Phenolic compounds of selected lentil cultivars and their contribution to antioxidant activity

Lentils are a significant legume crop for the human diet due to their plentiful phytochemical and nutritional value as well as potential health benefits. Soluble (free and esterified) and insoluble-bound phenolic compounds were extracted from 6 different lentil cultivars, namely CDCSB-2, 3493-6, CDC invincible, CDC green land, maxim and black lentils using 70% acidic acetone (0.5% acetic acid). Each fraction was separately assayed to determine its total phenolic, flavonoid and condensed tannin (proanthocyanidins) contents. The antioxidant activities of these phenolic fractions were estimated using trolox equivalent antioxidant capacity (TEAC), reducing power, 1,1-diphenyl-2-picrylhydrazyl (DPPH) and hydroxyl radical scavenging activities using electron paramagnetic resonance (EPR) spectroscopy. Generally there was a noticeable difference in the total content of phenolics, flavonoids and condensed tannins (proanthocyanidins) between soluble (free and esterified), and insoluble-bound phenolic extracts. However, there was a strong and positive correlation between total phenolic, flavonoid, condensed tannin (proanthocyanidin) contents and antioxidant activity of lentil extracts. Seventeen phenolic compounds belonging to several different classes, namely hydroxybenzoic acids, hydroxycinnamic acids, flavan-3-ol (monomers, dimers and trimers), flavone, and flavonol were positively and/or tentatively identified and quantified using high performance liquid chromatography-diode array detection-electrospray ionization-mass spectrometry (HPLC-DAD-ESI- MSn). Among all phenolic compounds catechin, epicatechin and procyanidins B were the predominant flavonoids that were detected in all fractions of all tested samples. However, some phenolics were present only in the bound form. The total content of flavonoids was up to 2000- and 84-fold higher than that found for hydroxybenzoic acids and hydroxycinnamic acids, respectively. Furthermore, the quantification of flavonoids using HPLC showed higher contents of insoluble-bound and esterified phenolics for CDC green land and black lentils. In addition, these two samples exhibited the highest antioxidant properties among all lentils tested. These results suggest that flavonoids from lentils contribute most to the antioxidant activity as evaluated by several methods and were the major extractable phenolics in the tested lentil samples.