Study On The Synthesis,Chemical Stability Of Tocored,and Its Relevance With Color Reversion In Edible Vegetable Oils

Color is an intuitive and important factor that affects consumers'purchase of edible oils.Effectively inhibiting the color reversion in edible oils during storage,transportation,and use is an urgent problem for the oil industry.There is sufficient evidence to show that the color reversion is essentially caused by the oxidative instability of the oil system under the combined actions of internal and external factors.The ortho-quinone derivative of the endogenous?-tocopherol in the oil—tocored is one of the key substances for color reversion.The research on the oxidative stability of tocored helps to reveal the mechanism of color reversion.Thus,based on the synthesis and structure identification of tocored,the present study systematically studied the antioxidant activity of tocored,investigated its oxidative stability in the lipid matrixes and its influencing factors,and then correlated with the phenomenon of color reversion.The main findings are as follows:First of all,using mixed tocopherols as raw materials,high-purity tocored was successfullyobtained for the first time by using concentrated nitric acid oxidation method to synthesize,and thin-layer chromatography?TLC?and high pressure preparative liquid chromatography to separate and purify.The results showed that?-tocopherol with a purity of 96%could be obtained from the Mixed Tocopherols products by reversed-phase high pressure preparative liquid chromatography,and further reacted with concentrated nitric acid in anhydrous ethanol solution.The 85%crude tocored through three times of TLC was further separated by reverse phase high pressure preparative liquid chromatography to obtain 98%pure tocored.UV-Vis spectroscopy,proton nuclear magnetic resonance spectroscopy,and ultra-high-performance liquid chromatography tandem quadrupole time-of-flight mass spectrometry?UPLC-QTOF-MS?identified and confirmed that the product obtained was tocored.Then,ultra-performance convergence chromatography tandem quadrupole time-of-flight mass spectrometry?UPC²-QTOF-MS?and high performance liquid chromatography tandem diode array detector?HPLC-PDA?were applied to investigate the influence of external factors on the changes in the structure of tocored,and clarified the relationship between them and the color changes of the substrates.The results showed that the thermal instability of tocored was temperature-dependent within the range of conventional oil refining temperatures?50?150oC?,and follows a formula"?=1×10^-5e^0.0335T"(?is the rate,mg·kg^-1·min^-1;T is the temperature,K),and the thermal instability activation energy?E_a?is 38.54 k J/mol,indicating that thermal instability of tocored was prone to occur and unstable to heat;The color gradually deepened with the increasing heating time,and part of the tocored dimers formed.When ultraviolet?UV?irradiated tocored in solvents,the degradation rates were affected by the types of ultraviolet light with?_UVC>?_UVA;the degradation rates of tocored in different solvents were also different,such as?_methanol>?_hexane;tocored was unstable to light.During UV irradiation,the color of the solvent matrixes gradually became lighter because of that tocored was decomposed into small molecules.Next,the DPPH method,FRAP method,Rancimat method and Schaal oven method wereused to systematically study the antioxidant activity of tocored.The results showed that the activity of tocored in scavenging DPPH free radicals was significantly lower than that of?-tocopherol in the pure solvent matrix,which is consistent with the conclusion of the FRAP method.In the stripped corn oil matrix,Rancimat method found that the induction times of tocored increased with the increase of concentrations?0?1000 mg/kg?,but it was shorter than?-tocopherol.when tocored compared with the same concentration of?-tocopherol,an antagonistic effect was observed;it is found by Schaal oven method?55oC,50 days?that tocored can significantly inhibited the formation of primary oxidation products such as peroxides and conjugated dienes and secondary oxidation products such as aldehydes.With the increase in concentrations?100?500 mg/kg?,the oxidation level of the matrix increased,and there is a phenomenon of"loss of antioxidant efficiency",but it was still significantly better than the?-tocopherol.In short,the antioxidant activity of tocored in the lipid matrixes at low temperature?<60oC?was significantly better than that of?-tocopherol,wherease the antioxidant activity of tocored at high temperatures?110?120oC?was significantly weaker than that of?-tocopherol.In the accelerated oxidation experiment,the color of the lipid matrixes gradually became lighter,and the contents of tocored were significantly reduced,indicating that tocored probably acted as an antioxidant to participate in the lipid oxidation process,causing its own consumption.This is also in consistent with the phenomenon that the content of tocored in the oil recolor gradually decreases.Finally,in the lipid matrixes?methyl palmitate,methyl oleate and methyl linoleate?,the oxidation stability of tocopherol was studied,and UPC²-QTOF-MS was used to initially explore the mechanism of its color transformation,and its correlation with oil color recovery.The results showed that the color of the lipid matrix was in a concentration-dependent relationship with tocored,that is,the higher the concentration of tocored,the higher the color of the matrix.The instability of tocored in the lipid matrix was closely related to the degree of unsaturation of the matrixes.The instability of tocored in methyl linoleate was the fastest,followed by methyl oleate,and methyl palmitate was the slowest.In the same lipid matrix,the rate of instability of tocored increased significantly with temperatures increase?90?120oC??p<0.05?.The kinetic stability study showed that the instability of tocored more likely followed the zero-order reaction kinetic model,and the study of thermodynamic stability shows that the instability of tocored was an endothermic reaction,which was consistent with the relevant parameters of kinetic stability study.UPC²-QTOF-MS was used to preliminarily separate the reaction products of tocored and fatty acid methyl esters,and the antioxidant activity mechanism was speculated as tocored competed with oxygen and couples with lipid free radicals directly,delaying the spread of lipid oxidation chain reaction.The ortho-quinone carbonyl structure in the associated conjugate was destroyed,and its conjugated structure was not extended,resulting in a decrease in the color of the matrixes.Further,subsequent storage experiments in the dark found that the reaction products of tocored and oxidized fatty acid methyl esters were color-reversion precursors.In summary,the present study obtained high-purity tocored for the first time,furtherconducted the study of the conversion mechanism of tocored under external factors,and systematically studied its antioxidant activity,and preliminarily explored its antioxidant mechanism in the lipid matrixes.Further,the potential molecular mechanism of the color change of the substrate was clarified,and the reaction conversion paths of tocored were expanded,which would expand and enrich the basic research on the mechanism of color reversion in edible oils.