Comparison of strategies to overcome the inhibitory effects in high-gravity fermentation of lignocellulosic hydrolysates

High-gravity (HG) technology aims at generating final ethanol concentrations above 50 kg m−3 in order to reduce the cost of the distillation step. The generation of higher amounts of inhibitors during the pretreatment step is one of the challenges that accompany the increase in initial dry matter. Detoxification of spruce hydrolysate, adaptation of the cells before fermentation, supplementation with nutrients, and washing of solids were the strategies compared in this study. They represent different approaches to cope with the inhibitory effects, and we compared their efficiencies using a thermotolerant strain of Saccharomyces cerevisiae at temperatures from 30 °C up to 40 °C. lute acid-pretreated spruce used as substrate in this study was not fermentable under HG conditions (200 g kg−1 water-insoluble solids) when no improvement method was applied. In HG simultaneous saccharification and fermentation at 30 °C combined with a 24 h pre-hydrolysis step, the detoxification of pretreated spruce with reducing agent (Na2S2O4) gave the best result with an ethanol yield of 57% (on total sugars) of the maximum theoretical and a volumetric productivity of 1.58 g dm−3 h−1. In HG separate hydrolysis and fermentation, nutrients supplementation gave better final ethanol yields than detoxification of the material, reaching an ethanol yield of about 60% of the theoretical (on total sugars). The results obtained, showed an increase in severity of inhibitory effects with temperature increase. Improved cell viability was observed when detoxified material was used and also when yeast extract addition was coupled with adaptation of the cells to the hydrolysate.