Postharvest quality, soluble phenols, betalains content, and antioxidant activity of Stenocereus pruinosus and Stenocereus stellatus fruit

Postharvest Biology and Technology 111 (2016) 69–76

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Postharvest quality, soluble phenols, betalains content, and antioxidant activity of Stenocereus pruinosus and Stenocereus stellatus fruit Leticia García-Cruz a , Salvador Valle-Guadarrama a,∗ , Yolanda Salinas-Moreno b , César del Carmen Luna-Morales a a b

Universidad Autónoma Chapingo, Mexico-Texcoco km 38.5, Texcoco de Mora 56230, Mexico, Mexico Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias, Tepatitlán-Lagos de Moreno km 8, Tepatitlán de Morelos, Jalisco, Mexico

a r t i c l e

i n f o

Article history: Received 3 April 2015 Received in revised form 8 July 2015 Accepted 10 July 2015 Keywords: Stenocereus pruinosus Stenocereus stellatus Pitaya fruit Postharvest behavior

a b s t r a c t Fruit of the Stenocereus genus have good acceptance and high potential in the fresh fruit market. However, their use is limited to regional production areas due to they are highly perishable and the postharvest behavior has not been studied. The objective of the work was to characterize morphological, physiological, physical, and chemically, red and orange fruit of Stenocereus pruinosus, and red and white fruit of Stenocereus stellatus in postharvest. Storage during 10 d at 24 ◦ C was carried out and shelf life was estimated in six days. Respiration rate suggested non-climacteric behavior. Color, acidity, pH, soluble phenols content, and betalains content distinguished fruit. These variables, besides total soluble solids, remained without significant changes throughout the storage period, but weight loss, firmness, and total sugar content experimented modification. Antioxidant activity was highest in red fruit. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Pitaya is the name of species of the Stenocereus genus, which develop as columnar cacti in arid and semi-arid areas, such as those located at the Rio Balsas basin and Tehuacan Valley in the central region of Mexico. In addition to fruit consumption, these plants are used as building materials, fodder, living fences, and fuel (Parra et al., 2008). Important commercial species are included in this genus, like Stenocereus queretaroensis (F. A. C. Weber) Buxbaum, Stenocereus griseus (Haworth) Buxbaum, Stenocereus pruinosus (Otto) Buxbaum, and Stenocereus stellatus (Pfeiffer) Riccobono (García-Suárez et al., 2007), whose fruit have weight ranging from 20 to 200 g and have palatable flesh with different tonalities, with small, soft and edible seeds, and also with deciduous spines on the epidermis. S. pruinosus and S. stellatus have several aspects that need to be attended. The harvest index is not well defined and producers take into account the color change, the epidermis brightness, and detachment of spines for cutting fruit. Furthermore, shelf life is short and it is reported that varies between four and five

∗ Corresponding author. Tel.: +52 595 952 1629; fax: +52 595 952 1629. E-mail address: [email protected] (S. Valle-Guadarrama). http://dx.doi.org/10.1016/j.postharvbio.2015.07.004 0925-5214/© 2015 Elsevier B.V. All rights reserved.

days (Armella et al., 2003). Pitaya fruit exhibit attractive colors in skin and flesh, which is derived from betalains presence (Kimler et al., 1970). In the case of S. pruinosus, García-Cruz et al. (2013) found that betalains and phenolic compounds confer fruit high antioxidant activity at harvest, but characteristics during postharvest life are unknown. With respect to S. stellatus, reports describing such properties in fruit have not been published. Currently, public and private institutions recognize the need to promote consumption of fruit and vegetables (Nepal et al., 2012; Pivonka et al., 2011), due to the observed benefit of ingesting phytonutrients that can mitigate the oxidative stress caused by the overproduction of free radicals (Wang et al., 2011), which reduce the risk of several sickness like cardiovascular diseases, diabetes, and cancer (Wootton-Beard and Ryan, 2011). Consequently, it is necessary, first, to identify plant sources that can provide the type of phytonutrients mentioned and, secondly, to characterize in them changes that occur naturally during postharvest life, in order to have elements that support the development of technologies to preserve the potential use of plant products. In that context, the objective of the work was to characterize morphological, physiological, physical, and chemically, fruit of red and orange variants of S. pruinosus, and fruit of red and white variants of S. stellatus in postharvest.

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2. Materials and methods

hue angle, chroma, and lightness. Epicarp thickness was measured with a Vernier caliper and was expressed in cm.

