Research progress on the effects of postharvest storage methods on melon quality

Effects of different storage methods on the sensory quality of melon fruits

Influence on melon firmness

Firmness is an indicator that reflects the quality changes and degree of softening of fruits during postharvest storage (Ahmad et al., 2023) and is closely related to the cell wall (CW) structure (Fig. 1). Cell walls are mainly composed of pectin, cellulose, hemicellulose, and glycoproteins (Uluisik & Seymour, 2020). CWs are present in all plants and provide mechanical strength by acting as an exoskeleton, regulating osmotic pressure, and controlling cell adhesion during cell expansion. In addition, CWs function as a diffusion barrier, protecting and preventing plasma membrane disruption and acting as a barrier against pathogens and herbivores.

The main reason for fruit softening is the destruction of the CWs (Pott, Vallarino & Osorio, 2020). As the product organs mature, the network structure of CWs continuously disintegrates, and the original pectin in the CWs gradually transforms into soluble pectin. The CW structure becomes loose, and ultimately, the firmness decreases (Posé et al., 2019). During storage, the firmness of melon fruits decreases with the progression of fruit ripening. Firmness is strongly correlated with sugar content, and the acoustic response of melon fruits is an important parameter for estimating firmness, soluble solid content (SSC), grade and internal quality (Sun et al., 2010).

Softening is a complex process often associated with the degradation of fruit cell wall components and involves an increase in water-soluble pectin and a decrease in insoluble and covalently bound pectin (Yan et al., 2023). According to Le Nguyen et al’s (2019) detection of acoustic vibrations in melon fruits, conventional gaseous 1-methylcyclopropylene (1-MCP) fumigation and liquid 1-MCP microbubble treatment for 20 to 30 min can delay the softening of ‘Donatello’ melons. Ergun et al. (2005) reported that during storage at 20 °C, 1.5 µL/L 1-MCP gas treatment of ‘Galia’ melon for 24 h delayed fruit softening, extended the shelf life of the yellow harvest to 11 days, and extended the shelf life of the green harvest to 20 days.

Treating melon fruits with an aqueous calcium solution before or after harvesting could increase the content of calcium ions in cells and improve the stability of pectin (Silveir et al., 2011). Zhang et al. (2022b) studied the ‘Golden Queen’ hami melon, which was treated with a 1.5% chitosan solution for 10 min before refrigeration at 3 °C, and the results revealed that chitosan treatment significantly reduced fruit softening. Hatami et al. (2019) harvested Dudam melon 21 days after flowering, and long-term storage at a low temperature of 13 °C was conducive to maintaining its firmness. Tappi et al. (2016) used empty dipping (VI) as a processing operation, which can better maintain the texture of processed melons during storage after VI treatment.

Ethylene treatment significantly accelerates the decomposition of cell wall membranes (CWMs) and covalently bound pectin (CSP), promotes an increase in the water-soluble pectin (WSP) content, accelerates the degradation of hemicellulose and cellulose, promotes the decomposition of ion-bound pectin (ISP) at the later stage of storage, and accelerates the decrease in fruit firmness. During storage, treatment with the ethyl synthesis inhibitor (AVG) 1-MCP and low temperatures can significantly delay the degradation of the CWM and CSP components and inhibit the increase in the WSP content. It is beneficial for maintaining the stable content of ISP, hemicellulose and cellulose; reducing the decomposition rate; and helping to maintain the firmness of fruit (Zhang et al., 2022a; Zhang et al., 2022b).

