ORIGINAL ARTICLES
Improved Accuracy in Determining Optimal Harvest Time for Pitaya (Hylocereus undatus) Using the Elasticity Index
2013 Volume 82 Issue 4 Pages 354-361
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2013 Volume 82 Issue 4 Pages 354-361
This study was conducted to determine whether the optimal harvest time of pitaya (Hylocereus undatus) could be identified more accurately using the elasticity index as determined by a nondestructive resonant-vibration method. Seven-year-old pitaya grown in beds filled with sand in an unheated greenhouse were used, and pollinated fruit in July (Jul.-PF) and September (Sep.-PF) were harvested at intervals of 4 days during days 20–36 or 20–40 days, respectively, after anthesis. The second resonant frequency, the elasticity index, and flesh firmness decreased with ripening; however, values for Jul.-PF decreased faster than those for Sep.-PF. During the final harvest time, both groups of pollinated fruits had similar values for second resonant frequency (330 Hz), the elasticity index (60×105), and flesh firmness (7 N·cm−2). The a-value of peel color for Jul.-PF and Sep.-PF increased rapidly during days 24–32 and 28–36, respectively, after anthesis and thereafter showed almost constant values. Total sugar content in Jul.-PF increased rapidly until 28 days after anthesis and then increased slightly. Total sugar content in Sep.-PF increased later than in Jul.-PF. Organic acid in Jul.-PF and Sep.-PF decreased rapidly until 28 and 36 days, respectively, after anthesis and thereafter decreased slowly. In Sep.-PF that were harvested at the optimal time, the second resonant frequency declined as fruit size increased, but the elasticity index was not affected by fruit size. High positive correlations were found among the second resonant frequency, the elasticity index, and flesh firmness in both groups of pollinated fruit. Considering a comprehensive suite of parameters including pulp rate, peel color, sugar, organic acid, flesh firmness, and occurrence of cracking, the optimal harvest time was estimated to be 36 and 40 days after anthesis for Jul.-PF and Sep.-PF, respectively, and the elasticity index of fruit at optimal harvest time was 62 × 105 and 73 × 105, respectively. This study showed that the optimal harvest time of pitaya could be determined more accurately by measuring the elasticity index of on-tree fruit.
Pitaya (Hylocereus undatus Britt & Rose) is a climbing cactus that is native to tropical forest regions in Mexico and Central and South America (Mizrahi et al., 1997); it has been cultivated in Vietnam, Nicaragua, Columbia, Israel, and other areas (Merten, 2003). Pitaya cultivation in Japan began in Okinawa Prefecture more than 20 years ago. In 2010, the total growing area was 48 ha, and the total production was 388 t; more than 90% of the production occurred in Okinawa Prefecture (2013 statistical data of Ministry of Agriculture, Forestry and Fisheries, Japan: the survey included the production movement of special fruit), where it can be grown in the open field because temperature conditions are suitable year-round.
Originally, pitaya fruits had low acidity and less aroma, but high sugar content and were moderate fleshy, and the taste was delicious if the fruit were harvested at the proper time. In addition, because several functional ingredients, such as plant fibers, are abundant in the fruit, consumption is expected to increase. However, the eating quality is poor when the fruit are harvested too early, and the flesh becomes poor and fruit cracking occurs when harvest is delayed. Hence, the fruit must be harvested at the optimal time to produce non-defective fruit.
Harvest time for pitaya has conventionally been determined using the change in peel color and days after anthesis. Fumuro et al. (2007) reported that 42–43 days after anthesis was suitable for harvesting pollinated fruits in July by considering the content of sugar and organic acid, and the occurrence of fruit cracking; however, Nomura et al. (2005) indicated that the a*-value of peel color and total soluble solids (TSS) content could not be used alone as a maturity index, but that the a*-value and cumulative temperature after pollination could be used to determine the optimal harvest time. Flower bud differentiation in pitaya requires long days and average temperatures above 20°C (Ogata et al., 2007). On the experimental farm used here, anthesis and fruit setting usually occur approximately six times from June until October, and fruit are harvested from August until November every year. The number of days from anthesis until harvest is usually 35–50, but fruit that mature in autumn require more days than fruit that mature in summer. Therefore, determination of the exact harvest times throughout the year could be difficult when using a single variable, such as the change in peel color or days after anthesis. In addition, domestic pitaya have generally been grown in the open in subtropical areas, and the introduction of greenhouse cultivation that allows heating and photoperiod control in winter is required for production to expand into temperate areas and for year-round fruit production. When this occurs, determination of the optimal harvest time may become even more important.
