Grass pea seeds of 30 varieties from Poland, Slovakia, Czech, Russia, Ukraine, Hungary, Bulgaria, India, Iran, Tunisia, Ethiopia, Canada, and Chile were investigated. Phenolic compounds were extracted from seeds into 80% (v/v) methanol. The total phenolics compounds content of the extracts was determined using Folin-Ciocalteu's phenol reagent. The antioxidant activity was determined using ABTS and FRAP assays. Total phenolic contents ranged from 2.72 to 10.7 mg/g extract and from 35.9 to 144.7 mg/100 g seeds. The extracts and seeds were characterized using Trolox equivalent antioxidant capacity values of 0.014–0.060 mmol Trolox/g extract and 0.188–0.866 mmol Trolox/100 g seeds, and FRAP values of 0.188–0.866 mmol Fe²⁺/g extract and 0.541–1.398 Fe²⁺/100 g seeds. The total phenolics content of grass pea extract was correlated with the results of the ABTS (r = 0.854) and FRAP (r = 0.958) assays. A similar correlation existed between the results of both assays (r = 0.890). Phenolic acids were the dominant phenolic compounds of two cultivars of grass pea. Syringic and sinapic acids had the highest amounts while ferulic, p-hydroxybenzoic, protocatechuic, and p-coumaric acids were present in much lower quantities. Phenolic acids were present in the form of free compounds, esters, and glycosides.

Phenolic compounds were extracted into 80% (v/v) acetone. The crude extract so obtained was separated into fraction of low-molecular-weight compounds (Fraction Ⅰ) and a fraction of condensed tannins (Fraction Ⅱ) using Sephadex LH-20 column chromatography with ethanol and acetone/water (1:1, v/v) as respective mobile phases. The antioxidants activities of the crude extract and fractions thereof was investigated in an emulsion system and using ABTS, DPPH, and reducing power assays. Fraction Ⅱ exhibited a higher antioxidant efficacy than those of the crude extract and Fraction Ⅰ. In the crude extract, 21 compounds, namely phenolic acids (gallic, p-hydroxybenzoic, protocatechuic, p-coumaric, ferulic), flavonoids (catechin, epigallo catechin, kaempferol, apigenin, and luteolin derivatives), gallate procyanidins, vanillin, protocatechuic aldehyde, and tryptophan were identified by HPLC-DAD-MS. Gallate procyanidin dimer, gallate procyanidins, and acetylated kaempferol hexose were the major phenolics present in the extract.