Polyvinyl alcohol-based composite film, method for preparing the same, and use thereof

Abstract

The application belongs to the field of film materials for food, and provides a polyvinyl alcohol-based composite film and a preparation method and application thereof.The preparation method of the polyvinyl alcohol-based composite film comprises the following steps: (1) providing an extractant, wherein the extractant comprises a eutectic solvent and water, and the eutectic solvent is formed by beta-cyclodextrin and glycolic acid in a molar ratio of 1:(45-55); (2) extracting propolis powder by using the extractant to obtain a propolis extract; (3) uniformly mixing a polyvinyl alcohol aqueous solution, the propolis extract and an optional zeolitic imidazolate framework material to obtain a film-forming solution; and (4) preparing a film from the film-forming solution to obtain the polyvinyl alcohol-based composite film.The polyvinyl alcohol-based composite film not only has high transparency and good processability, but also has excellent barrier properties, antioxidant properties and antibacterial activity.

Description

Technical Field

[0001] This invention belongs to the field of food film materials, specifically, it provides a polyvinyl alcohol-based composite film, its preparation method, and its application. Background Technology

[0002] Food packaging materials are usually made from traditional petroleum-based plastics. Although petroleum-based plastics have excellent processing performance and mechanical strength, they are difficult to degrade in the natural environment. Over time, they have accumulated a serious ecological burden and environmental pressure. Therefore, developing high-performance biodegradable packaging materials based on renewable resources has become a key path to solve plastic pollution and promote the green transformation of the packaging industry.

[0003] Currently, biodegradable packaging materials are mainly divided into three categories: the first is natural polymer-based materials, including starch, cellulose, lignin, chitosan, and protein. These materials are widely available, highly renewable, and have excellent biocompatibility, but they generally have shortcomings such as insufficient mechanical properties, poor water resistance, and limited barrier properties, which restrict their large-scale application. The second is microbially synthesized materials, with polyhydroxyalkanoates (PHA) as a typical example. These materials have complete biodegradability and good biocompatibility, but their production costs are high and their processing performance is limited, making it difficult to meet the needs of large-scale industrialization. The third is chemically synthesized biodegradable polymer materials, including polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), and polyvinyl alcohol (PVA). These materials combine biodegradability with excellent processing performance.

[0004] Polyvinyl alcohol (PVA) is considered a food packaging substrate with great application potential due to its excellent film-forming properties, high transparency, superior biocompatibility, and good biodegradability. However, traditional PVA films still have significant performance defects: on the one hand, its molecular chains are rich in hydroxyl groups, making it overly hydrophilic and resulting in insufficient water vapor barrier properties, making it difficult to effectively prevent moisture migration in fresh food; on the other hand, PVA films themselves do not possess active functions such as antioxidant, antibacterial, and UV shielding, which cannot meet the long-term preservation requirements of high-value-added foods, especially perishable aquatic products with high moisture and high protein content, thus limiting its application in the field of active packaging. Summary of the Invention

[0005] To address the aforementioned problems in the existing technology, the present invention aims to provide a polyvinyl alcohol-based composite film, its preparation method, and its applications. The polyvinyl alcohol-based composite film prepared by the present invention maintains high transparency and good processability while also possessing excellent barrier properties, antioxidant properties, and antibacterial activity.

[0006] In a first aspect, the present invention provides a method for preparing a polyvinyl alcohol-based composite film, comprising the following steps: (1) Provide extraction agent The extractant includes a eutectic solvent and water, wherein the water content in the extractant is 25% to 40% by mass, and the eutectic solvent is formed by β-cyclodextrin and glycolic acid in a molar ratio of 1:(45 to 55). (2) The propolis powder was extracted with the extractant at a temperature of 60-80°C for 40-80 min to obtain a propolis extract containing a eutectic solvent and propolis active components; (3) Mix the aqueous solution of polyvinyl alcohol with the propolis extract and the optional zeolite imidazole ester framework (ZIF) material to obtain a film-forming solution; (4) The film-forming liquid is used to form a film to obtain a polyvinyl alcohol-based composite film.

[0007] In a second aspect, the present invention provides a polyvinyl alcohol-based composite film prepared by the preparation method described in the first aspect of the present invention.

[0008] Thirdly, the present invention provides the application of the polyvinyl alcohol-based composite film described in the second aspect of the present invention in the preparation of food preservation film or food packaging film.

[0009] The present invention has the following beneficial effects: This invention successfully introduces natural propolis extract into a polyvinyl alcohol matrix. The resulting composite film possesses high transparency, good processability, excellent antibacterial and antioxidant properties, and is also safe for use in food preservation.

[0010] Specifically, the preparation method of this invention uses an aqueous eutectic solvent system as the extractant to extract propolis powder. This eutectic solvent is composed of β-cyclodextrin and glycolic acid, which is not only green and non-toxic with good biocompatibility, but also significantly enhances the dissolution and mass transfer of flavonoids and other polyphenolic active substances in propolis, greatly improving the extraction efficiency and stability of active ingredients in propolis, and obtaining a propolis extract (DPT) including the eutectic solvent (DES) and propolis active components. After introducing DPT into a polyvinyl alcohol (PVA) membrane system, the DES component in DPT can act as a plasticizer to improve the toughness of the PVA film; the propolis active components stably present in DPT can stably impart excellent antioxidant and antibacterial activities to the film. More importantly, DPT has excellent compatibility with the PVA matrix and can be uniformly dispersed in the system without phase separation, which does not affect the film-forming properties and transparency of PVA itself, and has biocompatibility, and can replace traditional inorganic nano-functional fillers. In some preferred solutions, an appropriate amount of ZIF material is introduced into the DPT-modified PVA film system. Since ZIF material has good interfacial compatibility with both PVA and DPT, the three can be uniformly compounded without obvious phase separation. This allows the propolis active components, eutectic solvent and ZIF material to form a highly efficient synergistic effect, achieving simultaneous improvement in toughening, light transmission, antibacterial, antioxidant and barrier properties.

[0011] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0012] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 The images shown are scanning electron microscope (SEM) image (A), infrared spectrum (B), and particle size distribution (C) of the ZIF-L material prepared in Example 1. Figure 2 The figures show a comparison of the temperature stability (A) and light stability (B) of the propolis extracts from Preparation Example 2 and Comparative Preparation Example 4, respectively. Figure 3 The ultraviolet-visible light transmittance curves of the films of Comparative Examples 1-2, Example 1, and Examples 4-8 are shown. Figure 4 SEM images of the films of Comparative Examples 1-2, Examples 1, and Examples 4-8; Figure 5 Comparison charts of tensile strength (A) and elongation at break (B) of the films of Comparative Examples 1-2, Example 1 and Examples 4-8, and physical photographs and macroscopic mechanical property verification charts (C) of the film of Example 4. Figure 6 The FT-IR spectrum (A), XRD spectrum (B), TG test results (C), and DTG test results (D) of the thin films of Comparative Examples 1-2, Example 1, and Examples 4-8 are shown. Figure 7 Comparative graphs show the test results of surface contact angle (A), water vapor transmission rate (B), and DPPH free radical scavenging rate (C) of the films of Comparative Examples 1-2, Example 1, and Examples 4-8; Figure 8 The images show the antibacterial properties of the films of Comparative Examples 1-2, Examples 1 and Examples 4-8 (A), the survival rate of the film of Example 4 after co-culturing with L929 cells for 24 hours (B), and the staining results of live and dead cells (C). Figure 9 Images (A) showing the application of PE film, PVA film and the film of Example 4 on oysters for preservation, and graphs (B) showing the changes in weight loss and pH value of oysters after different preservation days; Figure 10 A to C are comparative graphs of TVB-N, TBARS, and TVC of PE film, PVA film, and film of Example 4 after different days of oyster preservation, respectively. D to E are analysis graphs of flavor characteristics of PE film, PVA film, and film of Example 4 after different days of oyster preservation, respectively. Detailed Implementation

[0013] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0014] The "scope" disclosed in this invention is defined in the form of a lower limit and / or an upper limit, whereby a given scope is defined by selecting a lower limit and / or an upper limit. This scope may or may not include endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form an undefined scope, and any lower limit can be combined with other lower limits to form an undefined scope, similarly, any upper limit can be combined with any other upper limit to form an undefined scope. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and can be combined with any other point or single value, or with other lower or upper limits, to form an undefined scope.

