Efficient antibacterial and antioxidant composite hydrogel film as well as preparation method and application thereof

Abstract

The invention discloses an efficient antibacterial and antioxidant composite hydrogel film as well as a preparation method and application thereof. The efficient antibacterial and antioxidant composite hydrogel film comprises a film-forming substrate with polyvinyl alcohol and chitosan as main components and a ZIP-8 composite material which is dispersed in the film-forming substrate and is loaded with curcumin. The composite hydrogel film provided by the invention has excellent and synergistic antibacterial and antioxidant dual effects, and can effectively inhibit microbial growth and lipid / protein oxidation; the fresh-keeping agent is suitable for fresh keeping of high-fat and easily-oxidized pork, can prolong the shelf life compared with a common PE film, can better maintain the color and texture of the pork, and reduces juice loss; zIF-8 is used for packaging curcumin to realize pH-dependent controlled release and prolong the active action time, and the film also has a pH-responsive color change effect and can be used for real-time visual monitoring of food freshness.

Description

Technical Field

[0001] This invention belongs to the field of fresh meat preservation technology, specifically relating to a composite hydrogel film for food preservation, its preparation method, and its application. Background Technology

[0002] Food is susceptible to spoilage due to microbial contamination and its own biochemical reactions during storage, transportation, and sales. Meat (such as pork), rich in water, protein, and fat, spoils particularly rapidly, primarily through the proliferation of microorganisms, fat oxidation and rancidity, and discoloration of myoglobin. This not only causes significant economic losses but also poses food safety risks. Therefore, developing efficient and safe food preservation technologies is crucial for ensuring food quality and reducing waste.

[0003] Traditional food packaging materials (such as petroleum-based plastic films like polyethylene and polypropylene) primarily provide physical insulation, offering limited functionality and failing to actively inhibit spoilage within food. Furthermore, their non-degradability poses serious environmental problems. To meet consumer demand for high-quality, long-shelf-life foods, active packaging and smart packaging technologies have emerged. Active packaging adds functional components (such as antibacterial agents and antioxidants) to packaging materials, enabling them to actively interact with food or the environment, thereby delaying quality deterioration. Among these, hydrogel films loaded with natural active ingredients (such as polyphenolic compounds) have become a research hotspot due to their excellent biocompatibility, biodegradability, and functional designability.

[0004] Chitosan (CS) and polyvinyl alcohol (PVA), two common biodegradable polymers, are frequently used as matrices for hydrogel films. Chitosan itself possesses broad-spectrum antibacterial properties, but its films often exhibit weak mechanical strength and poor moisture barrier properties. PVA, on the other hand, can form strong and tough films with good oxygen barrier properties, but lacks biological activity. Blending these two materials is considered an effective modification strategy. Furthermore, to endow films with stronger targeting functions, researchers have attempted to introduce metal-organic framework materials (such as ZIF-8) as nanocarriers to encapsulate natural antibacterial and antioxidant agents (such as polyphenols). ZIF-8 possesses a high specific surface area and pH-responsive decomposition characteristics, enabling the protection and controlled release of active ingredients.

[0005] However, existing composite thin film technologies still have many limitations: (1) Insufficient functional synergy: Most studies only focus on the single properties of the film (such as antibacterial or antioxidant properties), while meat spoilage is the result of the combined action of microorganisms and oxidation reactions, lacking the ability to simultaneously and efficiently inhibit these two spoilage pathways.

[0006] (2) Lack of real-time monitoring capability: Existing active packaging usually does not have the function of indicating the freshness of food, and consumers cannot intuitively judge the real-time quality status of the contents, which reduces the level of intelligence of the packaging; (3) Uncontrollable release of active substances: Although the carrier properties of ZIF-8 are utilized, there is a lack of systematic optimization of the release kinetics of active substances (especially polyphenols) in complex food systems, their compatibility with the film matrix and their long-term effectiveness, which can easily lead to early burst release or late failure.

[0007] (4) Performance balance challenge: The addition of active ingredients often has a negative impact on the mechanical properties and barrier properties of the film. How to maintain or even enhance its physical integrity while giving it high bioactivity is a technical challenge. (5) Weak application specificity: Many studies focus on fruit preservation, while films designed for high-fat, near-neutral pH meat systems (such as pork) lack the ability to inhibit lipid oxidation and the effectiveness in specific spoilage microbial environments. Summary of the Invention

[0008] Therefore, the purpose of this invention is to provide a composite hydrogel film that integrates high-efficiency antibacterial and antioxidant properties, intelligent visual monitoring, pH-controlled release, excellent mechanical properties and physical barrier, and environmental friendliness, so as to overcome the technical problems existing in the above-mentioned existing composite films.

[0009] The above-mentioned objective of this invention is achieved through the following technical solution: The first aspect of the present invention is to provide a highly efficient antibacterial and antioxidant composite hydrogel film, comprising a film-forming matrix mainly composed of polyvinyl alcohol and chitosan, and a ZIP-8 composite material loaded with curcumin dispersed in the film-forming matrix.

[0010] In one optional embodiment, the mass ratio of polyvinyl alcohol, chitosan, ZIF-8, and curcumin is (40-60):(20-30):(5-15):(1-5).

[0011] A second aspect of the present invention is to provide a method for preparing a highly efficient antibacterial and antioxidant composite hydrogel film, comprising the following steps: (1) Preparation of film-forming matrix Chitosan was dissolved in dilute acetic acid solution to prepare a chitosan solution. Polyvinyl alcohol was dissolved in deionized water and heated and stirred until completely dissolved to prepare a polyvinyl alcohol solution. The chitosan solution and polyvinyl alcohol solution were mixed in a mass ratio of 1:(1-3), and plasticizer and crosslinking agent were added. The mixture was then stirred for 30-45 minutes to obtain a film-forming matrix mixed solution. (2) Preparation of ZIF-8 composite material loaded with curcumin A zinc source solution was prepared by dissolving zinc salts, and a ligand solution was prepared by dissolving imidazole organic ligands in methanol, adding curcumin, and stirring until completely dissolved. The zinc source solution was mixed with the ligand solution, stirred and reacted, centrifuged, and the precipitate was collected. After washing and drying, the ZIF-8 composite material loaded with curcumin was obtained. (3) Preparation of composite hydrogel films ZIF-8 composite material is added to the film-forming matrix mixture solution and ultrasonically mixed to obtain a film-forming solution. The film-forming solution is then used to form a composite hydrogel film.

