Classification of vegetable Tannin

Plant polyphenols can be classified into the following three groups based on their structural characteristics: 

• Hydrolysable Tannins, with subgroup gallotannins
• Condensed tannins and
• Complex tannins

1. Hydrolysable Tannin

The so-called 'hydrolysable' or 'Pyrogallol' tannins are the saccharide-based compounds, in which the aliphatic hydroxyls are esterified by carboxylate species carrying pyrogallol group (1,2,3-trihydroxybenzene). 

1,2,3-trihydroxybenzene

The tannin are typically based on glucose, but there are variations in which derivatives of glucose can be the central moiety (moiety: a part or functional group of a molecule), such as sucrose.

They can be separated into two groups: the gallotannins and the ellagitannins.
In each case, the properties dominated by the ease of hydrolysis of
the ester linkages and the reactivity of  the phenolic hydroxyl groups.

The glucose core IS esterified only with gaffic actd. less

common polyphenols.

The comrnon gallete groups can be esterified themselves, at thew phenolic hydroxyls: this is called depslde esterification (two or moro monocyclic aromatic units linked by an ester bond).

2. Condensed tannin

The hydroxyl in the 3-pcsition in the central C-ring may be cis or trans 'with the B ring. as exemplified by gallocatecnin(Fig. 31).

The condensed tannins are believed to be more closely related to flavans and anthocyanidin group of compounds. which are nonhydrolysable.

In condensed tannins, mono, di, and trihydric phenolic nuclei are present, probably directly linked through heterocyclic ring containing oxygen.