Moisture migration
Water moving within a bake and out of it — the second staling mechanism, and the one that explains why a crust softens while the crumb firms.
Professional consensus
Agreed by professional bakers and baking-science texts, with an accepted mechanism, though not isolated by controlled experiment.
A freshly baked loaf is not uniformly wet. The crumb holds a great deal of water and the crust, having been driven above the boiling point at its surface, holds very little. Water moves down that gradient: outward from crumb to crust, and then from crust to the air if nothing stops it.
This single process explains two changes that seem contradictory. The crust softens and goes leathery because it is gaining water. The crumb firms because it is losing it — and, separately and simultaneously, because its starch is retrograding. A baker who knows only about retrogradation cannot explain the crust; one who knows only about drying cannot explain why a sealed loaf still stales.
WHAT DRIVES IT IS NOT HOW MUCH WATER EACH PART HOLDS. It is water activity — roughly, how available that water is to move — and the difference matters because the two do not track each other. A syrup and a biscuit can hold wildly different amounts of water at the same water activity, and sitting in contact they will exchange none of it. Get this wrong and you predict a soggy base every time a wet filling meets pastry, which is not what happens.
The direction of the gradient is what decides whether a product wants to be sealed or left open, and it is why no single storage rule can be right for all baking. A crisp biscuit is drier than the air around it, so it gains water and goes soft. A loaf is wetter than the air, so it loses water and goes hard. Storing them the same way harms one of them.
What is actually established
Water activity sets the direction, and moisture content does not
Where two materials of unequal WATER ACTIVITY are in contact, water moves from the higher to the lower until they equalise. The gradient that drives it is a difference in chemical potential, which for foods is expressed as water activity — not as how much water each holds. Two foods can sit at very different moisture contents, be at the same water activity, and exchange nothing at all. This is why bread and biscuits stale in opposite directions, and why storing them in the same tin makes both worse at once.
SPRINT 15: the last of three Tier A claims held on an unread text, and verifying it CORRECTED IT. The old wording — “water moves from wherever it is more available to wherever it is less” — reads naturally as moisture content, and moisture content is the wrong variable. The thesis models the transport with water activity as the driving term precisely because a moisture-content gradient can persist at equilibrium while a water-activity gradient cannot. The practical consequence is not decorative: a filling that is wetter than a pastry case will not necessarily wet it, and matching water activity is how commercial multi-component products stop it happening.
RecordedP. Mavrou, University of Surrey, Paula Figoni, John Wiley & Sons
A sealed loaf still stales
Preventing water loss does not prevent staling, because retrogradation proceeds regardless. Sealing changes which mechanism dominates rather than stopping the process — it protects the crumb from drying and destroys the crust by keeping its water in.
Follows from the retrogradation finding rather than being separately measured. Attached as context, not as direct support.
RecordedJ. F. Aguirre, C. A. Osella, C. R. Carrara, H. D. Sánchez and M. del P. Buera, Starch/Stärke
What you can do with this
Decide what you are protecting before you decide how to store something. A crusty loaf you will finish today should sit cut-side down on a board, because sealing it ruins the crust you paid for. The same loaf on day two is better sealed, because by then the crust is already lost and the crumb is what remains. Crisp biscuits need an airtight container to keep water out; soft ones need one to keep water in — the same container serving two opposite purposes.
What is not settled
How fast migration proceeds depends on the product’s own moisture gradient, on ambient humidity and on the packaging, none of which a general statement can pin down. The relative contribution of migration and retrogradation to perceived staling has been measured for particular products rather than established in general, so BakeHQ describes both mechanisms and does not assign them percentages. Claims that a specific ingredient or wrapping 'keeps bread fresh’ usually address only one of the two.
Related
Variables that move this
Storage temperature
Storage decisions trade the two staling mechanisms against each other, which is why no single rule serves both a crusty loaf and a crisp biscuit.
Covered in
How to store baked things, and why the rules differ
The second mechanism, and the one that explains why biscuits and bread need opposite storage.
Storing bread, and the trade-off you cannot avoid
Why crust and crumb want opposite storage.
Reheating: what it can undo, and what it cannot
The half of staling heat does nothing lasting about, and the reason a crust needs its water driven off rather than added.
Chewy, crisp or cakey: what actually decides a cookie’s texture
Why storage undoes texture in both directions, and why an airtight tin does opposite jobs for a crisp and a chewy cookie.
Techniques this explains
The humid environment is the second effect — it stops the surface drying and skinning, which is why a cheesecake baked this way cracks less.
Causes this explains
It was in the oven too long
Water leaves a hot cake for as long as it stays hot, and the edges have the most surface to lose it from.
The loaf was wrapped or bagged while its crust was still cooling
Water moves from the crumb to the crust continuously; in open air it leaves, and in a bag it accumulates.
The sugar never dissolved
Dissolution needs available water, and an icing has very little of it — which is why the same sugar dissolves readily in a syrup and not in a buttercream.