How Dilute Acid Pretreatment Produces a Highly Digestible Cellulosic Substrate

The structure and chemical composition of plant biomass has evolved to make the cellulose recalcitrant to hydrolysis by cellulase enzymes. Contributing factors to the recalcitrance of lignocellulosic biomass include the epidermal tissue of the plant body, the organization and types of cells present in plants, the difficulty of liquid penetration into the plant cell wall, lignification, the diversity of hemicellulose structure, and cellulose structure. Enzymatic conversion of lignocellulosic biomass requires an effective pretreatment to enable efficient cellulase action. Dilute acid pretreatment substantially increases the accessibility of the cellulose to hydrolytic enzymes and decreases the recalcitrance of the cellulose to hydrolysis. Using a variety of tools, we have developed a better understanding of how the pretreatment process generates a highly digestible cellulosic substrate. Characteristics such as cellulose accessibility, crystallinity, morphology and molecular weight are monitored to ascertain pretreatment effectiveness in addition to other factors including substrate porosity and particle size. Microscopic imaging can indicate changes in lignin and xylan distribution in the plant cell wall that are also important. Higher yields of fermentable sugars, at lower severities, with decreased generation of undesired products and inhibitors, are becoming a reality based on pretreatments designed using the fundamental knowledge being acquired with these developing techniques.