[0012] In an optional embodiment, in step (2), the zinc salt is Zn(NO3)2·6H2O, the imidazole organic ligand is 2-methylimidazolium, and the mass ratio of the zinc salt, the imidazole organic ligand, and curcumin is (0.5~2.5):(1~4):(0.05~0.25).

[0013] In one optional embodiment, in step (1), the temperature for heating and stirring is 90-120°C, and the stirring time is 1-2 hours.

[0014] In an optional embodiment, in step (1), the plasticizer is glycerol and the crosslinking agent is anhydrous citric acid, and the mass ratio of glycerol to anhydrous citric acid is (0.01~0.03):(0.05~0.2).

[0015] In an alternative embodiment, in step (3), the mass of the ZIF-8 composite material is (0.1 to 0.25).

[0016] In one alternative embodiment, in step (3), the ultrasonic treatment is performed for 30 to 45 minutes.

[0017] In one alternative embodiment, in step (3), the film-forming process involves casting the film-forming solution onto a flat plate and drying it at a temperature of 40–60°C for 48–60 hours.

[0018] A third aspect of the present invention is to provide the application of the highly efficient antibacterial and antioxidant composite hydrogel film or the composite hydrogel film prepared by the preparation method described above in the preservation of chilled fresh meat.

[0019] Compared with the prior art, the technical solution of the present invention has the following advantages: (1) The composite hydrogel film of the present invention has excellent and synergistic antibacterial and antioxidant dual effects, which can effectively inhibit microbial growth and lipid / protein oxidation.

[0020] (2) This invention is applicable to the preservation of pork that is high in fat and easily oxidized. It can extend the shelf life compared to using ordinary PE film, while better maintaining the color and texture of pork and reducing juice loss.

[0021] (3) The composite hydrogel film of the present invention utilizes ZIF-8 to encapsulate curcumin to achieve pH-dependent controlled release, prolonging the active time. Furthermore, the film also has pH-responsive color change function, which can be used for real-time visual monitoring of food freshness.

[0022] (4) Through the synergistic effect of chitosan, polyvinyl alcohol, ZIF-8 and curcumin, the physical and mechanical properties and barrier properties of the film can be enhanced in a synergistic way, providing fresh food with a stronger and more isolated physical protection from the external environment, ensuring the structural integrity and barrier performance of the film in the cold chain high humidity environment, and avoiding functional failure caused by swelling or rupture.

[0023] (5) Using chitosan and polyvinyl alcohol as film-forming matrices, the film can be completely biodegraded in the soil in the later stage, which overcomes the environmental pollution problem of traditional petroleum-based plastic packaging and conforms to the green and sustainable packaging development trend. Attached Figure Description

[0024] Figure 1 These are SEM images of ZIF-8, EGCG@ZIF-8, Cur@ZIF-8, and Ant@ZIF-8 of the present invention. Figure 2 The FTIR spectra of ZIF-8, EGCG@ZIF-8, Cur@ZIF-8, and Ant@ZIF-8 of this invention are shown below. Figure 3 The XRD spectra of ZIF-8, EGCG@ZIF-8, Cur@ZIF-8, and Ant@ZIF-8 of this invention are shown below. Figure 4 SEM images and elemental distribution diagrams of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown. Figure 5 The FTIR spectra of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown below. Figure 6The XRD patterns of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown below. Figure 7 The stress-strain curves of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown. Figure 8 The tensile strength test results are for the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention. Figure 9 The results of the elongation at break test of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown. Figure 10 The water contact angle test results are for the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of this invention. Figure 11 The results of water vapor permeability tests for the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of this invention are shown. Figure 12The oxygen permeability test results of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown below. Figure 13 The antibacterial test results are for the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention. Figure 14 The results of DPPH free radical scavenging tests are for the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention. Figure 15 The results of ABTS radical scavenging tests on the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown. Figure 16 The biocompatibility test results of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention are shown below. Figure 17 The color change test results of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant at different pH values ​​during pork preservation are shown below. Figure 18 The results show the release rate of polyphenols from the composite hydrogel film PVA-CS-EGCG at different pH values. Figure 19The results show the release rate of polyphenols from the composite hydrogel film PVA-CS-Cur at different pH values. Figure 20 The results show the polyphenol release rate of the composite hydrogel film PVA-CS-Ant at different pH values. Figure 21 The results of the polyphenol release rate test of the composite hydrogel film PVA-CS-EGCG@ZIF-8 of the present invention at different pH values ​​are shown. Figure 22 The results of the polyphenol release rate test of the composite hydrogel film PVA-CS-Cur@ZIF-8 of the present invention at different pH values ​​are shown. Figure 23 The results of the polyphenol release rate test of the composite hydrogel film PVA-CS-Ant@ZIF-8 of the present invention at different pH values ​​are shown. Figure 24 The present invention describes the appearance changes of pork during the preservation process using the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant. Figure 25 The effects of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention on the total sulfur content of pork during the preservation process; Figure 26 The effects of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention on the carbonyl content of pork during the preservation process; Figure 27 The effects of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention on the TVB-N value of pork during the preservation process; Figure 28The effects of the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant of the present invention on the TBARS value of pork during the preservation process; Figure 29 The present invention relates to the effects of composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant on microbial metabolism during pork storage. Detailed Implementation

[0025] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0026] Example 1 This invention provides a highly efficient antibacterial and antioxidant composite hydrogel film, the preparation method of which includes the following steps: (1) Preparation of film-forming matrix PVA-CS 1 g of chitosan CS was dissolved in 50 mL of 1% acetic acid solution to prepare a chitosan solution. 2 g of polyvinyl alcohol (PVA) was dissolved in 50 mL of deionized water and heated to 90 °C and stirred for 1 hour to prepare a polyvinyl alcohol solution. The chitosan solution and polyvinyl alcohol solution were mixed, and then 0.03 g of glycerol and 0.12 g of anhydrous citric acid were added. The mixture was then stirred for 30 min to obtain a film-forming matrix mixture solution.

