What is starch gelatinization -- and why does bread go stale?
Starch granules in flour absorb water and swell when heated, which is a big part of what actually "sets" a baked good's crumb. Staling afterward isn't the crumb drying out -- it's that same starch slowly reorganizing itself back toward its original, firmer state.
The kitchen answer
Baking sets a crumb by swelling its starch with heat and water. Staling is that process partly reversing itself over time as the starch cools and settles -- which is why sealing bread in a bag slows staling a little, but never stops it.
What's actually happening
Raw starch exists as tightly packed granules that are mostly insoluble in cold water. Heat them in the presence of water past roughly 60–70°C and the granules absorb water, swell dramatically, and their ordered internal structure breaks down -- that's gelatinization, and it's what turns a runny batter into a set, sliceable crumb (and what thickens a starch-based sauce or custard).
As gelatinized starch cools and sits, it doesn't stay in that swollen, disordered state -- the starch molecules (particularly amylose) slowly realign and partially re-crystallize, a process called retrogradation. That re-crystallization physically squeezes out some of the water the starch was holding, which is what actually firms up and dries out the crumb over a day or two -- not simple moisture evaporation, which is why stale bread can feel dry even sealed in an airtight bag.
What does that mean for your recipe
Putting bread in the fridge to "keep it fresh longer" backfires: retrogradation actually happens faster at typical refrigerator temperatures than at room temperature, because that range happens to suit starch recrystallization well. Room temperature (wrapped, to control moisture loss) or the freezer (cold enough to essentially halt the process) both keep bread fresher for longer than the fridge does.
What can go wrong
- Bread stale within a day even sealed well: retrogradation, not evaporation -- sealing keeps moisture in the bag but doesn't stop the starch from re-crystallizing and squeezing that moisture out of the crumb itself.
- Refrigerated "to stay fresh" bread staling faster than counter bread: refrigerator temperatures are close to ideal for starch retrogradation -- counterintuitive, but real.
- Sauce or custard too thin: starch never reached its gelatinization temperature range, or wasn't held there long enough for full granule swelling.
- Sauce turns gummy or breaks down on reheating: the same retrogradation/re-swelling cycle repeating with each heating and cooling round.
Try it yourself
One loaf, three storage conditions:
- Cut a loaf of bread into three pieces.
- Store one at room temperature (wrapped), one in the fridge, one in the freezer, for 24 hours.
- Bring all three back to room temperature and compare firmness.
Watch for: the fridge piece often feeling notably firmer and staler than the room-temperature piece, despite feeling like the "safer" storage choice -- direct, edible proof retrogradation has its own temperature preference.
Go deeper
Retrogradation proceeds fastest for amylose (the linear starch molecule), over minutes to hours, which is why crumb firms up quickly at first -- while amylopectin (the branched molecule) recrystallizes far more slowly, over days. That's roughly the difference between "this bread firmed up overnight" and "this loaf is properly stale a week later": two separate molecules retrograding on two separate timescales.