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Food Lipids: Chemistry, Nutrition, and Biotechnology

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present a possible risk factor for coronary heart disease [6], some trans conjugated<br />

fatty acids in ruminant fat have been reported to have several beneficial physiological<br />

effects in experimental animals [7]. trans-Vaccenic acid (trans-11-18:1), which is the<br />

major trans fatty acid isomer present in meat <strong>and</strong> dairy products from ruminants<br />

[8,9], has been shown to be converted to cis-9,trans-11-18:2, a conjugated linoleic<br />

acid (CLA) isomer [10–12], by the action of �9 desaturase present in mammalian<br />

tissue [13,14]. The relationship of trans-18:1 fatty acid isomers <strong>and</strong> CLA isomers is<br />

an active area of research today [15].<br />

Even after several years of research, there are still several laboratories in the<br />

world that have active research programs trying to improve the methodology to<br />

determine trans fatty acids in food products. The quantitation <strong>and</strong> identification of<br />

trans fatty acid isomers is difficult because of the wide range of positional monoene,<br />

diene, <strong>and</strong> triene fatty acid isomers present in hydrogenated oils. Moreover, cis positional<br />

isomers are also present, <strong>and</strong> commercial chromatographic st<strong>and</strong>ards are<br />

lacking for many fatty acid isomers.<br />

Analytical procedures used to quantify <strong>and</strong> identify fatty acids have been reviewed<br />

[5,16,17]. This chapter discusses the newest developments in analytical methods,<br />

including infrared (IR), Raman, <strong>and</strong> nuclear magnetic resonance (NMR) spectroscopy,<br />

gas chromatography (GC), high-performance liquid chromatography<br />

(HPLC), supercritical fluid chromatography (SFC), <strong>and</strong> mass spectrometry (MS). The<br />

analysis of trans fatty acid isomers is extremely challenging <strong>and</strong> complex. Thus<br />

various combinations of techniques have effectively been used to determine the concentration<br />

<strong>and</strong> confirm the identity of individual trans <strong>and</strong> cis isomers. Improved<br />

methods are still needed to accurately <strong>and</strong> conveniently determine total trans content<br />

as well as specific fatty acid isomers in hydrogenated oils.<br />

II. ANALYSIS USING SPECTROSCOPY<br />

A. Infrared Spectroscopy<br />

Infrared spectroscopy is a widely used technique for determining nonconjugated trans<br />

unsaturation in both natural <strong>and</strong> processed fats <strong>and</strong> oils. It is not applicable to materials<br />

that have functional groups with absorption b<strong>and</strong>s close to 966 cm �1 , which<br />

is the strong absorption b<strong>and</strong> arising from the C—H deformation about a trans double<br />

bond. This absorption b<strong>and</strong> is absent in natural vegetable oils that are composed of<br />

saturated fatty acids <strong>and</strong> fatty acids with only cis-unsaturated double bonds. For<br />

increased accuracy, oil samples are usually converted to methyl esters prior to analysis.<br />

This eliminates interfering absorptions associated with the carboxyl groups of<br />

free fatty acids <strong>and</strong> the glycerol backbone of triacylglycerols.<br />

1. Conventional IR Methods<br />

The early Official Method of the American Oil Chemists’ Society (AOCS), Cd 14-<br />

61 [18] for the determination of trans fatty acid concentrations in fats <strong>and</strong> oils, was<br />

based on a comparison of absorption at 966 cm �1 of st<strong>and</strong>ards <strong>and</strong> unknowns. Samples<br />

<strong>and</strong> st<strong>and</strong>ards were diluted in carbon disulfide <strong>and</strong> placed in an absorption cell<br />

so that the transmittance or absorbance could be measured in an infrared spectrophotometer.<br />

The quantitation of the trans concentration was based on Beer’s law:<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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