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. 2026 Oct 15;40:104642. doi: 10.1016/j.fochx.2026.104642

Quality deterioration characteristics and metabolomic analysis of nano-packaged Venus Golden apples under dynamic temperature fluctuations

Xiaodong Zheng a,b,⁎, Shaoxiang Pan a,b, Zhen Wang a,b, Qi Guo a,b, Wenyu Du a,b, Yixuan Fan a,b, Xuemei Liu a,b, Xinhuan Yan a,b, Lei Qin c,⁎⁎, Ye Song a,b,⁎
PMCID: PMC13697270  PMID: 42935827

Abstract

Temperature fluctuations are a major cause of apple quality deterioration during distribution, but their effects on nano-packaging performance and fruit metabolism remain unclear. Long-term cold-stored Venus Golden apples (VG) were evaluated using quality, physiological, and untargeted metabolomic analyses. Different packaging treatments were compared, and NPF-packaged fruit was stored at constant 10 °C, constant 20 °C, or fluctuating 10–20 °C. NPF effectively delayed changes in color, texture, sugars, organic acids, and antioxidant capacity. However, temperature fluctuations weakened its preservation efficacy and intensified browning, softening, and oxidative damage. Metabolomic analysis revealed disturbances in alanine, aspartate and glutamate metabolism, galactose metabolism, and arginine biosynthesis, accompanied by accelerated depletion of sugars, organic acids, and amino acids. Nine candidate biomarkers of quality deterioration, including L-glutamic acid, γ-aminobutyric acid, and D-sorbitol, were identified. These findings provide insights for cold-chain temperature management, nano-packaging optimization, and metabolite-based quality monitoring.

Keywords: Venus Golden apples (VG), Nano-packaging, Temperature fluctuation, Quality deterioration, Metabolomics

1. Introduction

In the global supply chain system, postharvest apples that enter the distribution chain after prolonged cold storage face a significant risk of quality deterioration (Hasan et al., 2024). Although modern cold chain logistics systems are continuously optimized, environmental temperature fluctuations remain difficult to completely avoid during actual loading, unloading, transportation, and retail processes (Liu, Lv, et al., 2023). This dynamic thermal stress not only accelerates fruit senescence, browning, and texture degradation, but may also disturb internal metabolic balance, thereby shortening shelf life and reducing commercial value (Chen et al., 2023). However, most current postharvest preservation studies focus primarily on ideal distribution scenarios under constant low temperatures. A systematic understanding of fruit quality changes and metabolic responses induced by temperature fluctuations when fruit enters the distribution stage after long-term cold storage remains relatively limited (Liu, Wang, et al., 2023).

Notably, fruits subjected to prolonged cold storage exhibit a specific state of physiological adaptation. Subsequent temperature fluctuations during distribution can further exacerbate metabolic disturbances, thereby compromising the efficacy of preservation technologies. Modified atmosphere packaging based on nanomaterials has received widespread attention in the postharvest preservation of fruits and vegetables due to its excellent gas barrier properties and ability to regulate the packaging microenvironment (Shan et al., 2023). In recent years, numerous studies have demonstrated that nano-packaging can effectively reduce fruit respiration and water loss and alleviate oxidative damage, thereby maintaining fruit quality and extending shelf life (Martínez-González et al., 2020; Sharma et al., 2025). However, existing nanomaterial-based packaging has been evaluated primarily under constant-temperature conditions, and its preservation efficacy under dynamic cold-chain conditions remains unclear. Temperature fluctuations can alter fruit respiration and metabolic activity while disrupting the stability of the package microenvironment. As a result, the protective effects achieved under constant-temperature conditions may not be maintained throughout commercial distribution (Jafarzadeh et al., 2021; Nath et al., 2022; Liu, Wang, et al., 2023). Therefore, it is essential to elucidate the changes in fruit quality, key metabolic responses, and potential deterioration biomarkers in nano-packaged fruit under fluctuating temperature conditions. A better understanding of these responses will provide a basis for evaluating the suitability of nanomaterial-based packaging for cold-chain applications and for optimizing its practical use.

From a metabolic regulatory perspective, maintaining postharvest apple quality relies on the coordinated regulation of carbon and nitrogen metabolism, osmotic regulation, and reactive oxygen species (ROS) scavenging systems (Brizzolara et al., 2020). Previous studies have shown that the GABA shunt, arginine-polyamine metabolism, and tryptophan-related metabolic pathways play crucial roles in plant stress adaptation and quality maintenance (Xu, Li, et al., 2022). Under environmental imbalances induced by temperature fluctuations, fruits may activate compensatory stress responses and accelerate the conversion of key substrates, including sugars, organic acids and amino acids. However, it remains unclear whether temperature fluctuations disrupt the balance of these metabolic networks in nano-packaged apples and which key pathways predominantly govern the adaptive response of the fruit. Therefore, untargeted metabolomics offers a powerful approach for systematically characterizing apple quality changes and metabolic reprogramming under temperature fluctuations. It may help clarify how dynamic temperature fluctuations influence the preservation efficacy of nano-packaging and provide insights into the underlying metabolic mechanisms.

