Double-layer composite preservative film as well as preparation method and application thereof in food preservation

Abstract

The invention discloses a double-layer composite preservative film as well as a preparation method and application thereof in food preservation. The preparation method of the double-layer composite preservative film comprises the following steps: (1) preparing the air-conditioning outer film; (2) preparing a slow-release inner membrane: mixing procyanidine, polyvinyl alcohol, polyethylene glycol and deionized water, stirring, and defoaming to obtain an inner membrane solution; and coating the surface of the outer film with the inner film liquid, and drying to obtain the double-layer composite preservative film. The modified diatomite is used as a raw material for preparing the air-conditioning outer film. The water-soluble characteristics of polyvinyl alcohol and polyethylene glycol are utilized, slow release of procyanidine is accurately controlled, and the antioxidant and antibacterial effects are achieved. According to the double-layer composite preservative film, through the synergistic effect of controlled atmosphere of the outer film and slow release of active substances of the inner film, the fruit and vegetable preservation effect is remarkably improved, the shelf life is prolonged, and the double-layer composite preservative film is suitable for sealed packaging and preservation of fruit and vegetable foods.

Description

Technical Field

[0001] This invention relates to the field of food preservation materials technology, specifically to a double-layer composite preservation film, its preparation method, and its application in food preservation. Background Technology

[0002] Preservation of fruits and vegetables is a crucial aspect of the food industry. For individually packaged fruits and vegetables, the preservation function of the packaging material directly impacts the preservation effect. Currently, modified atmosphere packaging (MAP) technology inhibits food respiration and microbial growth by adjusting the gas composition inside sealed packaging (e.g., increasing carbon dioxide concentration). However, it suffers from low carbon dioxide adsorption efficiency and short effective time. Meanwhile, numerous studies have shown that preservative materials with added active substances (such as antibacterial agents and antioxidants) can achieve good preservation results. However, a significant challenge lies in controlling the release rate of these active substances, leading to excessively rapid release and loss of long-lasting preservation effects. Therefore, seeking a preservation technology and material that combines the advantages of MAP and slow-release of active substances is of paramount importance.

[0003] Existing modified atmosphere packaging technologies generally involve precise perforation of polymer films using high-energy particle beams, incorporation of gas-selective microparticles, and chemical modification of the composite film. Among these, incorporating gas-selective microparticles is a good way to maintain long-lasting carbon dioxide selectivity in the film, offering better gas selectivity compared to micropores, and a simpler preparation process compared to chemical modification of composite films. On the other hand, there are many methods for the sustained release of active substances, among which utilizing the material's own porosity variation characteristics is a good approach to achieving both modified atmosphere and sustained release.

[0004] Chinese patent CN202411281636.8 discloses a composite biodegradable modified atmosphere film and its preparation and application. The modified atmosphere film prepared by this method can rapidly reduce the oxygen content and create a low-respiration environment. However, this method relies on the rapid oxygen reduction of the intermediate layer, which depends on the consumption reaction (redox reaction) between micro-nano iron powder and oxygen. Once the iron powder is exhausted, the oxygen reduction ability will fail and it will be unable to maintain a low-oxygen environment continuously.

[0005] Chinese patent CN202411281636.8 uses another technical route. The food preservation film uses polybutylene adipate terephthalate as the base and is compounded with copolyester. Through the molecular structure and aggregation state design of the material itself, it achieves differentiated control of CO2 and O2 permeability. However, the synthesis threshold of copolyester is high, which makes the whole preparation process complex and requires high precision. Summary of the Invention

[0006] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the purpose of this invention is to provide a method for preparing a double-layer composite preservation film. The inner film utilizes the water-soluble properties of polyvinyl alcohol and polyethylene glycol to precisely control the slow release of proanthocyanidins, thereby exerting antioxidant and antibacterial effects. Furthermore, by utilizing the synergistic effect of the double-layer structure, the modified atmosphere of the outer film and the slow release of the inner film work together to significantly improve the preservation effect and extend the shelf life of fruits and vegetables.

[0007] Another object of the present invention is to provide a double-layer composite food preservation film.

