Volatile compounds as indicators of conjugated and unconjugated linoleic acid thermal oxidation

In this research, volatiles arising from heating of conjugated (CLA) and unconjugated (LA) linoleic acid, in different chemical forms (free acids, methyl esters, and homogeneous TAGs) have been studied. A solid phase micro-extraction (SPME) coupled with GC-MS was used to analyze the volatile compounds produced during heating at 180 degrees C for different times (15, 30, 45, and 60min). Numerous types of volatiles have been identified such as aldehydes, furan fatty acids (FA), alcohols, methyl esters, and methyl-oxoacids. Results showed that aldehydes were the main volatile compounds both in CLA and LA, even if some differences were found between conjugated and unconjugated systems. In particular, heptanal and trans-2-octenal were detected mainly in CLA forms, while trans-2-heptenal in LA. Among furan FA, 2-pentyl-furan content was higher in LA than in CLA at all treatment times, while 2-hexyl-furan was detected in free fatty acid and methyl ester forms of CLA and it was never detected in LA model systems. Among alcohols, 1-octen-3-ol was found only in LA forms. Methylated forms of CLA and LA provided high contents of methyl esters, among which methyl octanoate was the most represented. Therefore heptanal, trans-2-octenal, 2-hexyl-furan could be considered as characteristic compounds produced during CLA-rich sample oxidation. Practical applications: Nowadays CLA, known for its interesting beneficial health effects both on animals and humans, is widely used as functional component in numerous foods and nutritional supplements. Besides the more common TAG form, CLA is used also as free fatty acids and methyl esters. To date, studies on CLA oxidation are scarce and the results are sometimes contradictory. For this reason, the comparison of the volatile compounds obtained from different model systems of CLA and LA could be useful to deepen knowledge on secondary oxidation products of some chemical forms of interest for pharmaceutical formulae and CLA-rich oils. In fact, it is very important to know the secondary products typical of CLA- and LA-based matrices to identify characteristic oxidation compounds of CLA and LA products.