Production of citric acid by Aspergillus niger using cane molasses in a stirred fermentor

Abstract

The present investigation deals with the kinetics of submerged citric acid fermentation by Aspergillus niger using blackstrap molasses as the basal fermentation media. A laboratory scale stirred fermentor of 15-L capacity having working volume of 9-L was used for cultivation process and nutritional analysis. Among the 10 stock cultures of Aspergillus niger, the strain GCBT7 was found to enhance citric acid production. This strain was subjected to parametric studies. Major effects were caused due to oxygen tension (1.0 l/l/min), pH value (6.0) and incubation temperature (30ºC). All fermentations were carried out following the growth on 150 g/l raw molasses sugars for 144 hours. Ferrocyanide (200 ppm) was used to control the trace metals present in the molasses medium. Ammonium nitrate (0.2%) was added as nitrogen source. Maximum citric acid production (99.56 ± 3.5a g/l) was achieved by Aspergillus niger GCBT7. The dry cell mass and sugar consumption were 18.5 and 96.55 g/l, respectively. The mycelia were intermediate round pellets in their morphology. The specific productivity of GCBT7 (qp = 0.074 ± 0.02a g/g cells/h) was several folds higher than other strains. The specific production rate and growth coefficient revealed the hyperproducibility of citric acid using mutant GCBT7.