Since the flesh of pitaya becomes soft when it matures, nondestructive measurements of flesh firmness might allow us to determine the optimal harvest time. However, no previous study has assessed if flesh firmness can be used to determine the optimal harvest time. When fruit is subjected to vibrations, resonances occur at specific frequencies within the fruit, and the second resonant frequency has been shown to be closely related to flesh firmness (Taniwaki and Sakurai, 2010). Recent research showed that fruit resonance could be nondestructively measured using a laser Doppler vibrometer (LDV), and LDVs have been used to study the ripening of fruit (Muramatsu et al., 1997, 1999; Murayama et al., 2006; Terasaki et al., 2006). Because the second resonant frequency declines as fruit size increases, the elasticity index, which minimizes the effect of fruit weight, is used as an indicator of flesh firmness (Cooke, 1972; Terasaki et al., 2001). In addition, positive correlations between the elasticity index, as determined by nondestructive resonance vibration, and flesh firmness, as determined by destructive methods, were reported in research on a variety of fruits, such as pears (Murayama et al., 2006), apples (Motomura et al., 2004) and kiwifruit (Muramatsu et al., 1999; Terasaki et al., 2001). However, nondestructive evaluations of flesh firmness in pitaya by resonance vibration have not been reported.
This study was conducted to determine whether the optimal harvest time of pitaya (Hylocereus undatus) can be identified more accurately by measuring the elasticity index using fruits harvested in two seasons.
In 2011, forty-seven 7-year-old white pitaya (red-skinned with white flesh) grown in concrete beds (45 cm wide, 10 cm deep, 20 m long) filled with sand in an unheated greenhouse (137 m2) at the experimental farm of Kinki University (Wakayama Prefecture) were used for this investigation. Two beds separated by 2.2 m were used, and the lateral branches of trees planted at 60 cm spacing were trained with 1.2-m-high trellises. To prevent sunburn, the plants were covered with blue cheesecloth to provide 50% shading. The greenhouse was ventilated by a fan when the internal air temperature reached 30°C to maintain it below 35°C. Both the side windows and skylights were kept open from April to November. Irrigation was conducted once each day by automatic irrigation equipment, and liquid fertilizer (N : P2O5 : K; = 6 : 6 : 6) was applied in April and June at a rate of 1 g·m−2. Artificial pollination was performed between 23:00 and 23:30 from July 21 until July 23 (the mid-point of anthesis was July 22, and these fruits will be expressed as Jul.-PF) and from September 8 until September 10 (the mid-point of anthesis was September 9, and these fruits are expressed as Sep.-PF). All of the flowers that blossomed outside of these periods were resected with scissors. The average fruit number per plant was 1.3 in the former and 1.5 in the latter period.
Time course of changes in the second resonant frequency, the elasticity index, and fruit quality (Experiment 1)Jul.-PF were harvested at 20 (Aug. 11), 24 (Aug. 15), 28 (Aug. 19), 32 (Aug. 23), 36 (Aug. 27), and 40 (Aug. 31) days after anthesis (Jul. 22), and Sep.-PF were harvested at 20 (Sep. 29), 24 (Oct. 2), 28 (Oct. 6), 32 (Oct. 10), 36 (Oct. 14), 40 (Oct. 18), and 44 (Oct. 22) days after anthesis (Sep. 9). Six fruits per harvest were used in the study and the second resonant frequency, the elasticity index, and fruit quality were determined.
Effects of fruit size on the second resonant frequency, elasticity index, and fruit quality (Experiment 2)Sep.-PF were harvested at 41 days after anthesis and sorted by size into small (<350 g), medium (350–450 g), and large (> 450 g) categories, and the effects of fruit size on the second resonant frequency, the elasticity index, and fruit quality were investigated using eight fruits per category. The average weights of the fruit used in this experiment were small: 223 ± 25 g (± standard deviation), medium: 359 ± 34 g, and large: 548 ± 113 g.
