In this study, citric acid (CA) production by autochthonous Candida zeylanoides 7.12 was investigated and optimized. Response surface methodology (RSM) was used for the analysis of simultaneous effects of the chosen factors and 2 experiment designs were applied. In the first experimental design, the effects of initial pH value (5.5, 6.0 and 6.5), fermentation time (4, 5 and 6 days) and initial glucose concentration (125, 150 and 175 g/L) on CA production were investigated. Initial pH value was adjusted periodically with NaOH. Results of the statistical analysis showed that the model was found to be not applicable sufficiently to the chosen data. A second experimental design was employed at the same levels of glucose concentration and fermentation time by disabling the pH factor. pH level was kept at 6.5 with CaCO3. Results of the statistical analysis showed that the fit of the model was good and the lack of fit was not significant (P > 0.05). The highest CA concentration of 11.36 g/L was obtained after 6 days of fermentation with an initial glucose concentration of 125 g/L. The results indicated that initial glucose concentration and fermentation time were important parameters for CA production by C. zeylanoides 7.12 and this strain could be used for future studies.
Keywords: Autochthonous; Candida zeylanoides; Optimization; Organic acid; Response surface methodology.
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