Synergistic effects of inhibitors and osmotic stress during high gravity bioethanol production from steam-exploded lignocellulosic feedstocks

High gravity second-generation bioethanol production (solid loading ≥30% w w−1) suffers from prolonged fermentation times and low ethanol productivities due to high concentrations of toxic by-products (inhibitors) and high monosaccharide concentrations. To better understand inhibition during high gravity bioethanol production, this study focused on ethanol production from a variety of different steam-exploded feedstocks with different inhibitor concentrations. The effects of inhibitors on enzymatic hydrolysis and fermentation were investigated by selective removal of inhibitors by means of biological detoxification. The degree of lignocellulose depolymerization and inhibitor formation during steam explosion varied significantly between different feedstock types. Subsequent enzymatic hydrolysis and fermentation indicated that lignocellulosic inhibitors had no significant effect on cellulolytic enzymes, but showed strong inhibition on fermentation performance. Biological detoxification of furan aldehydes and organic acids had no impact on monosaccharide yield during enzymatic hydrolysis, but reduced the fermentative lag phase by up to 48 h. While the detoxification significantly improved fermentation performance, the maximal ethanol yield of 52.6 g L−1 with wheat straw or miscanthus could not be increased. A key finding of this study was that the effect of lignocellulosic inhibitors is greatly increased in environments with high osmotic stress. This synergistic effect is hypothesized to be caused by an inhibitor-induced redox imbalance, impeding the cells’ ability to counteract osmotic stress.