## 3 Results and Discussion
### Raw BSG Characterization and Pretreatment
A high content of protein was also observed which can be also used as carbon source. As expected, calcium, magnesium and sodium were detected as they are comprised in BSG constitution [1; 3]. A high content of sulfate was also detected, which resulted from the use of sulfuric acid for BSG hydrolysis. Moreover, acetic acid (HAc), furfural and 5-HMF were also detected at low concentrations (Table 3). Furfural and 5-HMF result from the degradation of hexoses and pentoses during the hydrolysis procedure [24]. HAc usually results from the release of the acetyl groups attached to the hemicellulose backbone [25]. Nevertheless, such compounds are \begin{table}
\begin{tabular}{l c c} \hline  & **[Sugars]** & **Hydrolysis Efficiency** & **Overall Hydrolysis Efficiency a** \\  & **(g L\({}^{-1}\))** & **(\%)** & **(\%)** \\ \hline
**1 st hydrolysis** & 32.4 \(\pm\) 2.6 & 56.0 & \\
**2 nd hydrolysis** & 16.4 \(\pm\) 2.9 & 64.6 & 84.5 \\ \hline \end{tabular} a  
\end{table} 
Table 2: Monosaccharide content and efficiency of BSG acid hydrolysis. \begin{table}
\begin{tabular}{l c c} \hline  & **Before** & **After** \\  & **Neutralization** & **Neutralization** \\ \hline
**Calcium** (g L\({}^{-1}\)) & 1.12 \(\pm\) 0.04 & 4.60 \(\pm\) 0.33 \\ \hline
**M magnesium** (g L\({}^{-1}\)) & 1.76 \(\pm\) 0.03 & 0.38 \(\pm\) 0.04 \\ \hline
**Sodium** (mg L\({}^{-1}\)) & 5.06 \(\pm\) 0.07 & 4.90 \(\pm\) 0.01 \\ \hline
**Sulfate** (g L\({}^{-1}\)) & 66.6 \(\pm\) 2.9 & 4.61 \(\pm\) 1.8 \\ \hline
**Furfural** (g L\({}^{-1}\)) & 0.71 \(\pm\) 0.07 & 0.26 \(\pm\) 0.05 \\ \hline
**5-HMF** (mg L\({}^{-1}\)) & 70 \(\pm\) 30 & 30 \(\pm\) 5 \\ \hline
**Acetic acid** (g L\({}^{-1}\)) & 1.13 \(\pm\) 0.14 & 1.52 \(\pm\) 0.001 \\ \hline
**Glucose** & & \\ (g L\({}^{-1}\)) & 3.47 \(\pm\) 0.19 & 3.35 \(\pm\) 0.31 \\ \cline{2-3} (gCOD L\({}^{-1}\)) & 3.68 \(\pm\) 0.20 & 3.55 \(\pm\) 0.33 \\ \hline
**Xylose** & & \\ (g L\({}^{-1}\)) & 12.4 \(\pm\) 2.8 & 9.20 \(\pm\) 0.23 \\ (gCOD L\({}^{-1}\)) & 13.2 \(\pm\) 2.9 & 9.75 \(\pm\) 0.24 \\ \hline
**Arabinose** & & \\ (g L\({}^{-1}\)) & 6.53 \(\pm\) 0.60 & 4.94 \(\pm\) 0.06 \\ \cline{2-3} (gCOD L\({}^{-1}\)) & 6.92 \(\pm\) 0.64 & 5.23 \(\pm\) 0.06 \\ \hline
**Total protein** (g L\({}^{-1}\)) & 29.6 \(\pm\) 1.9 & 19.7 \(\pm\) 2.1 \\ \hline
**NH\({}_{4}\)+** (gN L\({}^{-1}\)) & 0.69 \(\pm\) 0.05 & 0.64 \(\pm\) 0.001 \\ \hline
**PO\({}_{4}\)3\({}^{-}\)** (mgP L\({}^{-1}\)) & 90 \(\pm\) 10 & \textless{}2 \\ \hline
**COD** (gO\({}_{2}\) L\({}^{-1}\)) & 56.8 \(\pm\) 0.7 & 54.6 \(\pm\) 0.8 \\ \hline
**C:N:P ratio** & & \\ (C-mol:N-mol:P-mol) & 100:31:60 & 100:33:2 \\ \hline \end{tabular}
\end{table} 
Table 3: Characterization of BSG hydrolysate before and after neutralization. potentially inhibitory to the fermentation process [25]. Generally, furfural is produced from the degradation of pentose sugars (xylose and arabin) and 5-HMF results from the degradation of hexose sugars (glucose). In this work, low concentrations of furfural and 5-HMF were detected (0.71 \(\pm\) 0.07 g L\({}^{-1}\) and 70 \(\pm\) 30 mg L\({}^{-1}\), respectively), not likely affecting the potential performance of the hydrolysate fermentation. Since a strong acidic pH of the hydrolyses (resulting from the acid hydrolysis) might negatively impact the fermentation stage, neutralization with Ca(OH)\({}_{2}\) was performed [20; 24]. As expected, neutralization led to an increase of the calcium content of BSG hydrolysate (Table 3). Concomitantly, the content in sulfate decreased as it was removed in the form of calcium sulphate precipitate [24]. Xylose and arabin concentrations decreased after neutralization with Ca(OH)\({}_{2}\) (Table 3). This sugar decrease can be explained by the fact that the hydrophilic property of sulphate groups in the calcium sulphate precipitate promotes the adsorption of part of the water and monosaccharides present in the neutralized acid hydrolysates to the precipitate [26].