Baking powder, soda or corn starch should be stored in a dry cupboard away from heat and excess moisture. You should only measure with a dry utensil and replace the lid promptly after each use. If you use a utensil that has any moisture on it, you can cause a reaction inside the can and your baking powder may not work properly. Storage in a refrigerator or freezer is not recommended, as the condensation from your refrigerator can also cause moisture to form inside the can, causing a reaction.