Nanocellulose-based slow-release preservative film and preparation method thereof

Abstract

The invention relates to the technical field of food packaging, in particular to a nanocellulose-based slow-release preservative film and a preparation method thereof. According to the method, the preservative film with a multi-layer structure is prepared through a multi-layer composite film structure (a modified pullulan layer, a nanocellulose / essential oil slow-release layer and a chitosan layer), so that collaborative optimization of charge adsorption, slow-release bacteriostasis and mechanical strength of the preservative film is realized. Wherein the modified pullulan is subjected to positive charge modification, the gradient dialysis process is combined, the film-forming property and stability of the pullulan are remarkably improved, the prepared preservative film has enough mechanical strength and excellent antibacterial performance, and the preservation effect can be more lasting through the slow release performance of the preservative film.

Description

Technical Field

[0001] The present invention relates to the technical field of food packaging, and particularly relates to a nano-cellulose-based slow-release fresh-keeping film and a preparation method thereof. Background Art

[0002] With the progress of society and the enhancement of people's awareness of food safety and environmental protection, natural polymer biomaterials are increasingly widely used in the field of food packaging. However, the food packaging films prepared from existing single natural polymer biomaterials have significant defects in mechanical properties, mechanical performance, and antibacterial activity, which limit their effects in practical applications.

[0003] Regarding the specific defects of the food packaging films prepared from single natural polymer biomaterials, they are manifested in the following aspects: Firstly, the mechanical properties are poor. Currently, many films made of natural polymer materials are prone to rupture, and their mechanical strength is not high, unable to meet the basic requirements of food packaging; specifically, the deficiency in mechanical performance is manifested in the poor tensile strength and elongation at break of these films, which are easily damaged during transportation and storage; Secondly, the antibacterial activity is limited. The antibacterial properties of natural polymer materials themselves are weak, and it is difficult to effectively inhibit the growth of microorganisms during food storage, resulting in food spoilage.

[0004] Based on the above deficiencies existing in the actual application of the current packaging films, it limits the application of the packaging films and cannot bring better packaging effects. Therefore, it is necessary to further improve the current packaging films to meet the actual needs in view of the above deficiencies. Summary of the Invention

[0005] The purpose of the present invention is to solve the problem that the food packaging films prepared from single natural polymer biomaterials in the prior art have significant defects in their mechanical properties, mechanical performance, and antibacterial activity, and thus propose a preparation method of a nano-cellulose-based slow-release fresh-keeping film. By using this preparation method, a slow-release fresh-keeping film can be prepared.

[0006] Specifically, the present invention adopts the following technical solutions: A preparation method of a nano-cellulose-based slow-release fresh-keeping film, the preparation method comprising the following steps: Step 1: First, modify pullulan polysaccharide to make it a positively charged polysaccharide; Step 2: Dissolve the positively charged pullulan polysaccharide obtained after the modification treatment in step 1 in water to prepare an aqueous solution with a mass concentration of 1.0 wt% - 5.0 wt%, and then pour the aqueous solution into a film-forming tray and dry it into a film at 30 - 45 °C; Step 3: Adsorb plant essential oil with antibacterial function using a nanofibrillated cellulose mixture with a mass concentration of 1.0 wt% - 3.0 wt% to obtain an antibacterial mixture; Step 4: Dry the antibacterial mixture obtained in Step 3 on the film formed in Step 2 at 30 - 45 °C to form a film, and form a composite film with the film in Step 2; Step 5: Dry a 1.0 wt% - 3.0 wt% chitosan solution on the composite film formed in Step 4 at 30 - 45 °C to form a film, and finally obtain a composite film with a multi-layer structure, which is the nanofibrillated cellulose-based slow-release fresh-keeping film.

[0007] The fresh-keeping film of the present invention modifies neutral polysaccharides to make them carry active functional groups, which can combine with negatively charged nanofibrillated cellulose, and form a dense network structure through layer-by-layer self-assembly, increasing mechanical properties.

