A kinetic study of the autoxidation reaction in olive oil triacylglycerols stored in darkness at different temperatures (25, 40, 50, 60 and 75 7C), in absence of pro-and antioxidant compounds to avoid confounding effects, is described. After the induction period (IP) the decrease in the oxidizing substrate and the formation of primary oxidation products followed a pseudo-zero-order kinetic, and the calculated E a from the Arrhenius equation for the formation of hydroperoxides was 32.1 kJ?mol 21 . The formation of secondary oxidation products followed a pseudo-first-order kinetic whose rate reaction constant also increased exponentially with temperature. The first oxidation index to exceed the upper limit in the EU regulations was PV, followed by K 232 and K 270 . The time required reaching these limits and the rancidity threshold showed a potential dependence on temperature, and therefore with accelerated storage at 75 7C, POO shelf-life in ambient conditions (25 7C) can be predicted. Finally, there was a good linear relationship between the time required to reach the rancidity threshold and the IP of the formation of the 2,4-decadienal, and hence this instrumental determination could be useful to measure sensory recognition of the rancid defect in POO.
This study proposes an accelerated test performed at mild temperature (40-60 7C) to measure oxidative stability and estimate the potential shelf life of extra-virgin olive oil (EVOO). The kinetic behavior of normalized oxidation indices (PV, K 232 and K 270 ) and the oxidizing substrate [unsaturated fatty acids (UFA)] during storage of different virgin olive oil samples in darkness and at different temperatures (25-60 7C) is reported for the first time. PV and K 232 followed an apparent pseudo zero-order kinetics (R 2 .0.951) at all the experimental temperatures in all samples, whereas the evolution of K 270 apparently better fitted a pseudo first-order kinetics (R 2 .0.926). The temperature-dependent kinetics of the oxidation indices and the UFA were well described by the linear Arrhenius equation between 25 and 60 7C (0.960, R 2 ,0.999, p ,0.05). The best correlation between loss of PUFA and increase of oxidation product indices was K 232 (0.581, R 2 ,0.924). The time required to reach the upper limits for PV, K 232 and K 270 established for the EVOO category in the current EU legislation correlated well with temperature using a potential equation, making it possible to set up an accelerated stability test at temperatures below 60 7C to estimate the potential shelf life under normal storage temperature conditions.
Oxidative stability should be one of the most important quality markers of edible oils; nevertheless, it is not recognized as a legal parameter. The results reported in this study highlight the differences in the olive oil oxidation process under Rancimat accelerated conditions with respect to long-term storage at room temperature and clearly show the lack of correlation between shelf life and the Rancimat induction period. A better correlation, although not yet satisfactory, was found when the same oxidation end-point was used in both assays. The parameter K 270, a marker of secondary oxidation products, was the first index to reach the established upper legal limit under Rancimat conditions, whereas at 25 degrees C it was an index of primary oxidation products ( K 232). Furthermore, the ratio of oxidation rate at Rancimat conditions to oxidation rate at 25 degrees C was more than double for secondary oxidation products compared with primary ones. Notable differences were also observed in degradation rates of the different unsaturated fatty acids and in rates of formation of polar oxidation compounds. Moreover, under the Rancimat conditions antioxidants such as o-diphenols and alpha-tocopherol rapidly depleted, and when they had practically disappeared, there was a sharp increase in oxidation indices, such as peroxide value, and in oxidation products. At 25 degrees C, on the other hand, the depletion was much lower.
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