Preparation method of double-layer composite film for walnut kernel preservation

Abstract

The present application relates to a kind of carboxymethyl cellulose / sodium alginate double-layer composite preservative film preparation method and application, the composite preservative film at least includes the first layer composite film and the second layer composite film of laminated arrangement;Wherein, the first layer preservative film is carboxymethyl cellulose / sodium alginate composite film, and contain nano zinc oxide and activated carbon;The second layer composite film is carboxymethyl cellulose / sodium alginate composite film, and contain citric acid.The carboxymethyl cellulose / sodium alginate composite preservative film of the present application can absorb walnut kernel oxidation rancidity volatile flavoring material, reduce the transmittance of ultraviolet, inhibit walnut kernel autoxidation and reach better walnut kernel preservation effect.

Description

Technical Field

[0001] This invention relates to the field of food packaging materials technology, specifically to a carboxymethyl cellulose / sodium alginate double-layer composite preservation film and its preparation method. Background Technology

[0002] Walnuts are rich in oils, primarily unsaturated fatty acids such as linoleic acid and linolenic acid. The double-bond structure of these unsaturated fatty acids makes them chemically reactive and prone to reacting with oxygen. Under light (especially in the 280-315nm ultraviolet band) and oxygen, the free radical chain reaction rate of walnuts is significantly higher than that of other nuts. This oxidation process is an auto-oxidative chain reaction that, once started, continues and gradually accelerates. Therefore, during storage, when oxygen is present in the packaging, the unsaturated fatty acids will oxidize, not only causing problems such as excessive peroxide value and acid value (common indicators of deterioration), but also generating low-molecular-weight volatile aldehydes and ketones with off-odors, such as hexanal and nonanal. This leads to a sharp decline in sensory quality and reduces the nutritional value of the walnuts.

[0003] Traditional single-barrier packaging materials (such as polyethylene and aluminum-plastic composite films) can only delay primary oxidation and cannot simultaneously solve the following key problems: ① UV-induced lipid photooxidation; ② continuous accumulation of secondary oxidation products (such as malondialdehyde); ③ the enrichment effect of volatile odor substances in the headspace of the packaging. These three problems together lead to a unique deterioration trajectory of walnut kernels during storage: "stable acid value in the early stage - sudden increase in odor in the middle stage - quality collapse in the later stage," becoming a technical bottleneck restricting the development of the industry. At the same time, these materials have poor environmental performance and are difficult to degrade. Composite packaging materials are expensive, such as aluminum-plastic composites and paper-plastic composites, which increase product prices. Moreover, recycling and disposal are complicated, and the separation of different materials is difficult, which easily leads to resource waste and environmental pollution. These problems affect the quality maintenance, cost control, and market expansion of walnut kernel products.

[0004] Natural polymer materials generally possess many excellent properties. Among them, carboxymethyl cellulose can form a uniform, continuous film with a certain strength. This film can adhere tightly to the surface of the food being preserved, providing a physical barrier to prevent contamination and damage from external factors such as dust, microorganisms, and oxygen. Furthermore, carboxymethyl cellulose can be decomposed by microorganisms in the natural environment, causing no pollution. Sodium alginate is no exception; it has excellent film-forming properties, good biocompatibility, and strong thermal stability.

[0005] Nano-zinc oxide possesses antibacterial properties, inhibiting microorganisms and protecting walnut kernels from contamination. It also blocks ultraviolet radiation, preventing oxidation and enhancing the mechanical properties of the film. Nano-activated carbon, with its numerous pores and large specific surface area, adsorbs odors produced by the walnut kernels themselves, improving their quality. Simultaneously, it adsorbs harmful gases such as carbon dioxide and ethylene, slowing down the respiration of the kernels and facilitating storage. Furthermore, nano-activated carbon regulates humidity, preventing mold growth due to abnormal moisture levels. The composite electrolyte precipitation process allows nano-zinc oxide to uniformly combine and distribute on the surface and within the pores of the activated carbon powder, expanding the specific surface area, providing more adsorption sites, optimizing the diffusion path of the adsorbate, and accelerating the adsorbate transfer rate. Moreover, the nano-zinc oxide and activated carbon composite can be well dispersed in the sodium alginate and carboxymethyl cellulose matrix, filling the gaps between matrix molecules to form a denser filling structure. This enhances the overall density of the film, improving its tensile and tear resistance, and increasing its mechanical strength.

[0006] The "external oxygen barrier - internal absorption" composite membrane design achieves hierarchical regulation of physical barriers. The outer layer blocks oxygen to reduce the generation of primary oxidation products, while the inner layer PAC / Nano-ZnO dynamically adsorbs and catalytically degrades odor-causing substances such as hexanal and nonanal that have already been generated. This forms a two-level defense of "blocking - adsorption", which can effectively prevent the oxidation of oils and other components in walnut kernels, avoid the accumulation of unpleasant flavors in the packaging, delay the oxidation and deterioration process of walnut kernels, and improve the consumer's eating experience. Summary of the Invention

[0007] This invention addresses the unique oxidative rancidity of walnut kernels by innovatively constructing a dual-layer synergistic mechanism of "external oxygen barrier and internal adsorption": ① The outer layer, a carboxymethyl cellulose / sodium alginate / citric acid composite membrane, forms a dense network structure through citric acid cross-linking, controlling its water vapor permeability below 5.33 g / (m²·48h) and reducing its oxygen permeability coefficient by 67% compared to PE membranes, effectively blocking external oxygen penetration; ② In the inner layer, a carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon composite membrane, nano-ZnO is directionally loaded into the mesopores of activated carbon through electrochemical deposition, forming "adsorption-catalysis" micro-reaction units. The pores of activated carbon can specifically capture small molecule volatiles such as hexanal and nonanal. This dual-level protection system reduces the rate of acid value increase in walnut kernels by 64% and delays the peak concentration of volatile aldehydes by 14 days during accelerated storage experiments (60℃, 28 days).

[0008] To achieve the above objectives, the present invention provides a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film, wherein the composite food preservation film comprises at least a first composite film and a second composite film stacked together. The first layer of the plastic wrap is a carboxymethyl cellulose / sodium alginate composite film, which contains activated carbon and nano zinc oxide; the second layer of the composite film is a carboxymethyl cellulose / sodium alginate / citric acid composite film.

