Modelling of aerobic growth of Saccharomyces cerevisiae in a pH-auxostat

A model for growth and overflow metabolism of Saccharomyces cerevisiae was applied to simulate continuous cultivations in a pH-auxostat. The concentra tions of glucose, biomass and ethanol are controlled by the flow ratio r be tween fresh medium and titrant solution, both of which are pH-regulated. A critical value of r could be derived, below which the culture becomes subst rate depleted, resulting in a stop-flow condition with retained biomass but without growth. At r-values slightly above the critical value the pH-auxos tat is substrate limited and unstable. Further increase of the r value resu lts in a stable continuous culture growing at mu(max). Thus, the pH-auxosta t complements the chemostat in the growth range at or close to mu(max). Eve n at mu(max) conditions, the ethanol concentration can be controlled at a l ow level.