It would be an understatement to say that staling is a complicated phenomenon. Entire books and extensive review articles have been written on this topic alone. Recently, however, thanks to the application of modern molecular methods, including infrared, near infrared, and nuclear magnetic resonance spectroscopy, x-ray crystallography, and various microscopic techniques, a clearer understanding of the details involved in staling has emerged. Simply stated, staling refers to the increase in crumb firmness that makes the bread undesirable to consumers. In addition, staling is associated with an increase in crust softness and a decrease in fresh bread flavor.
Staling is basically a starch structure and moisture migration problem. The reactions that eventually lead to staling actually start when the bread is baked, as starch granules in the dough begin to adsorb water, gelatinize, and swell. The amylose and amylopectin chains separate from one another and become more soluble and less ordered.Then, when the bread is cooled, these starch molecules, and the amylopectin, in particular, slowly begin to re-associate and re-crystallize (see below).This process, called retrogradation, results in an increase in firmness due to the rigid structures that form. Amylose retrogrades rapidly upon cooling, while amylopectin retogrades slowly. It is the slow retrogradation of amylopectin that is now thought to be primarily associated with staling of the crumb. Furthermore, moisture migration from starch to gluten and from crumb to crust make the crumb dryer and more firm. Although staling is an inevitable process, a number of strategies have been adopted to delay these reactions and extend the shelf-life of bread (Box 8-6).
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