Samples of oils of different degrees of unsaturation, namely palm olein, olive oil, highlinoleic sunflower oil, high-oleic sunflower oil, rapeseed oil and soybean oil, were heated at 180°C for 2, 4, 6, 8 and 10 h in the presence or absence of their natural antioxidants. Also, tocopherol-stripped oils were supplemented with a-tocopherol (500 mg kg À1 ), d-tocopherol (500 mg kg À1 ) or a mixture of a-, b-, g-and d-tocopherols (250 mg kg À1 each) and heated under the same conditions. Losses of tocopherols and formation of polymeric triacylglycerols were followed. Total polar compounds were also evaluated after 10 h of heating. Results demonstrated that tocopherols were lost very rapidly, in the expected order, with a-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 10 h of heating were not dependent on the remaining content of antioxidants.
INTRODUCTIONIn a previous study we stressed the interest in defining the action of natural antioxidants, particularly concerning the relationships between loss of antioxidants and formation of new compounds at the high temperature of processes such as frying. 1 With this aim we initiated a series of experiments to determine the influence of tocopherols on the alteration of the lipid substrate without the influence of minor uncontrolled compounds normally present in the oils, which might exert an antioxidant or pro-oxidant effect. We selected three model systems constituted by monoacid triacylglycerols, ie triolein, trilinolein and a 50:50 mixture of the two, of clearly differentiated degrees of unsaturation, and both loss of tocopherols and triacylglycerol degradation were analysed after heating at frying temperature for different periods of time. The results obtained indicated that, under the conditions used, addition of tocopherols had a protective action at high temperature, particularly when a mixture of the four