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

Natural Antioxidants In Foods 337

of a FRS to donate a hydrogen to a free radical can some- associated with the prevention of diseases such as cancer
times be predicted from standard one electron reduction and atherosclerosis. Plant foods high in phenolics include
potentials (E ). If a compound has a reduction potential cereals, legumes, and other seeds (e.g., sesame, oats, soy-
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lower than the reduction potential of a free radical found beans, and coffee); red-, purple-, and blue-colored fruits
in a food or biological tissue (e.g., fatty acid based per- (e.g., grapes, strawberries, and plums); and the leaves of
oxyl radical), it can donate hydrogen to that free radical herbs and bushes (e.g., tea, rosemary, and thyme). Many
unless the reaction is kinetically unfeasible. For exam- natural phenolics are capable of inhibiting oxidative reac-
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ple, FRS including α-tocopherol (E = 500 mV), urate tions. However, because phenolics have such a wide array
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(E = 590 mV), catechol (E = 530 mV), and ascorbate of chemical structures, it is not surprising that antioxidant
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(E =282mV)allhavereductionpotentialsbelowperoxyl activities and health benefits vary greatly. Knowledge of
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radicals (E = 1000 mV, a common free radical in lipid antioxidant activity, antioxidant mechanisms, and health
oxidation reactions) and therefore can convert the peroxyl benefits of plant phenolics is just beginning to be under-
radical to a hydroperoxide through hydrogen donation. stood. This section focuses on the best studied of the plant
The efficiency of an antioxidant FRS is also dependent phenolics.
on the energy of the resulting antioxidant radical. If a FRS Tocopherols and tocotrienols are a group of phenolic
produces a low energy radical then the likelihood of the FRS isomers (α, β, δ and γ ; see Fig. 1 for the structure
FRS radical to promote the oxidation of other molecules is of α-tocopherol) originating in plants and eventually end-
lower and the oxidation reaction rate decreases. Phenolics ing up in animal foods via the diet. Interactions between
are effective FRS because phenolic free radicals have low tocopherols and fatty acid peroxyl radicals lead to the for-
energy due to delocalization of the free radical thoughout mation of fatty acid hydroperoxides and several resonance
the phenolic ring structure. Standard reduction potentials structures of tocopheroxyl radicals. Tocopheroxyl radicals
can again be used to help illustrate this point. Radicals on can interact with other compounds or with each other to
α-tocopherol (E = 500 mV) and catechol (E = 530 mV) form a variety of products. The types and amounts of these
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have lower reduction potentials than polyunsaturated fatty products are dependent on oxidation rates, radical species,
acids (E = 600 mV), meaning that their radicals do not lipid state (e.g., bulk vs. membrane lipids), and tocopherol
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posses high enough energy to effectively promote the oxi- concentrations.
dation of unsaturated fatty acids. Effective phenolic aniox- Under condition of low oxidation rates in lipid mem-
idants FRS also produce radicals that do not react rapidly brane systems, tocopheroxyl radicals primarily convert to
withoxygentoformhydroperoxidesthatcouldautoxidize, tocopherylquinone. Tocopherylquinone can form from the
thus depleting the system of antioxidants. Antioxidant hy- interaction of two tocopheroxyl radicals leading to the for-
droperoxides are also a problem because they can decom- mation of tocopherylquinone and the regeneration of toco-
pose into radicals that could promote oxidation. Thus, if pherol. Tocopherylquinone can also be regenerated back
antioxidant hydroperoxides did form, this could result in to tocopherol in the presence of reducing agents (e.g.,
consumption of the antioxidant with no net decrease in ascorbic acid). An additional reaction that can occur is
free radicals numbers. the interaction of two tocopheroxyl radicals to form toco-
Antioxidant radicals may undergo additional reactions pherol dimers.
that remove radicals from the system, such as reactions Tocopherol is found in plant foods especially those high
with other antioxidant radicals or lipids radicals to form in oil. Soybean, corn, safflower, and cottonseed oil are
nonradical species. This means that each FRS is capable good sources of α-tocopherol as are whole grains (in par-
of inactivating at least of two free radicals, the first being ticular wheat germ) and tree nuts. All tocopherol isomers
inactivated when the FRS interacts with the initial oxidiz- are absorbed by humans, but α-tocopherol is preferen-
ing radical, and the second, when the FRS radical interacts tially transfered from the liver to lipoproteins, which in
with another radical via a termination reaction to form a turn transports α-tocopherol to tissues. For this reason,
nonradical product. α-tocopherol is the isomer most highly correlated with
Phenolic compounds that act as antioxidants are vitamin E activity.
widespread in the plant kingdom. Plant phenolics can be Tea is an important source of dietary antioxidants for
classifiedassimplephenolics,phenolicacids,hydroxycin- humans because it is one of the most common beverages
namic acid derivatives, and flavonoids. In addition to the in the world with annual consumption of over 40 liters/
basic hydroxylated aromatic ring structure of these com- person/year. Phenolics in tea are mainly catechin deriva-
pounds, plant phenolics are often associated with sugars tives, including catechin (Fig. 1), epicatechin, epicatechin
and organic acids. The consumption of natural plant phe- gallate, gallocatechin, epigallocatechin gallate, and
nolics have been estimated to be up to 1 g per day. Overall, epigallocatechin. Tea originates from leaves harvested
the presence of phenolics in the diet has been positively from the bush, Camellia sinensis. Processing of tea leaves