Effects of Modified Atmosphere Packaging on Postharvest Physiology and Quality of ‘Meizao’ Sweet Cherry (Prunus avium L.)

Abstract

Sweet cherry (Prunus avium L.) is becoming increasingly popular in China, but its postharvest quality deteriorates significantly during harvest storage and transport. Here, we investigated the efficiency of different modified atmosphere packaging (MAP) treatments on the quality and physiology of ‘Meizao’ sweet cherry during 60 days of cold storage (0 ± 0.5 °C). Fruits were sealed in four types of MAP low-density polyethylene (LDPE) liners (PE20, PE30, PE40, and PE50), with unsealed 20 μm LDPE packaging bags used as the control. Our findings demonstrated that PE30 packaging established an optimal gas composition (7.0~7.7% O₂ and 3.6~3.9% CO₂) that effectively preserved ‘Meizao’ sweet cherry quality. It maintained the fruit color, firmness, soluble solid content (SSC), titratable acidity (TA), and vitamin C (Vc) content while simultaneously delaying deteriorative processes such as weight loss, pedicel browning, and fruit decay. These results indicate that PE30 was the most suitable treatment for preserving the quality of ‘Meizao’ sweet cherries during cold storage. Furthermore, physiological research showed that significant inhibition of respiration rate was achieved by PE30, accompanied by maintained activities of antioxidant enzymes (CAT, POD, and SOD), which consequently led to reduced accumulations of ethanol and malondialdehyde (MDA) during cold storage. To date, no systematic studies have investigated the physiological and biochemical responses of ‘Meizao’ to different thickness-dependent LDPE-MAP conditions. These observations highlight the power of the optimized PE30 packaging as an effective method for extending the fruit storage life, delaying postharvest senescence, and maintaining fruit quality of ‘Meizao’ sweet cherry.

1. Introduction

Sweet cherry (Prunus avium L.), one of the earliest maturing deciduous fruit trees in Rosaceae family, is extensively consumed worldwide and valued for its health-promoting properties [1]. Due to its high quality and significant market value, the production and cultivation area of the sweet cherry have been expanding rapidly in China. In recent years, its production has reached 35,789.45 tons in 2023 (FAOSTAT, http://faostat.fao.org accessed on 1 March 2025); moreover, plantation and greenhouse cultivation are primarily distributed across 23 provinces (including autonomous regions and municipalities), with approximately three-quarters concentrated in the Round-Bohai Bay region [2,3]. Numerous cultivars with diverse fruit characteristics are available, such as ‘Meizao’, ‘Hongdeng’, ‘Rainier’, ‘Brooks’, ‘Lapins’, ‘Summit’, etc. [4]. Among these, ‘Meizao’ is the most popular sweet cherry variety in China, with significant potential for further development. It is characterized by attractive traits such as mid-early maturity, larger fruit size, dark red color, excellent taste, high quality, and premium market value [5]. The respiration rate of ‘Meizao’ cherries progressively increases from ~135 to ~380 mg CO₂/(kg·h) during cold storage [6]; however, because of its high respiratory activity and susceptibility to mechanical damage, rapid softening and decay occur after harvest, leading to a limited shelf life (2 days at 20 °C or 21–28 d under cold storage) and significant postharvest losses [7].

Multiple postharvest treatments have been used to extend the cold storage life and preserve the quality of fresh cherries, e.g., coatings with chitosan [8], Aloe vera, or thymol [9], fumigated application of methyl salicylate [6], packaging with modified atmosphere [10], etc. Among these, modified atmosphere packaging (MAP) has been significant advantages for various sweet cherry cultivars beyond ‘Meizao’ in many countries due to its simplicity, recyclability, and low cost [11]. MAP is able to maintain an atmosphere over the product with lower oxygen, high carbon dioxide, and moisture content, and these atmosphere conditions are conductive to reducing respiration rate and water loss; thus, the storage life of the fruit is prolonged [11]. However, the single permeability of MAP films cannot meet the preservation requirements of different cherry varieties [12]; moreover, in traditional equilibrium systems, an imbalanced gas composition in MAP can lead to CO₂ injury, resulting in fruits decay, pericarp browning, and off-flavor production [13]. Therefore, a suitable packaging film with appropriate CO₂ and O₂ permeability is needed to effectively control the fruit respiration rate, microbiological changes, enzymatic activity, and oxidation [14,15].

Different types of polymeric film materials or film thickness influence the gas concentration within packaging, which can extend the storage life and maintain the quality of fresh fruits [16]. For sweet cherries, early studies have shown that packaging in microperforated, biaxially oriented polypropylene (BOPP) film (40 μm in thickness) combined with antagonistic yeast could extend the storage time of ‘Ambrunés’ sweet cherries up to 35 days [17]. Additionally, Koutsimanis et al. [18] replaced macroperforated low-density polyethylene (LDPE) bags with PLA cups and PLA peelable microperforated lidding films, which extended the shelf life of ‘Skeena’ sweet cherries by 6 days. However, compared to BOPP and PLA, polyethylene (PE) is more commercially viable based on its characteristics of high strength and toughness, stable storage properties, lower production costs, and wide range of applications. Recent studies have demonstrated that LDPE films with varying thicknesses can effectively reduce fruit rot and maintain the titratable acidity (TA), total soluble solids (TSS), and vitamin C content in bell pepper [19], as well as alleviation of chilling injury in kiwifruit [20] with different film thicknesses, whereas to date, no systematic studies have investigated the effects of LDPE film and its film thickness on the preservation of ‘Meizao’ sweet cherry. Therefore, the objective of this study was to evaluate different thicknesses of LDPE films combined with cold storage on postharvest physiology and quality of ‘Meizao’ sweet cherry.

