Tannins Background

Tannins have been defined as water-soluble polyphenolic compounds with molecular weights between 500 and 3000 that have the ability to precipitate alkaloids, gelatin, and other proteins (1). The usefulness of such a definition stems from the original interest in these substances in the tanning process, that is, the conversion of hides to leather, and is enhanced by the recent recognition of other important chemical and physical properties. The tanning of hides has been practiced since around 1500 b.c. The process involves the cross-linking of adjacent collagen chains with polyphenol attachment at several points. The molecular weight limitations apply because a minimum molecular size is required to engage protein chains; too large a molecule cannot easily diffuse into the collagen fibers (2). The plant polyphenols that exhibit the property of tannage also interact with other proteins, peroxides, and many other biological molecules. This has broad implications for fields as diverse as plant evolution, animal nutrition, other aspects of animal physiology, food technology, and, possibly, human therapeutic applications. The significance of the plant polyphenols referred to as tannins goes far beyond their role in the production of leather.

Tannin molecules are complex. Not all have been completely characterized or even recognized. Basically, there are two types: proanthocyanidins, often referred to as condensed; and glucose polyesters of gallic or hexahydroxy-diphenic acids, also known as hydrolyzable. The polyphenols are secondary metabolites and not integral parts of the primary growth, energy producing, or reproductive processes of the plant.

Examples of the two major types of tannins are shown in Figure 1.

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