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. 2025 Aug 28;30:102951. doi: 10.1016/j.fochx.2025.102951

Effects of low-density polyethylene inner liner on keeping the postharvest quality in ‘Fuji’ apple during storage

Cen Li a, Jia-Qi Li a, Qing Yue a, Xiao-Ling Ding a, Rui Li b, Xiao-Lin Ren a,⁎, Cui-Hua Liu a,⁎
PMCID: PMC12418996  PMID: 40934033

Abstract

Apples are prone to superficial scald after long-term cold storage. We studied the effects of low-density polyethylene (LDPE) inner liners with 10, 20, 30, and 40 perforations of 150 μm on the quality and superficial scald of the ‘Fuji’ apple. The LDPE liner with 10 perforations (LDPE-10) formed microenvironments with lower concentrations of O2 (15.96 %–17.9 %) and higher concentrations of CO2 (0.59 %–1.19 %). Compared with the control (CK), the ethylene release rate of apples packed in LDPE-10 was reduced by more than 31.47 %, and the weight loss of the fruits in LDPE-10 was significantly reduced by 41.41 %. Notably, the LDPE-10 reduced the incidence of superficial scald and internal flesh browning by 16.45 % and 1.68 %, respectively. Lower contents of O2− reflected a better antioxidant capacity of fruit treated by LDPE-10 and MDA which resulted from more antioxidant secondary metabolites, including phenols, GABA, GSH and ASA, plus with higher activities of anti-oxidases, including POD and CAT, and together with lower PPO activity. This work provides an effective way and a theoretical foundation for improving the traditional mechanical refrigeration of apple fruits.

Keywords: Malus domestica, Modified atmosphere packaging, Weight loss, Physiological disorder, Anti-oxidation

Highlights

  • •

    LDPE inner liners form micro-environmental parameters with lower O₂ and higher CO₂ than PE inner liners.

  • •

    The LDPE-10 inner liner respectively reduced the ethylene release rate and the weight loss of the fruits by 31.47 % and 41.41 %.

  • •

    LDPE-10 inner liner reduced the incidence of superficial scald and internal flesh browning by 16.45 % and 1.68 %, respectively.

  • •

    LDPE-10 better maintained membrane integrity, and weakened oxidation in fruits.

  • •

    BA content in LDPE-10 was consistently higher than in PE during storage. The effect of BA on apple preservation deserves further study.

1. Introduction

‘Fuji’ (Malus × domestica Borkh.) apples with a high nutrition content, low susceptibility to disease, and good storage quality are widely cultivated worldwide (Tanaka et al., 2018). According to the Food and Agriculture Organization (2023), postharvest loss remains more than one-third each year for apple fruits, mainly caused by weight loss and physiological disorders, among which superficial scald and pulp browning are severe (Populin et al., 2023).

Superficial scald manifests as brown spots on the peel. As the disease progresses, the initial spots gradually connect into pieces, and the colour of the spots deepens (Lurie & Watkins, 2012). This seriously affects the appearance qualities and commercial values of apple fruits. However, superficial scald's precise physiological and molecular basis has not been fully elucidated.

Some hypotheses are oriented to explain the occurrence and development of superficial scald, such as the antioxidants and antioxidant enzymes hypothesis and the α-farnesene oxidation hypothesis. To some extent, the symptoms of superficial scald are similar to the characteristics of oxidative damage (Lurie & Watkins, 2012), and it was previously shown that the antioxidant system was concerning the occurrence of superficial scald and pulp browning (Ahn et al., 2007). The antioxidant system of the browning tissue of fruit was abnormal, and the production of reactive oxygen species increased, which damaged the cell membrane (Franck et al., 2007). Taking measures to reduce superficial scald and pulp browning and improving the antioxidant system of fruits are crucial for improving fruit quality.

In the postharvest industry, controlled atmosphere storage (CA) is a principal means for controlling the concentration of O2 and CO2 in the storage facilities. CA can effectively maintain the fruit quality (Bodbodak & Moshfeghifar, 2016), but its high cost and management technology limit its application. Low-temperature storage is widely used for apple storage instead of CA in China, and the inner liners are widely used to reduce the water loss of apple fruits during low-temperature storage (Du et al., 2024).

In addition to fruit postharvest water retention, the inner liner can also form an ideal microenvironment for decreasing the respiration of fruits and vegetables with a high concentration of CO2 and a low concentration of O2 due to its different gas permeability and permselectivity for CO2 and O2. As a result, the post-ripening of the horticultural products was delayed (Cai et al., 2022; Guo et al., 2015; Qu et al., 2022); the quality of fresh-cut products was better kept; the growth of microorganisms after harvest was slowed down; and storage time was elongated. The key points for inner liner application were to select a suitable film to obtain an appropriate air atmosphere in the bag. Many new films have been invented recently, and low-density polyethylene (LDPE) film is one of them. It was suggested that the LDPE liner can effectively maintain the quality of fruits (Dwibedi et al., 2024). For instance, LDPE has been used as modified atmosphere packaging (MAP) to create a more suitable micro-atmosphere during the storage of ‘Bartlett’ pears. It was demonstrated that LDPE was good at preserving the quality and extending the shelf life of these fruits. Meanwhile, MAP could also significantly reduce the respiration rate and ethylene production of pears, thereby delaying the ripening process and maintaining their texture and flavor (Kumar et al., 2020). However, to the best of our knowledge, the effect of LDPE microporous liner on the storage quality of apples at low temperatures has never been reported.

In this study, the ‘Fuji’ apple was employed to reveal the effects of the LDPE liner on the fruit quality and superficial scald during storage and shelf life (SL). To achieve this, it was subjected to various LDPE inner liners (LDPE-10, 20, 30, 40). These treatments were used to assess their effect on physiological indexes, and the incidence of superficial scald and internal flesh browning of fruit. Additionally, the volatile compounds, phenolic compounds, and antioxidant enzyme activities were evaluated to gain insights into the influence of the LDPE liner on the overall apple quality. This study aimed to reduce the apple postharvest loss under the condition of cold storage by upgrading the inner liner at a lower cost than CA.

