. This means that cooking with the shiny side out would make your food cook more slowly.
But it does not really matter much -- the effect is small.
Aluminum foil used for wrapping food, either while cooking or for safe storage of food, generally has a bright finish on one side and dull or mat finish on the other side. The manufactures of the foil do not provide any instructions about which side of the foil should be the outer side when wrapping the food. Based on this. I believe that it does not make significant difference to the the quality of cooking or preservation of food, whichever side is the outer side, although the points about better absorption of heat by surfaces with dull finish is theoretically correct.
Another consideration for decision on keeping a particular side up is the appearance. Some people may prefer the bright side to be visible from outside while others may prefer the dull side.
Finally the question of hygiene can also be an important criteria. The aluminum foil rolls come with the dull side in and bright side up. In this situation the brighter side of the foil in the roll is more likely to be contaminated wit other substances in surrounding. Thus it may be safer to keep the bright side on the outside.
I love this question because when I first started cooking I always thought that the shiny side had to be the side that was up. However, after teaching science and testing the sides as well as engaging my students in research on aluminum foil, I can honestly say that it does not matter which side is up.
Both sides work the same. People often think that because there is a more polished side that one particular side is for use. The only difference is that the smooth shinier side is more reeflective than the other side, but it does not matter in relationship to food or heat retention.
Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass.
It is nearly impossible to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny.
This difference in the finish has led to the perception that favouring a side has an effect when cooking. While many believe that the different properties keep heat out when wrapped with the matte finish facing out, and keep heat in with the matte finish facing inwards, the actual difference is imperceptible without instrumentation.
I'm pretty sure it doesn't matter, both sides should be the same.
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