Repeated-batch xylitol bioproduction using yeast cells entrapped in polyvinyl alcohol-hydrogel.

Xylose-to-xylitol conversion was investigated in a bench-scale bioreactor using Candida guilliermondii cells entrapped within polyvinyl alcohol-hydrogel beads in a system operated in repeated-batch mode with cell recycling. Yeast-viable cells were immobilized in the support using the freezing-thawing method. Bioconversion assays were performed in a stirred tank reactor operated at 400-rpm agitation speed, 30 degrees C temperature, and 1.04-vvm air flow rate. The system was explored during six successive cycles, and a small decrease in the conversion performance in the fifth cycle was observed, but the biocatalytic activity of the microorganism was recovered in the sixth cycle after washing the particles. During the process, the hydrogel beads maintained their shape and size without appreciable deterioration. Xylitol production, yield factor, and volumetric productivity increased with progressive recycling of cells and achieved their maximum values (P(F) = 39.7 g l(-1); Y(P/S) = 0.77 g g(-1); Q(P) = 0.53 g l(-1) h(-1), respectively) after the third cell recycling, probably because of cells' adaptation to the medium.