High ethanol production under optimal aeration conditions and yeast composition in a very high gravity fermentation from sweet sorghum juice by Saccharomyces cerevisiae

• Nitrogen and aeration are important for high ethanol production in very high gravity fermentation.
• Response surface methodology was successfully used for optimization of aeration in very high gravity ethanol fermentation.
• Under the optimal aeration rate and time, yeast produced up to 16% (v/v) of ethanol.
• Trehalose accumulation in yeast related to ethanol stress during the fermentation.
• Aeration promoted ergosterol synthesis, resulting in an increase in ethanol tolerance.

Aeration is one of the significant factors affecting yeast physiology and impacts ethanol production. The aim of this study was to optimize the aeration rate and time for high levels of ethanol production under very high gravity (VHG) fermentation by Saccharomyces cerevisiae NP 01 using a factorial design based on response surface methodology (RSM). The changes in intracellular composition (trehalose and ergosterol) during the fermentation were also investigated. The ethanol production medium was sweet sorghum juice containing 280 g l−1 of total sugar and 3.45 g l−1 of urea. The fermentation was carried out at 30 °C in a 2‐L fermenter. The results showed that the optimal aeration rate and aeration time for the VHG fermentation were 0.31 vvm and 12.01 h, respectively. Under the optimal conditions, the ethanol concentration (PE), productivity (Qp) and yield (Yp/s) were 127.80 g l−1 (16.20%, v/v), 2.66 g l−1 h−1 and 0.49 g g −1, respectively, whereas the PE values were 119.64 and 79.74 g l−1 under no aeration with urea (positive control) and without urea (negative control) supplementation, respectively. The trehalose contents under optimal, positive and negative control conditions were 200, 192 and 154 mg g DCW (dry cell weight)−1, respectively. The ergosterol contents under the optimal conditions were 3.35 and 5.74 fold higher than those of positive and negative controls, respectively. It can be conclude that ethanol fermentation efficiency is markedly related to the degree of aeration, trehalose accumulation and ergosterol synthesis in the plasma membranes of yeast cells.