Branched chain fatty acids (iso- and anteiso-fatty acids) are common minor compounds of the lipids found in dairy products and fish (<1-3%) and major fatty acids of Gram-positive bacteria. Their presence in food has been associated with bacterial sources. Bacterial lipids usually consist of polar lipids and virtually no triacylglycerides while food lipids are predominantly composed of triacylglycerides. In this study, we examined the difference in the iso- or anteiso-fatty acid content and composition of neutral and polar lipids in fish and cheese samples. Neutral lipids (triacylglycerides) were separated from the polar lipids (phospholipids) by means of solid-phase extraction (SPE). Deuterium-labeled internal standards were used to verify the successful performance of the accelerated solvent extraction and particularly the SPE. The separated lipid fractions were transformed into their corresponding fatty acid methyl esters, and the concentrations of seven iso- and anteiso-fatty acids were determined by means of gas chromatography coupled to electron ionization mass spectrometry operated in the selected ion monitoring mode (GC/EI-MS-SIM). No coelutions of branched chain fatty acids with other fatty acids were obtained on the medium polar column used for quantification. The branched chain fatty acid content of 17 cheese and 7 fish samples ranged between 0.2% and 1.9% in polar lipids and between 0.1% and 1.7% in neutral lipids. The concentration of total branched chain fatty acids in fish was 2-10 times lower than that found in cheeses, and the relative distribution of iso-17:0 and iso-15:0 increased compared to their anteiso homologues. While branched chain fatty acids in polar lipids of cheese constituted only approximately 1% of the content in total lipids, their contribution in fish was significantly higher (6% to >30%).

Anteiso fatty acids (aFA) are substituted with a methyl group on the antepenultimate carbon of the straight acyl chain. This feature leads to a stereogenic center. The 12-methyltetradecanoic acid (a15:0) and the 14-methylhexadecanoic acid (a17:0) are the most common aFA found in food, although they occur only in very small quantities. In this study we used gas chromatography in combination with a chiral stationary phase to determine the enantiomeric distribution of both a15:0 and a17:0 in the neutral and polar lipids of aquatic food samples and cheese. The best suited column was selected out of four custom-made combinations of heptakis(6-O-tert-butyldimethylsilyl-2,3-di-O-methyl)-beta-cyclodextrin (beta-TBDM) with different amount and polarity of an achiral polysiloxane. After separation of polar and neutral lipids of the food samples by solid phase extraction, fatty acid methyl esters were prepared and the fatty acid methyl esters were fractionated by reversed phase high performance liquid chromatography. Measurements of fractions high in aFA by enantioselective GC/MS in the selected ion monitoring mode verified the dominance of the (S)-enantiomers of a15:0 and a17:0 in both lipid fractions. However (R)-enantiomers were detectable in all samples. The relative proportion of the (R)-enantiomers was up to fivefold higher in the polar lipids than in the neutral lipids. The higher proportions in the polar lipids indicate that microorganisms might be involved in the formation of (R)-aFA.

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Monounsaturated fatty acids (MUFAs) are important constituents of dietary fats. While cis-configurated isomers belong to the classic fatty acids of food, trans-fatty acids are suspected to pose a risk to human health. In either case, a thorough assessment of both positional and geometrical isomers of MUFAs is an important task in food and life sciences. For this purpose, a method suitable for routine analysis was developed. After lipid extraction and conversion of fatty acids into corresponding fatty acid methyl esters, cis and trans isomers of MUFAs were separated on silver-ion-impregnated cartridges. Fractions containing either cis- or trans-MUFAs were determined by gas chromatography-electron ionization mass spectrometry in the selected ion monitoring (GC/EI-MS-SIM) mode using [M-32](+) as quantification ions and [M-74](+) as well as M(+) as qualifier ions. A total of 14 MUFAs were available as reference standards, but a total of 40 MUFAs (22 cis and 18 trans isomers) were identified with high selectivity in samples of cheese, goat fat, human milk, and cod liver oil. The 18:1 and 16:1 isomers contributed most to both the cis- and trans-MUFAs. Application of internal standards allowed for the quantification of MUFAs only in the food samples. The amount of trans-fatty acids was determined to be 0.9- 4.3 g/100 g, with the lowest levels in human milk fat and the highest levels in Roquefort cheese. After exclusion of oleic acid, the concentrations of trans- and cis-MUFAs were at the same level in samples from ruminants and human milk fat.

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