2. • Definition: Tannins are naturally occurring
complex organic compounds possessing
polyphenols of high molecular weight which
generally have astringent properties.
• They are considered as secondary metabolite in all trees and plant can shows
astringent and antioxidant property that binds to and precipitates proteins and
various other organic compounds including amino acids and alkaloids. They
Present in plants, ex. grapes, cranberry, hamamelis and tea leaves.
• The term tannin (from tanna, an Old High
German word for oak or fir tree, as in
Tannenbaum) refers to the use of wood tannins
from oak in tanning animal hides into leather ?
(Precipitate animal proteins in hides and
converting them into leather; Tanning Industry).
3. Examples of common tannins:
Gallic acid: Galls, rhubarb and most drugs that contain gallitannins
Catechins: Catechu, cocoa and most drugs containing condensed
tannins.
Chlorogenic acid: Coffee particularly when unroasted.
Ipecacuanhic acid: Ipecacuanha.
4. Classification of tannins
I- True tannins: High molecular weight from
about 1000 to 5000 amu.
• Give positive goldbeater's skin test
a. Hydrolysable tannins (Pyrogallol tannins)
b. Condensed tannins (catechol tannins)
c. Complex tannins
II- Pseudotannins: (low molecular weight )
• Give negative goldbeater's skin test
• Concentrated solutions, give precipitates with gelatin
5. I. True tannins
1- Hydrolysable tannins (Pyrogallol tannins)
- Hydrolyzed by acids or enzymes such as tannase (rumen bacteria).
- Formed from several molecules of gallic and hexahydroxydiphenic acids united
by ester linkages to a central sugar (mainly glucose).
- On dry distillation; Gallic acid and other components are converted into
pyrogallol.
- Their solutions turn blue with iron salts like gallic acid
Ellagic acid
gallic acid
- Types:
- Gallitannin: On hydrolysis, it gives gallic acid and glucose ex.
Rhubarb rhizome, clove buds, red rose petals, galls, hamamelis
leaves and chestnut bark.
- Ellagitannin: On hydrolysis it gives ellagic acid and glucose
ex. Pomegranate rind and bark, eucalyptus leaves, myrobolan and
oak bark.
6.
7. 2- Condensed tannins (catechol tannins): polymeric flavan-3-ol
- not readily hydrolyzed and no sugar moiety.
- On dry distillation they yield catechol.
- their solutions turn to green with ferric chloride like catechol.
- On treatment with acids or enzymes converted into phlobaphenes
(Phlobatannins).
- Phlobaphenes give the characteristic red color to many drugs such as:
Barks: e.g. Cinnamon, hamamelis and cinchona.
Roots and rhizomes: e.g. krameria and male fern.
Flowers: e.g. lime and hawthorn.
Seeds :e.g. cocoa ,kola and areca.
Leaves: e.g. hamamelis, hawthorn and tea ,especially green tea .
Extracts and dried juices: catechu and Indian kino.
8.
9. 3- Complex tannins
• These represent a group of tannins that are biosynthesized from both
hydrolysable tannin and condensed tannin. Mostly C- glucoside ellagitannin
and flavono-ellagitannin.
• Members of Combretaceae, Myrtaceae and Fagaceae
10. Qualitative identification of tannins:
• Ferric chloride; (FeCl3) + tannins : Bluish black colour with hydrolysable
tannin. Greenish black colour with catechol.
• Gelatin test: Solution of tannins (about 5
-
1
% ) precipitate 1
% Solution of
gelatin containing l0 % sodium chloride Gallic acid and other pseudotannins
also precipitate. gelatin if the solutions are sufficiently concentrated.
• Bromine water: only condensed tannin gives ppt. with bromine water.
Gold beater's skin test
• Gold beater's skin test: a soak a small piece of
Gold beater's skin membrane in 2% hydrochloric acid
(HCL). Rinse with distilled water, place the piece for
5 minutes in the tannin solution to be tested. Wash
with distilled water and transfer to 1% ferrous sulfate
(FeSO4) solution: a brown or black colour in the skin
denote the presence of tannins.
11. Quantitative determination of tannin:
1. Hide powder method: The difference in the dry weight of the extract
before and after treatment with hide powder is taken as a measure for
tannin content.
2. Copper acetate method: Gravimetric method using copper as precipitant.
12. - Uses of Tannins:
• Astringent, haemostatic, antidiarrheal, potent antioxidant, an
antidote in alkaloid and heavy metals toxicity.
• Reduce cell damage as antiaging agent, tannins act as an
antioxidant against free radicals.
• Natural astringent, they help remove excess oil from and
constricting pores without drying out the skin.
• Coffee chlorogenic acid is slimming and protect from diseases
related oxidative stress (diabetes and cardiovascular diseases).
• Treatment of burns.
• Anti-diarrheals although not recommended ?? as they usually
delay elimination of bacterial toxins from the body. ex. albumin
tannate.
• Antidote in poisoning by heavy metals, alkaloids and certain
glycosides due to their precipitation as tannates.
• Skin toner: as general but Hamamelis is common cosmetics
ingredient and especially in treatment of Hemorrhoids and varicose
veins.
• Anti-inflammatory properties minimize redness and inflammation.
