Litchi modified atmosphere preservation method combining physical field and chemical sterilization

Abstract

The present application relates to the field of litchi preservation and storage, in particular to a litchi modified atmosphere preservation method combining physical field and chemical sterilization. The main steps of the method include: screening, cleaning and pre-cooling the picked fresh litchi, then wrapping it with temperature-sensitive polyurethane modified atmosphere bag or modified atmosphere film, sterilizing it with physical field, adding ClO2 slow-release antibacterial microcapsules, sealing the litchi and storing it at room temperature. The method uses physical field, ClO2 slow-release antibacterial microcapsules and spontaneous modified atmosphere preservation in combination, which has obvious synergistic effect on litchi preservation. Fresh litchi can be stored for 9-12 days at room temperature (25-30 DEG C), and the storage time of fresh litchi at 4 DEG C can reach 35-40 days.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of litchi preservation and storage, in particular to a litchi modified atmosphere preservation method combining physical field and chemical sterilization. BACKGROUND

[0002] Litchi (Litchi / lychee) is native to China and is a specialty fruit widely cultivated in subtropical regions of southern China. It has high economic value and is one of the most competitive fruits in the international market for China. However, litchi has a short hanging period on the tree, and within 1-2 days after harvesting, it will show signs of decay and browning. It is not resistant to storage and transportation, and according to incomplete statistics, the postharvest loss rate of litchi is as high as 20%. At room temperature, the shelf life of litchi is only 3-5 days, and after 5 days, it begins to discolor and lose flavor, which seriously hinders the development of the litchi industry.

[0003] Currently, the preservation methods for postharvest litchi include chemical preservation, ozone preservation, modified atmosphere packaging preservation, irradiation preservation, new intelligent modified atmosphere packaging, edible coating preservation, microencapsulated natural antibacterial agent, conventional low-temperature preservation, low-temperature plasma preservation, etc. During transportation, ice blocks or ice packs combined with foam boxes are most commonly used for fresh litchi. The cost of ice block preservation is much lower than that of cold storage preservation, but it also results in a shorter preservation period of litchi, which cannot meet the market needs of long-distance transportation. To date, there is no universally applicable and cost-effective long-term preservation technology for litchi worldwide. Chemical preservation and modified atmosphere preservation have been widely used in litchi preservation, and multiple methods are often combined to achieve better results. Plasma refers to an ionized gas with a certain density of charged particles. Plasma sterilization is a new technology with broad-spectrum sterilization, low temperature, high efficiency, and no residue. It is a cold sterilization technology and is widely used in industries and medical fields. Plasma sterilization can completely sterilize fresh fruits and vegetables and fresh-cut vegetables, without secondary pollution, and can better maintain the sensory quality and nutritional ingredients. Chinese patent CN202310946702.8 discloses a hydrophobic antibacterial PE preservation film prepared by a film blowing process and applied to litchi preservation. Chinese patent CN202310214798.9 discloses a litchi preservation and storage method based on electrostatic field combined with low-temperature treatment. Since current litchi preservation is mostly assisted by low-temperature and chemical preservation, the preservation cost is high, and it is not suitable for the fluctuation of temperature and gas environment during actual transportation and sales of litchi. The preservation effect is not good, and there is an urgent need to find a litchi preservation method that can spontaneously adjust the gas composition and inhibit bacteria and moisture.

[0004] Therefore, it is crucial to develop a self-regulating modified atmosphere film that can inhibit the respiratory intensity of litchi postharvest gas environment and apply it to litchi preservation in combination with physical and chemical sterilization methods to prolong the shelf life of litchi. SUMMARY

[0005] The present application aims at the problems of high cost and poor preservation effect under fluctuating temperature in the current low-temperature assisted chemical preservation of litchi, and provides a self-regulated breathable and moisture-permeable packaging film, combined with ozone sterilization and ClO2 slow-release antibacterial, to achieve the extension of the shelf life of litchi at room temperature, and provide a reference for the development of litchi storage and preservation technology.

