Analysis of the mechanism of improving retrogradation rate of sweet potato starch by ultrasonic wave in method of micrograph

The retrogradation rate of sweet potato has been improved by the treatment of ultrasonic wave. The effects of the treatment of ultrasonic wave on morphology of sweet potato starch were investigated by analyzing the micrographs of sweet potato starch at different stages such as before-pasting, after-pasting, after-autoclaving, after-aging. The distribution of sweet potato granules in water had become more evenly by ultrasonic wave treatment at stage of pre-pasting and the attractive force among different granules decreased, thus it was easier for those granule balls to absorb water and to swell. The breakage of granules increased by the ultrasonic wave treatments after being pasted for a short time, but for a long time, the starch film was built and the aggregation of amylopectin were strengthened. The breakage of all starch balls, the network structure formation by graft amylopectin, the disappearance of gelling and the increase of retrogradation rate happened when the ultrasonic wave treatments were carried out after autoclaving. The gelatinized starch was firstly melted by the ultrasonic wave treatments after aging, then the stiff starch was hardened and the hydrolysis resistance of sweet potato starch was enhanced by long time treating.