Adaptation of Saccharomyces cerevisiae in a concentrated spent sulphite liquor waste stream for increased inhibitor resistance

Abstract

The fermentation of spent sulphite liquor (SSL) from the pulping of hardwoods is limited by the combination of xylose, the primary fermentable sugar and high concentrations of microbial inhibitors that decrease the yeast fermentation ability. The inhibitor resistance phenotypes of xylose-capable Saccharomyces cerevisiae strains were therefore enhanced by combining rational engineering for multi-inhibitor tolerance, with adaptation in concentrated hardwood SSL as selective pressure. The adapted strains were assessed in fermentations with 60-80% v/v concentrated SSL under industrially relevant fermentation conditions. During adaptation, strains produced ethanol concentrations between 11.0 and 15.4 g/L in the range of that reported in literature. The adapted TFA40 and TP50 strains displayed enhanced inhibitor resistance phenotypes and were able to ferment xylose-rich SSL at pH below 5, exhibiting improved ethanol yields relative to the reference strain. Using yeast extract and peptone as nitrogen source in concentrated SSL fermentations further improved ethanol yields. However, strains exhibited a trade-off between resistance and ethanol productivity, indicating a carbon/energy cost for the expression of this inhibitor tolerance phenotype. Key points• Achieved fermentation of xylose-rich hardwood spent sulphite liquor at pH below 5.0 • Adaptation of xylose-capable S. cerevisiae in concentrated spent sulphite liquor • Adapted strains exhibited enhanced inhibitor resistance phenotypes.

Reducing Carbon Intensity of Food and Fuel Production Whilst Lowering Land-Use Impacts of Biofuels

“…Currently, sulfite liquor waste streams from wood pulping processes are the only lignocellulosic streams available in the volumes needed to support the tonnage scale of industrial Saccharomyces production. Recent research has shown that molecular engineering coupled with environmental adaptation can deliver S. cerevisiae strains that grow on, and ferment spent sulfite liquors from hardwoods [72,73]. At this stage, we envisage that lignocellulosic substrates would be useful in producing yeast biomass where yeast is being applied in the manufacture of nonpotable ethanol or animal feed applications, but its immediate role in the production of yeast for baking or potable alcohol might be more contentious.…”