Physicochemical Properties and Structure of Annealed Sweet Potato Starch: Effects of Enzyme and Ultrasound

Three annealing (ANN) treatments on the physicochemical properties and structure of sweet potato starch are investigated in this work, including pure ANN (PA), enzymatical ANN (EA), and ultrasonically enzymatic ANN (UEA). Experimental results indicated the effects of different treatments on the starch are in the order: UEA > EA > PA, which are reflected by the decrease of the relative crystallinity, short-range molecular order, enthalpy (Delta H), and the increase of peak viscosity and swelling power. X-ray diffraction showed that the reduction of relative crystallinity of starch after UEA is up to 9.25%, however, this decrease after EA is 4.9%. Thermal analysis suggested that the Delta H of starch with UEA is reduced up to 8.27%, and after EA it is 6.34%. The oil absorption rate of starch after UEA increases by a maximum of 23.35%, whereas with EA treatment it increases by only a maximum of 21.75%. Ultrasound and enzyme make tiny holes on starch granules. ANN does not change the appearance of granules suspended in deionized water, however it brings out tiny holes on some starches in phosphate buffer solution. In conclusion, EA is more effective than PA for modifying starch, and ultrasound promotes the EA process.