A method for preparing polyvinyl alcohol / phenethyl caffeate composite film by a biomimetic lignification process

Abstract

The present invention discloses a method for preparing a polyvinyl alcohol / caffeic acid phenethyl ester composite film through a biomimetic lignification process, which comprises the following steps: preparing a polyvinyl alcohol solution, and after it is cooled, adding an acetone or ethanol solution of caffeic acid phenethyl ester into the polyvinyl alcohol solution, and uniformly dispersing caffeic acid phenethyl ester through stirring to obtain a dispersion; adding an enzyme solution into the dispersion to catalyze the in-situ polymerization of caffeic acid phenethyl ester in the polyvinyl alcohol matrix to obtain a PVA / pCAPE film-forming solution, casting the solution in a mold, and drying to obtain a modified polyvinyl alcohol film material. This application adopts a simple and green biomimetic design strategy, and in the PVA matrix, HRP or rCWPO-C / H2O2 is used to catalyze the in-situ polymerization of CAPE, so that the polymerized CAPE (pCAPE) is uniformly distributed in the PVA matrix and forms extensive hydrogen bond and covalent bond interactions with PVA, preparing a film material with good ultraviolet shielding, water vapor barrier, antibacterial property, mechanical property and recycling property, and using it for food fresh-keeping packaging to extend its shelf life.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of polymer thin film materials, and particularly relates to a preparation method of a polyvinyl alcohol / caffeic acid phenethyl ester composite film by mimicking the lignification process in natural cell walls. Background Art

[0002] In recent years, diseases caused by food spoilage, mold, etc. have become a major health problem of common concern to people. Packaging materials are crucial for food preservation, mold prevention, and antibacterial properties. During food storage, moisture penetrates through the packaging material and contacts the food, leading to chemical / enzyme reactions, structural changes, and microbial growth of the food. Ultraviolet (UV) irradiation can cause oxidation and photodegradation of food. Organic UV absorbers such as cinnamate, oxybenzone, and avobenzone can effectively enhance the UV shielding ability of packaging materials, but they are prone to migrate in the matrix, thus causing harm to the human body and the environment. The UV shielding effect of inorganic UV absorbers such as TiO2, CeO2, and ZnO is limited. Microbial infection is likely to lead to the occurrence of food-borne diseases, and antibiotics and silver antibacterial agents are often used to improve the antibacterial properties of materials, but they pose threats to the environment and the human body. In addition, traditional food packaging materials are difficult to degrade and cause serious environmental pollution after being discarded. Therefore, it is of great significance and challenging to develop biodegradable, healthy and environmentally friendly packaging materials with excellent preservation, antibacterial, and UV shielding functions.

[0003] Due to the potential toxic effects of synthetic additives, the development of multifunctional packaging materials based on biomass-based additives for preservation, antibacterial, and anti-UV has received extensive attention. Currently, commonly used biomass-based additives include resveratrol, curcumin, plant polyphenols, chitosan, etc. Caffeic acid phenethyl ester (CAPE) is the main biomass component in propolis. Caffeic acid phenethyl ester endows propolis with good antibacterial, antioxidant, and anti-inflammatory activities, etc. At the same time, due to the polyphenol structure of CAPE, propolis has UV blocking and antioxidant properties. Many literatures have reported the preparation of bioactive materials using CAPE for wound repair, antibacterial, drug delivery, etc. However, the use of CAPE to prepare multifunctional packaging materials with preservation, antibacterial, and anti-UV functions has not been reported yet; in addition, as a natural small molecule, caffeic acid phenethyl ester has problems such as instability, photocytotoxicity, and poor water solubility when directly used, which seriously restrict its practical application in packaging materials. It has been reported that polymerizing CAPE into nanoparticles can significantly improve its instability and photocytotoxicity (Patent CN 111603396 B), but this method first polymerizes CAPE to form nanoparticles and then adds them to the matrix after drying. The process is complex, the nanoparticles are difficult to disperse evenly, and it is difficult to form strong interfacial interactions with the matrix.

