Experimental design

Because pineapple guava is a perennial crop, 10 trees per element plot (^{Fernández et al., 2010}) and two plots per farm were studied, for a total of 40 trees, planted at 4 χ 4 m. The aim was to record fruit growth and development from anthesis to harvest along with the weather conditions. A total of 300 fruits free of defects and mechanical damage were collected during two harvests per plot and per farm. The fruits were transported to the laboratory for disinfection with a solution of 1 mL L^-1 sodium hypochlorite. The fruits were differentiated per plot and site of origin for each harvest and stored at 18 ± 1°C (76 ± 5% RH, 90 fruits by 11 d) and 5 ± 1°C (87 ± 5% RH, 210 fruits by 31 d), taking into account that this fruit species can stand temperatures as low as 1.7°C (^{Valderrama et al., 2005}).

Measured variables

The measured quality attributes included weight loss (WL), skin and pulp firmness, total soluble solids (TSS), total titratable acidity (TTA) and epidermal color (hue angle; °h). To determine WL, the weight variation during the storage of five samples from two fruits was determined based on a gravimetric method using an analytical precision balance, Precisa XT220A, with a capacity of 220 g and 0.0001 g precision (Precisa Instruments, Zurich, Switzerland). The skin and pulp firmness were quantified with a Brookfield CT3-4500 texturometer (Brookfield Engineering, Middleboro, MA, USA). Two readings per fruit were performed with a TA39 probe at a precision of ± 0.5%. The NTC 4624 Technical Standard (Icontec, 1999a) was used for the measurements of TSS with an Eclipse refractometer (Bellingham Stanley, Tunbridge Well, UK) using a scale from 0 to 32 and a precision of 0.2 °Brix. The TTA was determined following the NTC 4623 Technical Standard (Icontec, 1999b). The maturity ratio (MR), defined as the ratio between TSS and TTA (TSS/TTA), was determined. The epidermal color was measured using a Minolta CR-400 chroma meter (Konica Minolta, Ramsey, NJ, USA). The quality attributes of the fruits were evaluated after 1, 3, 5, 7, 9 and 11 d of fruit storage at 5 and 18°C. The quality attributes were also evaluated after 15, 19, 23 and 31 d of fruit storage at 5°C. The statistical design was entirely random, with five replicates per trial.

In order to analyze the behavior of each quality parameter and its variation over time, the statistical program IBM-SPSS v.20 (SPSS Inc., Chicago, IL, USA) was used to perform a correlation analysis between the fruit quality parameters. The data were analyzed with descriptive statistics, and the standard deviation (SD) was used as a measure of dispersion. An analysis of variance and comparison of means test (Tukey's tests) were performed for the fruit-quality characteristics during storage for each study location and harvest.

Skin and pulp firmness

The pineapple guava fruit skin and pulp firmness had the same trends in behavior over time for both storage conditions and both locations, with initially high values that decreased during ripening (Tab. 2). The skin firmness was always higher than the pulp firmness for the same storage period.

TABLE 2 Analysis of means for skin and pulp firmness (N) of pineapple guava fruits during post-harvest under two storage conditions. 

T-1: Tenjo location, harvest 1; T-2: Tenjo location, harvest 2; S.F.-1: San Francisco location, harvest 1; S.F.-2: San Francisco location, harvest 2. Different lowercase letters in the rows show statistical differences according to the Tukey's test (P<0.05).

At the beginning of storage, the skin firmness of the pineapple guava fruits showed mean values of 15.2 ± 1.6 N for San Francisco and 12.5 ± 3.1 N for Tenjo, with values decreasing over time. For the fruits stored at 5°C, the skin firmness at the end of the storage period reached average values of13.8 ± 2.7 N for fruits coming from San Francisco and 7.8 ± 2.0 N for those coming from Tenjo, whereas the "San Francisco fruits" stored at 8°C showed average values of 9.3 ± 1.8 N and the "Tenjo fruits" showed mean values of 4.2 ± 0.9 N at the end of the storage period.

The pulp firmness had average values of 5.8 ± 2.0 N in fruits from San Francisco and 6.5 ± 3.1 N in fruits from Tenjo at the beginning of storage, at both temperatures. At the end of the storage period, the pulp firmness of the fruits stored at 5°C reached values of 1.7 ± 1.0 N in the fruits from San Francisco and 1.2 ± 0.5 N in the fruits from Tenjo, whereas the "San Francisco fruits" stored at 18°C had mean values of 0.8 ± 0.3 N and the "Tenjo fruits" showed average values of 0.5 ± 0.2 N.