[0027] (2) Preparation of Curcumin-ZIF-8 composite material loaded with curcumin Dissolve 1.0 g Zn(NO3)2·6H2O in 5 mL deionized water, dissolve 2.0 g 2-methylimidazole in 10 mL methanol, add 0.1 g curcumin, stir to dissolve, mix the two solutions, stir to react for 2 h, centrifuge to collect the precipitate, wash three times with methanol, and vacuum dry to obtain the curcumin-loaded ZIF-8 composite material Curcumin-ZIF-8 (Cur-ZIF-8 or Cur@ZIF-8).

[0028] (3) Preparation of composite hydrogel films 0.15 g of ZIF-8 composite material was added to the film-forming matrix mixture solution and ultrasonically treated for 30 min to obtain the film-forming solution. The film-forming solution was cast onto a plate and dried at 45℃ for 48 h to obtain the composite hydrogel film PVA-CS-Cur@ZIF-8.

[0029] Example 2 (1) Preparation of film-forming matrix 0.5 g of chitosan CS was dissolved in 50 mL of 1% acetic acid solution to prepare a chitosan solution. 2.5 g of polyvinyl alcohol (PVA) was dissolved in 50 mL of deionized water and heated to 90 °C and stirred for 1 hour to prepare a polyvinyl alcohol solution. The chitosan solution and polyvinyl alcohol solution were mixed, and then 0.02 g of glycerol and 0.05 g of anhydrous citric acid were added. The mixture was then stirred for 30 min to obtain a film-forming matrix mixture solution.

[0030] (2) Preparation of ZIF-8 composite material loaded with curcumin 2.5 g Zn(NO3)2·6H2O was dissolved in 5 mL of deionized water, 1.0 g 2-methylimidazole was dissolved in 10 mL of methanol, 0.3 g curcumin was added, and the mixture was stirred to dissolve. The two solutions were then mixed and stirred for 2 h. The precipitate was collected by centrifugation, washed three times with methanol, and dried under vacuum to obtain the curcumin-loaded ZIF-8 composite material.

[0031] (3) Preparation of composite hydrogel films 0.25 g of ZIF-8 composite material was added to the film-forming matrix mixture solution and ultrasonically treated for 30 min to obtain the film-forming solution. The film-forming solution was cast onto a plate and dried at 45℃ for 48 h to obtain the composite hydrogel film.

[0032] Example 3 (1) Preparation of film-forming matrix 1.5 g of chitosan CS was dissolved in 50 mL of 1% acetic acid solution to prepare a chitosan solution. 3.0 g of polyvinyl alcohol (PVA) was dissolved in 50 mL of deionized water and heated to 90 °C and stirred for 1 hour to prepare a polyvinyl alcohol solution. The chitosan solution and polyvinyl alcohol solution were mixed, and then 0.01 g of glycerol and 0.2 g of anhydrous citric acid were added. The mixture was then stirred for 30 min to obtain a film-forming matrix mixture solution.

[0033] (2) Preparation of ZIF-8 composite material loaded with curcumin Dissolve 0.5 g Zn(NO3)2·6H2O in 5 mL of deionized water, dissolve 3.5 g 2-methylimidazole in 10 mL of methanol, add 0.2 g curcumin, stir to dissolve, mix the two solutions, stir to react for 2 h, centrifuge to collect the precipitate, wash three times with methanol, and vacuum dry to obtain the curcumin-loaded ZIF-8 composite material.

[0034] (3) Preparation of composite hydrogel films 0.1 g of ZIF-8 composite material was added to the film-forming matrix mixture solution and ultrasonically treated for 30 min to obtain the film-forming solution. The film-forming solution was then cast onto a plate and dried at 45℃ for 48 h to obtain the composite hydrogel film.

[0035] Comparative Example 1 The only difference between this comparative example and Example 1 is that curcumin is replaced with epigallocatechin gallate (EGCG) to obtain the ZIF-8 composite material EGCG-ZIF-8 (EGCG@ZIF-8) loaded with epigallocatechin gallate. The ZIF-8 composite material is then mixed with the film-forming matrix to obtain the composite hydrogel film PVA-CS-EGCG@ZIF-8. The amounts of other raw materials and the preparation process remain unchanged.

[0036] Comparative Example 2 The only difference between this comparative example and Example 1 is that curcumin is replaced with anthocyanin to obtain anthocyanin-loaded ZIF-8 composite material Anthocyanin-ZIF-8 (Ant-ZIF-8 or Ant@ZIF-8). The ZIF-8 composite material is mixed with the film-forming matrix to obtain the composite hydrogel film PVA-CS-Ant@ZIF-8. The amount of other raw materials and the preparation process remain unchanged.

[0037] Comparative Example 3 This comparative example provides a polyvinyl alcohol-chitosan PVA-CS membrane. The specific steps of the preparation process are as follows: Step (1) of Example 1: the obtained film-forming matrix mixture solution is used as the film-forming solution and cast onto a plate. It is then dried at 45°C for 48 hours to obtain a hydrogel film PVA-CS.

[0038] Comparative Example 4 The difference between this comparative example and Example 1 is that curcumin is not added. Instead, dissolved Zn(NO3)2·6H2O and 2-methylimidazole are directly mixed with the film-forming matrix mixture solution. The amounts of other raw materials and the preparation process remain unchanged, resulting in the composite hydrogel film PVA-CS-ZIF-8.

[0039] Comparative Example 5 The difference between this comparative example and Example 1 is that Zn(NO3)2·6H2O and 2-methylimidazole are not added. Instead, the dissolved curcumin is directly mixed with the film-forming matrix mixture solution. The amount of other raw materials and the preparation process remain unchanged to obtain the composite hydrogel film PVA-CS-Cur.

[0040] Comparative Example 6 The difference between this comparative example and Comparative Example 1 is that Zn(NO3)2·6H2O and 2-methylimidazole are not added. Instead, the dissolved epigallocatechin gallate (EGCG) is directly mixed with the film-forming matrix mixture solution. The amounts of other raw materials and the preparation process remain unchanged, resulting in a composite hydrogel film PVA-CS-EGCG.

[0041] Comparative Example 7 The difference between this comparative example and comparative example 2 is that Zn(NO3)2·6H2O and 2-methylimidazole are not added. Instead, the dissolved anthocyanin is directly mixed with the film-forming matrix mixture solution. The amounts of other raw materials and the preparation process remain unchanged, resulting in the composite hydrogel film PVA-CS-Ant.

