water. Tannins have the ability to precipitate different compounds like gelatin,
alkaloid, protein, glycoside, skin hide and heavy metal.
• Reaction with salts or bases: Tannins yield purple, violet or black precipitates
with iron compounds. They are precipitated by number of metallic salts notably
potassium dichromate, and lead acetate and sub acetate. Tannins are precipitated
by copper acetate solution. Tannins when mixed with potassium ferricyanide in
the presence of ammonia, it produce deep red colour (Wei and Xingjin 1989).
• When it is boiled with acetic anhydride, it is converted into a crystalline acetyl
derivative, which melts at 137 °C (Ramakrishnan and Krishnan 1994).
• Carcinogenicity: Prolonged use of tannins can cause cancer. An evidence about
the carcinogenic effect of Areca catechu was observed by typical use of it that
resulted in oral and esophageal cancer (Hernandez et al. 2017).
• Anti-oxidizing properties: Tannins can show their anti-oxidant property because
of the large number of the hydroxyl (OH) groups present in it (Bagyalakshmi
et al. 2019).
21.7 Chemical Tests of Tannins
The presence and the amount of tannins can be clarified with the help of different
chemical tests of tannins. It can be divided into two groups, like, qualitative tests
and quantitative test. By qualitative test we can understand about the presence of
different tannins and by the quantitative tests, the amount of the tannin present in
the sample can be determined.
21.7.1 Qualitative Tests
1. Goldbeater's skin test: This is the major qualitative test for the confirmation of
true tannins based on the astringent property of tannins. Goldbeater’s shin is the
untanned hide (membrane) prepared from the intestine of ox. The gold-beater’s
skin test is originally described by Atkinson and Hazleton (Honorá Price 1924).
The first stage of performing this test is to make the Gold-beater’s skin to soak
in 2% HCl and make the skin hide permeable for tannin. After that, the skin is
rinsed with distilled water and is dipped in the tannin solution for 5 min. After
this step, the skin is washed with water and is treated with 1% ferrous sulphate
solution. The brown or black colouration of skin gives the confirmation about
the presence of tannins. Pseudo tannins don’t give positive result to this test
(Baig and Anand).
2. Water test: Tannins create a light yellow to dark brown discoloration in the
water. A simple test for tannins involves filling a clear glass with water and
letting it sit overnight. If the colour settles to the bottom of the glass, the
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alkaloid, protein, glycoside, skin hide and heavy metal.
• Reaction with salts or bases: Tannins yield purple, violet or black precipitates
with iron compounds. They are precipitated by number of metallic salts notably
potassium dichromate, and lead acetate and sub acetate. Tannins are precipitated
by copper acetate solution. Tannins when mixed with potassium ferricyanide in
the presence of ammonia, it produce deep red colour (Wei and Xingjin 1989).
• When it is boiled with acetic anhydride, it is converted into a crystalline acetyl
derivative, which melts at 137 °C (Ramakrishnan and Krishnan 1994).
• Carcinogenicity: Prolonged use of tannins can cause cancer. An evidence about
the carcinogenic effect of Areca catechu was observed by typical use of it that
resulted in oral and esophageal cancer (Hernandez et al. 2017).
• Anti-oxidizing properties: Tannins can show their anti-oxidant property because
of the large number of the hydroxyl (OH) groups present in it (Bagyalakshmi
et al. 2019).
21.7 Chemical Tests of Tannins
The presence and the amount of tannins can be clarified with the help of different
chemical tests of tannins. It can be divided into two groups, like, qualitative tests
and quantitative test. By qualitative test we can understand about the presence of
different tannins and by the quantitative tests, the amount of the tannin present in
the sample can be determined.
21.7.1 Qualitative Tests
1. Goldbeater's skin test: This is the major qualitative test for the confirmation of
true tannins based on the astringent property of tannins. Goldbeater’s shin is the
untanned hide (membrane) prepared from the intestine of ox. The gold-beater’s
skin test is originally described by Atkinson and Hazleton (Honorá Price 1924).
The first stage of performing this test is to make the Gold-beater’s skin to soak
in 2% HCl and make the skin hide permeable for tannin. After that, the skin is
rinsed with distilled water and is dipped in the tannin solution for 5 min. After
this step, the skin is washed with water and is treated with 1% ferrous sulphate
solution. The brown or black colouration of skin gives the confirmation about
the presence of tannins. Pseudo tannins don’t give positive result to this test
(Baig and Anand).
2. Water test: Tannins create a light yellow to dark brown discoloration in the
water. A simple test for tannins involves filling a clear glass with water and
letting it sit overnight. If the colour settles to the bottom of the glass, the
21 Tannins and Polyphenols Extracted from Natural Plants …
733