2. Materials and Methods

2.1. Sweet Cherry Fruit and Experimental Design

Sweet cherry (Prunus avium L.cv. ‘Meizao’) fruits were hand-harvested from eight-year-old sweet cherry trees in a commercial orchard located in Lvshunkou district of Dalian City, Liaoning Province. All fruits were harvested at commercial maturity stage based on uniform size and external purple-red color. After being packed in Styrofoam boxes, these harvested fruits were immediately transported to the laboratory of IPCAAS (Xingcheng, China). They were then subjected to forced air until the pulp temperature reached 0~2 °C and maintained at (0 ± 0.5) °C for 24 h.

After sorting for freedom from damage or defects, fruits with pedicels were placed into LDPE packaging bags (28.0 × 26.0 cm) of four different thicknesses (produced by national engineering and technology research center for preservation of agricultural products (Tianjin, China)). Unsealed 20 μm LDPE packaging bags were used as the control. The four liners with thicknesses of 20 μm, 30 μm, 40 μm, and 50 μm were labeled as PE20 (oxygen transmission rate (O₂TR): 11,000 cm³/(m²·d); carbon dioxide transmission rate (CO₂TR): 64,000 cm³/(m²·d)), PE30 (O₂TR: 10,000 cm³/(m²·d); CO₂TR: 54,000 cm³/(m²·d)), PE40 (O₂TR: 8000 cm³/(m²·d); CO₂TR: 48,000 cm³/(m²·d)), and PE50 (O₂TR: 7000 cm³/(m²·d); CO₂TR: 35,000 cm³/(m²·d)), respectively. Each bag contained 500 g of fruit, was sealed using a sealing machine, and then placed in a fresh-keeping corrugated box (produced by Hubei Zhihe Printing Industry Co., Ltd., Jingmen, China). Each carton contained three bags of fruit (3 × 500 g = 1500 g). The sweet cherries were then stored at 0 °C with a relative humidity (RH) of 90% in darkness for 60 days. Physical and biochemical analyses were conducted at 0, 20, 40, and 60 d of storage. The experiment was repeated twice with three replicates.

2.2. The Change in O₂ and CO₂ Gas Composition (%) in Different MAPs

The gas composition (oxygen and carbon dioxide) inside the packages was determined using a gas analyzer (PBI Dansensor, Ringsted, Denmark). Gas analysis was performed by inserting a needle attached to the gas analyzer through an adhesive seal fixed on the top cover. A volume of 15 mL of gaseous sample was extracted from the headspace using an airtight syringe connected to the analyzer. Immediately after completing the gas analysis, the gas composition inside the LDPE bags was measured every 5 days during the first 30 days and 10 days thereafter.

2.3. Weight Loss

The initial weight of the fruits (W₀) was measured using a digital scale with a precision of 0.01 g (Huazhi, LCD-A600, Putian, China). Subsequently, on days 20, 40, and 60, the fruits (W_i) were weighed again. The weight loss (WL) of the fruits was calculated based on the weight at the beginning of each measurement period and determined as a percentage using the following equation (Equation (1)):

$$\mathrm{The} \mathrm{weight} \mathrm{loss} \left(\mathrm{WL}\right) = {\displaystyle \frac{{\mathrm{W}}_0-{\mathrm{W}}_{\mathrm{i}}}{{\mathrm{W}}_0}}\times 100$$

(1)

where W₀ represents the initial weight of fruits on day 0, and W_i represents the weight of fruits on the sampling day (i = 20, 40, and 60).

2.4. Pedicel Freshness, Respiration Rate, and Fruit Decay

The pedicel freshness was evaluated based on the browning percentage of the entire length of the pedicel. Each pedicel was assessed for freshness at 20-day intervals on a continuous scale from 0 to 5, in which 0, fully brown pedicel; 1, browning > 2/3 of the pedicel; 2, 1/2~2/3 browning; 3, 1/3~1/2 browning; 4, browning < 1/3 of the pedicel; and 5, no browning and dehydration. A pedicel preservation index (PPI) was then calculated using the following equation (Equation (2)):

$$\mathrm{PPI}={\displaystyle \frac{\left({\mathrm{X}}_0\times 0\right) + ({\mathrm{X}}_1\times 1) + \left({\mathrm{X}}_2\times 2\right) + \left({\mathrm{X}}_3\times 3\right) + \left({\mathrm{X}}_4\times 4\right) + \left({\mathrm{X}}_5\times 5\right)}{({\mathrm{X}}_0 +{ \mathrm{X}}_1 +{ \mathrm{X}}_2 +{ \mathrm{X}}_3 +{ \mathrm{X}}_4 +{ \mathrm{X}}_5)\times 5}}\times 100$$

(2)

in which X₀, X₁, X₂, X₃, X₄, and X₅ represent the number of fruits with pedicel browning scores of 0, 1, 2, 3, 4, and 5, respectively. Each treatment investigated 20 fruits, and the experiment was repeated twice with three replicates per treatment.