[0042] Effect verification

[0043] I. Characterization of ZIF-8 composite materials (a) Scanning Electron Microscopy (SEM) Curcumin-ZIF-8 (Cur-ZIF-8), EGCG-ZIF-8, and Anthocyanin-ZIF-8 (Ant-ZIF-8) prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively, were thoroughly ground. The ground powder was then uniformly dispersed on the surface of a metal thin film and subjected to gold plating. Subsequently, the surface morphology and microstructure of the samples were observed using a scanning electron microscope (Verios G4 UC, Thermo Fisher Scientific, Czech Republic) under high vacuum conditions with an accelerating voltage of 10 kV. The results are as follows: Figure 1 As shown in the figure, EGCG-ZIF-8, Cur-ZIF-8, and Ant-ZIF-8, like ZIF-8, all have clear, smooth surfaces and sharp edges and corners. This indicates that the ZIF-8 framework still maintains structural integrity after encapsulating EGCG, curcumin, or anthocyanin.

[0044] (II) Fourier Transform Infrared Spectroscopy (FTIR) Analysis Curcumin-ZIF-8 (Cur@ZIF-8), EGCG-ZIF-8 (EGCG@ZIF-8), and Anthocyanin-ZIF-8 (Ant@ZIF-8) prepared in Examples 1, 1, and 2 were used as test samples and measured using Fourier Transform Infrared Spectroscopy (FTIR) (Nicolet IS5, Thermo Fisher, USA). The spectral acquisition range was 400 to 4000 cm⁻¹. -1 The resolution is 4 cm. -1 Each sample was scanned 32 times. The results are as follows: Figure 2 As shown in the figure.

[0045] As shown in the figure, the FTIR spectra of EGCG@ZIF-8, Cur@ZIF-8, and Ant@ZIF-8 exhibit characteristic peaks of ZIF-8, rather than characteristic peaks of polyphenols. For example, the hydroxyl absorption peak of EGCG appears in the 3300-3600 cm⁻¹ range. -1 The absorption peak of curcumin's OH bond stretching vibration appears at 3505 cm⁻¹. -1 The aromatic ring CH bend of anthocyanin appears at 1000 cm. -1 Nearby. These peaks indicate that EGCG, curcumin, and anthocyanins were successfully encapsulated within the ZIF-8 framework.

[0046] (III) X-ray diffraction (XRD) analysis The XRD patterns of Curcumin-ZIF-8 (Cur@ZIF-8), EGCG-ZIF-8 (EGCG@ZIF-8), and Anthocyanin-ZIF-8 (Ant@ZIF-8) prepared in Example 1, Comparative Example 1, and Comparative Example 2 were determined using an X-ray powder diffractometer (D8 Advance, Bruker, Germany). The test conditions were 40 kV and 40 mA, with a scan rate of 0.02° / min in the 2θ region from 5° to 60°. The results are as follows: Figure 3 As shown.

[0047] As shown in the figure, ZIF-8, EGCG@ZIF-8, Cur@ZIF-8, and Ant@ZIF-8 all exhibit strong diffraction peaks at 7.34, 10.37, 12.74, and 18.05 nm. However, no characteristic diffraction peaks of polyphenols were observed in the XRD patterns of EGCG@ZIF-8, Cur@ZIF-8, and Ant@ZIF-8. These results indicate that EGCG, curcumin, and anthocyanin are not attached to the surface of ZIF-8 but are instead encapsulated within it. Furthermore, the encapsulated polyphenols do not alter the structure of ZIF-8.

[0048] In the following tests, unless otherwise specified, the composite hydrogel films PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Ant@ZIF-8, PVA-CS, PVA-CS-ZIF-8, PVA-CS-Cur, PVA-CS-EGCG, and PVA-CS-Ant prepared in Example 1 and Comparative Examples 1-7 were used as the test samples.

[0049] II. Characterization of Composite Hydrogel Films (a) Scanning Electron Microscopy (SEM) The samples were observed using scanning electron microscopy (SEM), and their elemental composition was analyzed using energy-dispersive X-ray spectroscopy (EDS; Xplore 30, Oxford Instruments, UK). The results are as follows: Figure 4 As shown in the figure, the PVA-CS and PVA-CS-ZIF-8 films have uniform and smooth surfaces with no obvious cracks or pores, but slight cross-sectional protrusions are observed. In contrast, obvious cracks or pores are observed on the surfaces of the PVA-CS-EGCG, PVA-CS-Cur, and PVA-CS-Ant hydrogels. The PVA-CS-EGCG@ZIF-8, PVA-CS-Cur@ZIF-8, and PVA-CS-Ant@ZIF-8 films with the addition of ZIF-8 regained a uniform and smooth morphology. Among them, PVA-CS-Cur@ZIF-8 exhibited the best uniformity and smoothness.

[0050] The distribution of C, O, N, and Zn elements was further examined to assess whether the addition of PVA-CS hydrogel affected the morphology of the composite material. The results showed that the elemental distributions of PVA-CS-EGCG@ZIF-8, PVA-CS-Cur@ZIF-8, and PVA-CS-Ant@ZIF-8 were very similar to those of PVA-CS and PVA-CS-ZIF-8, indicating that the composite was uniformly dispersed in the hydrogel.

[0051] (II) Fourier Transform Infrared Spectroscopy (FTIR) Analysis Fourier transform infrared spectroscopy (FTIR) (Nicolet IS5, Thermo Fisher, USA) was used to test the sample, with the spectral acquisition range from 400 to 4000 cm⁻¹. -1 The resolution is 4 cm. -1 Each sample was scanned 32 times. The results are as follows: Figure 5 As shown in the figure, it can be seen that in the range of 500-4000 cm -1Within the range of 1500–1750 cm⁻¹, characteristic absorption peaks corresponding to CS and PVA were observed in all hydrogel films. All samples showed absorption peaks in the 1500–1750 cm⁻¹ range. -1 All peaks show the stretching vibration band of amide II and the C=O vibration peak of the ester bond located at 1710 cm⁻¹. -1 The results indicate that hydrogen bonds are formed between the hydroxyl and amino groups in CS and the hydroxyl groups in PVA. Furthermore, during film formation, the carboxyl groups in CS and PVA undergo cross-linking and esterification reactions, forming ester bonds, which are formed at 1710 cm⁻¹. -1 This is confirmed by the C=O peak at [location missing]. Notably, after loading ZIF-8 and polyphenols, the [value missing] at 1500-1750 cm⁻¹ [value missing]. -1 The increased absorption peak area within the range indicates enhanced interaction between the NH groups in the CS and PVA matrices and the phenolic groups in the support. Furthermore, no new support-related peaks appeared in the spectrum of the composite hydrogel, suggesting that the PVA-CS matrix effectively interacted with and encapsulated the composite support during film formation.

