Briefly, the dragon fruit peels will be dried using three methods, which are oven dry, vacuum dry, and microwave dry. The color, phenolic content, betacyanin content, antioxidant activity of the dragon fruit powder for each drying method and fresh peels will be compared. The water activity of the powder will be determined. All the measurement will be performed in triplicate.
Absolute ethanol, Folin-Ciocalteu reagent, sodium carbonate, gallic acid, distilled water, Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) 97%, TPTZ (2,4,6-tripyridyl-s-triazine) >98%, ferric chloride hexahydrate (FeCl3 . 6H2O) sodium acetate trihydrate, methanol, hydrochloric acid (HCl).
Convection oven, Vacuum oven, Microwave oven, grinder, homogenizer, Buchner funnel, rotary evaporator, water bath, UV-spectrophotometer, Labscan, XE, Hunter Lab. Inc., USA, AquaLab Series 3 Water Activity Meter.
Dragon fruit (Hylocereus polyrhizus) will be obtained from different stalls in Pasar Tani FAMA, Serdang.
5 kg of dragon fruit will be obtained. Then the fruits will be washed and peeled. The peels will be washed thoroughly to remove dirt and unwanted/spoilage part.
The dragon fruit peels will be oven-dried in a convection oven, vacuum-dried and microwave dried. 500g peels will be used for each drying method.
For oven drying, the peels will be dried in an oven at 55 oC until the peels reached a constant weight. This temperature is chosen because according to Larrauri (1999), drying temperature below 65oC able to avoid changes in the functional properties and in the content of polyphenols, tannins, anthocyanidins and proteins.
For vacuum drying, the process will be carried out according to Krokida, Maroulis, and Saravacos (2001) with slight modification. Vacuum drying is will be carried out using vacuum dryer at 65oC ± 0.2oC. and 33 mbar ± 3%, until constant weight.
In microwave drying, the peels will placed in the middle of the turntable of a commercial microwave oven at 540W (Inchuen, Narkrugsa, & Pornchaloempong, 2010).
Time taken for drying will be measured and recorded.
All the dried samples will be grinded using a grinder mill and sieved to obtain a powder particle size.
Furthermore, freeze-drying is known to have high extraction efficiency because ice crystals formed within the plant matrix can rupture cell structure, which allows exit of cellular components and access of solvent, and consequently better extraction(Asami, Hong, Barrett, &Mitchell, 2003). (Effects of different drying methods on the antioxidant properties of leaves and tea of ginger specie).
3.4 Extraction of Samples
Dried peels for each drying method will be extracted by a method reported by Dewanto, Wu, Adom, & Liu (2002) with slight modification. Briefly, the dry equivalent of 100 g of peel powder or fresh peel will be blended with 200ml 80% ethanol solution and homogenize for 5 min using a Virtis 45 homogenizer. The slurry will be filtered through Whatman No. 1 filter paper in a Buchner funnel under vacuum. The filter cake is washed twice with 15 mL of ethanol solution. The filtrate is evaporated using a rotary evaporator at 45 °C until less than 10% of the initial volume remained. The extract is frozen at -40 °C until analysis.
3.5 Total Phenolics Content
Total phenolic content will be analyzed, using the Folin-Ciocalteu method described by Sato et al. (1996) with some modification. Briefly, the plant extract will be dispersed in ethanol to yield 1 mg/ml of test solution. An aliquot of 1 ml of test solution will be diluted with 9 ml of distilled water. Afterwards, 200 μl Folin-Ciocalteu reagent and 600 μl of 2% sodium carbonate are added. The mixture is allowed to stand for 2 h at room temperature before the absorbance was measured spectrophotometrically at 750 nm. Gallic acid is used as the standard for the calibration curve. Total phenolic content of the sample will be expressed as gallic acid equivalent concentration (mg/ml).
3.6 Determination of total betacyanin content in samples
Absorbance values of samples are measured using a spectrophotometer at 538 nm against a blank of Standard Distilled Water (SDW). The absorbance obtained is then used to calculate the total betalain concentration using following formula (Herbach et al., 2007).
A = Absorbance
DF = Dilution Factor
MW = Molecular weight of betanin (550 g mol-1)
= Molar extinction coefficients (60,000 L mol-1 cm in H2O
l = Path length of cuvette = 1 cm
3.7 Ferric reducing antioxidant potential assay (FRAP)
The ability to reduce ferric ions will be measured using a method described by Benzie and Strain (1996) and Wong, Leong, and Koh (2006), with slight modification. An aliquot (200µl) of sample extract will add to 3 ml of FRAP reagent (10 parts of 300 mM sodium acetate buffer at pH 3.6, 1 part of 10 mM TPTZ solution and 1 part of 20 mM FeCl3 . 6H2O solution). The 300 mM acetate buffer will be prepared by mixing 3.1g of sodium acetate trihydrate (C2H3NaO2_3H2O) with 16ml glacial acetic acid and brought to 1 L with distilled water. The TPTZ solution will be prepared by making a solution of 10mM TPTZ in 40 mM HCl. The reaction mixture is incubating at 37oC in water bath for 30 minutes. The increase in absorbance at 593nm is measured. The antioxidant capacity based on the ability to reduce ferric ions of the extract is expressed as µmol Trolox equivalent per g of fresh weight.
3.8 Color Measurement
The color of dried powder and fresh peel will be using a Hunter Colour Measuring System (Labscan, XE, Hunter Lab. Inc., USA).
3.9 Water Activity of dried peels
AquaLab Series 3 Water Activity Meter will be used to determine the water activity of the samples.
