The influence of hexoses addition on the fermentation of D-xylose in Debaryomyces hansenii under continuous cultivation

The effect of hexoses (glucose and galactose) addition to the feed xylose m ineral medium of Debaryomyces hansenii chemostat cultures grown at a consta nt dilution rate of 0.055 h(-1) was studied. Xylitol was the major product detected amongst all tested conditions. The maximal values for xylitol yiel d and volumetric productivity (0.56 gg(-1) xylose and 0.21 gl(-1)h(-1), res pectively) were obtained fora glucose/xylose feeding ratio of 10%, showing that the addition of small amounts of glucose, but not galactose, enhanced the xylitol production. A xylitol yield increase of 30%, compared with the sole xylose-containing feed medium, was observed. It was found that the oxy gen requirement for D. hansenii growth is lower under glucose compared with xylose. Ethanol and glycerol were only produced for glucose/xylose feeding ratio above 30%. The byproducts accumulation was correlated with glucose m etabolism, because a direct relationship between the increase of ethanol (a nd glycerol) concentration and the increase of glucose in the feed medium w as found. (C) 2000 Elsevier Science Inc. All rights reserved.