Relationships among the second resonant frequency, the elasticity index, and flesh firmnessUsing data from the 127 fruits used in this study, correlations among the second resonant frequency, the elasticity index, and flesh firmness were investigated. The maximum, minimum, and average fruit weights were 506, 256, and 352 g, respectively.
Simple linear regression analysis was performed, with the second resonant frequency and the elasticity index included as explanatory variables (X) and flesh firmness included as the target variable (Y).
Measuring the second resonant frequency and the elasticity indexThe second resonant frequency was measured using portable vibration measurement equipment (Vp-2; Seibutsu-Sindo-Kenkyusyo, Hiroshima, Japan). The speaker and receiver were attached to the equator of the fruit by sandwiching them between fingers. Vibrations (0–3 kHz in swept sine wave signal, l0 s in measurement time, 1.35 Hz in resolution) generated by a PC were applied to the fruit from the speaker, vibrations from the fruit were detected by the receiver, and the corresponding voltage signals were transferred to the PC. The resonant frequency of the fruit was determined by fast Fourier transformation (FFT) of the acquired signal. The elasticity index (EI) was calculated using the following formula (Cooke, 1972; Terasaki et al., 2001):
In this paper, not only the elasticity index but also the second resonant frequency is shown in the figure, because the second resonant frequency will enable to faster estimation of the maturation rate of fruit almost equal in weight than the elasticity index.
Fruit quality and temperature measurementFruit weight, peel color, flesh firmness, and TSS, sugar, and organic acid content were determined. Peel color (L-, a-, and b-values) was measured using a color-difference meter (CR-400; Konica-Minolta, Tokyo, Japan) at the center of the equator of the fruit. To measure flesh firmness, a 3-cm-diameter patch of peel was horizontally removed using a sharp knife, and firmness was determined using a Magness-Taylor-type fruit penetrometer (FT011, FT327; Effegi, Florence, Italy) mounted on a plunger (11.3 mm diameter). The plunger penetrated the flesh and the maximum force was recorded when it penetrated 7 mm into the flesh through the cut surface. Measurements were performed on both sides of the fruit and the average value was calculated. Flesh was collected from the center of the equator on both sides of the fruit. Next, the juice from the fruit was squeezed and filtered through gauze, and TSS, sugar content, and titratable acidity were determined. TSS was determined using a refractometer (PAL-1; Atago, Tokyo, Japan) and titratable acid was determined by the titration method with 0.1 M NaOH to a phenolphthalein endpoint and converted to malic acid content. The juice was diluted 20-fold with distilled water and then filtered through a syringe filter (0.45 μm pore size); the sugar content and composition were determined using a sugar analyzer (SU300; DKK-TOA, Tokyo, Japan) via pulsed amperometry. The anion exchange column method (Zook and LaCourse, 1995) was used to determine the contents of sucrose, glucose, and fructose. Pulp rate was calculated by dividing the pulp weight by the fruit weight. The air temperature in the greenhouse was recorded every hour from July 22 until October 23 using a temperature data logger (Ondotori-Jr.; T&D, Nagano, Japan); the temperature sensor was placed under the trellis and was not exposed to direct sunlight. The average values of average, maximum and minimum temperature for four days from anthesis until 40 or 44 days after anthesis were calculated. The cumulative temperature was calculated during the fruit-growing season by summing the daily average temperatures.
Statistical analysisData obtained in this study were subjected to analysis of variance, Tukey-Kramer’s multiple-range test and Student’s t-test.
Time-course changes in temperature during the experiment are shown in Figure 1. The average temperature from the mid-point of anthesis until the first and last days of harvest were 28.1 ± 1.7°C and 28.4 ± 1.6°C in Jul.-PF, and 25.8 ± 3.0°C and 23.1 ± 3.6°C in Sep.-PF, respectively; temperatures in Sep.-PF were 2.3 and 5.3°C lower than in Jul.-PF, respectively.
Time-course changes of air temperature for four days from anthesis until harvest time in the greenhouse. Jul. 22 and Sep. 9 are the mid-points of the pollination periods, respectively; the former was pollinated from Jul. 21 to Jul. 23, and the latter was pollinated from Sep. 8 to Sep. 10. The thick arrows indicate the harvest times of pitaya.
