The fruit called pitaya/dragon fruit is a cactaceae originally from Tropical and Subtropical America which belongs to a group of fruits considered promising for cultivation in Brazil. Until recently, this fruit was unknown and has come to represent a growing niche in the exotic fruit market due to appreciation of the organoleptic characteristics when eaten raw or inserted in gastronomy [1]. The red pitaya can be grouped into four botanical genera: Stonecereus Briton & Rose, Cereus MiLL., Selenicereus (A. Beger) Riccob and Hylocereus Briton & Rose [2]. The variability of the species is mainly related to size and color and production time [3]. The most common and commercialized species are: Selenicereus megalanthus , red yellow pitaya with white flesh known as "Colombian red pitaya"; Hylocereus polyrhzius , red pitaya with red peel and pulp; Hylocereus undatus, red pitaya with white pulp [4]. The Selenicereus setaceus species, also known as the red pitaya of the cerrado is commonly found in Brazil and presents small thorny fruit [5].

With many dark edible seeds, approximately 3 mm in diameter, the apparently juicy red pitaya pulp presents a sweet tasting, gelatinous consistency when ripe, which is usually consumed fresh or processed in the form of ice cream, juice, jelly, wine and salad [6]. In some regions of South America, the pulp is used in beverages, as successfully occurs in Brazilian restaurants where it is served in chunks, together with champagne [7]. The red pitaya shell has great potential to be used as natural pigment due to the presence of betacyanin [8], and presents possible antioxidant activity [9]. In Brazil, there are a few farming areas dedicated to the pitaya cultivation, in the state of São Paulo, mainly in Catanduva County where production occurs from December to May, with an average annual yield of 14 tons of fruit per hectare [1]. The high cost charged for the kilo of the fruit, which can range from ten to sixty reais (R$) in Brazil, depending on the season and demand, makes planting this fruit very attractive [10]. Therefore, this study aims to characterize the chemical-physicochemical properties and the antioxidant activity and to profile the fatty acids in the pulp and peel of red pitaya [Hylocereus undatus (Haw.) Britton & Rose] grown in Brazil.

Obtaining raw material

A selection of healthy-looking red pitaya fruit, free of bruises, rotting and odors was obtained from a producer in Brasília - Federal District, between January and February 2013.

Physical-chemical analysis of red pitaya

The duplicate physical-chemical analyses of the red pitaya pulp were carried out at the Laboratory of Physicochemical and Food Analysis - School of Pharmacy, Federal University, Goiás. The results were expressed by calculating the mean values obtained in each analysis. Moisture, ash, protein, lipids, crude fiber and carbohydrates were analyzed in the pulp and peel of the fruit, following the methodology proposed by the AOAC [11]. To calculate the total calories (Kcal), the following factors were used: four for protein and carbohydrates and nine for lipids, according to the methodology described by Ferreira and Graça [12]. The hydrogenionic potential (pH) was performed according to the methodology No. 017/IV of the Adolfo Lutz Institute [13], in which an electrometric method in water was used. The reducing (glucose) sugars and non-reducing (sucrose) sugars were quantified based on the Somogyi and Nelson method [14] adapted by Pereira and Campos [15]. Readings were taken using a spectrophotometer UV-VIS (510 nm wavelength). The analysis of the titratable acidity, soluble solids and ascorbic acid (vitamin C) were also carried out with methodology based on redox reaction [16].

Determination of Mineral

To determine mineral levels, the atomic absorption spectrophotometry technique (GBC Brand Spectrophotometer, Model 932AA) was used, in duplicate, at the physical-chemical analysis laboratory of the Food Research Center, School of Veterinary Medicine, Federal University of Goiás. Research on aluminium, arsenic, lead, cobalt, copper, chromium, cadmium, calcium, iron, phosphorus, magnesium, manganese, mercury, nickel, potassium, selenium, sodium, and zinc was conducted, in accordance with the presence of specific cathode lamps for each mineral. To determine sodium and potassium levels, flame photometry was used in a photometer (Model DM-6, Digimed brand). The results were expressed by calculating the mean values obtained for each analysis.

