Oxidative Patterns And Kinetics Of Edible Oils With Different Fatty Acid Compositions

Edible vegetable oils constitute lots of unsaturated fatty acids. Oxidation of unsaturated fatty acids is the major cause of lipid rancidity. Presently, there is no unified oxidation model for edible oil, and huge losses of economy and hazards on human body are big troubles. In this study, Eight vegetable oils with different fatty acid profiles(palm oil, peanut oil, camellia oil, rapeseed oil, sunflower oil, corn oil, perilla oil and soybean oil), and eighteen blend oils with different ratios of saturated, monounsaturated and polyunsaturated fatty aicds(SFA:MUFA:PUFA=1:3:2, 1:3.5:2 and 1:4:2), different ratios of n-6/n-3 fatty acids(n-6/n-3=1:1, 3:1, 5:1, 6:1, 8:1 and 10:1), were chosen to carry on Schaal oven accelerated oxidation assay. It was aimed to study the kinetics and correlations among inherent influence factors like fatty acid compositions and tocopherols, and oxidation indices like acid value, peroxide value, thiobarbituric acid value, p-anisidine value, aldehydes and ketones, in order to supply references and suggestions for manufacture, transportation, storage and consumption of edible oils.The main results are as follows:(1) Linoleic acid levels in sunflower oil, soybean oil, corn oil, peanut oil, palm oil and camellia oil were decreasing during accelerated oxidation. Is was found that there were linear correlations between the linoleic acid levels and oxidation times. Also exponential correlations between them were found. Linolenic acid levels in perilla oil, soybean oil and camellia oil were decreasing during accelerated oxidation. Similarly, both linear and exponential correlations between the linolenic acid levels and oxidation times were found. Linear correlation indicated that oxidation of fatty acid was applied to a zero-order kinetic model, and exponential correlation indicated the oxidation was a first-order kinetic model. Correlation coefficients between predictive values and experiment values of models were over 0.8, so the models were reasonable. However, kinetic models were different among different types of oils.(2) Fatty acid distributions in the Sn-1(3) and Sn-2 postion of triglycerides were different among different vegetable oils. And the oxidation stabilites of fatty aicds were also different in different positions. It was found that the oxidation stabilities of linoleic acid in the Sn-1(3) and Sn-2 positions of sunflower oil and soybean oil were similar. However, for camellia oil, palm oil, peanut oil and corn oil, linoleic acid in Sn-1(3) postion may be more unstable than in Sn-2 position. For perilla oil and soybean oil, linolenic acid in Sn-2 position may be more unstable than in Sn-1(3) postion. Hence, the oxidation stabilities of linoleic and linolenic acids from different oils in three acyl positions were different.(3) The oxidation stabilities of tocopherols were different among different oils, one is due to the differences in homologues, and the other is effect of fatty acid composition. Sunflower oil, peanut oil, palm oil and camellia oil had more than 88% of α-tocopherol, and the order of decrease rate during accelerated oxidation was as follows: sunflower oil > peanut oil > palm oil > camellia oil. The four oils had very low level of linolenic acid(<1%), and the order of linoleic acid level was just same as the order of decrease rate of α-tocopherol. Hence, when linolenic acid level is low(<1%), and α-tocopherol was the major homologue(≥88%), α-tocopherol may decrease more quickily in oils with higher linoleic acid level.(4) Acid value, peroxide value, thiobarbituric acid value are the common oxidation indices for oil. When n-3 polyunsaturated fatty acid(PUFA) level is low(<1%), and α-tocopherol is the major homologue(≥88%), oil may be more unstable in higher n-6PUFA. Because peroxide value increased more significantly and α-tocopherol decreased more dramatically in higher n-6 PUFA level of sunflower oil. However, when the percentage of γ-and δ-tocopherol increased, such as corn oil and soybean oil, oil stabilities were improved. When the level of n-3PUFA increased, such as perilla oil, peroxide value may not increase, but thiobarbituric acid value will increase.(5) Simultaneous determination of 20 aldehydes and 5 ketones within the C2-C10 range was carried out using HPLC-QqQ-MS technique. Close relationships among the amounts of aldehyde carbonyls and the initial contents of oleic, linoleic and α-linolenic acids were revealed by Principal Component Analysis. Octanal, octanone, nonanal, nonanone, decanal, decanone and 2-decenal were the oxidation indexes linked to the initial content of oleic acid. Pentanal, hexanal, hexanone, heptanal, 2-propenal, 2-heptenal, 2-octenal, 2-nonenal and 2, 4-decadienal were the key carbonyls in close association with the initial content of linoleic acid. Ethanal, acetone, propanal, butanal, 2-pentenal, 2-hexenal, 2, 4-heptadienal and 2, 4-nonadienal were the key markers closely related with the initial content of α-linolenic acid. The results provide a complete picture of secondary oxidation products in edible oils and possible source of parent fatty acids.(6) Nonanal in camellia oil(oleic acid mainly) increased significantly, propanal in perilla oil(linolenic acid mainly) increased significantly, hexanal and nonenal in sunflower oil(linoleic acid mainly) increased significantly. These indicators had good linear correlations with their corresponding total oxidation value(TOTOX) and the amounts of total nonpolar carbonyls in oils(R2 > 0.8) during accelerated oxidation. According to the change patterns of peroxide value, TOTOX, total nonpolar carbonyls and oxidation indicator, the corresponding order of oxidation stability was uniformly sunflower oil < camellia oil < perilla oil < palm oil. Hence, the yield of indicator was a key index for evaluating the oxidation state of vegetable oil. In addition, determination of indicator was a novel approach to monitor oil oxidation. They played an important role as TOTOX or peroxide value in the oxidation assessment of lipid.(7) Both molecular synchronous and three dimensional(3D) fluorescence spectrometry were used to check the possibility to monitor changes in edible oils during oxidation. Principal Component Analysis plot of synchronous fluorescence intensity(λex = 320-700 nm) clearly showed the oxidative evolution of oils over heating time. High saturated or monounsaturated oils exhibited good regression results between peroxide values and fluorescence intensity(R2 = 0.973 for 400 nm in palm oil; R2 = 0.956 for 370 nm in camellia oil). High diunsaturated oil exhibited good regression results between nonpolar carbonyl compounds and fluorescence intensity(R2 = 0.970 for 370 nm in sunflower oil). High triunsaturated oil exhibited good regression results between p-anisidine value and fluorescence intensity(R2 = 0.938 for 665 nm in perilla oil). In conclusion, the differences of fatty acid compositions played key rules in the formation of oxidation products and the evolution of fluorescence spectra.(8) Eighteen blend oils were blended using vegetable oil, according to a required confection condition(SFA:MUFA:PUFA were 1:3:2. 1:3.5:2 and 1:4:2; n-6/n-3 were 1:1, 3:1, 5:1, 6:1, 8:1 and 10:1). During Schall oven accelerated oxidation assay, PUFA and tocopherol levels were decreasing, primary and secondary oxidation products were increasing. Samples with higher n-6PUFA, had more decrease of α-tocopherol and more increase of peroxide value. However, not all the samples with higher n-6PUFA, had more increase of perocide value. That was likely due to the higher level of n-3PUFA, which increased the thiobarbituric acid value.