Abstract
This article is devoted to the study of the impact of macro- and microelement concentrations on citric acid biosynthesis using the mutant strain Aspergillus niger R5/4 under submerged fermentation conditions. Laboratory experiments were conducted by varying the levels of NH₄NO₃, KH₂PO₄, MgSO₄·7H₂O, ZnSO₄·7H₂O, FeSO₄·7H₂O, and CuSO₄·5H₂O in the nutrient medium. It was found that a balanced ratio of these elements has a significant effect on the fungal metabolism and morphology, as well as on citric acid productivity. The maximum citric acid yield (69.18 g/L) was achieved after 168 hours of incubation at 30°C and pH 5.0 with the following concentrations: NH₄NO₃ – 3 g/L, KH₂PO₄ – 2 g/L, MgSO₄·7H₂O – 0.3 g/L, ZnSO₄·7H₂O – 1 mg/L, FeSO₄·7H₂O – 12 mg/L, CuSO₄·5H₂O – 70 mg/L. High productivity was associated with the formation of compact mycelial pellets with diameters less than 0.5 mm. Exceeding threshold concentrations of certain microelements, particularly zinc and iron, led to decreased citric acid output, likely due to toxic effects and disruption of physiological processes. The results obtained may be used to optimize nutrient media composition in the context of industrial-scale citric acid production through biotechnological methods.