Bioconversion of green agricultural herbaceous biomass to ethanol

Biofuels can offer an alternative to fossil fuels in the context of climate change and fossil reserves depletion. Plant biomass consists mainly of structural (cell wall) polysaccharides, but contains also reserve polysaccharides (starch and polyfructans) and soluble sugars, that can be converted by micro-organisms. We examined the feasibility of a simple ethanol fermentation process directly from crude, not sterilized grasses with the commonly used yeast Saccharomyces cerevisae. Crude and steam- and/or polysaccharidase pretreated grass was fermented with S. cerevisiae for 40 hours at 40°C. Ethanol and volatile fatty acids were analyzed by GC-FID. The results show that S. cerevisae can produce ethanol directly from green herbaceous biomass with an ethanol concentration in the range 8–16 g(EthOH)/L without enzyme treatment. Polysaccharidase hydrolysis can increase the ethanol concentration by 20–100%. As crude, not sterilized substrate was used, the endogenous microbial flora was responsible for the production of lactic acid and other volatile fatty acids (mainly acetate and butyrate). The soluble sugar conversion to ethanol was 60–100% of the theorical yield. Enzymatic treatments did not increase significantly the availability of metabolizable sugars for S. cerevisiae. Grass harvest time had also no significant influence on ethanol production. Future work will focus on the characterization of the enzymatic hydrolysis and the release of soluble sugars