## 3 Results and Discussion
### Xylitol Fermentation from EOP Acid Hydrolysate  
_C. boidinii_ was not able to produce xylitol from the EOP acid hydrolysate without a previous detoxification step (data not shown). The inhibitor compounds contained in this hydrolysate (as described in Section 3.2), mainly furans, acetic acid, and phenolic compounds, are seemingly the underlying reason for this finding. Moreover, a possible synergistic effect between them could also take place, significantly increasing the inhibition of _C. boidinii_[34]. In this work, two different detoxification methods were applied: Activated charcoal and ion-exchange resin. The resulting detoxified hydrolysates were fermented by _C. boidini_ in order to evaluate its fermentability. Sugar consumption, as well as xylitol, ethanol, and cell production values attained during the fermentation process of EOP hydrolysates, are displayed in Figure 3 for each detoxification method. As observed, the maximum xylitol concentration was achieved at 96 h fermentation for the two detoxification methods applied. Although, at this fermentation time, a small xylose content remained without being consumed (between 16-19%). In all cases, _C. boidinii_ was able to consume almost the totality of this sugar at the end of the fermentation (Figure 3). However, after 96 h fermentation, no significant increase in xylitol production (<1.5%) was achieved in any case. [PERSON] et al. [35] also reported good results in the fermentation of hemicellulose hydrolysates of corn fibre by _C. boidinii_, after treatment with activated charcoal. The authors focused on the removal of phenolic compounds by the treatment applied, which reduced almost 96% of the total initial content. In the same way, [PERSON] et al. [32] proved the success of using a detoxification treatment with organic solvent (ethyl acetate) in the fermentation, by _D. hansenii_ and _C. guilliermondii_, of rapeseed straw hydrolysates containing toxic compounds (mainly acetic acid and phenolic compounds), although in a higher concentration than in EOP hydrolysate. This detoxification method reached a 62% reduction of the acetic acid level. By comparing fermentations with the different detoxified hydrolysates, the highest xylitol concentration (Figure 3) and yield (Table 2) were achieved with ion-exchange resin (6.0 vs. 5.2 g/L and 0.43 vs. 0.36 g/g, respectively). However, as seen in Table 2, similar xylitol productivities and cell growths were obtained in both cases (0.07 g/L/h and 0.04 g/g, respectively). The best fermentation results are most likely attained for ion-exchange resin detoxified hydrolysate were due to the lowest levels of acetic acid (3.5 vs. 5.4 g/L) and phenolic compounds (0.6 vs. 1.3 g/L) found in this case compared to activated charcoal detoxified hydrolysate (Table 1). Therefore, the ion-exchange resin provides advantages over activated charcoal treatment by achieving a more effective removal of inhibitory compounds, particularly in the case of acetic acid. In relation to the inhibition by acetic acid, the literature reported that the presence of acetic acid concentrations higher than 3-3.5 g/L in liquors can negatively affect the xylitol fermentation process by typical microorganisms, such as _C. guilliermondii_[36], _C. magnoliae_ TISTR 5663 [37], or _D. hansenii_[32]. Considering the presence of phenolic compounds, according to [PERSON] et al. [38], _C. athensensis_ growth is only possible when the levels of these types of compounds are reduced below 1 g/L, resulting also in a more prolonged lag phase. Moreover, a possible synergistic effect between acetic acid and phenolic compounds can also take place in the fermentation of activated charcoal detoxified hydrolysate, which has been previously reported in these types of xylitol producing microorganisms [34]. Even though acetic acid was generated at the onset of the fermentation with ion-exchange resin detoxified hydrolysate, _C. boidinii_ was able to consume this compound once the xylose concentrations were low (data not shown). According to [PERSON] et al.