Acidolysis Reactions Lead to Esterification of Endogenous Tocopherols and Compromised Oxidative Stability of Modified Oils

Abstract

For the first time, a possible mechanism responsible, in part, for the removal of endogenous antioxidants through the formation of tocopheryl esters during acidolysis reactions is proposed and confirmed. Tocopherols in the oils were found to react with carboxylic acids present in the medium, thus leading to the formation of tocopheryl esters that do not render any stability to the resultant modified oils as they lack any free hydroxyl groups on the phenolic ring of the molecule. Tocopheryl oleate, used as a standard, was synthesized through the reaction of acyl chloride of oleic acid with alpha-tocopherol (m/z 695.5 as evidenced by mass spectrometry). Subsequently, lipase-assisted esterification of alpha-, gamma-, and delta-tocopherols with oleic acid was carried out, and corresponding tocopheryl esters were isolated. In a real acidolysis reaction system involving docosahexaenoic acid single-cell oil and capric acid, high-performance liquid chromatography-mass spectrometry analysis demonstrated the presence of several tocopheryl esters. These included tocopheryl esters of myristic acid, namely, alpha-tocopheryl myristate, m/z 641.1, gamma-tocopheryl myristate, m/z 627.1, and delta-tocopheryl myristate, m/z 613.1, as well as those of palmitic acid, namely, alpha-tocopheryl palmitate, m/z 669.1, gamma-tocopheryl palmitate, m/z 655.1, and delta-tocopheryl palmitate, m/z 641.1. The mixture also contained different species of tocopheryl oleates, namely, alpha-tocopheryl oleate, m/z 695.5, gamma-tocopheryl oleate, m/z 681.1, and delta-tocopheryl oleate, m/z 667.2. Esters produced from reactions of docosahexaenoic acid and tocopherols were also detected, namely, alpha-tocopheryl docosahexaenoate, m/z 738.7, and delta-tocopheryl docosahexaenoate, m/z 710.7.