[0006] The purpose of the present application is achieved by the following technical solutions:

[0007] A litchi modified atmosphere preservation method combining physical field with chemical sterilization, comprising the following steps:

[0008] (1) screening, cleaning and pre-cooling treatment of picked fresh litchi;

[0009] (2) wrapping the litchi treated in step (1) with a temperature-sensitive polyurethane modified atmosphere bag or film;

[0010] (3) sterilizing the litchi wrapped in step (2) with a physical field; when the litchi is wrapped with a temperature-sensitive polyurethane modified atmosphere bag or film, the litchi is not completely sealed to ensure that the litchi is in full contact with the physical field treatment environment;

[0011] (4) sealing the litchi after adding ClO2 slow-release antibacterial microcapsules and citric acid microcapsules and storing the litchi at room temperature.

[0012] Preferably, the temperature-sensitive polyurethane modified atmosphere bag or film in step (2) is prepared by the following steps:

[0013] (2-1) mixing polyols and isocyanate under an inert gas atmosphere and reacting at 80-95 DEG C for 1-3h;

[0014] (2-2) adding chain extender, isocyanate and viscosity regulator and continuing to react for 1-3h to obtain polyurethane liquid;

[0015] (2-3) spreading the polyurethane liquid into a film, heating and curing to obtain the temperature-sensitive polyurethane modified atmosphere film;

[0016] (2-4) heat sealing the modified atmosphere film into a preservation bag by using a sealing machine to obtain the temperature-sensitive polyurethane modified atmosphere bag.

[0017] Preferably, in step (2-1), the polyols are polyethylene glycol and castor oil, and the isocyanate is 1,6-hexamethylene diisocyanate;

[0018] In step (2-2), the chain extender is 1,4-butanediol, the isocyanate is 1,6-hexamethylene diisocyanate, and the viscosity regulator is dimethylformamide;

[0019] In step (2-3), the temperature for the heat curing is 70-90℃, and the time is 15-25h.

[0020] Preferably, in step (2-1), the mass ratio of 1,6-hexamethylene diisocyanate, polyethylene glycol and castor oil is (3-4):(8-10):(4-5);

[0021] In step (2-2), the molar ratio of total -OH in the chain extender and polyol to -NCO in the isocyanate is 1:1; a viscosity modifier is added to make the solid content of the system 25-35%;

[0022] In step (2-3), the thickness of the obtained temperature-sensitive polyurethane modified atmosphere film is 80±5μm.

[0023] Preferably, the ClO2 slow-release antibacterial microcapsule or citric acid microcapsule in step (4) is prepared by the following steps: dissolving the polyurethane liquid in step (2-2) and the core material in a mixed solvent as an oil phase; dissolving polyvinyl alcohol in water as an aqueous phase; slowly adding the oil phase to the aqueous phase under stirring, and filtering, washing and drying after solidification to obtain the ClO2 slow-release antibacterial microcapsule or citric acid microcapsule; wherein the core material is NaClO2 or citric acid.

[0024] Preferably, the mixed solvent is a mixture of ethanol and chloroform in a volume ratio of 1:3-1:5; the mass concentration of polyvinyl alcohol in water is 0.5-2%; the volume ratio of the oil phase to the aqueous phase is 5-10:34; the mass concentration of polyurethane in the oil phase is 8-10%, and the mass concentration of the core material is 1-2%, and the mass ratio of the core material to polyurethane is 1-2:10.

[0025] Preferably, the physical field sterilization treatment in step (3) is using a physical field sterilization equipment integrating ozone and plasma, and the litchi in the open modified atmosphere bag is sterilized for 5-10min under the conditions of a plasma power of 25-30W and an ozone concentration of 1-3mg / L.

[0026] Preferably, the screening and cleaning in step (1) is to screen litchi free of pests and mechanical damage, and to retain a small part of branches and leaves.

[0027] The precooling treatment is precooling for 1-3h in an environment of 8-10℃ to ensure that the temperature of all litchi skins is lower than 15℃.

[0028] Preferably, in step (2), the size of the modified atmosphere bag is set to 52cm*50cm for 2.5kg of litchi, and 82cm*70cm for 5kg of litchi.

[0029] Preferably, in step (4), the added amounts of the ClO2 slow-release antibacterial microcapsules and the citric acid microcapsules are 3-5 g / bag, respectively, and the room temperature is 25-30 °C.

