Antibacterial material of chitosan-coated modified paper-based sushi meal box and preparation method thereof

Abstract

This invention belongs to the technical field of functional food packaging materials, specifically relating to a chitosan-coated modified paper-based antibacterial material for sushi lunch boxes and its preparation method. The antibacterial material comprises a food-grade white cardboard or sugarcane bagasse pulp molded paper base and an antibacterial coating thereon. The coating is formed by hot air curing of a coating liquid, which includes chitosan, ε-polylysine, tea polyphenols, beeswax microemulsion, and citric acid. All components are food-grade substances listed in GB 2760-2024. Through the synergistic design of a ternary food-grade antibacterial network of chitosan, ε-polylysine, and tea polyphenols, combined with the bifunctional coupling of citric acid amidation crosslinking and hydrophobic modification of beeswax microemulsion, a single process of scraping or roller coating combined with hot air curing at 100°C~120°C for 5 min~10 min simultaneously achieves a water contact angle greater than 100°, a Kit oil resistance grade of 12, and an antibacterial rate greater than 99% against both Escherichia coli and Staphylococcus aureus.

Description

Technical Field

[0001] This invention belongs to the technical field of functional food packaging materials, specifically relating to chitosan-coated modified paper-based sushi lunchbox antibacterial material and its preparation method. Background Technology

[0002] Sushi, a typical ready-to-eat fresh food, is composed of raw fish slices, rice, seaweed, and other ingredients rich in protein and carbohydrates. Even under refrigeration at 4°C, it remains susceptible to contamination by pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes. Furthermore, the oxidation and discoloration of fish fat and the formation of unsaturated aldehydes and ketones, which produce off-flavors, further shorten the product's shelf life. The typical shelf life of commercially available sushi is generally less than 24 hours. This short shelf life severely restricts the logistics radius and inventory turnover efficiency of sushi in convenience stores, ready-to-eat delivery services, and fresh food e-commerce channels. Extending the shelf life of sushi while ensuring food safety has long been a technological challenge in this field.

[0003] Current sushi containers are primarily made of petroleum-based plastics such as polypropylene, polystyrene, and polyethylene terephthalate. While these materials offer good water and oil resistance, they also present significant problems such as white pollution, non-biodegradability, and monomer migration. Under the national dual-carbon strategy and plastic restriction order, replacing plastic containers with paper-based materials such as food-grade white cardboard and sugarcane bagasse molded paper boxes has become an industry transformation trend. However, paper-based materials have two inherent defects: firstly, paper-based materials, with cellulose as their main component, are highly hydrophilic, rapidly absorbing moisture and softening when in direct contact with water- or oil-containing sushi ingredients, significantly reducing their mechanical strength; secondly, paper-based materials lack active antibacterial functions, failing to inhibit the proliferation of pathogenic bacteria within the container, and instead becoming a carrier for microbial attachment and reproduction due to their porous structure. The core technological bottleneck in replacing plastic containers with paper-based sushi containers lies in how to construct a functional coating on the paper-based surface that combines five functions—water resistance, oil resistance, antibacterial properties, antioxidant properties, and slow-release properties—while meeting food contact compliance requirements.

[0004] Several technical attempts have been made in this field to address the aforementioned technical problems, but all have significant shortcomings. Chinese patent application CN114561827B, entitled "A Fully Biomass-Based Wax Emulsion Waterproof Coating and Its Preparation Method," discloses a technical solution of spraying a wax emulsion onto the surface of a molded pulp plate and then heat-treating it at 75°C~100°C for 15~120 minutes to obtain a waterproof plate. While the coating exhibits good hydrophobicity, it completely lacks antibacterial properties, and the excessively long heat treatment time is detrimental to industrial production efficiency. Chinese patent application CN116159188A, entitled "Chitosan-Citrate / Polyvinyl Alcohol Dual-Network Hydrogel Scaffold and its Preparation Method and Uses", discloses the preparation of a dual-network hydrogel scaffold using polyvinyl alcohol, chitosan and citric acid as raw materials through a three-stage temperature program of 80°C / 100°C / 140°C for cartilage repair. Although it involves the cross-linking reaction of citric acid and chitosan, the application field is biomedical scaffolds rather than food packaging coatings. Moreover, its process temperature is too high and the time is too long, making it completely unsuitable for paper-based short-time curing processes.

[0005] In the research of antibacterial paper, Tankhiwale et al. disclosed a method for preparing antibacterial paper by grafting cerium ammonium nitrate and acrylamide onto cellulose filter paper and then loading it with nano-silver ions. Yang Fei et al. disclosed a method for preparing antibacterial paper with a silver loading of 5.38% by ion exchange. Shankar et al. reported in 2021 a cast film for strawberry preservation using a composite of chitosan, essential oil, and silver nanoparticles. Kim and Lee et al. disclosed a method for preparing antibacterial paper by coating paper with Nisin and chitosan in combination with vinyl acetate polymer. The core antibacterial components of the above schemes either rely on metal antibacterial agents such as nano-silver or silver ions, or use a single antibacterial peptide or essential oil as the active ingredient, and have three common shortcomings. First, the status of nano-silver in the positive list of coatings and coatings for food contact materials and products in GB 4806.10-2025 is unclear, and there is a lack of long-term safety data on the migration behavior of silver ions in food simulants, which poses a continuous compliance risk for commercial applications. Second, a single antibacterial component cannot simultaneously cover the broad-spectrum antibacterial needs of both Gram-positive and Gram-negative bacteria. For example, Nisin is highly effective against Gram-positive bacteria but has limited effectiveness against Gram-negative bacteria such as Escherichia coli. Third, none of the above solutions simultaneously address the five performance requirements of antibacterial, antioxidant, water-resistant, oil-resistant, and sustained-release properties. They often require multiple layers of coating, resulting in complex processes, high costs, and a high risk of interfacial peeling.

[0006] In the field of ternary compound preservatives, the 22nd issue of "Food Industry" in 2020 reviewed and reported on the application of various ε-polylysine-based compound preservative technologies in the preservation of aquatic products, such as compound preservatives of nisin and ε-polylysine hydrochloride, compound preservatives of ε-polylysine, rosmarinic acid and chitosan, and edible coatings of dragon fruit peel extract, gelatin and ε-polylysine. The results showed that the ternary food-grade antibacterial components have significant effects in synergistic antibacterial activity. However, all the technical solutions described in this review are liquid preservatives that are directly sprayed or impregnated on the surface of aquatic products. From the perspective of chemical composition formulation, they fall under the category of direct use of food additives and are subject to the GB 2760-2024 standard for the use of food additives in food safety supervision. In contrast, this invention addresses the morphology of coatings on the surface of paper-based packaging materials, which falls under the GB4806.10-2025 standard for coatings and coatings for food contact materials and products in food safety supervision. The two types of technical solutions are fundamentally different in terms of morphology, mass transfer mechanism, contact relationship with food, and regulatory affiliation. Liquid preservative technology cannot be directly transferred to solid coating applications. A series of new technical issues need to be addressed, such as component compatibility reconstruction, curing reaction control, reshaping of the sustained-release behavior of active components, and migration compliance verification.

