Determination of antioxidant activity in free and hydrolyzed fractions of millet grains and characterization of their phenolic profiles by HPLC-DAD-ESI-MSn

Abstract

Over 50 phenolic compounds belonging to several classes, namely, phenolic acids and their derivatives, dehydrodiferulates and dehydrotriferulates, flavan-3-ol monomers and dimers, flavonols, flavones and flavanonols in four phenolics fractions of several whole millet grains (kodo, finger (Ravi), finger (local), foxtail, proso, little and pearl millets) were positively or tentatively identified using high performance liquid chromatography (HPLC) and HPLC-tandem mass spectrometry (MSn). Total phenolic content (TPC) was determined colourimetrically using Folin–Ciocalteu reagent. Antioxidant and antiradical activities of phenolic fractions were estimated using oxygen radical absorbance capacity (ORAC), 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, and singlet oxygen scavenging activity. In addition, antioxidant activities of phenolic fractions were determined in a β-carotene linoleate emulsion system. Insoluble bound fraction of kodo millet showed the highest phenolic content as well as antioxidant activity in the in vitro test systems employed. In general, hydroxycinnamic acids and their derivatives were the main contributors (87–98%) to the TPC of insoluble bound phenolic fraction of millet varieties examined. Flavonoids were predominantly present in the free phenolic fractions of two finger millet varieties and included catechin, epicatechin, gallocatechin, epigallocatechin, and procyanidin dimers B1 and B2 among others. The degree of esterification or etherification of phenolics with sugar residues in the soluble fraction changed with the variety. Thus millet grains may serve as a viable functional food ingredient and a source of natural antioxidants.