[0004] The lignification of plant cell walls can be traced back 43 million years ago. Plants migrated from the ocean to the terrestrial environment through lignification. In the cell wall, peroxidase or oxidase catalyzes the free radical polymerization of lignin monomers in the polysaccharide matrix. This process enables lignin to form strong covalent and non-covalent bonds with the polysaccharide matrix, enhancing the cell wall. This process can not only evenly disperse natural small molecules in the substrate, but also effectively avoid the instability of natural small molecules. At the same time, the polymerized lignin polyphenol structure endows wood with excellent functions such as ultraviolet shielding and antioxidant properties. Inspired by this, researchers have developed multifunctional membranes and coatings with excellent performance, such as lignin / cellulose membranes with synchronous enhancement and toughening, protective membranes, antibacterial and antioxidant coatings, etc. Therefore, based on the structural characteristics of phenethyl caffeate, simulating the lignification process is expected to develop multifunctional packaging materials with freshness preservation, antibacterial, and anti-ultraviolet properties. Summary of the Invention

[0005] Object of the Invention: Inspired by the natural cell wall lignification process, this study adopted a simple and green bionic design strategy. In the PVA matrix, in-situ polymerization of CAPE was catalyzed by HRP or rCWPO-C / H2O2, enabling the polymerized CAPE (pCAPE) to be evenly distributed in the PVA matrix and form extensive hydrogen bonds and covalent bond interactions with PVA, to prepare a membrane material with good ultraviolet shielding, water vapor barrier, antibacterial properties, mechanical properties, and recycling performance, and to use it for food freshness preservation packaging to extend its shelf life.

[0006] To solve the above technical problems, the present invention discloses a method for preparing a polyvinyl alcohol / phenethyl caffeate composite membrane by bionic lignification process, which is characterized by including the following steps:

[0007] (1) Prepare a polyvinyl alcohol (PVA) solution. After it cools, add an acetone or ethanol solution of phenethyl caffeate (CAPE) to the polyvinyl alcohol solution, and stir to evenly disperse phenethyl caffeate to obtain a dispersion;

[0008] (2) Add an enzyme solution to the dispersion obtained in step (1) to catalyze the in-situ polymerization of phenethyl caffeate in the polyvinyl alcohol matrix to obtain a PVA / pCAPE film-forming solution. Cast the film-forming solution in a mold and dry it to obtain a modified polyvinyl alcohol membrane material.

[0009] Among them, in step (1), the PVA solution is prepared by the following steps: Swell PVA particles in water, preferably for 1 - 2 hours, heat to 80 - 95 °C and stir for 2 - 3 h to fully dissolve it, with a stirring rate of 600 - 1500 rmp, and cool the solution to below 28 °C to obtain a PVA solution.

[0010] In step (1), the acetone / ethanol solution of phenethyl caffeate (CAPE) is prepared by the following steps: Dissolve phenethyl caffeate in acetone or ethanol to obtain a phenethyl caffeate solution with a concentration range of 10 - 100 mg / mL.

[0011] Preferably, in step (1), the polyvinyl alcohol solution is kept under stirring at a stirring rate of 6000 - 1500 rmp. After adding the acetone or ethanol solution of phenethyl caffeate (CAPE), keep stirring at room temperature for 10 - 30 minutes.

[0012] Preferably, the enzyme used is any one of horseradish peroxidase (HRP) or recombinant cationic peroxidase (rCWPO - C).

[0013] Among them, the recombinant cationic peroxidase (rCWPO - C) is obtained by protein folding and purification from Escherichia coli containing the cationic cell wall - binding peroxidase (CWPO - C) gene sequence. It has the same structure and function as natural CWPO - C. Natural CWPO - C is purified from Populus tremula callus but has a low yield. The DNA accession number of natural CWPO - C in SignalP ((http: / / www.cbs.dtu.dk / services / SignalP / )) is: AB210901. For its detailed information, see "The cationic cell - wall - peroxidase having oxidationability for polymeric substrate participates in the late stage of lignification of Populus alba L" published by Shinya Sasaki et al. (Plant Mol Biol (2006) 62:797–807).

[0014] The enzyme solution is prepared by the following steps: Dissolve the enzyme in water to prepare an enzyme solution with a concentration of 0.5 - 1 mg / mL, and the final concentration of the enzyme solution in the system is 0.3 - 1 unit / mL.

[0015] Preferably, during in - situ polymerization, a certain amount of H2O2 is added to the system, and the final concentration of H2O2 in the system is 10 - 20 mmol.

[0016] Preferably, in step (2), keep the stirring rate in step (1), and sequentially add the enzyme solution and the H2O2 aqueous solution. Heat the reaction system to 25 - 28 °C and keep stirring for 3 - 5 hours to polymerize phenethyl caffeate in - situ; after the reaction is completed, heat the system to 80 - 100 °C and keep it for 5 - 30 minutes to inactivate the enzyme and stop the reaction.

