Introduction

South Korea is one of the major producers of Asian pears (Pyrus pyrifolia Nakai), with a total cultivated area of 9,607 ha and 202,700 metric tons (M/T) of production in 2023. The most commonly cultivated varieties are ‘Niitaka’, ‘Wonhwang’, ‘Shinhwa’, ‘Whasan’, ‘Chuhwang’, and ‘Whangeumbae’. Of these varieties, ‘Wonhwang’ accounts for approximately 4.4% of the total cultivation area, second only to ‘Niitaka’ (KREI 2023). In 2022, a total volume of 26,274 M/T of fresh Korean pears were exported, generating approximately $74,359,000 in revenue (KATI 2023a). The main export destinations for pears are the United States (48.4%), Taiwan (28.4%), and Vietnam (12.6%), with exports to these three countries accounting for approximately 89.4% of all exports (KATI 2023b).

‘Wonhwang’ is an early-season cultivar with relatively low storability, which leads to quality deterioration and physiological disorders occurring in the flesh and core during distribution periods after low-temperature export. These disorders are not easily observable based on the outside appearance, making it difficult to predict them accurately (Moon et al. 2008).

It has been reported that the occurrence of physiological disorders of ‘Wonhwang’ pears, including core browning, flesh browning, pithiness, water soaking, and flesh spot decay, increase rapidly when the harvest period is delayed (Lee and Chun 2011), with such disorders also strongly affected by the temperature during transportation and distribution (Oh et al. 2010).

Core browning is a symptom of browning in the core area and is reportedly caused by delayed harvesting and exposure to high concentrations of carbon dioxide in the storage environment (Lee and Chun 2011; Li et al. 2022). Fruits with core browning have been shown to have high levels of ethanol accumulation, which may be related to anaerobic respiration in the tissues (Hong et al. 2004).

Flesh browning mainly manifests as browning around vascular bundle cells, and it has been reported that the Maillard reaction and lipid peroxidation reactions induced by oxidative stress due to increased reducing sugar concentrations during the maturation period lead to the development of disorders, such as flesh browning and water soaking in pears (Fukuoka et al. 2022).

Pithiness is a physiological disorder in which a portion of the flesh becomes spongy and depleted of cellular content, resulting in a specific gravity below 1.0 and a low weight relative to the size. This disorder is reportedly associated closely with late harvest, low alcohol-insoluble solids, and a deficiency in the level of cell-wall-binding calcium (Cho et al. 2010), with symptoms tending to increase during cold storage and being more pronounced in mature fruits than in immature fruits (Shim et al. 2007).

Asian pears belong to the same pome fruit family as apples but exhibit different respiration patterns at maturity (Kitamura et al. 1981; Lee and Chun 2011). Early-season pears such as ‘Wonhwang’ are known to be a typical climacteric type of fruit with a low but gradual increase in ethylene production (Hong et al. 2004; Choi et al. 2015).

When pears are exported, they are packed according to the size of the fruit for convenience and quality maintenance during the export and distribution process and are categorized into six grades based on an overall weight of 5 kg, with either six, seven, eight, nine, ten, or eleven fruits in export-only paper cartons. Asian pears tend to increase in weight as the harvest time is delayed; therefore, larger fruits are available, but respiration and ethylene production increase significantly during storage at room temperature (Oh et al. 2010; Lee et al. 2011). However, few studies have been conducted on the occurrence of physiological disorders in Asian pears of different fruit sizes during the post-export shelf life.

Therefore, this study sought to compare the quality indices and occurrence of various physiological disorders in ‘Wonhwang’ Asian pears packaged at different sizes according to export standards under simulated cold-storage and room-temperature distributions.

Materials and Methods

Fruits, storage and shelf-life conditions

The pear fruit was selected by the APC Export Ship Department of the Cheonan Bae Horticultural Agricultural Cooperative, located in Seonghwan-eup, Seobuk-gu, Cheonan-si, and Chungcheongnam-do. The fruits were sorted on August 15, 2023, packed on August 17, 2023, and transported to Chungnam National University’s pomology laboratory. The fruits were packed in 5 kg export-only pear cartons with seven, eight, none, or ten fruits. In this study, we refer to these as Size 7, 8, 9, and 10, respectively. We used 18 boxes per treatment, for a total of 72 boxes in the experiment. All fruits were weighed and loaded onto 110 cm × 110 cm × 180 cm pallets (Nova Corporation, Gwangju, Korea) covered with zippered storage pallet covers, undergoing simulated transport for 30 days in cold storage at 1°C, after which the boxes were transferred to a constant-temperature room at 25°C for three weeks for simulated marketing.

