Background : Biomass recalcitrance is governed by various molecular and structural factors but there is still a lack of understanding on the interplay between these multiscale factors. In this study, hot water pretreatment (HWP) was applied to maize stem internode in order to highlight the impact of polymer interactions and their ultrastructure on the accessibility and recalcitrance of the lignocellulosic biomass. The impact of HWP were analysed at different scales, from the polymers ultrastructure or water mobility to the cell wall organization by combining complementary compositional, spectral and NMR analysis.
Result : HWP increased the kinetics and yield of saccharification. The chemical characterization showed that HWP altered cell wall composition with a loss of hemicelluloses (up to 45% for HWP 40 min) and of ferulic acid cross-linking, associated with a lignin enrichment. The lignin structure was also altered (up to 35% decrease of β-O-4 bonds), associated to a slight depolymerization/repolymerization depending on the treatment time. The increase of the T 1 ρ H , T HH and specific surface area (SSA) showed that the cellulose environment was looser after pretreatment. These changes were related to an increase of the accessibility of more constrained water to the cellulose across 5–15 nm pore size range.
Conclusion : The loss of hemicelluloses and changes in polymers structural features induced by HWP leads to reorganization of the lignocellulosic matrix. These modifications led to an increase in the SSA and a water redistribution allowing an increase in the accessibility of cellulases and an enhancement of the hydrolysis. Interestingly, the lignin content had no negative effect on the enzymatic hydrolysis but a higher lignin condensed state seemed to promote saccharification. The environment and organization of lignin is thus more important to consider than its concentration to explain cellulose accessibility. Elucidating the interactions between polymers is the key to understanding LB recalcitrance and to find the best severity conditions to optimise HWP in sustainable biorefineries.