2.1. Plant material Fruit of S. pruinosus with red (SpR) and orange (SpO) flesh, and fruit of S. stellatus with red (SsR) and white (SsW) flesh were used, all grown at Tepexi de Rodríguez, Puebla, Mexico (18◦ 35 46 N; 97◦ 55 48 W; 1644 masl). In this region, average seasonal temperatures during harvest period vary between 20 and 25 ◦ C. Physiological condition of fruit corresponded to commercial maturity, which occurred when spines were easily released and skin became bright. At the moment of harvest 10 fruit of each variant were measured in terms of equatorial diameter (De ), polar diameter (Dp ), and number of areoles present. Further, a shape index (Si ) was calculated as Si = De /Dp . 2.2. Experimental organization Batches of 80 fruit of each variant (SpR, SpO, SsR, and SsW) were weighed, disinfected by immersion in NaClO solution (100 mg kg−1 ), and placed at 24 (±2) ◦ C, which was considered representative of ambient thermal conditions at harvest site. In addition, the relative humidity (HR) quickly reached a level of 90% inside the storage room. Fruit of each variant were sampled daily during 10 d in order to assess weight loss, respiration rate, epicarp and flesh color, epicarp and flesh firmness, epicarp thickness, total soluble solids, titratable acidity, total sugars content, betalains and total soluble phenols content, and antioxidant activity. All measures were performed in triplicate, and the experimental unit included two fruit.

2.3. Physiological variables Weight of each experimental unit was evaluated with a digital scale (Ohaus, USA) and weight loss was calculated, in percentage, in relation to the initial condition. Respiration rate was evaluated ˜ with a static method (Hernández-Munoz et al., 2008). The experimental unit was placed in an  airtight  container for 45 min and the CO2 concentration change yCO2 was determined. To do this, gaseous samples of 3 mL were obtained from the headspace of recipients and placed in glass tubes of 7 mL with 4 mL of a solution of sodium bicarbonate containing bromothymol blue as indicator. Agitation was applied during 15 s and absorbance was measured with a spectrophotometer (Perkin Elmer Lambda 25 UV/Vis, USA) at 615 nm. Mixtures of CO2 ranging from 0.2 to 12.5% in concentration were used to prepare a standard curve, which was used to quantify CO2 concentration in samples. Then, based on experimental unit weight (mfr , kg), container free volume (VL , m3 ), and −1 −1 elapsed  time (t,s),  respirationrate (R, ␮g kg s ) was calculated  as R = 1 × 106 yCO2 VL PM / (R (T + 273.15) mfr t) , where P is atmospheric pressure (77993 Pa), T is temperature (24 ◦ C), M is molecular weight of CO2 (44.01 g mol−1 ), and R is the ideal gas constant (8.314 Pa m3 mol−1 K−1 ).

2.4. Physical variables Firmness was measured at two points of fruit equatorial region and was expressed in Newtons (N) as the average of both determinations. To do this, a texture analyzer (TA-XT2i; Stable Micro Systems, UK) was used with a needle element for epicarp and a conical one for flesh, this last having 2.6 cm at the base and angle of 75◦ , with routines where samples were deformed up to 5 mm at a velocity of 5 mm s−1 . Color was evaluated with a Hunter Lab colorimeter (Mini Scan XE Plus 45/0-L, USA) and was expressed in

2.5. Chemical variables Total soluble solids (TSS, ◦ Brix) were measured in juice of flesh with a portable Abbe refractometer (Atago Co., Japan). pH was measured with a portable potentiometer (Hanna Instruments, Romania) in extracts obtained by grinding 5 g of flesh with 50 mL of distilled water. Acidity was assessed by titration of the same extract with NaOH 0.01 N (AOAC, 1999). Total sugar content was evaluated with the method of antrona (Witham et al., 1971). 2.6. Betalains and total soluble phenols content The method described by Wu et al. (2006) was applied to obtain a pitaya fruit methanolic extract. Two grams of macerated flesh were placed in an Erlenmeyer flask with 20 mL of 80% aqueous methanol (v/v). Samples were sonicated during 10 min in a Branson® bath (USA), stirred in dark during 20 min at room temperature, and centrifuged at 2200 × g during 10 min (Hettich Zentrifugen, Germany). Supernatant was recovered and the residue was subjected to a similar second extraction. Supernatants were pooled, filtered (Whatman paper no. 4), and concentrated to dryness at 40 ◦ C, in a rotary evaporator (Laborata 4010, Germany). Residues were re-suspended in 10 mL of 80% methanol solution, and stored in amber vials at −20 ◦ C. Concentrations of betacyanins and betaxanthins were determined with the method of Castellanos-Santiago  and Yahia  (2008)byspectrophotometry  and the calculation B = (1000) ADf WVm / εms ı , where B is

