6, 326 [ 19551. 52 M. van den Tempel, J. Colloid Sci. 16,284 [1961]. 53 A. J. Haighton, Fette . Seifen . Anstrichmittel 65, 479 119633. 54 A. J. Haighton, J. h e r . Oil Chemists' SOC. 42,27 [1965]. Danksagung rung der Arbeit. Eingegangen am 7. Juli 1988. Der H. Wilhelm Schaumann-Stiftung danken wir fiir die Forde-High Performance Size Exclusion Chromatography of Polar Compounds in Heated and Non-Heated Fats Fat Sci. Technol. 90. Jahrgang Nr. 8 1988

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

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.

The formation and evolution of monoepoxy fatty acids, arising from oleic and linoleic acids, were investigated in olive oil and conventional sunflower oil, representatives of monounsaturated and polyunsaturated oils, respectively, during thermoxidation at 180 degrees C for 5, 10, and 15 h. Six monoepoxy fatty acids, cis-9,10- and trans-9,10-epoxystearate, arising from oleic acid, and cis-9,10-, trans-9,10-, cis-12,13-, and trans-12,13-epoxyoleate, arising from linoleic acid, were analyzed by gas chromatography after oil derivatization to fatty acid methyl esters. Considerable amounts, ranging from 4.29 to 14.24 mg/g of oil in olive oil and from 5.10 to 9.44 mg/g of oil in sunflower oil, were found after the heating periods assayed. Results showed that the monoepoxides quantitated constituted a major group among the oxidized fatty acid monomers formed at high temperature. For similar levels of degradation, higher contents of the monoepoxides were found in olive oil than in sunflower oil. Ten used frying oils from restaurants and fried-food outlets in Spain were analyzed to determine the contents of the monoepoxides in real frying oil samples. Levels ranged from 3.37 to 14.42 mg/g of oil. Results show that, for similar degradation levels, the monoepoxides were more abundant in the monounsaturated oils than in the polyunsaturated oils.

The objective of this work was to study the evolution of oxidation in sunflower oils differing in unsaturation degree during long-term storage at room temperature. For this purpose, a combination of adsorption and size-exclusion chromatographies was used for quantification of oxidized triacylglycerol (TG) monomers, dimers, and polymers. Conventional sunflower oil, genetically modified high-oleic sunflower oil, and a 1:1 mixture of the two were used. Results showed that oxidized TG monomers were the only group of oxidation compounds increasing during the early oxidation stage, and an excellent correlation was found between amounts of oxidized TG monomers and PV during the induction period, independently of the degree of oil unsaturation. Both the rate of formation and the amount of oxidized TG monomers accumulated at the end of the induction period increased as the unsaturation degree of the oils tested was higher. The end of the induction period was marked by the initiation of polymerization and exhaustion of tocopherol. Therefore, the concomitant determination of oxidized TG monomers and polymerization compounds provided a complete picture of the oxidation process.

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The incorporation of Chia (Salvia hispanica L.) in the formulation of certain foods may be particularly desirable from a nutritional and healthy point of view. The effect of addition of chia flour on the nutritional properties and the formation of process contaminants in wheat flour-based biscuits was investigated. Higher percentage of chia flour in the formula increased the antioxidant capacity, phenolic compounds, protein, fiber and polyunsaturated fatty acids content, then resulting in a nutritionally enhanced product. However levels of process contaminants were also increased and thus acrylamide, hydroxymethylfurfural and furfural ranged between 151 and 1188 mg/kg, 22.8-71.4 mg/kg and 1.3-5.6 mg/kg, respectively, when chia was added in a range of 0-20% of the total weight. In parallel, the formation of dicarbonyl compounds, such as methylglyoxal and glyoxal, were significantly increased with addition of 5%. Lipid oxidation, particularly polymerization compounds, was accelerated in chia enriched biscuits, which decreased the shelf-life of the product by promoting a rapid rancidity under accelerated storage conditions. Therefore, although nutritional properties are improved by the incorporation of chia into the biscuits, the increase in the content of process contaminants and the extent of the lipid oxidation should be carefully considered in a context of risk/benefit.