Antioxidant Activity of Gallic Acid as Affected by an Extra Carboxyl Group than Pyrogallol in Various Oxidative Environments

The antioxidant activity of gallic acid and pyrogallol is studied by the DPPH• scavenging assay and the extent of oxidation inhibition of the triacylglycerols of various unsaturated oils (olive, sunflower, and soybean) and their oil-in-water emulsions. The gallic acid molecules with an electron-donating carboxylate anion are significantly stronger DPPH• scavengers than pyrogallol molecules (IC50 = 11.4 vs. 20.2 μM). Regarding the prevention of the triacylglycerols from oxidation, gallic acid is of higher activity than pyrogallol in the bulk oils (86.1–1244.3 vs. 73.4–331.6) but of lower activity in the O/W emulsions (13.0–74.9 vs. 17.4–207.9), depending on their different antioxidant behaviors arisen from the free radical chain mechanism of action as well as interfacial phenomena. Practical Applications: The present study clarifies the role of the carboxyl group in gallic acid antioxidant activity, antioxidant activity as a function of the free radical chain mechanism as well as interfacial phenomena, critical concentration of reverse lipid hydroperoxide micelles as affected by the antioxidant activity, and the effect of the degree of lipid unsaturation on the antioxidant activity of gallic acid and pyrogallol. Comparative antioxidant performance of gallic acid and pyrogallol to scavenge 1,1-diphenyl- 2-picrylhydrazyl radical (DPPH•) in a methanolic system and to inhibit peroxyl radical (LOO•) in triacylglycerols of olive, sunflower, and soybean oils and their oil-in-water (O/W) emulsions.