Thirty fruits from each replicate bag were placed into hermetically sealed glass containers (2.35 L) equipped with a rubber stopper for 1 h. Prior to this, the fruits were equilibrated in air at 20 °C for 5 h. CO₂ production was quantified using an SP-9890 gas chromatograph (produced by Shangdong Lunan Ruihong Chemical Instrement Co., Ltd., Tengzhou, China), equipped with a thermal conductivity detector (TCD). Results were expressed as CO₂ content in mg/(kg·h).

The decay rate of the sweet cherries was assessed at 20-day intervals and expressed as the percentage of rotted fruits relative to the total number of fruits (20 fruits) in each tray. If the development of mycelium on the fruit surface was observed, the fruit was considered rotten.

2.5. Fruit Color Characteristics

Pericarp color was measured using a Chroma Meter (CR-400, Konica Minolta, Inc., Tokyo, Japan), and the CIE parameters L*, a*, and b* were determined for twenty fruits per treatment. Measurements were taken on opposite sides of equatorial part of each fruit. The results were expressed in L* and C*. Here, L* defines the lightness, a* represents the red/greenness, and b* represents the blue/yellowness. Chromatic correction (C*) was calculated as the following equation (Equation (3)) [13]:

$${\mathrm{C}}^{*} = {({{\mathrm{a}}^{*}}^2 + {{\mathrm{b}}^{*}}^2)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$

(3)

2.6. Fruit Quality Parameters

Fruit firmness (FF) was measured on two opposite sides of each sweet cherry using a Fruit Texture Analyzer (Model GS-15, Güss Manufacturing (Pty) Ltd., Strand, South Africa) interfaced with a computer and equipped with an 8.0 mm diameter probe [21]. The results were expressed as the force-deformation ratio (kg/cm²). Soluble solids content (SSC) was determined in duplicate using the juice obtained by squeezing 20 fruits from each treatment with a temperature-compensated digital refractometer (PR-101, Atago Co., Ltd., Tokyo, Japan). Titratable acid (TA) content was determined in duplicate from the same juice using Metrohm automatic titration (model 808 Titrando, Herisau, Swetzerland) with NaOH titration to pH 8.2. For TA analysis, 3 g of juice was diluted in distilled water to a final volume of 30 mL. Vitamin C (Vc) content was determined using a modified 2, 6-dichlorophenolindophenol titrimetric method with a Metrohm automatic titration (model 808 Titrando, Herisau, Swetzerland) [22]. Homogenized samples from 20 fruits per treatment were prepared by mixing 100 g with 100 mL of 2% oxalic acid solution, followed by filtration. A 30 mL aliquot of the filtrate was titrated with 1.0% 2, 6-dichlorophenol-indophenol solution, with the endpoint identified by a persistent pink color for 15 s. The titrant was calibrated with 0.1% ascorbic acid solution. Results were expressed as mg ascorbic acid (AsA) equivalents per 100 g fresh weight (FW) (mg/100 g FW).

2.7. Analysis of Ethanol and MDA Produced

Here, 5 mL of the fruit juice, extracted from 20 fruits, was mixed with 1.335~1.350 g NaCl powder and 1 mL distilled water in 20 mL vials sealed with crimp capes and Teflon-coated septa (Gerstel, Inc., Baltimore, MD, USA). Ethanol content was analyzed using a GC-2010 gas chromatography (Shimadzu, Kyoto, Japan) [23].

Malondialdehyde (MDA) level was measured according to a slight modified method from Wang & Long [10]. Briefly, 1 g of frozen fruit was homogenized in 5 mL of 10% (w/v) trichloroacetic acid (TCA) and centrifuged at 10,000× g for 20 min at 4 °C. Then, 2 mL aliquot of the supernatant was mixed with 2 mL 10% TCA containing 0.67% (w/v) thiobarbituric acid (TBA), heated to 100 °C for 20 min, quickly cooled, and centrifuged at 10,000× g for 10 min. Absorbance of the supernatant was measured at 532, 450, and 600 nm using a UV-T9 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., Beijing, China). MDA content was expressed as μmol/kg FW according to Sharafi et al. [24].

2.8. Enzyme Extraction and Activity Assays

Triplicate samples of fruit tissue (5 g FW) were homogenized in 5 mL of 50 mmol/L Na₃PO₄ buffer (pH 7.5) containing 5 mmol/L dithiothreitol (DTT) and 5% polyvinyl polypyrrolidone (PVPP) using a cold mortar and pestle. Then, the homogenate was centrifuged at 12,000× g for 30 min at 4 °C, and the supernatant was collected as enzyme extract and stored at −80 °C for further analysis.

