3-22: Controlling pH during ethanol production from xylose with xylose isomerase and Saccharomyces cerevisiae co-immobilized in calcium alginate

Xylose, a plentiful sugar in lignocellulosic biomass, can be converted to ethanol by unmodified Sacharomyces cerevisiae if previously isomerized to xylulose. The isomerization can be catalyzed by xylose isomerase (XI), and a process of simultaneous isomerization and fermentation (SIF) leads to the displacement of the chemical equilibrium, allowing full conversion of xylose to xylulose. The biocatalyst for the SIFs contains XI immobilized on chitosan, with baker's yeast and calcium carbonate co-immobilized in calcium alginate gel particles. The purpose of this study was to investigate the influence of the concentration of enzyme and yeast on ethanol productivity and selectivity. Increasing concentrations of yeast were tested: 5, 10 and 17%, as well as of enzyme: 5, 10 and 20% of the biocatalyst mass composition. The SIF experiments were carried out for 24 h, at pH 5.6 and 35°C, in a shaker, under stirring. The reactor was comprised by medium, containing 65 g.L^-1 of xylose and biocatalyst (1:1 w/w). The pH was controlled at 5, and in less than 24 h all sugar was consumed. The main products were ethanol and xylitol, and the biocatalyst containing 10% of yeast and 20% of enzyme achieved 98% of conversion within 11 h, providing a yield of 0.35± 0.02g_ethanolxg_xylulose^-1, with productivity of 2.07± 0.17g.L^-1.h^-1 and ethanol/xylitol selectivity of 2.42±0.01. Productivity and ethanol yield increased with yeast concentration, while selectivity increased with enzyme concentration.