The pH influence on continuous citric acid secretion was investigated in
Candida oleophila
ATCC 20177 (var.) under NH
4+ limiting state steady conditions, using glucose. Highest citric acid concentration of 57.8 g/l, citrate/isocitrate ratio of 15.6, space-time yield of 0.96 g/(l x hr) and biomass specific productivity of 0.041 g/(g x hr) were obtained at pH 5 and 60 hrs residence time. Only 22.8 g/l (39.4%) and a ratio of 9.9 were achieved at pH 6 pH and 12.4 g/l (21.5%) and a ratio of 3.7 at pH 3. Under non producing conditions, in excess of nitrogen, biomass concentration increased at raising pH. An iron concentration of 200 ppm was determined in biomass of
C. oleophila at pH 5, compared with only 26 ppm found at pH 3 (factor 7.7). Intra- and extracellular concentrations of citrates and glucose confirmed the existence of a high specific, pH dependent active transport system for citrate secretion, while isocitrate isn't a high-affine substrate, displaying a strong correlation with ATP/ADP ratio. Differences between extra- and intracellular concentration of citrate higher than 1 and up to about 60 were determined. The active transport systemfor citrate excretion appears to be the main speed-determining factor in citrate overproduction by yeasts.
