Virgin olive oils were subjected to simulated common domestic processing, including frying, microwave heating, and boiling with water in a pressure cooker. The impact of these processes on polyphenol content and physicochemical characteristics of oils was assessed. Thermal oxidation of oils at 180 degrees C caused a significant decrease in hydroxytyrosol- and tyrosol-like substances. In contrast, oils heated for 25 h still retained a high proportion of the lignans 1-acetoxypinoresinol and pinoresinol. Thermal oxidation also resulted in a rapid degradation of alpha-tocopherol and the glyceridic fraction of oils. Microwave heating of oils for 10 min caused only minor losses in polyphenols, and the oil degradation was lower than that in thermoxidation assays. Again, lignans were the least affected polyphenols and did not change during microwave heating. Boiling a mixture of virgin olive oil and water in a pressure cooker for 30 min provoked the hydrolysis of the secoiridoid aglycons and the diffusion of the free phenolics hydroxytyrosol and tyrosol from the oil to the water phase. Losses of polyphenols were detected only at pH lower than 6. Moreover, alpha-tocopherol and the glyceridic fraction of oils were not modified during this process. It is worth noting that all the heating methods assayed resulted in more severe polyphenols losses and oil degradation for Arbequina than for Picual oil, which could be related to the lower content in polyunsaturated fatty acids of the latter olive cultivar. These findings may be relevant to the choice of cooking method and olive oil cultivar to increase the intake of olive polyphenols.

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

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In deep‐fat frying the food is completely surrounded by the frying fat or oil and different events occur within a few minutes: dehydration of food surface, absorption of fat, formation of flavour compounds, development of surface colour, etc. Due to the drastic conditions applied during deep‐frying, the frying fat also undergoes degradation. Although much work has been done on modifications of used frying fats and oils under different conditions, changes in the fried substrate have been much less studied. Particularly, there is minimal information on some physical and chemical aspects of the interactions between frying fats and fried foods. In this paper the main changes in the frying fat due to the nature of the food fried in it as well as modifications in the food as a consequence of the fat or oil used as heat transfer medium are reviewed. Fat absorption and lipid exchanges are the main physical changes involved. Chemical reactions include interaction between food constituents and oxidised lipids as well as hydrolysis of frying fats due to food moisture.

This work was aimed at studying lipid oxidation in dried microencapsulated oils (DMOs) during long-term storage. Samples were prepared by freeze-drying of emulsions containing sodium caseinate and lactose as encapsulating components. Evaluation of lipid oxidation was approached by quantitative analysis of nonvolatile lipid oxidation products and tocopherol. Lipid oxidation products were analyzed by separation of polar compounds by adsorption chromatography followed by HPSEC with refraction index detection for quantitation of oxidized triglyceride monomers, dimers, and oligomers. The analytical method applied enabled the detection of different oxidative patterns between the free and encapsulated oil fractions. The free oil fraction of DMOs showed a typical oxidative pattern for oils in continuous phase, which consisted of a clear induction period, in which hydroperoxides (oxidized triglyceride monomers) accumulated, before oxidation accelerated. The end of the induction period was marked by the total loss of tocopherol and the initiation of polymerization. On the contrary, the encapsulated oil showed a pattern characteristic of a mixture of oils with different oxidation status. Thus, high contents of advanced oxidation compounds (polymerization compounds) were detected when the antioxidant (tocopherol) was still present in high amounts. It is concluded that the encapsulated oil was comprised of oil globules with very different oxidation status. The results obtained in this study gave evidence of heterogeneous aspects of lipid oxidation in a dispersed-lipid food system.