Molecular and Morphological Aspects of Annealing-Induced Stabilization of Starch Crystallites

A unique series of potato (mutant) starches with highly different amylopectin/amylose (AP/AM) ratios was annealed in excess water at stepwise increasing temperatures to increase the starch melting (or gelatinization) temperatures in aqueous suspensions. Small-angle X-ray scattering (SAXS) experiments revealed that the lamellar starch crystals gain stability upon annealing via thickening for high-AM starch, whereas the crystal surface energy decreases for AM-free starch. In starches with intermediate AP/AM ratio, both mechanisms occur, but the surface energy reduction mechanism prevails. Crystal thickening seems to be associated with the cocrystallization of AM with AP, leading to very disordered nanomorphologies for which a new SAXS data interpretation scheme needed to be developed. Annealing affects neither the crystal internal structure nor the spherulitic morphology on a micrometer length scale.