Bread becomes stale by drying out
November 19, 2011
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Stale bread – yuck. The crumb (the part inside) gets hard and stiff and the crust loses any crispness it might have had. Most people attribute this to drying out, but the opposite is in fact true. The bread is actually absorbing moisture, as shown by an increase in weight as the loaf goes from fresh to stale. The moisture absorbed by the crumb causes the starch granules to crystallize, hardening the bread. This is why the fridge is a bad place to store bread, even when it is well-wrapped, because low temperatures speed up the starch crystallization process (although freezing bread is fine because starch crystals don’t form at freezer temperatures). It’s also why a brief visit to the oven can improve stale bread, because the heat drives out some moisture and helps melt the starch crystals.
Bread can dry out, of course, but that’s another matter.