Tannins are generally taken to include a whole group of substances which have certain chemical and physical properties in common but which are ostensively not related structurally. To reach a satisfactory definition it is necessary to consider which chemical and physical characters are essential to give tannins their necessary properties. Swain and Bate-Smith (1962) defined as tannins "all naturally occurring substances which have chemical and physical properties akin to those which are capable of making leather. This means that they would be water-soluble phenolic compounds, have molecular weights lying between 500 and 3,000, and besides giving the usual phenolic reactions, have special properties such as the ability to precipitate alkaloids, gelatin and other proteins." Walker (1975) considers that their molecular weight can reach 5,000, and that tannins contain sufficient phenolic hydroxyl groups to permit the formation of stable cross-links with proteins, and as a result cross-linking enzymes may be inhibited. Because of these protein-binding properties, they are of considerable importance in food processing, fruit ripening, manufacture of cocoa, wine, etc... Their occurrence is usually associated with the woody habit and they are rarely found in herbaceous angiosperms. The presence of tannins in trees and woody shrubs serves to render the material bitter or astringent and therefore less palatable to grazing animals.

Tannins are classified into two broad groups: the hydrolysable and the condensed or non-hydrolysable tannins. The hydrolysable tannins are usually compounds containing a central core of glucose or other polyhydric alcohol esterified with gallic acid (gallotannins) or hexahydroxydiphenic acid (ellagitannins). The condensed ones are mostly flavolans or polymers of flavan-3-ols (catechins) and/or flavan 3:4-diols (leucoanthocyanidins). They are more resistant to breakdown. Quite frequently the tannins extracted from a plant bears the characteristics of both groups. The condensed tannins are characteristic of most of the important tanning material, such as wattle or "mimosa", quebracho, mangrove (see picture below) and hemlock. Chestnut wood (Castanea sativa and C. dentata) and dry myrobalan fruits (Terminalia chebula) are the most important tanning material for the hydrolysable group.

In the Vegetable Kingdom, tannins are more prevalent among the Angiosperms. The Gymnosperms have classes in which tannin is well developed, familiar examples being Pinus, Picea and Tsuga. Normally the Monocotyledons are poorly represented in species rich in tannin. The palm family (Palmae) however affords an exception and tannin is well developed in some species. Among the Dycotyledons, there are many families in which tannins occur very freely, the most notable being the Leguminosae (e.g. black wattle), Anacardiaceae (e.g. quebracho), Combretaceae (e.g. myrabolan), Rhizophoraceae (e.g. mangroves), Myrtaceae (e.g. Eucalyptus) and Polygonaceae (e.g. canaigre). The family Myristicaceae (to which the nutmeg belongs) is of special interest on account of the distinctive tanniferous tubes that occur in the rays of the wood of all species. Plants rich in tannin occur both in temperate and in tropical or subtropical climates. Most of the commercially important tanning materials are products of warm countries. A recent research carried out in Australia (Yazaki, 1997) has shown that Acacia storyi, which is grown in Queensland, has a high polyflavanoid content and exceptionally high tannin purity. This makes A. storyi a potential tanning-producing species.

Tannin may occur in almost any part of a plant - root, stem or trunk, bark, leaves, fruit and even hairs. It may occur either in isolated individual cells, in groups or chains of cells (more common) or in special cavities or sacs. In living plant tissue, tannin is present chiefly in solutions in the vacuoles. As the cell ages and loses its protoplasmatic contents, the tannin commonly becomes absorbed in the cell wall. Certain special plant structures may be rich in tannins, particularly those associated with movement. Tannins are often to be found in gland cells, the cells of pulvini (swollen leaf stalk bases) and in the tissue caused through pathological conditions, e.g. in plant galls. Certain plant galls constitute the richest source of tannins in the Vegetable Kingdom. The young, actively-growing tissue of plants is also liable to be very rich in tannins. Some species may contain 50 % tannin in dried young material. However, in general the highest concentration of tannin in normal healthy plants is to be found in the bark.

Tannins or solutions of tannins are known to be toxic to many plants. Concentrations of tannins as low as 0.1% or 0.8 % have been shown to retard the growth of a large number of parasitic fungi. Bianco and Savolainen (1997) described harmful effects of tannins to human healthy. Some of the wood tannin fractions are tumorigenic and they contain complex groups of conjugated and substituted phenols with more than 500 individual congeners being identified. Oak dust particles are classified among the most toxic. Their identification may be possible by newly developed chromatographic methods for flavonoids and phenolic acids in the isolated wood tannin fractions (Conde et al, 1995). Chromatographic analysis is very useful for taxonomic purposes and it can also be applied to monitoring of workplaces with notable hardwood dust exposure.

Tannins have been used in a broad range of fields, e.g. manufacture of leather, food processing, etc... More recently, research have been undertaken in order to determine the suitability of tannins in wood preservatives formulations, as adhesives for wood panel, etc... In fact,, in Slovenia for example, the amount of tannins which can be produced could be 50 % of the amount of adhesives used in the wood panels industry there, most of it coming from bark of Spruce (Tisler et al, 1998). Another interesting use of tannins is in preservation treatments for wood. Their relative low toxicity prevents their use as wood preservative on their own. However, they can fix biocides because of their excellent chelating properties. Co-impregnation in a two-step treatment of copper, zinc and boron with tannins has been examined, and the treated wood met the European standard for protection against rots (Scalbert et al, 1998). The use of boric acid plus tannin extract in an one-step fixation process has also been studied very recently (Thevenon et al., 1998). This shows the interest that is still generated by tannins and their multiple applications.

Bianco, M.A. & Savolainen, H. (1997). Phenolic acids as indicators of wood tannins. Science of the Total Environment 203:1, 79-82.

Conde, E., Cadahia, E. & Garcia-Vallejo, MC. (1995). A chromatographic method for analysis of oak wood tannins. Chromatographia 41: 657-663.

Haslam, E. (1966). Chemistry of Vegetable Tannins. Academic Press London and New York.

Howes, F.N. (1953). Vegetable Tanning Materials. London Butterworths Scientific Publications.

Ribéreau-Gayon, P. (1972). Plant Phenolics. Oliver & Boyd Edinburgh.

Scalbert, A., Cahill, D., Dirol, D., Navarrete, M.A., Troya, M.T., Leempur, M. van, De Troya, M.T. (1998). A tannin/copper preservation treatment for wood. Holzforschung, 52:2, 133-138.

Thevenon, M.F., Pizzi, A., Haluk, J.P. (1998). One-step tannin fixation of non-toxic protein borates wood preservatives. Holz als Roh und Werkstoff 56:1, 90.

Tisler, V., Devjak, S., Merzelj, F. (1998). Possibility of tannin production in Slovenia. Holzforschung und Holzwertung 50:1, 11-13.

Walker, J.R.L. (1975). The Biology of Plant Phenolics. Edward Arnold Lt.

Yazaki, Y. (1997). Acacia storyi: a potential tannin-producing species. Australian Forestry 60:1 24-28.

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