One important mechanism is the migration of moisture from the starch granules into the interstitial spaces, degelatinizing the starch. The starch amylose and amylopectin molecules realign themselves causing recrystalisation. This results in stale bread 's leathery, hard texture. Additionally, pleasant "fresh '' flavor is lost to the air, and often unpleasant flavor is absorbed from it as well, especially in a confined space with other food such as when in a refrigerator.
why does starchy food like bread become hard and stale when it dries out
the migration of moisture from the starch granules into the interstitial spaces, degelatinizing the starch