A polylactic acid-tea polyphenol antioxidant fresh-keeping coating and a preparation method thereof

Abstract

This invention belongs to the field of food packaging and functional polymer materials, and discloses a polylactic acid-tea polyphenol antioxidant and preservation coating and its preparation method. The coating introduces carboxyl active sites on the polylactic acid molecular chain, which then react chemically with the phenolic hydroxyl or aromatic ring active sites on the tea polyphenol molecule to form a covalently immobilized polylactic acid-tea polyphenol complex. This method effectively solves the problems of easy migration and oxidative failure of tea polyphenols, significantly improving the antioxidant stability and durability of the coating. The coating not only possesses excellent antioxidant function but also slows down the release of tea polyphenols, extending the shelf life of food. The coating is transparent and biodegradable, suitable for green packaging of fruits, vegetables, meat, and baked goods, meeting food safety and environmental protection requirements. The preparation process uses green solvents, avoiding organic solvent residues, and is simple. Furthermore, the coating exhibits strong stability under different storage conditions.

Description

Technical Field

[0001] This invention belongs to the field of food packaging and functional polymer materials, specifically relating to a polylactic acid-tea polyphenol antioxidant and preservation coating and its preparation method. Background Technology

[0002] With the development of the food industry, the demand for food preservation is increasing, especially in technologies that preserve the nutritional components of food, extend shelf life, and prevent oxidation and spoilage. Traditional preservation coatings typically employ composite systems of natural or synthetic polymers with functional substances such as antioxidants and antibacterial agents. However, existing coating technologies generally suffer from several problems, such as poor coating stability, rapid degradation of antioxidant properties, and easy migration or peeling, limiting their effectiveness in food preservation.

[0003] Polylactic acid (PLA), a polymer with good biodegradability, has been widely used in the food packaging industry. PLA not only reduces environmental pollution but also possesses good mechanical properties and transparency. However, pure PLA films themselves lack strong antioxidant properties and cannot effectively resist oxidative degradation during food storage. Tea polyphenols, a class of naturally derived polyphenolic compounds, have significant antioxidant, antibacterial, and anti-inflammatory effects and are widely used in food preservation, cosmetics, and pharmaceuticals. Due to their strong antioxidant properties, tea polyphenols have great potential for application in food preservation. However, tea polyphenol molecules are easily oxidized and volatile, and their active ingredients often rapidly escape from the coating, resulting in a lack of long-lasting preservation effects. Therefore, how to effectively immobilize tea polyphenols in coating materials and maintain their activity has become a major challenge in food preservation technology. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a polylactic acid-tea polyphenol antioxidant and preservative coating and its preparation method. This method involves introducing carboxyl, hydroxyl, or epoxy active sites onto the polylactic acid molecular chain, which then react with the phenolic hydroxyl groups or aromatic rings of tea polyphenols to form a covalently grafted structure. This coating exhibits excellent antioxidant properties and stability, extending the shelf life of food and reducing oxidation and spoilage problems.

[0005] The objective of this invention can be achieved through the following technical solutions: A polylactic acid-tea polyphenol antioxidant and freshness-preserving coating comprises the following raw materials in parts by weight: 80-120 parts modified polylactic acid, 5-30 parts tea polyphenols, 0.5-5 parts coupling catalyst, 0.1-2 parts thickener, 0.05-1 parts vitamin E, and 0.1-2 parts polyvinyl alcohol; The modified polylactic acid molecular chain contains carboxyl active sites, which react with the phenolic hydroxyl groups or aromatic rings of tea polyphenols to form a covalent graft structure.

[0006] More preferably, the preparation method of the modified polylactic acid specifically includes the following steps: S101. Mix polylactic acid and maleic anhydride at a mass ratio of 100:5, place them in a reaction vessel, and fill the reaction vessel with nitrogen to ensure an inert atmosphere; S102. Heat the reactor to 150–200 °C and maintain this temperature for 2–6 hours. After the reaction is complete, cool the reactants to room temperature and wash the product with anhydrous ethanol to remove unreacted maleic anhydride residues. S103. Dissolve the washed modified polylactic acid in a mixed solvent of ethanol and water, and further dissolve it under stirring to obtain a clear and transparent modified polylactic acid solution.

