Enhanced Enzymatic Hydrolysis of Sorghum Stalk by Supercritical Carbon Dioxide and Ultrasonic Pretreatment.

Sorghum was pretreated by sole ultrasound or supercritical carbon dioxide (scCO2), as well as the method combining both to intensify enzymatic hydrolysis. The effect of the time (1-5 h) and temperature (30-70 °C) on ultrasonic pretreatment was investigated, and the best condition was determined as 5 h and 50 °C with the EH sugar yield of 33.69%. The influence of the time (6-48 h), temperature (40-80 °C), and pressure (15-25 MPa) on scCO2 pretreatment were also discussed in this study. The optimum condition of scCO2 pretreatment was identified as 60 °C, 20 MPa, and 36 h with the EH sugar yield of 43.57%. Compared with the sole ultrasonic or scCO2 pretreatment, scCO2 associated with the subsequent ultrasonic pretreatment did not show significant improvement in sugar yield. However, 30 MPa was an extremely effective pressure, which led to 45.50% EH sugar yield with 60 °C, 6-h pretreatment. Finally, the change of the microscopic structures of the sorghum stalk after the pretreatment was investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD).