In general, it does not matter which side of an aluminum pan is up when using it for cooking. Aluminum pans typically have the same material and finish on both sides, so there is no specific "up" or "down" side.
However, there are a few exceptions to consider:
Nonstick coating: Some aluminum pans may have a nonstick coating applied to one side. In such cases, it is important to use the pan with the nonstick coating facing up. This allows for easy food release and prevents scratching or damaging the coating.
Ridged or textured surface: Certain aluminum pans, such as grill pans or roasting pans, may have a ridged or textured surface on one side. The ridges or texture are designed to enhance cooking or create grill marks. In these cases, you would typically use the side with the ridges facing up to cook your food.
For regular, flat-bottomed aluminum pans that do not have any specific coatings or textures, you can use either side interchangeably. Just ensure that the pan is clean and properly preheated before cooking to achieve the desired results.