Introduction

In Korea, apples (Malus domestica Borkh.) rank among the most popular fruits, with consumers seeking them out for their daily consumption and as part of a healthy diet (KREI, 2021; USDA, 2021). Consequently, better quality apples are in high demand and have excellent marketability. The commercial quality standard of apples is generally determined based on internal and external fruit attributes (Djekic et al., 2019; Grabska et al., 2023). Fruit texture, sugar levels, acidity, and nutrient contents are considered internal attributes, whereas the fruit shape, size, color, and lack of defects are considered external attributes (Jaeger et al., 2018; Jaeger et al., 2023). A higher quality of apples creates better economic value. Therefore, the occurrence of defects, decay, and quality reduction after harvest and during storage can cause downgrading and postharvest losses.

‘Colorpple’ (‘Yoko’ × ‘Senshu’) and ‘Manhong’ (‘Hongso’ × ‘Gamhong’) are new apple cultivars which have been recently registered in Korea for commercial production, in 2019 and 2022, respectively. Generally, while apples can be stored for a relatively long time, they are climacteric fruits, meaning that the plant hormone ethylene regulates their ripening and postharvest preservation (Barry and Giovannoni, 2007). Thus, controlling ethylene production is an appealing strategy for delaying ripening and preserving the quality of apples after harvest. Research on postharvest preservation technologies for apples has included analyses of changes in fruit physiological characteristics in association with fruit ripening during storage, though as new cultivars, the postharvest ripening and handling mechanisms of ‘Colorpple’ and ‘Manhong’ apples are not yet well known.

The application of ethylene inhibitors is an effective way to control ethylene production during ripening and thus to extend the storage life of apples. Among ethylene inhibitors, 1-methylcyclopropene (1-MCP) has been used extensively as an ethylene action and perception inhibitor to extend the storage life of many horticultural crops, and the impacts of 1-MCP on the quality parameters of crops during storage has been reported (Dias et al., 2021). Specifically, in fruits, 1-MCP maintained fruit texture (Win et al., 2021; Kwon et al., 2022), slowed sugar accumulation and organic acid reduction (Hu et al., 2017; Cai et al., 2023; Wu et al., 2023), induced disease resistance and reduced fruit decay (Li et al., 2017; Zhang et al., 2021), inhibited volatile and aroma compound emissions (Shu et al., 2020; Lv et al., 2021), maintained fruit color and slowed chlorophyll degradation (Lv et al., 2020; Ha et al., 2023), and delayed core browning caused by reactive oxygen species (Dong et al., 2015). However, controlling fruit ripening and quality using 1-MCP involves accounting for many factors, including the cultivar, species, maturity, concentration, and treatment exposure duration (Zhang et al., 2020; Dias et al., 2021), as well as the storage temperature, conditions, and storage duration (Lee et al., 2017; Jia et al., 2018; Wang et al., 2020).

Therefore, this study analyzed the postharvest physiological characteristics and storage quality of ‘Colorpple’ and ‘Manhong’ apples at warm and cold storage temperatures. Other research objectives included extending the storage life of apples with a 1-MCP treatment and evaluating the physiological responses of the apple cultivars to 1-MCP during storage at warm and cold temperatures.

Materials and Methods

Fruit Samples

‘Colorpple’ and ‘Manhong’ apples were harvested from an experimental field at the Apple Research Institute, Sobo-myeon, Gunwi-gun, Republic of Korea. The fruit samples for the ‘Colorpple’ apples were collected from five-year-old apple trees (M.9 rootstock) and the fruit samples for the ‘Manhong’ apples were collected from six-year-old apple tress (M.9 rootstock). Apples with an approximate score of 7.0 on the starch pattern index (SPI) according to Cornell’s SPI score chart method (Blanpied and Silsby, 1992) were harvested on October 30, 2022 and November 1, 2022. The morphological characteristics of the ‘Colorpple’ and ‘Manhong’ apples were assessed (Fig. 1). Subsequently, uniform fruits lacking defects were selected and divided into two treatment groups for the two storage temperature experiments. In total, 153 fruits were selected for the two experiments. Each experiment consisted of 72 fruits (36 fruits per treatment group), and the remaining nine fruits were used for the analysis at harvest.

