21.3 Classification of Tannins
Chemically, tannins are high molecular weight compounds, which are soluble in
water and can be broadly classified in various types according to their complexity of
their chemical nature or according to the structural moieties or according to the
behavior on dry distillation or on the action of enzyme tannase. Firstly, on the basis
of the chemical nature or better to say, according to the functionality, tannins are
classified as “true tannins’ and “pseudo tannins”. In case of this first divisional
group, true tannins are those, which is having high molecular weight and give
positive result in Goldbeater’s skin test (Rangari 2008), so they can be used as
tanning agents. Whereas, the latter is of low molecular weight, do not respond to the
Goldbeater’s skin test i.e. they cannot be used as the tanning agents. The basic
nucleus of true tannins and pseudo tannins are also in contrast like, true tannins are
made of gallic acid or egallic acid derivatives but the basic nucleus of pseudo
tannins are chlorogenic acid or catechins (Shah 2009). Catechins, and chlorogenic
acid from cocoa and Nux vomica respectively are two common examples of
pseudotannins (Ashok and Upadhyaya 2012). True tannins are further classified as
hydrolysable tannins, condensed tannins and complex tannins, according to their
behavior in acid hydrolysis. Hydrolysable tannins are those, which upon hydrolysis
by acids like HCl or H 2 SO 4, yield gallic acid or ellagic acid. Since, it is a pyrogallol
type of tannin, further dry distillation of gallic acid and ellagic acid gives pyrogallol
(Ramakrishnan and Krishnan 1994). On the other hand, condensed tannins are
resistant to hydrolysis by acids. Instead of hydrolysis, they depolymerize or
decompose into red or brown colored pigment called phlobaphene (Foo and
Karchesy 1989). So, the hydrolysable tannins comprise gallotannins and hexahydroxydiphenic acid made polyesters, whereas condensed tannins contain favan-3-ol
nuclei (proanthocyanidins) (De Bruyne et al. 1999). Complex tannins are mixtures
of both, hydrolysable and condensed tannins. But according to modern studies,
structural characteristics leads to categorise tannins into four major groups i.e.
gallotannins, ellagitannins, complex tannins and condensed tannins (Khanbabaee
and Van Ree 2001).
21.3.1 Gallotannins
Gallotannins are one of the compounds, belonging to the subclass of hydrolysable
tannins because it can be hydrolysed by mineral acids and yield gallic acid. It is a
polymer, obtained from the condensation of the carboxy group (–COOH) of gallic
acid and its polymeric derivatives with the hydroxyl groups (–OH) of a carbohydrate (mainly glucose). In case of partial substitution with galloyl unit (1), the
remaining –OH groups of polyol can make substitutions with various other residues
or remain un-substituted (Mueller-Harvey 2001).
21 Tannins and Polyphenols Extracted from Natural Plants …
723
Chemically, tannins are high molecular weight compounds, which are soluble in
water and can be broadly classified in various types according to their complexity of
their chemical nature or according to the structural moieties or according to the
behavior on dry distillation or on the action of enzyme tannase. Firstly, on the basis
of the chemical nature or better to say, according to the functionality, tannins are
classified as “true tannins’ and “pseudo tannins”. In case of this first divisional
group, true tannins are those, which is having high molecular weight and give
positive result in Goldbeater’s skin test (Rangari 2008), so they can be used as
tanning agents. Whereas, the latter is of low molecular weight, do not respond to the
Goldbeater’s skin test i.e. they cannot be used as the tanning agents. The basic
nucleus of true tannins and pseudo tannins are also in contrast like, true tannins are
made of gallic acid or egallic acid derivatives but the basic nucleus of pseudo
tannins are chlorogenic acid or catechins (Shah 2009). Catechins, and chlorogenic
acid from cocoa and Nux vomica respectively are two common examples of
pseudotannins (Ashok and Upadhyaya 2012). True tannins are further classified as
hydrolysable tannins, condensed tannins and complex tannins, according to their
behavior in acid hydrolysis. Hydrolysable tannins are those, which upon hydrolysis
by acids like HCl or H 2 SO 4, yield gallic acid or ellagic acid. Since, it is a pyrogallol
type of tannin, further dry distillation of gallic acid and ellagic acid gives pyrogallol
(Ramakrishnan and Krishnan 1994). On the other hand, condensed tannins are
resistant to hydrolysis by acids. Instead of hydrolysis, they depolymerize or
decompose into red or brown colored pigment called phlobaphene (Foo and
Karchesy 1989). So, the hydrolysable tannins comprise gallotannins and hexahydroxydiphenic acid made polyesters, whereas condensed tannins contain favan-3-ol
nuclei (proanthocyanidins) (De Bruyne et al. 1999). Complex tannins are mixtures
of both, hydrolysable and condensed tannins. But according to modern studies,
structural characteristics leads to categorise tannins into four major groups i.e.
gallotannins, ellagitannins, complex tannins and condensed tannins (Khanbabaee
and Van Ree 2001).
21.3.1 Gallotannins
Gallotannins are one of the compounds, belonging to the subclass of hydrolysable
tannins because it can be hydrolysed by mineral acids and yield gallic acid. It is a
polymer, obtained from the condensation of the carboxy group (–COOH) of gallic
acid and its polymeric derivatives with the hydroxyl groups (–OH) of a carbohydrate (mainly glucose). In case of partial substitution with galloyl unit (1), the
remaining –OH groups of polyol can make substitutions with various other residues
or remain un-substituted (Mueller-Harvey 2001).
21 Tannins and Polyphenols Extracted from Natural Plants …
723