*Article* **Optimized Bioconversion of Xylose Derived from Pre-Treated Crop Residues into Xylitol by Using** *Candida boidinii*

**Soma Bed ˝o <sup>1</sup> , Anikó Fehér 1 , Panwana Khunnonkwao <sup>2</sup> , Kaemwich Jantama <sup>2</sup> and Csaba Fehér 1,\***


**Abstract:** Crop residues can serve as low-cost feedstocks for microbial production of xylitol, which offers many advantages over the commonly used chemical process. However, enhancing the efficiency of xylitol fermentation is still a barrier to industrial implementation. In this study, the effects of oxygen transfer rate (OTR) (1.1, 2.1, 3.1 mmol O2/(L × h)) and initial xylose concentration (30, 55, 80 g/L) on xylitol production of *Candida boidinii* NCAIM Y.01308 on xylose medium were investigated and optimised by response surface methodology, and xylitol fermentations were performed on xylose-rich hydrolysates of wheat bran and rice straw. High values of maximum xylitol yields (58–63%) were achieved at low initial xylose concentration (20–30 g/L) and OTR values (1.1–1.5 mmol O2/(L × h)). The highest value for maximum xylitol productivity (0.96 g/(L × h)) was predicted at 71 g/L initial xylose and 2.7 mmol O2/(L × h) OTR. Maximum xylitol yield and productivity obtained on wheat bran hydrolysate were 60% and 0.58 g/(L × h), respectively. On detoxified and supplemented hydrolysate of rice straw, maximum xylitol yield and productivity of 30% and 0.19 g/(L × h) were achieved. This study revealed the terms affecting the xylitol production by *C. boidinii* and provided validated models to predict the achievable xylitol yields and productivities under different conditions. Efficient pre-treatments for xylose-rich hydrolysates from rice straw and wheat bran were selected. Fermentation using wheat bran hydrolysate and *C. boidinii* under optimized condition is proved as a promising method for biotechnological xylitol production.

**Keywords:** wheat bran; rice straw; acidic hydrolysis; fermentation; aeration; detoxification
