Abstract: Oxidized mung bean starch was prepared with sodium hypochlorite as an oxidant and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as a catalyst under ultrasonic treatment and its structure and properties were analyzed. Using one-factor-at-a-time method, the optimal preparation conditions were determined to be molar ratio between oxidant and glucose units in starch 1.5:1, and pH 8.5–9.0. Subsequently, the effect of ultrasonic treatment on the structure and properties of oxidized starch prepared using the optimized conditions was studied. The results showed that at an ultrasonic frequency was 25 kHz, the highest degree of oxidation and the smallest particle size were obtained. The results of scanning electron microscopy (SEM) showed that the original starch granules were smooth and intact, while the oxidized starch granules were broken into smaller particles with pores and dents being observed on the surface. Fourier transform infrared spectra showed that obvious carboxyl absorption peaks occurred in the range of 1 613–1 637 cm-1, indicating that the starch was successfully oxidized. The X-ray diffraction results showed that the diffraction peaks of the starch did not change in shape and position after oxidization, suggesting no change in the starch crystal type. However, the starch properties such as viscosity, particle size, charge, solubility and transmittance changed greatly after oxidation, which will be beneficial to the application of mung bean starch.

Key words: 2,2,6,6-tetramethyl-piperidine-1-oxyl, oxidized mung bean starch, ultrasonic frequency, structural analysis