Fig. 1.

Morphological characteristics of ‘Colorpple’ (A) and ‘Manhong’ (B) apples at harvest.

Treatments and Storage Condition

For the treatment groups, fruits were treated with 1 µL·L^-1 1-methylcyclopropene gas (SmartFresh^TM, AgroFresh, Yakima, WA, USA) in an enclosed container at 20°C for 24 h. The temperature of the control group was maintained at 20°C. In the first experiment, the fruits were stored at a warm temperature (20 ± 1°C) at 70 ± 3% relative humidity (RH) for 40 days to evaluate the shelf life of both cultivars. In the second experiment, fruits were stored at a cold temperature (0 ± 1°C) at 90 ± 3% RH for eight months to evaluate the storability of apple cultivars during long-term cold storage for year-round consumption. Changes in fruit physiological attributes were measured at ten-day intervals at the warm temperature and at two-month intervals plus one day at a shelf life temperature of 20°C for the cold temperature condition. Due to the limited access of fruit samples from the experimental trees, a large number of fruit samplings could not be used in this study. However, in both experiments, nine fruits (three fruits per replicate) per treatment were used for all analyses at each storage interval.

Fruit Quality Evaluation

External quality factors, such as the fruit weight loss, skin color, skin greasiness, and fruit decay, were assessed. All individual fruits were weighed before and after storage, and the difference was considered the fruit weight loss during storage. The fruit skin color was measured in three equatorial regions of the fruit skin using a chroma meter (CR-400, Konica Minolta, Tokyo, Japan), with the color values expressed as L* (lightness), a* (green-red), and b* (blue-yellow). Fruit skin greasiness was assessed according to Yang et al. (2017a) using a point scoring system (0–3), where 0 indicated no greasiness, 1 indicated slight greasiness, 2 indicated moderate greasiness, and 3 indicated severe greasiness. Fruit decay was assessed as previously described by Liu et al. (2024). Fruits with necrotic spots over 1 cm in diameter were designated as rotten fruits. The fruit decay rate was calculated as follows: fruit decay (%) = (number of rotten fruits/ total number of fruits) × 100.

Internal quality factors, in this case the flesh firmness, soluble solid content (SSC), and titratable acidity (TA) were assessed. Fruit skins were peeled at three places on the equatorial region and flesh firmness was measured by inserting the 11-mm probe of a flesh firmness tester (FT-327, TR Co., Forlì, Italy) into each apple thus tested (Win et al., 2022). Next, juice samples were extracted from each fruit and were subjected to SSC and TA analyses. The SSC was analyzed using a refractometer (PR-201, Atago, Tokyo, Japan). TA was analyzed using the malic acid reduction method by titrating apple juice samples with NaOH to pH 8.1 (Win et al., 2022).

Ethylene Production and Respiration Rate Determination

The fruits were individually isolated in individual containers (1.6 L) for 1 h, and two headspace gas samples (1 mL each) were collected using syringes. To measure the amount of ethylene produced, each gas sample (1 mL) was inserted into a gas chromatograph (G1540A, Hewlett-Packard Co., Wilmington, NC, USA) equipped with a flame ionization detector and a Porapak Q column (80/100 2-m, RASTEK, Bellefonte, PA, USA). For the respiration rate measurements, each gas sample (1 mL) was inserted into a gas chromatograph (6890N, Agilent Technology Inc., Santa Clara, CA, USA) equipped with a thermal conductivity detector and a Porapak Q column. During both the ethylene production and respiration rate measurements, the temperatures were set to 100°C and 250°C for the injector and detector, respectively, with an oven temperature of 70°C. Helium with a flow rate of 20 mL·min^-1 was used as a carrier gas. The amounts of ethylene produced and the respiration rates were calibrated with internal standards (Yoo et al., 2021).

Statistical Analysis

A completely randomized design was used for this study. Each treatment included three replicates and each replicate consisted of three apple fruits. All analyses were performed with SPSS (Version 26, IBM SPSS, Armonk, NY, USA). All data were subjected to an analysis of variance (ANOVA), and the least significant difference (LSD) test was used to determine the mean difference at the p < 0.05 significance level.

