Method for regulating and controlling storage quality of sweet cherry fruits through cooperation of low-voltage electrostatic field and melatonin under plasma pretreatment

Abstract

The invention relates to a method for regulating and controlling the storage quality of sweet cherry fruits through cooperation of a low-voltage electrostatic field and melatonin under plasma pretreatment, and belongs to the technical field of fruit preservation. The regulation and control method comprises the following steps: S1, pretreatment: performing plasma treatment on sweet cherries; s2, melatonin treatment: dissolving melatonin powder in ultrapure water to prepare a fresh-keeping solution, then putting the pretreated sweet cherry fruits in the fresh-keeping solution for soaking treatment, and then draining on a sterile filter screen; and S3, low-voltage electrostatic field treatment: putting the sweet cherry fruits treated by melatonin into a low-voltage electrostatic field, and carrying out low-voltage electrostatic field treatment. On the basis of low-temperature normal-pressure plasma (CAP) pretreatment creatively, a low-voltage electrostatic field (LVEF) and exogenous melatonin (MT) are cooperatively applied, and the regulation and control effect of the composite treatment on the fruit storage quality is systematically evaluated.

Description

Technical Field

[0001] This application relates to the field of fruit preservation technology, and in particular to a method for regulating the storage quality of sweet cherry fruit by synergistic use of low-voltage electrostatic field and melatonin under plasma pretreatment. Background Technology

[0002] Sweet cherries ( Prunus avium Sweet cherries (L.) are a type of fruit with high respiratory metabolism but not climacteric growth. Harvesting during the hot and humid season makes them prone to browning, rapid aging, softening, and rotting, resulting in poor storage and transportability and a short shelf life. Under normal temperature conditions, sweet cherries lose their economic value within 5-7 days, with post-harvest losses reaching 20-50%. Currently, various chemical, physical, and biological preservation methods have been applied to improve post-harvest fruit quality and extend shelf life, including 1-methylcyclopropene (1-MCP), tea polyphenols, CaCl2, low temperature, irradiation, and modified atmosphere packaging. However, chemical preservatives may leave fungicide residues, low-temperature storage can easily lead to chilling injury, and some treatment technologies have problems such as high investment costs and complex technical management. Furthermore, they may pose potential risks during storage or cause physiological disorders in the fruit, thus affecting its sensory characteristics.

[0003] Cold atmospheric plasma (CAP) generates reactive oxygen species (ROS), reactive nitrogen species (RNS), ultraviolet light (UV), positive and negative ions, and free electrons, which can cause biological and oxidative damage to pathogens that cause fruit rot. Studies have shown that CAP can effectively extend the shelf life of fruits such as mulberries, bayberries, and blueberries, slow down the loss of active ingredients, maintain their quality, and maximize the preservation of the nutritional and sensory characteristics of the food without producing toxic byproducts. Melatonin (N-Acetyl-5-methoxytryptamine, MT) is widely present in plants and animals and can participate in regulating biological processes such as reproductive growth, vegetative growth, and postharvest ripening and senescence of fruits. Exogenous melatonin has been widely used to delay browning and senescence in postharvest peaches, pears, and cherries, maintaining fruit quality. Existing research shows that single preservation technologies often have limitations, while the synergistic application of multiple technologies can leverage complementary advantages and significantly improve preservation effects. Electrostatic field preservation technology, as a novel physical preservation method, offers advantages such as low energy consumption and no chemical residues. Studies have shown that electrostatic field treatment can extend the shelf life of fruits and vegetables by altering their surface charge state, promoting water evaporation, inhibiting microbial growth, and reducing oxidation rates. High-voltage electrostatic field (HVEF) and low-voltage electrostatic field (LVEF) technologies are suitable for different types of fruit and vegetable preservation needs, offering advantages such as environmental friendliness, high efficiency, and safety. Related research indicates that HVEF can effectively maintain the TSS content of fruits, inhibit respiration, and significantly reduce fruit decay rates during refrigeration. LVEF can inhibit ethylene synthesis, delay fruit ripening and senescence, and enhance disease resistance by regulating the fruit's antioxidant system. Zhou et al. and Wang et al. applied LVEF to lychee and bamboo shoots, respectively, and both concluded that the treated fruits and vegetables effectively maintained their quality and extended their shelf life compared to the control group.

[0004] Given that low-temperature atmospheric pressure plasma can improve the hydrophilicity of fruit surfaces and sterilize them, low-pressure electrostatic fields can regulate water and respiratory metabolism, and melatonin can activate endogenous antioxidant systems, these three factors have a potential synergistic basis in terms of physical barriers, metabolic regulation, and oxidative defense. Therefore, this study attempts to construct a novel preservation strategy that combines low-temperature atmospheric pressure plasma pretreatment with low-pressure electrostatic fields and exogenous melatonin. This method helps to fully explore its application potential in fruit and vegetable preservation, providing a simple, low-energy-consumption, and chemical-residue-free new technical pathway for postharvest preservation of sweet cherries. This technology shows good application potential in improving the postharvest quality of sweet cherries and is expected to provide a new technical approach for fruit and vegetable preservation. Summary of the Invention

[0005] In view of this, this application provides a method for regulating the storage quality of sweet cherry fruit by combining low-voltage electrostatic field with melatonin under plasma pretreatment. It innovatively combines low-temperature atmospheric pressure plasma (CAP) pretreatment with low-voltage electrostatic field (LVEF) and exogenous melatonin (MT) to systematically evaluate the regulatory effect of this combined treatment on the storage quality of the fruit, which can effectively overcome the defects of the existing technology.