betacyanins or betaxanthins content (mg kg−1 ), A is absorbance (538 nm for betacyanins or 483 nm for betaxanthins), Df is dilution factor, W is molecular weight (550 g mol−1 for betanin and 308 g mol−1 for indicaxanthin), Vm is volume of mixture (m3 ), ε is molar extinction coefficient (6000 m3 mol−1 m−1 for betanin and 4800 m3 mol−1 m−1 for indicaxanthin), ms is mass of sample (kg), and ı is the length of the cell (0.01 m). Betacyanins and betaxanthins contents were added and results were expressed as total betalains content in mg kg−1 . Determination of total soluble phenols (TSP) was carried out with the Folin–Ciocalteu (FC) method (Singleton and Rossi, 1965), where a mixture of 125 ␮L of FC reagent and 125 ␮L of water reacted with 100 ␮L of pitaya extract in assay tubes during 6 min. After, neutralization was applied with 1.25 mL of solution of Na2 CO3 (19%) and volume was adjusted to 3.0 mL with distilled water. Mixtures were shaken on a vortex and placed in darkness for 90 min to achieve stabilization. Centrifugation (Hermle Z200 equipment, Labortechnik, Germany) at 15300 × g was applied during 10 min to removed turbidity and absorbance was determined (Perkin Elmer Lambda 25 UV/Vis, USA) at 760 nm. A standard curve was prepared with gallic acid to express TSP content in mg of gallic acid equivalents per kilogram of fresh weight (mg kg−1 ). 2.7. Antioxidant activity

ABTS (2,2 -azino-bis(3-ethylbenzothiazoline-6-sulphonic acid)) at concentration of 7 mmol L−1 was combined in proportion 2:1 with potassium persulfate (K2 S2 O8 ; PP) at concentration of 2.45 mmol L−1 . Mixture was placed in darkness during 16 h to allow free radical generation and it was after diluted with a buffer of phosphates at pH of 7.4 until absorbance was 0.7 at 734 nm (Wu et al., 2006). Aliquots of 200 ␮L of methanolic extract reacted with 2800 ␮L of ABTS-PP solution during 7 min and absorbance was measured every minute. The reduced ABTS was calculated as %ABTS = (A0 − An ) 100/A0 , where A0

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and An were blank and sample absorbances, respectively. Trolox ((±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid) at different concentrations (50–300 ␮mol L−1 ) was used to prepare a standard curve and antioxidant capacity was expressed in mmol of Trolox equivalents per kilogram of sample (mmol kg−1 ).

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were reported as the average of determinations made. Besides, analyses of variance, complemented with means comparison routines (Tukey, 0.05), were applied to data at each particular sampling time. All evaluations were performed with the program SAS (SAS Institute Inc, 1999). 3. Results and discussion

2.8. Data analysis 3.1. Morphological attributes General comparison of fruit type characteristics was carried out with analysis of variance based on a mixed model (Littell et al., 2006), where fruit variants (SpR, SpO, SsR, and SsW) were considered as a variation factor of fixed effects and storage time as a variation factor of random effects. Physical and chemical variables were evaluated in each fruit of the experimental units and results

Fruit of S. pruinosus (Sp) showed oval shape, with polar and longitudinal diameters greater than those of S. stellatus (Ss), which exhibited more spherical geometry, with shape index values close to unity (Fig. 1, Table 1). Fruit of Sp showed from 24 to 28 areoles, and this number was different (P ≤ 0.05) to which was found in Ss,

Fig. 1. Appearance and color attributes of epicarp (A and C) and flesh (B and D) of fruit of red (SpR) and orange (SpO) variants of S. pruinosus and red (SsR) and white (SsW) variants of S. stellatus stored after harvest during 10 d at 24 ◦ C. Different letters indicate general significant difference and an asterisk points out that at least one material was different from the rest at the specific sampling time. Error bars correspond to standard error. HSD is honest significant difference (Tukey, 0.05). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Morphological and physical characteristics of fruit of red (SpR) and orange (SpO) variants of Stenocereus pruinosus and fruit of red (SsR) and white (SsW) variants of Stenocereus stellatus at the moment of harvest. Characteristic

