What should you do?
This isn’t crucial:
For most cooking tasks, such as wrapping food or lining pans, you can use either side without noticeably affecting the result.
When frying or baking:
If you want to reflect heat away from the food (for example, when cooking delicate foods), you can use the shiny side facing outwards. Conversely, the blunt side can be oriented towards the food when you want to retain more heat and keep the food crisp (for example, when cooking vegetables).
Bottom line:
While there are slight differences, in most cases it is acceptable to use both sides for cooking. Choose the glossy or matte side depending on your preference, but rest assured that it won’t dramatically affect your food!
