Preparation method of double-layer composite film for preserving walnut kernels

Abstract

The invention relates to a preparation method and application of a carboxymethyl cellulose / sodium alginate double-layer composite preservative film. The composite preservative film at least comprises a first-layer composite film and a second-layer composite film which are arranged in a stacked mode. Wherein the first layer of preservative film is a carboxymethyl cellulose / sodium alginate composite film and contains nano zinc oxide and activated carbon; the second-layer composite film is a carboxymethyl cellulose / sodium alginate composite film and contains citric acid. The carboxymethyl cellulose / sodium alginate composite preservative film disclosed by the invention can absorb oxidative rancidity volatile flavor substances of the walnut kernels, reduce the ultraviolet transmittance, inhibit autooxidation of the walnut kernels and achieve a relatively good preservation effect of the walnut kernels.

Description

Technical Field

[0001] The present invention relates to the technical field of food packaging materials, and particularly relates to a carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film and a preparation method thereof. Background Art

[0002] Walnuts are rich in oils, mainly unsaturated fatty acids such as linoleic acid and linolenic acid. The double-bond structure of these unsaturated fatty acids makes their chemical properties active and prone to react with oxygen. Under the action of light (especially the 280-315nm ultraviolet band) and oxygen, the rate of the free radical chain reaction of walnuts is significantly higher than that of other nut products. This oxidation process is an auto-oxidation chain reaction, and once it starts, it will continue and gradually accelerate. Therefore, during storage, when there is oxygen in the package, the unsaturated fatty acids will be oxidized, not only resulting in problems such as excessive peroxide value and acid value, but also generating low-molecular-weight volatile aldehyde and ketone odor substances such as hexanal and nonanal, leading to a sharp decline in sensory quality and also reducing the nutritional value of walnuts.

[0003] Traditional single-barrier packaging materials (such as polyethylene and aluminum-plastic composite films) can only delay primary oxidation and cannot synchronously solve the following key problems: ① ultraviolet-induced lipid photo-oxidation reaction; ② continuous accumulation of secondary oxidation products (such as malondialdehyde); ③ enrichment effect of volatile odor substances in the package headspace. These three problems together lead to a unique deterioration trajectory of walnuts during storage, namely "stable acid value in the early stage - sudden increase in odor in the middle stage - quality collapse in the later stage", which has become a technical bottleneck restricting the industrial development. At the same time, these materials have poor environmental protection and are difficult to degrade. Composite packaging materials have high costs, such as aluminum-plastic composite and paper-plastic composite, which increase the product price, and their recycling and treatment are complex, with difficult separation of different materials, easily causing resource waste and environmental pollution. These problems affect the quality maintenance, cost control, and market expansion of walnut products.

[0004] Natural polymer materials generally have many excellent properties. Among them, carboxymethyl cellulose can form a uniform, continuous and relatively strong film. This film can closely adhere to the surface of the fresh-keeping object, providing a physical barrier to prevent external factors such as dust, microorganisms and oxygen from polluting and damaging the object. In addition, carboxymethyl cellulose can be decomposed by microorganisms in the natural environment and will not cause environmental pollution. Sodium alginate is no exception. It has good film-forming performance, good biocompatibility and strong thermal stability.

[0005] Nano-zinc oxide has antibacterial properties, can inhibit microorganisms, protect walnut kernels from contamination, and can block ultraviolet rays to prevent the oxidation of walnut kernels. It can also enhance the mechanical properties of the film. Nano-activated carbon has a large number of pores and a large specific surface area, can adsorb the peculiar smell generated by walnut kernels themselves, improve the quality of walnut kernels, and at the same time adsorb harmful gases such as carbon dioxide and ethylene, slow down the respiration of walnut kernels, and is conducive to storage. In addition, nano-activated carbon can adjust the humidity to prevent walnut kernels from mildewing due to abnormal humidity. The composite electrolyte precipitation can uniformly combine and distribute nano-zinc oxide on the surface and pores of activated carbon powder, expand the specific surface area, provide more adsorption sites, optimize the diffusion path of adsorbates inside, and accelerate the mass transfer rate of adsorbates. Moreover, the combination of nano-zinc oxide and activated carbon can be better dispersed in the sodium alginate and carboxymethyl cellulose matrix, fill the gaps between matrix molecules, form a more compact filling structure, thereby enhancing the overall density of the film, improving its tensile and tear resistance, and enhancing the mechanical strength.

[0006] Through the design of the "outer oxygen barrier - inner absorption" composite film, the physical barrier is hierarchically regulated. The outer layer blocks oxygen to reduce the generation of primary oxidation products, and the inner layer of PAC / Nano-ZnO dynamically adsorbs and catalytically degrades odor substances such as hexanal and nonanal that have been generated, forming a two-stage defense of "blocking - adsorption". This can effectively prevent the oxidation of components such as oils in walnut kernels, avoid the accumulation of bad flavors in the package, delay the oxidation and deterioration process of walnut kernels, and improve the consumer's eating experience. Summary of the Invention

[0007] In view of the uniqueness of the oxidative rancidity of walnut kernels, the present invention innovatively constructs a double-layer synergistic mechanism of "outer oxygen barrier - inner adsorption": ① The outer layer of carboxymethyl cellulose / sodium alginate / citric acid composite film forms a dense network structure through citric acid cross-linking. Its water vapor transmission rate is controlled below 5.33 g / (m²·48h), and the oxygen permeability coefficient is reduced by 67% compared with the PE film, effectively blocking the penetration of external oxygen; ② In the inner layer of carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon composite film, the directional loading of nano-ZnO in the mesopores of activated carbon is realized by electrochemical deposition method to form an "adsorption - catalysis" micro-reaction unit. The pores of activated carbon can specifically capture small molecule volatiles such as hexanal and nonanal. This two-stage protection system reduces the acid value increase rate of walnut kernels by 64% in the accelerated storage experiment (60°C, 28 days), and delays the peak concentration of volatile aldehyde substances by 14 days.

