Univ. of Georgia researchers develop new higher-yielding yeast strain for improved economics of pine-to-ethanol processes
Researchers at the University of Georgia have developed an industrial strain of the yeast Saccharomyces cerevisiae which produces ethanol much more rapidly than its parent in fermentations of pretreated pine. The final strain produced ethanol at more than 90% of the maximum theoretical yield after 120 hours of fermentation.
In an open access paper published in the journal Biotechnology for Biofuels, Gary Hawkins and Joy Doran-Peterson suggested that their results demonstrate that fermentations of pretreated pine containing liquid and solids, including any inhibitory compounds generated during pretreatment, are possible at higher solids loadings than previously reported in the literature. The fermentations demonstrated reduced inoculum sizes and shortened process times, thereby improving the overall economic viability of a pine-to-ethanol conversion process.
While softwoods are the dominant source of lignocellulosic biomass in the Northern hemisphere, one challenge to their use—particularly acute with pine—as a feedstock for ethanol production is that the pretreatment process produces inhibitory compounds detrimental to growth and/or metabolic activity of the fermenting organisms.
Inhibitors fall into three general categories: aromatic compounds; furan derivatives; and weak aliphatic acids. These inhibitors act through a variety of mechanisms to reduce ethanol production efficiency. Although inhibitory compounds produced by pretreatment may be removed before fermentation to increase ethanol production, this increases overall production costs.
Thus, the authors, note, it is important to use a fermenting organism that is able to tolerate these compounds, especially at the high solids loadings required for industrial fermentations to produce the ethanol concentrations necessary for cost-effective distillation.
The researchers worked with an industrial Saccharomyces yeast strain, XR122N, currently used in corn-ethanol fermentations. They selected sulfur dioxide-pretreated pine wood as the substrate, because of the high level of inhibitory compounds found in this feedstock. To generate a strain with improved tolerance of inhibitory compounds found in pretreated pine, XR122N was evolved using SO2-pretreated pine directly, without separating the liquid from the solids and without ameliorating the toxic compounds, rather than using a single inhibitory compound such as furfural for directed evolution.
They also subjected the strain to additional evolutionary adaptation at high solids loadings in order to increase ethanol concentrations in the fermentation.
Adaptation by preculturing of the industrial [parent strain] yeast XR122N and the evolved strains in 7% w/v pretreated pine solids prior to inoculation into higher solids concentrations, improved fermentation performance of all strains compared to direct inoculation into high solids. Growth comparisons between XR122N and AJP50 in model hydrolysate media containing inhibitory compounds found in pretreated biomass, revealed AJP50 exited lag phase faster under all conditions tested.
This ability is due, in part, to AJP50 rapidly converting furfural and hydroxymethylfurfural to their less toxic alcohol derivatives and recovering from reactive oxygen species damage more quickly than XR122N. Under industrially relevant conditions of 17.5% w/v pretreated pine solids loading, additional evolutionary engineering was required to decrease the pronounced lag phase. Using a combination of adaptation by inoculation first into a solids loading of 7% w/v for 24 h, followed by a 10% v/v inoculum (approximately 1 g/L dry cell wt) into 17.5% w/v solids, the final strain (AJP50) produced ethanol at >80% of the maximum theoretical yield after 72 h of fermentation and reached >90% of the maximum theoretical yield after 120 h of fermentation.
Gary M. Hawkins and Joy B. Doran-Peterson (2011) A strain of Saccharomyces cerevisiae evolved for fermentation of lignocellulosic biomass displays improved growth and fermentative ability in high solids concentrations and in the presence of inhibitory compounds. Biotechnology for Biofuels 4:49 DOI: 10.1186/1754-6834-4-49