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).