Stimulation of citric acid production by heat shock promoting <i>Aspergillus niger</i> using nitrogen enriched <i>Dioscorea bulbifera</i>

High cost of substrate and how to stimulate citric acid are among the factors that mitigate citric acid production. Exploiting a cheap substrate for citric acid production will be a viable alternative in order to reduce the cost of citric acid production. Therefore, the aim of this research was to evaluate the potential of wild Dioscorea bulbifera tubers for citric acid production. Experimental procedures were designed to determine the effect of different concentrations of wild Dioscorea bulbifera, different nitrogen sources and effect of different concentrations of nitrogen sources on citric acid production. The data obtained were analyzed using one- way analysis of variance (ANOVA). The results of the experiment showed that wild Dioscorea bulbifera tuber was able to produce citric acid. The concentration of acid produced increased from 5 % to 20 % citric acid up to 8.5 ±1.0 g/lafter 96 hours of fermentation. Among the different nitrogen sources used to supplement wild Dioscorea bulbifera, the medium with ammonium nitrate produced the highest citric acid concentration of 10.5 ±1.5 g/l after 96 hours of fermentation. Meanwhile, when the concentration of the amonium nitrate were varied, 0.4 % was the optimum for the maximum citric acid concentration of 14.8 ±2.0 g/l after 96 hours of fermentation (P< 0.05). In conclusion, Dioscorea bulbifera tubers is suitable for citric acid production and supplementation of the medium with ammonium nitrate as a nitrogen source had a positive effect on the yield.