Fuzzy control of ethanol concentration its application to maximum glutathione production in yeast fed‐batch culture

Abstract

A fuzzy logic controller (FLC) for the control of ethanol concentration was developed and utilized to realize the maximum production of glutathione (GSH) in yeast fedbatch culture. A conventional fuzzy controller, which uses the control error and its rate of change in the premise part of the linguistic rules, worked well when the initial error of ethanol concentration was small. However, when the initial error was large, controller overreaction resulted in an overshoot. An improved fuzzy controller was obtained to avoid controller overreaction by diagnostic determination of “glucose emergency states” (i.e., glucose accumulation or deficiency), and then appropriate emergency control action was obtained by the use of weight coefficients and modification of linguistic rules to decrease the overreaction of the controller when the fermentation was in the emergency state. The improved fuzzy controller was able to control a constant ethanol concentration under conditions of large initial error. The improved fuzzy control system was used in the GSH production phase of the optimal operation to indirectly control the specific growth rate μ to its critical value μ_c. In the GSH production phase of the fed‐batch culture, the optimal solution was to control μ to μ_c in order to maintain a maximum specific GSH production rate. The value of μ_c also coincided with the critical specific growth rate at which no ethanol formation occurs. Therefore, the control of μ to μ_c could be done indirectly by maintaining a constant ethanol concentration, that is, zero net ethanol formation, through proper manipulation of the glucose feed rate. Maximum production of GSH was realized using the developed FLC; maximum production was a consequence of the substrate feeding strategy and cysteine addition, and the FLC was a simple way to realize the strategy. © 1993 John Wiley & Sons, Inc.