Tannins in Fruits and Vegetables: Chemistry and Biological Functions

Tannins are highly hydroxylated compounds that include a large diversity of chemical structures. Tannins are widespread in plant foods, particularly in legume seeds, cereal grains, fruits, nuts, and vegetables, as well as in different beverages such as wine, tea, and cider. Currently, the interest in tannins as bioactive food components is increasing, especially due to their wide range of biological activities and to the increased incidence of deadly infectious and chronic degenerative diseases. Tannins have been classified according to their monomeric units into three main groups: hydrolyzable tannins (HTs), proanthocyanidins or condensed tannins (PAs), and complex tannins. This chapter focuses on the first two groups. Most studies in literature only deal with lower molecular weight tannins, and most of them are about easily extracted tannins. HTs are frequently quantitated by measuring the free hydrolysis products, i.e. gallic acid and ellagic acid.