## 3 Results and Discussion
### Optimization of K. pastoris Cultivation on BSG Hydrolysate  
#### 3.4.1 Detoxification of BSG Hydrolysate  
Aiming to improve cell growth and xylitol production from BSG, several treatments were applied to the raw hydrolysate (Figure S1). As mentioned above, the pH of the initial hydrolysate (Figure S1A) was adjusted to pH 5 using Ca(OH)\({}_{2}\) (Figure S15B), which is the optimum pH for _K. pastoris_ cultivation [56]. Afterwards, the raw hydrolysate was concentrated (Figure S1C) and detoxified (Figure S1D) to increase its sugars concentration and decrease the content in toxic hydrolysis degradation products, respectively. The BSG hydrolyses presented a brown coloration, with the color being intensified after the concentration treatment.  
As shown in Table 4, adjusting the pH to 5 had no significant effect on the hydrolysate's composition, since only a slight reduction in the concentration of the different components was noticed, namely the loss of xylose and arabinose.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Raw Hydrolysate** & **Banana Peels** & **BSG** & **Corncobs** & **Grape Pomace** & **Grape Stalks** & **Sawdust** \\ \hline Initial glucose (g/L) & 8.2 \(\pm\) 0.6 & 6.4 \(\pm\) 2.3 & 2.3 \(\pm\) 0.4 & 1.7 \(\pm\) 0.5 & 8.0 \(\pm\) 0.7 & 1.7 \(\pm\) 0.2 \\ Glucose consumed (g/L) & 8.2 \(\pm\) 1.5 & 4.9 \(\pm\) 0.9 & 0.7 \(\pm\) 0.1 & 1.7 \(\pm\) 0.3 & 6.7 \(\pm\) 1.2 & 0.1 \(\pm\) 0.0 \\ Initial xylose (g/L) & 1.2 \(\pm\) 1.1 & 8.9 \(\pm\) 2.6 & 8.2 \(\pm\) 1.5 & 3.1 \(\pm\) 1.3 & 5.5 \(\pm\) 1.1 & 10.8 \(\pm\) 1.9 \\ Xylose consumed (g/L) & 1.0 \(\pm\) 0.4 & 2.3 \(\pm\) 0.9 & 2.2 \(\pm\) 0.8 & 1.7 \(\pm\) 0.6 & 3.1 \(\pm\) 1.2 & 0.5 \(\pm\) 0.2 \\ Initial arabinose (g/L) & 1.4 \(\pm\) 0.3 & 6.2 \(\pm\) 2.1 & 0.9 \(\pm\) 0.2 & 0.9 \(\pm\) 0.3 & 1.9 \(\pm\) 0.7 & 0.4 \(\pm\) 0.1 \\ Arabinos consumed (g/L) & 0.4 \(\pm\) 0.1 & 0.4 \(\pm\) 0.1 & 0.1 \(\pm\) 0.0 & 0.2 \(\pm\) 0.1 & 0.4 \(\pm\) 0.1 & 0.1 \(\pm\) 0.0 \\ CDW (g/L) & 15.18 \(\pm\) 0.33 & 5.70 \(\pm\) 0.94 & 13.70 \(\pm\) 0.15 & 7.08 \(\pm\) 0.17 & 14.58 \(\pm\) 0.19 & 0.92 \(\pm\) 0.09 \\ Xylitol (g/L) & 0.06 \(\pm\) 0.01 & 1.51 \(\pm\) 0.07 & 0.81 \(\pm\) 0.05 & 0.36 \(\pm\) 0.07 & 0.78 \(\pm\) 0.04 & 0.02 \(\pm\) 0.01 \\ Arabidopsis (g/L) & 0.02 \(\pm\) 0.01 & 0.90 \(\pm\) 0.09 & 0.21 \(\pm\) 0.03 & 0.21 \(\pm\) 0.02 & 0.34 \(\pm\) 0.04 & 0.07 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Substrate consumption, cell dry weight (CDW), xylitol and arabitol production, after 120 h of cultivation.  
As expected, upon concentrating, the concentrations of glucose, xylose, and arabin increased (11.71 \(\pm\) 2.16, 15.22 \(\pm\) 5.55, and 10.30 \(\pm\) 2.96 g/L, respectively), but as a side effect, the concentration of degradation products also increased (up to 1.14 g/L) (Table 4). Therefore, to reduce the concentration of toxic compounds, a detoxification treatment using activated charcoal was applied to the concentrated hydrolysate. Activated charcoal was reported as the most effective method for the removal of furan derivates [49]. As shown in Table 4, the detoxification treatment allowed HMF to be completely removed, and furfural concentration was reduced by 96%. Nevertheless, the removal rate of acetic acid was significantly lower (37%), with the concentration obtained (0.72 \(\pm\) 0.05 g/L) being similar to the value present in the raw hydrolysate (0.78 \(\pm\) 0.06 g/L). In addition, the detoxification treatment also resulted in losses of sugars, which were reduced by 14-18% (Table 4). These results are comparable to those reported by [PERSON] et al. [57], namely a high removal of furfural and HMF (92% and 68%, respectively), along with a reduction of 13% of the sugars concentrations. Moreover, the removal of acetic acid was inefficient with a removal rate of 17%.