Strain Isolation and Study on Process Parameters for Xylose-to-Xylitol Bioconversion

ABSTRACT

A xylitol producing yeast was isolated from soil and identified as Candida tropicalis W103 by 18S r DNA gene sequence analysis and physiological characteristics. The optimal fermentation conditions for Candida tropicalis W103 were: 35°C, pH 4.5 and 120 g l⁻¹ initial xylose concentration. However, the maximum yield and productivity of xylitol were obtained at K_La 16.5 h⁻¹ and 18.3 h⁻¹, respectively. A two-stage aeration strategy (0–24 h, K_La 18.3 h⁻¹; after 24 h, shift K_La to 16.5 min⁻¹) was applied in the fermentation to get higher xylitol yield and productivity. After 60 h in batch fermentations, both the xylitol concentration and xylose consumption reached the maximum, obtaining 87.1 g l⁻¹ of xylitol with 1.45 g l⁻¹ h⁻¹ productivity and 0.72 g g⁻¹ xylose yield. Fed-batch fermentation with 120 g l⁻¹ initial xylose and regulating xylose concentration to 40–55 g l⁻¹ during 24–96 h was performed to reach a higher productivity of 1.82 g l⁻¹ h⁻¹, and xylitol concentration of 218.7 g l⁻¹.

Keywords:

• Candida tropicalis
• fermentation
• isolation
• xylitol
• xylose