A loaf sealed in a bag still turns firm and crumbly within days, so staling cannot simply be moisture escaping. The change is happening inside the starch itself.

Baking rearranges the starch

Flour is mostly starch, held in granules as tightly ordered crystalline structures that are hard and insoluble at room temperature.

During baking, water and heat force those structures apart, the granules swell and the starch takes on a soft, disordered form that traps water.

That disordered state is what gives fresh bread its soft, elastic crumb, and it is also thermodynamically unstable.

The starch slowly reorders itself

From the moment a loaf cools, the starch molecules begin drifting back towards their ordered arrangement, a process known as retrogradation.

As they recrystallise they squeeze out the water they were holding, and the crumb becomes firm and crumbly rather than tender.

The water is not lost from the loaf; it moves within it, which is why a sealed loaf stales while a bag full of moisture sits around it.

Refrigeration accelerates it

Retrogradation runs fastest at temperatures a little above freezing, which is squarely the range inside a domestic refrigerator.

Bread kept chilled therefore stales considerably faster than bread left at room temperature, despite the intuition that cold preserves food.

Freezing has the opposite effect, since it takes the loaf below the range where the process can proceed and effectively pauses it.

Warming reverses part of the change

Heating stale bread supplies enough energy to break the newly formed crystalline structure apart again, and the crumb softens noticeably.

The recovery is temporary, because the loaf begins recrystallising as soon as it cools, and each cycle leaves it drier than before.

Moisture genuinely lost through the crust cannot be restored, which is why an old loaf revives less convincingly each time it is warmed.

Why loaves differ in how fast they stale

Fats, sugars and emulsifiers in a recipe interfere with the starch reordering, which is why enriched breads stay soft far longer than lean ones.

Commercial bakers use ingredients that slow the process deliberately, and the difference in shelf life between a plain crusty loaf and a soft packaged one is largely this.

A crust that has gone soft while the crumb has gone hard is the same mechanism seen from both sides, as water migrates outward from the interior.