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Food Lipids: Chemistry, Nutrition, and Biotechnology

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B. Gas Chromatography/IR Spectroscopy<br />

GC provides useful information concerning the total fatty acid composition of hydrogenated<br />

oils. The overlap of some of the trans with cis monoene peaks makes it<br />

difficult to get accurate GC determinations of total cis <strong>and</strong> total trans monoene<br />

content of food products. Accurate determination of these isomers is essential because<br />

the latest food labeling regulations (<strong>Nutrition</strong> Labeling <strong>and</strong> Education Act<br />

(NLEA) [96]) permit cis monounsaturates to be listed on a food label. A combined<br />

GC <strong>and</strong> IR method to determine cis <strong>and</strong> trans fatty acid monoenes that coelute on<br />

GC columns was developed [68]. This method was adopted as an AOAC official<br />

method [97] upon completion of a collaborative study [57]. In this method, the total<br />

trans content of the oil was first determined by modifying a published IR procedure<br />

[23] that used a two-component calibration plot. Then the weight percentages of all<br />

the trans diene (18:2t <strong>and</strong> 18:2tt) <strong>and</strong> triene (18:3t) isomers were determined by<br />

means of a highly polar 50- or 100-m capillary GC column. The weight percentage<br />

of trans monoenes (18:1t) was then determined by the following formula with the<br />

appropriate correction factors (0.84 <strong>and</strong> 1.74):<br />

IR trans = %18:1t � (0.84 � %18:2t) � (1.74 � %18:2tt)<br />

� (0.84 � %18:3t) (5)<br />

After the trans monoenes had been calculated, the cis monoenes were determined<br />

by finding the difference between the total monoenes determined by GC <strong>and</strong> the<br />

trans monoenes that were calculated:<br />

%18:1c = (total %18:1t by GC � 18:1c by GC) � (%18:1t by calculation) (6)<br />

C. Silver Ion Chromatography/NMR<br />

As already discussed, 13 C NMR spectroscopy can be an effective tool for the identification<br />

of fatty acid isomers. This tool is most useful if purified fatty acid fractions<br />

are first obtained. Silver ion chromatography on both TLC plates <strong>and</strong> HPLC columns<br />

have been used to obtain purified fractions for 13 C NMR analysis. Hydrogenated<br />

soybean oil was separated into six b<strong>and</strong>s with preparative silver ion TLC [48]. The<br />

isomers in each b<strong>and</strong> were identified by capillary GC <strong>and</strong> 13 C NMR analysis. The<br />

presence of specific trans diene isomers was confirmed by observing their unique<br />

chemical shifts. Recently, silver ion HPLC was used to obtain a purified trans monoene<br />

fraction from hydrogenated soybean oil [98]. 13 C NMR spectroscopic analysis<br />

of this fraction confirmed the presence of the minor �6 <strong>and</strong> �7 trans monoene<br />

positional isomers. The presence of the �5 trans monoene positional isomer was<br />

inferred. This HPLC fraction had been analyzed by capillary GC (see Sec. V.B), but<br />

several minor isomers could not be identified.<br />

V. ANALYSIS USING HYPHENATED TECHNIQUES<br />

The lack of st<strong>and</strong>ards for many fatty acids <strong>and</strong> their isomers <strong>and</strong> the problem of<br />

coelution can lead to the misidentification of fatty acids in gas chromatograms. Recent<br />

publications misidentified trans monoene positional isomers [90,98]. Another<br />

study determined the level of trans,trans 18:2 isomers in margarine to be an order<br />

of magnitude too high (3% instead of 0.3%) also because of GC peak misidentifi-<br />

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

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