*4.2.8.1 Application*

Flavonoids are generally found in many fruits and vegetables. When human increasingly consumed it, flavonoids have been linked with a decrease in the incidence of diseases such as prostate [56, 57] or breast cancer [58, 59].

#### *4.2.9 Tannins*

Tannins are relatively high-molecular compounds, which comprise the third essential group of phenolics and can be divided into condensed and hydrolysable tannins. Condensed tannins are produced by the polymerization of flavonoid units. The mainly studied condensed tannins are based on flavan-3-ols: (−)-epicatechin and (+)-catechin. Hydrolysable tannins are heterogeneous polymers containing phenolic acids, in particular, gallic acid (3,4,5 trihydroxyl benzoic acid) and simple sugar [42, 55].

#### *4.2.9.1 Application*

Because of tannin features, such as being the potential metal ion chelators, protein-precipitating agents, and biological antioxidants, tannins have different effects on biological systems. As a consequence of the diverse tannins biological roles and structural variation, it has been difficult to modify models that would let a precise prediction of their effects in any system. Therefore, the tannin structure modification and activity relationship are important to predict their biological effect [42].