2. Materials and methods

2.1. Plant materials and treatments

The ‘Fuji’ apples were harvested 185 days after full bloom in the autumn of 2021 from Xunyi Apple Experimental Station in Xianyang City (108°08′ E, 34°57′ N), Shaanxi Province, China. After harvest, the 250 kg of healthy fruits in uniform size without damage were selected and transferred to the Postharvest Biology Laboratory of the Horticulture College in Northwest A&F University. After dissipating the field heat for 12 h, the fruits were stored at 0 ± 0.5 °C, 90 % RH, in an assembly cold storage which adopts automatic temperature control system.

These fruits were divided into 25 groups, with 10 kg of fruits in each group. Each group was randomly assigned across the five inner liner treatments (four LDPE liners and the control of PE liner). After being packed in the corresponding inner liner with 100 g dried vermiculite adsorbed with saturated KMnO4 in a breathable, non-woven bag, which was used for decreasing ethylene concentration (Shorter et al., 1992), these fruits were put into commercial plastic baskets (49 × 27 × 34 cm) and then transferred to the low-temperature storage facility for long-term storage. In detail, among five groups used for individual treatment, 3 groups were randomly selected to investigate the micro-environmental parameters in inner liners and the occurrence of superficial scald. The other fruit groups were used to determine the ethylene production rate and respiratory rate, and the samples were collected after 45, 90, 120, 135, 150, and 165 days of storage (DOS). The peel was carefully separated and ground in liquid nitrogen, and then the samples were stored at −80 °C until analyses of the compounds. After 165 DOS, 50 healthy fruits were selected to investigate the incidence of superficial scald and internal flesh browning during the SL at 20 °C for 7 days. Three biological replicates were prepared for all the above experiments.

The thickness, length and width of the LDPE inner liner were 0.025 mm, 70 cm, and 60 cm, respectively (Fig. 1). The four kinds of perforation liners with different numbers of perforations were applied here, including LDPE-10 (10 perforations), LDPE-20 (20 perforations), LDPE-30 (30 perforations), LDPE-40 (40 perforations) liner, and the diameter of all the microhole was 150 μm. The polyethylene (PE) inner liner with 8 holes (diameter = 1.5 cm) on its two sides was most widely used in the Shaanxi Province (As the first big province for apple production in China, Shaanxi Province produced 13.027 million tons in 2022 (https://www.stats.gov.cn/sj/ndsj/2023/indexch.htm) was selected as control. The thickness, length and width of the polyethylene (PE) inner liner were 0.02 mm, 70 cm, and 60 cm, respectively.

Fig. 1.

Fig. 1

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Polyethylene (PE) inner liner and low-density polyethylene (LDPE) inner liner. Note: PE: control group, polyethylene liner; 10: LDPE-10 group, 10 perforations low-density polyethylene liner group; 20: LDPE-20 group, 20 perforations low-density polyethylene liner group; 30: LDPE-30 group, 30 perforations low-density polyethylene liner group; 40: LDPE-40 group, 40 perforations low-density polyethylene liner group. The same as below.

2.2. Analysis the O2, CO2 and ethylene contents in the liner

A portable gas detector (SKY6000, Shenzhen Yuande Technology Co., LTD.) was used to determine the volume fraction of CO2 and O2 in the liner. The detector tube was put into the liner, the concentrations of CO2 and O2 were recorded, and 1 mL of gas was extracted from the liner and used to determine the ethylene concentration with the assistance of a gas chromatograph (FuliGC9790II, Zhejiang, China).

2.3. Determination of physiological indexes of apple fruits

The ethylene production was measured according to Li, Shi, et al. (2023). The CO2 infrared gas analyzer (Telaire 7001, Goleta, CA, USA) was used to test the fruit respiratory rate, recording the CO2 concentrations at 40 and 60 min, respectively. Three fruits were sealed in a 9.7 L pot for 1 h, and the headspace gas was collected to measure the fruit ethylene production using a gas chromatograph (FuliGC9790II, Zhejiang, China) coupled with a flame ionization detector.

The methods of fruit texture and coloration index were measured by a texture analyzer (TA-XT ExpressCl, Stable Micro Systems Co. Ltd., Surrey, United Kingdom) equipped with an SMS-P/2 cartridge and a CR-400 reflectance colorimeter (Minolta, Osaka, Japan), respectively, as described by Yan et al. (2020). The peels of 9 samples were measured and 45 L*a*b* values were obtained. The change in colour values was evaluated using the following equation (Beyaz et al., 2010):

∆E∗=∆L∗2+∆a∗2+∆b∗21/2.

The rate of weight loss was determined based on 9 fruits in each group. The contents of soluble solid and titrable acid were determined with a digital refractometer (Pocket PAL-1, Atago, Tokyo, Japan) and an apple acidity meter (GMK-835F, Korean) by taking the pulp on two opposite sides of the equator.

2.4. Investigation of superficial scald and internal flesh browning

Superficial scald is manifested as brown patches on the peel (Fig. 2A). The incidence was recorded visually and calculated as a percentage of the total number of fruits affected per group (Cáceres et al., 2016; Dias et al., 2022).

Fig. 2.

Fig. 2

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Superficial scald (A) and internal brown flesh (B) in ‘Fuji’ apple. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Incidence of superficial scald (%) = (Number of superficial scald fruits with brown spots on the peel/total number of fruits) × 100 %.

At the end of SL, the fruit was cut into two pieces longitudinally, and the flesh browning (Fig. 2B) each group's was investigated.

Incidence of browning (%) = (Number of fruit with brown flesh/total number of fruits) × 100 %.

2.5. Collection and analysis of volatile compounds

The volatile compounds were analyzed according to Li, Shi, et al. (2023) with minor modifications. The frozen peel (0.3 g) was transferred into a 40 mL vial containing 2 mL of saturated sodium chloride solution and 10 μL internal standard (cyclohexanone: Milli-Q water = 1: 12,000, v/v). After thorough mixing, the samples were incubated at 40 °C for 10 min, then a 2 cm SPME fiber (DVB/CAR/PDMS; Supelco, Inc., Bellefonte, PA, USA) was extracted for 30 min at an agitation speed of 250 rpm, then thermally desorbed at a GC–MS (7890BAgilent Technologies, Palo Alto, CA, USA) injector port (230 °C) for 3 min. The mass spectrum conditions were consistent with the method according to Li, Shi, et al. (2023) without modification.

