Cysteine addition strategy for maximum glutathione production in fed-batch culture of Saccharomyces cerevisiae

Summary

A good cysteine addition method that could increase the specific glutathione (GSH) production rate (G9_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 ϱ_G about twofold compared to a culture without cysteine. The increase in ϱ_G by the shot method can be achieved without growth inhibition if the cysteine dose is maintained at 0.7 mml·g^-1 cell or less. The positive effect on ϱ_G (at every specific growth rate, µ) 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 µ and ϱ_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 µ, and a shot of cysteine just at the start of the GSH production phase. In other words, the cysteine shot time and the µ switching time should be the same. For a total feeding time of 10 h, both the switching time of µ and cysteine shot time were calculated to be about 6.4 h.