Relative autoxidative and photolytic stabilities of tocols and tocotrienols

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Abstract
The relative stabilities of selected individual tocols and tocotrienols and of equimolar mixtures of either α‐ plus γ‐ or α‐ plus δ‐ tocopherols were determined in methyl myristate and methyl linoleate during autoxidation and photolysis. Solutions containing 0.05% of the appropriate tocopherol(s) or tocotrienols were subjected to UV light (254 nm) or to a flow of 4.3 ml/min of oxygen, both at 70 C. Tocopherols (T) and tocotrienols (T−3) were determined by gas chromatography without preliminary separation or purification. Under photolytic conditions, stabilities in increasing order in methyl myristate were γ‐T−3<α‐T−3<δ‐T<α‐T <γ‐T<5,7‐T<β‐T and in methyl linoleate were α‐T<α‐T−3≤γ‐T−3≤β‐T≤5,7‐T <γ‐T<δ‐T. A solvent effect on the initial rate of photolysis was observed for 5‐methyl substituted tocols but not for the tocols with an unsubstituted 5‐position or for the tocotrienols. Under autoxidative conditions, stabilities in increasing order in methyl myristate were α‐T=α‐T−3 <β‐T−3<γ‐T−3<δ‐T−3<γ‐T<δ‐T=β‐T and in methyl linoleate were α‐T<α‐T−3 <γ‐T−3<β‐T<γ‐T<δ‐T. Tocopherols were much more stable during autoxidation in methyl myristate than they were in methyl linoleate. In mixtures, there was no significant protection of α‐tocopherol by either γ‐ or δ‐tocopherol under any of the conditions used. However, α‐tocopherol was highly effective in protecting γ‐ and δ‐tocopherols in methyl myristate during both photolysis and autoxidation and in methyl linoleate during photolysis. During autoxidation in methyl linoleate, α‐tocopherol protection of γ‐ and δ‐ tocopherols after 24 hr was slight tough measurable.