[0015] Unless otherwise specified, all embodiments and optional embodiments of the present invention may be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present invention.

[0016] The first aspect of the present invention provides a method for preparing a polyvinyl alcohol-based composite film, comprising the following steps: (1) Providing an extractant: The extractant comprises a eutectic solvent (DES) and water; (2) Preparation of propolis extract: The propolis powder is extracted with the extractant to obtain a propolis extract containing a eutectic solvent and active components of propolis; (3) Preparation of film-forming solution: Mix the aqueous solution of polyvinyl alcohol with the propolis extract and the optional zeolite imidazole ester framework structural material evenly to obtain the film-forming solution; (4) Preparation of composite film: The film-forming liquid is used to form a film to obtain a polyvinyl alcohol-based composite film.

[0017] In this invention, the eutectic solvent (DES) is formed by β-cyclodextrin (β-CD, as a hydrogen bond acceptor) and glycolic acid (as a hydrogen bond donor), wherein the molar ratio of β-cyclodextrin (β-CD) to glycolic acid is 1:(45~55), for example 1:45, 1:47, 1:48, 1:50, 1:51, 1:52, or 1:55. Controlling the relative amounts of both within this range avoids insufficient dissolution due to excessive β-CD addition and alleviates the irregular network structure and decreased extraction efficiency caused by too low a β-CD proportion. If the β-CD proportion is too high, precipitation will occur, preventing the formation of a transparent and stable eutectic solvent; if the β-CD proportion is too low, the inherent hydrogen bond donor-acceptor network of the eutectic solvent will be disrupted, forming irregular molecular entanglements and affecting extraction efficiency. Preferably, the molar ratio of β-CD to glycolic acid is 1:(48~52), which further improves the stability of propolis active ingredients in DES.

[0018] In this invention, the water content (i.e., water content) is 25% to 40% based on the total mass of the extractant, for example, 25%, 30%, 32%, 35%, 38%, or 40%. Because DES has excessively high viscosity, it suffers from high mass transfer resistance and is difficult to operate. Adding an appropriate amount of water can significantly reduce the viscosity of the extractant system and improve the extraction efficiency of propolis.

[0019] In some embodiments, step (1) includes mixing β-cyclodextrin, glycolic acid, and water under heating conditions to obtain the extractant. β-CD mixed with glycolic acid at high temperature yields a transparent DES, wherein the heating temperature is 70-85°C, for example, 70°C, 75°C, 78°C, 80°C, or 85°C.

[0020] In step (2), the propolis powder can be obtained by freeze-drying, pulverizing, and sieving propolis. The freeze-drying temperature can be -80℃ to -30℃, the freeze-drying time can be 1 to 3 days, and the sieve mesh size can be 40 to 100 mesh. As an example, the method for preparing the propolis powder includes: pre-freezing propolis overnight in a freezer at -80℃, drying it in a freeze dryer at -40℃ for 48 hours, then pulverizing it with a high-speed grinder, sieving it through a 40-mesh sieve, sealing it, and freezing it in a freezer at -80℃.

[0021] This invention does not specifically limit the source of propolis; it can be any natural propolis formed by worker bees collecting secretions from plant buds, bark, or resin, and then transforming these secretions themselves, such as, but not limited to, poplar propolis. Propolis is rich in flavonoids, mainly including myricetin, quercetin, syringin, kaempferol, and galangin. These flavonoids possess antioxidant and antibacterial activities and are key active components in the preparation of functional food preservation packaging films. The eutectic solvent of this invention is a natural product extractant; using it to extract propolis powder can efficiently dissolve and separate flavonoids while maintaining the high stability of these active ingredients.

[0022] In step (2), based on 1g of the propolis powder, the amount of the extractant used is 25~35mL, for example 25mL, 30mL, 32mL or 35mL. This dosage range can ensure the efficient dissolution and extraction of active components in propolis, and can also avoid the negative impact on the antibacterial properties of the film due to excessive eutectic solvent (DES) causing the relative content of active components in the propolis extract (DPT, i.e., the components in the extract excluding water).

[0023] In step (2), the extraction temperature is 60-80℃, for example, 60℃, 65℃, 70℃, or 75℃. The extraction can be carried out under stirring conditions, and the extraction time is typically 40-80 min, for example, 40 min, 50 min, 60 min, 65 min, 70 min, or 80 min. As the extraction time increases, the total flavonoid extraction yield gradually increases; however, with further extension of the extraction time, the extraction yield tends to stabilize or slightly decrease (this may be due to the degradation of some active components caused by prolonged heating). Therefore, the extraction time is preferably 50-70 min, and more preferably 50-60 min.

[0024] In some embodiments, step (2) includes: adding propolis powder and extractant into a pressure-resistant bottle and stirring evenly, then placing it in a water bath and heating and extracting under stirring conditions; after extraction, centrifuging to separate solid and liquid, collecting the supernatant, thus obtaining a propolis extract containing a eutectic solvent and propolis active components.

[0025] In some embodiments, the active ingredient in the propolis extract obtained in step (2) is 110~120 mg / g based on its total flavonoid content, for example 110.0 mg / g, 112.0 mg / g, 113.5 mg / g, 114.5 mg / g, 115.5 mg / g, 118.5 mg / g or 119.0 mg / g.

[0026] In step (3), polyvinyl alcohol is used as the matrix of the composite film. To improve film-forming properties and reduce the film's sensitivity to humidity, the polyvinyl alcohol used preferably has a number-average degree of polymerization of 1600-1800, such as 1650, 1700, 1750, or 1800, and a degree of alcoholysis of not less than 98 mol%, such as 98-99 mol%. In addition, mixing polyvinyl alcohol with propolis extract in the form of an aqueous solution can effectively reduce the risk of phase separation and significantly improve the compatibility between the PVA matrix and propolis extract (DPT): if PVA is directly mixed with the extract, white precipitate or uneven suspension system is likely to occur, making it difficult to form a continuous and uniform film. As some examples, the concentration of the aqueous solution of polyvinyl alcohol is 0.02-0.08 g / mL, such as 0.04 g / mL, 0.05 g / mL, or 0.07 g / mL.

[0027] In some embodiments, the amount of propolis extract relative to 100 parts by weight of the polyvinyl alcohol can be 30 to 50 parts by weight, for example, 30, 35, 40, 45, or 50 parts by weight. Furthermore, introducing an appropriate amount of DPT into the PVA matrix can impart antibacterial properties to the film while maintaining good mechanical strength and film-forming properties. If the amount of propolis extract is too low, it may result in insufficient bioactivity of the film and poor antibacterial and antioxidant effects; if the amount is too high, it may affect the film-forming quality. Preferably, the amount of the extract relative to 100 parts by weight of the polyvinyl alcohol is 35 to 45 parts by weight.

[0028] In step (3), "optional" means that the zeolite imidazole ester framework (ZIF) material may or may not be added. In some preferred examples, the addition of the zeolite imidazole ester framework material in step (3), which is introduced into the PVA and DPT composite system as a functional nanofiller, can improve the overall performance of the film.

[0029] In some embodiments, the Z-average particle size of the zeolite imidazole ester framework material is 90-150 nm, for example, 110 nm, 115 nm, 120 nm, 140 nm, or 150 nm. The Z-average particle size of the present invention can be obtained by dynamic light scattering (DLS) analysis.

[0030] In step (3), the amount of the zeolite imidazole ester skeleton structural material relative to 100 parts by mass of the polyvinyl alcohol can be 0 to 1 part by mass, for example 0.1 part by mass, 0.3 part by mass, 0.5 part by mass, 0.7 part by mass, 0.8 part by mass or 1.0 part by mass, preferably 0.3 to 0.7 parts by mass.

[0031] In some embodiments, the zeolite imidazole ester framework material is a layered zeolite imidazole ester framework material (ZIF-L). ZIF-L has a two-dimensional layered structure with unique pores between the layers, which makes it superior to the common three-dimensional structure ZIF-8 in terms of molecular transport. More preferably, the zeolite imidazole ester framework material is a layered zeolite imidazole ester framework material formed by the coordination of zinc ions with 2-methylimidazolium.

[0032] As examples, ZIF-L can be prepared as follows: Zinc salt (e.g., zinc chloride) and 2-methylimidazole are dissolved separately in deionized water. The resulting two solutions are mixed and stirred. After the reaction is complete, the mixture is centrifuged, washed with water, and vacuum dried to obtain a white powder, ZIF-L. The molar ratio of 2-methylimidazole to zinc salt can be (3~5):1, for example, 4:1 or 4.2:1, and the reaction time can be 12~24 h, for example, 15 h, 19 h, or 22 h.

