Multiple Steady States in Continuous Cultivation of Yeast

Abstract

The occurance of multiple steady states in continuous cultivation of yeast was investigated experimentally and a simplified model is presented that accounts for the behaviour. The model is a modification of the bottle-neck model which explains steady state data over a wide range of dilution rates as well as part of the dynamic behaviour. The modified model accounts for the possibility that an inhibitory substance decreases the respiratory bottle-neck. Ethanol itself has such an inhibitory effect. Other candidates are various organic acids that in small concentrations influence the respiration of the yeast cell. The model was analysed with respect to multiple steady states and a certain range of dilution rates was found where the culture can show a steady state both with ethanol production as well as without, depending on the initial state. An experiment was designed to reveal the two steady states within as short time as possible. Results from experiments in laboratory scale with Saccharomyces cerevisiae are shown and compared with simulations. Ethanol in the culture was measured on-line using a mass spectrometer and off-line GC measurements were made of organic acids.