Agriculture Reference
In-Depth Information
most characteristic feature of carotenoids is the
long series of conjugated double bonds forming
the central part of the molecule. This gives
them their shape, chemical reactivity, and light-
absorbing properties.
Carotenoids perform important functions in
plants. They provide pigmentation essential for
photosynthesis, reproduction, and protection.
They provide yellow color in whole-grain fl our.
They may also act as antioxidants in lipid environ-
ments of many biological systems through their
ability to react with free radicals and form less reac-
tive products. Carotenoid radicals are stabilized by
delocalization of unpaired electrons over the con-
jugated polyene chain of the molecule, allowing
addition of other functional groups to many sites
on the radicals (Britton 1995). Carotenoids are
especially powerful against singlet oxygen gener-
ated from lipid peroxidation or radiation. β-Caro-
tene, α-carotene, and β- cryptoxanthin have
provitamin A activity. Zeaxanthin and lutein are
the major carotenoids in the macular region (yellow
spot) of the retina in humans.
Carotenoids commonly found in wheat grains
are lutein, zeaxanthin, β-cryptoxanthin, β-
carotene, and α-carotene (Britton 1995; Adom et
al., 2003, 2005). Generally, lutein is the carot-
enoid present in the highest concentration in
wheat, followed by zeaxanthin, and then β-cryp-
toxanthin. In the study mentioned previously
(Adom et al., 2003) involving 11 wheat geno-
types—which included a synthetic wheat and a
combination of red or white, hard or soft, and
winter or spring cultivars—lutein, zeaxanthin,
and β-cryptoxanthin contents varied signifi cantly
among cultivars. Lutein content varied from
26.41 ± 1.40 to 143.46 ± 6.67 μg 100 g
−1
grain,
amounting to a 5.4-fold difference. Zeaxanthin
content varied from 8.70 ± 0.75 to 27.08 ± 0.54 μg
100 g
−1
grain, amounting to a threefold difference.
The β-cryptoxanthin content varied from 1.12 ±
0.13 to 13.28 ± 0.43 μg 100 g
−1
grain, amounting
to a 12-fold difference. The synthetic wheat
experimental line in that study, W7985, gave the
lowest carotenoid concentrations of any of the
genotypes. Such large genotypic differences in
carotenoid content may open up new opportuni-
ties for breeding wheat cultivars with higher
nutritional value (Adom et al., 2003).
Vitamin E
Vitamin E is the generic term used to describe
a family of eight lipid-soluble antioxidants
with two types of structures, the tocopherols
(α-tocopherol, β-tocopherol, γ-tocopherol, δ-
tocopherol) and tocotrienols (α-tocotrienol,
β-tocotrienol, γ-tocotrienol, and δ-tocotrienol).
Their basic structures comprise a 6-hydroxychro-
man group and a phytyl side chain made of iso-
prenoid units. The chroman group may be
methylated at different positions to generate dif-
ferent compounds with vitamin activity. Both
tocopherol and tocotrienol structures are similar
except tocopherols contain saturated phytol side
chains whereas the tocotrienols have three
carbon-carbon double bonds in the phytol side
chain.
Vitamin E compounds are found in many
foods including whole grains, where they are
mostly present in the germ fraction. The concen-
tration of vitamin E compounds in whole grains
are: 75 mg kg
−1
dry weight (DW) total tocopherol
in soft wheat and barley (
Hordeum vulgare
L.); 33
to 43 mg kg
−1
DW β-tocotrienol in wheat;
45 mg kg
−1
DW γ-tocopherol in maize; and 56
and 40 mg kg
−1
DW α-tocotrienol in oat and
barley, respectively (Panfi li et al., 2003). The
most important functions of vitamin E in the
body are antioxidant activity and maintenance of
membrane integrity. The free hydroxyl group on
the aromatic ring is responsible for the antioxi-
dant properties. The hydrogen atom from this
group can be donated to free radicals, resulting
in a resonance-stabilized vitamin E radical.
Vitamin E has also been shown to play a role in
immune function, in DNA repair, and other
metabolic processes (Traber 1999).
Lignans
Lignans are a group of dietary phytoestrogen
compounds that are made up of two coupled
C6C3 units. The common plant lignans in
the human diet include secoisolariciresinol,
