ABSTRACT
The aim of this study was to compare the rate of production of citric acid by wild and mutant strains of Aspergillus niger, determine the effect mutation of A. niger has on the production of citric acid, and to device a better and more economical means of citric acid yield. Six isolates of Aspergillus niger (Brywild, Potywild, Onywild, Brymutant, Potymutant, and Onymutant) were screened for citric acid production using submerged fermentation. This process was done by adding the following to each nine 250ml conical flasks respectively: 100ml of water, 14g of sugar, 0.25g of MgSO4.7H2O, 0.3g of KH2PO4, 0.25g of NH4NO3. After the mixture in each conical flask was stirred vigorously to dissolve, to ensure sterility, the conical flasks containing the mixture were autoclaved and allowed to cool properly. Then, 3ml of 107 spores of the specific strain (both wild and mutant) was used to inoculate each flask, which was then incubated for a period of 10 days. During the incubation period, there was periodic hand shaking of the conical flask in place of a mechanical shaker at different time intervals. Wild and mutant strains of A. niger were cultivated on a PDA plate for a period of seven days, measurement of the size of the organisms were taken daily. The measurement showed that the growth of the mutant was generally slow when compared with the wild strains. The mutant strains produced higher yield of citric acid than the wild. It can be concluded that mutagenesis of local strains of Aspergillus niger from bread, potato, and onion samples using sodium azide will cause an increase in citric acid production, and also have a negative effect on the growth of the organism.