Second resonant frequencies and elasticity index values are shown in Figure 2. No differences were found between the two groups of pollinated fruit in the second resonant frequency at 20 days after anthesis (values were ～630 Hz), and the values decreased to ～330 Hz in the fully ripened stage (Fig. 2A). The second resonant frequency of Jul.-PF declined rapidly until 32 days after anthesis, but the subsequent decline was slight. A rapid initial decline was also observed in Sep.-PF until 36 days after anthesis, but the subsequent decline was slow. The second resonant frequency of Jul.-PF declined faster than that of Sep.-PF, and was significantly lower at 24, 28, and 32 days after anthesis. The elasticity index values of the two groups of pollinated fruit showed similar time-course changes to the second resonant frequency, having values of ～160×105 at 20 days after anthesis and declining to ～60×105 at the fully ripened stage (Fig. 2B). The elasticity index of Jul.-PF declined rapidly until 32 days after anthesis and subsequent changes were small. Similarly, in Sep.-PF, a rapid decline occurred until 36 days after anthesis, and the subsequent decline was slow. The elasticity index of Jul.-PF declined faster than that of Sep.-PF, and was significantly lower at 24, 28, 32, and 40 days after anthesis.
Effects of pollination time on time-course changes in the second resonant frequency (A) and the elasticity index (B) in pitaya. Jul. 22 and Sep. 9 are the mid-points of the pollination periods, respectively; the former was pollinated from Jul. 21 to Jul. 23, and the latter was pollinated from Sep. 8 to Sep. 10. Vertical bars represent ± SD. NS, *, **, and *** indicate nonsignificant and significant differences at P = 0.05, P = 0.01, and P = 0.001, respectively, from t-tests.
Fruit qualities are shown in Figure 3. Weight increased gradually in both groups of pollinated fruit after anthesis, reaching 420–430 g by the final harvest time (Fig. 3A). The pulp rate was ～30% at 20 days after anthesis, and no difference was found between the two pollinated fruit groups (Fig. 3B). Thereafter, the pulp rate increased until 36 and 40 days after anthesis, reaching levels of 80% and 76% in Jul.-PF and Sep.-PF, respectively. The a-value of peel color in Jul.-PF was ～−12; it increased rapidly during 24–32 days after anthesis and then was almost constant at 33–34 (Fig. 3C). In Sep.-PF, the a-value was ～−12, similar to that of Jul.-PF; it increased rapidly during 28–36 days after anthesis and then was almost constant at 35–36. The a-value of Jul.-PF was significantly higher than that of Sep.-PF at 28 and 32 days after anthesis. The b-value of Jul.-PF was 21–22 during days 20–24 after anthesis; it decreased until 32 days after anthesis and then was almost constant at ～4 (Fig. 3D). The b-value of Sep.-PF was ～17 at 20 days after anthesis; it increased at 24 days after anthesis, decreased until 36 days after anthesis and thereafter was almost constant at 4, similar to Jul.-PF. The b-value of Jul.-PF was significantly lower than that of Sep.-PF at 28 and 32 days after anthesis. The L-value in both groups of pollinated fruit was ～50 at 20 days after anthesis, with no difference between the groups; it increased at 24 days after anthesis and then decreased, having a value ～40 at the final harvest time (Fig. 3E). The L-value of Jul.-PF was significantly lower than that of Sep.-PF at 28 and 32 days after anthesis. Flesh firmness declined as fruit ripening progressed, following a pattern similar to the second resonant frequency and the elasticity index, having values of ～7 N·cm−2 in the fully ripened stage in both groups of pollinated fruits (Fig. 3F). Flesh firmness in Jul.-PF was significantly lower than in Sep.-PF at 20–32 days after anthesis. The TSS of Jul.-PF increased rapidly until 28 days after anthesis and then increased slightly until 36 days after anthesis, reaching a value of ～15.3% by the final harvest time (Fig. 3G). The TSS of Sep.-PF increased rapidly until 36 days after anthesis and then remained fairly constant at ～13.8% until the final harvest time. TSS values in Jul.-PF were significantly higher than those of Sep.