Fresh-keeping method, system and composition for prolonging shelf life of picked lettuce

Abstract

The invention belongs to the technical field of agricultural product preservation, and discloses a lettuce preservation method which comprises the following steps: raw material selection and pretreatment: harvesting fresh lettuce with proper maturity, removing outer leaves and impurities, washing surface floating dust with clear water, and draining for later use; preparation of a melatonin solution: dissolving melatonin in a small amount of methanol, and diluting with distilled water to a target concentration; spraying the picked lettuce with a melatonin aqueous solution, and draining the lettuce subjected to spraying treatment; and storing the drained lettuce in a normal temperature environment of 20-25 DEG C or a refrigeration environment of 4 + / -1 DEG C. The method can prolong the shelf life of the lettuce and maintain the freshness, nutrition and sensory quality of the lettuce.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of agricultural product preservation, and particularly relates to a preservation method, system and composition for prolonging the shelf life of postharvested lettuce. BACKGROUND

[0002] As a leafy vegetable, lettuce (Lactuca sativa L.) is prone to wilting, yellowing and rotting due to water loss, oxidation, microbial infection and vigorous respiration after harvesting, resulting in a short shelf life. In traditional preservation techniques, chemical fungicides (such as chlorine preparations) are not in line with food safety requirements due to the risk of residues. Although simple low-temperature refrigeration can prolong the shelf life, the cost of cold chain transportation is high, and leaf yellowing cannot be completely avoided. At present, there is no safe and low-cost special preservation technology for lettuce due to its characteristics of wilting and yellowing after harvesting.

[0003] Melatonin (N-acetyl-5-methoxytryptamine) is a natural indole amine substance widely present in plants, which has functions of antioxidant, free radical scavenging, anti-aging and regulation of plant stress resistance. Studies have shown that exogenous melatonin treatment can significantly reduce the weight loss rate of fruits and vegetables after harvesting, inhibit respiration intensity and ethylene release, and reduce active oxygen (ROS) accumulation by increasing antioxidant enzyme activity (such as SOD, POD, CAT) to protect cell membrane integrity, thereby prolonging the shelf life. For example, the application of melatonin in lettuce (a close relative of lettuce) shows that a concentration of 100 μmol / L can effectively delay the decline of color, hardness and nutritional components. However, there is no in-depth research report on the systematic application of melatonin in postharvested lettuce preservation.

[0004] In the current patent field, melatonin is mainly used in plant fertilizers (such as drought-resistant cultivation), fruit preservation (such as peaches and passion fruits) or composite preservatives (such as cabbage), but there is still a blank in the preservation method of different concentrations of melatonin for postharvested lettuce storage. Therefore, it is urgent to develop an efficient and safe lettuce preservation technology to fully utilize the natural advantages of melatonin for preservation. SUMMARY

[0005] The technical problems to be solved by the present application are that lettuce is prone to wilting and yellowing within 1-2 days after harvesting, and the shelf life at room temperature is short. The existing technology has safety hazards or high cost. The present application solves the problems by using 10-200 μmol / L melatonin aqueous solution for spraying treatment combined with room temperature (20-25°C) or cold storage (4±1°C) regulation to prolong the shelf life and maintain the quality.

[0006] The present application is implemented as follows: a lettuce preservation method, which comprises:

[0007] S1: raw material selection and pretreatment: collect fresh lettuce with appropriate maturity, remove outer leaves and impurities, rinse the surface dust with clean water, and drain for standby;

[0008] S2: preparation of melatonin solution: dissolve melatonin in a small amount of methanol, and then dilute with distilled water to the target concentration;

[0009] S3: post-harvest lettuce is sprayed with melatonin aqueous solution, and the sprayed lettuce is drained;

[0010] S4: the drained lettuce is stored in a normal temperature environment of 20-25℃ or a refrigerated environment of 4±1℃.

[0011] Further, the concentration of the melatonin solution is 10-200 μmol / L.

[0012] Further, the draining in S4 is placing the lettuce in a ventilated environment with a relative humidity of 45-55%, and draining until no free water is visible on the surface.