[0052] (III) X-ray diffraction (XRD) analysis The samples were analyzed using an X-ray powder diffractometer (D8 Advance, Bruker, Germany) under the following conditions: 40 kV and 40 mA, scanning from 5° to 60° in the 2θ region, at a scan rate of 0.02° / min. The results are as follows: Figure 6 As shown in the figure, all hydrogel groups exhibit two broad diffraction peaks corresponding to the crystalline regions of chitosan and polyvinyl alcohol, confirming that these hydrogel films have an amorphous structure. The loading of ZIF-8 and polyphenols increased the peak width of PVA-CS, indicating that the support (the complex of polyphenols and ZIF-8) successfully crosslinked with the hydrogel film while maintaining its crystalline integrity.

[0053] II. Mechanical property testing of composite hydrogel films

[0054] The mechanical properties of hydrogel films, including flexibility and strength, tensile strength (TS), and elongation at break, are key factors affecting their functionality and service life. These properties were tested at 25°C and 60% relative humidity. Samples were molded to standard dimensions (10 mm × 60 mm). The initial gauge length was set to 40 mm, and the stretching rate was set to 20 mm / min. -1 The film thickness was measured at five different locations using vernier calipers, and the average value was used for subsequent calculations. The results are as follows: Figure 7-9 As shown, Figure 7 For stress-strain curve testing; Figure 8 Tensile strength; Figure 9 Elongation at break.

[0055] from Figure 7 As can be seen, compared with other groups, PVA-CS-ZIF-8, PVA-CS-EGCG@ZIF-8, PVA-CS-Cur@ZIF-8, and PVA-CS-Ant@ZIF-8 exhibit significantly higher tensile toughness and hardness. This trend was further confirmed by... Figure 8 The tensile strength (TS) data shown support this finding. The tensile strength of PVA-CS hydrogel was 2.09 ± 0.23 MPa, while that of PVA-CS-ZIF-8 was 4.93 ± 0.14 MPa. The mechanical properties of PVA-CS-EGCG@ZIF-8, PVA-CS-Cur@ZIF-8, and PVA-CS-Ant@ZIF-8 were 5.61 ± 0.42 MPa, 7.89 ± 0.41 MPa, and 8.92 ± 0.31 MPa, respectively. These results indicate that ZIF-8 primarily enhances the mechanical properties of the material, and the synergistic effect of the polyphenols further amplifies this effect.

[0056] from Figure 9 The results show that the elongation at break of PVA-CS-EGCG@ZIF-8, PVA-CS-Cur@ZIF-8, and PVA-CS-Ant@ZIF-8 were 458.53±8.79%, 555.29±25.23%, and 447.57±10.42%, respectively, significantly higher than that of PVA-CS hydrogel and hydrogels loaded only with ZIF-8 or ZIF-8 and polyphenols. Among these, the elongation at break of PVA-CS-Cur@ZIF-8 was approximately 100% higher than that of PVA-CS-EGCG@ZIF-8 and PVA-CS-Ant@ZIF-8. This indicates that the synergistic effect of curcumin and ZIF-8 can improve the elongation at break of the composite film.

[0057] In summary, only PVA-CS-Cur@ZIF-8 has the best overall performance, meaning that the PVA-CS-Cur@ZIF-8 composite film has the best mechanical properties.

[0058] III. Three-phase contact angle test of composite hydrogel films The water contact angle (WCA) of a hydrogel film reflects its surface wettability, which affects its water affinity and potential functional applications. Films with a WCA < 90 are considered hydrophilic, while films with a WCA > 90 are considered hydrophobic.

[0059] The contact angles of hydrogel film samples were determined using an optical contact angle analyzer (OCA25, Dataphysics GmbH, Germany). A 4 μL drop of deionized water was placed on the film surface using the pendant drop method, and the contact angles on both sides of the droplet were measured using the SCA20 program. The results are as follows: Figure 10As shown in the figure, the WCA values ​​of PVA-CS, PVA-CS-EGCG, PVA-CS-Cur, and PVA-CS-Ant are all below 90, indicating that these hydrogel membranes are hydrophilic. Although the WCA of hydrogels containing ZIF-8 increases significantly, its value remains within the hydrophilic range, indicating that the addition of ZIF-8 increases WCA without impairing hydrophilicity.

[0060] IV. Water vapor permeability (WVP) and oxygen permeability testing of composite hydrogel films

[0061] The barrier properties of hydrogel films are determined by measuring water vapor transmission rate (WVP) and oxygen transmission rate (OP). Lower WVP values ​​indicate reduced moisture transfer from food to the external environment, thus improving preservation.

[0062] The water vapor transmission rate (WVP) of the hydrogels was evaluated under simulated drying conditions. Each hydrogel membrane was securely sealed to the mouth of a flask and placed in a desiccator containing a saturated salt solution at room temperature. Weighing was performed every 24 hours for 7 consecutive days. Oxygen transmission rate was measured using a Mocon OX-TRAN 2 / 10 instrument (MOCON, USA). Results are as follows: Figure 11 and Figure 12 As shown.

[0063] from Figure 11 The results show that the WVP value of PVA-CS decreased significantly after the addition of polyphenols, while the WVP value of the hydrogel membrane containing ZIF-8 was lower than that of the PVA-CS group after the addition of polyphenols. Furthermore, the WVP value of PVA-CS-Cur@ZIF-8 loaded with curcumin was lower than that of all other groups.

[0064] from Figure 12 As can be seen, the OP value of the hydrogel film shows a similar trend to that of WVP. This correlation indicates that the loading of polyphenols and ZIF-8 can increase the density of the polymer network in the hydrogel. This dense network structure not only enhances the mechanical properties of the film but also blocks the diffusion of small molecules, thereby reducing the permeability of water and oxygen. Among these, PVA-CS-Cur@ZIF-8 exhibits the lowest water and oxygen permeability, indicating its optimal preservation effect.