[0008] Another aspect of the present invention is to provide the application of the above-mentioned double-layer composite preservation film in food preservation.

[0009] The objective of this invention is achieved through the following technical solution: This invention provides a method for preparing a double-layer composite food preservation film, comprising the following steps: (1) Preparation of modified atmosphere outer membrane; (2) Preparation of sustained-release inner membrane: Proanthocyanidins, polyvinyl alcohol, polyethylene glycol and deionized water are mixed, stirred and defoamed to obtain inner membrane liquid; the inner membrane liquid is coated on the surface of the outer membrane and dried to obtain double-layer composite food preservation film.

[0010] Preferably, the preparation of the modified atmosphere outer membrane in step (1) specifically includes: Diatomaceous earth was modified by mixing tetraethylenepentamine (TEPA), polyethyleneimine (PEI), and sorbitol oleate. After modification, the diatomaceous earth was centrifuged and the precipitate was washed to obtain modified diatomaceous earth. The modified diatomaceous earth was mixed with an aqueous solution of carboxymethyl cellulose and sodium alginate, and then degassed by magnetic levitation stirring to obtain an outer membrane solution. An outer membrane was prepared using the outer membrane solution.

[0011] Preferably, the weight ratio of diatomaceous earth, tetraethylenepentamine (TEPA), and polyethyleneimine (PEI) in step (1) is (1.8~2.2):(1.3~1.7):1; the weight of sorbitol oleate is 20-30% of that of diatomaceous earth.

[0012] Preferably, the weight ratio of carboxymethyl cellulose, sodium alginate, and deionized water is 1:(2.8~3.2):(280~320); the amount of modified diatomaceous earth added is 0.5~0.7% of the mass of the outer membrane solution.

[0013] Preferably, the weight ratio of polyvinyl alcohol, polyethylene glycol and deionized water in step (2) is (2.8~3.2):1:(180~220); the amount of proanthocyanidins added is 3-5% of the total weight of polyvinyl alcohol and polyethylene glycol.

[0014] Preferably, the water content of the outer membrane fluid is 90-95 wt%, and the water content of the inner membrane fluid is 90-95 wt%.

[0015] Preferably, the outer membrane is dried at a constant temperature of 50-60℃ for 5-6 hours; the inner membrane is dried at a constant temperature of 35-45℃ for 5 hours.

[0016] The present invention also provides a double-layer composite food preservation film, comprising a modified atmosphere outer film and a slow-release inner film; the inner film comprises proanthocyanidins, polyvinyl alcohol and polyethylene glycol.

[0017] Preferably, the outer membrane comprises diatomaceous earth, tetraethylenepentamine (TEPA), polyethyleneimine (PEI), and sorbitol oleate.

[0018] Preferably, when the double-layer composite preservation film is used for preserving fruits and vegetables, the fruits and vegetables are placed in a box with one open side, and the double-layer composite preservation film is tightly wrapped around the open side, with the outer film facing outward and the inner film adhering to the packaging box for sealing. The inner film utilizes the water-soluble properties of polyvinyl alcohol and polyethylene glycol. When the inner film comes into contact with the water vapor generated by the respiration of the fruits and vegetables, its structure collapses, thereby controlling the slow release rate of proanthocyanidins and prolonging the antioxidant and antibacterial effects.

[0019] Preferably, in the process of modifying diatomite in step (1), the mixture is stirred at a temperature of 40-60℃ for 10-14 hours.

[0020] Preferably, the centrifugation speed in step (1) is 6000-8000 r / min and the time is 8-15 min.

[0021] Preferably, the degassing is performed by vacuum degassing or static degassing, and the time is 4-5 hours.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects: (1) The double-layer composite preservation film of the present invention utilizes the water-soluble properties of polyvinyl alcohol and polyethylene glycol in the inner film. When it comes into contact with water vapor generated by the respiration of fruits and vegetables, the structure collapses, which precisely controls the slow release rate of proanthocyanidins and prolongs the antioxidant and antibacterial effects.

