• From a chemical point of view it is difficult to define tannins since the term encompasses some very diverse oligomers and polymers.
• It
might be
said that the tannins are a heterogeneous group
of
high molecular weight polyphenolic compounds with
the capacity to form reversible and irreversible
complexes with proteins (mainly), polysaccharides
(cellulose, hemicellulose,
pectin, etc.), alkaloids, nucleic acids and
minerals, etc.
• On
the
basis of their
structural
characteristics it is therefore possible to divide the tannins into four
major groups: Gallotannins, ellagitannins,
complex tannins, and
condensed tannins.
(1) Gallotannins
are
all those tannins in which galloyl units or their meta-depsidic
derivatives are bound to diverse polyol-, catechin-, or triterpenoid units.
(2) Ellagitannins are
those tannins in which at least two galloyl units are C–C coupled to each other, and
do not contain a glycosidically
linked catechin
unit.
(3) Complex tannins are
tannins in which a catechin unit
is bound glycosidically to a
gallotannin or an
ellagitannin
unit.
(4) Condensed tannins are
all oligomeric and polymeric
proanthocyanidins
formed by linkage of
C-4
of one catechin with
C-8 or C-6 of the next
monomeric
catechin.
Tannins are found commonly in fruits such as grapes,
persimmon, blueberry, tea, chocolate, legume forages,
legume trees like Acacia spp., Sesbania
spp.,
in grasses i.e;
sorghum, corn,
etc.
Activity of Tannins
• In
medicine, especially in Asian (Japanese and Chinese) natural healing, the tannin-containing plant
extracts
are used as astringents, against diarrhoea,
as diuretics, against stomach and duodenal tumours,
and
as antiinflammatory,
antiseptic,
antioxidant and
haemostatic
pharmaceuticals.
• Tannins
are
used in the dyestuff industry
as
caustics for cationic dyes (tannin
dyes), and
also in the production of inks (iron gallate ink). In the food
industry tannins
are used to
clarify wine,
beer, and fruit juices.
Other industrial
uses
of tannins include textile dyes, as antioxidants
in
the fruit juice, beer, and wine industries,
and as coagulants in rubber Production.
• Several
health benefits have been recognized for the intake of tannins and some
epidemiological associations with the decreased frequency of chronic diseases
have been established.
Recently the
tannins have attracted scientific
interest,
especially due to the increased incidence of deadly illnesses such as AIDS and various
cancers. The search
for new lead compounds
for
the development of novel pharmaceuticals
has
become increasingly important,
especially as the biological action of tannin-containing plant
extracts has been well documented.
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