2-29: High gravity and high cell density enhances the fermentation of the hexose sugars present in softwood hydrolysates

Over the past 30 years, the production of ethanol from sugar and starch feedstocks has developed into mature industries with an annual global production of more than 80 billion liters of ethanol. Final ethanol concentrations of 6-20% v/v can be achieved commercially from sugar and starch feedstocks with >90% yield, while “cellulosic ethanol” typically reaches ~4% v/v ethanol with >80% yield. Fermentation of the sugars obtained after steam pretreatment and enzymatic hydrolysis of softwoods is problematic because of both the generally low sugar concentrations that can be supplied and the presence of naturally occurring and process derived inhibitors.

In this work, a strategy of using high gravity/high consistency and high cell density approach for soft wood Douglas fir hydrolysates was investigated, to see if it might better cope with inhibition. Several yeast strains were assessed with Tembec T1, T2 and Lallemand L4 proving to be the most promising in terms of ethanol productivity and yield.

High glucose concentrations that were provided enabled the faster and quantitatively higher removal of hydroxymethylfurfural (HMF) and this observed “boosting effect” was more pronounced with the three superior strains. It was apparent that a high cell density approach improved ethanol production by all of the evaluated yeast strains at high sugar concentrations while enhancing inhibitor reduction. The addition of trace amounts of nutritional supplements and sodium sulfite appeared to both detoxify the biomass derived sugar stream and result in the complete fermentation of all of the hexose sugars to close to theoretical yields of ethanol.