Study of the biotechnological production of xylitol in a fluidized bed reactor using sugarcane bagasse and immobilized cells: evaluation of operational parameters and economical viability

Xylitol is being distinguished for its application in the industries of food, odontology and pharmacy; furthermore, it presents a potential use in other industrial segments (textiles and chemicals). The different benefits of xylitol will open doors for new selling areas which will lead to its growth in the international market of polyols and alternative sweeteners. The present work had as an objective the contribution in the development of a technically and economically viable technology for the production of xylitol starting from the hemicellulosic hydrolysate of sugarcane bagasse, using a fluidized bed bioreactor with yeast cells of Candida guilliermondii FTI 20037 immobilized in a natural support of calcium alginate. To evaluate the technical viability of this biotechnological process, they were realized simple batch fermentations according to a factorial design 23 with three central points. Furthermore, it was evaluated the influence of the variables, fluidization flux, hydrolysate concentration factor and air flux in the process yield (Yp/s) and volumetric productivity (Qp). According to the obtained results it was observed that, only an increase in the fluidizations flux exercised a positive influence in process yield and volumetric productivity. This fact is due to a better oxygen transfer to the inside of the immobilization support, resulting in a higher xylose consumption and xylitol production. The biotechnological process used in this work resulted in, a final concentration of xylitol of 34 g/L starting from an initial concentration of xylose of 49 g/L, a yield (Yp/s) of 0.7 g/g (corresponding to 76 % of bioconversion efficiency) and a volumetric productivity (Qp) of 0.44 g/L.h, after 72h of fermentation. Also they were realized repeated batch fermentations with the recycle of the immobilized cells, using the optimized fermentation conditions as indicated by the statistical analysis previously done. It was verified that, the yield (Yp/s) and the volumetric productivity (Qp) of the process have presented small variations throughout the 6 repeated batch fermentations (B1-B6), with an average final production of 31,5 g/L of xylitol. On the other hand, at the end of the batch fermentation B7 it was observed a decrease of 44% in the final concentration of the produced xylitol (17 g/L) and 28% in the final number of viable immobilized cells (3.4 x1010 mL/cells) in comparison with the batch fermentations B1-B6 (average value of 4.7x1010 mL/cells), after 72h of fermentation. This reduction in the growth rate of the immobilized cells can be explained by the possible diffusion and accumulation of insoluble substances originating from the hemicellulosic hydrolysate, during the 7 repeated batch fermentations, into the interior of the immobilization support resulting in limitations in xylose transference from the fermentation medium into the encapsulated cells. With the objective to evaluate the production cost of xylitol, it was realized a technicaleconomical study for the production of a xylitol syrup with 80% of purity, using hemicellulosic hydrolysate from sugarcane bagasse in a pilot plant with the capacity to process 1 tons of bagasse. According to the results obtained in this study, it was observed that the biotechnological process for xylitol production has shown to be economically viable with a payback period of 24 months and a TIR of 51. 7%, considering that the selling price of xylitol syrup (80% of purity) was estimated to be R$ 211.60 in the internal market. Aiming to reduce the selling cost of xylitol syrup and increase its competitiveness in relation to other polyols found in the market, they were suggested modifications in some stages of the process used in this work (increase in the hydrolysis efficiency to 80% and the utilization of ionic exchange resins in the treatment of the hemicellulosic hydrolysate). The modified process resulted in a reduction in the selling price of xylitol syrup, being this value estimated in R$ 113.10 corresponding to only 28% of the selling price of crystallized xylitol in the internal market (R$ 402.50). The biotechnological production of xylitol has shown to be economically promising for future implantation at industrial level. (AU)