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Agrociencia

versión On-line ISSN 2521-9766versión impresa ISSN 1405-3195

Agrociencia vol.47 no.8 México nov./dic. 2013

 

Fitociencia

 

Symptoms and sensitivity to chilling injury of pitahaya (Hylocereus undatus (haw.) britton & rose) fruits during postharvest

 

Síntomas y sensibilidad al daño por frío de frutos de pitahaya (Hylocereus undatus (haw.) britton & rose) durante la postcosecha

 

Rosendo Balois-Morales1, Cecilia B. Peña-Valdivia2*, V. Baruch Arroyo-Peña3

 

1 Unidad de Tecnología de Alimentos, Secretaria de Investigación y Posgrado. Universidad Autónoma de Nayarit. México. (rosendobm@colpos.mx).

2 Botánica, Campus Montecillo. Colegio de Postgraduados. 56230. Montecillo, Estado de México.* Author for correspondence. (cecilia@colpos.mx).

3 The University of Kansas, Department of Ecology and Evolutionary Biology, Lawrence, Kansas 66045. (vbap@ku.edu).

 

Received: February, 2013.
Approved: November, 2013.

 

Abstract

Dragon fruit or pitahaya (Hylocereus undatus (Haw.) Britton & Rose) is an exquisite and exotic fruit with attractive aroma and flavor, and a characteristic red-skinned color. Its production and supply for regional and foreign markets is limited to its storage life. The objective of this study was to evaluate physiological alterations and sensitivity to chilling injuries during postharvest of pitahaya. Fruits harvested at the Tehuacán Valley, Puebla, México, were stored at three postharvest temperatures: 3, 7, and 11 ±1 °C, for 7, 14, and 21 d and under these conditions plus a 4 d period at 22 ±1 °C. Evaluated fruits were then contrasted with fresh fruit (control 1), and non-cold stored fruits maintained 4 d at 22 °C (control 2). Variables were epicarp color, fruit firmness, total soluble solids (TSS), titratable acidity, and vitamin C content in the mesocarp; and a maturity index was calculated. The experimental design was completely randomized, with 10 repetitions, where one fruit was the experimental unit. Results were analyzed using ANOVA and treatment means were compared with the Tukey test. In order to recognize symptoms of chilling injury a multivariate data analysis of Principal-Component (PC) was also carried out. Control 1 fruits was red, with relatively low-lightness and bright suitably index of L* 38, hue angle 38 and chroma 39. The color of control 2 fruits significantly changed compared to control 1 (L* 46, hue angle 6, and chroma, 53). Fruit on cold storage maintained color, firmness (5 N), TSS (9.7 °Brix), acidity (0.37 g malic acid g-1), maturity index (42 °Brix g malic acid g-1) and vitamin C content (0.78 mg 100 g-1) partially stable. Changes depending on temperature and time of storage did not follow identifiable trends. Chilling injury in pitahaya fruits does not occur progressively, and is higher under cold and room storage temperature. PC analysis indicated that 7 d of storage partially maintained fruits quality, regardless of the extra 4 d time at room temperature; thus, implying that postharvest handling of pitahaya fruits may extend its shelf life several days.

Keywords: acidity, color, firmness, total soluble solids, vitamin C.

 

