Litchi preservative and preparation method thereof

Abstract

The invention provides a fresh keeping agent for litchis and a preparation method of the fresh keeping agent for litchis. Comprising the following components in percentage by weight: 0.5%-1.2% of carboxymethyl chitosan, 0.2%-0.5% of sodium alginate, 0.02%-0.1% of nano-selenium, 0.8%-1.5% of thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres, 0.1%-0.3% of citral-malonic anhydride copolymer, 0.8%-1.5% of glycerol and the balance of deionized water. According to the litchi preservative provided by the invention, through a triple synergistic mechanism of physical barrier, chemical preservation and intelligent response, the preservation effects that the weight loss rate is remarkably reduced and the browning degree is obviously improved are achieved, and the litchi preservative is completely free of sulfur residues and meets the international food safety requirements.

Description

Technical Field

[0001] The present invention relates to the technical field of agricultural product preservation, and specifically, to a litchi preservative and a preparation method thereof. Background Art

[0002] As a fruit rich in nutrients, litchi not only contains a large amount of nutrients such as sugars and vitamins, but also has a unique flavor and delicate texture, and has both medicinal value and health care effects, and is deeply loved by consumers. However, litchi matures in the high-temperature and high-humidity summer, and its post-harvest physiological metabolism is vigorous, and it is extremely easy to show the phenomenon of water loss and browning. Research shows that the litchi pericarp contains rich polyphenol oxidase (PPO) and peroxidase (POD), and these enzymes will catalyze the oxidation reaction of phenolic quinone substances and anthocyanins, resulting in fruit browning. At the same time, the high-sugar characteristic of litchi makes it an ideal culture medium for pathogenic bacteria, and it is extremely easy to cause spoilage and deterioration. Therefore, the post-harvest preservation of litchi faces the dual challenges of browning and rotting, which seriously affect its commercial value and nutritional quality.

[0003] Currently, the commonly used litchi preservation methods, such as sulfur dioxide fumigation, although can effectively inhibit browning, but will cause the sulfite residue in the pulp to exceed the standard (>10 ppm), and there are potential food safety hazards; the chitosan coating technology is limited by problems such as narrow antibacterial spectrum and poor film-forming performance, and it is difficult to meet the actual preservation needs. Especially in the high-temperature and high-humidity storage and transportation environment, traditional preservatives often show the situation of "premature failure" or "inadequate release": natural antibacterial components such as plant essential oils have strong volatility, and the activity loss exceeds 50% within 48 hours; although chemical fungicides have a long effective period, they are easy to induce pathogenic bacteria to produce drug resistance, and regions such as the European Union have implemented strict residue limits (≤0.05 mg / kg) on agents such as imazalil. These problems restrict the international development of the litchi industry, and there is an urgent need to develop a new preservation technology with both safety and long-term effectiveness. Summary of the Invention

[0004] The present invention provides a litchi preservative and a preparation method thereof. The litchi preservative significantly extends the post-harvest preservation period of litchi through nano-material controlled release technology, pH-responsive release and multi-component synergistic action.

[0005] To achieve the above object, in the first aspect, the present invention provides the following technical solution: A litchi preservative, characterized in that, by mass percentage, it comprises the following components: carboxymethyl chitosan 0.5%-1.2%, sodium alginate 0.2%-0.5%, nano-selenium 0.02%-0.1%, thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres 0.8%-1.5%, citral-maleic anhydride copolymer 0.1%-0.3%, glycerol 0.8%-1.5%, and the balance is deionized water.

