Effect of volume of culture medium on enhanced citric acid productivity by a mutant culture of Aspergillus niger in stirred fermentor.

AIMS

The present study deals with the effect of volume of culture medium on enhanced citric acid productivity by a mutant culture of Aspergillus niger.

METHODS AND RESULTS

A laboratory scale stirred fermentor of 15-l capacity was employed for all microbial cultivations. Blackstrap molasses, a by-product of sugar industries is easily and abundantly available for its exploitation as a carbon source in fermentation processes. The parental culture of A. niger was improved by mutation using ultraviolet radiations and N-methyl N-nitro N-nitroso guanidine, i.e. mutagen MNNG. Six MUV and eight MNNG-treated mutant strains were isolated after extensive screening and optimization. Mutant strain of A. niger MNNG-2 showed enhanced citric productivity (87.60 g l-1) over the parental strain BTL-45 (19.53 g l-1) and other mutant derivatives (49.85 g l-1 citric acid in case of mutant MUV-5 and 76.82 g l-1 in case of mutant MNNG-7). The optimal sugar level was found to be 150 g l-1 (optimum volume of the medium, 60%) after 6 days of inoculation, which is economically significant. Specific productivity of the mutant culture MNNG-2 (qp = 0.057 g/g cells h-1) was several folds higher than other strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of the present study are of commercial level. All kinetic parameters including yield coefficients and volumetric rates revealed the hyper-producibility of citric acid by mutant MNNG-2 using blackstrap molasses as the basal medium in stirred fermentor.