Antioxidant properties of thymol, carvacrol, and thymoquinone and its efficiencies on the stabilization of refined and stripped corn oils
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The antioxidant activities of natural phenolic compounds (thymol, carvacrol, and thymoquinone) were compared with commercial antioxidants (alpha-tocopherol, BHT, and BHA) using DPPH center dot, conjugated diene (CD) in the linoleic acid emulsion, and ferric reducing power methods. Commercial antioxidants had higher DPPH center dot antiradical activity than natural phenolics. Thymoquinone (TQ) at 1000 ppm showed higher inhibition (65.7%) on DPPH center dot radicals than other natural phenolics at 1000 ppm (25.0% for thymol, and 18.3% for carvacrol). Carvacrol and thymol showed similar antioxidant activities compared with BHT and BHA in linoleic acid emulsion test at different concentrations, while TQ and alpha-tocopherol exhibited lower activity among analyzed samples. The results from reducing power test showed that natural phenolics were less effective than commercial antioxidants. The impacts of natural phenolics and BHT on the oxidative stabilities of refined and stripped corn oils were investigated using the Rancimat, Schaal oven, peroxide value (PV), CD (K232) andp-anisidine value (p-AV) methods. The loss in total tocopherols in refined corn oils was recorded during storage under Schaal oven conditions (60 degrees C). BHT showed a higher induction period (15.01 h) than phenolic compounds added to corn oils (3.88-5.69 h) and a control sample (2.82 h). According to the results of the Schaal oven test, BHT much better protected the refined and stripped corn oils from oxidation than natural phenolic compounds. Among phenolic compounds, TQ at 250 and 500 ppm exhibited high antioxidant potential in refined and stripped corn oils.