Coupling the pretreatment and hydrolysis of lignocellulosic biomass by the expression of beta-xylosidases

Thermochemical pretreatment and enzymatic hydrolysis are the areas contributing

most to the operational costs of second generation ethanol in lignocellulosic

biorefineries. The improvement of lignocellulosic enzyme cocktails has been significant

in the recent years. Although the needs for the reduction of the energy intensity and

chemical consumption in the pretreatment step are well known, the reduction of the

severity of the process strongly affects the enzymatic hydrolysis yield. To explore the

formulation requirements of the well known cellulolytic cocktail from Myceliophthora

thermophila on mild pretreated raw materials, this cocktail was tested on steam

exploded corn stover without acid impregnation. The low hemicellulose yield and

significant accumulation of xylobiose compared with the standard pretreated material

obtained with dilute acid impregnation evidenced a clear limitation in the conversion of

xylan to xylose. In order to complement the beta-xylosidase limitation, a selection of

enzymes was expressed and tested in this fungus. A controlled expression of

xylosidases from Aspergillus nidulans, Aspergillus fumigatus, and Fusarium oxysporum

allowed recovering hemicellulose yields reached with standard acid treated material.

The results underline the need of parallel development of the pretreatment process

with the optimization of the formulation of the enzymatic cocktails.