[0023] (2) The double-layer composite food preservation film of the present invention is prepared by using modified diatomaceous earth as raw material for the outer film. Diatomaceous earth itself has a porous structure and its surface is rich in silanol groups. At a modification temperature of 40-60℃, the amino groups in TEPA and PEI can undergo dehydration condensation reaction with silanol groups to form stable Si-OC bonds and Si-N bonds, grafting a large number of amine groups onto the pore surface of diatomaceous earth. These amine groups are basic groups and can undergo chemical adsorption reaction with acidic CO2 molecules, greatly increasing the CO2 adsorption sites per unit mass of diatomaceous earth. Compared with unmodified diatomaceous earth, the adsorption capacity is increased by 3-5 times.

[0024] (3) The double-layer composite preservation film of the present invention utilizes the synergistic effect of the double-layer structure, with the outer membrane modified atmosphere and the inner membrane slow release working together to significantly improve the preservation effect and extend the shelf life of fruits and vegetables. Attached Figure Description

[0025] Figure 1 This is a flowchart illustrating a method for preparing a double-layer composite food preservation film according to an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the double-layer composite food preservation film according to an embodiment of the present invention.

[0027] Figure 3 This is a comparison chart of the internal carbon dioxide content of the packaging of Examples 1-3 and Comparative Examples 1 and 2 of the present invention.

[0028] Figure 4 This is a comparison chart of the proanthocyanidin release rates of Examples 1-3 and Comparative Examples 1 and 2 of the present invention.

[0029] Figure 5 This is a comparison chart of the preservation effects of Example 1 and Comparative Examples 1 and 2 on lychees. Detailed Implementation

[0030] The present invention is further described below through specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0031] In the following examples / comparative examples, the steps for detecting the release rate of the active ingredient are as follows: Cut the double-layer composite plastic wrap prepared in the examples / comparative examples into 5cm×5cm sizes (ensuring consistent inner film area to reduce error). Accurately weigh the total amount of proanthocyanidins originally present in the inner film (denoted as M0, calculated based on the amount of proanthocyanidins added during preparation; for example, in Example 1, the amount of proanthocyanidins added to the inner film was 0.12g, i.e., M0=0.12g). Prepare a soaking solution simulating the inside of fruit and vegetable packaging (0.1% citric acid aqueous solution, pH=5.5, simulating the acidic environment produced by fruit and vegetable respiration). Take 50mL of the soaking solution and place it in a 50mL petri dish. Fix the cut plastic wrap sample (inner film facing the soaking solution, outer film facing outward, simulating the state of the inner film adhering to the fruit and vegetables in actual packaging) at the mouth of a centrifuge tube, ensuring that the inner film is completely in contact with the soaking solution and the outer film is not in contact with the soaking solution. After sealing the centrifuge tube, place it in a 25℃ constant temperature incubator. After soaking for 24 h, 48 h, and 72 h respectively (corresponding to the cumulative release rate of active ingredients detected in the examples at 72 h), 2 mL of the soaking solution was taken, filtered through a 0.22 μm organic filter membrane, and the concentration of proanthocyanidins in the soaking solution (denoted as C, unit: g / mL) was determined by high performance liquid chromatography (HPLC). The concentration was calculated according to the formula "Active ingredient release rate (%) = (C × V × n) / M0 × 100%", where: V is the volume of the soaking solution (50 mL = 0.05 L), n is the dilution factor (n = 1 if undiluted during detection), and M0 is the total amount of proanthocyanidins originally present in the inner membrane (g).

[0032] Example 1 The preparation method of the double-layer composite food preservation film in this embodiment is as follows: Figure 1 As shown, it includes the following steps: (1) Preparation of outer membrane: Weigh 2g of diatomaceous earth, 1.5g of TEPA and 1g of PEI, add 0.4g of sorbitol oleate (20% of the amount of diatomaceous earth added), mix and stir at 40℃ for 12 h; centrifuge the mixture at 6000 r / min for 10 min, discard the supernatant, wash the precipitate twice with ethanol, and then vacuum dry at 50℃ for 12 h, and store in the dark for later use. Weigh 1g of carboxymethyl cellulose and 3g of sodium alginate, add 200g of deionized water to make an aqueous solution. The total weight of the outer membrane matrix is ​​4g. Add 0.024g of the above-mentioned amine-modified diatomaceous earth at a ratio of 0.6%. The water content of the mixed system is 90wt%. The mixed system is magnetically suspended and stirred at 50℃ and 1000 r / min for 5h. After stirring, vacuum degassing is performed for 4h to obtain the outer membrane casting solution. The outer membrane casting solution is cast onto a polytetrafluoroethylene plate, evenly spread with a scraper, dried at a constant temperature of 50℃ for 5h, removed and naturally air-dried for 12h, and stored in the dark for later use.

