Ultrasound-assisted alkaline pretreatment to intensify enzymatic saccharification of Crotalaria juncea using a statistical method

Abstract

Sunn hemp as a non-woody fiber crop was selected to determine the effect of ultrasound-assisted alkali pretreatment on enzymatic hydrolysis. We hypothesized that ultrasound-assisted alkali pretreatment (US-NaOH) was more efficient than alkali pretreatment alone; maximum fermentable sugars were released in US-NaOH-pretreated biomass by enzymatic hydrolysis. In this study, sunn hemp was pretreated with NaOH and US-NaOH at 121 °C. A higher cellulose content of 67% was obtained in US-NaOH-pretreated biomass (1% NaOH, 10-min sonication, 121 °C) as compared with that of the alkali-pretreated biomass. After ultrasound-assisted alkaline pretreatment, the crystallinity index of cellulose and the solubilization of pentose and lignin increased from 39 to 53%, leading to a significant increase in enzyme accessibility to cellulose to release reducing sugars. Scanning electron microscopy showed that the biomass underwent erosion after ultrasound pretreatment. Response surface methodology was chosen to study the individual effects of variables on enzymatic hydrolysis and their inter-dependence. The quantity of reducing sugars released was optimized using a response surface methodology, and the experimental results with optimized process variables gave 5.17 mg/mL glucose and 2.08 mg/mL xylose.