Citric acid production by the yeasts Candida guilliermondii and Yarrowia lipolytica : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Biotechnology and Bioprocess Engineering at Massey University

Abstract

The aim of this thesis was to investigate the relationships, for a citric acid­ producing strain of yeast, among the growth rate, sugar uptake rate and the citric acid production rate, and to investigate the hypothesis that citric acid production occurs when the growth rate slows, but the sugar uptake rate is maintained. As previous experimental work in the Department of Process and Environmental Technology (formerly Biotechnology Department) of Massey University had been performed in shake flask cultures only, it was desired to scale-up the culture into a 21 laboratory scale batch culture, and then into a chemostat culture. The first yeast investigated, Yarrowia lipolytica IMK2, failed to successfully scale-up, so further investigations were performed using the yeast Candida guil!iermondii IMK1. Experiments were performed in shake flask culture to investigate the effect of using mixed carbon sources to adjust the carbon uptake rate, and hence the citric acid production rate, but no effect was noticed with the mixtures tested. Batch fermenter experiments were performed to investigate the effect of the culture pH, and the aeration rate, on citric acid production. The aeration rate was not observed to have an effect on the culture in the range tested (0.06 - 0.333 vvm), but the culture pH was observed to have an effect, with the maximum production occurring at pH 4.3, and no citric acid production occurring below pH 3.5. Chemostat culture experiments were performed to investigate the effect of culture pH and the specific growth rate on citric acid production. The specific

growth rate was observed to have a significant effect, with the specific citric acid production rate increasing as the growth rate decreased. The effect of the culture pH was found to vary with the growth rate, with the maximum production rate and yield occurring at pH 3.8, and a growth rate of 0.02 h-1 • From cultures where the glucose was exhausted from the medium, and therefore glucose was a limiting nutrient, the specific citric acid production rate was observed to decrease as the glucose uptake rate decreased. Thus, it could be concluded that the specific citric acid production rate increased as the growth rate decreased, provided that the sugar uptake rate remained high.