Abstract:Relationship between polymerised triacylglycerols formation and tocopherols degradation was studied during heating of four commercially accessible vegetable oils (rapeseed oil, classical sunflower oil, soybean oil and olive oil) on the heating plate with temperature 180°C. The content of polymerised triacylglycerols 6% (i.e. half of maximum acceptable content) was achieved after 5.3, 4.2, 4.1, and 2.6 hours of heating for olive oil, soybean oil, rapeseed oil and sunflower oil, respectively, while decrease in content of total tocopherols to 50% of the original content was achieved after 3.4, 1.6, 1.3, and 0.5 hours of heating for soybean oil, rapeseed oil, sunflower oil and olive oil, respectively. Because of the high degradation rate of tocopherols, decrease in content of total tocopherols to 50% of the original content was achieved at content of polymerised triacylglycerols 0.6%, 1.9%, 2.8% and 4.9% for olive oil, rapeseed oil, sunflower oil and soybean oil, respectively, i.e. markedly previous to the frying oil should be replaced.

Réblová Z., Fišnar J., Tichovská D., Doležal M., Joudalová K. (2012): Effect of temperature and oil composition on the ability of phenolic acids to protect naturally present α-tocopherol during the heating of plant oils. Czech J. Food Sci., 30: 351-357.The ability of phenolic acids (ferulic, gallic, protocatechuic, and sinapic; 600 mg/kg) to protect naturally present a-tocopherol was tested during the heating of sunflower oil on a hot plate set at 120, 150, 180, 210, or 240°C, and during the heating of rapeseed, olive and soybean oils on a hot plate set at 180°C. In all the studied conditions, a-tocopherol was significantly protected only by gallic acid. This phenolic acid prolonged the half-life of a-tocopherol (calculated as the time needed for the a-tocopherol content to decrease to 50% of the original value) typically two-to four-fold. Hence the ability of phenolic acids to protect a-tocopherol in bulk oils does not markedly depend on the experimental conditions as is seen in antioxidant activity, i.e. in the ability of antioxidants to protect fatty acids.