Functional food packaging film and method for preparing the same

Abstract

The application belongs to the technical field of food packaging film, and particularly relates to a functional food packaging film and a preparation method thereof. The preparation method of the functional food packaging film is as follows: dissolving chitosan in an acetic acid solution, adding an ethanol dispersion solution of nano zinc oxide and epsilon-polylysine, and then adding glycerol to obtain a bottom layer film solution; dissolving sodium alginate in deionized water, and then adding a citric acid-sodium citrate buffer pair, an ethanol-water mixed solution of red cabbage anthocyanin, a polyphenol crosslinking agent and a cinnamyl aldehyde-beta-cyclodextrin inclusion compound in sequence to obtain a top layer film solution; casting the bottom layer film solution to obtain a bottom layer film, spraying a calcium ion solution on the surface of the bottom layer film, and then casting the top layer film solution to obtain the functional food packaging film. The functional food packaging film prepared by the application has the functions of antibiosis, antioxidation and freshness indication, realizes long-acting and sustained release of antibiosis and antioxidation activities, and realizes threshold type jump indication of food spoilage, and is suitable for industrialized production.

Description

Technical Field

[0001] This invention belongs to the field of food packaging film technology, specifically relating to functional food packaging films and their preparation methods. Background Technology

[0002] Food spoilage is a major cause of food safety problems and economic losses. Fresh meat and seafood, in particular, are susceptible to microbial contamination and oxidation during storage and transportation, leading to discoloration, flavor deterioration, and nutrient loss. In recent years, functional food packaging films have received widespread attention. Unlike traditional packaging films that only serve a barrier and seal function, functional food packaging films incorporate specific active ingredients or structural designs into the film matrix, giving the packaging itself functions such as preservation, antibacterial properties, antioxidant properties, indication, and nutritional supplementation, making them more suitable for fresh food packaging.

[0003] Among numerous natural polymer substrates, chitosan (CS) and sodium alginate (SA) have become the main materials for preparing food packaging films due to their wide availability, biodegradability, good biocompatibility, and recognition as food-grade safe materials. Chitosan molecules are rich in amino groups, possessing antibacterial activity and readily forming films, while sodium alginate molecules contain carboxyl groups, exhibiting crosslinking and gas barrier properties. The composite preparation of chitosan / sodium alginate bilayer films has become a research hotspot in active food packaging.

[0004] However, chitosan / sodium alginate bilayer membranes also have the following shortcomings: (1) The interlayer bonding mainly relies on electrostatic attraction, which makes it easy to peel off in high humidity environments and has poor mechanical stability; (2) Functional additives are mostly produced by direct blending, which has problems such as explosive release, easy inactivation during storage, and mutual interference between components; (3) Poor water resistance; (4) The indicator function lacks threshold design, and the color gradient makes it difficult for consumers to judge the critical point of spoilage; (5) The preparation process mostly adopts layer-by-layer casting, which has a long drying time, long preparation cycle, and low efficiency.

[0005] Patent CN122167832A discloses a pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate. The method involves mixing a carboxymethyl chitosan solution and a sodium alginate solution, adding anthocyanins (with indicator function) and magnolol (with antibacterial function), and then casting to form a composite monolayer film with both indicator and antibacterial functions. While this method can solve the technical problem of low interlayer bonding in bilayer films, both carboxymethyl chitosan and sodium alginate are highly hydrophilic polysaccharide polymers. The resulting film matrix has extremely high hygroscopicity, leading to very poor water resistance and water barrier properties in the composite film. Furthermore, intermolecular interactions may occur between magnolol and anthocyanins during film formation, affecting their respective functions.

[0006] Patent CN116836422A discloses a chitosan / alginic acid dialdehyde-cinnamaldehyde / chitosan-tea polyphenol three-layer composite membrane. This membrane oxidizes the hydroxyl groups of sodium alginate to aldehyde groups, which then react with the amino groups of chitosan in a Schiff base reaction, thereby increasing the interlayer bonding force. Simultaneously, the three-layer membrane structure enables the slow release of cinnamaldehyde and tea polyphenols. However, the alginate dialdehyde synthesis process used in this method is complex and costly. Furthermore, the preparation of the entire three-layer membrane requires repeated casting and drying, resulting in a very long process cycle, low production efficiency, and unsuitability for industrialization. In addition, the three-layer composite membrane prepared by this method only focuses on improving antioxidant and antibacterial properties, offering relatively limited functionality.

[0007] To address the aforementioned issues, there is an urgent need for a functional food packaging film that features strong interlayer bonding, more comprehensive functionality, rational functional zoning, controllable release kinetics of functional additives, and suitability for industrialization. This would overcome the shortcomings of existing chitosan / sodium alginate bilayer films in terms of structural stability, functional synergy, and industrial feasibility. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a functional food packaging film that simultaneously possesses antibacterial, antioxidant, and freshness indication functions. It also achieves long-term and continuous release of antibacterial and antioxidant activities, as well as threshold-based indication of food spoilage. The present invention also provides a preparation method that is simple to operate, has low production costs, and is suitable for industrial production.

[0009] The method for preparing the functional food packaging film of the present invention includes the following steps:

[0010] (1) Preparation of bottom membrane solution: Chitosan was dissolved in acetic acid solution, and ethanol dispersion of nano zinc oxide and ε-polylysine were added. After stirring evenly, glycerol was added and degassing was performed to obtain bottom membrane solution.

