Kinetic studies on citric acid production by Aspergillus niger. II. The two-stage process.

A two-stage process of submerged citric acid fermentation with replacement of growth medium by fermentation medium has been worked out. The optimum composition of mineral nutrients and pH of the fermentation medium of the second stage of the process were determined. An addition of 0.5 g/l of NH4NO3 as nitrogen source and 0.1 g/l of MgSO4-7H2O as magnesium source ensured effective conversion of sucrose to citric acid. An addition to KH2PO4, on the other hand, was definitely unfavourable as it considerably reduced the product yield. The medium for the second stage of fermentation should be acidified to about pH 2.2, while the water used for washing the mycelium from the remains of the growth medium should have a pH of 2.5--3.5. Under these conditions, with an initial sucrose concentration of 100 g/l, after 132 hr fermentation at 26 degrees up to 90 g/l of citric acid was obtained, which corresponds to a productivity of over 16 g/l. day. The highest activity for citric acid formation was found in three- or four-day-old mycelium.