Wolff, RL; Bayard, CC; Fabien, RJ
The successive steps of an integrated analytical procedure
aimed at the accurate determination of butterfat fatty acid composition, including trans-18:1
acid content and profile, have been carefully checked. This sequential procedure includes:
dispersion of a portion of butter in hexane/isopropanol (2:1, vol/vol) with anhydrous Na2SO4,
filtration of aliquots of the suspension through a microfiltration unit, subsequent preparation
of fatty acid isopropyl esters (FAIPE) with H2SO4 as a catalyst, and analysis of total FAIPE by
capillary gas-liquid chromatography (GLC). Isolation of trans-18:1 isomers was by silver-ion
thin-layer chromatography (Ag-TLC), followed by extraction from the gel of combined saturated and
trans-monoenoic acids with a biphasic solvent system. Analysis of these fractions by GLC allows
the accurate quantitation of trans-18:7 acids with saturated acids (14:0, 16:0, and 18:0) as
internal standards. A partial insight in the distribution of trans-18:1 isomers can be obtained
by GLC on a CP Sil 88 capillary column (Chrompack, Middelburg, The Netherlands). All steps of the
procedure are quite reproducible, part of the coefficients of variation (generally less than 3%,
mainly limited to butyric and stearic acids) being associated with GLC analysis (injection,
integration of peaks) and, to a lesser extent, to FAIPE preparation. FAIPE appear to be of
greater practical interest than any other fatty acid esters, including fatty acid methyl esters
(FAME), for the quantitation of short-chain fatty acids, because peak area percentages,
calculated by the integrator coupled to the flame-ionization detector, are almost equal
(theoretically and experimentally) to fatty acid weight percentages and do not require correction
factors. With this set of procedures, we have followed in detail the seasonal variations of fatty
acids in butterfat, with sixty commercial samples of French butter collected at five different
periods of the year. Important variations occur around mid-April, when cows are shifted from
forage and concentrates during winters spent in their stalls to fresh grass in pastures. At this
period, there is a decrease of 4:0-14:0 acids and of 16:0 (-2 and -6%, respectively), while 18:0
and cis- plus trans-18:1 acids rise suddenly (2 and 5%, respectively). These modifications then
progressively disappear until late fall or early winter. Other variations are of minor
quantitative importance. Although influenced by the season, the content of 18:2n-6 acid lies in
the narrow range of 1.2-1.5%. Trans-18:1 acids, quantitated by GLC after Ag-TLC fractionation,
are at their highest level in May-June (4.3% of total fatty acids), and at their lowest level
between January and the end of March (2.4%), with a mean annual value of 3.3%. The proportion of
vaccenic (trans-11 18:1) acid, relative to total trans-18:1 isomers, is higher in spring than in
winter, with intermediate decreasing values in summer and fall, which supports the hypothesis
that the level of this isomer is linked to the feed of the cattle, and probably to the amount of
grass in the feed.