Fruit quality analysis and assessment of physiological disorders

The fruit weights were measured using an electronic balance (CB-2000, A&D Systems, Seoul, Korea), and the length and diameter of the fruit were measured using Vernier calipers (IP-67, Mitutoyo, Tokyo, Japan). The weight loss during storage was expressed as a percentage by subtracting the post-storage weight from the pre-storage weight, with both measured using an electronic balance (CB-2000; A&D Systems, Seoul, Korea).

The firmness of the fruit was measured with a rheometer (TMS-Pro, Food Technology Corp., Philadelphia, USA) using an 8 mm flat-tipped probe after removal of the pericarp from the equatorial side of the fruit. The measurements were recorded as the maximum pressure, expressed in Newtons (N), until the probe penetrated the fruit to a depth of 5 mm at a speed of 100 mm·min^-1. The soluble solids content was determined with a refractometer (PR-32α, Atago, Tokyo, Japan) after cutting 1 cm thick slices from the same part of the equatorial side of the fruit and juicing the flesh using four layers of cheese cloth. The titratable acidity was measured by diluting 5 mL of the collected juice in 35 mL of distilled water, titrating to pH 8.3 with 0.1 NaOH, and calculated on the basis of malic acid.

Skin color was determined using a chroma meter (CR-410, Minolta, Tokyo, Japan) to examine the L* (lightness), a* (redness), and b* (yellowness) values, which were then converted to a hue angle using the formula h_ab = tan^-1 (b*/a*). The hue angle specifies the polar coordinates that define the color and is based on four basic psychological colors: red (0°), yellow (90°), green (180°), and blue (270°).

Ethylene production and the respiration rates of the fruits were measured by placing the fruits in a 3.4 L airtight container (HPL848, LocknLock, Seoul, Korea), two fruits in three replicates, and leaving them at 25°C for 3 h. The gas inside the airtight container was collected in 10 mL aliquots with a syringe and analyzed by gas chromatography (YOUNGIN Chromass, YL6500 GC System, Anyang-si, Korea). The column used was a Porapak N (60–80 mesh, ID; 2 mm, OD; 0.125 in, length; 12 ft, material; UltiMetal, Gwangmyeong-si, Korea). Ethylene was analyzed using a flame ionization detector (FID). O₂ was analyzed with a thermal conductivity detector (TCD), and the respiration rate was calculated. The oven temperature was set to 70°C, and the FID and TCD temperatures were set to 250°C and 150°C, respectively. The carrier gas was helium (He) which flowed at 20 mm·min^-1, and the combustion gas was hydrogen (H₂) at 50 mm·min^-1.

Physiological disorders were investigated by visual observations of browning, pithiness, and water-soaking of cut sections based on the equatorial plane of the fruit, and the incidence was expressed as a percentage according to the presence or absence of a disorder. The degree of disorder severity in each case was scored using a six-level index (0; 0%, 1; 1–19%, 2; 20–39%, 3; 30–59%, 4; 60–79%, 5; 80% or more) according to the area of occurrence, with these values then converted into a percent disorder index (PDI) by dividing the sum of the total occurrence index by the number of samples and the highest occurrence index and multiplying by 100 (Lee et al. 2018).

Extraction of the ethanol insoluble solids (EIS) was performed by placing 20 g of fresh tissue in 80 mL of 80% ethanol, crushing the tissue with a homogenizer, heating for 20 min, filtering through a Miracloth (Calbiochem, USA) to collect the residue, washing with 80% ethanol and acetone, and drying in a 35°C drying oven for 48 h before weighing it (Nunan et al. 1997).

