On-site cellulase production for cost-effective manufacturing of 2nd-generation biofuels

The techno-economic evaluations of processes for ethanol production from lignocellulosic biomass demonstrate that reducing the cost of the enzymes responsible for cellulose breakdown (cellulases) is a prerequisite to bring the ethanol cost to values close to that of ethanol produced from starch. A very attractive option is to perform the production of enzymes on-site in order to substitute expensive and pure carbon sources by waste streams generated by the process. At present, industrial cellulases are mainly produced by a filamentous fungus, Trichoderma reesei, because of its high secretion capacity. During its growth, the suspension of T. reesei exhibits complex flow properties with shear thinning characteristics and an increase of the viscosity that decreases the oxygen transfer rate. The integration of the cellulase production step in a lignocellulosic biorefinery was studied by determining the impacts on the metabolism of Trichoderma reesei of the available sugar mixtures and the inhibitory compounds generated by the acidic pretreatment of the lignocellulosic biomass. Various fermentation strategies have been tested to overcome the deleterious effect of inhibitory compounds and a successful approach has been defined to produce cellulases at a concentration higher than 100 g/L with low oxygen transfer rate. The crude fermentation broth containing the produced enzymes has been tested to hydrolyze pretreated wheat straw without any previous separation. The obtained performances were comparable with those obtained with "pure" enzymes. These results clearly illustrate the benefits of an on-site cellulase production, using locally available lignocellulosic biomass, for a cost-effective production of 2nd-generation biofuels.