Written by: Dr. Jacquie Jacob, University of Kentucky
Tannins are defined as "phenolic compounds of high molecular weight." A phenol is a phenyl (-C6H5) bonded to a hydroxyl (-OH) group. Polyphenols, including tannins, have many of these phenols connected together.
Tannins are responsible for the astringent taste of some leaves, fruits, and wines. Tannins are found in plant leaves, bark, fruit, wood, and roots. They have been closely linked with plant defense mechanisms against ruminant animals, birds, and insects. They act as anti-nutritional factors when included in the the diet of animals. Tannins have the ability to bind protein. Digestive proteins can bind with dietary tannins, making the proteins unavailable to the animal.
Diets high in tannin content have been found to reduce growth rate of animals. Brown sorghums, for example, are high in tannins, reducing their suitability as a feedstuff for animal diets.