Gamma radiation was used to induce citric acid overproduction mutations in some of the locally isolated strains of Aspergillus niger. Comparison between the parent and mutant strains was studied. The study revealed that, the most two isolates potent in the acid production were identified as A. niger F1 and F6. Modified Czapek’s medium proved to be the best medium for citric acid production. As for the influence of irradiation of A. niger F1 and F6 strains using gamma rays in different doses (25 – 200 k rad) on the production of citric acid, one mutant obtained from 175 k rad treatment for A. niger F1 and another mutant obtained from 100 k rad treatment for A. niger F6. These two mutants (A. niger MF1 and A. niger MF6) increased the citric acid production as compared with the parent strains. The best sugar concentration of beet molasses was when the dilution of sugar reached 15% from the basal medium. Addition of potassium ferrocyanide to beet molasses, to precipitate excess heavy metals, at a concentration of 0.15% gave the highest conversion coefficient and yield of citric acid for parent and mutant strains. The addition of alcohols, namely methanol or ethanol (4%) to beet molasses medium greatly stimulated citric acid production. Methanol addition (4%) gave the best result of conversion coefficient and yield of citric acid for A. niger F1 and F6 as well as MF1 and MF6, followed by ethanol treatment (4%) which gave an increase in citric acid as compared to control. The proper time for addition of methanol was found to be immediately at the beginning of fermentation.
Key words: citric acid, Aspergillus niger, mutation, γ-radiation beet molasses, alcohol.
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