In practice, no. The shiny and dull sides result from the manufacturing process and have no measurable effect on cooking; it does not matter which side faces the food. The only exception is specialty foil with a nonstick coating: the coated side is clearly marked on the package and should face the food. Otherwise, foil has no right or wrong side. Checking the foil thickness is more useful than choosing a side.
The difference comes entirely from manufacturing. Aluminum cannot be rolled into its final thinness as a single layer because it would tear at that thickness, so two layers are placed together and rolled thinner at the same time. The outer surfaces that touch the rollers come out polished-looking and shiny, while the inner surfaces that touch each other remain dull. In other words, the dull side is not specially manufactured; it is simply the side that did not touch the rollers. The two sides have exactly the same chemical composition, thickness, and behavior when heated. The same manufacturing process does not apply to wax paper or parchment paper; those are treated differently on each side, and the package indicates which side to use.
A common claim is that the shiny side reflects heat and the dull side absorbs it. This is not entirely wrong from a physics perspective, but the scale of the effect is misunderstood: the difference in reflectivity is very small and has no measurable effect on cooking time at oven temperatures. Foil transfers heat mainly through contact, not radiation. Testing the same recipe with each of the two sides facing the food gives the same result. Turning the foil into a rule distracts from the things that really make a difference: a tight seal and the foil’s thickness.
Three things matter most. First is thickness: thin foil tears easily in the oven, so use heavy-duty foil for heavy items and long cooking times. Second is the seal; if the edges are not folded over twice to close them, steam escapes and foil-wrapped steaming does not work. Third, the foil should be stretched taut over the pan; wrinkled foil leaves air pockets underneath, and food cooks more slowly at those spots. With nonstick-coated foil, the coated side should face the food, as stated on the package. Lining the bottom of an oven tray with foil is not recommended; the foil reflects heat back from below and disrupts the oven’s airflow.
Two cautions apply regardless of which side faces the food. Do not put aluminum foil in a microwave; it reflects the waves, can spark, and can damage the appliance. Also, do not leave foil in contact with highly acidic or very salty foods for a long time; this can create small holes and gray spots in the foil and give the food a metallic taste. Store dishes with tomato sauce, lemon, or brine in a glass container instead of foil. Apart from these cautions, which side faces where truly does not matter.
A Note for Readers
This content is for informational purposes only and is not a substitute for medical advice. Consult your doctor before making any decision about your health.