2.6. Determination of MDA content, the production rate of O2−, H2O2 and GABA content

Malondialdehyde was determined by spectrophotometry (Draper et al., 1993). In brief, 0.5 g frozen peel tissue was centrifuged with 5 mL trichloroacetic acid (10 %, m/v) at 10000 rpm for 20 min. The supernatant added two milliliters of thiobarbituric acid: TCA: H2O (0.6 %: 10 %: 89.4 %). The mixture was heated at 100 °C for 15 min and then quickly cooled in an ice bath. After centrifugation at 10,000 rpm for 20 min, the supernatant was determined spectrophotometrically by measuring the absorbance at λ = 532 and 450 nm.

H2O2 and O2− were extracted by SA-2-G and H2O2–2-Y kits (Suzhou Keming Biological Company), respectively. The extraction and detection were done as described in the instructions of the corresponding kits. The contents of H2O2 and O2− were measured by UV–Vis spectrophotometry at λ = 405 and 530 nm, respectively.

The γ-aminobutyric acid (GABA) content was determined based on the Berthelot reaction. After mixing 0.5 g frozen tissue with 2.5 mL lanthanum chloride solution (0.05 mol/L) for 30 s, the supernatant was centrifugated at 10,000 rpm for 20 min. After that, the supernatant was added to a mixed solution consisting of 400 μL potassium hydroxide solution (1 mol/L), 100 μL phosphate buffer (0.1 mol/L, pH 9), 800 μL phenol (6 %, m/v), and 1 mL sodium hypochlorite(5 %, v/v). After centrifugation at 10,000 rpm for 20 min, the supernatant was measured at λ = 645 nm (Yu et al., 2023).

2.7. Determination of GSH content, GR activity and ASA content

The glutathione (GSH) content was detected according to the modified method of Yao et al. (2021). Frozen peel tissue (0.5 g) was centrifuged with 2.5 mL trichloroacetic acid solution (5 %, w/v) at 10,000 rpm for 20 min. The supernatant was added to 0.5 mL 5,5’-Dithiobis-(2-nitrobenzoic acid) (4 mmol/L) and 0.5 mL sodium phosphate solution (0.1 mol/L, pH 6.8) at 412 nm to determine absorbance.

The glutathione reductase activity (GR) was detected according to the modified method of Yao et al. (2021). Frozen peel tissue (0.5 g) was centrifuged with 1 mL sodium phosphate solution (0.1 mol/L, pH 7.5) at 10,000 rpm for 20 min. The supernatant was taken and added to 2.7 mL sodium phosphate solution (0.1 mol/L, pH 7.5), 100 μL GSSG solution (5 mmol/L) and 40 μL NADPH (4 mmol/L) solution at 340 nm to determine absorbance. The results of GR activity were expressed as U, which meant the absorbance change per minute.

The ascorbic acid (ASA) content was detected according to the modified method of Yao et al. (2021). Frozen peel tissue (0.3 g) was centrifugated with 3 mL trichloroacetic acid solution (0.1 mol/L, pH 7.5) at 10,000 rpm for 20 min. The supernatant was added to 1 mL trichloroacetic acid solution (5 %, m/v), 1 mL ethanol solution, 1 mL BP-ethanol solution (5 %, m/v) and 0.5 mL FeCl3-ethanol solution (5 g/L). After mixing evenly, the absorbance of the mixture was measured at λ = 534 nm.

2.8. Determination of phenolic compounds

Frozen peel tissue (0.1 g) and 1 mL extraction liquid (methanol: water: formic acid = 25: 24: 1, v/v/v) were mixed in the 2 mL centrifuge tube, which was subjected to ultrasound-assisted extraction (SB25–12 DTD, Ningbo Xinzhi Biotechnology Co., LTD.) for 20 min (440 HZ, 100 W). Then, the tube was placed in a shaker under a shaking speed of 150 rpm for 20 min at 20 °C. After centrifugation at 4 °C under 10,000 rpm for 15 min, 1 mL of the supernatant was filtered with a 0.22 μm organic nylon filter, and 10 μL of the extracted solution was injected into Liquid chromatography-quadrupole ion trap-mass spectrometry (TQ-8050NX, Scientific Export, Carlsbad, USA).

With an ultraviolet detector (Agilent Technology, Palo Alto, CA, USA) and an AQ-C18 column (150 mm × 4.6 mm, 5 μm, Shimadzu InertSustain, Tokyo, Japan). The flow rate was 0.7 mL/min, and the column temperature chamber was 25 °C. The mass spectra conditions were as follows: ionization voltage-4.5 kV, heater gas temperature of ion source TurboIonSpray probe 600 °C.

2.9. Evaluation of antioxidant enzyme activities

The activity of catalase (CAT), superoxide dismutase (SOD), peroxidase (POD) and polyphenol oxidase (PPO) was measured by UV–Vis spectrophotometry according to the modified method described by Li, Xiong, et al. (2023).

For detecting CAT activity, 0.5 g frozen peel tissue was centrifuged with 1 mL mixed solution at 10,000 rpm for 20 min, and the mixed solution consisted of 0.144 g dithiothreitol, 10 g polyvinylpyrrolidone and 200 mL of sodium phosphate solution (0.1 mol/L, pH 5.5). The absorbance at 240 nm was measured by adding 100 μL supernatant to 2.9 mL H2O2 solution (20 mmol/L).

For detecting SOD activity, 0.5 g frozen peel tissue was centrifuged with 1 mL mixed solution at 10,000 rpm for 20 min, and the mixed solution was composed of 77 mg dithiothreitol, 5 g polyvinylpyrrolidone and 100 mL sodium phosphate solution (0.1 mol/L, pH 7.8). Collected the supernatant and added the 3 mL reaction solution, which was made of 1.7 mL sodium phosphate solution (0.1 mol/L, pH 7.8), 300 μL methionine(130 mmol/L), 300 μL nitroblue tetrazolium chloride (750 μmol/L), 300 μL EDTA-Na2 (100 μmol/L), and 300 μL riboflavin (20 μmol/L). Absorbance was recorded at λ = 560 nm.

