Aspergillus niger accumulates citric acid when: (1) glucose is high (substrate excess), (2) pH is low (~2.0–2.5, suppressing competing organic acids), (3) Mn²⁺ is severely limited (inhibits isocitrate dehydrogenase and aconitase, blocking TCA cycle beyond citrate), and (4) nitrogen is limited (reducing biosynthetic demands). The combination causes citrate to accumulate. Annual production exceeds 1.5 million tonnes, mostly for food and beverage use.