[0006] The present invention innovatively constructs an intelligent responsive fresh-keeping system to achieve efficient fresh-keeping of litchi through multiple synergistic mechanisms. This system has the following technical advantages: (1) Using nano-selenium as the core component, it can not only effectively scavenge reactive oxygen species to delay browning, but also strongly inhibit the growth of pathogenic bacteria. By compounding nano-selenium with carboxymethyl chitosan, a stable electrostatic network can be formed, which not only enhances the strength of the fresh-keeping film but also protects the activity of nano-selenium; (2) Encapsulating thymol essential oil with mesoporous silica nanoparticles (MSN) to construct a pH-responsive slow-release system. When litchi begins to deteriorate (pH drops to about 4.5), the pore structure of MSN changes to achieve controlled release of the essential oil. Among them, carvacrol can specifically inhibit the key metabolic enzymes of pathogenic bacteria and, in synergy with nano-selenium, significantly improve the antibacterial effect; (3) Introducing a citral-maleic anhydride copolymer as an intelligent responsive switch. This polymer can rapidly release antibacterial active components in an acidic environment and, in synergy with the MSN slow-release system, forms a dual-responsive mechanism to ensure the precise release of antibacterial components when they are most needed; (4) Carboxymethyl chitosan and sodium alginate form a three-dimensional network film through intermolecular hydrogen bonds, and a physical barrier layer is constructed by combining with the plasticizing effect of glycerol.

[0007] In any of the above technical solutions, further, the nano-selenium is prepared by reducing sodium selenite with green tea polyphenols, the reaction temperature is 40 - 60 °C, and the reaction time is 1 - 3 h.

[0008] In any of the above technical solutions, further, the molar ratio of citral to maleic anhydride in the citral-maleic anhydride copolymer is 1:0.8 - 1.2, and the molecular weight is 2000 - 5000 Da.

[0009] In any of the above technical solutions, further, the particle size of the thymol essential oil / mesoporous silica nanoparticle slow-release microspheres is 45 - 55 nm, the pore diameter is 3 - 5 nm, and the loading rate is 20% - 30%. The preferred ratio and molecular weight can balance the pH response speed and film performance. Beyond the range, it will cause too fast or delayed release, affecting the fresh-keeping effect.

[0010] In any of the above technical solutions, further, the mass ratio of thymol essential oil to mesoporous silica nanoparticles is 1:3 - 4. The preferred ratio can ensure an appropriate loading rate and stable slow-release performance. Too high a ratio will lead to serious initial burst release, and too low a ratio will affect the antibacterial persistence.

[0011] Compared with the prior art, the technical solutions provided by the present invention have the following technical effects:

[0012] The preservation system provided by the present invention realizes the preservation effect of significantly reducing the weight loss rate and significantly improving the degree of browning through a triple synergistic mechanism of "physical barrier - chemical preservation - intelligent response", and completely contains no sulfur residue, meeting the international food safety requirements. Compared with traditional preservation technologies, this solution has the advantages of comprehensive functions, intelligent response, and long-lasting effects, providing a new solution for the preservation of perishable fruits such as litchi.

[0013] In a second aspect, the present application also provides a preparation method of the above-mentioned litchi preservative, comprising the following steps:

[0014] S1. Mix a sodium selenite solution with a concentration of 0.05 - 0.2 mol / L and a green tea polyphenol extract in a volume ratio of 1:3 - 7, react at 40 - 60 °C for 1 - 3 h, then perform centrifugation, washing, and vacuum drying to obtain the nano-selenium.

[0015] S2. Prepare mesoporous silica nanoparticles by a method, immerse them in a thymol essential oil ethanol solution, perform ultrasonic treatment for 30 - 60 min, and then perform vacuum drying to obtain the thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres.

[0016] S3. Mix citral and maleic anhydride in a molar ratio of 1:0.8 - 1.2, add p-toluenesulfonic acid catalyst, react at 70 - 90 °C for 4 - 8 h, cool, precipitate and purify with ether, and perform vacuum drying to obtain the citral - maleic anhydride copolymer.

[0017] S4. Weigh each component according to the ratio, dissolve the carboxymethyl chitosan in an acetic acid solution, add sodium alginate and stir until dissolved, then add nano-selenium, thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres, citral - maleic anhydride copolymer, and glycerol, perform homogeneous emulsification, adjust the pH to 5.8 - 6.2, supplement deionized water, and sterilize to obtain the litchi preservative.

[0018] In any of the above technical solutions, further, in step S1, the preparation method of the green tea polyphenol extract comprises the following steps: Mix green tea powder and an ethanol aqueous solution with a concentration of 50% - 70% in a solid-liquid ratio of 1:10 - 20 (g / mL), perform ultrasonic extraction at 40 - 60 °C for 30 - 60 min, centrifuge to take the supernatant, and concentrate to a polyphenol content of 50 - 100 mg / mL.