[0007] More preferably, the chemical bonding reaction is one or more of esterification, Schiff base bonding, and free radical grafting.

[0008] More preferably, the coupling catalyst is one or more of p-toluenesulfonic acid, dicyclohexylcarbodiimide, and benzoyl peroxide.

[0009] More preferably, the thickener is one or more of sodium carboxymethyl cellulose, xanthan gum, and gelatin.

[0010] More preferably, the coating thickness is 0.01–0.1 mm, and the coating can maintain its antioxidant effect for at least 6 months under storage conditions.

[0011] A method for preparing a polylactic acid-tea polyphenol antioxidant and preservative coating includes the following steps: S1. Add the modified polylactic acid to a mixed solvent of ethanol and water, stir until completely dissolved, and obtain a transparent polylactic acid solution; S2. Add tea polyphenols and coupling catalyst to polylactic acid solution, stir and react at 60–80℃ for 3–5 hours to form a covalent grafted structure; S3. After cooling to room temperature, wash the reaction product with anhydrous ethanol to remove unreacted tea polyphenols and catalyst. Then add thickener, vitamin E and polyvinyl alcohol and stir evenly to obtain a stable coating solution. S4. Apply the coating solution evenly to the pretreated substrate surface and dry it to form a transparent and uniform polylactic acid-tea polyphenol antioxidant and preservation coating.

[0012] More preferably, in step S1, the polylactic acid is dissolved at 40–60°C for 2–4 hours to ensure that the polylactic acid is completely dissolved and no precipitation occurs.

[0013] More preferably, in step S4, after coating, vacuum drying is performed at a temperature controlled at 50–60°C for 4–6 hours to improve the adhesion and stability of the coating.

[0014] More preferably, the coating has excellent water resistance, and after immersion in water for 72 hours, the coating retains at least 80% of its antioxidant properties and does not undergo significant degradation or failure.

[0015] The beneficial effects of this invention are: This invention effectively solves the problems of tea polyphenol volatilization and oxidation failure in traditional preservation coatings by chemically bonding tea polyphenols to the active sites on the modified polylactic acid molecular chain to form a stable covalent graft structure. This chemical bonding method ensures the long-term stability of tea polyphenols in the coating and enables the gradual release of its antioxidant components, continuously exerting a preservation effect during food storage. Compared with traditional physical blending or surface coating techniques, the coating of this invention not only effectively improves antioxidant performance but also extends the shelf life of food, making it particularly suitable for easily oxidized fruits, vegetables, meats, and baked goods.

[0016] Furthermore, using modified polylactic acid (PLA) as the coating substrate provides both excellent mechanical strength and transparency, while also meeting environmental requirements because PLA is a biodegradable material. This allows the coating of the present invention to improve food preservation performance while also possessing environmentally friendly characteristics, reducing the environmental burden of plastic packaging. The coating's antioxidant properties enable it to effectively prevent food oxidation, discoloration, and flavor loss for a longer period, significantly improving food preservation. Attached Figure Description

[0017] The invention will now be further described with reference to the accompanying drawings.

[0018] Figure 1 The graph shows the changes in storage time and antioxidant rate of the antioxidant preservative coatings prepared in Examples 1-3 and Comparative Examples 1-2 of this invention. Figure 2 This is a comparison diagram of the tensile peel strength of the antioxidant preservation coatings prepared in Examples 1-3 and Comparative Examples 1-2 of the present invention; Figure 3 This is a comparison chart showing the water resistance test results of the antioxidant preservation coatings prepared in Examples 1-3 and Comparative Examples 1-2 of the present invention. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 I. Preparation of Modified Polylactic Acid Take 10 g of polylactic acid and add it to a reaction vessel. Add 0.5 g of maleic anhydride and purge the reaction vessel with nitrogen to ensure an inert atmosphere and prevent oxygen from interfering with the reaction. Heat the reaction vessel to 170°C and maintain this temperature for 4 hours. After the reaction is complete, cool the reactants to room temperature. Wash the product with 50 ml of anhydrous ethanol to remove unreacted maleic anhydride residue. Remove the washing liquid by filtration or centrifugation to ensure pure modified polylactic acid. Add the washed modified polylactic acid to a mixed solvent of ethanol and water (ethanol:water volume ratio 7:3), stir and heat to 60–70°C to ensure complete dissolution of the polylactic acid, resulting in a clear and transparent modified polylactic acid solution.

