Modification of brewer’s spent grain after sc-CO2 extraction: improvement of sugar and phenolic compounds release

Abstract

Brewer ́s spent grain (BSG) is the solid by-product generated in breweries after the mashing and wort filtration process. It comprises about 85 % of the total by-products, generating approximately 20 kg per 100 L [1]. BSG presents a valuable chemical composition with a high content of protein and carbohydrates, 5 % of lipids and an important source of phenolic compounds. Among the different techniques proposed to valorize BSG, supercritical CO2 (sc-CO2) has become a promising technology to process biomass. The main aim of this work was to assess the effect of sc-CO2, after BSG oil extraction, on the subsequent enzymatic hydrolysis to hydrolyze the polysaccharide fraction into monomeric sugars by comparing the sugar yields of sc-CO2 and non-scCO2 treated BSG. After sc-CO2 treatment at 40 MPa and 80 oC, the raffinate obtained was subjected to enzymatic hydrolysis by cellulase at different enzyme dose. At the sc-CO2 extraction the carbohydrate fraction remained in the raffinate phase after extraction. Glucose yield increased with enzyme concentration for non-treated and sc-CO2 treated BSG. Higher yields of glucose were obtained for sc-CO2 treated compared to non-treated sc-CO2 for all the enzymes concentrations. The percentage increase in glucose yield for sc-CO2 treated and non-treated BSG was 8, 14 and 18 % for the three cellulase concentrations essayed in this work, 0.25, 0.5 and 1 %, respectively. The hydrolysis kinetics for xylose and arabinose have been also determined for non-treated and sc-CO2 treated BSG at 1 % of cellulase dose. Although not big differences were observed in the final sugar yield in the hydrolysates, the initial hydrolysis rate were significant higher for the sc-CO2 treated BSG than for the untreated samples. The higher enzymatic hydrolysis rate and yield obtained in the raffinate-BSG after sc- CO2 treatment compared with non-treated BSG could be attributed partially to the removal

of the lipid fraction. As it has been described in literature [2], fats and oils could influence the susceptibility of carbohydrates to enzymes. This improvement was also due to surface morphology modification. The structural and chemical changes of untreated and sc-CO2 treated BSG was assessed by scanning electron microscopy and X-ray powder diffraction (XRD). Although the XRD pattern after sc-CO2 treatment indicated that the pre-treatment was not strong enough to modify the BSG crystallinity, the raffinate exhibited an irregular porosity and lamellar structure. sc-CO2 broke partially some structural barriers allowing a better enzyme access. Furthermore, some phenolic compounds were determined in the enzymatic hydrolysates at the end of the hydrolysis by using 1 % of cellulase (Table 1). It was observed that for p-coumaric acid and ferulic acid, a concentration 31 and 24 % higher, respectively, was obtained in the BSG hydrolysates after sc-CO2 treatment, while for vanillin similar concentration was obtained in both hydrolysates. The concentration of p-coumaric acid and vanillin after cellulase hydrolysis was lower than the values previously reported for the same BSG by alkaline, xylanase (1%) and subcritical water hydrolysis [3], [4]. Nevertheless, for ferulic acid, only a higher value was reached for alkaline hydrolysis.