[0019] In any of the above technical solutions, further, in step S2, the concentration of the thymol essential oil ethanol solution is 10 wt% - 15 wt%.

[0020] In any of the above technical solutions, further, in step S3, the concentration of the acetic acid solution is 0.5 wt% - 3.0 wt%.

[0021] In any of the above technical solutions, further, in step S4, the rotation speed of the homogenization is 6000 - 12000 rpm, and the time is 10 - 30 min.

[0022] In a third aspect, the present invention also provides an application of the above litchi preservative in litchi preservation. The application is to spray the litchi preservative on the surface of litchi within 12 h after harvesting to form a 10 - 20 μm transparent film, and the dosage is 30 - 80 mL / kg. Detailed implementation manners

[0023] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0024] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not used to limit the present invention.

[0025] The terms "comprise" and "have" and any variations thereof in the present invention are intended to cover non - exclusive inclusion. For example, a process, method, apparatus, product or equipment that includes a series of steps is not limited to the listed steps or modules, but optionally further includes steps not listed, or optionally further includes other steps inherent to these processes, methods, products or equipment.

[0026] The term "and / or" in the present invention is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B is specifically understood as: A and B can be included simultaneously, A can exist alone, or B can exist alone, and any of the above three situations can be satisfied.

[0027] For those conditions not specified in the following examples, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0028] Example 1

[0029] This example provides a litchi preservative, which includes the following components by mass percentage: carboxymethyl chitosan 0.8%, sodium alginate 0.3%, nano - selenium 0.05%, thymol essential oil / mesoporous silica nanoparticle sustained - release microspheres 1.0% (mass ratio 1:3.5), citral - maleic anhydride copolymer 0.2% (molar ratio 1:1, molecular weight 3500 Da), glycerol 1.0%, and the balance is deionized water.

[0030] It is prepared by the following method:

[0031] (1) Mix green tea powder with an ethanol aqueous solution with a concentration of 60% according to a solid-liquid ratio of 1:15 (g / mL), perform ultrasonic extraction at 50 °C for 45 min, centrifuge to obtain the supernatant, and concentrate it to a polyphenol content of 80 mg / mL to obtain the green tea polyphenol extract;

[0032] (2) Mix a 0.1 mol / L sodium selenite solution with the green tea polyphenol extract according to a volume ratio of 1:5, react at 50 °C for 2 h, then centrifuge at 12000 rpm for 30 min, wash 3 times with deionized water, and vacuum dry at 50 °C to obtain the nano selenium;

[0033] (3) Dissolve 1.0 g of cetyltrimethylammonium bromide in 100 mL of deionized water, add 2 mL of ammonia water, dropwise add 3 mL of tetraethyl orthosilicate under stirring, react at 30 °C for 8 h, centrifuge and calcine at 550 °C for 5 h to obtain mesoporous silica nanoparticles. The particle size is measured by dynamic light scattering DLS to be 50 nm, and the pore size is calculated by nitrogen adsorption-desorption BET method to be 3.5 nm;

[0034] (4) Immerse the above mesoporous silica nanoparticles in a thyme essential oil ethanol solution (concentration 12 wt%), perform ultrasonic treatment for 45 min, and vacuum dry at 50 °C for 18 h to obtain the thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres with a loading rate of 25%;

[0035] (5) Mix citral and maleic anhydride according to a molar ratio of 1:1, add 0.5% p-toluenesulfonic acid catalyst, react at 80 °C for 6 h, precipitate and purify with ether after cooling, and vacuum dry to obtain the citral-maleic anhydride copolymer;

[0036] (6) Weigh each component according to the ratio, dissolve the carboxymethyl chitosan in an acetic acid solution (concentration 1.5 wt%), add sodium alginate and stir until dissolved, then add nano selenium, thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres, citral-maleic anhydride copolymer and glycerol, homogenize at 9000 rpm for 20 min, adjust the pH to 6.0, supplement deionized water, filter and sterilize to obtain the litchi preservative.