[0021] II. Preparation of Polylactic Acid-Tea Polyphenol Antioxidant Preservative Coating The polylactic acid-tea polyphenol antioxidant and preservative coating comprises the following raw materials in parts by weight: 80 parts modified polylactic acid, 5 parts tea polyphenols, 0.5 parts coupling catalyst, 0.1 parts thickener, 0.05 parts vitamin E, and 0.1 parts polyvinyl alcohol; The preparation steps are as follows: Take 80 g of modified polylactic acid and add it to a reaction vessel. Add 50 ml of a mixed solvent of ethanol and water, with a volume ratio of ethanol to water of 7:3. Heat the reaction vessel to 40°C and stir until the polylactic acid is completely dissolved to obtain a transparent solution. Add 5 g of tea polyphenols and 0.5 g of p-toluenesulfonic acid, stir evenly, and then heat to 60°C and maintain this temperature for 3 hours. After the reaction is complete, cool the reaction solution to room temperature, wash the reaction product with 50 ml of anhydrous ethanol to remove unreacted tea polyphenols and coupling catalyst, and filter to remove impurities to ensure that pure polylactic acid-tea polyphenol covalent grafts are obtained. Then add 0.1 g of sodium carboxymethyl cellulose, 0.05 g of vitamin E, and 0.1 g of polyvinyl alcohol, stir evenly, and obtain a stable coating solution. The obtained coating solution was uniformly coated on the surface of the pretreated polylactic acid film, and the coating thickness was controlled to be 0.01–0.1 mm. After coating, vacuum drying was performed at a temperature of 50°C for 4 hours until the coating was completely dry, forming a transparent and uniform polylactic acid-tea polyphenol antioxidant and freshness-preserving coating.

[0022] Example 2 The preparation method of modified polylactic acid is the same as that in Example 1.

[0023] The preparation method of polylactic acid-tea polyphenol antioxidant and preservative coating is as follows: The polylactic acid-tea polyphenol antioxidant and freshness-preserving coating comprises the following raw materials in parts by weight: 120 parts modified polylactic acid, 30 parts tea polyphenols, 5 parts coupling catalyst, 2 parts thickener, 1 part vitamin E, and 2 parts polyvinyl alcohol. The preparation steps of the polylactic acid-tea polyphenol antioxidant and preservation coating are the same as in Example 1.

[0024] Example 3 The preparation method of modified polylactic acid is the same as that in Example 1.

[0025] The preparation method of polylactic acid-tea polyphenol antioxidant and preservative coating is as follows: The polylactic acid-tea polyphenol antioxidant and preservative coating comprises the following raw materials in parts by weight: 100 parts modified polylactic acid, 15 parts tea polyphenols, 2 parts coupling catalyst, 1.25 parts thickener, 0.6 parts vitamin E, and 1.25 parts polyvinyl alcohol; The preparation steps of the polylactic acid-tea polyphenol antioxidant and preservation coating are the same as in Example 1.

[0026] Comparative Example 1 The preparation method of modified polylactic acid is the same as that in Example 1.

