Xylose fermentation by Scheffersomyces stipites for 2nd generation bioethanol: Optimization of bioprocess aeration strategy

Abstract

Hardwood spent sulfite liquor (HSSL) is a by-product from pulping industry with huge potential for second generation bioethanol production, owing to its high sugars content, namely xylose and glucose. Scheffersomyces stipitis can use this substrate since it is able to ferment either hexoses or pentoses. Nevertheless, this yeast requires microaerophilic conditions for ethanol production, so the establishment of optimal aeration conditions is the main challenge for a successful ethanol production. The objective of this work was the establishment of an aeration strategy that allowed an efficient bioethanol production from HSSL by S. stipitis C4, an adapted strain to this substrate. Fermentations were carried out in a bioreactor using two aeration stages, with dissolved oxygen tension (DOT, %) control in the first aeration stage and with oxygen limitation in the second aeration stage. With this strategy, DOT control at high values in the first aeration stage allowed to obtain high specific growth rates and biomass yields. In addition, the DOT control at high values in the first stage also contributed to increase the ethanol productivity during the second aeration stage. In turn, the oxygen limitation in the second aeration stage stimulated the ethanol production and suppressed the growth of S. stipitis C4 in this second stage. This approach of two aeration stages resulted in a better control between the biomass and ethanol production. Using 60% HSSL, a conversion to ethanol of 74.4% was reached. The two stages aeration approach is a promising strategy to obtain second generation bioethanol from HSSL.