Enrichment and bioactivities of polyphenols of crude extract by deep eutectic solvent extraction from foxtail millet bran

The adsorption and desorption capabilities of eight types of macroporous adsorption resins (MARs) for polyphenols from foxtail millet bran (FMB) extract were investigated. The D101 resin demonstrated the highest adsorption and desorption capacity, followed by the AB-8, DM301 and ADS-17 resins. The reaction kinetics for the adsorption of FMB polyphenols onto MARs accorded with a pseudo-first-order model (R2 > 0.99), with an apparent adsorption rate constant (K) of 1.225 × 10−2 g/mg·min. The adsorption process of FMB polyphenols onto D101 resin could be best described with the Langmuir adsorption isotherm (R2 > 0.99). The phenolic fractions were separated using silica gel column chromatography, and their total phenolic content (TPC), antioxidant activity, total flavonoid content (TFC), anti-glycosylation activity and acetylcholinesterase and α-glucosidase inhibitory activities were compared. Fraction 4 had the highest abundance of phenolic compounds, with N′-p-coumaroyl-N″-feruloylspermidine (461.65 ± 0.79 μg/g) and N′,N″-di-feruloylspermidine (578.46 ± 2.56 μg/g) being the predominant phenolics, the best acetylcholinesterase inhibitory activity and anti-glycosylation activity. Fraction 5 contained the highest apigenin-C-dihexoside, and therefore showed the best performance of α-glucosidase inhibitory activity. Fractions 4 and 5 displayed high antioxidant, acetylcholinesterase inhibitory activity and anti-diabetic potential, indicating that these two fractions should be targeted for further high-value utilisation of free polyphenols from FMB.