Effects of high power ultrasound on the enzymolysis and structures of sweet potato starch.

BACKGROUND: The general enzymatic method to produce reducing sugar is firstly by liquefaction and subsequently followed by saccharification of starch. This method results in lower yields, consumes high energy and time. Therefore, the present study evaluated a new approach to produce 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 studied.

RESULTS: The optimum ultrasound pretreatment conditions were 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 SPS conversion rate of 59.10%, which was improved by 56.35% compared with that of the control. The results of pasting properties and thermal analysis showed that ultrasound pretreatment decreased the peak viscosity (PV), breakdown temperature (BT), setback viscosity (SV), gelatinization range (Tc -T0 ) and enthalpy of gelatinization (ΔH) of SPS significantly (p < 0.05) by 12.1%, 7.6%, 6.6%, 18.8% and 44.4%, respectively. FT-IR 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 reduced by 15.0%. Scanning electron microscopy showed that ultrasound destroyed the surfaces and the linkages between starch granules.

CONCLUSION: Prior to simultaneous liquefaction and saccharification of sweet potato starch, high power ultrasound pretreatment is a promising way to improve the conversion rate of starch. This article is protected by copyright. All rights reserved.