Finger millet (Eleusine coracana) polyphenols: Investigation of their antioxidant capacity and antimicrobial activity

Abstract

Finger millet (Eleusine coracana) or ragi, one of the important minor cereals is a rich source of several phytochemicals and among them polyphenols are the most important because of their nutraceutical potentials. The seed coat-rich fraction (SCF) of the millet contains over 70% of polyphenols of the millet. Treated with different polar and non-polar solvents as such and after acidification with HCl for the extraction, it was observed that the polar solvents extracted more (1.7 to 7.4%) than the non-polar solvents (0.28 to 1.54%) while the 1% HCl–methanol extracted the highest proportion of polyphenols (13%). Fractionation of extracted polyphenols by HPLC revealed the presence of a large number of constituent phenolics. The inhibitory concentration (IC50) values for the antioxidant activity of the polyphenols determined in terms of 1, 1-diphenyl-2-picrylhydrazyl (DPPH), hydroxyl and 2, 2’- azinobis – 3 – ethylbenzothiazoline – 6 – sulfonate (ABTS) radicals scavenging activities of acidified methanolic extract, were 63, 109, and 114 μg/mL phenolic, respectively. The polyphenols extracted from finger millet seed coat using 1% HCl –methanol was fractionated by reverse phase– high performance liquid chromatography (RP-HPLC) and the constituent phenolics were characterized by electrospray ionization mass spectrometry (ESI-MS) and NMR. The phenolics identified were the benzoic acid derivatives (gallic, protocatechuic, p-hydroxybenzoic, vanillic, and ferulic acids) and cinnamic acid derivatives (syringic, trans-cinnamic and p-coumaric acids); and also a flavonoid compound namely, quercetin. The millet polyphenols showed proliferation inhibitory activities on Escherichia coli, Bacillus cereus, Staphylococcus aureus, Listeria monocytogenes, Streptococcus pyogenes, Proteus mirabilis, Pseudomonas aeruginosa, Serratia marcescens, Klebsiella pneumonia, and Yersinia enterocolitica. Quercetin exhibited substantial inhibition of the growth of these pathogenic bacteria revealing the potential nutraceutical properties of the millet polyphenols. From the observation, it could be inferred that finger millet polyphenols could be used as natural source of antioxidants especially for minimizing the risk of diseases arising from oxidative deterioration.