Quantification of branched chain fatty acids in polar and neutral lipids of cheese and fish samples.

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%).