The Application Of Electron Spin Resonance Spectroscopy In Oxidative Stability Evaluation Of Edible Oil

Edible oil is the important nutrient substance and food for human beings, also widely used in the pharmaceuticals industry. But the auto oxidation of oil occurring under room temperature can affect its quality severely. Especially the oil which contains a lot of unsaturated fatty acids, like peanut oil, can be oxidized more easily. Although there have been several methods to study the oxidative stability of edible oil, such as Oven test and Rancimat. Yet they are all based on detecting the final products of oxidation, which make them studying the oxidation indirectly. While Electron Spin Resonance（ESR） spectroscopy can directly detect the free radicals, making it no doubt in the detection of free radicals area with unparalleled advantages, since the oxidation process is chain reaction of free radicals. This study was focused on the auto oxidation of edible oil, establishing the method of evaluation of oil’s oxidative stability, and then studying the mechanism of oxidation based on a perspective of free radical by ESR. The studies were summarized as follows:（1） Firstly the differences of spectra were showed between spin trap PBN and DMPO, determining that PBN was used to study the amount of free radicals（amount of spins） and DMPO was used to analyze the types of free radicals. Their adding ways and the amounts used were also optimized, that the PBN was added into oil for 3% and DMPO was added in oil with toluene for 0.1 mol/L.（2） Then, the evaluation methodology on oil oxidative stability was established and its conditions were optimized. Firstly optimizing the microwave power showed that around 4 mW（17 dB attenuation） was appropriate for ESR signal; The Boltzmann fitting method was used to determine the Induction Period of oils, which was very convenient; To get a rapid and definite Induction Period, heating the oil under 90°C was the best choice. There were superior correlation between the Induction Periods from ESR and Rancimat method, and the value of r was 0.961 and 0.995 respectively, during detecting different antioxidants by adding into peanut oil or several different edible oils, indicating that to study the oxidative stability of oils, the ESR method established in this study was reliable.（3） The effects of Cu²⁺ and Fe²⁺ on the free radicals in oxidation process were studied. Both Cu²⁺ and Fe²⁺ showed the capacity of accelerating the production of free radicals, and Cu²⁺ had a stronger effect than Fe²⁺; The types of free radicals were fitted, including peroxyl（DMPO-?OOL）, alkoxyl（DMPO-?OL）, alkyl（DMPO-?L） and oxidized DMPO adduct.（4） The free radical signals of four main fatty acids existed in peanut oil were compared. Using PBN as the spin trap, two saturated fatty acids, stearic acid and palm oil were found very stable and no free radical appeared; While two unsaturated fatty acids, oleic acid and linolic acid produced large amounts of free radicals, and using DMPO as spin trap, four types of free radicals were determined, the majority one was the alkyl（DMPO-?L）.（5） The difference of spectra was compared between peanut oil under 90°C and 120°C. The fitting results showed that the spectrum under 90°C was mainly alkoxyl（DMPO-?OL） and unknown radical whose hyperfine coupling constants were =14.73 G, =2.12 G, determined to be the C-center free radical. While the spectrum under 120°C mainly contained alkyl（DMPO-?L）.