Enhancement of converting corn stalk into reducing sugar by ultrasonic-assisted ammonium bicarbonate pretreatment

•Ultrasonic-assisted ammonium bicarbonate pretreatment of corn stalk was proposed.•Ultrasonic time, temperature and liquid/solid mass ratio were investigated.•Optimal conditions were temperature 42 °C, time 11 min and liquid/solid ratio 12:1.•The highest yield of reducing sugar increased by 355%. In order to improve the yield of reducing sugar from corn stalk, ultrasonic-assisted ammonium bicarbonate pretreatment of corn stalk was proposed. Three ultrasonic factors (time (0–30 min), temperature (30–60 °C) and liquid/solid mass ratio (5–20)) were optimized by response surface experiment. The optimal conditions of ultrasonic pretreatment were obtained (liquid/solid mass ratio is 12:1, temperature is 42 °C and time is 11 min). The highest saccharification rate of corn stalk was of 82.61%, which was remarkably increased by 355% compared to the control group.