5.2.3.1 Efficiency of Cellulases

Cellulase utilization plays a crucial role considering the global costs of the bio — mass-to-ethanol process. The cellulases available for ethanol industry account for 36 to 45% of the costs of bioethanol produced from lignocellulosic materi­als. According to different evaluations (Cardona and Sanchez, 2007; Reith et al., 2002), a 30% reduction in capital costs and 10-fold decrease in the cost of current cellulases are required for this process to be competitive in relation to ethanol produced from starchy materials. These analyses evidence the need of improving the cellulase performance in the following aspects:

• Increase of thermal stability

• Improvement of the binding to cellulose

• Increase of specific activity

• Reduction of the nonspecific binding to lignin

The increase of the thermal stability of cellulases is important since the tem­perature increase implies an increase of the cellulose hydrolysis rate (Mielenz, 2001). The augment of specific activity, in turn, can lead to significantly reduced costs. It is estimated that a 10-fold increase in specific activity could lead to nearly 16 cents (U. S.) savings per liter of ethanol produced. Among the strategies to attain the increase of specific activity are the increase in the efficiency of active sites through protein engineering or random mutagenesis, augment of thermal tolerance, improvement in the degradation of the crystalline structure of cellu­lose, enhancement of the synergism among the cellulases from different sources, and reduction of nonspecific bindings (Cardona and Sanchez, 2007; Sheehan and Himmel, 1999).

In general, the costs of cellulases are considered high. According to prelimi­nary evaluations of the National Renewable Energy Laboratory (NREL) cited by Tengerdy and Szakacs (2003), the cost of cellulase production in situ by sub­merged culture is U. S.$0.38/100,000 FPU. Hence, cellulase costs make up 20% of ethanol production costs, assuming them at U. S.$1.5/gallon. On the other hand, commercial cellulase cost (U. S.$16/100,000 FPU) is prohibitive for this process. In contrast, these authors indicate that the cost for producing cellulases by solid — state fermentation of corn stover would be U. S.$0.15/100,000 FPU that would correspond to U. S.$0.118/gal EtOH, i. e., nearly 8% of total costs. The analysis of process integration via simulation of not only the technological scheme, but also the costs structure, can provide key elements allowing a deep evaluation of all of these alternatives in order to choose the more convenient option for in situ cellu — lase production. On the other hand, the mathematical description of cellulase pro­duction can allow the definition of useful relationships to assess the performance and quality of the production of such enzymes using different process analysis approaches. These approaches undoubtedly will contribute to the definition of strategies to lower the costs of cellulases. This is the case of cellulase production by Trichoderma reesei using kinetic and neural networks approaches (Tholudur et al., 1999), which allowed the optimization of operating conditions considering two performance indexes based on the estimated protein value and volumetric productivity.