[0009] Preferably, in the first composite membrane, the mass ratio of carboxymethyl cellulose to sodium alginate is 1:3, and the mass ratio of nano zinc oxide / activated carbon composite to film-forming matrix is ​​0.11~0.33; in the second composite membrane, the mass ratio of carboxymethyl cellulose to sodium alginate is 1:1, and the mass ratio of citric acid to film-forming matrix is ​​0.2~0.8.

[0010] Preferably, the carboxymethyl cellulose / sodium alginate composite preservation film further includes glycerol as a plasticizer in the first and / or second composite film layers, with a mass ratio of glycerol to the film-forming matrix of 0.5.

[0011] Preferably, the carboxymethyl cellulose / sodium alginate double-layer composite preservation film is a cast film, wherein the first composite film and / or the second composite film are cast films.

[0012] In this invention, the thicknesses of the first composite film and the second composite film can be set between 0.10 mm and 0.40 mm. Further, the thickness of the first composite film is 0.10-0.20 mm, and the thickness of the second composite film is 0.20-0.40 mm.

[0013] Preferably, the first composite film and / or the second composite film are cast films, which have the advantages of high production efficiency, good uniformity of composite films, and stable performance.

[0014] Another aspect of the present invention provides a method for preparing a carboxymethyl cellulose / sodium alginate composite food preservation film, comprising the following steps: Prepare mixed membrane solutions of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon and mixed membrane solutions of carboxymethyl cellulose / sodium alginate / citric acid; A mixed film solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon is cast into a film and dried; then a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid is cast onto its surface and dried to obtain the carboxymethyl cellulose / sodium alginate double-layer composite preservation film.

[0015] Furthermore, the preparation method of the carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution is as follows: S1. Dissolve carboxymethyl cellulose in water at 55-65℃ and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 14-16.0 g / L, for later use; dissolve sodium alginate in water at 55-65℃ and stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 40.0-50.0 g / L, for later use. S2 involves immersing activated carbon particles in a 10-15% nitric acid solution to remove metallic impurities and other contaminants from the activated carbon surface. This is combined with microwave-ultrasound synergistic technology (300W-400W / 40kHz ultrasound, reaction at 75-85℃ for 20-40 min) to achieve rapid and uniform loading of nano-zinc oxide within the activated carbon pores. The activated carbon is then washed with deionized water until clean, and dried in an oven at approximately 150-115℃ until excess moisture is removed. Finally, the activated carbon is mixed with a 3-7wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 25-35 min, and dried at 55-65℃ for later use. S3 Zinc nitrate and sodium nitrate are dissolved in deionized water to form an electrolyte with concentrations of 0.25~0.35 mol / L and 0.10~0.20 mol / L, respectively. Hexadecyltrimethylammonium bromide (CTAB) is used as a dispersant with a concentration of 0.02-0.04 mol / L to inhibit ZnO agglomeration. The activated carbon treated in step S2 is immersed in the electrolyte, and the electrolyte temperature is controlled at 35~45℃. Combined with a magnetic stirring speed (200~400 rpm), the mass transfer efficiency is enhanced and concentration polarization is reduced. A constant voltage of -1.2V was applied for 10-20 min to reduce Zn²⁺ to metallic zinc, which was then loaded into the pores of activated carbon. Subsequently, the reaction was switched to +0.5V for 5-10 min to oxidize and generate ZnO nanoparticles. This process was repeated three times to achieve uniform composite formation. Finally, the nano-zinc oxide / activated carbon composite was ultrasonically cleaned with deionized water for 2-5 min to remove loose particles. The composite was then vacuum dried at 55-65℃ and annealed in an oven at 230-260℃ to obtain the nano-zinc oxide / activated carbon composite for later use. S4 mixes carboxymethyl cellulose and sodium alginate solution by mass, and adds nano zinc oxide / activated carbon conjugate to obtain the first layer composite film forming solution with a nano zinc oxide / activated carbon conjugate content of 0.0033~0.0099.

[0016] Preferably, carboxymethyl cellulose is dissolved in water at 60°C and stirred until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L, for later use. Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. Activated carbon particles were soaked in a 10% nitric acid solution to remove metallic impurities and other contaminants from the surface. A microwave-ultrasound co-precipitation technique (300W / 40kHz ultrasound, 80℃ reaction for 30 min) was then used to achieve rapid and uniform loading of nano-zinc oxide within the pores of the activated carbon. The carbon was washed with deionized water until neutral and dried at 110℃. The activated carbon was then mixed with a 5wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 30 min, and dried at 60℃ to improve conductivity.

[0017] Zinc nitrate and sodium nitrate were dissolved in deionized water at concentrations of 0.30 mol / L and 0.15 mol / L, respectively, to form the electrolyte. Hexadecyltrimethylammonium bromide (CTAB) was used as the dispersant at a concentration of 0.03 mol / L. The treated activated carbon was first immersed in the electrolyte and magnetically stirred (200 rpm). A constant voltage of -1.2V was applied for 10 min to reduce Zn²⁺ to metallic zinc, which was then loaded into the pores of the activated carbon. The reaction was then switched to +0.5V for 5 min to oxidize and generate ZnO nanoparticles. This process was repeated three times to achieve uniform composite formation. Finally, the carbon was ultrasonically cleaned with deionized water for 2 min to remove loose particles. The resulting ZnO / activated carbon composite was then vacuum dried at 60℃ and annealed in an oven at 250℃. Equal masses of carboxymethyl cellulose and sodium alginate solutions were mixed, and a nano-zinc oxide / activated carbon composite with a content of 0.0033–0.0099% was added to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon.

[0018] Furthermore, the preparation method of the carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution is as follows: Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which was then set aside. Equal masses of carboxymethyl cellulose and sodium alginate solution are mixed, and citric acid is added to obtain the carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution with a citric acid content of 0.003~0.012%.

[0019] Furthermore, the plasticizer is glycerin.

[0020] Furthermore, during film casting, the amount of the carboxymethyl cellulose / sodium alginate / citric acid mixed film solution is greater than that of the carboxymethyl cellulose / sodium alginate / nano-activated carbon / nano-zinc oxide mixed film solution. Preferably, the ratio of the carboxymethyl cellulose and sodium alginate mixed film solution to the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed film solution is 1~3:1. At this ratio, the composite membrane exhibits good UV blocking and opacity. When the ratio exceeds 3:1, the water vapor permeability of the membrane is relatively high, while the performance is best at 2:1.

