An radicals. In human body, antioxidant capabilities can

An
antioxidant is defined as “any substance 
that  delays,  prevents 
or  removes  oxidative 
damage to  a  target 
molecule” (Halliwell and
Gutteridge, 2007). These could be divided to endogenous compounds and
exogenous compounds. The endogenous compounds are naturally synthesized in the
human body, while the exogenous ones are produced by plants and are consumed
through food by humans. Antioxidants play vital functions in a
cell and combat free radicals. Free radicals (atoms with an unpaired electron)
cause oxidation stress to various micro-molecules and macro-molecules present
in an organism. During oxidative stress the electrons are transferred   from one molecular atom to another, with the
molecule that is losing electrons getting oxidized (Manda et al., 2009).  Presence
of antioxidant compounds in various plant species showed their activity by
scavenging the dangerous effect of active oxygen species (AOS), including free
radicals and non-free radical species. These free radicals are highly unstable
atoms develops in human body due to oxidative stress, this leads to cellular
damage in the body (Figure I) mainly because of lipid, protein and DNA damage
due to imbalance of reactive oxygen species and antioxidant compounds. These
free radicals can be scavenged by the protective role of natural bioactive/antioxidant
compounds such as carotenoids, flavonoids, phenolics acid, polyphenols,
triterpenoids, vitamin E, vitamin C, and thereby reduce the risk of chronic and
degenerative disorders (Uddin et
al.,2008). An antioxidant has a characteristic ability to trap
free radicals. In human body, antioxidant capabilities can be divided into
three classes; those include the role of enzymes which initially controls the free
radical production, the second one dietary antioxidants derived from the diet
includes various fruits and vegetables. The third class of antioxidants is replenishers,
these stops the free radical formation by donating proton to the unstable atoms
in oxidative stress condition (Ernster etal., 1981). Natural antioxidants are found
in all parts of plants and play vital role in quenching pathways of singlet and
triplet oxygen and these include polyphenols (catechins and flavonoids), phenolics
acids, tocopherol, dietary glutathione and endogenous metabolites. The principal
antioxidants of plant origin comprise of nutrient-derived antioxidants like
ascorbic acid (vitamin C), tocopherol and tocotrienols (vitamin E),
carotenoids, and other low molecular weight compounds such as glutathione and
lipoic acid. Antioxidant enzymes, e.g., super oxide dismutase, glutathione
peroxidase, and glutathione reductase, have a major role in the body by
catalysing free radical quenching reactions. Metal binding proteins, such as
ferritin, lactoferrin, albumin, and ceruloplasmin that sequester free iron and copper
ions that are capable of catalyzing oxidative reactions. The exploration of new potent bioactive/antioxidant
compounds is a new area of drug discovery, that addresses hitherto unmet
pharmaceuticals needs and the tropical tuber crops have been bounteous in producing
diverse group of bioactive compounds with high antioxidant activity. The goal
of this review is to focus on potential antioxidants present in the tropical
tuber crops and compare them with the antioxidant rich plant sources that have
been well documented as well as throw light on natural sources, structure of
different antioxidants, biosynthesis of antioxidants, nutritive value and
medicinal properties of antioxidants and their bioavailability.Sources of antioxidantsThe plant kingdom provides many natural antioxidants. They include
several fruits, vegetables and root and tuber crops. Vegetables are good
sources
of bioactive compounds such as anthocyanins,
flavonoids, carotenoids, vitamins and other polyphenolics (Lugasi et
al., 2003). Researchers have identified several
bioactive compounds in vegetable crops with functional qualities and make new
discoveries surrounding the complex benefits of phytochemicals such as lycopene
in tomatoes, gingerol in ginger, organosulphur compounds in Allium sp.,
omega-3fatty acids in cucurbitaceous vegetable seeds etc. There has been
increasing consumer awareness in recent years regarding the vegetables with
physiologically-active specific nutraceuticals. Among vegetables salads,
endives, spinach, garlic, onion, horseradish and broccoli (in fact, the entire
cabbage group), green peppers, tomatoes, dill, thyme, basil and root crops,
artichoke and peppergrass etc. are few examples. Several processed products
with nutraceutical role are paprika, tomato products and green pepper pastes
etc are already commercialized. Such products include food supplements, dietary
supplements, value added processed vegetables as well as non-food supplements
such as tablets, soft gels, capsules etc (Rai et al., 2012).ChilliChillies
are the dried red fruits of the genus Capsicum and family Solanaceae.
The two-important species are Capsicum annum L. and C. frutenscens L.,
the former being widely grown and economically important as domesticated species.
India is the largest producer and exporter of chillies. Pungency and color are
the two important quality attributes in chillies. The red color of chillies is
due to the presence of the carotenoid pigment like capsanthin (major, 35%),
zeaxanthin, violaxanthin, cryptoxanthin, ?-carotene etc. These pigments are
present in chillies mainly in the esterified form and to a small extent in
non-esterified forms. The pungency of chillies is due to the presence of
capsaicin C18H23NO3, fat soluble compound,
flavorless, colourless, and odourless but gives taste i.e. hotness, that
account for approximately 90% of the pungency. Tomato Lycopene
is the vibrant red pigment and fat soluble carotenoid prominently present in
tomato fruits. It is a major carotenoid in tomato (Lycopersicon esculentum Mill.)
fruit. It is an intermediate for the biosynthesis of other carotenoids and is
found in moderate to high concentrations in foods such as tomato, watermelon
and capsicum. Moreover, tomato fruit has recently gained attention in relation
to the prevention of some human diseases. This interest is due to presence of
carotenoids, particularly lycopene, which appears to be an active compound in
the prevention of cancer and cardiovascular diseases and in slowing down cellular
aging owing to its high antioxidant and antiradical power. AmaranthsOne
cup of amaranth leaves that are cooked, boiled, and
drained is reported to meet 90% vitamin C daily value requirement, 73% vitamin
A, 28% calcium and 17% iron. Amaranths have excellent
nutritional value because of their high content of essential micronutrients
such as Beta-carotene, iron, calcium, vitamin C and folic acid. Amaranthus tricolor and A. caudatus
are used externally to treat inflammations, and internally as a diuretic. Because
of its low production costs, amaranth is one of the cheapest dark-green leafy
vegetables in tropical markets and is often described as the poor man’s
vegetable (Varalakshmi, 2004).

Garlic

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