Based on this, VG were selected as the experimental material because of their high commercial value, distinctive quality attributes, and susceptibility to quality deterioration during distribution. Their quality changes and metabolic responses during distribution after long-term cold storage were investigated under dynamic temperature fluctuations. Specifically, this study aimed to: (1) determine how temperature fluctuations affect quality maintenance in apples packaged with nano-packaging film (NPF); (2) identify the key metabolic pathways involved in changes in metabolic homeostasis and quality deterioration under fluctuating temperatures; and (3) identify candidate biomarkers of quality deterioration. We hypothesized that temperature fluctuations would impair quality maintenance in NPF-packaged apples by disrupting the antioxidant enzyme system and the balance between carbon and nitrogen metabolism. This study aims to provide a scientific basis for optimizing cold-chain temperature management of VG after long-term storage and for enhancing the preservation efficacy of NPF packaging.

2. Materials and methods

2.1. Experimental materials and treatments

VG at commercial maturity (approximately 180 days after full bloom) were harvested from the orchard of Weihai Cuihong Fruit Co., Ltd. Upon harvest, the fruits were transported to the laboratory. Fruits with diseases, insect pests, mechanical damage, or inconsistent sizes were eliminated. Finally, healthy fruits with uniform coloration and an average individual fruit weight of 220 ± 15 g were selected for the experiment.

The packaging materials used in this study included: nano-coating fluid (NCF), nano-packaging film (NPF) and conventional polyethylene film (PE). The NCF was a 2% carboxylated cellulose nanocrystal dispersion (Model: CNC-MY2ON, 5 L, Foshan Sailuna Technology Co., Ltd., China). Before use, the dispersion was stirred on a magnetic stirrer at 500 rpm for 50 min. The fruits were then completely immersed in the dispersion for 1 min. After removal, the coated fruits were air-dried in a clean room until the surface moisture had completely evaporated, which required approximately 30 min. The NPF was a polyethylene-based nano-Ag modified blend film (Model: FMSX-SXV, 40 cm × 40 cm, thickness of 35 μm, Shanghai Fuming New Material Technology Co., Ltd., China) with an oxygen transmission rate (OTR) of 9000–10,000 cm3/(m2·24 h·0.1 MPa) and a water vapor transmission rate (WVTR) of 23 g/(m2·24 h). It had a tensile strength of 33.83–38.93 MPa, an elongation at break of 618–799%, a light transmittance greater than 88%, and an average nano‑silver particle size of 20–50 nm. The NPF was selected as a representative nano-packaging material because of its commercial availability and potential practical applications. Compared with conventional PE film, it has the potential to regulate the packaging microenvironment. Although similar functional nanomaterials have been used to preserve vegetables and other agricultural products, the suitability of this film for VG cold-chain distribution remains to be evaluated. A conventional PE film was used as the control. It was a standard PE film measuring 40 cm × 40 cm (Model: 6095, Laiyang Fuxing Packaging Technology Co., Ltd., Laiyang, China).

The fruits were randomly divided into 4 treatment groups: the CK group (unpackaged control), the PE group (packaged in traditional PE bags), the NCF group (immersed in nano-coating fluid for 1 min and air-dried naturally without packaging), and the NPF group (packaged in nano-packaging film). Each treatment consisted of 3 biological replicates, with 18 fruits per replicate.

2.2. Logistics simulation and temperature settings

To simulate the commercial cold storage phase prior to distribution, all VG were stored before packaging in a cold room at 0 ± 0.5 °C and 90%–95% relative humidity for 3 months (November 5 to February 5) prior to packaging treatment. A subsequent 35-day distribution cycle was then simulated. All treatments were placed in a precision temperature- and humidity-controlled incubator (Model: HW-600, Shandong Xinggu Medical Technology Co., Ltd., China), and the relative humidity was maintained at 85% ± 5%.

To systematically evaluate the preservation efficacy of nano-packaging under different temperature conditions, the experiment was divided into two stages. The first stage involved packaging-material screening. The effects of the CK, PE, NCF, and NPF treatments on fruit quality were evaluated at a constant 10 °C to identify the most effective packaging treatment. The results indicated that NPF provided the best overall preservation performance and was therefore selected for the second stage. The second stage involved the simulation of dynamic temperature fluctuations. Three temperature treatments were established for fruits packaged with NPF packaging: constant 10 °C (simulating cold chain distribution), constant 20 °C (simulating ambient-temperature retail conditions), and 10–20 °C fluctuating-temperature (between 10 °C and 20 °C in 24-h cycles to simulate dynamic temperature fluctuations during transportation, distribution and retail). To assess the effect of packaging under fluctuating temperatures, 10–20 °C unpackaged (UP) group was included for comparison with the NPF-packaged group.

2.3. Analytical methods

Regular sampling was conducted on days 0, 7, 14, 21, 28, and 35 of the simulated distribution period. Separate sampling time points were designated for metabolomic analysis.

