Oxygen and temperature effect on continuous citric acid secretion in Candida oleophila

ABSTRACT

The influence of air saturation and temperature on continuous citric acid secretion was studied in chemostat cultures of Candida oleophila ATCC 20177 (var.). Simultaneous measurements of intra- and extracellular concentration of glucose, citric and isocitric acid confirmed the involvement of a specific active transport system in citrate secretion, favouring citric acid over isocitrate. An optimum air oxygen saturation of 20 percent and temperature of 30-31ºC were determined for the continuous citric acid secretion. The highest values of citric acid concentration (98 g/L), citrate to isocitrate ratio (33.31), volumetric citric productivity (1.8 g/(L x h)), and specific citric acid productivity (0.1 g/(g x h)), were reached at 20 percent air saturation at a residence time of 54 hrs by the experiment's lowest biomass of 18 g/L. The highest isocitic acid volumetric productivity (55.6 mg/(L x h)) and specific productivity (0.99 mg/(g x h)) were identified at 50 percent, instead. The fastest citrate excretion rate of the generic product of 0.046 g/(g*h) was found at 30-31ºC. A concentration ratio between extra- and intracellular concentration of citrate of up to 9 was identified. The highest extra-/intracellular ratio of citrate and lowest intracellular concentrations of glucose, citric and isocitric acid were determined at optimum air saturation as a consequence of active citrate export.