Morphological attributes at harvest Equatorial diameter (cm) Polar diameter (cm) Shape index Weight (g) Areoles number Epicarp physical attributes at harvest Thickness (cm) Firmness (N) Hue angle (degrees) Chroma Lightness (%) Flesh physical attributes at harvest Firmness (N) Hue angle (degrees) Chroma Lightness (%)

Fruit

HSD

SpR

SpO

SsR

SsW

6.46 a (±0.18) 7.59 b (±0.31) 0.86 b (±0.02) 171.15 a (±16.40) 24 a (±2)

6.05 ab (±0.25) 8.73 a (±0.34) 0.70 c (±0.02) 195.46 a (±16.20) 28 a (±3)

5.34 b (±0.08) 5.63 c (±0.11) 0.95 a (±0.02) 91.26 b (±3.78) 14 b (±4)

5.62 b (±0.13) 5.91 c (±0.16) 0.95 a (±0.02) 127.08 b (±5.79) 18 b (±3)

0.61 a (±0.08) 2.04 a (±0.31) 22.87 c (±1.45) 30.93 b (±2.84) 20.82 c (±1.16)

0.61 a (±0.04) 1.59 a (±0.15) 35.28 b (±1.41) 43.21 a (±1.69) 27.85 b (±1.34)

0.53 a (±0.11) 2.37 a (±0.08) 14.93 d (±0.56) 11.36 c (±0.49) 15.70 c (±0.24)

0.43 a (±0.10) 2.45 a (±0.09) 88.50 a (±1.93) 7.01 c (±0.34) 41.99 a (±0.78)

1.74 a (±0.03) 46.98 c (±2.56) 12.07 c (±0.77) 33.88 b (±1.90)

1.57 ab (±0.12) 61.22 b (±2.48) 24.12 a (±0.52) 39.95 ab (±0.97)

1.87 a (±0.12) 18.17 d (±2.35) 13.53 c (±0.43) 38.05 b (±0.52)

1.14 b (±0.13) 107.59 a (±0.89) 19.33 b (±0.37) 45.37 a (±0.79)

0.71 1.05 0.08 43.39 6 0.18 1.01 7.90 9.32 5.41 0.59 12.10 3.02 6.48

Data in parenthesis correspond to standard deviation. Different letters indicate significant difference. HSD is honest significant difference (Tukey, 0.05).

where fruit had between 14 and 18 areoles (honest significant difference [HSD] equal to 6). This information may be used as basis to select packaging materials for handling fruit during postharvest storage (Al-Said et al., 2009). Weight at harvest ranged from 171.2 to 195.5 g in Sp and from 91.3 to 127.1 g in Ss, with significant difference (P ≤ 0.05) between both species (HSD = 43.4; Table 1). During storage, weight loss reached 25–30% after 10 d and was higher in variants of Sp than in those of Ss (P ≤ 0.05; Fig. 3A). In addition, the relative humidity of the storage room increased quickly to 90%, which suggested that weight loss may be due to high transpiration rate of fruit, and differences between species could be originated by differences in areoles number. The loss found in the present work was greater than weight reduction observed in other cacti fruit like red pitahaya (Hylocereus undatus; 3.0% in 12 d; Osuna-Enciso et al., 2011) and yellow pitahaya (Selenicereus melaganthus; 7.8% in 15 d; Rodríguez et al., 2005), which also suggests higher transpiration in fruit of pitaya. Transpiration is among causes of postharvest losses (Ben-Yehoshua and Rodov, 2003) and it is typified as a water transport phenomenon from within the product to surrounding air, through plant structures like epidermis and cuticle (Maguire et al., 2001; Nobel, 2009). In the present work, beginning from the exterior to the interior, layers of wax, cuticle, epidermal cells, and parenchymatous cells were found in pitaya fruit (Fig. 2). Besides, although studies of the areoles structure and the possible presence of stomata were not carried out, in other cacti species like Hylocereus undatus, Hylocereus polyhrizus, and Selenicereus megalantus a high number of stomata was found on fruit peel, these appearing open during night and closed during most of the daytime period, and fruit shriveling in such materials has been attributed to this characteristic (Mizrahi, 2014). The epicarpic region of fruit had, in general, similar thickness (P > 0.05) in all the variants, with values at harvest of 0.61 (±0.08), 0.61 (±0.14), 0.53 (±0.11), and 0.43 (±0.10) cm, in SpR, SpO, SsR, and SsW, respectively. However, such dimension diminished during storage and resulted, after 10 d, about 55% thinner than initial condition (P ≤ 0.05; Fig. 2), which could favored weight loss because, if water exchange occurs in fruit epicarp in a similar way as in plastic films, as thickness was smaller the water vapor permeability was higher, due to this latter depends on thickness (Banks et al., 1995). On other hand, the epicarp thickness reduction could be associated both with cell wall degradation and with a dehydration phenomenon.