[0033] (2) Inner membrane preparation: Weigh 3g of polyvinyl alcohol and 1g of polyethylene glycol, add 200g of deionized water, and then add 0.12g of proanthocyanidins (3% of the total weight of polyvinyl alcohol and polyethylene glycol). The water content of the mixed system is 90wt%. Stir magnetically at 25℃ and 1000 r / min for 3 h. After stirring, let stand for 4 h to remove bubbles and obtain the inner membrane casting solution. Coat the inner membrane casting solution evenly on the surface of the outer membrane, dry at 35℃ for 5 h, and then let it air dry naturally for 12 h to obtain a double-layer composite preservation film.

[0034] The principle of the double-layer composite food preservation film in this embodiment is as follows: Figure 2 As shown, the outer membrane is prepared using modified diatomaceous earth as raw material. Diatomaceous earth itself has a porous structure, and its surface is rich in silanol groups. At a modification temperature of 40-60℃, the amino groups in TEPA and PEI can undergo dehydration condensation reactions with silanol groups to form stable Si-OC and Si-N bonds, grafting a large number of amine groups onto the pore surface of diatomaceous earth. These amine groups are basic groups and can undergo chemical adsorption reactions with acidic CO2 molecules, significantly increasing the CO2 adsorption sites per unit mass of diatomaceous earth. Compared with unmodified diatomaceous earth, the adsorption capacity is increased by 3-5 times. The inner membrane utilizes the water-soluble properties of polyvinyl alcohol and polyethylene glycol. Upon contact with water vapor generated by the respiration of fruits and vegetables, the structure collapses, precisely controlling the slow-release rate of proanthocyanidins and prolonging the antioxidant and antibacterial effects.

[0035] The prepared composite film was used to package and preserve fruits and vegetables. The release of active ingredients from the composite film under different humidity gradients was measured, as shown in Table 1. The carbon dioxide content inside the packaging was 82.5% (see Table 1). Figure 3 The release rate of active ingredients was 68.2% (see...). Figure 4 ).

[0036] Table 1

[0037] The double-layer composite preservation film of this embodiment is used for the preservation of fruits and vegetables. When the fruits and vegetables are placed in a box with one side open, the double-layer composite preservation film is tightly wrapped around the open side, with the outer film facing outward and the inner film adhering to the packaging box for sealing. By utilizing the synergistic effect of the double-layer structure, the modified atmosphere of the outer film and the slow release of the inner film work together to significantly improve the preservation effect and extend the shelf life of fruits and vegetables.

[0038] Example 2 (1) Preparation of outer membrane: Weigh 4g of diatomaceous earth, 3g of TEPA and 2g of PEI, add 1.5g of sorbitol oleate (25% of the amount of diatomaceous earth added), mix and stir at 50℃ for 12 h; centrifuge the mixture at 7000 r / min for 10 min, discard the supernatant, wash the precipitate twice with ethanol, and then vacuum dry at 50℃ for 12 h, and store in the dark for later use. Weigh 2g of carboxymethyl cellulose and 6g of sodium alginate, add 400g of deionized water to make an aqueous solution. The total weight of the outer membrane matrix is ​​8g. Add 0.048g of the above-mentioned amine-modified diatomaceous earth at a ratio of 0.6%. The water content of the mixed system is 92wt%. The mixed system is magnetically suspended and stirred at 50℃ and 1000 r / min for 5 h. After stirring, let it stand for 4.5 h to remove bubbles and obtain the outer membrane casting solution. Cast the outer membrane casting solution onto a polytetrafluoroethylene plate, spread it evenly with a scraper, dry it at a constant temperature of 55℃ for 5.5 h, remove it and air dry it naturally for 12 h, and store it in the dark for later use.