[0030] The advantages of the present application over the prior art are as follows:

[0031] 1) The use of temperature-sensitive polyurethane modified atmosphere film for litchi preservation can better adapt to self-adaptive fruit and vegetable preservation in a temperature fluctuating environment. Litchi has a high postharvest respiration rate, and a high CO2, low O2, and high humidity gas environment can inhibit its respiration rate, prevent water loss and browning, and water-soaking browning. In addition, fresh litchi is extremely susceptible to pathogen infection during storage, leading to mold or rot. Therefore, it is crucial to have a synergistic antibacterial effect under modified atmosphere preservation. Compared to passive modified atmosphere preservation, spontaneous modified atmosphere has environmental adaptability and can dynamically adjust the gas environment during litchi storage to inhibit litchi respiration (3% to 10% O2, 4% to 7% CO2, 85-95% relative humidity).

[0032] 2) The use of physical field sterilization equipment for postharvest litchi sterilization relies on the generation of ozone and plasma to effectively kill microorganisms on the litchi skin. Ozone is commonly used in litchi processing products, and 5-10 min of ozone treatment on fresh litchi can significantly slow down the increase in fruit skin browning index, maintain total suspended solids (TSS) content and total acid (TA) content. Ozone treatment can also effectively inhibit the activity of fruit skin polyphenol oxidase (PPO), induce the increase of superoxide dismutase (SOD) activity, and maintain the integrity of fruit skin cytoplasmic membrane, but it cannot induce the increase of peroxidase (POD) activity and accelerates the accumulation of membrane lipid peroxidation product malondialdehyde (MDA). Therefore, ozone treatment alone is not an ideal method for storing litchi, and it should be considered to combine ozone treatment with other preservation methods to achieve better results.

[0033] 3)ClO2 slow-release antibacterial microcapsules were prepared using temperature-sensitive polyurethane and polyvinyl alcohol. Compared with gaseous and liquid ClO2, the slow-release antibacterial microcapsules have high safety performance, are easier to store and transport, can exert long-term antibacterial effect, and have better preservation effect. ClO2 has strong oxidizing property, can effectively inhibit fruit and vegetable browning and microbial growth, has multiple effects of color protection and respiration inhibition, and can also reduce the ethylene release of postharvest fruits and vegetables. It is an ideal fruit and vegetable preservative. However, gaseous and liquid ClO2 are not stable, and high-concentration gaseous ClO2 also has a certain risk of explosion, which greatly increases the difficulty of practical application of ClO2. NaClO2 (sodium chlorite) is solid at room temperature and can release ClO2 under acidic conditions, is easy to transport and store, has high safety performance, and can release antibacterial effect during use. Based on this, pre-treated litchi is sterilized by ozone, stored in an environment-adaptive breathable and moisture-permeable modified atmosphere film, and a certain amount of ClO2 slow-release antibacterial agent is added, which can improve the postharvest quality of litchi, effectively delay litchi browning, significantly increase the soluble solids of litchi, and significantly reduce the respiration rate, thereby prolonging the shelf life of litchi.

[0034] 4) The use of physical fields, ClO2 slow-release antibacterial microcapsules, and spontaneous modified atmosphere preservation has obvious synergistic effect on litchi preservation. Fresh litchi can be stored for 9-12 days at room temperature (25-30°C), and for 35-40 days at 4°C. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 : Change of gas content during litchi preservation in each example.

[0036] Figure 2 : Change of humidity during litchi preservation in each example.

[0037] Figure 3 : Change of total bacterial count during litchi preservation in each example.

[0038] Figure 4 : Change of total mold count during litchi preservation in each example.

[0039] Figure 5 : Change of VC content during litchi preservation in each example.

[0040] Figure 6 : Change of soluble solids content during litchi preservation in each example.

[0041] Figure 7 : Change of weight loss rate during litchi preservation in each example.

[0042] Figure 8 : Change of water content during litchi preservation in each example.

[0043] Figure 9Hardness change of each embodiment during litchi preservation process.

[0044] Figure 10 Browning index change of each embodiment during litchi preservation process.

[0045] Figure 11 Good fruit rate change of each embodiment during litchi preservation process.

[0046] Figure 12 Sensory change of each embodiment during litchi preservation process. DETAILED DESCRIPTION

[0047] The application will be further described in conjunction with the embodiments and the accompanying drawings.

