A kinetic model of overflow metabolism in Saccharomyces cerevisiae was used for simulation of aerobic fed‐batch cultivations. An inhibitory effect of ethanol on the maximum respiration of the yeast was observed in the experiments and included in the model. The model predicts respiration, biomass, and ethanol formation and the subsequent ethanol consumption, and was experimentally validated in fed‐batch cultivations. Oscillating sugar feed with resulting oscillating carbon dioxide production did not influence the maximum respiration rate, which indicates that the pyruvate dehydrogenase complex is not involved as a bottleneck causing aerobic ethanol formation. © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 60: 474–482, 1998.

If a microorganism has a growth coupled production or consumption of acid or alkali, it is possible to use the pH-auxostat as a means of control in continuous fermentation. In using the pH-auxostat, it is possible to separate the inlet substrate flow in two different streams. These will both be pH controlled, with one main flow, consisting of nutrients and a second minor but concentrated flow, of acid or alkali. Hereby, it is possible to vary the difference in pH between the fermentor and the inlet medium. This pH difference is proportional to the steady-state cell mass concentration.(1,2) It is shown that by separating the inlet flow in two different streams and cultivating without any substrate limitation, the maximum growth rate may be obtained while the cell mass concentration will be controlled. This will also give the possibility to reach high cell mass concentrations at micro(max) without the risk of wash-out. A modified expression, based on hydrogen, of the steady-state bio-mass concentration, X, is developed as X = Y(X/H x [FHin/(FHin +FMin)] x (CHin -CHFerm) where Y(X/H) is the yield coefficient of cell mass per acid produced. The indexes Hin and Min refer to the inflows of alkali and medium, respectively; C(Hin) is the inlet concentration of hydrogen ions. The boundary condition for the cell mass shows that S(in) > X/Y(X/S), where S(in) is the medium substrate concentration and Y(X/S) is the yield of biomass per consumed substrate. It is shown that when the cell mass concentration exceeds this value, the flow stops. The applicability of the pH-auxostat method is then verified from different experiments. It is hereby used to detect a deviation from the maximal growth rate showing effects on the microbial physiology. With Escherichia coli used as the model organism, the effect on the growth rate of temperature and high concentration of ammonia were investigated.

A model for growth and over¯ow metabolism of Saccharomyces cerevisiae was applied to simulate continuous cultivations in a pH-auxostat. The concentrations of glucose, biomass and ethanol are controlled by the¯ow ratio r between fresh medium and titrant solution, both of which are pH-regulated. A critical value of r could be derived, below which the culture becomes substrate depleted, resulting in a stop-¯ow condition with retained biomass but without growth. At r-values slightly above the critical value the pH-auxostat is substrate limited and unstable. Further increase of the r value results in a stable continuous culture growing at l max . Thus, the pH-auxostat complements the chemostat in the growth range at or close to l max . Even at l max conditions, the ethanol concentration can be controlled at a low level.

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