Retrogradation

Noun

1. Motion in a retrograde manner.
2. Decline, degradation.
3. A reaction that takes place in gelatinized starch when the amylose and amylopectin chains realign themselves, largely responsible for bread becoming stale.

   "The major reason that bread stales is not moisture loss, but rather a process called retrogradation, in which the starch molecules in the bread crystallize."