When you store open bags of flour in the pantry, it invites air, moisture, and sunlight, which are all accelerants for deterioration that occurs slowly over time.

These elements create chemical reactions in the flour as air and oxygen, especially, interact with the fat content of the flour grain. This interaction is responsible for spoilage.

Even when bags are left open in a dry, cool pantry, this exposure invites moisture, which causes clumpiness, and even pests. Flour that goes bad also has a musty scent.

To solve this problem, store your flour in a simple mason jar with a well-fitting lid, a food canister, or a large Ziploc bag.

So long as you keep your flour well sealed up and away from sunlight, versus in the paper bag it was sold in, you can prevent the vast majority of potential spoilage issues.