[0037] Example 2

[0038] This example provides a litchi preservative, which, by mass percentage, comprises the following components: carboxymethyl chitosan 1.0%, sodium alginate 0.4%, nano selenium 0.08%, thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres 1.2% (mass ratio 1:3), citral-maleic anhydride copolymer 0.25% (molar ratio 1:0.9, molecular weight 4500 Da), glycerol 1.2%, and the balance being deionized water.

[0039] It is prepared by the following method:

[0040] (1) Mix green tea powder with an ethanol aqueous solution with a concentration of 65% according to a solid-liquid ratio of 1:18 (g / mL), perform ultrasonic extraction at 55 °C for 50 min, centrifuge to obtain the supernatant, and concentrate it to a polyphenol content of 95 mg / mL to obtain the green tea polyphenol extract;

[0041] (2) Mix a 0.15 mol / L sodium selenite solution with the green tea polyphenol extract according to a volume ratio of 1:4, react at 55 °C for 2.5 h, then centrifuge at 12000 rpm for 30 min, wash 3 times with deionized water, and vacuum dry at 50 °C to obtain the nano selenium;

[0042] (3) Dissolve 1.0 g of cetyltrimethylammonium bromide in 100 mL of deionized water, add 2 mL of ammonia water, dropwise add 3 mL of tetraethyl orthosilicate under stirring, react at 30 °C for 8 h, centrifuge and then calcine at 550 °C for 5 h to obtain mesoporous silica nanoparticles;

[0043] (4) Immerse the above mesoporous silica nanoparticles in a thymol essential oil ethanol solution (concentration 14 wt%), perform ultrasonic treatment for 50 min, and vacuum dry at 50 °C for 18 h to obtain the thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres, with a loading rate of 28%;

[0044] (5) Mix citral and maleic anhydride according to a molar ratio of 1:0.9, add 0.8% p-toluenesulfonic acid catalyst, react at 85 °C for 5 h, cool and precipitate and purify with ether, and vacuum dry to obtain the citral-maleic anhydride copolymer;

[0045] (6) Weigh each component according to the ratio, dissolve the carboxymethyl chitosan in an acetic acid solution (concentration 3 wt%), add sodium alginate and stir until dissolved, then add nano selenium, thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres, citral-maleic anhydride copolymer and glycerol, homogenize at 10000 rpm for 25 min, adjust the pH to 5.9, supplement deionized water, filter and sterilize to obtain the litchi preservative.

[0046] Example 3

[0047] This embodiment provides a litchi preservative, which, by mass percentage, comprises the following components: 0.6% of carboxymethyl chitosan, 0.2% of sodium alginate, 0.03% of nano selenium, 1.3% of thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres (mass ratio 1:4), 0.15% of citral-maleic anhydride copolymer (molar ratio 1:1.1, molecular weight 2800 Da), 1.3% of glycerol, and the balance being deionized water.

[0048] It is prepared by the following method:

[0049] (1) Mix green tea powder with an ethanol aqueous solution with a concentration of 55% according to a solid-liquid ratio of 1:12 (g / mL), perform ultrasonic extraction at 45 °C for 35 min, centrifuge to obtain the supernatant, and concentrate it to a polyphenol content of 60 mg / mL to obtain the green tea polyphenol extract;

[0050] (2) Mix a 0.08 mol / L sodium selenite solution with the green tea polyphenol extract according to a volume ratio of 1:6, react at 45 °C for 1.5 h, then centrifuge at 8000 rpm for 15 min, wash 3 times with deionized water, and vacuum dry at 50 °C to obtain the nano selenium;

[0051] (3) Dissolve 1.0 g of cetyltrimethylammonium bromide in 100 mL of deionized water, add 2 mL of ammonia water, dropwise add 3 mL of tetraethyl orthosilicate under stirring, react at 30 °C for 8 h, centrifuge and then calcine at 550 °C for 5 h to obtain mesoporous silica nanoparticles;

[0052] (4) Immerse the above-mentioned mesoporous silica nanoparticles in a thymol essential oil ethanol solution (concentration 10 wt%), perform ultrasonic treatment for 35 min, and vacuum dry at 50 °C for 18 h to obtain the thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres, with a loading rate of 22%;