Catalase (CAT) activity was measured following the method of Chen et al. [25]. The reaction system contained 0.1 mL of supernatant and 2.9 mL of H₂O₂ solution (20 mmol/L). The absorbance was recorded every 30 s for more than 5 min at 240 nm. The CAT activity was expressed as U/g FW based on fresh weight, and one unit (U) was defined as the change of 0.01 per min in absorbance.

Peroxidase (POD) activity was determined using the method of Chen et al. [25]. The reaction mixture consisted of 50 mmol/L phosphate buffer (pH 7.0), 12 mmol/L H₂O₂, 14 mmol/L guaiacol, and 100 μL of enzymatic extract in a total volume of 3 mL. POD activity was expressed as U/g FW based on fresh weight, where one unit (U) was expressed when the absorbance increased 0.01 per min at 470 nm.

Superoxide dismutase (SOD) activity was assayed spectrophotometrically by measuring the 50% inhibition of nitro blue tetrazolium (NBT) photochemical reduction according to the method of Pan et al. [26], with slight modifications. The reaction system (3 mL) contained 50 mmol/L sodium phosphate buffer (pH 7.8), 13 mmol/L methionine, 75 μmol/L nitro blue tetrazole (NBT), 10 μmol/L EDTA, 2 μmol/L riboflavin, and 0.1 mL of an enzyme extract. After mixing, placing the control tube in darkness, and illuminating the remaining mixed tubes with 4000 lx for 15 min, we then measured its absorbance at 560 nm. SOD activity was expressed as U/g FW based on fresh weight, and one unit (U) was defined as the amount of enzyme requited to inhibit 50% of NBT reduction.

2.9. Statistical Analysis

All data were expressed as the mean ± standard deviation (SD) of three analytical replicates per MAP treatment at each storage time point. For each storage time, statistical analysis among MAP treatments was performed using one-way analysis of variance (ANOVA) following Duncan’s multiple range test at a 95% confidence interval for mean comparisons. This approach allowed for independent assessment of treatment effects at each specific storage interval without assuming interaction effects between treatment and storage duration. All analyses were conducted using SPSS software (version 22.0, IBM Corp., Armonk, NY, USA).

3. Results

3.1. Changes in O₂ and CO₂ Gas Composition (%) in MAP

During storage, sweet cherries consume O₂ and release CO₂, leading to changes in the gas composition within the packages. Our results showed that the O₂ concentration reached equilibrium by day 40 of storage. At equilibrium, the internal O₂ levels stabilized at 9.0%, 7.5%, 4.6%, and 4.4% for PE20, PE30, PE40, and PE50 bags, respectively (Figure 1A). The O₂ concentration in PE20 bags was significantly higher than all other treatments (p < 0.05), while PE30 showed intermediate O₂ levels, significantly different from PE40 and PE50 (p < 0.05). No significant difference was observed between PE40 and PE50 bags (p ≥ 0.05). Regarding CO₂ accumulation, all MAP treatments exhibited a gradual increase during the initial 40-day storage period. At equilibrium, the CO₂ concentrations reached 3.07%, 3.90%, 4.58%, and 7.20% in PE20, PE30, PE40, and PE50 bags, respectively (Figure 1B). Significant differences were observed among all MAP treatments (p < 0.05), with the PE50 maintaining consistently higher CO₂ levels throughout the storage period than all other treatments at each storage time point (p < 0.05). These results demonstrate that gas adjustment capability followed the order PE50 > PE40 > PE30 > PE20, indicating that the thickness of MAP bags significantly affects the changes in O₂ and CO₂ concentration during cold storage of sweet cherries.

3.2. Weight Loss, Pedicel Freshness, and Decay Ratio

Weight loss is primarily associated with water evaporation and respiration [27]. For each specific MAP thickness treatment, fruit weight loss increased progressively during storage (

Table 1. Effects of different MAP treatments on weight loss, pedicel freshness, and fruit decay of sweet cherry during cold storage.

Attribute	Treatments	Storage Time (d)
		20	40	60
Weight loss (%)	Control	12.2 ± 1.60 a	17.1 ± 2.50 a	33.4 ± 3.71 a
	PE20	5.3 ± 1.52 b	5.5 ± 1.94 b	10.3 ± 3.81 b
	PE30	0.4 ± 1.34 c	1.2 ± 1.39 c	4.0 ± 3.36 d
	PE40	1.5 ± 1.19 c	5.8 ± 1.61 b	15.5 ± 3.53 b
	PE50	1.6 ± 1.13 c	6.9 ± 2.78 b	8.5 ± 0.00 b
	Control	83.1 ± 1.93 c	76.3 ± 2.97 c	68.9 ± 1.63 d
Pedicel preservation index (PPI)	PE20	96.7 ± 1.14 a	92.7 ± 1.95 a	88.8 ± 0.87 a
	PE30	94.0 ± 1.65 ab	93.3 ± 1.15 a	91.1 ± 1.78 a
	PE40	92.9 ± 1.93 b	87.8 ± 2.10 b	78.3 ± 2.87 c
	PE50	94.3 ± 1.51 ab	88.7 ± 1.44 b	83.8 ± 2.96 b
	Control	4.9 ± 0.40 a	17.3 ± 2.15 a	20.9 ± 2.19 a
Decayed (%)	PE20	3.5 ± 0.82 b	9.3 ± 1.80 b	15.3 ± 2.17 b
	PE30	1.8 ± 0.30 c	2.7 ± 0.44 c	7.2 ± 1.18 d
	PE40	2.1 ± 0.36 c	9.5 ± 2.35 b	13.4 ± 3.65 bc
	PE50	2.3 ± 0.30 c	5.0 ± 1.37 c	10.9 ± 1.45 cd