Results

Experiment-1: Physiological Characteristics of ‘Colorpple’ and ‘Manhong’ Apples under a Warm Temperature Condition

For the untreated fruits, the maximum ethylene production level and maximum respiration rate were observed at 40 days (7.51 µL·kg^-1·h^-1) and 10 days (13.65 mL·kg^-1·h^-1) in the ‘Colorpple’ (Fig. 2A and 2C) and at 30 days (0.79 µL·kg^-1·h^-1) and 40 days (10.35 mL·kg^-1·h^-1) in the ‘Manhong’ apples (Fig. 2B and 2D). However, compared to the untreated fruits, significant ethylene production levels were observed after 30 and 40 days of storage in the treated ‘Colorpple’ (Fig. 2A) apples and after 40 days of storage in the treated ‘Manhong’ apples (Fig. 2B). Except for the ‘Manhong’ apples after 20 and 30 days of storage, the respiration rates were significant in the untreated fruits compared to the treated fruits throughout the storage duration in both cultivars (Fig. 2C and 2D). 1-MCP did not reduce fruit weight loss in either cultivar at any time during the storage periods (Fig. 2E and 2F).

Fig. 2.

Ethylene production (A and B), respiration rate (C and D), and weight loss (E and F) of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a warm temperature (20 ± 1°C) for 40 days. Values are presented as the mean ± standard error (n = 9).

Except for the ‘Colorpple’ apples after 30 days of storage, 1-MCP did not slow the reduction of flesh firmness during storage in both cultivars (Fig. 3A and 3B). TA was not significantly different until 30 days of storage, but a higher TA was observed after 40 days in the 1-MCP-treated fruits when compared to the untreated fruits of both cultivars (Fig. 3C and 3D). 1-MCP did not affect SSC at any time during the storage period in both cultivars (Fig. 2E and 2F). In addition, the SSC/TA ratio was not found to be significant throughout the storage period in either apple cultivar (Fig. 2G and 2H).

Fig. 3.

Firmness (A and B), titratable acidity (TA) (C and D), soluble solid content (SSC) (E and F), and SSC/TA ratio (G and H) of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a warm temperature (20 ± 1°C) for 40 days. Values are presented as the mean ± standard error (n = 9).

In ‘Colorpple’, a higher L* value was observed in untreated fruits than in treated fruits at 20 and 30 days of storage (Fig. 4A). The a* value of the 1-MCP-treated fruits was higher than that of the untreated fruits after 30 and 40 days of storage (Fig. 4C). The b* values were not significantly different during the storage periods (Fig. 4E). In the ‘Manhong’ case, 1-MCP did not affect the skin color values at any time during the storage period (Fig. 4B, 4D and 4F).

Fig. 4.

Fruit skin color L* (A and B), a* (C and D), and b* (E and F) values of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a warm temperature (20 ± 1°C) for 40 days. Values are presented as the mean ± standard error (n = 9).

Fruit skin greasiness increased with the storage time in both cultivars (Fig. 5A and 5B). In ‘Colorpple’, a higher (slight to intermediate) greasiness level was observed in untreated fruits at 30 and 40 days compared to 1-MCP-treated fruits (Fig. 5A). In ‘Manhong’, a higher greasiness level was observed in untreated fruits only at 20 days of storage compared to treated fruits (Fig. 5B). A low percentage of fruit decay, without significance, was observed in both untreated and treated ‘Colorpple’ fruits at 30 days and at 30 and 40 days in the ‘Manhong’ apples (Fig. 5C and 5D).

Fig. 5.

Fruit skin greasiness (A and B) and fruit decay rate (C and D) of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a warm temperature (20 ± 1°C) for 40 days. Values are presented as the mean ± standard error (n = 9). Skin greasiness was scored as 0 = no greasiness, 1 = slight greasiness, 2 = moderate greasiness, and 3 = severe greasiness. Regarding the fruit decay rate, fruits with necrotic spots over 1 cm in diameter were designated as rotten fruits.

