ABSTRACT
The biochemical mechanism by which Aspergillus niger accumulates citric acid has attracted the interest of researchers since the late 1930s when the optimization of this accumulation to give a commercial process began. In this sense, the various theories which have been proposed to explain the accumulation of citric acid in such high yields also reflect the general biochemical knowledge at the time the respective research was done. In view of the high input into this research through more than 50 years it is therefore rather disappointing that there is still no explanation of the biochemical basis of this process which would consistently explain all the observed factors influencing this fermentation. Reasons for this are manifold. First, citric acid is only accumulated when several nutrient factors are present, either in excess (i.e. sugar concentration, H+, dissolved oxygen), or at suboptimal levels (trace metals, nitrogen and phosphate), and thus is subject to multifactorial influence. Hence it is unlikely that single biochemical events are solely responsible for citric acid overflow.