[0033] In some embodiments, step (3) includes mixing an aqueous solution of polyvinyl alcohol with propolis extract and optional ZIF nanoparticles, stirring at 300-600 rpm for 1-3 hours, and then performing ultrasonic treatment for 15-50 minutes. The above stirring conditions can effectively overcome the high viscosity of the PVA solution, allowing the propolis extract and any ZIF nanoparticles present to be uniformly dispersed in the system; the subsequent ultrasonic treatment can remove air bubbles in the mixture and improve the film quality.

[0034] In step (4), the film can be prepared by casting. As a preferred example, step (4) includes: pouring the film-forming solution into a mold and allowing it to flow naturally, then drying it at 40-50°C until the moisture is completely evaporated, thus obtaining the composite film. During this drying and film-forming process, the moisture is removed by heating and evaporation, ultimately forming a uniform PVA-based composite film composed of PVA, DPT, and optionally ZIF.

[0035] In some embodiments, the thickness of the formed polyvinyl alcohol-based composite film is 50 to 150 μm, for example 70 μm, 80 μm, 90 μm, 98 μm, 100 μm, 120 μm or 130 μm.

[0036] A second aspect of the present invention provides a polyvinyl alcohol-based composite film prepared by the preparation method described in the first aspect of the present invention. Specifically, the composite film comprises a polymer matrix (polyvinyl alcohol) and an active antibacterial component (propolis extract) dispersed therein, and optionally nanoparticles (ZIF, preferably ZIF-L).

[0037] In the composite film of this invention, propolis extract (DPT) imparts excellent antioxidant and antibacterial properties to the film. In a preferred embodiment, an appropriate amount of ZIF-L is further introduced. Its surface imidazole rings can form hydrogen bonds with the hydroxyl groups of PVA, achieving initial uniform dispersion. The β-CD in the extract can form hydrogen bonds with both PVA and ZIF-L, and can also encapsulate the hydrophobic components of propolis, creating steric hindrance around ZIF-L and reducing particle aggregation. Glycolic acid acts as a molecular bridge, anchoring ZIF-L within the polymer network, and the active components of propolis also stabilize the dispersion of ZIF-L in the system. This allows both ZIF-L and DPT to be uniformly and stably distributed within the PVA matrix, thereby improving the film's toughness (improving processability) and antibacterial and antioxidant activity, and imparting suitable optical and air permeability.

[0038] A third aspect of the present invention provides the application of the polyvinyl alcohol-based composite film described in the second aspect of the present invention in the preparation of food preservation films or food packaging films. As described above, this composite film possesses high transparency, processability, antibacterial and antioxidant properties, and exhibits significant preservation effects. It can be used for active packaging of fresh meat, aquatic products, fruits and vegetables, and cooked foods, and can also be used as a post-harvest biodegradable preservation film for fruits and vegetables, effectively inhibiting microbial growth and lipid oxidation, and reducing food spoilage.

[0039] The following describes embodiments of the present invention. These embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0040] Preparation Example 1 This preparation example is used to illustrate the synthesis method of the ZIF-L material used in the following embodiments and comparative examples.

[0041] 2.5 mmol of zinc chloride (ZnCl2) was dissolved in 45 mL of deionized water to obtain an aqueous solution of zinc chloride; 10 mmol of 2-methylimidazole (2-MI) was dissolved in 45 mL of deionized water to obtain an aqueous solution of 2-methylimidazole. The two solutions were mixed and stirred at 300 rpm for 19 h at room temperature. The resulting mixture was then centrifuged at 8000 rpm for 10 min, washed three times with 20 mL of deionized water, and finally dried under vacuum at 60 °C for 20 h to obtain a white powder, the target product ZIF-L.

[0042] Figure 1SEM images of A show that ZIF-L exhibits a narrow, flat, leaf-like structure with sharp ends, a morphology derived from Zn. 2+ The zeolite imidazole ester skeleton is formed through coordination with the nitrogen atom of the imidazole. Figure 1 In the FT-IR spectrum of B, the main absorption peaks are attributed to the vibrational mode of the 2-methylimidazole ligand: 600~800 cm⁻¹ -1 It is an out-of-plane bending vibration of the imidazole ring, 900~1350cm -1 It is an in-plane bending vibration, 1350~1500cm -1 For the stretching vibration of the ring skeleton; 1115cm -1 994cm -1 and 760cm -1 The locations correspond to the bending vibrations of the CH bond and the torsional and bending vibrations of the CN bond, respectively, at 1570 cm. -1 The point is a CN stretching vibration, 2922 cm. -1 and 3134cm -1 These correspond to the stretching vibrations of aliphatic and aromatic CH bonds, respectively. Furthermore, 2-methylimidazole exhibits a stretching vibration at 1852 cm⁻¹. -1 The NH characteristic peak at the location almost completely disappears in ZIF-L, indicating successful coordination. Figure 1 C shows the dynamic light scattering particle size analysis results of ZIF-L, which reveals that the average particle size (Z-average) of ZIF-L is 138.1 nm. This result indicates that the synergistic effect of zinc chloride and 2-methylimidazole in the synthesis process effectively controls the nucleation and crystallization of ZIF-L nanoparticles.

[0043] Preparation Examples 2-4 illustrate the propolis extract and its preparation method used in the following examples. The method for preparing the propolis powder is as follows: the original propolis sample (poplar propolis) was pre-cooled overnight at -80°C, then freeze-dried at -40°C for 48 hours, ground, and sieved through a 40-mesh sieve to obtain propolis powder.

[0044] Preparation Example 2 β-Cyclodextrin (β-CD) and glycolic acid (GA) were mixed with water at a molar ratio of 1:50 and stirred at 80°C until a uniform and transparent eutectic solvent system was formed to obtain an extractant with a water content of 30%.

[0045] Take 0.5g of propolis powder and 15mL of extraction solvent and add them to a 100mL pressure-resistant bottle. Place the bottle in a 70℃ constant temperature water bath and stir for 60min. After extraction, centrifuge the product and collect the liquid fraction to obtain the propolis extract containing the eutectic solvent and the active components of propolis, denoted as DPT-1.

[0046] Comparative Preparation Example 1 Propolis extract was prepared according to the method of Preparation Example 2, except that β-cyclodextrin was replaced with α-cyclodextrin (α-CD), and the prepared propolis extract was denoted as DPT-d1.

[0047] Comparative Preparation Example 2 Propolis extract was prepared according to the method of Preparation Example 2, except that the eutectic solvent used was formed by α-cyclodextrin and gluconic acid (GluA) in a molar ratio of 1:50. The prepared propolis extract was denoted as DPT-d2.

[0048] Comparative preparation example 3 Propolis extract was prepared according to the method of Preparation Example 2, except that glycolic acid was replaced with gluconic acid. The prepared propolis extract was denoted as DPT-d3.

[0049] Comparative preparation example 4 Ethanol and water are mixed and stirred until homogeneous to obtain an ethanol solution with a water content of 30%.

[0050] Take 0.5g of propolis powder and 15mL of the above ethanol solution and add them to a 100mL pressure-resistant bottle. Place the bottle in a 70℃ constant temperature water bath and stir for 60min. After extraction, centrifuge the obtained product and collect the liquid fraction to obtain the propolis extract containing ethanol-propolis active components. This propolis extract is designated as DPT-d4.

[0051] Test Example 1 This test example is used to characterize the performance of the propolis extract prepared in the above preparation examples and the comparative preparation examples.

[0052] 1. Determination of total flavonoid content According to GB / T20574-2006, the total flavonoid content was determined by the NaNO2-Al(NO3)3 colorimetric method: 10 mg of rutin reference standard was accurately weighed, placed in a 25 mL volumetric flask, dissolved in anhydrous ethanol and diluted to the mark, and shaken well to obtain the rutin standard stock solution.