[0065] V. Antibacterial properties, antioxidant activity and biocompatibility testing of composite hydrogel films

[0066] (a) Antibacterial test The antibacterial activity of the hydrogel membrane against Staphylococcus aureus and Escherichia coli was evaluated using the inhibition zone diameter method. 200 μL of bacterial suspension (10... 5The sterilized hydrogel sample (CFU / mL) was evenly spread onto an agar plate. A well was then punched in the center of the agar plate. The sterilized hydrogel sample was placed in the well and incubated at 37°C for 24 hours. Antimicrobial activity was assessed by measuring the diameter of the inhibition zone formed around the hydrogel sample.

[0067] The results are as follows Figure 13 As shown in the figure, the diameters of the inhibition zones of the PVA-CS-Cur@ZIF-8 membranes were 17.89 mm and 21.52 mm, respectively, indicating the strongest antibacterial activity. This suggests that curcumin and ZIF-8 synergistically enhance the antibacterial effect.

[0068] (ii) Antioxidant test The antioxidant activity of hydrogel membrane samples was evaluated using commercially available DPPH and ABTS free radical scavenging kits. 0.1 g of the hydrogel membrane sample was weighed and 1 mL of 80% methanol extract was added. The sample was then sonicated at 60°C for 30 min at 200-300 W. After sonication, the sample was centrifuged at 12000 rpm for 10 min at room temperature, and the supernatant was collected. The DPPH or ABTS working solution was mixed with the supernatant, and the absorbance of the mixture at 517 nm (DPPH) and 734 nm (ABTS) was measured. The free radical scavenging rate was calculated using the formula provided in the kit. The results are shown below. Figure 14 and Figure 15 As shown, the PVA-CS-Cur@ZIF-8 membrane exhibited the best antioxidant performance among all membranes, with a DPPH scavenging rate of 0.226 μg Trolox / mL and an ABTS scavenging rate of 294.557 μg Trolox / mL. This indicates that the synergistic effect of curcumin and the ZIF-8-loaded hydrogel demonstrates a significant preservation ability. (iii) Biocompatibility testing The biocompatibility of the hydrogel membrane was evaluated based on cell viability assays using Caco-2 cells and a CCK-8 assay kit. Sterile hydrogel samples (0.1 g / mL) were immersed in complete culture medium at 37°C for 24 hours, followed by filtration to obtain the hydrogel extract. The extract was then added to a mixture containing Caco-2 cells (cell density = 10⁻⁶). 5 Cells were placed in 96-well plates (cells / mL). The plates were incubated at 37°C under a 5% CO2 atmosphere for 24 hours, and cell viability was determined using a CCK-8 assay kit. Results are as follows: Figure 16 As shown.

[0069] from Figure 16The results showed that the cell viability after co-incubation with PVA-CS for 24 and 48 hours was 94.23 ± 2.12% and 93.41 ± 5.23%, respectively. In contrast, the cell viability of the ZIF-8-loaded hydrogel membrane group decreased significantly to 79.31 ± 4.53% and 71.59 ± 6.25%. This is mainly due to the Zn content in ZIF-8. 2+ Ions can induce reactive oxygen species (ROS) and other free radicals in mitochondria during cellular stress, leading to cell death. For the same reason, the cell viability of the PVA-CS-EGCG@ZIF-8, PVA-CS-Cur@ZIF-8, and PVA-CS-Ant@ZIF-8 groups was also lower than that of the PVA-CS-EGCG, PVA-CS-Cur, and PVA-CS-Ant groups. Although ZIF-8 affected cell viability, it was not considered cytotoxic according to criteria because the cell viability did not fall below 70%. All groups showed values ​​exceeding 70% at 48 hours, indicating good biocompatibility of these hydrogel composite films.

[0070] VI. pH-responsive color change and controlled release capabilities of composite hydrogel films One of the most notable characteristics of protein-rich foods (such as meat and dairy products) during the breakdown process is a significant increase in pH levels, which can be used to monitor the freshness of meat.

[0071] (a) pH-responsive color change ability The hydrogel films from each group were immersed in buffer solutions with different pH values ​​(from 5.0 to 7.5), and the color changes were observed. The color results of the hydrogels at different pH values ​​(from 5.0 to 7.5) are shown below. Figure 17 As shown in the figure, as the surrounding environment changes from acidic to alkaline, PVA-CS-EGCG@ZIF-8 gradually changes from colorless to brown, PVA-CS-Cur@ZIF-8 changes from pale yellow to orange-red, and PVA-CS-Ant@ZIF-8 changes from red to light red, and then to bluish-purple. The color gradient in the PVA-CS-Cur@ZIF-8 composite hydrogel film is more pronounced than that of other samples, indicating that the PVA-CS-Cur@ZIF-8 composite hydrogel film is more sensitive to pH changes and better reflects the freshness of meat.

[0072] (ii) The ability of the composite hydrogel film to release EGCG, curcumin and anthocyanins at different pH levels The membranes prepared in Examples 1, 1, 2, and 5-7 were used as test samples. Five grams of each sample were immersed in PBS buffer containing 15% ethanol at pH values ​​of 5.0, 6.0, 7.0, or 8.0. The samples were then shaken at 100 r / min in the dark at room temperature. Samples were removed at predetermined time points, and absorbance at 273 nm, 423 nm, and 515 nm was measured using a UV-Vis spectrophotometer. The cumulative release of EGCG, curcumin, and anthocyanins from the hydrogel was calculated using a standard curve.

[0073] The results are as follows Figure 18-23 As shown, from Figure 21-23 As can be seen, compared with hydrogels without ZIF-8, hydrogels containing ZIF-8 showed a significantly lower release rate at the same pH level. Furthermore, the release rate of polyphenols in each type of hydrogel was inversely proportional to pH. Figure 18-20 As can be seen from this, in hydrogels without ZIF-8, H under acidic conditions + The glycosidic bonds of CS can be attacked, causing bond breakage and accelerating polyphenol release. In contrast, in hydrogel membranes containing ZIF-8, ZIF-8 rapidly dissociates under acidic conditions, triggering rapid polyphenol release, while maintaining high stability under neutral or alkaline conditions. This stability helps maintain the structural integrity of EGCG, curcumin, and anthocyanins, ensuring a more sustained release. Since the pH of meat increases with storage time, polyphenol-ZIF-8 hydrogels offer a significant advantage in extending the freshness of meat products during storage.

