Citric acid production and morphology of Aspergillus niger as functions of the mixing intensity in a stirred tank and a tubular loop bioreactor

• Published in: Biochemical engineering journal 1998-12, Vol.2 (3), p.197-205
• Main Authors: Papagianni, M., Mattey, M., Kristiansen, B.
• Format: Article
• Language: English
• Subjects: Aspergillus niger; Citric acid; Loop bioreactor; Morphology; Relative mixing time
• ISSN: 1369-703X; 1873-295X
• DOI: 10.1016/S1369-703X(98)00032-1

The relationship between Aspergillus niger morphology and citric acid production was investigated in two reactor systems with different configurations, a tubular loop and a stirred tank bioreactor, with operating volumes of 6 and 8 dm 3, respectively. Morphology was quantified by image analysis. In each system, morphology, characterized by the parameter P (mean convex perimeter of clumps), and citric acid production, were agitation-dependent and closely linked. Increased agitation caused a reduction of clump sizes and results when both reactors demonstrate that the parameter P should not exceed a threshold value in order to achieve increased productivities. The results obtained from the two reactors were in agreement, both qualitatively and quantitatively. Reducing the fundamentally different mixing conditions of the two bioreactors to the order of the dimensionless mixing parameter relative mixing time ( τ m), results showed that the loop simulated the stirred tank. Also, relationships valid for one system accurately described the results obtained from the other system, demonstrating the validity of the relationship between morphology and productivity for the particular fermentation, regardless of the reactor type. Previous attempts to evaluate the use of loop configurations as scale-up tools and their performance as bioreactors, neglected the morphology of the producer micro-organisms. This study demonstrated the close link between morphology and productivity for citric acid production by A. niger, and identified a morphology parameter that was used successfully to characterize the process performance.