CYSTEINE ADDITION STRATEGY FOR MAXIMUM GLUTATHIONE PRODUCTION IN FED-BATCH CULTURE OF SACCHAROMYCES-CEREVISIAE

A good cysteine addition method that could increase the specific glutathione (GSH) production rate (rho(G)) was investigated and utilized to maximize total GSH production in fed-batch culture of Saccharomyces cerevisiae. The single-shot addition of cysteine was a better method compared to a continuous method that maintained a constant cysteine concentration in the reactor. The shot method increased rho(G) about twofold compared to a culture without cysteine. The increase in rho(G) by the shot method can be achieved without growth inhibition if the cysteine dose is maintained at 0.7 mmol.g-1 cell or less. The positive effect on rho(G) (at every specific growth rate, mu) was saturated when the cysteine shot concentrations was 3 mM or more. A simple model was developed consisting of mass balance equations and the relationship between mu and rho(G), for the single cysteine shot addition method. From this model an optimal operating strategy was determined to maximize total GSH production in fed-batch culture. This optimal operation consisted of separating the process into phases of (1) cell growth and (2) GSH production, through a bang-bang profile control of mu, and a shot of cysteine just at the start of the GSH production phase. In other words, the cysteine shot time and the mu-switching time should be the same. For a total feeding time of 10 h, both the switching time of mu and cysteine shot time were calculated to be about 6.4 h.