Revealing the Changes in Topochemical Characteristics of Poplar Cell Wall During Hydrothermal Pretreatment

Abstract

Understanding of ultrastructural organization related to the biomass recalcitrance is critical for optimizing the deconstruction of cell wall to release fermentable sugars. In this study, the changes in topochemical characteristics of poplar cell wall pretreated with liquid hot water at 170 °C for 5, 10, 20, 30, and 40 min were investigated. The hydrothermal pretreatment (HTP) caused the alterations of surface morphological and chemical features in different cell wall sublayers in a time-dependent manner. Scanning electron microscopy observations revealed a reduction in re-deposited lignin droplets in the middle layer of secondary wall (S2 layer) after 20 min of pretreatment. Confocal Raman microscopy images exhibited that the loss of lignin concentration in the S2 layer was more than that in the compound middle lamella (CML) after 30 min of pretreatment, whereas the drop of hemicellulose concentration exhibited an opposite trend. Meanwhile, the decline of lignin and hemicellulose concentrations was accompanied by the increase in relative concentration of cellulose, indicating the exposure of embedded cellulose. The higher yield of enzymatic hydrolysis obtained from the poplars after 30 min of pretreatment were 38 %, indicating that the removal of lignin and hemicelluloses from the S2 layer and CML is likely to be important factors in increasing enzymatic hydrolysis of poplar. This study is expected to potentially promote understanding of ultrastructural features hindering cell wall deconstruction, making it effective to convert lignocellulosic biomass into sugars.