[0053] Accurately pipette 0.0 mL, 0.5 mL, 1.0 mL, 1.5 mL, 2.0 mL, 2.5 mL, and 3.0 mL of rutin standard stock solution into separate 10 mL volumetric flasks. Add 0.3 mL of 5% sodium nitrite solution to each flask, let stand for 6 min, then add 0.3 mL of 10% aluminum nitrate solution, let stand for 6 min, then add 4 mL of 4.3% sodium hydroxide solution, and dilute to the mark with water. Shake well. Perform a full-wavelength scan; a maximum absorption peak is observed at approximately 510 nm. Plot a curve with the absorbance measured at 510 nm as the ordinate and the rutin concentration as the abscissa, and derive the regression equation for the standard curve.

[0054] Accurately measure 1 mL of propolis extract into a 10 mL volumetric flask, and dilute to volume with anhydrous ethanol. Subsequent procedures should be performed according to the standard curve method for determining flavonoid content. Calculate the total flavonoid content of the propolis extract based on the regression equation of the rutin standard curve. Test each sample three times.

[0055] The test results are shown in Table 1.

[0056] Table 1

[0057] As shown in Table 1, the extraction of propolis powder using an aqueous extractant formed by DES constructed with β-CD and GA in a 1 / 50 (molar ratio) yielded an extract with a much higher content of active components (total flavonoids) than that of other DES and ethanol extractants.

[0058] 2. Stability Analysis Propolis extracts DPT-1 and DPT-d4 were compared, and their thermal and photostability were evaluated. For thermal stability, each extract sample was placed in two temperature environments under light-protected conditions: 25°C (room temperature) and 60°C (accelerated degradation conditions). For photostability, each extract sample was subjected to three light conditions: complete darkness, indoor natural light, and direct sunlight. Direct sunlight was simulated by placing the sample on a windowsill exposed to ample sunlight, while the indoor natural light and darkness-protected control groups were placed in the same laboratory environment (25°C). All samples were placed against a uniform white background to ensure consistent reflection conditions and temperature. Stability analysis involved recording the retention rate of total flavonoids (TFC) every 2 hours over a 12-hour monitoring period, with three sets of tests per sample. Results are as follows: Figure 2 As shown.

[0059] like Figure 2 As shown in Figure A, after storage at 25°C, the total flavonoid retention rate of all samples exceeded 90%, indicating that the DES formed by β-CD and GA performed better than ethanol in maintaining the stability of flavonoids and significantly reduced their degradation rate during storage. When the storage temperature was increased to 60°C, the degradation process of flavonoids in DPT-1 was significantly slower, indicating that it provided a protective environment for flavonoids under thermal stress conditions. This may be attributed to the hydrogen bonding and π–π stacking interactions between DES and flavonoids. In this process, the hydrogen bonding between the solvent and the hydroxyl groups in the flavonoids, as well as the π–π stacking interactions between the solvent and the aromatic rings in the flavonoids, jointly promoted molecular stabilization. In addition, the hydrophobic cavity provided by β-cyclodextrin in DES may form a non-covalent host-guest bond with flavonoids, which can be used to enhance the stability of flavonoids. Figure 2As can be seen from B, under different light conditions, DPT-1 also shows higher flavonoid stability compared to DPT-d4.

[0060] The following examples illustrate the polyvinyl alcohol-based composite film and its preparation method of the present invention. The polyvinyl alcohol used is PVA-1799, with a degree of alcoholysis of 98%~99% (mol / mol).

[0061] Example 1 Add 5g of polyvinyl alcohol to 95mL of deionized water and stir at 650r / min at 95℃ until the solution is clear and transparent to obtain a PVA solution.

[0062] Add 2g of propolis extract DPT-1 (the amount of extract is 40% of the mass of PVA) to the above PVA solution, and stir magnetically at 300rpm for 2h at room temperature to mix evenly. Then, sonicate for 30min to remove air bubbles to obtain the film-forming solution.

[0063] Pour 50g of the mixture into a mold (16.5cm×11.5cm×0.5cm, the same below), place it in an oven and dry at 45℃ for 24h to obtain a polyvinyl alcohol-based composite film, denoted as PVA / DPT40.

[0064] Examples 2-3 Polyvinyl alcohol-based composite films were prepared according to the method of Example 1, except that the amount of propolis extract DPT-1 was adjusted to 1.5g and 2.5g, respectively, which means that the amount of extract was 30% and 50% of the mass of PVA. The prepared composite films were successively named PVA / DPT30 and PVA / DPT50.

[0065] Example 4 Add 5g of polyvinyl alcohol to 95mL of deionized water and stir at 650r / min at 95℃ until the solution is clear and transparent to obtain a PVA solution.

[0066] 2g of propolis extract DPT-1 and 25mg of ZIF-L were added to the above PVA solution (the amounts of DPT-1 and ZIF-L were 40% and 0.5% of the mass of PVA, respectively). The mixture was magnetically stirred at 300rpm for 2 hours at room temperature to ensure uniform mixing. Then, it was sonicated for 30 minutes to remove air bubbles and obtain the film-forming solution.

[0067] Pour 50g of the mixture into a mold and dry it in an oven at 45℃ for 24h to obtain a composite film, denoted as PVAD / ZIF0.5.

[0068] Examples 5-8 Polyvinyl alcohol composite films were prepared according to the method of Example 4, except that the amount of ZIF-L was adjusted so that the added mass was 0.1%, 0.3%, 0.7% and 1.0% of the mass of PVA, respectively. The prepared composite films were successively named PVAD / ZIF0.1, PVAD / ZIF0.3, PVAD / ZIF0.7 and PVAD / ZIF1.0.

[0069] Comparative Example 1 A composite film was prepared using the PVA solution from Example 4 as the film-forming liquid, denoted as PVA.

[0070] Comparative Example 2 The composite film was prepared according to the method of Example 4, except that the extraction solution DPT-1 was not added. The prepared composite film is designated as PVA / ZIF0.5.

[0071] The composite films of the above embodiments and comparative examples, along with their main raw materials and addition amounts, are shown in Table 2.

[0072] Table 2

[0073] Test Example 2 The test examples are used to evaluate the performance of the films in the above embodiments and comparative examples.

[0074] 1. Thin Film Characterization 1) Optical properties The color of the film was measured using a portable colorimeter (TaiShuang TS7700, Shenzhen Sanenshi Co., Ltd.). Data was recorded from five random locations on the film, and the results are denoted as L. * (Brightness), a * (Red / Green), b * (Yellow / Blue), and take the average value of each, and then calculate the difference in color (ΔE) according to Equation 1.

[0075] Formula 1 Where ΔE represents color difference; L0, a0, and b0 are the color parameters of the standard white board; and L*, a*, and b* are the color parameters of the film.

[0076] 2) Thickness Five points were randomly selected on the film, and the thickness of the film was measured using a 0.001mm digital micrometer (awt-chy01, Henan Evit Electronic Technology Co., Ltd.).

[0077] 3) Ultraviolet transmittance The UV-Vis transmittance of the film in the 200–900 nm range was evaluated using a UV-Vis spectrophotometer (Q-6, Shanghai Yuan'an).

[0078] 4) Scanning electron microscope Different thin film samples were cut into 100mm × 10mm pieces using a cutting tool, and their cross-sections were obtained by liquid nitrogen embrittlement. A thin layer of gold was then sputtered onto each piece. The surface and cross-sectional morphology of the different composite films were observed using SEM (Phenom Pure, Phenom Scientific Instruments (Shanghai) Co., Ltd.) at an accelerating voltage of 5kV.

[0079] 5) Fourier transform infrared spectroscopy (FT-IR) The wavenumber scan range was analyzed using an FT-IR spectrometer (Nicolet iS50 FT-IR) with a range of 4000–400 cm⁻¹. -1 Infrared spectra of all thin films, resolution 4 cm⁻¹ -1 .

[0080] 6) X-ray diffraction (XRD) Wide-angle diffraction tests were performed using an X-ray diffractometer (Bruker, Germany), and XRD patterns were recorded at a scan rate of 5° / min within the diffraction angle range of 5° to 55° (2θ).

[0081] 7) Thermogravimetric analysis The thermal stability of the membrane was measured using a TGA instrument (STA 449 F3 / F5, NETZSCH, Germany), with the temperature increased from 30°C to 600°C at a rate of 10°C / min, and the test atmosphere was nitrogen.

[0082] 8) Mechanical properties The tensile strength (TS) and elongation at break (EAB) of the membrane were measured using a tensile testing machine (AI-7000-SJ, China Railway High Speed ​​Technology Co., Ltd.). The sample size was 100 mm × 10 mm, the initial and standard distances between the clamps were set to 50 mm, and the tensile speed was 50 mm / min. Each sample was tested three times.

