branched]] (with 4 6 bounds - dotted line). Condensed tannins (or polyflavonoid tannins, catechol-type tannins, non-hydrolyzable tannins or flavolans) are polymers formed by the condensation of flavans. They do not contain sugar residues.
Different types of condensed tannins exist such as the proanthocyanidins, prodelphinidins, profisetinidins, proguibourtinidins or prorobinetidins.
one particular type of condensed tannin, found in grape are proanthocyanidins, which are polymers of 2 to 50 (or more) flavonoid units joined by carbon-carbon bonds. These are not susceptible to being cleaved by hydrolysis.
While hydrolyzable tannins and most condensed tannins are water soluble, some very large condensed tannins are insoluble.
Condensed tannins from Lithocarpus glaber leaves have a potent free radical scavenging activity. Condensed tannins can be found in Prunus sp.
Condensed tannins can be characterised by a number of techniques including depolymerisation, asymmetric flow field flow fractionation or small-angle X-ray scattering.
Depolymerisation reactions are mainly analytical techniques but it is envisaged to used them as means to produce molecules for the chemistry industry derived from waste products such as bark from the wood industry or pomaces from the wine industry.
The butanol hydrochloric acid iron assay for depolymerisation of condensed tannins has limitation.
Non oxidative chemical depolymerisation
The condensed tannins can nevertheless undergo acid-catalyzed cleavage in the presence of (an excess of) a nucleophile like phloroglucinol (reaction called phloroglucinolysis), benzyl mercaptan (reaction called thiolysis), thioglycolic acid (reaction called thioglycolysis) or cysteamine. These techniques are generally called depolymerisation and give informations such as average degree of polymerisation or percentage of galloylation. These are SN1 reactions, a type of substitution reaction in organic chemistry, involving a carbocation intermediate under strongly acidic conditions in polar protic solvents like methanol. The reaction leads to the formation of free and derivated monomers that can be further analyzed. The free monomers correspond to the terminal units of the condensed tannins chains.
Reactions are generally made in methanol, especially thiolysis, as benzyl mercaptan has a low solubility in water. They involve a moderate (50 to 90 C) heating for a few minutes. Epimerisation may happen.
Phloroglucinolysis can be used for instance for proanthocyanidins characterisation in wine or in the grape seed and skin tissues.
Thioglycolysis can be used to study proanthocyanidins or the oxidation of condensed tannins. It is also used for lignin quantitation. Reaction on condensed tannins from Douglas fir bark produces epicatechin and catechin thioglycolates.
Condensed tannins from Lithocarpus glaber leaves have been analysed through acid-catalyzed degradation in the presence of cysteamine.
es:Tanino condensado fr:Tanin condens