Bread kept in a refrigerator becomes firm and dry noticeably faster than bread left out. The reason is that staling is a structural change, not simply drying.
Staling is starch retrogradation
During baking, starch granules absorb water and swell in a process called gelatinisation, which is what turns dough into the soft open structure of a crumb.
As the loaf cools and ages, those starch molecules gradually reassociate into a more ordered crystalline arrangement, squeezing out the water they had taken up.
The crumb becomes firm and crumbly because its internal structure has reorganised, not because the water has left the loaf.
The process has a temperature optimum
Retrogradation runs fastest at temperatures a little above freezing, which is precisely the range a domestic refrigerator maintains.
At room temperature it proceeds more slowly, and below freezing the molecules lack the mobility to rearrange at all.
So chilling accelerates staling while freezing halts it, which is why the freezer preserves bread well and the fridge does not.
Reheating reverses part of the change
Warming stale bread supplies enough energy to disrupt the crystalline structure that has formed, and the crumb softens again.
The improvement is temporary, because cooling allows the same reorganisation to resume, generally faster than the first time.
This is why yesterday's loaf revives in a warm oven and why the revived loaf declines quickly afterwards.
A brief spell of steam followed by dry heat works best, because the surface needs rehydrating while the interior needs warming through to the point where the structure loosens.
Moisture loss is a separate problem
Water does migrate outward over time, moving from the crumb into the crust, which is why a crisp crust softens as the interior firms.
Wrapping tightly slows that migration but also traps humidity against the crust, trading crispness for a longer-lasting interior.
Paper does the opposite, keeping the crust dry while the loaf dries out faster overall, which is why bakery bags and plastic suit different priorities.
Formulation changes the timeline
Fats, sugars and eggs interfere with starch reassociating, which is why enriched breads and cakes stay soft considerably longer than a lean loaf.
Commercial bakeries add enzymes and emulsifiers that act on the same mechanism, extending softness across a supply chain that needs days rather than hours.
Sourdough resists staling partly through its acidity and hydration, which is one practical reason a naturally leavened loaf remains pleasant on the third day.
Slicing accelerates everything, since every cut face exposes crumb to air, which is why an uncut loaf outlasts a sliced one bought on the same morning.