Antioxidants are substances that can prevent or delay oxidative damage of lipids, proteins and nucleic acids by reactive oxygen species, which include reactive free radicals such as superoxide, hydroxyl, peroxyl, alkoxyl and non- radicals such as hydrogen peroxide, hypochlorous, etc. They scavenge radicals by inhibiting initiation and breaking chain propagation or suppressing formation of free radicals by binding to the metal ions, reducing hydrogen peroxide, and quenching superoxide and singlet oxygen (Shi et al., 2001). The most abundant antioxidants in fruits are polyphenols and Vitamin C, Vitamins A, B and E and carotenoids are present to a lesser extent in some fruits. These polyphenols, most of which are flavonoids, are present mainly in ester and glycoside forms (Fleuriet and Macheix, 2003).
Antioxidant in Dragon Fruit
The fruits of Hylocereus cacti, known as red pitaya or pitahaya, have recently drawn much attention of growers worldwide, not only because of their red-purple colour and economic value as food products, but also for their antioxidative activity from the betacyanin contents (Wybraniec and Mizrahi, 2002).
In cacti, the most important fruit pigments are the betacyanins and betaxanthins (Wybraniec et al., 2001). Betalains, composed of red-violet betacyanin and yellow
betaxanthins, are water-soluble pigments that provide colours in flowers and fruits. The known betacyanin pigments of Hylocereus polyrhizus flesh are betanin, phyllocactin (60-O-malonylbetanin), and a recently discovered betacyanin, hylocerenin (5-O-[60-O- (300-hydroxyl- 300-methyl-glutaryl)-b-D-glucopyranoside) (Wybraniec and Mizrahi, 2002; Wybraniec et al., 2001). Betacyanin, with 5-O-glycosides (betanin) or 6-O- glycosides, is commonly detected in plants. More-complicated esterification of 5-O- glycosides with hydrocinnamic acids such as ferulic or p-coumaric acids (Strack et al.,
1993) and malonic acid (Kobayashi et al., 2000; Minale et al., 1996) have also been studied.