The analysis of means (Tab. 2) indicated that, during storage, the differences were related to storage conditions and place of origin. However, differences were not observed in fruits from the same locations. The firmness behavior of the pineapple guava fruits during post-harvest depended on the storage temperature and fruit firmness at the time of harvest, which is influenced by the climatic conditions of the cultivation site. The weather conditions at the origin site influenced durability in postharvest and invariably affected the internal and external quality of the fruits and their storage capacity (^{Moretti et al., 2010}).

The loss of firmness in fruit skin and pulp (Tab. 2) during storage was higher for fruits produced at the elevated altitude (Tenjo), with a lower average temperature and higher solar radiation; thus, a higher number of calendar d and lower thermal time (GDD) were required to proceed from anthesis to harvest (Tab. 1).

The average loss of skin firmness of the pineapple guava fruits from Tenjo and San Francisco was 37.1 and 8.2% for the fruits stored at 5°C, and 66.0 and 38.5% for the fruits stored at 18°C, respectively.

Contents of total soluble solids, total titratable acidity and maturity ratio

For the pineapple guava fruits stored at 18°C, the TSS (°Brix) and maturity ratio (MR) increased and TTA (% of citric acid) decreased with ripening, whereas the fruits stored at 5°C showed little variation of those parameters throughout the storage time (Tab. 3). The fruits stored at a higher temperature showed more changes because of the exponential increase in the speed of enzymatic reactions with increasing temperatures (^{Wills et al., 2007}).

TABLE 3 Analysis of means for total soluble solids (TSS) and total titratable acidity (TTA) in pineapple guava fruits under two storage conditions. 

T-1: Tenjo location, harvest 1; T-2: Tenjo location, harvest 2; S.F.-1: San Francisco location, harvest 1; S.F.-2: San Francisco location, harvest 2. Different lowercase letters in the rows show statistical differences according to the Tukey's test (P<0.05).

The TTS content showed average values of 10.8 ± 0.9 °Brix for San Francisco and 12.6 ± 1.1 °Brix for Tenjo at the beginning of storage for the two temperatures. Although the TSS in the fruits stored at 5°C showed an increasing trend, the variation was not significant, especially for the fruits from San Francisco, which was shown by comparing the values recorded at the beginning and end of the study (Tab. 3).

At the end of the storage period, the fruits reached mean values of 10.9 ± 0.7 °Brix for San Francisco and 14.2 ± 0.9 °Brix for Tenjo. The TSS of the fruits stored at 18°C increased as the fruits ripened, especially in the fruits from Tenjo. At the end of the storage period, mean TSS values of 12.0 ± 0.9 °Brix for San Francisco and 16.5 ± 0.7 °Brix for Tenjo fruits were observed.

The TTA registered average values of 1.69 ± 0.18% for San Francisco and 1.80 ± 0.20% for Tenjo at the beginning of storage at both temperatures. Although the TTA in the fruits stored at 5°C showed low variation, the TTA tended to decrease during the last day of storage (Tab. 3) reaching final mean values of 1.42 ± 0.15% for San Francisco and 1.53 ± 0.19% for Tenjo. The TTA of the fruits stored at 18°C decreased as they ripened (Tab. 3), especially in fruits from Tenjo, with average values of 1.23 ± 0.25% for San Francisco and 0.78 ± 0.27% for Tenjo at the end of the storage period.

The MR increased for the two storage conditions (Fig. 1) and was elevated at the highest temperature. The MR showed average values of 6.6 ± 1.1 for San Francisco and 7.1 ± 1.2 for Tenjo at the beginning of storage for both temperatures. Although the MR at 5°C showed little variation, it generally tended to increase during the last 8 d of storage, reaching average values of 7.9 ± 0.9 for fruits from San Francisco and 9.6 ± 2.2 for fruits from Tenjo. The MR of the fruits stored at 18°C increased as the fruits ripened, presenting average values of 10.3 ± 2.6 for San Francisco and 23.9 ± 8.1 for Tenjo at the end of storage, with high dispersion for the latter location.

