RESUMEN
Continuous ethanol fermentation using very high gravity medium containing 280 g/L glucose, 5 g/L yeast extract and 3 g/L peptone was run at the dilute rates of 0.006 h(-1), 0.012 h(-1), 0.017 h(-1), 0.024 h(-1) and 0.032 h(-1) (based on the total working volume) in a combined bioreactor system composed of a stirred tank and three-stage tubular bioreactors in series. Oscillations marked by big fluctuations of residual glucose, ethanol and biomass were observed at the dilution rate of 0.012 h(-1). The Hopf Bifurcation theory was used to analyze and predict the occurring of these oscillations and the dilution rates that incited oscillations. Theoretical analysis revealed that oscillations can occur at designated specific growth rates and was validated by experimental results. The benefits of oscillations for the fermentation system were also discussed by comparing the fermentation results with those without oscillations.