EFFECT OF FERROCYANIDE ON GROWTH AND CITRIC ACID PRODUCTION BY ASPERGILLUS NIGER

Ferrocyanide at concentrations of less than 30 p.p.m. (the amount tolerated in citric acid fermentation of beet molasses) had no measurable effect on citric acid production or on the oxidation of glucose or Krebs cycle compounds by resting cells of Aspergillus niger or on the growth rate of this organism during submerged fermentation of beet molasses. Concentrations above 30 p.p.m., however, stimulated citric acid formation in resting cells, but markedly inhibited cell development in growing cells. This inhibition of growth was the main cause of the detrimental effect of high concentrations of ferrocyanide on citric acid formation in molasses; good growth throughout the fermentation was essential to high acid yield, inhibition of growth could be released at any time during the fermentation by addition of sufficient ZnSO4 to reduce the ferrocyanide content to below 30 p.p.m. No evidence that ferrocyanide favors citric acid accumulation by blocking a reaction in the Krebs cycle was found.