Effect of lignin on recalcitrance of lignocellulose

Abstract: Lignin has been considered as an important factor to affect the recalcitrance of lignocellulose. Understanding the effect of lignin on the recalcitrance of lignocellulose make sense to building plants by gene modification, screening of enzymes and optimizing pretreatment conditions for biofuels production. Study on the effect of lignin on the recalcitrance of lignocellulose has been done by many researchers. This review examines the effect of lignin content and structure on the recalcitrance of lignocellulose primarily on the effect of the initial lignin content, the residual lignin content, and ratio of different constitutive units, the interconnected bond and functional group. In addition, the effect of isolated lignin on the recalcitrance of simulated lignocellulose was reviewed. General, higher initial lignin content make higher recalcitrance of lignocellulose for the same species. Relationship of them for different species was similar but the regularity is poorer. Removal of lignin from the lignocellulose can improve the enzymatic hydrolysis of biomass. The residual lignin content was negatively correlated to the recalcitrance of lignocellulose within certain limits. Beyond this limit, further removal of lignin had nonsense to the improvement of enzymatic hydrolysis of lignocellulose. The structure of the lignin effect on the recalcitrance of lignocellulose too. Firstly, the constituent of lignin, mainly the content/ proportion of syringyl units(S) / guaiacyl units (G), has influence on the inhibition of lignin to the enzymatic hydrolysis of cellulose. In addition, the content of H also affects the inhibition. Secondly, the interconnected bond between monomers of lignin impacts the recalcitrance of lignocellulose. The type of bonds may affect the complexity of lignin removal. Moreover, it may affect the contact of lignin with cellulose. Thirdly, the functional group of lignin, mainly the hydroxyl group (-OH), carboxylic group (-COOH) can affect the inhibition of lignin on the recalcitrance of lignocellulose. Lignin with high phenolic hydroxyl group was easier to be removed. On the other hand, the phenolic hydroxyl group of lignin intensifies the invalid adsorption of cellulase on the lignin. The aliphatic hydroxyl group can affect the hydrogen bond between cellulase and lignin. Carboxylic group of lignin can reduce the invalid adsorption of cellulase on the lignin. In order to suppress interference, many studiers isolated lignin from biomass, and study the impact of isolated lignin on the cellulose. Since the origin, species and methods of isolation were different, the structure of lignin was different. As a result, the effect of isolated lignin on the enzymatic hydrolysis of cellulose was different. Though many studies have been done, it is far from recognizing the exact effect of lignin on the recalcitrance of lignocellulose. Because there are various lignocellulose and lignin, the study conclusions always result from their substrate. It is difficult to summarize the law applied to all lignocellulose. Verification of lignin effect on the recalcitrance of lignocellulose need further studies and advance in the In situ detection technology.