[0074] VII. Test on the Preservation Effect of Composite Hydrogel Film on Pork

[0075] (a) Impact on the physicochemical properties (appearance) of pork during storage Used uniform and fresh pork tenderloin samples. Pork samples of similar weight were divided into ten groups and placed in plastic cups. The blank control group received no treatment, and the positive control group (PE) was sealed with plastic film. The experimental groups were sealed with the following materials: pure PVA-chitosan (CS) composite membrane (PVA-CS group), PVA-CS membrane containing ZIF-8 (PVA-CS-ZIF-8 group), PVA-CS membrane loaded with EGCG, curcumin, or anthocyanins (PVA-CS-EGCG group, PVA-CS-Cur group, PVA-CS-Ant group), and PVA-CS membrane loaded with EGCG-ZIF-8, Cur-ZIF-8, or Ant-ZIF-8 (PVA-CS-EGCG@ZIF-8 group, PVA-CS-Cur@ZIF-8 group, PVA-CS-Ant@ZIF-8 group).

[0076] All samples were stored at 4°C, and their physicochemical parameters (correlation with freshness) were measured on days 0, 1, 3, 5, 7, and 9. The changes in appearance of the fresh pork during the 9-day storage period are shown below. Figure 24 As shown. From Figure 24 As can be seen, all pork samples showed varying degrees of discoloration and lipid separation as storage time increased.

[0077] To quantitatively assess these changes, color changes in pork tenderloin samples during storage were measured using a colorimeter, and the differences in color parameters (ΔL * (light / dark), Δa * (red / green), and Δb * (yellow / blue)) before and after storage were calculated. Each sample was measured three times, and the average value was taken. The results are shown in Table 1 below.

[0078] Table 1. Color changes of pork during storage.

[0079]

[0080] As shown in Table 1 above, the L* values ​​of all groups decreased with increasing storage time, indicating that the pork samples gradually darkened. On day 9, the L* values ​​of PVA-CS-EGCG@ZIF-8 and PVA-CS-Ant@ZIF-8 were not significantly different from those of PE, while only the L* value of the PVA-CS-Cur@ZIF-8 sample (45.39±0.08) was significantly higher than that of the other groups. This indicates that PVA-CS-Cur@ZIF-8 can effectively delay the spoilage and discoloration of pork. Similarly, the a* and b* values ​​of the PVA-CS-Cur@ZIF-8 group decreased more slowly, indicating that it can also reduce the loss of red and yellow color caused by oxidation.

[0081] (II) Impact on oxidation indicators during pork storage 1. Thiol and carbonyl content The total sulfhydryl and carbonyl content in meat proteins is an important indicator of protein oxidation. During storage, protein oxidation converts thiol groups into disulfide bonds, leading to a decrease in total sulfhydryl content. Simultaneously, protein oxidation also generates free radicals, which attack amino acid side chains, resulting in an increase in carbonyl content.

[0082] Commercially available kits were used to assess the degree of protein oxidation in pork samples from different treatment groups by detecting the total thiol and total carbonyl groups. The specific procedures were as follows: 0.1 g of pork tissue was weighed, and 1 mL of pre-chilled extraction buffer was added. The mixture was homogenized on ice. The homogenate was centrifuged at 10,000 rpm for 10 minutes at 4°C, and the supernatant was collected for subsequent analysis. After adding the colorimetric reagent according to the kit instructions, the absorbance was measured at 412 nm (total thiol) and 370 nm (total carbonyl) wavelengths using a spectrophotometer. Finally, the thiol and carbonyl groups were calculated using the formulas provided in the kit instructions. The results are shown below. Figure 25 and 26 As shown, from Figure 25 As can be seen, the total thiol content gradually decreased with the extension of storage time in all groups. By day 9, the total thiol content of the PVA-CS-ZIF-8 (32.29 ± 0.59 μmol / g), PVA-CS-EGCG@ZIF-8 (44.60 ± 0.15 μmol / g), PVA-CS-Cur@ZIF-8 (45.91 ± 0.64 μmol / g), and PVA-CS-Ant@ZIF-8 (43.02 ± 1.55 μmol / g) groups were significantly higher than those of the other groups.

[0083] from Figure 26 The results showed that the observed carbonyl content trend was opposite to that of the thiol content, with all groups gradually increasing over time. On day 9, the carbonyl content in the PVA-CS-ZIF-8 (2.71±0.14 nmol / mg protein), PVA-CS-EGCG@ZIF-8 (1.89±0.05 nmol / mg protein), PVA-CS-Cur@ZIF-8 (1.67±0.08 nmol / mg protein), and PVA-CS-Ant@ZIF-8 (1.93±0.08 nmol / mg protein) groups was significantly lower than that in the other groups. Among these, PVA-CS-Cur@ZIF-8 had the lowest carbonyl content, indicating that PVA-CS-Cur@ZIF-8 has a superior antioxidant effect compared to the other groups.

[0084] 2. Total volatile basic nitrogen (TVB-N) Changes in TVB-N (total free amine nitrogen) concentration can be used as an indicator of meat freshness.

[0085] Mix 20g of chopped pork tenderloin with 100ml of deionized water, stir for 30 minutes, and then filter. Take 5ml of the filtrate and alkalize it with 5ml of magnesium oxide solution (10g / L), followed by steam distillation using a Kjeldahl apparatus for 5 minutes. Place the collected distillate in a flask containing 10ml of boric acid solution (20g / L) and 2-3 drops of 0.1% methyl red-methylene blue, and finally titrate with 0.005 mol / L sulfuric acid standard solution.

[0086] The results are as follows Figure 27 As shown, the TVB-N content in all groups was negligible during the initial storage period. However, the TVB-N content gradually increased with prolonged storage. According to national standards, the TVB-N content in fresh meat should not exceed 15 mg / 100g. By the fifth day of storage, the TVB-N content in the control group and the PVA-CS group were 16.80±0.36 mg / 100g and 15.51±0.39 mg / 100g, respectively, exceeding the limit and indicating the onset of spoilage. In contrast, by the ninth day of storage, the TVB-N content in all groups reached the limit, except for the PVA-CS-Cur@ZIF-8 group, whose TVB-N content remained below the limit (12.15±0.79 mg / 100g), indicating that the hydrogel film had the strongest preservation ability.

