Cellulose accessibility determines the rate of enzymatic hydrolysis of steam-pretreated spruce.

Spruce chips steam-pretreated at various conditions, according to a central composite design, were used for investigating the influence of pretreatment conditions on enzymatic hydrolysis, accounting for the individual effects of pretreatment temperature (194-220 °C), time (3-11 min) and sulfur dioxide uptake (0.7-2.5%). The materials were analyzed for several surface characteristics, including IR absorption, enzyme adsorption capacity, total surface area, cellulosic surface area, and cellulosic pore sizes. This work showed a clear correlation between rate of enzymatic hydrolysis and specific surface area. Although the lignin content of the particle surface increased at higher pretreatment temperature and residence time, the initial rate of enzymatic hydrolysis increased. Enzyme adsorption measurements and staining methods revealed that the higher rate of hydrolysis of these materials was due to increased accessibility of the cellulose. An accessible cellulose fraction is thus more important than a low surface lignin content for the enzymatic hydrolysis of steam-pretreated spruce.

Effect of mixing on enzymatic hydrolysis of steam-pretreated spruce: a quantitative analysis of conversion and power consumption.

BACKGROUND: When scaling up lignocellulose-based ethanol production, the desire to increase the final ethanol titer after fermentation can introduce problems. A high concentration of water-insoluble solids (WIS) is needed in the enzymatic hydrolysis step, resulting in increased viscosity, which can cause mass and heat transfer problems because of poor mixing of the material. In the present study, the effects of mixing on the enzymatic hydrolysis of steam-pretreated spruce were investigated using a stirred tank reactor operated with different impeller speeds and enzyme loadings. In addition, the results were related to the power input needed to operate the impeller at different speeds, taking into account the changes in rheology throughout the process. RESULTS: A marked difference in hydrolysis rate at different impeller speeds was found. For example, the conversion was twice as high after 48 hours at 500 rpm compared with 25 rpm. This difference remained throughout the 96 hours of hydrolysis. Substantial amounts of energy were required to achieve only minor increases in conversion during the later stages of the process. CONCLUSIONS: Impeller speed strongly affected both the hydrolysis rate of the pretreated spruce and needed power input. Similar conversions could be obtained at different energy input by altering the mixing (that is, energy input), enzyme load and residence time, an important issue to consider when designing large-scale plants.

Rheological characterization of dilute acid pretreated softwood.

Large-scale bioethanol production from lignocellulosic biomass will require high solids loading in the enzymatic hydrolysis step. However, slurries of pretreated lignocelluloses are complex fluids due to the fibrous nature, especially at high concentrations of water insoluble solids (WIS). A prerequisite for dealing with transport issues and for developing efficient full-scale processes is a fundamental understanding of the flow properties of pretreated lignocellulose. A comprehensive rheological characterization of dilute acid pretreated spruce has been carried out in this study, accounting for the effects of WIS concentration, particle size distribution (PSD), and the degree of enzymatic hydrolysis. The rheology of pretreated spruce slurries was found to be strongly dependent on the WIS concentration. The storage modulus (G'(LVR)) and yield stress showed typical power-law dependencies on volume fraction and WIS content. Milling of the pretreated material resulted in significantly higher yield stress and viscosity, likely due to narrower PSD, which suggests that the strength of the network of the coarsest fibers determines the rheology of these materials to a large extent. During enzymatic hydrolysis, yield stress and viscosity decreased dramatically, partly due to decreasing WIS content, but possibly also due to changes in fiber properties such as the chemical composition.

Controlled feeding of cellulases improves conversion of xylose in simultaneous saccharification and co-fermentation for bioethanol production.

Simultaneous saccharification and fermentation (SSF) is an interesting option for ethanol production from lignocellulosic materials. To meet desired overall yields during ethanol production from lignocellulosic materials, it is important to use both hexoses and pentoses. This can be achieved by co-fermentation of sugars in SSF, so called SSCF (simultaneous saccharification and co-fermentation), using genetically modified yeast strains. However, high concentration of glucose in the pretreated material makes xylose utilization challenging due to competitive inhibition of sugar transport. The present work demonstrates a new approach for controlling the glucose release rate from the enzymatic hydrolysis by controlling the addition of enzymes in SSCF using spruce as the raw material. Enzyme kinetics and yeast sugar uptake rates for a recombinant xylose utilizing strain of Saccharomyces cerevisiae, TMB3400, were determined in a real hydrolyzate medium. A simplified model for glucose release and uptake was created, to be used as a tool for control of the glucose concentration in a SSCF process. With help of this model, an SSCF process with efficient co-utilization of glucose and xylose was successfully designed. The results showed that the total xylose uptake could be increased from 40% to as much as 80% by controlling the enzyme feed.