Statistically based design of experiment (DOE) was applied to the optimization of synthetic medium composition for oxalic acid (OA) production by Aspergillus niger in shake-flask cultures. The Box-Wilson Central Composite Circumscribed quadratic model (CCC) was used to search the optimum levels of macroelements influencing mycelia growth and OA production. Among these variables, N, P, Mg, were found to be significant factors that affected oxalic acid production. The optimum medium composition was found to be as follows: sucrose 150 g dm^-3, NH₄NO₃ 1.3 g dm^-3, KH₂PO₄ 0.3 g dm^-3, MgSO4·7H₂O 0.64 g dm^-3. Under optimal culture conditions, the maximum oxalic acid dihydrate amounted to 85.7 g dm^-3 with confidence level 95%.

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