[0011] (2) Preparation of top membrane solution: Sodium alginate was dissolved in deionized water, and then citric acid-sodium citrate buffer pair, ethanol-water mixed solution of red cabbage anthocyanin, polyphenol crosslinking agent, and cinnamaldehyde-β-cyclodextrin inclusion complex were added in sequence. The mixture was homogenized by ultrasonication and degassed to obtain the top membrane solution.

[0012] Among them, the cinnamaldehyde-β-cyclodextrin inclusion complex is cinnamaldehyde with β-cyclodextrin encapsulated on its surface;

[0013] (3) Casting and molding: The bottom film liquid is cast into the mold and dried at 30-50℃ for 4-8 hours to obtain the bottom film; calcium ion solution is evenly sprayed on the surface of the bottom film, and after standing for 1-3 minutes, the top film liquid is cast and dried at 30-50℃ for 4-8 hours to obtain the double film.

[0014] (4) Post-processing: After the double-layer film is placed in a constant temperature and humidity environment for equilibration, the functional food packaging film is obtained.

[0015] In step (1), based on 100 parts of chitosan, the amount of nano zinc oxide added is 2-3 parts, the amount of ε-polylysine added is 0.5-1.5 parts, and the amount of glycerol added is 30-45 parts.

[0016] Preferably, the degree of deacetylation of chitosan is 80-95%, and the molecular weight is 150-300 kDa; the average particle size of nano zinc oxide is 20-30 nm; and the molecular weight of ε-polylysine is 3000-4500 Da.

[0017] In this invention, two antibacterial adjuvants, nano zinc oxide and ε-polylysine, are added simultaneously. On the one hand, this can improve the antibacterial effect. ε-polylysine is used to disrupt the integrity of the cell membrane, and zinc oxide can more easily enter the cell to generate reactive oxygen species, thereby achieving a synergistic antibacterial effect. On the other hand, it can reduce the amount of nano zinc oxide added, avoiding the increase in membrane brittleness caused by excessive addition.

[0018] In step (1), the concentration of the acetic acid solution is 1-1.5 vol.%; the mass-volume ratio of chitosan to acetic acid solution is 1 g: (40-60) mL.

[0019] In step (1), the mass-volume ratio of nano zinc oxide to ethanol in the ethanol dispersion of nano zinc oxide is 1 g: (50-150) mL.

[0020] In step (2), based on 100 parts of sodium alginate, the amount of citric acid-sodium citrate buffer added is 5-8 parts, the amount of red cabbage anthocyanin added is 1-3 parts, the amount of polyphenol crosslinking agent added is 1-3 parts, and the amount of cinnamaldehyde added to the cinnamaldehyde-β-cyclodextrin inclusion complex is 5-8 parts.

[0021] The primary function of the citric acid-sodium citrate buffer pair is to adjust the pH range of the indicator's color-changing threshold, ensuring that the food packaging film maintains a relatively stable color during the food's freshness period (pH below the color-changing threshold). When the concentration of spoilage markers exceeds the safety threshold (pH above the color-changing threshold), the film color difference changes drastically within 2 hours, achieving an upgrade from gradual color change to a sudden color change, thus avoiding consumer misjudgment of gradual color changes. Red cabbage anthocyanins, as an indicator, can rapidly respond with color change within 2 hours when volatile amines released from food spoilage diffuse into the food packaging film through gas-phase diffusion and exceed the buffer capacity. The polyphenol crosslinking agent provides both crosslinking enhancement and additional antioxidant functions. If its dosage is too low, the film crosslinking degree is insufficient, resulting in severe swelling of the top layer film; if it is too high, the film becomes over-crosslinked and brittle, and the excessively dark color interferes with the observation of anthocyanin color changes. Cinnamaldehyde in the cinnamaldehyde-β-cyclodextrin inclusion complex has both antibacterial and antioxidant effects, but it also has the disadvantages of being insoluble in water, having a strong odor, and being easily volatile. This invention encapsulates cinnamaldehyde and adds it to the casting solution, which effectively isolates oxygen and light, improves the stability of cinnamaldehyde, and achieves long-term release. At the same time, the water solubility of the cinnamaldehyde-β-cyclodextrin inclusion complex ensures that cinnamaldehyde is evenly distributed in the membrane matrix, avoiding membrane defects caused by oil phase separation.

[0022] In step (2), the pH value of the citric acid-sodium citrate buffer pair is 5.5-6.2, and the molar ratio of citric acid to sodium citrate is (1-1.5):1.

[0023] The pH value of the citric acid-sodium citrate buffer pair determines the position of the indication threshold window. The pH range of the citric acid-sodium citrate buffer pair can be adjusted according to the critical pH value for spoilage of fresh food. For example, the pH value of fresh pork products during their fresh period is approximately 5.4-5.8, and the critical pH value for spoilage is approximately 6.3-6.6. During the fresh period of fresh pork products (pH < 6.2), red cabbage anthocyanins remain locked in the purplish-red range. When spoilage markers cause the environmental pH to exceed 6.2, red cabbage anthocyanins will turn blue-green within 2 hours, achieving a clear threshold-based alarm. This indication threshold window is also applicable to fresh products with similar critical pH values ​​for spoilage.

