## 2 Materials and Methods
### Analytical Methods  
BSG hydrolysate and anaerobic reactors samples were characterized in terms of: sugars, namely, xylose (Xyl), arabinose (Ara) and glucose (Glu) concentration; organic acids (HOrgs), including lactic HLac, acetic HAc, propionic HPro, butyric HBut, valeric HVal and isovaleric HNov acids concentration; ethanol (EtOH); ammonia (NH\({}_{4}\)\({}^{+}\)); phosphate (PO\({}_{4}\)\({}^{3-}\)); and total protein content, as described by [PERSON] et al. [15]. Briefly, sugars, HOrgs and EtOH of filtered samples (VWR, spin filter 0.2 \(\mu\)m) were quantified by high performance liquid chromatography (HPLC) using a Hitachi High-Tech chromatograph (Hitachi High-Tech, Tegama, Japan) equipped with a RI detector, a Micro-Guard Cation H Refill Cartridge #1250129 pre-column (BioRad, Hercules, CA, USA) and an Aminex HPX-87H column (BioRad, Hercules, CA, USA; column temperature 30 \({}^{\circ}\)C, 0.01 N H\({}_{2}\)SO\({}_{4}\) eluent, flow rate 0.5 mL min\({}^{-1}\)). The sugars and HAc concentrations were calculated through a standard calibration curve (31-1000 mg L\({}^{-1}\) of each sugar, HOrgs and EtOH). NH\({}_{4}\)\({}^{+}\) and PO\({}_{4}\)\({}^{3-}\) contents were determined by colorimetry, as implemented in a flow segmented analyzer (Skalar 5100, Skalar Analytical, Breda, The Netherlands) [16]. The total protein content was determined using the colorimetric method described by [PERSON] et al. [17]. Bovine serum albumin (BSA) standards (0-100 mg L\({}^{-1}\)) were used for the protein content determination.  
BSG hydrolysate was characterized in terms of elements, furfural and 5-HMF content. Briefly, calcium, magnesium and sodium were quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES) through an Ultima ICP spectrophotometer (Horiba Jobin-Yvon, Bensheim, Germany) equipped with a RF generator (40.68 MHz), a monochromator (1.00 m Czerny-Turner), AS500 automatic sampler and Concomitant Metals Analyzer (CMA) device. Sulphate (SO\({}_{4}\)\({}^{2-}\)) content was determined by HPLC (ICS-3000, Dionex, Sunnyvale, CA, USA) through an ionPAC AS16 column coupled to a conductivity detector using NaOH 22 mM as eluent at 1.5 mL min\({}^{-1}\) flow rate. Furfural and 5-HMF contents were also analyzed by a Surveyor HPLC system (Thermo Finnigan, San Jose, CA, USA) with a UV/Vis detector (Accela \(\lambda\) = 280 nm) through an Aminex 87H column (BioRad 30 \({}^{\circ}\)C) using H\({}_{2}\)SO\({}_{4}\) 10 mM as eluent with a 0.6 mL min\({}^{-1}\) flow rate.  
The anaerobic reactors samples were also characterized in terms of chemical oxygen demand (COD), gas content, and biomass concentration. COD was measured using cuvette test kits LCK 914 (5-60 gO\({}_{2}\) L\({}^{-1}\); Hach-Lange, Dusseldorf, Germany). The composition of the gas produced (CO\({}_{2}\), H\({}_{2}\), O\({}_{2}\), N\({}_{2}\) and CH\({}_{4}\)) was performed using a gas chromatograph (Agilent Technologies 7890B, Agilent Technologies Co., Ltd., Shanghai, China) equipped with a thermal conductivity detector, 50m of CP-Molsieve 5A and 25m Po-raBOND Q columns using the method described by [PERSON] et al. [8]. Biomass concentration was determined according to the volatile suspended solids concentration (VSS) procedure described in standard methods [13].