[0007] In summary, there is an urgent need in this field for a novel antibacterial coating material and its preparation method that uses food-grade natural components as the antibacterial basis, completely avoids the compliance risks of metal antibacterial agents, can simultaneously achieve five functions in a single coating: broad-spectrum antibacterial, antioxidant, water-resistant, oil-resistant, and slow-release, and is suitable for short-term industrial coating processes for paper-based sushi lunch boxes. This would fundamentally solve the three major industry pain points of paper-based sushi lunch boxes: short shelf life, high compliance risks, and complex multi-layer coating manufacturing processes. Summary of the Invention

[0008] To address the technical problems of existing technologies, such as the lack of antibacterial function in paper-based sushi containers, insufficient water and oil resistance, high compliance risks of active components, and the difficulty in simultaneously achieving five functions in a single-layer coating, the present invention aims to provide a chitosan-coated modified antibacterial material for paper-based sushi containers and its preparation method. Through the synergistic design of three food-grade antibacterial components—chitosan, ε-polylysine, and tea polyphenols—combined with the bifunctional coupling of citric acid amidation crosslinking and hydrophobic modification of beeswax microemulsion, a comprehensive performance is simultaneously achieved in a single-coating process using scraping or roller coating combined with short-time hot air curing. This results in a broad-spectrum antibacterial rate greater than 99%, a water contact angle greater than 100°, a Kit oil resistance grade of 12, a DPPH free radical scavenging rate greater than 85%, and an extension of the shelf life of refrigerated sushi at 4°C from 24 h to 48 h~72 h. Furthermore, all components of the coating comply with the GB 2760-2024 standard for the use of food additives and the GB 4806.10-2025 compliance requirements for food contact coatings.

[0009] To achieve the above-mentioned objectives, the present invention adopts the following technical solution. The present invention provides a chitosan-coated modified paper-based sushi container antibacterial material, wherein the antibacterial material comprises a paper-based substrate and an antibacterial coating applied to the paper-based substrate; the paper-based substrate is food-grade white cardboard or sugarcane bagasse pulp molded paper box with a basis weight of 250 g / m³. 2 ~350 g / m 2 The antibacterial coating is formed by hot air curing of a coating solution. The coating solution includes a film-forming matrix, a polycationic antimicrobial peptide, a polyphenolic antioxidant and antibacterial agent, a hydrophobic modifying component, and a crosslinking agent. The film-forming matrix is ​​chitosan with a degree of deacetylation greater than 90% and a molecular weight of 100,000 Da to 300,000 Da, dissolved in a 1% to 2% (v / v) aqueous acetic acid solution. The mass fraction of chitosan in the coating solution is 1.5% to 3%. The polycationic antimicrobial peptide is ε-polylysine, with a mass fraction of 0.2% to 0.6% in the coating solution. The polyphenolic antioxidant and antibacterial agent is tea polyphenol, with a mass fraction of 0.5% to 1.5% in the coating solution. The hydrophobic modifying component is a beeswax microemulsion with a solid content of 20% to 30% and a particle size of 100 nm to 300 nm. nm, wherein the beeswax in the coating solution has a dry basis mass fraction of 10%~20%; the crosslinking agent is citric acid, and the mass ratio of citric acid to chitosan is 1:10~1:5; the dry coating weight of the antibacterial coating is 10 g / m 2 ~20 g / m 2 The hot air curing temperature is 100°C~120°C and the time is 5 min~10 min.

[0010] This invention also provides a method for preparing the above-mentioned antibacterial material, comprising the following steps: S1 dissolving the chitosan in the aqueous acetic acid solution to prepare a chitosan solution; S2 preparing the ε-polylysine, tea polyphenols, beeswax microemulsion, and citric acid into aqueous solutions or dispersions respectively; S3 mixing the chitosan solution from S1 with the component solutions or dispersions obtained in S2 according to the mass ratio, and stirring at 40°C~50°C for 1 h~2 h to obtain a uniform coating liquid; S4 coating the coating liquid onto the surface of the paper-based substrate by scraping or roller coating, controlling the dry coating amount to be 10 g / m². 2 ~20 g / m 2 S5 involves hot air drying and curing the coated paper substrate at 100°C~120°C for 5 min~10 min to complete the amidation crosslinking of citric acid and chitosan and the coating film formation, thereby obtaining the target antibacterial material.

[0011] Compared with existing technologies, the beneficial effects of this invention are as follows. First, this invention uses only food-grade natural components listed in the GB 2760-2024 standard for the use of food additives as antibacterial active substances, completely avoiding the compliance risks and migration safety disputes of metal antibacterial agents such as nano-silver in food contact scenarios. All components meet the migration limit requirements of GB 9685 under the migration test conditions of GB 31604.1 food simulants. Second, the ternary food-grade antibacterial network of chitosan, ε-polylysine, and tea polyphenols achieves non-additive synergistic antibacterial activity through a four-path parallel attack: polycation electrostatic adsorption, antibacterial peptide membrane perforation, polyphenol reactive oxygen species oxidation, and polyphenol-metal ion chelation. The antibacterial rate against Escherichia coli and Staphylococcus aureus is greater than 99%, significantly better than any binary combination. The introduction of tea polyphenols simultaneously endows the coating with an antioxidant capacity of DPPH free radical scavenging rate greater than 85%, effectively inhibiting lipid oxidation and discoloration of sushi fish fillets. Third, the synergistic design of beeswax microemulsion and citric acid amidation crosslinking simultaneously achieves dual functions of water resistance (water contact angle greater than 100°) and oil resistance (Kit grade 12) in a single coating. Furthermore, the coating maintains an antibacterial rate of over 95% after 30 days at 25°C and 90% relative humidity, demonstrating excellent slow-release stability. Fourth, the process of blade coating or roller coating combined with 100°C~120°C hot air curing for 5~10 minutes is compatible with existing paper printing and coating production lines, requiring no additional equipment modifications and exhibiting high industrial adaptability. Fifth, considering the aforementioned synergistic effects, this invention extends the shelf life of fresh sushi from the existing 24 hours to 48~72 hours under 4°C refrigeration conditions, a 2 to 3-fold increase, significantly expanding the logistics radius and channel adaptability of sushi products. Attached Figure Description

[0012] Figure 1 This is a scanning electron microscope image of the cross-section of the antibacterial coating obtained in Example 1 of the present invention.

[0013] Figure 2 This is a high-magnification scanning electron microscope image of the spherical microdomain distribution of beeswax microemulsion within the antibacterial coating obtained in Example 1 of the present invention.

[0014] Figure 3 This is a bar chart comparing the water contact angle and Kit oil resistance rating of Examples 1-3 and Comparative Examples 1-5 of the present invention.

[0015] Figure 4 This is a bar chart comparing the antibacterial rates of Examples 1-3 and Comparative Examples 1-5 against Escherichia coli and Staphylococcus aureus.

[0016] Figure 5 This is a line graph comparing the shelf-life extension effect of fresh sushi under 4°C refrigeration conditions between Example 1 and Comparative Examples 1-5 of the present invention. Detailed Implementation

[0017] The technical solution and beneficial effects of the present invention will be further explained below with reference to embodiments and comparative examples. The ε-polylysine used in the following examples is food-grade ε-polylysine hydrochloride (CAS 25104-18-1, average degree of polymerization 25~35, purity greater than or equal to 95%, conforming to the food additive specifications listed in GB 2760-2024, with a maximum usage of 0.15 g / kg in baked goods and 0.25 g / kg in cooked meat products); the tea polyphenols are food-grade tea polyphenols (total catechin content greater than or equal to 80%, EGCG content greater than or equal to 50%, conforming to the food additive specifications listed in the GB 2760-2024 announcement on expanded use, with a maximum usage of 0.4 g / kg in oily foods); the chitosan is food-grade chitosan; the beeswax microemulsion is self-made using polyglycerol fatty acid ester (E475) and lecithin (E322) as emulsifying stabilizers; citric acid, glacial acetic acid, and polyvinylpyrrolidone are all food-grade and purchased from Aladdin Reagent (Shanghai) Co., Ltd. The food-grade white cardboard used has a basis weight of 250 g / m². 2 ~350 g / m 2 The raw materials were purchased from APP (Asia Pulp & Paper) Co., Ltd.; the bagasse pulp molded paper boxes were custom-made and supplied by Guangxi Fengtang Biochemical Co., Ltd. The bacterial strains used were Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 6538, purchased from the China General Microbiological Culture Collection Center.