[0017] Preferably, the addition amount of phenethyl caffeate is 1-10 wt% of polyvinyl alcohol.

[0018] The present application further provides an application of the above-prepared polyvinyl alcohol / phenethyl caffeate composite film in the preparation of functional films with freshness preservation, antibacterial, and ultraviolet resistance functions.

[0019] Beneficial effects: Compared with the prior art, the present technology has the following advantages:

[0020] (1) The reaction temperature is close to room temperature and the reaction is completed in one step. The reaction is catalyzed by HRP and H2O2, with a small usage amount and no impact on the performance of the composite film. The reaction does not produce waste, and has the characteristics of simplicity, high efficiency, and greenness.

[0021] (2) The in-situ polymerization strategy adopted in this study enables the uniform dispersion of CAPE polymer in the PVA matrix, and extensive hydrogen bond and covalent bond interactions are formed with PVA, significantly improving the surface hydrophilicity, water vapor barrier property, and mechanical properties of the film.

[0022] (3) The prepared PVA / CAPE composite film has good ultraviolet barrier performance, water vapor barrier performance, antibacterial property, and recycling performance. Description of the Drawings

[0023] Figure 1 Showing the preparation process and design concept of the film material described in this patent

[0024] Figure 2 Showing the transmission electron micrographs of the samples in Comparative Example (a) and Example 4 (b);

[0025] Figure 3 Showing the water vapor barrier mechanism diagram of the prepared film material;

[0026] Figure 4 Showing the effect diagrams of using the samples in the comparative example and Example 4 for strawberry freshness preservation;

[0027] Figure 5 Showing the mass loss rate of strawberries during the freshness preservation process. Detailed Embodiments

[0028] Inspired by the natural lignification process, this patent proposes a simple strategy for preparing biodegradable, healthy, and environmentally friendly packaging films. By mimicking the lignification process of plant cell walls, a multifunctional packaging film is developed by in situ polymerization of CAPE in a polyvinyl alcohol (PVA) matrix using horseradish peroxidase (HRP) or cationic peroxidase (rCWPO-C) / H2O2 catalysis. PVA was chosen as the matrix material due to its biodegradability, low cost, non-toxicity, and good oxygen barrier properties. HRP is the most commonly used enzyme for in vitro catalysis of lignin monomers to produce artificial lignin, and it has a good catalytic effect on CAPE. However, HRP only catalyzes G units, resulting in a relatively low molecular weight artificial lignin. Therefore, this patent also utilizes rCWPO-C, which catalyzes substrates containing G, S, and H units and produces high-molecular-weight artificial lignin. This reaction occurs at near-room temperature, is completed in a single step, and produces no wastewater or exhaust gas. It is simple, efficient, and environmentally friendly, offering a simple and efficient new approach for the development of multifunctional membrane materials for preservation, antibacterial properties, and other applications.

[0029] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0030] In the following examples, horseradish peroxidase (HRP) was purchased from Sigma, and cationic peroxidase (rCWPO-C) was donated from another laboratory. The DNA accession number of its sequence in SignalP (http: / / www.cbs.dtu.dk / services / SignalP / ) is AB210901. The preparation method of rCWPO-C is disclosed in "Identification of Tyr74 and Tyr177 as substrate oxidation sites in cationic cell wall-bound peroxidase from Populusalba L", FEBS Journal 279 (2012) 348–357.

[0031] Example 1

[0032] 1) Disperse 1 gram of PVA particles (degree of hydrolysis: 98.0%-98.8%; molecular weight: 105 kDa) in 20 mL of deionized water and allow to swell for 2 hours. The solution is then heated to 90°C and stirred at 800 rpm for 2 hours to completely dissolve the PVA particles. The resulting PVA solution is cooled to approximately 25°C at room temperature.

[0033] 2) Dissolve 100 mg of CAPE in 5 mL of acetone to obtain a CAPE / acetone solution with a concentration of 20 mg / mL.

[0034] 3) Place the dissolved PVA on a magnetic stirrer, maintain the stirring speed at 1000 rpm, use a pipette to add 0.5 mL of the CAPE / acetone solution to the PVA solution, and continue stirring for 10 minutes.

[0035] 4) Prepare an aqueous solution of HRP at 1 mg / mL and an aqueous solution of H2O2 at 30 wt%.

