Short communication Content of conjugated linoleic acid in neutral and polar lipid fractions of milk of different ruminant species Giovanna Contarini * , Valeria Pelizzola, Milena Povolo Centro di Ricerca per le Produzioni Foraggere e Lattiero-Casearie (CRA – FLC), Via A. Lombardo 11, 26900 Lodi, Italy article info Article history: Received 2 April 2008 Received in revised form 9 September 2008 Accepted 1 October 2008 abstract 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 frac- tionated 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 frac- tions (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. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Conjugated linoleic acid (CLA) is produced by both the microbial bio-hydrogenation of linoleic and linolenic acids in the rumen, and the desaturation of vaccenic acid in mammalian tissues. As a consequence, milk of ruminants and the corresponding dairy products are good sources of CLA isomers (Ledoux and Laloux, 2006) and particularly of the isomer rumenic acid (cis-9, trans-11), which is the most prevalent one (Kay et al., 2004). Variation in milk CLA content is linked to endogenous (animal breed, genotype, lactation and pregnancy stages) or exogenous (feeding and envi- ronmental) factors. Diet is the most important influence on milk CLA content, which can be naturally enhanced by fresh pasture feeding (Bauman et al., 2003), or through the use of specific dietary formulations including oil seeds or fish oil (Collomb et al., 2006). Among the different species of ruminants, ewes’ milk shows the highest CLA content (1.17% in ovine milk fat and 0.70, 0.64% in bovine and caprine milk fat, respectively), even when the three ruminant species are fed similar forages (Banni et al., 1996; Jahreis et al., 1999). To our knowledge, all the studies conducted on milk and dairy products determined the CLA content by analysing the whole lipid matrix. Therefore, the aim of this research was to investigate the content of CLA in neutral and polar milk lipid fractions. For this purpose, different chromatographic techniques were applied to samples of lipids from bovine, ovine and caprine milk. 2. Materials and methods Bulk milk of the three different species was sampled three times in one month (May) from processing plants of local dairy compa- nies. Holstein cows’ milk was sampled from bulk tanks where the production of about 140 farms adopting the Unifeed system was daily collected. Ovine and caprine milk was sampled from bulk tanks of a company producing both Pecorino and Caprino cheeses, daily collecting milk from 92 farms breeding Sarda ewes, and 12 farms breeding Camosciata and Saanen goats, under semi-exten- sive systems. In order to avoid the occurrence of any lipolytic processes, the milk was analysed on the day of sampling. The lipid fraction was extracted with chloroform–methanol (2:1 v/v) according to the method of Folch et al. (1957) and divided into three aliquots. To evaluate the CLA content of the whole lipid fraction, the first aliquot (100 mg) was dissolved in 5 mL of a solution of methyl C22:0 in hexane (5 mg mL 1 ) and transmethylated by applying the procedure described in ISO-IDF (2002). To evaluate the CLA content of the glyceride fractions, thin-layer chromatography (TLC) on silica-gel precoated plates (Merck, Darmstadt, Germany) was applied to the second aliquot. According to the method of Mariani et al.(1990), 1 mL of the internal standard solution, containing mono-docosanoin (0.04 mg mL 1 ), di-docosanoin (0.13 mg mL 1 ) * Corresponding author. Tel.: þ39 37145011; fax: þ39 37135579. E-mail address: giovanna.contarini@entecra.it (G. Contarini). Contents lists available at ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj 0958-6946/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2008.10.013 International Dairy Journal 19 (2009) 342–344