[0018] Example 1

[0019] This embodiment uses an intermediate parameter combination to prepare a chitosan-coated modified paper-based sushi container antibacterial material. The specific steps are as follows: Step 1, Substrate preparation. A substrate with a basis weight of 300 g / m³ is selected. 2Food-grade white cardboard was cut into standard samples 30 cm long and 20 cm wide. These samples were pre-dried in a 40°C oven for 30 min to remove adsorbed moisture, and their initial weight was measured for later use. The second step involved preparing the chitosan solution. 2.25 g of chitosan with a degree of deacetylation greater than 90% and a molecular weight of 200,000 Da was weighed and added to 97.75 g of a 1.5% (v / v) acetic acid aqueous solution. The solution was stirred at 40°C with magnetic stirring for 2 h until the chitosan was completely dissolved, yielding 100 g of a 2.25% (w / v) chitosan solution. The solution pH was 4.2–4.5, and the transmittance at 550 nm was greater than 95%. The third step involved preparing an ε-polylysine aqueous solution. 0.4 g of ε-polylysine hydrochloride was weighed and added to 9.6 g of deionized water. The solution was stirred at room temperature for 10 min until completely dissolved, yielding 10 g of a 4% (w / v) ε-polylysine stock solution.

[0020] Step 4: Preparation of tea polyphenol aqueous solution. Weigh 1.0 g of tea polyphenols and add them to 9.0 g of preheated deionized water at 50°C. Stir for 15 min until completely dissolved to obtain 10 g of a 10% (w / w) tea polyphenol stock solution. The solution is a pale yellow to yellowish-brown clear liquid. Step 5: Preparation of beeswax microemulsion. Weigh 15 g of yellow beeswax and melt it in an 85°C water bath. Add 1.5 g of polyglycerol fatty acid ester and 0.5 g of lecithin as emulsifying stabilizers. Stir at high speed at 85°C until the emulsifiers are completely dissolved. Separately, take 83 g of 80°C deionized water and slowly pour it into the molten wax phase while stirring. Homogenize three times using a high-pressure homogenizer at 60 MPa pressure to obtain 100 g of a stable beeswax microemulsion with a solid content of 25% and an average particle size of 200 nm (polydispersity index less than 0.2 as determined by dynamic light scattering). Step 6: Preparation of citric acid aqueous solution. Weigh 0.3 g of citric acid (citric acid / chitosan mass ratio 1:7.5), add it to 9.7 g of deionized water, and stir at room temperature for 5 min until completely dissolved to obtain 10 g of citric acid solution with a mass fraction of 3%.

[0021] Step 7: Coating solution mixing. Add 100 g of the chitosan solution obtained in Step 2 to a 500 mL beaker. Then, under stirring conditions in a 45°C water bath, sequentially add 10 g of the ε-polylysine stock solution from Step 3, 10 g of the tea polyphenol stock solution from Step 4, 60 g of the beeswax microemulsion from Step 5 (corresponding to 15 g of dry beeswax, accounting for 15% of the dry basis of the coating solution), and 10 g of the citric acid solution from Step 6. Stir at 500 r / min for 90 min to obtain approximately 190 g of a uniform and stable coating solution. The coating solution is a pale yellow to light brown emulsion, showing no obvious stratification or precipitation within 24 h of standing. Its viscosity (Bruchner viscometer at 25°C) is approximately 85 mPa·s, suitable for blade coating. Step 8: Coating and curing. Use a bar coater to evenly coat the surface of the white cardboard obtained in Step 1 with the coating solution. The wet coating amount is controlled to 60 g / m² by adjusting the bar specifications. 2 The corresponding dry coating amount is 15 g / m². 2 Immediately after coating, the sample was placed in a forced-air drying oven and cured with hot air at 110°C for 7.5 min to complete the citric acid-chitosan amidation crosslinking and coating film formation. After cooling to room temperature, the sample was placed in a standard environment of 25°C and 50% relative humidity for 24 h to equilibrate, obtaining the antibacterial material of this embodiment, denoted as sample E1.

[0022] The antibacterial material obtained in this invention uses a paper-based substrate as the base layer, with an antibacterial coating uniformly applied to the paper surface. The coating's internal structure comprises a multi-component synergistic structure consisting of a chitosan network, ε-polylysine, tea polyphenols, beeswax microemulsion, and citric acid crosslinking points. The appearance characteristics of sample E1 are as follows: the coating surface is smooth and uniform, pale yellow and translucent, without bubbles, cracks, or particle protrusions. The results of cross-sectional scanning electron microscopy (SEM) observation are as follows... Figure 2 As shown, the coating thickness was measured to be approximately 15 μm to 18 μm, which is consistent with the designed dry coating amount of 15 g / m². 2 The coating adheres well to the paper fibers, exhibiting a tight bond with no obvious interfacial peeling or porosity. High-magnification SEM observation results are as follows: Figure 1 As shown, spherical beeswax microdomains of 100 nm to 300 nm are uniformly dispersed in the chitosan matrix within the coating, with a microdomain spacing of 200 nm to 500 nm and a microdomain distribution density of approximately (8 to 12) × 10⁻⁶. 9 pcs / cm 3 This is consistent with design expectations. Fourier transform infrared (FTIR) spectroscopy shows that at 1640 cm⁻¹... -1 A distinct absorption peak of amide I was observed at 1550 cm⁻¹. -1 The presence of an amide II absorption peak at 3400 cm⁻¹ confirms that the citrate carboxyl group has undergone an amidation reaction with the chitosan amino group; -1 The characteristic hydrogen bond peak at that location is red-shifted by approximately 18 cm⁻¹ compared to pure chitosan.-1 This confirms that the phenolic hydroxyl groups of tea polyphenols and the amino groups of chitosan form hydrogen-bonded complexes. 13 The solid-state NMR spectrum showed a new amide carbonyl characteristic peak at δ=175.2 ppm, with a peak integral ratio corresponding to a citric acid-chitosan amidation conversion rate of approximately 42%, falling within the preferred range of 30%–60%. This ensures both the water resistance of the crosslinked network and avoids brittleness caused by excessive crosslinking. The coating exhibited a zeta potential of +42 mV in pH 6.0 buffer, within the preferred range of +30 mV–+50 mV, confirming the formation of a synergistic cation charge array.

[0023] The key performance test results of sample E1 are as follows; for a comprehensive performance comparison, please refer to [link / reference]. Figures 3-5 As shown. The water contact angle test (using a German Krüss DSA-100 contact angle meter, droplet volume 5 μL, averaged at 5 different locations) yielded a result of 110.5° ± 2.1°, far exceeding the design target of 100°, and the contact angle decreased by less than 5° within 24 hours, demonstrating excellent hydrophobic stability, as detailed below. Figure 3 As shown. The Kit oil resistance test (referring to TAPPI T-559 standard, using Kit reagents from No. 1 to No. 12 for step-by-step testing) resulted in Kit level 12, reaching the highest test level. The antibacterial rate test (referring to GB / T 21510-2008 and ISO22196:2011 standards, with an initial inoculum concentration of 1×10⁻⁶) 6 The results (CFU / mL, contact time 24 h, contact temperature 37°C) showed an antibacterial rate of 99.7% against Escherichia coli ATCC 25922 and 99.8% against Staphylococcus aureus ATCC 6538, both exceeding the design target of 99%. The results were compared with those of the control group. Figure 4 As shown. DPPH free radical scavenging rate test (refer to the Brand-Williams method, 1 cm coated sample). 2 Immersed in 5 mL of 0.1 mmol / L DPPH ethanol solution, reacted at room temperature in the dark for 30 min (absorbance measured at 517 nm), the result was 88.3%, exceeding the design target of 85%. The shelf life test result for sushi refrigerated at 4°C was 72 h, as shown in the control result. Figure 5 As shown.

