Study of individual trans- and cis-16 : 1 isomers in cow, goat, and ewe cheese fats by gas-liquid chromatography with emphasis on the trans-Delta 3 isomer

Low-temperature gas-liquid chromatography (GLC) was applied to study the di stribution profiles of isomeric trans- and cis-hexadecenoic acids in rumina nt (cow, goat, and ewe) milk fat after their fractionation by argentation t hin-layer chromatography (Ag-TLC. The fat was extracted from cheeses (12 sa mples of each species), the most common foods made with goat and ewe milks. The predominant trans-16:1 isomer is palmitelaidic acid (the Delta 9 isome r), but it does not exceed one-third of the total group, which itself repre sents 0.17% (cow), 0.16% (goat), and 0.26% (ewe) of the total fatty acids. The trans-Delta 3 16:1 isomer, which is reported for the first time in rumi nant lipids and which likely comes from the animals' feed, is present at a level of ca. 10% of the trans-16:1 acid group. Otherwise, all isomers with their ethylenic bond between positions Delta 4 and Delta 14 are observed in the three species studied, roughly showing the same relative distribution pattern. Quantitatively, the trans-16:1 isomers only represent ca. 5% of th e sum of the trans-16:1 plus trans-18:1 isomers, and they appear of little importance in comparison. It is inferred from this and recent studies that some previously reported data that were established for consumption assessm ents dealt in fact mainly with iso-17:0 acid, which was confused with land added to) trans-Delta 9 (palmitelaidic) acid; consequently, these results w ere large overestimates. Regarding the cis-16:1 acids, the Delta 9 isomer i s the prominent constituent as expected, but the second-most important isom er is the Delta 13 isomer. It does not appear that trans-16:1 isomers are f rom ruminant milk fats of great nutritional importance as compared with tra ns-18:1 isomeric acids. As for trans18:1 isomers, the combination Ag-TLC/GL C is a necessary procedure to quantitate trans-16:1 acids accurately and re liably. Ag-TLC allows removal of interfering branched 17:0 acids and cis-16 :1 acids, and low-temperature GLC permits an accurate measurement of all in dividual isomers most of which with baseline resolution.