POD activity was detected using the modified method of Li, Xiong, et al. (2023). 0.5 g frozen peel tissue was centrifuged with 1 mL mixed solution at 10,000 rpm for 20 min, and the mixed solution was composed of 340 mg macrogol 6000, 4 g polyvinylpyrrolidone, 1 mL Triton X-100 and 100 mL sodium phosphate solution (0.1 mol/L, pH 5.5). The supernatant was added to 3 mL guaiacol solution (25 mmol/L) and 200 μL H2O2 solution (0.5 mol/L), and the absorbance at λ = 470 nm was measured.

For detecting PPO activity, 0.5 g frozen peel tissue was centrifuged with 1 mL mixed solution at 10,000 rpm for 20 min, and the mixed solution was composed of 340 mg macrogol 6000, 4 g polyvinylpyrrolidone, 1 L Triton X-100 and 100 mL sodium phosphate solution (0.1 mol/L, pH 5.5). The supernatant was added to 4 mL sodium phosphate solution (50 mmol/L, pH 5.5) and 1 mL catechol solution (0.5 mol/L), then the absorbance at λ = 420 nm was measured.

One U of CAT, SOD, POD and PPO activity was denoted as the enzymatic amount that causes absorbance change per minute. The results of CAT, SOD, POD and PPO were expressed as U, too.

2.10. Statistical analysis

SAS software (SAS Institute, Cary, NC, USA) was used to conduct a one-way analysis of variance (ANOVA). TBtools (https://github.com/CJ-Chen/TBtools/releases.) was used for data normalization heat mapping. The correlation analysis network was established by Origin 2023 (v9.8.0.200, OriginLab Inc., Hampton, MA) based on the data set of ethylene and volatile compounds with the method of Pearson correlations. The data were analyzed using ANOVA (P < 0.05) or independent Student's t-tests (P < 0.05; P < 0.01) in IBM SPSS Statistics version 16.0. The data were presented as the biological triplicates' mean ± standard deviation (SD) of biological triplicates.

3. Results

3.1. Changes of O2, CO2 and ethylene contents in the liners

As shown in Table S1, the gas composition in the packages varied dynamically during low-temperature storage. However, the volume fraction of O2 in all five liners was relatively stable (Fig. 3A). Furthermore, the volume fraction of O2 in all four LDPE inner liners was significantly lower than the control. Detailedly, the volume fraction of O2 in the LDPE-10 ranged from 15.96 % (10 d) to 17.76 % (50 d) in the initial 50 DOS, and then it showed little change, which was always significantly lower than the other groups. The O2 volume fraction values of LDPE-20, LDPE-30, and LDPE-40 were similar and relatively stable (about 19.8 %). As for the control group, the O2 volume fraction remained at 20.9 % in the initial period of storage (10 d - 40 d). Then, the O2 volume fraction slightly dropped to 20.3 % after 50 days and remained stable until the end of storage.

Fig. 3.

Fig. 3

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Effects of different liners in the concentrations of O2 (A), CO2 (B) and ethylene (C).

Different from the changing pattern of O2, the CO2 volume fractions in the LDPE inner liners were all significantly higher than that of the control with a stable level of 0.1 % (Fig. 3B). For 4 LDPE groups, the CO2 volume fractions in LDPE-10 were generally higher than the others at most times, but the CO2 concentration in the LDPE-10 fluctuated widely during storage. It reached the peak value of 1.19 % after 10 DOS and decreased to 0.59 % after 50 DOS when no significant difference was detected among the four treatments. However, the volume fraction of CO2 in LDPE-10 was significantly higher again than in LDPE-30 and LDPE-40 during the storage period of 60–110 days and 150–170 days. The values of LDPE-30 and LDPE-40 were similar, fluctuating between 0.37 % and 0.76 %, while the range of the values of LDPE-20 ranged from 0.54 to 0.90 %.

During the initial 110 DOS, the concentrations of ethylene in all liners were generally low (Fig. 3C). Especially the ethylene level in the LDPE-10 ranged from 0.08 to 0.89 nL/L which was significantly lower than that of other groups (0.15–3.84 nL/L). During 120–170 DOS, ethylene concentration in the LDPE-10 increased initially and then decreased, while ethylene concentration in LDPE-20, LDPE-30 and LDPE-40 increased continuously. At 170 DOS, the ethylene concentration in LDPE-10 was 1.79 nL/L which was significantly lower than that of control (3.03 nL/L), while that of LDPE-20, LDPE-30 and LDPE-40 were 5.55 nL/L, 12.57 nL/L, and 10.27 nL/L, respectively. Compared with PE, the ethylene production of the fruits in LDPE-10 was significantly reduced by 60.74 %.

In summary, the LDPE liners effectively improved the constitutions of gas in their inner microenvironment with lower levels of O2, ethylene, and higher levels of CO2 for most of the storage time. The LDPE-10 liner presented the most remarkable effects.

3.2. Effects of liners on the qualities of fruits

Ethylene production and respiration rate are important for evaluating the physiological state of postharvest fruits (Table S2. A and S2. B). With the elongation of storage, the ethylene release rate of each group increased gradually (Fig. 4A), but the value of LDPE-10 was consistently lower than that of other groups. After 165 DOS, the ethylene production rate of the LDPE-10 reached the maximum value of 8.74 μL/kg/h, while maximum values of the control and LDPE-20 presented at 165 DOS with 14.39 μL/kg/h and 13.19 μL/kg/h, respectively. The ethylene release rate of LDPE-30 and LDPE-40 peaked at 135 DOS with 16.20 μL/kg/h and 14.2 μL/kg/h, respectively. During SL, the ethylene release rates were significantly alleviated by LDPE treatments. On the 7th day at room temperature, the ethylene release rate of the LDPE-10 was 21.13 μL/kg/h, which was only 53.66 % of the control.

Fig. 4.

Fig. 4

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Effects of different liners on the ethylene release rate (A), respiration rate (B) and weight loss rate (C) of fruits.

During low-temperature storage, the respiration rates of the LDPE-10, LDPE-20 (except at 90 DOS), LDPE-30 and LDPE-40 at 45, 90 DOS, and LDPE-30 at 120 DOS were significantly higher than the control (Fig. 4B). During SL time, the respiration rates of all fruits increased sharply. However, no significant difference was detected among the five groups at the 7th SL.