[0006] This application provides a method for regulating the storage quality of sweet cherry fruits by synergistic effect of low-voltage electrostatic field and melatonin under plasma pretreatment, comprising the following steps:

[0007] S1. Pretreatment: The sweet cherries are subjected to plasma treatment, then the sweet cherry fruits are washed with ultrapure water and placed on a stainless steel screen to drain the surface moisture at room temperature. Then, they are laid in a single layer in a sterile container. The container is connected to the plasma jet outlet through a glass tube. The fruits are laid in a single layer to obtain the pretreated sweet cherry fruits.

[0008] S2. Melatonin treatment: Melatonin powder is dissolved in ultrapure water to prepare a preservation solution. Then, the pretreated sweet cherry fruits are placed in the preservation solution for soaking. After that, they are drained on a sterile filter to obtain melatonin-treated sweet cherry fruits.

[0009] S3. Low-voltage electrostatic field treatment: Sweet cherry fruits treated with melatonin are placed in a low-voltage electrostatic field for treatment.

[0010] Preferably, in step S1, the plasma treatment is performed using an atmospheric pressure plasma jet device, and the conditions for plasma treatment are: radio frequency power supply output voltage of 12 kV, frequency of 20 kHz, processing power of 200 W, and gas flow rate of 15 mL·min. -1 The processing time is 2.5 minutes.

[0011] Preferably, in step S1, the ambient temperature of the plasma treatment is 25 ± 1 °C and the relative humidity is 55%.

[0012] Preferably, in step S2, the concentration of the preservation solution is 100 μmol·L⁻¹. -1 .

[0013] Preferably, in step S2, the specific conditions for the soaking treatment are: soaking at 25 °C for 5 min.

[0014] Preferably, in step S3, the specific process of the low-voltage electrostatic field treatment is as follows: a low-voltage electrostatic field device is set up 80 mm above the melatonin-treated sweet cherry fruit.

[0015] Preferably, in step S3, the output voltage of the low-voltage electrostatic field device is 200 V.

[0016] Compared with the prior art, this application has the following advantages:

[0017] 1. To address the problem of short shelf life caused by browning, softening, and rotting after harvest in sweet cherries, this application innovatively combines low-temperature atmospheric pressure plasma (CAP) pretreatment with low-voltage electrostatic field (LVEF) and exogenous melatonin (MT), systematically evaluating the regulatory effect of this combined treatment on fruit storage quality. Results showed that after 20 days of storage, the LVEF+MT group had significantly higher lightness (L*) and red-green axis chromaticity index (a*) of 48.59 and 13.22, respectively, compared to other groups, and the lowest total color difference (ΔE) (5.14). The LVEF+MT group maintained a fruit firmness of 988 g (a decrease of 19.1%), significantly lower than the CK group (43.6%). The peel penetration reached 243.8 g. The fruit weight loss rate was only 5.3%, indicating that the mechanical strength of the peel was effectively maintained. The browning rate of the fruit stalk (19.67%) and the browning index of the fruit (3.65%) were reduced by 80.3% and 78.3% respectively compared with the control group. The fruit weight loss rate (4.42%) and the fruit rot rate (6.43%) were reduced by 59.7% and 65.7% respectively compared with the control group. The soluble solids (TSS) content remained stable at 18.0%, and the pH value fluctuated the least (final value 3.77). Principal component analysis (PCA) showed that the LVEF+MT group exhibited the highest comprehensive score at all storage time points, indicating its significant advantage in maintaining quality in multiple dimensions. This application provides a new, efficient, safe, and environmentally friendly technical approach for postharvest preservation of sweet cherries, which has important theoretical value and application prospects. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in this application or the prior art, the drawings used in the description of this application or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 A flowchart of the experiment for treating sweet cherries with low-pressure electrostatic field synergistic with melatonin under low-temperature plasma pretreatment;

[0020] Figure 2 Images showing the changes in the appearance of sweet cherries during storage under different preservation treatments;

[0021] Figure 3 Figures showing the color changes of sweet cherries during storage under different preservation treatments (aL*, ba*, cb*, dC*, eH).o (f.ΔE);

[0022] Figure 4 The graph shows the changes in the browning rate of the fruit stalk (a) and the browning index of the fruit in sweet cherries during storage under different preservation treatments;

[0023] Figure 5 The graph shows the changes in fruit weight loss (a) and fruit rot rate (b) of sweet cherries under different preservation treatments during storage;

[0024] Figure 6 The graph shows the changes in TSS content (a) and pH (b) of sweet cherries under different preservation treatments during storage.