[0013] Further, in S4, the relative humidity of the normal temperature storage environment is 50-70%, and the relative humidity of the 4℃ refrigerated storage environment is 85-95%.

[0014] Further, after draining in step S3, before storage in step S4, the lettuce is placed in a breathable packaging container.

[0015] The application provides an application of melatonin in preparing a preservative for prolonging the shelf life of post-harvest lettuce, and the preservative is an aqueous solution, wherein the application concentration of melatonin is 10-200 μmol / L, and the lettuce is sprayed.

[0016] In combination with the above technical solutions and solved technical problems, the technical solution to be protected by the application has the following advantages and positive effects:

[0017] The application can prolong the shelf life, prolong the normal temperature storage period by 1-2 times (up to 3-5 days) compared with the control, prolong the 4℃ refrigerated storage period to more than 10 days, significantly reduce the rot rate, maintain the quality, maintain the leaf greenness (improve the chlorophyll retention rate), brittleness (slow down the loss of hardness), and sensory quality (reduce yellowing, browning, and peculiar smell), and is simple to operate, low in cost, and easy to industrialize.

[0018] The application preserves the lettuce by regulating the physiological metabolism of the plant. According to experimental data, the weight loss rate of fresh-cut lettuce treated with 50 μmol / L melatonin for one week can be reduced by about 15% under normal temperature conditions, and by about 20% at 4°C. The loss of chlorophyll content is reduced by 7 SPAD units and 5 SPAD units, respectively, and the MDA content is reduced by about 15 nmol / mg and 12 nmol / mg, respectively, proving its significant advantage in preservation.

[0019] In the field of fresh e-commerce and retail, the shelf life of lettuce can be extended, and the loss during transportation and storage can be reduced, especially suitable for e-commerce cold chain distribution, and the product competitiveness can be improved.

[0020] In the catering industry and household consumption, it can facilitate the bulk storage of lettuce in restaurants and reduce the daily purchase frequency. Small fresh-keeping devices (such as refrigerator modules equipped with this technology) for household use can improve the lettuce preservation experience and reduce waste.

[0021] In the agricultural industry chain, it is applied from the moment of post-harvest treatment, throughout the whole chain of processing, transportation, and sales, promoting the standardization of the lettuce industry and helping the value-added of agricultural products.

[0022] The application effectively solves the core problems of browning, rapid nutrient loss, and short shelf life of fresh-cut lettuce during storage by regulating the physiological metabolism of the plant. The main mechanisms include: reducing the weight loss rate; by enhancing the activity of antioxidant enzymes such as SOD and POD, removing active oxygen free radicals, and reducing the MDA content, thereby maintaining the integrity of the cell membrane; in addition, melatonin can also significantly retain nutrients such as chlorophyll, allowing the lettuce to maintain a high soluble solid content during storage.

[0023] Traditional preservation technologies such as cold chain have formed a mature supply chain system, and the market may prefer this "risk-free" solution. However, data shows that the preservation effect of melatonin treatment under cold storage conditions is better than simple low-temperature storage. For example, fresh-cut lettuce treated with 50 μmol / L melatonin and stored at 4°C for 7 days has a significant effect on leaf greenness, brittleness, and sensory quality compared to the control group, proving its significant advantage in inhibiting browning. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 For: Sensory photos of lettuce treated with different concentrations of melatonin at room temperature (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0025] Figure 2Figure 1 is a photograph of the sensory evaluation of the lettuce treated with different concentrations of melatonin at room temperature (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0026] Figure 3 Figure 2 is a graph showing the effect of different concentrations of melatonin on the weight loss rate of the lettuce at room temperature (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0027] Figure 4 Figure 3 is a graph showing the effect of different concentrations of melatonin on the weight loss rate of the lettuce at 4°C (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0028] Figure 5 Figure 4 is a graph showing the effect of different concentrations of melatonin on the chlorophyll content of the lettuce at room temperature (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0029] Figure 6 Figure 5 is a graph showing the effect of different concentrations of melatonin on the chlorophyll content of the lettuce at 4°C (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0030] Figure 7 Figure 6 is a graph showing the effect of different concentrations of melatonin on the nitrogen content of the lettuce at room temperature (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0031] Figure 8 Figure 7 is a graph showing the effect of different concentrations of melatonin on the nitrogen content of the lettuce at 4°C (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0032] Figure 9 Figure 7 is a graph showing the effect of different concentrations of melatonin on the MDA content of lettuce at room temperature (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT);

