Valorization of Brewer's Spent Grain Through Ultrasound-Assisted Extraction of Phenolic Compounds Using Deep Eutectic Solvents.

Brewer's Spent Grain (BSG) is a residue produced in large quantities of breweries. BSGs have a high nutritional value and are a source of bioactive compounds, suggesting that this residue can be reused to produce high-value products with industrial applications. Among the bioactive compounds, the BSGs have high concentrations of phenolic compounds that are in high demand in the cosmetic and food industry. In this work, an ultrasound-assisted extraction using deep eutectic solvents (DESs) was evaluated to recover phenolic compounds from BSG. A screening of four DESs and their mixtures with 25% water (v/v) identified DES choline chloride and oxalic acid (ChCl:OA) and DES choline chloride and lactic acid (ChCl:LA) as the most effective solvents, achieving phenolic compound yields of 0.201 and 0.172 mg GAE/L, respectively, during the initial screening. To further enhance the extraction efficiency, a two-step serial extraction process was employed. The first step used hexane or ethanol to remove oils and sugars, which reduced the impurities and increased the selectivity of DES in the second step. The highest phenolic compound yields were obtained with ethanol-ChCl:OA and hexane-ChCl:OA systems, achieving 0.513 and 0.516 mg GAE/L, respectively, at 30 °C. Notably, ChCl:OA, the most viscous DES among those tested, demonstrated the highest extraction efficiency, suggesting that its chemical composition and interactions with the BSG matrix may have compensated for its higher viscosity. Serial ultrasound-assisted extraction effectively removed oils and sugars from the BSG sample and increased the efficiency of the DES in recovering phenolic compounds.