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Food Lipids: Chemistry, Nutrition, and Biotechnology

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tures <strong>and</strong> further reduce the antioxidant radical’s ability to participate in propagation<br />

reactions.<br />

The most commonly used primary antioxidants in foods are synthetic compounds.<br />

Examples of important primary phenolic antioxidants include butylated hydroxyanisole<br />

(BHA), butylated hydroxytoluene (BHT), propyl gallate (PG), <strong>and</strong> tertiary<br />

butylhydroquinone (TBHQ). However, a few natural components of food also<br />

act as primary antioxidants <strong>and</strong> are commonly added to foods. Tocopherols are the<br />

most commonly used natural primary antioxidants. Carotenoids are another group of<br />

natural compounds that have primary antioxidant activity, although the mechanism<br />

differs from the phenolics.<br />

2. Secondary Antioxidants<br />

Secondary, preventive, or type 2, antioxidants act through numerous possible mechanisms.<br />

These antioxidants slow the rate of oxidation by several different actions,<br />

but they do not convert free radicals to more stable products. Secondary antioxidants<br />

can chelate prooxidant metals <strong>and</strong> deactivate them, replenish hydrogen to primary<br />

antioxidants, decompose hydroperoxides to nonradical species, deactivate singlet<br />

oxygen, absorb ultraviolet radiation, or act as oxygen scavengers. These antioxidants<br />

are often referred to as synergists because they promote the antioxidant activity of<br />

type 1 antioxidants. Citric acid, ascorbic acid, ascorbyl palmitate, lecithin, <strong>and</strong> tartaric<br />

acid are good examples of synergists. Some of the more important types of<br />

secondary antioxidant mechanism are discussed in the following sections.<br />

a. Chelators. Several heavy metals with two or more valence states (Fe, Cu, Mn,<br />

Cr, Ni, V, Zn, Al) promote oxidation by acting as catalysts of free radical reactions.<br />

These redox-active transition metals transfer single electrons during changes in oxidation<br />

states.<br />

Two mechanisms of oxidation promotion by metals have been proposed. Metals<br />

are believed to either interact with hydroperoxides or to react directly with lipid<br />

molecules. Metals are able to promote oxidation by interacting directly with unsaturated<br />

lipids [Eq. (13)] <strong>and</strong> lowering the activation energy of the initiation step of<br />

autocatalysis. However, because of thermodynamic constraints, spin barriers, <strong>and</strong> an<br />

extremely slow reaction rate, this direct interaction of metals with lipid moieties is<br />

not the main mechanism of metal catalysis [7–9].<br />

(n�1)� n� �<br />

M � RH → M � H � R� (13)<br />

Metals are known to interact with hydroperoxides <strong>and</strong> promote oxidation.<br />

Moreover, it is thought that a metal–hydroperoxide complex forms <strong>and</strong> subsequently<br />

decomposes to produce free radicals. Metals enhance the rate of decomposition of<br />

hydroperoxides <strong>and</strong> the generation of free radicals. Two metal–hydroperoxide reactions<br />

are possible.<br />

(n�1)� n� �<br />

M ROOH → M � H � ROO� (14)<br />

n� (n�1)� �<br />

M � ROOH → M � OH � RO� (15)<br />

Reaction (15) is less significant in aqueous solution, since metals in their lower<br />

oxidation states accelerate hydroperoxide degradation more than metals in their<br />

higher oxidation states [10]. Even trace amounts of these metals promote electron<br />

transfer from lipids or hydroperoxides because the reactions [Eqs. (14) <strong>and</strong> (15)] can<br />

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

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