[0033] Figure 10 Figure 8 is a graph showing the effect of different concentrations of melatonin on the MDA content of lettuce at 4°C (A: control group, B: 10 μmol / L MT, C: 50 μmol / L MT, D: 100 μmol / L MT, E: 150 μmol / L MT, F: 200 μmol / L MT). DETAILED DESCRIPTION

[0034] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to examples. It should be understood that the specific examples described herein are only used to explain the present application and do not limit the present application.

[0035] In the existing field of fresh food preservation, the shelf life of lettuce under room temperature and cold storage conditions is limited mainly due to the increase in cell membrane permeability, accumulation of active oxygen free radicals and continuous rise in browning enzyme activity. Traditional chemical preservatives (such as sodium hypochlorite, sodium benzoate, etc.) can delay spoilage to some extent, but there are problems of residual risk and limited inhibition effect. Therefore, how to effectively slow down the quality deterioration of postharvest lettuce while ensuring food safety has become a key technical difficulty in industrial application.

[0036] Melatonin, as a naturally occurring indole molecule in plants, has been proven to have the functions of scavenging active oxygen, enhancing stress resistance and regulating gene expression. In the postharvest treatment process of fruits and vegetables, melatonin can effectively reduce the membrane lipid peroxidation level by regulating the activity of antioxidant enzyme systems (such as superoxide dismutase, catalase, ascorbate peroxidase, etc.), thereby delaying the damage to cell membrane integrity and reducing the leakage of permeating substances. Based on this mechanism of action, spraying melatonin aqueous solution directly on postharvest lettuce tissue becomes an innovative preservation approach with food safety and functionality.

[0037] In the application process, dissolving melatonin in a small amount of methanol and diluting it into an aqueous solution of appropriate concentration can ensure its stability and sprayability. After spraying treatment, the lettuce is drained to avoid the risk of microbial growth caused by free water residue, while the melatonin molecules can penetrate into the cell surface layer and part of the tissue, starting the plant's own antioxidant and stress response pathways. This process is different from simply covering with exogenous preservatives, but it achieves endogenous protection by inducing physiological activity.

[0038] In terms of storage conditions, the method simultaneously considers the normal temperature flow field scenario and the cold chain transportation system. In the normal temperature storage at 2025℃, the melatonin can inhibit the increase of the respiration rate and the acceleration of the water loss rate caused by the temperature, so as to maintain the greenness and the brittleness of the leaves; under the cold storage condition of 4±1℃, the melatonin further strengthens the protection of the membrane structure and delays the cold damage and the degradation of chlorophyll induced by the low temperature. The compatibility makes it suitable for the centralized preservation in the cold storage at the production site and also plays a role in the normal temperature shelf at the retail end.

[0039] Meanwhile, in the implementation process, the concentration of the spraying liquid is controlled in the range of 10200 μmol / L, so as to ensure the preservation effect and avoid the waste of resources. The research shows that the too low concentration is not enough to effectively start the antioxidant enzyme system, and the too high concentration may cause the reverse effect. The concentration setting of the method is reasonably optimized and is suitable for the application and popularization of different varieties and batches of lettuce.

[0040] The technical route proposed in the application not only solves the residue problem of the traditional chemical preservative, but also avoids the high energy consumption limitation of simply relying on the cold chain. The mechanism is based on the endogenous regulation of plants, has the characteristics of green, environmental protection and safety, can meet the current demand of the fresh supply chain for "harmlessness, low cost and sustainability", and is expected to be widely applied in postharvest processing, wholesale circulation and retail links, significantly prolong the shelf life of lettuce and improve the market value.

