All over tannin

Edwin Haslam's detailed synthesis of current understanding of plant polyphenolics comprises nine chapters that discuss the structure and biosynthesis of polyphenols, the molecular basis of their bio-activity, and characteristics such as their astringency, pigmentation, medicinal activity and tanning properties.

Chapter one discusses the structure, stereochemistry and biosynthesis of the main groups of polyphenols, and begins to explore the concept of molecular recognition among these so-called "'vegetable tannins". Molecular recognition is discussed in greater depth in the second chapter, including an overview of various types of bonding and the roles of different bonds in polyphenol aggregation, solvation and other molecular interactions. These issues, including the importance of hydration patterns, are examined further in chapter three.

The remaining chapters refer back to the concepts discussed in these early discussions. Chapter four looks at taste, bitterness and astringency, properties commonly associated with the plant polyphenols. It begins with a discussion of the arguments for and against chemical co-evolution, before moving on to such issues as food choice in herbivores, the potential anti-nutritional and phytotoxic effects of plant phenolics, and the role of polyphenols in taste and flavour in a range of alcoholic and non-alcoholic beverages.

The changes in these properties - taste and astringency - that occur during fruit maturation are discussed in chapter five, which explores the idea that it is not so much polyphenol content but the size of the polyphenols present that is responsible for these changes.

Chapter six discusses the nature and chemistry of plant phenols responsible for flower pigmentation. It introduces the role of such carbohydrates and metal ions as well as other relevant phenomena such as co-pigmentation, self-association and the effects of cellular pH. The chapter concludes with a discussion of the molecular basis of pigmentation, and the possibility of using DNA technology to develop, for example, a blue rose.

Chapter seven reviews the medicinal properties of polyphenols, revisiting issues of molecular recognition within the context of anti-HIV activity and free-radical scavenging. It is disappointing that this section only briefly mentions the more traditional aspects of polyphenol-based medicinals such as their bacteriocidal, molluscicidal and wound-healing properties.

The final two chapters discuss the oxidative polymerisation of tannins and their role in industry, first examining this process in relation to the fermentation of tea and cocoa, and then addressing this same issue within the context of leather-making. This final chapter begins with a discussion of the history of leather-making and the nature, biosynthesis and fibrillogenesis of collagen. The text concludes with a discussion of the molecular basis of the tanning process and the durability of leathers based on the use of different tannins, followed by a bibliography of 700 references; an indication of the book's depth of detail.

While difficult to read - alternating between discussions of molecular structure and intermolecular interactions and quotations from Pliny, Robbie Burns and David Lodge - this book offers a rich source of information on a little-understood group of plant products. Although the level of detail and lack of introductory explanation as to the nature and occurrence of plant polyphenols render the book more suitable to the experienced researcher than the novice, the historical discussions of polyphenol research will be of interest to students of plant chemistry.

Catherine Cotton, formerly lecturer in ethnobotany, Roehampton Institute, London, is based in Amsterdam.