Bread goes stale because its starch recrystallises, not because it dries out

The obvious explanation fails three tests

Stale bread feels dry, so drying out is the natural assumption, and it is wrong in a way that is easy to demonstrate. Bread sealed in a bag, losing essentially no water, stales on much the same schedule as bread left out. Bread kept in a refrigerator, where the air is cold and the loaf is enclosed, stales markedly faster than bread on the counter. And a stale loaf warmed through in an oven comes back soft for an hour or so, which no amount of water actually leaving could allow.

What all three point to is an internal rearrangement rather than a loss. The water is still in there. It has simply stopped being where the texture needs it to be, because the starch around it has changed state.

Drying does happen alongside it, of course, and a loaf left cut side up on a bench for a day is genuinely both stale and dry. The two processes are simply independent, and confusing them leads to the wrong remedies — most obviously to wrapping bread tightly in plastic, which prevents the drying and does nothing about the staling while comprehensively ruining the crust.

Gelatinisation, and the slow reversal that follows

During baking, starch granules absorb water and swell, their ordered crystalline regions melt apart, and the long glucose chains inside spread out into a soft hydrated gel. That transformation is what turns raw dough into crumb. It is also, thermodynamically speaking, not where those molecules would rather be, and from the moment the loaf leaves the oven they begin drifting back towards an ordered arrangement.

This is retrogradation. The shorter, straighter amylose chains recrystallise within hours, which accounts for the initial firming as a loaf cools and why bread cut hot behaves differently from bread cut cold. The branched amylopectin molecules realign far more slowly over the following days, pulling water out of the surrounding gel and into rigid crystalline regions where it no longer softens anything. The loaf has the same water and a fraction of the tenderness.

Temperature sets the speed, and the fridge sits at the worst point

Retrogradation needs the molecules mobile enough to move and cold enough to want to settle into order, and that combination peaks a little above freezing. A refrigerator is almost exactly there, which is why a loaf kept in one goes stale in a day or two while the same loaf on a bench takes several. It is a genuinely counterintuitive result and it is well established.

A freezer, by contrast, arrests the process almost completely, because below the glass transition of the starch-water system the molecules can’t rearrange at all. Bread frozen while fresh and thawed later is close to what went in. So the correct storage order is counter for a day or two, freezer for anything beyond that, and the fridge for essentially nothing except keeping mould off in a hot damp week — which is a real reason, just an expensive one in texture.

Heat undoes part of it

Warming stale bread above roughly sixty degrees melts the amylopectin crystallites that formed during storage, releasing the water they had locked up and restoring much of the softness. Ten minutes in a moderate oven does it. This is refreshing rather than repair, because the amylose fraction that set early is far more heat-stable and doesn’t come back, and because staling resumes from the moment the bread cools again, generally faster than the first time.

A loaf can be refreshed this way once, tolerably twice, and after that the accumulated water lost to evaporation each round leaves you with something genuinely dry. At that point the honest destinations are breadcrumbs, a bread pudding or croutons, all of which are recipes built for exactly this state.

Why enriched bakes and cakes keep longer

Fat, sugar and emulsifiers all slow retrogradation. Fat coats starch and gets in the way of crystal formation. Sugar competes for water and interferes with the ordering directly. Mono- and diglycerides, which is what most commercial anti-staling agents are, insert themselves into the amylose helix and physically obstruct it. This is most of why a brioche stays soft for days while a lean baguette is past its best by evening, and it is also most of why industrial sliced bread has the shelf life it does.

The crust is a separate story running in the other direction. A fresh crust is crisp because it is dry, and water migrating outward from the crumb over the following hours softens it into leather while the interior firms. Reheating drives that water off again, which is why a refreshed loaf recovers both textures at once and why sealing a crusty loaf in plastic destroys the crust while doing nothing at all for the crumb.