Experiment-2: Physiological Characteristics of ‘Colorpple’ and ‘Manhong’ Apples under a Cold Temperature Condition

In the untreated fruits, the maximum ethylene production level and highest respiration rate were observed at six months (27.78 µL·kg^-1·h^-1) and at eight months (23.70 mL·kg^-1·h^-1), respectively, in the ‘Colorpple’ (Fig. 6A and 6C) apples and at eight months (40.87 µL·kg^-1·h^-1 and 21.18 mL·kg^-1·h^-1) in the ‘Manhong’ apples (Fig. 6B and 6D). Except for ethylene production in the ‘Manhong’ apples at two months, higher ethylene production levels and respiration rates were observed in the untreated fruits compared to the 1-MCP-treated fruits during storage in both cultivars (Fig. 6A-6D). Neither cultivar demonstrated inhibited weight loss as a result of the 1-MCP treatment during the storage durations (Fig. 6E and 6F).

Fig. 6.

Ethylene production (A and B), respiration rate (C and D), and weight loss (E and F) of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a cold temperature (0 ± 1°C) for eight months and then kept at 20°C for one day. Values are presented as the mean ± standard error (n = 9).

Higher flesh firmness was observed in the 1-MCP-treated fruits compared to the untreated fruits after four to eight months of storage in both cultivars (Fig. 7A and 7B). Similarly, a higher TA was observed in the 1-MCP-treated fruits after four to eight months of storage when compared to untreated fruits of both cultivars (Fig. 7C and 7D). Higher SSC values in ‘Colorpple’ apples and lower SSC values in ‘Manhong’ apples were observed in the untreated fruits compared to the treated fruits at four months of storage, but no differences were observed between treatments at other storage intervals (Fig. 7E and 7F). In both cultivars, a significantly higher SSC/TA ratio was observed in untreated fruits compared to treated fruits after six to eight months of storage (Fig. 7G and 7H).

Fig. 7.

Firmness (A and B), titratable acidity (TA) (C and D), soluble solid content (SSC) (E and F), and SSC/TA ratio (G and H) of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a cold temperature (0 ± 1°C) for eight months and then kept at 20°C for one day. Values are presented as the mean ± standard error (n = 9).

Fruit skin color values were not affected by 1-MCP in either cultivar during the storage period (Fig. 8A-8F). After 8 months of storage, significant severe and moderately-severe skin greasiness was observed, especially in untreated ‘Colorpple’ and ‘Manhong’ apples while slightly-moderate skin greasiness scores were observed in apples treated with 1-MCP (Fig. 9A and 9B). A very low percentage of fruit decay was observed in the 1-MCP-treated ‘Colorpple’ apples after eight months (Fig. 9C), but no fruit decay occurred in the ‘Manhong’ apples throughout the storage period (Fig. 9D).

Fig. 8.

Fruit skin color L* (A and B), a* (C and D), and b* (E and F) values of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a cold temperature (0 ± 1°C) for eight months and then kept at 20°C for one day. Values are presented as the mean ± standard error (n = 9).

Fig. 9.

Fruit skin greasiness (A and B) and fruit decay rate (C and D) of ‘Colorpple’ and ‘Manhong’ apples after being treated with 1-MCP and stored at a cold temperature (0 ± 1°C) for eight months and then kept at 20°C for one day. Values are presented as the mean ± standard error (n = 9). Skin greasiness was scored as 0 = no greasiness, 1 = slight greasiness, 2 = moderate greasiness, and 3 = severe greasiness. Regarding the fruit decay rate, fruits with necrotic spots over 1 cm in diameter were designated as rotten fruits.

