Abstract
Modelling studies were performed on a fermentation system using Saccharomyces cerevisiae NCYC 754. The production of fermentation product and cytochrome P-450 were studied under semi-anaerobic condition in batch cultures. The fermentation was carried out in a 5-litre fermenter and controlled at constant set-points which had been optimized by an earlier worker with respect to enzyme yield. An unstructured model was established to describe the biomass profile which comprised two growth phases; however the system did not demonstrate the classical diauxic growth as expected. Furthermore, against the general belief that glucose is the limiting substrate of the system; the maximum wet biomass seemed to depend on the concentration of peptone and yeast extract in the fermentation broth. Growth kinetics indicated that a second substrate was utilized before glucose metabolism began in spite of the presence of high levels of glucose. Luedeking and Piret type models, combined with ethanol inhibition, were derived to describe the profile of ethanol and cytochrome P-450 concentration. Later, it was demonstrated that a close correlation exists between initial glucose and cytochrome P-450 concentration. Viable count by agar plating techniques was used to test the proposed biomass model. The results were in line with the proposed model, even though the cell viability profile in the system was rather low. The Taguchi method was used to seek out the noise factor in the system, and optimize the operating conditions for a particular performance statistic. Contrary to earlier findings, the stirrer speed was found to have little effect on the yield of cytochrome P-450.