Hydrolyzable Gallotannin

Depending on their structures, tannins are defined as hydrolyzable (gallotannins and ellagitannins) or condensed (monomers, dimers, oligomers, and polymers of flavan-3-ols). Condensed tannins are also known as proanthocyanidins [3,47]. Proanthocyanidins can be divided into propelargonidins, based on the hydroxylation pattern of the A- and B-rings [3]. Of these, procyanidins constitute the most common subclass of flavonoids in foods, and prodelphinidins and propelargonidins are also present [48,49]. 

The most widely used method is based on the reaction between potassium iodate and hydrolyzable tannins (Hartzfeld and others 2002). This method provides a good estimate for gallotannins but underestimates the content of ellagitannins. 

Other analytical assays proposed for the quantification of hydrolyzable tannins in plant materials include the rhodanine assay for the estimation of gallotannins (Berardini and others 2004) and sodium nitrate for the quantitative determination of ellagic acid (Wilson and Flagerman 1990). 

Degradation of Gallotannins. Different investigations on tannase revealed that this enzyme was not equally efficient on all hydrolyzable tannins. This was particularly true for yeast, whose tannase was effective only on tannic... 

Chart 8.1.S Shikimates (skeletons) widespread on land (5max=57, av=43 5//ftnax=0.48, av=0.39). Tannins lignans epigallocathechin gallate (Asian black tea. Camellia sinensis L.) Kuntze, Theaceae, Ang. MI) procyanidins and prodelphinidins (widespread in plants) soluble hydrolyzable tannins gallotannin (myrobalan. East Indian trees, Terminalia spp., oak, Quercus spp., Lithocarpus spp., and sumac, Rhus spp.) ellagitannin (Myrtaceae). 

Complex tannins are defined as tannins in which a catechin unit (1.39) is bound glycosidically to either a gallotannin or an ellagitannin unit. As the name implies, the structure of these compounds can be very complex. An example is Acutissimin A (1.99). This is a flavogallonyl unit bound glucosidically to Cl, with an additional three hydrolyzable ester bonds to a D-glucose-derived open-chain polyol. 

C.I. Natural Brown 6 (constitution not available), basic structure gallic acid (gallotannin 1), is based on a large variety of vegetable products having a high content of hydrolyzable tannin, which mostly form gallic acid when hydrolyzed. 

Hartzfeld, P.W. Forkner, R. Hunter, M.D. Hagerman, A.E. 2002. Determination of hydrolyzable taimins (gallotannins and ellagitannins) after reaction with potassium iodate. J. Agric. Food Chem. 50 1785-1790. 

Fig. 7.2 Subgroups of hydrolyzable tannins gallotannin, ellagitannin, dehydroellagitannin, C-glucosidic ellagitannin and complex tannin.

Tannins are a broad class of complex phenolic compounds that are comprised of two chemical groups the hydrolyzable tannins (gallotannins) and the condensed tannins (proanthocyanidins). Tannins bind to and precipitate proteins, producing the astringent activity of tannin-containing herbs. Tannins are natural components of many herbs and common foods, and some tannins are used in the processing of foods, alcoholic beverages, and medicines. 

Jacob and Pignal [25] reported the growth and hydrolytic action of tannins by 6 strains of yeast which were isolated from tanning liquors and xylophagous insects in culture media, containing various concentrations of tannic acid. Hydrolyzable tannins, except gallotannins, were not hydrolyzed. 

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