Comparative studies on individual isomeric 18 : 1 acids in cow, goat, and ewe milk fats by low-temperature high-resolution capillary gas-liquid chromatography

The trans- as well as the cis-18:1 isomer profiles were established in cow, goat, and ewe cheese fats, with the assumption that these are representati ve of the corresponding milks. Argentation thin-layer chromatography was co mbined with low-temperature high-resolution gas-liquid chromatography on 10 0-m highly polar capillary columns, thus adding precision to earlier data f or these species. Despite differences in the absolute content of trans-18:1 isomers between species, the relative profiles were essentially similar. E xcept for the minor trans Delta6-Delta8 group, all trans-18:1 isomers with their ethylenic bonds between positions Delta4 and Delta 16 (including the resolved critical pair Delta 13/Delta 14) were separated and quantitated in dividually. As expected, vaccenic (trans Delta9-18:1) acid was the main iso mer, accounting for as much as 37 to 50% of the total fraction. It was obse rved that the goat trans-18:1 isomer profile was usually rather close to th at of cows in winter (barn feeding), whereas that of the ewe shows a season al dependence. The trans-18:1 profile of ewe milk fats from this study rese mbles that of cows in the transition period between winter and summer (past ure) feeding. Regarding the cis-18:1 acid fraction, two isomers (oleic and cis-vaccenic acids) accounted for ca. 97% of that fraction for the three sp ecies, with the cis-Delta 12 isomer ranked third. The analytical procedure employed here appears a convenient alternative to oxidative-based procedure s (generally ozonolysis), taking less time and alleviating some drawbacks o f the latter procedure.