[0021] Furthermore, the drying conditions for the carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution are: drying temperature 55℃, drying time 8 h; the drying conditions for the carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution are: drying temperature 65℃, drying time 10 h. In this invention, the carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution and the carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution are dried under different conditions because the amount of carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution is relatively large, thus requiring a higher drying temperature.

[0022] The beneficial effects of this invention are: 1. Method of obtaining the product: Through physical synergy and electrochemical deposition, the deposition cycle is ≤65 min (measured data from Example 1). An innovative "external oxygen barrier - internal adsorption" dual-layer synergistic mechanism is constructed, which reduces the rate of increase of acid value in walnut kernels by 64% (accelerated experimental data from Example 1), and delays the appearance of peak concentration of volatile aldehydes by 14 days (see Example 1). Figure 5 Because nano-zinc oxide not only adsorbs onto the surface of activated carbon, but also removes metallic impurities and other contaminants from the activated carbon surface through acid washing, this process increases the number of oxygen-containing functional groups (such as carboxyl, hydroxyl, and aldehyde groups) on the surface, providing more binding sites for zinc oxide deposition. In the mixed electrolyte system, sodium nitrate acts as a supporting electrolyte, increasing the solution conductivity, reducing the system resistance, and making the current distribution more uniform, resulting in a more stable bond between it and the activated carbon. The matrix molecules in the film-forming solution also prevent the detachment of nano-zinc oxide to some extent due to the spatial limitation of the pores. The electrochemical deposition process ensures a tight bond between ZnO and activated carbon, and the tensile strength of the composite film reaches 18.99 MPa (data from Example 1), an increase of 11% compared to Comparative Example 6. The nanoparticle binding energy is increased by 42% (see Table 2 for van der Waals force comparison), effectively inhibiting particle detachment.

[0023] 2. By employing a natural polymer matrix and a casting method for synergistic enhancement, the carboxymethyl cellulose / sodium alginate system achieves a biodegradability rate of 92% (test data from GB / T19277), reducing environmental impact compared to PE films. The casting process enables highly efficient double-layer film composite production, reaching 8.7 m² / h (measured in Example 2), a 35% improvement over traditional layer-by-layer coating methods. The resulting composite film possesses both excellent mechanical properties (elongation at break 55.12%) and barrier properties (water vapor transmission rate 5.33 g / (m²·48h)), as shown in Table 1 (Example 1 data).

[0024] 3. By adjusting the ratio of carboxymethyl cellulose and sodium alginate in the inner and outer hybrid films and adding other functional components, this invention can impart different properties to the preservation film, such as enhanced mechanical strength, reduced film thickness, reduced water vapor permeability (see Table 1), and improved film antioxidant properties (see Table 2). Figure 2In particular, it can absorb oxygen around walnut kernels during storage, adsorb off-odor substances produced by walnut kernels, slow down the oxidation reaction of oils and proteins in walnut kernels, and reduce the rate of oxidation and spoilage. Attached Figure Description

[0025] Figure 1 Acid value, peroxide value and malondialdehyde content of volatile flavor compounds in walnut kernels under accelerated oxidation conditions.

[0026] Figure 2 Comparison of ·OH and DPPH free radical scavenging rates of different films.

[0027] Figure 3 Material Studio simulation of mixed electrolyte (a) and Material Studio simulation of ZnO-supported activated carbon (b), as well as the convergence of the electrolyte (c) and energy changes (d).

[0028] Figure 4 Material Studio simulation of a single electrolyte (a) and Material Studio simulation of ZnO-supported activated carbon (b), as well as the convergence of the electrolyte (c) and energy changes (d).

[0029] Figure 5 The total peak area and relative peak area ratio of volatile flavor compounds in walnut kernels under accelerated oxidation conditions. Detailed Implementation

[0030] The present invention will be further described below with reference to specific embodiments. The present invention will be described in further detail, but is not limited to these embodiments.

[0031] Example 1 This embodiment provides a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Preparation of sodium alginate solution 2 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which was then set aside. (4) Pretreatment of activated carbon Activated carbon was acid-washed using 10% nitric acid under microwave-ultrasound synergistic co-precipitation (300W / 40kHz ultrasound, 80℃ for 30 min). After acid washing, the activated carbon was thoroughly washed with deionized water to remove residues from the acidic solution. After washing, the activated carbon was dried in an oven at 110℃. The treated activated carbon was then mixed with a 5wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 30 min, and dried at 60℃ to improve conductivity for later use. (5) Grafting of zinc oxide onto the surface of activated carbon Zinc nitrate and sodium nitrate were dissolved in deionized water at concentrations of 0.30 mol / L and 0.15 mol / L, respectively, to form the electrolyte. Hexadecyltrimethylammonium bromide (CTAB) was used as the dispersant at a concentration of 0.03 mol / L. The treated activated carbon was first immersed in the electrolyte and magnetically stirred (200 rpm). Then, a constant voltage of -1.2 V was applied for 10 min to reduce Zn²⁺ to metallic zinc, which was then loaded into the pores of the activated carbon. Subsequently, the reaction was switched to +0.5 V for 5 min to oxidize and generate ZnO nanoparticles. This process was repeated three times to achieve uniform composite formation. Finally, the carbon was ultrasonically cleaned with deionized water for 2 min to remove loose particles, and then vacuum dried at 60℃ and annealed in an oven at 250℃ to obtain the ZnO / activated carbon composite for later use. (6) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Then, nano zinc oxide / activated carbon conjugate was added and stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon with a nano zinc oxide / activated carbon conjugate content of 0.0033.

[0032] (7) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / citric acid Carboxymethyl cellulose and sodium alginate solution 2 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Citric acid was then added and stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid with a citric acid content of 0.003%.

[0033] (8) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite food preservation film Take 30 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon and place it in a 15×15 cm immersion chamber. 2A film was cast onto a polytetrafluoroethylene (PTFE) sheet and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. Then, 70 mL of a mixed film solution of carboxymethyl cellulose (CMC), sodium alginate, and citric acid was added to the surface, and the film was cast again and dried in a constant temperature and humidity chamber at 65°C and 50% humidity for 10 hours. The film was then peeled off, yielding a carboxymethyl cellulose (CMC) / sodium alginate (Sodium alginate) double-layer composite food preservation film.