[0039] (2) Inner membrane preparation: Weigh 6g of polyvinyl alcohol and 2g of polyethylene glycol, add 400g of deionized water, and then add 0.32g of proanthocyanidins (4% of the total weight of polyvinyl alcohol and polyethylene glycol). The water content of the mixed system is 92wt%. The mixture is magnetically stirred at 25℃ and 1000 r / min for 3 h. After stirring, vacuum degassing is performed for 4.5 h to obtain the inner membrane casting solution. The inner membrane casting solution is uniformly coated onto the surface of the outer membrane and dried at a constant temperature of 40℃ for 5 h. After removal, it is naturally air-dried for 12 h to obtain a double-layer composite preservation film. The prepared composite film is used to package and preserve fruits and vegetables. The release of active ingredients in the composite film of this embodiment under different humidity gradients is shown in Table 2. The carbon dioxide content inside the packaging is 85.1% (see Table 2). Figure 3 The release rate of active ingredients was 70.4% (see...). Figure 4 ).

[0040] Table 2

[0041] Example 3 (1) Preparation of outer membrane: Weigh 6g of diatomaceous earth, 4.5g of TEPA and 3g of PEI, add 1.8g of sorbitol oleate (30% of the amount of diatomaceous earth added), mix and stir at 60℃ for 12 h; centrifuge the mixture at 8000 r / min for 10 min, discard the supernatant, wash the precipitate twice with ethanol, and then vacuum dry at 50℃ for 12 h, and store in the dark for later use. Weigh 3g of carboxymethyl cellulose and 9g of sodium alginate, add 600g of deionized water to make an aqueous solution. The total weight of the outer membrane matrix is ​​12g. Add 0.072g of the above-mentioned amine-modified diatomaceous earth at a ratio of 0.6%. The water content of the mixed system is 95wt%. The mixed system is magnetically suspended and stirred at 50℃ and 1000 r / min for 5h. After stirring, let it stand for 5h to remove bubbles to obtain the outer membrane casting solution. Cast the outer membrane casting solution onto a polytetrafluoroethylene plate, spread it evenly with a scraper, dry it at a constant temperature of 60℃ for 6h, remove it and air dry it naturally for 12h, and store it in the dark for later use.

[0042] (2) Inner membrane preparation: Weigh 9g of polyvinyl alcohol and 3g of polyethylene glycol, add 600g of deionized water, and then add 0.6g of proanthocyanidins (5% of the total weight of polyvinyl alcohol and polyethylene glycol). The water content of the mixed system is 95wt%. The mixture is magnetically stirred at 25℃ and 1000 r / min for 3 h. After stirring, vacuum degassing is performed for 5 h to obtain the inner membrane casting solution. The inner membrane casting solution is uniformly coated onto the surface of the outer membrane and dried at a constant temperature of 45℃ for 5 h. After removal, it is naturally air-dried for 12 h to obtain a double-layer composite preservation film. The prepared composite film is used to package and preserve fruits and vegetables. The release of active ingredients in the composite film of this embodiment under different humidity gradients is shown in Table 3. The carbon dioxide content inside the packaging is 87.3% (see Table 3). Figure 3 The release rate of active ingredients was 72.6% (see...). Figure 4 ).