[0024] In step (2), the volume ratio of ethanol to water in the ethanol-water mixed solution of red cabbage anthocyanins is 3:7-5:5, and the mass-volume ratio of red cabbage anthocyanins to the ethanol-water mixed solution is 1g:(50-100)mL.

[0025] In step (2), the polyphenol crosslinking agent is at least one of tannic acid and gallic acid.

[0026] In step (2), the preparation method of cinnamaldehyde-β-cyclodextrin inclusion complex is as follows: β-cyclodextrin is dissolved in deionized water to prepare a solution with a concentration of 1-2 wt.%, heated to 40-60℃, and cinnamaldehyde is added dropwise for inclusion. After inclusion, the temperature is lowered to below 4℃ for crystallization. Then the crystals are separated, washed with deionized water at a temperature not higher than 4℃, and dried under vacuum at 30-40℃ to obtain cinnamaldehyde-β-cyclodextrin inclusion complex; wherein the mass ratio of cinnamaldehyde to β-cyclodextrin is 1:(10-15).

[0027] This invention incorporates cinnamaldehyde into the top membrane to impart antibacterial properties. However, both cinnamaldehyde and red cabbage anthocyanins are active functional components, and there is a potential risk of chemical interaction between them. The aldehyde group of cinnamaldehyde can undergo a nucleophilic addition reaction with the phenolic hydroxyl group of anthocyanins, leading to premature degradation of anthocyanins. This invention prepares cinnamaldehyde as a cinnamaldehyde-β-cyclodextrin inclusion complex, utilizing the hydrophobic cavity of β-cyclodextrin to encapsulate the cinnamaldehyde molecule, physically shielding its aldehyde group and significantly reducing the probability of direct contact and reaction with anthocyanins. Simultaneously, a polyphenol crosslinking network is introduced. The β-cyclodextrin in the red cabbage anthocyanin and cinnamaldehyde-β-cyclodextrin inclusion complex can form a hydrogen bond network with the polyphenol crosslinking agent and sodium alginate, fixing them within the crosslinked segments. This improves the membrane's density, mechanical strength, and water-blocking properties. Furthermore, the hydrogen bond anchoring effectively prevents the migration and detachment of both components within the membrane, ensuring the long-term sustained release of red cabbage anthocyanins and cinnamaldehyde. The addition of the citrate-sodium citrate buffer pair can stabilize the pH of the membrane microenvironment at 5.5-6.2, which not only avoids the irreversible fading of red cabbage anthocyanins caused by the continuous acidification of cinnamaldehyde oxidation products (cinnamic acid), but also ensures that anthocyanins are within the optimal pH window for color development.

[0028] In step (3), the calcium ion solution is at least one of calcium chloride solution and calcium lactate solution; the calcium ion concentration of the calcium ion solution is 2-3 wt.%; and the spraying amount of the calcium ion solution is 10-15 mL / m².

[0029] This invention involves spraying a calcium ion solution onto the bottom membrane surface and then casting the top membrane solution. Ca²⁺ not only coordinates and crosslinks with the G units of sodium alginate but also forms ionic bridges with the amino groups of chitosan, creating a three-dimensional ionic crosslinking network at the membrane interface. This enhances the interlayer bonding strength. Furthermore, the coordination and crosslinking of Ca²⁺ with the G units of sodium alginate significantly improves the water resistance of the top membrane and the interlayer interfaces. In addition, after casting the top membrane solution, citrate ions form soluble complexes with some Ca²⁺, changing the release of Ca²⁺ from an explosive to a slow-release process. This results in a more uniform crosslinking process, avoiding membrane wrinkles and microcracks caused by localized over-crosslinking.

[0030] In step (4), the temperature of the constant temperature and humidity environment is 20-30℃, the humidity is 40-60%RH, and the equilibration time in the constant temperature and humidity environment is 12-24h. Equilibration in the constant temperature and humidity environment for a period of time can stabilize the moisture content of the food packaging film at 5-10wt%, thereby improving the dimensional stability and performance of the food packaging film.

[0031] The present invention also provides a functional food packaging film prepared by the above preparation method, comprising a bottom film and a top film, wherein the bottom film comprises chitosan, nano zinc oxide and ε-polylysine; the top film comprises sodium alginate, citrate-sodium citrate buffer pair, red cabbage anthocyanin, polyphenol crosslinking agent and cinnamaldehyde-β-cyclodextrin inclusion complex; and a calcium ion solution is sprayed between the bottom film and the top film.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] (1) In this invention, ZnO and ε-polylysine are loaded onto the bottom membrane, and red cabbage anthocyanins and cinnamaldehyde-β-cyclodextrin inclusion complex are loaded onto the top membrane. Red cabbage anthocyanins are located on the top membrane and directly contact the spoilage gases (such as NH3 and amines) released from the food, so that they can respond quickly in the early stage of spoilage. ZnO and ε-polylysine are dispersed in the bottom membrane and slowly diffused and released through the swelling of the membrane matrix. They can maintain an effective antibacterial concentration in the early stage and the development stage of spoilage. At the same time, ZnO and red cabbage anthocyanins are distributed in different membrane layers, avoiding the risk of photocatalytic degradation of red cabbage anthocyanins by ZnO. Cinnamaldehyde is dispersed in the top membrane after being included by β-cyclodextrin, which gives the top membrane antibacterial properties. After the bottom membrane swells, it can further achieve long-term sustained release of antioxidant and antibacterial functions. The partitioning and differentiated release kinetic design of multiple functional components is precisely matched with the spoilage law of food.