Influence on melon weight loss rate and decay rate

Melon fruits are divided into respiratory climacteric and nonrespiratory climacteric types. During postharvest storage, water is lost from the fruit tissue of respiratory climacteric sweet melons, and the activities of various hydrolytic and respiratory enzymes increase sharply. Fruit respiration is accelerated, and related nutrients are decomposed, thus increasing the weight loss rate of the fruit. Zhang et al. (2022a; 2022b) studied three Hami melon varieties stored at 20 ± 1 °C and 25% relative humidity (RH) for 18 days. The weight loss results indicated that the storage quality of ‘Chougua’ was obviously better than that of ‘Xizhoumi 17’ and ‘Jinhuami 25’. The application of rapeseed meal enzymatic hydrolysate as a natural preservative in postharvest storage and the preservation of cantaloupe can effectively reduce the quality loss rate of cantaloupe (Wang et al., 2017). Kim et al. (2010) reported that a low temperature ranging from 7 to 10 °C and a high RH ranging from 90 to 95% are suitable environmental conditions for delaying weight loss in fresh oriental melon fruits. Controlled atmosphere (CA) storage or modified atmosphere (MA) packaging can be used as supplemental treatments. Research has shown that, compared to separate treatments of 0.75 g/L chitosan and 0.05 mL/kg ethanol, composite treatment further delays the loss of melon quality (Sun et al., 2023). Moreover, experiments have shown that the combination of postharvest chitosan coating and low-temperature (5 °C) storage treatment has a more prominent effect on delaying the rate of fruit weight loss (Wang et al., 2016).

The decay index is an important index of the effect of fruit storage and preservation (Zhang et al., 2023a; Zhang et al., 2023b; Zhang et al., 2023c). A previous study revealed that when melons were stored at 20 ±1 °C and 25% RH for 18 days, compared with those of ‘Xizhoumi 17’ and ‘Jinhuami 25’, the decay rate of ‘Chougua’ significantly decreased in the later stage of storage (Zhang et al., 2022a; Zhang et al., 2022b). Gal et al. (2006) stored Galia melon for 15 days at 5 °C and for 3 days at 20 °C. The results showed that the optimal treatment with 1-MCP for suppressing the ripening of fruits harvested at the green/yellow stage of maturity was 300 nl l⁻¹ for 24 h at 20 °C, and the decay rate of fruits after treatment was significantly lower than that of the control group. However, it has been also reported that disease severity can be severe in both control and 1-MCP-treated melons (Sun et al., 2010). Hoberg et al. (2003) conducted postharvest and end-of-year storage and marketing simulations. After 16 days of storage at 5 °C and an additional 3 days at 20 °C, only cultivar ‘C-8’ had a poor general appearance due to its relatively high decay incidence compared to those of the cultivars ‘5080’, ‘Ideal’ and ‘7302’.

Influence on melon color indices

The color and luster of fruits are the most intuitive manifestations during storage and are important indicators that affect their commercial value (Afonso et al., 2023). Many studies have been conducted on the preservation of melon fruit color. Du et al. (2016) used ‘Xizhou Mi 25’ as the test material and cooled it with 0 °C ice water for 120 min and then refrigerated it at 6 °C for 6 h after harvest, which slowed the color change during storage and effectively maintained the color of Hami melons. Among the two low-temperature treatment conditions of 10 °C and 15 °C, the 10 °C treatment can effectively delay the color change in melon fruit flesh, maintain fruit brightness, and maintain the original color of the fruit (Shao, 2019). Yao et al. used the melon variety ‘Xizhou Mi 25’ as the test material and continuously irradiated it with red, blue, and white monochromatic weak light (30 lx) using light-emitting diodes at 7 °C. Red light has the greatest effect on maintaining the color of melon skin (Yao et al., 2023). In the study of Agehara et al. (2018), 1-MCP immersion had a small effect on the initial color transition from green to light yellow, but it delayed the subsequent development to orange, extending the fruit shelf life with a desirable ripe skin color. Watkins (2006) confirmed that 1-MCP treatment could inhibit the synthesis and accumulation of pigments and delay the color transformation of fruits. Jin et al. (2013) studied the effects of ethanol steam treatment (0.5 mL/kg and 3 mL/kg) at 23 °C on the postharvest storage quality of ‘Oriental’ melon fruits. The effects of ethanol vapor treatment on skin color intensity and color change in melons were dependent on the dose of ethanol, with a better appearance of melons at 0.5 mL/kg than at 3 mL/kg. The use of flusilazole fungicide to treat fruits results in greater changes in the total color difference ΔE during storage (Du et al., 2015).

