Optimization of Citric Acid Production by Aspergillus niger and Candida tropicalis for Solid State Fermentation Using Banana Peel Substrate

Abstract

Increasing cost of production and global demand for citric acid is driving research towards optimizing process conditions to yield very high quantity of the organic acid using abundant cheap substrates and selected microorganisms. Consequently, this study was designed to optimize the production of citric acid by making use of banana (Musa acuminata) peels (agro-waste) through means of solid state fermentation involving Aspergillus niger (Model A) and Candida tropicalis (Model B). In this study, a two-level, five-variable full factorial design of response surface methodology (RSM) comprising 32 experimental runs for each model were used to develop a statistical model for the optimization of fermentation conditions which include: pH, glucose, zinc, ammonium chloride and methanol. The results obtained indicate that a second order polynomial model fitted adequately and statistically significant (p<0.0001) and (p<0.0410) for Model A and B, respectively. The optimum values of the variables were: pH 4; glucose 5% w/v; zinc 2% w/v; ammonium chloride 0.5% w/v; and methanol 3% v/v. Under these conditions, the concentration of citric acid produced were 97.6 g/L with a pH of 3.85 using Aspergillus niger and 113.6 g/L with a pH of 3.45 using Candida tropicalis at 10days fermentation period. Experimental validation of the model indicated that no difference exist between the predicted and the actual yield results. Therefore, utilization of low-cost agro-waste banana peel which serve as suitable substrate for optimization of citric acid production is advocated because of their advantages such as income generation, reduction in environmental problems posed by food-waste disposal and public health hazards associated with it.