Food Lipids: Chemistry, Nutrition, and Biotechnology

containing plant phenolics. Carnosic acid, carnosol, and rosmarinic acid are the major
antioxidant phenolics in rosemary extracts (Fig. 7) [18]. Crude rosemary extracts
have been found to inhibit lipid oxidation in a wide variety of food products including
meats, bulk oils, and lipid emulsions [18–20]. Utilization of phenolic antioxidants
from crude herb extracts such as rosemary is often limited by the presence
of highly flavorful monoterpenes. Use of more purified forms of herbal phenolics is
restricted by both economic and regulatory hurdles.
E. Carotenoids
Carotenoids are a diverse group (>600 compounds) of yellow to red polyenes consisting
of 3 to 13 double bonds and in some cases 6 carbon hydroxylated ring
structures at one or both ends of the molecule [21]. Carotenoids may be important
biological antioxidants and are thought to play a role in controlling oxidatively induced
diseases such as cancer and atherosclerosis [22]. The antioxidant properties
of carotenoids depend on environmental conditions and the nature of oxidation catalyst.
Carotenoids can be effective antioxidants in the presence of singlet oxygen
(see Sec. III-A: Control of Prooxidant Metals). However, when peroxyl radicals are
the initiating species, the antioxidant efficiency of carotenoids depends on oxygen
concentrations.
�-Carotene, the most extensively studied carotenoid antioxidant, reacts with
lipid peroxyl radicals, resulting in the formation of a carotenoid radical. Burton and
Ingold [23] found that under conditions of high oxygen tension, the antioxidant
activity of �-carotene is diminished. They proposed that increasing oxygen results
in increased formation of carotenoid peroxyl radicals, thus favoring autoxidation of
�-carotene over inactivation of lipid peroxyl radicals. Under conditions of low oxygen
tension, the lifetime of the carotenoid radical is long enough to permit reaction
with another peroxyl radical, thus forming a nonradical species and effectively inhibiting
oxidation by removing radicals from the system.
Incubation of �-carotene with peroxyl radical generators in organic solvents at
high (atmospheric) oxygen tensions leads to addition reactions to form carotenoid–
peroxyl adducts (Fig. 8). Addition of a peroxyl radical to the cyclic end group or
the polyene chain followed by loss of an alkoxyl radical leads to the formation of
5,6- and 15,15�-epoxides. Elimination of the alkoxyl radical from the 15,15� positions
can also cause cleavage of the polyene chain, resulting in formation of aldehydes.
Since the formation of �-carotene epoxides from the addition of peroxyl radicals
results in the formation of an alkoxyl radical, the net change in radical number is
zero; thus an antioxidant effect is not expected [24].
�-Carotene is capable of donating an electron to peroxyl radicals to produce a
�-carotene cation radical and a peroxyl anion. The �-carotene cation radical is resonance
stabilized and does not readily react with oxygen to form peroxides. However,
the �-carotene cation radical appears to be strong enough to oxidize other lipophilic
hydrogen donors, including tocopherols and ubiquinone [24]. Additional research is
needed to identify the oxidation products that form from carotenoids under low
oxygen partial pressures. Identification of these products may help determine the
exact mechanism by which carotenoids act as free radical scavengers when oxygen
concentrations are low. Such knowledge would make it easier to predict when carotenoids
will exhibit antioxidant activity.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.