Slow-release antibacterial and antioxidant composite film and application thereof in preservation of livestock and poultry prefabricated vegetables

Abstract

The invention discloses a slow-release antibacterial and antioxidant composite film and application thereof in fresh keeping of livestock and poultry prefabricated vegetables. The device sequentially comprises a first outer layer, a functional inner layer and a second outer layer from top to bottom, the first outer layer and the second outer layer both contain polyvinyl butyral and ethyl cellulose; the functional inner layer contains ZIF <->; and 8, cinnamyl aldehyde, starch, glycerol and chitosan. Dissolving polyvinyl butyral and ethyl cellulose in absolute ethyl alcohol, heating and uniformly stirring to obtain an outer layer membrane solution; the preparation method comprises the following steps: respectively dissolving starch and chitosan in deionized water and an acetic acid solution to prepare a starch solution and a chitosan solution, uniformly mixing the two solutions, and then adding glycerol and ZIF <->; stirring uniformly at room temperature to obtain an inner-layer film solution; and pouring the outer-layer membrane liquid and the inner-layer membrane liquid to obtain the composite membrane. The composite film provided by the invention has slow-release antibacterial and antioxidant functions, and can be used for fresh-keeping of livestock and poultry prefabricated vegetables.

Description

Technical Field

[0001] This invention relates to the field of food packaging technology, specifically to a slow-release antibacterial and antioxidant composite film and its application in the preservation of pre-cooked meat and poultry dishes. Background Technology

[0002] Food packaging film materials are particularly important in food transportation and storage, as they can effectively block the external environment and resist microbial invasion, thereby ensuring food quality. Currently, the most commonly used packaging films on the market are polyethylene or polyvinyl chloride plastic films. The difficulty in degrading these plastic films leads to "white pollution," a serious threat to the natural environment and an increasing global nuisance. Developing bioactive and environmentally friendly food packaging has become a key focus of food preservation research.

[0003] Metal-organic frameworks (MOFs) are materials with well-organized porous structures formed by the combination of metal cations and organic ligands. MOFs exhibit beneficial properties such as high surface area, controllable pore size, functional flexibility, ease of modification, and good biocompatibility. ZIF-8 (Zn-MOF) is a subclass of MOFs, composed of Zn²⁺ ions and pyrazole or its derivatives, and is notable for its multifunctionality. Zn-MOFs, due to their high porosity, UV absorption, and good dispersibility in aqueous media, can be used as multifunctional materials in the packaging field. Cinnamaldehyde is a natural antibacterial and antioxidant agent, generally considered safe and possessing strong food preservation potential. Therefore, antibacterial materials combining MOFs and cinnamaldehyde have been gradually developed. However, the use of cinnamaldehyde is limited by its instability, volatility, and strong odor, which may lead to unacceptable sensory properties. Patent application CN 121203319 A discloses an ethylene adsorption and pH / pectinase dual-response intelligent antibacterial composite membrane, its preparation method, and its application, using ZIF-8@cinnamaldehyde@pectin as the functional carrier. This composite membrane achieves efficient adsorption of ethylene released during fruit and vegetable storage through the porous structure of ZIF-8. Simultaneously, it utilizes the naturally occurring acidic environment during fruit and vegetable ripening and spoilage to synergistically trigger the intelligent sustained release of cinnamaldehyde loaded on ZIF-8. Cinnamaldehyde can form a dual-channel intelligent antibacterial system with the natural antibacterial properties of chitosan in the membrane substrate and the metal ion antibacterial properties of ZIF-8. However, the high volatility of cinnamaldehyde leads to rapid loss of the effective components, and the hydrophobic properties of the essential oil make its dispersion in aqueous systems difficult. These shortcomings severely restrict its large-scale application in the food industry. Therefore, it is necessary to develop a composite membrane that contains both MOF materials and cinnamaldehyde, which can avoid the strong odor emitted by cinnamaldehyde, the rapid volatilization of cinnamaldehyde, and the difficulty in dispersion in aqueous systems; thus enabling it to slow-release and inhibit bacteria. Summary of the Invention

