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.
Highlights
Several cheap forestry and agricultural waste streams, as well as energy crops, are available for being applied as feedstocks for bioethanol production (Bhatia et al, 2017)
In order to obtain steam-exploded liquors with high inhibitor concentrations, samples of Phragmites australis, Cynara cardunculus, and Saccharum officinarum bagasse pre-treated by applying different conditions resulting in increasing severity factors (LogR0) were investigated in this study (Table 1)
Seven S. cerevisiae strains, namely, Fm17, Fm89, Fm90, Fm96, M2n, MEL2, and YI30, were previously described for their potential in various bioethanol applications (Favaro et al, 2013a, 2014; Jansen et al, 2018). These strains were further characterized with the final aim of assessing their promise to be used for lignocellulosic ethanol production
Summary
Several cheap forestry and agricultural waste streams, as well as energy crops, are available for being applied as feedstocks for bioethanol production (Bhatia et al, 2017). Such biomasses need to be pre-treated to make the cellulose more accessible to the following enzymatic hydrolysis aimed to release fermentable sugars. During steam explosion, possible inhibitors of fermentations such as phenolic compounds, furans, or weak acids are released decreasing the final ethanol yields (García et al, 2014; Morales et al, 2017)
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