The degradation process of vegetable oil in storage and usage involves oxidation, cis-trans isomerization and free fatty acids production. Several approaches have been developed in order to measure the oxidative stability of oils, inducing the degradation by conventional heating, microwave¹ and ultrasound energy². The commonest methods for establishing the oxidative stability of oils are the active oxygen method (AOM) and the Rancimat method, based upon conventional heating. However, these methods are time consuming and afford only information on a specific change in the oil. Aging oils by ultrasound energy has been proved more effective and reduces over 100-fold experimental time³.

On the other hand, Fourier Transform Infrared Spectrometry (FTIR) affords a lot of structural information in order to evaluate the degree of unsaturation of edible oils and assesses the quality of the final product. Moreover, specific spectral changes are shown in aged oils, which can by unambiguously due to compositional changes as cis-trans isomerization, hydroperoxides and carbonyl compound formation^3,4. Furthermore, Attenuated Total Reflectance (ATR) avoids the use of organic solvent, and then reduces waste production.

In this work, the changes in the (ATR-FTIR) spectra of eighteen edible vegetable oils during oxidation were measured to explore if the oxidation process (induced by ultrasound energy) follows the same pattern in the different types of oil under study. The aim of this study was to monitor the oxidation process in the different samples and to characterize the spectral changes occurring lipid oxidation processes. Thus, it highlights the capability of Fourier Transform Infrared Spectrometry (FTIR) to provide information about the chemical processes taking place during lipid oxidation in a rapid and experimentally simple way.

Keywords: oil, ultrasound, ATR-FTIR Spectroscopy

Acknowledgements: The authors would like to thank the financial support of the Spanish Government (projects n. DPI2002-04305-C02-01, PTR1995-0581-OP-02-01 and PROFIT2004-23-122). This work has been undertaken as part of the EU sponsored COST programme (Action D32, working group D32/005/04, "Microwaves and Ultrasound Activation in Chemical Analysis"). C.D also thanks "Vicerrectorado de Relaciones Institucionales y Cooperación Internacional" of University of Alicante for her scholarship.

[1] Canizares, M.P.; Garcia, J.A.; Luque de Castro, M.D. (2004), Analytical and Bioanalytical Chemistry 378: 479-483

[2] Canizares-Macias, M.P.; Garcia-Mesa, J.A.; Luque de Castro, M.D. (2004), Analytica Chimica Acta. 502: 161-166

[3] Guillen, M.D.; Cabo, N. (1999), Journal of Agricultural and Food Chemistry 47: 709-719

[4] Guillen, M.D.; Cabo, N. (2000), Journal of the Science of Food and Agriculture 80: 2028-2036

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