Although there is a growing interest
Although there is a growing interest in the biological production of xylitol from renewable materials (Albuquerque et al., 2014, Dhar et al., 2016), at present, most of the xylitol is produced on commercial scale by traditional chemical approach that include hydrogenation of pure d-xylose derived from the 67 7 sale (Ur-Rehman et al., 2015). Even though d-xylose can be obtained from cheap lignocellulosic (de Albuquerque et al., 2015, Zabed et al., 2016), the traditional chemical route of xylitol production is very costly due to some drawbacks and economic issues. Firstly, it requires high cost Raney-nickel catalyst together with high pressure (10–15 atm) and elevated temperature of up to 130 °C (Cheng et al., 2014). Moreover, chemical synthesis of xylitol employs not only the expensiveness process, but also requires high costs and high amounts of water for purification of xylose from hemicellulose hydrolysates, in addition to the requirement of costly refining process for downstream xylitol recovery (Albuquerque et al., 2014, Pal et al., 2016). What is more, chemical synthesis of xylitol is not environmentally friendly and is energy consuming (Dasgupta et al., 2017, Li et al., 2015). Alternatively, microbial biotransformation of xylose into xylitol has been investigated using several yeast strains such as Debaryomyces hansenii, Candida tropicalis and Candida parapsilosis (Li et al., 2016). This bioconversion route is further found to be challenging because of the requirement of hydrolysis of hemicellulose and purification of xylose from the hydrolysate as it contains microbial inhibitors (Li et al., 2016). Compared to xylose, glucose could be a good candidate for xylitol production in the perspectives of the technology and economy (Mayer et al., 2002). However, natural microorganisms cannot convert glucose to xylitol directly due to the lack of comprehensive metabolic pathway and hence, it is necessary to employ a two-step biotransformation process, in which glucose is first converted into d-arabitol by yeasts that undergoes subsequent biotransformation into xylitol by microorganisms (Li et al., 2016, Qi et al., 2014). This two-step biotransformation system is therefore technologically complex and time consuming. As a result, although d-arabitol is a relatively costly substrate compared to glucose, it has some promising advantages and can be used for xylitol production by the most efficient xylitol producing strains, such as Gluconobacter sp., which do not have the metabolic system to convert glucose into d-arabitol, in addition to the fact that utilization of d-arabitol as substrate reduces process step and risk of contamination (Zhang et al., 2013, Qi et al., 2015). Additionally, d-arabitol can be produced from various renewable sources such as glucose, glycerol and xylose by simple microbial biotransformation (Kordowska-Wiater, 2015, Yoshikawa et al., 2014, Zhu et al., 2010). Two key enzymes play important roles in the bioconversion of xylitol from d-arabitol, namely d-arabitol dehydrogenase (ArDH) and NADH-dependent xylitol dehydrogenase (XDH), respectively (Qi et al., 2014). In an earlier study, we developed a high xylitol yield strain by expressing xdh in E. coli BL21 from Gluconobacter oxydans CGMCC1.49, where the mixed culture of recombinant and G. oxydans strains produced 0.83 g/g of xylitol from d-arabitol in the presence of exogenous NADH (Qi et al., 2016). In another study, we have constructed two recombinant strains, namely BL21-xdh and BL21-ardh that contained novel xdh and ardh, respectively, from Gluconobacter sp. JX-05 and produced 26.1 g/L of xylitol with a yield of 0.87 g/g in a mixed culture of both strains using exogenous NADH (Qi et al., 2017).