STALE bread may seem like it has simply dried out, but staling is actually a complex process that still isn’t fully understood.
In the 19th century, the French chemist Jean-Baptiste Boussingault noted that stale bread can be refreshed by putting it in the oven, and showed that bread will still go stale if it is hermetically sealed and doesn’t lose any moisture.
In fact, staling is to do with the chemistry of starch, which is found in flour and consists of two kinds of sugar molecules, amylose and amylopectin. Raw starch has a rigid, crystalline structure, but it …
Source: Humans - newscientist.com