-PF at any sampling time. Glucose contents of Jul.-PF and Sep.-PF increased rapidly until 36 and 40 days after anthesis, respectively, reaching values ～8% by the final harvest time (Fig. 3H). Glucose content in Jul.-PF was significantly higher at 20, 24, and 28 days after anthesis and was lower at 40 days after anthesis than in Sep.-PF. Fructose content in Jul.-PF increased rapidly until 28 days after anthesis and then increased slightly (Fig. 3I). Fructose content in Sep.-PF increased during 28–36 days after anthesis and then remained constant. The fructose contents of Jul.-PF and Sep.-PF were ～5.9% and 4.6% at the final harvest time, respectively. Fructose content was significantly higher in Jul.-PF than in Sep-PF during every sampling period. Total sugar content in Jul.-PF increased until 28 days after anthesis and then increased slightly, and the content in Sep.-PF increased rapidly until 36 days after anthesis and then remained fairly constant (Fig. 3J). Total sugar contents were ～13.6% and 13.0% in Jul.-PF and Sep.-PF, respectively, during the final harvest time. Regarding the organic acid content, no difference was observed between the two groups of pollinated fruits at 20 days after anthesis (～2.1% in both groups) (Fig. 3K). However, in Jul.-PF, the organic acid content decreased rapidly until 28 days after anthesis and then continued to decrease slowly. In Sep.-PF, the organic acid content decreased rapidly until 36 days after anthesis and then it decreased gradually. The organic acid content in Jul.-PF was significantly lower than that of Sep.-PF at 28 and 32 days after anthesis. Fruit cracking occurred at 40 and 44 days after anthesis in Jul.-PF and Sep.-PF, respectively (data not shown).
Effects of pollination time on time-course changes in fruit weight and fruit quality in pitaya. Jul. 22 and Sep. 9 are the mid-points of the pollination periods, respectively; the former was pollinated from Jul. 21 to Jul. 23, and the latter was pollinated from Sep. 8 to Sep. 10. Vertical bars represent ± SD. NS, *, **, and *** indicate nonsignificant and significant differences at P = 0.05, P = 0.01, and P = 0.001, respectively, from t-tests.
The second resonant frequency declined as fruit size increased, but the elasticity index was not affected by fruit size, having almost constant values of ～74 × 105 (Table 1). No significant differences were observed among fruit size categories in fruit color, flesh firmness, organic acid, and pulp rate, but TSS tended to be low in small fruit.
| Fruit size | Fruit weight (g) | Second resonant frequency (Hz) | Elasticity index (×105) | Peel color | Peel thickness (mm) | Percentage of pulp (%) | Flesh firmness (N·cm−2) | TSS (%) | Organic acid (%) | ||
|---|---|---|---|---|---|---|---|---|---|---|---|
| L | a | b | |||||||||
| Smallz | 223cy | 449a | 74.1a | 40.6a | 35.2a | 4.5a | 2.0b | 74.4a | 7.4a | 12.3b | 0.15a |
| Medium | 359b | 379b | 73.3a | 39.2a | 36.6a | 3.5a | 2.2b | 75.7a | 8.0a | 13.6a | 0.17a |
| Large | 548a | 330c | 74.1a | 38.8a | 36.1a | 3.3a | 2.7a | 74.5a | 7.9a | 13.4ab | 0.17a |
High positive correlations were found among the second resonant frequency, the elasticity index, and flesh firmness in both groups of pollinated fruits (Fig. 4). The correlation coefficients between the second resonant frequency or the elasticity index and flesh firmness were 0.922 or 0.905 for Jul.-PF and 0.863 or 0.912 for Sep.-PF, respectively.
Relationships among the second resonant frequency, the elasticity index, and flesh firmness in pitaya. A: pollinated fruit on Jul. 22 (n = 58), B: pollinated fruit on Sep. 9 (n = 69). ***, significant at P = 0.001.
Ohata and Sakurai (2011) and Takahashi et al. (2010) measured the elasticity index of grapes on the vine and plums on the tree, respectively, and reported that the values decreased with fruit maturation, following similar time courses of change to those found in pitaya (Fig. 2). Comparing the two groups of pollinated pitaya fruits, the elasticity index of Jul.-PF declined faster than that of Sep.-PF, which suggests that maturation progressed faster in Jul.-PF than in Sep.-PF because of the higher average temperature from anthesis to harvest time (Fig. 1).
