Direct GC analysis of the fatty acid compositions of conjugated linoleic acid and its L-menthyl esters.

Commercially available conjugated linoleic acid (CLA) is a mixture of two main isomers. Fractionation of the two isomers was performed by a lipase-catalyzed esterification of CLA with L-menthol. In this study, a GC analytical method was developed to simultaneously determine the degree of esterification and fatty acid (FA) compositions of CLA in the free fatty acid (FFA) and ester forms without separation of the FFA and the ester. The methylation of the oil phase of the reaction mixture was performed using trimethylsilyldiazomethane in a mixture of toluene/methanol. Only FFA was quantitatively methylated, whereas the other compounds were little changed. A GC analysis using a polar column was performed to simultaneously determine the degree of esterification and the fatty acid compositions.