Effect of refining process on antioxidant capacity, total phenolics and prooxidants contents in rapeseed oils

Four modified procedures: 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS)), cupric reducing antioxidant capacity (CUPRAC) and ferric reducing antioxidant power (FRAP) were used for determination of antioxidant capacities (AC) of rapeseed oils from different stages of refining process through three periods of production. The obtained DPPH, ABTS, CUPRAC and FRAP values of rapeseed oils ranged between 126 and 586, 400–1998, 455–1913, 72–291 μmol TE 100 g−1, respectively. The AC correlated with the content of total phenolics (TPC = 5.4–43.2 mg sinapic acid (SA) 100 g−1) in the studied oils (r = 0.9222–0.9821). The refining process of rapeseed oils decreased the AC by about 60% and TPC by above 80%. Moreover, amounts of prooxidant metals, iron, copper and manganese in the analyzed rapeseed oils were determined by inductively coupled plasma mass spectrometry (ICP-MS) after microwave digestion. The concentrations of Fe, Cu and Mn in rapeseed oil samples varied from 229.4 to 753.0, 2.2–15.8, 1.8–13.5 μg kg−1, respectively, that is below the Polish legal requirements.