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 T2 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 β-glucosidase) or mannose (a stereoisomer of glucose that does not end-product inhibit β-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 water-cellulose and water-solute interactions that ultimately increases diffusion resistances and decreases saccharification rates.