[0083] 9) Water contact angle (WCA) The hydrophilicity / hydrophobicity of the thin film surface was measured using a contact angle meter (SDC-100, Shengding Precision Co., Ltd.), images were captured using a camera, and the WCA was measured by semicircle fitting analysis. Each sample was tested three times.

[0084] 10) Water vapor transmission rate (WVT) Different film samples were cut into circles and placed in permeation cups, and reacted in a water vapor transmission rate tester (AuToW806, Guangzhou Biaoji Packaging Equipment Co., Ltd.) for 24 hours. The results were recorded. Each sample was tested three times.

[0085] 2. Functional testing of thin films 1) Antioxidant properties Weigh 0.1 g of the sample film and mix it with 2.5 mL of 0.2 mmol / L 1,1-diphenyl-2-trinitrophenylhydrazine (DPPH) ethanol solution and 2.5 mL of deionized water. Incubate in the dark for 30 min, and measure the absorbance at 517 nm (zero the sample solution with 2.5 mL of anhydrous ethanol and 2.5 mL of sample solution, subtracting the influence of the sample's own color). Calculate the free radical scavenging rate according to Equation 2. Test each sample three times.

[0086] Formula 2 In Equation 2, X: DPPH free radical scavenging rate (%); A1: Absorbance of the film and DPPH; A2: Absorbance of the thin film and the ethanol solution; A0: Absorbance of DPPH and ethanol solutions.

[0087] 2) Antibacterial properties Staphylococcus aureus CMCC(B)26003 and Escherichia coli ATCC25922 were selected as test subjects, and the antibacterial activity of the active membrane was evaluated by the inhibition zone method. Cut discs of the membrane (6 mm in diameter) were placed on LB solid medium containing 100 μL of the target bacterial solution and incubated at 37 °C for 12 h. The diameter of the inhibition zone was observed and measured. Each sample was tested three times.

[0088] 3) Cell compatibility Cytotoxicity testing was performed on the membrane samples by immersing 1.5 mm diameter membranes in deionized water for 12 h and adjusting the concentration of L929 cell suspension in logarithmic growth phase to 1 × 10⁻⁶. 5 CFU / mL. 10 μL of the membrane soaking solution was directly added to L929 cells. After incubation for 24 h, the culture medium was removed, and 100 μL of 10% (v / v) CCK-8 solution was added. After further incubation for 4 h, the absorbance of the sample was measured at a characteristic wavelength of 450 nm using a microplate reader. Deionized water was used as the control group, and cells from the soaking solution sample without the membrane were used as the blank control group. The effect of the material on the viability of L929 cells was evaluated using the CCK-8 reagent. Live and dead cells were observed by staining, and cell viability (%) was calculated according to Equation 3. Each sample was tested three times.

[0089] Formula 3 In the formula: A sample A0: Absorbance of the added thin film sample; A0: Absorbance of the blank group.

[0090] 3. Oyster preservation experiment PVAD / ZIF0.5 composite film was selected as the test material for actual shelf-life testing. Fresh Fujian oysters were purchased from Jimei Agricultural Market (Xiamen, China). After opening the shells, the oysters were rinsed with sterile saline solution and dried with absorbent paper. All oysters were divided into three groups: oysters packaged in plastic wrap (PE), oysters individually packaged in PVA film (PVA), and oysters packaged in film (PVAD / ZIF0.5). All samples were stored at 4°C for 12 days. Each sample was tested three times.

[0091] 1) Weight loss measurement The weight loss of oysters during storage was calculated using a gravimetric method. The weight loss rate X (%) was calculated according to Formula 4: Formula 4 In the formula, M0: oyster mass on day 1, g; M1: oyster mass on day n, g.

[0092] 2) pH value measurement The pH value of oysters was determined according to GB 5009.237-2016. 2.0 g of the minced sample was weighed and added to 20 mL of deionized water. The mixture was stirred for 1 min using a vortex mixer (VORTEX-GENIEZ, Beijing Shuanglian Jiangtong Experimental Equipment Co., Ltd.) to ensure uniform dispersion. After standing for 30 min, the mixture was centrifuged at 8000 r / min for 5 min at 4℃. The filtrate was collected in a beaker, and the pH value was measured using a pH meter (FE28-Standard, Mettler Olaf). Before use, the pH meter was calibrated with standard buffer solutions at pH values ​​of 2.00, 4.01, 7.00, 9.21, and 11.00.

[0093] 3) Thiobarbituric acid reaction (TBARS) determination The absorbance was determined according to the spectrophotometric method in GB 5009.181-2016. Weigh 1.0 g of the minced sample and add 10 mL of 7.5% trichloroacetic acid solution. Vortex for 30 s, let stand for 10 min, and centrifuge at 8000 r / min for 5 min at 4℃ to obtain the supernatant. Take 5 mL of the supernatant and add 5 mL of 0.02 mol / L thiobarbituric acid (TBA) solution. React in a 90℃ water bath for 30 min, then cool to room temperature and measure the absorbance at 532 nm. The malondialdehyde (MDA) content in the sample was calculated based on the MDA standard curve and is abbreviated as TBARS value.

[0094] A standard curve was prepared using 1,1,3,3-tetraethoxypropane (TEP) standard solution. The results are expressed as milligrams of malondialdehyde (MDA / 100g) in oyster meat.

[0095] The content of TBARS in the sample is calculated according to Equation 5: Formula 5 In the formula, X: TBARS content in the sample, mg MDA / 100g; c: Malondialdehyde concentration corresponding to the standard curve, μg / mL; V: Volume of the sample solution after final volume adjustment, mL; m: Sample mass (g); 1000: Conversion factor.

[0096] 4) TVB-N Measurement The volatile basic nitrogen content in oysters was determined using the automatic Kjeldahl nitrogen analyzer method (GB 5009.228-2016). 4.0 g of minced sample was weighed and added to 30 mL of deionized water. The mixture was stirred for 1 min using a vortex mixer. After soaking the resulting mixture for 30 min, it was centrifuged at 8000 r / min for 5 min at 4℃. The supernatant was filtered, and 30 mL of the liquid sample was transferred to a distillation tube. 0.4 g of magnesium oxide was then added to the distillation tube, which was connected to the still. The volume of boric acid receiving solution was set to 30 mL, and distillation was carried out for 3 min. The receiving solution was titrated with 0.05 mol / L hydrochloric acid standard titration solution. The volatile basic nitrogen content in the sample was calculated according to Equation 6: Formula 6 In the formula, X: the content of volatile basic nitrogen in the sample, mg / 100g; V1: The volume of hydrochloric acid standard titration solution consumed by the test solution, in mL; V2: The volume of standard hydrochloric acid titration solution consumed by the reagent blank, in mL; c: Concentration of the hydrochloric acid standard titration solution, mol / L; 14: The mass of nitrogen equivalent to titrating 1.0 mL of standard hydrochloric acid solution, in g / mol; m: Sample mass, g.

[0097] 5) Total bacterial count determination Total bacterial count was determined according to GB 4789.2-2022. Total viable count (TVC) was determined using the plate count agar (PCA) method. 1.0 g of oyster meat was weighed and added to 9 mL of sterile physiological saline to prepare a 10-fold plate count. -1 Diluent was used to continuously dilute the sample homogenate and apply it to plate counting agar plates. The plates were incubated at 37°C for 12 h. The results are expressed as log CFU / g.

[0098] 6) Electronic nose measurement The odor characteristics of oyster samples were analyzed using a sensor array (DQ0204, Beijing Yinuoweiteng Technology Development Co., Ltd.). 2.0 g of oyster sample was weighed into a 20 mL headspace vial and allowed to stand at room temperature for 30 min to allow the odor to fill the headspace vial. The washing time was set to 60 s, and the testing time to 100 s. Data from the 83rd to 85th s after stabilization were collected, and principal component analysis was performed using the sensor described in Table 3.

[0099] Table 3

[0100] 4. Statistical Analysis Correlation analysis of the samples was performed using IBM SPSS Statistics 19, and the results are expressed as mean ± standard deviation. Data visualization and plotting were performed using Origin 2021 software.

