Monday, May 4, 2009
12-16
How Pretreatment Can Overcome the Natural Recalcitrance of Biomass to Cellulase Hydrolysis
David K. Johnson, William S. Adney, Roman Brunecky, Shi-You Ding, Bryon S. Donohoe, Todd B. Vinzant, and Michael E. Himmel. Chemical and Biosciences Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
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. Pretreatment conditions cover the entire range from low to high pH, moderate to high temperatures, and minutes to weeks. The main job of pretreatment in a biorefinery is to overcome the natural recalcitrance of biomass, increasing access of enzymes to cellulose, and increasing the ease with which the cellulose can be hydrolyzed. Using a variety of tools, we have developed a better understanding of how pretreatment processes can generate highly digestible cellulosic substrates. Characteristics such as cellulose accessibility, crystallinity, morphology and molecular weight can indicate pretreatment effectiveness. Other factors include substrate porosity and particle size. Microscopic imaging can indicate changes in lignin and xylan distribution in the plant cell wall that can also be important. This paper will summarize our current understanding of how pretreatment can overcome the natural recalcitrance of biomass to cellulase hydrolysis.