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Foam disruption by agitation-the stirring as foam disruption (SAFD) technique-was scaled up to pilot and production scale using Rushton turbines and an up-pumping hydrofoil impeller, the Scaba 3SHP1. The dominating mechanism behind SAFD-foam entrainment-was also demonstrated at production scale. The mechanistic model for SAFD defines a fictitious liquid velocity generated by the (upper) impeller near the dispersion surface, which is correlated with complete foam disruption. This model proved to be scalable, thus enabling the model to be used for the design of SAFD applications. Axial upward pumping impellers appeared to be more effective with respect to SAFD than Rushton turbines, as demonstrated by retrofitting a 12,000 l bioreactor, i.e. the triple Rushton configuration was compared with a mixed impeller configuration from Scaba with a 20% lower ungassed power draw. The retrofitted impeller configuration allowed 10% more broth without risking excessive foaming. In this way a substantial increase in the volumetric productivity of the bioreactor was achieved. Design recommendations for the application of SAFD are given in this paper. Using these recommendations for the design of a 30,000 l scale bioreactor, almost foamless Escherichia coli fermentations were realised.

In large-scale aerobic fed-batch processes, cells are exposed to local zones of high glucose concentrations that can also cause local oxygen limitations at high cell densities. The mRNA levels of four stress genes (clpB, dnaK, uspA, and proU) and three genes responding to oxygen limitation or glucose excess (pfl, frd, and ackA) were investigated in an industrial 20-m(3) Escherichia coli process and in a scale-down reactor with defined high-glucose and low-oxygen zones. The mRNA levels of ackA and proU were high during the batch growth phase, but declined drastically when glucose became limited, whereas the mRNA levels of the other stress genes were relatively constant throughout the process. In the industrial-scale reactor, the stress gene mRNA levels were, in most cases, highest in the middle part and at the top of the reactor, where the substrate was fed. Cells passing through the high glucose zone of the scale-down reactor had elevated mRNA levels for the oxygen limitation genes and had also elevated heat-shock gene mRNA levels. Both responses to stress occurred within seconds. The approach presented in this study offers a tool for monitoring process-related changes in the transcriptional regulation of genes., (Copyright 1999 John Wiley & Sons, Inc.)