Manufacture of citric acid by submerged fermentation

In the process of producing citric acid by submerged aerobic fermentation with a citric acid-producing microorganism in a sterilized carbohydrate-containing growth medium containing complexable substances and assimilable heavy metals, the improvement which comprises adding to the medium not later than its sterilization, a first amount of ferrocyanide ions in an amount necessary to complex only one-third to two-thirds of the total amount of complexable substances and assimilable heavy metals of the medium, and adding to the medium after sterilization of the medium an additional amount of ferrocyanide ions in an excess over the amount sufficient to complex the remaining assimilable heavy metals and the buffering capacity of the medium but said excess amount being insufficient to block the respiratory system of the microorganism.