[0101] Results Analysis 1. Thin Film Optical Analysis Highly effective UV blocking capabilities can effectively protect food from oxidative damage. Ultraviolet radiation (200-400nm) easily induces lipid oxidation and nutrient degradation in food; therefore, the development of packaging materials with UV-shielding capabilities is of significant practical importance. For example... Figure 3 As shown in the ultraviolet-visible transmission spectrum, the pure PVA film (Comparative Example 1) exhibits high transmittance in both the ultraviolet and visible light regions, indicating that it is almost completely transparent and does not have ultraviolet blocking ability. However, after introducing propolis extract (DPT) and ZIF-L nanofiller, the transmittance of the composite film in the 200~400nm ultraviolet band is significantly reduced, and it has a significant ultraviolet light blocking effect.

[0102] from Figure 3 It can be observed that when DPT (PVA / DPT40) or ZIF-L (PVA / ZIF0.5) is added alone, the transmittance of the film in the ultraviolet region is significantly lower than that of pure PVA, indicating that it has a certain ultraviolet blocking effect. The composite system of DPT and ZIF-L (PVAD / ZIF series) shows a more significant ultraviolet shielding effect: in the core ultraviolet band of 200~350nm, its transmittance is significantly lower than that of the single filler system, and the transmittance of the whole band is even lower and close to 0%. This synergistic feature directly confirms the synergistic enhancement effect of the composite system. The reason may be that: (1) DPT is rich in flavonoids and phenolic acid compounds, and the aromatic rings and conjugated double bonds in its molecules can directly absorb ultraviolet energy through chemical action to achieve ultraviolet light shielding; (2) the two-dimensional layered structure of ZIF-L can act as a physical barrier to reflect and scatter incident ultraviolet light, thus constructing a physical blocking path.

[0103] 2. Film color and thickness The average test results for film color and thickness are shown in Table 4.

[0104] Table 4

[0105] Based on the data in Table 4, it can be seen that the L of pure PVA film * The value was the highest (average was 91.38), but a * The value is 1.31, b * The value was as low as -10.94, exhibiting a noticeable bluish tint with a slight reddish hue, resulting in an unnatural color tone. However, after introducing DPT (see Examples 1-3), the thin film L... * The value remains at a high level, maintaining good brightness and transparency; at the same time, a * Significantly reduced, b * The increase indicates that the addition of DPT effectively corrected the color cast problem of pure PVA, making the film's tone more neutral and its visual appearance purer, meeting the core requirement of transparency for food packaging. Based on this, after compounding with ZIF-L, the L of the composite films in Examples 4-8... * The values ​​remained stable at 90.93~91.06, and a* and b* remained at 0.22~0.39 and -7.65~-7.87 respectively. ΔE was within a controllable range of 3.29~3.70, indicating that the film, after being compounded with DPT and ZIF-L, still maintained a uniform appearance with no obvious color difference or appearance defects, demonstrating good film uniformity and stability. Furthermore, thickness test results showed that the film thickness increased slightly compared to pure PVA film with the addition of DPT and / or ZIF-L.

[0106] In summary, compared with pure PVA film, the embodiments of adding DPT alone and combining DPT with ZIF-L successfully optimized the film color tone and improved color uniformity while maintaining high transparency and high brightness appearance, without affecting the visual display requirements of food packaging.

[0107] 3. Microstructure Analysis Figure 4 SEM images show the surface (first and second rows) and cross-sectional (third and fourth rows) microstructures of different films. The surface morphology results show that the surface of pure PVA film is smooth and intact overall, without obvious voids, cracks or protrusions. After introducing DPT and / or ZIF-L, the surfaces of each composite film still maintain a relatively flat and intact state, without significant phase separation or macroscopic defects, demonstrating good film formation and processing stability.

[0108] The cross-sectional morphology further shows that the cross-sectional shape of PVA / DPT40 changes very little compared to pure PVA, indicating that DPT has good compatibility with the PVA matrix. After further compounding ZIF-L with DPT to form the PVAD / ZIF series system, the overall cross-section still maintains a relatively dense structural characteristic, and no obvious defects such as pores, cracks, bubbles, or droplets were observed. This confirms that ZIF-L can be successfully integrated into the PVA / DPT matrix, forming a continuous and homogeneous composite system together with PVA and DPT. No obvious particle agglomeration or phase separation was observed in any of the composite films, fully demonstrating that ZIF-L and DPT can achieve uniform diffusion and dispersion in the PVA matrix, and possess excellent interfacial compatibility with the PVA matrix, thus constructing a compact and uniform microstructure for the composite films.

[0109] 4. Mechanical Performance Analysis The average test results of tensile strength and elongation at break are shown in Table 5.

[0110] Table 5

[0111] Mechanical properties are the core indicators that determine the practicality of food packaging materials. Appropriate tensile strength and fracture toughness can ensure that food is protected from external damage during transportation and storage. Tensile strength (TS) represents the maximum stress that a material can withstand during stretching, while elongation at break (EAB) reflects its flexibility. The elongation at break of the PVA / DPT composite films in Examples 1-3 is significantly higher than that of pure PVA films, indicating that their flexibility and elasticity are improved. This improvement is mainly attributed to two factors: (1) the natural eutectic solvent (DES) in DPT acts as a plasticizer, promoting molecular chain movement; (2) the hydrogen bonding between the propolis extract component and the PVA chain enhances the film's extensibility. As the DPT content increases, TS decreases, while EAB increases. This negative correlation may stem from the reconstruction of intermolecular interactions within the polymer matrix: the plasticizing effect of DPT improves the fluidity of the molecular chains, while hydrogen bonding and possible cross-linking form a denser spatial structure. This synergistic effect improves flexibility but slightly reduces tensile strength. At a DPT addition of 40%, an optimal balance between strength and flexibility is achieved.

[0112] Combined with Table 5 Figure 5The test results from A to B show that the pure PVA film (Comparative Example 1) has the highest tensile strength, but its elongation at break is very low, and its toughness is insufficient, failing to meet the requirements for packaging materials. Compared with pure PVA film, the tensile strength of all composite films with added DPT and / or ZIF-L (Examples 1 to 8) decreased, but still met the basic mechanical performance requirements for non-load-bearing food packaging (such as food storage bags and cling films). Simultaneously, the elongation at break of all composite films was significantly improved, exhibiting superior flexibility and resistance to deformation. Furthermore, the introduction of a DPT and ZIF-L compound system (PVAD / ZIF series composite films) further optimized the overall mechanical properties: an appropriate amount of ZIF-L synergistically improved the elongation at break of the composite film, and there was no significant difference in tensile strength under different ZIF-L addition amounts, indicating stable mechanical properties of the system. This performance improvement stems from multiple synergistic mechanisms: a strong hydrogen bond network is formed between the PVA matrix, DPT, and ZIF-L, enhancing interfacial interactions; simultaneously, DES in DPT, as a green plasticizer, inserts its hydroxyl groups into the PVA molecular chains, weakening interchain forces and enhancing chain segment mobility, thus achieving plasticization and toughening. The composite film exhibits a significantly increased elongation at break and a moderate decrease in tensile strength, greatly enhancing its resistance to deformation and effectively protecting packaged food from mechanical stress damage during transportation and storage.

[0113] Figure 5 Physical testing of C shows that the PVAD / ZIF0.5 composite film not only has excellent flexibility and tensile strength, but can also stably support a weight of 5kg, and can still maintain structural stability in a bent state, proving that it meets the mechanical requirements of actual food packaging.

[0114] 5. FT-IR, XRD, TG, and DTG analysis like Figure 6 As shown in Figure A, all films are in the range of 3600~3000 cm⁻¹. -1 A broad absorption band appears within the range, corresponding to the stretching vibration of the hydroxyl group (-OH) in the PVA molecule, which is a characteristic absorption peak of PVA. 2928 cm⁻¹ -1 and 1093cm -1 The absorption peaks at 1721 cm⁻¹ are attributed to the stretching vibrations of the CH and CO bonds in the PVA backbone, respectively. -1 The weak absorption band at 1575 cm⁻¹ corresponds to the C=O stretching vibration of the incompletely hydrolyzed acetate groups in PVA. After the addition of DPT and ZIF-L, the characteristic absorption peaks of PVA were retained, and no new covalent bond signals appeared. -1 The weak absorption peaks observed at 1400–1600 cm⁻¹ are attributed to the C=C stretching vibration of the aromatic ring. -1The skeletal vibration of the imidazole ring within the region was significantly enhanced, confirming that propolis polyphenols and ZIF-L were successfully loaded into the polymer network. (1424 cm⁻¹) -1 (-CH2 bending vibration), 1142cm -1 and 851cm -1 A slight shift in the peak position at the (CC stretching vibration) indicates good chemical compatibility between the imidazole ring of ZIF-L and the hydroxyl group of PVA. In summary, these results show that ZIF-L and DPT achieve efficient interfacial bonding with the PVA matrix through non-covalent interactions such as hydrogen bonding and van der Waals forces, thus enhancing the functional properties of the film while maintaining structural stability.