FIGURE 1 Variation of the maturity ratio (TSS/TTA) of pineapple guava fruits stored at T₁ (5°C) and T₂ (18°C). 

The analysis of means (Tab. 3) indicated that storage conditions and place of origin differed for TSS and TTA. The behavior of TSS, TTA, and MR in the pineapple guava fruits during post-harvest depended on the storage temperature and TSS and TTA values at the time of harvest, which were influenced by weather conditions during fruit growth. During storage, the fruits from the highest location (Tenjo) with an elevated accumulated solar radiation and lower mean temperature and relative air humidity (Tab. 1) had higher levels of TSS and MR, but lower TTA. These results indicate that pineapple guava fruits produced in cold climates have a better flavor than those from warm climates.

The values listed in Table 3 indicate that the TTA in the pineapple guava fruits did not show differences at the time of harvest, which could help determine the influence of climatic conditions on this parameter, but differences were observed at the end of the storage period, especially for fruits stored at the highest temperature (18°C). The TTA was higher in the fruits produced at the lowest altitude (San Francisco), with less accumulated solar radiation and higher average temperatures and relative air humidity (Tab. 1).

Weight loss and color change

The weight loss (WL) in the pineapple guava fruits increased for both storage conditions (Fig. 2A). For fruits stored at 5°C, the WL at the end of the storage period showed a mean value of 8.48 ± 1.91°% for San Francisco and 5.94 ± 0.75% for Tenjo, whereas those stored at 18°C presented a mean value of 20.37 ± 1.60% for fruits from San Francisco and 13.01 ± 1.98% for fruits from Tenjo. For the storage conditions (equal altitude), it was observed that, at a lower temperature, the relative humidity was lower (sensitive cooling of the air) and, therefore, the vapor pressure deficit was lower, which is the cause of the moisture loss of the product (weight loss). Transpiration and substrate consumption through respiration are the primary causes of WL of fruits during post-harvest ripening (^{Saladié et al., 2007}).

The analysis of means indicated that differences in WL occurred between storage conditions and cultivation location. The fruits stored at 5°C did not show significant differences for the site of origin or harvest when compared with fruits stored at 18°C. However, for the two storage conditions, the WL was lower in the fruits from the highest location (Tenjo), where the accumulated radiation is greater and mean temperature and relative humidity are lower (Tab. 1).

Color change occurred because of chlorophyll degradation and pigments synthesized such as anthocyanins and carotenoids (^{Mishra and Gamage, 2007}). The color measured as the °h represented the color or tonality that varies from 0° in pure red to 180° in pure green (^{Hernández et al., 2007}). The value of the °h of the pineapple guava fruits did not show a clear trend during the first d of storage, but decreased over time for the two storage conditions (Fig. 2B). The analysis of means indicated that statistically significant differences did not occur in the °h for the pineapple guava fruits during the last d of storage for the different sites and harvests.

FIGURE 2 Variation in weight loss (A) and hue angle (B) of pineapple guava fruits stored at T₁ (5°C) and T₂ (18°C). 

Skin and pulp firmness

Different authors (^{Osterloh et al., 1996}; ^{Parra-Coronado et al., 2006}) have indicated that decreased firmness values during fruit ripening are determined by propectin, the agglutinating substance within cells that gives turgidity to fruits and degrades along with pectin substances. This degradation process changes the texture and consistency of fruits, producing the characteristic softening during the ripening process. The decrease in firmness over the storage period has been reported by various authors for other fruits of the Mirtaceae family, such as guava (^{Solarte et al., 2010}), champa (Campomanesia lineatifolia) (^{Álvarez et al., 2009}), and Araza (Eugenia stipitata) (^{Hernández et al., 2007}). Firmness is an important fruit characteristic of consumption quality. When designing packaging and transport during the harvest and post-harvest periods (Parra and Fischer, 2013), the criteria set by ^{Márquez et al. (2007)} should be considered; these authors indicated that the load that a fruit can support is equivalent to 70% of its durability.