[0008] To achieve the above object, the present invention provides a carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film, and the composite fresh-keeping film at least includes a first composite film and a second composite film arranged in layers; Among them, the first layer of fresh-keeping film is a carboxymethyl cellulose / sodium alginate composite film and contains a combination of activated carbon and nano-zinc oxide; the second composite film is a carboxymethyl cellulose / sodium alginate / citric acid composite film.

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

[0010] Preferably, in the carboxymethyl cellulose / sodium alginate composite fresh-keeping film, a plasticizer glycerol is further included in the first composite film and / or the second composite film, and the mass ratio of glycerol to the film-forming matrix is 0.5.

[0011] Preferably, in the carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film, the first composite film and / or the second composite film is a cast film.

[0012] In the present 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 is a cast film, which has the advantages of high production efficiency, good uniformity of the composite film, and stable performance.

[0014] On the other hand, the present invention provides a method for preparing a carboxymethyl cellulose / sodium alginate composite fresh-keeping film, including the following steps: Prepare a carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed film solution and a carboxymethyl cellulose / sodium alginate / citric acid mixed film solution; Cast the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed film solution into a film and dry it; then cast the carboxymethyl cellulose / sodium alginate / citric acid mixed film solution on its surface into a film and dry it to obtain the carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film.

[0015] Further, the preparation method of the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed film solution is as follows: S1 Dissolve carboxymethyl cellulose in water at 55 - 65°C, stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 14 - 16.0 g / L, and set aside; dissolve sodium alginate in water at 55 - 65°C, stir and ultrasonically treat until completely dissolved to obtain a sodium alginate solution with a concentration of 40.0 - 50.0 g / L, and set aside; S2 Immerse the activated carbon particles in a 10 - 15% nitric acid solution to remove metal impurities and other contaminants on the surface of the activated carbon. Combine the microwave - ultrasonic synergistic technology (300W - 400W / 40kHz ultrasonic, react at 75 - 85°C for 20 - 40 min) to achieve rapid and uniform loading of nano - zinc oxide in the pores of the activated carbon; wash with deionized water until neutral. After washing, dry the activated carbon in an oven at about 150 - 115°C until the excess water is removed; mix the activated carbon with a 3 - 7wt% polypyrrole (PPy) ethanol solution, ultrasonically disperse for 25 - 35 min, and dry at 55 - 65°C for later use; S3 Dissolve zinc nitrate and sodium nitrate in deionized water to form an electrolyte with concentrations of 0.25 - 0.35mol / L and 0.10 - 0.20mol / L respectively. Cetyltrimethylammonium bromide (CTAB) is used as a dispersant with a concentration of 0.02 - 0.04mol / L to inhibit ZnO agglomeration; immerse the activated carbon treated in step S2 into the electrolyte, control the electrolyte temperature at 35 - 45°C, and combine with a magnetic stirring rate (200 - 400 rpm) to enhance the mass transfer efficiency and reduce concentration polarization. Apply a constant voltage of - 1.2V for deposition for 10 - 20 min to reduce Zn² + to metallic zinc and load it into the pores of the activated carbon. Subsequently, switch to +0.5V and react for 5 - 10 min to oxidize and generate ZnO nanoparticles, and cycle three times to achieve uniform composite; finally, ultrasonically clean with deionized water for 2 - 5 min to remove loose particles, and vacuum - dry at 55 - 65°C and anneal in an oven at 230 - 260°C to obtain the nano - zinc oxide / activated carbon composite for later use; S4 Mix equal masses of carboxymethyl cellulose and sodium alginate solutions, and add the nano - zinc oxide / activated carbon conjugate to obtain a film - forming solution of the first - layer composite film with a nano - zinc oxide / activated carbon conjugate content of 0.0033 - 0.0099.

[0016] Preferably, dissolve carboxymethyl cellulose in water at 60°C, stir until completely dissolved to obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for later use; Dissolve sodium alginate in water at 60°C, stir and ultrasonically treat until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L for later use; Immerse the activated carbon particles in a 10% nitric acid solution to remove metal impurities and other contaminants on the surface of the activated carbon. Combine the microwave - ultrasonic synergistic co - precipitation technology (300W / 40kHz ultrasonic, react at 80°C for 30 min) to achieve rapid and uniform loading of nano - zinc oxide in the pores of the activated carbon; wash with deionized water until neutral and dry at 110°C. Mix the activated carbon with a 5wt% polypyrrole (PPy) ethanol solution, ultrasonically disperse for 30 min, and dry at 60°C to improve conductivity.

[0017] Zinc nitrate and sodium nitrate were dissolved in deionized water to form an electrolyte with concentrations of 0.30 mol / L and 0.15 mol / L respectively. Cetyltrimethylammonium bromide (CTAB) was used as a dispersant with a concentration of 0.03 mol / L. First, the treated activated carbon was immersed in the electrolyte. After magnetic stirring (200 rpm), a constant voltage of -1.2 V was applied for 10 min to reduce Zn² + to metallic zinc and deposit it in the pores of the activated carbon. Subsequently, the voltage was switched to +0.5 V for 5 min to oxidize and generate ZnO nanoparticles, and the cycle was repeated three times to achieve uniform composite. Finally, it was ultrasonically cleaned with deionized water for 2 min to remove loose particles, vacuum dried at 60 °C, and annealed in an oven at 250 °C to obtain the ZnO / activated carbon composite. A carboxymethyl cellulose and sodium alginate solution were mixed in equal mass, and the content of nano-zinc oxide / activated carbon conjugate was 0.0033 - 0.0099 to obtain the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution.

[0018] Furthermore, the preparation method of the carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution is as follows: Carboxymethyl cellulose was 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 standby. Sodium alginate was dissolved in water at 60 °C and stirred and ultrasonically treated until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L for standby. The carboxymethyl cellulose and sodium alginate solutions were mixed in equal mass, and citric acid was 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 glycerol.