[0115] The crystallization characteristics before and after the addition of DPT and ZIF-L were analyzed by XRD. Figure 6 As shown in Figure B, the main diffraction peak of PVA was visible at 19.5° in all samples, confirming the semi-crystalline nature of PVA, i.e., some PVA molecular chains arranged to form ordered regions, coexisting with amorphous PVA. The characteristic peak of PVA remained constant in all composite films, indicating that the crystal structure of the matrix was not affected by the addition of ZIF-L.

[0116] The thermal stability test results of the thin film are as follows Figure 6 As shown in C and 6D, the thermal decomposition of the film mainly consists of three weight loss stages: the first stage (below 200°C) shows a relatively gentle downward trend in the TG curve, attributed to the evaporation of adsorbed water in the film; the second stage, the rapid decline in weight loss (200~450°C), corresponds to the drastic weight loss of the film, caused by the breakage of functional groups on polymer side chains, the structural degradation of the polymer backbone, and the thermal decomposition of organic linkers in the ZIF-L nanofiller; the third stage (>450°C) corresponds to the oxidation of more stable crystalline parts, hydrocarbon backbone, bio-derived compounds, and the degradation of the ZIF-L framework. The thermal decomposition temperature of the film exceeds 150°C, indicating that the materials all possess a certain degree of thermal stability. Compared with pure PVA film (thermal decomposition temperature of 291°C), the thermal decomposition temperature of the composite film is slightly higher (see...). Figure 6 The D (temperature range of 332-355℃) is improved, reflecting enhanced heat resistance. This improvement stems from the strong interfacial hydrogen bonds formed between ZIF-L, DPT, and PVA, which restrict molecular chain movement and increase the activation energy required for thermal degradation.

[0117] 6. Analysis of WCA, WVT, and DPPH free radical scavenging rates The average test results of the thin film water contact angle (WCA), water vapor transmission rate (WVT), and DPPH free radical scavenging rate are shown in Table 6.

[0118] Table 6

[0119] Combination Figure 7 As shown in Table 6, the pure PVA film exhibits strong hydrophilicity and almost no antioxidant activity (DPPH radical scavenging rate of only 3.83%). In contrast, the WCA of the PVA / DPT40 film significantly increases to approximately 69.30°, effectively reducing the surface free energy and optimizing surface wettability. Simultaneously, its radical scavenging rate is greatly enhanced to 63.06%, primarily due to the hydrogen-donating capacity of the phenolic hydroxyl groups in propolis, which efficiently neutralizes DPPH radicals, endowing the film with excellent antioxidant activity. Conversely, the addition of ZIF-L alone to the composite film PVA / ZIF0.5 reduces the water contact angle to 34.4°, possibly due to the presence of polar functional groups on the MOF surface and the capillary effect generated by its two-dimensional porous structure. Furthermore, when the DPT addition reaches 50%, the WCA of the resulting PVA / DPT50 film decreases, possibly because the composite film surface becomes rougher and more protrusions appear.

[0120] By introducing an appropriate amount of ZIF-L into the PVA / DPT system, synergistic performance enhancement and precise control were further achieved. The PVAD / ZIF0.5 composite film achieved the highest WCA in this series, indicating that the optimal ratio of ZIF-L and DPT formed a dense hydrogen bond network, effectively shielding hydrophilic groups. Simultaneously, the surface roughness introduced by the nanoparticles synergistically enhanced the anti-wetting properties with the hydrophobic components. Regarding antioxidant performance, the combination of DPT and ZIF-L exhibited a significant synergistic enhancement effect, with the free radical scavenging rate of PVAD / ZIF0.5 reaching a peak of 70.29%. This may be attributed to the high specific surface area and porous structure of ZIF-L, which protectively encapsulates polyphenols, reducing the ineffective consumption of active ingredients. Furthermore, its metal active sites can assist in the catalytic reduction of free radicals, resulting in superior antioxidant effects.

[0121] In terms of water vapor barrier properties, the water vapor transmission rate (WVT) of all composite films was higher than that of pure PVA films, and the WVT of PVAD / ZIF composite films showed a moderate upward trend with increasing ZIF-L content. This characteristic has significant practical value for food packaging: the abundant hydroxyl and other strongly polar groups in the film can effectively regulate the exchange of moisture inside and outside the packaging by forming hydrogen bonds with water molecules, avoiding excessively high or low local humidity, thereby inhibiting microbial growth, delaying food spoilage, and providing an ideal preservation environment for fresh foods that require moisture balance regulation.

[0122] In summary, the introduction of DPT solves the problems of lack of antioxidant capacity and unsuitable hydrophilicity of pure PVA, while the appropriate amount of ZIF-L further enhances the antioxidant activity and surface wetting properties of the film.

[0123] 7. Antibacterial activity and biocompatibility The average test results for antibacterial properties are shown in Table 7.

[0124] Table 7

[0125] The antimicrobial properties of food packaging films are a key indicator for preventing food spoilage, maintaining food quality, and extending shelf life. This study investigated the antimicrobial zone experiment (…). Figure 8 The antibacterial activity of each film against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was systematically evaluated using data from Tables A to C and Table 7.

[0126] The results showed that the PVA film had no antibacterial activity against either bacterium, with an inhibition zone diameter of 0. However, the antibacterial effect of the composite film was significantly improved after the introduction of DPT or ZIF-L. Specifically, the PVA / DPT40 showed inhibition zone diameters of approximately 14.87 mm and 19.50 mm against *Escherichia coli* and *Staphylococcus aureus*, respectively, demonstrating good natural antibacterial activity. Furthermore, the DPT / ZIF-L combination system (PVAD / ZIF series) exhibited a significant synergistic effect; the inhibition zone diameter of the composite film continuously increased with increasing ZIF-L content, while the inhibitory effect against *Staphylococcus aureus* steadily increased with increasing ZIF-L content. All films showed better inhibitory effects against *Staphylococcus aureus* than against *Escherichia coli*, mainly due to the difference in bacterial cell wall structure: *Escherichia coli*, as a Gram-negative bacterium, has a double-membrane structure and a lipopolysaccharide barrier, resulting in strong resistance; while the peptidoglycan layer of *Staphylococcus aureus* is relatively loose, making it easier for active substances to penetrate and destroy, thus resulting in a more significant antibacterial effect.

[0127] The excellent antibacterial properties of the PVAD / ZIF composite film stem from the synergistic effect of a dual mechanism: on the one hand, Zn released from the ZIF-L framework... 2+ Electrostatic interactions can disrupt the integrity of bacterial cell membranes, leading to leakage of intracellular substances and inhibiting their growth and reproduction. On the other hand, the flavonoids and phenolic acids in DPT can interfere with bacterial metabolic enzyme systems, further enhancing the antibacterial effect. Simultaneously, the porous structure of ZIF-L not only enables the production of Zn... 2+ Its controlled, sustained release also serves as a highly efficient carrier library for DPT, promoting uniform dispersion and sustained release of active ingredients, thereby ensuring a long-lasting and stable antibacterial effect. Against Escherichia coli, PVAD / ZIF0.5 exhibited the best synergistic antibacterial effect, confirming this addition amount as the optimal loading ratio for synergistic action.

[0128] Biocompatibility is a crucial prerequisite for the safe application of food packaging materials. The cytotoxicity of the PVAD / ZIF0.5 composite film was assessed using the CCK-8 assay and live / dead cell fluorescence staining. Table 7 and... Figure 8Results B showed that after co-culturing the membrane with L929 cells for 24 hours, the cell survival rate was as high as 96.28%, which was not significantly different from the control group, indicating that it had no obvious cytotoxicity.

[0129] Figure 8 The results of live and dead cell fluorescence staining of C further confirmed that cells in both the blank group and the PVAD / ZIF0.5 experimental group showed strong green fluorescence (survival), and no red death fluorescence was detected. The cell nucleus staining was completely superimposed with the green fluorescence, indicating that the cells had good viability and no significant damage.