Resumen

La pitahaya (Hylocereus undatus (Haw.) Britton & Rose) es una fruta exquisita y exótica con aroma y sabor atractivos, y color de piel rojo característico. Su producción y suministro para los mercados regionales y extranjeros son limitados por su vida de anaquel. El objetivo de este estudio fue evaluar las alteraciones fisiológicas y la sensibilidad al daño por frío en la pitahaya durante la postcosecha. Frutos cosechados en el Valle de Tehuacán, Puebla, México, se almacenaron a tres temperaturas postcosecha: 3, 7 y 11 ±1 °C, por 7, 14 y 21 d, y en estas condiciones más un periodo de 4 d a 22 ±1 °C. Los frutos evaluados se compararon con frutos frescos (testigo 1), y los frutos almacenados sin frío se conservaron 4 d a 22 °C (testigo 2). Las variables fueron color del epicarpio, firmeza del fruto, sólidos solubles totales (SST), acidez titulable y contenido de vitamina C en el mesocarpio; y se calculó el índice de madurez. El diseño experimental fue completamente aleatorio, con 10 repeticiones, y un fruto fue la unidad experimental. Los resultados se analizaron con ANDEVA y las medias de los tratamientos se compararon con la prueba de Tukey. Para reconocer los síntomas del daño por frío se realizó un análisis multivariado de componentes principales (CP). Los frutos del testigo 1 fueron rojos, con luminosidad relativamente baja y un índice de brillantez adecuada de L* 38, ángulo de tonalidad 38 y croma 39. El color de los frutos del testigo 2 cambió significativamente en relación al testigo 1 (L* 46, ángulo de tonalidad 6 y croma 53). Los frutos en almacenamiento frío conservaron color, firmeza (5 N), SST (9.7 oBrix), acidez (0.37 g ácido málico g-1), índice de madurez (42 oBrix g ácido málico g-1) y contenido de vitamina C (0.78 mg 100 g-1) parcialmente estables. Los cambios que dependieron de la temperatura y el tiempo de almacenamiento no siguieron tendencias identificables. El daño por frío en frutos de pitahaya no ocurre progresivamente, y es mayor con almacenamiento frío y temperatura ambiente. El análisis CP indicó que 7 d de almacenamiento mantienen la calidad de los frutos parcialmente, independiente de los 4 d extra a temperatura ambiente; esto implica que el manejo postcosecha de los frutos de pitahaya puede extender la vida de anaquel por varios días.

Palabras clave: acidez, color, firmeza, sólidos solubles totales, vitamina C.

 

INTRODUCTION

There is an increasing global demand for traditional and rare fruits, which increases the gastronomic diversity as they provide new flavors, aromas, colors, and attractive appearance for consumers (Ortiz-Hernández and Carrillo-Salazar, 2012). This is the case of several edible fruits from the cactus families and they are eaten fresh and have high commercial value (Corrales-García, 2003). The pitahaya (Hylocereus undatus (Haw.) Britton & Rose), a cactaceae native to México, Central, and South America, is a species naturally distributed in dry tropical climates, which produces a rare, attractive, and exquisite fruit. This is a complex fruit, which develops from the inferior ovary, has a roundlike or ovoid shape, and covered with bracts; when ripe, develops a characteristic red epicarp color, white mesocarp, with embedded small black seeds (Figure 1).

Because of the increasing global demand of exotic fruits, pitahaya commercialization has enhanced its cultivation, as well as its economic and agronomic potential (Centurión et al., 2008). Canada, Japan, USA and the EU annually import 1105 t. In México, its price is variable and can reach up to US $ 6.1 kg-1, while in international markets it fluctuates between 22 and 26 US $ kg-1; which suggests that its cultivation is profitable (Centurión et al., 2008).

Fruit commercialization is affected by quality drop due to inappropriate post-harvest handling, which largely determines the economic outcome of fruit and vegetable marketing (Toivonen and Hodges, 2011). Pitahaya is a nonclimacteric fruit, with best flavor when harvested at full red color (Nerd et al., 1999), showing fast postharvest quality deterioration under natural environmental conditions, decreasing physical and chemical quality and its overall appearance; thus, its maximum commercial life may be less than 8 d (Centurión et al., 1999). However, the useful life span can be increased by storing fruits at low temperatures (Ortiz-Hernández and Carrillo-Salazar, 2012). Information about cooling conditions for optimal conservation is needed to increase its lifespan, which will allow reaching markets away from production sites. Yet, chilling sensitivity, expressed as quickly firmness loss, was reported in pitahaya fruits at room temperature after 7 d of storage at 6-13 °C (Corrales-García, 2003). Plants may develop chilling injury as physiological, biochemical and cellular disorders when exposed to low but non-freezing temperatures. The full manifestations of stress become apparent only after fruits are replaced on non-chilling temperatures (Toivonen and Hodges, 2011). The aim of this research was to evaluate the symptoms and sensitivity to chilling injury during postharvest of pitahaya fruits.

