Effects of initial pH on biological synthesis of xylitol using xylose-rich hydrolysate.

Sugarcane bagasse, an agricultural residue plentiful in Brazil, was utilized for xylitol production by a biotechnological process. A medium fermentation prepared with this xylose-rich biomass at an oxygen transfer volumetric coefficient of 10/h1 and different initial pH values was inoculated with cells of Candida guilliermondii FTI 20037. The maximum values of xylitol and cell volumetric productivities (Qp = 0.56 g/[L.h] and Qx = 0.11 g/[g.h]), xylitol yield factor (YP/S = 0.79 g/g), and xylose uptake rate (qs = 0.197 g/[g.h]) were attained at pH 7.0 without further pH control. The results show that the yeast performance was influenced by the pH, an important bioengineering parameter in this fermentation process.

Inhibition of microbial xylitol production by acetic acid and its relation with fermentative parameters.

Precipitated sugarcane bagasse hemicellulosic hydrolysate containing acetic acid was fermented by Candida guilliermondii FTI20037 under different operational conditions (pH 4.0 and 7.0, three aeration rates). At pH 7.0 and kLa of 10 (0.75 vvm) and 22.5/h (3.0 vvm) the acetic acid had not been consumed until the end of the fermentations, whereas at the same pH and kLa of 35/h (4.5 vvm) the acid was rapidly consumed and acetic acid inhibition was not important. On the other hand, fermentations at an initial pH of 4.0 and kLa of 22.5 and 35/h required less time for the acid uptake than fermentations at kLa of 10/h. The acetic acid assimilation by the yeast indicates the ability of this strain to ferment in partially detoxified medium, making possible the utilization of the sugarcane bagasse hydrolysate in this bio-process. The effects on xylitol yield and production are reported.