## 3 Results and Discussion
### Xylitol Fermentation from Synthetic Media  
The aim of the fermentation tests carried out in synthetic media (SM1 and SM2, see Section 2.4) was to evaluate how the presence of glucose affects xylitol production by _C. boidinii_. Moreover, these experiments were useful in estimating the highest xylitol yield and productivity that could be obtained in the fermentation of EOP hydrolysate in the absence of inhibitor compounds. The influence that sugars other than xylose (especially glucose) exert in xylose-to-xylitol bioconversion by yeast has been previously studied. For instance, [PERSON] et al. [28], evaluated the influence of glucose and arabinose on the biological production of xylitol by _Candida guilliermondii_. The authors found that glucose decreased the xylose uptake rate by 30%, while arabinose did not affect xylose consumption. The negative effect of glucose has been suggested to originate from a partial inhibition of the xylose reductase of the yeast [29; 30; 31]. Figure 2 shows sugar uptake and xylitol, ethanol, and cell production during the fermentation of synthetic media (SM1 and SM2). As seen, in both media, the highest xylitol concentrations were achieved for fermentation times as short as 48 h. At this fermentation time, most xylose had already been consumed by _C. boidinii_ (xylose consumption \(>84\%\)), and without that, the subsequent uptake of remaining xylose resulted in higher xylitol production. Moreover, in absence of glucose (SM2 medium), significant xylitol consumption after 48 h fermentation was observed, which could be used for the growth of the microorganism. In this context, a slightly higher cell yield was obtained in SM2 medium compared with SM1 medium (0.08 vs. 0.05 g/g). By comparing the two synthetic media (Figure 2 and Table 2), it can be appreciated that the highest xylitol concentration and yield values (11.9 g/L, and 0.58 g/g, respectively) were obtained when glucose was not present in the synthetic medium (SM2), supporting the observation of glucose's negative effect on xylose metabolism by _C. boidinii_. [PERSON] et al. [31] also attained a lower xylitol concentration in the fermentation by _C. boidinii_ with a synthetic media containing glucose and xylose than when only xylose was present (39.4 vs. 59.3 g xylitol/L). These results were obtained in fermentation runs under fed-batch conditions, which the authors claim is a suitable technique to improve productivities and yields in microbial fermentation. This behaviour was also reported by [PERSON] et al. [32] in xylitol fermentation of synthetic media by _Debaryomyces hansenii_ and _Candida guilliermondii_, well-known microorganisms used for xylitol production. As seen in Table 1, raw EOP acid hydrolysate and SM1 synthetic medium contain a glucose/xylose ratio above 21%. Glucose/xylose ratios \(>10\%\) have been reported to negatively affect xylose transportation into the strain, inhibiting the action of the most important enzymes, such as the xylose reductase, of the yeast [33]. On the contrary, glucose/xylose ratios \(<10\%\) could enhance the enzyme action and, thus, the xylitol production. On the other hand, xylitol fermentation by _C. boidinii_ of SM1 synthetic medium also afforded knowledge of the maximum xylitol yield and productivity values that could be obtained in the fermentation of the EOP acid hydrolysate if any inhibitor compound was present. In this way, as seen in Table 2, maxima values for xylitol yield and productivity of 0.52 g/g and 0.22 g/L/h were achieved, respectively.