Comparison of oxidative stability for the thermally-oxidized vegetable oils using a DPPH method

The 2,2-diphenyl picrtlhydrazyl (DPPH) method, which can be used to predict the oxidative stability of edible oils, was previously reported by our research group. Not only free radical scavenging antioxidants but also radicals from oxidized oils are capable of reacting with DPPH radicals, thereby reducing the absorbance of DPPH. In this study, the optimum sample size of edible oils for the DPPH method was determined, and the oxidation of the edible oils was monitored via DPPH, coupled with other conventional methods. The optimum sample size was determined as 1.5g using soybean oil. Soybean, corn, virgin and refined olives oils were thermally oxidized for 3 hr at 180°C and analyzed via DPPH, conjugated dienoic acid (CDA) value, and ρ-anisidine value (ρ-AV) protocols. Soybean and corn oils were found to be more sensitive to thermal oxidation than virgin and refined olive oils, on the basis of the CDA value and ρ-AV measurements. The DPPH method can indicate the inherent radical scavenging activity of unoxidized sample, the time required levels of free radical scavenging compounds, required more time for the consumption of inherent antioxidants, and also manifested steeper antioxidant degradation rates than olive oils, based on the results of DPPH analysis. The DPPH method, accompanied by other conventional methods, may prove useful in predicting the degree of oxidation of vegetable oils.