Fermentation kinetics of Zymomonas mobilis at high ethanol concentrations: Oscillations in continuous cultures

Substrate-limited continuous culture results at 47 g/L ethanol show that the maintenance factor and the yield factor of an unstructured maintenance model are lower compared to the values at 23 g/L ethanol. Computing the results according to a structured two-compartment model predicts an enhanced turnover rate of the K-compartment (RNA fraction) by ethanol, resulting in a lower steady-state amount of K-compartment. This is in agreement with experimentally determined RNA fractions. The parameters of both models respond qualitatively in the same way to elevation of the ethanol concentration as to elevation of the temperature. In product-inhibited continuous cultures, at ethanol concentrations above 55 g/L, nearly sustained oscillations in biomass, substrate, and products were observed. The maximum ethanol concentration achieved in these continuous cultures was 70 g/L. The oscillations could be described by a structured mathematical model, in which ethanol inhibits the maximum specific growth rate indirectly by inhibiting the synthesis of an internal growth-rate-determining compound.