[0034] Example 2 This embodiment provides a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Preparation of sodium alginate solution 2 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which was then set aside. (4) Pretreatment of activated carbon Activated carbon was acid-washed using 10% dilute hydrochloric acid under microwave-ultrasound synergistic co-precipitation (300W / 40kHz ultrasound, 80℃ reaction for 30 min). After acid washing, the activated carbon was thoroughly washed with deionized water to remove residues from the acidic solution. After washing, the activated carbon was dried in an oven at 110℃. The treated activated carbon was then mixed with a 5wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 30 min, and dried at 60℃ to improve conductivity for later use. (5) Grafting of zinc oxide onto the surface of activated carbon Zinc nitrate and sodium nitrate were dissolved in deionized water at concentrations of 0.30 mol / L and 0.15 mol / L, respectively, to form the electrolyte. Hexadecyltrimethylammonium bromide (CTAB) was used as the dispersant at a concentration of 0.03 mol / L. The treated activated carbon was first immersed in the electrolyte and magnetically stirred (200 rpm). Then, a constant voltage of -1.2 V was applied for 10 min to reduce Zn²⁺ to metallic zinc, which was then loaded into the pores of the activated carbon. Subsequently, the reaction was switched to +0.5 V for 5 min to oxidize and generate ZnO nanoparticles. This process was repeated three times to achieve uniform composite formation. Finally, the carbon was ultrasonically cleaned with deionized water for 2 min to remove loose particles, and then vacuum dried at 60℃ and annealed in an oven at 250℃ to obtain the ZnO / activated carbon composite for later use. (6) Preparation of mixed film solution of carboxymethyl cellulose / sodium alginate / nano activated carbon / nano zinc oxide Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min, and then a mixture of nano zinc oxide and nano activated carbon was added. The mixture was stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed film solution with a nano zinc oxide / activated carbon conjugate content of 0.0066.

[0035] (7) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / citric acid Carboxymethyl cellulose and sodium alginate solution 2 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Citric acid was then added and stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid with a citric acid content of 0.006%.

[0036] (8) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite food preservation film Take 50 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon and place it in a 15×15 cm immersion chamber. 2 A film is cast onto a polytetrafluoroethylene (PTFE) sheet and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. Then, 50 mL of a mixed film solution of carboxymethyl cellulose (CMC), sodium alginate, and citric acid is added to the surface, and the film is cast again and dried in a constant temperature and humidity chamber at 65°C and 50% humidity for 10 hours. The film is then peeled off, yielding a carboxymethyl cellulose (CMC) / sodium alginate (Sodium alginate) double-layer composite food preservation film.

[0037] Example 3 This embodiment provides a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Preparation of sodium alginate solution 2 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which was then set aside. (4) Pretreatment of activated carbon Activated carbon was acid-washed using 10% nitric acid under microwave-ultrasound synergistic co-precipitation (300W / 40kHz ultrasound, 80℃ for 30 min). After acid washing, the activated carbon was thoroughly washed with deionized water to remove residues from the acidic solution. After washing, the activated carbon was dried in an oven at 110℃. The treated activated carbon was then mixed with a 5wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 30 min, and dried at 60℃ to improve conductivity for later use. (5) Grafting of zinc oxide onto the surface of activated carbon Zinc nitrate and sodium nitrate were dissolved in deionized water at concentrations of 0.30 mol / L and 0.15 mol / L, respectively, to form the electrolyte. Hexadecyltrimethylammonium bromide (CTAB) was used as the dispersant at a concentration of 0.03 mol / L. The treated activated carbon was first immersed in the electrolyte and magnetically stirred (200 rpm). Then, a constant voltage of -1.2 V was applied for 10 min to reduce Zn²⁺ to metallic zinc, which was then loaded into the pores of the activated carbon. Subsequently, the reaction was switched to +0.5 V for 5 min to oxidize and generate ZnO nanoparticles. This process was repeated three times to achieve uniform composite formation. Finally, the carbon was ultrasonically cleaned with deionized water for 2 min to remove loose particles, and then vacuum dried at 60℃ and annealed in an oven at 250℃ to obtain the ZnO / activated carbon composite for later use. (6) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min, and then a mixture of nano zinc oxide and nano activated carbon was added. The mixture was stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon with a nano zinc oxide / activated carbon conjugate content of 0.0099.

[0038] (7) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / citric acid Carboxymethyl cellulose and sodium alginate solution 2 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Citric acid was then added and stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid with a citric acid content of 0.009%.

[0039] (8) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite food preservation film Take 70 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon and place it in a 15×15 cm immersion chamber. 2A film was cast onto a polytetrafluoroethylene (PTFE) sheet and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. Then, 30 mL of a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid was added to the surface, and the film was cast again and dried in a constant temperature and humidity chamber at 65°C and 50% humidity for 10 hours. The film was then peeled off, yielding a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film.

[0040] Comparative Example 1 This comparative example provides a pure carboxymethyl cellulose / sodium alginate composite film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Preparation of carboxymethyl cellulose / sodium alginate mixed membrane solution Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate mixed film solution.

[0041] (4) Preparation of pure carboxymethyl cellulose / sodium alginate composite film Take 100 mL of carboxymethyl cellulose / sodium alginate mixed membrane solution and place it in a 15×15 cm membrane. 2 A film is cast onto a polytetrafluoroethylene (PTFE) plate and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. The film is then peeled off to obtain a pure carboxymethyl cellulose / sodium alginate composite film.

[0042] Comparative Example 2 This comparative example provides a carboxymethyl cellulose / sodium alginate / activated carbon composite film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Dissolve the activated carbon powder in water by stirring and set aside; (4) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano activated carbon Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Activated carbon sample solution was added, and the mixture was stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate / activated carbon mixed membrane solution with an activated carbon content of 0.0003. (5) Preparation of carboxymethyl cellulose / sodium alginate / activated carbon composite film Take 100 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / activated carbon and place it in a 15×15 cm immersion chamber. 2 A film is cast onto a polytetrafluoroethylene (PTFE) plate and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. The film is then peeled off to obtain a carboxymethyl cellulose / sodium alginate / activated carbon composite film.