During low-temperature storage, the weight loss rate of fruits increased gradually (Table S2. C). However, the 4 LDPE groups effectively reduced the weight loss rates (Fig. 4C). Of them, the LDPE-10 and LDPE-20 had the best performances, and their weight loss rates were 0.41 % at the end of storage, significantly lower than that of LDPE-30 (0.66 %), LDPE-40 (0.65 %), and PE (0.99 %).

Additionally, there were no significant differences in pulp firmness, titratable acid, soluble solids, ∆E* among all the five groups of fruits, including the PE group and the four LDPE groups (Table S3). At the same time, the fruit texture values (peel firmness, pulp crunchiness and pulp firmness) were relatively stable during storage, and there was no significant difference among the 5 above groups (Table S4).

3.3. Effects of liners on superficial scald and internal flesh browning of fruits

The superficial scald began to appear on 150 DOS (Fig. S1), and the incidence of fruits in the PE, LDPE-20, and LDPE-40 were 8.49 %, 7.79 %, and 6.14 %, respectively. Notably, the incidence of fruits in LDPE-10 and LDPE-30 was only 1.78 % and 2.63 %, respectively. After that, the incidence of superficial scald increased rapidly, and the incidence of fruits in the PE, LDPE-20, and LDPE-30 rose to 33.31 %, 25.91 %, and 31.53 % on the 165 DOS, respectively. Meanwhile, LDPE-10 and LDPE-40 significantly alleviated the incidence of superficial scald (16.86 % and 18.42 % on 165 DOS, respectively).

Superficial scald could be induced by long-term cold storage (Lurie & Watkins, 2012). To determine the effect of liners on the incidence of superficial scald during SL after 165 DOS at 25 °C, the healthy fruits were employed (Fig. 5). At 6 h of SL, superficial scald did not appear in LDPE-10. At this moment, the incidences of LDPE-20, 30 and 40 were 6.25 %, 4.44 % and 7.41 %, respectively, while 13.33 % of the fruits in the PE group showed the symptoms of superficial scald. Superficial scald did not appear in the LDPE-10 until 12 h of SL. At this time, the incidences in the LDPE-10, LDPE-20, LDPE-30 and LDPE-40 were 3.33 %, 12.5 %, 13.33 % and 20.37 %, respectively, but the incidence in the PE group rose rapidly to 31.11 %. The incidence increased rapidly from 12 h to 72 h. At 72 h of SL, the incidence of each group was 51.11 % (PE), 33.33 % (LDPE-10), 47.92 % (LDPE-20), 60 % (LDPE-30), and 42.59 % (LDPE-40), respectively. By the end of SL (168 h), the incidence in LDPE-10 was the lowest (43.33 %), while LDPE-20, LDPE-30, LDPE-40, and PE were as high as 56.25 %, 66.67 %, 51.85 %, and 51.11 %, respectively. In summary, the incidence of superficial scald in the LDPE-10 was always lower than in the other groups, suggesting that the LDPE-10 liner could significantly reduce the incidence of superficial scald.

Fig. 5.

Fig. 5

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Effects different liners on the incidence of superficial scald of fruit during shelf life.

At the end of SL, the internal flesh browning rates of the LDPE-10 and LDPE-20 were 7.41 % and 7.14 %, respectively (Fig. S2). The LDPE-30 was the highest, reaching 17.86 %. The internal flesh browning rate of the LDPE-40 was 8.82 %, close to that of the PE (9.09 %). The results indicated that fruits' internal flesh browning rate could be reduced by LDPE-10 and LDPE-20 liners.

In summary, the LDPE-10 inner liner was the most suitable for ‘Fuji’ long-term cold storage. It most effectively reduced the O2 content of the microenvironment in the liner, and its CO2 content was higher than that of the other groups. The ethylene release rate of the fruits in LDPE-10 was significantly reduced, and its weight loss rate was reduced to 0.41 %, which was 0.99 % in the PE liner group, without affecting the titratable acid, soluble solids, and fruit texture. It also effectively reduced the internal flesh browning rate and the incidence of superficial scald. Therefore, the mechanism of the LDPE-10 inner liner for keeping the postharvest qualities in the ‘Fuji’ apple was studied, and the fruits packed in LDPE-10 and PE inner liner were employed in the following experiments.

3.4. Effects of LDPE-10 liner on MDA content, O2−production rate, H2O2 and GABA content in fruits

It is generally believed that the content of MDA reflects the degree of oxidative stress in plants (Liu et al., 2021). Generally, MDA content increased with the elongation of storage, but its content of the LDPE-10 was consistently lower than that of PE (Fig. 6A). Especially at 45 and 150 DOS, the MDA content of the LDPE-10 was 78.53 % and 83.76 % of PE group, respectively.

Fig. 6.

Fig. 6

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Effects of different liners on the MDA content (A), O2− production rate (B) and H2O2 content (C) of fruit. Note: *indicates the level of significant difference: P <0.05.

When fruits were subjected to oxidative stress, many reactive oxygen species were produced and accumulated (Gill & Tuteja, 2010). In the study, the production rate of O2− generally increased during storage, while LDPE-10 significantly decreased the production rate of O2− (Fig. 6B). On 150 DOS, the production rate of O2− in the LDPE-10 was 81.96 % lower than of the PE. Additionally, the content of H2O2 also increased during storage. However, its contents in fruits of the LDPE-10 and PE were similar without significant differences (Fig. 6C).

In summary, the peroxidation could be somewhat inhibitted by the LDPE-10 inner liner, producing less MDA and O2− during long-term storage.

3.5. Effects of LDPE-10 on the contents of GSH, ASA, and GABA and the activity of GR in fruits

The GSH content of fruits in both LDPE-10 and PE reached their peaks on 45 DOS, with the values of 137.93 μmol/g and 128.09 μmol/g, respectively (Fig. 7A). After that, the GSH content decreased rapidly in the fruits packed with PE liner, while the decline rate of GSH content was delayed by LDPE-10 liner. Correspondingly, the GSH content of the LDPE-10 was 1.34 and 1.33 times that of the PE group at 120 DOS and 135 DOS, respectively. Then, GSH content sharply decreased, and no significant difference in GSH content was detected between the two groups on 150 DOS.

Fig. 7.

Fig. 7

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Effects of different liners on the contents of GSH (A), ASA (B), and GABA (C), and GR activity (D) of fruit. Note: *indicates the level of significant difference: P <0.05.

