Pressurized liquid extraction of brewer’s spent grain: Kinetics and crude extracts characterization

Highlights

• High extraction yields of BSG were obtained with pressurized water + EtOH solvent.

• Extracts with high levels of total phenolic compounds were obtained.

• Water + EtOH provided high initial extraction rates and short times of process.

• Compressed propane is a suitable solvent as a prior defatting step.

Abstract

In this study, extraction yield of valuable compounds from brewer’s spent grain (BSG) using pressurized liquid extraction (PLE) was investigated varying temperature, solvent type, and flow rate at a constant pressure. The results were compared with Soxhlet extractions using the total phenolic compounds (TPC), total flavonoid compounds (TFC), antioxidant activity (AA), reducing sugars (RS), and total reducing sugars (TRS) as indices. The highest PLE extraction yield was 20.1 wt%, at 120 °C, 2 mL/min using ethanol to water volume ratio of 0.5, at 10 MPa. The TPC, TFC, and AA content were favored by water and water/ethanol extractions and temperature increase. The highest AA was obtained with water at 120 °C and 4 mL/min achieving 9944, 4769 and 4096 µmol TE/100 g of BSG extract, respectively. PLE was capable of present high extraction yields maintaining the RS and TRS in the BSG matrix. In addition, BSG was defatted with compressed propane before the extraction with PLE, showing that compressed propane is highly efficient in recovering the lipid fraction from BSG. The results demonstrated a great potential of water and different EtOH/Water solvent ratio at pressurized conditions for obtaining bio-compounds of BSG in a crude extract with expressive biological activity.

Introduction

One of the most significant challenges of the 21st century is the sustainable reuse and valorization of biomass from agro-industrial residue. Food and beverage industries are responsible for the production of large amount of residues, such as peels, seeds, pits, pulps, press cakes, and leaves that can be used as raw materials to obtain high added-value products [1]. Thus, agro-industrial residues’ valorization has emerged as a critical strategy for an economic and environmental-friendly production within the concept of biorefinery [2]. In this context, the brewing sector with billions of liters of beer produced annually is responsible for large amounts of solid residues, among them brewer spent grain, solid hop residue, and surplus yeasts [3].

Brewer’s spent grain (BSG) represents 85% of the by-products generated by the beer industry. It constitutes the solid residue from the initial stage of the brewing process, resulting from the grinding and barley grains cooking in the mashing stage. The residue consists of barley grains, husks, and endosperm, and it is classified as a heterogeneous material composed mainly of cellulose (12–25%), hemicellulose (19–42%), lignin (8–28%), proteins (14–31%) and extractable, such as lipids (5–13%) [4], [5], [6], [7], [8].

BSG is usually destined for animal feed due to its high nutritional value, low cost, and soil disposal. However, studies explored the possibility to enhance this residue as a matrix in the production of compounds such as xylitol, lactic acid and extractable bio-compounds such as sugars, proteins, antioxidants, phenolics, flavonoids, vitamins, and minerals for chemical, pharmaceutical, and food industries [3], [9], [10].

Studies have described the extraction of bio-compounds from BSG by conventional solid–liquid extraction processes, such as maceration, infusion, and Soxhlet extraction [9], [10], [11]. However, alternative extraction methodologies based on compressed fluids have been proposed to efficiently obtain and recovery of different compounds, such as extraction of spent coffee grounds oil using high-pressure CO₂ [12], high-pressure extraction of caffeine [13], pressurized hot water extraction of pectin [14], pressurized liquid extraction combined with dispersive liquid–liquid micro extraction of endocrine disrupting compounds from cheese [15], compressed extraction of total phenolics and flavonoids content in hops [16].

