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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Propagation:<br />

–> R • � O 2 → ROO •<br />

Termination:<br />

ROO • � RH → ROOH � R •<br />

⎢<br />

• •<br />

R � R → RR (4)<br />

• •<br />

R � ROO → ROOR (5)<br />

• •<br />

ROO � ROO → ROOR � O 2<br />

(6)<br />

The mechanism of lipid autoxidation has been postulated by Farmer et al. [1],<br />

Bol<strong>and</strong> <strong>and</strong> Gee [2], <strong>and</strong> Bateman et al. [3]. The propagation step in the autoxidation<br />

process includes an induction period when hydroperoxide formation is minimal [4,5].<br />

The rate of oxidation of fatty acids increases in relation to their degree of unsaturation.<br />

The relative rate of autoxidation of oleate, linoleate, <strong>and</strong> linolenate is in the<br />

order of 1:40–50:100 on the basis of oxygen uptake <strong>and</strong> 1:12:25 on the basis of<br />

peroxide formation [6]. Therefore, oils that contain relatively high proportions of<br />

polyunsaturated fatty acid (PUFA) may experience stability problems. The breakdown<br />

products of hydroperoxides, such as alcohols, aldehydes, ketones, <strong>and</strong> hydrocarbons,<br />

generally possess offensive off-flavors. These compounds may also interact<br />

with other food components <strong>and</strong> change their functional <strong>and</strong> nutritional properties<br />

[7].<br />

II. MEASUREMENT OF OXIDATIVE RANCIDITY<br />

There are various methods available for measurement of lipid oxidation in foods.<br />

Changes in chemical, physical, or organoleptic properties of fats <strong>and</strong> oils during<br />

oxidation may be monitored to assess the extent of lipid oxidation. However, there<br />

is no uniform <strong>and</strong> st<strong>and</strong>ard method for detecting all oxidative changes in all food<br />

systems. The available methods to monitor lipid oxidation in foods <strong>and</strong> biological<br />

systems may be divided into two groups. The first group measures primary oxidative<br />

changes <strong>and</strong> the second determines secondary changes that occur in each system.<br />

A. Primary Changes<br />

1. Changes in Reactants<br />

Methods that measure primary changes of lipids may be classified as those that<br />

quantify loss of reactants (unsaturated fatty acids). Measurement of changes in fatty<br />

acid composition is not widely used in assessing lipid oxidation because it may<br />

require total lipid extraction from food <strong>and</strong> subsequent conversion to derivatives<br />

suitable for gas chromatographic analysis. Separation of lipids into neutral, glycolipid,<br />

phospholipid, <strong>and</strong> other classes may also be necessary. However, it has been<br />

proven that this method serves as a useful technique to identify class of lipids <strong>and</strong><br />

fatty acids that are involved in the oxidative changes [8,9] <strong>and</strong> also to assess lipid<br />

oxidation induced by different metal complexes that afford a variety of products<br />

[10]. On the other h<strong>and</strong>, changes of fatty acid composition cannot be used in more<br />

saturated oils because this indicator reflects only the changes that occur in unsatu-<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.<br />

(2)<br />

(3)

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