Food Lipids: Chemistry, Nutrition, and Biotechnology

where
A = abc (1)
1
A = absorbance = log
transmittance
a = absorptivity
b = path length
c = trans concentration
Disadvantages of using this method include (1) the need to make methyl ester
derivatives at trans levels less than 15%, (2) the use of the toxic, volatile solvent
carbon disulfide, (3) the high bias found for triacylglycerols, and (4) low accuracy
obtained for trans levels less than 5%. Therefore, there has been a great deal of
interest in improving methods to determine trans fatty acid concentrations.
A study was conducted that showed that the AOCS method produced trans
values that were 2–3% too high, while derivatized methyl esters produced values
that were 1.5–3% too low [19]. In 1965 correction factors were suggested that were
incorporated into AOAC method 965.35 [20]. The percentage of trans was calculated
with a procedure similar to AOCS method Cd 14-61. Several formulas using correction
factors were proposed to calculate trans concentration as methyl esters or
triacylglycerols. Different formulas were also used for oils containing long chain or
short/medium chain fatty acids.
In a 1969 study [21], the concentration of trans fatty acids was determined
based on the ratio of the IR absorptions at 965 and 1170 cm �1 . In 1971 Firestone
and Huang [22] proposed a dual-beam differential spectrophotometry procedure with
a zero trans-containing vegetable oil in the reference cell. This procedure, which
resulted in a 1–2% high bias for both triacylglycerols and methyl esters, was the
first successful attempt to eliminate the sloping background of the trans infrared
band.
In 1982 a two-component calibration curve was proposed [23] in an attempt
to overcome some of the drawbacks of the earlier procedures. A calibration curve
was developed by means of different levels of the trans monoene, methyl elaidate,
and methyl linoleate dissolved in carbon disulfide. The calibration standards and the
test samples in carbon disulfide were scanned against a carbon disulfide background
and recorded from 900 to 1500 cm �1 . After a baseline had been drawn as a tangent
from about 935 and 1020 cm �1 at the peak minima, the corrected absorbance of the
calibration standards trans peaks, at 966 cm �1 , was obtained. The baselines for the
test samples spectra were obtained by overlaying the spectra of the calibration standards
spectra at corresponding concentrations. This method compensates for the low
bias of other methods and eliminates the need for correction factors.
An absorption band–height ratioing procedure was used with an attenuated
total reflection (ATR) cell, which allowed the use of neat samples and eliminated
the need to rely on volatile toxic solvents [24]. The introduction of Fourier transform
infrared spectroscopy (FTIR) instruments also facilitated more accurate and rapid
determination of trans fatty acids. A laser light source was used for wavelength
accuracy. Interferometers were used that allowed all wavelengths of light to be measured
simultaneously. Since FTIR spectrometers are computerized, multiple spectral
scans could be averaged in a few seconds. An FTIR spectrometer equipped with a
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.