 

MATERIALS AND METHODS

Plant material

Pitahaya fruits, with the typical harvesting red color covering 80-100 % of the epicarp, were harvested at Tehuacán Valley, Puebla, México (18° 10' 12" N and 97° 21' 24" W, climate BS1hw "(w)(i') (g), 19 °C mean annual temperature, and 480 mm mean annual precipitation). Up to 20 kg of fruits were placed in disinfected plastic boxes, and transported (about 20 h) to the Colegio de Postgraduados, Montecillo Campus, Estado de México, México. A batch of 170 fruits with no apparent damage were separated into five groups of 34 fruits each (treatments): 1) one group was immediately analyzed (control 1 without storage); 2) another was kept 4 d at room temperature (22± 1 °C; control 2 without cold storage) and analyzed; 3) other was stored at 3±1 °C (90-95 % RH); 4) other was stored at 7± 1 °C (90-95 % RH); 5) this was stored at 11 ±1 °C (90-95 % RH). The fruits stored at chilling temperatures were tested after 7, 14, and 21 d at storage, plus an additional second evaluation after the extra 4 d period at room temperature. The 4 d period intended to simulate a common commercialization scenario, where fruits remain in shelf after storage at low temperatures, to be exhibited to consumers for purchase, and then placed in fruit bowls before being consumed.

Experimental design and statistical analysis

The experimental design was completely randomized with 10 repetitions, and each fruit was an experimental unit. Data were analyzed via ANOVA, and treatment means were compared with the Tukey test (p≤0.05). To identify physical and chemical characteristics influenced by the cold storage, or symptoms of chilling injury in the pitahaya fruits, multivariate data analysis of Principal-Component (PC) were carried out: one PC with measurements data of cold storage fruit and control 1; other PC of cold storage fruit with the extra 4 d period at room temperature and the control 2 data. These were carried out using SAS/STAT (version 9).

Methodology

Two opposite zones (without bracts) at the equatorial region of the fruit were chosen to evaluate epicarp color quantified as reflected color in the Hunter (L*a*b*) color space, with a Hunter Lab colorimeter (D25-PC2, Hunter Laboratory, Inc., Reston, VA, USA). The a* and b* values were converted to hue angle (H=tan-1(b*/a*)) and chroma (C=(a*2+b*2)1/2). Firmness was determined using a Force Five (FDV-30) texturemeter equipped with a conical support of 0.8 mm diameter. The equatorial zone of fruit was chosen to test shell strength to penetration, and results were expressed as N. The total soluble solids content (TSS) was determined in fruits juice according to AOAC official method (1990). The juice was extracted from 20 g of mesocarp, filtered through a sieve to remove seeds and fruit debris. TSS were determined in °Brix using a PR-100 refractometer (ATAGO, USA) (0-32 %) calibrated with distilled water. Acidity and vitamin C content was determined in fresh mesocarp according to AOAC official method (1990) and results were expressed as percentage of malic acid and as mg ascorbic acid 100 g-1 mesocarp.

 

RESULTS AND DISCUSSION

Color

Fruit color, before storage (control 1), was red (L* 38), with a relatively low-lightness (hue angle 38) and suitably bright vivid coloration (chroma 39) (Figure 1A and white bars in Figure 2 A, C, D). Producers use color to decide time for harvest, but there is no formal methodology to evaluate this characteristic, resulting in nonstandard maturity levels.

Color values are adequate parameters to describe the pitahaya fruit color at harvest time, as others plant tissues; however, during postharvest handling, mainly at room temperature, tissue senescence and oxidative stress negatively affect mesocarp quality (Balois et al., 2007, 2008). Thus, color measurement could be used to determine optimal harvesting dates and optimal temperature for fruit storage, mainly because color is an important factor in the perception of pitahaya's fruit quality.

