Evaluation Of The Oxidation Stability Of Oil By Chronoamperometry

Electrochemical Analysis, a fast developing, sensitive and convenient analytical method, has been applied in the research of the quality of oil. In the process of oil oxidation, peroxide value(POV) is gradually changed with the increase of the concentration of lipid peroxide and it has been used as a test index. In this dissertation, two kinds of different modified electrodes were prepared and used to measure the degree of the oxidation of oil by chronoamperometry. The induction times and the oxidation reaction rate constants under different temperatures were obtained by dynamic analysis of the process of oil oxidation. According to the dynamic parameters and Arrhenius equation, dynamic models of oxidation of different kinds of oil were built to evaluate the oxidation stability and predict the shelf life of oil at room temperature(25℃).The main research contents are described as follows:Graphene oxide(GO) and graphene modified electrode(rGO/GCE) were prepared and their characteristics were studied. Based on the electrocatalytic activity of rGO, chronoamperometry was used to directly and sensitively measure the oxidation current caused by the change of the concentration of lipid peroxide. Therefore, POVs of oil at different oxidation degrees could be obtained within 100 s. Lauroyl peroxide was selected as a model compound and its linear range and detection limit were 0.1-10.0 mmol/L and 0.032 mmol/L,respectively. Based on the analysis of induction times of olive oil, soybean oil and almond oil which obtained by national standard method(GB) or by chronoamperometry, the oxidation stability of oils was soybean oil> olive oil> almond oil. F-test and t-test statistics results also showed that there was no significant difference between GB and the proposed method.A easy-renewable, convenient preparable, low material consumable and structure stable electrochemical sensor, Fe3O4 nanoparticles modified expanded graphite paste electrode(EG/Fe3O4-PE), was constructed. Fe3O4 nanoparticles, as a stable enzyme mimetic sensor, could catalytze the reduction of lipid peroxide. The sensor could be used to detected the reductive current of lauroyl peroxide. The linear range and detection limit of lauroyl peroxide were 5.0×10-6-1.0×10-2 mol/L and 1.7 μmol/L, respectively. According to the Lineweaver-Burk equation, the apparent michaelis constant(Km) of the Fe3O4 NPs was 0.68 mmol/L. The peroxide values of different kinds of oil were determined by chronoamperometry. F-test and t-test statistics results showed that there was no significant difference between GB and the proposed method.In addition, the variation of peroxide values of olive oil, soybean oil, blend oil, almond oil, sunflower oil and rapeseed oil during the process of primary oxidation under different temperatures by the proposed method with EG/Fe3O4-PE were thoroughly studied. According to POV-t curves of six kinds of oil, the induction times of these oils were obtained. The oxidation rate constants of these oil under different temperatures could be obtained with dynamics analysis of the oxidation process of oil. Both the induction time and the oxidation rate constants of these oils revealed the same order of oxidation stability, that was soybean oil >blend oil >rapeseed oil >olive oil >sunflower oil >almond oil. Therefore, the induction time and oxidation rate constant had close relationship with oxidation stability of oil.Dynamic models of oxidation of oil were construted based on the dynamic parameters and Arrhenius equation to evaluate the oxidation stability and predict the shelf life of oils at ambient temperature(25℃). Based on GB, the obtained shelf lives of olive oil, soybean oil, blend oil, sunflower oil, rapeseed oil and almond oil were 91 d, 168 d, 115 d, 61 d, 105 d, 53 d, repestively. Meanwhile, the shelf lives of six kinds of oil obtained by chronoamperometry were 95 d, 163 d, 118 d, 56 d, 103 d, 52 d, repestively. In conclusion, chronoamperometry could simply, sensitively and accurately determine the POVs, evaluate the oxidation stability and predict the shelf life of oil.