Rheological properties of corn stover slurries during fermentation by Clostridium thermocellum

Abstract

BackgroundMilling during fermentation, termed cotreatment, has recently been proposed as an alternative to thermochemical pretreatment as a means to increase the accessibility of lignocellulosic biomass to biological attack. A central premise of this approach is that partial solubilization of biomass changes the slurry’s physical properties such that milling becomes more impactful and more feasible. A key uncertainty is the energy required to mill partially fermented biomass. To inform both of these issues, we report rheological characterization of small-particle, corn stover slurries undergoing fermentation by Clostridium thermocellum.ResultsFermented and unfermented corn stover slurries were found to be shear-thinning and well described by a power law model with an exponent of 0.10. Plastic viscosity of a slurry, initially at 16 wt.% insoluble solids, decreased as a result of fermentation by a factor of 2000, with the first eightfold reduction occurring in the first 10% of carbohydrate conversion. Large amplitude oscillatory shear experiments revealed only minor changes to the slurry’s rheological fingerprint as a result of fermentation, with the notable change being a reduction in the critical strain amplitude needed for the onset of nonlinearity. All slurries were found to be elastoviscoplastic, with the elastic/viscous crossover at roughly 100% strain amplitude.ConclusionsWhereas prior biomass rheology studies have involved pretreated feedstocks and solubilization mediated by fungal cellulase, we report results for feedstocks with no pretreatment other than autoclaving and for solubilization mediated by C. thermocellum. As observed in prior studies, C. thermocellum fermentation results in a dramatic decrease in viscosity. The magnitude of this decrease, however, is much larger starting with unpretreated feedstock than previously reported for pretreated feedstocks. LAOS measurements provide a detailed picture of the rheological fingerprint of the material. Viscosity measurements confirm the hypothesis that the physical character of corn stover slurries changes dramatically during fermentation by C. thermocellum, and indicate that the energy expended on overcoming slurry viscosity will be far less for partially fermented corn stover than for unfermented corn stover.

High-solids enzymatic hydrolysis of ball-milled corn stover with reduced slurry viscosity and improved sugar yields

“…The apparent viscosity decreased as the hydrolysis process progressed. The decrease of viscosity could be explained by the decreased insoluble solid content and the modified particle properties as hydrolysis progressed [18,19]. The viscosity during EH was used to estimate the energy consumption for mixing according to the theory in "Methods" section, and this is discussed in the next section.…”

“…The effects of solid loading, composition and particle morphology (including particle size, size distribution and aspect ratio) on the rheological properties of the biomass slurries have been reported [12][13][14][15][16]. Studies have also looked at the evolution of the rheological behavior during the course of saccharification or fermentation [17][18][19]. In general, biomass slurries containing smaller particles exhibit lower viscosity and yield stress under same solids loading [14,15,20], which implies that reducing particle size may potentially improve the rheology of high-solids slurry and therefore reduce the energy consumption for mixing.…”

Toward low-cost biological and hybrid biological/catalytic conversion of cellulosic biomass to fuels

“…For laminar flow, relevant for high-solids lignocellulose slurries, the mass transfer coefficient is proportional to the viscosity to the À0.5 power. Because the viscosity of biomass slurries at high solids is in general markedly higher than that of water, 70 the mass transfer coefficients for such slurries are typically much lower than that for water. Aeration provides an illustrative example of the impact of slurries on processing costs.…”

“…[148][149][150][151] However, the profound physical changes that accompany biologically-mediated plant cell wall polysaccharide deconstruction may enable much-reduced energy requirements for milling at an intensity sufficient to enhance solubilization. [68][69][70] In experiments involving fermentation without milling and milling without fermentation followed by a second fermentation, Paye et al 136 found that ball milling for five minutes nearly doubled solubilization of senescent switchgrass by C. thermocellum compared to two fermentations without milling. Balch et al 62 found a similar enhancement of senescent switchgrass solubilization for C. thermocellum fermentation with continuous ball milling as compared to an unmilled control.…”

“…64 Strategies to ameliorate these challenges have been investigated, including partial hydrolysis in advance of introducing solids into the vessel in which fermentation occurs, 65 and fed-batch operation. 64,66,67 Both of these strategies take advantage of the dramatic liquefaction that accompanies biologically-mediated solubilization of lignocellulose (e.g., viscosity reduction by about 30-fold for pretreated lignocellulose 35,36 and as much as 2000-fold for unpretreated lignocellulose), 70 with most of this reduction occurring during the first 10% of the reaction. Even with such strategies, solids handling constraints limit product concentrations for lignocellulosic feedstocks to lower values than can be achieved with starch-rich feedstocks.…”