[0048] 1) Preparation of temperature-sensitive polyurethane modified atmosphere film

[0049] 1,6-hexamethylene diisocyanate (HDI) was used as the hard segment, polyethylene glycol (PEG-1500) and castor oil (CO) were used as the polyol soft segment, and 1,4-butanediol (BDO) was used as the chain extender. N2 was continuously introduced into a three-necked flask, 9 g of polyethylene glycol, 4.15 g of castor oil, and 2.36 g of 1,6-hexamethylene diisocyanate were added, and after stirring and mixing the reaction at 80°C for 2 h, 0.721 g of 1,4-butanediol and 1 g of 1,6-hexamethylene diisocyanate were added to ensure that the molar ratio of -OH and -NCO was 1:1. During this period, an appropriate amount of dimethylformamide (DMF) was added to adjust the solution viscosity, and the solid content was maintained at about 30%. After 2 h, a transparent viscous liquid (polyurethane) was obtained, which was poured onto a glass plate and spread into a film by a doctor blade. The film was placed in an 80°C oven and cured for 19 h to form a stable milky white film with a thickness of 80±5 μm. Differential scanning calorimetry (DSC) analysis combined with X-ray diffraction (XRD) analysis showed that the polyurethane film had significant modified atmosphere temperature-sensitive properties at room temperature (20-40°C), that is, the permeability of the film to CO2, O2, and H2O(g) changed with temperature. The prepared polyurethane film was heat-sealed into modified atmosphere bags with specifications of 52 cm*50 cm and 82 cm*70 cm using a hand pressure type plastic film sealing machine, and was applied to litchi preservation. Due to the temperature-sensitive properties of the film, the gas environment in the bag could quickly reach the appropriate composition for litchi preservation under temperature fluctuations.

[0050] 2) Physical field sterilization of litchi

[0051] The physical field sterilization equipment independently designed and prepared in the laboratory was used to sterilize litchi in modified atmosphere bags. The physical field equipment integrates ozone sterilization and low-temperature plasma sterilization. The instrument generates low-temperature plasma through dielectric barrier discharge, which contains active oxygen and active nitrogen and other bacteriostatic active substances, and can perform non-thermal sterilization on the surface of litchi. After sterilization for 5-10 min at a plasma power of 25-30 W, the surface microorganisms of litchi can be effectively killed, the agricultural residues can be reduced, the enzyme activity can be passivated, and the softening can be delayed, thereby prolonging the shelf life. During the operation of the large low-temperature plasma equipment, a certain concentration of ozone will be released. Based on this, the sterilization equipment can collect and generate ozone, and further use an ozone concentration of 1-3 mg / L to sterilize litchi. It is determined that the plasma combined with ozone sterilization can effectively kill the bacteria and mold on the surface of litchi, thereby reducing the rotting rate and mold rate of fresh litchi during storage.

[0052] 3) Preparation of ClO2 slow-release antibacterial agent

[0053] 20 g of the polyurethane (PU) prepared in step 1) and a certain amount of NaClO2 were dissolved in 200 mL of a mixture of ethanol and chloroform at room temperature as an oil phase, a certain amount of polyvinyl alcohol (PVA) was dissolved in a certain volume of ultrapure water at 85°C in a water bath under stirring as an aqueous phase. After the PVA solution was cooled, the oil phase mixture was slowly added to it under high-speed stirring in a fume hood, and then high-speed stirring was continued. After solidification and molding, the microcapsule solid was filtered, washed with anhydrous ethanol and ultrapure water several times, and then air-dried at room temperature to obtain the microcapsule with PU as the wall material and NaClO2 as the core material.

[0054] The preparation process was optimized by single-factor test with the encapsulation rate as the index. The optimal process was as follows: the mass concentration of PVA was 0.5%, the volume ratio of oil phase to aqueous phase was 70:340, the mass ratio of core to wall material was 1.5:10, and the volume ratio of ethanol to chloroform in the oil phase was 1:4. At this time, the NaClO2 microcapsule prepared had the highest encapsulation rate, which could reach 69.68%. It can be known from the results of Fourier infrared spectroscopy and thermogravimetric analysis that NaClO2 was successfully encapsulated into the polyurethane wall material. The microcapsule prepared was milky white and spherical, the average particle size was 576.73 μm, and the particle size distribution was concentrated in 450-650 μm.

[0055] Citric acid was used as the water-soluble core material, and citric acid microcapsules were prepared by the same method. The ClO2 release behavior of the microcapsules was determined. It was found that after the citric acid microcapsules and NaClO2 microcapsules were mixed, the released NaClO2 reacted with citric acid and water to release ClO2 gas.