Means in a column followed by a different letter for the same storage period differ significantly at p < 0.05 by Duncan’s multiple range tests. Data are accompanied by the mean ± SD.

). PE30 packaged fruits exhibited the lowest weight loss (approximately 4.0%) compared to other treatments (8.5–15.5%) and the control (33.4%) after 60 days. Moreover, PE30 packaging consistently demonstrated the lowest weight loss over 20-day intervals, indicating its effectiveness in reducing water evaporation and respiration, thereby preserving sweet cherry weight.

Pedicel freshness, a key quality indicator of preservation [28], varied significantly among treatments at 20-day intervals (Table 1). During cold storage for 60 days, all sealed packaging maintained significantly higher pedicel freshness than the control (p < 0.05); PE20 and PE30 treatments showed the highest pedicel freshness compared to PE40 and PE50 (p < 0.05), with no significant difference between PE40 and PE50 except when stored for 60 days (p ≥ 0.05). These results suggested that PE30 and PE20 packaging effectively delayed pedicel aging and water loss.

Fruit decay rate is closely related to microbial activity and environment conditions [27]. PE30 treatment maintained the lowest decay rate (with only 7.2% after 60 days) compared to other treatments (15.3%, 13.4%, and 10.9% after 60 days), while the control exhibited the highest decay rate (20.9% after 60 days). These results demonstrate that PE30 packaging effectively inhibited pathogenic activity and maintained benefit environment conditions, thereby exhibiting the lowest fruit decay rate.

3.3. Fruit Color

Fruit color is a critical parameter influencing consumer acceptance of sweet cherries and is considered a key indicator of quality and maturity [29]. The lightness value (L*), representing pericarp lightness [13], gradually decreased from the onset of cold storage across all treatments (Figure 2A). No significant differences in L* values were observed among the four films after 20 days of cold storage. However, after 40 and 60 days, the L* values of fruits in PE30 packaging were significantly higher than those in PE40 and PE50 packaging. Both PE30 and PE20 packaging demonstrated superior performance in maintaining fruit lightness. The chroma (C*), which represents color intensity or saturation [30], decreased significantly from harvest to 60 days of storage at 0 °C. Significant differences were observed among the four treatments and the control. The C* value of fruits in PE30 packaging was significantly higher (p < 0.05) than those in PE40 and PE50 throughout the storage period, while no significant difference was noted between PE30 and PE20 (p ≥ 0.05) (Figure 2B). These results indicate that PE30 was the most effective treatment for maintaining the C* values of sweet cherries during cold storage.

3.4. Fruit Firmness, SSC, TA, and Vc Content

To evaluate the impact of film thickness on fruit texture and flavor, we examined fruit firmness, soluble solids content (SSC), titratable acidity (TA), and vitamin C (Vc) content across the four types of packaging bags (

Table 2. Effects of different MAP treatments on fruit firmness, SSC, TA, and Vc content of sweet cherry during cold storage.

Storage Time (d)	Treatments	Firmness (kg/cm2)	Soluble Solid (%)	Titratable Acid (%)	VC Content (mg/100 g FW)
0		2.53 ± 0.14	15.58 ± 0.23	0.550 ± 0.002	7.78 ± 0.15
	Control	1.74 ± 0.10 b	13.15 ± 0.27 b	0.383 ± 0.004 b	7.28 ± 0.12 d
20	PE20	2.10 ± 0.11 a	13.24 ± 0.67 b	0.384 ± 0.006 b	8.39 ± 0.04 a
	PE30	2.19 ± 0.19 a	14.49 ± 0.33 a	0.398 ± 0.005 a	8.18 ± 0.07 b
	PE40	1.93 ± 0.10 ab	13.57 ± 0.71 ab	0.381 ± 0.002 b	7.50 ± 0.16 c
	PE50	2.07 ± 0.23 a	13.93 ± 0.39 ab	0.376 ± 0.007 c	6.84 ± 0.05 e
	Control	1.63 ± 0.08 b	13.34 ± 0.21 d	0.262 ± 0.002 d	5.03 ± 0.17 c
40	PE20	1.99 ± 0.03 a	13.58 ± 0.20 cd	0.278 ± 0.002 b	6.54 ± 0.03 a
	PE30	2.01 ± 0.16 a	14.60 ± 0.21 a	0.297 ± 0.003 a	5.78 ± 0.09 b
	PE40	1.87 ± 0.12 a	14.21 ± 0.24 ab	0.268 ± 0.000 c	5.61 ± 0.08 b
	PE50	1.92 ± 0.07 a	13.90 ± 0.35 bc	0.257 ± 0.000 d	4.98 ± 0.08 c
	Control	1.55 ± 0.06 d	13.80 ± 0.26 d	0.169 ± 0.002 b	4.55 ± 0.22 b
60	PE20	1.82 ± 0.07 ab	13.82 ± 0.23 d	0.166 ± 0.005 b	5.61 ± 0.66 a
	PE30	1.90 ± 0.10 a	15.40 ± 0.19 a	0.216 ± 0.002 a	4.56 ± 0.16 b
	PE40	1.67 ± 0.08 cd	14.86 ± 0.18 b	0.169 ± 0.001 b	3.63 ± 0.08 c
	PE50	1.75 ± 0.06 bc	14.21 ± 0.12 c	0.169 ± 0.002 b	3.62 ± 0.10 c

