Optimization of growth conditions and continuous production of inulinase using immobilized Aspergillus niger cells

Abstract

Aim The aim of the study was the optimization of growth conditions for the production of inulinase as well as the continuous production of the enzyme in an airlift bioreactor using Aspergillus niger cells. Methods First, inulinase production by A. niger cells, using different carbon and nitrogen sources, was studied on a shake flask level. Second, the cells were adsorbed onto different carriers, and their production over several successive batches was tested. Finally, the economically-favorable continuous production of inulinase by A. niger cells immobilized onto linen fibers was carried out in an airlift bioreactor using crude inulin juice as the fermentation medium. Results Although all tested substances resulted in the biosynthesis of certain amounts of inulinase enzyme, the highest titer of 163.5 U/ml was obtained when the producing cells were incubated for 96 h at 27πC and 180 rpm in a fermentation medium containing both inulin and peptone as sole carbon and nitrogen sources, respectively. Moreover, when the cells of the tested microorganism were adsorbed onto different carriers, especially linen fibers, their productivity was also successfully maintained, to different extents, for five successive batches. However, as commercially pure inulin is very expensive and available in only small quantities, the fermentation medium was later substituted by a crude inulin extract obtained by mechanical crushing and filtration of Jerusalem artichoke tubers. The crude inulin juice was able to sustain inulinase production during the second batch cultivation of A. niger cells that were immobilized by their adsorption onto linen fibers to a satisfactory level of about 122 U/ml. Furthermore, the use of the previously mentioned crude inulin preparation was also compared with the use of either complete or minimal media, composed solely of 1% pure inulin, for the continuous production of inulinase enzyme by A. niger cells that were immobilized in their maximum production phase and packed inside an external loop airlift bioreactor. The results of this experiment were very encouraging as, using this technique, an inulinase production of about 838 U/ml over an incubation period of 48 h was obtained compared with a production of about 996 and 1013 U/ml, which resulted from the use of either minimized or complete media, respectively, for the same incubation period. Conclusion The method adopted in this study for inulinase production is simple, economic, time saving, and nontoxic to the microorganism. Moreover, the loaded linen fiber pads are reusable.