Does it cook faster? slower? Does it matter?
"The difference in appearance between dull and shiny is due to the
foil manufacturing process. In the final rolling step, two layers of
aluminum 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."
So the answer is that shiny or dull has no affect on the cooking or storing of food but....
now the real question....how do you use it.....shiny side out or facing the food?