[0056] Example 1

[0057] The fresh litchi is immediately screened for no disease and insect pests and mechanical damage, and a small part of branches and leaves are reserved. The screened litchi is placed in a 10°C environment for precooling treatment for 2h, to ensure that the temperature of all litchi skins is lower than 15°C. After the pre-cooled litchi with consistent maturity, uniform size and color is dried, it is packed in a temperature-sensitive polyurethane modified atmosphere bag for batch packaging treatment. The litchi is placed flat to reduce mutual extrusion and collision to cause juice leaching, accelerate browning and rot. The size of the modified atmosphere bag is set to 52cm*50cm for 2.5kg of litchi, and 82cm*70cm for 5kg of litchi. The packaged litchi is stored at room temperature (25-30°C), and the changes of O2, CO2 content, relative humidity, water content, water loss rate, ascorbic acid, soluble solids, browning rate and good fruit rate in the bag are measured at 0d, 3d, 5d, 7d, 9d and 12d, respectively.

[0058] Example 2

[0059] The fresh litchi is immediately screened for no disease and insect pests and mechanical damage, and a small part of branches and leaves are reserved. The screened litchi is placed in a 10°C environment for precooling treatment for 2h, to ensure that the temperature of all litchi skins is lower than 15°C. After the pre-cooled litchi with consistent maturity, uniform size and color is dried, it is packed in a temperature-sensitive polyurethane modified atmosphere bag for batch packaging treatment. The litchi is placed flat to reduce mutual extrusion and collision to cause juice leaching, accelerate browning and rot. The size of the modified atmosphere bag is set to 52cm*50cm for 2.5kg of litchi, and 82cm*70cm for 5kg of litchi. The packaged litchi is stored at room temperature (25-30°C), and the changes of O2, CO2 content, relative humidity, water content, water loss rate, ascorbic acid, soluble solids, browning rate and good fruit rate in the bag are measured at 0d, 3d, 5d, 7d, 9d and 12d, respectively.

[0060] Example 3

[0061] The fresh litchi is immediately screened out without disease and insect pests and mechanical damage, and a small part of branches and leaves are reserved, the screened litchi is placed in a 10℃ environment for precooling treatment for 2h, and the temperature of all litchi skins is ensured to be lower than 15℃. After the surface moisture of the pre-cooled litchi is wiped dry, the litchi is packed in a temperature-sensitive polyurethane modified atmosphere bag for batch packaging treatment, and the litchi is placed flat to ensure that the litchi in the bag reduces mutual extrusion and collision to cause juice leaching, accelerate browning and rot. The size of the modified atmosphere bag is set to 52cm*50cm to hold 2.5kg of litchi, and 82cm*70cm to hold 5kg of litchi. The bagged litchi is placed in an open physical field sterilization equipment, the plasma power is set to 30W, the ozone generator is set to an air inlet amount of 3mL / min, a voltage of 3.8V, an air inlet adjustment of 10g / h, and a fixed ozone treatment amount of 2.5mg / L, and the litchi is sterilized for 5min each time, then immediately taken out, heat sealed and stored at room temperature (25-30℃). The changes of O2, CO2 content, relative humidity, water content, water loss rate, ascorbic acid, soluble solids, browning rate and good fruit rate in the bag are measured at 0d, 3d, 5d, 7d, 9d and 12d, respectively.

[0062] Example 4

[0063] The fresh litchi is immediately screened out without disease and insect pests and mechanical damage, and a small part of branches and leaves are reserved, the screened litchi is placed in a 10℃ environment for precooling treatment for 2h, and the temperature of all litchi skins is ensured to be lower than 15℃. After the surface moisture of the pre-cooled litchi is wiped dry, the litchi is packed in a temperature-sensitive polyurethane modified atmosphere bag for batch packaging treatment, and the litchi is placed flat to ensure that the litchi in the bag reduces mutual extrusion and collision to cause juice leaching, accelerate browning and rot. The size of the modified atmosphere bag is set to 52cm*50cm to hold 2.5kg of litchi, and 82cm*70cm to hold 5kg of litchi. The bagged litchi is placed in an open physical field sterilization equipment, the plasma power is set to 30W, the ozone generator is set to an air inlet amount of 3mL / min, a voltage of 3.8V, an air inlet adjustment of 10g / h, and a fixed ozone treatment amount of 2.5mg / L, and the litchi is sterilized for 5min each time, then immediately taken out, heat sealed and stored at room temperature (25-30℃). The changes of O2, CO2 content, relative humidity, water content, water loss rate, ascorbic acid, soluble solids, browning rate and good fruit rate in the bag are measured at 0d, 3d, 5d, 7d, 9d and 12d, respectively.

