A critical evaluation of the analytical techniques in the photodegradation monitoring of edible oils

The performance of the widely used analytical techniques in monitoring the changes from light on edible oils was compared. Flax, soybean, peanut, sunflower, olive and corn oils were subdued to forced irradiation and analyzed by IR spectroscopy, UV-visible spectroscopy and HPLC. The stressing test was executed in agreement with international rules and the samples were analyzed at time intervals up to 540 min. The results recorded by the spectrometric techniques were completely different. All IR spectra recorded along the test were superimposable, indicating full stability of all oils even after multivariate elaboration by principal component analysis. In contrast, UV spectra showed a constant variation, stating a clear photolability. HPLC-DAD determination of the main fatty acids confirmed the photodegradation of the samples, showing a clear decrease in their concentration. This study provides information on the photostability of various edible oils, which could be useful for their proper storage. Moreover, the results clearly indicate that the IR spectrometry is not appropriate to evaluate the photostability of the oils. On the contrary, UV spectrometry is able to provide a rapid evaluation of the degradation degree and HPLC an effective assessment on the degradation of some important components such as the unsaturated fatty acids.