Enhancing enzymatic digestibility of switchgrass by microwave-assisted alkali pretreatment

Abstract Pretreatment of lignocellulosic materials to disrupt their recalcitrant structures is a crucial step in cellulosic ethanol production. Most pretreatment processes require a high temperature reaction, which is often achieved through conventional heating. In this study, microwave-based heating was used to pretreat switchgrass, which was then hydrolyzed by cellulase enzymes. When switchgrass was soaked in water and treated by microwave, total sugar (xylose + glucose) yield from the combined treatment and hydrolysis was 34.5 g/100 g biomass, equivalent to 58.5% of the maximum potential sugars released. This yield was 53% higher than that obtained from conventional heating of switchgrass. To further improve the sugar yield, switchgrass was presoaked in different concentrations of alkali solutions and then treated by microwave or conventional heating. With alkali loading from 0.05 to 0.3 g alkali/g biomass, microwave pretreatment resulted in a higher sugar yield than from conventional heating, with the highest yield (90% of maximum potential sugars) being achieved at 0.1 g/g of alkali loading. Scanning electron microscope images revealed that the advantage of microwave over conventional heating was due to the disruption of recalcitrant structures. Finally, the effects of temperature, solid content, and treatment time on microwave pretreatment of switchgrass were investigated. At optimal conditions of 190 °C, 50 g/L solid content, and 30 min treatment time, the sugar yield from the combined pretreatment and hydrolysis was 58.7 g/100 g biomass, equivalent 99% of potential maximum sugars. The results demonstrate that microwave-assisted alkali treatment is an efficient way to improve the enzymatic digestibility of switchgrass.