[0020] Furthermore, when casting the film, the amount of the carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution is more than that of the carboxymethyl cellulose / sodium alginate / nano-activated carbon / nano-zinc oxide mixed membrane solution. Preferably, the dosage ratio of the carboxymethyl cellulose and sodium alginate mixed membrane solution to the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution is 1 - 3:1. At this time, the composite membrane has good ultraviolet blocking and opacity. When the ratio exceeds 3:1, the water vapor transmission rate of the membrane is relatively large, and the performance is the best when the ratio is 2:1.

[0021] Furthermore, the drying conditions for the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed film solution are as follows: drying temperature of 55 °C and drying time of 8 h; the drying conditions for the carboxymethyl cellulose / sodium alginate / citric acid mixed film solution are: drying temperature of 65 °C and time of 10 h. In the present invention, different drying conditions are adopted for the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed film solution and the carboxymethyl cellulose / sodium alginate / citric acid mixed film solution because the amount of the carboxymethyl cellulose / sodium alginate / citric acid mixed film solution is relatively large, so a higher drying temperature is required.

[0022] Advantages of the present invention: 1. The acquisition method combines physical synergy and electrochemical deposition, with a deposition period ≤ 65 min (measured data in Example 1). An innovative "outer oxygen barrier - inner adsorption" double-layer synergistic mechanism is constructed, reducing the acid value increase rate of walnut kernels by 64% (accelerated experiment data in Example 1), and delaying the peak concentration of volatile aldehyde substances by 14 days (see Figure 5 ). Since nano-zinc oxide is not only adsorbed on the surface of activated carbon but also undergoes pickling to remove metal impurities and other pollutants on the surface of activated carbon. This process can increase the number of surface oxygen-containing functional groups (such as carboxyl, hydroxyl, aldehyde groups, etc.), 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, making the current distribution more uniform, and its combination with activated carbon will be relatively more stable. Matrix molecules in the film-forming solution will also prevent the detachment of nano-zinc oxide to a certain extent due to the spatial limitation of pores. The electrochemical deposition process makes ZnO and activated carbon bind tightly, and the tensile strength of the composite film reaches 18.99 MPa (data in Example 1), which is 11% higher than that of Comparative Example 6. The binding energy of nanoparticles increases by 42% (see the comparison of van der Waals forces in Table 2), effectively inhibiting particle detachment.

[0023] 2. The combination of natural polymer matrix and casting method achieves synergistic enhancement. The biodegradation rate of the carboxymethyl cellulose / sodium alginate system reaches 92% (test data of GB / T19277), reducing the environmental load compared to PE films. Through the casting process, efficient composite of double-layer films is realized, and the production efficiency reaches 8.7 m² / h (measured in Example 2), which is 35% higher than that of the traditional layer-by-layer coating method. The obtained composite film has both excellent mechanical properties (elongation at break of 55.12%) and barrier properties (water vapor transmission rate of 5.33 g / (m²·48 h)), see the data of Example 1 in Table 1.

[0024] 3. By adjusting the proportions of carboxymethyl cellulose and sodium alginate in the inner and outer mixed films and adding other functional components, the present invention can endow the fresh-keeping film with different properties, such as enhancing mechanical strength, reducing film thickness, decreasing water vapor transmission rate (see Table 1), and improving film antioxidant properties (see Figure 2), etc. In particular, it can absorb the oxygen around the walnut kernels during storage, adsorb the odor substances generated by the walnut kernels, slow down the oxidation reaction of the oil and protein in the walnut kernels, and reduce the rate of oxidative deterioration. Description of the Drawings

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

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

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

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

[0029] Figure 5 Total peak area and relative peak area ratio of volatile flavor substances in walnut kernels under accelerated oxidation conditions. Detailed Implementation Modes

[0030] The present invention will be further described below in conjunction with specific embodiments. A more detailed description of the present invention is given, but it is not limited to these embodiments.

[0031] Example 1 This example provides a carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film, and its preparation method includes the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in water at 60°C, stir until completely dissolved, and obtain a carboxymethyl cellulose solution with a concentration of 15.0 g / L for standby; (2) Preparation of sodium alginate solution 1 Dissolve sodium alginate in water at 60°C, stir and ultrasonicate until completely dissolved, and obtain a sodium alginate solution with a concentration of 45.0 g / L for standby; (3) Preparation of sodium alginate solution 2 Dissolve sodium alginate in water at 60°C, stir and ultrasonicate until completely dissolved, and obtain a sodium alginate solution with a concentration of 15.0 g / L for standby; (4) Pretreatment of activated carbon The activated carbon was pickled with 10% nitric acid under the synergistic co-precipitation of microwave and ultrasound (300W / 40kHz ultrasound, reacting at 80°C for 30 min). After pickling, the activated carbon was washed thoroughly with deionized water to remove the residues in the acidic solution. After washing, the activated carbon was dried in an oven at 110°C. The treated activated carbon was mixed with a 5wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 30 min, and dried at 60°C to enhance conductivity, and then reserved for use; (5) Grafting zinc oxide on the surface of activated carbon Zinc nitrate and sodium nitrate were selected and dissolved in deionized water to form an electrolyte with concentrations of 0.30 mol / L and 0.15 mol / L respectively. Cetyltrimethylammonium bromide (CTAB) was used as a dispersant with a concentration of 0.03 mol / L. First, the treated activated carbon was immersed in the electrolyte. After magnetic stirring (200 rpm), a constant voltage of -1.2 V was applied for 10 min to reduce Zn² + to metallic zinc and load it into the pores of the activated carbon. Subsequently, the voltage was switched to +0.5 V and reacted for 5 min to oxidize and generate ZnO nanoparticles. The cycle was repeated three times to achieve uniform composite. Finally, it was ultrasonically cleaned with deionized water for 2 min to remove loose particles, vacuum dried at 60°C and annealed in an oven at 250°C to obtain the ZnO / activated carbon composite, and then reserved for use; (6) Preparation of carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution The carboxymethyl cellulose and sodium alginate solutions 1 were mixed in equal mass, and glycerol with a mass ratio of 0.5 to the film-forming matrix was added, and stirred for 30 min. The nano-zinc oxide / activated carbon conjugate was added and stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution with a nano-zinc oxide / activated carbon conjugate content of 0.0033.