[0053] (5) Mix citral and maleic anhydride according to a molar ratio of 1:1.1, add 0.3% of p-toluenesulfonic acid catalyst, react at 75 °C for 7 h, cool and then precipitate and purify with ether, and vacuum dry to obtain the citral-maleic anhydride copolymer;

[0054] (6) Weigh each component according to the ratio, dissolve the carboxymethyl chitosan in an acetic acid solution (concentration 0.5 wt%), add sodium alginate and stir until dissolved, then add nano selenium, thymol essential oil / mesoporous silica nanoparticle sustained-release microspheres, citral-maleic anhydride copolymer and glycerol, homogenize at 8000 rpm for 15 min, adjust the pH to 6.1, supplement deionized water, filter and sterilize to obtain the litchi preservative.

[0055] Comparative Example 1

[0056] The litchi preservative provided in this comparative example includes the following components by mass percentage: 1.0% of carboxymethyl chitosan, 0.5% of sodium alginate, 1.5% of glycerol, and the balance is deionized water.

[0057] Comparative Example 2

[0058] The difference between the litchi preservative provided in this comparative example and that in Example 1 is that: thymol essential oil / MSN (1.0%) is replaced by free thymol essential oil (0.25%, equivalent to the essential oil content in the sustained-release microspheres), and the other components, component contents, and method steps are the same as those in Example 1.

[0059] Comparative Example 3

[0060] The difference between the litchi preservative provided in this comparative example and that in Example 1 is that: citral-maleic anhydride copolymer (0.2%) is replaced by citral (0.2%), and the other components, component contents, and method steps are the same as those in Example 1.

[0061] Experimental Example

[0062] The preservatives of Examples 1-3 and Comparative Examples 1-3 were tested for their preservation performance.

[0063] Test conditions:

[0064] Litchi variety: Feizixiao

[0065] Storage temperature: 25±1°C (simulating normal temperature transportation)

[0066] Relative humidity: 85±5%

[0067] Each group has 30 litchis, repeated 3 times

[0068] Browning index: Calculate the browning area of the fruit peel according to the following browning grade standard. Grade 1: green color; Grade 2: slight browning, less than 1 / 4 of the fruit surface; Grade 3: 1 / 4-1 / 2 of the fruit surface is browned; Grade 4: 1 / 2-3 / 4 of the fruit surface is browned; Grade 5: more than 3 / 4 of the fruit surface is browned. Browning index calculation formula: Browning index = ∑(browning grade × proportion of fruits in each browning grade to the total fruits).

[0069] Weight loss rate: Measure the weight loss changes of fresh fruits during post-harvest storage.

[0070] Bacteriostatic rate: Determined with reference to GB 4789.2-2016. Experimental strain: Peronophythora litchi, prepared into a 10 6 CFU / mL bacterial solution. Take 100 μL of the bacterial solution and spread it on a PDA plate. Place Oxford cups and add 200 μL of 10-fold diluted preservative, and culture at 28°C for 48 hours.

[0071] Shelf life: Sensory evaluation + Microbiological limit method

[0072] Standard: Termination condition: The shelf life is determined to end when any of the following situations occur: Browning index ≥ grade 3; Soluble solid content decreases by > 20%. Record the spoilage quantity of 30 litchis in each group daily, and calculate the average salable days.

[0073] VC retention rate (HPLC method):

[0074] Sample treatment: Take 5 g of pulp + 25 mL of 3% metaphosphoric acid, homogenize at 12000 rpm for 15 min, and filter through a 0.22 μm filter membrane.

[0075] Chromatographic conditions: Chromatographic column: C18 (4.6×250 mm, 5 μm); Mobile phase: 0.1% oxalic acid aqueous solution; Flow rate: 1.0 mL / min; Detection wavelength: 245 nm; Column temperature: 30 °C; Injection volume: 20 μL; The storage results for 7 days are shown in Table 1, and the blank control is without any treatment.

