## 3 Results and Discussion
### Ethanol Sub-Production  
In both fermentations of EOP hemicellulose hydrolysates and synthetic media by _C. boidini_, ethanol was produced as a secondary product, due to the consumption of hexoses (like glucose, galactose, and mannose). In relation to the synthetic media fermentations (Figure 2 and Table 2),  
Figure 3: Fermentation of EOP acid hydrolysate by _C. boidini_ after detoxification by activated charcoal; and (**a**) ion-exchange resin (**b**). as expected, the lowest ethanol production was obtained in absence of glucose (SM2 synthetic media). _C. boidini_ was able to produce a small amount of ethanol (1.3 g/L) by consumption of galactose and mannose with a yield of 0.05 g/g. In the two synthetic media, the highest ethanol production was achieved at 48 h of fermentation (Figure 2), having consumed the totality of glucose (in SM1) and most of the galactose and mannose (84%, in both SM1 and SM2) by this time. As seen in Figure 2a, glucose was the first hexose completely consumed by the yeast, being exhausted in only 24 h of fermentation. This same behaviour was also appreciated in the fermentation of EOP hemicellulose hydrolysates, but in this case, glucose was not totally consumed until 48 h of fermentation, probably due to the presence of toxic compounds (Figure 3). Moreover, based on the same reason, the maximum ethanol concentrations were attained at fermentation times as long as 96 h. By comparing the ethanol results achieved with EOP hemicellulose hydrolysates detoxified by activated charcoal and ion-exchange resin, slightly higher ethanol concentrations (2.6 versus 2.0 g/L, Figure 3) and yields (0.12 versus 0.08 g/g, Table 2) were obtained for ion-exchange resin detoxification. This is most likely due to the lowest toxic compound concentrations (mainly acetic acid and phenolic compounds) contained in the ion-exchange resin detoxified hydrolysate (Table 1). On the other hand, as seen in Figures 2 and 3, glucose, xylose, galactose, and mannose were consumed simultaneously by _C. boidini_, being able to produce both xylitol and ethanol at the same time. However, as expected, the main product generated was xylitol, being produced in much higher levels than ethanol. Regarding arabinose, _C. boidini_ was not able to consume this sugar, neither in synthetic media nor in EOP hemicellulose hydrolysates fermentations. This behaviour of _C. boidini_ was also reported by [PERSON] et al. [35] in the xylitol fermentation of corn fibre hydrolysates. Arabinose was not consumed in the fermentation of rice straw hydrolysates by a recombinant _Saccharomyces cerevisiae_ YPH499 strain [45]. Moreover, as can be appreciated in Figures 2 and 3, once most of the sugars were used up by _C. boidini_, the yeast started to consume xylitol and, above all, ethanol in both synthetic media and EOP hemicellulose hydrolysate fermentations, being employed probably to cell growth. The use of ethanol as a second carbon source has also been reported by [PERSON] et al. [46] in single and mixed substrate fermentations by _D. [PERSON]_. In this case, fermentation tests were carried out in a glucose-containing media, and ethanol was used when the glucose concentration in the media was close to 3 g/L. In this work, _C. boidini_ first consumed galactose and mannose, and once all hexose sugars were depleted, the ethanol concentration started to decrease. Partial consumption of ethanol and xylitol after depletion of the main carbon sources (xylose and glucose) was found in the fermentation of rapeseed straw hemicellulose hydrolysates by _C. guilliermondii_[32]. To sum up, the results of the present work prove the suitability of the yeast _C. boidini_ for the production of xylitol from EOP hemicellulose hydrolysate, previous a detoxification step by activated charcoal or ion-exchange resin treatment. This is a novel and valuable finding for increasing the value of all sugars contained in EOP biomass that are separated into the different streams generated after pretreatment, in this case, an acid pre-treatment.