[0025] Figure 7 Principal component analysis loading diagrams for the storage quality indicators of sweet cherry fruit (a.PC1PC2, b.PC3PC2, c.PC1PC3).

[0026] Figure 8 A comprehensive score chart of quality indicators for sweet cherries under different preservation treatments during storage. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0028] Unless otherwise specified, the experimental methods used in the embodiments of this application are all conventional methods.

[0029] In the following examples and comparative examples, unless otherwise specified, all raw materials can be prepared by commercial purchase or conventional methods.

[0030] 1. Materials and Methods

[0031] 1.1 Materials, Reagents and Equipment

[0032] In May 2024, this application selected 'Rainier' sweet cherry fruits cultivated in a facility at the Yanghe New Area High-Efficiency Agricultural Park in Suqian City, Jiangsu Province as samples. Fruits at commercial maturity (approximately 90% ripeness) were collected and pre-cooled in an ice box to reduce the fruit's respiration rate, and then transported back to the laboratory within approximately 1.5 hours. Fruits with intact stems, uniform size and color, and free from pests, diseases, and mechanical damage were selected and randomly divided into four groups of 45 kg each, further subdivided into three replicates (15 kg per replicate).

[0033] The reagents and instruments used in the experiment are as follows: melatonin (98% purity, Shanghai Maclean Biochemical Technology Co., Ltd.); atmospheric pressure plasma system (model UNI-SCPT10, Beijing Zhongji Huasheng Machinery Equipment Import & Export Co., Ltd.); low-voltage electrostatic field generator (model Bx2000, Zhejiang Chili Technology Co., Ltd.); aluminum discharge plate (Zhejiang Chili Technology Co., Ltd.); texture analyzer (model TA-300W, Shanghai Tengba Instrument Technology Co., Ltd.); spectrophotometer (model DS-210, Hangzhou Caipu Technology Co., Ltd.); saccharimeter (model PAL-106, Beijing Jinkelida Electronic Technology Co., Ltd.); pH meter (model PHS-25, Shanghai Leici Instrument Co., Ltd.); electronic balance (model FA-E, Changzhou Lucky Electronic Equipment Co., Ltd.); refrigerator (model MC-5L416, Guangzhou Hezhong Biotechnology Co., Ltd.).

[0034] 1.2 Experimental Design

[0035] Pretreatment: Sweet cherries were treated using an Atmospheric Pressure Plasma Jet (APPJ) device. The RF power supply output voltage was 12 kV, the frequency was 20 kHz, the processing power was 200 W, and the gas flow rate was 15 mL·min. -1 The treatment time was 2.5 min. Sweet cherry fruits were washed with ultrapure water and placed on a stainless steel sieve to drain surface moisture at room temperature. They were then laid in a single layer in a sterile container. The container was connected to the plasma jet outlet via a glass tube. The fruits were laid in a single layer with a 1 cm gap to ensure uniform plasma coverage of the fruit surface. The treatment environment temperature was 25 ± 1 °C, and the relative humidity was 55%. This pretreatment kills microorganisms on the fruit surface and inactivates key browning enzymes such as polyphenol oxidase, thus providing a clean initial surface. It also enhances the hydrophilicity of the fruit peel, promoting the adhesion and penetration of the subsequent melatonin solution.

[0036] Control group (CK): Sweet cherry fruit received no treatment. Melatonin treatment group (MT): Melatonin powder was prepared with ultrapure water to a concentration of 100 μmol·L⁻¹. -1 The sweet cherries were soaked in a preservative solution at 25 °C for 5 minutes, then drained on a sterile filter for 60 minutes. Low-voltage electrostatic field treatment group (LVEF): A low-voltage electrostatic field device with an output voltage of 200 V was placed 80 mm above the sweet cherries. Low-voltage electrostatic field + melatonin (LVEF+MT) synergistic treatment group: The melatonin-treated sweet cherries were placed in the low-voltage electrostatic field.

[0037] The fruit samples were placed in transparent, perforated PET containers (12 cm × 9 cm × 3 cm) and stored in a refrigerator at 4 ± 0.5 °C and 90 ± 2% relative humidity for 20 days. Fruit samples were randomly selected every 4 days for testing. Fifty fruits were used to determine fruit texture, color, single fruit weight, soluble solids (TSS) content, and pH value. Thirty fruits were used to calculate the browning index, and the decay rate of 100 fruits was randomly calculated each time. The overall experimental procedure is as follows: Figure 1 As shown, the appearance changes of sweet cherries during storage under different preservation treatment conditions are as follows: Figure 2 As shown.

[0038] 1.3 Test Methods

[0039] 1.3.1 Fruit color

[0040] The color of the fruit was detected using a spectrophotometer. Twenty fruits with uniform surface color were randomly selected from each experiment for measurement, and the average value was taken. The fruit color index in this application includes: lightness (L). * ), red-green axis chromaticity index (a * ), Yellow-Blue axis chromaticity index (b * ), chromaticity (C) * ), Hue angle (H) o ), total color difference (ΔE).

