Classification And Antioxidant Activity Of Polyphenols

Polyphenols are a group of natural compounds with phenolic structural features (C. Rice-Evans, Miller, & Paganga, 1997; C. A. Rice-Evans, Miller, & Paganga, 1996; Tripoli et al. , 2005; Tsao, 2010; Zheng & Wang, 2001) with two or more hydroxyl groups on the aromatic ring(s) (Cartea et al. , 2010). They are secondary metabolites that plants produce to protect themselves from other organisms (Tsao, 2010). Polyphenols are strong antioxidants that complement and add to the functions of antioxidant vitamins and enzymes (Tsao, 2010) as a defense against oxidative stress caused by excess reactive oxygen species (Lima et al. , 2014). Dietary phenolics or polyphenols constitute one of the most numerous and widely distributed groups of natural products in the plant kingdom. More than 8000 phenolic structures are currently known (Cartea et al. , 2010; Garcia-Salas et al. , 2010; Tsao, 2010) and among them over 4000 flavonoids have been identified (Tsao, 2010).

Fruits and vegetables are rich sources of polyphenols and majority of polyphenols in plants exist as glycosides with different sugar units and acylated sugars at different positions of the polyphenol skeletons (Tsao, 2010). Polyphenols are classified into; flavonoids, polyphenolic amides, non-flavonoid polyphenols (Cartea et al. , 2010). Phenolic acids are non-flavonoid polyphenolic compounds which can be further divided into two main types, benzoic acid derivatives that include Protocatechuic acid, Vanillic acid, Gallic acid, Syringic acid, and cinnamic acid derivatives that includes Caffeic acid, Chlorogenic acid, sinapic acid Cryptochlorogenic acid, Neochlorogenic acid, p-coumaric acid and ferulic acid basing on C1–C6 and C3–C6 backbones (Dai & Mumper, 2010; Tsao, 2010). The antioxidant activity of phenolics depends on the numbers and positions of the hydroxyl groups in relation to the carboxyl functional group (Garcia-Salas et al. , 2010). Thus the higher the degree of hydroxylation the higher the antioxidant activity of phenolic acids. Polyphenols also modulate the activity of a wide range of enzyme and cell receptors and in addition to having antioxidant properties, polyphenols have several other specific biological actions in preventing and or treating diseases (Dai & Mumper, 2010). Fruits and vegetables contain many free phenolic acids (Tsao, 2010).

These polyphenols are produced in plants as secondary metabolites via the shikimic acid (shikimate) pathway and phenylalanine ammonialyase (PAL) is the key enzyme catalyzing the biosynthesis of phenolics from phenylalanine amino acid (Cartea et al. , 2010) and the acetic acid path way (Giada, 2013). Vegetables contain phenolics varying from simple (for example phenolic acids, anthocyanins) to highly polymerized substances (for example tannins) in different quantities (Dai & Mumper, 2010). Polyphenols affect the sensory attributes of vegetables, they determine the colour, taste, aroma and flavor and also responsible for the astringent and bitter taste (Garcia-Salas et al. , 2010; Lima et al. , 2014).