Color of fruits after 4 d at 22 °C (control 2; white bars in Figure 2 B, D and F) changed (p≤0.05) compared to control 1, as shown by its higher values on L*, chroma and hue angle. Thus, control 2 was significantly less dark, developed a more vivid coloration, but was slightly less red than control 1. These changes indicated that epicarp color is quite unstable on pitahaya fruits after harvest, and may be the result of fruit maturity. Still, some physiological characteristics, like activity of antisenescence enzymatic system i.e. catalase-superoxide dismutase (EC. 1.11.1.6 - EC. 1.15.1.1), and phenol content did not change (Balois-Morales et al., 2007 and 2008) in batches of fruits under similar conditions compared to those in the present study.

Cold storage maintained the fruit epicarp color partially stable, and changes depending on temperature and time of storage did not follow identifiable trends. L* is (lightness) an indicator of tissue darkening (from 0 to 100, from black to white); after 7 d in cold storage, fruits were significantly less dark than control 1, but fruits stored for longer time (Figure 2 A) and all cold treatments with an extra period of 4 d at 22 °C were also darker than control 1 (Figure 2 B). Hue angle did not change in several cold storage treatments compared to control 1, but drastically decrease (p≤0.05) after 14 and 21 d at 3 oC (Figure 2 C). Fruits on cold storage after 4 d at 22 °C also decrease their hue angle compared to control 1 (Figura 2 C-D). With two exceptions, cold stored treatments and those with an extra period of 4 d at 22 °C developed a more vivid coloration than control 1, as evidenced by higher (p≤0.05) values of chroma, even after 14 and 21 d in storage (Figure 2 E-F). This indicates that among the color values in harvested, fully red, ripe pitahaya fruits, hue angle is one of the most affected characteristics by cold storage; color changes were higher by the combination of cold and room temperature storage, and each of these changes did not occur progressively during storage.

Lightness (L*), hue angle and chroma contribute all together to the total color perception, and they are partially related among them. Brawner and Warmund (2008) calculated the L* + hue angle + chroma sum (LHC sum) of husk and kernel color of several walnut cultivars to compare the accuracy of visual sorting with quantitative measurement. Warmund (2008) used the LHC sum to evaluate the effect of delayed hulling of fruits on kernel color at successive harvest dates. In order to evaluate the color changes on pitahaya fruit during storage the LHC sum was calculated (Figure 3). Total color changes of fruits increased or did not change during cold (3-11 °C) storage with storing time (7-21 d), but decayed after the longer storage (14-21 d) at the lower temperature (3 °C) (Figure 3 A), and also with cold storage plus 4 d at room temperature (Figure 3 B). The varying responses of L*, hue angle and chroma during storage time in the last group of treatments (Figure 2 B, D and F) can observed as a whole gradual (linear) color decrease, of the LHC sum, from day 7 to d 21 at cold storage with the extra period at room temperature (Figure 3 B). Color alterations on pitahaya epicarp were enhanced by the combined effect of cold and room temperatures, probably due to chilling injury, because plants native to tropical and subtropical climates, like pitahaya, may develop symptoms slowly during the actual chilling period, that are expressed much more clearly and quickly once the tissue is returned to non-chilling conditions (Lukatkin et al., 2012). This is also, the most likely scenario on the actual consuming process most fruits go through in most households.

Pigments such as betalains, e.g. betacianins (red or red-violet color) and betaxantins (yellow) are responsible for the red pigmentation in the pitahaya epicarp (Wybraniec and Mizrahi, 2002). These pigments are unstable and get discolored when fruits are exposed to light, extreme temperature and standard dry air (Soriano et al., 2007). This pigments instability helps to explain changes of the pitahaya's external color, after some days at 22 °C (control 2). These are evidence of chilling injury, mainly once the fruits were returned to non-chilling conditions.