Determination of simple profile and fatty acid complex

The complete profile of fatty acids was performed at the Food Research and Analysis Food, School of Veterinary Medicine, Federal University of Goiás (CPA - EV - UFG). All analyses were performed in duplicate and the results were expressed by mean results. Oil was extracted from red pitaya pulp to perform fatty acid analysis following methodology proposed by Bligh and Dyer [17] and qualitative and quantitative identification followed the Visentainer and Franco [18] protocol. The ethereal extract of pulp underwent the methylation of fatty acids technique, according to the methodology described by Antoniosi Filho (1995), and transesterification followed the Maya technique [19]. In order to quantify and determine fatty acid, we used Focus Gas Chromatograph (Focus Model GC Finningan) equipped with a flame ionization detector (FID) and fused silica capillary columns Restek 2560 RT of 100 m long × 0.25 mm of internal diameter containing 0.2 μm. As carrier gas, hydrogen was used at a flow rate of 2.0 mL per minute, and the makeup gases used to maintain the flame detector were synthetic air, hydrogen and nitrogen at rates of 300, 30 to 28 mL per minute, respectively. The injection volume was 1 μL and the split ratio was 2/98. The retention time and peak area, as well as area rate values (normalization method), were obtained using the Chrom Quest 4.1 Software. The identification and quantification of fatty acids (%) were measured by using a calibration curve made with the assistance Fatty Acid Methyl esters standards (Sigma - FAME Mix C4-C24).

Determination of antioxidant potential

The analyses of red pitaya pulp and shell antioxidant activity were performed in triplicate, at the physical-chemical analysis lab at the Goiania Federal Institute - Urutaí Campus. The antioxidant activity was determined according to the methodology proposed by Borguini [20], upon evaluating the non-fractionated watery extract of the sample using the DPPH method (2,2-diphenyl picrylhydrazyl) following the technique described by Brand-Williams et al. (1995). A spectrophotometer UV-VIS (Spectrum brand, model SP 2000 UV wavelength of 517 nm) was used for the readings. To evaluate antioxidant activity, the values observed in the spectrophotometer were inserted in the formula:

% discoloration = {1- [Abs sample + Abs blank) / control × 100}

The antioxidant activity of the samples was evaluated at 0, 1, 2, 3, 4, 5, 10, 15 and 20 minutes after initiating the reaction with DPPH.

Physical-chemical analysis of red pitaya (Hylocereus undatus Haw)

The results of physical-chemical analyses of red pitaya pulp are expressed in Table 1. The red pitaya sample from Brasília - DF, Brazil, had high moisture content (86.03%), Table 1. Results reported by Le Bellec [21] corroborate the results of this study, since, according to the authors, this level ranges between 82 and 88%. Le Bellec [21] found lower protein values (between 0.3 and 1.5%) in red pitaya than the values found in this study (2.27%).

Table 1: Physical-chemical analysis of red pitaya pulp (Hylocereus undatus Haw ).

In a study by Barbosa [22], the authors found 5.47% and 4.81% protein values, respectively, in xique-xique (Cereus gounellei ) and mandacaru fruits (Cereus jamacaru ), also belonging to the Cactaceae family, which are superior to the values found in the present study. In accordance with Rufino [23], the pH values of red pitaya pulp range from 4.3 to 4.7, which are lower than the pH (5.05) found in this study.

According to Baruffaldi and Oliveira [24] and Silva and Alves [25], pH values significantly affect the growth of microorganisms, since lowacid foods, with pH above 4.0, are susceptible to growth of Clostridium botulinumstrains, toxin producers. According to Sim [26], Salmonella spp. may grow on freshly harvested red pitaya under inadequate storage conditions at room temperature. Considering that the optimum pH for the growth of Salmonella is between 6.5 and 7.5 [27], the use of low temperatures during storage of red pitaya would extend shelf life and contribute to consumption safety. Brunini and Cardoso [28] studied red dragon fruit storage at 13°C for 25 days and observed an increase in pH value from 4.60 to 5.8, consistent with results presented in this study. Studies on dragon fruit storage conditions show that an 8°C storage temperature is best to keep the same quality attributes [29]. The acid values found in the red pitaya was (1.82%), superior to those obtained by Oliveira [30] with 0.21% acidity values in the fruit of the mandacaru cactaceous (Cereus jamacaru ). In research conducted by Nerd et al. [31], the authors found that the titratable acidity below 1% explains the pleasant flavor and sweetness red pitaya fruit. Total soluble solids are used as the maturity index for some fruit and indicate the amount of substances that are dissolved in the juice, constituted mostly by sugars [32]. The amount of soluble solids found in samples of red pitaya (11.40 °Brix) corroborate the results reported by Wu [33] in red pitaya pulp (11.1 °Brix). For Wanitchang [34], fruits with °Brix reading higher than 12% and 13% have better acceptance for consumption. The crude fiber found in red pitaya was 1.15%, a result similar to that found in the pulps of xique-xique cacti (Cereus gounellei ) and mandacaru (Cereus jamacaru ) 1.13% and 1.45%, respectively [22]. With the results obtained in this study, it holds true that red pitaya is a food with low energy value and a good source of crude fiber, thus contributing to a healthy diet. Additionally, it displays low lipid content (0.16%). The caloric value (53.68 Kcal 100/g) found in the pulp was higher than the caloric values quoted by Le Bellec [35], for the Hylocereus undatus and Hylocereus costaricensis species, corresponding to 37.9 kcal and 41.7 kcal respectively. The ash content which corresponds to the inorganic or mineral fraction of foods was 0.75%. According to Gondim [35], fruits are considered the main sources of minerals in human diet, found in higher levels in the fruit bark than in the edible parts. In a study conducted by Silva [36], the authors found 0.72% mineral values in "coroa-de-frade" fruit (Melocactus zehntneri ), which was close to the value found in the present study for red pitaya. Morton [37], working with "ora-pro-nobis" cactaceous, found an ash content of 0.6%, which was lower than the red pitaya fruit content found in the present investigation. The red pitaya pulp showed higher total solids (13.97%) than the 11.82% value found by Queiroz [38] for the fig-of- India pulp, which is also a cactaceous.