Pressurized liquids are widely used in obtaining bio-compounds from natural sources and in the thermochemical fractionation of lignocellulosic biomass. At high temperatures, the solvents used in PLE have unique properties due to the change in the dielectric constant, density, viscosity, and diffusivity due to intermolecular forces' rupture, such as van der Waals, hydrogen bonds, and dipole interactions. High pressures are used to maintains the solvent at a liquid state while temperatures over the solvent boiling point are applied. However, several studies indicated that pressure had no significant effects on extract yields and bioactive compounds recovery. Even so, pressures between 5 to 10 MPa are applied to reduces the occurrence of air bubbles inside the solid matrix, increasing the analyte solubility and desorption kinetics [17], [18], [19]. Moreover, at high temperatures and elevated pressures, the mass transfer is improved, the solubility and diffusivity of solutes are increased causing the analyte-matrix bonds to break, and the surface tension of solvents and viscosity decrease allowing the solvent to penetrate the solid matrix easily, solvating the components of interest and accelerating the extraction rates [19], [20], [21]. The most used fluids are subcritical water and pressurized aqueous ethanol (PAE) since they do not generate residues from the extracts' neutralization, they are non-toxic and allow the preservation of chemical and thermolabile bio-compounds. Moreover, the combination of these two polar solvents provides a solvent-medium with thermodynamic excess properties (e.g., excess volume of mixture (density), dielectric constant, viscosity, etc.) that favor the interaction and extraction of different classes of compounds when compared to the extracts obtained using the stand-alone solvents.

Alonso-Riaño et al. [22] proposed subcritical water as a hydrolytic medium to recover and fractionate the protein fraction and phenolic compounds from craft BSG, varying the temperature from 125 to 185 °C at a constant flow rate of 4 mL/min. They obtained 78 % as the maximum yield of solubilized protein at 185 °C and phenolic recovery, while the maximum level of free amino acids was reached at 160 °C with a value of 55 mg free amino acids/g_protein-BSG. Torres-Mayanga et al. [23] studied the production of C-5 sugars from the BSG hydrolysis, varying the reaction temperature (140, 160, 180, and 210 °C), flow rate (10 and 20 mL min⁻¹), and the solvent/feed ratio (S/F: 64, 80 and 112), and they obtained a maximum yield of reducing sugars of 5.84 g per 100 g of feed and the maximum total reducing sugar yield of 35.11 g per 100 g of feed, at the optimal operating conditions of 210 °C, 20 mL min⁻¹ water flow rate, and S/F of 64. Those authors reported that arabinose was the most abundant identified sugar product, with a maximum yield of approximately 3.1 g per 100 g of feed. Benito-Román et al. [6] evaluated pressurized aqueous ethanol extraction of β-glucans and phenolic compounds from waxy barley, under conditions of temperature (135–175 °C), extraction time (15–55 min), and ethanol content (5–20%) and they obtained 51% β-glucan extraction yield with a molecular weight of 500–600 kDa and 5 mg GAE/ g barley at mild conditions of 151 °C, 21 min of extraction using 16% ethanol in water. Therefore, the literature has reported an excellent potential for pressurized water and ethanol to obtain different add-value products from BSG. However, different parameters as solvent concentration, extraction kinetics, and the characterization of a broader class of compounds still need to be studied. In addition, as the BSG presents a high content of lipids, around 5–13% as above, a fractionating extraction strategy using a first extraction stage with a nonpolar solvent can be interesting to produce crude extract fractions, e.g., lipid fraction and a phenolics-rich fraction that can be used in different industries with different and specific applications. Compressed propane has been successfully applied to extract lipids from different raw materials producing solvent-free crude extracts and allowing fast extraction rates [24], [25], [26], [27]. However, to the best of our acknowledge, BSG defatting with compressed propane has not been reported in the literature. Thus, it was considered a valuable comparison to study PLE extractions of BSG using compressed propane as a prior defatting step.

This study aims to recover a broader class of compounds from BSG evaluating different extractions parameters using water, ethanol, and different ethanol to water ratios as pressurized solvents for the best operating condition to achieve the highest extraction yield and compounds recovery. In addition, compressed propane is also evaluated as an initial step to recover the lipid contend in the BSG. Total phenolic compounds (TPC), total flavonoids compounds (TFC), antioxidant activity (AA), reducing sugars (RS), and total reducing sugars (TRS) were also quantified in the BSG extracts to evaluate the potential use of the PLE technique to recovery from BSG compounds with bioactive properties.