| Starch is a kind of widespread and natural renewable resources. Its structure and properties have certain limitations, which limit its wide application. Ultrasonic treatment can lead to the fracture of starch molecular chemical bond, and make the structure and properties of starch change. As a new type of starch modification technology, ultrasound has attracted more and more attention. It is rich in sweet potato starch resources in our country, but these resources are not used efficiently. In this paper, sweet potato starch was used as the raw material, and effects on the structure and properties of sweet potato starch under different frequencies ultrasound in the water system and acid-alcohol system were investigated at the same treating conditions else, which lay the foundation for the modified starch and the application of ultrasonic technology. The main research results were as follows.The structure of starch was investigated respectively by using the scanning electron microscope (SEM), infrared spectrometer (FT-IR), X-ray diffraction and UV-Vis spectrophotometer. SEM analysis suggested that the native starch granules had the round and polygonal granular shape, and the surface was relatively smooth and complete, there were no holes and cracks. It was obvious that many inhomogeneous pores were visible on the surface of the granules when the starch suspensions were treated with different frequencies ultrasounds. FT-IR indicated that with ultrasonic treatments, the original structure of starch was not destroyed, but its crystalline index decreased. X-ray diffraction indicated that the crystal form of starch being treated by ultrasound was not destroyed, but its characteristic diffraction peak intensity was significantly reduced, and the relative crystallinity decreased. Starch-iodine complex analysis showed these different frequencies ultrasounds destroyed amylopectin, and the amylose content increased. Compared to single frequency and dual-frequency ultrasounds, tri-frequency ultrasound caused more obvious changes to the starch structure.The ultrasonic effects on properties of sweet potato starch were investigated by analyzing the viscosity, transmittance and solubility. The results showed that the peak viscosity of native starch was 447.0 BU. In the water system, the peak viscosity reached the minimum when the starch being treated for 60min by tri-frequency ultrasound, which was 17.67% lower than that of the native starch. In the acid-alcohol system, the peak viscosity of starch with tri-frequency ultrasound was 61.30% lower than that of the native starch. In addition, the transmittance and solubility of starch increased by ultrasound in certain time. The transmittance and solubility of native starch were respectively 6.6% and 8.35%. In the water system, the starch being treated by tri-frequency ultrasound for 30min showed the maximum transmittance and was 12.5%.The solubility reached the maximum when the starch being treated for 60min by tri-frequency ultrasound, which was 11.41%.In the acid-alcohol system, the starch being treated by tri-frequency ultrasound for 30min showed the maximum transmittance and was 34.5%. The solubility reached the maximum when the starch being treated for 60min by tri-frequency ultrasound, which was 37.86%.The mechanism of multi-frequency was studied by iodine release of KI solution, visual observation of foil and cavitation bubble movement. The main research was that the cavitational yield of multi-frequency ultrasound from big to small sequence was tri-frequency ultrasound, dual-frequency ultrasound and single frequency ultrasound. It meant that the tri-frequency ultrasound could produce the most amounts of cavitational events at the same time. |
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