The ASA content showed a downward trend during storage (Fig. 7B), and the ASA content of the LDPE-10 was significantly higher than that of the PE group from 90 DOS - 150 DOS, which showed that the LDPE-10 liner significantly inhibited the degradation of ASA.

In recent years, the roles of GABA in plants have received widespread attention as an inhibitor of the formation of advanced lipoxidation end-products (Song et al., 2010). The contents of GABA in fruits of LDPE-10 and PE decreased firstly and then showed an overall upward trend (Fig. 7C). However, the minimum contents of GABA in LDPE-10 and PE appeared at 45 DOS and 90 DOS, and their values were 24.11 μg/g and 17.12 μg/g, respectively. Importantly, the GABA content of LDPE-10 was higher than PE's from 90 to 150 DOS. In particular, the GABA content of LDPE was 2.49 times that of PE at 150 DOS.

Similar to the changing GSH content trends, GR activity increased first, followed by a decrease (Fig. 7D). GR activity in LDPE-10 and PE reached the highest values of 11.09 U/g and 9.59 U/g at 45 and 90 DOS, respectively. During storage, GR activity in LDPE-10 was significantly higher than PE at 45 DOS but lower than PE at 90 and 150 DOS.

In summary, GSH, ASA, and GABA contents in LDPE-10 were generally higher than in PE.

3.6. Effects of LDPE-10 liner on the constitutions of volatile compounds in fruits

Ninety-one volatile compounds were detected (Table S5), including 10 alcohols, 10 aldehydes, 56 esters, 3 ketones, 6 terpenes, 4 furans and 2 benzenoids. Of these volatiles, the ester volatile content was the maximum, accounting for more than 68 % of the total volatile content. Correspondingly, the changing trends of total volatiles (Fig. 8A) and esters (Fig. 8D) were very similar. Besides esters, alcohols and aldehydes were the second and third highest proportion of volatiles, up 3.83 %-16.07 % and 1.05 % - 10.82 %, respectively. Furthermore, the contents of total volatiles (Fig. 10A), esters (Fig. 10B), and alcohols (Fig. 8C) showed general upward trends. In contrast, the aldehyde content showed a decreasing trend after harvest (Fig. 8D). Moreover, the content of these compounds in LDPE-10 was consistently lower than that in PE except aldehyde contents.

Fig. 8.

Fig. 8

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Effects of different liners on the volatile compounds content in fruits. Note: *indicates the level of significant difference: P <0.05.

Fig. 10.

Fig. 10

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Effect of different liners on the content of phenolic substances in fruits.

The total volatiles and esters increased in content from 0 DOS-90 DOS, decreased slightly at 120 DOS, and then continued to increase until 150 DOS. At 150 DOS, the total volatile contents in LDPE-10 and PE were 318,511.57 μg/kg and 552,940.90 μg/kg, respectively. Ester contents in LDPE-10 and PE were 259,023.66 μg/kg and 466,147.66 μg/kg at 150 DOS, respectively.

The content of alcohol in fruits of LDPE-10 and PE showed an overall upward trend, and the alcohol content in the PE group was also higher than that in LDPE-10 from 45 to 150 DOS. The maximum values of LDPE-10 and PE were 18,629.76 μg/kg and 29,685.20 μg/kg, respectively, and they showed on 150 DOS (Fig. 8C).

The content of volatile aldehydes showed a downward trend. Interestingly, the content of aldehydes in LDPE-10 was significantly higher than that in the PE from 45 to 135 DOS (Fig. 8D). The content of aldehydes in the LDPE-10 and PE reached the peak at 45 DOS (10,864.31 μg/kg and 9587.11 μg/kg, respectively). Thereafter, the aldehyde content in LDPE-10 fluctuated, while it generally decreased in PE. On 120 DOS, the difference between LDPE-10 and PE was the largest. At this time, the aldehyde content was 9762.44 μg/kg in LDPE-10, which is 2.05 times of PE. After 120 DOS, aldehyde content decreased in LDPE-10, while it remained stable in PE. Finally, the contents of aldehydes in LDPE-10 and PE were similar, and the values were 6151.68 μg/kg and 5799.48 μg/kg, respectively.

Pearson correlation analysis showed a high correlation between the ethylene release rate and the contents of various volatiles (Fig. 9). Detailedly, the ethylene release rate was positively correlated with total volatiles, ester content and alcohol content (P < 0.001), but negatively correlated with aldehyde content (P < 0.05). It was indicated that the LDPE-10 liner might indirectly affect volatile production by reducing the ethylene release rate.

Fig. 9.

Fig. 9

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Heatmap of correlation between ethylene release rate and volatile compounds contents in fruits during storage. Note: *indicates the level of significant difference:P <0.05, **indicates the level of significant difference: P <0.01, ***indicates the level of significant difference:P <0.001.

3.7. Effect of LDPE-10 on phenols in fruits

In this study, 29 phenols were detected (Table S6). According to the changing patterns during storage, these phenolic compounds were clustered into four groups (Fig. 10).

Albiflorin, gallic acid and proanthocyanidin B1 belonging to Group A were accumulated abundantly at harvest, and the content of albiflorin was significantly higher in LDPE-10 than those in PE at the beginning stage of storage (45 and 90 DOS).

Methyl gallate, luteolin, astragalin and benzoic acid (BA) were included in Group B. Of these compounds, methyl gallate and luteolin remained stable in content during storage. At the same time, the contents of astragalin and BA in fruits packed with LDPE-10 were significantly different from PE at 45 DOS. In detail, astragalin accumulated more in PE, but BA accumulated more in LDPE-10. More importantly, the content of BA in the LDPE-10 was always higher than that in PE during storage, and the differences were significant from 90 to 150 DOS.

The contents of the compounds in Group C (PGG, protocatechuic acid, ferulic acid, chlorogenic acid, quercetin, rutin, prunin, luteoloside, naringin, isorhamnetin-3-O-neohesperidoside) gradually increased with the elongation of storage. At 150 DOS, the contents of isorhamnetin-3-O-neohesperidoside and naringin in PE were significantly higher than in PE. The rutin content of LDPE-10 was significantly higher than that of the PE group at 120 DOS. The prunin content in PE was higher at 45 DOS and 120 DOS, but its content of LDPE-10 was significantly higher at 90 DOS and 135 DOS.