[0041] Through CIE L * a * b * The calculation is performed in a uniform color space, and the chromaticity (C) is calculated. * The calculation formula for ) is as follows:

[0042] (1)

[0043] H o The calculation formula is as follows:

[0044] (2)

[0045] The formula for calculating ΔE is as follows:

[0046] (3)

[0047] In the formula, ΔL * =L t * –L1 * , Δa * =a t * –a1 * , Δb * =bt * –b1 * L1 * a1 * b1 * For the sample stored on day 0, L t * a t * b t * The sample is stored on day t.

[0048] 1.3.2 Fruit texture (whole texture and puncture)

[0049] Individual sweet cherry fruits were placed on the texture analyzer test platform, and full texture profile analysis (TPA) was performed using a cylindrical probe (P / 36) with a diameter of 36 mm. The test parameters were set as follows: pre-test speed 1 mm·s. -1 Test speed 1 mm·s -1 The uplink speed after testing was 2 mm / s. -1 The pulp deformed by 25% under pressure, with a 4-second interval between two compressions. Texture characteristics (firmness, elasticity, chewiness, and resilience) were calculated using texture characteristic curves. Twenty fruits of uniform size and color were selected from each treatment group for parallel experiments. The test head was changed, and the sweet cherry fruits were placed on the texture analyzer's test plate to determine the penetration force. The test procedure was set to a puncture test, using a 2 mm P / 2E needle probe. Puncture parameters were set as follows: compression interval 4 s, trigger force 3 g, puncture depth 3 mm, and other parameters were the same as for the TPA test.

[0050] 1.3.3 Browning rate of fruit stem (BRS) and browning index of fruits (BIF)

[0051] The formula for calculating the browning rate of fruit stalks is as follows:

[0052] (4)

[0053] The browning index of fruits was determined using a statistical method. Fruits were transversely cut along the equator, and the browning level was determined by the proportion of browned area on the cut surface: Level 0, no browning; Level 1, browned area ≤ 1 / 10; Level 2, browned area between 1 / 4 and 1 / 2; Level 3, browned area between 1 / 2 and 3 / 4; Level 4, browned area ≥ 3 / 4. Multiple fruits were randomly selected each time, and the calculation formula is as follows:

[0054] (5)

[0055] 1.3.4 Weight loss rate of fruits and fruit decay rate

[0056] The sweet cherries were weighed every 4 days, using the following formula:

[0057] (6)

[0058] Formula for calculating fruit rot rate (η):

[0059] (7)

[0060] In the formula: N is the total number of samples, and n is the number of rotten samples.

[0061] 1.3.5 Total soluble solids (TSS) content and pH value of fruit pulp

[0062] Before each experiment, the saccharimeter was zeroed with distilled water. Every four days, the juice of sweet cherry fruits was collected for measurement to obtain data on the change of TSS of the fruit with storage time.

[0063] Crush the sweet cherries to extract the juice, insert the pH meter probe into a small beaker containing the cherry juice, ensure good electrode contact, and read the pH value after the display reading stabilizes.

[0064] 1.4 Data Processing

[0065] All indicators were measured three times in succession, and the results are expressed as mean ± standard error. Statistical analysis was performed using SPSS software. One-way ANOVA combined with Duncan's multiple variance test was used for significance analysis, with P < 0.05 indicating statistical significance. Data visualization was performed using Origin (2021) software.

[0066] 2. Results and Discussion

[0067] 2.1 Changes in fruit color

[0068] Changes in the color of sweet cherries can reflect the effects of different preservation treatments on their appearance quality. The lightness (L) of the control group... * The color index (a) decreased from an initial value of 50.36 to 42.12 after 20 days of storage, a drop of 16.3%, indicating a significant darkening of the fruit. * After peaking at 16.62 over 12 days, the chromaticity index (b) quickly fell back to 10.58.* The chromaticity (C) also shows a trend of first increasing and then decreasing. * The color saturation of the fruit in the CK group increased from an initial value of 25.21 to 31.56 on day 12, then decreased to 25.19 on day 20. The total color difference (ΔE) increased to 8.3, indicating a decrease in fruit saturation and hue shift in the CK group. The LVEF group effectively delayed the decline in color saturation. * The value decreased, remaining at 47.93 after 20 days. * and b * At 20 days, the values ​​reached 12.81 and 25.97 respectively, both significantly higher than the CK group, with a ΔE of only 4.67, indicating that the LVEF group could better maintain the stability of fruit brightness and color. The MT group also showed a certain preservation effect, with L... * It is 46.50, a * It is 11.64, b * The value was 25.72, and the ΔE was 5.01. Although the overall effect was slightly inferior to the LVEF group, it was significantly better than the CK group. The LVEF+MT group performed best in all color indicators, with L... * The value reached 48.59 after 20 days of storage, significantly higher than other treatment groups. Figure 3 a). a * The value reached 22.18 at 12 days, significantly higher than other groups at the same time, and remained at 13.22 at 20 days, demonstrating the strongest ability to maintain a red color. Figure 3 b). b from the same period * The value was higher than other groups throughout the entire storage process, reaching 26.79 at 20 days. Figure 3 c). C * The value reached 35.63 at 12 days, which was 1.13 times that of the CK group. Figure 3 d). H o It shows a trend of first decreasing and then increasing, with a reddish hue. Figure 3 e). Its total color difference (ΔE) was only 5.14 on day 20, significantly lower than that of the CK group (P < 0.05). Figure 3 f) indicates that the synergistic treatment effectively slowed down the color deterioration.