[0004] In view of the above-mentioned prior art, the purpose of this invention is to provide a slow-release antibacterial and antioxidant composite film and its application in the preservation of prepared livestock and poultry dishes. This invention uses starch, chitosan, glycerol, and ZIF-8@CIN as raw materials to prepare a composite film with slow-release antibacterial and antioxidant functions, which can be used for the preservation of prepared livestock and poultry dishes.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a slow-release antibacterial and antioxidant composite membrane, comprising, from top to bottom, a first outer layer, a functional inner layer, and a second outer layer; both the first and second outer layers contain polyvinyl butyral and ethyl cellulose; the functional inner layer contains ZIF-8@cinnamaldehyde, starch, glycerol, and chitosan.

[0006] Preferably, the thickness of both the first outer layer and the second outer layer is 0.05 mm.

[0007] Preferably, the thickness of the functional inner layer is 0.06 mm.

[0008] Preferably, the sustained-release antibacterial and antioxidant composite membrane is prepared by the following method: (1) Dissolve polyvinyl butyral and ethyl cellulose in anhydrous ethanol, heat and stir until homogeneous to obtain the outer membrane solution; (2) Dissolve zinc nitrate hexahydrate and 2-methylimidazole in methanol to obtain zinc nitrate hexahydrate solution and 2-methylimidazole solution respectively. Pour the zinc nitrate hexahydrate solution into the 2-methylimidazole solution, stir and dry to obtain ZIF-8. Put ZIF-8 into a methanol solution of cinnamaldehyde, heat and stir and dry to obtain ZIF-8@CIN. Dissolve starch in deionized water, add glycerol and gelatinize to obtain gelatinized starch solution. Dissolve chitosan in acetic acid solution to obtain chitosan solution. Mix the two solutions evenly, then add ZIF-8@CIN and stir evenly at room temperature to obtain inner membrane solution. (3) The first outer layer is obtained by pouring the outer membrane liquid, and after drying at room temperature, the inner membrane liquid is poured to obtain the functional inner layer. After standing at room temperature, the second outer layer is obtained by pouring the outer membrane liquid. After drying, the second outer layer is obtained.

[0009] Preferably, in step (1), the ratio of the amount of polyvinyl butyral, ethyl cellulose and anhydrous ethanol added is 6 g: 4 g: 125 mL; the heating temperature is 50 °C and the time is 1 h.

[0010] Preferably, in step (2), the molar ratio of zinc nitrate hexahydrate to 2-methylimidazolium is 1:8, the stirring time is 1 hour, and the drying time is 12 hours.

[0011] Preferably, in step (2), the ratio of the amount of ZIF-8 nanoparticles to the methanol solution of cinnamaldehyde added is 1 g: 10 mL; the heating and stirring temperature is 50°C and the time is 5 h.

[0012] Preferably, the volume ratio of the starch solution to the chitosan solution is 5:4; the mass ratio of the starch to the chitosan is 3:1; the gelatinization temperature is 90°C and the time is 30 min; the amount of glycerol added accounts for 35% of the total mass of starch and chitosan; and the amount of ZIF-8@CIN added accounts for 2-6% of the total mass of starch and chitosan.

[0013] Preferably, in step (3), the room temperature drying time is 2 hours; the room temperature standing time is 48 hours.

[0014] In this invention, the amino groups of chitosan bind a large number of hydrogen ions in the acetic acid solution, so ZIF-8 will not be decomposed in the inner membrane solution of this invention.

[0015] A second aspect of the present invention provides the application of a slow-release antibacterial and antioxidant composite film in the preservation of pre-cooked meat and poultry dishes.

[0016] The beneficial effects of this invention are: (1) The cinnamaldehyde added in this invention has broad-spectrum antibacterial and antioxidant properties, which can effectively inhibit the growth of microorganisms in the packaging and extend the shelf life. ZIF-8@cinnamaldehyde, starch, glycerol and chitosan are used as the functional inner layer. The first and second outer layers, as well as the starch, glycerol and chitosan in the functional contents, can prevent cinnamaldehyde from emitting a strong odor and prevent cinnamaldehyde from volatilizing rapidly.