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

[0033] (8)Preparation of carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film Take 30 mL of the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution on a 15×15 cm 2The film was cast on a polytetrafluoroethylene plate, dried for 8 h in a constant temperature and humidity chamber at 55°C and 50% humidity, and then 70 mL of a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid was added to the surface, cast into a film, dried for 10 h in a constant temperature and humidity chamber at 65°C and 50% humidity, and then the film was removed. The carboxymethyl cellulose / sodium alginate double-layer composite cling film was obtained.

[0034] Example 2 This embodiment provides a carboxymethyl cellulose / sodium alginate double-layer composite cling film, and the preparation method thereof comprises the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in 60°C water 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 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which is set aside; (3) Preparation of sodium alginate solution 2 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which is set aside; (4) Pretreatment of activated carbon The activated carbon was acid-washed with 10% dilute hydrochloric acid under the action of microwave-ultrasonic synergistic coprecipitation (300W / 40kHz ultrasound, 80℃ reaction for 30min). After acid washing, the activated carbon was fully washed with deionized water to remove the residue in the acid solution. After washing, the activated carbon was dried in an oven at 110℃. The treated activated carbon was mixed with 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 on the surface of activated carbon Zinc nitrate and sodium nitrate were dissolved in deionized water with concentrations of 0.30 mol / L and 0.15 mol / L, respectively, to form an electrolyte. Hexadecyltrimethylammonium bromide (CTAB) was used as a dispersant with a concentration of 0.03 mol / L. The treated activated carbon was first immersed in the electrolyte, and after magnetic stirring (200 rpm), a constant voltage of -1.2 V was applied for 10 min to deposit Zn² + The ZnO / activated carbon composite was reduced to metallic zinc and loaded in the pores of activated carbon, and then switched to +0.5 V for 5 min to oxidize and generate ZnO nanoparticles, and the reaction was repeated three times to achieve uniform composite. Finally, the composite was ultrasonically cleaned with deionized water for 2 min to remove loose particles, vacuum dried at 60 °C, and annealed in an oven at 250 °C to obtain a ZnO / activated carbon composite for later use. (6) Preparation of carboxymethyl cellulose / sodium alginate / nano-activated carbon / nano-zinc oxide mixed membrane solution Mix equal masses of carboxymethyl cellulose and sodium alginate solution, add glycerol in a mass ratio of 0.5 to the film-forming matrix, stir for 30 min, add nano zinc oxide and nano activated carbon mixed solution, stir for 30 min, and obtain a carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution with a nano zinc oxide / activated carbon conjugate content of 0.0066.

[0035] (7) Preparation of carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution Mix equal amounts of carboxymethyl cellulose and sodium alginate solution, add glycerol at a mass ratio of 0.5 to the film-forming matrix, and stir for 30 minutes. Add citric acid and stir for 30 minutes to obtain a carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution with a citric acid content of 0.006.

[0036] (8) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite cling film Take 50 mL of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution and place it in a 15×15 cm 2 The film was cast on a polytetrafluoroethylene plate, dried for 8 h in a constant temperature and humidity chamber at 55°C and 50% humidity, and then 50 mL of a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid was added to the surface, cast into a film, dried for 10 h in a constant temperature and humidity chamber at 65°C and 50% humidity, and then the film was removed. The carboxymethyl cellulose / sodium alginate double-layer composite cling film was obtained.

[0037] Example 3 This embodiment provides a carboxymethyl cellulose / sodium alginate double-layer composite cling film, and the preparation method thereof comprises the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in 60°C water 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 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which is set aside; (3) Preparation of sodium alginate solution 2 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which is set aside; (4) Pretreatment of activated carbon The activated carbon was pickled with 10% nitric acid under the synergistic co-precipitation of microwave and ultrasound (300 W / 40 kHz ultrasound, reacting at 80 °C for 30 min). After pickling, the activated carbon was thoroughly washed with deionized water to remove the residues in the acidic solution. After washing, the activated carbon was dried in an oven at 110 °C. The treated activated carbon was mixed with a 5 wt% polypyrrole (PPy) ethanol solution, ultrasonically dispersed for 30 min, and dried at 60 °C to improve conductivity and set aside; (5) Grafting of zinc oxide on the surface of activated carbon Zinc nitrate and sodium nitrate were selected and dissolved in deionized water to form an electrolyte with concentrations of 0.30 mol / L and 0.15 mol / L respectively. Cetyltrimethylammonium bromide (CTAB) was used as a dispersant with a concentration of 0.03 mol / L. First, the treated activated carbon was immersed in the electrolyte, and after magnetic stirring (200 rpm), a constant voltage of -1.2 V was applied for 10 min to reduce Zn² + to metallic zinc and load it into the pores of the activated carbon. Subsequently, the voltage was switched to +0.5 V for 5 min to oxidize and generate ZnO nanoparticles, and the cycle was repeated three times to achieve uniform composite; finally, it was ultrasonically cleaned with deionized water for 2 min to remove loose particles, vacuum dried at 60 °C and annealed in an oven at 250 °C to obtain the ZnO / activated carbon composite and set aside; (6) Preparation of carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution The carboxymethyl cellulose and sodium alginate solutions 1 were mixed in equal mass, glycerol with a mass ratio of 0.5 to the film-forming matrix was added, and stirred for 30 min. Then, the nano-zinc oxide and nano-activated carbon mixed solution was added and stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution with a nano-zinc oxide / activated carbon conjugate content of 0.0099.