[0087] 3. Thiobarbituric acid reactants (TBARS) TBARS content is another important indicator of lipid oxidation in meat. One gram of meat sample was homogenized with 15 mL of a mixed solution containing 7.5% thiobarbituric acid (TBA) and 0.1% ethylenediaminetetraacetic acid (EDTA) and filtered. One mL of the filtrate was mixed with 1.0 mL of a 20 mM TBA aqueous solution and heated in a boiling water bath for 20 minutes. After cooling, the absorbance at 532 nm was measured using a microplate reader (SpectraMaxiD3, Molecular Equipment Corporation, USA). A calibration curve was established using a 1,1,3,3-tetraethoxypropane (TEP) standard solution. TBARS values ​​are expressed as milligrams of malondialdehyde (MDA) per kilogram of meat sample (mg MDA / kg).

[0088] The results are as follows Figure 28As shown in the figure, the trend of TBARS values ​​is consistent with that of TVB-N. When lipid oxidation intensifies, ketone body accumulation reaches 1.0 mg MDA / kg, a level typically associated with spoilage and unpleasant odors. On day 9 of storage, the TBARS value of the control group was 1.17 ± 0.09, exceeding this threshold, indicating significant lipid oxidation and spoilage. Other groups also exceeded 0.6 mg MDA / kg. Only PVA-CS-Cur@ZIF-8, PVA-CS-EGCG@ZIF-8, and PVA-CS-Ant@ZIF-8 maintained a low level (close to 0.5 mg MDA / kg), indicating that the synergistic effect of polyphenols and ZIF-8 can effectively delay the oxidation of proteins and lipids in meat, thereby extending its shelf life and preventing the development of unpleasant flavors during storage. In summary, only pork tenderloin treated with PVA-CS-Cur@ZIF-8 showed the best preservation effect.

[0089] VIII. Effect of Composite Hydrogel Film on Total Viable Bacterial Count (TVC) of Pork During Storage

[0090] TVC (Total Volatility and Currency) value can serve as a key indicator for assessing meat freshness and determining spoilage time. Studies have shown that when the TVC value exceeds 6 Log (CFU), the meat is considered spoiled and unsuitable for consumption.

[0091] Mix 25g of pork sample with 225ml of sterile physiological saline in a sterile beaker and homogenize for 2 minutes. Prepare 10-fold serial dilutions of the homogenate, and spread an appropriate amount of each dilution onto PCA (Polydioxanone) agar. Incubate the plates at 37°C for 48 hours, then count the colonies to determine the total viable count.

[0092] The results are as follows Figure 29 As shown in the figure, the TVC value of the control group (6.75±0.09 Log(CFU)) exceeded this threshold on the 7th day of storage. On the 9th day of storage, except for the PVA-CS-Ant@ZIF-8 group (5.82±0.06 Log(CFU)) and the PVA-CS-Cur@ZIF-8 group, all other groups exceeded this threshold. Among them, the PVA-CS-Cur@ZIF-8 group maintained around 5.5 Log(CFU) after 9 days, indicating that PVA-CS-Cur@ZIF-8 can extend the spoilage period of pork and has a good preservation effect.

[0093] Although the present invention has been described using the above preferred embodiments, it is not intended to limit the scope of protection of the present invention. Any changes and modifications made by those skilled in the art to the above embodiments without departing from the spirit and scope of the present invention shall still fall within the scope of protection of the present invention.

Claims

1. A highly efficient antibacterial and antioxidant composite hydrogel film, characterized in that, It includes a film-forming matrix with polyvinyl alcohol and chitosan as the main components, and a ZIP-8 composite material loaded with curcumin dispersed in the film-forming matrix.

2. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 1, characterized in that, Includes the following steps: (1) Preparation of film-forming matrix Chitosan was dissolved in dilute acetic acid solution to prepare a chitosan solution. Polyvinyl alcohol was dissolved in deionized water and heated and stirred until completely dissolved to prepare a polyvinyl alcohol solution. The chitosan solution and polyvinyl alcohol solution were mixed, and plasticizer and crosslinking agent were added. Then the mixture was stirred for 30-45 minutes to obtain a film-forming matrix mixture solution. (2) Preparation of ZIF-8 composite material loaded with curcumin A zinc source solution was prepared by dissolving zinc salts, and a ligand solution was prepared by dissolving imidazole organic ligands in methanol, adding curcumin, and stirring until completely dissolved. The zinc source solution was mixed with the ligand solution, stirred and reacted, centrifuged, and the precipitate was collected. After washing and drying, the ZIF-8 composite material loaded with curcumin was obtained. (3) Preparation of composite hydrogel films ZIF-8 composite material is added to the film-forming matrix mixture solution and ultrasonically mixed to obtain a film-forming solution. The film-forming solution is then used to form a composite hydrogel film.

3. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (1), the mass ratio of polyvinyl alcohol to chitosan is (2-3):(0.5-1.5).

4. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (2), the zinc salt is Zn(NO3)2·6H2O, the imidazole organic ligand is 2-methylimidazolium, and the mass ratio of the zinc salt, imidazole organic ligand and curcumin is (0.5~2.5):(1~4):(0.1~0.3).

5. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (1), the heating and stirring temperature is 90-120℃, and the stirring time is 1-2h.

6. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (1), the plasticizer is glycerol and the crosslinking agent is anhydrous citric acid. The mass ratio of glycerol to anhydrous citric acid is (0.01-0.03):(0.05-0.2).

7. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (3), the mass of the ZIF-8 composite material is (0.1 to 0.25).

8. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (3), the ultrasonic treatment lasts for 30 to 45 minutes.

9. The method for preparing the highly efficient antibacterial and antioxidant composite hydrogel film according to claim 2, characterized in that, In step (3), the film-forming process involves casting the film-forming solution onto a flat plate and drying it at a temperature of 40–60°C for 48–60 hours.

10. The application of the high-efficiency antibacterial and antioxidant composite hydrogel film according to claim 1 or the composite hydrogel film prepared by any one of claims 2-9 in the preservation of chilled fresh meat.

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