For a calcium analysis, the equatorial surface of the fruit was cut into 1-cm-wide slices to match the width of the fruit core. The flesh in the core, the flesh close to the core, and the flesh close to the skin were cut into cubes, immersed in liquid nitrogen, and stored in a cold storage at ‒80°C. After 72 h of drying at ‒50°C in a freeze-dryer, 0.5 g aliquots of the crushed flesh were dry-painted in a painting furnace at 500°C and the painted samples were stored in a 0.01 N HCl solution (Choi 1994). Elements were analyzed using an atomic absorption spectrometer (AA-7000, Shimadzu, Japan). For the phenolics determination, an amount of 10 mg of each freeze dried sample was extracted with 2 mL of 80% (v/v) methanol in a 15 mL tube. The resulting powder was sonicated for 1 h at 35°C and then centrifuged at 12,000 ×g for 10 min. The supernatant was filtered with a 0.45 µm PTFE syringe filter and analyzed using a HPLC apparatus (Agilent Technologies, 1260 Infinity II, CA, USA). Four phenolic compounds, in this case arbutin, catechin hydrate (Sigma Chemical, St. Louis, MO, USA), 4-hydroxybenzoic acid, and chlorogenic acid, were identified and quantified at 280 nm using a C18 column (250 mm × 4.6 mm, 5 µm, Agilent Technologies, 1260 Infinity II, CA, USA).

Statistics

Statistical analyses were conducted using Python (version 3.10.12). The pandas (version 1.5.3) and Numpy (version 1.23.5) libraries were used for data processing and analysis. One-way and two-way ANOVAs were conducted using the ols and anova_lm functions of the StatsModels library (version 0.14.0). Tukey’s HSD test was performed using the MultiComparison and pairwise_tukeyhsd functions of the statistical models, and Duncan’s multiple comparison test was performed using SPSS (Ver. 26, SPSS Inc., IBM, Armonk, NY, USA). For all tests, the significance level was set to 5%.

Results and Discussion

Change in fruit quality

The changes in the quality indices of the pear fruit samples were compared at seven-day intervals during 30 days of simulated export transportation at 1°C followed by 21 days of shelf life at 25°C.

The average weights of the fruit by size were 747.0 g, 648.0 g, 572.7 g, and 514.8 g for Sizes 7, 8, 9, and 10, respectively, showing a clear weight difference by size (Table 1). The fruit length and diameter were greatest at Size 7 and decreased with an increase in the number of fruits per paper carton (Table 1). Across all sizes, the fruit shape ratio (L/D) was 0.82–0.83, indicating a somewhat oblate shape.

The rate of weight loss during the shelf life period after cold storage tended to increase with time. The overall rates were not significantly different between the different sizes at 2.03–2.63%, 2.98–3.73%, and 4.95–6.26% after seven, 14, and 21 days of shelf life, respectively (Table 2).

Firmness gradually decreased in all fruit sizes with an increase in the shelf life after cold storage. After seven days of shelf life, the firmness values of Sizes 7, 8, 9, and 10 were 31.55, 29.95, 31.39, and 27.32 N, respectively, and at 14 days, the corresponding firmness values decreased to 27.18, 31.02, 28.84, and 26.43 N. After 21 days of shelf life, firmness decreased rapidly, with nearly a loss of merchantability. The firmness value of the Size 9 samples was 12.07 N, which was relatively high compared to the outcomes of 9.69 N, 9.59 N and 8.31 N for Sizes 7, 8 and 10, respectively, but the difference was not statistically significant (Table 2). Therefore, it was estimated that the duration of merchantability at room temperature for ‘Wonhwang’ pears based on the firmness of the fruit was within 14 days.

The soluble solids content (SSC) tended to increase for all sizes as the shelf life progressed, increasing by approximately 1°Brix during the shelf life period compared to that before storage. In comparison of the titratable acidity levels (TA), the smallest fruit (Size 10) had the lowest TA content (0.20 %) when tested before storage, which decreased rapidly during the shelf life period to a significantly lower TA content compared to the other fruit sizes. In contrast, fruits of Sizes 7, 8, and 9 showed less of a difference in the TA content during the shelf life period compared to the Size 10 fruits. These results suggest that the internal quality indicators, SSC and TA, may show different values for different sizes but the differences are vague enough that this indicators are unlikely to be an assessment of size.

The examination of skin color differences revealed a slight decrease in lightness (L*) with longer shelf life periods for all sizes. Redness (a*) tended to increase gradually with the shelf life regardless of the fruit size, whereas yellowness (b*) remained largely unchanged until 21 days of shelf life. The hue angle (H°) decreased significantly with the shelf life regardless of the fruit size, from an average of 79° before storage to 73° after 21 days of shelf life (Table 3). These results indicate that the hue angle change can be used as an aging indicator for Asian pears with yellow skin because this factor decreases gradually during the senescence process with an extension of the shelf life (Kim et al. 2011; Lee et al. 2014; Oh et al. 2010).