3.2. Physiological behavior Fruit of the evaluated variants of S. pruinosus and S. stellatus showed respiratory activity with some variations during first six days of storage, but without a clear trend (Fig. 3B). Based on this type of behavior, fruit of the Stenocereus genus have been classified as non-climacteric (Armella et al., 2003). However, from the sixth day the respiratory activity showed an upward trend, which coincided with visual symptoms of fungal growth, which, although they were slight, suggested the end of the useful life of fruit and the onset of senescence. On average, the respiratory activity (␮g kg−1 s−1 ) was (19.3 ± 7.3; SsW) > (13.2 ± 4.5; SsR) = (11.1 ± 3.4; SpO) = (9.5 ± 3.6; SpR). The variability in respiratory activity was high and it is believed that, because of this, significant contrast was only confirmed between SsW and (SpO, SpR, SsR) and between SsR and SpR. On other hand, results were higher than those reported by Armella et al. (2003) for fruit of S. griseus (1.1–2.8 ␮g kg−1 s−1 ), although similarity was found with fruit of Hylocereus spp. (OsunaEnciso et al., 2011; Rodríguez et al., 2005). 3.3. Physical attributes The epicarp firmness had values at harvest of 2.04 (±0.31), 1.59 (±0.15), 2.37 (±0.08), and 2.45 (±0.09) N, in SpR, SpO, SsR, and SsW, respectively, and such mechanical property was higher in Ss than in Sp during almost all storage (P ≤ 0.05). Besides, values indicated that pitaya fruit are, in general, materials of soft consistency, which make them susceptible to mechanical damage. Therefore, as protection is one of the expected functions of packaging (Robertson, 2012), the softness of pitaya fruit should be considered in selecting a package for proper management through marketing channels. During storage, the fruit of S. stellatus remained with firmness approximately constant during the first six days, but from there a continuous decrease in the mechanical strength was observed, reaching values close to 1 N. This behavior contrasted with the fruit of S. pruinosus, where firmness diminished steadily during the first six days to values between 0.26 and 0.43 N, and then had an irregular behavior, increasing to values ranging between 0.70 and 1.30 N (Fig. 3C), which was associated with an epicarp drying phenomenon originated by water loss. In the case of flesh, firmness values of 1.74 (±0.03), 1.57 (±0.12), 1.87 (±0.12), and 1.14 (+0.13) N, were found at harvest in fruit of SpR, SpO, SsR, and SsW, respectively. In addition, during storage this mechanical property

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Fig. 2. Left: microphotograph of the epidermal region of a fruit of S. stellatus obtained with scanning electron microscopy. Right: epicarp thickness variation of fruit of red (SpR) and orange (SpO) variants of S. pruinosus and red (SsR) and white (SsW) variants of S. stellatus stored after harvest during 10 d at 24 ◦ C. Different letters indicate general significant difference. An asterisk points out that at least one material was different from the rest at the specific sampling time and ns indicates non-significant difference. Error bars correspond to standard error. HSD is honest significant difference (Tukey, 0.05).

Fig. 3. Cumulative weight loss (A), respiration rate (B), epicarp firmness (C), and flesh firmness (D) of fruit of red (SpR) and orange (SpO) variants of S. pruinosus and red (SsR) and white (SsW) variants of S. stellatus stored after harvest during 10 d at 24 ◦ C. Different letters indicate general significant difference. An asterisk points out that at least one material was different from the rest at the specific sampling time and ns indicates non-significant difference. Error bars correspond to standard error. HSD is honest significant difference (Tukey, 0.05).

