Comparative analysis of lignin chemical structures of sugarcane bagasse pretreated by alkaline, hydrothermal, and dilute sulfuric acid methods

To understand the lignocellulose properties of sugarcane (Saccharum sp.) bagasse with a view to its application to produce fermentable sugars, we comparatively analyzed lignocellulose properties, especially lignin chemical structures, of bagasse before and after alkaline, hydrothermal, and dilute sulfuric acid pretreatments. Wet chemical analyses and two-dimensional nuclear magnetic resonance spectroscopy determined that lignocellulose from untreated bagasse contained similar levels of guaiacyl (G) and syringyl (S) lignins with lower levels of p-hydroxyphenyl (H) and tricin (T) lignins. Alkaline pretreatment preferentially removed S and T lignins from bagasse, whereas the hydrothermal and dilute sulfuric acid pretreatments had the effect of enriching lignins (except T lignin) by reducing xylans and arabinans. Our data indicated that the alkaline and dilute sulfuric acid pretreatments relatively enriched β-5 linkages, while reducing β-O-4 and β-β substructures, in the residual lignin polymers. In contrast, hydrothermal pretreatment reduced all three major linkage types.