Yield in continuous aerobic bacterial fermentation

Presented is a mathematical model for the continuous-flow steady-state bacterial culture which permits the experimental determination of carbon transfer rates within the system by use of radioactive tracer techniques. The transfer rates are specific for hydraulic loading rates, feed concentrations, type of organism, and substrate, and were incorporated within the existing theoretical description of the growth kinetics in order to elucidate the yield relationships. The carbon transfer rate of cells to soluble organic substrate was observed to exhibit a minimum value at or near dilution rate D = 0.5 hr.−1. A maximum effective yield coefficient, Y, was observed at the same value of D. At dilution rates greater and less than D = 0.5 hr.−1, the cell–substrate transfer rate increased, and effective yield coefficient was observed to decrease. The former showed increases of 50–200%, and the latter exhibited decreases of the order of 10%. The magnitude of these variations would seem to be significant in industrial fermentation processes which utilize continuous microbiological cultures. In light of these findings, the results of other researchers were shown to exhibit maximum effective yield at similar dilution rates or process loadings.