Membrane-Based Downstream Processing of Microbial Xylitol Production

Xylitol is a sugar alcohol used as a sweetener in the food industry. Xylitol can be produced from D-xylose using a fermentation process, but it then needs to be separated from the other components of the fermentation broth (e.g., metabolic products, residual substances, biomass cells, and mineral salts), before being purified as xylitol crystals. Therefore, to obtain high purity xylitol, various separation processes are required. One very promising downstream processing method is membrane separation. This study evaluated membrane-based processes for the separation of biomass cells and other impurities, determined the concentration of xylitol produced from Debaryomyces hansenii yeast fermentation broth, and proposed a polysulfone ultrafiltration (UF) membrane for biomass-cell separation followed by polyamide nanofiltration (NF) to remove low-molecular-weight compounds (e.g., acetic acids) from sugars. The effects of operating pressure were examined using a fermentation broth model solution. The results showed that a higher pressure caused a higher permeate flux; however, the permeate flux’s rate flow decreased over time due to concentration polarization, and fouling in the UF and NF membranes. Nevertheless, at all pressures, UF achieved a 99% rejection of biomass cells. In addition, microscope analysis showed that no biomass cells were detected in the permeates of UF. The resulting NF concentrates revealed high xylitol retention and a beneficially lower concentration of acetic acids. The operating pressures of the UF test conditions were 1 barg and 1.5 barg, illustrating that, at a pressure of 5.5 barg, the experiments achieved reasonably high xylitol retention (above 90%) indicating negligible losses of sugar in the permeate port. Moreover, this was proven to be a feasible way to concentrate xylitol up to three times from the initial concentration of the model fermentation broth (MFB). Therefore, the results demonstrated that a two-stage combination of UF and NF is a promising system for the downstream processing of microbial xylitol production.

Biomass cells; Fermentation broth; Nanofiltration; Ultrafiltration; Xylitol

The results showed that the UF membrane can remove biomass cells from fermentation broth. UF showed great performance for the retention of biomass cells, in that no cells were observed in the UF permeate. An increase in the applied pressure did not significantly increase the steady-state UF fluxes of the UF permeate. Increasing the applied NF pressure tended to increase xylitol retention and increased the initial permeate flux, despite the faster decrease in the flux due to the resulting concentration polarization and membrane fouling. 

The optimum test conditions were achieved by applying a UF pressure of 1 barg and an NF pressure of 5.5 barg, giving a 3.3-times xylitol concentration of the feed solution. Overall, the combination of both the UF and NF processes was shown to be a promising process configuration to purify and concentrate xylitol obtained from a fermentation process.

This research was funded by the Directorate of Higher Education, Indonesian Ministry of National Education (DIKTI) under the scheme for Fundamental Research.

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