Abstract
The general enzymatic method for producing reducing sugar is liquefaction followed by saccharification of starch. This method results in lower yields, consuming high energy and time. Therefore, the present study evaluated a new approach for producing reducing sugar from sweet potato starch (SPS), including simultaneous liquefaction (by α-amylase) and saccharification (by glucoamylase) of SPS pretreated with high power ultrasound. The effects of ultrasound parameters on the conversion rate of SPS and mechanism were investigated.
The optimum ultrasound pretreatment conditions were a frequency of 20 kHz, SPS concentration of 125 g L
, temperature of 30 °C, pulsed on-time of 3 s, pulsed off-time of 5 s, power density of 8 W mL
and sonication time of 15 min. The ultrasound assisted enzymolysis resulted in a SPS conversion rate of 59.10%, which was improved by 56.35% compared to the control. The results of pasting properties and thermal analysis showed that ultrasound pretreatment decreased the peak viscosity, breakdown temperature, setback viscosity, gelatinization range (T
- T
) and enthalpy of gelatinization (ΔH) of SPS significantly (P < 0.05) by 12.1%, 7.6%, 6.6%, 18.8% and 44.4%, respectively. Fourier-transform infrared spectroscopy indicated that ultrasound damaged the ordered structures and crystallization zone. This was confirmed by X-ray diffraction analysis, which showed that the relative crystallinity was reduced by 15.0%. Scanning electron microscopy showed that ultrasound destroyed the surfaces and the linkages between starch granules.
Prior to simultaneous liquefaction and saccharification of SPS, high power ultrasound pretreatment is a promising method for improving the conversion rate of starch. © 2020 Society of Chemical Industry.