The effects of water interactions in cellulose suspensions on mass transfer and saccharification efficiency at high solids loadings

Abstract

Water is essential to the hydrolysis and conversion of lignocellulosic materials as it is both the medium through which enzymes diffuse to and products diffuse away from the reaction sites and a reactant in the hydrolysis reaction of the glycosidic bonds within the polysaccharides. However, little is known about how water interactions with the biomass change with solids content and how this affects mass transfer resistances during high solids saccharification. Nuclear magnetic resonance spectroscopy measurements of the T 2 relaxation times of water in cellulose suspensions were used to demonstrate that increases in solids content led to increases in the physical constraint of water in the suspensions. Moreover, the addition of either glucose (a monosaccharide which end-product inhibits b-glucosidase) or mannose (a stereoisomer of glucose that does not end-product inhibit b-glucosidase) further increased water constraint at all solids contents. The presence of either monosaccharide constrained water and inhibited saccharification rates to similar extents. This observation, coupled with the absence of cellobiose produced in the reactions, demonstrated that the presence of soluble sugars can negatively impact saccharification efficiency simply by increasing water constraint in cellulose suspensions before impacting enzyme activity. Furthermore, results are presented that demonstrate strong correlations between water constraint in cellulose suspensions with diffusivities of enzyme and monosaccharides within the system. These results are discussed in the context of increased viscosity of the aqueous fraction in the suspension resulting from increased watercellulose and water-solute interactions that ultimately increases diffusion resistances and decreases saccharification rates.

Relationship between Crystallinity Index and Enzymatic Hydrolysis Performance of Celluloses Separated from Aquatic and Terrestrial Plant Materials

“…In fact, a further increase in enzyme loading up to 250 or 300 FPU/g led to little further increase or even decrease in the sugar yield from the hydrolysis of these celluloses (data not shown). A possible reason may be that an increased enzyme loading results in increased medium viscosity, and the increased viscosity makes the diffusion of enzymes to the substrate and the leaving of products from the substrate vicinity more difficult, decreasing the efficiency of the hydrolysis reaction (Roberts et al 2011). As a whole, under an enzyme loading of 200 FPU/g, the hydrolysis reactions are no longer limited by enzyme availability so that the hydrolysis performance is truly determined by the structure of the cellulose materials.…”

Effective diffusivities of BSA in cellulosic fiber beds measured with magnetic resonance imaging

“…It was also observed that order of magnitude differences in adsorption resulted in quantitatively similar differences in effective diffusivities. In addition, Roberts et al (2011) found that increasing solids content decreased the effective diffusion coefficients of proteins in bacterial cellulose substrates. This study will continue the work and discussions presented in Tozzi et al (2012) by using more substrates and a different diffusing molecule-in this case bovine serum albumin (BSA).…”

“…Solids content has been correlated with increases in yield stress and viscosity, and concomitant decreases in hydrolysis yields (Bennington et al 1990;Hodge et al 2008;Lavenson et al 2011a;Stickel et al 2009;Um and Hanley 2008). Diffusion rates in cellulosic fiber suspensions-specifically the time scales of diffusion of smaller molecules-have only been recently studied (Roberts et al 2011;Tozzi et al 2012;Wong and Reeve 1990). Diffusion in these high solids cellulose suspensions is poorly understood, especially diffusion rates of protein and enzyme molecules.…”

“…Diffusion was found to be heavily dependent on the strength of surface adsorption of the diffusing molecules. In addition, diffusion rates are also affected by solids content, fiber characteristics, and geometry (Roberts et al 2011;Tozzi et al 2012).…”

A process for energy-efficient high-solids fed-batch enzymatic liquefaction of cellulosic biomass

“…Thus one possibility is that adding all the endo-acting enzymes at the beginning of the reaction gave more time for the full loading of endoglucanases to hydrolyze the substrate. Additionally, the longer exposure to the substrate added earlier in the reaction may allow for more release of constrained water, thus decreasing viscosities in the soluble phase and facilitating mass transfer of the same endoglucanases to biomass in subsequent additions (Roberts et al, 2011). The addition of new biomass to an ongoing biomass hydrolysis reaction (as in fed-batch mode) has been shown to adversely affect glucose yields (Rosgaard Fig.…”