[0008] In the fresh-keeping film of the present invention, by adding polysaccharides with antibacterial effects, a dense network structure is formed through layer-by-layer self-assembly with modified polysaccharides and nanofibrillated cellulose, which is used to adsorb plant essential oil inside, extend the slow-release effect, and effectively solve the problems of fast volatilization and short effective time of plant essential oil.

[0009] Regarding the technical solution of the present invention, a design scheme of a multi-layer structure is mainly adopted. Specifically, functional synergy is achieved through a three-layer composite structure (modified pullulan polysaccharide layer, nanofibrillated cellulose / essential oil layer, chitosan layer).

[0010] In the prior art, single or double-layer membranes (such as chitosan-pullulan polysaccharide composite membranes) are used. However, membranes without positive charge modification obviously have defects. In the present invention, the combination method of using positively charged modified pullulan polysaccharide as the bottom layer, nanofibrillated cellulose adsorbing essential oil as the slow-release layer, and chitosan as the outer antibacterial layer is an unconventional technical operation in the art. The present invention achieves the purpose of solving the current technical problems through the functional complementarity (charge adsorption, essential oil slow release, antibacterial enhancement) of each layer, which requires a specific order and process support.

[0011] Preferably, in Step 1, the modification treatment of pullulan polysaccharide specifically includes the following steps: A) Activation of pullulan polysaccharide. Specifically, dissolve pullulan polysaccharide powder in dimethyl sulfoxide and stir evenly, then add carbonyldiimidazole under continuous stirring to prepare a mixed solution. The mass concentration of pullulan polysaccharide is 1.0 wt% - 5.0 t%, and the mass concentration of carbonyldiimidazole is 0.5 wt% - 2.5 wt%. Activate at 25 °C for 2 hours; B) In the product of step A), add N,N-dimethyl-1,3-propanediamine and 3-dimethylamino-1-propanamine. After addition, the mass concentration of N,N-dimethyl-1,3-propanediamine is 0.03 wt% - 0.25 wt%, and then stir and react at room temperature for 48 hours; C) Pour the product obtained in step B) into acetone and recover the precipitate by filtration; D) Dissolve the precipitate obtained in step C) in distilled water, first dialyze with acidic water, then dialyze with alkaline water to remove residual N,N-dimethyl-1,3-propanediamine, and finally dialyze with distilled water until completely deionized; E) Recover the polymer by freeze-drying the product obtained in step D), which is the modified positively charged pullulan polysaccharide.

[0012] Pullulan polysaccharide has always had a place in the research plan of food preservative films. Its significant advantages are good gas barrier properties, and almost no oxygen and carbon dioxide can pass through; in addition, its aqueous solution has a low viscosity and is not affected by temperature, pH, metal ions, etc. It has good film-forming properties, adhesiveness and antioxidant properties. However, pullulan polysaccharide is a neutral polysaccharide, which is not charged and does not have any antibacterial activity. The film formed by it has disadvantages such as being fragile and having strong hydrophilicity, which limits its application in the field of food packaging. Therefore, the present invention modifies pullulan polysaccharide.

[0013] In step B), the present invention grafts amino groups onto activated pullulan polysaccharide to improve its functional activity.

[0014] Preferably, in step B), the molar ratio of 3-dimethylamino-1-propanamine to pullulan polysaccharide is 4.12:1.

[0015] Preferably, the thickness of the film formed after drying in step 2 is 50 - 100 μm.

[0016] Preferably, the thickness of the film formed after drying in step 4 is 100 - 200 μm for the composite thickness of two layers of film.

[0017] Preferably, the thickness of the film formed after drying in step 5 is 150 - 300 μm for the thickness of three layers of film.

[0018] Preferably, the adsorption ratio of the plant essential oil in step 3 is 1.0 wt%.

