Abstract
Biosynthesis of citric 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 wehatbran. 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 citric acid production by 1·4–1·9 times. However, only iron, copper and zinc were effective in combination. Citric acid production with this combination was 49·21 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.
Published Version
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