Abstract: PVA and sodium alginate were mixed to entrap Candida tropicalis cells. The effect of PVA concentration on phosphate tolerance, mechanical strength, morphology and xylitol production of the immobilized cell pellets was discussed. The results indicated that increasing PVA concentration could effectively improve phosphate tolerance of the immobilized cell pellets. All the cell pellets had good mechanical strength before fermentation, which dramatically decreased at lower PVA concentrations but showed good stability at the PVA concentration of 60 g/L, even after fermentation. Transmission scanning microscopic observations showed obvious rupture and holes as well as the resulting free cells on the surface of the immobilized cell pellets. The amount of free cells did not exhibit a decrease with increasing mechanical strength. Instead, the smallest amount of free cells was observed when PVA concentration was 50 g/L. Therefore, PVA concentration could greatly affect the distribution, morphology and xylitol production of Candida tropicalis cells. When PVA concentration was lower, mounting cells could be observed on the surface and in the center of the pellets in full round or oval shape. Increasing PVA concentration resulted in a decrease of the cell amount in the center of the pellets. When PVA concentration was higher than 60 g/L, the cells were mainly located in the outer layer with the aggregation and partially deformed cells in the caves. Xylose consumption and xylitol accumulation were also significantly affected by PVA concentration. When PVA concentration was in the range of 40－60 g/L, the yield of xylitol from xylose could reach up to 0.6 g/g in a slow xylose consumption manner.

Key words: polyvinyl alcohol, sodium alginate, Candida tropicalis, xylitol