On observing the amount of vitamin C in red pitaya (0.84 mg.100 g^-1), we found out that is was lower than the amounts found by Beltrán-Orozco [39], which were a mean 13 mg. 100 g^-1 in red pitaya of the gender Stenocereus. On the other hand, Choo and Yong [40] found a mean concentration of vitamin C equal to 32.65 and 31.05 mg. 100 g^-1 in red pitaya Hylocereus polyrhizus and Hylocereus undatus , respectively. In research conducted by Mahattanatawee [41], the authors found values of vitamin C equal to 55.8 and 13 mg.100 g^-1 in red pitaya Hylocereus sp ., cv. Red Jaina (red pitaya with red pulp) and Hylocereus sp ., cv. David Bowie (red pitaya with white pulp), respectively. It can be seen that vitamin C levels may vary according to species, cultivar and origin.

Determination of minerals present in red pitaya pulp is expressed in Table 2. The elements found in red pitaya pulp have great importance for human consumption. The body requires more than 100 mg/day of calcium and magnesium macronutrients, while it requires less than 100 mg/day of manganese, copper, zinc and iron.

Table 2: Minerals found in red pitaya (Hylocereus undatus ) cultivated in Brazil.

The mineral contents found in 100 g of red pitaya pulp with the exception of manganese, do not meet the recommended daily allowances for adults, which according to the World Health Organization are 1200 mg of calcium, 230-420 mg of magnesium, 1.8 to 2.3 mg of manganese, 11 mg of zinc for men and 8 mg for women [42]. Among the mineral elements analysed, potassium showed higher concentration, followed by manganese, chromium, sodium, calcium, and at a lower concentration, phosphorous. When Stinzing [43], characterized red pitaya pulp (Hylocereus undatus ) chemically, the following concentrations were verified: calcium (23 mg. 100 g^-1), potassium (320 mg. 100 g^-1), magnesium (265 mg. 100 g^-1) sodium (33 mg. 100 g^-1). Moreover, in a study by Rodrigues [44], red pitaya from the cerrado showed levels of copper (1.1 mg. 100 g^-1), zinc (0.3 mg. 100 g^-1), and iron (2.9 mg. 100 g^-1), which were all higher than those found in the present study.

The determination of fatty acids present in red pitaya pulp is displayed in Table 3.

Table 3: Fatty acids present in red pitaya pulp (Hylocereus undatus ) cultivated in Brazil.

On Table 3, observation showed that the predominant fatty acid in red pitaya pulp Hylocereus undatus is linoleic acid, which represents 50.869% of the total fatty acids in the fruit, followed by oleic acid with 21.551% and palmitic acid with 12.632 % of the total fatty acids. This mono and polyunsaturated composition of fatty acids is paramount to health, forasmuch as these acids contribute to reducing low-density lipoprotein fractions and very low density, both responsible for increased serum cholesterol [45]. Lipid components, especially fatty acids, play important roles in the structure of cell membranes and in metabolic processes. In humans, linoleic and alpha-linolenic acids are necessary to maintain cell membranes, brain function and the transmission of nerve impulses under normal conditions.