[0036] 5) Take the solution from step 3), place it in a water bath, maintain the temperature at 28 °C, and the stirring rate at 800 rpm. Sequentially add 30 μL of the HRP solution and 30 μL of the H2O2 solution, and keep stirring for 3 hours to polymerize CAPE.

[0037] 6) After reacting for 3 hours, heat it to 90 °C, keep stirring for 5 minutes, and the stirring rate is 800 rpm.

[0038] 7) Pour the reacted mixture into a mold with a side length of 10 cm, place it in a vacuum oven, and keep it dry at 40 °C to form a film. The desiccant is phosphorus pentoxide powder.

[0039] 8) The resulting film is PVA / CAPE 1%.

[0040] Test the surface water contact angle, water vapor barrier property, mechanical property, ultraviolet shielding property, and transparency of the prepared PVA / LNP@Cu 1% film. The test results are listed in Table 1.

[0041] Example 2

[0042] 1) Disperse 1 g of PVA particles (degree of hydrolysis: 98.0% - 98.8%; molecular weight: 105 KDa) into 20 mL of deionized water and swell for 2 hours. Then heat it to 90 °C and stir for 2 hours to completely dissolve the PVA particles. The stirring rate is 800 rpm. Place the obtained PVA solution at room temperature and cool it to about 25 °C.

[0043] 2) Dissolve 100 mg of CAPE in 5 mL of acetone to obtain a CAPE / acetone solution with a concentration of 20 mg / mL.

[0044] 3) Place the dissolved PVA on a magnetic stirrer, maintain the stirring speed at 1000 rpm, use a pipette to add 1 mL of the CAPE / acetone solution to the PVA solution, and continue stirring for 10 minutes.

[0045] 4) Prepare an aqueous solution of HRP at 1 mg / mL and an aqueous solution of H2O2 at 30 wt%.

[0046] 5) Take the solution in step 3) and place it in a water bath. Keep the temperature at 28 °C and the stirring rate at 800 rpm. Add 30 μL of HRP solution and 30 μL of H2O2 solution successively, and keep stirring for 3 hours to polymerize CAPE.

[0047] 6) After reacting for 3 hours, heat it to 90 °C, keep stirring for 5 minutes, and the stirring rate is 800 rpm.

[0048] 7) Pour the reacted mixture into a mold with a side length of 10 cm, place it in a vacuum oven, and keep it at 40 °C to dry into a film. The desiccant is phosphorus pentoxide powder.

[0049] 8) The obtained film is PVA / CAPE 2%.

[0050] Test the surface water contact angle, water vapor barrier property, mechanical property, ultraviolet shielding property and transparency of the prepared PVA / LNP@Cu 2% film. The test results are listed in Table 1.

[0051] Example 3

[0052] 1) Disperse 1 g of PVA particles (degree of hydrolysis: 98.0% - 98.8%; molecular weight: 105 KDa) into 20 mL of deionized water and swell for 2 hours. Then heat it to 90 °C and stir for 2 hours to completely dissolve the PVA particles. The stirring rate is 800 rpm. Place the obtained PVA solution at room temperature and cool it to about 25 °C.

[0053] 2) Dissolve 100 mg of CAPE in 5 mL of acetone to obtain a CAPE / acetone solution with a concentration of 20 mg / mL.

[0054] 3) Place the dissolved PVA on a magnetic stirrer, keep the stirring speed at 1000 rpm, and use a pipette to add 2.5 mL of CAPE / acetone solution to the PVA solution, and continue stirring for 10 minutes.

[0055] 4) Prepare a 1 mg / mL aqueous HRP solution and a 30 wt% aqueous H2O2 solution.

[0056] 5) Take the solution in step 3) and place it in a water bath. Keep the temperature at 28 °C and the stirring rate at 800 rpm. Add 30 μL of HRP solution and 30 μL of H2O2 solution successively, and keep stirring for 3 hours to polymerize CAPE.

[0057] 6) After reacting for 3 hours, heat it to 90 °C, keep stirring for 5 minutes, and the stirring rate is 800 rpm.

[0058] 7) Pour the reacted mixture into a mold with a side length of 10 cm, place it in a vacuum oven, and keep it at 40 °C to dry into a film. The desiccant is phosphorus pentoxide powder.

[0059] 8) The obtained film is PVA / CAPE 5%.