When comparing ethylene production by fruit size during the shelf life period, none of the fruit sizes showed detectable ethylene (FID minimum level of GC detection, 2.0 pg C/sec) at seven days of shelf life, with ethylene production gradually increasing up to 21 days of shelf life (Fig. 1A). After 14 days of shelf life, smaller fruits tended to have higher ethylene production levels. Ethylene production in the smallest fruit (Size 10) was highest compared to the other sizes, with values of 0.23 and 0.35 µL·kg^-1·h^-1 at 14 and 21 days of shelf life, respectively.

The respiration rate of the fruit was found to increase gradually during the shelf life period within the range of 6–10 mg·kg^-1·h^-1 (Fig. 1B). Changes in the respiration rate during the shelf lifetimes were relatively small for the Size 8 and 9 fruits, indicating that the respiration rate remained stable during the shelf life after 30 days of cold storage. Similar to ethylene production, smaller fruit sizes were associated with higher respiration rates; however, the differences were not statistically significant. In particular, ethylene production for the largest fruit (Size 7) increases during the shelf life period, suggesting that larger fruits are often grown for more time and may have a higher degree of maturity. These fruits have tissues that age more rapidly during storage, leading to increased ethylene production. The smallest fruit (Size 10) had the highest average respiration rate during the shelf life period. It should be noted that small fruits have a higher surface area-to-volume ratio, which increases respiration through the lenticel and epidermis, leading to higher respiration and transpiration rates (Díaz-Pérez 1998; Ustun et al. 2023).

Fig. 1.

Changes in ethylene production (A) and the respiration rate (B) by size grade during 30 days of cold storage followed by 21 days of shelf life in ‘Wonhwang’ pears. The vertical bars represent the standard error of the mean (n=3). *Size 7, 8, 9, 10 refer to the fruit sizes that fit into a 5 kg export-only pear carton with seven, eight, nine and ten (avg. 514.8 g FW) fruit per container, respectively.

Overall, the smallest (Size 10) fruit samples showed higher ethylene production and respiration rates than the other samples after cold storage, resulting in higher consumption rates of respiratory substances, such as organic acids and sugars, and more rapid declines in quality factors (Table 2).

Occurrence of physiological disorders

The occurrence of physiological disorders in the fruits during storage at 25°C for 21 days was compared after a simulated exportation period of 30 days under 1°C cold storage for fruits in the Size 7, 8, 9, and 10 groups. These results showed that the main physiological disorders were core browning, flesh browning, water soaking, and pithiness.

The incidence of core browning was evident after 21 days of shelf life, and the severity was calculated as a PDI. Size 10 had the highest index at 14.4, followed by Size 7 at 11.9, Size 9 at 2.7, and Size 8 at 1.2 (Fig. 2A). Flesh browning disorder occurred after 14 d of shelf life, and the severity of flesh browning increased consistently with extended shelf lifetimes. The PDI indexes for Size 8 and Size 9 examined after 21 days of shelf life were 0.56 and 2.73, significantly lower than the values of 14.4 for Size 10 and 11.9 for Size 7 (Fig. 2B). Pithiness symptoms initially appeared at 21 days, the last shelf life day, and were most severe in the largest fruit (Size 7), whereas the Size 9 fruit samples showed the mildest outcome in this regard (Fig. 2C). Water soaking in the flesh occurred in a pattern similar to that of the flesh browning symptoms. This disorder increased sharply at the end of the shelf life, with the smallest Size 10 fruits having the highest severity value of 15.2, while Sizes 8 and 9 had significantly lower severity values of 1.9 and 0.9, respectively (Fig. 2D).

Fig. 2.

Comparison of the severity of internal physiological disorders by size grade during 30 days of cold storage followed by 21 days of shelf life in ‘Wonhwang’ pears. A: Core browning, B: Flesh browning, C: Pithiness, D: Water soaking. The vertical bars represent the standard error of the mean (n=4). Different letters between treatments indicate a significant difference according to Tukey’s multiple range test (p < 0.05). *Size 7, 8, 9, 10 refer to the fruit sizes that fit into a 5 kg export-only pear carton with seven, eight, nine and ten (avg. 514.8 g FW) fruit per container, respectively.