[0130] In summary, the introduction of DPT endows PVA films with natural antibacterial activity, while the ZIF-L compound utilizes Zn... 2+ The dual mechanism of releasing bioactive substances achieves a synergistic enhancement of antibacterial properties, and the prepared composite film maintains excellent biocompatibility, fully meeting the safety requirements for food packaging materials.

[0131] 8. Analysis of the Preservation Effect of Film 1) Appearance, weight loss, pH To test its application effect in food packaging, oysters were packaged using PVAD / ZIF0.5 composite film, and their appearance, weight loss, pH value, volatile basic nitrogen (TVB-N) content, thiobarbituric acid reactants (TBARS), and total bacterial count (TVC) were measured.

[0132] Figure 9 A shows the differences in the appearance of oysters throughout the storage process, with all oyster samples showing a decrease in weight ( Figure 9 B). The polyethylene (PE) packaging control group maintained a high relative humidity within the packaging, thus reducing shrinkage; however, the sample surface exhibited a sticky, sensory deterioration characteristic during the later stages of storage. In contrast, the PVA film experienced accelerated physical weight loss and surface drying due to moisture dissipation. The pH value during storage showed a decreasing-then-increasing trend because the action of endogenous enzymes produced a large amount of acidic substances, leading to a decrease in pH. Subsequently, under the action of microorganisms, proteins and amino acids within the microorganisms were decomposed into substances such as ammonia, resulting in a gradual increase in pH. Figure 9 As shown in Figure C, after 2 days of storage, the pH values ​​of all oyster groups showed a slight decreasing trend. Higher pH values ​​generally indicate more severe spoilage. By the end of storage, the pH value of the PVAD / ZIF0.5 experimental group was significantly lower than that of the other packaging treatment groups.

[0133] 2) TVB-N, TBARS, TVC, Electronic Nose Analysis To systematically evaluate the actual preservation performance of PVAD / ZIF0.5 composite film, PE packaging and PVA film were used as controls. Volatile basic nitrogen (TVB-N), thiobarbituric acid reactants (TBARS), total bacterial count (TVC), and flavor and sensory characteristics of refrigerated oysters were monitored throughout the entire process. The results are as follows: Figure 10 As shown.

[0134] Volatile basic nitrogen (TVB-N) is produced by the combined action of spoilage bacteria and endogenous enzymes, and is widely considered a key indicator of the freshness and quality of aquatic products. During storage, the proteins in aquatic products are decomposed by microorganisms, producing alkaline nitrogenous substances such as ammonia and amines, thereby increasing the TVB-N content. When the TVB-N value exceeds 30 mg / 100g, it indicates product spoilage. Figure 10 A showed that the TVB-N content increased continuously after the start of the experiment. The TVB-N value of the PVA control group rose to 33.36±1.14 mg / 100g on day 8, and the TVB-N value of the PE control group rose to 40.26±1.6 mg / 100g on day 10, exceeding the critical freshness limit, while the experimental group maintained its quality. This demonstrates that the PVAD / ZIF0.5 composite film can delay protein degradation and extend the shelf life of oysters.

[0135] Lipid peroxidation is also an important factor limiting sensory properties; the changes in TBARS values ​​in each group are shown in the figure. Figure 10 B. With prolonged storage, the TBARS values ​​of all groups increased, indicating continued oxidation of unsaturated fatty acids in the samples. Starting from day 8, the TBARS value of the PVA group was higher than that of other packaging treatment groups. In contrast, the TBARS value of oyster samples packaged with PVAD / ZIF0.5 composite film increased slowly during refrigeration, indicating that this composite film treatment effectively inhibits lipid oxidation in the samples, thereby achieving a long-lasting preservation effect.

[0136] Changes in quality are closely related to the growth and reproduction of microorganisms. Figure 10 C shows the changes in TVC content in samples under different storage conditions. A TVC value ≤ 5 log CFU / g indicates freshness, a TVC value between 5 and 6 log CFU / g indicates sub-freshness, and a TVC value exceeding 6 log CFU / g is considered spoiled and inedible. At the end of storage, the TVC values ​​of the PE control group, PVA control group, and PVAD / ZIF0.5 experimental group were 6.37 log CFU / g, 6.16 log CFU / g, and 5.80 log CFU / g, respectively. Only the experimental group's TVC did not exceed the recommended limit.

[0137] An electronic nose is an electronic system that uses the response patterns of a set of gas sensors to identify odors. It can analyze the overall flavor characteristics of food. Differences in the electronic nose signal can reflect changes in flavor compounds; the type and intensity of the main sensors change with storage time. Figure 10 As shown in Figures D-F, with prolonged storage, the content of benzene, nitrogen oxides, and long-chain alkanes in the PE and PVA control groups generally increased, leading to increased sensor response values ​​for W1C, W5S, and W2W. The production of sulfides and methyl compounds in oysters gradually increased. At the end of storage, only the W1W and W1S values ​​in the PVAD / ZIF0.5 experimental group were lower than those in the two control groups, indicating that the unpleasant odors caused by the experimental group treatment were limited. This helps maintain the freshness of the product.

[0138] In summary, the PVAD / ZIF0.5 composite film, through its synergistic effects of anti-oxidation, antibacterial properties, and oxygen barrier, can effectively inhibit protein decomposition, lipid oxidation, and microbial proliferation in oysters, while also delaying the development of undesirable flavors. Its overall preservation effect is significantly better than that of traditional PE and PVA packaging, and it has high application value in the field of active packaging for fresh aquatic products.

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for preparing a polyvinyl alcohol-based composite film, characterized in that, Includes the following steps: (1) Provide extraction agent The extractant includes a eutectic solvent and water, wherein the water content in the extractant is 25% to 40% by mass, and the eutectic solvent is formed by β-cyclodextrin and glycolic acid in a molar ratio of 1:(45 to 55). (2) The propolis powder was extracted with the extractant at a temperature of 60-80°C for 40-80 min to obtain a propolis extract containing a eutectic solvent and propolis active components; (3) Mix the aqueous solution of polyvinyl alcohol with the propolis extract and the optional zeolite imidazole ester framework structural material evenly to obtain a film-forming solution; (4) The film-forming liquid is used to form a film to obtain a polyvinyl alcohol-based composite film.

2. The preparation method according to claim 1, characterized in that, In the eutectic solvent, the molar ratio of β-cyclodextrin to glycolic acid is 1:(48~52); Step (1) involves mixing β-cyclodextrin, glycolic acid, and water at a temperature of 70-85°C to form a transparent liquid, thereby obtaining the extractant.

3. The preparation method according to claim 1, characterized in that, In step (2), the amount of the extractant used is 25-35 mL relative to 1 g of the propolis powder; and / or The extraction was carried out under stirring conditions for 50-70 minutes.

4. The preparation method according to claim 1, characterized in that, The active ingredients in the propolis extract, calculated by their total flavonoid content, are 110-120 mg / g; and / or The polyvinyl alcohol has a number-average degree of polymerization of 1600-1800 and a degree of alcoholysis ≥98mol.

5. The preparation method according to claim 1, characterized in that, In step (3), the concentration of the aqueous solution of polyvinyl alcohol is 0.02~0.08 g / mL; The amount of propolis extract used is 30-50 parts by weight relative to 100 parts by weight of the polyvinyl alcohol, and the amount of zeolite imidazole ester framework structural material used is 0-1.0 parts by weight.

6. The preparation method according to claim 1, characterized in that, In step (3), the amount of propolis extract used is 35-45 parts by weight relative to 100 parts by weight of the polyvinyl alcohol, and the amount of zeolite imidazole ester skeleton structural material used is 0.3-0.7 parts by weight.

7. The preparation method according to any one of claims 1-6, characterized in that, The zeolite imidazolium ester framework material is a layered zeolite imidazolium ester framework material formed by the coordination of zinc ions and 2-methylimidazolium; and / or The Z-average particle size of the zeolite imidazole ester framework material is 90~150nm.

8. The preparation method according to any one of claims 1-6, characterized in that, Step (4) includes: pouring the film-forming liquid into a mold and drying it at 40~50°C to evaporate the water, thereby forming the polyvinyl alcohol-based composite film.

9. A polyvinyl alcohol-based composite film prepared by the preparation method according to any one of claims 1-8.

10. The application of the polyvinyl alcohol-based composite film according to claim 9 in the preparation of food preservation film or food packaging film.

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