Means in a column followed by a different letter for the same storage period differ significantly at p < 0.05 by Duncan’s multiple range tests. Data are presented as the mean ± SD.

). For each MAP thickness treatment, fruit firmness consistently decreased during storage, while MAP packaged fruits maintained higher firmness compared to the control. Among treated fruits, no significant differences in firmness were observed during the first 20 and 40 days. However, at 60 days, fruit firmness in PE20 and PE30 treatments was significantly higher than in other packaging treatments (p < 0.05), indicating that PE20 and PE30 packaging retained better fruit firmness. Throughout storage, fruits in PE30 and PE40 packaging exhibited the highest SSC values, while the control and PE20 showed lower SSC values, with a gradual increase observed in all treatments. Moreover, PE30-treated fruits had the highest TA levels. By the end of cold storage, no significant differences in TA were detected between MAP treatments and the control, except for PE30. The most effective maintenance of V_C was observed in PE20 treated fruits during whole storage, followed by PE30. In contrast, PE40 and PE50 treatments showed significantly lower Vc retention capacities. These above results indicate that PE30 was the most optimal treatment for retaining the fruit texture and flavor, including fruit firmness, SSC, TA, and Vc content (Table 2).

3.5. Fruit Respiratory Rate

It has been reported that postharvest treatments greatly impact fruit respiration, which, in turn, affects fruit senescence [24,31]. No distinct respiratory peak was observed in any of the treatments throughout the storage period (Figure 3). After 20 days of storage, the respiratory rate of fruits in all sealed packaging treatments was significantly lower than that of the control. At 40 and 60 days of storage, PE50-treated fruits exhibited the highest respiration rate among all treatments, including the control. Notably, the respiratory rate of PE30-treated fruits showed a remarkable decline by the 60th day, reaching 107.3 ± 4.4 mg/(kg·h), which was significantly lower than others (p < 0.05). The results indicating that PE30 treatment effectively inhibited fruit respiratory activity and delayed metabolic changes and senescence, thereby extending the storage period.

3.6. Fruit Ethanol and MDA

To evaluate the anaerobic respiration (fermentation) and lipid peroxidation of the fruits during storage, we measured the ethanol and malondialdehyde (MDA) contents of treated fruits and the control. For each MAP treatment, ethanol content consistently increased over storage time in varying degrees (Figure 4A). At each detection time point, PE50-treated fruits exhibited the highest ethanol content. In contrast, PE30 and PE40 treatments effectively inhibited higher ethanol accumulation compared to the control and PE20 treatment.

MDA content, a key indicator of lipid peroxidation caused by oxidative stress in fruits and vegetables [21,31], is linked to senescence and disturbs membrane integrity and physiological metabolism [32]. We observed that MDA content increased in non-treated fruits during storage (Figure 4B), indicating that oxidative damage continuously occurred in fruits. Interestingly, PE30 treatments significantly inhibited the MDA accumulation and consistently exhibited the lowest MDA levels compared to other treatments and the control. Conversely, PE50-treated fruits displayed the highest MDA content, more than twice as high as PE30-treated at the same time point. These results reveal that PE30 treatment demonstrated the highest efficacy in mitigating oxidative damage, helping to maintain the quality of fruit and prolong the storage period.

