Structural and retrogradation properties of sonicated high-amylose corn starch with temperature-cycling treatment

Abstract

In this study, high-amylose corn starch (HACS) was sonicated, and then retrograded by isothermal storage and temperature-cycling for 16 days. After the sonication, the retention time of both amylopectin (AP) and amylose (AM) peak increased in gel permeation chromatography because of the reduced molecular weight of starch. Percentages of α-1,4 glycosidic bonds in 1H NMR spectroscopy decreased slightly as the sonication time increased. Breakdown of AM double helices provided more space to form iodine-starch complexes, resulting in a higher apparent amylose content in sonicated starch. Crystallites of AP collapsed gradually, and those of AM disappeared in differential scanning calorimetry after the sonication. Though HACS samples maintained B-type polymorph, decreases in major peak intensities and relative crystallinity were observed by X-ray diffractometry during sonication. 13C CP/MAS NMR spectroscopic analysis revealed that the proportion of double helical structure also reduced with the extended sonication time. A gradual decrease in average DP determined by high-performance anion-exchange chromatography could be caused by the vulnerability of outer branched chains to sonication. S6, which was sonicated for 60 min, and then retrograded with native HACS, showed the most significant difference in structural properties. Nucleation was accelerated by the numerous AM nuclei formed by disruption of most AM double helices during sonication. Hydrolysis of α-1,4 linkage in amorphous region reduced the length of AM random coil, which was transformed to AM single helix fast and made AM double helices by combining with other single helices broken by sonication. Also, disintegration of cluster structure in AP by hydrolysis of amorphous region led to dense arrangement of AP in propagation step. Thus, transition enthalpy, major peak intensities, relative crystallinity, proportion of double helical structure, and RS contents were elevated during the retrogradation of S6. Moreover, compared with isothermal storage, both nucleation and propagation were promoted by temperature-cycling, and consequently resulted in a higher extent of retrogradation. Higher nucleation and propagation rates were also shown in sonicated and temperature-cycling treated groups using an irreversible consecutive reaction model, supplementing the Avrami kinetics model. S6-TC16 which was subjected to both sonication and temperature-cycling for 16 days, therefore, revealed the most striking retrogradation characteristics. Sonication treatment induced the proper structural changes for the retrogradation. Structural and retrogradation characteristics and RS contents also changed to a great extent in temperature-cycling compared with isothermal storage. This study provided the basic understanding regarding the retrogradation of sonicated starch and a kinetic model for temperature-cycled retrogradation. Moreover, RS content obtained suggests a new practical application of sonicated starch.