Insights into various pretreatments on recalcitrance in Populus Trichocarpa mutants

The natural recalcitrance of biomass is a critical issue for the viable biomass to biofuel process that is considered to be largely due to the complex physical/chemical structures of plant cell walls. A pretreatment process that can effectively reduce biomass recalcitrance is generally required as the first step for the biological conversion of lignocellulosic biomass to biofuels. In this study, two Populus Trichocarpa mutants that demonstrated distinct high and low recalcitrance were selected to subject to four types of pretreatment processes: dilute acid, hydrothermal, lime and organosolv pretreatments at typical conditions between 140 - 190 °C. The effects of pretreatment type on sugar release performance of poplar mutants and the structural features related to high/low sugar release resulted from various pretreatments were investigated. The accessibility of pretreated poplar mutants was estimated with Simon’s stain technique. Cellulose, hemicellulose and lignin were isolated from the pretreated poplar and the detailed chemical structure changes were characterized. The cellulose crystallinity was measured using solid state nuclear magnetic resonance (NMR). The molecular weights of cellulose, hemicellulose and lignin were analyzed by gel permeation chromatography (GPC). Lignin structural features were characterized by one/two-dimensional NMR techniques including ¹³C, ³¹P, HSQC and HMBC. The results of chemical structural changes and their relationship to the observed high/low recalcitrance of poplar mutants were discussed with a perspective to future improvement of biomass for enhanced enzymatic hydrolysis.