Differential Release of Biologically Active Cell-Wall-Bound Phenolics from Brewer’s Spent Grain Using Conventional and Deep Eutectic Solvents

Bound phenolics (BP) are potentially valuable bioactive compounds that covalently cross-link hemicellulose and lignin in the cell wall of the abundant waste product brewer’s spent grain (BSG). In the present study, the release of BP compounds was compared using conventional solvent-alkali (acetone (AC) and sodium hydroxide (NaOH)) extraction and deep eutectic solvents (DESs) composed of choline chloride and maleic acid (CCM), glycerol (CCG), and urea (CCU) and recovered as enriched fractions after solid-phase purification using solid-phase extraction cartridges. Analysis by mass spectrometry (LC-DAD-QTOF-MS) identified that ferulic and p-coumaric acids and ferulate dimers were the most abundant BP released in aglycon form by NaOH hydrolysis with concentrations of 23.51, 3.90, and 6.19 mg/g of dry BSG, respectively. However, a pentose glycosylated ferulic acid, most likely ferulic acid arabinoside (6.76 mg/g), was the main compound identified in CCM extracts. This finding is notable because it suggests a role for CCM to selectively cleave ether linkages between xylan and arabinose side chain residues of cell wall arabinoxylans, therefore releasing compounds not obtainable by using conventional alkali hydrolysis. Total phenolics ranged from 3.90 to 25.80 mg/g BSG, and high phenolic-rich extracts from NaOH and CCM exhibited antioxidant and anti-inflammatory activity by significantly reducing the expression of interleukins Il6, Il1b, and cyclooxygenase Cox2 in lipopolysaccharide-activated murine macrophages in a dose-dependent manner. A concentration of 100 μg/mL effectively reduced some inflammatory markers by 90% and 60% from NaOH and CCM extracts, respectively. This work identifies the potential to extract new and diverse bioactive compounds from BSG and will be of interest for valorizing abundant lignocellulosic waste products.