No difference was observed between the two groups in fruit weight at final harvest time, but the pulp rate in Jul.-PF was 4% higher than that of Sep.-PF, suggesting that the peel in Jul.-PF was thinner than that of Sep.-PF (Fig. 3A, B). The L-, a-, and b-values of peel color indicate brightness and the color directions of red and yellow, respectively. The a-values of Jul.-PF and Sep.-PF increased rapidly during 24–32 and 28–36 days, respectively, after anthesis and then stabilized (Fig. 3C). The time course of change in Jul.-PF was highly consistent with the results of Nomura et al. (2005).
Flesh firmness declined faster in Jul.-PF than in Sep.-PF, suggesting that maturation progressed faster in Jul.-PF than in Sep.-PF, following a pattern similar to the elasticity index (Fig. 3F).
Because the concentration of sugar differs considerably among tissues in pitaya fruit (Inoue et al., 2001; Kataoka et al., 2006; Nomura et al., 2005), the pulp was minced and stirred thoroughly and then squeezed to separate the juice from the pulp before we measured the sugar content. As a result, both groups of pollinated fruits contained ～8% glucose and ～5% fructose, and we did not detect any sucrose (Fig. 3H, I). These values were highly consistent with the results of Inoue et al. (2001). Regarding the time course of changes in sugar content in Jul.-PF, the value increased rapidly during days 20–28 after anthesis following a pattern that was similar to the time courses reported by Kataoka et al. (2006) and Nomura et al. (2005).
Around 90% of organic acid in pitaya fruit consists of malic acid and the rest is citric acid with minute amounts of ascorbic acid (Inoue et al., 2001; Nomura et al., 2005). The organic acid content in both groups of pollinated fruit at 20 days after anthesis was relatively high at ～2.1%, but had declined to ～0.2% by the full-ripe stage (Fig. 3K).
The TSS/acid ratio is an important parameter that affects the taste of fruit (Kubo, 2002), and values of 10–20 are generally considered suitable in satsuma mandarin. Both groups of pollinated fruit had TSS values of 14–15% at the full-ripe stage, and the TSS/acid ratio values were ～70. When the TSS/acid ratio is too high, pitaya fruit can have a bland taste regardless of the high sugar content. In addition, this is thought to be one of the reasons why the sweetness of glucose, which is present in large amounts in pitaya fruit, is lower (0.6–0.7) when that of sucrose is assumed to be 1.0 (Murakami et al., 2011).
The optimal harvest time should be determined by considering the pulp rate (thickness of peel), peel color, sugar, organic acid, flesh firmness, and the occurrence of cracking. Because a water core appears in the fruit and eating quality becomes poor when fruit are overripe, harvest must be completed before fruit cracking occurs. Considering the comprehensive set of parameters, optimal harvest times were estimated to be 36 and 40 days after anthesis for Jul.-PF and Sep.-PF, respectively. In this study, fruit parameters at optimal harvest time were 80% pulp rate, 15.7% TSS, 0.22% organic acid, and 34 for the a-value for Jul.-PF, and 76%, 13.8%, 0.20%, and 36, respectively, for Sep.-PF (Fig. 3). The number of days required before harvest was 6–7 days less in this study than reported by Fumuro et al. (2007). One explanation for this difference is that the anthesis date reported by Fumuro et al. (2007) was ～20 days earlier than the date reported in this study; as a result, the average temperature from anthesis to harvest time was 0.9°C lower than that in Jul.-PF in this study. Because the peel color of white pitaya changes markedly from green to strong red with fruit maturation, the change in peel color provides an indication of harvest time. However, the number of days from the change in peel color until the optimal harvest time could change depending on the time of fruit maturation. Therefore, it is difficult to use the number of days after anthesis or the change in peel color alone to determine the optimal harvest time.