[0060] The surface water contact angle, water vapor barrier property, mechanical property, ultraviolet shielding property and transparency of the prepared PVA / LNP@Cu 5% film were tested, and the test results are listed in Table 1.

[0061] Example 4

[0062] 1) 1 g of PVA particles (degree of hydrolysis: 98.0% - 98.8%; molecular weight: 105 KDa) were dispersed in 20 mL of deionized water and swollen for 2 hours, then heated to 90 °C and stirred for 2 hours to completely dissolve the PVA particles. The stirring rate was 800 rpm, and the obtained PVA solution was placed at room temperature and cooled to about 25 °C.

[0063] 2) 100 mg of CAPE was dissolved in 5 mL of acetone to obtain a CAPE / acetone solution with a concentration of 20 mg / mL.

[0064] 3) The PVA solution was placed on a magnetic stirrer, and the stirring speed was maintained at 1000 rpm. 5 mL of the CAPE / acetone solution was taken with a pipette and added to the PVA solution, and stirring was continued for 10 minutes.

[0065] 4) An aqueous solution of HRP at 1 mg / mL and an aqueous solution of H2O2 at 30 wt% were prepared.

[0066] 5) The solution in step 3) was placed in a water bath, the temperature was maintained at 28 °C, and the stirring rate was 800 rpm. 30 μL of the HRP solution and 30 μL of the H2O2 solution were added successively, and stirring was maintained for 3 hours to polymerize CAPE.

[0067] 6) After reacting for 3 hours, it was heated to 90 °C and stirred for 5 minutes, and the stirring rate was 800 rpm.

[0068] 7) The reaction mixture was poured into a mold with a side length of 10 cm and placed in a vacuum oven. It was dried into a film at 40 °C, and the desiccant was phosphorus pentoxide powder.

[0069] 8) The obtained film is PVA / CAPE 10%.

[0070] 9) To characterize the recyclability of the film, the prepared PVA / CAPE 10% film was cut into pieces, swollen with 20 mL of water for 2 hours, then heated to 90 °C and stirred for 2 hours, and the stirring rate was 800 rpm. It was formed into a film by the same method as in step 7), and the prepared film was PVA / CAPE 10%-R1; the dissolution and film-forming process were repeated three times, and the prepared film was PVA / CAPE 10%-R3.

[0071] The surface water contact angle, water vapor barrier property, mechanical property, ultraviolet shielding property and antibacterial property of the prepared PVA / LNP@Cu-10% film were tested, and the test results are listed in Table 1. The water vapor transmission rate, ultraviolet shielding, transparency and mechanical property of the prepared PVA / CAPE10%-R1 and PVA / CAPE10%-R3 were tested, and the test results are listed in Table 2.

[0072] Example 5

[0073] 1) 1 g of PVA particles (degree of hydrolysis: 98.0%-98.8%; molecular weight: 105 KDa) were dispersed in 20 mL of deionized water and swollen for 2 hours, then heated to 90 °C and stirred for 2 hours to completely dissolve the PVA particles, the stirring rate was 800 rpm, and the obtained PVA solution was placed at room temperature and cooled to about 25 °C.

[0074] 2) 100 mg of CAPE was dissolved in 5 mL of acetone to obtain a CAPE / acetone solution with a concentration of 20 mg / mL.

[0075] 3) The PVA solution was placed on a magnetic stirrer, and the stirring speed was maintained at 1000 rpm. 5 mL of the CAPE / acetone solution was taken with a pipette and added to the PVA solution, and stirring was continued for 10 minutes.

[0076] 4) An rCWPO-C aqueous solution with a concentration of 1 mg / mL and a 30 wt% H2O2 aqueous solution were prepared.

[0077] 5) The solution in step 3) was placed in a water bath, the temperature was maintained at 28 °C, and the stirring rate was 800 rpm. 30 μL of the rCWPO-C solution and 30 μL of the H2O2 solution were added in sequence, and stirring was maintained for 3 hours to polymerize CAPE.

[0078] 6) After reacting for 3 hours, it was heated to 90 °C and stirred for 5 minutes, and the stirring rate was 800 rpm.

[0079] 7) The reacted mixture was poured into a mold with a side length of 10 cm, placed in a vacuum oven, and dried into a film at 40 °C. The desiccant was phosphorus pentoxide powder.

[0080] 8) The obtained film was PVA / CAPE10%-C.

[0081] The surface water contact angle, water vapor barrier property, mechanical property, ultraviolet shielding property and transparency of the prepared PVA / LNP@Cu10%-C film were tested, and the test results are listed in Table 1.

