Retrogradation

In baked starch foods, starch is softened and swollen by heat. As the bread cools and then sits, the long amylose and branched amylopectin chains drift back together and re-form tighter, partly crystalline structures. This process is called retrogradation.

The result is staling: the crumb loses its softness and becomes firmer and drier. Research on bread finds that amylopectin retrogradation is the major factor in bread staling, with the amylose-lipid complex contributing little after the first day. Storage temperature also matters, since cool temperatures speed the staling process, which is why bread firms up quickly in the refrigerator.

Bakers slow it with a fat-rich recipe, added emulsifiers or enzymes, and good wrapping.