fell steadily to 0.45 (±0.24), 0.40 (±0.12), 0.66 (±0.18), and 0.26 (+0.16) N, respectively (Fig. 3D), confirming the fragility of this type of fruit and, although variants of S. stellatus had the highest and lowest flesh firmness values (P ≤ 0.05), the observed differences have actually little practical importance. Besides, the observed postharvest firmness reduction was similar to data reported for prickly pear fruit (Ochoa-Velasco and Guerrero-Beltrán, 2014) and red pitahaya fruit (Osuna-Enciso et al., 2011; Zahid et al., 2013). Softening is a phenomenon associated with degradation of cell wall components (Vicente et al., 2007) during fruit ripening (Paul et al., 2012). In the present work, although pitaya fruit were collected in consumption maturity, the firmness variation indicated that polysaccharides modification in cells middle lamella and primary wall could continue even in senescence phase. However, firmness is affected by both cell wall structure and cell turgor pressure (Brummell, 2006; Smith et al., 2003), and the high cumulative weight loss during storage suggested that turgor loss could also have an important role in reducing mechanical consistency of fruit tissue.

Color was the most distinctive feature of pitaya fruit and ranged between red and yellow tones of CieLAB space (McGuire, 1992), with contrasting hue angle average values in epicarp (HSD = 4.20) of 22.87◦ (±1.45◦ ) in SpR, 35.28◦ (±1.41◦ ) in SpO, 14.93◦ (±0.56◦ ) in SsR, and 88.50◦ (±1.93◦ ) in SsW (Fig. 1A). In the case of flesh, all variants also differed in hue angle, with mean values of 46.98◦ (±2.56◦ ), 61.22◦ (±2.48◦ ), 18.17◦ (±2.35◦ ), and 103.59◦ (±0.89◦ ) in SpR, SpO, SsR, and SsW, respectively (HSD = 12.10; Fig. 1B). The fruit epicarp chroma varied between 7.0 and 43.0, and in descending order average values were 43.21 ± 1.69 (SpO), 30.93 ± 2.84 (SpR), 11.36 ± 0.49 (SsR), and 7.01 ± 0.34 (SsW), with significant difference between all (HSD = 4.32; Fig. 1A). In flesh, the highest chroma was found in SpO (24.12 ± 0.52) and SsW (19.33 ± 0.37) and the lowest in SsR (13.53 ± 0.43) and SpR (12.07 ± 0.77), with significant difference between the first two and the last two (HSD = 3.02; Fig. 1B). Finally, the highest lightness both in epicarp and flesh was found in fruit of the white variant (SsW; 41.99% ± 0.78%; 45.37% ± 0.79%), followed by fruit of orange variant (SpO; 27.85% ± 1.34%; 39.95% ± 0.97%), and red variants SpR (20.82% ± 1.16%; 33.88% ± 1.90%) and SsR

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Fig. 4. Behavior of total soluble solids (SST, A), total sugar content (B), titratable acidity (C), and pH (D) of fruit of red (SpR) and orange (SpO) variants of S. pruinosus and red (SsR) and white (SsW) variants of S. stellatus stored after harvest during 10 d at 24 ◦ C. Different letters indicate general significant difference. An asterisk points out that at least one material was different from the rest at the specific sampling time and ns indicates non-significant difference. Error bars correspond to standard error. HSD is honest significant difference (Tukey, 0.05).

(15.70% ± 0.24%; 38.05% ± 0.52%), respectively, with significant difference between all fruit type (Fig. 1C and 1D). Besides, none of the color attributes of epicarp or flesh registered significant variation (P > 0.05) during storage. Color of pitaya fruit is derived from betalains presence (García-Cruz et al., 2013), and these pigments, unlike anthocyanins do not give bright colors (Stintzing and Carle, 2004). The hue angle in flesh of SpR and SsR indicated that such variants have a dark-red tonality, and the difference between them lies in the lower lightness of SsR flesh, which makes this latter appear darker (Fig. 1). On other hand, based on hue angle, color of SsW is yellow; however, due to the low value of chroma, fruit flesh acquires a grayish appearance. 3.4. Chemical attributes 3.4.1. Total soluble solids, sugars, titratable acidity, and pH Total soluble solids (TSS) ranged from 9 to 11 ◦ Brix, and between fruit types there was significant contrast (P ≤ 0.05) only in relation to the SpO variant, where an slightly increment and subsequent decrement was observed (Fig. 4A). However, this was attributed to variability of tested materials, because it was considered normal that TSS were approximately constant during storage, due to the non-climacteric character of fruit (Reid, 2002). In addition, results were lower than those reported for other cacti fruit like prickly pear (13.6–13.9 ◦ Brix; Ochoa-Velasco and GuerreroBeltrán, 2014) and yellow pitahaya (20.0 ◦ Brix; Nerd and Mizrahi, 1999). Fruit variants had similar total sugar content (P > 0.05; Fig. 4B), and this ranged from 72.0 to 99.0 g kg−1 at harvest and decreased to 47.0–53.0 g kg−1 at the end of storage, with significant difference between values of first four days and those of last six days (P ≤ 0.05). On other hand, the acid content remained without significant change in all variants along the storage (P > 0.05; Fig. 4C), which, jointly with sugar content, suggests that carbohydrates could be the major substrate for respiratory activity (Taiz and Zeiger, 2006), and although rate of this physiological process exhibited relatively low values, it seems that these were high enough to cause

