Most cellulase enzymes are relatively unstable at high temperatures. The maximum activity for most fungal cellulases and |3-glucosidase occurs at 50±5°C and a pH 4.5- 5 (Taherzadeh & Karimi, 2007; Galbe & Zacchi, 2002). Usually, they lose about 60% of their activity in the temperature range 50-60 °C and almost completely lose activity at 80°C (Gautam et al., 2010). However, the enzymes activity depends on the hydrolysis duration and on the source of the enzymes (Tengborg et al., 2001). In general, cellulases are quite difficult to use for prolonged operations.

As mentioned before, the enzyme production costs mainly depend on the productivity of the enzymes-producing microbial strain. Filamentous fungi are the major source of cellulases and mutant strains of Trichoderma (T. viride, T. reesei, T. longibrachiatum) have long
been considered to be the most productive (Gusakov et al., 2007; Galbe & Zacchi, 2002). Preparations of cellulases from a single organism may not be highly efficient for the hydrolysis of different feedstocks. For example, Thrichoderma reesei produces endoglucanases and exoglucanases in large quantities, but its P-glucosidase activity is low, resulting in an inefficient biomass hydrolysis. For this reason, the goal of the enzymes producing companies has been to form cellulases cocktails by enzymes assembly (multienzyme mixtures) or to construct engineered microrganisms to express the desired mixtures (Mathew et al., 2008). Enzyme mixtures often derive from the co-fermentation of several micro-organisms (Ahamed & Vermette, 2008; Kabel et al., 2005; Berlin et al., 2007), (Table 4). All the commercial cellulases listed in table 4 have an optimal condition at 50°C and pH of 4.0-5.0. More recently, some enzymes producers have marked new mixtures able to work in a higher temperature ranging from 50 to 60°C (Table5).

In 2010, new enzymes were produced by two leading companies, Novozymes and Genencor, supported by the USA Department of Energy (DOE). Genencor has launched four new blends: Accelerase®1500, Accelerase®XP, Accelerase®XC and Accelerase®BG.

Accelerase®1500 is a cellulases complex (exoglucanase, endoglucanase, hemi-cellulase and P-glucosidase) produced from a genetically modified strain of T. reesei. All the other Accelerase are accessory enzymes complexes: Accelerase®XP enhances both xylan and glucan conversion; Accelerase®XC contains hemicellulose and cellulase activities; Accelerase® BG is a P-glucosidase enzyme. In February 2010, Genencor has developed an enzyme complex known as Accellerase®Duet which is produced with a genetically modified strain of T. reesei and that contains not only exoglucanase, endoglucanase, P- glucosidase, but includes also xylanase. This product is capable of hydrolyzing lignocellulosic biomass into fermentable monosaccharides such as glucose and xylose (Genencos, 2010)[3]. Similarly, Novozymes has produced and commercialized two new enzymatic mixtures: cellic Ctec, and cellic Htec. Cellic CTec is used in combination with Cellic HTec and this mixture is capable to work with a wide variety of pretreated feedstocks, such as sugarcane bagasse, corn cob, corn fiber, and wood pulp, for the conversion of the carbohydrates in these materials into simple sugars (Novozyme, 2010)[4].

In order to meet the future challenges, innovative bioprocesses for the production of new generation of enzymes are needed. As already described, conventional cellulases work within a range of temperature around 50°C and they are typically inactivated at temperatures above 60-70 °C due to disorganization of their three dimensional structures followed by an irreversible denaturation (Viikari et al., 2007). Some opportunities of process improvement derive from the use of thermostable enzymes.