title: 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.
abstract: Commercial cheese products were analyzed for their composition and content of conjugated linoleic acid (CLA) isomers. The total lipids were extracted from cheese using petroleum ether/diethyl ether and methylated using NaOCH3. The fatty acid methyl esters (FAME) were separated by gas chromatography (GC), using a 100-m polar capillary column, into nine minor peaks besides that of the major rumenic acid, 9c,11t-octadecadienoic acid (18:2), and were attributed to 19 CLA isomers. By using silver ion-high performance liquid chromatography (Ag+ -HPLC), CLA isomers 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 of the 9c,11t-18:2 (78-84%). The FAME of total cheese lipids were fractionated by semipreparative Ag+ -HPLC and converted to their 4,4-dimethyloxazoline derivatives after hydrolysis to free fatty acids. The geometrical configuration of the CLA isomers was confirmed by GC-direct deposition-Fourier transform infrared, and their double bond positions were established by GC-electron ionization mass spectrometry. Reconstructed 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,13c-18:2 and 12c,14t-18:2, and their trans,trans and cis,cis geometric isomers. Minor amounts of 8,10-, and 10,12-18:2 were also found. The predicted elution orders of the different CLA isomers on long polar capillary GC and Ag+ -HPLC columns are also presented.
authors.affiliation: U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Washington, DC 20204, USA.
authors.forename: N
authors.initials: N
authors.lastname: Sehat
authors.forename: J K
authors.initials: JK
authors.lastname: Kramer
authors.forename: M M
authors.initials: MM
authors.lastname: Mossoba
authors.forename: M P
authors.initials: MP
authors.lastname: Yurawecz
authors.forename: J A
authors.initials: JA
authors.lastname: Roach
authors.forename: K
authors.initials: K
authors.lastname: Eulitz
authors.forename: K M
authors.initials: KM
authors.lastname: Morehouse
authors.forename: Y
authors.initials: Y
authors.lastname: Ku
mesh_terms: D002611:Cheese / Q000032:analysis*; D002849:Chromatography, Gas / Q000379:methods*; D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D007536:Isomerism; D019787:Linoleic Acid / Q000032:analysis / Q000737:chemistry*; D008055:Lipids / Q000737:chemistry; D013058:Mass Spectrometry / Q000379:methods; D008745:Methylation; D012834:Silver; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D008055:Lipids; D012834:Silver; D019787:Linoleic Acid