reduction in the total sugar content. The average sugar content of S. pruinosus fruit was 57.1 (±13.9) g kg−1 in SpR and 66.7 (±19.8) g kg−1 in SpO, while in S. stellatus content of 60.7 (±20.3) g kg−1 was found in SsR and 68.6 (±18.5) g kg−1 in SsW. These values ˜ were similar to those reported by Yánez-López et al. (2005) for S. griseus (76.0–103.0 g kg−1 ), suggesting that such content could be a characteristic feature of the Stenocereus genus. On other hand, titratable acidity (TA) clearly differentiated S. pruinosus from S. stellatus. Fruit of SpO and SpR had low acidity (from 0.14 to 0.17%), while SsR and SsW had average values of 0.60 and 0.48%, respectively, with significant difference between these latter (P ≤ 0.05; Fig. 4C). In fruit of Hylocereus spp. acidity is mainly caused by malic acid, although ascorbic, dehydroascorbic, citric, lactic, and oxalic are also present (Esquivel et al., 2007). In the case of Selenicereus megalanthus citric and ascorbic acids are predominant (Rodríguez et al., 2005). However, in Stenocereus species this type of characterization has not been attended yet. TSS/TA ratio was another feature that clearly differentiated S. pruinosus from S. stellatus, since values ranged between 56.23 and 73.83 were found in the former and between 15.99 and 19.52 in the latter. The TSS/TA ratio is commonly associated with sweetness of fruit (Magwaza and Opara, 2015), and obtained values coincided with the common perception of consumers of pitaya fruit, who qualify those of S. pruinosus as sweet and those of S. stellatus as acid materials (data not shown). In consistent form with above results, the pH was distinct between species (P ≤ 0.05). SpR and SpO had values of 5.70 (±0.49) and 5.80 (±0.49), respectively, which were higher (P ≤ 0.05) than pH of SsR (4.19 ± 0.24) and SsW (4.39 ± 0.15). Besides, differences between variants of each species were not found (Fig. 4D). In addition, pH remained also unchanged throughout the storage, confirming the constancy of acid content in fruit. pH values of variants of S. stellatus were similar to which has been reported for pitahaya (Rodríguez et al., 2005; Esquivel et al., 2007), while values found in S. pruinosus resembled more to those reported for red prickly pear fruit (Ochoa-Velasco and Guerrero-Beltrán, 2014).

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Fig. 5. Total soluble phenols content (A), betalains content (B), and antioxidant activity (C) of fruit of red (SpR) and orange (SpO) variants of S. pruinosus and red (SsR) and white (SsW) variants of S. stellatus stored after harvest during 6 d at 24 ◦ C. Different letters indicate general significant difference and an asterisk points out that at least one material was different from the rest at the specific sampling time. Error bars correspond to standard error. HSD is honest significant difference (Tukey, 0.05).