[0024] Example 2

[0025] This embodiment uses a combination of lower endpoint parameters to verify the performance feasibility of the invention under the lower boundary conditions of the parameters. The specific steps are as follows: The substrate is selected with a basis weight of 250 g / m³. 2Food-grade white cardboard was used, and the cutting and pretreatment were the same as in Example 1. The chitosan solution was prepared by adding 1.5 g of chitosan with a deacetylation degree greater than 90% and a molecular weight of 100,000 Da to 98.5 g of a 1% (v / v) acetic acid aqueous solution, stirring at 40°C for 2 h to dissolve, resulting in 100 g of a 1.5% (w / w) chitosan solution. The concentration of the ε-polylysine stock solution was adjusted to 2% (preparation method same as in Example 1, but the amount added was changed to 0.2 g of ε-PL added to 9.8 g of deionized water). The concentration of the tea polyphenol stock solution was adjusted to 5% (the amount added was changed to 0.5 g of tea polyphenol added to 9.5 g of deionized water). The solid content of the beeswax microemulsion was adjusted to 20% (preparation method same as in Example 1, but the amount added was changed to 12 g of yellow beeswax added to 88 g of aqueous phase). The citric acid / chitosan mass ratio was adjusted to 1:10, i.e., the amount of citric acid added was 0.15 g.

[0026] The coating solution was mixed using the same 45°C water bath stirring process as in Example 1, with the stirring time extended to 120 min to ensure thorough homogenization of the components at the lower concentration parameters. The viscosity of the coating solution was approximately 62 mPa·s. Coating was performed using a wire rod coater, with the wire rod specifications adjusted to control the dry coating amount at 10 g / m². 2 (corresponding to a wet coating amount of approximately 40 g / m²) 2 After coating, the sample was dried and cured with hot air at 100°C for 5 min. After cooling and equilibration, sample E2 was obtained.

[0027] Appearance and characterization of sample E2: The coating is pale yellow and translucent, with a thickness of approximately 10 μm to 12 μm. FTIR and... 13 CNMR analysis showed that the citric acid-chitosan amidation conversion rate was approximately 31%, falling within the lower limit of the 30%–60% range. The coating zeta potential was +32 mV, within the lower limit of the preferred range. The beeswax microdomain distribution density was lower than that of Example 1, approximately (5–7) × 10⁻⁶. 9 pcs / cm 3 Key performance test results: water contact angle 102.3°±1.8°, just above the design target of 100°; Kit oil resistance level 12; antibacterial rate against Escherichia coli 99.1%, antibacterial rate against Staphylococcus aureus 99.2%, both just reaching the 99% target; DPPH removal rate 85.2%, just reaching the 85% target.

[0028] Example 3

[0029] This embodiment uses a combination of upper-end parameters to verify the performance of the present invention under the upper-boundary conditions of the parameters. The specific steps are as follows. The substrate is selected with a basis weight of 350 g / m³. 2Food-grade sugarcane bagasse pulp molded paper boxes. The chitosan solution used was prepared by adding 3.0 g of chitosan (with a deacetylation degree greater than 90% and a molecular weight of 300,000 Da) to 97.0 g of a 2% (v / v) acetic acid aqueous solution and stirring at 40°C for 2 h to dissolve. Considering the significant increase in viscosity of the high molecular weight chitosan solution, the stirring time was extended to 3 h to ensure complete dissolution. The concentration of the ε-polylysine stock solution was adjusted to 6% (0.6 g ε-PL added to 9.4 g water). The concentration of the tea polyphenol stock solution was adjusted to 15% (1.5 g tea polyphenol added to 8.5 g preheated water at 50°C). The solid content of the beeswax microemulsion was adjusted to 30% (20 g yellow beeswax added to 80 g aqueous phase; high-pressure homogenization pressure increased to 80 MPa to ensure a particle size limit of 300 nm). The citric acid / chitosan mass ratio was adjusted to 1:5, i.e., 0.6 g citric acid was added.

[0030] The coating solution was mixed using a 50°C water bath with stirring (slightly higher than in Example 1 to reduce the viscosity of the high-concentration component) for 90 min. The coating solution viscosity was approximately 145 mPa·s, suitable for roller coating processes (higher viscosity is more suitable for continuous roller coating production). This example uses roller coating, and the dry coating amount was controlled to be 20 g / m² by adjusting the roller coating parameters. 2 After coating, the sample was dried and cured with hot air at 120°C for 10 min. After cooling and equilibration, sample E3 was obtained.

[0031] Appearance and characterization of sample E3: The coating is deep yellow and translucent, with a thickness of approximately 18 μm to 22 μm. FTIR and... 13 CNMR analysis showed that the citric acid-chitosan amidation conversion rate was approximately 58%, falling within the upper limit of the 30%–60% range. The coating zeta potential was +48 mV, within the upper limit of the preferred range. The beeswax microdomain distribution density was higher than in Example 1, approximately (12–16) × 10⁻⁶. 9 pcs / cm 3 Key performance test results: water contact angle 118.7°±2.3°, significantly higher than the design target; Kit oil resistance grade 12; antibacterial rate against Escherichia coli 99.5%, antibacterial rate against Staphylococcus aureus 99.6%; DPPH removal rate 92.1%. It is worth noting that although the water contact angle and DPPH removal rate of Example 3 are better than those of Example 1, the shelf life extension (60 h) is slightly lower than that of Example 1 (72 h). This is presumably because excessive cross-linking (58% close to the upper limit) makes the coating slightly brittle, which may form micro-cracks under the micro-dynamic stress of sushi packaging, affecting the sustained-release stability of the antibacterial components.

[0032] Comparative Example 1

[0033] Comparative Example 1, which omits ε-polylysine, was used to verify the irreplaceable role of ε-polylysine in the ternary antibacterial network of this invention. Except for the absence of ε-polylysine stock solution in the coating solution (the corresponding positions were replaced with an equal amount of deionized water to maintain a consistent total concentration in the coating solution), all other raw materials, proportions, and processes were identical to those in Example 1. The resulting sample was designated C1. Test results showed that C1 had a water contact angle of 108.9°, a Kit oil resistance rating of 12, a DPPH removal rate of 87.5%, and a coating thickness of 14 μm–17 μm, which were not significantly different from Example 1. However, its antibacterial performance was significantly reduced; the antibacterial rate against Escherichia coli decreased from 99.7% in Example 1 to 88.4%, and the antibacterial rate against Staphylococcus aureus plummeted from 99.8% to 72.6%. The shelf life of sushi refrigerated at 4°C was shortened from 72 hours to 36 hours. The results confirm that ε-polylysine is the key contributor to the broad-spectrum antibacterial activity against Gram-positive bacteria (typically Staphylococcus aureus) in this invention. The antibacterial ability of the chitosan and tea polyphenol binary system against Gram-positive bacteria is insufficient to meet the 99% target. The addition of ε-polylysine achieves a nonlinear synergistic transition of 1+2=99% or more.

