Lignosulfonate and elevated pH can enhance enzymatic saccharification of lignocelluloses

Abstract

BackgroundNonspecific (nonproductive) binding (adsorption) of cellulase by lignin has been identified as a key barrier to reduce cellulase loading for economical sugar and biofuel production from lignocellulosic biomass. Sulfite Pretreatment to Overcome Recalcitrance of Lignocelluloses (SPORL) is a relatively new process, but demonstrated robust performance for sugar and biofuel production from woody biomass especially softwoods in terms of yields and energy efficiencies. This study demonstrated the role of lignin sulfonation in enhancing enzymatic saccharification of lignocelluloses – lignosulfonate from SPORL can improve enzymatic hydrolysis of lignocelluloses, contrary to the conventional belief that lignin inhibits enzymatic hydrolysis due to nonspecific binding of cellulase.ResultsThe study found that lignosulfonate from SPORL pretreatment and from a commercial source inhibits enzymatic hydrolysis of pure cellulosic substrates at low concentrations due to nonspecific binding of cellulase. Surprisingly, the reduction in enzymatic saccharification efficiency of a lignocellulosic substrate was fully recovered as the concentrations of these two lignosulfonates increased. We hypothesize that lignosulfonate serves as a surfactant to enhance enzymatic hydrolysis at higher concentrations and that this enhancement offsets its inhibitive effect from nonspecific binding of cellulase, when lignosulfonate is applied to lignocellulosic solid substrates. Lignosulfonate can block nonspecific binding of cellulase by bound lignin on the solid substrates, in the same manner as a nonionic surfactant, to significantly enhance enzymatic saccharification. This enhancement is linearly proportional to the amount of lignosulfonate applied which is very important to practical applications. For a SPORL-pretreated lodgepole pine solid, 90% cellulose saccharification was achieved at cellulase loading of 13 FPU/g glucan with the application of its corresponding pretreatment hydrolysate coupled with increasing hydrolysis pH to above 5.5 compared with only 51% for the control run without lignosulfonate at pH 5.0. The pH-induced lignin surface modification at pH 5.5 further reduced nonspecific binding of cellulase by lignosulfonate.ConclusionsThe results reported in this study suggest significant advantages for SPORL-pretreatment in terms of reducing water usage and enzyme dosage, and simplifying process integration, i.e., it should eliminate washing of SPORL solid fraction for direct simultaneous enzymatic saccharification and combined fermentation of enzymatic and pretreatment hydrolysates (SSCombF). Elevated pH 5.5 or higher, rather than the commonly believed optimal and widely practiced pH 4.8-5.0, should be used in conducting enzymatic saccharification of lignocelluloses.

pH‐Induced Lignin Surface Modification to Reduce Nonspecific Cellulase Binding and Enhance Enzymatic Saccharification of Lignocelluloses

“…The results are corroborated by our previous studies [24,25] that used lignocellulosic substrates produced from a softwood and a hardwood pretreated by different processes (i.e., dilute acid, alkaline, and SPORL). These results contradict the well-established concept that the optimal pH is 4.8-5.0 for the enzymatic hydrolysis of lignocelluloses using Trichoderma reesi cellulase based on optimization with pure cellulose.…”

Enhanced Enzymatic Hydrolysis of Chitosan by Surfactant Addition

“…At the reaction time 24h, the addition of Tween 80 could increase the formation of reducing sugars approximately two times higher than without Tween 80. On the contrary, as reported in a previous publication, surfactants actually did not consistently enhance the enzymatic hydrolysis of pure cellulose [31][32][33]. However, the addition of surfactant in the pretreatment could improve the conversion of cellulose [16][17][18][19][20].…”

Pilot-scale demonstration of SPORL for bioconversion of lodgepole pine to bioethanol and lignosulfonate

“…An elevated pH of 5.5, higher than the commonly used pH of 4.8 -5.0 can reduce nonproductive cellulase binding to lignin and enhance lignocellulose saccharifi cation ( Lan et al 2013b ;Lou et al 2013 ;Wang et al 2013 ). The CTec3 enzyme loading was 10 FPU g -1 glucan.…”