Flavonoids represent a large group of biologically active plant compounds that have been shown to possess health benefits to humans who consume them regularly. Many flavonoids can become polymerised into much larger molecules as a result of metabolism within plants or during processing in the manufacture of particular foods such as black tea. These larger polyermerised molecules are called tannins and represent an important group of biologically active molecules (figure 1). Tannins have the chemical property that they are able to precipitate proteins and have been used historically to treat burns topically because of their ability to form a protein-tannate film that protects the wound. A number of groups of tannins are recognised including the condensed tannins, the derived tannins and the hydrolysable tannins.
Figure 1 Tannin subclasses and their characteristics1
Condensed tannins are also referred to as proanthocyanidins and are one of the most important groups of tannins because they are found widely in a number of commonly eaten foods such as apples, berries, chocolate, red wine and nuts. The most important proanthocyanidins in human nutrition are the procyanidins, prodelephinidins and propelargonidins, which are polymerised epicatechins, epigallocatechins and epiafzelechins respectively. Derived tannins are a group of compounds formed during the manufacture of certain produces such as red wine, coffee and black and oolong tea. The most widely known compounds of this group are the theoflavins present in tea. Hydrolysable tannins are present in foods such as grapes and wine and consist of gallic acid or ellagic acid to which a sugar or quinic acid is esterified. Hydrolysable tannins are not well researched and little is known about their health effects.
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1Beecher, G. R. 2003. Overview of dietary flavonoids: nomenclature, occurrence and intake. Journal of Nutrition. 133: 3248S-3254S
