Integrated technology for Consolidate Bioprocessing from sugarcane straw using recombinant yeast

Abstract

Second generation (2G) ethanol is produced from lignocellulosic materials, which, despite representing an abundant source of energy, are still underused, especially its hemicellulosic fraction. In the last decades, many efforts have been carried out seeking to make viable the use of this fraction of the biomass in the context of the Biorefinery concept, which foresees the total and integrated use of lignocellulosic materials. On the other hand, the high costs associated with enzymatic cocktails can compromise the economic viability of the industrial process for 2G ethanol production. In this sense, ConsolidatedBioprocessing (CBP) stands out as an emerging technology, which enables the production of 2G ethanol using both cellulosic and hemicellulosic fractions of biomass. In CBP, the production of enzymes, hydrolysis of biomass and fermentation of released sugars occur simultaneously in a single reactor. Although the construction of some recombinant strains for application in CBP is reported in the literature, the focus has been on Genetic Engineering, while the development and optimization of "Consolidated Bioprocessing Engineering" has not been explored. Therefore, despite the availability of efficient strains for CBP, fermentations are long and result in low concentrations of ethanol, showing the need to develop appropriate cultivation strategies to enhance the performance of recombinant yeasts built for CBP. In this sense, the present project proposes to study the CBP applied to sugarcane straw, a residue from the sugarcane industry, using a high performance recombinant strain of Saccharomyces cerevisiae (capable of secreting seven hydrolytic enzymes), thus contributing to the development of a process for 2G ethanol production with high productivity and easy scalability and application in industrial conditions. Initially, the sugarcane straw will be subjected to hydrothermal pre-treatment, generating the hemicellulose hydrolysate (liquid fraction)nd cellulignin (solid fraction). Part of the cellulignin obtained will be further subjected to freezing as a complementary pre-treatment, to evaluate its modification effect on the structure of the solid material. Experiments to assess the performance of CBP in different formulations of culture media, containing the hemicellulose hydrolysate mixed with the solid fractions resulting from the hydrothermal pre-treatment, combined or not with freezing, will be carried out in mini-reactors, at 35 oC and initial pH 5.0. From the results obtained in the experiments in minireactors, the most promising medium formulation will be identified and employed in validation studies using a bioreactor equipped with a module for solid fraction confinement, which will be operated in simple and repeated batches. The experiments will be accompanied by the measurement of carbon dioxide production, by the analysis of the concentration of sugars and products in the supernatant (high performance liquid chromatography) and by the characterization of the solid materials generated before and after the CBP.