[0034] Comparative Example 2

[0035] Comparative Example 2 was a scheme lacking tea polyphenols, used to verify the irreplaceable role of tea polyphenols in antioxidation and auxiliary antibacterial activity. Except for the absence of tea polyphenol stock solution in the coating solution (replacing it with an equal volume of deionized water), all other conditions were identical to those in Example 1. The resulting sample was designated C2. Test results showed that C2 had a water contact angle of 107.6°, a Kit oil resistance rating of 12, an antibacterial rate of 95.3% against Escherichia coli, and an antibacterial rate of 96.1% against Staphylococcus aureus (slightly lower than Example 1 but still close to 99%). However, the DPPH free radical scavenging rate plummeted from 88.3% in Example 1 to 24.7% (a decrease of approximately 64 percentage points), and the shelf life of sushi refrigerated at 4°C was shortened from 72 hours to 30 hours. The results confirm that tea polyphenols are the main contributor to the antioxidant capacity of the coating, while chitosan and ε-polylysine have almost no significant free radical scavenging ability. Moreover, the lack of tea polyphenols not only leads to a decrease in antioxidant capacity, but also significantly affects shelf life. It is speculated that this is because unsaturated aldehydes and ketones, which are off-flavor substances produced by lipid oxidation of sushi fish fillets, accumulate rapidly without antioxidant protection. Even if microbial proliferation is effectively inhibited, sensory degradation will reach the end of shelf life before microbial contamination.

[0036] Comparative Example 3

[0037] Comparative Example 3, which lacks the beeswax microemulsion, was used to verify the core role of the beeswax microemulsion in hydrophobic modification. Except for the absence of beeswax microemulsion in the coating solution (replacing it with an equal amount of deionized water, and moderately adjusting the total solids content of the coating solution to maintain comparable coating thickness), all other conditions were identical to those in Example 1. The resulting sample was designated C3. The test results showed that the water contact angle of C3 plummeted to 68.2° (far below the target of 100°, indicating that the coating almost completely lost its hydrophobicity), the Kit oil resistance rating dropped from 12 to 6 (severely insufficient oil resistance), and the coating showed significant swelling after immersion in deionized water for 10 minutes, with localized peeling starting after 30 minutes. Although the antibacterial properties remained intact in the initial tests (99.0% antibacterial rate against Escherichia coli and 99.2% against Staphylococcus aureus), the shelf life of sushi refrigerated at 4°C was only extended to 28 hours. This was because the coating rapidly absorbed water and softened in the moist food medium, losing its physical barrier effect. The antibacterial components migrated into the food more quickly, resulting in a loss of the coating's functional durability. This result confirms that beeswax microemulsion is an essential component for the water- and oil-resistant dual-function of this invention, and its absence triggers a chain reaction of coating failure.

[0038] Comparative Example 4

[0039] Comparative Example 4 is a scheme lacking citric acid crosslinking agent, used to verify the indispensable role of citric acid amidation crosslinking in the coating's water solubility and sustained-release stability. Except for the absence of citric acid solution in the coating solution (replacing it with an equal volume of deionized water), all other conditions were identical to those in Example 1. The resulting sample was designated C4. The test results showed that C4 had an initial water contact angle of 105.7°, a Kit oil resistance grade of 12, an antibacterial rate of 99.4% against Escherichia coli, an antibacterial rate of 99.6% against Staphylococcus aureus, and a DPPH removal rate of 87.9%, with initial performance close to that of Example 1. However, it performed extremely poorly in the coating durability test. After immersing the coating in deionized water for 24 hours, the coating coverage loss reached 38.5% (the corresponding data for Example 1 was less than 5%), the water contact angle dropped to 42.3°, and in the GB31604.1 food simulant migration test, the migration amount of ε-polylysine reached 0.18 mg / kg (far exceeding the limit of 0.05 mg / kg set by this invention), and the migration amount of tea polyphenols reached 0.35 mg / kg (exceeding the limit of 0.1 mg / kg). The shelf life of sushi refrigerated at 4°C was only 36 hours. The results confirm that citric acid amidation crosslinking is a key reaction for maintaining the integrity of coatings in hydrated food environments, preventing excessive migration of active components, and ensuring food contact compliance. Non-crosslinked coatings, even if they have good initial performance, are not practically valuable.

[0040] Comparative Example 5

[0041] Comparative Example 5, with the curing temperature reduced to 80°C, was used to verify the necessity of the 100°C~120°C hot air curing temperature window of the present invention. Except for the hot air drying curing temperature being reduced from 110°C in Example 1 to 80°C (curing time maintained at 7.5 min), all other conditions were exactly the same as in Example 1. The resulting sample was designated C5. 13 The solid-state NMR spectrum showed that the citric acid-chitosan amidation conversion rate was only 12.5%, far lower than the 42% of Example 1 and the lower limit of the preferred range of 30%~60% in this invention, confirming that the temperature of 80°C is insufficient to drive the effective amidation reaction between the carboxyl groups of citric acid and the amino groups of chitosan. The test results showed that the water contact angle of C5 was 95.3° (not reaching the target of 100°), the coating weight loss was 41.2% after immersion in deionized water for 24 h, the water contact angle decreased to 38.7°, and the shelf life of sushi refrigerated at 4°C was 30 h. These results confirm that the 100°C~120°C curing temperature window of this invention is a necessary selection based on the kinetics of the citric acid-chitosan amidation reaction. Insufficient temperature leads to incomplete crosslinking reaction, and the coating's water resistance and the sustained-release ability of the active components do not meet the design requirements.

[0042] The key performance testing methods involved in the embodiments and comparative examples of this invention are detailed below. First, coating thickness measurement. A Hitachi S-4800 field emission scanning electron microscope (Japan) with an accelerating voltage of 5 kV was used to observe the cross-section of the coating sample. The coating thickness was measured at five different locations and the average value was taken. Second, coating chemical structure characterization. A Thermo Fisher Nicolet iS50 Fourier transform infrared spectrometer (USA) in attenuated total reflectance (ATR) mode was used with a scanning range of 4000 cm⁻¹. -1 ~400 cm -1 4 cm resolution -1 The FTIR spectrum of the coating surface was acquired through 32 scans. A Bruker AVANCE III HD 400 MHz solid-state nuclear magnetic resonance spectrometer (Swiss) was used. 13 C CP / MAS mode, magic angle rotation frequency 10 kHz, collecting coating powder. 13 The citric acid-chitosan amidation conversion rate was calculated by integrating the characteristic peaks of the amide carbonyl group in the C solid-state NMR spectrum within the range of δ = 170 ppm to 180 ppm.

[0043] Third, water contact angle test. A German Krüss DSA-100 contact angle meter was used, employing the seated drop method. The droplet volume was 5 μL, and images were taken within 5 seconds of droplet deposition. The Young-Laplace method was used to fit the contact angle value. Measurements were taken at 5 random locations on the surface of the coated sample, and the average value was recorded. The contact angle decay was also recorded after 24 hours. Fourth, Kit oil resistance rating test. Referring to the TAPPI T-559 standard, a Kit reagent (prepared from castor oil, toluene, and n-heptane in a specific ratio, with 12 grades, the higher the grade number, the stronger the resistance to oily media) was used. Testing was conducted step by step starting with reagent number 1. If the reagent droplet did not penetrate the coating surface within 15 seconds, that grade was recorded as qualified. Testing continued to higher grades until penetration occurred. The highest qualified grade was taken as the Kit oil resistance rating of the sample. Fifth, antibacterial rate test. Referring to GB / T 21510-2008 "Test Method for Antibacterial Properties of Nano-Inorganic Materials" and ISO 22196:2011 "Determination of Antibacterial Activity of Plastic Surfaces", Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 6538 were used as indicator bacteria, with an initial bacterial concentration of 1×10⁻⁶. 6 CFU / mL was inoculated onto the surface of coated sample (5 cm × 5 cm), covered with an equally sized sterile film, and incubated at 37°C and relative humidity greater than 90% for 24 h. After elution with SCDLP elution buffer, the sample was serially diluted and plated for counting. The control group was a blank paper base without antibacterial coating. The antibacterial rate was calculated according to the formula: antibacterial rate (%) = (number of bacteria in control group - number of bacteria in sample group) / number of bacteria in control group × 100%. The average value was taken from three parallel trials.

