Application of kaolin to improve citric acid production by a thermophilic Aspergillus niger

Abstract

Citric acid production by a thermophilic strain of the filamentous fungus Aspergillus niger IIB-6 in a medium containing blackstrap cane molasses was improved by the addition of kaolin to the fermentation medium. The fermentation was run in a 7.5-l stirred bioreactor (60% working volume). The optimal sugar concentration was found to be 150 g/l. Kaolin (1.0 ml) was added to the fermentation medium to enhance volumetric production. The best results in terms of product formation were observed when 15 parts per million (ppm) kaolin was added 24 h after inoculation. With added kaolin, citric acid production was enhanced 2.34-fold, compared to a control fermentation without added kaolin. The length of incubation to attain this product yield was shortened from 168 to 96 h. The comparison of kinetic parameters showed improved citrate synthase activity of the culture (Y _p/x=7.046 g/g). When the culture grown at various kaolin concentrations was monitored for Q _p, Q _s, and q _p, there was significant improvement in these variables over the control. Specific production by the culture (q _p=0.073 g/g cells/h) was improved several fold. The addition of kaolin substantially improved the enthalpy (ΔH _D=74.5 kJ/mol) and entropy of activation (ΔS=−174 J/mol/K) for citric acid production, free energies for transition state formation, and substrate binding for sucrose hydrolysis. The performance of fuzzy logic control of the bioreactor was found to be very promising for an improvement (≈4.2-fold) in the production of citric acid (96.88 g/l), which is of value in commercial applications.