[0043] Comparative Example 3 This comparative example provides a carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon composite film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Dissolve activated carbon and nano zinc oxide in water by stirring, and set aside; (4) Preparation of mixed film solution of carboxymethyl cellulose / sodium alginate / nano activated carbon / nano zinc oxide Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min, and then a mixture of nano zinc oxide and activated carbon was added. The mixture was stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon with nano zinc oxide and activated carbon contents of 0.003 and 0.0003, respectively.

[0044] (5) Preparation of carboxymethyl cellulose / sodium alginate composite preservation film Take 100 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon and place it in a 15×15 cm immersion chamber. 2 A film is cast onto a polytetrafluoroethylene (PTFE) plate and dried in a constant temperature and humidity chamber at 55°C and 50% for 8 hours. The film is then peeled off to obtain a carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon composite film.

[0045] Comparative Example 4 This comparative example provides a carboxymethyl cellulose / sodium alginate / citric acid composite film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 2 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which was then set aside. (3) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / citric acid Carboxymethyl cellulose and sodium alginate solution 2 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Citric acid was then added and stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid with a citric acid content of 0.003%.

[0046] (4) Preparation of carboxymethyl cellulose / sodium alginate / citric acid composite film Take 100 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / citric acid and place it in a 15×15 cm membrane. 2 A film is cast onto a polytetrafluoroethylene (PTFE) plate and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. The film is then peeled off to obtain a carboxymethyl cellulose / sodium alginate / citric acid composite film.

[0047] Comparative Example 5 This embodiment provides a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film, the preparation method of which includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use. (2) Preparation of sodium alginate solution 1 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which was then set aside. (3) Preparation of sodium alginate solution 2 Sodium alginate was dissolved in water at 60°C, stirred, and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which was then set aside. (4) 1 g of nano zinc oxide was suspended in 100 mL of water and sonicated for 40 min. In another flask, the activated carbon sample was suspended in 20 mL of water and sonicated for 20 min. Then, the activated carbon was added to the nano zinc oxide suspension with continuous stirring, and the resulting mixture was heated at 80 °C until the water was completely evaporated. The remaining solid was dried overnight in an oven at 110 °C to obtain the nano zinc oxide / activated carbon composite for later use. (5) Preparation of mixed film solution of carboxymethyl cellulose / sodium alginate / nano activated carbon / nano zinc oxide Carboxymethyl cellulose and sodium alginate solution 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min. Then, nano zinc oxide / activated carbon conjugate was added and stirred for 30 min to obtain a mixed film solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon with a nano zinc oxide / activated carbon conjugate content of 0.0033.

[0048] (6) Preparation of carboxymethyl cellulose / sodium alginate mixed membrane solution Carboxymethyl cellulose and sodium alginate solution 2 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added. The mixture was stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate mixed film solution.

[0049] (7) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite food preservation film Take 30 mL of a mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon and place it in a 15×15 cm immersion chamber. 2 A film is cast onto a polytetrafluoroethylene (PTFE) sheet and dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 hours. Then, 70 mL of a mixed film solution of carboxymethyl cellulose (CMC) and sodium alginate is added to the surface, and the film is cast again and dried in a constant temperature and humidity chamber at 65°C and 50% humidity for 10 hours. The film is then peeled off. This yields a carboxymethyl cellulose (CMC) / sodium alginate double-layer composite food preservation film.

[0050] Comparative Example 6 This comparative example provides a carboxymethyl cellulose / sodium alginate double-layer composite food preservation film, based on Example 1, with the difference being that in step (5), activated carbon is grafted with zinc oxide in a single electrolyte: Zinc nitrate was dissolved in deionized water at a concentration of 0.30 mol / L to form a single electrolyte. The treated activated carbon was immersed in the electrolyte and magnetically stirred (200 rpm). A constant voltage of -1.2V was applied for 10 min to reduce Zn²⁺ to metallic zinc, which was then loaded into the pores of the activated carbon. Subsequently, the reaction was switched to +0.5V for 5 min to oxidize and generate ZnO nanoparticles. This cycle was repeated three times to achieve uniform composite formation. Finally, the composite was ultrasonically cleaned with deionized water for 2 min to remove loose particles, and then vacuum dried at 60℃ and annealed in an oven at 250℃ to obtain the ZnO / activated carbon composite for later use. The subsequent preparation method is completely consistent with that in Example 1.

[0051] Performance testing 1. The tensile strength and elongation at break of the film were determined using a universal testing machine in accordance with GB13022-91 "Test Method for Tensile Properties of Plastic Films". The thickness of the film was determined using a thickness gauge.

[0052] (1) The tensile strength (TS) is calculated as shown in Formula 1: TS=Fmax (L×W) (Formula 1) TS represents tensile strength (MPa), Fmax is the maximum tensile force that the membrane can withstand when it breaks, L represents the width of the membrane, and W represents the thickness of the membrane.

[0053] (2) The calculation of elongation at break (EB%) is shown in Formula 2: EB%=(L-L0) / L0(Formula 2) L represents the initial length of the membrane (mm), and L0 represents the length of the membrane after stretching (mm).

[0054] 2. The water vapor transmission rate of the membrane was tested according to GB1307-88 "Test Method for Water Vapor Permeability of Plastic Films and Sheets - Cup Method". Specifically, dried anhydrous calcium chloride was added to the permeation cup, 3 mm from the mouth. The mouth was then tightly sealed with the prepared plastic wrap and paraffin wax. The cup was placed in a desiccator containing a saturated sodium chloride solution at the bottom. After 48 hours, the weighing bottle was removed and weighed. This process was repeated until the difference in mass increase between two consecutive weighings was less than 5%. Three weighings were performed, and the average value was calculated as shown in Formula 3. WVP=Δm×d / (A×Δt×Δp) (Formula 3) Where WVP represents the water vapor transmission coefficient [g / (m 2 ·48 h)]; d represents the thickness of the composite membrane (mm); Δm represents the mass increase after stabilization (g); A represents the area of ​​the cut membrane (cm²) 2), where Δt represents the time interval between each measurement, which was 48 h in this experiment, and Δp represents the water vapor pressure difference (Pa) fixed on both sides of the composite membrane. The experimental temperature was 25℃ and the relative humidity was maintained at 95%.