Twelve phenols were clustered in Group D, including hyperin, myricetin, phloridzin, isorhamnetin-3-O-glucoside, proanthocyanidin B2, catechin, dicumarol, isorhamnetin, ethyl gallate, kaempferol, and paeonol. These compounds were extremely low in fruits packed with PE at 45 DOS, but accumulated abundantly in LDPE-10 at 150 DOS. Furthermore, the contents of catechin, myricetin, isorhamnetin, and dicumarol in PE were significantly lower than those in LDPE-10 at 45 DOS. The content of paeonol in LDPE was also significantly higher than that in the PE group at 150 DOS. Only ethyl gallate in LDPE-10 was significantly higher than PE at 45 DOS. Overall, the phenols detected in the study were accumulated more in the LDPE-10 in general.

3.8. Effects of LDPE-10 on the activities of PPO, CAT, POD and SOD in fruits

In brief, PPO activity increased at first and then decreased during storage. However, PPO activity of fruits packed in the LDPE-10 was always significantly lower than that of PE during 45–150 DOS (Fig. 11A). The peak value of the LDPE-10 was 10.80 U/g at 90 DOS which was 45.49 % of the maximum value (23.74 U/g) of fruits packed in PE at 135 DOS. At 150 DOS, the PPO activity of the LDPE-10 was 70.25 % that of the PE. In summary, the LDPE-10 inner liner inhibited the PPO activity of fruits.

Fig. 11.

Fig. 11

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Effects of different liners on the activities of PPO (A), CAT (B), POD (C) and SOD (D) in fruits. Note: *indicates the level of significant difference: P <0.05.

During 45–120 DOS, CAT activities in both LDPE-10 and PE kept stable, and there was no significant difference between LDPE-10 and PE (Fig. 11B). At 135 DOS, the activity of CAT in PE reached the lowest value (10.99 U/g), while that in LDPE-10 boosted to 46.49 U/g, which was 4.29 times that in the PE. Thereafter, the CAT activity of PE increased sharply, but the activity of the LDPE-10 remained stable. However, the CAT activity was still 1.36 times higher than PE at 150 DOS.

During low-temperature storage, the activity of POD had been increasing (Fig. 11C). From 45 DOS to 135 DOS, there was no significant difference in POD activities between LDPE-10 and PE. At 150 DOS, the POD activity of the LDPE-10 was 13.24 U/g, which was 1.54 times that of the PE.

The SOD activity of fruits increased at first. Then it decreased during low-temperature storage (Fig. 11D). The peak values of PE (5.40 U/g) and LDPE-10 (5.29 U/g) appeared on 90 and 120 DOS, respectively, and no significant difference was detected between them.

To sum up, the LDPE-10 inner liner decreased the activity of PPO, whereas LDPE-10 increased the activities of CAT and POD.

4. Discussion

Previous studies have shown that suitable liners can effectively prolong the storage time and improve the postharvest qualities of horticultural products (Cai et al., 2022; Li, Xiong, et al., 2023; Qu et al., 2022). In this study, the effects of four LDPE inner liners on keeping the postharvest qualities of the ‘Fuji’ apple were compared, and it was shown that they significantly reduced the weight loss rates of fruits during storage. At the end of storage, the weight loss rates of fruits packed in LDPE-10 and LDPE-20 were only 41.41 % of that in PE.

In addition, we found that LDPE increased the respiration rate of fruits during storage, but the contents of SSC and TA were not affected. However, it was reported that polyolefin liner decreased the SSC level of Lycium barbarum fruits during low-temperature storage (Ozturk et al., 2019), but the TA content in cherry packaged with 3-layer films of polyethylene/polyamide/polyethylene was kept at a high level, and the SSC content was not affected (Khorshidi et al., 2011). These results showed that different inner liners have different effects on different horticultural products.

Superficial scald and internal flesh browning were closely related to membrane integrity (Sidhu et al., 2023). When cells were subjected to oxidative stress, reactive oxygen species were overproduced, including superoxide radicals (O2·−), hydroxyl radical (OH·), per hydroxy radical (HO2·), alkoxy radicals (RO·), hydrogen peroxide (H2O2), and singlet oxygen (1O2) (Gill & Tuteja, 2010), these reactive oxygen species could cause lipid peroxidation and damage cell membranes (Sidhu et al., 2023). As one of the major products of membrane lipid peroxidation, MDA would further aggravate the specific damage to the cell membrane, resulting in superficial scald or internal flesh browning (Feng et al., 2018). In this study, the LDPE-10 liner significantly reduced the occurrence of fruits' superficial scald and internal flesh browning, and the MDA content and O2− production rate in LDPE-10 were significantly lower than in PE.

As natural antioxidants, phenols can reduce oxidative damage (Akbari et al., 2022). Previous studies showed that superficial scald was related to decreased phenol content (Ju et al., 1996). In this study, the content of phenols in fruits packed in LDPE-10 liner was significantly higher than that in PE, especially BA. Of these phenols, the content of BA in LDPE-10 was as high as 0.51 μg/g, which was 2.68 times that in PE. Perhaps the most studied plant, BA is considered to be one of the key endogenous signals involved in defense response activation, due to its strong antibacterial activity and high stability, it was reported that BA was widely used as a food preservative (Widhalm & Dudareva, 2015), and the effect of BA on apple preservation is worthy of further study. Phenolic compounds could be oxidated to brown quinone under PPO, resulting in browning (Piretti et al., 1994). Here, both PPO activity and the incidence of superficial scald were reduced by LDPE-10.

