Effect of ammonium ion concentration and application to fed-batch culture for overproduction of citric acid

Abstract

Abstract In citric acid fermentation by Aspergillus niger, nutrients especially the nitrogen source have a great influence on productivity. Citric acid has been known to be produced when the nitrogen source was the limiting condition. In the present study, ammonium nitrate was employed as a nitrogen source and batch cultures were carried out under various concentrations of ammonium nitrate. The specific growth rate was inhibited and the biosynthesis of citric acid was delayed at high concentrations of ammonium nitrate. The specific citric acid production rate was the highest when intracellular ammonium ion concentration was between 2 and 3 mmol/g cell. However, its production was stopped when intracellular ammonium ion concentration decreased below 1 mmol/g cell. This knowledge was applied to the fed-batch culture, where maximum citric acid concentration was almost two times higher than that of a conventional batch culture and no morphological change was observed during the feeding period.