Effects of ultrasonic combined with hydroxypropylation on structural and physicochemical properties of Foxtail millet (Jin gu 21) starch

Foxtail millet is a specialty grain mainly containing starch. In this research, the physicochemical properties of foxtail millet starch were modified using combined ultrasonic (0, 100 W, 200 W, 300 W, 400 W and 500 W) and hydroxypropylation with 20% propylene oxide (based on starch weight) to evaluate the effects of these modifications. The native foxtail millet starch granules exhibited a glabrous surface and were polygonal or spherical. The hydroxypropylation treatment slightly degraded the surface of the native foxtail millet starch granule, while ultrasonic-hydroxypropylation treatment resulted in depressions and cracks of the surfaces of the native foxtail millet starch granule. X-ray diffraction pattern showed that the foxtail millet starch was of crystal type "A". Notably, ultrasonic-hydroxypropylation and hydroxypropylation treatments did not affect the crystal shape of foxtail millet starch but decreased the relative crystallinity. Furthermore, ultrasonic-hydroxypropylation treatment blurred the polarized cross structure of foxtail millet starch and increased the content of amylose. Ultrasonic-hydroxypropylation treatment also resulted in a wider range of granule size distribution and increased the average granule size of foxtail millet starch. Hydroxypropylation and ultrasonic-hydroxypropylation treatments increased the transparency, peak viscosity, and swelling power of foxtail millet starch. Additionally, the treatments decreased retrogradation and pasting temperature of foxtail millet starch. Compared with single hydroxypropylation modification, ultrasonic-hydroxypropylation complex modification significantly improved the physicochemical properties of foxtail millet starch. Notably, ultrasonic power impacted the physicochemical properties of ultrasonic-hydroxypropylated foxtail millet starch. Specifically, the millet starch (UH-200) treated with 200W ultrasonic combined with hydroxypropylation exhibited the best physicochemical properties, with a significantly higher transmittance (24.62%) and lower retrogradation (14.47%) compared to the other groups. These findings indicate that ultrasonic combined with hydroxypropylation can modify the physicochemical properties of foxtail millet starch , thus improving its industrial utilization.