Continuous enzymatic production of xylitol with simultaneous coenzyme regeneration in a charged membrane reactor.

We have developed a new process for the production of xylitol from D-xylose by enzyme technology. An NADH-dependent xylose reductase (XR) from Candida tenuis catalyzes the reduction of xylose, which is coupled to enzymatic oxidations of D-glucose or D-xylose by glucose dehydrogenase from Bacillus cereus to make achievable an up to 10,000-fold regeneration of NADH per cycle of discontinuous conversion. Using a simple kinetic model as a tool for process optimization, suitable conditions with regard to performance and stability of the multi-component reaction system were established, and 300 g/L of substrate could be converted in yields above 96% in one single batch reaction. Due to selective and over 98% complete retention of the native coenzyme by negatively charged nanofiltration membranes used in a continuously operated enzyme reactor, a specific productivity of 80 g xylitol per liter, day, and kilounit of XR was maintained over the 150-h reaction time with only a single dosage of NADH. (c) 1996 John Wiley & Sons, Inc.