Abstract
Peroxide value and the determination of conjugated dienes by UV absorption at 232 nm are normally applied alternatively for the evaluation of primary oxidation compounds. The objective of this study was to define the relationship between both indexes during oil oxidation in sunflower oils with different degree of unsaturation. The oils studied were conventional sunflower oil (SO) and high oleic sunflower oil (HOSO). For comparative purposes, methyl linoleate, whose hydroperoxides are conjugated diene hydroperoxides, was also studied. Samples were oxidized at 40 °C in the dark. In order to accelerate oxidation at 40 °C, the natural antioxidants present in the oils were removed, and a high surface to oil volume ratio (0.8 cm -1 ) was applied to guarantee total air availability. Linear regressions with high correlation coefficients between peroxide values and conjugated dienes were found for the three lipid systems studied (0.9988, 0.9991 and 0.9977 for methyl linoleate, SO and HOSO, respectively). Significant differences in the slopes of the lines were found (0.0974, 0.0854 and 0.0503 for methyl linoleate, SO and HOSO, respectively), indicating the formation of non-conjugated hydroperoxides from oleic acid in the oils, even in the oxidation of SO with a high degree of unsaturation. Consequently, only peroxide value would be reliable for the evaluation of primary oxidation compounds in oils of different degree of unsaturation, unless adequate calibration lines (peroxide value versus conjugated dienes) are applied.
Highlights
Lipid oxidation takes place through a set of autocatalytic reactions that produce a high number of new compounds
It should be noted that the main objective of this study was to know the intrinsic relationship between two analytical determinations, which should not be affected by oxidation conditions
It can be observed that high Peroxide value (PV) were obtained after only 50 hours of storage under the conditions applied (40 °C, S/V ϭ 0.8 cmϪ1)
Summary
Lipid oxidation takes place through a set of autocatalytic reactions that produce a high number of new compounds. Hydroperoxides, the primary oxidation compounds, decompose giving rise to a variety of secondary oxidation products. The difficult evaluation of the oxidation status in oils requires the application of more than one analytical method for a better understanding of the level of both primary and secondary oxidation compounds present in a certain sample (Frankel, 2005). The analysis of primary oxidation compounds is always carried out either by the peroxide value, normally determined by titration of the iodine released from the reaction of peroxides with potassium iodide, or, alternatively, by the determination of specific extinction at 232 nm, since the hydroperoxides of polyunsaturated fatty acids, the most susceptible fatty The results are not interpreted due to the varying significance of the methods applied (peroxide value, conjugated dienes, anisidine value, level of pentane or hexanal, TBARS, UV extinction at 270 nm, etc.), and to the unspecified conditions of oxidation storage, in particular, the surface to oil volume ratio, which influences the availability of air.
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