Food Lipids: Chemistry, Nutrition, and Biotechnology

levels in excess of 50% of the total fat for two of three shortenings analyzed. A
1992–1995 Danish study of margarine and shortening [155] also found a decreased
level of trans fatty acids. The most significant decrease was from 9.8% trans of the
total fat in 1992 compared with 1.2% in 1995 for margarines with a linoleic acid
content of 20–40%. There was a 40–90% decrease in trans levels when the 1995
results were compared to an earlier (1985) Danish survey [156].
VII. CONCLUSIONS
As long as the nutritional effects of trans fatty acids are debated, there will be an
important need to accurately determine the total trans content of food products by
simple and rapid methods. As reviewed in this chapter, the standard infrared methods
have been recently revised to increase their accuracy and reliability in the measurement
of low levels of trans in food products. Other studies are now in progress to
directly determine the trans content of neat fats and oils by means of an infrared
single-beam ratioing procedure, with attenuated total reflection cells to increase accuracy
and analysis speed.
Also, the nutritional effects of most positional and/or geometric isomers of fatty
acid have not been extensively studied. The identification and quantitation of individual
fatty acid isomers will be a critical step in any future study designed to determine the
nutritional properties of various cis and trans geometric and positional isomers with
one or more double bonds. The complex isomer mixture inevitably contains overlapping
cis and trans GC peaks that make identification difficult. GC on long, highly
polar columns will continue to be an important method for separating fatty acid isomers.
However, as discussed in this chapter, GC alone cannot separate all fatty acid
isomers in hydrogenated oils. Therefore, combined and/or hyphenated techniques are
necessary to separate and identify individual fatty acid isomers. The recent introduction
of commercial silver ion HPLC columns should increase the use of these columns,
which can be combined with other techniques, such as reversed phase HPLC, SFC,
GC, NMR, FTIR, MS, and MS-MS. An increased use of MS-MS in lipid analysis is
expected. The introduction of the more sensitive GC-DD-FTIR system and the use of
DMOX derivatives, which enhance GC resolution while stabilizing the charge during
electron ionization of fatty acids, are other developments that should allow accurate
confirmation of individual fatty acid geometric and positional isomers.
Analyzing the total trans content and the individual isomers in complex
mixtures in both natural and hydrogenated oils is challenging. With the introduction
of some of the new methods discussed in this chapter, there is now an opportunity
to meet this challenge. Work on improving these methods is continuing. Although
an in-depth discussion of each method is beyond the scope of this chapter, we have
attempted to indicate some of the difficulties of analyzing trans fatty acid isomers
while referring the reader to some exciting new developments that are now available
to lipid scientists. The references provided will help the serious analytical chemist
find an in-depth discussion of these methods.
REFERENCES
1. R. P. Mensink and M. B. Katan. Effect of dietary trans fatty acids on high-density and
low-density lipoprotein cholesterol levels in healthy subjects. N. Engl. J. Med. 323:439
(1990).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.