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Nutritional Effects of Tannins in Animal Feeds

The term ‘tannin’ is difficult to define precisely, but the essential property of tannins is that they are naturally-occurring plant polyphenols which combine with proteins and other polymers such as cellulose, hemicellulose and pectin, to form stable complexes. In traditional tanning the collagen chains of hides are crosslinked by tannins to give a durable leather resistent to microbial attack. Tannins proper comprize a relatively small part of a large and diverse group of plant phenolics which range from simple C,-C, phenolic acids such as gallic and coumaric acids, through the C,, flavanoids to the highly-polymerized inert lignins. The molecular weight of tannins is usually in the range 500-3000, requirements being that the tannin is soluble and small enough to orientate itself suitably between the protein chains, but has sufficient phenolic groups to crosslink efficiently under suitable conditions of concentration and pH (Goldstein & Swain, 1963). Much confusion exists in the older literature because simple phenolics such as chlorogenic acid which do not have tannin properties in terms of protein precipitation or astringency react with the much-used Folin-Dennis reagent in the same way as true tannins. At the upper end of the molecularweight scale the phenolic polymers become less able to align themselves between protein components as they increase in size. They become less astringent and reactive towards proteins, their solubility decreases and eventually they become bonded to cell-wall components, when they may be measured along with lignin. Although tannins are chemically a diverse and ill-defined group it is usual to divide them into two types (a) the hydrolysable and (b) the condensed tannins. The hydrolysable tannins consist of a carbohydrate moiety in which the hydroxyl groups are esterified to gallic acid