3.7. Antioxidant Enzyme Activity

To investigate the effects of different film thickness treatments (PE20, PE30, PE40, and PE50) on antioxidant enzyme activities, we evaluated the activities of CAT, POD, and SOD in sweet cherries during 60 days of cold storage. The results revealed significant variations in enzyme activities among the treatments, highlighting the influence of film thickness on postharvest physiological processes (Figure 5). CAT, POD, and SOD activities displayed a similar trend, with an initial increase followed by a gradual decline; however, the rate and timing of these changes varied significantly across treatments. As seen in Figure 5A, CAT activities increased steadily during the first 40 days of storage, peaking in PE30-treated fruits with a value of up to 200 U/g, which was significantly higher than other treatments. After 40 days, CAT activity declined rapidly, and by the 60th day, no significant differences were observed among the PE20, PE30, PE40, and PE50 treatments. However, all treatments maintained significantly higher CAT activity compared to the control (p < 0.05). POD activity decreased steadily throughout storage in PE20 and PE50 treatments, as well as the control, suggesting that thinner or thicker films impaired the antioxidant defense system of fruits. In contrast, it initially increased until day 40, followed by a decline at day 60 in PE30 and PE40 treatments, and higher POD activity was exhibited in PE30-treated fruits compared to other treatments (Figure 5B). SOD activities showed no significant differences among PE20, PE30, PE40, and PE50 treatments before 40 days of storage. By 60 days, PE30-treated fruits exhibited the highest SOD activity, followed by PE20 and PE40 (Figure 5C). The above investigations demonstrate that PE30 performed best in the preservation of antioxidant enzyme activities (CAT, POD, and SOD) in sweet cherries during cold storage, highlighting its potential to mitigate oxidative stress and extend shelf life.

4. Discussion

The MAP technique, relaying on gas exchange modeling within the package, has been widely applied for extending the storage and transportation life of fruits [33]. Optimization of gas mixtures has been the general focus for improving fruit storage. Sweet cherry is unique among the stone fruits due to its higher tolerance to CO₂ concentrations [34], maintaining good quality under atmosphere with O₂ levels below 10% and CO₂ above 15% [10]. Gas transmission in MAP is partly subject to film thickness, with reduced thickness enhancing permeability [35]. In this study, O₂ concentrations decreased with increasing packaging thickness for ‘Meizao’ sweet cherry, whereas CO₂ concentrations showed the opposite trend (Figure 1). Furthermore, different O₂ and CO₂ levels have been used for storing various cherry varieties [12,31]. For the ‘Meizao’ sweet cherry, MAP with a 30 μm thickness (PE30) balanced the gases’ permeability, maintaining optimal gas levels of 7%~9% O₂ and 3%~4% CO₂. This thickness (PE30) was significantly superior to the other treatment in reducing decay rates, inhibiting respiratory activities, and maintaining higher fruit quality, probably due to optimized gas exchange and inhibited microbial activity during cold storage. As previously reported, MAP with lower O₂ (5–8%) reduced fruit respiration rate and maintained flavor in ‘Bing’ and ‘Sweetheart’ cultivars for 6 weeks under 0 °C storage [10]; 50 μm LDPE bags (2.8% O₂ and 8.8% CO₂) could preserve the marketable quality of red-ripe ‘Burlat’ sweet cherries for 3 weeks under 2 °C storage [13]. However, improper MAP application can accelerate fruit senescence by increasing respiration rates, ethanol accumulation, and membrane lipid peroxidation [23]. Excessive CO₂ (above 30%) or low O₂ (below 1%) can cause browning and off-flavors [36]. Here, core browning in the PE50-treated fruit after 60 days of cold storage was linked to higher CO₂ levels, leading to increased ethanol and MDA contents (Figure 4). The results were consistent with findings in apples [37] and ‘Hongdeng’ sweet cherry [34]. These observations highlight the importance of gas composition stability, which varies with MAP thickness. However, further research on the underlying mechanisms may enhance MAP efficiency for sweet cherry storage.

Fruit weight loss is a major physiological disorder during postharvest storage, primarily caused by water evaporation through evapo-transpiration and respiration, which depends on the relative humidity surrounding the fruit [14]. In this study, PE30-treated fruits exhibited significantly lower weight loss compared to the control and other treatments (Table 1), demonstrating that MAP technology extends cold storage life and reduces weight loss by minimizing gas exchange and water evaporation [38]. Meanwhile, MAP-treated fruits maintained significantly higher pedicel freshness than the control (Table 1), attributed to reduced water losses [28]. Furthermore, pericarp color, a key visual indicator of fruit freshness, is influenced by color difference indices [39]. The L* and C* values of sweet cherry pericarp color gradually declined during storage, but PE30 treatment delayed the decline (Figure 2), indicating that suitable MAP conditions retarded color development during storage and shelf life [30]. Previous studies indicate that fruit lightness increases during growth due to chlorophyll breakdown and carotenoids or anthocyanin accumulation [40,41]. Moreover, co-pigmentation with organic acids and high temperatures can enhance anthocyanin color intensity [42]; anthocyanins are also sensitivity to factors like pH, metal ions, enzymes, ascorbic acid, and sugars [43]. So, whether the decline in L* and C* values during storage results from carotenoid accumulation surpassing anthocyanin and chlorophyll degradation or from anthocyanin fading and interactions with other compounds requires further investigation.

