Oxidation at elevated temperatures: competition between α-tocopherol and unsaturated triacylglycerols
✍ Scribed by Tom Verleyen; Afaf Kamal-Eldin; Revilija Mozuraityte; Roland Verhé; Koen Dewettinck; Andre Huyghebaert; Wim De Greyt
- Publisher
- John Wiley and Sons
- Year
- 2002
- Tongue
- English
- Weight
- 97 KB
- Volume
- 104
- Category
- Article
- ISSN
- 1438-7697
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✦ Synopsis
Introduction
Vegetable oils have different susceptibilities towards oxidative degradation due to differences in their fatty acid unsaturation and the varying type and contents of antioxidants. Tocopherols are the most important natural antioxidants present in vegetable oils . Although it is agreed that scavenging of peroxyl radicals is the main mechanism responsible for tocopherol action, it does not explain all effects observed during lipid oxidation studies .
Previous literature reports indicated faster tocopherol degradation in saturated than in unsaturated triacylglycerol matrices . These findings prompted us in a previous study to investigate the effect of triolein : tripalmitin in mixtures of different ratios on the rate of degradation of α-tocopherol, using a model design . The results from this study were in agreement with those obtained in an earlier study and a mechanism, based on competitive oxidation between tocopherols and triacylglycerides where unsaturated fatty acyl moieties compete with tocopherols in reactions with initiating alkoxyl radicals, was proposed to explain this phenomenon. Polar tocopherol oxidation products were identified in heat-treated triacylglycerols suggesting the participation of peroxyl radicals in tocopherol degradation .
In another study, reported that tocopherol losses were not affected by relative levels of triolein and trilinolein during simulated deep-frying experiments at 180 °C. In the present study, four oils (viz. palm, high oleic sunflower (HOSO), sunflower, and flaxseed oil) were selected for a detailed kinetic study on α-tocopherol degradation at two temperatures; viz. 180 °C and 240 °C. The formation of polar triacylglycerol oxidation products is mandatory for confirming the mechanism hitherto proposed for the competitive oxidation between α-tocopherol and unsaturated acyl groups . The degradation of α-tocopherol was finally studied in purified triacylglycerols derived from nine dietary oils and fats in order to investigate the previously observed phenomenon in triacylglycerols featuring a wide range of unsaturation.
Materials and methods
2.1 Materials and reagents
Commercial samples of refined corn, HOSO, olive, soybean, and sunflower oil were purchased in a local supermarket while samples of refined coconut, palm, flaxseed, and tallow oil were kindly donated by Extraction De Smet (Edegem, Belgium). Pure α-tocopherol and HPLC-grade solvents (petroleum ether, n-hexane, 2-propanol, tetrahydrofuran) were purchased from Merck (Darmstadt, Germany), activated aluminia and silica gel 60 for chromatography from Sigma Chemical Company (St. Louis, MO). All chemicals and reagents were of analytical grade and were used without further purification.