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Physicochemical and Antioxidant Activities of Spray-dried Pitaya Fruit Powder

Pitaya commonly known as dragon fruit is very popular in China due to its intense color, constituent minerals, vitamins, and antioxidant properties. In the present study, physiochemical properties and antioxidant activities of fruit powder from two pitaya cultivars (namely red flesh and white flesh) and fruit peel were observed. Compared with the fruit powder of fruit flesh, the fruit powder made from fruit peel showed a higher antioxidant activity. The current study provides insights to produce spray-dried pitaya fruit powders that could potentially be used as functional food ingredients in various food fields.


Introduction
Pitaya (Hylocereu undatus (Haw.)), also as known as dragon fruit, is of a member of the cactus family. The fruit described associated with the green color of the immature fruit, and the 'dragon-like' appearance of the 'scales' or bracts on the fruit surface. Besides of its special appearance, pitaya also known to be rich in nutritional value, especially rich in betalains which meet the increasing trade interest for antioxidant products and natural food colorants [1]. According to the color of pitaya fruit flesh, three types of fruit, namely white-fleshed, red-fleshed and yellow-fleshed, exists in production and market [2]. But the white-and red-fleshed fruit is very usual in market. It is difficult to have a long-time shelf time because it ripe in hot season.
Spray drying is a well-known technique used in food industries since 1940s. It transforms a material from liquid to a dried powder state by spraying into a hot drying gas medium. A appropriate hot temperature is important to this technique. The product can be made in the form of powder, granules and agglomerates [3]. For most fruit or fruit juice, various antioxidants exist inside that a sensitive to high temperature. So the quality and sensory of product may lose especially vitamin C, βcarotene, flavors and aroma. A lot of studies were reported that compared the influence of different drying methods on different quality attributes of fruits and vegetables. However, no studies have been conducted to evaluate the effect of drying methods on the color, and antioxidant. Thus, the objective of the present work was to investigate the influence of spry-drying method on the color change and the antioxidant activity of pitaya fruit.

Plant Materials and Sampling
Fruits of pitaya were purchased from a local supermarket in Zhanjiang city, Guangdong Province. Uniform fruit without visible defects were selected as the experimental materials. Two cultivars were chosen in our study, namely red-fleshed and white fleshed. The flesh and peel were separated by hand. The puree of pitaya fruit was extracted by a juicer. The puree was kept in fridge at 4℃ for further use.

Drying Experiment
The puree was blend for 5 min to a uniform consistency. The spry-dryer used in our study is DFRPprying tower (Dafeng, Wuxi). The dryer conditioned for 1 h before starting the experiment. The tapwater was pumped through the atomizer with the dryer inlet and outlet temperature set at 180 and 80℃, respectively. The puree was pumped into the dryer chamber by a pump at a flow rate of 0.5 ml/s. The air temperature of dryer inlet and outlet were 180℃ and 90℃, respectively. These temperatures were determined by our previous work.

Determination of powder color
Pitaya fruit powder color was measured by a colorimeter (MiniScan XE Plus, HunterLab, VA, USA), which provided CIE L*, a*, and b* values. L* represents the relative lightness of color with a range from 0 to 100, being small for dark color and large for light color. Both a* and b* scales extend from -60 to 60. Negative a* values indicates greenness and positive for redness, while b* is negative for blueness and positive for yellowness. These values were then converted to a hue angle h° = arctan [b*/a*] and a chroma value C= [(a*)2+(b*)2]1/2 which quantified the intensity or purity of the hue [4]. The values of hue angle 0°, 90°, 180°, and 270° represent red purple, yellow, bluish green, and blue, respectively. Chroma value indicates the intensity or color saturation.

Antioxidant activity measurement of pitaya fruit-powder
Two methods to measure antioxidant activity were applied in our study, namely DPPH free radical scavenging assay and Ferric reducing/antioxidant power assay (FRAP assay). The free radical scavenging activity of fruit extracts was measured according to DPPH method [5]. A 100 μl aliquot of fruit extract was mixed with 3.9ml of 60 μM DPPH methanolic solution. The reaction for scavenging DPPH radicals was kept in the dark at room temperature for 6 h. The absorbance was measured by a UV-visible spectrophometer (SP-752N, Shanghai Spectrum Instrument Co., LID, China) at 515 nm, against a blank of methanol without DPPH. Results were expressed as percentage of inhibition of the DPPH radical. Percentage of inhibition of the DPPH radical was calculated according to the equation [(Abs control-Abs sample)×100/ Abs control]. Trolox was used as the standard. Results were expressed as mmol Trolox equivalents/100 g fresh weight.
The FRAP assay was performed according to the method described by Guo [6], with some modification. FRAP reagent was made from 1mM TPTZ and 2 mM ferric chloride in 300 mM sodium acetate buffer (pH 3.6). A 200μl aliquot of fruit extract was mixed with 1.8ml FRAP reagent, the absorbance was measured after a 10 mins reaction using a UV-visible spectrophometer (SP-752N, Shanghai Spectrum Instrument Co., LID, China) at 595 nm, against a blank of methanol. The antioxidant capacity of the fruit extract was determined by its ability to reduce ferric to ferrous ions in the FRAP reagent. The standard curve was constructed using concentration of Trolox from 0 to 1 000 μM and the linear range was used for calculation. FRAP assay results were expressed on a fresh weight basis as mmol Trolox equivalents/100 g fresh weight.
2.5. Particle shape SEM (Philips XL-30 ESEM; MEMS and Nanotechnology Exchange, VA, USA) was used to analyze the particle shape of the fruit powder.

Maintaining the Integrity of the Specifications
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The Antioxidant Activity of Different Powder
DPPH free radical scavenging assay and Ferric reducing/antioxidant power assay were used in our study to determine the antioxidant activity. The white-fleshed sample showed a lower antioxidant activity by both methods. The peel powder had the highest antioxidant activity by DPPH method (5.25), while the peel sample showed a lower antioxidant activity by FRAP method (1.90). In summary, the red samples of fruit powder showed a higher antioxidant activity than white sample, regardless of measurement method (Table 1).

The Correlation Between fruit Color and Antioxidant Activity in Pitaya Fruit Powder
Different correlations were detected in our study, as shown in table 2. Interestingly, the different antioxidant activity measurement method showed the similar antioxidant activities to the pitaya fruit powder. Both of DPPH and FRAP showed a negative correlation with lightness (-0.582, and -0.947). However, positive correlations between antioxidant activities and a*values, and chroma (C) were detected in our study. The finding of the present study may be attributed to the different contents of total phenolic compounds in red-fleshed fruit, white-fleshed fruit, and peel. Similar results were also report by Lee [7]. Tenore pointed out that the loss of phenolic compounds including gallic acid, caffeic acid and p-coumaric acid during spray drying result in the reduction in radical scavenging activity of fruit [8].

The Powder Morphology
Both red and white flesh fruit powders in the present study showed spherical particle morphology with smooth surfaces and no surface cracks. Powders of fruit peel showed an irregular shape (Figure 1). Agglomerate is a particle composed of individual grains of material bound together by sub-micron dust. And agglomerate was observed in both red and white flesh fruit powders. Similar particle morphology has been reported for spray-dried blackberry [9] and cactus pear powders [10].

Conclusion
In conclusion, pitaya fruit powder made by spray drying can be used as an excellent food resource. In spite of the different antioxidant activity of fruit powder, the higher antioxidant activity was observed in fruit powder from fruit peel. The current study provides insights to produce spray-dried pitaya fruit powders that could potentially be used as functional food ingredients or coloring agents in various food applications.