Thermally Induced Isomerisation Kinetics of the 9c11t and 10t12c Conjugated Linoleic Acids in Triacylglycerols as Studied by FT-IR Spectrometry Aided by Gas Chromatography

Abstract

Isomerisation kinetics of the 9c11t and 10t12c Conjugated Linoleic Acids (CLA) in triaclglycerols at isothermal conditions (250, 280 and 325oC) has been studied by infrared spectroscopy. Fifteen micro liter portions of the glycerides were placed in micro glass ampoules and sealed under nitrogen. Several glass tubes containing the same triacylglycerols were then subjected to thermal treatment. The glass tubes were removed at regular time intervals, cut open, and a part of the contents in each glass tube analysed by infrared spectroscopy using a single reflectance attenuated total internal reflectance crystal accessory. Then the rest of the products in each of the glass tubes were analysed by gas chromatography after derivatisation. The gas chromatographic results show that the 9t11c and 10t12c CLA isomers undergo a quick [1,5] sigmatropic transformations and form 8c10t and 11c13t respectively. These then form more stable 8t10t, 9t11t and 10t12t, 11t13t isomers respectively. Formation of the other CLA isomers is in relatively small quantities for shorter thermal induction times. The chromatographic profiles also show that the thermal induction reaction can be considered as a reaction starting from the pair of isomers formed through the [1.5] sigmatropic rearrangements. The intensity of one of the characteristic absorptions at 944 cm-1 arising from the out-of-plane =C-H deformation in the CLAs was used as the quantitative measure of the remaining triaclglycerol in the heated samples. The activation energies determined for the isomerisation reactions are 25.500.30 and 25.250.70 kcal/mol for the triacylglycerols containing 9c11t (8t10c) and 10t12c (11c13t) fatty acid isomers respectively.