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

338 Natural Antioxidants In Foods

involves either blanching to produce green tea or fer- 294–1625 µg/g product. Genistein and daidzein are ab-
menting to produce oolong or black tea. The fermentation sorbed into human plasma from products such as tofu
process allows polyphenol oxidase enzymes to react and soy-based beverages. Bioavailability is low, with only
with the catechins to form the condensed polyphenols 9–21% of the isoflavones being absorbed. Over 90% of the
that are responsible for the typical color and flavor absorbed isoflavones are removed from the plasma within
of black teas. Green tea leaf extracts contain 38.8% 24 hours.
phenolics on a dry weight basis with catechins contribut- Herbs and spices often contain high amount of pheno-
ing over 85% of the total phenolics. Condensation of lic compounds. For example, rosemary contains carnosic
catechins can decrease their solubility; therefore black acid, carnosol, and rosmarinic acid. Crude rosemary ex-
tea extracts contain less phenolics (24.4%) of which tracts are a commercially important source of natural phe-
17% are catechins and 70% are condensed polyphenols nolic antioxidant additives in foods meats, bulk oils, lipid
(thearubigens). Extraction of phenolics with water from emulsions, and beverages.
the leaves of rooibos (Aspalathus linearis) resulted in
increased antioxidant activity with increasing extraction
temperature and time, suggesting that brewing techniques B. Ascorbate
could influence the antioxidant phenolic content of teas. Ascorbic acid (vitamin C; Fig. 2) acts as a water-soluble
Ingestion of dietary phenolics from tea has been asso- free radical scavenger in both plant and animal tissues.
ciated with cancer prevention, and absorption of dietary Like phenolics, ascorbate (E = 282 mV) has a reduction
◦
tea phenolics has been reported. potential below peroxyl radicals (E = 1000 mV) and thus
◦
Grapes and wines are also significant sources of di- can inactivate peroxyl radicals. In addition, ascorbate’s re-
etary phenolic antioxidants. Grapes contain a wide variety duction potential is lower than the α-tocopherol radical
of phenolics including anthocyanins, flavan-3-ols (cate- ◦
(E = 500 mV), meaning that ascorbate may have an ad-
chin), flavonols (quercetin and rutin), and cinnamates (S- ditional role in the regeneration of oxidized α-tocopherol.
glutathionylcaftaric acid). As with many fruits, the ma- Interactions between ascorbate and free radicals result in
jority of grape phenolics are found in the skin, seeds, the formation of numerous oxidation products. Although
and stems (collectively termed pomace). During extrac- ascorbate seems to primarily play an antioxidant role in
tion of juice, the pomace is left in contact with the juice living tissues, this is not always true in food systems.
for varying times in order to produce products of vary- Ascorbate is a strong reducing agent especially at low pH.
ing color, with increasing contact time resulting in in- When transition metals are reduced, they become very ac-
creased phenolic extraction and, thus, darker color. There- tive prooxidants that can decompose hydrogen and lipid
fore, white grape juices and wines have lower phenolics peroxides into free radicals. Ascorbate also causes the re-
contents (119 mg of gallic acid equivalents/L) than red lease of protein-bound iron (e.g., ferritin), thus promot-
wines (2057 mg of gallic acid equivalents/L). As would ing oxidation. Therefore, ascorbate can potentially exhibit
be expected, red grape juice and wines have greater antiox- prooxidative activity in the presence of free transition met-
idant capacity due to their higher phenolic content. Both als or iron-binding proteins. This does not typically occur
grape juice and wines have been suggested to have posi- in living tissues due to the tight control of free metals by
tive heath benefits, however, their phenolic compositions systems that prevent metal reduction and reactivity. How-
are not the same due to differences in juice preparation ever, in foods the typical control of metals can be lost
and changes in phenolic composition that occurs during by processing operations that cause protein denaturation.
both fermentation and storage. Thus in some foods, ascorbate my act as a prooxidant and
The primary phenolics in soybeans are classified as accelerate oxidative reactions.
isoflavones. Included among the soybean isoflavones are Ascorbate is found in numerous plant foods including
daidzein (Fig. 1), genistein, and glycitein, and the glycoso- green vegetables, citrus fruits, tomatoes, berries, and pota-
lated counterparts daidzin, genistin, and glycitin. Unlike
toes. Ascorbate can be lost in foods due to heat processing
the phenolics in grapes and tea, soybean isoflavones are as- and prolonged storage. Transition metals and exposure to
sociated with proteins and, therefore, are found in soy flour air will also cause the degradation of ascorbic acid.
and not in soybean oil. The concentration of isoflavones
in soybeans varies with the environmental conditions
under which the beans were grown. In addition, isoflavone C. Thiols
concentrations in soy-based foods are altered during food
1. Glutathione
processing operations such as heating and fermentation.
Beside whole soybeans, isoflavones are found in soy milk, Glutathione (Fig. 2) is a tripeptide (γ -glutamyl-cysteinyl-
tempeh, miso, and tofu at concentrations ranging from glycine) where cysteine can be in either the reduced or