Production of citric acid by immobilized Aspergillus niger using a rotating biological contactor (RBC)

Production of citric acid by Aspergillus niger was studied in a rotating biological contactor (RBC) consisting of plastic disks mounted on a horizontal shaft with polyurethane foam (PUF), as a porous biomass support, attached on each side of the RBC disks. Mycelia of A. niger formed the bio®lm of immobilized cells on the surface of the PUF. The RBC±PUF system was operated with a rotational speed of 10 rev/min during the stage of bio®lm formation. The mature bio®lm was exposed to the fermentation medium and the air space alternately. The results showed that the volumetric productivity obtained with the RBC±PUF system (0.896 g/l h) was almost three times higher than that obtained with a stirred-tank fermenter (0.33 g/l h). The immobilized bio®lm was active for over 8-cycle periods of citric acid production with repetitive use without loss of bioactivity