[0034] (2) The present invention introduces a citrate-sodium citrate buffer pair into the top layer film to lock the microenvironment inside the food packaging film in a weakly acidic range. When the food is fresh, the film color remains basically unchanged. When the alkaline volatiles produced by spoilage cause the pH inside the film to exceed the threshold, the anthocyanins undergo a significant change, and the color changes from purple-red to blue-green, achieving a clear visual judgment. At the same time, citrate ions form soluble complexes with Ca²⁺, which changes the release and cross-linking process of Ca²⁺ from an explosive type to a slow-release type, avoiding the wrinkles and microcracks on the film surface caused by local excessive cross-linking. The calcium citrate complex remaining in the film is a food-grade calcium source and has a certain nutritional fortification effect.

[0035] (3) In this invention, a polyphenol crosslinking agent is introduced into the top layer film, and calcium ions are introduced between the film layers. Ca²⁺ forms a three-dimensional crosslinking network of calcium ion bridges between the film layers. The phenolic hydroxyl groups of the polyphenol crosslinking agent and the carboxyl / hydroxyl groups of sodium alginate form a hydrogen bond network, which further enhances the compactness of the top layer film. The synergistic effect of the two can greatly improve the bonding force between the film layers and the water resistance of the top layer film. In addition, the phenolic hydroxyl groups of the polyphenol crosslinking agent and the aldehyde groups of cinnamaldehyde can form π-π stacking and hydrogen bonds, which can improve the retention rate of cinnamaldehyde in the food packaging film and prolong the slow release time of cinnamaldehyde. Moreover, it has antioxidant properties, which can further enhance the antioxidant properties of the food packaging film.

[0036] (4) The functional food packaging film prepared by the present invention has antibacterial, antioxidant and freshness indication functions, and realizes the long-term continuous release of antibacterial and antioxidant activities, as well as the threshold jump indication of food spoilage. It also enhances the interlayer bonding force, water resistance and mechanical properties of the film, and is suitable for fresh products, especially fresh pork for preservation and intelligent monitoring of freshness.

[0037] (5) The membrane matrix materials (chitosan and sodium alginate) and functional additives used in this invention are all food-grade raw materials and the addition amount complies with the relevant provisions of GB 2760-2024 "National Food Safety Standard for the Use of Food Additives" and GB 9685-2016 "Standard for the Use of Additives in Food Contact Materials and Products". The preparation process adopts conventional solution blending and casting film formation method, which does not require high temperature and high pressure or special equipment. It is simple to operate, has a short preparation cycle, and is suitable for industrial production. Detailed Implementation

[0038] The present invention will be further described below with reference to embodiments, but is not limited to the specific embodiments listed herein. Unless otherwise specified, the process methods used in the embodiments are conventional methods in the art. Unless otherwise specified, the raw materials used in the embodiments are commercially available conventional raw materials, or can be prepared using existing technologies.

[0039] Example 1

[0040] A method for preparing a functional food packaging film includes the following steps:

[0041] (1) Preparation of the bottom membrane solution:

[0042] Add 2g of chitosan (85% degree of deacetylation, 200kDa molecular weight) to 80mL of acetic acid solution (1vol.%), stir to dissolve, and obtain chitosan solution;

[0043] Take 0.06 g of nano zinc oxide with an average particle size of 20 nm, disperse it in 6 mL of anhydrous ethanol, and sonicate for 30 min to obtain an ethanol dispersion of nano zinc oxide.

[0044] The ethanol dispersion of nano zinc oxide and 0.02 g of ε-polylysine (molecular weight 4000 Da) were added to the chitosan solution and stirred for 30 min. Then, 0.8 g of glycerol was added and stirred for another 20 min. The mixture was allowed to stand to remove bubbles and the bottom film solution was obtained.

[0045] (2) Preparation of the top membrane solution:

[0046] Add 3g of sodium alginate (G / M=1.2, molecular weight 200kDa) to 150mL of deionized water, stir to dissolve, and obtain sodium alginate solution;

[0047] Add 0.06g of red cabbage anthocyanin to 3mL of ethanol-water mixture (ethanol:water = 4:6, v / v), stir to dissolve, and obtain ethanol-water mixture of red cabbage anthocyanin;

[0048] 1.8 g of β-cyclodextrin was dissolved in deionized water to prepare a solution with a concentration of 1.5 wt.%. The solution was heated to 50 °C, and 0.15 g of cinnamaldehyde was added dropwise. The mixture was stirred for 2 h to carry out inclusion. After inclusion, the solution was cooled to below 4 °C to carry out crystallization. The crystals were then separated, washed with deionized water at a temperature not higher than 4 °C, and dried under vacuum at 40 °C to obtain the cinnamaldehyde-β-cyclodextrin inclusion complex.

[0049] 0.18 g of citrate-sodium citrate buffer (molar ratio 1:1.5, pH 6.2), the above red cabbage anthocyanin ethanol-water mixed solution, and 0.06 g of tannic acid were added to sodium alginate solution and stirred for 10 min. Then, cinnamaldehyde-β-cyclodextrin inclusion complex was added, and the mixture was sonicated at 40 kHz for 15 min. After standing to remove bubbles, the top layer film solution was obtained.

