Investigation of hemicellulosic hydrolysate inhibitor resistance and fermentation strategies to overcome inhibition in non-saccharomyces species

Abstract

Sugarcane bagasse, residue of first-generation ethanol fermentation, is a rich substrate for the second-generation (2G) ethanol. However, the processes used to release sugars from such material produce inhibitory compounds. The present study aims to elucidate the effects of inhibitory compounds present in hemicellulosic hydrolysates (acetic acid, furfural, 5-hydroxymethylfurfural and vanillin) on the yeast Spathaspora passalidarum and to compare its performance with that of the most commonly used yeast for 5-carbon consumption, Scheffersomyces stipitis. In addition, fermentation strategies such as using adapted strains and increasing the initial cell concentration were used to address the presence of inhibitors in hemicellulosic hydrolysates. The results showed that acetic acid and furfural are the strongest inhibitors of S. passalidarum. Interestingly, although ethanol production by S. passalidarum was strongly affected by all the considered inhibitors, the final concentrations of ethanol reached by this yeast were similar to those obtained by S. stipitis, even though S. stipitis was less inhibited. The adapted S. stipitis showed better performance than the wild type, resulting in a 25% increase in xylose consumption and a 5% increase in yield. Although increasing the initial concentration of cells improved the performance of the adapted S. stipitis, the performance of the S. passalidarum was better. The understanding of the behaviour of these yeasts in the presence of inhibitors allows to establish process strategies such as deciding when the use of high cell density is advantageous or selecting a methodology for the evolutionary adaptation of strains with the goal of higher performance in fermentation.