Method for continuously producing citric acid by dual hollow fiber membrane bioreactor

Abstract

Aspergillus niger B60 was immobilized in a dual hollow fiber bioreactor to produce citric acid continuously. The fungi proliferated well in the interstitial region formed by a parallel arrangement of three microporous polypropylene hollow fibers contained within a silicone tube. Excessive fungal cell growth could be controlled by supplying a nitrogen-deficient medium at the production stage, and also uniform cell distribution in the reactor could be obtained by changing the direction of medium flow. With pure oxygen aeration and nitrogen-deficient medium, volumetric productivity reached 1.62 g/l.h at a residence time of 4.02 h, which corresponded to a 27-fold increase over that of shake-flask fermentation. When the residence time was increased to 20.1 h, citric acid at a concentration of 26 g/l was continuously produced, with a yield of 80-90% and a volumetric productivity of 1.3 g/l.h. This represents a significant improvement in final concentration, yield and the volumetric productivity over the equivalent values of the corresponding batch formentation, which were 18 g/l, 40% and 0.06 g/l.h, respectively.