[0082] Comparative Example

[0083] 1 g of PVA was swollen in 20 mL of deionized water for 2 h. The swollen PVA suspension was heated to 90 °C and stirred for 2 h to fully dissolve it. The stirring rate was 800 rmp. The fully dissolved PVA solution was poured into a square plastic mold with a side length of 10 cm and placed in a vacuum oven for drying at 40 °C to obtain a pure PVA membrane material. It was cut into pieces and redissolved into a membrane 1 or 3 times to obtain PVA-R1 and R3 membrane materials respectively.

[0084] Performance comparison table of the prepared modified membrane materials in Table 1

[0085]

[0086] Performance comparison table of the recycled membrane materials in Table 2

[0087]

[0088] Figure 1 The figure shows the schematic diagram of the design strategy of the present invention. From Example 1 to Example 4, as the addition amount of CAPE increases, the water contact angle on the membrane surface gradually increases, and at the same time, the water vapor transmission rate gradually decreases. After in-situ polymerization of CAPE in the PVA matrix, particles with a diameter of 300 - 1100 nm are uniformly dispersed in the membrane ( Figure 2 ), which makes the path for water vapor molecules to pass through longer, improving the barrier performance. In addition, the surface of the modified membrane becomes hydrophobic, making it difficult to be wetted by water molecules, further reducing the water vapor transmission rate. This mechanism of action is shown in Figure 3 . The prepared membrane has good ultraviolet shielding effect, antibacterial property, transparency and mechanical strength, and is suitable for the field of fresh-keeping packaging. In addition, the prepared membrane has good recycling performance. The water vapor transmission rate of the recycled membrane further decreases. While completely shielding ultraviolet rays, the transparency is increased to 70%, and the tensile strength is higher than that of the original membrane material. However, the toughness decreases slightly after recycling three times. In this study, the prepared membrane was used for strawberry fresh-keeping, which can extend its fresh-keeping period from 2 days to 7 days. The fresh-keeping effect is significantly better than that of commercial polyethylene (PE) fresh-keeping film and unmodified PVA film. The fresh-keeping results are shown in Figure 4 and Figure 5 .

Claims

1. A method for preparing a polyvinyl alcohol / phenethyl caffeate composite film by a biomimetic lignification process, characterized in that, It includes the following steps: (1) Prepare a polyvinyl alcohol solution. After it cools down, add an acetone or ethanol solution of phenethyl caffeate to the polyvinyl alcohol solution, and stir to uniformly disperse phenethyl caffeate to obtain a dispersion. The addition amount of phenethyl caffeate is 1-10 wt% of polyvinyl alcohol; (2) Maintain the stirring rate in step (1), and sequentially add the enzyme solution and the H2O2 aqueous solution to the dispersion obtained in step (1). Heat the reaction system to 25-28 °C and keep stirring for 3-5 hours to in-situ polymerize phenethyl caffeate; after the reaction ends, heat the system to 80-100 °C and keep it for 5-30 minutes to inactivate the enzyme and stop the reaction, obtaining a PVA / pCAPE film-forming solution. Cast the film-forming solution in a mold and dry it to obtain a modified polyvinyl alcohol membrane material. Among them, the enzyme used is horseradish peroxidase or recombinant cationic peroxidase.

2. The method according to claim 1, wherein In step (1), the polyvinyl alcohol solution is prepared by the following steps: swell polyvinyl alcohol particles in water, heat to 80-95 °C and stir for 2-3 h to fully dissolve it, and the stirring rate is 600-1500 rpm. Cool the solution to below 28 °C to obtain a polyvinyl alcohol solution.

3. The method according to claim 1, wherein In step (1), the acetone / ethanol solution of phenethyl caffeate is prepared by the following steps: dissolve phenethyl caffeate in acetone or ethanol to obtain a phenethyl caffeate solution, and its concentration range is 10-100 mg / mL.

4. The method according to claim 1, wherein The enzyme solution is prepared by the following steps: dissolve the enzyme in water to prepare an enzyme solution with a concentration of 0.5-1 mg / mL, and the final concentration of the enzyme solution in the system is 0.3-1 unit / mL.

5. Application of the polyvinyl alcohol / phenethyl caffeate composite film prepared by the method according to any one of claims 1-4 in the preparation of functional films with freshness preservation, antibacterial, and ultraviolet resistance functions.

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