[0050] (3) Casting and molding:

[0051] The bottom film solution was cast into a polytetrafluoroethylene mold and dried at 40°C for 6 hours to obtain the bottom film. A 2.5 wt.% calcium chloride solution was uniformly sprayed onto the surface of the bottom film at a spraying amount of 10 mL / m². After spraying, the film was allowed to stand for 2 minutes. The top film solution was then cast and dried at 40°C for 8 hours. The film was then peeled off to obtain the double-layer film.

[0052] (4) Post-processing:

[0053] The double-layer film is placed in a constant temperature and humidity chamber at 25℃ and 50%RH for 24 hours to equilibrate, thus obtaining the functional food packaging film.

[0054] Example 2

[0055] A method for preparing a functional food packaging film includes the following steps:

[0056] (1) Preparation of the bottom membrane solution:

[0057] Add 2g of chitosan (85% degree of deacetylation, 200kDa molecular weight) to 120mL of acetic acid solution (1.5 vol.%), stir to dissolve, and obtain chitosan solution;

[0058] Take 0.04 g of nano zinc oxide with an average particle size of 20 nm, disperse it in 6 mL of anhydrous ethanol, and sonicate for 30 min to obtain an ethanol dispersion of nano zinc oxide.

[0059] The ethanol dispersion of nano zinc oxide and 0.03 g of ε-polylysine (molecular weight 4000 Da) were added to the chitosan solution and stirred for 30 min. Then 0.9 g of glycerol was added and stirred for another 20 min. The mixture was allowed to stand to remove bubbles and the bottom film solution was obtained.

[0060] (2) Preparation of the top membrane solution:

[0061] Add 3g of sodium alginate (G / M=1.2, molecular weight 200kDa) to 150mL of deionized water, stir to dissolve, and obtain sodium alginate solution;

[0062] Add 0.09g of red cabbage anthocyanin to 9mL of ethanol-water mixture (ethanol:water = 3:7, v / v), stir to dissolve, and obtain ethanol-water mixture of red cabbage anthocyanin;

[0063] 2.4 g of β-cyclodextrin was dissolved in deionized water to prepare a solution with a concentration of 1 wt.%, heated to 40 °C, and 0.24 g of cinnamaldehyde was added dropwise. The mixture was stirred for 2 h to carry out inclusion. After inclusion, the mixture was cooled to below 4 °C to carry out crystallization. The crystals were then separated, washed with deionized water at a temperature not higher than 4 °C, and dried under vacuum at 50 °C to obtain the cinnamaldehyde-β-cyclodextrin inclusion complex.

[0064] 0.24 g of citrate-sodium citrate buffer (molar ratio 1:1.2, pH 5.8), the above red cabbage anthocyanin ethanol-water mixed solution, and 0.09 g of gallic acid were added to sodium alginate solution and stirred for 10 min. Then, cinnamaldehyde-β-cyclodextrin inclusion complex was added, and the mixture was sonicated at 40 kHz for 15 min. After standing to remove bubbles, the top layer film solution was obtained.

[0065] (3) Casting and molding:

[0066] The bottom film solution was cast into a polytetrafluoroethylene mold and dried at 30°C for 8 hours to obtain the bottom film. A 3 wt.% calcium lactate solution was uniformly sprayed onto the surface of the bottom film at a spraying amount of 15 mL / m². After spraying, the film was left to stand for 3 minutes. The top film solution was then cast and dried at 30°C for 8 hours. The film was then peeled off to obtain a double-layer film.

[0067] (4) Post-processing:

[0068] The double-layer film is placed in a constant temperature and humidity chamber at 20℃ and 40%RH for 24 hours to equilibrate, thus obtaining the functional food packaging film.

[0069] Example 3

[0070] A method for preparing a functional food packaging film includes the following steps:

[0071] (1) Preparation of the bottom membrane solution:

[0072] Add 2g of chitosan (85% degree of deacetylation, 200kDa molecular weight) to 80mL of acetic acid solution (1.0 vol.%), stir to dissolve, and obtain chitosan solution;

[0073] Take 0.06 g of nano zinc oxide with an average particle size of 20 nm, disperse it in 3 mL of anhydrous ethanol, and sonicate for 30 min to obtain an ethanol dispersion of nano zinc oxide.

[0074] The ethanol dispersion of nano zinc oxide and 0.01 g of ε-polylysine (molecular weight 4000 Da) were added to the chitosan solution and stirred for 30 min. Then 0.6 g of glycerol was added and stirred for another 20 min. The mixture was allowed to stand to remove bubbles and the bottom film solution was obtained.

[0075] (2) Preparation of the top membrane solution:

[0076] Add 3g of sodium alginate (G / M=1.2, molecular weight 200kDa) to 150mL of deionized water, stir to dissolve, and obtain sodium alginate solution;

[0077] Add 0.03g of red cabbage anthocyanin to 2mL of ethanol-water mixture (ethanol:water = 5:5, v / v), stir to dissolve, and obtain ethanol-water mixture of red cabbage anthocyanin;

[0078] 2.7 g of β-cyclodextrin was dissolved in deionized water to prepare a solution with a concentration of 2 wt.%, heated to 60 °C, and 0.18 g of cinnamaldehyde was added dropwise. The mixture was stirred for 2 h to carry out inclusion. After inclusion, the mixture was cooled to below 4 °C to carry out crystallization. The crystals were then separated, washed with deionized water at a temperature not higher than 4 °C, and dried under vacuum at 60 °C to obtain the cinnamaldehyde-β-cyclodextrin inclusion complex.

