Abstract
BackgroundVHG fermentation is a promising process engineering strategy aiming at improving ethanol titer, and thus saving energy consumption for ethanol distillation and distillage treatment. However, sustained process oscillation was observed during continuous VHG ethanol fermentation, which significantly affected ethanol fermentation performance of the system.ResultsSustained process oscillation was investigated in continuous VHG ethanol fermentation, and stresses exerted on yeast cells by osmotic pressure from unfermented sugars and ethanol inhibition developed within the fermentation system were postulated to be major factors triggering this phenomenon. In this article, steady state was established for continuous ethanol fermentation with LG medium containing 120 g/L glucose, and then 160 g/L non-fermentable xylose was supplemented into the LG medium to simulate the osmotic stress on yeast cells under the VHG fermentation condition, but the fermentation process was still at steady state, indicating that the impact of osmotic stress on yeast cells was not the main reason for the process oscillation. However, when 30 g/L ethanol was supplemented into the LG medium to simulate the ethanol inhibition in yeast cells under the VHG fermentation condition, process oscillation was triggered, which was augmented with extended oscillation period and exaggerated oscillation amplitude as ethanol supplementation was increased to 50 g/L, but the process oscillation was gradually attenuated when the ethanol supplementations were stopped, and the steady state was restored. Furthermore, gas stripping was incorporated into the continuous VHG fermentation system to in situ remove ethanol produced by Saccharomyces cerevisiae, and the process oscillation was also attenuated, but restored after the gas stripping was interrupted.ConclusionsExperimental results indicated that ethanol inhibition rather than osmotic stress on yeast cells is one of the main factors triggering the process oscillation under the VHG fermentation condition, and in the meantime gas stripping was validated to be an effective strategy for attenuating the process oscillation.
Highlights
Very high gravity (VHG) fermentation is a promising process engineering strategy aiming at improving ethanol titer, and saving energy consumption for ethanol distillation and distillage treatment
Process oscillation associated with continuous VHG ethanol fermentation Previous studies indicated that continuous ethanol fermentation with the Low gravity (LG) medium by S. cerevisiae was at steady state, but process oscillation developed under VHG ethanol fermentation conditions [5]
The process oscillation was effectively attenuated when gas stripping was incorporated into the continuous VHG ethanol fermentation system to in situ remove ethanol produced by yeast cells, which further validated the impact of ethanol inhibition in yeast cells on the fermentation process, and provides an effective strategy for its attenuation
Summary
VHG fermentation is a promising process engineering strategy aiming at improving ethanol titer, and saving energy consumption for ethanol distillation and distillage treatment. Low ethanol concentration in the effluent makes downstream processes such as ethanol distillation and stillage treatment more energy-intensive, when the stillage is treated by the multi-evaporation process that consumes 40-45% of the total thermal energy [3]. To address this issue, VHG fermentation with mash containing total sugars in excess of 250 g/L was developed [4], but sustained oscillation was observed with process parameters including sugar, ethanol and biomass concentrations as the operation was extended [5]. The underlying mechanisms for the process oscillations were identified to be the synchronization of the population metabolism and cell cycles under specific physiological and culture conditions, due to the asymmetrical budding growth nature of S. cerevisiae [15,16,17]
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