[0055] 3. Physicochemical index testing of the preservative effect of the membrane on walnut kernels Acid value (AV) was determined by titration according to GB5009.229—2016.

[0056] Peroxide value (PV) was determined by titration according to GB5009.227—2016.

[0057] The content of malondialdehyde (MDA) was determined according to GB5009.181—2016, using spectrophotometry.

[0058] 4. Antioxidant activity test of the membrane DPPH scavenging rate was determined by mixing the sample membrane with 5.0 mL (0.2 mmol / L) of DPPH methanol solutions at concentrations of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mg / mL. Each solution was incubated in the dark at 25 °C for 60 min, and the absorbance at 517 nm was measured (A1). The absorbance of the DPPH methanol solution was denoted as A0, and A2 was the absorbance of the sample membrane in the methanol solution. The scavenging rate was calculated using Equation 4.

[0059] DPPH removal rate = [A0 - (A1 - A2)] / A0 × 100% (Formula 4) ·OH scavenging rate: A certain mass of membrane was sequentially added to 1.0 mL of 1.0 mmol / L ferrous sulfate solution and 2.0 mL of 3.0 mmol / L salicylic acid solution. After mixing, 1.0 mL of 3.0 mmol / L H2O2 was added to initiate the reaction. The mixture was quickly mixed and heated in a 37 ℃ water bath for 30 min. The final sample concentrations were 0.5, 1.5, 2.5, 3.5, and 4.5 mg / mL. After the reaction, the samples were centrifuged at 12000 r / min for 10 min, the supernatant was removed, and the absorbance was measured at 510 nm (A1). The absorbance measured according to the above steps for the group without sample membrane was A0, and the absorbance measured according to the above steps for the group using deionized water instead of salicylic acid solution was A2. The ·OH scavenging rate was calculated according to Formula 5: OH removal rate = [A0 - (A1 - A2)] / A0 × 100% (Formula 5) 5. Verification of adsorption kinetics of activated carbon and electrolyte (1) Constructing an activated carbon substrate model To create a graphene layer, open the Build module, select Graphene Sheet, and use Layer Tools to replicate 3 layers of graphene, stacking them to form a microporous structure (pore size 1-2 nm).

[0060] (2) Electrolyte environment setup Example 1 (containing NaNO3, Zn(NO3)2 / CTAB), solvent: water molecules: a water box (1000 H2O molecules) was created using the Amorphous Cell module. Solutes added: Zn(NO3)2 (0.3 mol / L), NaNO3 (0.15 mol / L), and CTAB (0.03 mol / L) were added according to their concentration ratios. Comparative Example 6 (without NaNO3 / CTAB) Add only Zn(NO3)2 (0.3 mol / L) and H2O, and follow the same steps as above. Place activated carbon in the electrolyte box, and place the constructed activated carbon model in the center of the electrolyte box.

[0061] (3) Molecular dynamics (MD) simulation steps Equilibrium phase (NPT ensemble), module: Forcite→Molecular Dynamics; force field: COMPASS III (select electrostatic and van der Waals interactions); ensemble: NPT (temperature 298K, pressure 1 atm, temperature control method: Berendsen); step size: 1 fs, total duration: 500 ps; monitor system density and temperature fluctuations (ensure fluctuations <5%); export the equilibrium structure file (for subsequent adsorption simulations).

[0062] 6. Test on the effect of membrane on the volatile flavor compounds of walnut kernels A certain amount of walnut kernels were sealed in PE packaging film and in packaging film prepared based on Example 1. They were placed in a constant temperature oven at 60°C for accelerated oxidation. An equal amount of unpackaged walnut kernels was added as a control experiment. Samples were taken out every 7 days for gas chromatography-mass spectrometry analysis to analyze the changes in flavor substances of walnut kernels.

[0063] Headspace solid-phase microextraction: Weigh 2.0 g of walnut kernels and place them in a headspace vial. Equilibrate for 20 min in a 50℃ water bath. Insert a 50 / 30 μm DVB / CAR / PDMS extraction head for adsorption for 30 min. Desorb at 250℃ for 5 min through the injection port.

[0064] Gas chromatography-mass spectrometry (GC-MS) conditions: DB-WAX column (30 m × 0.25 mm × 0.25 μm); initial column temperature 40 °C (hold for 2 min); then increased to 200 °C at 5 °C / min, and to 300 °C at 20 °C / min (hold for 8 min); splitless injection at 1 mL / min; injector temperature 230 °C; helium (purity ≥99.999%) as carrier gas. EI ion source and transfer line temperature 240 °C; electron energy 70 eV; scan range 40–450 m / z; solvent delay 4 min.

[0065] Volatile substances were identified by comparing the retention index (RI) of C7-C30 n-alkane standards and the standard mass spectrometry library (NIST17). 5 μL of cyclohexanone was added as an internal standard before extraction, and the concentration of each substance was calculated by semi-quantitative analysis.

[0066] Performance test results Table 1 shows the film thickness, mechanical properties, and water vapor permeability of the composite films prepared in Comparative Examples 1-6 and Examples 1-3.

[0067] Table 1 shows the film thickness, mechanical properties, and water vapor transmission coefficient of the composite membranes prepared in Examples 1-3 (Comparative Examples 1-6). As can be seen from Table 1, the addition of contents to the film increases the film thickness. Specifically, the addition of activated carbon increases the elongation at break, while the addition of nano-zinc oxide enhances the tensile strength and reduces the water vapor transmission rate. This is because nano-zinc oxide can be uniformly dispersed and exists within the film, exhibiting good interfacial interaction with carboxymethyl cellulose and sodium alginate, thus acting as a filler in the film.

[0068] Figure 1 The results showed that with increasing oxidation days, the acid value, peroxide value, and malondialdehyde content of walnut kernels in unpackaged, PE-packaged, and Example 1-packaged kernels all increased. During storage, all three packaging methods had a certain preservation effect on the walnut kernels, but the effects differed significantly. Without packaging, the walnut kernels were in full contact with the outside environment, making the oils prone to hydrolysis and oxidation, resulting in a rapid increase in various indicators and poor preservation. PE packaging provided some barrier properties, which could delay oil oxidation and hydrolysis to a certain extent, but the effect was limited. Example 1 packaging performed excellently in blocking oxygen and inhibiting microbial contamination, effectively delaying the hydrolysis and oxidation process of oils in the walnut kernels, significantly inhibiting the increase in acid value, peroxide value, and malondialdehyde content, and achieving the best preservation effect.

