Evidence for nitrogen catabolite repression during citric acid production by Aspergillus niger under phosphate-limited growth conditions.

Experiments performed in batch fermentation under phosphate-limited growth conditions showed that the citric acid yield was inversely related to the excess nitrogen concentration in the medium. Results from chemostat culture confirmed a negative relationship between the citric acid yield and both the specific growth rate and the nitrogen consumption rate. This is evidence for nitrogen catabolite repression. A fed-batch fermentation performed under dual phosphate/nitrogen limitation produced results very similar to those from a culture limited by nitrogen alone. There is no advantage in maintaining an excess of phosphate during citric acid production and the process will therefore be more economic when operated under dual limitation conditions.