Structural variations of rice starch affected by constant power microwave treatment

Although lots of work has reported the structural variations of starch in microwave treatment, most of them are detected in the environment with non-constant microwave power and inhomogeneous heating, and the results are always in poor repeatability. In this study, the equipment with constant microwave power (CPM) and homogeneous heating was designed. And the phase transition of multi-scale structure of rice starch (30% moisture content) caused by CPM treatments with two heating modes, namely rapid microwave heating (RWH) and slow microwave heating (SWH) were investigated systematically. SEM results showed that the surface of starch granules after CPM treatment were rough and broken, and the damage caused by RWH was more distinct than that by SWH. SAXS, XRD and ¹³C NMR results revealed that the CPM treatment decreased the degree of crystallinity and content of double helices of starch. Moreover, the influence of RWH on the variation of starch granules was greater than that of SWH, which can be attributed to the intensive friction and collision as well as the rapid evaporation of water in RWH treatment. Specifically, it exhibited greater destruction on the linkage of starch and the internal crystalline region in RWH treatment than SWH treatment, thereby resulting in more obvious damages on the lamellar and morphological structure of rice starch. In conclusion, CPM equipment has improved the problems of uneven heating and poor experimental repeatability. After CPM treated starch, the molecular structure of starch was destroyed, which provides a useful method to modify properties of starch.