Phenolics comprise a large complex group of compounds which occur in many fruits and vegetables, but are not found in food of animal origin. Often, as in the case of flavonoids, they occur in high concentrations, and are consumed in relatively large quantities. Many phenolics have therefore been tested for carcinogenesis, often with conflicting results. Reviews of the phenolic compounds which occur in plant foods have been published (Singleton 1981; Deshpande et ah 1984; Natori et al. 1987). As several phenolics occur in high concentrations in plant products which have been implicated in esophageal cancer, for example red wine, tea and bracken, they are very relevant to the etiology of the disease.
Simple phenolics occur as di- and tri-hydroxyphenols, usually as derivatives of cinnamic acid or benzoic acid (Fig. 7.1). One or more of the hydroxyl groups may be substituted, as with ferulic acid. Two or more monomeric phenols may condense together, as with the formation of ellagic acid from two molecules of gallic acid, and of chlorogenic acid from caffeic acid and a derivative of gallic acid (Fig. 7.2).
The flavonoids comprise a large family of phenolic plant pigments with chemical structures based on that of flavone or flavonol (Fig. 7.3). Commonly occurring derivatives consumed by man are catechin and anthocyanidin, both found more often in condensed forms as in tannins, and quercetin, which occurs mainly in the form of its glucoside, rutin (Fig. 7.4).