21.5 Extraction Process of Tannins
As tannins show its solubility in water, alcohols, acetone, ethyl acetate etc., the
solvents which are commonly used to extract different tannins (better say hydrolysable tannin) from natural sources are: aqueous solutions of methanol, ethanol, or
acetone and also ethyl acetate. Nonpolar organic solvents like benzene, n-hexane,
petroleum ether, chloroform, dichloromethane etc. are usually used in pre-treatment
of sample for the purpose of removing lipids and chlorophyll. These non-polar
solvents with low extraction strengths are also beneficial in preventing enzymatic
reactions of tannins (Arapitsas 2012). In case of handling tannins with low
molecular weight or in the processing of matrices containing large amounts of
enzymes (i.e., bark or fruit), extraction is done with methanol treatment, while the
use of acetone is preferentially done with tannins of higher mol. Wt. But the
temperature should be taken into account as higher temperatures for extended times
results in hydrolysis of the galloyl moiety and it may force ellagitannins releasing
ellagic acid (Okuda et al. 1989). In addition, extraction with alcohols like ethanol
and methanol may produce ethyl or methyl esters of gallic acid, respectively
(Mueller-Harvey 2001) (Okuda et al. 1989). Not only the extraction solvents, but
also the other factors like temperature, pH, solvent/material ratio etc. contributes in
the extraction procedure. Some of the extraction process applied in the extraction of
tannins from different plants are shortly described below:
1. Classic extraction is done via boiling the solvent. The optimal conditions for
extracting Assam green tea polyphenols lies under maintaining sample and water
ratio (1:20), with a pH of 4–5, utilizing fresh green tea leaves (Wati et al. 2009).
2. Pressurized liquid extraction is another way of extracting tannins, treating
with the solvent at an elevated temperature (usually between 50 and 200 °C) and
under high pressure (between 10 to 15 Mpa). Herein, the best recoveries were
obtained by way of employing this technique for releasing catechin and epicatechin from grape seeds, as well as from non-fermented tea leaves,
medium-fermented tea leaves, and fermented tea leaves. Such release comes
about within 10 min (Piñeiro et al. 2004).
3. Supercritical carbon dioxide extraction is applicable using supercritical carbon dioxide (SC–CO 2 ) with critical point at 31.1 °C and under a pressure of
73.8 bar. But while performing this technique, one condition should be taken
care of. Normally, carbon dioxide (CO 2 ) is not a suitable solvent for extracting
polyphenols because of its nonpolar nature, hence, to overcome this problem, a
polar organic solvent such as a solution of ethanol and water, can be used during
extraction or before the extraction (Lang and Wai 2001). Ethanol is used in this
technique, as a co-solvent, which was successfully utilized in the extraction of
green tea catechins (Gadkari et al. 2015).
4. Microwave-assisted extraction is done with solvent that is directly heated by
microwaves. The optimal performance of tea polyphenols extraction was
obtained under a microwave intensity of 600 W, a microwave radiation time of
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As tannins show its solubility in water, alcohols, acetone, ethyl acetate etc., the
solvents which are commonly used to extract different tannins (better say hydrolysable tannin) from natural sources are: aqueous solutions of methanol, ethanol, or
acetone and also ethyl acetate. Nonpolar organic solvents like benzene, n-hexane,
petroleum ether, chloroform, dichloromethane etc. are usually used in pre-treatment
of sample for the purpose of removing lipids and chlorophyll. These non-polar
solvents with low extraction strengths are also beneficial in preventing enzymatic
reactions of tannins (Arapitsas 2012). In case of handling tannins with low
molecular weight or in the processing of matrices containing large amounts of
enzymes (i.e., bark or fruit), extraction is done with methanol treatment, while the
use of acetone is preferentially done with tannins of higher mol. Wt. But the
temperature should be taken into account as higher temperatures for extended times
results in hydrolysis of the galloyl moiety and it may force ellagitannins releasing
ellagic acid (Okuda et al. 1989). In addition, extraction with alcohols like ethanol
and methanol may produce ethyl or methyl esters of gallic acid, respectively
(Mueller-Harvey 2001) (Okuda et al. 1989). Not only the extraction solvents, but
also the other factors like temperature, pH, solvent/material ratio etc. contributes in
the extraction procedure. Some of the extraction process applied in the extraction of
tannins from different plants are shortly described below:
1. Classic extraction is done via boiling the solvent. The optimal conditions for
extracting Assam green tea polyphenols lies under maintaining sample and water
ratio (1:20), with a pH of 4–5, utilizing fresh green tea leaves (Wati et al. 2009).
2. Pressurized liquid extraction is another way of extracting tannins, treating
with the solvent at an elevated temperature (usually between 50 and 200 °C) and
under high pressure (between 10 to 15 Mpa). Herein, the best recoveries were
obtained by way of employing this technique for releasing catechin and epicatechin from grape seeds, as well as from non-fermented tea leaves,
medium-fermented tea leaves, and fermented tea leaves. Such release comes
about within 10 min (Piñeiro et al. 2004).
3. Supercritical carbon dioxide extraction is applicable using supercritical carbon dioxide (SC–CO 2 ) with critical point at 31.1 °C and under a pressure of
73.8 bar. But while performing this technique, one condition should be taken
care of. Normally, carbon dioxide (CO 2 ) is not a suitable solvent for extracting
polyphenols because of its nonpolar nature, hence, to overcome this problem, a
polar organic solvent such as a solution of ethanol and water, can be used during
extraction or before the extraction (Lang and Wai 2001). Ethanol is used in this
technique, as a co-solvent, which was successfully utilized in the extraction of
green tea catechins (Gadkari et al. 2015).
4. Microwave-assisted extraction is done with solvent that is directly heated by
microwaves. The optimal performance of tea polyphenols extraction was
obtained under a microwave intensity of 600 W, a microwave radiation time of
21 Tannins and Polyphenols Extracted from Natural Plants …
731