[0069] Studies have shown that electrostatic fields can not only create a weak electric field on the fruit surface, altering cell membrane potential and water distribution, but also inhibit transpiration, thereby reducing fruit shrinkage and darkening caused by water loss (L). *Melatonin not only reduces the concentration of oxidative stress (ESD) but also inhibits microbial growth and enzyme activity, delaying physiological metabolism and oxidative damage in fruit tissues. Furthermore, melatonin not only activates antioxidant enzyme systems such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), reducing malondialdehyde (MDA) accumulation and protecting cell membrane integrity, but also regulates the phenylpropanoid metabolic pathway, inhibiting the activity of key enzymes such as phenylalanine ammonia-lyase (PAL) and cinnamic acid-4-hydroxylase (C4H), reducing the oxidative polymerization of phenolic substances, thereby inhibiting discoloration. This application uses the LVEF+MT group, demonstrating the synergistic effect of physical regulation and biological antioxidants, forming multiple protective effects. This treatment not only strengthens the protection of fruit cell membrane structures but also significantly delays the degradation of pigments such as anthocyanins and flavonoids by scavenging reactive oxygen species (ROS), maintaining the L-color of the fruit. * a * b * The stability of indicators such as [missing information].

[0070] 2.2 Changes in fruit texture

[0071] Fruit texture properties are not only key sensory indicators determining consumer preference, but also decisive factors affecting the shelf life and transportation of sweet cherries. As shown in Table 1, the control group (CK) exhibited a continuous softening trend throughout the storage period. Hardness decreased significantly from 1220 g on day 0 to 688 g on day 20, a decrease of 43.6%. Springiness decreased from 0.703 to 0.313, a decrease of 55.4%. Chewiness decreased from 406 g to 210 g. Peel penetration force decreased from 258 g to 183 g, a decrease of 29.1%. The simultaneous decrease in these indicators suggests that fruit dehydration leads to a reduction in cell turgor pressure, while cell wall components (pectin, hemicellulose, and cellulose) are enzymatically degraded, weakening intercellular bonds, reducing the mechanical strength of the peel, and increasing the risk of cracking and damage during storage and transportation. In contrast, both low-voltage electrostatic field treatment and exogenous melatonin treatment significantly slowed the rate of fruit texture deterioration. On day 20, the hardness of the LVEF and MT groups remained at 888 g and 845 g, respectively, with loss rates of 27.2% and 30.8%, significantly lower than the 43.6% loss in the control (CK) group. Under the same storage conditions, the elasticity, chewiness, and peel penetration of the LVEF and MT groups were all higher than those of the CK group. The LVEF+MT group showed a significant synergistic effect on all textural parameters. On day 20, the hardness of the LVEF+MT group remained at 988 g, with a loss of only 19.1%, significantly higher than that of the LVEF group (888 g) and the MT group (845 g). During the same period, the springiness of the fruit remained stable at 0.580, significantly better than the samples treated with a single preservation method. Chewiness (259 g), resilience (0.203), and cohesiveness (0.559) were all the highest among the four groups, indicating that the pulp could quickly rebound and maintain high structural integrity during chewing. Peel penetration force was 244 g, a 5.3% decrease from day 0, but still higher than the LVEF group (227 g) and the MT group (220 g), showing that the peel's mechanical barrier was more robust and could effectively resist collisions and microbial contamination during cold chain transportation.

[0072] In summary, the LVEF+MT group effectively delayed the deterioration of sweet cherry fruit through both physical and biochemical pathways. On one hand, LVEF promotes electron transport and water molecule resonance in the internal respiratory system of sweet cherries, influencing enzyme activity, slowing down redox reactions, and reducing the depolymerization of pectin and hemicellulose. On the other hand, exogenous melatonin activates the antioxidant enzyme system in the fruit (such as SOD and CAT), directly scavenging free radicals, inhibiting the activity of browning-related enzymes (such as PPO and PAL), and promoting the accumulation of antioxidants such as phenols, thereby reducing oxidative damage and maintaining cell membrane integrity. The synergistic effect of these two factors not only maintains the firmness, elasticity, and chewiness of the flesh but also significantly improves the strength and cohesiveness of the peel.

[0073] Table 1. Changes in the texture properties of sweet cherries under different storage periods and preservation treatments.

