Comparative studies on structure and physiochemical changes of millet starch under microwave and ultrasound at the same power.

As two novel starch modification technologies, the comparative study between microwave (MC) and ultrasound (UC) is of important significance for their applications. The objective of this work is to compare the changes of millet starch with MC and UC, focusing on the viscosity, swelling power (SP), transparency (TR), short-range molecular order (FT-IR spectrum), thermal properties (DSC), long-range molecular order (X-ray diffraction), in vitro enzymatic digestibility, and morphological properties. MC changed the type of the pasting curves form B-type to C-type, and the maximum decrease of the peak viscosity and relative crystallinity of MC starch was 74.9% and 100%, however, these changes for UC starch were only 14.8% and 23.2%. MC decreased the SP, TR, DSC, and ratio of absorbances at 1047/1022 cm, but UC caused opposite trend of these properties. Both MC and UC increased the in vitro enzymatic digestibility by 26.7% and 13.3%, respectively. MC also destroyed the original appearance of the starch granules and formed smaller and lamellar gel blocks. In conclusion, MC had greater influence on starch structure and properties than UC, even in a short period.