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demand for H. polyrhizus and its perceived higher antioxidant properties. There is no report on the comparison of antioxidant properties of H. polyrhizus and H. undatus grown in Malaysia. Mahattanatawee et al. [8] investigated the antioxidant activity of selected tropical fruits grown in South Florida, U.S.A. and reported that the antioxidant activity of red pitaya was higher than that of white pitaya. Recently, Kim et al. [15] investigated the antioxidant and antiproliferative activities of red and white pitaya grown in Jeju Island, Korea and reported that the peel extract and pulp extract of white pitaya had higher bioactivity index values that those of red pitaya. The objective of this study was to investigate the antioxidant properties of fruit (peel and pulp) and pulp of H. polyrhizus and H. undatus. Antioxidant contents of the two species of Hylocereus fruits were measured using ascorbic acid and total phenolic contents. Antioxidant activities of the two species of Hylocereus fruits were investigated using free radical scavenging activity and ferrous ion chelating activity. MATERIALS AND METHODS Samples H. polyrhizus (red dragon fruit) and H. undatus (white dragon fruit) were obtained from the local markets. All fruits were analyzed within one week after acquisition. Triplicate analysis from different fruit samples were carried out. Chemicals and solvents Folin-Ciocalteu'

s reagent (2N), gallic acid, L-ascorbic acid and 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical were purchased from Sigma-Aldrich (U.S.A.). Ethanol, iodine 0.1N, sulfuric acid and sodium carbonate were purchased from Merck (Germany). Potato starch was purchased from Bendosen Laboratory Chemicals (Norway). Iron (II) sulphate 7-hydrate and ferrozine iron hydrate were purchased from Acros Organics (Belgium). Sample preparation The sample preparation was based on the method of Lim et al. [16] with slight modification. Sample of fruits (peels and pulps) and pulps for H. polyrhizus and H. undatus were prepared. The fruits were washed and wiped dry. All fruits were cut into a few portions. The portions (20 g) were crushed to a paste-like state for approximately

2 min (with intermittent stops to minimize heating) using a Waring blender. The homogenized sample was transferred into a 100-mL Wee Sim Choo et al Adv. Appl. Sci. Res., 2011,

2 (3): 418-425

420 Pelagia Research Library volumetric flask and 50% ethanol was added up to the mark. The mixture was shaken manually for

10 min, followed by centrifugation at

1500 * g. The extracts were stored at -20?C. All tests were performed within a week. Determination of ascorbic acid content The ascorbic acid content was determined using the iodine titration method [17] with slight modification. Starch indicator solution was prepared by mixing 1g of starch with 200mL of boiling water. The solution must immediately removed from heat and left for cool. Each 25mL of fresh juice sample was transferred into a 250mL Erlenmeyer flask. Twenty-five milliliter of 2N sulfuric acid was added, mixed, diluted with water (50 mL) and starch indicator solution (3 mL) was added. The solution was directly titrated with 0.001N standardized iodine solution. A blank titration was performed prior to titration of each sample. One mL of 0.001N iodine was equivalent to 88.06 ?g ascorbic acid. Determination of total phenolic content Total phenolic content was determined according to the method of Lim et al. [16]. Samples (0.3 mL) were measured into test tubes followed by 1.0 mL of Folin-Ciocalteu'

s reagent (diluted

10 times with water) and 1.2mL of sodium carbonate (7.5% w/v). The tubes were vortexed, covered with parafilm and allowed to stand for