Exploring grape marcs as trove for new thermo- and inhibitor-tolerant yeasts for second generation bioethanol.

摘要

Abstract\udBackground: Robust yeasts with high inhibitor, temperature, and osmotic tolerance remain a crucial requirement\udfor the sustainable production of lignocellulosic bioethanol. These stress factors are known to severely hinder\udculture growth and fermentation performance.\udResults: Grape marc was selected as an extreme environment to search for innately robust yeasts because of its\udlimited nutrients, exposure to solar radiation, temperature fluctuations, weak acid and ethanol content. Forty newly\udisolated Saccharomyces cerevisiae strains gave high ethanol yields at 40°C when inoculated in minimal media at\udhigh sugar concentrations of up to 200 g/l glucose. In addition, the isolates displayed distinct inhibitor tolerance in\uddefined broth supplemented with increasing levels of single inhibitors or with a cocktail containing several\udinhibitory compounds. Both the fermentation ability and inhibitor resistance of these strains were greater than\udthose of established industrial and commercial S. cerevisiae yeasts used as control strains in this study. Liquor from\udsteam-pretreated sugarcane bagasse was used as a key selective condition during the isolation of robust yeasts for\udindustrial ethanol production, thus simulating the industrial environment. The isolate Fm17 produced the highest\udethanol concentration (43.4 g/l) from the hydrolysate, despite relatively high concentrations of weak acids, furans,\udand phenolics. This strain also exhibited a significantly greater conversion rate of inhibitory furaldehydes compared\udwith the reference strain S. cerevisiae 27P. To our knowledge, this is the first report describing a strain of S. cerevisiae\udable to produce an ethanol yield equal to 89% of theoretical maximum yield in the presence of high concentrations of\udinhibitors from sugarcane bagasse.\udConclusions: This study showed that yeasts with high tolerance to multiple stress factors can be obtained from\udunconventional ecological niches. Grape marc appeared to be an unexplored and promising substrate for the isolation\udof S. cerevisiae strains showing enhanced inhibitor, temperature, and osmotic tolerance compared with established\udindustrial strains. This integrated approach of selecting multiple resistant yeasts from a single source demonstrates\udthe potential of obtaining yeasts that are able to withstand a number of fermentation-related stresses. The yeast\udstrains isolated and selected in this study represent strong candidates for bioethanol production from\udlignocellulosic hydrolysates.