OPTIMIZATION OF A 2-STAGE RECOMBINANT FERMENTATION PROCESS - THE DILUTION RATE EFFECT

The effects of dilution rates on the performance of a two-stage fermentation system for a recombinant Escherichia coli culture were studied. Dilution rate determines the apparent or averaged specific growth rate of a heterogeneous population of cells in the recombinant culture. The specific growth rate affects the genetic parameters involved in product formation in the second stage, such as plasmid stability, plasmid content, and specific gene expression rate. Kinetic models and correlations were developed for these parameters based on experimental data. Simulations of plasmid stability in the first stage showed that for longer fermentation periods, plasmid stability is better at higher dilution rates. However, the plasmid content is lower at these dilution rates. The optimal apparent specific growth rate for maximum productivity in the second stage was determined using two methods: (1) direct search for a constant specific growth rate, and (2) dynamic optimization using the maximum principle for a time-dependent specific growth rate profile. The results of the calculations showed that the optimum constant apparent specific growth rate for maximum over-all productivity is 0.40 h-1. This coincides with the optimal specific growth rate for maximum plasmid content in the expressed stage. A 3.5% increase in overall productivity can be obtained by using a linear time dependent apparent specific growth rate control, mu-2(t) = 0.35 + 0.0007t, in the course of the fermentation time.