[0079] 0.15 g of citric acid-sodium citrate buffer (molar ratio 1:1, pH 5.5), the above red cabbage anthocyanin ethanol-water mixed solution, and 0.03 g of tannic acid were added to sodium alginate solution and stirred for 10 min. Then, cinnamaldehyde-β-cyclodextrin inclusion complex was added, and the mixture was sonicated at 40 kHz for 15 min. After standing to remove bubbles, the top layer film solution was obtained.

[0080] (3) Casting and molding:

[0081] The bottom film solution was cast into a polytetrafluoroethylene mold and dried at 50°C for 4 hours to obtain the bottom film. A 2 wt.% calcium chloride solution was uniformly sprayed onto the surface of the bottom film at a spraying amount of 12 mL / m². After spraying, the film was allowed to stand for 1 minute. The top film solution was then cast and dried at 50°C for 4 hours. The film was then peeled off to obtain a double-layer film.

[0082] (4) Post-processing:

[0083] The double-layer film is placed in a constant temperature and humidity chamber at 30℃ and 60%RH for 12 hours to equilibrate, thus obtaining the functional food packaging film.

[0084] Comparative Example 1

[0085] The only difference between this comparative example and Example 1 is that ε-polylysine is not added when preparing the bottom membrane solution; the other components and steps are exactly the same.

[0086] Comparative Example 2

[0087] The only difference between this comparative example and Example 1 is that the citrate-sodium citrate buffer pair is not added when preparing the top layer film solution; the other components and steps are exactly the same.

[0088] Comparative Example 3

[0089] The only difference between this comparative example and Example 1 is that tannic acid is not added when preparing the top film solution; the other components and steps are exactly the same.

[0090] Comparative Example 4

[0091] The only difference between this comparative example and Example 1 is that cinnamaldehyde-β-cyclodextrin inclusion complex was not added when preparing the top layer film solution; the other components and steps are exactly the same.

[0092] Comparative Example 5

[0093] The only difference between this comparative example and Example 1 is that, in preparing the top film solution, the cinnamaldehyde-β-cyclodextrin inclusion complex was replaced with 0.15g of cinnamaldehyde; the other components and steps are exactly the same.

[0094] Comparative Example 6

[0095] The only difference between this comparative example and Example 1 is that, during casting, calcium chloride solution is not sprayed onto the surface of the bottom film, and the top film liquid is cast directly; the other components and steps are exactly the same.

[0096] The functional food packaging films (bottom film thickness approximately 0.09 mm, top film thickness approximately 0.06 mm) prepared in each embodiment and comparative example were subjected to performance tests, and the test methods are as follows:

[0097] (1) Antibacterial performance test:

[0098] The inhibition rate of Escherichia coli and Staphylococcus aureus was determined according to the standard GB / T 21510-2008. Food packaging film samples (2cm×2cm) were attached to agar plates coated with bacterial solution and incubated at 37℃ for 24h before measuring the diameter of the inhibition zone.

[0099] (2) Mechanical property testing:

[0100] Tensile strength and elongation at break were determined according to standard GB / T 1040.3-2006 (dumbbell-shaped specimen, tensile rate 50 mm / min).

[0101] (3) Interlayer adhesion test:

[0102] The interlayer peel strength was determined according to standard GB / T 8808-1988 (15 mm wide strip specimen, tensile rate 50 mm / min).

[0103] (4) Water-blocking performance test:

[0104] The water vapor transmission rate was determined according to the standard GB / T 1037-2021. The food packaging film was sealed in a moisture-permeable cup containing 5 mL of deionized water and weighed continuously for 7 days at 25℃ and 90%RH. The mass loss rate during the stable period was calculated.

[0105] (5) Red cabbage anthocyanin retention rate test:

[0106] A food packaging film sample (2cm×2cm) was immersed in 4wt.% acetic acid simulant and placed at 4℃ for 10 days. The absorbance of the immersion solution was measured at 520nm, and the anthocyanin retention rate of red cabbage was calculated.

[0107] The results are shown in Table 1.

[0108] Table 1 Performance test results of each embodiment and comparative example

[0109]

[0110] As can be seen from Table 1, the food packaging films prepared in Examples 1-3 have good antibacterial properties, mechanical properties, interlayer bonding strength, water resistance and red cabbage anthocyanin retention rate.

[0111] Compared with Example 1, Comparative Example 1 did not add ε-polylysine, and the diameter of its inhibition zone against Escherichia coli and Staphylococcus aureus was smaller, indicating that ε-polylysine and ZnO have a synergistic antibacterial effect.

[0112] Comparative Example 2, without the addition of the citric acid-sodium citrate buffer pair, showed a slight decrease in the mechanical properties, water resistance, and interlayer bonding of the food packaging film. This may be because the lack of citrate ions to form soluble complexes with Ca²⁺ led to explosive cross-linking of Ca²⁺, resulting in excessive local cross-linking on the film surface and the formation of an interfacial brittle layer.

