A powder formulation of aged garlic extract was heated at 100 °C for 1 day to obtain higher antioxidant activity determined with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging (ARS) and oxygen radical absorbance capacity (ORAC) assays. Activity-guided fractionation afforded 12 new in vitro antioxidative Maillard-type products, α-[(2-formyl-5-hydroxymethyl)pyrrol-1-yl]arginine (3), 4-[7-hydroxy-6-(hydroxymethyl)-7,8-dihydro-6 H-pyrano[2,3- b] pyrazine-3-yl]butane-1,2,3-triol (4), 4-[6-(1,2-dihydroxyethyl)-6,7-dihydro-furo[2,3- b]pyrazin-3-yl]-butane-1,2,3-triol (5), α-[(2-formyl-5-hydroxymethyl)-pyrrol-1-yl] aspartic acid (12), 1-[5-(1,2-dihydroxyethyl)-2-oxotetrahydrofuran-3-yl]-5-(hydroxymethyl)-1 H-pyrrole-2-carbaldehyde (14), 4-(6-ethyl-2-pyrazinyl)-1,2,3-butanetriol (17), α-[(2-formyl-5-hydroxymethyl)pyrrol-1-yl] glutamic acid (19), ( S)-1-[(5-hydroxymethyl)furan-2-yl]methyl]-5-oxopyrrolidine-2-carboxylic acid (20), 3-hydroxy-1 H-[{5-(hydroxymethyl)furan-2-yl}methyl]-2,5-dioxo-3-pyrrolidine acetic acid (21), ( E)-4-(5-methylpyrazin-2-yl)but-3-ene-7,2-diol (23), 4-acetyl-6-(hydroxymethyl)picolinic acid (24), ( E)-4-(6-methylpyrazin-2-yl)but-3-ene-1,2-diol (26) and 14 known compounds, 1, 2, 6-11, 13, 15, 16, 18, 22 and 25, which were characterized via 1D/2D-NMR, CD spectroscopy, and mass spectrometry. ARS and ORAC activities of these antioxidants ranged from 0.01 to 0.49 μmol TE/μmol and from 0.01 to 3.50 μmol TE/μmol, respectively. Additionally, plausible formation pathways for the new organic acid-type products (15, 20, and 21) were proposed based on proving their generation in model reactions detected via liquid chromatography-mass spectrometry (LC-MS/MS).
Keywords: Maillard reaction; aged garlic extract; antioxidant; citric acid; glutamic acid.