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

[0039] (8) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite fresh-keeping film Take 70 mL of the carboxymethyl cellulose / sodium alginate / nano-zinc oxide / activated carbon mixed membrane solution in a 15×15 cm 2The film was cast on a polytetrafluoroethylene plate, dried for 8 h in a constant temperature and humidity chamber at 55°C and 50% humidity, and then 30 mL of a mixed film solution of carboxymethyl cellulose / sodium alginate / citric acid was added to the surface, cast into a film, dried for 10 h in a constant temperature and humidity chamber at 65°C and 50% humidity, and then the film was removed. The carboxymethyl cellulose / sodium alginate double-layer composite cling film was obtained.

[0040] Comparative Example 1 This comparative example provides a pure carboxymethyl cellulose / sodium alginate composite film, and its preparation method comprises the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in 60°C water 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 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which is set aside; (3) Preparation of carboxymethyl cellulose / sodium alginate mixed membrane solution The carboxymethyl cellulose and sodium alginate solution were mixed in equal weights, and glycerol was added in a mass ratio of 0.5 to the film-forming matrix, and stirred for 30 min to obtain a carboxymethyl cellulose / sodium alginate mixed membrane 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 2 The film was cast on a polytetrafluoroethylene plate, dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 h, and then the film was 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, and its preparation method comprises the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in 60°C water 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 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which is set aside; (3) Dissolve the activated carbon powder in water by stirring and set aside; (4) Preparation of carboxymethyl cellulose / sodium alginate / nano-activated carbon mixed membrane solution Mix equal masses of carboxymethyl cellulose and sodium alginate solution 1, add glycerol with a mass ratio of 0.5 to the film-forming matrix, and stir for 30 min. Add the activated carbon sample solution and stir for 30 min to obtain a carboxymethyl cellulose / sodium alginate / activated carbon mixed film solution with an activated carbon content of 0.0003; (5)Preparation of carboxymethyl cellulose / sodium alginate / activated carbon composite film Take 100 mL of the carboxymethyl cellulose / sodium alginate / activated carbon mixed film solution and cast it into a film on a 15×15 cm 2 polytetrafluoroethylene plate, dry it in a constant temperature and humidity box at a temperature of 55 °C and a humidity of 50% for 8 h, and then peel off the film. The carboxymethyl cellulose / sodium alginate / activated carbon composite film can be obtained.

[0043] Comparative Example 3 This comparative example provides a carboxymethyl cellulose / sodium alginate / nanozinc oxide / activated carbon composite film, and its preparation method 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 standby; (2)Preparation of sodium alginate solution 1 Dissolve sodium alginate in water at 60 °C, stir and ultrasonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L for standby; (3)Dissolve activated carbon and nanozinc oxide in water by stirring for standby; (4)Preparation of carboxymethyl cellulose / sodium alginate / nanoactivated carbon / nanozinc oxide mixed film solution Mix equal masses of carboxymethyl cellulose and sodium alginate solution 1, add glycerol with a mass ratio of 0.5 to the film-forming matrix, stir for 30 min, add the nanozinc oxide and activated carbon mixed solution, and stir for 30 min to obtain a carboxymethyl cellulose / sodium alginate / nanozinc oxide / activated carbon mixed film solution with nanozinc oxide and activated carbon contents of 0.003 and 0.0003 respectively.

[0044] (5)Preparation of carboxymethyl cellulose / sodium alginate composite fresh-keeping film Take 100 mL of the carboxymethyl cellulose / sodium alginate / nanozinc oxide / activated carbon mixed film solution and cast it into a film on a 15×15 cm 2 polytetrafluoroethylene plate, dry it in a constant temperature and humidity box at a temperature of 55 °C and a humidity of 50% for 8 h, and then peel off the film. The carboxymethyl cellulose / sodium alginate / nanozinc oxide / activated carbon composite film can be obtained.

[0045] Comparative Example 4 This comparative example provides a carboxymethyl cellulose / sodium alginate / citric acid composite film, and its preparation method comprises the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in 60°C water 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 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which is set aside; (3) Preparation of carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution Mix equal amounts of carboxymethyl cellulose and sodium alginate solution, add glycerol at a mass ratio of 0.5 to the film-forming matrix, and stir for 30 minutes. Add citric acid and stir for 30 minutes to obtain a carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution with a citric acid content of 0.003.

[0046] (4) Preparation of carboxymethyl cellulose / sodium alginate / citric acid composite film Take 100 mL of carboxymethyl cellulose / sodium alginate / citric acid mixed membrane solution and place it in a 15×15 cm 2 The film was cast on a polytetrafluoroethylene plate, dried in a constant temperature and humidity chamber at 55°C and 50% humidity for 8 h, and then the film was 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 cling film, and the preparation method thereof comprises the following steps: (1) Preparation of carboxymethyl cellulose solution Dissolve carboxymethyl cellulose in 60°C water 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 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 45.0 g / L, which is set aside; (3) Preparation of sodium alginate solution 2 Dissolve sodium alginate in 60°C water, stir and sonicate until completely dissolved to obtain a sodium alginate solution with a concentration of 15.0 g / L, which is set aside; (4) Suspend 1 g of nano-zinc oxide in 100 mL of water and ultrasonicate for 40 min. In another flask, suspend the activated carbon sample in 20 mL of water and ultrasonicate for 20 min. Then add the activated carbon to the nano-zinc oxide suspension under continuous stirring, and heat the resulting mixture at 80 °C until the water is completely evaporated. Dry the remaining solid in an oven at 110 °C overnight to obtain a nano-zinc oxide / activated carbon combination for later use; (5) Preparation of mixed membrane solution of carboxymethyl cellulose / sodium alginate / nano activated carbon / nano zinc oxide Mix equal amounts of carboxymethyl cellulose and sodium alginate solution, add glycerol at a mass ratio of 0.5 to the film-forming matrix, and stir for 30 minutes. Add nano zinc oxide / activated carbon conjugate and stir for 30 minutes to obtain a carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution with a nano zinc oxide / activated carbon conjugate content of 0.0033.

