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.
Highlights
Biotechnological valorization of lignocellulosic residues derived from the agro-industrial sector is of great importance in many countries with intense agriculture in order to deal with the increasing demand of the society for energy and materials, the mitigation of greenhouse gas emissions and waste production and the development of a bio-based circular economy [1]
This study revealed the main factors affecting the xylitol production of C. boidinii and provided validated models to predict xylitol yield and productivity depending on the initial xylose concentration and oxygen transfer rate (OTR)
In order to investigate the effects of initial xylose concentration and OTR on xylitol production, statistical analysis of the results of the designed experiments was performed (Table 2)
Summary
Biotechnological valorization of lignocellulosic residues derived from the agro-industrial sector is of great importance in many countries with intense agriculture in order to deal with the increasing demand of the society for energy and materials, the mitigation of greenhouse gas emissions and waste production and the development of a bio-based circular economy [1] For such a sustainable bio-based economy, biorefineries that provide integrated facilities to produce a wide range of bio-products and bioenergy from biomass residues within a zero waste approach are considered as main pillars [2]. Wheat bran and rice straw are the main lignocellulosic by-products of wheat-milling and rice-harvesting processes, with an estimated global production of 150 and 730 million tonnes per year, respectively [12,13] They contain a considerable amount of xylan-type hemicellulose fractions besides cellulose and lignin [14,15]. Due to the relatively high xylan content of wheat bran and rice straw, one of the most promising platform chemicals that can be produced from them in a biorefinery is xylitol [17,18]
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