2.3.1. Color and texture evaluation

Following the method of Zheng et al. (2025), a colorimeter (CR-400, Konica Minolta, Japan) was used to measure the color parameters L* and b* of the peel and flesh to reflect changes in lightness and the degree of yellowing. Texture was assessed using a texture analyzer (TA.XT Plus, Stable Micro Systems, UK) equipped with a P/50 stainless-steel cylindrical probe (50 mm diameter). The test parameters were set as follows: pre-test speed, 5 mm/s; test speed, 5 mm/s; post-test speed, 5 mm/s; compression distance,10 mm; compression time, 5 s; and trigger force, 5 g. Hardness, crispness, springiness and chewiness were recorded and calculated. For each fruit, four symmetrical tissue cubes (2 cm × 2 cm × 2 cm each) were cut from the equatorial region for measurement.

2.3.2. Determination of sugars and organic acids

Total soluble solids (TSS) were measured using a handheld refractometer (PAL-1, ATAGO, Japan). Titratable acidity (TA) was determined by the acid-base titration method, referencing Ferreira et al. (2024). Major sugars (fructose, glucose, sucrose, sorbitol) and organic acids (malic acid, citric acid, quinic acid, fumaric acid) were extracted and quantitatively analyzed using high-performance liquid chromatography (HPLC) (Zheng et al., 2025).

2.3.3. Antioxidant enzyme activity determination

A specific amount of fruit flesh tissue was homogenized with pre-cooled extraction buffer and centrifuged. The activities of polyphenol oxidase (PPO), peroxidase (POD) and catalase (CAT) were determined using a UV–Vis spectrophotometric method by dynamically monitoring absorbance changes at specific wavelengths, according to the method described by Zhang et al. (2025).

2.3.4. Untargeted metabolomic analysis

Samples were collected on days 0, 14, 21, 28, and 35 during the VG distribution period. The samples were rapidly quenched in liquid nitrogen to terminate metabolic activity and then ground into a uniform powder. Exactly 70 mg of freeze-dried powder was weighed for metabolite extraction. The extraction protocol was adapted from Zheng et al. (2025). Metabolites were extracted using a methanol/water/methyl tert-butyl ether (MTBE) partitioning system. After centrifugation and phase separation, the extracts were dried under reduced pressure and reconstituted in 50% acetonitrile. Metabolomic analysis was performed using an ultra-high-performance liquid chromatography system coupled to a high-resolution Orbitrap mass spectrometer (UHPLC–Orbitrap MS). Chromatographic separation was achieved on an Acquity UPLC HSS T3 column (2.1 × 100 mm, 1.8 μm) maintained at 25 °C. The mobile phases consisted of water containing 0.1% formic acid (mobile phase A) and acetonitrile (mobile phase B), with a flow rate of 0.5 mL/min. The gradient program was as follows: 1%–99% B from 1 to 8 min, 99% B from 8 to 10 min, followed by a return to the initial conditions and re-equilibration until 12 min. The injection volume was 2 μL. The mass spectrometer was operated in positive electrospray ionization (ESI) mode over an m/z range of 60–900. The main source parameters were as follows: capillary temperature, 380 °C; spray voltage, 3.6 kV; sheath gas, 60 arbitrary units; and auxiliary gas, 20 arbitrary units.

One quality-control (QC) sample was injected after every six analytical samples to monitor instrumental stability and analytical reproducibility. Raw mass spectrometry data were processed using MS-DIAL software. Compound identification was performed by matching the detected features to an in-house database based on exact mass, isotope distribution, and MS/MS spectra. Compounds with a match score exceeding 85% were considered reliably identified metabolites.

2.4. Statistical analysis

Three biological replicates were used to assess conventional physicochemical and physiological parameters, while five independent biological replicates were used for metabolomic analysis. Conventional data were subjected to one-way analysis of variance (ANOVA) using SPSS version 26.0. Duncan's multiple range test was used to determine significant differences among treatments (P < 0.05). After mean-centering, the metabolomic data were imported into SIMCA-P version 14.1 for principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA). Differential metabolites were identified based on a variable importance in projection (VIP) > 1 and P < 0.05. Candidate biomarkers associated with quality changes were further selected by integrating their temporal variation patterns and correlations with quality parameters. Metabolites showing distinguishable differences from the 10 °C constant-temperature treatment and pronounced temporal changes were retained. In addition, candidate metabolites were required to be significantly correlated with at least two quality parameters (P < 0.05). Data visualization was performed using Origin 2021 and online plotting platforms. Time-series color difference data and the relative abundances of differential metabolites were Z-score normalized, and hierarchical clustering heatmaps were generated using the R package pheatmap.

3. Results and discussion

3.1. Effects of nano-packaging on postharvest quality and metabolic characteristics of VG

3.1.1. Effect of nano-packaging on VG phenotype and texture maintenance

As shown in Fig. 1A, VG color and texture gradually deteriorated over time under all packaging treatments, although the rates of change differed markedly among treatments. Color changed rapidly during the first 14 d, as indicated by a decrease in the L* and an increase in the b*, suggesting that this period was critical for the loss of lightness and the development of yellowing. Over time, both the CK and PE groups exhibited significant decreases in lightness and increases in yellowing in both the peel and flesh, and the differences among treatments became more pronounced as the distribution period progressed. In contrast, the NCF and NPF treatments showed smaller decreases in L* and slower increases in b*, indicating that the nano-packaging treatments effectively preserved fruit lightness and color. Notably, the differences among treatments were established primarily during the first 14 d, after which the rates of change decreased substantially. This protective effect was not attributable to changes in temperature. Instead, nano-packaging may have indirectly delayed chlorophyll degradation, carotenoid exposure, and phenolic oxidation by reducing water loss and regulating the internal gas composition and fruit respiration rate (Martínez-González et al., 2020; Si et al., 2022).

