Mechanistic insights into the liquefaction stage of enzyme‐mediated biomass deconstruction

Abstract

Effective enzyme-mediated viscosity reduction, disaggregation, or "liquefaction," is required to overcome the rheological challenges resulting from the fibrous, hygroscopic nature of lignocellulosic biomass, particularly at the high solids loadings that will be required for an economically viable process. However, the actual mechanisms involved in enzyme-mediated liquefaction, as determined by viscosity or yield stress reduction, have yet to be fully resolved. Particle fragmentation, interparticle interaction, material dilution, and water-retention capacity were compared for their ability to quantify enzyme-mediated liquefaction of model and more realistic pretreated biomass substrates. It was apparent that material dilution and particle fragmentation occurred simultaneously and that both mechanisms contributed to viscosity/yield stress reduction. However, their relative importance was dependent on the nature of the biomass substrate. Interparticle interaction and enzyme-mediated changes to these interactions was shown to have a significant effect on slurry rheology. Liquefaction was shown to result from the combined action of material dilution, particle fragmentation, and alteration of interactions at particle surfaces. However, the observed changes in water retention capacity did not correlate with yield stress reduction. The relative importance of each mechanism was significantly influenced by the nature of the biomass substrate and its physicochemical properties. An ongoing challenge is that mechanisms, such as refining, which enhance enzyme accessibility to the cellulosic component of the substrate, are detrimental to slurry rheology and will likely impede enzyme-mediated liquefaction when high substrate concentrations are used.

Synergistic Enzyme Cocktail to Enhance Hydrolysis of Steam Exploded Wheat Straw at Pilot Scale

“…It is an "auto catalyzed" process, where acetic acid released from hemicellulose and self-ionization of water at elevated temperatures causes a drop in pH which acts as a catalyst (Sharma et al, 2015b). SE has been reported to break down biomass structure and develop pores for higher enzymatic accessibility (van der Zwan et al, 2017). During SE most of the hemicellulose is hydrolyzed to soluble sugar monomers and some fraction of cellulase also gets depolymerized into glucose while, a small fraction of lignin is dissolved and/or redistributed (Kumar and Murthy, 2011;Agrawal et al, 2013b).…”

“…A few published reports are discussed here inbrief which might be referred for further details. Takano and Hoshino (2018) who optimized the commercial enzyme reagents for hydrolysis of the alkali pretreated rice straw have reported that a combination of "Cellulase Onozuka 3S, " "Cellulase T Amano 4, " and "Pectinase G Amano" improved the hydrolysis efficiency with a yield of 94%. Soleimani and Ranaei-Siadat (2017) reported that an optimize enzyme cocktail optimized via response surface methodology involved enzymes, including Exo-1,4-β-D-glucanases II (CBHII), Endo-1,4-β-D-glucanases II (EGII) and 1,4-β-D-glucosidases I (BGI) improved the degradation of pretreated bagasse.…”

Toward an understanding of the increase in enzymatic hydrolysis by mechanical refining

“…The analysis of microscope images showed that refining caused separation of cells, surface fibrillation, internal delamination and generation of fines. These changes collectively contributed to increased accessible surface area and enhance enzymatic hydrolysis [ 7 , 11 – 13 ]. Fiber quality analyzer and laser diffraction were used to measure particle size of unrefined and refined biomass.…”

“…Water retention values, dye adsorption and differential scanning calorimetry were used to access the effect of mechanical refining on biomass surface area. Refining processes increased cell wall porosity and surface area, which contributed to increase the exposure of carbohydrates to cellulolytic enzymes [ 9 , 10 , 12 , 13 , 15 , 16 ]. The effect of mechanical refining on cellulose crystallinity was also evaluated.…”

Enzyme-Mediated Lignocellulose Liquefaction Is Highly Substrate-Specific and Influenced by the Substrate Concentration or Rheological Regime

“…It has been shown that, during enzymatic hydrolysis, the viscosity or yield stress of lignocellulose slurries decreases in a process termed "liquefaction, " which occurs through various mechanisms including material dilution, particle fragmentation and modification of interparticle interactions (Roche et al, 2009;Thygesen et al, 2014;van der Zwan et al, 2017). To deal with the high viscosities/yield stresses of concentrated lignocellulose slurries, a separate enzymatic liquefaction stage similar to that used in starch-based biorefineries appears promising (Szijártó et al, 2011a), reducing viscosity/yield stress prior to further saccharification.…”