[0041] The embodiment of the application provides a lettuce preservation method, which comprises the following steps:

[0042] S1: raw material selection and pretreatment: fresh lettuce with appropriate maturity is collected, the outer leaves and impurities are removed, the surface dust is washed with clean water, and then the lettuce is drained for standby;

[0043] S2: preparation of melatonin solution: the melatonin is dissolved in a small amount of methanol, and then distilled water is used for dilution to a target concentration;

[0044] S3: the postharvested lettuce is sprayed with the melatonin aqueous solution, and then the sprayed lettuce is drained;

[0045] S4: the drained lettuce is stored in a normal temperature environment of 2025℃ or a cold storage environment of 4±1℃.

[0046] Further, the concentration of the melatonin solution is 10 μmol / L200 μmol / L.

[0047] Further, the draining in S4 is that the lettuce is placed in a ventilated environment with a relative humidity of 4555%, and then the lettuce is drained until no free water is visible on the surface.

[0048] Further, in the S4, the relative humidity of the normal temperature storage environment is 50-70%, and the relative humidity of the 4℃ refrigeration storage environment is 85-95%.

[0049] Further, after the draining in the step S3, before the storage in the step S4, the step of placing the lettuce in a breathable packaging container is further included.

[0050] The application provides application of melatonin in preparation of a preservative for prolonging postharvest shelf life of lettuce, the preservative is an aqueous solution, wherein the application concentration of the melatonin is 10 μmol / L-200 μmol / L, and the lettuce is subjected to spraying treatment.

[0051] Embodiment 1

[0052] Raw material selection and pretreatment: fresh lettuce with appropriate maturity, no disease and insect pests, and no mechanical damage is selected, the outer leaves and impurities are removed, and the surface floating dust is washed with clean water and then drained for use.

[0053] Melatonin solution preparation: melatonin is dissolved in an appropriate amount of methanol (or other safe cosolvent), and then distilled water is used for dilution to the target concentration. The preferred concentration range is 10-100 μmol / L (for example, 10, 50, 100 μmol / L), which can be adjusted according to actual storage requirements.

[0054] Normal temperature preservation: the treated lettuce is wrapped with a breathable preservative bag or a preservative film, and is stored in an environment with a temperature of 20±5℃ and a relative humidity of 50-70%.

[0055] Embodiment 2

[0056] The materials and steps are the same as those in Embodiment 1, and finally the treated lettuce is also packaged and stored in a refrigeration environment with a temperature of 4±1℃ and a relative humidity of 85-95%.

[0057] Index determination: after treatment with different concentrations of melatonin, the phenotypes of the lettuce in each group are recorded, and the weight loss rate, chlorophyll content, nitrogen content and MDA content are counted.

[0058] Experiment and analysis

[0059] (1) Sensory analysis of lettuce

[0060] For example, Figure 1 and Figure 2 It can be seen that with the increase of storage time, the leaves of fresh lettuce gradually turn yellow and rot, and the leaves of the control lettuce have the highest rot degree, and the result obtained by using 50 μmol / L melatonin is the best. This may be because melatonin has significant antioxidant effect, can reduce the accumulation of active oxygen, and thus can improve the sensory quality of the lettuce during storage.

[0061] (2) Analysis of weight loss rate

[0062] From Figure 3 and Figure 4 It can be seen that the weight loss rate of lettuce increases with the extension of storage time. However, the weight loss rate of the control group is significantly higher than that of the melatonin treatment group during storage. In addition, the weight loss rate of the 50 μmol / L melatonin treatment group is lower than that of other melatonin treatment groups.

[0063] (3) Chlorophyll content analysis

[0064] During storage, lettuce leaves are prone to yellowing due to chlorophyll degradation. The color change of green leafy vegetables is closely related to chlorophyll content. As shown in Figure 5 , 6 the chlorophyll content of the control group and the treatment group showed a downward trend, and compared with the control group, the 50 μmol / L melatonin treatment maintained a relatively good color and delayed the occurrence of yellowing.