Based on these results, physiological disorder assessments during shelf life at 25°C showed that the incidences of core browning, flesh browning, and pithiness were relatively high in the largest Size 7 fruits, while the incidences of core browning, flesh browning, and water-soaking symptoms were relatively high in the smallest Size 10 fruits. Incidences of physiological disorders were relatively low in the Size 8 and 9 fruits compared to those of other sizes, indicating that fruits of these sizes are more likely to maintain their quality level during their distribution after export.

However, the differences in the quality levels and instances of physiological disorders during cold storage and the shelf life periods among the different export size standards appear to be related to the differences in the cell wall and calcium quantities, as these components form the skeleton of the fruit. First, a comparison of the ethanol-insoluble substances (EIS), including the cell wall components that support the fruit morphologically and resist vacuole turgor pressure, showed that the contents were similar for Sizes 8, 9, and 10, with almost identical values of 20.3, 20.0, and 20.1 mg·g^-1 FW, respectively. The lowest value of 17.5 mg·g^-1 FW was obtained for the largest plant size, Size 7 (Table 4), with this considered to be due to excessive water uptake at maturity.

In addition, the EIS contents, calculated as the total amount per individual fruit, were similar for Size 7 and Size 8 at 13.07 g/fruit and 13.12 g/fruit, respectively, while Size 9 showed a value of 11.43 g/fruit and the smallest, Size 10, was lowest at 10.32 g/fruit (Table 4). These results likely stem from the fact that smaller fruits had fewer cells that formed during the cell division process in the 30 days after flowering. Accordingly, less cell wall formation took place. As a result, the Size 7 fruits had similar total EIS content levels per fruit but significantly lower EIS contents per unit weight compared to the corresponding outcomes for Size 8. Therefore, it was considered desirable to set the export limit fruit weight for this variety at a smaller size as opposed to Size 7.

When examining the calcium contents of the fruit samples, the largest fruit group, Size 7, had an average flesh calcium content of 0.69 mg·L^-1, which was relatively low compared to those of the other sizes. In general, approximately 50% of the calcium is absorbed during cell division, and it continues to be absorbed during the fruit ripening stage. However, it has been reported that the calcium concentration in the fruit decreases as the fruits become larger and remain very low during rapid enlargement (Zheng et al. 2006). Therefore, it is thought that the largest fruit group in this study (Size 7) had a larger area of fresh tissue, which further diluted the distribution of calcium and resulted in a lower calcium concentration in this case (Table 4). The flesh closer to the core region tended to have a higher calcium content than the outer flesh closer to the pericarp regardless of the fruit size (data not shown). Therefore, the relatively low cell wall component levels and tissue calcium concentrations in the largest group, Size 7, indicate that these fruits are structurally vulnerable to physiological disorders occurring in the flesh, such as the pithiness disorder found in Asian pears (Moon et al. 2013). In contrast, small fruits of Size 10 are less competitive than larger fruits with regard to nutrient uptake during early development, resulting in a relatively low level cell wall formation (Tamura 2017) or a low calcium content, which increases watercore symptoms, such as the appearance of waterlogged fleshy tissue (Tanaka et al. 1992).

Conclusions

Among the export pear sizes examined in this study, ethylene production and respiration rates increased continuously with extending periods of cold storage and longer shelf lifetimes. Size 10 was associated with higher ethylene production and a higher respiration rate, most likely due to the higher material consumption level and more rapid aging rate. The firmness of the Size 9 fruit retained highest during shelf life of 21 days at room temperature. In addition, physiological disorders such as ‘core browning’, ‘flesh browning’, and ‘water soaking’ were more common in the Size 10 and Size 7 cases compared to the Size 8 and 9 cases during 21 days of shelf life after 30 days of cold storage. Also, Size 7 had the lowest EIS and calcium contents compared to the other sizes, suggesting that larger fruits have diluted calcium concentrations and are more susceptible to physiological disorders.

Considering both the change in the fruit quality and the incidences of physiological disorders, fruit that are too large or too small are not suitable for export. Thus, based on the results here, it is necessary to establish a fruit production system that does not exceed eight to nine fruits per 5 kg carton, which corresponds to an average of 570–650 g each, to extend the dignity maintenance period during local distribution after export.