Effects of alpha- and gamma-tocopherols on formation of hydroperoxides andtwo decomposition products from methyl linoleate

The antioxidant effects of alpha- and gamma-tocopherols (at 0, 10, 100, 500 , and 1000 ppm) were evaluated in a model system based on the autoxidation of methyl linoleate in bulk for 4 d at 40 degrees C. Samples were collected every 24 h and analyzed for the 9 cis,trans, 9 trans,trans, 13 cis,trans, and 13 trans,trans isomers of hydroperoxide, hydroxy, and ketodiene oxidati on products by high-performance liquid chromatography. Results showed that both alpha- and gamma-tocopherols are effective hydrogen donors as evidence d by their abilities to inhibit the formation of hydroperoxides, hydroxy co mpounds, and ketodienes and the cis,trans to trans,trans isomerization of h ydroperoxides. Compared with gamma-tocopherol, alpha-tocopherol was a more efficient antioxidant at very low concentrations (10 ppm) but a less effici ent antioxidant at the high concentrations (100-1000 ppm). This paradoxical behavior is explained on the basis of differences in ease of hydrogen dona tion between the two tocopherol homologs. Although alpha-tocopherol shows s ome loss of efficiency with increasing concentration, it is not a prooxidan t when compared to the control void of antioxidants.