Optimization of citric acid production by a mutant strain of Yarrowia lipolytica in natural fermentation medium containing carrot juice

In this study, growth and citric acid production of a mutant strain, Y. lipolytica K-168 was investigated in a natural fermentation medium containing carrot juice. In this medium, interactive effects of selected independent variables; initial total sugar concentration, initial pH, initial concentration of undersized semolina protein concentrate and temperature, on the growth and citric acid production of the yeast were investigated. An experimental design including 30 experiments was conducted by using the method of central composite design. Modelling the effects of these independent variables on maximum citric acid concentration, maximum citric acid production rate, citric acid yield, the ratio of maximum citric acid concentration to maximum isocitric acid concentration and specific growth rate were performed by response surface methodology. The variations of all of the responses with the independent variables were defined by a quadratic model. According to the results of variance analysis, initial total sugar concentration was found as one of the most important responses affecting citric acid production. The effect of initial pH was found as insignificant for the responses related to citric acid production, while it was found as significant for specific growth rate. It was determined that the effect of temperature changed by the initial sugar concentration. The linear effect of initial concentration of undersized semolina protein concentrate was found as significant for all of the responses except the ratio of maximum citric acid concentration to maximum isocitric acid concentration. It was determined that most of the responses related to citric acid production were adversely affected by high concentrations of this component. However, the specific growth rate was found to increase by the increase in initial concentration of undersized semolina protein concentrate. By the numeric optimization performed; the conditions with 190.83 g/L initial sugar concentration, 5.90 initial pH, 0.07 g/L initial concentration of undersized semolina protein concentrate and 27.86oC were determined as optimum conditions. In the experiment in which optimum conditions were used together, maximum citric acid concentration, maximum citric acid production rate, citric acid yield, the ratio of maximum citric acid concentration to maximum isocitric acid concentration and specific growth rate were found as; 80.53 g/L, 0.374 g/L.h, 42.21%, 9.04 and 0.017 h-1, respectively. These values were determined as very similar to the predicted values by the optimization