[0017] (2) This invention constructs a packaging material with multiple functions of "slow release-antibacterial-antioxidant", which effectively prolongs the antibacterial, antioxidant and stability properties of cinnamaldehyde. The preparation method of the slow release antibacterial and antioxidant composite film is simple and does not require complicated processes or harmful solvents. Attached Figure Description

[0018] Figure 1 a) shows the changes in thiobarbituric acid (TBA) values ​​of the composite film prepared in Examples 1-5 and the ordinary PE film after storing braised pork for a period of time; b) shows the changes in thiobarbituric acid (TBA) values ​​of the composite film prepared in Comparative Examples 1-3 and Example 1 after storing braised pork for a period of time. Figure 2 a) shows the change in volatile basic nitrogen (TVB-N) values ​​of the composite film prepared in Examples 1-5 and the ordinary PE film after storing the packaged braised pork for a period of time; b) shows the change in volatile basic nitrogen (TVB-N) values ​​of the composite film prepared in Comparative Examples 1-3 and Example 1 after storing the packaged braised pork for a period of time. Figure 3 a) shows the changes in total bacterial count of the composite film prepared in Examples 1-5 and ordinary PE film after storing braised pork for a period of time; b) shows the changes in total bacterial count of the composite film prepared in Comparative Examples 1-3 and Example 1 after storing braised pork for a period of time. Figure 4 : The release rate of cinnamaldehyde in a 50% ethanol simulated solution of the composite membranes prepared in Comparative Examples 1-3 and Example 1. Detailed Implementation

[0019] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0020] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0021] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0022] Example 1: Preparation of a sustained-release antibacterial and antioxidant composite membrane (1) Dissolve 2.975 g of zinc nitrate hexahydrate and 6.569 g of 2-methylimidazole in 200 mL of methanol to obtain zinc nitrate hexahydrate solution and 2-methylimidazole solution, respectively. Pour the zinc nitrate hexahydrate solution into the 2-methylimidazole solution, stir at room temperature for 1 h, and dry at 60 °C for 12 h to obtain ZIF-8; put 1 g of ZIF-8 into 10 mL of cinnamaldehyde methanol solution, heat and stir at 50 °C for 5 h, and dry at 60 °C for 12 h to obtain ZIF-8@CIN; 1 g of chitosan was dissolved in a 2% (volume fraction) aqueous solution of glacial acetic acid to obtain a 1% (w / w) chitosan-acetic acid solution. 3 g of corn starch was added to distilled water to obtain a 3% (w / w) starch solution. The solution was heated to 90 °C with magnetic stirring and gelatinized for 30 min. Then, 1.05 mL of glycerol was added, and the mixture was stirred for another 30 min to obtain a gelatinized starch solution. The chitosan solution and starch solution were mixed in a 5:4 ratio to obtain a starch-chitosan solution. 5% (w / w) of ZIF-8@CIN (based on the total mass of starch and chitosan) was added, and the mixture was magnetically stirred for 2 h to obtain the inner membrane solution.

[0023] (2) Dissolve 6 g of polyvinyl butyral and 4 g of ethyl cellulose in 125 mL of anhydrous ethanol and stir at 50 °C for 1 h to obtain a uniform outer membrane solution.

[0024] (3) Pour the outer membrane solution into a plastic petri dish (15 cm × 15 cm), tilt the petri dish to allow the membrane solution to flow naturally and dry at room temperature for 2 hours to obtain the first outer layer. Then, pour the inner membrane solution onto the first outer layer into a plastic petri dish (15 cm × 15 cm) and dry at room temperature for 48 hours to obtain the functional inner layer. Finally, pour another layer of outer membrane solution onto the functional inner layer to obtain the second outer layer. After drying, a slow-release antibacterial and antioxidant composite membrane is prepared and named ZIF-8@CIN-5 membrane. The thickness of the outer membrane is 0.05 mm; the thickness of the slow-release, antibacterial, and antioxidant functional layers is 0.06 mm.

