The effects of screw elements on enzymatic digestibility of corncobs after pretreatment in a twin-screw extruder

Extrusion of lignocellulosic biomasses can be an effective physical continuous pretreatment method towards bioethanol production. Extrusion processes would typically precede enzymatic cellulose saccharification in a given pretreatment. The screw configuration determines the functionality of the extruder and different screw elements (conveying, kneading and reverse) can have different effects on enzymatic digestibility. This study investigates the effects of individual functional screw elements on enzymatic digestibility after extrusion. It was found that extrusion enhanced enzymatic digestibility under all tested condition. Reverse and kneading screw elements however resulted in lignin re-distribution over the cellulose fibres, blocked pores (shown via SEM) and resulted in less digestible biomass, compared to conveying screw elements. Lignin removal via NaOH reversed this effect and the highest digestibility was found for biomass extruded in the presence of reverse screw elements (the most severe conditions tested in this study), indicating that lignin redistribution can counteract otherwise positive effects of extrusion pretreatment.