Fate of p -hydroxycinnamates and structural characteristics of residual hemicelluloses and lignin during alkaline-sulfite chemithermomechanical pretreatment of sugarcane bagasse.

Background: Preparing multiple products from lignocellulosic biomass feedstock enhances the profit and sustainability of future biorefineries. Grasses are suitable feedstocks for biorefineries as they permit a variety of possible by-products due to their particular chemical characteristics and morphology. Elucidating the fate of p -hydroxycinnamates (ferulates-FAs and p -coumarates- p CAs) and major structural components during bioprocessing helps to discriminate the sources of recalcitrance in grasses and paves the way for the recovery of p -hydroxycinnamates, which have multiple applications. To address these subjects, we assessed sugarcane bagasse biorefining under alkaline-sulfite chemithermomechanical (AS-CTM) pretreatment and enzymatic saccharification.

Results: The mass balances of the major bagasse components were combined with 2D-NMR structural evaluation of process solids to advance our understanding of sugarcane bagasse changes during biorefining. AS-CTM pretreatment provided a high yield and thoroughly digestible substrates. The pretreated material was depleted in acetyl groups, but retained 62 and 79% of the original lignin and xylan, respectively. Forty percent of the total FAs and p CAs were also retained in pretreated material. After pretreatment and enzymatic hydrolysis, the residual solids contained mostly lignin and ester-linked p CAs, with minor amounts of FAs and non-digested polysaccharides. Saponification of the residual solids, at a higher alkali load, cleaved all the ester linkages in the p CAs; nevertheless, a significant fraction of the p CAs remained attached to the saponified solids, probably to lignin, through 4- O ether-linkages.

Conclusion: AS-CTM pretreatment provided soundly digestible substrates, which retain substantial amounts of xylans and lignin. Acetyl groups were depleted, but 40% of the total FAs and p CAs remained in pretreated material. Ester-linked p CAs detected in pretreated material also resisted to the enzymatic hydrolysis step. Only a more severe saponification reaction cleaved ester linkages of p CAs from residual solids; nevertheless, p CAs remained attached to the core lignin through 4- O ether-linkages, suggesting the occurrence of an alkali-stable fraction of p CAs in sugarcane bagasse.