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Food Lipids: Chemistry, Nutrition, and Biotechnology

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to 18. Octadecadienoic acids have been reported to contain conjugated double bonds<br />

at positions 7,9; 8,10; 9,11; 10,12; 11,13; <strong>and</strong> 12,14 (counting from the carboxyl end<br />

of the molecule) in chemically prepared CLA mixtures or natural products (10–13).<br />

The positional conjugated diene isomers can occur in one or more of the following<br />

four geometric configurations: cis,trans; trans,cis; cis,cis; or trans,trans; which would<br />

give 24 possible isomers of CLA (10). Many of these isomers were found in commercially<br />

available preparations of CLA produced under alkaline conditions from<br />

vegetable oils containing a high concentration of LA (14).<br />

The most common CLA isomer found in meat from ruminant species <strong>and</strong> bovine<br />

dairy food products is octadeca-c9,t11-dienoic acid (15), even though minor components,<br />

such as the t7,c9, t8,c10, t10,c12, t11,c13, c11,t13, <strong>and</strong> t12,t14 isomers, <strong>and</strong><br />

their cis,cis, trans,trans isomers were also reported in these products (11,12). Two<br />

trivial names have been proposed for the c9,t11 isomer: bovinic acid (16) <strong>and</strong> rumenic<br />

acid (17). The name bovinic acid is considered to be too restrictive for CLA<br />

because the c9,t11 isomer is not only produced in the rumen of the bovine but also<br />

produced by other ruminant animals by the same mechanism.<br />

The CLA in ruminant meat <strong>and</strong> dairy products is believed to be formed by<br />

bacterial isomerization of LA <strong>and</strong> possibly �-linolenic acid (18:3n-3) from grains<br />

<strong>and</strong> forages to the c9,t11-18:2 in the rumen of these animals (1,2,18). CLA may also<br />

be formed during cooking <strong>and</strong> processing of foods (18). Presently CLA mixtures <strong>and</strong><br />

pure isomers are available from various commercial sources. The composition of<br />

CLA products should be carefully checked before they are used in research work<br />

since the isomeric distribution varies largely between manufacturers <strong>and</strong> even between<br />

batches made by the same manufacturer (19,20).<br />

III. ANALYSIS OF CLA IN FOOD AND BIOLOGICAL SAMPLES<br />

As research continues on the actions of CLA, it is critical to have accurate compositional<br />

analyses of food <strong>and</strong> biological samples to correctly interpret data from<br />

investigations. The analysis of CLA in food <strong>and</strong> other biological samples follows<br />

the general guidelines for the determination of fatty acid composition with particular<br />

emphasis on the derivatization methods. The conjugated double bonds in CLA are<br />

less stable than the methylene-interrupted double bonds in LA. It is reported that the<br />

stability of CLA is similar to that of arachidonic acid (20:4n-6) <strong>and</strong> docosahexaenoic<br />

acid (DHA, 22:6n-3) (21,22). Therefore, preventing isomerization <strong>and</strong> oxidation of<br />

these labile fatty acids is critical for a successful analysis.<br />

The most common analyzed form of fatty acids is the fatty acid methyl ester<br />

(FAME). Both acid <strong>and</strong> alkaline catalysts can be used to prepare FAME for gas<br />

chromatography (GC) analysis. However, since acid catalysts, such as BF3, will<br />

change the double-bond configuration in conjugated dienes <strong>and</strong> generate artifact CLA<br />

isomers (23–25), their use is not recommended in the analysis of CLA.<br />

The alkaline catalysts perform best for lipids containing fatty acids with unique<br />

conjugated diene structures. Isomerization <strong>and</strong> artifacts are not produced when sodium<br />

methoxide or tetramethylguanidine (TMG) are used as transesterification<br />

agents; however, they do not methylate free fatty acids <strong>and</strong> N-linked (amide bond)<br />

fatty acids as those found in sphingolipids. Therefore, these methods are not suitable<br />

for samples with high acid values (high free fatty acid content). There is no single<br />

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

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