[0074]

[0075] Note: Data results are the mean ± standard deviation of three replicate experiments. Different letters at the same time point indicate significant differences between treatments (P < 0.05).

[0076] 2.3 Changes in fruit stalk browning rate and fruit browning index

[0077] During fruit senescence, the fruit's own antioxidant system gradually weakens its ability to scavenge reactive oxygen species (ROS), leading to ROS accumulation and oxidative stress. This process damages DNA, RNA, proteins, and lipids, ultimately resulting in overripe fruit. The browning rate of the fruit stalk in the CK group showed a typical "exponential" upward trend. Figure 4 a) By day 4, the browning rate exceeded 10%, reaching 36.5% by day 12, and as high as 99.95% by day 20, almost completely browned. The browning index of the fruit also continued to rise in tandem. Figure 4 (b) The browning rate increased from 2.67% on day 8 to 16.83% on day 20, a 6.3-fold increase. This indicates that phenolic substances in the fruit stalk and pulp are catalyzed and oxidized by endogenous polyphenol oxidase (PPO) and peroxidase (POD), leading to irreversible browning. On day 20, the browning rate of the fruit stalk in the single-treatment LVEF group and MT group decreased to 30.67% and 28.48%, respectively. Although the difference between the two was not significant (P>0.05), it was much lower than that in the CK group. In terms of fruit browning index, the MT group performed better than the LVEF group, with 6.83% and 11.74% on day 20, respectively, indicating that MT had a more significant inhibitory effect on PPO activity and phenolic substrate accumulation. However, neither single technology could completely block the browning chain reaction, and the browning rate of the fruit stalk remained at about 30%, which was insufficient to meet the strict requirements for maintaining the green color of the fruit stalk.

[0078] On the 20th day of storage, the browning rate of fruit stalks in the LVEF+MT group decreased to 19.67%, which was 80.3% lower than that in the CK group (P<0.05), and about 10% lower than that in the single preservation treatment. Figure 4 a). The fruit browning index was only 3.65%, a decrease of 78.3% compared to the CK group, and significantly lower than the LVEF group and MT group, which were preserved using a single method. Figure 4 (b) The LVEF+MT group significantly delayed the browning index and stalk browning rate of the fruit. This is because LVEF creates a negative ion environment around the sweet cherries, inhibiting microbial growth and achieving a preservation effect. MT, on the other hand, directly removes superoxide anions (·O2) from the fruit. - The LVEF+MT group effectively inhibited the release of reactive oxygen species such as hydroxyl radicals (·OH) and hydrogen peroxide (H2O2). The synergistic effect of the LVEF+MT group delayed the browning critical point by approximately 8 days, significantly extending the shelf life of sweet cherries.

[0079] 2.4 Changes in fruit weight loss rate and fruit decay rate

[0080] The low-temperature, atmospheric-pressure plasma used in this application can reduce the microbial load on the fruit surface, decrease the fruit decay rate, and increase its surface hydrophilicity, which is beneficial for the subsequent adhesion of preservatives. Similar preservation mechanisms have been verified in other fruits and vegetables: Hu et al. used low-temperature, atmospheric-pressure plasma treatment on blueberries, which inhibited... Botrytis cinerea The spores germinated, and within ≤15 min, the effects on fruit firmness, pH, and Oxidation-Reduction Potential (ORP) were minimal, with only slight darkening of color. This indicates that short-duration low-temperature atmospheric pressure plasma can achieve surface disinfection without significantly damaging the tissue structure, similar to the treatment in this study, further supporting the effectiveness of short-duration, high-power low-temperature atmospheric pressure plasma treatment for fruit safety and preservation.

[0081] like Figure 5As shown, the fruit weight loss rate in the CK group exhibited an exponential upward trend, with a gradual increase at the beginning and a steep increase at the end. During days 0-8, the fruit weight loss rate was only 1.85%; however, starting from day 12, the rate increased rapidly, reaching 10.99% by day 20, a 5.9-fold increase compared to day 8. The fruit rot rate exceeded 1% by day 4, jumped to 10.32% by day 12, and reached 18.77% by day 20. The fruit weight loss rate and rot rate were highly positively correlated (Pearson correlation coefficient r = 0.987, P ≤ 0.01), indicating that the destruction of the waxy layer of the pericarp and the abnormal stomatal regulation during the later stages of storage exacerbated water loss, thus providing favorable conditions for pathogen infection. Notably, during the same storage period, the fruit weight loss rate and fruit rot rate in the LVEF and MT groups were lower than those in the CK group. This phenomenon suggests that a low-voltage electrostatic field may delay fruit rot by inhibiting respiration intensity, reducing water loss, and decreasing the abundance of pathogenic fungi. Meanwhile, MT, as an antioxidant, can directly scavenge free radicals and regulate membrane lipid metabolism and phenolic synthesis, thereby reducing oxidative damage. By day 20 of storage, the LVEF+MT group controlled fruit weight loss and fruit decay rates at 4.42% and 6.43%, respectively, which were 59.7% and 65.7% lower than the CK group, and approximately 5% lower than the single preservation treatment group. This result indicates that the low-voltage electrostatic field and exogenous melatonin treatment formed a good complementary effect in time and space, and were more effective than internationally used preservation methods such as 1-MCP and tea polyphenols in reducing cherry fruit weight loss and decay rates. 1-MCP can block the ethylene signal transduction pathway, effectively delaying the ripening, softening, abscission, and senescence process of sweet cherries. In the preservation of non-climacteric fruits like sweet cherries, MT is more effective and reasonable than 1-MCP. Tea polyphenol treatment performed well in reducing decay rates, but may leave a bitter taste. Liu et al. studied the effects of HVEF on the physicochemical properties and fungal composition of sweet cherries during cold storage. They found that HVEF effectively maintained the soluble solids (TSS) content of cherry fruits, inhibited cherry respiration, and reduced the incidence of rot in sweet cherries. Although LVEF uses a lower voltage in this application, it performs equally well in maintaining firmness and color, and consumes less energy, making it suitable for large-scale promotion. Therefore, this study demonstrates that LVEF synergistically enhances preservation efficacy under low-temperature, ambient-pressure plasma conditions using MT. This method avoids chemical reagent residues and exhibits good performance in maintaining sensory quality.

