Fate and Role of Ammonium Ions during Fermentation of Citric Acid by Aspergillus niger

ABSTRACT Stoichiometric modeling of the early stages of the citric acid fermentation process by Aspergillus niger revealed that ammonium ions combine with a carbon-containing metabolite inside the cell, in a ratio 1:1, to form a nitrogen compound which is then excreted by the mycelium. High-performance liquid chromatography analysis identified glucosamine as the product of the relationship between glucose and ammonium during the early stages of the citric acid fermentation process. Slightly acidic internal pHs, extremely low ammonium ion concentrations inside the cell, and glucosamine synthesis come into direct contradiction with the earlier theory of the ammonium pool inside the cell, regarded as responsible for inhibition of the enzyme phosphofructokinase. At later fermentation stages, when the mycelium is involved in a process of fragmentation and regrowth, the addition of ammonium sulfate leads to a series of events: the formation and secretion of glucosamine in elevated amounts, the short inhibition of citrate synthesis, growth enhancement, the utilization of glucosamine, and finally, the enhancement of citric acid production rates. Obviously, the enzymatic processes underlining the phenomena need to be reexamined. As a by-product of the citric acid fermentation, glucosamine is reported for the first time here. Suitable process manipulations of the system described in this work could lead to successful glucosamine recovery at the point of its highest yield before degradation by the fungus occurs.