Flavolans (Condensed Tannins)
acid and its positional isomer gentisic acid, which are referred to as benzoic acid derivatives (Figure 7.2). To increase the solubility and target the phenolics to specific parts of the plant, and to prevent their enzymatic and chemical degradation, phenolic phytochemicals are often esterified with sugars and other chemical components such as quinic acid through the hydroxyl groups of the phenolic ring. The esters are sugars and phenolic phytochemicals are called glycosides. A majority of glycosides contain glucose, but the glycosides of phenolics with galactose, sucrose, and rhamnose have also been described (11).
Flavonoids and biphenyls are another important class of phenolic phytochemicals that are especially rich in fruits and legumes. Flavonoids like quercetin constitute the most abundant group of phenolic phytochemicals. Their common structure consists of two aromatic rings linked through three carbons that usually form an oxygenated heterocycle; this type of chemical arrangement is often described as diphenylpropanes (C6-C3-C6) (2,11). Structural variations within the rings resulting in an alteration in the extent of hydroxyl-ation, methylation, isoprenylation, dimerization, and glycosylation (producing O- or C-glycosides) subdivide the flavonoids into several families: flavonols, flavones, isofla-vones, antocyanidins and others (Figure 7.2).
Phenolic phytochemicals are ubiquitous in nature and are especially rich in fruits (12,13). Fruits such as berries (14,15) are known to be good sources of phenolic antioxidants having protective health benefits. Fruit beverages such as wines have been shown to
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