[0043] Table 3

[0044] Comparative Example 1 (1) Outer membrane preparation: Weigh 2g of unmodified diatomaceous earth and add 0.4g of sorbitol oleate (20% of the diatomaceous earth addition). Mix and stir at 40℃ for 12 h. Subsequent centrifugation (6000 r / min, 10 min), ethanol washing, and drying are the same as in Example 1. Weigh 1g of carboxymethyl cellulose and 3g of sodium alginate, add 200g of deionized water, and add 0.024g of unmodified diatomaceous earth at a ratio of 0.6%. The water content of the mixed system is 90wt%. Subsequent magnetic levitation stirring, degassing, casting, and drying are the same as in Example 1. (2) Inner membrane preparation: Weigh 3g of polyvinyl alcohol and 1g of polyethylene glycol, add 200g of deionized water, and then add 0.12g of proanthocyanidins (3% of the total weight of polyvinyl alcohol and polyethylene glycol). The water content of the mixed system is 90wt%. The mixture is magnetically stirred at 25℃ and 1000r / min for 3 h, and then allowed to stand for 4 h to remove bubbles, obtaining the inner membrane casting solution. The inner membrane casting solution is evenly coated onto the surface of the outer membrane, dried at a constant temperature of 35℃ for 5 h, and then naturally air-dried for 12 h to obtain a double-layer composite preservation film. The prepared composite film is used to package and preserve fruits and vegetables. The release of active ingredients in the composite film of this comparative example under different humidity gradients is shown in Table 4. The carbon dioxide content inside the packaging is 28.3% (see Table 4). Figure 3 The release rate of active ingredients was 67.2% (see...). Figure 4 ).

[0045] Table 4

[0046] Comparative Example 2 (1) Preparation of outer membrane: Weigh 2g of diatomaceous earth, 1.5g of TEPA and 1g of PEI, add 0.4g of sorbitol oleate (20% of the amount of diatomaceous earth added), mix and stir at 40℃ for 12 h; centrifuge the mixture at 6000 r / min for 10 min, discard the supernatant, wash the precipitate twice with ethanol, and then vacuum dry at 50℃ for 12 h, and store in the dark for later use. Weigh 1g of carboxymethyl cellulose and 3g of sodium alginate, add 200g of deionized water to make an aqueous solution. The total weight of the outer membrane matrix is ​​4g. Add 0.024g of the above-mentioned amine-modified diatomaceous earth at a ratio of 0.6%. The water content of the mixed system is 90wt%. The mixed system is magnetically suspended and stirred at 50℃ and 1000 r / min for 5h. After stirring, vacuum degassing is performed for 4h to obtain the outer membrane casting solution. The outer membrane casting solution is cast onto a polytetrafluoroethylene plate, evenly spread with a scraper, dried at a constant temperature of 50℃ for 5h, removed and naturally air-dried for 12h, and stored in the dark for later use.

[0047] (2) Inner membrane preparation: Weigh 4g of polyethylene glycol, add 200g of deionized water, and add 0.12g of proanthocyanidins (4% of the weight of polyvinyl alcohol). The water content of the mixture is 90wt%. The mixture is magnetically stirred at 25℃ and 1000 r / min for 3 h. After stirring, it is allowed to stand for 4 h to remove bubbles, and the inner membrane casting solution is obtained. The inner membrane casting solution is uniformly coated on the surface of the outer membrane and dried at 35℃ for 5 h. After removal, it is naturally air-dried for 12 h to obtain a double-layer composite preservation film. The prepared composite film is used to package and preserve fruits and vegetables. The release of active ingredients of the composite film in this embodiment under different humidity gradients is shown in Table 5. The carbon dioxide content in the packaging is 80.5% (see Table 5). Figure 3The release rate of the active ingredient was 15.7% (see...). Figure 4 ).

[0048] Table 5

[0049] Figure 5 This image shows a comparison of the preservation effects on lychees using a control group, Example 1, Comparative Example 1, and Comparative Example 2. The control group used commercially available ordinary polyethylene preservation film, with other conditions identical to the experimental group. All four samples were preserved simultaneously. Due to the lack of modified atmosphere packaging and antibacterial properties, the control group showed a browning rate exceeding 60% after 5 days of storage, with some areas already rotting. Example 1, Comparative Example 1, and Comparative Example 2 all showed lower levels of browning and spoilage than the control group, with Example 1 demonstrating the best effect among the three. After 8 days of storage, the control group showed a browning rate exceeding 90% and a rotting rate exceeding 50%, essentially losing its commercial value. Example 1, however, showed a browning rate below 20% and a rotting rate below 5%, demonstrating a significant preservation effect. Comparative Examples 1 and 2, lacking modified atmosphere packaging and antibacterial properties respectively, while exhibiting some preservation effect, could not reach the same level as Example 1. In summary, the multi-faceted comparison using the control group, Comparative Example 1, and Comparative Example 2 clearly verifies the advantages of the double-layer composite preservation film of this invention under the synergistic effect of modified atmosphere packaging and slow-release technology.

