Analytical evaluation of polyunsaturated fatty acids degradation during thermal oxidation of edible oils by Fourier transform infrared spectroscopy

The oxidative deterioration of polyunsaturated fatty acids (PUFAs) in culin ary oils and fats during episodes of heating associated with normal usage ( 80-300 degrees C, 20-40 min) has been monitored by Fourier transform infrar ed spectroscopy (FTIR). The thermal oxidation of PUFAs is a free radical ch ain reaction, in which hydroperoxides are generally recognized as the prima ry major products. Hydroperoxides of PUFAs are easily decomposed into a ver y complex mixture of secondary products with the decrease in unsaturation. The oxidative advance of PUFAs during heating was studied by the determinat ion of unsaturation percentage at different temperatures and heating times. oils frequently used in food frying such as olive oil, sunflower oil, corn oil and seeds oil (sunflower, safflower and canola seed) were studied. The results show there is a decrease in unsaturation starting at 150 degrees C and becoming more pronounced at temperatures around 250 degrees C. The fol lowing variations were found in the unsaturation percentage, expressed as m ethyl linoleate, between the original sample and the sample heated at 300 d egrees C for 40 min: olive oil (19-6%), sunflower oil (29-12%), corn oil (2 8-18%) and seeds oil (23-11%). This variation in unsaturation grade provide s evidence of the transformation of essential PUFAs and subsequent decrease in the oils' nutritional value. The internal standard method is suitably p recise when the n-valeronitrile is used as standard as shown by the 1-2% re lative standard deviation (R.S.D.) found for seven replicates. (C) 1999 Els evier Science B.V. All rights reserved.