Compared with PE, the LDPE-10 inner liner also increased the GABA and GSH contents in fruits, delaying the decrease of ASA. The ASA-GSH cycle is also one of the important antioxidant systems in plants, and the ASA-GSH cycle is an effective way to remove H2O2 (Zeng et al., 2008). Under the catalysis of GR, GSSG was reduced to GSH. Increasing GR activity could improve the stress resistance and maintain plant cell membrane stability (Qin et al., 2018). In addition, GABA could enhance the ASA-GSH loop (Yang et al., 2017) and increase the content of ASA (Zhu et al., 2022). GABA was ubiquitous in organisms, and endogenous GABA accumulation was another way to enhance biotic and abiotic stress resistance (Ramos-Ruiz et al., 2019). Meanwhile, GABA treatment could enhance the antioxidant capacity of plants to abiotic stress as well also enhance plants' antioxidant capacity to abiotic stress (Li et al., 2014). As an effective free radical quenchant, POD could reduce the epidermis damage caused by biotic and abiotic stress (Zhao et al., 2020). Both SOD and CAT were able to improve resistance to various abiotic stresses by scavenging in cells (Gao et al., 2015). The PPO function is also indispensable in the context of stress. It has been shown that browning caused by phenol oxidation can be reduced by effectively inhibiting PPO activity (Tinello and Lante, 2018). In this study, LDPE-10 inner liner significantly increased the activity of CAT and POD in ‘Fuji’ apple, and the activity of CAT and POD reached the maximum at 150 DOS, 46.97 U/g and 13.24 U/g, respectively. In brief, POD and CAT might play an important role in the enzymatic anti-oxidation of apples after harvest. Higher anti-oxidase activities, including POD and CAT, and lower PPO activity can catalyze the oxidation of phenols to brown quinones.

CAT and POD are critical antioxidant enzymes in apple fruits, which scavenge ROS such as H₂O₂ and O₂−, thereby mitigating cellular oxidative damage. When the activities of CAT and POD are enhanced, the oxidative stress level within the fruit is reduced, preserving cellular structure and function. This process retards senescence and extends the postharvest shelf life of apples (Sharma & Nath, 2016).

PPO is one of the key enzymes responsible for apple browning. Decreasing the activity of PPO can mitigate the occurrence of browning reactions, prevent cell structure damage and nutrient loss caused by browning. This helps maintain the integrity and freshness of apple fruits and extends their shelf life (Hamdan et al., 2022).

The volatile profile of postharvest apples was greatly affected by the storage environment and period (Echever et al., 2004). Many researchers studied the effects of low temperature and CA storage on the volatile constitutions of apple fruits (Su et al., 2019), and it was shown that CA storage reduced the production of total volatile contents (Mattheis et al., 2005). Correspondingly, lower O2 concentration and higher CO2 concentration inhibited the production of volatile flavor substances in apples (Chen et al., 2021), including volatile esters, such as butyl acetate and hexyl acetate (Mattheis et al., 2005), which agreed with the decrease of the contents of total volatiles and esters in fruits packed with LDPE-10 in which lower O2 and higher CO2 were also formed. It was worth pointing out that the volatile production was strongly correlated with ethylene release (Defilippi et al., 2005), and LDPE-10 significantly inhibited the ethylene release in this study. Therefore, the decrease of volatile production in LDPE-10 might be the consequence of a lower ethylene release rate, which was caused by lower O2 and higher CO2 in LDPE-10.

5. Conclusion

The LDPE-10 inner liner formed microenvironments with lower concentrations of O2 and higher concentrations of CO2, significantly reducing the ethylene release rate without affecting fruit quality in ‘Fuji’ apple. Compared with fruit packed in PE perforated liner, the fruit weight loss rate was reduced to 41.41 % by LDPE-10. Notably, the incidence of superficial scald was reduced during cold storage, and the initial occurrence time of superficial scald was delayed during SL by LDPE-10. Meanwhile, LDPE-10 also reduced the rate of internal flesh browning at the end of SL. The lower occurrences of physiological disorders might be caused by the stronger substantial antioxidant capacity of fruits packed in LDPE-10. Lower contents of O2− reflected a better antioxidant capacity of fruit treated by LDPE-10 and MDA which resulted from more antioxidant secondary metabolites, including phenols, GABA, GSH and ASA, plus with higher activities of anti-oxidases, including POD and CAT, and together with lower PPO activity. (Fig. 12).

Fig. 12.

Fig. 12

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Diagram of the insights into the effect of 10 perforations low-density polyethylene liner on keeping the postharvest quality in ‘Fuji’ apple. Blue means down, and red means up. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Based on the findings of this study, under the condition of unchanging the cold storage, the fruits packed in liner had more excellent substantial antioxidant capacity and a lower incidence of superficial scald and internal flesh browning without affecting fruit qualities. In the future, we can conduct qRT-PCR analysis of target genes associated with oxidative stress and antioxidant activity. This would add a molecular dimension to the findings and make the study more robust. In addition, the effect of BA on apple preservation is worthy of further study. These points could contribute to the development of more effective post-harvest technologies for apple preservation.

CRediT authorship contribution statement

Cen Li: Writing – original draft. Jia-Qi Li: Writing – review & editing, Conceptualization. Qing Yue: Writing – review & editing. Xiao-Ling Ding: Writing – review & editing. Rui Li: Writing – review & editing. Xiao-Lin Ren: Writing – review & editing. Cui-Hua Liu: Writing – review & editing.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Ina Terstiege reports a relationship with AstraZeneca that includes: employment and equity or stocks. Anna Aagaard reports a relationship with AstraZeneca that includes: employment and equity or stocks. Kristina Berggren reports a relationship with AstraZeneca that includes: employment and equity or stocks. James Bird reports a relationship with AstraZeneca that includes: employment and equity or stocks. Iain A. Cumming reports a relationship with AstraZeneca that includes: employment and equity or stocks. Lotta Hidestal reports a relationship with AstraZeneca that includes: employment and equity or stocks. Petra Johannesson reports a relationship with AstraZeneca that includes: employment and equity or stocks. Pernilla Korsgren reports a relationship with AstraZeneca that includes: employment and equity or stocks. Karl-Johan Leuchowius reports a relationship with AstraZeneca that includes: employment and equity or stocks. Sara Lundqvist reports a relationship with AstraZeneca that includes: employment and equity or stocks. James S. Scott reports a relationship with AstraZeneca that includes: employment and equity or stocks. Yafeng Xue reports a relationship with AstraZeneca that includes: employment and equity or stocks. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2023YFD2301000), the Key S&T Special Projects of Shaanxi Province, China (2020zdzx03-05-01), the China Agricultural Research System for Apple (CARS-27).

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2025.102951.

Contributor Information

Xiao-Lin Ren, Email: renxl@nwsuaf.edu.cn.

Cui-Hua Liu, Email: liuch@nwafu.edu.cn.

Appendix A. Supplementary data

Supplementary material

mmc1.docx (243KB, docx)

Data availability

Data will be made available on request.

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