SummaryTrans fatty acids (TFA) formed during biohydrogenation by ruminant animals were quantified in cows' milk fat by means of Fourier transform i.r. (FTIR) spectroscopy. When samples were analysed as the derived methyl esters, a spectral subtraction technique resulted in TFA values that were not biased by the unspecific absorption of intact triacylglycerols or influenced by the shape of the trans band. Austrian milk fat samples contained 20–50 g TFA/kg. Results obtained by analysis of TFA as triacylglycerols compared favourably with those obtained by using methyl esters. The findings of the FTIR method were further substantiated by applying argentation chromatography in combination with capillary gas chromatography to the same set of samples. Besides trans monoenoic acids, non-methylene interrupted cis, trans octadecadienoic acids contributed significantly to the total trans content. Two novel TFA, trans-11, cis-15 octadecadienoic and cis-9, trans-13 octadecadienoic acids, were identified in milk fat by means of their retention behaviour on argentation chromatography and gas chromatography as well as by an oxidative degradation procedure.