[0025] Example 2 The difference from Implementation 1 is that in step (2), the amount of ZIF-8@CIN added accounts for 6% of the total mass of starch and chitosan (the mass ratio of starch to chitosan is 3:1), while the other steps and parameters are the same. The resulting film is named ZIF-8@CIN-6 film.

[0026] Example 3 The difference from Example 1 is that in step (2), the amount of ZIF-8@CIN added accounts for 4% of the total mass of starch and chitosan (the mass ratio of starch to chitosan is 3:1), while the other steps and parameters are the same. The resulting film is named ZIF-8@CIN-4 film.

[0027] Example 4 The difference from Example 1 is that in step (2), the amount of ZIF-8@CIN added accounts for 3% of the total mass of starch and chitosan (the mass ratio of starch to chitosan is 3:1), while the other steps and parameters are the same. The resulting film is named ZIF-8@CIN-3 film.

[0028] Example 5 The difference from Example 1 is that in step (2), the amount of ZIF-8@CIN added accounts for 2% of the total mass of starch and chitosan (the mass ratio of starch to chitosan is 3:1), while the other steps and parameters are the same. The resulting film is named ZIF-8@CIN-2 film.

[0029] Comparative Example 1 The difference from Example 1 is that in step (3), the inner membrane liquid is poured into a plastic petri dish (15 cm × 15 cm) and dried at room temperature for 48 hours to obtain the functional inner layer, which is named CS membrane.

[0030] Comparative Example 2 The difference from Example 1 is that ethyl cellulose is not added in step (2) to obtain the outer membrane solution. The final composite membrane is named PVB membrane.

[0031] Comparative Example 3 The difference from Example 1 is that in step (2), polyvinyl butyral is not added to obtain the outer membrane solution. The final composite membrane is named EC membrane.

[0032] Comparative Example 4 The difference from Example 1 is that the outer membrane solution obtained in step (2) was added to the inner membrane solution in step (1) to obtain a composite membrane solution. The composite membrane solution was poured into a plastic petri dish (15 cm × 15 cm) and left to dry at room temperature for 48 hours, but film formation was not observed. Experiments revealed that when the outer membrane solution was mixed with the inner membrane solution, the two were incompatible, resulting in severe delamination and preventing film formation.

[0033] Test case The composite membranes prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to relevant performance tests.

[0034] (1) pH determination: Based on the film products prepared in Examples 1-5 and Comparative Examples 1-3, the cooled braised pork samples were heat-sealed and stored at room temperature for 21 days. The pH of the braised pork samples was measured every 3 days. On day 0, the pH of the samples was measured first, and then the samples were divided into groups 1-5 of Examples 1-5 and groups 1-3 of Comparative Examples 1-3 (the tests for the remaining groups were conducted in the same manner). In a sterile environment, 10 g of uniformly textured braised pork (purchased from Zhang Xiumei Braised Pork Shop in Tai'an, Shandong) was placed in a homogenizing bag (sterile) containing 90 mL of 0.85% sterile physiological saline. The homogenizing bag was then tapped with a small hammer to thoroughly mix the contents. Finally, the pH value was measured using a handheld pH meter. The results are expressed as the average of three measurements. The results are shown in Tables 1 and 2. Table 1. pH changes during storage of braised pork packaged in composite films prepared in Examples 1-5. Table 2. pH changes during storage of braised pork packaged with composite films prepared in Comparative Examples 1-3 and Example 1. Note: The experiments in Tables 1 and 2 were conducted simultaneously. For ease of comparison, they are divided into two tables. ZIF-8@CIN-5 in Tables 1 and 2 are the experimental results of Example 1.

[0035] The results are shown in Tables 1 and 2. During storage, the pH value of braised pork in ZIF-8@CIN packaging showed an upward trend, with the pH value of the PE-packaged braised pork changing more slowly than that in PE packaging. The pH value of the PE-packaged braised pork reached 6.77 on day 21 of storage, while the pH value of the ZIF-8@CIN-5 film-packaged braised pork was 6.59 on day 21. The CS film, due to its high initial release of cinnamaldehyde, showed a slow pH increase, but the pH value increased significantly with prolonged storage. Its shelf life was not as long as that of multi-layer films.

