The effect of alkali pretreatment on biomass surface properties affecting cellulase accessibility to cellulose

In the biochemical conversion of lignocellulosic biomass, a major goal of pretreatment is to maximize the accessibility of cellulose to cellulases. Assessing changes to biomass surface properties, therefore, can provide insights into pretreatment efficacy. Here, we present a study on the effect of alkali pretreatment on biomass surface properties affecting cellulose accessibility. Rice straw was pretreated with sodium hydroxide followed by conditioning to pH 5-6 by: 1) water washing, and 2) neutralization with hydrochloric acid then washing with water. Alkali pretreatment increased cellulose accessibility to a purified Trichoderma reesei Cel7A and improved overall cellulase digestibility of rice straw. Lignin and xylan were mostly removed by the pretreatment and surface analyses by ToF-SIMS confirmed the predominance of polysaccharides on the pretreated rice straw surfaces. Further probing with water protons by ¹H-Nuclear Magnetic Resonance relaxometry showed that alkali pretreatment increased the specific surface area and porosity of rice straw. Taken together, the results indicate that alkali pretreatment improved rice straw digestibility by increasing porosity, specific surface area and surface cellulose content to increase cellulase access to cellulose. Acid neutralization of the alkali pretreated rice straw, however, decreased gains in digestibility and T. reesei Cel7A accessibility to the cellulose fraction. The surface analyses suggest that the losses are because acid neutralization precipitated a chemically modified form of lignin on the rice straw surfaces and decreased gains in specific surface area and porosity of the pretreated rice straw. In this presentation, we elaborate on the surface analytical methods and the results of this study.