Food Lipids: Chemistry, Nutrition, and Biotechnology

methyl elaidate weight equivalents (g)
% trans = � 100 (4)
sample weight (g/10 mL of CS )
The major advantage of this method is its accuracy at low trans levels. However,
methyl ester derivatization and the use of carbon disulfide are still required.
More accurate results than those produced by the current official methods are
claimed for an IR procedure, which uses a partially hydrogenated vegetable oil
methyl ester mixture as the calibration standard [29]. The improved results were
attributed to the assortment of trans monoene and polyene isomers in the calibration
standard with different absorbtivities relative to that of methyl elaidate.
2. ‘‘Ratioing’’ of Single-Beam FTIR Spectra
Figure 1 indicates that the 966 cm �1 trans band is only a shoulder at low levels.
This is due to the overlap of the trans band with other broad bands in the spectrum,
which produces a highly sloped background that diminishes the accuracy of the trans
analysis. Many reports in the literature have proposed changes to the procedures
above, including minor refinements to major modifications aimed at overcoming
some of the limitations already discussed. These studies have resulted in the development
of procedures that use spectral subtraction to increase accuracy, as well as
means of analyzing neat samples to eliminate the use of solvents.
Mossoba et al. [30] recently described a rapid IR method that uses a Fourier
transform infrared spectrometer equipped with an attenuated total reflection cell for
quantitating trans levels in neat fats and oils. This procedure measured the 966 cm �1
trans band as a symmetric feature on a horizontal background. The ATR cell was
incorporated into the design to eliminate one potential source of error: the weighing
of test portions and their quantitative dilution with the volatile CS 2 solvent. The high
bias previously found for triacylglycerols has been attributed to the overlap of the
trans infrared band at 966 cm �1 with ester group absorption bands. Errors for the
determination of trans concentrations below 5% that resulted from this overlap could
result in relative standard deviation values greater than 50% [31]. The interfering
absorption bands were eliminated, and baseline-resolved trans absorption bands at
966 cm �1 were obtained by ‘‘ratioing’’ the FTIR single-beam spectrum of the oil or
fat being analyzed against the single-beam spectrum of a reference material (triolein,
a mixture of saturated and cis-unsaturated triacylglycerols or the corresponding unhydrogenated
oil). This approach was also applied to methyl esters. Ideally, the
reference material should be trans-free oil that has an otherwise similar composition
to the test sample being analyzed.
The simplified method just outlined allowed the analysis to be carried out on
neat analytes that are applied directly to the ATR crystal with little or no sample
preparation. With this method, the interference of the ester absorptions with the 966
cm �1 trans band and the uncertainty associated with the location of the baseline were
eliminated. Figure 2 shows the symmetric spectra that were obtained when different
concentrations of methyl elaidate (ME) in methyl oleate (MO) were ‘‘ratioed’’ against
methyl oleate. A horizontal baseline was observed, and the 966 cm �1 band height
and area could be readily measured. The minimum identifiable trans level was 0.2%,
and the lower limit of quantitation was 1% in hydrogenated vegetable oils.
Further refinement of this procedure by means of single-bounce, horizontal
attenuated total reflection (SB-HATR) infrared spectroscopy was recently reported
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.
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