The logical culmination of reaction-reaction integration for the transformation of biomass into ethanol is the consolidated bioprocessing (CBP), known also as direct microbial conversion (DMC). The key difference between CBP and the other strategies of biomass processing is that only one microbial commu­nity is employed for both the production of cellulases and fermentation, i. e., cellulase production, cellulose hydrolysis, and fermentation are carried out in a single step (see Figure 9.2). This configuration implies that no capital or operation expenditures are required for enzyme production within the process. Similarly, part of the substrate is not deviated for the production of cellulases, as shown in Figure 9.7 (compare to the more complex configurations depicted in Figure 7.6 for the SHF process with the addition of commercial cellulases, and Figure 9.4 for SSF of biomass with in situ production of cellulases). Moreover,

TABLE 9.4

Integration of Reaction-Reaction Processes by Simultaneous Saccharification and Co-Fermentation (SSCF) for Fuel Ethanol Production from Different feedstocks

Source: Modified from Cardona, C. A., and O. J. Sanchez. 2007. Bioresource Technology 98:2415-2457. Elsevier Ltd. Note: SHCF = separate hydrolysis and co-fermentation.

FIGURE 9.7 Simplified diagram of the integrated process for fuel ethanol production from lignocellulosic biomass by consolidated bioprocessing (CBP).

the enzymatic and fermentation systems are entirely compatible (Cardona and Sanchez, 2007).

The extended concept of CBP involves four biologically mediated transforma­tions: (1) the production of saccharolytic enzymes (cellulases and hemicellulases); (2) the hydrolysis of carbohydrate components present in pretreated biomass to form sugars; (3) the fermentation of hexose sugars (glucose, mannose, and galac­tose); and (4) the fermentation of pentose sugars (xylose and arabinose). These four transformations occur in a single step. In this case, a dedicated process for production of cellulases is not required to make CBP a highly integrated configu­ration (Cardona and Sanchez, 2007; Lynd et al., 2005). This process is conceptu­ally depicted in Chapter 6, Figure 6.5 for the case of ethanol production from lignocellulosic biomass.

Process integration through CBP represents a considerable improvement of technologies for conversion of lignocellulosic biomass into ethanol. The enhance­ment of the conversion technology contributes by far the most reduction of etha­nol production costs (Cardona and Sanchez, 2007). According to projections of Lynd (1996), the reduction of production costs due to an advanced configuration involving the CBP is three times greater than the reduction related to the scale economy of the process and 10 times greater than the reduction associated with a lower cost of the feedstock. This diminish would be accomplished thanks to the reduction of more than eight times in the costs of biological conversion (Lynd et al., 1996). Lynd et al. (2005) reported the comparative simulation of SSCF and CBP processes assuming aggressive performance parameters intended to be rep­resentative of mature technology. Their results indicate that production costs of

ethanol for SSCF reach US4.99 cents/L including the costs of dedicated cellulase production, whereas CBP gives total costs of only US1.11 cents/L demonstrating the potential effectiveness of this process configuration.

Most studies on CBP of biomass contemplate the use of the thermophilic bacterium Clostridium thermocellum, which is employed for cellulase produc­tion, cellulose hydrolysis, and glucose fermentation. In addition, the bacterium Thermoanaerobacter thermosaccharolyticum can be co-cultured along with C. thermocellum to allow the simultaneous conversion of pentoses obtained from hemicellulose hydrolysis into ethanol (Cardona and Sanchez, 2007; Wyman, 1994). In particular, the CBP using C. thermocellum showed a substrate conversion 31% higher than a system using T. reesei and S. cerevisiae. South et al. (1993) tested the continuous CBP of cellulose into ethanol using C. thermocellum and showed, under very specific conditions with a residence time of 0.5 d, higher con­versions than a continuous SSF process. Some filamentous fungi such as Monilia sp., Neurospora crassa, and Paecilomyces sp. are also able to transform cellulose into ethanol (Szczodrak and Fiedurek, 1996). Nevertheless, this technique faces the problem of the low tolerance of clostridia to ethanol and the reduction in the ethanol yield due to the formation of acetic acid and salts of other organic acids like lactates (Baskaran et al., 1995; Cardona and Sanchez, 2007; McMillan, 1997; Wyman, 1994). This means that the final ethanol concentration is low in compari­son with the traditionally used yeasts (0.8 to 60 g/L) with very large cultivation times of 3 to 12 d (Szczodrak and Fiedurek, 1996).

To date, there is no microorganism known that can exhibit the whole com­bination of features required for the development of a CBP, as the one shown in Figure 6.5 (Chapter 6). However, there are realistic expectations about the possibility of overcoming the limitations of current CBP organisms. In Section 6.3.2.2, the main strategies for developing engineered microorganisms that can be used in technological configurations involving CBP were disclosed. In this way, the huge possibilities of CBP are based on the development of genetically modified microorganisms allowing such a high degree of reaction-reaction inte­gration that can make possible the direct conversion of pretreated lignocellu — losic biomass into ethanol at elevated yields under industrial conditions. Some examples of CBP, not only of lignocellulosic materials but also of starch, are presented in Table 9.5.