Technical Assessment of Cellulosic Ethanol Production Using β-Glucosidase Producing Yeast Clavispora NRRL Y-50464

Abstract

Reducing the cost of cellulosic ethanol production, especially the use of expensive exogenous cellulose hydrolytic enzymes such as cellulase and β-glucosidase, is a critical challenge and vital for a sustainable advanced biofuel industry. Here, we report a novel ethanologenic yeast strain Clavispora NRRL Y-50464 that produces sufficient innate β-glucosidase enzyme activity for cellulosic ethanol production by simultaneous saccharification and fermentation (SSF). In a bottled SSF, strain Y-50464 produced 40.44 g/L ethanol from pure cellulose within 72 h at a conversion rate of 0.04 g/L/h, applying conventional cellulase without supplementary β-glucosidase. Ethanol conversion from delignified corn stover by Y-50464 showed significantly higher titers and rates at various solids loading levels than that from conventional pretreated corn stover with over 40 to 60 % improved efficiency in a bottled SSF. However, the bottled SSF was inefficient for mixing higher levels of cellulose feedstock and should be replaced by a more suitable experimental apparatus. In a 2-L bioreactor SSF using conventional dilute acid pretreated corn stover, strain Y-50464 produced 32 g/L ethanol from 20 % solids loading at 48 h applying cellulase alone without addition of β-glucosidase. This represented a conversion rate of 0.088 g/L/h, the highest rate so far for cellulosic ethanol production from lignocellulosic materials. Elimination of β-glucosidase in cellulose-to-ethanol fermentation would be expected to reduce cost of cellulose conversion. The robustness, fast growth rate, and the capability of producing both ethanol and β-glucosidase illustrated the potential of strain Y-50464 as a potential candidate biocatalyst for advanced biofuel production from lignocellulosic biomass.