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Trans-octadecenoic acid composition of goat milk fat was studied by using silver-ion thin layer chromatography combined with gas-liquid chromatography. This analytical procedure also was used to investigate the effect of diet on trans-C18:1 fatty acid content in goat milk. Thirty-two goats were used in a 2 x 2 factorial arrangement and treatments. Two groups of goats received alfalfa hay at either a high or a low level of forage and two other groups received Rumiluz (dehydrated alfalfa from France Luzerne, Châlon en Champagne, France) at either a high or a low level of forage. Trans-C18:1 isomer proportions (relative to total fatty acids) were, respectively, 2.02% for the Rumiluz low-level group and 1.75% for the Rumiluz high-level group versus 1.71% for the alfalfa low-level group and 1.21% for the alfalfa high-level group. Goats fed on Rumiluz thus produced relatively higher levels of trans-C18:1 fatty acids than animals fed alfalfa hay. The results also showed that production of trans-C18:1 fatty acids increased when the level of forage in the diet decreased. Moreover, goat milk trans-C18:1 composition appeared similar to the cow milk profile. Vaccenic acid, trans-11-C18:1, was the major component and represented about 36.2% of total trans-C18:1 isomers.

Adequate analytical tools are required for labelling food products for trans fatty acid (TFA) contents. The choice of the fatty acid (FA) derivatisation is a crucial step in obtaining good results, in particular with short-chain and conjugated FA. Several gas chromatographic and infrared spectrophometric methods have been developed for analysing TFA in foods, some of which have been validated as official methods after collaborative studies. Complete analysis of total FA often requires the use of complementary high-performance liquid chromatography methods. This paper reviews the analytical methods developed for identifying and quantifying TFA in foods and presents the official methods of analysis. Neither of these analytical methods enables a TFA naturally occurring in basic food to be distinguished from a TFA formed during an industrial process.

The increasing concern for consumed fat by western populations has raised the question of the level and the quality of fat intake, especially the composition of fatty acids (FA) and their impact on human health. As a consequence, consumers and nutritionists have requested updated publications on FA composition of food containing fat. In the present study, fourteen different kinds of edible oils (rapeseed, olive, hazelnut, argan, groundnut, grape seed, sesame, sunflower, walnut and organic walnut, avocado, wheat germ, and two combined oils) were analysed for FA determination using a BPX-70 60 m highly polar GC column. Oils were classified according to the classification of Dubois et al. (2007Dubois et al. ( , 2008. Monounsaturated FA (MUFA) group oils, including rapeseed, olive, hazelnut, and avocado oils, contained mainly oleic acid (OA). Groundnut and argan oils, also rich in MUFA, showed in addition high linoleic acid (LA) contents. In the polyunsaturated (PUFA) group, grape seed oil presented the highest LA content while sunflower, sesame, and wheat germ oils showed noticeable MUFA amounts in addition to high PUFA contents. Walnut oils, also rich in LA, showed the highest linolenic acid (ALA) content. The n-6/n-3 ratio of each oil was calculated. Trans-FA (TFA) was also detected and quantified. Results were compared with the data published during the past decade, and the slight discrepancies were attributed to differences in origin and variety of seed-cultivars, and in seed and oil processes.

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Most of fatty acid double bonds are in cis configuration, but some processes may isomerise double bonds from cis to trans configuration and from their naturally occurring positions in the carbon chain. Since some fatty acids with trans double bond(s) are suspected to have adverse effects, several countries have regulated the inclusion of trans fatty acid (TFA) content labelling. The French governmental authorities requested AFSSA to propose recommendations regarding food labelling. This first article summarises the discussion on a regulatory TFA definition and reviews the occurrence and origins of TFA in foods. From a chemical point of view, a TFA is any kind of fatty acid that has one or more double bond(s) in trans configuration, but restricted TFA definitions have been proposed for food labelling purposes. These definitions exclude fatty acids with conjugated double bonds on different criteria which are commented on in this paper. Taking into account the goal of TFA labelling and the potential harmful properties of some trans conjugated fatty acids, the working group proposes the chemical definition for TFA labelling purposes. The main TFA natural origin is ruminal biohydrogenation, and therefore TFA are present in dairy products and ruminant meat. TFA are also found in domestically or industrially processed foods due to hydrogenation or heat treatment effects on oils and fats. Although the TFA molecules produced by both natural and industrial processes are often quite similar, TFA distribution profiles and levels are very different from one case to another.

Abstract. This review describes analytical methods developed for the separation, identification and quantitation of trans-isomers of unsaturated fatty acids. Although these methods are applicable to a wide variety of food matrices, we have particularly emphasised the analysis of trans unsaturated fatty acids in milk fat. Various methods are described such as infrared spectroscopy, direct gas chromatography, gas chromatography in combination with either thin-layer chromatography or liquid chromatography both impregnated with silver nitrate and gas chromatography coupled with mass spectrometry. The field of application, the advantages and the drawbacks of each method are evaluated and discussed.