Nanostructure of Potato Starch,Part I: Early Stages of Retrogradation of Amorphous Starch in Humid Atmosphere as Revealed by Simultaneous SAXS and WAXS

Crystallization experiments on amorphous, injection-molded starch in a humid atmosphere are reported. The crystallization mechanisms have been studied using simultaneous SAXS and WAXS during a temperature stepwise increase. In contrast to the crystallization of linear synthetic polymers, in starch the WAXS peaks are observed at low temperature before the appearance of the SAXS maximum. The initial state of crystallization is dominated by the amylose (AM) component of the potato starch alone. After the initial formation of large (16 nm) uncoordinated individual crystallites, stacks of lamellae and finally, an insertion of thinner lamellae within the stacks are observed. Results indicate that only if all AM is converted into a semicrystalline structure and if the secondary starch network of double helices of AM and amylopectin (AP) is molten by a temperature increase above 70°C, crystallization of AP also occurs. Because the AM crystals act as nuclei for the AP component, a common superstructure is developed. Within a spherulite, alternating AM and AP lamellae develop radially from the center of the AP molecule. Results suggest that the AM is distributed inhomogeneously with respect to the AP molecules, leaving approximately one half of the AP fraction free, which means not crystallized to a spherulitic structure together with AM.