Assignment of Milk Fat Fatty Acid Propyl Esters by GC-FID Analysis with the Aid of Ag-ion Solid-phase Extraction.

The recovery of short-chain fatty acids (FAs) in milk fat (MF) is improved when the analysis of the FA composition of MF by gas chromatography (GC) is conducted with the propyl or butyl ester derivatives, instead of the methyl esters. However, this approach complicates the detection of minor FAs, such as the minor positional isomers of 16:1, which represent <0.2% of the total content. In addition, the standards of these minor esters are not commercially available. In this study, with the aim to identify minor FAs, the fatty acid propyl esters (FAPEs) of MF were fractionated by Ag-ion solid phase extraction (Ag(+)-SPE) and analyzed by GC using a DB-23 capillary column. FAPEs were successfully fractionated mainly according to the degree of unsaturation by adjusting the elution conditions of the Ag(+)-SPE, and the minor FAPEs were easily determined without the aid of standard compounds. For example, by comparison of the GC profile of the saturated Ag(+)-SPE fraction with that of the original MF, minor FAs, such as iso-, anteiso-, and saturated FAs of 15:0 and 17:0, were expected to be eluted in this order. In addition, 16:1 propyl ester was co-eluted with iso 17:0 propyl ester under the GC conditions used in this study, as confirmed by the detection of the corresponding molecular ions (296 and 312, respectively) by GC-MS. Moreover, 9c,11t-conjugated linoleic acid was found to elute between 18:3 and 20:0. To the best of our knowledge, this is the first report suggesting that the peak observed before that of cis-12:1 corresponds to trans-12:1. In conclusion, Ag(+)-SPE fractionation of FAPEs contributed to the identification of minor FAs in MF without the use of standard compounds.