Influence of encapsulating conditions on alginate and nutrient supplementation on xylose fermentation by recombinant Saccharomyces cerevisiae

Abstract

The exploitation of biomass hemicellulosic fraction is important to economically enable the industrial production of second generation bioethanol (2G). Wild strains of Saccharomyces cerevisiae do not metabolize xylose, but recombinant strains, capable of efficiently assimilate this sugar under fermentative conditions, are available. However, the implementation of an industrial process to produce 2G ethanol using recombinant strains relies on developing fermentation strategies proper for obtaining ethanol in high concentrations from hemicellulosic concentrated hydrolysates, containing inhibitory compounds. Innumerous studies describe the protection effect provided by alginate gel, whilst others mention that supplementation with lipids and nutrients rich in proteins can contribute to a higher tolerance to ethanol. Therefore, the main objectives of this proposal are evaluate the influence of alginate gel microcapsules characteristics, enriched (or not) with specific nutrients, on the performance of xylose fermentation process by immobilized recombinant yeast and identify the main factors that contribute to increase the ethanol productivity. The experiments will be carried out under fermentative conditions, in sealed microreactors with an exit for CO2 release, containing high load (10 % in mass) of recombinant yeast S. cerevisiae MDS130, using medium YPX (yeast extract, peptone and xylose), supplemented (or not) with ethanol. The influence of 7 factors (sodium alginate concentration and particles diameter, as well as the presence of yeast extract, peptone, chitosan, lipids and glycerol in the microcapsules) will be studied according to the Plackett-Burman experimental design methodology. The fermentations will be tracked by the mass loss associated with CO2 release. At the end of the fermentations, xylose, ethanol and xylitol concentrations (high performance liquid chromatography) and cell viability (counting cells stained by methylene blue under microscope) will be determined. Six responses (viability, ethanol final concentration, yield, conversion, productivity and selectivity in ethanol) will be evaluated under all conditions studied and the main significative effects will be identified through statistical analysis. The most promising conditions will still be validated in repeated batches with YPX medium and hemicellulose hydrolysate.