[0065] (4) Nitrogen content analysis

[0066] During the preservation process, the change of nitrogen content can reflect the degree of consumption of nutrients in the plant body. If the nitrogen content decreases rapidly, it may mean that the plant is aging rapidly and the preservation effect is not good. As shown in Figure 7 , 8 the nitrogen content of the control group and the treatment group showed a downward trend, and compared with the control group, the 50 μmol / L melatonin treatment maintained a relatively low nitrogen loss.

[0067] (5) MDA content analysis

[0068] As shown in Figure 9 , 10 the MDA content of lettuce increased continuously during the whole storage period, but the 10, 50, and 100 μmol / L melatonin treatment groups were lower than the control group. The effect of the 50 μmol / L melatonin treatment group was the most significant during the whole storage period, indicating that melatonin effectively controlled the degree of cell membrane damage, thereby delaying the decay of lettuce.

[0069] Example 3: Melatonin aqueous solution spraying preservation treatment

[0070] This example provides a method for extending the shelf life of postharvest lettuce. Dissolve melatonin in a small amount of methanol, then dilute with distilled water to a final concentration of 50 μmol / L to prepare an aqueous preservative. After harvesting fresh lettuce, remove the outer leaves and wash off the dust, then evenly spray the melatonin aqueous solution with a sprayer to ensure that a thin layer of liquid film is formed on the surface. After treatment, place it in a clean and ventilated condition for natural draining, and then enter the cold storage stage.

[0071] Example 4: Application of high concentration melatonin

[0072] This example provides a high-concentration melatonin fresh-keeping treatment. The concentration of the melatonin solution is set to 200 μmol / L, and the same preparation and spraying method as in Example 1 is used. After spraying, the lettuce is stored at room temperature of 25°C to verify the effect of melatonin on extending the shelf life.

[0073] The test shows that the untreated lettuce under room temperature conditions wilts and yellows after about 3 days, while the treated group can still maintain good appearance quality after 7 days, the leaf water loss rate is reduced by 15%, and the chlorophyll content remains at a high level, indicating that melatonin also exhibits obvious fresh-keeping effect under high-temperature storage and transportation conditions.

[0074] Example 5: Melatonin composition without preservatives

[0075] This example provides a lettuce fresh-keeping composition. Melatonin is taken and prepared into a 100 μmol / L aqueous solution, the dilution medium is distilled water, and no bacteriostatic agent, preservative or organic acid substance is added to ensure that the composition is single and safe.

[0076] The composition does not produce chemical residue risk to the lettuce after spraying. Under refrigeration conditions at 4°C, the lettuce treated with the composition still maintains a high sensory score after 7 days, and the leaf erectness is significantly improved compared with the control group, proving that a simple formula can achieve ideal fresh-keeping effect.

[0077] Example 6: Application of lettuce fresh-keeping treatment system

[0078] This example provides a system for lettuce fresh-keeping treatment. The system includes a melatonin solution preparation unit (automatic metering and mixing), a spraying unit (fine atomizing nozzle), a draining unit (blower ventilation device) and a storage unit. The storage unit is provided with a temperature control module and a humidity adjustment module, which can be maintained at 4±1°C and 90% relative humidity.

[0079] The system is used for whole-process treatment of post-harvest lettuce. The experimental results show that the lettuce treated by the system can maintain 1 week without obvious yellowing or water loss under refrigeration environment, and the leaf structure is complete, which is significantly better than the control of conventional water mist spraying and ordinary refrigeration.

[0080] Example 6: Comparison test under low temperature and room temperature conditions

[0081] This example compares and verifies the effect of melatonin treatment under different storage conditions. The concentration of the melatonin aqueous solution is 20 μmol / L, and the process of spraying-draining-storing is used. The storage conditions are room temperature (25°C) and refrigeration (4°C).

[0082] The results show that, under normal temperature conditions, the shelf life of the treated group of lettuce is extended from 3-4 days to 6-7 days; under refrigerated conditions, the treated group of lettuce can maintain freshness for more than 14 days, while the control group shows obvious yellowing and water loss after about 7 days. This example demonstrates that melatonin can play a fresh-keeping role under different temperature conditions, enhancing the universality of application.