Fig. 1.

Fig. 1

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Changes in fruit color and texture attributes of VG under different packaging treatments during the 35-day distribution period. (A) Heatmap of colorimetric parameters (L* and b*) for fruit skin and flesh. (B) Firmness. (C) Crispness. (D) Springiness. (E) Chewiness.

Texture measurement results showed temporal patterns consistent with those of the color parameters (Fig. 1B-E). Hardness, crispness, and chewiness all decreased with increasing distribution time. Hardness declined earliest and to the greatest extent, whereas springiness changed more slowly. Compared with the CK and PE treatments, both NPF and NCF significantly delayed the decreases in all texture parameters, with NPF consistently maintaining the highest texture levels. For example, on day 35, hardness and crispness in the NPF group decreased by only 12.7% and 6.8%, respectively, which were markedly lower than the corresponding decreases of 25.3% and 16.6% observed in the CK group. Hardness and crispness primarily reflect cell turgor and cell wall integrity, while chewiness reflects the overall mechanical properties of fruit tissue (Shi et al., 2022). The smaller decreases in these parameters under NPF treatment suggest that nano-packaging effectively delayed cell wall degradation and tissue softening, thereby maintaining the structural stability of the flesh.

Color and texture exhibited highly consistent temporal patterns, characterized by an initial phase of rapid deterioration followed by a slower decline. A clear quality-preservation gradient was observed, with NPF showing the strongest effect, followed by NCF. These results indicate that nano-packaging not only preserved the external appearance of VG but also maintained tissue structural integrity.

3.1.2. Regulatory effect of nano-packaging on primary metabolites and antioxidant enzyme activities

The consumption of primary metabolites directly affects the postharvest shelf life and quality of fruit (Brizzolara et al., 2020). As shown in Fig. 2, both TSS and TA gradually declined during distribution under all packaging treatments, whereas nanocomposite packaging effectively retarded their decline. For TSS (Fig. 2A), CK showed a 7.9% decrease after 35 days, which was significantly greater than those of NCF (5.0%) and NPF (6.5%), while no significant difference was observed between NCF and NPF. These results indicate that nano-packaging, particularly NCF, was more effective in maintaining TSS. TA exhibited a similar trend (Fig. 2B). After 35 days, the TA reduction in NPF was only 33.3%, significantly lower than that in the CK (44.4%), whereas PE and NCF showed intermediate values with no significant difference between them. This suggests that NPF was the most effective treatment for delaying TA degradation. Overall, both nano-packaging treatments effectively maintained sugar and acid levels during distribution, although their effects differed slightly. NCF showed a slight advantage in preserving TSS, whereas NPF was more effective in maintaining TA, which may be associated with differences in their gas permeability and their ability to regulate fruit respiratory metabolism.

Fig. 2.

Fig. 2

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Effects of different packaging treatments on (A) total soluble solids (TSS), (B) titratable acidity (TA), (C) PPO, (D) POD, and (E) CAT during 35 days of storage.

Changes in antioxidant enzyme activities showed patterns consistent with those of the sugar and acid substrates. As shown in Fig. 2C–E, PPO and POD activities increased more rapidly in the CK and PE groups, whereas the NPF treatment maintained lower PPO and POD activities at several sampling points. PPO is primarily involved in the oxidation of phenolic substrates, whereas POD participates in peroxide metabolism and tissue browning. Excessive increases in the activities of PPO and POD generally indicate intensified phenolic oxidation and oxidative damage (Wang et al., 2023; Wang, Cao, et al., 2024). Therefore, the lower PPO and POD activities observed in the NPF group were consistent with its higher L* and slower color deterioration. The temporal pattern of CAT activity differed from those of PPO and POD. The NPF treatment generally maintained higher CAT activity, and the differences between NPF and the CK and PE treatments became more pronounced during the middle and late stages. CAT directly decomposes H₂O₂ and is one of the key enzymes involved in reactive oxygen species scavenging in fruit tissues (Liu, Zhang, Wu, et al., 2024). The higher CAT activity in NPF-treated fruit therefore indicates a greater capacity for H₂O₂ removal, which may help alleviate membrane lipid peroxidation and cellular structural damage.

Overall, nano-packaging exerted a synergistic preservation effect by slowing the consumption of respiratory substrates, including soluble sugars and organic acids, and maintaining antioxidant enzyme homeostasis through the suppression of PPO and POD activities and the preservation of relatively high CAT activity. As important carbon sources for postharvest respiration, sugars and organic acids are continuously consumed during distribution, and their rapid depletion is closely associated with tissue senescence and cellular energy imbalance (Tijero et al., 2021).

