Commercial Synthesis of Citric Acid Citric acid is used as a flavoring agent in soft drinks, fruit juices, and many other foods. Worldwide, the market for citric acid is valued at hundreds of millions of dollars per year. Commercial production uses the mold Aspergillus niger, which metabolizes sucrose under carefully controlled conditions. (a) The yield of citric acid is strongly dependent on the concentration of FeCl3 in the culture medium, as indicated in the graph. Why does the yield decrease when the concentration of Fe+ is above or below the optimal value of 0.5 mg/L? Yield of citric acid (%) 90 80 70 60 50 1 L 4 2 3 [FeCl3] (mg/L). 5 (b) (e) Does the commercial process require the culture medium to be aerated that is, is this a fermen- tation or an aerobic process? Explain Write the sequence of reactions by which A. niger synthesizes citric acid from sucrose. Write an equation for the overall reaction.