[0050] Those skilled in the art will readily understand that the above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a double-layered composite preservative film, characterized by, It comprises the following steps: (1) preparation of the modified atmosphere outer film; (2) preparation of the slow-release inner film: mixing procyanidins, polyvinyl alcohol, polyethylene glycol and deionized water, stirring and then defoaming to obtain an inner film solution; coating the inner film solution on the surface of the outer film and drying to obtain a double-layer composite preservative film.

2. The method of claim 1, wherein the bi-layer composite fresh-keeping film is prepared by coating a first layer of the biodegradable polymer on a substrate, and then coating a second layer of the biodegradable polymer on the first layer. The preparation of the modified atmosphere outer film in step (1) is specifically as follows: Mixing diatomite, tetraethylenepentamine TEPA, polyethyleneimine PEI and sorbitan oleate to modify the diatomite, centrifuging after the modification, washing the precipitate to obtain modified diatomite; Mixing the modified diatomite with an aqueous solution of carboxymethyl cellulose and sodium alginate, magnetically suspending and stirring, and then defoaming to obtain an outer film solution; and preparing an outer film using the outer film solution.

3. The method of claim 2, wherein the bi-layer composite fresh-keeping film is prepared by coating a solution of the second layer onto the first layer. The weight ratio of diatomite, tetraethylenepentamine TEPA and polyethyleneimine PEI in step (1) is (1.8-2.2):(1.3-1.7):1; and the weight of sorbitan oleate is 20-30% of the weight of diatomite.

4. The method of claim 2, wherein the bi-layer composite fresh-keeping film is prepared by coating a solution of the second layer onto the first layer. The weight ratio of carboxymethyl cellulose, sodium alginate and deionized water is 1:(2.8-3.2):(280-320); and the addition amount of modified diatomite is 0.5-0.7% of the mass of the outer film solution.

5. The method of claim 1, wherein the bi-layered composite fresh-keeping film is prepared by coating a solution of the second layer on the first layer. The weight ratio of polyvinyl alcohol, polyethylene glycol and deionized water in step (2) is (2.8-3.2):1:(180-220); and the addition amount of procyanidins is 3-5% of the total weight of polyvinyl alcohol and polyethylene glycol.

6. The method of claim 2, wherein the bi-layered composite fresh-keeping film is prepared by coating a solution of the second layer on the first layer. The water content of the outer film solution is 90-95wt%, and the water content of the inner film solution is 90-95wt%.

7. The method of claim 5, wherein the bi-layer composite fresh-keeping film is prepared by coating a solution of the second layer onto the first layer. The outer film is dried at a constant temperature of 50-60℃ for 5-6h; and the inner film is dried at a constant temperature of 35-45℃ for 5h.

8. A double layer composite preservative film, characterized by, It comprises a modified atmosphere outer film and a slow-release inner film; the inner film comprises procyanidins, polyvinyl alcohol and polyethylene glycol.

9. The dual layer composite preservative film according to claim 8, wherein, The outer film comprises diatomite, tetraethylenepentamine TEPA, polyethyleneimine PEI and sorbitan oleate.

10. Use of the double-layer composite preservative film according to claim 8 or 9 for food preservation, characterized in that, When the double-layer composite preservative film is used for fruit and vegetable food preservation, the fruit and vegetable food is placed in a box with one open side, the double-layer composite preservative film is tightly wrapped around the open side with the outer film facing outward and the inner film adhering to the packaging box, and then the packaging is sealed; the water-soluble properties of polyvinyl alcohol and polyethylene glycol are utilized, the inner film collapses after encountering the water vapor generated by the respiration of the fruit and vegetable food, thereby controlling the slow-release rate of procyanidins and prolonging the antioxidant and antibacterial effects.

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