Studies on dimerisation of tocopherols under the influence of methyl linoleate peroxides.

The stability of plant oils is related to the level of polyunsaturated fatty acids and the presence of native antioxidants--especially tocopherols. During storage, lipids or the fat products undergo oxidation and tocopherol dimers and trimers are formed. These compounds possess reducing and antioxidant properties and participate in oxidation clearly inhibiting this process. In the present study, the correlation between levels of peroxides formed during autoxidation of methyl linoleate and simultaneous decomposition of tocopherols was examined. The peroxide value was investigated. Quantities of decomposed tocopherols and formation of their dimers were determined by high-performance liquid chromatography (HPLC). Mass spectrum analysis confirmed thatthe analysed compounds were dimes. Dimerisation of gamma-T begins at the smaller quantity of the methyl linoleate peroxides than dimerisation of delta-T. At the beginning of methyl linoleate autoxidation dimerisation of gamma-T in relation to its loss was smaller. The quantity of gamma-T dimers with ether bonds in total dimers pointed to faster binding of phenoxy radicals than transformation into the phenyl ones. delta-T dimers with phenyl bonds constitute about 65% of the total. The quantity of peroxides in methyl linoleate, necessary for quantitative and qualitative changes of homologous tocopherols, decreased from delta-T to alpha-T.

Effect of menadione (vitamin K3) addition on lipid oxidation and tocopherols content in plant oils.

The effect of added menadione (vitamin K3) on stored corn and wheat germ oil on the dissolution and dimerization of natural tocopherols and autoxidation of triacylglycerols was investigated. Samples of corn and wheat germ oils pure and with added menadione were stored at +20 degrees C in brown and transparent glass bottles. During storage their peroxide value, changes of the content of fatty acids and dissolution of tocopherols were measured. Destruction of individual tocopherols in tested oils with menadione stored in both dark and transparent glass bottles was greater than in the oil samples without menadione. However, the degradation of individual tocopherols was different in each oil. Addition of menadione to the both oils resulted in accelerated the process of autoxidation of these oils. The corn oil stored in transparent bottles was oxidized faster than the one stored in brown bottles. The pro-oxidant activity of menadione was additionally activated by light. In contrast, in the case of germ oil, the process of autoxidation was very fast regardless of sort of container. Addition of menadione to the plant oils influenced the dissolution of natural tocopherols but did not influence the dimerization of tocopherols. As from the experiment, the addition of menadione to the oils decreased their nutritive value.