Page 112 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN010B-472 July 16, 2001 15:41

340 Natural Antioxidants In Foods

Although lipoic acid has been found in numerous bio- gen by a transfer of energy from singlet oxygen to the
logical tissues, reports on its concentrations in foods are carotenoid, resulting in an excited state of the carotenoid
scarce. Lipoic acid is detectable in wheat germ (0.1 ppm) and ground state, triplet oxygen. Harmless transfer of en-
but not in wheat flour. It has been detected in bovine liver ergy from the excited state of the carotenoid to the sur-
kidney and skeletal muscle. Oral administration of lipoic rounding medium by vibrational and rotational mecha-
acid (1.65 g/kg fed) to rats for five weeks resulted in el- nisms then takes place. Nine or more conjugated double
evated levels of the thiol in liver, kidney, heart, and skin. bonds are necessary for physical quenching, with the pres-
When lipoic acid was added to diets lacking in vitamin E, ence of six carbon oxygenated ring structures at the end
symptoms typical of tocopherol deficiency were not ob- themoleculeincreasingtheeffectivenessofsingletoxygen
served suggesting that lipoic acid acts as an antioxidant in quenching.
vivo. However, lipoic acid was not capable of recycling vi- In foods, light will activate chlorophyll, riboflavin, and
tamin E in vivo, as determine by the fact that α-tocopherol heme-containing proteins to high energy excited states.
concentrations are not elevated by dietary lipoic acid in vi- These photoactivated molecules can promote oxidation
tamin E deficient rats. by direct interactions with an oxidizable compounds to
produce free radicals, by transferring energy to triplet oxy-
gen to form singlet oxygen or by transfer of an electron to
D. Carotenoids
triplet oxygen to form the superoxide anion. Carotenoids
Carotenoids are a chemically diverse group (>600 differ- inactivate photoactivated sensitizers by physically absorb-
ent compounds) of yellow to red colored polyenes consist- ing their energy to form the excited state of the carotenoid
ing of 3–13 conjugates double bonds and in some cases, that then returns to the ground state by transfer of energy
six carbon hydroxylated ring structures at one or both ends into the surrounding solvent.
of the molecule. ß-Carotene is the most extensively stud-
ied carotenoid antioxidant (Fig. 2). ß-Carotene will react
with lipid peroxyl radicals to form a carotenoid radical.
II. METAL CHELATORS
Whether this reaction is truly antioxidative seems to de-
pend on oxygen concentrations, with high oxygen con-
A. Ethylene Diamine Tetraacetic Acid
centrations resulting in a reduction of antioxidant activity.
The proposed reason for loss of antioxidant activity with Transition metals will promote oxidative reactions by
increasing oxygen concentrations involves the formation hydrogen abstraction and by hydroperoxide decompo-
of carotenoid peroxyl radicals that autoxidizes into addi- sition reactions that lead to the formation of free radi-
tional free radicals. Under conditions of low oxygen ten- cals. Prooxidative metal reactivity is inhibited by chela-
sion, the carotenoid radical would be less likely to autoox- tors. Chelators that exhibit antioxidative properties in-
idize and thus could react with other free radicals thereby hibit metal-catalyzed reactions by one or more of the fol-
forming nonradical species with in a net reduction of rad- lowing mechanims: prevention of metal redox cycling;
ical numbers. occupation of all metal coordination sites thus inhibit-
The major antioxidant function of carotenoids in foods ing transfer of electrons; formation of insoluble metal
is not due to free radical scavenging but instead is through complexes; stearic hinderance of interactions between
its ability to inactivate singlet oxygen. Singlet oxygen metals and oxidizable substrates (e.g., peroxides). The
is an excited state of oxygen in which two electrons in prooxidative/antioxidative properties of a chelator can of-
the outer orbitals have opposite spin directions. Initiation ten be dependent on both metal and chelator concen-
of lipid oxidation by singlet oxygen is due to its elec- trations. For instance, ethylene diamine tetraacetic acid
trophillic nature, which will allow it to add to the double (EDTA) can be prooxidative when EDTA:iron ratios are
bonds of unsaturated fatty acids leading to the formation ≤1 and antioxidative when EDTA:iron is ≥1. The proox-
of lipid hydroperoxides. Carotenoids can inactivate sin- idant activity of some metal-chelator complexes is due
glet oxygen by both chemical and physical quenching. to the ability of the chelator to increase metal solubility
Chemical quenching results in the direct addition of sin- and/or increase the ease by which the metal can redox
glet oxygen to the carotenoid, leading to the formation cycle.
of carotenoid breakdown products and loss of antioxi- The most common metals chelators used in foods con-
dant activity. A more effective antioxidative mechanism of tain multiple carboxylic acid (e.g., EDTA and citric acid)
carotenoids is physical quenching. The most common en- or phosphate groups (e.g., polyphosphates and phytate).
ergy states of singlet oxygen are 22.4 and 37.5 kcal above Chelators are typically water soluble but many also exhibit
ground state. Carotenoids physically quench singlet oxy- some solubility in lipids (e.g., citric acid), thus allowing