Ability of Aspergillus niger to tolerate metal ions and minerals in a solid-state fermentation system for the production of citric acid.

Biosynthesis ofcihic acid by Aspergillus niger in molasses medium in submerged
and liquid surface fermentations is highly sensitive to the concentration of trace
elements present in the medium. Studies on citric acid production in wheatbran
medium in a solid-state fermentation system, however, showed no such influence
despite high concentrations of metal ions and total ash content in the wheatbran.
In contrast, supplementation of wheatbran with additional amounts of mineral
ions such as iron, copper, zinc, manganese, magnesium and phosphorous was
found to be necessary for increased production of citric acid. All the mineral ions
when used individually at an optimum level increased cim’c acid production by
1.4-1-9 times. However, only iron, copper andzinc were effective in combination.
Citric acid production with this combination was 4921 g/kg commercial
wheatbran at 48 h as against 22.49 g formed in the absence of any added metal
ions at the same fermentation time. These results indicate that the advantage of
solid-state fermentations in tolerating high concentrations of metal ions in the
production of citric acid is probably due to concentration gradients and impaired
transport of the metal ions in the conditions of limited water availability.