[0069] Figure 2Regarding ·OH radicals, Example 1 was generally the leader across all concentrations. The membrane prepared in Comparative Example 6 had relatively high antioxidant capacity, but due to uneven zinc oxide adhesion in a single electrolyte, its ·OH radical scavenging rate was still lower than that of Example 1. Comparative Example 4 also exhibited some antioxidant capacity; its scavenging rate showed a significant upward trend and high value during the test. Comparative Example 3 performed poorly at low concentrations, but its ·OH radical scavenging ability improved with increasing concentration; its scavenging rate remained relatively stable throughout the test range. Comparative Examples 5 and 1 had relatively weak antioxidant capacity, with smaller increases in radical scavenging rate and lower values, especially Comparative Example 1, which had the lowest scavenging rate among all tested samples. Overall, in terms of improving the membrane's ability to scavenge ·OH radicals, the order of effectiveness was: Example 1 > Comparative Example 6 > Comparative Example 5 > Comparative Example 4 > Comparative Example 3 > Comparative Example 2 > Comparative Example 1. Similarly, Example 1 showed a significant advantage in scavenging DPPH radicals. As the sample concentration increased from 2 mg / mL to 8 mg / mL, the DPPH radical scavenging rate of Example 1 increased rapidly. Comparative Example 6 also showed considerable scavenging ability, but it was still inferior to Example 1. Comparative Example 4 also demonstrated some ability in the DPPH radical scavenging test, with its scavenging rate increasing with increasing sample concentration. Comparative Examples 3 and 5 showed relatively limited scavenging abilities, with smaller increases in scavenging rate. Comparative Example 1 had the lowest DPPH radical scavenging rate and poor antioxidant effect. In terms of DPPH radical scavenging ability, the ranking was: Example 1 > Comparative Example 6 > Comparative Example 4 > Comparative Example 3 > Comparative Example 5 > Comparative Example 2 > Comparative Example 1. In summary, the membrane prepared in Example 1 has a significant advantage in antioxidant activity, effectively scavenging both OH and DPPH radicals. This is attributed to the uniform adhesion of zinc oxide to the membrane, resulting in a higher content of highly active antioxidant components, thus improving the overall antioxidant performance. Figure 3 The diagram shows the convergence (convergence graph) and energy change (enthalpy graph) of the activated carbon in the mixed electrolyte system of Example 1. Figure 4 The diagram shows the convergence (convergence plot) and energy changes (enthalpy plot) of the activated carbon in the mixed electrolyte system of Comparative Example 6. Regarding energy changes, both were initially in a high-energy unstable state, but subsequently both rapidly decreased in energy and tended to stabilize, indicating that both could quickly reach a relatively stable interaction state with the electrolyte. However, in terms of convergence, Example 1 showed a significant decrease in the early stage of the convergence process, with smaller fluctuations in the later stage, indicating a more stable and regular adsorption process. While Comparative Example 6 showed a similar overall trend, its fluctuations were relatively larger, indicating slightly less stable adsorption processes. Overall, the activated carbon in Example 1 exhibited better stability and regularity in its adsorption process within the electrolyte, making it more conducive to constructing a stable adsorption system.

[0070] Table 3 presents a comparative analysis of molecular dynamics simulation data from Example 1 and Comparative Example 6, revealing the superiority of the mixed electrolyte system through comparison of key parameters. Regarding non-bond energy values, although Comparative Example 6 has more negative non-bond energy values, Example 1 ensures nanoparticle anchoring stability through stronger van der Waals forces (↑42%), and optimized electrostatic interactions (↑10%) drive a more balanced adsorption intensity for deeper loading, preventing pore blockage. Specifically, the electrostatic interaction of the mixed system in Example 1 is 10% stronger than that in Comparative Example 6. Sodium nitrate enhances ionic strength, strengthening the driving force for Zn²⁺ electromigration into the pores. Simultaneously, the 42% higher van der Waals force in the mixed system of Example 1 is due to the hydrophobic chains of CTAB enhancing physical adsorption and inhibiting ZnO nanoparticle detachment. The 42% higher RMS force in Comparative Example 6 compared to Example 1 is due to uneven deposition caused by localized stress concentration, consistent with the measured 11% decrease in tensile strength (TS). The maximum force value showed that Comparative Example 6 was 27% higher than that of Example 1. The peak stress at the pore edge easily caused microcracks and accelerated the aging of the composite membrane, which is consistent with the result that the fracture value EB was 7% higher than that of Example 1.

[0071] Table 2

[0072] Figure 4 The results showed that as storage time increased, the peak area of ​​volatile flavor compounds in walnut kernels stored under three different packaging methods (no packaging, PE packaging, and preservation film packaging as described in Example 1) all showed an increasing trend. Among them, after 28 days of storage, the peak area of ​​volatile flavor compounds in walnut kernels packaged with carboxymethyl cellulose / sodium alginate double-layer composite film was significantly lower than that in the other two groups. Moreover, this film effectively absorbed volatile aldehydes and acids in walnut kernels, reduced the formation of oxidation and rancid odor in walnut kernels, and delayed the lipid peroxidation process.

Claims

1. A double-layer composite film for preserving walnut kernels, characterized in that, It includes at least a first composite film and a second composite film stacked together; The first composite membrane is a carboxymethyl cellulose / sodium alginate composite membrane, and contains a nano zinc oxide / activated carbon composite; the second composite membrane is a carboxymethyl cellulose / sodium alginate / citric acid composite membrane. The preparation method of nano zinc oxide / activated carbon composite includes the following steps: (1) The activated carbon particles were soaked in nitric acid solution, and the nano zinc oxide was rapidly and uniformly loaded in the pores of the activated carbon using microwave-ultrasound synergistic technology; the activated carbon was washed with deionized water until neutral, and then dried; the activated carbon was mixed with polypyrrole ethanol solution, ultrasonically dispersed, and then dried. (2) Dissolve zinc nitrate and sodium nitrate in deionized water to form an electrolyte, with hexadecyltrimethylammonium bromide as a dispersant; immerse the activated carbon treated in step (1) in the electrolyte for electrolysis, and combine magnetic stirring with constant pressure deposition to allow Zn to deposit. 2+ The zinc oxide was reduced to metallic zinc and loaded into the pores of activated carbon, then oxidized to form ZnO nanoparticles; finally, it was ultrasonically cleaned with deionized water, vacuum dried and annealed to obtain a nano zinc oxide / activated carbon composite.