Effects of different storage methods on compound contents in melon

Effects on the vitamin C content in melon

Vitamin C (Vc) is an important nutrient in fruits that contributes to the antioxidant capacity of plant cells by detoxifying reactive oxygen species (ROS) and free radicals (Rey, Zacarías & Rodrigo, 2020). Research has shown that temperature affects the Vc content in fruits, and as temperature decreases, the Vc content increases. Yousuf, Srivastava & Ahmad (2020) analyzed fresh-cut melons treated with different concentrations of lemon extract (0, 5, 10 and 15%) and coated them with 0 and 5% soy protein isolate (SPI), and the results showed that these treatments were effective at retaining vitamin C in melon samples (Kim et al., 2015). Research has shown that when different gas ratios (O₂:CO₂) are used to treat ‘Xizhou Mi 25’ Hami melons, the rate of change in the Vc content in the 6% CO₂+1% CO₂ and 3% O₂+1% CO₂ treatments is slower than that in the control group (Yousuf, Srivastava & Ahmad, 2020). Wang et al. (2023) used three different concentrations of melatonin to maintain the content of ascorbic acid, and the fruit treated with 0.5 mmol/L MT showed the greatest effect.

Influence on the titratable acidity content of melon

The titratable acid (TA) content is an important indicator used to reflect the flesh quality of fruits and has a significant impact on the intrinsic quality of fruits (Zhang et al., 2023a; Zhang et al., 2023b; Zhang et al., 2023c). Titrable acidity can serve as a substrate for respiration, and as storage time progresses, the titratable acidity gradually decreases, leading to a decrease in its content and quality. The main organic acid in cantaloupe is citric acid. Research has shown that 2 µL/L1-MCP fumigation for 24 h can be used to treat ‘Berserksin’ cantaloupe, which is stored in cold storage at 7 °C (85∼90% RH), which significantly inhibits the decrease in titratable acid content (Zhang, 2018). Moreover, 1-MCP treatment of ‘Xizhou Mi 25’ under storage at 5 °C effectively inhibited the titratable acid content of the fruit during postharvest storage of sweet melons. A study was conducted on the physiological changes and quality of ’Jiashigua’ fruit during storage using eight different modified atmosphere treatments. A gas ratio of 7% CO₂+5% O₂+88% N₂ was shown to better maintain the titratable acid content of the fruit (Zhang et al., 2011). After treating Jinxiangyu melon with hot water at 53 °C, the decrease in the titratable acid content has been found to be delayed, indicating that hot water treatment can better maintain the storage quality of the fruit (Zhang et al., 2010). Wang et al. (2012) reported the slowest decrease in titratable acid in fruits treated with 1-MCP combined with Na₂S₂O₅.

Effects on enzyme activities related to postharvest metabolism in melon

Polygalacturonase (PG) is an important hydrolase that breaks down pectin molecules by hydrolyzing 1,4- α-D-galactoside bonds in polygalacturonic acid in the CWs, thus disintegrating the CW structure and softening the fruit (Goukh & Elhassan, 2017). Pectin methylesterase (PME) is involved mainly in the demethylation of pectin substances in the cell walls and can remove methyl groups from pectin in the cell walls, thus accelerating the degradation of PG metabolism (Fig. 2) (Yu et al., 2023). Studies have shown that calcium treatment can effectively inhibit the degradation of cell wall substances (Liu et al., 2023a; Liu et al., 2023b). Moreover, PME demethylation products are precursor substrates that significantly inhibit the activities of enzymes related to fruit softening, such as PG, PME and β-glucosidase (Jeong et al., 2004). Under conditions of 20 ±1 °C and 25% RH, PME deesterification can be reduced by inhibiting PME activity in fruits, thus inhibiting the hydrolysis of the polygalacturonic acid main chain catalyzed by PG. In a study of thick skin melons, soaking melon in hot water at 53 °C for 3 min effectively inhibits the activities of polygalacturonase, pectinesterase, β-galactosidase and the endonuclide 1,4- β-D-glucanase; maintains the firmness of the fruit during storage; and increases the contents of suberin and callose (Yuan et al., 2013).

Peroxidase (POD) is an important oxidoreductase in fruit that can catalyze H₂O₂ to oxidize phenolic substances to produce quinones, and the quinones are further concentrated to form brown substances (Luo, Chen & Xie, 2011). Under postharvest conditions, fruits usually receive multiple signals at the same time, and ROS are easily destroyed (Karlia, Yonadita & Sony, 2020). Superoxide dismutase (SOD) and catalase (CAT), two major antioxidant enzymes, can effectively control the imbalance of ROS metabolism, thereby inhibiting oxidative stress during storage (Fig. 3) (Chen et al., 2015).

