Antioxidant Activity of Tannin Fractions Isolated from Buckwheat Seeds and Groats

Phenolic compounds were extracted with 80% (v/v) aqueous acetone from buckwheat seeds and groats. Tannin fractions were obtained from the crude extracts by Sephadex LH-20 column chromatography. Total phenolic contents of isolated fractions from buckwheat seeds and groats were 477 and 371 mg catechin equiv/g, respectively. The analyzed samples were characterized by electrophoretic separations using capillary zone electrophoresis. Both fractions exhibited strong antioxidant activity. The tannin fraction of buckwheat seeds reduced 1,1-diphenyl-2-picrylhydrazyl radicals (DPPH^·) and 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid)] diammonium salt radical cations (ABTS^·+) stronger than the fraction of buckwheat groats. The EC₅₀ values amounted to 0.019 and 0.020 mg while Trolox equivalent antioxidant capacity values were 4.06 and 3.55 mmol Trolox equiv/g for tannin fractions of buckwheat seeds and groats, respectively. Similarly, antioxidant activity of the tannin fraction of buckwheat seeds measured by photochemiluminescence assay, was higher than antioxidant activity noted for the fraction obtained from groats. However, both fractions inhibited oxidation to the same extent in applied lipid models: β-carotene-linoleic acid emulsion and l-α-lecithin liposomes. In β-carotene-linoleic acid emulsion system, 1 mg of tannin fractions exhibited similar antioxidant activity to 0.5 mg butylated hydroxyanisole (BHA).