Food Lipids: Chemistry, Nutrition, and Biotechnology

thin (0.1-mm) transmission flow cell was used to develop an automated procedure
for calculating the percentage of trans in fats and oils [25].
Postmeasurement spectral subtraction manipulations were used to correct for
the highly sloping background in the FTIR absorption spectrum of hydrogenated
vegetable oil fatty acid methyl esters [26]. Determination of trans content, however,
required additional IR measurement of an appropriate reference material and the
digital subtraction of this reference absorption spectrum from that of the test portion.
FTIR spectroscopy, in conjunction with a transmission flow cell, was used to rapidly
determine the cis and trans content of hydrogenated oil simultaneously [27].
The 1995 official method of the AOCS for determination of the trans fatty acid
Cd 14-95 [28] utilizes two standard curves [23]. The choice of curve depends on the
trans concentration. This method is reported to be accurate to determine the trans
content of fats with trans levels of 0.5% or greater. Test samples and standards are
converted to methyl esters, diluted in 10 mL of carbon disulfide, and then placed in
a transmission IR cell before measuring the transmittance or absorbance over the
range 1050–900 cm �1 in an infrared spectrophotometer. A typical infrared absorption
spectra for an oil containing both 2% and 70% trans is shown in Figure 1. The
baseline-corrected absorbance (A C) is determined by subtracting the absorbance of
the baseline at the peak maximum (A B) from the maximum absorbance at the peak
(A p). Two plots are then constructed by using standards to cover samples with low
levels of trans (1–10%) and moderate to high trans levels (10–70%), and two regression
equations are generated. The percentage of trans as methyl elaidate of the
unknown is then obtained by referring to the appropriate calibration data (�10% or
>10%) and solving the following equations:
A =(A � A ) (2)
C p B
A � intercept
C
Methyl elaidate weight equivalents (g) = (3)
slope
Figure 1 Infrared absorption spectra of fatty acid methyl esters containing 2% and 70%
trans. (From Ref. 28.)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.