[0064] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the examples, and any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods and shall be included in the protection scope of the present application.

Claims

1. A method for modified atmosphere preservation of litchi by physical field combined with chemical sterilization, characterized in that, It comprises the following steps: (1) screening, cleaning and pre-cooling treatment of picked fresh litchi; (2) wrapping the litchi treated in step (1) with temperature-sensitive polyurethane modified atmosphere bag or film; (3) sterilizing the litchi wrapped in step (2) with physical field; when wrapping the litchi with temperature-sensitive polyurethane modified atmosphere bag or film, the litchi is not completely sealed to ensure that the litchi is in full contact with the physical field treatment environment; the physical field sterilization treatment is carried out by using ozone and plasma integrated physical field sterilization equipment under the conditions of plasma power of 25-30 W and ozone concentration of 1-3 mg / L for 5-10 min; (4) sealing the litchi after adding ClO2 slow-release antibacterial microcapsules and citric acid microcapsules and storing it at room temperature; The temperature-sensitive polyurethane modified atmosphere bag or film in step (2) is prepared by the following steps: (2-1) mixing polyethylene glycol, castor oil and 1,6-hexamethylene diisocyanate under inert gas atmosphere, and reacting at 80-95℃ for 1-3h; (2-2) adding 1,4-butanediol, 1,6-hexamethylene diisocyanate and dimethylformamide, and continuing to react for 1-3h to obtain polyurethane liquid; (2-3) spreading the polyurethane liquid into a film, and heating and curing at 70-90℃ for 15-25h to obtain the temperature-sensitive polyurethane modified atmosphere film; (2-4) heat sealing the modified atmosphere film into a preservation bag by using a sealing machine to obtain the temperature-sensitive polyurethane modified atmosphere bag; The ClO2 slow-release antibacterial microcapsules or citric acid microcapsules in step (4) are prepared by the following steps: dissolving the polyurethane liquid in step (2-2) and the core material in a mixed solvent as an oil phase; dissolving polyvinyl alcohol in water as an aqueous phase; slowly adding the oil phase to the aqueous phase and stirring, and then filtering, washing and drying after solidification to obtain the ClO2 slow-release antibacterial microcapsules or citric acid microcapsules; wherein the core material is NaClO2 or citric acid.

2. The method of claim 1, wherein, In step (2-1), the mass ratio of 1,6-hexamethylene diisocyanate, polyethylene glycol and castor oil is (2-4):(8-10):(4-5); In step (2-2), the molar ratio of 1,4-butanediol and the total -OH in polyethylene glycol and castor oil to -NCO in 1,6-hexamethylene diisocyanate is 1:1; dimethylformamide is added to make the solid content of the system 25-35%; The thickness of the temperature-sensitive polyurethane modified atmosphere film obtained in step (2-3) is 80±5 μm.

3. The method of claim 1, wherein, The mixed solvent is a mixture of ethanol and chloroform in a volume ratio of 1:3-1:5; the mass concentration of polyvinyl alcohol in water is 0.5-2%; the volume ratio of the oil phase to the aqueous phase is 5-10:34; the mass concentration of polyurethane in the oil phase is 8-10%, the mass concentration of the core material is 1-2%, and the mass ratio of the core material to polyurethane is 1-2:

10.

4. The method of claim 1, wherein, The screening and cleaning steps in step (1) are to screen out litchi without pests and mechanical damage, and to retain a small part of branches and leaves; The pre-cooling treatment is pre-cooling treatment in an environment of 8-10℃ for 1-3h to ensure that the temperature of all litchi skins is lower than 15℃.

5. The method of claim 1, wherein, In step (2), the size of the modified atmosphere bag is set to 52cm*50cm for 2.5kg litchi, and 82cm*70cm for 5kg litchi.

6. The method of claim 1, wherein, In step (4), the adding amount of ClO2 slow-release antibacterial microcapsules and citric acid microcapsules is 3-5g / bag, and the room temperature is 25-30℃.

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