Aerobic fungal cell immobilization in a dual hollow‐fiber bioreactor: Continuous production of a citric acid

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Abstract
Aspergillus niger B60 was immobilized in a dual hollow‐fiber bioreactor (DHFBR) 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. Long‐term operation with nitrogen‐enriched medium was not possible due to expansion of the silicone tubes by continual cell growth. The fungal growth could be controlled by supplying a nitrogen‐deficient medium at the production stage. 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 fermentation, which were 18 g/L, 40%, and 0.06 g/L h, respectively.

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