Figure 1. Effect of different concentrations of ammonium nitrate (as nitrogen source for mycelial growth) on citric acid productivity by Aspergillus niger GCBT-7. All the fermentations were carried out at 30ºC following growth on 150 g/l initial sugar concentration. The initial pH of the molasses medium was kept constant at 6.0 throughout the fermentation period of 6-days. Yp/s = g citric acid produced/g substrate consumed, Qp = g citric acid produced/l/h, µ (h^-1) = specific growth rate, qs = g substrate consumed/g cells/h.