[0044] Sixth, DPPH free radical scavenging rate test. Prepare a 0.1 mmol / L DPPH (1,1-diphenyl-2-trinitrophenylhydrazine) anhydrous ethanol solution, and take a 1 cm sample of the coated specimen. 2 Immerse the sample in 5 mL of DPPH solution and react at room temperature in the dark for 30 min. Then remove the sample and measure the absorbance A at a wavelength of 517 nm using anhydrous ethanol as a reference. 样品 The absorbance after reacting with the DPPH solution that did not contact the sample was A. 空白 According to the formula, DPPH clearance rate (%) = (A 空白 -A 样品 ) / A 空白×100% calculation. Seventh, migration test. Referring to GB 31604.1-2015 "National Food Safety Standard General Rules for Migration Testing of Food Contact Materials and Articles" and GB5009.156-2016 General Rules for Pretreatment Methods, three food simulants were used: 10% ethanol aqueous solution (simulating aqueous food), 3% acetic acid aqueous solution (simulating acidic food), and olive oil (simulating oily food). After contact at 4°C for 10 days, the migration amount of ε-polylysine and tea polyphenols in the simulants was quantitatively determined by high performance liquid chromatography (HPLC), and the silver content was determined by inductively coupled plasma mass spectrometry (ICP-MS) (the formula of this invention does not contain silver and is used as a control reference). Eighth, sushi shelf life test. Commercially available fresh salmon sushi (25 g per sushi, initial total bacterial count less than 100 CFU / g) was placed in a sushi container made of the antibacterial material of this invention. Samples were taken periodically under refrigeration at 4°C to test the total bacterial count, TVB-N value, and sensory score. The test was conducted when the total bacterial count reached 10. 5 The shelf life end is defined as the point at which either the CFU / g or TVB-N value reaches 20 mg / 100g or the sensory score is below 6 points (out of 10).

[0045] Table 1 summarizes the key performance test data of Examples 1-3 and Comparative Examples 1-5 of the present invention.

[0046]

[0047] Table 1 shows the nonlinear synergistic effect of the ternary antibacterial network and bifunctional synergy of the present invention. The data will be analyzed in depth from four dimensions below.

[0048] The first dimension is the synergistic non-additive analysis of the three antibacterial components. Comparing Example 1 and Comparative Example 1, it can be seen that after removing ε-polylysine, the antibacterial rate against Gram-negative *Escherichia coli* decreased from 99.7% to 88.4% (a decrease of 11.3 percentage points), and the antibacterial rate against Gram-positive *Staphylococcus aureus* plummeted from 99.8% to 72.6% (a decrease of 27.2 percentage points). The decrease in *Staphylococcus aureus* was 2.4 times that of *Escherichia coli*, and this asymmetric decrease confirms the specific contribution of ε-polylysine to Gram-positive bacteria. Comparing Example 1 and Comparative Example 2, it can be seen that after removing tea polyphenols, although the antibacterial rates of the two bacteria only decreased slightly (by 4.4 and 3.7 percentage points, respectively), the DPPH scavenging rate plummeted from 88.3% to 24.7% (a decrease of 63.6 percentage points), confirming that the core function of tea polyphenols is antioxidation rather than direct antibacterial activity, but its antioxidant capacity is key to the protection of lipid oxidation in sushi fillets. The comparison of the two groups shows that chitosan, ε-polylysine, and tea polyphenols each play their respective roles. Chitosan provides basic membrane electrostatic disturbance, ε-polylysine specifically enhances membrane perforation against Gram-positive bacteria, and tea polyphenols provide antioxidant protection and auxiliary antibacterial properties. The three work together to achieve all-around performance that no single binary combination can match.

[0049] The second dimension is the analysis of the irreplaceability of the bifunctional components. While the absence of the beeswax microemulsion and citric acid crosslinking agent (Comparative Examples 3 and 4) did not directly affect the antibacterial rate and DPPH scavenging rate (the corresponding indicators for C3 and C4 still reached or approached the level of Example 1), it led to a complete collapse of the coating's functional durability in humid food environments. Comparative Example 3, due to the loss of the hydrophobic barrier, saw its water contact angle drop to 68.2° and its Kit grade drop to level 6, resulting in a sushi shelf life of only 28 hours. Comparative Example 4, due to the loss of crosslinking fixation, initially exhibited good performance, but after 24 hours of water immersion, the coating lost 38.5% of its initial properties, and the migration of active components exceeded the standard by 3.5 times, resulting in a sushi shelf life of only 36 hours. These two sets of data reveal the ingenuity of this invention's design: antibacterial and antioxidant properties are only instantaneous coating performance; the true determinant of product value is durability. Durability requires the combined protection of the hydrophobic barrier provided by the beeswax microemulsion and the network fixation provided by the citric acid crosslinking; neither can be lacking.

[0050] The third dimension is the justification of the parameter range boundaries. The detection data of the three examples show a clear characteristic of optimal performance in the middle and usable performance at both ends. The water contact angle increases monotonically from 102.3° in E2, 110.5° in E1, to 118.7° in E3, which is consistent with the trend of increasing hydrophobic component content and cross-linking degree. However, the shelf life shows a parabolic curve of first increasing and then decreasing from 48 h in E2, 72 h in E1, to 60 h in E3, with the optimal value being the middle value E1. The chemical mechanism of this phenomenon is explained as follows: at the upper end parameter, the citric acid-chitosan amidation conversion rate reaches 58%, which is close to over-cross-linking. The coating is slightly brittle and may form micro-cracks under the micro-dynamic stress of sushi packaging, affecting the sustained-release stability of the active component. At the same time, the compatibility boundary of the high-concentration component system is tighter, and the long-term storage stability is slightly weaker than the middle value.

[0051] The fourth dimension is the chemical kinetics demonstration of the curing temperature window. In Comparative Example 5, after lowering the curing temperature from 110°C to 80°C, 13 Solid-state NMR spectroscopy revealed a sharp drop in amidation conversion rate from 42% to 12.5%, a decrease in water contact angle from 110.5° to 95.3°, a dramatic increase in coating loss from less than 5% to 41.2% after 24 hours of water immersion, and a reduction in sushi shelf life from 72 hours to 30 hours. These results quantitatively confirm that the reaction rate of the citric acid-chitosan amidation reaction at 80°C is insufficient to achieve the necessary crosslinking degree within a short timeframe of 7.5 minutes. Based on the Arrhenius equation, the apparent activation energy of this amidation reaction is estimated to be approximately 80 kJ / mol. The increase in reaction rate constant from 80°C (353 K) to 110°C (383 K) increases the reaction rate constant by approximately 6.8 times, which corresponds precisely to the measured change in amidation conversion rate from 12.5% ​​to 42%. This kinetic analysis provides a theoretical basis for selecting the lower limit of the 100°C–120°C curing temperature window in this invention, confirming that this temperature window is not arbitrarily selected but rather derived from reaction kinetic optimization.