[0082] 2.5 Changes in soluble solids (TSS) content and pH value in fruit pulp

[0083] During the storage period of 0–16 days, the TSS content in the CK group increased from 15.1% to 17.6%, but fell back to 16.7% on day 20, a decrease of 2.2% from the peak. Figure 6a). The pH value briefly decreased on day 8, then continued to rise, reaching 4.09 on day 20. Figure 6 (b) The LVEF group significantly delayed the respiratory climacteric by inhibiting fruit membrane permeability and ATPase activity, resulting in a peak TSS content of 17.5% that remained until day 20 (17.7%), an increase of 6.0% compared to the CK group. The pH increase was compressed to 0.18, reaching 3.82 on day 20. The peak TSS content in the MT group was delayed until day 16 (17.7%), decreasing to 17.1% on day 20, with a final pH of 3.92. The peak TSS content in the LVEF+MT group was advanced to day 12 (18.5%), remaining at 18.0% on day 20, an increase of 7.8% compared to the CK group, with fluctuations of less than 0.5% from day 12 to 20. The pH fluctuation was minimal (0.16), with a final value of 3.77, 0.32 lower than the CK group, and a variation of only 0.16 from day 4 to 20, significantly lower than the single preservation treatment groups. This application aligns with existing research finding that melatonin treatment of the sweet cherry variety "Yanyang" can maintain fruit firmness, color, and sugar-acid ratio, while reducing quality loss and spoilage. This indicates that MT can achieve preservation by activating endogenous antioxidant enzymes in the fruit and regulating phenolic metabolism.

[0084] 2.6 PCA Evaluation Among Indicators

[0085] This application employs principal component analysis (PCA) to perform dimensionality reduction analysis on 18 evaluation indicators of sweet cherry fruit, including color (L). * a * b * C * H o The results of the following parameters were measured: ΔE (grain hardness, elasticity, chewiness, resilience, cohesion, and peel penetration); browning rate of fruit stalks; browning index of fruit; fruit weight loss; fruit rot rate; TSS (total salinity) and pH value. See Table 2 for the results. Figure 7 Table 2 shows that the cumulative variance contribution rate of the three principal components (PC1, PC2, and PC3) with eigenvalues ​​greater than 1 reached 84.916%, with PC1 contributing 53.544%, PC2 23.069%, and PC3 8.303%. This indicates that these three principal components can represent more than 84% of the information in the original 18 indicators, and therefore can be used reliably and efficiently to evaluate fruit quality. PC1, with a contribution rate of 53.544%, mainly reflects fruit texture and decay. This principal component showed high positive loadings (all greater than 0.7) on texture (firmness, elasticity, chewiness, resilience, peel penetration) as well as indicators such as fruit stalk browning rate, fruit browning index, fruit weight loss rate, and fruit decay rate. A high PC1 score indicates severe fruit quality deterioration. PC2, with a contribution rate of 23.069%, mainly reflects fruit color and some chemical components. PC2 in a *b * C * And it shows a high positive load in the TSS index, in H o The higher loading rate indicates that PC2 is closely related to fruit color development and sugar accumulation. PC3's contribution rate of 8.303% is relatively complex, and its implications are related to b. * H o The color and texture of the fruit are closely related to indicators such as cohesiveness, and may reflect certain specific patterns of change in fruit color and texture.

