Biosynthesis, Biodegradation, and Cellular Localization of Hydrolyzable Tannins

Abstract

Tannins occupy a prominent position among plant polyphenols, not only because of their widespread occurrence in the plant kingdom but also on grounds of their technological, ecological, and medical significance. Unfortunately, it is impossible to describe simply this large group of natural products on the basis of a common, general chemical structure. Like some other heterogeneous groups of secondary plant constituents (essential oils, for instance), several different criteria are required to define tannins, the decisive factor traditionally being their tendency to interact with aqueous solutions of proteins and other biological macromolecules to form insoluble precipitates. A prominent example of this property is tannage, i.e. the conversion of raw animal hides to leather. Other tannin effects regard haze formation in beer, the taste, palatability, and digestibility of wines, fruit, vegetables, and leaves (which is thought to deter herbivores), or the inactivation of enzymes, particularly microbial exoenzymes. These manifold effects cannot be ascribed to a particular tannin structure, except for the general requirement of a high molecular weight of 800–3,000 daltons and sufficient phenolic hydroxy groups to allow complexation.