Antioxidant activity of minor amounts of γ‐tocopherol in natural triacylglycerols

Abstract

The effects of minor amounts of γ-tocopherol on the oxidation of natural triacylglycerols (TAG) of rapeseed (RO) and butter oils (BO) were studied. Four different TAG materials were blended from chromatographically purified TAG that contained 100-25% of RO TAG. The RO TAG contained from <1 to 43 µg γ-tocopherol per gram of TAG, which corresponded to ≤6% of the total tocopherols in the original RO. The TAG were held at 40°C in the dark for 4 wk and followed at regular intervals by measurements of hydroperoxide formation by peroxide values and of secondary product formation by p-anisidine values at regular intervals. In all TAG, minor amounts of γ-tocopherol retarded oxidation. In RO TAG, concentrations as low as 11 µg/g of γ-tocopherol (1.5% of the total tocopherols in the original RO) were enough to decrease hydroperoxide and secondary product formation to 46 and 39%, respectively. The effect was even more important in TAG mixtures that contained BO TAG. There were no significant differences between oxidation of the RO TAG at 24 µg/g, the 75% RO TAG mixture at 11 µg/g, and the 50% RO TAG mixture at 3 µg/g of γ-tocopherol. Even at these minor levels, γ-tocopherol was a significant antioxidant, which is important in oxidation studies of purified model systems. JAOCS 74, 549-555 (1997).KEY WORDS: Antioxidant, natural triacylglycerols, γ-tocopherol.Tocopherols (TocH) are known to act as antioxidants by donating a hydrogen atom to chain-propagating peroxyl radicals (LOO·):and are consumed by the reaction of chromanoxy (Toc•) radicals with other peroxyl radicals or with each other (1). It is generally agreed that the order of hydrogen-donating power of different tocopherols decreases α > β > γ > δ, as was shown by Burton and Ingold (1). Studies of the antioxidative activity of tocopherols in vivo and in vitro, however, have given contradictory results, which have recently been reviewed by Kamal-Eldin and Appelqvist (2). The effectiveness of tocopherols as antioxidants depends highly on the characteristics of the oxidizing material, such as the phase of the lipid (3-4) and its oxidative status (5-6), and on experimental conditions, such as the concentrations of tocopherols (3,7) and the temperature (4,8).When the role of tocopherols in relation to edible oils and fats is studied, it is important to have complex natural triacylglycerols (TAG) as the oxidizing material. Methyl esters and monoacyl TAG are inappropriate substrates, because the TAG structure influences the oxidation (9-11). Natural TAG with various acyl groups must be purified from oils to get material without the pro-and antioxidant activities of minor compounds that are normally present in the oils.Several column-chromatographic methods (6,(11)(12)(13)(14), as well as activated carbon treatments (15), have been used to purify natural TAG from oils for antioxidant studies. Commercially available corn oil, stripped of tocopherols, has also been used (3,16). Data on the concentrations of important pro-and antioxidants of the TAG, however, are not always given...

Effects of α‐ and γ‐tocopherols on the autooxidation of purified sunflower triacylglycerols

“…Results indicate that both α-and γ-tocopherols acted as very efficient antioxidants and almost completely inhibited the peroxida- Days of Oxidation tion of p-SFO during the 7 d of oxidation when present at concentrations above 20 ppm (PF ≈ 1.00). These results are in accordance with previous findings that concentrations as low as 11 ppm of γ-tocopherol in p-RSO were enough to cause a marked inhibition of peroxide and secondary product formation (18). Further details on the differences between α-and γ-tocopherols can be summerized as follows: (i) α-tocopherol was significantly more effective than γ-tocopherol at low concentrations (1-20 ppm), (ii) both tocopherols were equally effective within the concentration range 40-200 ppm, and (iii) γ-tocopherol was sightly more effective than α-tocopherol at concentrations greater than 200 ppm and the difference became significant at concentrations higher than 1000 ppm.…”

“…Purified triacylglycerols are the most valid models to study oxidation of vegetable oils (18). Purified triacylglycerol fractions must be prepared from vegetable oils to provide the natural complexity of fatty acid distribution and to the eliminate anti-and prooxidant species normally present in the oils.…”

Oxidative stability of natural and randomized high‐palmitic‐and high‐stearic‐acid oils from genetically modified soybean varieties

“…However, not all tocopherols could be removed from the oil (17) and about 4 µg of total tocopherols per gram of oil remained. However, the purified oils were verified to have no other kinds of non-TAG components by the…”

The Composition of Hemp Seed Oil and Its Potential as an Important Source of Nutrition

“…This increased bioactivity does not however make a-tocopherol a more effective antioxidant; g-tocopherol inhibits phosphatidylcholinehydroperoxide formation more effectively at low peroxynitrite concentrations than does a-tocopherol (Wolf, 1997). g-Tocopherol has been shown to have significant antioxidant effects in vitro even at concentrations less than 50 ppm (Lampi, Hopia, & Piironen, 1997). In addition, g-tocopherol is overall more effective in protecting against coronary heart disease, as compared to a-tocopherol supplementation (Wolf, 1997).…”