Content of conjugated linoleic acid in neutral and polar lipid fractions of milk of different ruminant species

The aim of this research was to investigate the distribution of conjugated linoleic acid (CLA) in the neutral and polar milk lipid fractions of samples of bovine, ovine and caprine milk. Lipids were fractionated by thin-layer chromatography to obtain triglyceride, diglyceride and monoglyceride fractions. Phospholipids were separated by solid-phase extraction. The CLA content was quantitatively determined after transmethylation and addition of internal standard. As expected, 95-97% of the CLA was found in the triglyceride fraction. The main differences between the species were observed in CLA content of the diglyceride and phospholipid fraction, while the amount of CLA in the monoglycerides fraction was negligible. Cows' milk showed comparable contents of CLA in both diglyceride and phospholipid fractions (4.2 and 5.5 mg 100 g-1 of lipids, respectively). The level of CLA in phospholipids of ewes' (38 mg) and goats' (20 mg) milk lipids was four times higher than the level of CLA in diglycerides (7 and 5 mg, respectively). In terms of the phospholipid content, CLA accounted for 2-4% (calculated as a percentage of total fatty acids esterified in phospholipids) in caprine and ovine and less than 1% in bovine milk.