Water retention value predicts biomass recalcitrance for pretreated biomass: biomass water interactions vary based on pretreatment chemistry and reflect composition

Abstract

Processing of lignocellulosic biomass is complex due to the heterogeneity of the substrate, but also due to lengthy unit operations, which complicates process control including for enzymatic saccharification. Methods for predicting enzymatic saccharification yield based on the properties of the pretreated biomass would be advantageous to process optimization and control. Biomass-water interaction measurements provide a method for quickly predicting biomass recalcitrance. Correlating water retention value (WRV) and enzymatic saccharification yield (ESY) on pretreated biomass has shown promise, especially when assessing only single biomass types pretreated with one specific chemistry. However, with comparisons between different types of biomasses, predictive powers have been low. We investigate the effect of pretreatment chemistry on the predictive power of WRV, when keeping the biomass static. Wheat straw was pretreated with dilute acid, hydrothermal, or alkaline chemistries at five different temperatures. Furthermore, low field nuclear magnetic resonance was used to measure water constraint in the pretreated materials, to better understand how biomass-water interactions change with pretreatment severity and chemistry. We show that the correlation of WRV and ESY is highly pretreatment dependent, while WRV strongly predicts ESY within each pretreatment chemistry. While ESY and WRV correlated under all chemistries, the direction of the correlations were divergent, suggesting a more complex interplay between recalcitrance and biomass-water interactions. Using T2 relaxation profiles, reductions in hemicellulose composition was related to the decrease in size of the most constrained water population present in the pretreated biomasses for all chemistries, suggesting a new identification of this population of constrained water.