Relationship between changes in peroxide value and conjugated dienes during oxidation of sunflower oils with different degree of unsaturation

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.