The yeast Saccharomyces cerevisiae is not able to use xylose or xylitol as a carbon source for growth or fermentation {54). Hallbom et al. (55) obtained efficient conversion of xylose to xylitol by transforming S. cerevisiae with the gene encoding the xylose reductase (XR) gene {XYL1) of Pichia stipitis. Due to lack of xylitol dehydrogenase (XDH), the recombinant S. cerevisiae needs a co-carbon substrate to regenerate the cofactors and to gain maintenance energy. Hallbom et al. (56) studied the influence of cosubstrate and aeration on xylitol formation by the recombinant S. cerevisiae. With glucose and ethanol, the conversion yields were close to 1 g xylitol/ g consumed xylose. Decreased aeration increased the xylitol yield based on consumed cosubstrate, while the rate of xylitol formation decreased. Xylitol yields close to 100% could be obtained from a medium with a total xylose concentration corresponding to that of an industrial hemicellulose hydrolyzate by fed-batch cultivation of recombinant XYL1 expressing S. cerevisiae using ethanol as co-substrate (57). Recently, Roca et al. {58) investigated the effect of hydraulic residence time (1.3-11.3 h), substrate/cosubstrate ratio (0.5 and 1), recycling ratio (0.5 and 10), and aeration (anaerobic and oxygen limited conditions) on xylitol production by immobilized recombinant S. cerevisiae in a continuous packed-bed bioreactor.