For biofuel production processes to be more cost-effective, on-site enzyme production of cellulase and hemicellulase (or xylanase) using lignocellulosic feedstock has been suggested (Singhania et al. 2007). Although lignocellulosic materials have great potential as a feedstock for ethanol production, they could also serve as feedstock for the production of enzymes when pretreated effectively prior to use (Esterbauer et al. 1991; Singhania et al. 2007). Cellulolytic enzymes may be produced by a wide variety of bacteria and fungi. These microorganisms may be cultivated in a wide range of conditions, ranging from aerobic to anaerobic and mesophilic to thermophilic environments. Among bacteria, Cellulomonas fimi and Thermomonospora fusca have been extensively studied for cellulolytic enzyme production. However, anaerobic bacteria such as Clostridium thermocellum and Bacteroides cellulosolvens produce high cellulase activity with low enzyme productivity (Duff and Murray 1996). Under anaerobic conditions, low growth rates limit enzyme productivity (Duff and Murray 1996).

On the other hand, more than 14,000 fungi are known to produce cellulolytic enzymes including Aspergillus niger, Aspergillus awamori, Trichoderma reesei, Phanerochaete chrysosporium and Pleurotus sajor-cajuz. These fungi are a good source of cellulases, but, among these, Trichoderma is one of the best known for a variety of industrial applications such as in the textile, beverages, and biofuels industries. The genetics of T. reesei plays an important role in commercial celluloytic enzyme production. A series of genetic modifications of T. reesei produced T. reesei Rut C-30, resulting in better enzyme productivity. A genetically engineered strain of T. reesei increased both the enzyme production and removal of unwanted enzyme expressions at the same time. For example, the expression of cellulases was suppressed during the production of xylanases by T. reesei.

T. reesei Rut-C-30 is a mesophilic fungus, producing high cellulase activity with considerable amounts of xylanase and p-glucosidase amongst Trichoderma spp. (Tangnu et al. 1981, Table 3). A consortium of different enzymes (consisting of endoglucanase, exoglucanase (cellobiohydrolases) and p-glucosidase (cellobiase)) act synergistically to break down lignocellulosic material (Esterbauer et al. 1991). However, endoglucanase and p-glucosidase respectively comprise approximately 20-36% and 1% of total cellulases produced by Trichoderma spp. (Esterbauer et al. 1991; Xiao et al. 2004).

T. reesei could be cultivated in aerobic fermentation mode using either a solid-state fermentation (SSF) or a submerged culture fermentation (SCF). In commercial cellulolytic enzyme production, SCF is preferred over the SSF process due to easier control of temperature, pH, oxygen, and the moisture

gradient of the substrate. The main aim of SSF is to achieve higher substrate concentration for fermentation, whereas in SCF the substrate concentration is limited to approximately 8% of the solution requiring much greater amounts of water. With SCF processing substrate concentration, nutrient requirement, T. reesei biomass concentration, pH, temperature, aeration and agitation are important parameters that influence the productivity of cellulolytic enzymes.