Firmness

Firmness of the pitahaya fruit is one of the main quality characteristic that determines consumer acceptance. At the beginning of the study, firmness of control 1 was heterogeneous among samples (3.35.8 N), but its mean value (5 N) was adequate for harvest (Figure 4 A) according to Centurión et al. (2008). After 4 d at room temperature there was a small but significant firmness reduction (4.4 N); in contrast, cold storage increased it and higher storage temperature (7-11 °C) for longer time (1421 d) did not modify it, as compared to control 1. In general, 4 d at room temperature after cold storage did not change firmness compared to control 1, with two exceptions: at the lowest and highest storage temperatures, for the longest and shortest time (Figure 4 A-B). These results showed that firmness is a more stable characteristic than fruit color through cold storage periods, and after, at room temperature (Figures 2 and 3).

These results partially contrast with those of Nerd et al. (1999) and Corrales-García (2003), where pitahaya firmness did not change after 7 d at 6, 8 or 13 °C, but quickly decayed at room temperature after cold storage. For this reason, they are considered as sensitive to chilling injury during postharvest. However, fruit reactions to environments during postharvest may vary due to their developing place, influencing their physiological characteristics at harvest and morphological characteristics as the epicarp thickness (Centurión et al., 2008). Besides, significant interactions between temperature and storage time on the firmness of pitahaya fruits indicate that physiological alterations, e.g. , cell membranes damage and direct impact on firmness. This ultimately results in excessive and irreversible softening (Corrales-García and Canche-Canche, 2008). In the present study, chilling injury varied in speed and intensity and only the combination of lower temperature with longer storage time led to alterations in their mesocarp appearance, like darkening (Figure 5) and heterogeneity in firmness among experimental units within a treatment. Such were the case of treatments stored at 7 °C for 7 d (5.6-7.3 N, s.e. 0.31) and 14 d (4.3-N. s.e. 0.47; Figure 4 A).

Total soluble solids (TSS) and titratable acidity

The TSS in control 2 and control 1 (Figure 6 A-B) showed an interval ranging between 9 and 14 °Brix. This was similar to those reported in ripe pitahaya fruits (Castillo-Martínez et al., 2005; Centurión et al., 2008). The TSS increased (20 %) after 7 d of cold storage and only in fruits stored at 3 °C remained significantly (p≤0.05) high with 14 and 21 d of storage, compared to control 1. In contrast, fruits stored at 7 and 11 °C for 14 and 21 d, and then placed on room temperature, diminished TSS (Figure 6 A-B), probably as a result of chilling injury (Figure 5).

The TSS is a ripening index for some fruits, indicating the quantity of sugars making the greatest contribution, along other dissolved substances (such as acids and salts), to those found in fruits juice (Tasnim et al., 2010). Cold damages may be diverse; some plant species accumulate sugars and TSS at low temperatures as a response mechanism to it in order to increase cold tolerance (Mundree et al., 2002). Fruits of H. undatus and H. polyrhizus stored at 6 ±1.6 °C for one week, significantly increased sugars concentration and TSS simultaneously with color development (Nerd et al., 1999). Thus, here the minor mesocarp damage in some treatments (mostly at 3 °C) agree with TSS increase from 16 to 20 °Brix, as in pitaya fruits (Acanthocereus pitajaya sensu Croizat) stored 9 d at 2 °C (Dueñas et al., 2009).

Control 2 significantly reduced its acidity to less than half of control 1 (Figure 6 C-D). At all tested temperatures, 7 d of cold storage did not affect acidity, which increased in fruits stored 14 and 21 d at 3 °C compared to control 1, but did not increase after the 4 d extra time at room temperature. Storage at higher temperature during longer time did not decrease acidity (Figure 6 C), but the 4 d extra time at room temperature decreased it (p≤0.05) to 0.054 g 100 g-1 (Figure 6 D).