[0036] (2) Determination of thiobarbituric acid (TBA) value: The composite membranes prepared according to Examples 1-5 and Comparative Examples 1-3 were heat-sealed and packaged after cooling, and stored at room temperature for 21 days. The thiobarbituric acid (TBA) value was determined every 3 days. Weigh 5 g of braised pork sample and place it in a 100 mL stoppered conical flask. Accurately add 50 mL of trichloroacetic acid mixture, shake well, seal the flask, and place it on a constant temperature shaker at 50 ℃ for 30 min. Remove the flask, cool to room temperature, and filter it using double-layer quantitative slow filter paper. Discard the initial filtrate and keep the subsequent filtrate for later use. Accurately transfer 5 mL of the above filtrate into 25 mL stoppered colorimetric tubes. Take another 5 mL of trichloroacetic acid mixture as a sample blank, add 5 mL of 0.02 mol / L thiobarbituric acid (TBA) aqueous solution to each tube, stopper the tube, mix well, and heat in a boiling water bath for 20 min. Remove the tube and cool to room temperature. Adjust the zero point with the sample blank and measure the absorbance of the sample solution at 532 nm. Calculate the TBA value by comparing it with the TEP standard curve. The result is expressed as mg MDA / kg (based on meat sample) (MDA is malondialdehyde).

[0037] Preparation of the standard curve: Accurately weigh 0.315 g TEP, dissolve and dilute to 1000 mL to prepare a 100 μg / mL TEP standard stock solution, and refrigerate until use. Prepare a 10 μg / mL TEP working solution using 10 mL of the TEP standard stock solution. Add 0.01, 0.05, 0.1, 0.15, 0.25, 0.35, and 0.45 mL of the 10 μg / mL TEP working solution to 10 mL heating tubes, respectively, and bring the volume to 5.0 mL with deionized water. Then add 5.0 mL of 0.02 mol / L TBA solution to each tube, incubate in a boiling water bath for 20 min, cool to room temperature, and measure the absorbance at 532 nm. The results are shown below. Figure 1 .

[0038] The results are as follows Figure 1As shown, the TBA value of braised pork gradually increased throughout the storage process, indicating a continuous increase in fat oxidation. The TBA value of PE packaging increased rapidly, reaching 5.87 mg MDA / kg on day 9, while the TBA value of braised pork packaged with ZIF-8@CIN-5 film increased relatively slowly, reaching only 3.00 mg MDA / kg on day 21. This indicates that the addition of ZIF-8@CIN can slow down the oxidation of fat in braised pork, thus providing better preservation. The TBA values ​​of braised pork packaged with CS, PVB, EC, and ZIF-8@CIN film were relatively similar in the early stages of storage, but the ZIF-8@CIN film showed a longer-lasting preservation effect as the storage time increased.

[0039] (3) Determination of volatile basic nitrogen (TVB-N) value: Based on the composite membranes prepared in Examples 1-5 and Comparative Examples 1-3, the cooled braised pork samples were heat-sealed and stored at room temperature for 21 days. Every 3 days, the volatile basic nitrogen (TVB-N) value of the braised pork samples was determined. The TVB-N level of the samples was determined using a fully automated Kjeldahl nitrogen determination system according to national standard GB 5009.228-2016. 0.3 g of braised pork sample, 0.2 g of copper sulfate, and 3 g of magnesium oxide were placed in a digestion tube, and 10 mL of concentrated sulfuric acid was added for digestion. The solution was then distilled using a Kjeldahl nitrogen analyzer, with 2% boric acid as the receiving solution. After complete distillation, the distillate was titrated using an automatic potentiometric titrator (the titrant was a 0.1 mol / L hydrochloric acid standard solution). The above experiment was repeated three times, and the results were calculated using the following formula: X=(V1-V2)×C×14×100 / M; In the formula, X (mg / 100g) is the TVB-N content of beef; V1 (mL) is the volume of standard solution consumed by the sample; V2 (mL) is the volume of solution consumed without the sample (blank); m (g) is the sample weight; and C (mol / L) is the concentration of hydrochloric acid standard solution, 0.1 mol / L.

