Why do some breads stay moist and others like french bread go stale so quickly?
Staling takes place in the days following baking, and seems to involve the loss of moisture: the bread interior gets dry, hard, and crumbly. It turns out that bread will stale even when there’s no net loss of moisture from the loaf. This was shown in the landmark study of bread staling in 1852, when the Frenchman Jean-Baptiste Boussingault showed that bread could be hermetically sealed to prevent it from losing water, and yet still go stale. He further showed that staling is reversed by reheating the bread to 140° F/ 60° C: the temperature, we now know, at which starch gelates.
Staling is now understood to be a manifestation of starch retrogradation, the recrystallization, water migration out of the granules, and hardening that take place when cooked starch is then cooled (p. 548). The initial firming of the freshly baked bread loaf, which improves its ability to be sliced, is caused by the retrogradation of the simple straight-chain amylose molecules, and is essentially complete within a day of baking. The majority of starch molecules, the branched amylopectins within the granule, also retrograde. But thanks to their irregular structure, they form crystalline regions and expel water much more slowly, over the course of several days. This is the process responsible for the undesirable firming in texture after the bread has become sliceable.