3.1.3. Analysis of differential metabolite characteristics of VG under nano-packaging

To further elucidate the potential metabolic mechanisms by which nano-packaging maintains VG quality, this study identified a total of 159 metabolites via untargeted metabolomics (Table S1). Based on the OPLS-DA model, 35 key differential metabolites (VIP > 1) were identified. These metabolites were mainly classified as sugars and sugar alcohols, amino acids, organic acids, and other related compounds (Fig. 3A), and their changes were associated with carbon source utilization, carbon–nitrogen metabolic regulation, and stress responses. These findings suggest that the regulation of VG quality by NPF depends on coordinated changes in multiple metabolite classes and associated metabolic pathways.

Fig. 3.

Fig. 3

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Metabolomic profiling of VG under different packaging treatments. (A) Classification and proportion of the identified differential metabolites. (B) Heatmap illustrating the relative abundance changes of key differential metabolites across various packaging treatments and storage periods.

Heatmap analysis further revealed clear time-dependent changes in the VG metabolome under different packaging treatments (Fig. 3B). Compared with the CK and NCF treatments, NPF better maintained the relative abundances of several stress-responsive metabolites, including myo-inositol, GABA, and DL-arginine, during the middle and late stages of distribution. Although these metabolites belong to different metabolic classes, they are all closely associated with the maintenance of cellular homeostasis. Myo-inositol is associated with cellular osmoregulation, membrane lipid metabolism, and membrane structural stability. Its relative maintenance may help reduce cellular structural damage caused by water loss and osmotic stress (Hu et al., 2025). Arginine is not only an important component of protein and nitrogen metabolism, but also a precursor for polyamine and nitric oxide biosynthesis. It therefore contributes to membrane stabilization, reactive oxygen species regulation, and stress signaling (Gao et al., 2021). GABA is located at the intersection of carbon and nitrogen metabolism and is closely related to cellular pH regulation, reactive oxygen species homeostasis, and anaplerotic replenishment of the tricarboxylic acid cycle (Aghdam et al., 2022). The relative maintenance of these metabolites suggests that NPF may enhance fruit tolerance to distribution-related stress by strengthening osmoregulation, membrane stability, and antioxidant defense.

Changes in sugar alcohols, organic acids, and amino acids further reflected the metabolic reprogramming of VG during distribution. Sugar alcohols such as dulcitol, lactitol, and L-fucitol increased after day 21 under NPF treatment, peaked on day 28, and then declined. This rise-and-fall pattern may be associated with osmoregulation, cell wall polysaccharide degradation, and respiratory consumption during the later stage of distribution. It also indicates that NPF did not completely block senescence-related metabolism, but rather delayed and modulated its temporal progression. The maintenance of amino acids such as arginine may also help preserve the carbon and nitrogen skeletons required for stress responses. Notably, on day 28, the relative abundances of DL-phenylalanine and trigonelline were higher in the NCF treatment than in the PE and NPF treatments, suggesting that NCF may be more effective in retaining certain metabolites associated with flavor and stress responses.

Overall, NPF was more effective than the other treatments in maintaining the quality and metabolic homeostasis of VG during distribution. By modulating the temporal dynamics of key metabolites, including sugar alcohols, amino acids, and organic acids, NPF slowed the consumption of respiratory substrates, cell wall degradation, and disruption of carbon–nitrogen metabolism, thereby delaying quality deterioration. Because temperature fluctuations during commercial distribution may disturb the packaging microenvironment and aggravate metabolic imbalance, the following section further examines quality deterioration and metabolic responses in NPF-packaged VG under dynamic temperature conditions.

3.2. Quality deterioration characteristics and metabolic responses of nano-packaged VG under dynamic temperature fluctuations

As NPF exhibited the best overall preservation performance among the different packaging treatments, it was selected as the representative nano-packaging system for subsequent experiments under temperature fluctuation conditions. The preservation efficacy of NPF and its underlying mechanisms under dynamic temperature conditions were further investigated.

3.2.1. Dynamic temperature fluctuations exacerbate VG appearance and texture deterioration

Distribution temperature is a critical environmental factor influencing the preservation efficacy of packaging systems (Li et al., 2024). Overall, the appearance and texture of VG gradually deteriorated with increasing distribution time under all temperature treatments, although the extent of deterioration differed markedly among treatments (Fig. 4 and Fig. S2). VG maintained at a constant temperature of 10 °C consistently exhibited the best quality, whereas those stored at a constant 20 °C showed a continuous decline in quality. The most severe declines were found for the 10–20 °C group and its unpackaged (UP) counterpart, with the change magnitudes in both groups exceeding those at a constant 20 °C. Colorimetric analysis revealed clear differences in the changes in VG appearance among temperature treatments (Fig. 4A). As distribution progressed, the L* of both the peel and flesh gradually decreased, whereas the b* increased, indicating a progressive loss of lightness accompanied by yellowing and browning. Compared with the two constant-temperature treatments, the 10–20 °C treatment caused a more rapid decrease in L* and a more pronounced increase in b*. These changes became particularly evident after 28 day and were even greater than those observed under constant storage at 20 °C.

Fig. 4.

