Starch Retrogradation
The starch paste or solution obtained after the gelatinisation is not stable and generally produces a viscoelastic, firm and rigid gel. Structural transformation occurs during storage. As starch pastes are cooled and stored, the starch becomes progressively less soluble. In dilute solution, starch molecules will precipitate, with the insoluble material being difficult to redissolve by heating. The collective processes of dissolved starch becoming less soluble are called retrogradation.
Retrogradation of cooked starch involves constituent polymers, amylose and amylopectin, with amylose undergoing retrogradation at a much more rapid rate than does amylopectin. Upon cooling, the dispersed starch polysaccharides re-associate. Concentrated amylose solutions rapidly gel on cooling to room temperature. Subsequently, some of the amylose molecules slowly crystallize. The gel can only be melted at 160°C. The amylopectin with gelatinized granules can also crystallize but this association can be reversed by heating to 70°C. Thus after retrogradation of a starch-water mixture, a partially crystalline polymer system is again obtained.