So we buy our flour in bulk from a local grain mill. Since it's literally just grain that is grown and ground it has a lot of natural yeast and such in it. They reccomend freezing it if you're not going to use it fast. So I froze it for awhile then locked it up in a large food grade tub in my kitchen. And then I just unseal it when I need to use it and keep it clean and bug free. I imagine if you had your flour willy nilly in the pantry it could get contaminated easy, especially if you don't bake often