Fig. 4

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Changes in fruit color and texture attributes of NPF-packaged VG under different temperature treatments during 35 days of storage. (A) Heatmap of colorimetric parameters (L* and b*) for the skin and flesh. (B) Firmness; (C) Crispness. (D) Springiness. (E) Chewiness.

Texture changes followed a similar pattern and further confirmed the accelerating effect of temperature fluctuations on quality deterioration (Fig. 4B). Hardness, crispness, springiness, and chewiness all decreased over time, with significant differences among treatments. Among these parameters, hardness was the most sensitive to temperature variation. Compared with day 0, hardness decreased significantly by day 14 in the constant 20 °C, 10–20 °C, and 10–20 °C (UP) treatments. Notably, during the first 21 days, hardness, crispness, and springiness were generally higher in the nano-packaged samples under 10–20 °C conditions than in 10–20 °C (UP) samples, indicating that nano-packaging effectively delayed texture deterioration during the early distribution period. However, from day 28 onward, these parameters became similar between the two groups, suggesting that the protective effect of nano-packaging gradually weakened under prolonged temperature fluctuations. Temperature fluctuations may initially accelerate tissue water loss and reduce cell turgor, followed by disruption of cell wall structure, thereby causing a continuous decline in fruit hardness and overall textural quality. Consequently, nano-packaging was unable to fully counteract fluctuation-induced softening during the later stages of distribution.

Previous studies have suggested that repeated temperature fluctuations continuously disturb the equilibrium between the internal and external packaging environments, forcing fruit to repeatedly adjust its respiratory metabolism while intensifying water loss. Temperature fluctuations may also activate enzymes involved in cell wall degradation, thereby accelerating tissue softening and browning (Li et al., 2024; Ning et al., 2022; Su et al., 2022). In the present study, the deterioration of color and texture under the 10–20 °C regime showed a clear coordinated pattern. During the later stage of distribution, its adverse effects even exceeded those observed under constant storage at 20 °C, and most texture parameters no longer differed significantly between the nano-packaged and unpackaged treatments. These findings indicate that temperature fluctuation exceeded the capacity of nano-packaging to regulate the internal microenvironment and was therefore a key environmental factor weakening its preservation efficacy. The associated deterioration further suggests that fluctuating temperatures may have disrupted metabolic homeostasis within the fruit. Their effects on sugar and organic acid metabolism and the antioxidant system are examined further through subsequent untargeted metabolomic analysis.

3.2.2. Dynamic temperature fluctuations accelerate primary metabolite consumption and perturb antioxidant enzyme systems

Temperature is a key environmental factor regulating respiration and enzyme activity in postharvest fruit and directly affects the consumption and transformation of sugars and acids (Zhang, 2024). Compared with the constant 10 °C treatment, the decline rates of TSS and TA were significantly accelerated under both the 10–20 °C and constant 20 °C treatments (Fig. 5A and B). In particular, TA showed the greatest decrease under the 10–20 °C and 10–20 °C (UP) treatments, declining from 0.18% to 0.10%. Sugar composition analysis (Fig. S3) revealed that fructose and glucose decreased markedly under the 10–20 °C treatment on day 28 and were significantly lower than those in the two constant-temperature groups (10 °C and 20 °C). By contrast, sucrose declined most rapidly under the constant 20 °C treatment. Sorbitol was rapidly depleted during the early stage of distribution under all treatments, with its content declining below the detection limit by 28 d. This limited effect of packaging on sorbitol retention may be attributed to the coordinated developmental regulation of sorbitol metabolism. Sorbitol is primarily converted into fructose through the action of sorbitol dehydrogenase (SDH), which plays an important role in regulating sorbitol utilization during fruit development. Therefore, the rapid depletion of sorbitol observed in this study may be associated with its active metabolic conversion rather than packaging effects alone. In addition, sorbitol metabolic flux is constrained by overall carbon metabolic reprogramming, limiting the capacity of postharvest packaging to maintain sorbitol levels (Li et al., 2018). As shown in Fig. S4, the decrease in organic acids under the 10–20 °C treatment during the first 28 day was comparable to that under the constant 20 °C treatment. However, by 35 d, the decrease was greater than those in the other two groups. Malic acid, the predominant organic acid in VG, showed the most pronounced decline. These results indicate that temperature fluctuations significantly accelerated the consumption of sugar and organic acid substrates in VG during distribution.

Fig. 5.

Fig. 5

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Effects of different temperature treatments on the primary metabolism of NPF-packaged VG during 35 days of storage: changes in (A) total soluble solids (TSS), (B) titratable acidity (TA), (C) PPO, (D) POD, and (E) CAT during 35 days of storage.

Along with the accelerated depletion of these substrates, temperature fluctuations significantly disrupted the antioxidant enzyme system of the fruit. During distribution, PPO activity increased continuously (Fig. 5C). Under the 10–20 °C treatment, PPO activity increased sharply after 14 day and reached the highest value at the end of distribution, approximately 3.8 U/g. This value was significantly higher than that under the constant 10 °C treatment, which maintained consistently low PPO activity. POD activity initially increased and subsequently decreased (Fig. 5D), with peaks occurring on 21 day under among the 10–20 °C, 10–20 °C (UP) and constant 20 °C treatments. In contrast, the constant 10 °C treatment delayed the activity peak and significantly suppressed overall POD activity. By contrast, CAT, a key ROS-scavenging enzyme, showed a continuous decline in activity (Fig. 5E). The 10–20 °C treatment increased the activities of PPO and POD, which are associated with browning and senescence, while accelerating the loss of CAT activity. This imbalance in the antioxidant system significantly aggravated oxidative stress in the fruit, thereby accelerating tissue senescence and quality deterioration.

