Ratio of Malondialdehyde to Hydroperoxides and Color Change as an Index of Thermal Oxidation of Linoleic Acid and Linolenic Acid

Linoleic acid (LA) and linolenic acid (LNA), the major unsaturated fatty acids of the vegetable oils consumed in Korea, were oxidized in tricaprylin at different temperatures (37, 60, 90 or 120C) and times (3, 6, 9 or 12 h). Primary lipid oxidation products (LOPs) and breakdown secondary LOPs were measured, and color development by polymeric secondary LOPs was visually monitored. When comparing the ratios of malondialdehyde to hydroperoxides, the difference between LA and LNA was greater at 37 and 60C, and relatively smaller at 90 and 120C. It seemed that the hydroperoxides were mostly oxidized to breakdown secondary LOPs at 37 and 60C, but were preferentially polymerized at 90 and 120C. This was confirmed by the noticeable darkening of color observed at 90 and 120C. The results indicate that breakdown and polymeric secondary oxidations should be considered together for a more comprehensive estimation of unsaturated fatty acids oxidation. Practical Applications A model system simulating a thermal oxidation of unsaturated fatty acids demonstrated that the ratio of malondialdehyde to hydroperoxides gave the useful information on the oxidation progression from primary to breakdown secondary products. When color is considered together as the index of polymeric secondary oxidation, the ratio can be an index to predict an oxidative status of food lipid under thermal processing such as baking and frying.