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.

The lethal effects of gas sparging on hybridoma cells obtained from a chemostat culture were examined in a bubble column. Experiments were performed to identify and quantify the main hazardous event: bubble formation, bubble rising, or bubble breakup. The results indicate that bubble breakup is the main cause of cell death. The protective activity of the surfactant Pluronic F68 against sparging seems to result from a direct interaction with the cells rather than influencing bubble-liquid interface properties.

The inhibition of the maximum specific growth and fermentation rate of Zymomonas mobilis by ethanol was studied in turbidostat cultures at constant and stepwise changed ethanol concentrations. Up to 50 g/L ethanol, the inhibition kinetics can be approximated by a linear relationship between the specific growth rate and the ethanol concentration. Above this level, deviations from this linearity are observed. The specific fermentation rates were less inhibited by ethanol than was the specific growth rate. The maximum ethanol concentration achieved was 72 g/L.The response time for the adaptation of a turbidostat culture to step changes in the ethanol concentration was markedly dependent on the concentration level, the response time being large at high ethanol concentrations.

Zymomonas mobilis was grown in continuous cultures at 30 and 35 degrees C. The specific substrate consumption rates at 35 degrees C were higher than those at 30 degrees C. An unstructured mathematical model based on the linear equation for substrate consumption provided a statistically adequate description for cultures grown at 35 degrees C but not for cultures grown at 30 degrees C. A structured two-compartment model described growth and substrate consumption well at both temperatures. Some theoretical and practical aspects of the two-compartment model are discussed.

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