Oxidative modifications of conjugated and unconjugated linoleic acid during heating.

The oxidative stability of conjugated linoleic (CLA) and linoleic (LA) acids in different chemical forms (free acids, methyl esters and homogeneous triacylglycerols) was compared. All model systems were heated at 180°C for different times (15, 30, 45 and 60min). The primary oxidation products were evaluated by spectrophometric analysis, while the volatile compounds were determined by solid phase micro-extraction (SPME), coupled with gas chromatography-mass spectrometry (HRGC-MS). The isomer profile modifications were investigated by silver-ion high performance liquid chromatography (Ag(+)-HPLC) equipped with an UV detector. Generally, peroxide values decreased during the heating time. Among the volatiles, saturated aldehydes were the most represented compounds. Isomerization of cis,trans and trans,cis CLA to trans,trans isomers was observed mainly for the methyl form of CLA. The three different chemical forms of LA never showed isomerization phenomena.