High-cell density two-stage self-cycling fermentation system for the enhanced production of ethanol from steam-exploded poplar hydrolysate

The economics of lignocellulosic ethanol production by Saccharomyces cerevisiae remain a challenge which can be partially addressed by improving the performance of pretreatment optimization and fermentation systems, an aspect that is too often overlooked. In this study, we investigated how the combination of self-cycling fermentation (SCF) with continuous adapted feeding in a two-stage system could improve bioethanol production from steam-exploded poplar hydrolysates. This system benefited from the improved productivity associated with SCF and from the high titers obtained in fed-batch operation. The system consisted of a first fed-batch SCF stage, which led to approximately two-fold improvements in cell dry weight and low residual glucose contents (< 0.5 g/L), demonstrating efficient substrate utilizations by the yeast. The second high cell density SCF stage was initiated by adding a pulse feed to further enhance ethanol titer and productivity. The patterns of glucose consumption, ethanol production, and evolved gas flow rate were all reproducible between the SCF cycles. The two-stage fermentation approach led to final ethanol titers of ~ 11% (v/v) with improvements in productivity reaching 30%. Overall, this study presents a robust two-stage high cell density SCF system for lignocellulosic ethanol production and highlights the feasibility and potential of implementing it in biorefinery processes.