Citric acid yields of 98.7% (sugar consumption basis) were reached in shaker flasks with mutant UV-ET-71-15 of Aspergillus niger in a resin-treated sucrose medium of the following composition (g/100 ml): sucrose, 14.0; NH4NO3, 0.20; KH2PO4, 0.10; MgSO4·7H2O, 0.025; and (mg/liter): FeSO4, 0.15 to 0.75; ZnSO4, 0.10; and CuSO4, 0.01. Yields of 75% were obtained in medium with resin-treated clarified syrup and 68% with ferrocyanide-treated blackstrap molasses. Optimal conditions included selection of appropriate pellets as inoculum at 3%, pH of 4.5, temperature at 30 C, agitation at 250 rev/min, and fermentation time of 8 days. The mutant tolerated high concentrations of trace elements.
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