Understanding the effects of different residual lignin fractions in acid-pretreated bamboo residues on its enzymatic digestibility

Wenqian Lin,Jinlai Yang,Qiang Yong,Caoxing Huang,Yayue Zheng

doi:10.1186/s13068-021-01994-y

Wenqian Lin, Jinlai Yang + Show 3 more

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https://doi.org/10.1186/s13068-021-01994-y

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Abstract

BackgroundDuring the dilute acid pretreatment process, the resulting pseudo-lignin and lignin droplets deposited on the surface of lignocellulose and inhibit the enzymatic digestibility of cellulose in lignocellulose. However, how these lignins interact with cellulase enzymes and then affect enzymatic hydrolysis is still unknown. In this work, different fractions of surface lignin (SL) obtained from dilute acid-pretreated bamboo residues (DAP-BR) were extracted by various organic reagents and the residual lignin in extracted DAP-BR was obtained by the milled wood lignin (MWL) method. All of the lignin fractions obtained from DAP-BR were used to investigate the mechanism for interaction between lignin and cellulase using surface plasmon resonance (SPR) technology to understand how they affect enzymatic hydrolysisResultsThe results showed that removing surface lignin significantly decreased the yield for enzymatic hydrolysis DAP-BR from 36.5% to 18.6%. The addition of MWL samples to Avicel inhibited its enzymatic hydrolysis, while different SL samples showed slight increases in enzymatic digestibility. Due to the higher molecular weight and hydrophobicity of MWL samples versus SL samples, a stronger affinity for MWL (KD = 6.8–24.7 nM) was found versus that of SL (KD = 39.4–52.6 nM) by SPR analysis. The affinity constants of all tested lignins exhibited good correlations (r > 0.6) with the effects on enzymatic digestibility of extracted DAP-BR and Avicel.ConclusionsThis work revealed that the surface lignin on DAP-BR is necessary for maintaining enzyme digestibility levels, and its removal has a negative impact on substrate digestibility.

Highlights

During the dilute acid pretreatment process, the resulting pseudo-lignin and lignin droplets deposited on the surface of lignocellulose and inhibit the enzymatic digestibility of cellulose in lignocellulose
Influence of organic reagent extraction on the chemical composition of dilute acid‐pretreated bamboo residues Recently, Hansen solubility parameter (HSP) theory has become a popular tool for screening different organic solvent systems for efficient separation of lignin from biomaterial matrices [27]
The difference in lignin extraction yields was confirmed by Scanning electron microscopy (SEM) images of extracted dilute acid-pretreated bamboo residues (DAP-BR), in which lignin droplets were not observed on the surfaces of extracted DAP-BR (Additional file 1: Table S1)

Summary

Introduction

During the dilute acid pretreatment process, the resulting pseudo-lignin and lignin droplets deposited on the surface of lignocellulose and inhibit the enzymatic digestibility of cellulose in lignocellulose. How these lignins interact with cellulase enzymes and affect enzymatic hydrolysis is still unknown. [11] Various explanations for the low enzymatic digestibility efficiency have been proposed: (1) Dilute acid-pretreated bamboo residues still have inherently complex and dense structures in natural cell walls that show recalcitrance toward enzymatic hydrolysis [7]. Reported work mainly focuses on the macroscopic effects of these lignins on the enzymatic hydrolysis of pretreated biomass, and few studies have investigated the microcosmic effects of the interaction between these lignins and cellulase on enzymatic hydrolysis

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