Abstract

Aspergillus niger EB-3 was used to enhance citric acid production through optimization of nutritional parameters using banana stalk as carrier/inert support in solid state fermentation (SSF) of sugarcane molasses. The effects of moisture level, nitrogen sources, incubation time, metabolic inhibitors and metal complexing chemicals were studied on citric acid production in SSF. A. niger EB-3 mutant could produce 97.52 mg/ml citric acid with sugar utilization of 104.07 ± 1.98 mg/ml in 96 h by optimizing moisture level (60%) and nitrogen source (0.2% w/w ammonium nitrate). Addition of different surfactants to the medium increased fungal biomass, sugar utilization and citric acid production and Tween-80 was the best surfactant that caused maximum enhancement in citric acid yield at 0.2% concentration. The metabolic inhibitors like naphthol, resorcinol and cresol also enhanced citric acid production when used in lower concentrations, and cresol was the most effective inhibitor at 0.4% (w/w) level. Addition of metal complexing compounds also stimulated citric acid production by A. niger EB-3 and EDTA was the most effective metal chelator at 0.4% (w/w) level. Under optimum conditions, 112.42 mg/ml citric acid was produced with sugar utilization of 121.84 mg/ml.

Key words: Citric acid, hyperproducting mutant, Aspergillus niger EB-3, optimization, metabolic inhibitors, metal complexing compounds.

Abbreviation

SSF, Solid state fermentation.