Conjugated linoleic acid (CLA) isomers in heat-treated vegetable oils

The aim of this study was to quantify the conjugated linoleic acids (CLA) isomers present in frying oils and to elucidate their structures. For this purpose, used frying oils were collected in restaurants by the laboratoire Interregional de la Repression des Fraudes. Three samples of sunflower oils, one of peanut oil and one of rapeseed oil containing different levels of total polar compounds were analysed. The samples were converted to methyl esters using sodium methoxide and the CLA content was determinated by a combination of gas chromatography analysis on a 100 m CP Sil88 column and of silver-ion-high pressure liquid chromatography. The identification of the CLA isomers was carried out by gas chromatography coupled with mass spectrometry of the 4-methyl- 1,2,4-triazoline-3,5-dione adducts. The amount of polar compounds in the frying oils ranged from 30 to 53%. The quantities of CIA from 0.4 to 0.5%. The 9, 11 and 70, 12-18:2 were the major CIA isomers. Among them, the di-trans isomers represent about 50% of the total CLA.