Hydrogels based on u.v.-crosslinked poly(ethylene oxide) -: matrices for immobilization of Candida boidinii cells for xylitol production

Hydrogels based on high molecular weight poly(ethylene oxide) were synthesized by u.v.-irradiation of aqueous solutions in presence of the photoinitiator, (4-benzoylbenzyl)trimethylammonium chloride and different crosslinkers, poly(ethylene glycol), diacrylates and N,N'-methylenebisacrylamide. Candida boidinii cells were immobilized in these hydrogels and the gels were characterized in regards to gel fraction yield, degree of equilibrium swelling, shear storage and loss moduli. In addition, the number average molecular weight between crosslinks and the mesh size were estimated. The incorporated yeast cells considerably affected the viscoelastic properties of the gels. Immobilized C. boidinii cells were used for conversion of xylose to xylitol. Of the immobilized systems tested, only the system with poly(ethylene oxide) crosslinked with N,N'-methylenebisacrylamide exhibited xylitol production. The operational stability of this system was evaluated by seven repeated-batch runs performed in Erlenmeyer flasks in duration of 55 days. The progressive improvement of xylose consumption, up to 73.5%, stopped in the fifth cycle, after which it dropped to 42.7%. Although xylitol concentration never reached more than 4.2 g l(-1), xylitol was produced in each of the seven cycles. The cell leakage of 1.8 g l(-1) during the first 45 days, indicated very good stability of the system.