Steady-state stability and dynamic behavior of continuous ethanol fermentation at high cell densities

A single stage gas-lift tower fermenter was used as a model system to study the steady-state stability and the dynamic behavior of an ethanol-inhibited continuous culture of Saccharomyces uvarum growing at high cell density. Empirical equations describing yeast growth and ethanol production rates in steady-state were combined with a mathematical description of the fermenter to predict steady-state stability and dynamic behavior when subjected to three sequential step changes in dilution rate from D = 2.13 to 0.213 h −1. Linearization and vector analysis of the theoretical model showed that eigen-values were real and negative, indicating that the system was stable and overdamped. When subjected to a step change in dilution rate, the experimental system responded immediately with a smooth overdamped transient to the next steady-state. Steady-state was reached within 6 h following a doubling in residence time. No cyclic or damped oscillations of glucose, ethanol or yeast concentration were observed in the theoretical or experimental system. The theoretical model predicted experimental behavior within 1.75 g l −1 root mean square deviation for large concentration ranges (glucose 210 → 16 g l −1; ethanol 16 → 88 g l −1) over experimental periods up to 28 h. The value of Y p/s = 0.425, calculated from steady-state experiments, gave the most precise prediction of dynamic behavior over the three transients. No evidence of significant metabolic or mass transfer lags was observed.