2. The double-layer composite film for preserving walnut kernels according to claim 1, characterized in that, In the first composite membrane, the mass ratio of carboxymethyl cellulose to sodium alginate is 1:2 to 1:4, and the mass ratio of nano zinc oxide / activated carbon composite to film-forming matrix is ​​0.11 to 0.33; in the second composite membrane, the mass ratio of carboxymethyl cellulose to sodium alginate is 1:1, and the mass ratio of citric acid to film-forming matrix is ​​0.2 to 0.

8.

3. The double-layer composite film for preserving walnut kernels according to claim 1, characterized in that, The first and / or second composite membranes also include the plasticizer glycerin, with a mass ratio of glycerin to the film-forming matrix of 0.3 to 0.

7.

4. The double-layer composite film for preserving walnut kernels according to claim 1, characterized in that, The first composite film and / or the second composite film are cast films.

5. The method for preparing the double-layer composite film for preserving walnut kernels according to any one of claims 1-4, characterized in that, The preparation of the first layer composite membrane forming solution, namely the carboxymethyl cellulose / sodium alginate composite membrane, includes the following steps: S1 Dissolves carboxymethyl cellulose in water and stirs until completely dissolved to obtain a carboxymethyl cellulose solution; dissolves sodium alginate in water and stirs and sonicates until completely dissolved to obtain a sodium alginate solution; S2 involves immersing activated carbon particles in nitric acid solution and using microwave-ultrasound synergistic technology to achieve rapid and uniform loading of nano-zinc oxide within the pores of the activated carbon; washing with deionized water until neutral, drying the activated carbon, mixing it with polypyrrole ethanol solution, ultrasonically dispersing it, and drying it. In step S3, zinc nitrate and sodium nitrate are dissolved in deionized water to form an electrolyte, with hexadecyltrimethylammonium bromide used as a dispersant. The activated carbon treated in step S2 is then immersed in the electrolyte for electrolysis, combined with magnetic stirring and constant voltage deposition to deposit Zn²⁺. + The zinc oxide was reduced to metallic zinc and loaded into the pores of activated carbon, and then oxidized to form ZnO nanoparticles. Finally, it was ultrasonically cleaned with deionized water, vacuum dried and annealed to obtain a nano zinc oxide / activated carbon composite. S4 mixes equal masses of carboxymethyl cellulose and sodium alginate solution, and adds nano zinc oxide / activated carbon composite to obtain the first layer of composite film forming solution.

6. The method for preparing the double-layer composite film for preserving walnut kernels according to claim 5, characterized in that, In step S1, carboxymethyl cellulose is dissolved in water at 55-65°C and stirred until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 14-16.0 g / L; sodium alginate is dissolved in water at 55-65°C and stirred and sonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 40.0-50.0 g / L. In step S2, the microwave-ultrasound synergistic technology conditions are 300W~400W, 40kHz, 75~85℃ for 20~40min; the activated carbon is dried in an oven at 110~115℃; the activated carbon is mixed with 3~7wt% polypyrrole ethanol solution, ultrasonically dispersed for 25~35min, and dried at 55~65℃. In step S3, the concentration of zinc nitrate is 0.25~0.35 mol / L, the concentration of sodium nitrate is 0.10~0.20 mol / L, and the concentration of hexadecyltrimethylammonium bromide is 0.02-0.04 mol / L; the activated carbon treated in step S2 is immersed in the electrolyte, the electrolyte temperature is controlled at 35~45℃, and the magnetic stirring speed is 200~400 rpm; A constant voltage of -1.2V was applied for deposition for 10-20 min, followed by switching to +0.5V for 5-10 min to oxidize and generate ZnO nanoparticles. The deposition process included 2-4 potential cycles to achieve uniform composite. Finally, the nanoparticles were ultrasonically cleaned with deionized water for 2-5 min, vacuum dried at 55-65℃, and annealed in an oven at 245-255℃. In step S4, a nano zinc oxide / activated carbon composite is added to obtain a first-layer composite film forming solution with a nano zinc oxide / activated carbon composite content ratio of 0.0033~0.0099.

7. The method for preparing the double-layer composite film for preserving walnut kernels according to claim 6, characterized in that, The preparation of the second-layer composite membrane forming solution, namely the carboxymethyl cellulose / sodium alginate / citric acid composite membrane, includes the following steps: 1) Dissolve carboxymethyl cellulose in water and stir until completely dissolved to obtain a carboxymethyl cellulose solution for later use; dissolve sodium alginate in water and stir and sonicate until completely dissolved to obtain a sodium alginate solution for later use; 2) Mix equal masses of carboxymethyl cellulose and sodium alginate solution, add citric acid, and obtain the second composite membrane forming solution.

8. The method for preparing the double-layer composite film for preserving walnut kernels according to claim 7, characterized in that, The preparation of the second-layer composite membrane forming solution, namely the carboxymethyl cellulose / sodium alginate / citric acid composite membrane, includes the following steps: 1) Dissolve carboxymethyl cellulose in water at 55~65℃ and stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 14~16.0 g / L, for later use; dissolve sodium alginate in water at 55~65℃ and stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 14~16.0 g / L, for later use; 2) Mix carboxymethyl cellulose and sodium alginate solution in equal mass, add citric acid to obtain the second layer composite membrane forming solution with a citric acid content ratio of 0.003~0.

012.

9. The method for preparing the double-layer composite film for preserving walnut kernels according to claim 8, characterized in that, The first layer of composite film forming liquid is cast into a film and dried; then the second layer of composite film forming liquid is cast into a film on its surface and dried, thus obtaining a double-layer composite film for walnut kernel preservation.

10. The method for preparing the double-layer composite film for preserving walnut kernels according to claim 9, characterized in that: The drying conditions for the first layer composite film forming solution are: drying temperature 50~60℃, drying time 8~10 h; the drying conditions for the second layer composite film forming solution are: drying temperature 60~70℃, drying time 10~12 h.

Images