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

Abstract

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.

CELF pretreatment of corn stover boosts ethanol titers and yields from high solids SSF with low enzyme loadings

“…However, Figure 1 shows that SSF at 37°C still achieved higher ethanol production rates and yields than SHF at 55°C, supporting earlier findings that sugar inhibition has a greater impact on performance than temperature. 42 SSF also achieved higher ethanol concentrations in a shorter reaction time, especially at higher glucan loadings, further showing that reducing enzyme inhibition by sugars has a greater effect on performance than increasing temperatures.…”

Recent updates on lignocellulosic biomass derived ethanol - A review

“…It is worth noting that in addition to making (mesophilic/thermophilic) microbes capable of fermenting pentoses together with hexoses, research efforts are also underway to make microbes metabolize cellobiose and higher cellodextrins directly to ethanol and other valuable metabolites. Although the concept is not new, as it was shown by (Spindler et al, 1989) that by directly fermenting cellobiose, it is possible to achieve higher conversion and ethanol yields, Galazka et al (2010) recently reported a much higher conversion and yields by reconstituting the Neurospora crassa cellodextrins transporters system into S.cerevisiae. In another study, Ha et al (2010) engineered a yeast strain to co-ferment cellobiose, glucose, and xylose together; however, high glucose concentrations expected after enzymatic saccharification of pretreated solids at high solids loading suppressed the metabolism of xylose.…”

Silage Processing of Forage Biomass to Alcohol Fuel

“…Finally, the possible benefits of ensiling toward increased ethanol productivity were measured using adapted SSF procedures (35). Because the coarse, fibrous nature of the sweet sorghum used in this study posed severe mixing problems, static fermentation vessels were used as opposed to an agitated system that was standardized for dried and milled materials.…”