During pineapple guava fruit ripening, many enzymes are expressed, which modify the plasticity of cell walls, and one the main enzymes involved in this process is polygalacturonase (PG) (^{Õpik and Rolfe, 2005}). The activity of PG in pineapple guava is higher inside the mesocarp, which suggests that softening progresses from the inside to the outside of the fruit (^{Parra and Fischer, 2013}). This process is evidenced by the lower firmness value of the pulp as compared to that of the skin. For the storage conditions considered in this study, firmness decreased more rapidly for the pineapple guava fruits stored at the higher temperature (18°C). At day 11 of storage, the fruits with higher values of skin and pulp firmness were those stored at the lowest temperature (5°C) (Tab. 2). Similar results were reported by different authors for pineapple guava from clones 41 ('Quimba') and 8-4 (^{Rodríguez et al., 2006}; Parra and Fischer, 2013) as well as for guava (^{Solarte et al., 2010}) and pear (^{Parra-Coronado et al., 2006}).

The post-harvest physiological behavior of fruits and vegetables depends on their storage conditions, with temperature being the main factor (^{Parra-Coronado and Hernández-Hernández, 2008}). The decreasing intensity of respiration is the basis for extending the life of agricultural products through low temperatures, which reduces the speed of enzymatic activity (^{Wills et al., 2007}). The speed of enzymatic reactions increases exponentially with increases of temperature, and, for every 10°C temperature increase, physiological and chemical changes increase 2 to 3 times (Q₁₀) (^{Mishra and Gamage, 2007}). As the respiration rates increase, ethylene concentration is also increased, promoting the activation of enzymes that intervene in cell wall degradation, which causes loss of firmness and consequent softening of the fruit (^{Saladié et al., 2007}).

Although studies have not been performed to determine the influence of weather conditions on the post-harvest behavior of pineapple guava fruits, the results found in this study are consistent with observations by ^{Minas et al. (2018)}, who found that peach fruits produced at higher crop temperatures were firmer, whereas their water content was lower. Limited studies have been performed to determine the influence of precipitation and relative humidity on fruit firmness. In this study on pineapple guava fruits, harvests with greater accumulated rain (Tenjo-2, with 417 mm; and San Francisco-2, with 1,400 mm) and average relative humidity produced fruits with lower skin firmness for the same location although the pulp firmness did not show differences (Tab. 2). This result is consistent with that of ^{Gariglio et al. (2007}), who reported that a high RH could severely affect fruit quality, as observed in mandarin, which quickly lost fruit consistency.

Contents of total soluble solids, total titratable acidity, and maturity ratio

The pineapple guava fruits stored at higher temperatures showed a higher and accelerated increase in TSS because the speed of enzymatic reactions increases exponentially with rises in temperature (^{Wills et al., 2007}). Although there is no consensus on variations of TSS in pineapple guava fruit during post-harvest, these results are within the range reported by certain authors, who indicated that TSS values increase up to the climacteric process and subsequently decrease (^{Rodríguez et al., 2006}; ^{Parra and Fischer, 2013}). Other authors have indicated that TSS decreases during post-harvest or remains constant when fruits are stored at a low temperature (^{Velho et al., 2011}), with values between 5 and 15 °Brix. According to ^{Osterloh et al. (1996)}, these dissimilar behaviors were most likely influenced by varietal characteristics, plant age, and climatic and growing conditions to which the fruits were exposed. According to Rodríguez et al. (2006), the pineapple guava had high levels of starch at the time of harvest, which hydrolyzed during post-harvest ripening and caused the TSS to increase.

The TTA results in this study are similar to those observed by different authors. ^{Rodríguez et al. (2006)} observed that, in pineapple guava fruits, the TTA increases until the climacteric process and subsequently decreases, whereas other authors (^{Velho et al., 2011}) have reported that the TTA decreases during storage (4 to 23°C). Rodríguez et al. (2006) indicated that, in pineapple guava fruits (clones 8-4 and Quimba), acidity decreases during the ripening process because organic acids degrade during respiration. According to ^{Wills et al. (2007)}, organic acids generally degrade during the ripening stage because they are utilized in respiration or transformed into sugars; such changes are shown in Table 3 for fruits produced in Tenjo, which presented increased TSS concentrations during storage at 18°C.

The maturity index increased for the two storage conditions (Fig. 1) and was increased at higher temperature, which is consistent with reports for the majority of fruits (^{Parra-Coronado et al., 2006}; ^{Álvarez et al., 2009}). The MR was low and relatively constant when the product was stored at low temperatures and showed little dispersion because the starch hydrolysis was higher and more complete at low temperatures (^{Musacchi and Serra, 2018}). The highest values for the MR occurred with the highest storage time, lowest relative air humidity and highest temperatures.