[0027] The preparation method of polylactic acid-tea polyphenol antioxidant and preservative coating is as follows: The polylactic acid-tea polyphenol antioxidant and preservative coating comprises the following raw materials in parts by weight: 100 parts modified polylactic acid, 15 parts tea polyphenols, 1.25 parts thickener, 0.6 parts vitamin E, and 1.25 parts polyvinyl alcohol; The preparation steps are as follows: Take 100 g of modified polylactic acid and add it to a reaction vessel, along with 100 ml of a mixed solvent of ethanol and water. Heat the reaction vessel to 50°C and stir until the polylactic acid is completely dissolved to obtain a transparent solution. Add 15 g of tea polyphenols, stir evenly, and then heat to 60°C and maintain this temperature for 3 hours. After the reaction is complete, cool the reaction solution to room temperature and wash the reaction product with 50 ml of anhydrous ethanol to remove unreacted tea polyphenols and coupling catalysts, ensuring that a pure polylactic acid-tea polyphenol covalent graft is obtained. Then add 1.25 g of sodium carboxymethyl cellulose, 0.6 g of vitamin E, and 1.25 g of polyvinyl alcohol, and stir evenly to obtain a stable coating solution. Coat the obtained coating solution uniformly onto the surface of a pretreated polylactic acid film, controlling the coating thickness to be 0.01–0.1 mm. After coating, perform vacuum drying treatment at 55°C for 5 hours until the coating is completely dry, forming a transparent and uniform polylactic acid-tea polyphenol antioxidant and preservative coating.

[0028] Comparative Example 2 The preparation method of modified polylactic acid is the same as that in Example 1.

[0029] The preparation method of polylactic acid-tea polyphenol antioxidant and preservative coating is as follows: The polylactic acid-tea polyphenol antioxidant and preservative coating comprises the following raw materials in parts by weight: 100 parts modified polylactic acid, 15 parts tea polyphenols, 2 parts coupling catalyst, 0.6 parts vitamin E, and 1.25 parts polyvinyl alcohol; The preparation steps are as follows: Take 100 g of modified polylactic acid and add it to a reaction vessel, along with 100 ml of a mixed solvent of ethanol and water. Heat the reaction vessel to 50°C and stir until the polylactic acid is completely dissolved. Add 15 g of tea polyphenols and 2 g of p-toluenesulfonic acid, stir evenly, and then heat to 60°C and maintain this temperature for 3 hours. After the reaction is complete, cool the reaction solution to room temperature and wash the reaction product with 50 ml of anhydrous ethanol to ensure that a pure polylactic acid-tea polyphenol covalent graft is obtained. Then add 0.6 g of vitamin E and 1.25 g of polyvinyl alcohol, stir evenly, and obtain a stable coating solution. Coat the obtained coating solution uniformly onto the surface of the pretreated polylactic acid film, controlling the coating thickness to be 0.01–0.1 mm. After coating, perform vacuum drying treatment at 55°C for 5 hours until the coating is completely dry, forming a transparent and uniform polylactic acid-tea polyphenol antioxidant and preservative coating.

[0030] Performance testing 1. Antioxidant performance test The DPPH radical scavenging method was used to evaluate the inhibitory effect of the coating on free radicals. A 0.1 mM DPPH radical solution was prepared and dissolved in ethanol. The dried coating samples of Examples 1-3 and Comparative Examples 1-2 were immersed in the DPPH radical-containing solution for 30 min, 1 h, 2 h, and 4 h. The absorbance change of the solution at 517 nm was measured using a UV-Vis spectrophotometer, and the free radical scavenging rate of the coating was calculated according to the following formula: Among them, A 样本 A represents the absorbance of the coated sample at a specific time point. 对照 The absorbance of the control group without coating is shown in Table 1 below.

[0031] Table 1 Free radical scavenging rate As shown in Table 1, the free radical scavenging rates of Examples 1-3 were significantly higher than those of Comparative Examples 1 and 2, and the scavenging rate continued to increase over time, demonstrating that the coating possesses sustained and potent antioxidant capabilities. This indicates that the tea polyphenols in the coating, through the formation of covalent graft structures with the active sites on the polylactic acid molecular chains, can effectively stabilize the tea polyphenols and prolong their antioxidant effect, especially exhibiting excellent antioxidant performance during long-term storage. In contrast, the low scavenging rates shown by Comparative Examples 1 and 2 indicate that the coupling catalyst and thickener have a significant impact on the coating performance; the absence of these two components affects the stability and antioxidant effect of the tea polyphenols.

[0032] 2. Coating durability and antioxidant effect test The coated samples were stored at 30°C and 50% relative humidity for 6 or 12 months. The antioxidant properties of the coatings were tested periodically (every 1 month, 3 months, 6 months, and 12 months). The antioxidant capacity of the coatings after storage was measured using the DPPH free radical scavenging method and compared with the antioxidant effect of the initial coating. The results are shown in Table 2 below.

