An investigation of antioxidant capacity of fruits in Singapore markets

Abstract

The antioxidant capacity of a group of fruits obtained in the Singapore markets was investigated. A total of 27 fruit pulps were tested for their general antioxidant capacity based on their ability to scavenge 2,2'-azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) free radical. The contribution of L-ascorbic acid (AA) to the total antioxidant activity of fruits was investigated by using RP-HPLC. The antioxidant capacity of the fruit pulp was measured by monitoring the change of absorbance of the free radical solution at 414 nm in the test reaction mixture following addition of the fruit extract, as compared with AA. The results were expressed as mg of AA equivalents per 100 g, i.e. the quantity of AA required to produce the same scavenging activity as the extract in 100 g of sample (L-ascorbic acid equivalent antioxidant capacity, AEAC). Total antioxidant capacities of AA acid, trolox, hydroquinone, pyrogallol and several fruits were also evaluated based on its ability to scavenge the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical. Results obtained were compared with those of ABTS assay. Every mol of AA, trolox or hydroquinone, was found to reduce about 2 mol of ABTS radical cation or DPPH radical. However, 4 mol of DPPH radical or 7 mol of ABTS radical cation were scavenged by every mol of pyrogallol. A good correlation of AEAC was observed between the two methods. Both methods have been recommended to be useful tools to evaluate antioxidant capacities of fruits. According to the AEAC value of binary extract solution of fruits in the ABTS model, ciku shows the highest antioxidant capacity, followed by strawberry, plum, star fruit, guava, seedless grape, salak, mangosteen, avocado, orange, solo papaya, mango, kiwi fruit, cempedak, pomelo, lemon, pineapple, apple, foot long papaya, rambutan, rambutan king, banana, coconut pulp, tomato, rockmelon, honeydew, watermelon and coconut water. The AA contribution to AEAC of fruits varied greatly among species, from 0.06% in ciku to 70.2% in rambutan.