Background:
Softwoods are the dominant source of lignocellulosic biomass in the Northern hemisphere and have been investigated world-wide as a renewable substrate for cellulosic ethanol production. One challenge to using softwoods, particularly acute with pine, is that the pretreatment process produces inhibitory compounds detrimental to growth and metabolic activity of fermenting organisms. To overcome the challenge of bioconversion in the presence of inhibitory compounds, especially at high solids loading, a strain of Saccharomyces cerevisiae was subjected to evolutionary engineering and adaptation using pretreated pine wood (Pinus taeda).
Results:
An industrial strain of Saccharomyces cerevisiae, XR122N, was evolved using pretreated pine; the resulting daughter strain, AJP50, produced ethanol much more rapidly than its parent in fermentations of pretreated pine. Adaptation by preculturing of the industrial 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.
Conclusions:
Our 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. These fermentations demonstrated reduced inoculum sizes and shortened process times, thereby improving the overall economic viability of a pine-to-ethanol conversion process.Source:
http://www.biotechnologyforbiofuels.com/rss/