Sequential batch culture was conducted using the flocculent yeast cells in order to use repeatedly the cells. For the flocculation of yeast cells, various flocculating agents were tested in order to select the best one. Seven cationic salts and five positively charged polymers were tested in order to assess their flocculating ability with washed cell suspension. Of the 12 chemicals tested, cationic synthetic polyacrylamide copolymer (Zetag78) was shown to be most effective for the flocculation of Candida tropicalis. Flocculation ability of polyacrylamide copolymer was also investigated in complex fermentation medium. Cell flocculation was not affected by pH but by degree of dilution of medium, demonstrating that some component of complex medium, namely yeast extract, was the cause of inhibition of flocculation. Moreover, salt concentration of yeast extract used in the medium was found to be a critical factor for the flocculation. In order to enhance the xylitol production, the effect of dissolved oxygen concentration (DO) on the cell growth and xylitol production was quantitatively investigated, and an optimum DO concentration range was determined. In repeated batch cultures using a flask, the volumetric productivity of 1.42g/L/hr was obtained under oxygen limited condition with a xylitol yield of 68%. In case of 2.5L jar fermenter, the volumetric xylitol productivity of 1.5g/L/hr was obtained at optimal DO concentration with a xylitol yield of 70%. These results were repeatedly obtained up to 4∼6 batches using flocculating agent.
