ABSTRACT
Tannins represent the largest group of polyphenols. They are widely distributed in the bark of trees, insect galls, leaves, stems and fruit. Tannins were originally isolated from the bark and insect galls of oak trees. Tannins are high-molecular-weight compounds containing sufficient phenolic hydroxyl groups to permit the formation of stable cross-links with proteins, and as a result of this cross-linking enzymes may be inhibited. Hydrolysable tannins are derived from simple phenolic acids, particularly gallic acid , which is linked to a sugar by esterfication. The gallic acid groups are usually bonded to form dimers such as ellagic acid. Flavonoids are universal within the plant kingdom—they are the most common plant pigments next to chlorophyll and carotenoids. Experiments have proven flavonoids affect the heart and circulatory system and strengthen the capillaries. The presence of numerous aromatic hydroxyl groups on flavonoids enables them to easily attach to enzyme surfaces averting potent inhibition of some enzyme systems in humans.
