Optimization of xylitol production by Candida tropicalis in fermentation medium containing chestnut shell hydrolysate

In this study, a hemicellulosic hydrolysate prepared from chestnut shell was used for xylitol production by Candida tropicalis. Firstly, xylitol productions of two indigenous C. tropicalis strains were investigated in fortified chestnut shell hydrolysate. C. tropicalis M2 was selected at this stage. Then, effects of different nitrogen sources on xylitol production of this strain was investigated in the medium containing chestnut shell hydrolysate. Ammonium chloride was selected as nitrogen source and interactive effects of independent variables -initial xylose concentration, initial concentration of ammonium chloride and shaking rate- on the growth and xylitol production of C. tropicalis M2 were investigated. An experimental design including 18 experiments was conducted by using central composite design. Modelling the effects of these independent variables on maximum xylitol concentration, xylitol yield, maximum xylitol production rate, percentage of substrate conversion and spesific product formation rate was performed by response surface methodology. Variations of all of the responses with the independent variables were defined by a quadratic model except the percentage of substrate conversion. The variation of percentage of substrate conversion with the independent variables was defined by a linear model. According to the results of variance analysis, initial concentration of ammonium chloride was found to be the most important response affecting xylitol production, which was significant for all of the responses. The linear effect of the initial xylose concentration was found as significant for only maximum xylitol concentration. It was observed that the effect of shaking rate was very significant for the maximum xylitol concentration and percentage of substrate conversion. By the numeric optimization performed; the conditions with 80 g/L initial xylose concentration, 3.761 g/L initial concentration of ammonium chloride and 80 strokes/min of shaking rate were determined as optimum conditions. In the experiment in which optimum conditions were used together, maximum xylitol concentration, xylitol yield, maximum xylitol production rate, percentage of substrate conversion and spesicific product formation rate were found as; 24.50 g/L, 31.517%, 0.497 g/L.h, 97.35% and 0.06 g/L.h.N, respectively. These values were determined as very similar to the predicted values by the optimization.