Simulations Of Parameter Oscillations And Studies Of Attenuation Mechanism For Continuous VHG Ethanol Fermentation

Preliminary research in continuous very high gravity (VHG) ethanol fermentation reported sustainable parameter oscillations characterized by their long periods and high amplitudes. In this study, the dynamic behaviors of such a fermentation process were simulated by means of weighting treatment of the specific growth rate of yeast cells, which considered the lag response of yeast cells to the stress exerted by high ethanol concentration. Model parameters were estimated and validated by experimental data.Packing can effectively attenuate the parameter oscillations occurred during continuous VHG ethanol fermentation and the mechanism(s) behind the attenuation was assumed to be yeast cell immobilization. A bioreactor system composed of a CSTR and three-stage tubular bioreactors in series was established and the tubular bioreactors were packed with wood chip (5×5×10 mm), 90 mesh stainless net ring (φ10×l0 mm) and polyurethane (10×10×10 mm) which were significantly different in their surface properties and corresponding yeast cell immobilization roles through adsorption. It was found that the wood chip was the best in attenuating the parameter oscillations although its yeast cell immobilization role was between the two other packings, which indicated that, in addition to yeast cell immobilization, other factors also contribute to the attenuation of parameter oscillations. Further analysis revealed that the microenvironmental changes of the tubular bioreactors created by these different packings which inevitably changed the specific growth rate of yeast cells as well as the dilution rate of the tubular bioreactors might contribute to the attenuation of parameter oscillations.Furthermore, an industrial yeast strain, S. cerevisiae 6508, was used to study the impact of strain on oscillatory behaviors. It showed that oscillations appeared at different dilution rates for S. cerevisiae 6508 and ATCC 4126 as well as SPSC 01. In addition, the periods and amplitudes of oscillation were also observed to be different for these several strains.