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Food Lipids: Chemistry, Nutrition, and Biotechnology

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method that works optimally in all situations. The researcher needs to know the<br />

nature of the sample <strong>and</strong> select a suitable method accordingly.<br />

Various instruments are adopted to separate <strong>and</strong> characterize the CLA isomers.<br />

A gas chromatograph equipped with a polar 30- to 100-m capillary column <strong>and</strong> a<br />

flame ionization detector is the most widely used instrument to characterize CLA<br />

isomers. Generally, GC columns have a limited capacity to fully separate all geometric<br />

<strong>and</strong> positional isomers of CLA; therefore, it is impossible to identify every<br />

individual CLA isomer by conventional capillary GC analysis. Mossoba et al. (26)<br />

reported that 10 CLA peaks were resolved in a commercial CLA preparation with a<br />

gas chromatograph equipped with a 100-m column (SP 2560, Supelco Inc., Bellefonte,<br />

PA or CP-Sil 88, Chrompack, Bridgewater, NJ). In the aforementioned procedure<br />

the 10 peaks resolved may actually represent more than 15 CLA isomers. A<br />

more powerful approach has been the application of argentation (silver ion) highperformance<br />

liquid chromatography (Ag � -HPLC) using a UV detector at a wavelength<br />

of 233 nm. Sehat et al. (10) described such a method that resolves CLA<br />

isomers according to geometric configuration <strong>and</strong> position of the conjugated diene<br />

structure. Moreover, Mossoba et al. (27) reported that 16 isomers of a CLA sample<br />

were resolved by an Ag � -HPLC method. Although the Ag � -HPLC method is more<br />

powerful than GC in resolving CLA isomers, it cannot be used to quantify other<br />

fatty acids due to the limitation of UV detectors <strong>and</strong> HPLC columns. Therefore,<br />

ideally the two methods should be combined for CLA analysis, using GC for the<br />

general fatty acid analysis <strong>and</strong> Ag � -HPLC for the conjugated dienes. The results of<br />

these two procedures would provide a more complete profile of CLA isomers in food<br />

<strong>and</strong> biological samples.<br />

IV. CLA CONTENT IN FOOD PRODUCTS AND<br />

BIOLOGICAL SAMPLES<br />

Cheese is a chief source of dietary CLA in animal-derived products. The CLA concentrations<br />

in various dairy products (cheeses, milk, butter, buttermilk, sour cream,<br />

ice cream, <strong>and</strong> yogurt) range from 0.55 to 24 mg/g fat. The average CLA content in<br />

milk is about 10 mg/g milk fat (4,28). The largest variation in the amount of CLA<br />

is found in various natural cheeses ranging from 0.55 to 24 mg/g of fat. Seven CLA<br />

peaks that could represent nine isomers were present in dairy products; among these<br />

c9,t11, t10,c12, t9,t11, <strong>and</strong> t10,t12 accounted for more than 89% (18). The CLA<br />

content in cheeses is primarily dependent on the CLA content in the milk, which<br />

varies in CLA concentration due to seasonal variation, geography, nutrition of the<br />

cow, <strong>and</strong> management practices. In addition, CLA content of cheese, to a limited<br />

extent, is affected by the production process <strong>and</strong> maturation (29).<br />

Reported values for CLA content in beef muscle vary considerably from 1.2<br />

to 9.9 mg/g fat (1,18,30,31). Fats <strong>and</strong> meats from ruminant species are a rich natural<br />

source of CLA, <strong>and</strong> the reported values ranged from 2.7 to 5.6 mg CLA/g fat in<br />

lamb, veal, <strong>and</strong> beef. Fritsche <strong>and</strong> Fritsche (32) reported that the amount of the<br />

c9,t11, 18:2 isomer in beef averaged 0.76% of total FAME for fat samples from<br />

bulls <strong>and</strong> 0.86% for fat from steers. Minor isomers, e.g., t9,c11, c9,c11, <strong>and</strong> t9,t11,<br />

were also found in beef fat samples. Others have reported that the c9,t11 18:2 content<br />

in beef ranged from 1.7 to 6.5 mg/g fat (30) <strong>and</strong> 0.65% of total FAME in beef<br />

fillet (4).<br />

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

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