Natural aging occurs when freshly milled “green” flour is exposed to air for several weeks or more. By naturally aging flour, air is added to it. Air is a powerful additive, causing two main changes. First, it whitens the flour. Second, it strengthens the gluten that forms from flour.
Actually, the active ingredient in air is oxygen, which is considered an oxidizing agent. Oxygen oxidizes the carotenoid pigments in flour, changing their chemical structure so that they absorb less light. This makes the flour appear whiter and brighter. Oxygen also oxidizes gluten-forming proteins, allowing them to form stronger gluten. Yeast doughs made from aged flour are easier to handle than those made from green flour, because doughs with stronger gluten are less sticky and less likely to tear when stretched. The ability to stretch without tearing is especially important when gases expand during proofing and baking, resulting in a higher volume and finer crumb in the baked bread.