Loss of tocopherols and formation of degradation compounds at frying temperatures in oils differing in degree of unsaturation and natural antioxidant content

Samples of oils of different degrees of unsaturation, namely palm olein, olive oil, high-linoleic sunflower oil, high-oleic sunflower oil, rapeseed oil and soybean oil, were heated at 180degreesC for 2, 4, 6, 8 and 10 h in the presence or absence of their natural antioxidants. Also, tocopherol-stripped oils were supplemented with alpha-tocopherol (500 mg kg(-1)), delta-tocopherol (500 mg kg(-1)) or a mixture of alpha-, beta-, gamma- and delta-tocopherols (250 mg kg(-1) each) and heated under the same conditions. Losses of tocopherols and formation of polymeric triacylglycerols were followed. Total polar compounds were also evaluated after 10h of heating. Results demonstrated that tocopherols were lost very rapidly, in the expected order, with alpha-tocopherol being the least stable. Polymeric and polar compound formation during heating was inhibited to a variable extent, being more dependent on the natural content and type of tocopherols than on the degree of unsaturation of the oil. For example, polymeric and polar compound contents in soybean oil were significantly lower than those found in high-linoleic sunflower oil. However, the expected influence of the degree of unsaturation was evident when oils were unprotected or possessed identical initial antioxidant contents. Finally, levels of degradation compounds after 10h of heating were not dependent on the remaining content of antioxidants. (C) 2002 Society of Chemical Industry.