Also, these fatty acids play an active part in transferring atmospheric oxygen into blood plasma, in the synthesis of hemoglobin and cell division, and are called essential because they are not synthesized by the human body [46]. The MUFA/SFA and PUFA/SFA ratio (Table 4) in the red pitaya was 1.299 and 2.558, respectively, which represents an excellent result from a nutritional standpoint. We know that foods with values under 0.45 (MUFA/SFA and PUFA/SFA) are considered undesirable for humans, since it is correlated with the increase in blood cholesterol [47].

Table 4: Ratio of ω-3 to ω-6 fatty acids present in red pitaya pulp obtained in Brasilia-FD.

Antioxidant activity of red pitaya pulp and peel

Figure 1 shows the antioxidant activity determined by the DPPH assay in red pitaya pulp and peel. The results are expressed as EC50 (μg mL^–1), which corresponds to the amount of extract required to reduce DPPH radical by 50%, therefore, the smaller the EC50, the better the antioxidant capacity of the extract. Some studies have shown that the dragon fruit has good antioxidant capacity in vitro. However, this antioxidant potential may vary among different species and different origins [41]. For red pitaya, the following absorbance values were obtained: Methanol (0.048 nm), Control: 750 μL + 1.5 mL DPPH (0.449 nm), White: 750 μL + 1.5 mL of methanol (0.043 nm). For standard BHT solutions, a powerful synthetic antioxidant, antioxidant activity was observed after a 20 minute reaction period with DPPH 90.20%.

Figure 1: EC50 values (μg mL^-1) of aqueous extracts from red pitaya pulp and peel (Hylocereus undatus).

Both pulp and peel extract displayed ability to sequester DPPH free radicals. The highest antioxidant activity was found in the peel (445.2 μg mL^–1), compared to the lowest value exhibited by the pulp (1,266.3 μg mL^–1).

Vizzotto [48] evaluated the antioxidant potential of the pulp and peel of pitaya fruit by means of extraction in methanol and obtained EC50 values of 209.66 and 1,363.2 μg mL^–1 for pulp and peel respectively, which were different from the values found in this work. Nevertheless, the authors also observed higher antioxidant activity in the pitaya peel in methanol extract than in the pulp.

According to Wu [33], the total phenolic contents of pitaya pulp and peel are similar, as well as the flavonoid content, which suggests that pulp and peel are rich in polyphenols and good sources of antioxidants. Even though polyphenols, flavonoids and phenolic contents were not evaluated in this study, observation shows that the red pitaya peel harvested in Brasilia has higher antioxidant activity than the pulp, in other words, both are nutritionally interesting for human consumption. However, it is essential to investigate the possible presence of anti-nutritional substances in the peel before recommending its use to the population. In a study by Kim [49], the authors also noticed that polyphenols and flavonoid contents in the methanol extract of red pitaya peel and white pitaya peel were approximately three to five times higher than the content of these same antioxidants in the red and white pitaya pulp, respectively. In research conducted by Orazco [50-55], the authors observed antioxidant activity in red pitaya, white pitaya and yellow pitaya belonging to the Stenocereus stellatus gender and showed some variation in the antioxidant capacity of this fruit, in accordance with its species.[Table 5]

Table 5: Antioxidant capacity (TEAC) of different pitaya species obtained in research conducted by Orozco et al. (2009).

The pitaya pulp [Hylocereus undatus (Haw.) Britton & Rose] displayed high moisture content, which explains the low shelf life if stored at room temperature. The pulp showed reduced levels of lipids, proteins, vitamin C and sodium. Ordinarily, fruits have low levels of lipids and proteins. The concentration of ascorbic acid in fruit varies according to the type of cultivation, the stage of maturity and the conditions of cultivation, among others.

The low calorific value and the presence of micronutrients in fruit pulp is a great benefit for use in food, especially in low-calorie diets. The red pitaya grown in Brazil exhibits predominance of essential linoleic fatty acid (C18:2) followed by oleic acid (C18:1), important essential fatty acids with beneficial health effects.

Through the present study, weak antioxidant activity of red pitaya pulp was verified in comparison to the peel of the fruit, which suggests that the highest concentration of compounds with antioxidant activity is found in the peel. Therefore, we suggest that the fruit peels not be discarded in food preparation and emphasize that they be dried and valued as leftovers and fiber, rich in nutrients and bioactive compounds, import to the health of human beings.