[0019] A nano-cellulose-based slow-release preservative film, and the preservative film is prepared by the above preparation method.

[0020] The preservative film prepared by the present invention not only has stable mechanical properties and enhanced antibacterial properties, but also has slow-release properties.

[0021] Specifically, regarding the sustained-release performance, the adsorption of essential oils by nanocellulose and the multi-layer film structure may extend the release time of essential oils, and the freshness preservation effect is more durable compared to the single-layer film or the direct mixing system; Meanwhile, the synergistic antibacterial effect of the chitosan outer layer and plant essential oils significantly reduces the growth of microorganisms, and if the experimental data shows that its antibacterial rate is superior to that of existing single materials, the antibacterial performance is enhanced; In addition, the control of the thickness of each layer (such as the total thickness of 150 - 300 μm) may optimize the flexibility and strength of the film, avoiding the problem of easy breakage of traditional food wrap, and ensuring sufficient mechanical properties.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: The food wrap of the present invention adopts a composite film type, having good mechanical properties and mechanical strength, thereby improving the tear strength of the film, and enabling the prepared film to provide a better packaging effect with safety guarantee; Through the layer-by-layer self-assembly mode, the present invention adsorbs plant essential oils in a dense network structure, effectively extending the sustained-release effect of essential oils, and utilizing the antibacterial property of essential oils, thereby being able to improve the antibacterial performance of packaging by the principle of essential oil sustained release; The food wrap prepared by the present invention is loaded with plant essential oils having antibacterial effects, which have very strong antibacterial, antifungal and antioxidant activities, and can effectively inhibit the growth of microorganisms during the storage of fruits and vegetables, thereby extending the shelf life of fruits and vegetables; The food wrap prepared by the present invention uses natural biodegradable materials (pullulan, nanocellulose, chitosan), which conforms to the environmental protection trend, solves the environmental pollution problem of synthetic food wrap, and at the same time has both sustained-release and antibacterial functions, and has the potential for market application. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Examples of the present invention and the mechanical property effect diagrams are given.

[0024] Figure 2 Examples of the present invention and the antibacterial effect diagrams of the comparative examples are given.

[0025] Figure 3 Examples of the present invention and the sustained-release effect diagrams of plant essential oils of the comparative examples are given. DETAILED DESCRIPTION OF THE INVENTION

[0026] Now, the representative embodiments shown in the drawings will be further refined. It should be understood that the following description is not intended to limit the embodiments to a preferred embodiment. On the contrary, it is intended to cover alternative forms, modified forms and equivalent forms that may be included in the essence and scope of the described embodiments defined by the appended claims.

[0027] The present invention provides a nano-cellulose-based slow-release fresh-keeping film, which uses a new method to control food freshness. Specifically, this fresh-keeping film is prepared according to the following steps: Step 1: First, modify pullulan polysaccharide to make it a positively charged polysaccharide; The specific steps of the modification treatment are as follows: A) Activation of pullulan polysaccharide. Specifically, dissolve pullulan polysaccharide powder in dimethyl sulfoxide and stir evenly, then add carbonyldiimidazole under continuous stirring to prepare a mixed solution. The mass concentration of pullulan polysaccharide is 1.0 wt%-5.0 wt%, and the mass concentration of carbonyldiimidazole is 0.5 wt%-2.5 wt%. Activate at 25 °C for 2 hours; B) In the product of step A), add N,N-dimethyl-1,3-propanediamine and 3-dimethylamino-1-propanamine. The molar ratio of 3-dimethylamino-1-propanamine to pullulan polysaccharide is 4.12:1. After adding, the mass concentration of N,N-dimethyl-1,3-propanediamine is 0.03 wt%-0.25 wt%, and then stir and react at room temperature for 48 hours; C) Pour the product obtained in step B) into acetone, and recover the precipitate by filtration; D) Dissolve the precipitate obtained in step C) in distilled water, first dialyze with acidic water, then dialyze with alkaline water to remove residual N,N-dimethyl-1,3-propanediamine, and finally dialyze with distilled water until completely deionized; E) Recover the polymer by freeze-drying the product obtained in step D), which is the modified positively charged pullulan polysaccharide.