[0113] Comparative Example 3, without the addition of tannic acid, showed a significant decrease in tensile strength, an increase in water vapor permeability, a decrease in interlayer peel strength, and a red cabbage anthocyanin retention rate of only 41.2%. This indicates that the cross-linked network not only enhances the mechanical properties, water resistance, and interlayer bonding of the food packaging film, but also anchors red cabbage anthocyanins.

[0114] Comparative Example 4, without the addition of cinnamaldehyde-β-cyclodextrin inclusion complex, showed a significantly reduced inhibition zone against Escherichia coli and Staphylococcus aureus, indicating that cinnamaldehyde plays a key role in the antibacterial properties of the top film. At the same time, the tensile strength of the food packaging film decreased slightly, and the water vapor permeability increased slightly, indicating that β-cyclodextrin in the cinnamaldehyde-β-cyclodextrin inclusion complex has a certain effect on improving the mechanical properties and water resistance of the film.

[0115] In Comparative Example 5, replacing the cinnamaldehyde-β-cyclodextrin inclusion complex with free cinnamaldehyde reduced the anthocyanin retention rate of red cabbage to 52.4%, indicating that the inclusion complex effectively reduced the competitive effect of cinnamaldehyde on red cabbage anthocyanins and prolonged the sustained-release period of red cabbage anthocyanins. The inhibition zone of the food packaging film against Escherichia coli and Staphylococcus aureus was increased. This is because the lack of β-cyclodextrin inclusion resulted in a faster initial release rate of cinnamaldehyde, which enhanced the antibacterial activity within 24 hours, but was not conducive to the long-term effectiveness of the antibacterial effect. In addition, the mechanical properties and water resistance of the food packaging film were lower than those of Comparative Example 4. This may be because the free cinnamaldehyde was poorly dispersed in the aqueous phase, which negatively affected the film's density and cross-linking strength.

[0116] Comparative Example 6, without the application of calcium ion solution, exhibited an interlayer peel strength of only 1.0 N / cm and a water vapor transmission rate as high as 521 g / (m²·d), indicating that Ca²⁺ ion bridging plays a crucial role in improving interlayer adhesion. Simultaneously, the antibacterial properties of the food packaging film were slightly enhanced. This is because the lack of Ca²⁺ ion bridging accelerated the swelling of the food packaging film and slightly increased the initial release rate of the antibacterial agent. Furthermore, the mechanical properties of the food packaging film were significantly reduced under the condition of lacking Ca²⁺ ion bridging.

[0117] The functional food packaging films (bottom film thickness approximately 0.09 mm, top film thickness approximately 0.06 mm) prepared in Example 1 and Comparative Example 2 were tested for freshness indication performance using the following methods:

[0118] (1) pH response color change test: The food packaging film sample (2cm×2cm) was attached to a sealed container containing 50mL of buffer solution with different pH values ​​(pH 4.0, 5.0, 6.0, 6.5, 7.0, 8.0). After being placed at 25℃ for 2h, the film color ΔE was measured using a colorimeter (D65 light source, 10° viewing angle) (based on the film color at pH 4.0). The results are shown in Table 2.

[0119] (2) Gas phase diffusion test: The food packaging film was suspended above a sealed container containing 20 mmol / L putrescine solution without contacting the liquid. ΔE was measured every 30 minutes at 25°C for 4 hours to simulate the response of volatile amines during meat spoilage. The results are shown in Table 3.

[0120] Table 2. pH response color change test results of Example 1 and Comparative Example 2

[0121]

[0122] Table 3. Gas phase diffusion test results of Example 1 and Comparative Example 2

[0123]

[0124] As shown in Tables 2 and 3, the food packaging film of Example 1, due to the presence of a citric acid-sodium citrate buffer pair, has its internal microenvironment locked within a weakly acidic window. At pH ≤ 6.0, ΔE ≤ 2.5 (no significant color change). When pH > 6.5, ΔE rapidly jumps to over 9.1, exhibiting a threshold-like color change characteristic, precisely aligning with the TVB-N critical value for pork spoilage (pH ≈ 6.5). In contrast, the food packaging film of Comparative Example 2, lacking a citric acid-sodium citrate buffer pair, already shows ΔE of 8.9 at pH 6.0, reaching 8.6 within 60 minutes under putrescine vapor stimulation. This demonstrates a linear, gradual change without a threshold, failing to provide a clear alarm signal at the spoilage critical point.

[0125] The functional food packaging film prepared in Example 1 (bottom film thickness approximately 0.09 mm, top film thickness approximately 0.06 mm) was subjected to actual preservation verification using the following method:

[0126] Fresh chilled pork (100g / serving) was packaged separately with PE preservation film and the food packaging film of Example 1 of this invention, and stored at 4℃. Samples were taken on days 0, 3, 6, and 9 to test volatile basic nitrogen (TVB-N value) (GB 5009.228-2016), total bacterial count (GB4789.2-2022), and thiobarbituric acid reactants (TBARS value, expressed as malondialdehyde (MDA) equivalent) to comprehensively evaluate the shelf-life extension effect. The results are shown in Table 4.