[0052] Based on the above embodiments, comparative examples, and data analysis, the comprehensive synergistic mechanism of the antibacterial coating of the present invention can be summarized as a five-functional integrated model under a molecular-level adhesive architecture. The core of this model is that chitosan plays a triple role in the coating: film-forming matrix, basic antibacterial agent, and multi-component anchoring center. This is due to the densely distributed C2-position primary amino groups (NH2 / NH3) on the chitosan molecular chain. + The ε-NH3 group of ε-polylysine, consisting of hydroxyl (OH) groups at positions C3 and C6 and the C1-C4 β-glycosidic backbone, forms a group of functional sites that interact with other components in a multimodal manner. +A dual-cationic charge array is formed between the tea polyphenols and chitosan through electrostatic attraction and hydrogen bonding. This array maintains a stable +30 mV to +50 mV zeta potential in the hydrated state of the coating, providing a multi-site anchoring basis for the anionic sites (teichoic acid, lipopolysaccharide) on the bacterial cell surface. Multiple phenolic hydroxyl groups of tea polyphenols (EGCG molecules contain 8 phenolic hydroxyl groups) simultaneously act as hydrogen bond donors, forming a multi-hydrogen bond network with the amino / hydroxyl groups of chitosan and the amide carbonyl groups of ε-polylysine. This results in a stable, soft-locked complex at the molecular level. After the coating cures, this complex structure is further fixed by a hard-locked three-dimensional network formed by the cross-linking of citric acid tricarboxylic acid, achieving uniform distribution and slow, controllable release of the active components within the coating. During the coating curing process, beeswax microemulsions form spherical microdomains of 100 nm to 300 nm through particle aggregation. These microdomains are connected by a chitosan-ε-polylysine-tea polyphenol composite framework, forming a phase-separated structure in which hydrophobic microdomains and a hydrophilic framework coexist. This structure ensures that the water contact angle on the coating surface is greater than 100° (the hydrophobic microdomains aggregate on the surface) and also ensures the coating's ability to accommodate water-soluble antibacterial components (the hydrophilic framework penetrates the interior of the coating).

[0053] The core chemical reaction involved in this invention is the amidation reaction between the carboxyl group of citric acid and the primary amino group at the C2 position of chitosan. This reaction is carried out under hot air drying conditions at 100°C~120°C. The specific reaction pathway and mechanism are analyzed as follows. A citric acid molecule (2-hydroxy-1,2,3-propanetricarboxylic acid, molecular formula C6H8O7, molecular weight 192.12 g / mol) contains three carboxyl groups (two α-carboxyl groups and one β-carboxyl group) and one α-hydroxy group. Under the weakly acidic conditions of the coating solution (pH 4~5), citric acid mainly exists in a single-proton dissociation form (pKa1=3.13, pKa2=4.76, pKa3=6.40), while the primary amino group at the C2 position of chitosan exists as protonated NH3. + It exists in the form of (pKa approximately 6.5). The citrate carboxyl group (-COO) in the coating solution system... - ) and chitosan amino (-NH3) + First, ion-pair prepolymers are formed through electrostatic attraction, which is the starting point of the thermally induced amidation reaction.

[0054] When the coating enters the hot air drying and curing stage at 100°C~120°C, water vapor is continuously discharged, and the pH of the system increases (protonated amino groups are gradually deprotonated into free -NH2). The correspondence between citrate carboxyl groups and chitosan free amino groups transforms into the classic two-step amidation mechanism of nucleophilic addition-dehydration. In the first step, the chitosan free amino group (-NH2) acts as a nucleophile to attack the carbonyl carbon of the citrate carboxyl group, forming a tetrahedral transition state. In the second step, the protons in the transition state rearrange and remove one molecule of water, forming a stable amide bond (-CO-NH-). All three carboxyl groups of citrate can participate in this reaction. Since the steric hindrance of the α-carboxyl group is smaller than that of the β-carboxyl group, the β-carboxyl group and the two middle α-carboxyl groups react preferentially. Theoretically, a single citrate molecule can crosslink with up to three chitosan molecular chains to form a multi-branched crosslinked structure. The actual amidation conversion rate is affected by hot air temperature, time, and the rate of moisture diffusion within the coating. Under the conditions of 100°C~120°C / 5 min~10 min in this invention, the optimal crosslinking range of 30%~60% is achieved.

[0055] The effect of the degree of amidation crosslinking on coating performance exhibits a typical bell-shaped curve relationship. When the degree of crosslinking is too low (less than 30%), there is a lack of sufficient covalent bonds between chitosan molecular chains, making the coating prone to swelling or even dissolving in the hydrated state, resulting in insufficient water resistance and easy migration of active components. When the degree of crosslinking is moderate (30%~60%), the crosslinking network forms a three-dimensional continuous structure, ensuring both water resistance and sufficient chain segment flexibility, allowing for slow release of active components without explosive release. When the degree of crosslinking is too high (greater than 60%), the overly dense crosslinking network leads to increased coating brittleness, making it prone to forming microcracks under external forces, thus reducing the long-term stability of the coating. This bell-shaped curve pattern is clearly demonstrated in the comparison of the shelf life of Example 1 (conversion rate 42%) and Example 3 (conversion rate 58%). Although Example 3 has better water resistance, its shelf life is slightly shorter than that of Example 1, which is a result of excessive crosslinking introducing brittle cracks.

[0056] The synergistic antibacterial mechanism of the ternary antibacterial components involves four parallel pathways. The first pathway is chitosan-NH3. + +ε-polylysine-NH3 + The two polycationic macromolecules form a dense array of positive charges on the coating surface. When bacteria (regardless of whether they are Gram-positive or Gram-negative, bacterial cell surfaces are generally negatively charged) come into contact with the coating, electrostatic attraction anchors the bacteria to the coating surface. The anion sites on the membrane surface are neutralized, generating membrane potential perturbation, and the selective permeability of the membrane is lost. Pathway two is membrane permeation via the long side chain of ε-polylysine. The lysine side chain of ε-polylysine contains an amphiphilic structure of a hydrophobic methylene chain and a hydrophilic ε-amino group. This structure allows it to further insert into the membrane after anchoring the bacteria, forming transmembrane channels, leading to the permeation of membrane contents (K... + Mg2+ The leakage of substances such as nucleotides leads to osmotic imbalance and bacterial death. This mechanism is particularly effective against Gram-positive bacteria, whose cell walls (peptidoglycan + teichoic acid) have a relatively loose structure, making the amphiphilic structure of ε-polylysine easier to penetrate.

[0057] Pathway three involves the generation and oxidative damage of reactive oxygen species (ROS) in tea polyphenols. Catechins (EGCG, EGC, ECG, EC) in tea polyphenols contain multiple ortho- and olfactory hydroxyl groups, which, under aerobic conditions, undergo auto-oxidation to generate superoxide anions (O2). ·- ), hydrogen peroxide (H2O2) and hydroxyl radicals (·OH) and other reactive oxygen species (ROS) attack bacterial DNA, causing oxidative damage; attack the active sites of key metabolic enzymes (such as dehydrogenases and transaminases), causing enzyme inactivation; and attack membrane lipids, triggering a chain reaction of lipid peroxidation. Pathway four involves the metal chelation of tea polyphenols. The ortho-trihydroxy structure (gallates) on the B ring of catechins chelates with Fe... 2+ Cu 2+ When metal ions form stable chelates, they deprive the ferrous-sulfur proteins and ceruloplasmin in the bacterial respiratory chain of their required metal cofactors, thus disrupting the bacterial respiratory chain. The simultaneous action of these four pathways renders the bacterial defense mechanisms (efflux pumps, biofilm formation, and antioxidant enzyme systems) ineffective. Therefore, the ternary antibacterial network of this invention achieves an antibacterial rate greater than 99% against both *Escherichia coli* and *Staphylococcus aureus*, and this antibacterial rate is higher than that of any single component or binary combination. This aligns with the expected chemical mechanism that the parallel attack of four pathways leads to synergistic antibacterial efficiency exceeding the sum of their components.