Accumulated temperature is used as an indicator for determining the optimal harvest time by subtracting the growth limit temperature from the average temperature above the growth limit temperature during the period from anthesis until harvest (Yonemori, 2002). In this study, when the growth limit temperature was regarded as 10°C and accumulated temperature was calculated (degree-days) for both groups of pollinated fruits, the values were 677.4°C-days and 545.2°C-days for Jul.-PF and Sep.-PF, respectively (Fig. 1). The accumulated temperature for Jul.-PF was 132.2°C-days higher than that of Sep.-PF, which suggests that the temperature in the greenhouse in summer was excessive for the growth and maturation of pitaya fruit. These results show that it is difficult to determine the optimal harvest time from accumulated temperature alone because the accumulated temperature differs considerably between fruit grown under hot and mild temperatures in pitaya, which has several anthesis-fruit maturation cycles in 1 year. In particular, it appears that the maturation rate of pitaya fruit cultivated in the greenhouse was affected by high temperatures. Furthermore, it is more difficult to determine the optimal harvest time because the daily temperature range and the maximum temperature in the greenhouse tend to be higher than with open cultivation.
Because the pulp becomes soft with fruit maturation, flesh firmness is an important parameter for determining the optimal harvest time. Recently, a small, practical device that can measure the second resonant frequency of fruit in the field was developed (Sakurai, 2012), and the use of resonance vibration methods for the determination of optimal harvest time has been studied in Japanese apricot (Oe et al., 2013) and plum (Ohata and Sakurai, 2011). In experiment 1, the elasticity index of fruit at the optimal harvest time, as determined by multiple fruit parameters, was 61.7 × 105 and 72.6 × 105 in Jul.-PF and Sep.-PF, respectively (Fig. 2), and in experiment 2, the elasticity index was 73–74 × 105 regardless of fruit size in Sep.-PF (Table 1). Together, these results suggest that the elasticity index at optimal harvest time was ～73 × 105 in Sep.-PF and was slightly lower in Jul.-PF. The optimal harvest time for Jul.-PF was August 27, which was 36 days after anthesis, and the number of days above 30°C and 35°C, the maximum temperature, was 33 and 9 days, respectively, whereas 13 and 0 days, respectively, were recorded for Sep-PF. Therefore, changes in the physicochemical properties of the cells may have occurred under hot temperatures and thus flesh firmness decreased in Jul.-PF.
Positive correlations between the elasticity index, as determined by nondestructive resonance vibrations, and flesh firmness, determined by destructive methods, have been reported in fruits such as pears (Murayama et al., 2006), apples (Motomura et al., 2004), and kiwifruit (Muramatsu et al., 1999; Terasaki et al., 2001). In this study, a positive correlation between the elasticity index and flesh firmness, as measured by a Magness-Taylor-type fruit penetrometer, was found in pitaya fruit (Fig. 4), indicating that flesh firmness can be estimated from the elasticity index. However, the slope of the regression line for Jul.-PF was lower than that of Sep.-PF, and flesh firmness in Jul.-PF was lower than that of Sep-PF. If we assume that flesh firmness at the optimal harvest time is 8 N·cm−2 and use the regression formula in Figure 4, the elasticity index at the optimal harvest time is 61.3 × 105 and 71.8 × 105 for Jul.-PF and Sep.-PF, respectively. These values are consistent with the elasticity index values of fruits during the optimal harvest times determined by considering a variety of fruit parameters. However, because the elasticity index values of Jul.-PF and Sep.-PF were reduced by about 25 and 10%, respectively, if flesh firmness at the optimal harvest time is lower, reduced from 8 N·cm−2 to 7 N·cm−2, it is inappropriate to determine the optimal harvest time using flesh firmness alone. From these results, adopting the elasticity index of fruit at the optimal harvest time is appropriate, as determined by assessing parameters such as peel color, sugar, organic acid, flesh firmness, and the occurrence of cracking. The elasticity values at the optimal harvest were 62 × 105 for Jul.-PF and 73 × 105 for the Sep.-PF. However, because the period of harvest time in pitaya is considerably longer than in other fruit trees because it blooms throughout the year, fruit that are harvested from early August to mid-September should use the elasticity index value from Jul.-PF and fruit that are harvested from late September to November should use the Sep.-PF value. However, these criteria for optimal harvest time need further study.
Therefore, we conclude that the optimal harvest time can be determined more accurately by measuring the elasticity index of on-tree fruit in pitaya.
