IDENTIFICATION OF CONJUGATED LINOLEIC-ACID ISOMERS IN CHEESE BY GAS-CHROMATOGRAPHY, SILVER ION HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY AND MASS-SPECTRAL RECONSTRUCTED ION PROFILES - COMPARISON OF CHROMATOGRAPHIC ELUTION SEQUENCES

Commercial cheese products were analyzed for their composition and con tent of conjugated linoleic acid (CLA) isomers. The total lipids were extracted from cheese using petroleum ether/diethyl ether and methylat ed using NaOCH3. The fatty acid methyl esters (FAME) were separated by gas chromatography (GC), using a 100-m polar capillary column, into n ine minor peaks besides that of the major rumenic acid, 9c,11 t-octade cadienoic acid (18:2), and were attributed to 19 CLA isomers. By using silver ion-high performance liquid chromatography (Ag+-HPLC), CLA iso mers were resolved into seven trans,trans (5-9%), three cis/trans (10- 13%), and five cis,cis (<1%) peaks, totaling 15, in addition to that o f the 9c,11 t-18:2 (78-84%). The FAME of total cheese lipids were frac tionated by semipreparative Ag+-HPLC and converted to their 4,4-dimeth yl-oxazoline derivatives after hydrolysis to free fatty acids. The geo metrical configuration of the CLA isomers was confirmed by CC-direct d eposition-fourier transform infrared, and their double bond positions were established by CC-electron ionization mass spectrometry. Reconstr ucted mass spectral ion profiles of the m + 2 allylic ion and the m 3 ion (where m is the position of the second double bond in the parent conjugated fatty acid) were used to identify the minor CLA isomers in cheese. Cheese contained 7t,9c-18:2 and the previously unreported 11t , 13 c- 18:2 and 12 c, 14t-18:2, and their trans,trans and cis,cis geo metric isomers. Minor amounts of 8, 10-, and 10, 12-18:2 were also fou nd. The predicted elution orders of the different CLA isomers on long polar capillary GC and Ag+-HPLC columns are also presented.