[0048] (6) Preparation of carboxymethyl cellulose / sodium alginate mixed membrane solution Mix 2 equal masses of carboxymethyl cellulose and sodium alginate solution, add glycerol in a mass ratio of 0.5 to the film-forming matrix, and stir for 30 min to obtain a carboxymethyl cellulose / sodium alginate mixed membrane solution.

[0049] (7) Preparation of carboxymethyl cellulose / sodium alginate double-layer composite cling film Take 30 mL of carboxymethyl cellulose / sodium alginate / nano zinc oxide / activated carbon mixed membrane solution and place it in a 15×15 cm 2 The film was cast on a polytetrafluoroethylene plate, dried for 8 h in a constant temperature and humidity chamber at 55°C and 50% humidity, and then 70 mL of carboxymethyl cellulose / sodium alginate mixed film solution was added on the surface, cast into a film, dried for 10 h in a constant temperature and humidity chamber at 65°C and 50% humidity, and then the film was removed. The carboxymethyl cellulose / sodium alginate double-layer composite cling film was obtained.

[0050] Comparative Example 6 This comparative example provides a carboxymethyl cellulose / sodium alginate double-layer composite cling film, which is based on Example 1, except 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 after magnetic stirring (200 rpm), a constant voltage of -1.2 V was applied for 10 min to deposit Zn² +It was reduced to metallic zinc and loaded into the pores of activated carbon. Subsequently, it was switched to +0.5 V for 5 min of reaction to oxidize and generate ZnO nanoparticles. After three cycles, uniform composite was achieved. Finally, it was ultrasonically cleaned with deionized water for 2 min to remove loose particles, vacuum dried at 60 °C and annealed in an oven at 250 °C to obtain the ZnO / activated carbon composite for standby. The subsequent preparation method was exactly the same as that of Example 1.

[0051] Performance Test 1. According to GB13022-91 "Test Method for Tensile Properties of Plastic Films", the tensile strength and elongation at break of the film were measured with a universal testing machine, and the thickness of the film was measured with a thickness gauge.

[0052] (1) The tensile strength (TS) was calculated as shown in Formula 1: TS = Fmax / (L × W) (Formula 1) TS represents the tensile strength (Mpa), Fmax is the maximum tensile force borne by the film at break, L represents the width of the film, and W represents the thickness of the film.

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

[0054] 2. According to GB1307-88 "Test Method for Water Vapor Transmission Rate of Plastic Films and Sheets - Cup Method", the water vapor transmission rate of the film was tested. The specific operation was as follows: anhydrous calcium chloride after drying was added into the moisture permeation cup, 3 mm away from the bottle mouth. The bottle mouth was tightly tied with the prepared fresh-keeping film and sealed with paraffin. It was placed in a dryer with saturated sodium chloride solution at the bottom. After 48 h, the weighing bottle was taken out and weighed until the difference in mass increase between the previous and the next time was less than 5%. It was weighed three times and the average value was calculated. The calculation was as shown in Formula 3: WVP = Δm × d / (A × Δt × Δp) (Formula 3) Among them, WVP represents the water vapor transmission coefficient [g / (m 2 ·48 h)]; d represents the thickness of the composite film (mm); Δm represents the mass increase after stabilization (g); A represents the area of the cut film (cm 2 ), Δt represents the time interval between each measurement. In this experiment, it was taken as 48 h, and Δp represents the water vapor pressure difference fixed on both sides of the composite film (Pa). The experimental temperature was 25 °C, and the relative humidity was kept at 95%.

[0055] 3. Physicochemical index test on the fresh-keeping effect of the film on walnut kernels The acid value (AV) was determined by titration according to GB5009.229—2016.

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

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

[0058] 4. Test of antioxidant activity of the membrane DPPH scavenging rate: The sample membrane was mixed with DPPH methanol solutions (5.0 mL, 0.2 mmol / L) at concentrations of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mg / mL. Each solution was kept in the dark at 25 °C for 60 min, and then the absorbance (A1) at 517 nm was measured. The absorbance of the DPPH methanol solution was A0, and A2 was the absorbance of the sample membrane in the methanol solution. The scavenging rate was calculated according to formula 4.

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

[0060] (2) Setup of the electrolyte environment Example 1 (containing NaNO3, Zn(NO3)2 / CTAB), solvent water molecules: Use the Amorphous Cell module to create a water box (1000 H2O molecules). Add solutes: Add Zn(NO3)2 (0.3 mol / L), NaNO3 (0.15 mol / L), and CTAB (0.03 mol / L) according to the concentration ratio. Comparative Example 6 (without NaNO3 / CTAB) Only add Zn(NO3)2 (0.3 mol / L) and H2O, and the remaining steps are the same 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 Equilibration stage (NPT ensemble), module: Forcite → Molecular Dynamics; force field: COMPASS III (check electrostatic and van der Waals interactions); ensemble: NPT (temperature 298 K, pressure 1 atm, temperature control method: Berendsen); time step: 1 fs, total duration: 500 ps; monitor the density and temperature fluctuations of the system (ensure the fluctuations < 5%); export the equilibrated structure file (for subsequent adsorption simulations).

[0062] 6. Test on the effect of the film on the volatile flavor substances of walnut kernels Seal a certain amount of walnut kernels in PE packaging films and in the packaging films made based on Example 1 respectively, and place them in an incubator at 60 °C for accelerated oxidation. At the same time, put an equal amount of unpacked walnut kernels for a control experiment. Take out the samples every 7 days for gas chromatography - mass spectrometry analysis to analyze the changes in the 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 under a water bath condition of 50 °C, insert a 50 / 30 μm DVB / CAR / PDMS extraction head to adsorb for 30 min, and desorb at 250 °C in the injection port for 5 min.

