Which side of Aluminum Foil should I use, the shiny or the dull side?
Actually, it makes no difference which side of the aluminum foil you use—both sides do the same fine job of cooking, freezing and storing food. The difference in appearance between dull and shiny is due to the foil manufacturing process. In the final rolling step, two layers of foil are passed through the rolling mill at the same time. The side coming in contact with the mill's highly polished steel rollers becomes shiny. The other side, not coming in contact with the heavy rollers, comes out with a dull or matte finish.
Aluminium foil typically has a highly reflective side and a more matte side. This is a result of common manufacturing processes. As aluminium foil is easy to tear, the foil is sent through machines in pairs. The side where the aluminium foil was in contact with the other sheet is more matte than the exterior side. This unconformity of finish has led to the perception that favoring a side has an effect when cooking. While many believe that the shiny side's reflective properties keep heat in when wrapped on the interior and keep heat out when facing exterior, the actual difference is imperceptible without instrumentation.
The shiny side is slightly better reflector of heat. Face that side of the foil in the
appropriate direction. To keep things cold, put the shiny side on the outside -- that
will reflect incoming heat. To keep things warm, face the shiny side inward toward the
hot food -- to reflect the heat that is trying to escape back into the food.
Note that people often bake potatoes with the shiny side out -- that is because it makes
for a better (prettier) presentation. In reality, baking them that way reflects the
incoming heat on the outside, slightly slowing the cooking process.
Of course, in reality, which side of the foil faces where makes little difference. It does
make for lively conversation, though.
hope this helps :)