[0040] The results are as follows Figure 2As shown, braised pork packaged in PE preservation bags and ZIF-8@CIN film were compared to demonstrate the preservation effect and freshness of ZIF-8@CIN film on braised pork. After 6 days of storage, the TVB-N value of braised pork packaged in ZIF-8@CIN-5 film was 9.24 mg / 100g, still fresh; while the TVB-N value of braised pork packaged in PE film had reached 15.45 mg / 100g, indicating slight spoilage. After 21 days of storage, the TVB-N value of braised pork packaged in ZIF-8@CIN-5 film was 12.74 mg / 100g, still fresh; while the TVB-N value of braised pork packaged in PE film had reached 22.68 mg / 100g, indicating complete spoilage. The TVB-N values ​​of braised pork packaged in CS, PVB, EC, and ZIF-8@CIN film were relatively similar in the early stages of storage, but ZIF-8@CIN film showed a longer-lasting preservation effect as the storage time increased.

[0041] (4) Total bacterial count determination: The composite membranes prepared according to Examples 1-5 and Comparative Examples 1-3 were used to heat-seal the cooled braised pork samples and store them at room temperature for 21 days. The total bacterial count of the braised pork samples was determined every 3 days. The total putrefactive bacteria (TPA) in the samples were determined by plate culture according to GB 4789.2-2022. 1 g of braised pork sample was weighed and placed in a sterile homogenizing bag containing 9 mL of diluent. The sample was homogenized by tapping for 1 min to prepare a 1:10 sample homogenate. Then, 10-fold serial dilutions of the sample homogenate were prepared. 100 μL of the sample homogenate from two suitable dilutions was taken into a sterile petri dish and counted by plate count method.

[0042] The results are as follows Figure 3 As shown, the total bacterial count of all samples showed an increasing trend during storage. The braised pork pre-cooked dish packaged in PE film reached a total bacterial count of 10 on day 9. 4 The CFU / g level exceeded the national standard. Meanwhile, the total bacterial count of braised pork pre-cooked dishes packaged in ZIF-8@CIN-5 film remained at 10 on day 18. 4 Below CFU / g, the total bacterial count of braised pork packaged with ZIF-8@CIN-5 film changed slowly and remained relatively constant throughout the storage period, consistently at a low level. This indicates that the addition of ZIF-8@CIN significantly increased the antibacterial and antioxidant effects of the film, effectively inhibiting the growth of microorganisms in the prepared braised pork and achieving a good preservation effect. The total bacterial count of braised pork packaged with CS, PVB, EC, and ZIF-8@CIN films all increased slowly in the early stages of storage; however, with prolonged storage, ZIF-8@CIN film showed a more sustained preservation effect.

[0043] (5) To investigate the release of cinnamaldehyde from the composite membrane in a water-oil mixture, flat, undamaged films prepared in Comparative Examples 1-3 and Example 1 were selected and cut into square samples of 2cm x 2cm. These samples were immersed in 30ml of 50% ethanol simulated solution. At regular intervals, 4ml of the extract was taken out, and its absorbance at 291 nm was measured using a UV spectrophotometer. After the measurement, the extract was poured back in. The content of cinnamaldehyde in the composite membrane was calculated based on the plotted standard curve. The release efficiency formula is as follows: K=M t / M0×100; Where: K is the cinnamaldehyde release efficiency (%); M t The content of cinnamaldehyde in the 50% ethanol simulation solution at the sampling time point (μg); M0 represents the total cinnamaldehyde content (μg) in the sample film.

