Abstract
Olive and sunflower oils were heated at 180 °C for 5, 10 and 15 hours with the aim of defining the changes in the contents of fatty acid methyl esters (FAME) and triacylglycerols (TAG) under frying conditions. Differences between apparent and real loss of FAME or TAG are defined for an adequate interpretation of the changes taking place at high temperature. Such differences depend on the expression of the results and frequently result in erroneous conclusions. Results showed that from the normalized composition it could be deduced that only the most unsaturated FAME (C18:2) or TAG containing it was significantly altered. However, quantitative data indicated that all the unsaturated FAME were degraded in samples of olive oil and sunflower oil whose levels of alteration ranged from 14.6 to 35.4% polar compounds. With regard to TAG, their loss depended on both the number of double bonds and the concentration of the linoleyl group, which is the most unsaturated fatty acyl group in both oils.
Highlights
A large number of papers dealing with comparative performance of different oils and fats at frying temperatures are published every year
Polar fatty acid methyl esters (FAME) is a measure of the altered fatty acids and is considered a good measurement of thermoxidative degradation
The ratio between total polar FAME and polar compounds provides a measurement of the average of polar fatty acids present in polar TAG
Summary
A large number of papers dealing with comparative performance of different oils and fats at frying temperatures are published every year. The deficient interpretation is due to the form of expression of results, being the normalized composition one of the most used. This gives information on the percentages of each FAME or TAG on the whole analyzed. In fresh refined fats and oils, the normalized composition for FAME and TAG coincides in practice with the quantitative composition, as their contents are close to the total oil sample. When oils and fats are heated at frying temperatures a significant percentage of the fatty acyl groups of the TAG may alter to form oxidized TAG, dimeric and oligomeric TAG. The relative FAME composition given is that of the non-altered FAME fraction, which is the only one eluted in the GC analysis (Dobarganes and Pérez Camino, 1988; Dobarganes and Márquez-Ruiz, 2007)
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