[0127] Table 4 Actual preservation test results of Example 1

[0128]

[0129] As shown in Table 4, the TVB-N value of the food packaging film in Example 1 was 17.8 mg / 100g on the 9th day, which still met the requirements of GB2707-2016 for Grade II freshness (≤20~25 mg / 100g), while the PE preservation film group exceeded the standard (22.4 mg / 100g) on ​​the 6th day.

[0130] In Example 1, the total bacterial count of the food packaging film was 6.2 log CFU / g on day 9, which was far below the inedible threshold (7 log CFU / g), while the PE preservation film group reached 7.9 log CFU / g on day 6.

[0131] In Example 1, the TBARS value of the food packaging film was 0.68 mg MDA / kg on day 9, indicating that fat oxidation was effectively inhibited, while the PE preservation film group had reached 1.15 mg MDA / kg, indicating obvious spoilage.

[0132] The test results above show that the food packaging film of the present invention can extend the shelf life of chilled pork from about 5-6 days to more than 9 days, and has a long-lasting antibacterial and antioxidant effect.

[0133] In addition, during the testing process, the color change of the food packaging film of Example 1 was observed. On the 6th day, when TVB-N=14.1, the film color showed a clear change from purple-red to gray-green, giving a spoilage warning about 2-3 days earlier than the TVB-N exceeding the standard, thus realizing the synergy of the indication function and the preservation function.

Claims

1. A method for preparing a functional food packaging film, characterized in that: Includes the following steps: (1) Preparation of bottom membrane solution: Chitosan was dissolved in acetic acid solution, and ethanol dispersion of nano zinc oxide and ε-polylysine were added. After stirring evenly, glycerol was added and degassing was performed to obtain bottom membrane solution. (2) Preparation of top membrane solution: Sodium alginate was dissolved in deionized water, and then citric acid-sodium citrate buffer pair, ethanol-water mixed solution of red cabbage anthocyanin, polyphenol crosslinking agent, and cinnamaldehyde-β-cyclodextrin inclusion complex were added in sequence. The mixture was homogenized by ultrasonication and degassed to obtain the top membrane solution. Among them, the cinnamaldehyde-β-cyclodextrin inclusion complex is cinnamaldehyde with β-cyclodextrin encapsulated on its surface; (3) Casting and molding: The bottom film liquid is cast into the mold and dried at 30-50℃ for 4-8 hours to obtain the bottom film; calcium ion solution is evenly sprayed on the surface of the bottom film, and after standing for 1-3 minutes, the top film liquid is cast and dried at 30-50℃ for 4-8 hours to obtain the double film. (4) Post-processing: After the double-layer film is placed in a constant temperature and humidity environment for equilibration, the functional food packaging film is obtained.

2. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (1), based on 100 parts of chitosan, the amount of nano zinc oxide added is 2-3 parts, the amount of ε-polylysine added is 0.5-1.5 parts, and the amount of glycerol added is 30-45 parts.

3. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (1), the concentration of the acetic acid solution is 1-1.5 vol.%; the mass-volume ratio of chitosan to acetic acid solution is 1 g: (40-60) mL; In the ethanol dispersion of nano zinc oxide, the mass-to-volume ratio of nano zinc oxide to ethanol is 1 g: (50-150) mL.

4. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (2), based on 100 parts of sodium alginate, the amount of citric acid-sodium citrate buffer added is 5-8 parts, the amount of red cabbage anthocyanin added is 1-3 parts, the amount of polyphenol crosslinking agent added is 1-3 parts, and the amount of cinnamaldehyde added to the cinnamaldehyde-β-cyclodextrin inclusion complex is 5-8 parts.

5. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (2), the pH value of the citric acid-sodium citrate buffer pair is 5.5-6.2, the molar ratio of citric acid to sodium citrate is (1-1.5):1, and the polyphenol crosslinking agent is at least one of tannic acid and gallic acid.

6. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (2), the volume ratio of ethanol to water in the ethanol-water mixed solution of red cabbage anthocyanins is 3:7-5:5, and the mass-volume ratio of red cabbage anthocyanins to the ethanol-water mixed solution is 1g:(50-100)mL.

7. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (2), the preparation method of cinnamaldehyde-β-cyclodextrin inclusion complex is as follows: β-cyclodextrin is dissolved in deionized water to prepare a solution with a concentration of 1-2 wt.%, heated to 40-60℃, and cinnamaldehyde is added dropwise for inclusion. After inclusion, the temperature is lowered to below 4℃ for crystallization. Then the crystals are separated, washed with deionized water at a temperature not higher than 4℃, and dried under vacuum at 30-40℃ to obtain cinnamaldehyde-β-cyclodextrin inclusion complex; wherein the mass ratio of cinnamaldehyde to β-cyclodextrin is 1:(10-15).

8. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (3), the calcium ion solution is at least one of calcium chloride solution and calcium lactate solution; the calcium ion concentration of the calcium ion solution is 2-3 wt.%; and the spraying amount of the calcium ion solution is 10-15 mL / m².

9. The method for preparing the functional food packaging film according to claim 1, characterized in that: In step (4), the temperature of the constant temperature and humidity environment is 20-30℃, the humidity is 40-60%RH, and the equilibration time in the constant temperature and humidity environment is 12-24h.

10. A functional food packaging film, characterized in that: It is prepared by the method of preparing the functional food packaging film according to any one of claims 1-9.

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