Previous studies have shown that temperature fluctuations during distribution can repeatedly disturb respiratory and oxidative metabolism in fruit, resulting in the disruption of metabolic homeostasis (Zhang, 2024). In the present study, the 10–20 °C treatment accelerated the depletion of sugar and organic acid substrates, increased PPO and POD activities, and reduced CAT activity, indicating that temperature fluctuations intensified oxidative stress and disrupted primary metabolism. Notably, the deterioration of several quality and physiological parameters under the 10–20 °C treatment even exceeded that observed under the constant 20 °C treatment. This finding indicates that the effects of temperature fluctuation cannot be explained solely by cumulative exposure to high temperature. Instead, the periodic environmental stress induced by repeated shifts between low and high temperatures further weakened the capacity of nano-packaging to maintain fruit metabolic homeostasis.

3.2.3. Characteristics of differential metabolite changes under dynamic temperature fluctuations and their relationship with quality deterioration

Under different temperature treatments, the metabolic profiles of NPF-packaged VG displayed clear differences (Fig. 6A, Table S2). The 34 differential metabolites screened using OPLS-DA showed that distribution temperature and temperature fluctuations markedly affected sugar, organic acid, and amino acid metabolism, as well as stress responses in the fruit (Fig. 6B). Compared with the constant 10 °C treatment, the 10–20 °C treatment generally resulted in lower abundances of sucrose, citric acid, l-glutamine, L-valine, DL-phenylalanine, and several other metabolites. In particular, decreases in stress-related metabolites, including tryptophan derivatives and γ-aminobutyric acid (GABA), may have weakened stress signaling, antioxidant defense, and cellular homeostasis (Xu, Yang, et al., 2022; Ouyang et al., 2025). These findings indicate that temperature fluctuations accelerated the consumption of respiratory substrates and selected amino acids and disrupted the balance between carbon and nitrogen metabolism. The overall metabolic trend under the 10–20 °C treatment was similar to that observed at constant 20 °C and 10–20 °C (UP) treatments, with progressive decreases in sugars, organic acids, and several amino acids. However, D-(+)-galactose, D-allulose, dulcitol, and melezitose gradually accumulated during the later stage of distribution, suggesting that temperature fluctuations induced not only substrate depletion but also sugar conversion and redistribution of carbon flux.

Fig. 6.

Fig. 6

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Metabolomic profile and differential metabolite analysis of NPF-packaged VG under different temperature treatments. (A) Classification and quantity distribution of the 34 identified differential metabolites. (B) Clustering heatmap of relative abundance changes in key differential metabolites. (C) KEGG pathway enrichment of differential components. (D) Heatmap of Pearson correlations among key metabolites, color, texture, and antioxidant enzyme activities.

Correlation analysis further revealed close associations between metabolic changes and quality deterioration (Fig. 6D). PPO, POD, and CAT activities were highly significantly correlated with fruit lightness and browning-related color attributes, whereas sucrose, GABA, citric acid, and L-glutamic acid were significantly and positively correlated with hardness and crispness. The depletion of sugars, organic acids, and stress-related metabolites under fluctuating temperatures may therefore have impaired osmotic regulation, cellular structural stability, and antioxidant defense, thereby accelerating color and texture deterioration (Xin et al., 2021). Notably, the differences in several key metabolites between the NPF-packaged and unpackaged groups became markedly smaller under fluctuating temperatures, indicating that recurrent temperature variability compromised the effectiveness of nano-packaging in maintaining metabolic homeostasis and fruit quality.

Further pathway enrichment analysis of the differential metabolites (Fig. 6C) showed that the temperature fluctuation affected multiple core metabolic pathways in VG. Among them, alanine, aspartate, and glutamate metabolism, galactose metabolism, arginine biosynthesis, and D-amino acid metabolism showed the most significant enrichment, indicating that these pathways were highly sensitive to temperature fluctuations. Additionally, nitrogen metabolism, valine, leucine, and isoleucine biosynthesis, glyoxylate and dicarboxylate metabolism, and butanoate metabolism also underwent significant changes. Although glutathione metabolism and glycine, serine, and threonine metabolism showed relatively low enrichment, several differential metabolites involved in these pathways were also significantly downregulated under fluctuating temperatures. These pathway-level changes indicate that dynamic temperature fluctuations affected not only individual metabolites but also multiple core carbon–nitrogen metabolic and stress-defense pathways. Consequently, the metabolic network responsible for fruit quality maintenance was disrupted. Correlation analysis further indicated that macroscopic quality deterioration was closely associated with the depletion of key metabolites, changes in antioxidant enzyme activity, and reduced tissue structural stability. Together, these changes reflected a transition from metabolic homeostasis toward senescence and quality deterioration (Chen et al., 2023).