[0064] Gas chromatography - mass spectrometry conditions: The chromatographic column is DB - WAX (30 m × 0.25 mm × 0.25 μm). The initial column temperature is 40 °C (held for 2 min), then it is raised to 200 °C at a rate of 5 °C / min, and then raised to 300 °C at a rate of 20 °C / min (held for 8 min). Inject without splitting at a flow rate of 1 mL / min. The injection port temperature is 230 °C. Use helium as the carrier gas (purity ≥ 99.999%). The EI ion source and transfer line temperature are 240 °C; the electron energy is 70 eV; the scanning range is 40 - 450 m / z; the solvent delay is 4 min.

[0065] Volatile substances were identified by comparing the retention indices (RI) of n-alkane standards C7 - C30 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 method.

[0066] Performance test results Table 1 shows the film thickness, mechanical properties and water vapor transmission coefficient 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 films prepared in Comparative Examples 1 - 6 and Examples 1 - 3. It can be seen from Comparative Examples 1 - 6 in Table 1 that the addition of contents in the film will increase the film thickness. Among them, adding activated carbon can improve the elongation at break of the film, and adding nano-zinc oxide can enhance the tensile strength of the film and reduce the water vapor transmission rate of the film. This is because nano-zinc oxide can be evenly dispersed and exist in the film, and has good interfacial interaction with carboxymethyl cellulose and sodium alginate, playing a filling role in the film.

[0068] Figure 1 It shows that with the increase of oxidation days, the acid value, peroxide value and malondialdehyde content of walnuts without packaging, PE packaging and Example 1 packaging all show an upward trend. During storage, the three packaging methods all have a certain fresh-keeping effect on walnuts, but the effects are significantly different. Without packaging, walnuts are in full contact with the outside world, and the oil is easy to hydrolyze and oxidize, and the indicators rise rapidly, with a poor fresh-keeping effect. PE packaging has a certain barrier property, which can delay the oxidation and hydrolysis of oil to a certain extent, but the effect is limited. Example 1 packaging performs excellently in terms of oxygen barrier and inhibition of microbial contamination, can effectively delay the hydrolysis and oxidation process of the oil in walnuts, and significantly inhibit the increase of acid value, peroxide value and malondialdehyde content, with the best fresh-keeping effect.

[0069] Figure 2In terms of ·OH free radicals, at each content level, Example 1 mostly takes the leading position. The film prepared in Comparative Example 6 has relatively high antioxidant capacity. However, due to the uneven attachment of zinc oxide in a single electrolyte, its ·OH free radical scavenging rate is still lower than that of Example 1. Comparative Example 4 also has a certain antioxidant capacity. During the test, its scavenging rate shows an obvious upward trend and the value is relatively high. Comparative Example 3 performs poorly at low contents, but as the content increases, its ability to scavenge ·OH free radicals improves; within the entire test range, its scavenging rate shows relatively stable performance. Comparative Example 5 and Comparative Example 1 have relatively weak antioxidant capacity, with a small increase in the free radical scavenging rate and relatively low values. Especially for Comparative Example 1, its scavenging rate is the lowest among all the tested objects. Generally speaking, in terms of improving the scavenging ability of the film for ·OH free radicals, the effectiveness ranking is: Example 1 > Comparative Example 6 > Comparative Example 5 > Comparative Example 4 > Comparative Example 3 > Comparative Example 2 > Comparative Example 1. Similarly, Example 1 has a significant advantage in scavenging DPPH free radicals. As the sample content increases from 2 mg / mL to 8 mg / mL, the DPPH free radical scavenging rate of Example 1 increases rapidly. The scavenging ability of Comparative Example 6 is also considerable, but there is still a gap compared with Example 1. Comparative Example 4 also shows a certain ability in the DPPH free radical scavenging test, and its scavenging rate increases as the sample content increases. The scavenging abilities of Comparative Example 3 and Comparative Example 5 are relatively limited, with a small increase in the scavenging rate. The DPPH free radical scavenging rate of Comparative Example 1 is the lowest, and its antioxidant effect is poor. In terms of the ability to scavenge DPPH free radicals, the ranking is: Example 1 > Comparative Example 6 > Comparative Example 4 > Comparative Example 3 > Comparative Example 5 > Comparative Example 2 > Comparative Example 1. Based on the above two test results, it can be seen that the film prepared in Example 1 has obvious advantages in antioxidant activity and can effectively scavenge ·OH free radicals and DPPH free radicals. This is attributed to the uniform attachment of zinc oxide on the film, which makes the film contain more highly active antioxidant components, thus improving the overall antioxidant performance. Figure 3 Shows the convergence situation (Convergence - Convergence graph) and energy change (Enthalpy - Energy graph) of the activated carbon in Example 1 in the mixed - system electrolyte. Figure 4 Shows the convergence situation (Convergence - Convergence graph) and energy change (Enthalpy - Energy graph) of the activated carbon in Comparative Example 6 in the mixed - system electrolyte. In terms of energy change, both initially are in a high - energy unstable state, and then both can rapidly reduce energy and tend to be stable, indicating that both can quickly reach a relatively stable interaction state with the electrolyte. However, in terms of the convergence situation, the decline in the early stage of the convergence process of Example 1 is obvious, and the fluctuation is small in the later stage, and the adsorption process is more stable and more regular; although the overall trend of Comparative Example 6 is similar, the fluctuation is relatively large, and the stability of the adsorption process is slightly inferior. Generally speaking, the stability and regularity of the adsorption process of the activated carbon in Example 1 in the electrolyte are better, which is more conducive to constructing a stable adsorption system.

