Supercritical carbon dioxide extraction of brewer´s spent grain (BSG)

Abstract

Brewer’s spent grain (BSG) is one of the most important by-products in large and small- scale breweries. BSG is the solid residue generated after mashing and wort filtration

process, it is generated at an average rate of 20 kg per 100 L of beer and accounts for approximately 85 % of the total residues of the brewing process [4]. Nowadays, it is mainly used for animal feed (70 %), biogas production (10 %), or landfilled [5]. However, BSG presents a valuable chemical composition with a high content of protein and carbohydrates, as well as important quantities of phenolic compounds with potential bioactive properties. BSG also contains non-negligible amount of lipids (5 %) with more than 50 % being linoleic acid (C18:2 ω-6) [6]. Due to the valuable chemical composition of BSG, different techniques have been proposed to valorize this lignocellulosic biomass, such as enzymatic and chemical hydrolysis, ultrasound assisted extraction or microwave assisted extraction [7]. High pressure processing of biomass has been also proposed since it offers unique opportunities in the extraction and valorization of the bioactive compounds of BSG. Among the different high-pressure processes, the use of supercritical CO2 (sc-CO2) presents a great attractive since it is considered a green solvent and it presents gas-like (high diffusivities) and liquid-like (good solvation power) properties at supercritical conditions (Tc = 31.1 oC pc= 7.39 MPa). Sc-CO2 has been extensively studied as a green extracting agent over traditional organic solvents to valorize the lipophilic fraction of biomass [8]. From a biorefinery perspective and aiming at the integral valorization of BSG, sc-CO2 extraction and enzymatic hydrolysis processes have been applied to this by-product. First, the extraction of the lipophilic fraction of BSG with sc-CO2 has been systematically studied. The most influential extraction conditions, namely pressure and temperature, were varied from 20 to 40 MPa and from 313 to 353 K, respectively. A maximum yield of 5.70 ± 0.07 g /100 gBSG was obtained at 353 K and 40 MPa (see Graphical abstract, Fig. a)). High pressures and temperatures resulted in higher content of total phenolic and flavonoids compounds, as well as higher antioxidant capacity. Enzymatic hydrolysis was performed on samples after sc-CO2 treatment and non-treated BSG samples. Enzymatic hydrolysis was carried out at 323 K in an acetate buffer at pH=5 with a cellulase, 1,4-(1,3:1,4)-β-D-Glucan 4-glucanohydrolase, EC 3.2.1.4, from Aspergillus niger provided by Sigma-Aldrich.

The graphical abstract b) represents the glucose yield along enzymatic hydrolysis for sc- CO2 treated and non-treated BSG at different enzyme concentrations. An improvement of

the enzymatic hydrolysis yield by cellulase was observed in the sc-CO2 treated BSG compared to the non-treated. This improvement could be partially attributed to the removal of the lipid fraction and to morphological changes of BSG after sc-CO2. Based on this double benefit, sc-CO2 can play an important role on biomass valorization [8].