Decrease in acidity during fruits ripening occurs usually along with sugars increase (Prasanna et al., 2007). Titratable acidity dropped from 1.1 to 0.4 % of the malic acid content in pitahaya fruits during ripening, between 27 and 31 d after anthesis (when fruits are ready to harvest and have high visual consumer acceptance), even though TSS content showed no significant change (Centurión et al., 2008). After 28 d of storage at 8 °C acidity of pitahaya slices remained almost unchanged (Vargas et al., 2005). In contrast, acidity decreased in fruits of H. undatus and H. polyrhizus stored at 6 °C, between two and three weeks, and were then transferred to 20 °C (Nerd et al., 1999). Maturity index of control 1 in the present study was similar to that reported by Centurión et al. (2008), but acid content was lower (near 50 %). These results suggest that the present maturity index (ratio of TSS content over acidity) would be appropriate to determine pitahaya maturity and quality during postharvest, rather than SST or acidity separately.

Four days at room temperature increased maturity index (p≤0.05) of fruits more than twice (control 2 compared to control 1; Figure 6 E-F), which confirms a quick ripening of pitahaya fruit at that temperature. Cold storage (3-11 °C) up to a 21 d did not change (p>0.05) maturity index (Figure 6 E). Such conditions emulated an alternative shipping scenario for fresh pitahaya fruits, particularly from distant harvesting places. The additional 4 d period at room temperature after cold storage increased (p≤0.05) maturity index compared to control 1, but when cold storage was a 14 d period the increase was lower than control 2 (Figure 6 F).

The maturity index of some harvested fruits is used to show the effect on post-harvest response during marketing. This is because maturity of harvested fruits interacts with the required trucking time for orchards to reach markets. For this reason, some fruits should be harvested at less advanced stage of maturity (Casierra-Posada et al., 2004).

Vitamin C

The vitamin C content did not change after 4 d at room temperature (control 2), neither did at 7 d at 3 °C, but at 7 and 11 °C it significantly (p≤0.05) decreased. Longer storage to 3 °C increased vitamin content (almost 50 %) as compared to control 1. But with one exception, vitamin C content was similar to control 1 after longer storage time at higher storage temperatures (Figure 7 A-B). Fluctuations in vitamin C content due to cold storage changed after the additional 4 d at room temperature; 21 d at 3 and 11 °C increased vitamin C content (15 to 28 %) compared to control 1 (Figure 7 A-B). According to Lee and Kader (2000), species susceptible to low temperatures show significant vitamin C losses during cold storage. Still, results in the present study (Figure 7) showed that cold storage had little effect on pitahaya fruits, although 21 d of storage can increase vitamin C content.

Results of this study are partially different to those of Corrales-García (2003), who showed that vitamin C content of freshly harvested pitahaya fruits is 4 to 14 mg 100-1 g in mesocarp, and decreases during storage, to the point that they may become undetectable. In contrast, vitamin C content in the prsent study remained within the control intervals for all treatments. There is limited information to explain such differences, which might be due to the evaluated genotypes, environmental conditions during fruit development and cultivation site (soil type, nutrient availability).

The precise physiological function of vitamin C on plant cells is unclear (Kays and Paull, 2004), but it is used as a plant stress index. Temperature management after harvest is the most important factor to maintain vitamin C in fruits (Lee and Kader, 2000), because its loss is accelerated by high temperatures and long storage periods (Kays and Paull, 2004). In addition, the natural ripening process of the H. undatus fruits decreases significantly vitamin C (5 mg 100 g-1) between 20 and 31 d since flower opening (Centurión et al., 2008) and (7.0 mg 100 g-1) during the period of color changes in fruits (Nerd et al., 1999). A similar decrease (11.4 mg 100 g- 1) was observed in fruits of H. polyrhizus between 24 and 27 d after flowering (Nerd et al., 1999). The results in this study showed that storage at 11 °C constrain the natural content decline of vitamin C in pitahaya fruits during post-harvest.

Principal-component analysis (PCA)

Two PCA were carried out: one with eight explanatory variables of control 1 and nine cold (311 °C) storage treatments, for up to 21 d (section A on Table 1); and the second with the eight explanatory variables of a similar cold storage treatments followed by 4 d storage at room temperature (section B on Table 1) and control 2.

