The influence of zinc, iron, copper, and manganese on the production of citric acid by Aspergillus niger

demonstrated that zinc and iron were essential to citric acid production by Asper-gillus niger 724. It was also found that the addition of very small amounts of manganese or copper, or both, to a sucrose medium caused an increase in the amount of citric acid produced by stationary surface culture. However, experi-mental proof that both elements were essential for maximum production was not obtained. The present work was undertaken for the purpose of clarifying the status of copper and manganese in this fermentation. METHODS AND MATERIAL