The behavior of TSS, TTA, and MR in pineapple guava fruits during post-harvest depends on the TSS and TTA values of fruits at the time of harvest, which are influenced by weather conditions at the origin site during fruit growth. The TSS content during post-harvest was higher in the fruits produced in Tenjo. These results are consistent with ^{Kano (2015}), who indicated that the TSS content in watermelon fruits would be lower at higher temperatures. ^{Arah et al. (2015)} also observed that, in tomatoes, the TSS content was lower at higher temperatures and relative humidity, and also at lower light intensity. Possibly, growing temperatures that are too high result in a loss of photoassimilates as a result of elevated respiration rates of carbohydrates (^{Wills et al., 2007}; ^{Taiz et al., 2014}). However, when the higher temperature is still inside the optimum range of a fruit species, the warmer site can also promote the sugar translocation to the fruits, which was observed in Colombia in cape gooseberry (^{Fischer et al., 2007}) and banana passion fruits (^{Mayorga, 2016}) at the lower and warmer site. For accumulated solar radiation, ^{Martínez-Vega et al. (2008)} found similar results for pineapple guava belonging to clone 41 and indicated that the fruits with the lowest TSS values were those located within the inner-middle area of the canopy, which has a low incidence of luminous radiation. The authors supposed that the higher accumulated solar radiation at the high elevation site (Tenjo) favored photosynthetic performance and thus the TSS content (Taiz et al., 2014; Fischer et al., 2016). On the other hand, the cooler nights in Tenjo (located at a higher elevation than San Francisco) decreased the maintenance respiration of the fruits and their energy costs, increasing the positive carbon balance (^{Gariglio et al., 2007}) and so, contributing to the higher TSS. Also, the lower TSS content in the fruits from San Francisco could have been influenced by the higher precipitation rates at this location (Tab. 1), especially because ^{Osterloh et al. (1996)} observed that high precipitation diminishes TSS production, possibly because of cloudy weather conditions and temperature decreases.

The results found in this study are consistent with those of ^{Martínez-Vega et al. (2008)}, who observed that TTA increased slightly in pineapple guava fruits from less illuminated parts of the canopy.

Weight loss and color change

The weight loss (WL) in the pineapple guava fruits increased during both storage conditions (Fig. 2A), which is consistent with what ^{Rodríguez et al. (2006)} found for pineapple guava in clones 8-4 and Quimba and for champa fruits (^{Álvarez et al., 2009}). The WL was higher for the pineapple guava fruits stored at higher temperatures, a behavior also reported by ^{Parra-Coronado et al. (2006)} for pears. Rodríguez et al. (2006) found intermediate values compared with the results of this study for pineapple guava fruits of the 'Quimba' clone from the municipality of Vega (1,900 m a.s.l.) stored at ambient temperature (16.3°C, 65.1% RH); after 18 d in storage, the WL was 17.3%. ^{Valderrama et al. (2005)} indicated that pineapple guava fruits could be stored over long periods at low temperatures (1.7°C) with low WL and TSS.

The °h value of the green pineapple guava fruits did not show a clear trend during the first d of storage, but decreased over time for the two storage conditions (Fig. 2B). This undefined trend of °h of the pineapple guava fruit is consistent with reports by ^{East et al. (2009)}, who indicated that significant variations might not be observed in skin color among certain cultivars as the fruit ripens. In other pineapple guava cultivars, the °h decreased, representing a loss in green color (^{Velho et al., 2011}). The results found in this study show that the pineapple guava fruit does not change color because of the genetics of the fruits and only varies in shades of green. This result should not be used to establish the influence of weather conditions during the growth stage of fruits over this parameter at post-harvest.

To date, studies reporting differences between pineapple guavas indicating the influence of weather conditions on the behavior of fruit quality parameters during the post-harvest stage have not been carried out. To our knowledge, this is the first study carried out on the subject. Further research covering a wide range of pineapple guava varieties and environments is recommended. Also, the application of research facilities such as FACE (Free-Air Carbon dioxide Enrichment), installed through weather measuring equipment in a circle around the studied trees (Jones et al., 2014) could provide much more information on the direct influence of weather on the quality of fruits inside the FACE circle.