[0028] Step 2: Dissolve the positively charged pullulan polysaccharide obtained after the modification treatment in step 1 in water to prepare an aqueous solution with a mass concentration of 1.0 wt%-5.0 wt%. Then pour this aqueous solution into a film-forming tray and dry it to form a film at 30-45 °C. The thickness of the film formed after drying is 50-100 μm.

[0029] Step 3: Use a nano-cellulose mixture with a mass concentration of 1.0 wt%-3.0 wt% to adsorb plant essential oils with antibacterial functions to obtain an antibacterial mixture; the adsorption ratio of plant essential oils is 1.0 wt%.

[0030] Step 4: Dry the antibacterial mixture obtained in step 3 on the film formed in step 2 at 30-45 °C to form a film, and form a composite film with the film in step 2. The thickness of the film formed after drying is 100-200 μm.

[0031] Step 5: On the basis of the composite film formed in Step 4, dry the 1.0 wt% - 3.0 wt% chitosan solution at 30 - 45 °C to form a film, and finally obtain a composite film with a multi-layer structure, which is the nano-cellulose-based slow-release fresh-keeping film; the thickness of the film is 150 - 300 μm.

[0032] The following is a corresponding introduction with actual cases.

[0033] Example 1: The applicant provides a nano-cellulose-based slow-release fresh-keeping film, which is prepared according to the following steps: Step 1: First, modify pullulan polysaccharide to make it a positively charged polysaccharide. Step 2: Dissolve the positively charged pullulan polysaccharide obtained after the modification treatment in Step 1 in water to prepare an aqueous solution with a mass concentration of 1.0 wt%, then pour the aqueous solution into a film-forming tray and dry it at 30 - 45 °C to form a film; the thickness of the film formed after drying is 55 μm. Step 3: Use a 3.0 wt% nano-cellulose mixture to adsorb thymol essential oil to obtain an antibacterial mixture; the adsorption ratio of the plant essential oil is 1.0 wt%. Step 4: On the basis of the film formed in Step 2, dry the antibacterial mixture obtained in Step 3 at 30 - 45 °C to form a film, and form a composite film with the film in Step 2; the thickness of the film formed after drying is 127 μm. Step 5: On the basis of the composite film formed in Step 4, dry the 1.0 wt% chitosan solution at 30 - 45 °C to form a film, and finally obtain a composite film with a multi-layer structure, which is the nano-cellulose-based slow-release fresh-keeping film; the thickness of the film is 184 μm.

[0034] Example 2: A nano-cellulose-based slow-release fresh-keeping film is prepared according to the following steps: Step 1: First, modify pullulan polysaccharide to make it a positively charged polysaccharide. Step 2: Dissolve the positively charged pullulan polysaccharide obtained after the modification treatment in Step 1 in water to prepare an aqueous solution with a mass concentration of 3.0 wt%, then pour the aqueous solution into a film-forming tray and dry it at 30 - 45 °C to form a film; the thickness of the film formed after drying is 72 μm. Step 3: Use a 3.0 wt% nano-cellulose mixture to adsorb thymol essential oil to obtain an antibacterial mixture; the adsorption ratio of the plant essential oil is 1.0 wt%. Step 4: On the basis of the film formed in Step 2, dry the antibacterial mixture obtained in Step 3 at 30 - 45 °C to form a film, and form a composite film with the film in Step 2; the thickness of the film formed after drying is 141 μm. Step 5: Based on the composite film formed in Step 4, dry the 1.0 wt% chitosan solution at 30 - 45 °C to form a film, and finally obtain a composite film with a multi-layer structure, which is the nano-cellulose-based slow-release fresh-keeping film; the thickness of the film is 203 μm.