Discussion

The aim of this research was to prolong the storage duration of ‘Colorpple’ and ‘Manhong’ apples through a 1-MCP treatment while also assessing the physiological reactions of these apple cultivars to the chemical under both warm and cold storage conditions. High levels of ethylene production and high respiration rates were observed at both warm and cold storage temperatures, especially in the untreated fruits of both cultivars. At a warm temperature, higher production amounts of ethylene and a higher respiration rate were observed in ‘Colorpple’ as compared to ‘Manhong’. However, the amounts of ethylene produced and the respiration rates differed between the two apple cultivars. The different amounts of ethylene produced between ‘Colorpple’ and ‘Manhong’ apples were due to the different genotypic characteristics of each cultivar, concurring with previous reports (Jung and Watkins, 2014; Yoo et al., 2016). At both warm and cold temperatures, 1-MCP suppressed ethylene production and the respiration rate in both cultivars; 1-MCP could inhibit both internal and external ethylene production during at warm and cold storage temperatures in the two apple cultivars in this study, as observed in other apple cultivars (Yang et al., 2013; Yoo et al., 2021).

A meta-analysis also reported that 1-MCP is the most effective inhibitor of ethylene production and the respiration rate in both apples and European pears; ethylene could be reduced by more than 85% and the respiration rate could be reduced by 34% when using 1-MCP (Zhang et al., 2020). Additionally, Zhang et al. (2020) reported that the storage conditions strongly influenced the efficiency of 1-MCP and that an application of 1-MCP combined with cold storage more effectively inhibited ethylene production and respiration. Similar to this study, except for two months of storage in the ‘Manhong’ case, the ethylene inhibition effect of 1-MCP was observed throughout the duration of cold temperature storage (until eight months) in both apple cultivars. Respiration is a metabolic process that deteriorates fresh produce, and a lower respiration rate in fruits indicates that the fruit can be stored for relatively longer times (Fonseca et al., 2002). The reduced respiration rate observed in treated apples is considered to be due to reduced ethylene production levels, as respiration is an ethylene-triggered process.

Fruit texture, as determined by flesh firmness, is an indicator of fruit softening and generally decreases as the fruit ripens (Payasi et al., 2009). The reduction in flesh firmness is due to the degradation of the fruit texture caused by the dissolution of cell wall structures, a process regulated by ethylene (Ireland et al., 2014; Paniagua et al., 2014). Except ‘Colorpple’ apples after 30 days of storage, firmness did not demonstrate significant changes between the treatments during storage at a warm temperature in this study, indicating that firmness in both cultivars at a warm storage temperature was less affected by 1-MCP. However, firmness decreased sharply in the treated fruits, though this reduction was slowed at a cold temperature in the treated fruits. Therefore, a 1-MCP treatment is essential to maintain softening and to prevent degradation of the fruit cell wall for long-term storage at cold temperatures. Slower apple softening and the maintenance of softening-related cell wall structures with slower cell-wall-degrading enzyme activities by 1-MCP during cold storage have also been demonstrated in previous reports (Win et al., 2019; Win et al., 2021; Yoo et al., 2021). Unlike firmness, TA significantly decreased after 40 days in the untreated fruits of both cultivars at a warm temperature. Unlike in the warm temperature condition, TA showed a trend similar to that found the firmness outcome under cold temperature storage, and this value decreased in untreated fruits from four to eight months in both cultivars. Such a TA reduction in apples is generally due to reduced malate and citrate levels during ripening (Ma et al., 2018), and the slower reduction in fruit acidity by 1-MCP is due to the slower degradation of the malate content (Liu et al., 2016). A strong relationship between firmness and TA has also been reported for apples (Nyasordzi et al., 2013). Similar to this study, Win et al. (2019) demonstrated that firmness and TA showed a similar reduction trend during the apple ripening stages and that 1-MCP slowed these reductions during cold storage.

