Assessment of Antioxidant Functions of Food Using Electron Paramagnetic Resonance Spectrometry: Verification Using Vinegar Samples

The abilities of food products to quench superoxide (O₂ ^•-) and the hydroxyl radical (^•OH) and to directly reduce stable nitroxyl radicals were assessed in the present study using electron paramagnetic resonance (EPR)-based methods. The antioxidant functions of vinegar samples, including 1- and 2-year-fermented black vinegar, were also examined. The abilities of the vinegar products to quench O₂ ^•- were in the order of 2-year-fermented black vinegar ≈ 1-year-fermented black vinegar > standard vinegar. Standard vinegar, 1-year-fermented black vinegar, and 2-year-fermented black vinegar did not reduce the stable nitroxyl radical. Apple-flavored black vinegar, containing 300 mg/100 g of ascorbic acid, directly reduced the stable nitroxyl radical. The ^•OH-quenching ability of 2-year-fermented black vinegar was slightly stronger than that of 1-year-fermented and standard vinegars. ^•OH-quenching ability appeared to be due to the sugars and/or proteins present in sample vinegars, particularly apple-flavored black vinegar, which had a high carbohydrate content. Adding vinegar to the reaction mixture enhanced the reduction of TEMPOL in the presence of GSH at a hyperthermal temperature. The enhancement of the reduction of TEMPOL was in the order of apple-flavored black vinegar >> 2-year-fermented black vinegar > 1-year-fermented black vinegar > standard vinegar. The reduction of TEMPOL was attributed to the quenching of H₂O₂, except for apple-flavored black vinegar, which directly reduced TEMPOL. Collectively, the present results suggest that fermentation increases the antioxidant ability of vinegar. The combination of EPR methods described herein is feasible for visualizing the antioxidant activities of food samples.