After lignocellulose pretreatment, there are four biologically mediated events typically in the course of biological processing of cellulosic biomass: cellulase production, enzymatic cellulose hydrolysis, hexose fermentation, and pentose fermentation (Figure 16.2). Separate

SHF: Separate hydrolysis & fermentation CBP: Consolidated bioprocessing

SSF:Simultaneous saccharification & fermentation SSCF: Simultaneous saccharification & co-fermentation

Figure 16.2 Evolution of biomass processing configurations featuring enzymatic hydrolysis.

hydrolysis and fermentation (SHF) involves these four discrete process steps. Simultaneous saccharification and fermentation (SSF) consolidates cellulose hydrolysis and hexose fer­mentation. Simultaneous saccharification and co-fermentation (SSCF) combines cellulose hydrolysis, hexose fermentation, and pentose fermentation. Consolidated bioprocessing (CBP) integrates cellulase production, cellulose hydrolysis, with pentose and hexose fer­mentations in a single step (10, 15, 28).

Over the past few years, much effort has been devoted to reducing the cost of cellulase en­zyme production (24). Following greater than 20-fold cost reductions, cellulase production costs have recently been reported in the range of ~20 cents per gallon of cellulosic ethanol produced (29). These developments enable a variety of formerly infeasible industrial SSF and SSCF processes, but do not diminish the competitive potential of CBP by offering significantly lower costs than other processes.

CBP offers the potential for lower production costs, lower capital investment, and higher conversion efficiency as compared to the processes featuring dedicated cellulase produc­tion. CBP avoids costs for capital, substrate, other raw materials, and utilities associated with cellulase production. In addition, CBP could realize higher hydrolysis rates, and hence reduce reactor volume and capital investment as a result of enzyme-microbe synergy. CBP provides access to the use of thermophiles or other organisms with high activity cellu — lases. Moreover, cellulose-adherent cellulolytic microorganisms may successfully compete for products of cellulose hydrolysis with non-adhered microbes. Moreover, these microor­ganisms are likely to be less sensitive to contaminants, which could increase the stability of an industrial processes. Economic analysis suggests that the sum of 9.9 p7gal ethanol

for dedicated cellulase production and 9.0 у/gal for SSCF gives a total cost for biological processing of 18.9 c/gal, which is more than fourfold greater than the 4.2 c/gal projected for CBP (28).

Today, there are no CBP-enabling microorganisms suitable for industrial applications. CBP microorganism development can proceed via a native cellulose utilization strategy and a recombinant cellulose utilization strategy (Figure 16.3). The native cellulolytic strategy involves engineering product metabolism to produce desired products based on naturally cellulolytic microorganisms (e. g., Clostridium thermocellum). The recombinant cellulolytic strategy involves introducing heterologous cellulase genes into an organism whose product yield and tolerance credentials are well-established (e. g., Baker’s yeast Saccharomyces cere — visiea). Each strategy has its own advantages and challenges, and different strategies may well prove most advantageous for different products.