Evaluation of thermotolerant yeasts in controlled simultaneous saccharifications and fermentations of cellulose to ethanol

Simultaneous saccharification and fermentation (SSF) experiments were performed at selected temperatures (37, 41, and 43 degrees C) to obtain comprehensive material balance and performance data for several promising strains of thermotolerant yeast. Parameters measured were ethanol concentration, yeast cell density, and residual sugar and cellulose concentrations. The three yeasts Saccharomyces uvarum, Candida brassicae, and C. lusitaniae and two mixed cultures of Brettanomyces clausenii with S. cerevisiae (mixed culture I) and C. Iusitaniae with S. uvarum (mixed culture II) exhibited rapid rates of fermentation, high ethanol yields, strong viability, or high cellobiase activity. Overall, mixed culture II at 41 degrees C performed better than either component yeast by themselves because it combined a cellobiose fermenting capability with the high ethanol tolerance and rapid glucose fermentation of conventional industrial yeasts. Thus, the mixed cultures provide good initial rates by preventing buildup of cellobiose (a strong inhibitor of enzyme activity) while attaining high ultimate yields of ethanol for high cellulase concentrations. However, C. brassicae and S. uvarum gave similar results to mixed culture II at 37 degrees C.