Evaluating polymer interplay after hot water pretreatment to investigate maize stem internode recalcitrance

Background: Biomass recalcitrance is governed by various molecular and structural factors but the interplay between these multiscale factors remains unclear. In this study, hot water pretreatment (HWP) was applied to maize stem internodes to highlight the impact of the ultrastructure of the polymers and their interactions on the accessibility and recalcitrance of the lignocellulosic biomass. The impact of HWP was analysed at different scales, from the polymer ultrastructure or water mobility to the cell wall organisation by combining complementary compositional, spectral and NMR analyses. Results: HWP increased the kinetics and yield of saccharification. Chemical characterisation showed that HWP altered cell wall composition with a loss of hemicelluloses (up to 45% in the 40-min HWP) and of ferulic acid cross-linking associated with lignin enrichment. The lignin structure was also altered (up to 35% reduction in beta-O-4 bonds), associated with slight depolymerisation/repolymerisation depending on the length of treatment. The increase in T1 rho H and specific surface area (SSA) showed that the cellulose environment was looser after pretreatment. These changes were linked to the increased accessibility of more constrained water to the cellulose in the 5-15 nm pore size range. Conclusion: The loss of hemicelluloses and changes in polymer structural features caused by HWP led to reorganisation of the lignocellulose matrix. These modifications increased the SSA and redistributed the water thereby increasing the accessibility of cellulases and enhancing hydrolysis. Interestingly, lignin content did not have a negative impact on enzymatic hydrolysis but a higher lignin condensed state appeared to promote saccharification. The environment and organisation of lignin is thus more important than its concentration in explaining cellulose accessibility. Elucidating the interactions between polymers is the key to understanding LB recalcitrance and to identifying the best severity conditions to optimise HWP in sustainable biorefineries. Highlights Reorganisation of the lignocellulosic matrix after water was redistributed by HWP Lignin reorganisation plus increased condensation promoted enzymatic deconstruction Multiscale approach combining chemical, spectral and NMR analyses helped elucidate polymer interplay.

Automated summary

Hot water pretreatment (HWP) alters the composition and structure of biomass, increasing saccharification kinetics and yield. Changes in cell wall composition, lignin structure, and cellulose environment after HWP contribute to enhanced accessibility of cellulases and promote hydrolysis. Understanding the interactions between polymers is crucial for optimizing HWP in sustainable biorefineries.