[0044] The single-layer membrane without a waterproof layer exhibited the highest CS release rate in the initial stage, with a cumulative release of 49.30% of cinnamaldehyde, reaching near-equilibrium after 96 hours. Compared to the single-layer membrane, the three-layer membrane showed a relatively slower release rate. The PVB membrane had a higher release rate than the EC membrane, while the ZIF-8@CIN-5 membrane showed the slowest increase in cumulative release, reaching a relatively stable state after 216 hours. This indicates that the three-layer membrane slowed the release of cinnamaldehyde compared to the single-layer membrane, and the combined use of polyvinyl butyral and ethyl cellulose resulted in a better sustained-release effect.

[0045] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A sustained-release antibacterial and antioxidant composite membrane, characterized in that, From top to bottom, it comprises a first outer layer, a functional inner layer, and a second outer layer; both the first and second outer layers contain polyvinyl butyral and ethyl cellulose; the functional inner layer contains ZIF-8@cinnamaldehyde, starch, glycerol, and chitosan.

2. The sustained-release antibacterial and antioxidant composite membrane according to claim 1, characterized in that, The thickness of both the first outer layer and the second outer layer is 0.05 mm.

3. The sustained-release antibacterial and antioxidant composite membrane according to claim 1, characterized in that, The thickness of the functional inner layer is 0.06 mm.

4. The sustained-release antibacterial and antioxidant composite membrane according to claim 1, characterized in that, The sustained-release antibacterial and antioxidant composite membrane is prepared by the following method: (1) Dissolve polyvinyl butyral and ethyl cellulose in anhydrous ethanol, heat and stir until homogeneous to obtain the outer membrane solution; (2) Dissolve zinc nitrate hexahydrate and 2-methylimidazole in methanol to obtain zinc nitrate hexahydrate solution and 2-methylimidazole solution respectively. Pour the zinc nitrate hexahydrate solution into the 2-methylimidazole solution, stir and dry to obtain ZIF-8. Put ZIF-8 into a methanol solution of cinnamaldehyde, heat and stir and dry to obtain ZIF-8@CIN. Dissolve starch in deionized water, add glycerol and gelatinize to obtain gelatinized starch solution. Dissolve chitosan in acetic acid solution to obtain chitosan solution. Mix the two solutions evenly, then add ZIF-8@CIN and stir evenly at room temperature to obtain inner membrane solution. (3) The first outer layer is obtained by pouring the outer membrane liquid, and after drying at room temperature, the inner membrane liquid is poured to obtain the functional inner layer. After standing at room temperature, the second outer layer is obtained by pouring the outer membrane liquid. After drying, the second outer layer is obtained.

5. The sustained-release antibacterial and antioxidant composite membrane according to claim 4, characterized in that, In step (1), the ratio of polyvinyl butyral, ethyl cellulose and anhydrous ethanol added is 6 g: 4 g: 125 mL; the heating temperature is 50 ℃ and the time is 1 h.

6. The sustained-release antibacterial and antioxidant composite membrane according to claim 4, characterized in that, In step (2), the molar ratio of zinc nitrate hexahydrate to 2-methylimidazolium is 1:8, the stirring time is 1 hour, and the drying time is 12 hours.

7. The sustained-release antibacterial and antioxidant composite membrane according to claim 4, characterized in that, In step (2), the ratio of the amount of ZIF-8 nanoparticles to the methanol solution of cinnamaldehyde added is 1 g: 10 mL; the heating and stirring temperature is 50°C and the time is 5 h.

8. The sustained-release antibacterial and antioxidant composite membrane according to claim 4, characterized in that, The volume ratio of the starch solution to the chitosan solution is 5:4; the mass ratio of the starch to the chitosan is 3:1; the amount of glycerol added accounts for 35% of the total mass of starch and chitosan; and the amount of ZIF-8@CIN added accounts for 2-6% of the total mass of starch and chitosan.

9. The sustained-release antibacterial and antioxidant composite membrane according to claim 4, characterized in that, In step (3), the room temperature drying time is 2 hours; the room temperature standing time is 48 hours.

10. The application of the slow-release antibacterial and antioxidant composite film according to any one of claims 1 to 9 in the preservation of pre-cooked meat and poultry dishes.

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