[0058] The hydrophobic modification mechanism of beeswax microemulsions involves a dual process of phase separation within the coating and surface enrichment. The main component of beeswax is C. 30 C of alcohols 20 ~C 30 The fatty acid ester mixture has a melting point of 62°C to 65°C and remains in a molten state during the hot air curing stage at 100°C to 120°C. The microemulsion particles exhibit a certain migration ability within the coating. Due to the poor compatibility between beeswax and the chitosan-ε-polylysine-tea polyphenol hydration complex (beeswax is hydrophobic and nonpolar, while the complex is hydrophilic and strongly polar), based on the Marangoni effect and the principle of surface energy minimization, beeswax microemulsion particles spontaneously accumulate on the coating surface during drying, forming a hydrophobic gradient where beeswax microdomains preferentially distribute. Simultaneously, a certain density of beeswax microdomains (100 nm to 300 nm spherical, with a distribution density of approximately (8 to 12) × 10⁻⁶) is still retained within the coating. 9 pcs / cm 3(Data from Example 1) forms a physical barrier within the coating against the penetration of oily food media. This synergistic structure of surface enrichment and internal distribution is the key mechanism by which this invention achieves both a water contact angle greater than 100° (surface hydrophobicity) and Kit 12 level oil resistance (internal impermeability).

[0059] The above embodiments and comparative examples have provided a detailed description of the main technical solutions, synergistic mechanisms, and beneficial effects of the present invention. These embodiments are merely preferred implementations of the present invention and are not intended to limit the scope of protection of the claims. Without departing from the technical concept of the present invention, those skilled in the art can make conventional adjustments and equivalent substitutions to the specific parameters and operating steps in the embodiments, and all such adjustments and substitutions should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A chitosan-coated modified paper-based sushi container antibacterial material, characterized in that, The antibacterial material comprises a paper-based substrate and an antibacterial coating applied to the paper-based substrate; the paper-based substrate is food-grade white cardboard or bagasse pulp molded paperboard, and the basis weight of the paper-based substrate is 250 g / m³. 2 ~350 g / m 2 The antibacterial coating is formed by hot air curing of a coating liquid. The coating liquid includes a film-forming matrix, a polycationic antimicrobial peptide, a polyphenolic antioxidant and antibacterial agent, a hydrophobic modifying component, and a crosslinking agent. The film-forming matrix is ​​chitosan with a degree of deacetylation greater than 90% and a molecular weight of 100,000 Da to 300,000 Da. The chitosan is dissolved in a 1% to 2% (v / v) aqueous acetic acid solution, and the mass fraction of chitosan in the coating liquid is 1.5% to 3%. The polycationic antimicrobial peptide is ε-polylysine, and the mass fraction of ε-polylysine in the coating liquid is 0.2% to 0.6%. The polyphenolic antioxidant and antibacterial agent is tea polyphenol, and the mass fraction of tea polyphenol in the coating liquid is 0.5% to 1.5%. The hydrophobic modifying component is a beeswax microemulsion with a solid content of 20% to 30% and a particle size of 100 nm to 300 nm. nm, wherein the beeswax in the coating solution has a dry basis mass fraction of 10%~20%; the crosslinking agent is citric acid, and the mass ratio of citric acid to chitosan is 1:10~1:5; the dry coating weight of the antibacterial coating is 10 g / m 2 ~20 g / m 2 The hot air curing temperature is 100°C~120°C and the time is 5 min~10 min.

2. The antibacterial material according to claim 1, characterized in that, The ε-polylysine is ε-polylysine hydrochloride with an average degree of polymerization of 25-35 and a purity of ≥95%; the total catechin content of the tea polyphenols is ≥80%, of which the content of epigallocatechin gallate (EGCG) is ≥50%; the beeswax microemulsion uses polyglycerol fatty acid esters or lecithin as emulsifying stabilizers.

3. The antibacterial material according to claim 1, characterized in that, The mass fraction ratio of chitosan, ε-polylysine, and tea polyphenols in the coating solution is 5~10:1~2:1.5~3; the zeta potential of the antibacterial coating in a hydrated state at pH 6.0 is +30 mV to +50 mV; the tea polyphenols form a hydrogen-bonded complex with the amino groups of chitosan through phenolic hydroxyl groups, and the hydrogen-bonded complex shows a significant difference at 3400 cm⁻¹ in the Fourier transform infrared spectrum. -1 The characteristic hydrogen bond peak at that location is red-shifted by ≥15 cm⁻¹ relative to pure chitosan. -1 .

4. The antibacterial material according to claim 1, characterized in that, The beeswax microemulsion is uniformly dispersed in the cured antibacterial coating in the form of spherical microdomains of 100 nm to 300 nm. The spacing between the microdomains, as determined by scanning electron microscopy cross-sectional images, is 200 nm to 500 nm, and the distribution density of the microdomains is 5 × 10⁻⁶. 9 pcs / cm 3 ~16×10 9 pcs / cm 3 .

5. The antibacterial material according to claim 1, characterized in that, The citric acid and chitosan undergo an amidation crosslinking reaction during the hot air curing process, with an amidation crosslinking conversion rate of 30% to 60%. 13 The amide carbonyl peak at a chemical shift of 170 ppm to 180 ppm was determined by integrating the solid-state nuclear magnetic resonance spectrum; after being placed at 25°C and 90% relative humidity for 30 days, the antibacterial coating showed an antibacterial rate of greater than or equal to 95% against both Escherichia coli and Staphylococcus aureus.

6. The antibacterial material according to claim 1, characterized in that, Under the migration test conditions of food simulants in GB 31604.1-2015, the antibacterial coating was contacted with three food simulants—10% ethanol aqueous solution, 3% acetic acid aqueous solution, and olive oil—at 4°C for 10 days. The migration amount of ε-polylysine was less than or equal to 0.05 mg / kg, and the migration amount of tea polyphenols was less than or equal to 0.1 mg / kg. Furthermore, the antibacterial coating and the antibacterial material do not contain silver.

7. The method for preparing the antibacterial material according to claim 1, characterized in that, The preparation method includes the following steps: S1 Dissolving the chitosan in a 1%~2% (v / v) aqueous acetic acid solution and stirring at 40°C for 2~3 h until completely dissolved to obtain a 1.5%~3% (w / w) chitosan solution; S2 Preparing the ε-polylysine aqueous solution, the tea polyphenol aqueous solution, the beeswax microemulsion, and the citric acid aqueous solution respectively; S3 Mixing the chitosan solution obtained in S1 with the component solutions or dispersions obtained in S2 according to the mass ratio and stirring at 40°C~50°C for 1~2 h to obtain a uniform coating liquid; S4 Coating the coating liquid onto the surface of the paper substrate by scraping or roller coating, controlling the dry coating amount to be 10 g / m². 2 ~20 g / m 2 S5 The coated paper substrate is dried and cured by hot air at 100°C~120°C for 5 min~10 min to complete the amidation crosslinking of citric acid and chitosan and the coating film formation.

8. The preparation method according to claim 7, characterized in that, The preparation method of the beeswax microemulsion in step S2 is as follows: place yellow beeswax in a water bath at 80°C~85°C to melt it, add 5%~15% of polyglycerol fatty acid ester and 1%~5% of lecithin by weight of the beeswax as emulsifying stabilizers; separately take deionized water preheated at 80°C, and slowly pour the molten wax phase into the aqueous phase while stirring, and homogenize it 2~3 times under a pressure of 60 MPa~80 MPa using a high-pressure homogenizer to obtain a stable beeswax microemulsion with a solid content of 20%~30% and an average particle size of 100 nm~300 nm.

9. The preparation method according to claim 7, characterized in that, The mixing sequence in step S3 is as follows: In the chitosan solution of S1, the ε-polylysine aqueous solution, the tea polyphenol aqueous solution, the beeswax microemulsion and the citric acid aqueous solution are added sequentially under stirring conditions of 40°C to 50°C. The stirring speed is 300 r / min to 500 r / min, and the total stirring time is 60 min to 120 min. The resulting coating solution has a Brookfield viscosity of 60 mPa·s to 150 mPa·s at 25°C.

10. The preparation method according to claim 7, characterized in that, The antibacterial material is used to hold fresh sushi, sashimi, or sushi rolls refrigerated at 4°C, extending the shelf life of the fresh sushi from 24 hours to 48-72 hours.

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