[0033] Table 2 Free radical scavenging rates after different storage times As shown in Table 2, the coatings of Examples 1-3 maintained high antioxidant performance during storage. Even after 12 months, the antioxidant scavenging rate of the coatings remained above 80%, demonstrating the durability and stability of the coatings during long-term storage. This invention enhances the stability of tea polyphenols by covalently grafting them onto the active sites of the modified polylactic acid molecular chain, enabling them to continuously exert their antioxidant effects during storage. In contrast, Comparative Examples 1 and 2 showed lower antioxidant scavenging rates, especially after 6 months of storage, where the antioxidant effect significantly decreased.

[0034] 3. Water resistance test The water resistance of the coating was tested using an immersion method. The coating sample was completely immersed in room temperature water for 72 hours, and its stability was observed. During the test, the appearance of the coating was checked periodically, especially for any peeling, dissolution, discoloration, or loss of transparency. After immersion, the antioxidant properties of the coating were evaluated using the DPPH free radical scavenging method. The antioxidant rate of the coating after immersion was measured and compared with the antioxidant effect of the initial coating. The results are shown in Table 3 below.

[0035] Table 3 Water resistance test results As shown in Table 3, the coatings of Examples 1-3 maintained an antioxidant rate of over 80% after immersion for 72 hours, with no obvious peeling, dissolution, or discoloration, and good transparency, demonstrating excellent stability and antioxidant effect in the aquatic environment. This indicates that the present invention covalently grafts tea polyphenols onto the active sites of the polylactic acid molecular chain, thereby stabilizing the tea polyphenols in the coating, prolonging its antioxidant effect, and improving the water resistance and durability of the coating. In contrast, Comparative Example 1 (without coupling catalyst) and Comparative Example 2 (without thickener) showed lower antioxidant effects, and the coatings exhibited poor stability in water, showing peeling, dissolution, or discoloration, verifying the importance of coupling catalysts and thickeners for the water resistance and antioxidant properties of the coating.

[0036] 4. Coating adhesion test Take an appropriate amount of coating sample and apply it evenly to the pretreated substrate surface, ensuring uniform coating thickness. Then, attach both ends of the coating sample to the clamps of the tensile testing machine and adjust the clamps to ensure proper contact between the coating and the substrate. Next, apply gradually increasing tensile force and record the maximum tensile strength during the peeling process, i.e., the peel force between the coating and the substrate. The peeling speed was 5 mm / min, and the experiment was conducted at room temperature. The results are shown in Table 4 below.

[0037] Table 4 Coating adhesion results As shown in Table 4, the coatings of Examples 1-3 exhibited strong peel strength in the adhesion test, especially Example 3, which achieved a peel strength as high as 11.8 N / m, demonstrating good adhesion between the coating and the substrate. This indicates that the design using coupling catalysts and thickeners can significantly enhance the adhesion of the coating, ensuring its stability and durability in practical applications. In contrast, Comparative Examples 1 and 2 showed lower peel strengths, at 6.5 N / m and 7.0 N / m, respectively, indicating that the lack of these key components significantly reduced the adhesion between the coating and the substrate, making it prone to peeling.

[0038] 5. Mechanical strength test Take an appropriate amount of coating sample and ensure its dimensions meet the test requirements. Perform a tensile test on the coating sample using a tensile testing machine, and record the tensile strength, breaking strength, and elastic modulus. Specifically, the sample width is typically 1 cm and the length is 5 cm, with the tensile speed of the testing machine set to 5 mm / min. During the test, gradually increase the tensile force until the coating fractures, and record the maximum tensile force and the point of fracture.

[0039] For the compression test, the coated sample was placed in the clamp of the compression testing machine, and a gradually increasing compressive force was applied. The maximum compressive strength and the deformation of the sample during the compression process were recorded, and its elastic modulus was measured. The results are shown in Table 5 below.

