Citric Acid Fermentation and Heavy Metal Ions – I. Effects of Iron, Manganese and Copper

High yielding citric acid fermentations are strongly impeded by concentrations of manganese ions in the range above 1μg/l. At manganese concentrations between 0.5 μg/l and 2.0 μg/l, additions of copper (II) ions in amounts of around 1 mg/l enable successful fermentations with citric acid yields between 80 and above 90% as well as high productivities. Manganese concentrations between 2 and 4 μg/l require higher copper additions, up to 4 mg/l, but yields and productivities are mostly unsatisfactory. At manganese concentrations below 0.5 μg/l, additions of copper are unnecessary or even detrimental. At such manganese concentrations, fermentations without copper additions enable yields comparable to those obtained at slightly higher manganese concentrations and copper additions. It is concluded that fermentations with top yields and productivities require exact monitoring of manganese concentrations in the μg/l range and that of iron concentrations in the range below 1 mg/l. The effects of manganese described are most pronounced at sugar concentrations above 140 g/l, whereas at lower sugar concentrations, the system is less sensitive and may tolerate manganese concentrations up to 2 μg/l with low (0.8 mg/l) copper concentrations. Maximum sugar concentrations with already insufficient fermentation yields and productivities are 240 g/l for glucose, and 260 g/l for sucrose, respectively.