Feeding rate control algorithms for fed-batch fermentation have been developed and applied to the cultivations of Escherichia coli, Brevibacterium, and microorganisms inhibited by substrates.
To obtain high cell density of E. coli in fed-batch culture, the accumulation of acetic acid should be prevented by controlling the specific growth rate and dissolved oxygen concentration (DOC). An integral controller with a variable gain and a feedforward scheme was developed to control the specific growth rate. The specific growth rate was estimated from the carbon dioxide evolution rate (CER) measured by mass-spectrometer. Although the controller and the estimator of the specific growth rate had a simple structure, these showed satisfactory performances. Both of the estimated and the experimental specific growth rate coincide with the set point value well. An automatic tuning PID controller was employed to control the DOC by manipulating the agitation speed and the oxygen partial pressure in the inlet gas. The disturbing effect of feed rate changes was compensated for by using a feedforward scheme. The DOC was controlled well for the entire period of the culture in spite of the changes in the feed rate and the change of the input variable.
A mutant of Brevibacterium ammoniagenes which produces inosine 5-monophosphate (IMP) was cultivated in batch, fed-batch, and total cell recycle modes. The effects of carbon source (glucose and fructose), corn steep liquor, and adenine on the cell growth and the production of IMP were investigated in batch cultures. The concentration of carbon source was estimated with CER under the assumption that CER is proportional to the consumption rate of carbon source. The yield coefficient of CER for the consumption rate was calculated with off-line data at every sampling time and used for the next sampling time. Feeding rate was manipulated to control the carbon source concentration by supplying the amount of the assimilated carbon source calculated by the above method. With this feeding method, the culture time of fed-batch could be reduced to 58 hours as compared with 72 hours of the intermittent feeding method. High cell density could be obtained with total cell recycle culture. However, the productivity of IMP was not improved as compared with that of the fed-batch culture.
In order to maintain maximum growth rate in the culture of aerobic and substrate inhibited microorganisms, a fuzzy controller was developed and tested through simulation. Feeding rate was controlled to minimize DOC as DOC was minimized at maximum growth rate with fixed agitation speed and inlet gas flow rate. With this controller, the maximum growth rate could be sustained and the fluctuation of substrate concentration was reduced as compared with the DO-stat method, and thus the fermentation time was reduced.