## 3 Results and Discussion
### Optimization of K. pastoris Cultivation on BSG Hydrolysate
#### 3.4.2 Cultivation of _K. pastoris_ on Detoxifed BS Hydrolysate  
The concentrated BSG hydrolysate promoted biomass production (13.73 \(\pm\) 0.25 g/L), but not xylitol synthesis (Table 5). In fact, the production of xylitol (1.56 \(\pm\) 0.07 g/L) was similar to that obtained with the raw hydrolysate (1.51 \(\pm\) 0.07 g/L), which could be explained by the presence of high concentration of degradation products, especially HMF, which could affect xylitol synthesis [58].  
The detoxification treatment, on the other hand, resulted in a significant improvement of the production of biomass and xylitol. Due to the decreased concentration of toxic compounds (Table 4), the biomass production increased to 20.26 \(\pm\) 0.25 g/L, while xylitol production was improved to 3.97 \(\pm\) 0.10 g/L. These results are within a range of xylitol production (1.40 and 5.31 g/L) as reported by [PERSON] et al. [33] for the same _K. pastoris_ strain in batch bioreactor experiments using glucose (35-60 g/L) and xylose (10-35 g/L) mixtures. Analogous improvement was also reported for _Candida  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Type of** & & & & & & \\
**Hydrolysate** & **Glucose (g/L)** & **Xylose (g/L)** & **Arabinose (g/L)** & **Furfural (g/L)** & **HMF (g/L)** & **Acetic Acid** (g/L)** \\ \hline Initial & 6.44 \(\pm\) 1.18 & 8.85 \(\pm\) 3.23 & 6.19 \(\pm\) 1.78 & 0.18 \(\pm\) 0.03 & 0.12 \(\pm\) 0.02 & 0.78 \(\pm\) 0.06 \\ Raw (pH adjusted) & 6.44 \(\pm\) 1.19 & 8.13 \(\pm\) 2.96 & 5.72 \(\pm\) 1.65 & 0.15 \(\pm\) 0.02 & 0.15 \(\pm\) 0.02 & 0.76 \(\pm\) 0.06 \\ Concentrated & 11.71 \(\pm\) 2.16 & 15.22 \(\pm\) 5.55 & 10.30 \(\pm\) 2.96 & 0.23 \(\pm\) 0.04 & 0.30 \(\pm\) 0.05 & 1.14 \(\pm\) 0.08 \\ Detoxifed & 9.57 \(\pm\) 1.76 & 13.00 \(\pm\) 4.74 & 8.85 \(\pm\) 2.55 & 0.01 \(\pm\) 0.00 & 0.00 \(\pm\) 0.00 & 0.72 \(\pm\) 0.05 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Effect of treatments on the composition of brewers spent grain hydrolysates.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of BSG Hydrolysate** & **Concentrated** & **Detoxifed** \\ \hline Initial glucose (g/L) & 11.90 \(\pm\) 0.19 & 11.68 \(\pm\) 2.10 \\ Glucose consumed (g/L) & 11.90 \(\pm\) 0.19 & 11.43 \(\pm\) 2.06 \\ Initial xylose (g/L) & 14.18 \(\pm\) 1.04 & 15.65 \(\pm\) 2.65 \\ Xylose consumed (g/L) & 5.89 \(\pm\) 0.43 & 7.13 \(\pm\) 1.21 \\ Initial arabin (g/L) & 9.87 \(\pm\) 0.43 & 9.71 \(\pm\) 0.86 \\ Arabinose consumed (g/L) & 3.98 \(\pm\) 0.17 & 1.37 \(\pm\) 0.12 \\ CDW (g/L) & 13.73 \(\pm\) 0.25 & 20.26 \(\pm\) 0.25 \\ Xylitol (g/L) & 1.56 \(\pm\) 0.07 & 3.97 \(\pm\) 0.10 \\ Y\({}_{\text{xyl}}\)/s (g\({}_{\text{xylitol}}\)/g\({}_{\text{xylose}}\)) & 0.26 \(\pm\) 0.09 & 0.56 \(\pm\) 0.17 \\ Arabitol (g/L) & 0.43 \(\pm\) 0.05 & 0.82 \(\pm\) 0.05 \\ Y\({}_{\text{ARB}}\)/s (g\({}_{\text{srabitol}}\)/g\({}_{\text{srabinose}}\)) & 0.11 \(\pm\) 0.09 & 0.60 \(\pm\) 0.11 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Substrate consumption, cell dry weight (CDW), xylitol and arabitol production, yield of xylitol (Y\({}_{\text{xyl}}\)/s) and yield of arabitol (Y\({}_{\text{ARB}}\)/s) on a substrate basis, after 168 h of cultivation.  
detoxification of BSG concentrated hydrolysate revealed an increase of the xylitol/xylose yield from 0.37 to 0.55 g/g [54].  
The production of arabitol was also noticed, which explains the consumption of arabinose (Table 5). As for xylitol production, the detoxification treatment applied to the concentrated hydrolysate improved arabitol production from 0.43 \(\pm\) 0.05 to 0.82 \(\pm\) 0.05 g/L.  
Although the production of xylitol was lower than the values reported for processes based on the use of brewer's spent grain as feedstock for microbial cultivation (Table 6), the yield of xylitol on a substrate basis was within the range of those reported for _D. hansenii_ CCMI 941 (0.50 g/g) or _C. guilliermondii_ FTI 20037 (0.55-0.79 g/g). These results show that _K. pastoris_ is a promising candidate for the development of a bioprocess for xylitol production.