Determination of optimum composition of fermentation medium for citric acid production from watermelon rinds using response surface methodology

Abstract

A four variable Box-Behnken design (BBD) coupled with response surface methodology (RSM) was employed to optimise the composition of fermentation medium for the production of citric acid from watermelon rind using Aspergillus niger in solid state fermentation (SSF). The four variables studied were glucose (C6H12O6), diammonium hydrogen phosphate ((NH4)2HPO4), potassium dihydrogen phosphate (KH2PO4) and sodium chloride (NaCl). Statistical analysis of the results showed that all the components of the medium ((NH4)2HPO4, KH2PO4 and NaCl) apart from glucose had a significant effect on citric acid production (p<0.05). A statistical quadratic model was developed by applying regression analysis to the experimental data. The model was statistically significant (p<0.0001) and showed a good fit with the experimental data. Citric acid production was positively influenced by high levels of glucose and NaCl while the reverse was the case for KH2PO4 and (NH4)2HPO4. The optimised nutrient medium contained glucose, (NH4)2HPO4, KH2PO4 and NaCl in proportions of 23.40 g/L, 0.19 g/L, 0.19 g/L and 0.59 g/L respectively. In validating the model, no significant difference was observed between predicted and experimental results. This study has demonstrated the feasibility of producing citric acid from watermelon rinds in SSF.