Fresh bread has a soft crumb, yet after a day or two it becomes firm. This is often explained as the bread “drying out”. Moisture loss matters, but it is not the whole process: even sealed bread can go stale while its total water content changes relatively little.
During baking, starch granules in the flour absorb water, heat up and gelatinise, disrupting their earlier ordered structure. As the bread cools and is stored, starch molecules draw together and form more ordered regions again. This process is called starch retrogradation, with amylopectin playing an important role in the longer-term firming of the crumb. At the same time, water is redistributed among starch, proteins, crumb and crust. The soft elastic network becomes harder to deform, so the crumb feels dry and firm, while the crust may lose crispness as it absorbs moisture from inside.
Stale therefore does not simply mean dry. Heating can temporarily disrupt some ordered structures and redistribute water, making bread softer for a while, but it may firm again after cooling. Mould is microbial growth and a separate process; heating does not restore mouldy bread. Staling is a texture change produced by several interacting molecular processes, not a one-way journey of water out of the loaf.
https://pubmed.ncbi.nlm.nih.gov/33451240/
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