Xylitol production from D-xylose in a membrane bioreactor

Economical feasibility of xylitol production by microorganisms depends directly on the efficiency of conversion and productivity. A potential alternative to ensure this objective is continuous bioconversion. Xylitol production and separation methodology by a biotechnological route using the benefits of membrane technology were investigated. Xylitol produced by a strain of Candida guilliermondii was obtained using a synthetic medium containing D-xylose in a continuous membrane bioreactor. Moreover, hollow-fiber membranes were prepared and modules suitable for continuous fermentation medium sterilization were designed. The membrane that presented the best performance had maximum pore diameter of 0.2 µm and permeability of 42.9 L/(m2.h.bar) for a simulated medium. The best results obtained with this system for the continuous production of xylitol in a membrane bioreactor was attained with a dilution rate of 0.03 h!1, providing an efficiency of D-xylose conversion of 86% and productivity values up to 1.14 g of xylitol/L.h.

This work shows the benefits of using membrane technology in several stages of a bioprocess. The sterilization of fermentation medium using hollow-fiber membranes is an extremely feasible and practical alternative since it is easy to operate. However, characterization of the microfiltration module using real fermentation media is extremely important for module dimensioning. The yield and volumetric productivity of continuous bioconversion of xylose for xylitol production with dilution rate of 0.03 h!1 were 0.79 g of xylitol/g of xylose and 1.14 g of xylitol/L.h, respectively. In these conditions, steady state was reached after 25 h and was maintained by 78 h. The ultrafiltration membrane of 20 kDa used for cell recycle allowed the flow needs requested by system, and during the steadystate condition, supplied a permeate with a high degree of purity containing 38 g/L of xylitol, 2.5 g/L of xylose and depleted of the macromolecules of high molecular weight. This certainly will facilitate further developments of xylitol purification. Biological/membrane hybrid processes can contribute economically to the viability of xylitol production by the biotechnology route.