In their natural form, tannins are water-soluble phenolic compounds with molecular weights in the range of 300 to 500 and have the ability to precipitate gelatin, alkaloids, and proteins. Tannins are secondary metabolites widely distributed in plants: they are polymeric phenolic substances with astringency properties.
Tannins are classified into two classes. One group is comprised of hydrolysable polyhydric alcohol in which the hydroxyl groups are partially or completely esterified by gallic acid or similar kind of compounds. Three types of hydrolysable tannins: gallotannines, ellagitannines, and complex tannins (sugars derivatives—mainly glucose, gallic acid, and ellagic derivatives).
Hydrolysable tannins are mainly present in angiosperm and dicotyledons. Both gallotannins and ellagitannins may synthesize individually or in the form of a mixture in plants.
The other group is composed of tannins which are formed by condensation of phenolic compounds. Those are non-hydrolysable and are known as condensed tannins. Condensed tannins (also referred to as proanthocyanidins) are flavonoid units linked by carbon–carbon bonds that are not susceptible to cleavage by hydrolysis. These compounds are responsible for the intense pink, red, purple, or blue colors of many flowers, fruits, and leaves.
The antioxidant properties of tannins are widely utilized in the food and medical fields. Owing to its antioxidant capacity, such as preventing cardiovascular disease, cancer or osteoporosis, tannins have attracted much attention.
Tannins in plant food
