Can delignification decrease cellulose digestibility in acid pretreated corn stover?

Abstract

It has previously been shown that the improved digestibility of dilute acid pretreated corn stover is at least partially due to the removal of xylan and the consequent increase in accessibility of the cellulose to cellobiohydrolase enzymes. We now report on the impact that lignin removal has on the accessibility and digestibility of dilute acid pretreated corn stover. Samples of corn stover were subjected to dilute sulfuric acid pretreatment with and without simultaneous (partial) lignin removal. In addition, some samples were completely delignified after the pretreatment step using acidified sodium chlorite. The accessibility and digestibility of the samples were tested using a fluorescence-labeled cellobiohydrolase (Trichoderma reesei Cel7A) purified from a commercial cellulase preparation. Partial delignification of corn stover during dilute acid pretreatment was shown to improve cellulose digestibility by T. reesei Cel7A; however, decreasing the lignin content below 5% (g g -1 ) by treatment with acidified sodium chlorite resulted in a dramatic reduction in cellulose digestibility. Importantly, this effect was found to be enhanced in samples with lower xylan contents suggesting that the near complete removal of xylan and lignin may cause aggregation of the cellulose microfibrils resulting in decreased cellulase accessibility.

Proposed Nano-Scale Coalescence of Cellulose in Chemical Pulp Fibers During Technical Treatments

“…However, the hemicellulose removal cannot by itself explain the coalescence, since there seems to be no fibril coalescence under low-temperature alkaline conditions that also remove hemicelluloses (Fahlén and Salmén 2003). The additional removal of lignin combined with the hemicellulose removal seems to give the microfibrils even more possibility to coalesce (Ishizawa et al 2009). The coalescence during chemical pulping may also be due to the softening of the lignin network (Fahlén and Salmén 2003).…”

Relating Nanoscale Accessibility within Plant Cell Walls to Improved Enzyme Hydrolysis Yields in Corn Stover Subjected to Diverse Pretreatments

“…Reductions in cellulose accessibility resulting from cellulose aggregation induced by sufficient removal of noncellulosic components in the cell wall would explain both reduced hydrolysis kinetics, as well as limited cellulose hydrolysis. 30 The cellulase enzyme cocktail used in this study (i.e., CTec3) has been shown to contain a suite of accessory enzymes including xylanase and pectinase activities. 31 Furthermore, this cocktail is capable of effectively hydrolyzing pretreated biomass with low to moderate hemicellulose content without any additional accessory enzymes; 31 however, significantly lower yields are observed in pretreated feedstocks containing a substantial fraction of the original xylan, such as the AHP-only samples.…”

Structural Characteristics and Distribution of Lignin in Eucalyptus Globulus Pulps Obtained by a Combined Autohydrolsis/Alkaline Extraction Process for Enzymatic Saccharification of Cellulose

“…Lignin also tends to irreversibly bind the enzymes through hydrophobic interactions causing a loss in cellulases activities 14,15,16 .In addition, during the pretreatment step lignin suffer changes in its structure and the effects on the enzymatic hydrolysis of cellulose are not completely understood 17,18 . For instance, the chemical removal of lignin after dilute acid pretreatment reduced the cellulose conversion possibly due to the aggregation of cellulose microfibrills in bundles that hindered the enzymes penetration 6,19 . On the other hand, Zhu et al 20 removed xylans from hardwood species improving the enzymatic digestibility of the substrate due to dissolution of acetyl and uronic acid groups, while the lignin removal was not the most important factor for enhancing cellulose hydrolysis.…”

“…Autohydrolysis pretreatment fails to remove high amounts of lignin, but the structure and distribution of this macromolecule over and inside the cell walls is altered, and has been demonstrated that, in some cases, it can affect the enzymatic access to the cellulose 25 . Some authors have reported that after hydrothermal and diluted acid pretreatments, the glucose yields from enzymatic hydrolysis was higher even when the lignin removal was minimal 19,26 . Many researchers have indicated that the combination of processes such as autohydrolysis and alkaline washing could increase the accessibility of cellulose to the enzymatic hydrolysis by removing recalcitrant lignin, improving the cellulose digestibility 24,27,28 .…”