[0083] In a specific embodiment, the lettuce fresh-keeping composition is prepared by dissolving melatonin in distilled water, and the concentration of melatonin can be adjusted between 10 μmol / L and 200 μmol / L according to storage needs. The composition is a clear and transparent aqueous solution, which can be directly used for spraying treatment. Since it does not contain bacteriostatic agents, preservatives and organic acid compounds, the composition avoids the risk of chemical residues and changes in taste, ensuring the green, safe and pollution-free characteristics of lettuce during fresh-keeping treatment.

[0084] In another specific embodiment, the system for lettuce fresh-keeping treatment includes a melatonin solution preparation unit, a spraying unit, a draining unit and a storage unit connected in sequence. The solution preparation unit is equipped with a precision weighing device and a stirring container for dissolving melatonin powder in distilled water in proportion; the spraying unit can use an automatic atomizing nozzle to ensure uniform coverage of the lettuce leaves with the melatonin solution; the draining unit is a ventilation rack or a blowing device to quickly evaporate excess water and prevent decay caused by water retention.

[0085] In yet another embodiment, the storage unit is equipped with a temperature control module and a humidity adjustment module to maintain the storage environment temperature at 4±1℃ and the humidity at 85% to 95%. Under these conditions, the melatonin-treated lettuce can significantly delay the yellowing and wilting process of the leaves, inhibit respiration and chlorophyll degradation, thereby extending the shelf life and maintaining the commodity quality. This system can be widely applied in cold chain transportation and supermarket storage, etc., and has good practical application prospects.

[0086] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any modification, equivalent replacement and improvement made by those skilled in the art within the technical scope disclosed by the present application shall be covered within the protection scope of the present application.

Claims

1. A method for prolonging the shelf life of postharvested lettuce, characterized in that, The method comprises the following steps: Step 1: harvesting fresh lettuce with appropriate maturity, removing outer leaves and impurities, washing and draining; Step 2: dissolving melatonin in methanol, then diluting with distilled water to a predetermined concentration to obtain a melatonin aqueous solution; Step 3: uniformly spraying the melatonin aqueous solution on the surface of the lettuce; Step 4: storing the treated lettuce under constant temperature and humidity conditions.

2. The method of claim 1, wherein The concentration of the melatonin aqueous solution is 10-200 μmol / L.

3. A method for storing lettuce based on melatonin treatment, characterized by, The method comprises: spraying the melatonin aqueous solution on the lettuce; draining the lettuce in a ventilated environment with a relative humidity of 45-55% after spraying; storing the drained lettuce in a refrigerated environment with a temperature of 4±1℃ and a relative humidity of 85-95%.

4. The storage method according to claim 3, characterized by, The drained lettuce after the spraying treatment is packaged in a breathable packaging container and then refrigerated.

5. Use of melatonin for the preparation of a preservative for prolonging the shelf life of postharvested lettuce, characterized in that, The preservative is in the form of an aqueous solution, and the application method is surface spraying on postharvest lettuce.

6. Use according to claim 5, characterized in that, The application concentration of the melatonin is 10-200 μmol / L.

7. A composition for preserving lettuce, characterized by comprising: The method comprises: melatonin; the dilution medium is distilled water; the concentration of the melatonin in the dilution medium is 10-200 μmol / L; the composition is in the form of a sprayable aqueous solution.

8. The composition of claim 7, wherein, The composition does not contain bacteriostatic agents, preservatives and organic acid compounds.

9. A system for the preservation of lettuce, characterized in that, The method comprises: a melatonin solution preparation unit for preparing a melatonin aqueous solution with a specific concentration; a spraying unit for uniformly spraying the melatonin aqueous solution on the surface of the lettuce; a draining unit for removing surface water from the sprayed lettuce in a ventilated environment; a storage unit for storing the treated lettuce under preset temperature and humidity conditions.

10. The system of claim 9, wherein, The storage unit comprises a temperature control module and a humidity adjustment module for maintaining a storage environment with a temperature of 4±1℃ and a humidity of 85-95%.

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