Selection of Superior Yeast Strains for the Fermentation of Lignocellulosic Steam-Exploded Residues

Abstract

The production of lignocellulosic ethanol calls for a robust fermentative yeast able to tolerate a wide range of toxic molecules that occur in the pre-treated lignocellulose. The concentration of inhibitors varies according to the composition of the lignocellulosic material and the harshness of the pre-treatment used. It follows that the versatility of the yeast should be considered when selecting a robust strain. This work aimed at the validation of seven natural Saccharomyces cerevisiae strains, previously selected for their industrial fitness, for their application in the production of lignocellulosic bioethanol. Their inhibitor resistance and fermentative performances were compared to those of the benchmark industrial yeast S. cerevisiae Ethanol Red, currently utilized in the second-generation ethanol plants. The yeast strains were characterized for their tolerance using a synthetic inhibitor mixture formulated with increasing concentrations of weak acids and furans, as well as steam-exploded lignocellulosic pre-hydrolysates, generally containing the same inhibitors. The eight non-diluted liquors have been adopted to assess yeast ability to withstand bioethanol industrial conditions. The most tolerant S. cerevisiae Fm17 strain, together with the reference Ethanol Red, was evaluated for fermentative performances in two pre-hydrolysates obtained from cardoon and common reed, chosen for their large inhibitor concentrations. S. cerevisiae Fm17 outperformed the industrial strain Ethanol Red, producing up to 18 and 39 g/L ethanol from cardoon and common reed, respectively, with ethanol yields always higher than those of the benchmark strain. This natural strain exhibits great potential to be used as superior yeast in the lignocellulosic ethanol plants.

Keywords: bioethanol; cardoon; common reed; industrial yeast strains; steam explosion; sugarcane bagasse.

FIGURE 1

Fermentation performances in the presence of pre-hydrolysate Cc3 from Cynara cardunculus and Pa3 from Phragmites australis, supplemented with YNB containing 20 g/L of glucose, by Saccharomyces cerevisiae Fm17 (A,C) and Ethanol Red (B,D). S. cerevisiae Fm17 (E) and Ethanol Red (F) were inoculated also in the reference broth (YNB containing 20 g/L of glucose without any pre-hydrolysate). When necessary, acidity of the medium was adjusted to pH 5.0 with NaOH. The experiment was conducted in triplicate. Error bars correspond to the standard deviation of the means.

FIGURE 2

Fermentation performances of Saccharomyces cerevisiae strains in YNB broth containing 40 g/L of glucose with or without addition of pre-hydrolysate Cc3 from Cynara. cardunculus: Fm17 (A: supplemented with Cc3, C: reference broth not supplemented with Cc3) and Ethanol Red (B: supplemented with Cc3, D: reference broth not supplemented with Cc3). The acidity of the medium was adjusted to pH 5.0 with NaOH. The experiment was conducted in triplicate. Error bars correspond to the standard deviation of the means.

FIGURE 3

Fermentation performances of Saccharomyces cerevisiae strains in YNB broth containing 92 g/L of glucose with or without the addition of pre-hydrolysate Pa3 from Phragmites australis: Fm17 (A: supplemented with Pa3, C: reference broth not supplemented with Pa3) and Ethanol Red (B: supplemented with Cc3, D: reference broth not supplemented with Pa3). The acidity of the medium was adjusted to pH 5.0 with NaOH. The experiment was conducted in triplicate. Error bars correspond to the standard deviation of the means.