SSC, an indicator of sugar content, directly influences the taste of sweet cherries [44]. In this study, SSC changes in ‘Meizao’ cherries were minimal, decreasing by only 0.18%~2.24% during storage (Table 2), suggesting it is not a suitable index for evaluating preservation effects. Fruit firmness is a critical quality attribute, as excessive softening during storage increases susceptibility to mechanical damage and pathogen infection, significantly reducing fruit quality [45]. Here, PE30-treated ‘Meizao’ sweet cherries maintained higher firmness, along with higher SSC, TA, and VC contents (Table 2) and lower decay rates (Table 1), compared to other treatments and control. These results indicate that PE30 treatment effectively mitigates natural softening and quality deterioration during cold storage. Fruit softening is influenced by several factors such as cell wall degradation [46], ethylene production [47], and oxidative stress accumulation [48]. In accordance with the cell wall degradation mechanism of sweet cherries, fruit firmness showed a strong negative correlation with increased activities of three key hydrolytic enzymes (polygalacturonase, pectin methylesterase, and carboxymethylcellulase) [49]. Further research into the mechanisms of PE30 will expand its application to other fruit varieties. While PE30 effectively maintains ‘Meizao’ quality, commercial adoption requires cost–benefit analysis of LDPE films, compatibility with existing supply chains, and pilot-scale trials.

In addition to texture and flavor loss, water loss and gas imbalance can trigger browning and accelerate senescence [50]. We propose that decreased pericarp brightness and increased respiratory rates may also be early manifestations of the senescence and browning during fruit storage. Fruits and vegetables will produce large amounts of reactive oxygen species (ROS) in the process of their own metabolism and under conditions of external adversity and stress, and these ROS bursts are key biochemical mechanisms underlying postharvest peel browning and senescence [26,51]. If these substances cannot be cleared from the plant part in time, they will have severe toxic effects on its physiological metabolic reactions and postharvest quality [52]. The process of ROS scavenging in fruits is accompanied by reducing activities of ROS-scavenging enzymes (e.g., SOD, CAT, POD, and APX), decreasing levels of endogenous antioxidant substances (e.g., AsA and GSH), and increasing MDA content [53]. SOD is the first line of defense against oxidation in plants, removing excess superoxide anions in cells and separating them into H₂O₂ and O₂, while CAT and POD can remove this excess H₂O₂ [26]. In our study, CAT, POD, and SOD activities in sweet cherry fruits declined rapidly during the middle and late storage periods. In contrast, PE30 treatment effectively maintained higher antioxidant enzyme activities (Figure 5), thereby directly alleviating oxidative stress. The suppression of ROS accumulation subsequently reduced MDA levels (Figure 4)—a well-established marker of lipid peroxidation—preserving membrane integrity and cellular function [10]. Consequently, these protective effects maintained key postharvest quality attributes, including color stability, firmness retention, and decay resistance (Table 1 and Table 2). These findings suggest that PE30 alleviates oxidative stress in ‘Meizao’ fruit, reducing ROS-induced cell damage and enzymatic browning. Moreover, pathogen infections, such as those caused by Alternaria alternata and Botrytis cinerea, are major contributors to peel browning and fruit decay [53,54]. Early studies indicate that SOD, CAT, and APX activities are initially upregulated to reduce H₂O₂ accumulation in early infecting stage but decline in middle and late stages [55], which is consistent with our findings above. Thus, further research into ROS scavenging and antioxidant signaling mechanisms may provide a powerful promotion for extending sweet cherry cold storage.

Overall, this study first establishes PE30 LDPE as an optimal packaging solution for maintaining ‘Meizao’ sweet cherry quality and physiological parameters during 0 °C storage, positioning it as the preferred MAP technology for this cultivar. However, certain limitations should be noted. First, professional sensory evaluation (including appearance, texture, aroma, and taste) was not performed, which could reveal consumer-relevant defects not captured by physicochemical analysis. Second, the experimental scale was limited, focusing exclusively on one packaging type. Future research should address these aspects by incorporating sensory panel assessments and comparing multiple packaging materials under commercial-scale conditions.

5. Conclusions

The results of this study demonstrate that there were significant differences in fruit quality and physiology characteristics of ‘Meizao’ sweet cherry under various MAP treatments at 0 °C. PE30 packaging treatment effectively maintained higher pericarp brightness (L* value) and pedicel freshness while significantly delaying weight loss, fruit decay, the reduction of firmness, and TA and Vc contents. Furthermore, this treatment minimized the accumulation of ethanol and MDA, demonstrating its superior efficacy in preserving ‘Meizao’ sweet cherry quality during cold storage. Moreover, the PE30 treatment also exhibited the most pronounced effect on enhancing the activity of key antioxidant enzymes, including CAT, POD, and SOD. Activating of these enzymes likely contributed to the scavenging of ROS generated during postharvest senescence, thereby extending the storage potential of ‘Meizao’ sweet cherry. In contrast, PE50 packaging resulted in slight fruit browning and significantly higher respiration rates, ethanol accumulation, and MDA content compared to other treatments and the control. These observations suggest that PE50 packaging accelerated fruit senescence, potentially due to CO₂ concentrations exceeding 7.2%, which may induce CO₂ injury in ‘Meizao’ sweet cherry. These findings indicate that not all MAP applications are effective in delaying fruit senescence and extending the shelf life of sweet cherry fruits, highlighting the importance of optimizing packaging parameters for specific fruit varieties.