1-MCP treatment can effectively maintain the activities of SOD, ascorbate oxidase (APX) and POD and improve the activity of CAT in muskmelon during the middle stage of storage (4∼5 days). However, ethylene treatment inhibits the activity of reactive oxygen-scavenging enzymes in fruits, which diminishes the scavenging of reactive oxygen radicals. The aggregation of ROS leads to intracellular oxidative stress, which includes the peroxidation of unsaturated fatty acids. Peroxidation triggers a free radical chain reaction that disrupts the lipid structure of cell membranes, leading to cellular senescence and tissue aging (Zhang, 2018). Yuan et al. (2013) soaked melon in hot water at 53 °C for 3 min, and the results showed that the activity of POD significantly increased. Studies have shown that oxalic acid treatment can reduce the accumulation of ROS by stimulating the antioxidant system of melons. Among them, enzymes, such as SOD, POD and CAT, are important antioxidant enzymes in plant cells that can help to scavenge reactive oxygen radicals and maintain the intracellular redox balance. Melatonin treatment can simultaneously increase SOD, POD, CAT and APX activities, reduce O₂⁻ and H₂O₂ levels, delay postharvest senescence, and maintain melon fruit quality (Wang et al., 2018).

Ethylene is a key factor affecting fruit ripening and senescence (Lee et al., 2023a; Lee et al., 2023b). The purpose of delaying and controlling the ripening and senescence process of melon fruits can be achieved by regulating the release of ethylene and the activities of the key enzymes ACS and ACO in the ethylene biosynthesis pathway (Fig. 4) (Li et al., 2023). Yazhong et al. reported the effects of ethanol vapor treatment (0.5 mL/kg and 3 mL/kg) on the postharvest storage of melon fruits. Both treatments significantly (P < 0.05) suppressed the internal ethylene concentration (IEC) during storage at 23 °C, which was evident from the decreases in 1-amino-cyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities and the inhibition of ACC biosynthesis (Jin et al., 2013); moreover, the effect of the 0.5 mL/kg treatment was greater than that of the 3 mL/kg treatment. According to Li et al., 1-MCP treatment led to a decrease in autocatalytic ethylene synthesis, mainly due to the inhibition of ACS and ACO activities, and the inhibition of fruit softening by 1-MCP may be related to a decrease in ethylene production (Wang et al., 2017). Ayub et al. (1996) showed that melons that express the antisense 1-amino-cyclopropane-1-carboxylic acid (ACC) oxidase gene were produced by genetic engineering preservation treatment, indicating that melons expressed by the ACC oxidase gene can inhibit postripening and senescence of fruits and prolong their shelf life.

Effect on the membrane permeability of melon plants

Malondialdehyde (MDA) is produced by lipid peroxidation in tissues or organs of plants due to senescence or damage under stress conditions. The MDA content is an important indicator of the degree of fruit senescence (Khaliq et al., 2016; Carvalho et al., 2016). Wang et al. (2023) treated melon plants with 0.5 mmol/L melatonin to effectively reduce the content of malondialdehyde, thus significantly delaying aging and prolonging storage life. Rokayya Sami et al’s. (2021) treatment of melon with nanomaterials delayed the increase in MDA, demonstrating that nanotreatment can inhibit lipid peroxidation during storage. When the 1-MCP compound fungicide imazole is used to treat the Golden Red Treasure thick-skinned melon, the MDA content in the fruit remains low over time (Yin et al., 2015). The soaking of ‘Yujinxiang’ muskmelon with 4 mmol/L salicylic acid for 10 min induced an oxygen explosion in thick-skinned muskmelon fruits and inhibited MDA production (Yang et al., 2021). Compared with the initial storage values under three low-temperature (5 °C, 8 °C, and 12 °C) and room temperature conditions, the malondialdehyde content of melon stored at 5 °C for 28 days increases by approximately 1.5 times, significantly delaying the increase in malondialdehyde content (Tian et al., 2022).

Effects of different storage methods on the physiological respiratory activities of melon fruits