Fuel ethanol that is renewable and environmentally friendly has been produced around the world as an alternative to fossil fuels
. However, high production cost makes fuel ethanol heavily dependent on preferential policies and governmental subsidies, especially in the United States and China where fuel ethanol is produced mainly from grain-based feedstocks
. Since feedstock and energy consumption are the major cost, lignocellulosic biomass, due to abundance and low cost, particularly agricultural residues, has been intensively studied for the production of fuel ethanol, but challenges are to be addressed to make such a process economically competitive
. Meanwhile, very high gravity (VHG) fermentation can significantly increase ethanol titer in the fermentation broth, which not only saves energy consumption for ethanol distillation, but also reduces waste distillage discharged from the distillation system, and thus has garnered great attention
Apparently, ethanol-tolerant strains are prerequisite for more efficient ethanol production under VHG conditions in order to overcome stuck fermentation, in which significant sugars are present at the end of fermentation, and ethanol yield, the most important techno-economic aspect of fuel ethanol production, is compromised, correspondingly. Stuck fermentation results from severe ethanol inhibition in yeast cells
. When yeast cells flocculate, they can be retained and immobilized within fermentors for high cell density to improve ethanol productivity
. Moreover, ethanol tolerance of yeast flocs can be improved, since the morphological change caused by the flocculation consequently affects physiological functions of yeast cell membranes and intracellular metabolism
[5, 6], making yeast flocs more suitable for VHG fermentation. However, in situ monitoring the growth of yeast flocs and their fermentation performance under VHG conditions presents a challenge, since dissolved oxygen in the fermentation broth is undetectable under micro-oxygen conditions.
Redox potential (ORP) reflects electron activities during fermentations, which provides real-time information on the physiological status of cells
. Controlling ORP during fermentations has thus been developed as an effective method to enhance the production of desired metabolites such as 1,3-propanediol, citric acid and xylitol
[8–10]. Recent studies have shown that for VHG ethanol fermentation with non-flocculating Saccharomyces cerevisiae subjected to ORP control, high ethanol productivity and yield could be achieved
In this study, ORP profiles were monitored and analyzed for ethanol fermentation with the flocculating yeast. Furthermore, ORP control strategy was developed by controlling ORP at two different levels in order to improve ethanol productivity and yield for VHG ethanol fermentation with the flocculating yeast.