**5. Conclusions**

Xylitol fermentation by *C. boidinii* NCAIM Y.01308 was optimised by investigating the effects of oxygen transfer rate and initial xylose concentration applied. The fitted models of maximum xylitol yield, maximum xylitol volumetric productivity, and xylitol yield after 24 h were verified and they were successfully used to predict xylitol production on wheat bran hydrolysate. Wheat bran hydrolysate was found to be an outstanding substrate for *C. boidinii* NCAIM Y.01308 to produce xylitol. Meanwhile, xylitol fermentation on rice straw hydrolysates by *C. boidinii* NCAIM Y.01308 requires further investigations to be improved.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-439 5/11/1/79/s1. Figure S1: Fermentation profiles obtained during the designed experiments of xylitol production on semi-defined medium by *Candida boidinii* NCAIM Y.01308. Figure S2: Thin-layer chromatography analysis of rice straw hydrolysates. STD: standard (X1: xylose, X2: xylobiose, X3: xylotriose, X4: xylotetraose, X5: xylopentaose, and X6: xylohexaose).

**Author Contributions:** S.B. conducted experiments, analyzed the data and wrote the manuscript. A.F. conducted experiments. C.F. and K.J. conceived and designed the research, were responsible for funding acquisition and corrected the manuscript. P.K. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledge the projects of TNN\_16-123305 of the National Research, Development and Innovation Fund of Hungary, the project of VEKOP-2.3.2-16-2017-00013 supported by the European Union, the State of Hungary and the European Regional Development Fund for their financial support. This article is based on work from COST Action (CA18229, Yeast4Bio), supported by COST (European Cooperation in Science and Technology, www.cost.eu). The research was financed by the Higher Education Institutional Excellence Programme (NKFIH-1150-6/2019) of the Ministry of Innovation and Technology of Hungary, within the framework of the Biotechnology thematic programme of the University of Debrecen.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.

**Acknowledgments:** The authors are grateful to Gyermelyi Ltd. (Gyermely, Hungary), Suranaree University of Technology (Thailand), and National Collection of Agricultural and Industrial Microorganisms (Budapest, Hungary) for kindly providing research material; to Kinga Kóder for technical support and to Julie Johnson for linguistics editing.

**Conflicts of Interest:** The authors declare no conflict of interests.
