1 Introduction

Polymeric phenolics comprise a substantial portion of the phytochemicals in the human diet.1 They are often designated by the term tannin, as defined by Bate-Smith and Swain, which refers to water-soluble phenolic compounds having the ability to precipitate alkaloids, gelatin and other proteins.2 This property has been exploited when using plant extracts for converting raw animal hides and skins to durable leather for over 2000 years.3 It is also responsible for numerous characteristic features of tannins, including in particular astringency, which results from interactions with salivary proteins,4 ^,5 formation of haze in protein-rich beverages6 and antinutritional properties of tannin-rich animal feeds.7

Tannins are classically divided in two groups: hydrolysable tannins, deriving from gallic and ellagic acids, and condensed tannins which are also called proanthocyanidins because they release red anthocyanidin pigments when heated in acidic media. In addition, transformation products of plant polyphenolic compounds formed during food processing (especially in wine or tea) may bind to proteins, and thus be regarded as tannins. These derived phenolic compounds are called here tannin-like compounds as recently proposed.8 Although some foods may also contain hydrolysable tannins, polymeric dietary polyphenols belong mostly to the last two categories, which will be treated in this chapter.

There have been extensive reviews covering the nomenclature, structure and distribution of proanthocyanidins in plants9 ^,10 as well as the dietary significance of proanthocyanidins and tannin-like compounds.1 ^,8 Therefore, this chapter will address mostly analytical aspects, with special emphasis on the...