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Food Lipids: Chemistry, Nutrition, and Biotechnology

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II. INACTIVATION OF FREE RADICALS<br />

Antioxidants can slow lipid oxidation by inactivating or scavenging free radicals,<br />

thus inhibiting initiation <strong>and</strong> propagation reactions. Free radical scavengers (FRS) or<br />

chain-breaking antioxidants are capable of accepting a radical from oxidizing lipids<br />

species such as peroxyl (LOO�) <strong>and</strong> alkoxyl (LO�) radicals by the following reactions<br />

[2)]:<br />

LOO� or LO� �FRS → LOOH or LOPH � FRS�<br />

FRS primarily react with peroxyl radicals for several reasons: (a) because propagation<br />

is a slow step in lipid oxidation, meaning that peroxyl radicals are often found<br />

in the greatest concentration of all radicals in the systems; <strong>and</strong> (b) because peroxyl<br />

radicals have lower energies than radicals such as alkoxyl radicals [3] <strong>and</strong> thus react<br />

more readily with the low energy hydrogens of FRS than with polyunsaturated fatty<br />

acids, <strong>and</strong> (c) because FRS, being generally found at low concentrations, do not<br />

compete effectively with initiating radicals (e.g., �OH) [4]. A FRS thus inhibits lipid<br />

oxidation by more effectively competing with other compounds (especially unsaturated<br />

fatty acids) for peroxyl radicals.<br />

Chemical properties, including hydrogen bond energies, resonance delocalization,<br />

<strong>and</strong> susceptibility to autoxidation, will influence the antioxidant effectiveness<br />

of an FRS. Initially, antioxidant efficiency is dependent on the ability of the FRS to<br />

donate hydrogen to the free radical. As the hydrogen bond energy of the FRS decreases,<br />

the transfer of the hydrogen to the free radical is more energetically favorable<br />

<strong>and</strong> thus more rapid. The ability of a FRS to donate a hydrogen to a free radical can<br />

be predicted from st<strong>and</strong>ard one-electron reduction potentials [3].<br />

Any compound that has a reduction potential lower than the reduction potential<br />

of a free radical (or oxidized species) is capable of donating a hydrogen to that free<br />

radical unless the reaction is kinetically unfeasible (Table 1). For example, FRS<br />

including �-tocopherol (E�� = 500 mV), urate (E�� = 590 mV), catechol (E�� = 530<br />

mV), <strong>and</strong> ascorbate (E�� = 282 mV) all have reduction potentials below peroxyl<br />

radicals (E�� = 1000 mV) <strong>and</strong> are therefore capable of donating a hydrogen atom to<br />

the peroxyl radical to form a peroxide. St<strong>and</strong>ard reduction potentials can also be<br />

used to predict the ease with which a compound can donate its hydrogen to a radical.<br />

Table 1 St<strong>and</strong>ard One-Electron Reduction Potentials<br />

for Common Free Radical Processes<br />

Couple E�� (mV)<br />

HO�, H � /H2O 2310<br />

RO�, H � /ROH 1600<br />

ROO�, H � /ROOH 1000<br />

PUFA�, H � /PUFA-H 600<br />

Urate � �,H � /urate � -H 590<br />

Catechol-O�, H � /catechol-OH 530<br />

�-Tocopheroxyl�, H � �-tocopherol 500<br />

Ascorbate � �,H � /ascorbate �<br />

282<br />

Source: Adapted from Ref. 3.<br />

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

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