[0040] Table 5 Mechanical Strength Results As shown in Table 5, the coatings of Examples 1-3 exhibit excellent mechanical properties in terms of tensile strength, fracture strength, elastic modulus, and compressive strength. Example 3, in particular, shows superior performance in all indicators compared to Comparative Examples 1-2. This indicates that the coating of the present invention, through covalent grafting of tea polyphenols to the active sites on the modified polylactic acid molecular chains, not only enhances the stability of tea polyphenols but also improves the mechanical properties of the coating. The covalent grafting mechanism promotes the synergistic effect between tea polyphenols and polylactic acid, improving the coating's tensile, fracture, and compressive strength. Furthermore, the addition of the coupling catalyst and thickener allows the coating to maintain high mechanical strength and stability under moisture and external forces, preventing brittle fracture. In contrast, Comparative Examples 1 and 2 lack these key components, resulting in a significant decrease in the mechanical properties of the coating.

[0041] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0042] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A polylactic acid-tea polyphenol antioxidant and preservative coating, characterized in that, It contains the following raw materials in parts by weight: 80–120 parts modified polylactic acid, 5–30 parts tea polyphenols, 0.5–5 parts coupling catalyst, 0.1–2 parts thickener, 0.05–1 part vitamin E, and 0.1–2 parts polyvinyl alcohol; The modified polylactic acid molecular chain contains carboxyl active sites, which react with the phenolic hydroxyl groups or aromatic rings of tea polyphenols to form a covalent graft structure.

2. The polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 1, characterized in that, The preparation method of the modified polylactic acid specifically includes the following steps: S101. Mix polylactic acid and maleic anhydride at a mass ratio of 100:5, place them in a reaction vessel, and fill the reaction vessel with nitrogen to ensure an inert atmosphere; S102. Heat the reactor to 150–200 °C and maintain this temperature for 2–6 hours. After the reaction is complete, cool the reactants to room temperature and wash the product with anhydrous ethanol to remove unreacted maleic anhydride residues. S103. Dissolve the washed modified polylactic acid in a mixed solvent of ethanol and water, and further dissolve it under stirring to obtain a clear and transparent modified polylactic acid solution.

3. The polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 1, characterized in that, The chemical bond reaction is one or more of the following: esterification bond, Schiff base bond, and free radical grafting bond.

4. The polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 1, characterized in that, The coupling catalyst is one or more of p-toluenesulfonic acid, dicyclohexylcarbodiimide, and benzoyl peroxide.

5. The polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 1, characterized in that, The thickener is one or more of sodium carboxymethyl cellulose, xanthan gum, and gelatin.

6. The polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 1, characterized in that, The coating has a thickness of 0.01–0.1 mm and can maintain its antioxidant effect for at least 6 months under storage conditions.

7. A method for preparing a polylactic acid-tea polyphenol antioxidant preservative coating, wherein the polylactic acid-tea polyphenol antioxidant preservative coating is as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Add the modified polylactic acid to a mixed solvent of ethanol and water, stir until completely dissolved, and obtain a transparent polylactic acid solution; S2. Add tea polyphenols and coupling catalyst to polylactic acid solution, stir and react at 60–80℃ for 3–5 hours to form a covalent grafted structure; S3. After cooling to room temperature, wash the reaction product with anhydrous ethanol to remove unreacted tea polyphenols and catalyst. Then add thickener, vitamin E and polyvinyl alcohol and stir evenly to obtain a stable coating solution. S4. Apply the coating solution evenly to the pretreated substrate surface and dry it to form a transparent and uniform polylactic acid-tea polyphenol antioxidant and preservation coating.

8. The method for preparing the polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 7, characterized in that, In step S1, the polylactic acid is dissolved at 40–60°C for 2–4 hours to ensure complete dissolution without precipitation.

9. The method for preparing the polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 7, characterized in that, In step S4, after coating, vacuum drying is performed at a temperature of 50–60°C for 4–6 hours to improve the adhesion and stability of the coating.

10. The polylactic acid-tea polyphenol antioxidant and preservative coating according to claim 1, characterized in that, The coating has excellent water resistance. After being immersed in water for 72 hours, the coating retains at least 80% of its antioxidant properties and does not undergo significant degradation or failure.

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