The SSC of neither cultivar was affected by the 1-MCP application at a warm temperature. At a cold temperature, higher SSC values in ‘Colorpple’ and lower SSC values in ‘Manhong’ were observed in the untreated fruits compared to the treated fruits at four months of storage. Changes in the SSC in fruits are related to the starch-sugar conversion during ripening, which is influenced by ethylene (Doerflinger et al., 2015; Yuan et al., 2023). Zhang et al. (2020) reported that SSC was less affected by 1-MCP and its response to a 1-MCP treatment differed depending on the apple cultivar, although 1-MCP inhibited ethylene (Win et al., 2019). In addition, the SSC/TA ratio was higher in the untreated fruits than in treated fruits after six to eight months of cold storage. Similarly, a lower SSC/TA ratio was reported in 1-MCP-treated apples during cold storage (Yoo et al., 2021; Yoo et al., 2023). In the present study, 1-MCP did not affect the fruit weight loss in either cultivar at either of the storage temperatures tested. Fruit weight loss is a result of water loss in fruits, which is associated with fruit transpiration and respiration rates, and excessive water loss in fruits during storage can lead to a loss of the fruit texture, greater susceptibility to chilling injury, and membrane disintegration (Harker et al., 2019; Lufu et al., 2020). In addition, storage conditions such as the temperature, relative humidity, and storage duration strongly influence the changes in the fruit weight loss (Lufu et al., 2020). Win et al. (2019) found that 1-MCP did not reduce weight loss in ‘Summer King’, whereas it reduced the increased weight loss in ‘Green Ball’ apples at a cold temperature. Additionally, 1-MCP did not affect the weight loss in ‘RubyS’ apples (Win et al., 2022). Thus, the SSC and weight loss outcomes here after a 1-MCP treatment varied with cultivar and the storage condition, and the insignificant results observed in this study may be due to these reasons.

Fruit skin color is important for visual appearance, and fruit color changes during ripening, caused by the degradation of chlorophyll and the accumulation of carotenoids and anthocyanins in the fruit skin (Kapoor et al., 2022). In this study, skin color in ‘Manhong’ was not affected by 1-MCP at either temperature. In ‘Colorpple’, 1-MCP led to slightly higher L* (20 to 30 days) and a* (30 to 40 days) values at a warm temperature but not at a cold temperature. Previous studies reported that 1-MCP did not affect skin color L* and b* values in ‘Honggeum’ (Yoo et al., 2020) but did so in ‘RubyS’ apples (Win et al., 2022) at a cold temperature. Lv et al. (2020) reported that 1-MCP slowed fruit color reduction during warm storage. Therefore, the different responses of the color values to 1-MCP could vary with the cultivar and storage temperature.

A higher greasiness score is an indicator of fruit over-ripening, which is caused by the accumulation of waxy constituents in the apple skin, leading to economic losses (Yang et al., 2017a). In this study, lower greasiness scores were observed in the 1-MCP treated fruits of both cultivars at both storage temperatures. Similarly, Yang et al. (2017b) reported that 1-MCP suppressed skin greasiness in apples by reducing the increased concentrations of waxy constituents and the transcript levels of genes involved in skin greasiness. Slight fruit decay appeared after 30 days in both treatments in both cultivars at a warm temperature, and after eight months in the 1-MCP-treated ‘Colorpple’ case at a cold temperature. Zhang et al. (2021) reported that 1-MCP reduced disease incidence and fruit decay in pears. Interestingly, fruit decay did not appear in ‘Manhong’ apples at a cold temperature. Jia et al. (2018) reported that the storage temperature influences the fruit decay rate in pears and that fruit decay occurs more during warm storage. Hence, storing ‘Colorpple’ and ‘Manhong’ apples at a cold temperature could allow one potentially avoid the occurrence of fruit decay for long-term storage.

In conclusion, ethylene production, the respiration rate, and skin greasiness increased and flesh firmness and TA decreased during the apple ripening stages here. These results were more significant in untreated than in 1-MCP-treated fruits of both cultivars tested, especially at a cold storage temperature. However, 1-MCP affected weight loss, SSC, and fruit skin color less at both temperatures. Overall, the effects of the 1-MCP treatment varied depending on the apple cultivar and storage temperature. Fruit texture maintenance was less affected by 1-MCP, although the treatment reduced ethylene production at a warm temperature. 1-MCP improved the postharvest physiological characteristics and fruit quality of ‘Colorpple’ and ‘Manhong’ apples by retaining the fruit texture and reducing ripening-related ethylene production for up to eight months at a cold temperature. This study can provide information to apple growers and researchers with regard to postharvest handling methods for ‘Colorpple’ and ‘Manhong’ apples at warm and cold storage temperatures.