3.4.2. Total soluble phenols, total betalains content, and antioxidant activity Content of total soluble phenols (TSP, mg kg−1 ) was significantly higher in fruit of red variants, with the highest value occurring in SsR (707.7 ± 90.6), followed by SpR (535.9 ± 69.8) and variants SpO (421.6 ± 69.7) and SsW (351.6 ± 42.7), between which there was no significance in the difference (Fig. 5A). On other hand, such content remained without significant change throughout the storage in all variants (P > 0.05), which was considered normal since phenolic compounds are present in plant tissues regardless of physiological state and can increase, decrease or remain constant during storage (Al-Najada and Mohamed, 2014; Castellar et al., 2012; Herrera-Hernández et al., 2011). Nevertheless, reported values corresponded only to first six days, because it was considered that edible value was nil after that time, due to the observed fungal growth symptoms. On other hand, the content found was similar to which has been reported for fruit of Opuntia megacantha and Opuntia ficus-indica (Coria-Cayupán et al., 2011) and Hylocereus spp. (Wu et al., 2006). However, for other cacti species like O. stricta, O. undulata, and other varieties of O. ficus-indica higher concentrations of TSP have been reported (1646.0–2188.0 mg kg−1 ; Fernández-López et al., 2010; Castellar et al., 2012). The fruit of garambullo (Myrtillocactus geometrizans) ˜ (Opuntia matudae) have the reports and xoconostle cuaresmeno

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of highest TSP contents among cacti species, with values from 7400.0 to 10020.0 mg kg−1 (Herrera-Hernández et al., 2011) and 8500.0 mg kg−1 (Guzmán-Maldonado et al., 2010), respectively, although these data could be overestimated, because the colorimetric method used in their determination is not specific to phenols and other compounds such as sugars and vitamin C can also reduce the Folin–Ciocalteu reagent (Georgé et al., 2005). The content of total betalains remained also unchanged during storage (P > 0.05) and, similarly to TSP, it was significantly higher in red variants, with 571.3 (±66.9) and 559.9 (±65.1) mg kg−1 in SsR and SpR, respectively, than in SpO (424.6 ± 48.4 mg kg−1 ) and SsW (7.8 ± 2.0 mg kg−1 ) (Fig. 5B). Color exhibited by flesh of pitaya fruit is caused by presence of betacyanins and betaxanthins, which provide red-purple and yellow tonalities, respectively. The sum of contents of such groups results in the total betalains content (Azeredo, 2009). In SpR and SsR variants, although flesh showed red-magenta coloration, betaxanthins had higher content than betacyanins (1.7:1). The greatest concentration of betaxanthins occurred in SpO, where the proportion in relation to betacyanins was 9:1. Fruit of SsW variant contained almost zero betalains, which explained the white-yellowish color in flesh of such materials. Beetroot (Beta vulgaris) is among main source of betalains and up to 12,300 mg kg−1 have been reported (García-Cruz et al., 2013). Although pitaya fruit had lesser betalains concentration than beetroot, their content was higher than values found in other cacti fruit as prickly pear and garambullo (Yahia and Mondragon-Jacobo, 2011; Guzmán-Maldonado et al., 2010). It has been reported that betalains and phenolic compounds have beneficial health properties such as anti-inflammatory (Vidal et al., 2014), anticlastogenic (Madrigal-Santillán et al., 2013), and anticancer (Wu et al., 2006). Several studies have shown that they easily stabilize free radicals (Cai et al., 2003; Gandía-Herrero et al., 2013; Stintzing et al., 2005), which is derived from their antioxidant activity (Azeredo, 2009). In the present work, the highest antioxidant activity was found in fruit of SsR (6.68 ± 0.72 mmol kg−1 ), which contrasted with that found in fruit of SpR (4.91 ± 0.28 mmol kg−1 ) and this with SpO (3.46 ± 0.47 mmol kg−1 ) and SsW (2.93 ± 0.43 mmol kg−1 ), without significant difference between the latter two (Fig. 5C). Besides, antioxidant activity remained approximately constant during storage, as occurred with other quality attributes. Fruit of SsW had practically no betalains content, but their antioxidant activity was close to that found in SpO fruit. Furthermore, although SsR and SpR had similar betalains content the former had higher antioxidant activity. Therefore, the antioxidant activity of pitaya fruit could be conferred in a more extent by phenolic compounds than by betalains. 4. Conclusions Fruit of S. pruinosus and S. stellatus are different in weight, size (polar and equatorial diameters), shape, color, acidity, pH, phenols and betalains content, and antioxidant activity. Quality attributes such as total soluble solids, titratable acidity, pH, color, soluble phenols and betalains content, and antioxidant activity remain constant throughout the storage at 24 ◦ C. Shelf life was estimated in six days and changes in postharvest occur in respiration rate, weight loss, epicarp thickness, epicarp and flesh firmness, and total sugars content. Acknowledgement Author Leticia García-Cruz wishes to acknowledge the financial support received from Consejo Nacional de Ciencia y Tecnología of Mexico (CONACyT).

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