[0070] The data in Table 3 are the comparative analysis of the molecular dynamics simulation data of Example 1 and Comparative Example 6. The superiority of the mixed electrolyte system is revealed by comparing the key parameters. Regarding the non-bond energy value, although the non-bond energy value of Comparative Example 6 is more negative, Example 1 ensures the anchoring stability of nanoparticles through stronger van der Waals forces (↑42%), and the optimized electrostatic interaction (↑10%) drives a more balanced adsorption intensity for deep loading to avoid pore blockage. Specifically, the electrostatic interaction of the mixed system in Example 1 is 10% stronger than that in Comparative Example 6. Sodium nitrate increases the ionic strength and enhances the electro-migration driving force of Zn² + towards the interior of the pores; at the same time, the van der Waals force of the mixed system in Example 1 is 42% higher because the hydrophobic chain of CTAB enhances physical adsorption and inhibits the shedding of ZnO nanoparticles. The RMS force of Comparative Example 6 is 42% higher than that of Example 1, which is due to local stress concentration resulting in uneven deposition, consistent with the result that the measured tensile strength TS decreases by 11%. The value of the maximum force shows that Comparative Example 6 is 27% higher than Example 1. The stress peak at the pore edge is likely to cause microcracks and accelerate the aging of the composite membrane, which is consistent with the result that the fracture value EB is 7% higher than that of Example 1.

[0071] Table 2

[0072] Figure 4 It shows that with the increase of storage time, the peak areas of volatile flavor substances of walnuts stored in 3 different packaging methods (unpacked, PE-packaged, and packaged with the fresh-keeping film of Example 1) all show an upward trend. Among them, on the 28th day of storage, the peak area of volatile flavor substances of walnuts packaged with the carboxymethyl cellulose / sodium alginate double-layer composite film is significantly lower than the other two groups, and this film effectively absorbs aldehyde and acid volatile compounds in walnuts, reduces the formation of oxidation and rancid odors in walnuts, and delays the process of lipid peroxidation.

Claims

1. A double-layer composite film for preserving walnut kernels, characterized in that: At least comprising a first composite film and a second composite film which are stacked; The first film layer is a carboxymethyl cellulose / sodium alginate composite film, and contains activated carbon and nano zinc oxide; the second film layer is a carboxymethyl cellulose / sodium alginate / citric acid composite film.

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

3. According to the double-layer composite film for preserving walnut kernels according to claim 1, the first composite film and / or the second composite film further comprises a plasticizer glycerol, and the mass ratio of glycerol to the film-forming matrix is ​​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 is a cast film.

5. The method for preparing the double-layer composite film for preserving walnut kernels according to any one of claims 1 to 4, characterized in that: The preparation of the first layer of composite film-forming liquid, i.e., carboxymethyl cellulose / sodium alginate composite film, comprises the following steps: S1: dissolving carboxymethyl cellulose in water, stirring until completely dissolved, to obtain a carboxymethyl cellulose solution; dissolving sodium alginate in water, stirring and ultrasonicating until completely dissolved, to obtain a sodium alginate solution; S2: Soaking activated carbon particles in nitric acid solution, using microwave-ultrasound synergistic technology to achieve rapid and uniform loading of nano zinc oxide in the pores of activated carbon; washing with deionized water until neutral, drying the activated carbon after washing; mixing the activated carbon with polypyrrole ethanol solution, ultrasonically dispersing, and drying; S3: dissolving zinc nitrate and sodium nitrate in deionized water to form an electrolyte, and using hexadecyltrimethylammonium bromide as a dispersant; immersing the activated carbon treated in step S2 in the electrolyte for electrolysis, and applying constant pressure deposition combined with magnetic stirring to make Zn² + The zinc is reduced to metal and loaded in the pores of activated carbon, and then oxidized to generate ZnO nanoparticles; finally, it is ultrasonically cleaned with deionized water, vacuum dried and annealed to obtain a nano zinc oxide / activated carbon composite; S4: Mix carboxymethyl cellulose and sodium alginate solution in equal mass, add nano zinc oxide / activated carbon combination, and obtain the first layer composite membrane forming solution.

6. The method for preparing a double-layer composite film for preserving walnut kernels according to claim 5, characterized in that: In the 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 ultrasonicated until completely dissolved to obtain a sodium alginate solution with a concentration of 40.0-50.0 g / L; In the step S2, the microwave-ultrasound synergistic technical conditions are 300W~400W / 40kHz ultrasound, 75~85°C reaction for 20~40min; drying the activated carbon in an oven at 110~115°C; mixing the activated carbon with 3~7wt% polypyrrole ethanol solution, ultrasonically dispersing for 25~35min, and drying at 55~65°C; In the 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 to be 35-45°C, and the magnetic stirring rate is 200-400 rpm; Apply -1.2V constant voltage deposition for 10-20min, then switch to +0.5V for 5-10min oxidation to generate ZnO nanoparticles, the electrochemical deposition includes 2-4 potential cycles to achieve uniform composite; finally, use deionized water for ultrasonic cleaning for 2-5min, vacuum drying at 55-65℃ and annealing in an oven at 245-255℃; In the step S4, a nano zinc oxide / activated carbon combination is added to obtain the first layer composite membrane forming solution having a nano zinc oxide / activated carbon combination content of 0.0033-0.0099.

7. The method for preparing the double-layer composite film for preserving walnut kernels according to any one of claims 1 to 4, characterized in that: The preparation of the second layer composite film-forming liquid, i.e., the carboxymethyl cellulose / sodium alginate / citric acid composite film, comprises the following steps: 1) Dissolve carboxymethyl cellulose in water, stir until completely dissolved, obtain a carboxymethyl cellulose solution with a concentration of 1.54, and set aside; dissolve sodium alginate in water, stir and ultrasonicate until completely dissolved, obtain a sodium alginate solution, and set aside; 2) Mix equal amounts of carboxymethyl cellulose and sodium alginate solution, add citric acid, and obtain the second layer composite membrane forming solution.

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

012.

9. The method for preparing a 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 on the surface of the film and dried to obtain a double-layer composite film for preserving walnut kernels.

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

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