Integral valorization of agro-food biomass through pressurized fluids. Case study: Brewery Spent Grain (BSG)

Abstract

The biorefinery concept involves the valorization and conversion of biomass into different bioproducts, including energy, materials and chemicals that can replace products derived from fossil carbon sources. The integral valorization of biomass requires the extraction and fractionation of the different constituents thereof, extractables, lipids, proteins and structural components such as cellulose, hemicellulose and lignin. In this work, the use of emerging and clean technologies for the integral valorization of different types of biomass is proposed. First, the use supercritical CO2 (SCCO2) extraction is proposed for recovering the lipid fraction and ultrasonic assisted extraction to recover the hydrophilic fraction, to subsequently perform a fractionation and hydrolysis of the residual biomass using pressurized liquid water. Several types of biomass are being studied, both second and third generation. Among second generation biomass, the brewery spent grain (BSG), which accounts for 85% of the by-products generated in beer processing [1], has been selected as case study of this communication. The extraction of the lipid fraction with SCCO2 has been carried out in a 26.5 mL capacity extractor at different pressures (20-40 MPa) and temperatures (40-80 °C). The lipid fraction obtained has been characterized by determining its lipid profile and antioxidant capacity. The experiments of fractionation and hydrolysis in subcritical water (scW) have been carried out in a semi-continuous reactor, varying the extraction temperature. The different fractions obtained have been characterized by HPLC with two series detectors, UVD and RID, to determine the content in monomeric sugars and oligomers. Protein, free amino acids, total polyphenols and total organic carbon have been also determined. The composition of the BSG according to the biomass characterization protocols of NREL [2] was 21.1 % arabinoxylans, 25.6 % glucanes, 5.1 % soluble lignin, 10.5 % insoluble lignin, 1.2 % ashes, 16.7 % proteins, 5.6 % lipids and 14.4 % extractables. Worth noting the presence of insoluble lignin as well as the high content of arabinoxylans and glucanes, 10% of which were residual starch. The extraction curves obtained when studying the extraction of the lipid fraction of BSG with SCCO2, showed that the extraction rate and the extraction yield increased with increasing temperature and pressure, with the major fatty acid being linoleic acid. Regarding fractionation of the carbohydrate fraction, we have observed that, as temperature increases, hydrolysis increases. Figure 1a shows the sugars yield, including monomeric sugars and oligomers. Degradation of sugars due to the high residence times produces acids (Fig. 1b), hydroxymethylfurfural (HMF) and furfural (Fig. 1c). The treatment of biomass by scW, allows also recovering the entire protein fraction by increasing the temperature up to around 185ºC. In addition, partial hydrolysis of the protein fraction occurs, obtaining as major free aminoacids valine, aspartic acid, alanine and glycine. We can conclude that the fractionation of BSG through emerging and clean technologies allows an integral recovery of BSG, obtaining extracts with high antioxidant capacity. Pressurized water hydrolysis allows the recovery and fractionation of the carbohydrate and protein fraction.