Antioxidant activity of crude tannins of canola and rapeseed hulls

The antioxidant activity of crude tannins of canola and rapeseed hulls was evaluated by beta-carotene-linoleate, alpha,alpha-diphenyl-beta-picrylhydrazyl (DPPH) radical, and reducing power assays. Crude tannins were extracted from three samples of Cyclone canola (high-tannin) hulls and Kolner, Ligaret, and Leo Polish rapeseed (low-tannin) hulls with 70% (vol/vol) acetone. The total phenolic content in crude tannin extracts ranged between 128 and 296 mg of sinapic acid equivalents per 1 g of extract. The ultraviolet spectra of methanolic solution of canola extracts showed two absorption maxima (282 and 309 nm), whereas those of rapeseed extracts exhibited a single maximum (326 nm). Crude tannins isolated from canola hulls exerted significantly (P less than or equal to 0.025) greater antioxidant activity than those from rapeseed in all three assays. The scavenging effect of all crude tannins, at a dose of 1 mg, on the DPPH radical ranged from 35.2 to 50.5%. The reducing power of Cyclone canola hull extracts on potassium ferricyanide was significantly (P less than or equal to 0.0025) greater than that of rapeseed hull extracts, and the observed data correlated well (r = 0.966; P = 0.002) with the total content of phenolics present.