In the first PCA, the first three principal components (PCs) accounted for 79.61 % of the total variance (section A, Table 1). The PC1 accounted for 50.56 % of data variability, had high positive loadings on the acidity and vitamin C variables, and high negative loadings on the maturity index variable; this CP seems to evaluate the preponderance of fruits chemical composition over physical characteristics. According to it, cold storage increased acidity and vitamin C content, and confirmed a delay on fruit maturity which was one of the expected changes. The PC2 related to the stability of fruit quality during storage, as the second eigenvector had a high positive loading on the red coloration variable and a high negative loading on the TSS content. The PC3 confirmed the relevant effect that cold has during storage on color of fruit epicarp, because its highest positive loadings on lightness and chroma (Table 1, section A).

The PC2 and PC3 plotted against the PC1 (Figure 8 A-B) showed some overlap between treatments at each temperature for the evaluation times. Treatments at 3 °C for 14 and 21 d (positive values on CP1), and at 11 °C for 14 and 21 d (negative values on CP1) were the most dissimilar among cold stored treatments. Those stored at 7 °C were intermediate between them and the control 1. This confirms that physical and chemical changes of pitahaya fruits largely depend on temperature and storage time. PC3 plotted against PC2 (Figure 8 C) showed that changes of fruits after cold storage at 3-11 °C for longer time (21 d) were more heterogeneous than those for shorter time (7 d). Cold storage for 7 d and many others for 14 d formed tight clusters, partially surrounded by those stored for 21 d, besides of temperature, and some of those at 7 °C for 14 d. This suggests that a reduction of variation in the characteristics under study is a consequence of cold storage for less than 14 d.

In the second PCA, the first three PC explained 75.23 % of the variability (section B on Table 1).

The PCI had a higher and negative correlation with acidity and maturity index and likely represents an opposite contribution of storage environments to fruit maturity. The PC2 had a higher and positive correlation with quality and stability of fruit color conditioned on the storage environments, as the second eigenvector had a high positive loading on L* and chroma. These effects can be interpreted as chilling damage on fruits. Te PC3 had relevant influence on firmness, as an important characteristic determining pitahaya fruits to ripeness.

When the PC2 was plotted against the PCI and PC3, no overlap was observed between all cold stored treatments and control 2; PC2 enhanced the separation and was composed of two physical characteristics related to the color perception (L* and chroma), which promoted this independence in the first three PC. This contrasted widely with the results of the first PC analysis (A group Table 1 and Figure 8) and indicates that these differences were generated by the 4 d at room temperature after cold storage. But, as in the first analysis, PC2 and PC3 plotted against the PC1 show that longer time periods (14-21 d), under each evaluated low temperature, were different than those stored just for 7 d, independently of the extra 4 d at room temperature, because of the higher positive and negative correlation of PC1 with acidity and maturity index, in both analysis (Figure 9 A-B).

In contrast, when the PC3 was plotted against the PC2 all treatments on the second analysis, except control 2, formed a tight cluster (Figure 9 C). Tis indicates that physical and chemical characteristics of cold stored fruits tended to be homogenized after 4 d at room temperature, regardless of time and temperature of storage.

Among the eight variables evaluated after cold storage and after cold storage followed by 4 d at room temperature, firmness, in first case, and hue angle, TSS and vitamin C, in the second case, had low relative contribution to the first three PC (Table 3). This fact might be the result of similar effects of cold exposure and its combination with room temperature on pitahaya fruit characteristics. PC analysis show that it is possible to distinguish positive effects of cold storage and chilling damage, and show differences between cold conditions that increase the pitahaya's fruits shelf-life and those that produce chilling damage on them.

 

CONCLUSIONS

Chilling injury in pitahaya fruit does not occur in a linear progressive fashion and is higher when a combination of cold and room storage temperature occurs. Cold storage, at chill temperatures, for 7 d partially maintains fruits quality independently of an extra 4 d time at room temperature (average household managing scenario). Thus, managing the time and temperature of storage postharvest handling of pitahaya fruits can extend several days the shelf life.

 

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