Among the 34 differential metabolites, 16 key metabolites were selected because their temporal trajectories and response magnitudes differed significantly from those observed under constant storage at 10 °C. Correlation analysis showed that nine metabolites, including L-glutamic acid, GABA, and D-sorbitol, were significantly associated with at least two quality attributes (Fig. 6D). These compounds were therefore considered candidate metabolic markers of quality deterioration. However, their reliability as predictive indicators requires further validation through targeted quantification and independent sample sets. Further analysis showed that these candidate metabolites were mainly involved in amino acid, sugar alcohol, and organic acid metabolism. L-Glutamic acid and GABA participate in the glutamate–GABA pathway, which is closely associated with plant stress responses, ROS homeostasis, and carbon–nitrogen balance (Li et al., 2021). Sugar alcohols such as D-sorbitol, myo-inositol, and dulcitol may contribute to osmotic regulation and cellular stability, whereas changes in citric acid and D-(+)-galactose may reflect adjustments in respiratory metabolism and carbon allocation. Previous studies have shown that alterations in the metabolic networks of sugars, organic acids, amino acids, and polyols are closely associated with quality deterioration in apples during prolonged cold storage (Zhang et al., 2020; Wang, Meng, et al., 2024). These metabolites may be key molecular nodes linking temperature stress to quality deterioration and potential indicators of VG quality changes under cold-chain fluctuations.

The present findings provide valuable guidance for the commercial distribution of VG under fluctuating temperature conditions. Under the representative 10–20 °C temperature fluctuation regime, VG exhibited accelerated quality deterioration and metabolic disruption. These results suggest that minimizing temperature fluctuations and maintaining a stable low temperature close to 10 °C during commercial distribution can help preserve fruit quality. It should be noted that the fixed 24 h temperature fluctuation cycle used in this study was a standardized and reproducible experimental model. It did not fully replicate the complex temperature fluctuations encountered in commercial cold-chain logistics. Therefore, the present findings require further validation across different apple cultivars and under practical commercial cold-chain conditions. In addition, because only the NPF nano-packaging system was evaluated, comparative studies involving different nano-packaging systems are needed to assess the general applicability of the present findings. Future research should also validate the identified metabolic biomarkers and further elucidate the underlying molecular mechanisms.

4. Conclusion

In conclusion, nano-packaging, particularly NPF, effectively alleviated quality deterioration of VG during distribution following long-term cold storage, although its preservation efficacy was weakened under dynamic temperature fluctuations. Metabolomic analysis demonstrated that temperature fluctuations accelerated quality deterioration by disrupting carbon–nitrogen metabolic homeostasis and antioxidant defense, leading to coordinated alterations in sugar metabolism, amino acid metabolism, membrane lipid metabolism, and other stress-responsive pathways. These metabolic changes were closely associated with fruit softening, browning, and the depletion of primary metabolites. Furthermore, nine candidate metabolic biomarkers, including L-glutamic acid, GABA, and D-sorbitol, were identified as potential molecular indicators of quality deterioration under fluctuating temperatures. Collectively, these results advance our understanding of the metabolic basis of temperature fluctuation-induced quality deterioration and provide guidance for improving the preservation of long-term cold-stored apples during commercial distribution. Nevertheless, further validation is required across different apple cultivars and under practical commercial cold-chain conditions. Future studies should focus on validating the identified metabolic biomarkers and elucidating the underlying molecular mechanisms.

CRediT authorship contribution statement

Xiaodong Zheng: Writing – original draft, Supervision, Investigation, Funding acquisition, Formal analysis. Shaoxiang Pan: Writing – review & editing, Investigation, Data curation. Zhen Wang: Writing – review & editing, Validation, Methodology, Data curation. Qi Guo: Validation, Software, Investigation. Wenyu Du: Writing – review & editing, Methodology. Yixuan Fan: Visualization, Validation, Methodology. Xuemei Liu: Writing – review & editing, Investigation. Xinhuan Yan: Writing – review & editing, Investigation. Lei Qin: Writing – review & editing, Methodology, Investigation, Conceptualization. Ye Song: Supervision, Project administration, Conceptualization.

Funding sources

This project was financially supported by Key R&D Program of Shandong Province, China (2024TZXD063).

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Xiaodong Zheng reports financial support was provided by Key R&D Program of Shandong Province. 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.

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2026.104642.

Contributor Information

Xiaodong Zheng, Email: zhxd1106@163.com.

Lei Qin, Email: qinlei@dlpu.edu.cn.

Ye Song, Email: songye_sy@163.com.

Appendix A. Supplementary data

Supplementary material 1
mmc1.docx (4.3MB, docx)
Supplementary material 2

Metabolites in Apples Under Different Packaging and Temperature Treatments

mmc2.xls (73.9KB, xls)

Data availability

Data will be made available on request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary material 1
mmc1.docx (4.3MB, docx)
Supplementary material 2

Metabolites in Apples Under Different Packaging and Temperature Treatments

mmc2.xls (73.9KB, xls)

Data Availability Statement

Data will be made available on request.

Articles from Food Chemistry: X are provided here courtesy of Elsevier

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