Natural deep eutectic solvents for polyphenol extraction from brewer’s spent grain: comparative evaluation with conventional solvents

Brewer's spent grain (BSG), the primary by-product of the brewing process, represents a valuable source of phenolic compounds with significant antioxidant potential. Developing efficient and environmentally sustainable extraction methods is critical to promote the recovery and valorization of these bioactives. This study evaluated Natural Deep Eutectic Solvents (NADES) as green alternatives for extracting phenolic compounds from BSG. Different NADES compositions were compared to conventional acetone: water and methanol: water systems in terms of phenolic yield and antioxidant activity. Eight NADES formulations were prepared using choline chloride or betaine as hydrogen bond acceptors, combined with glycerol, lactic acid, glucose, or fructose as donors. Extractions were performed at 50 degrees C for 1 h. Total phenolic content (TPC) was determined using the Folin-Ciocalteu assay, antioxidant activity by the DPPH and the ABTS methods, and individual phenolic compounds were quantified by HPLC. Among the tested solvents, NADES-2 (choline chloride: lactic acid) and NADES-5 (betaine: glycerol) yielded the highest TPC values, 285.92 +/- 8.01 mg GAE/100 g DW and 283.55 +/- 7.09 mg GAE/100 g DW, respectively, outperforming the acetone: water system (247.86 +/- 7.69 mg GAE/100 g DW). These NADES also enabled superior recovery of key phenolics such as ferulic acid, p-coumaric acid, and catechin. The highest antioxidant activity tested by DPPH and ABTS were observed for NADES-2 and NADES-6 (betaine: lactic acid). NADES-2 and 6 also exhibited potent antibacterial activity against Salmonella enterica serovar Typhimurium LT2 and Listeria monocytogenes-OH. Overall, NADES demonstrated high efficiency for polyphenol extraction from BSG, highlighting their potential as green alternatives to conventional solvents for obtaining extracts suitable for use as functional and nutraceutical ingredients.