[0076] Table 1

[0077]

[0078] The experimental results show that the litchi preservative provided by the present invention has excellent preservation effects. After storage at 25 °C at room temperature for 7 days, the browning index of the optimal example is only 1.0 ± 0.2, the weight loss rate is controlled at 2.8 ± 0.5%, which is significantly better than the comparative example and the blank control. At the same time, it maintains a high antibacterial rate and VC retention rate, and the shelf life is extended to 9 days, which is about 5 times higher than that of the blank control (2 days). These data fully verify the effectiveness of the "physical barrier - chemical preservation - intelligent response" triple synergistic mechanism, indicating that the present invention has outstanding advantages in inhibiting browning, reducing water loss, maintaining nutritional quality and extending the shelf life, and provides a new solution for postharvest preservation of litchi.

[0079] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A litchi preservative, characterized in that: Calculated by mass percentage, the invention comprises the following components: 0.5%-1.2% of carboxymethyl chitosan, 0.2%-0.5% of sodium alginate, 0.02%-0.1% of nano-selenium, 0.8%-1.5% of thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres, 0.1%-0.3% of citral-malonic anhydride copolymer, 0.8%-1.5% of glycerol, and the balance is deionized water.

2. The lychee preservative according to claim 1, characterized in that The nano-selenium is prepared by reducing sodium selenite with green tea polyphenols, the reaction temperature is 40-60° C., and the reaction time is 1-3 hours.

3. The litchi preservative according to claim 1, characterized in that: The molar ratio of citral to malonic anhydride in the citral-malonic anhydride copolymer is 1:0.8-1.2, and the molecular weight is 2000-5000Da.

4. The litchi preservative according to claim 1, characterized in that: The thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres have a particle size of 45-55 nm, a pore size of 3-5 nm, and a loading rate of 20%-30%.

5. The litchi preservative according to claim 1 or 4, characterized in that: The mass ratio of the thyme essential oil to the mesoporous silica nanoparticles is 1:3-4.

6. A method for preparing the litchi preservative according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1, mixing 0.05-0.2 mol / L sodium selenite solution and green tea polyphenol extract in a volume ratio of 1:3-7, reacting at 40-60° C. for 1-3 h, and then centrifuging, washing, and vacuum drying to obtain the nano-selenium; S2, adoption The mesoporous silica nanoparticles are prepared by the method, immersed in a thyme essential oil ethanol solution, subjected to ultrasonic treatment for 30-60 minutes, and then vacuum dried to obtain the thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres; S3, mixing citral and malonic anhydride in a molar ratio of 1:0.8-1.2, adding p-toluenesulfonic acid catalyst, reacting at 70-90° C. for 4-8 hours, cooling and purifying with ether precipitation, and vacuum drying to obtain the citral-malonic anhydride copolymer; S4, weighing each component according to the ratio, dissolving the carboxymethyl chitosan in acetic acid solution, adding sodium alginate and stirring until dissolved, then adding nano-selenium, thyme essential oil / mesoporous silica nanoparticle sustained-release microspheres, citral-malonic anhydride copolymer and glycerol, homogenizing and emulsifying, adjusting the pH to 5.8-6.2, supplementing with deionized water, sterilizing, and obtaining the litchi preservative.

7. The preparation method according to claim 6, characterized in that: In step S1, the preparation method of the green tea polyphenol extract comprises the following steps: mixing green tea powder with an ethanol aqueous solution with a concentration of 50%-70% at a solid-to-liquid ratio of 1:10-20 (g / mL), ultrasonically extracting at 40-60°C for 30-60 minutes, centrifuging to obtain the supernatant, and concentrating to a polyphenol content of 50-100 mg / mL.

8. The preparation method according to claim 6, characterized in that: In step S2, the concentration of the thyme essential oil ethanol solution is 10wt%-15wt%.

9. The preparation method according to claim 6, characterized in that: In step S3, the concentration of the acetic acid solution is 0.5wt%-3.0wt%; and / or In step S4, the homogenization speed is 6000-12000 rpm and the time is 10-30 min.

10. Use of the litchi preservative according to any one of claims 1 to 5 or the litchi preservative prepared by the preparation method according to any one of claims 6 to 9 in litchi preservation, characterized in that: The application is to spray the litchi preservative on the surface of litchi within 12 hours after the litchi is harvested to form a 10-20 μm transparent film, and the dosage is 30-80 mL / kg.

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