Direct Ethanol Fermentation of Agricultural Waste Biomass by a Cellulolytic Enzyme-Expressing Yeast

Scientists from the Department of Chemical Science and Engineering and the Organization of Advanced Science and Technology, Kobe University (Japan) report the first "proof-of-concept" study for the direct fermentation of agricultural biomass (rice straw) to ethanol, using a "cellulolytic enzyme-expressing yeast without the addition of exogenous enzymes."

Conventionally, lignocellulosic feedstocks (such as rice straw) must undergo prior pretreatment and saccharification steps before these could be fermented to ethanol by yeasts. The saccharification step for conversion of cellulose into ethanol-fermentable sugars usually involves the addition of (relatively expensive) cellulose-hydrolyzing (cellulase) enzymes. The report of a direct utilization of lignocellulosic feedstocks by a recombinant yeast (harboring the genes for cellulolytic-enzyme expression), can be an important breakthrough in lowering the cost of cellulose-ethanol production.

In the development of the yeast strain, the researchers used a "cocktail δ-integration method" to optimize cellulase expression in two yeast strains of opposite mating types. They mated these strains to produce a diploid strain with enhanced cellulase expression. The diploid strain is reported to produce 7.6 g/l ethanol in 72 hours, with an ethanol yield that achieved 75% of the theoretical value. It also produced 7.5 g/l ethanol from pretreated rice straw in 72 hours.

The full results are published in the open-access journal, Biotechnology for Biofuels (URL above).