[0086] Table 2 Principal Component Analysis of Variance

[0087]

[0088] From the comprehensive score chart ( Figure 8 As can be seen, the overall score of the CK group continuously decreased with the extension of storage time, from 2.04 on day 0 to -5.99 on day 20, indicating that the fruit quality deteriorated continuously with storage time. In the early stage of storage (4 days), the LVEF group had a higher score (1.36), indicating that it could better maintain the early quality of the fruit. Although the score decreased in the later stage, it was still much higher than the CK group, indicating that its preservation effect was effective but gradually weakened over time. Throughout the storage period, the score of the MT group was always higher than that of the CK group, especially in the later stage (8-12 days), where it could still maintain a positive value, but its maximum effect was not as good as that of the LVEF+MT group. The LVEF+MT group had the best preservation effect. Its overall score was the highest throughout the storage period, especially in the later stage (16-20 days), where it far led other groups, indicating that LVEF+MT produced a significant synergistic effect, effectively delaying the decline in fruit firmness, decay and quality deterioration, and extending the shelf life.

[0089] 3. Conclusion

[0090] This application systematically evaluated the regulatory effects of various postharvest treatments on the storage quality of sweet cherries. The preservation effect was ranked as follows: LVEF+MT group > MT group > LVEF group > CK group. A novel synergistic preservation strategy combining low-temperature plasma (CAP) pretreatment, low-voltage electrostatic field (LVEF), and exogenous melatonin (MT) was constructed for the first time, significantly delaying postharvest quality deterioration in sweet cherries. The synergistic effect is attributed to the sterile, hydrophilic surface provided by low-temperature atmospheric pressure plasma, which creates channels for more uniform MT penetration. LVEF and MT, through a dual mechanism of physical barrier and antioxidant defense, synergistically inhibit water loss, cell wall degradation, and oxidative browning, achieving efficient maintenance of fruit color, texture, and shelf life. This green, chemical-free, and easy-to-operate composite preservation technology effectively overcomes the limitations of single methods, providing a feasible solution and theoretical support for postharvest preservation of non-climacteric fruits with high respiratory metabolism (such as sweet cherries). Future research could further analyze the dynamic effects of LVEF-MT interaction on the transcriptome, metabolome, proteome, and phenotype of sweet cherry fruit, clarify its signal transduction network, and promote the optimization of the adaptability of this technology in different varieties, harvest maturity, and cold chain systems, so as to accelerate its transformation from laboratory to industrial application and provide a reference for the application of postharvest loss reduction and green preservation technologies for fruits and vegetables.

[0091] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for regulating the storage quality of sweet cherry fruits by synergistic effect of low-voltage electrostatic field under plasma pretreatment and melatonin, characterized in that, Includes the following steps: S1. Pretreatment: The sweet cherries are subjected to plasma treatment, then the sweet cherry fruits are washed with ultrapure water and placed on a stainless steel screen to drain the surface moisture at room temperature. Then, they are laid in a single layer in a sterile container. The container is connected to the plasma jet outlet through a glass tube. The fruits are laid in a single layer to obtain the pretreated sweet cherry fruits. S2. Melatonin treatment: Melatonin powder is dissolved in ultrapure water to prepare a preservation solution. Then, the pretreated sweet cherry fruits are placed in the preservation solution for soaking. After that, they are drained on a sterile filter to obtain melatonin-treated sweet cherry fruits. S3. Low-voltage electrostatic field treatment: Sweet cherry fruits treated with melatonin are placed in a low-voltage electrostatic field for treatment.

2. The method for regulating the storage quality of sweet cherry fruit by low-voltage electrostatic field synergistic melatonin under plasma pretreatment according to claim 1, characterized in that, In step S1, the plasma treatment is performed using an atmospheric pressure plasma jet device. The conditions for plasma treatment are: radio frequency power supply output voltage of 12 kV, frequency of 20 kHz, processing power of 200 W, and gas flow rate of 15 mL·min. -1 The processing time is 2.5 minutes.

3. The method for regulating the storage quality of sweet cherry fruit by synergistic effect of low-voltage electrostatic field and melatonin under plasma pretreatment according to claim 1, characterized in that, In step S1, the ambient temperature of the plasma treatment is 25 ± 1 °C and the relative humidity is 55%.

4. The method for regulating the storage quality of sweet cherry fruit by synergistic effect of low-voltage electrostatic field and melatonin under plasma pretreatment according to claim 1, characterized in that, In step S2, the concentration of the preservation solution is 100 μmol·L⁻¹. -1 .

5. The method for regulating the storage quality of sweet cherry fruit by synergistic effect of low-voltage electrostatic field and melatonin under plasma pretreatment according to claim 1, characterized in that, In step S2, the specific conditions for the soaking treatment are: soaking at 25 °C for 5 min.

6. The method for regulating the storage quality of sweet cherry fruit by synergistic effect of low-voltage electrostatic field and melatonin under plasma pretreatment according to claim 1, characterized in that, In step S3, the specific process of the low-voltage electrostatic field treatment is as follows: a low-voltage electrostatic field device is set up 80 mm above the melatonin-treated sweet cherry fruit.

7. The method for regulating the storage quality of sweet cherry fruit by low-voltage electrostatic field synergistic melatonin under plasma pretreatment according to claim 6, characterized in that, In step S3, the output voltage of the low-voltage electrostatic field device is 200 V.

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