Effect of medium pH on chemical selectivity of oxalic acid biosynthesis by Aspergillus niger W78C in submerged batch cultures with sucrose as a carbon source

The pH of the medium is the key environmental parameter of chemical selectivity of oxalic acid biosynthesis by <i>Aspergillus niger</i>. The activity of the enzyme oxaloacetate hydrolase, which is responsible for decomposition of oxaloacetate to oxalate and acetate inside the cell of the fungus, is highest at pH 6. In the present study, the influence of pH in the range of 3-7 on oxalic acid secretion by <i>A. niger</i> W78C from sucrose was investigated. The highest oxalic acid concentration, 64.3 g dm^-3, was reached in the medium with pH 6. The chemical selectivity of the process was 58.6% because of the presence of citric and gluconic acids in the cultivation broth in the amount of 15.3 and 30.2 g dm^-3, respectively. Both an increase and a decrease of medium pH caused a decrease of oxalic acid concentration. The obtained results confirm that pH 6 of the carbohydrate medium is appropriate for oxalic acid synthesis by <i>A. niger</i>, but the chemical selectivity of the process described in this paper was high in comparison to values reported previously in the literature.