[0035] Example 3: A nano-cellulose-based slow-release fresh-keeping film is prepared according to the following steps: Step 1: First, modify pullulan polysaccharide to make it a positively charged polysaccharide. Step 2: Dissolve the positively charged pullulan polysaccharide obtained in Step 1 in water to prepare an aqueous solution with a mass concentration of 3.0 wt%, then pour the aqueous solution into a film-forming tray and dry it at 30 - 45 °C to form a film; the thickness of the film formed after drying is 70 μm. Step 3: Use a 1.0 wt% nano-cellulose mixed solution to adsorb thymol essential oil to obtain an antibacterial mixed solution; the adsorption ratio of the plant essential oil is 1.0 wt%. Step 4: Based on the film formed in Step 2, dry the antibacterial mixed solution obtained in Step 3 at 30 - 45 °C to form a film, and form a composite film with the film in Step 2; the thickness of the film formed after drying is 126 μm. Step 5: Based on the composite film formed in Step 4, dry the 1.0 wt% chitosan solution at 30 - 45 °C to form a film, and finally obtain a composite film with a multi-layer structure, which is the nano-cellulose-based slow-release fresh-keeping film; the thickness of the film is 179 μm.

[0036] In order to conduct a horizontal comparison of the actual effects of the fresh-keeping film prepared by the present invention, the following comparative examples are provided.

[0037] Comparative Example 1: A nano-cellulose-based fresh-keeping film different from the above method is prepared as follows: 1) Dissolve 1.0 wt% pullulan polysaccharide in water, pour it into a tray, and dry it at 30 - 45 °C to form a film. 2) Use 3.0 wt% nano-cellulose to adsorb 1.0 wt% thymol essential oil. 3) Pour the product obtained in Step 2) into the tray and dry it at 30 - 45 °C to form a film. 4) Pour 1.0 wt% chitosan solution into the tray and dry it at 30 - 45 °C to obtain a nano-cellulose-based slow-release fresh-keeping film with a thickness of 188 μm.

[0038] The difference between Comparative Example 1 and the actual operation of the present invention is obvious. The main difference is that: pullulan polysaccharide is not positively charged.

[0039] Comparative Example 2: A nanocellulose-based fresh-keeping film different from the above method is prepared as follows: Pour a 1.0 wt% chitosan solution into a tray and dry it at 30 - 45 °C to obtain a fresh-keeping film with a thickness of 52 μm.

[0040] The main difference between Comparative Example 2 and the actual operation of the present invention is that Comparative Example 2 directly uses an antibacterial fresh-keeping film prepared from a single natural macromolecule.

[0041] Perform mechanical property tests on the fresh-keeping films of Examples 1 - 3 and the corresponding Comparative Examples 1 - 2 provided in the present invention: The test method is as follows: Cut the film into rectangular pieces of 1 cm × 6 cm and measure using a texture analyzer. The test conditions are: The load cell is 25 N; Probe: A / MTG; Initial grip distance: 20 mm; Tensile rate: 2 mm / s; Use the following formula to measure the tensile strength (TS) of the film: .

[0042] In the formula: TS, tensile strength, MPa; F, the maximum tension borne at break, N; L, the thickness of the film, mm; M, the width of the film, mm.

[0043] Use the following formula to calculate the elongation at break (EB): .

[0044] In the formula: EB, elongation at break, %; L, the length of elongation when the film breaks, mm; L0, the original length of the film, mm.

[0045] The final measurement results are the mechanical property effects of the fresh-keeping film products of Examples 1 - 3 and Comparative Examples 1 - 2, as shown in Figure 1 .

[0046] According to Figure 1 It can be seen that the mechanical strength of the examples is generally stronger than that of the comparative examples because the modified pullulan polysaccharide can form a denser three-dimensional network structure with nanocellulose and chitosan through electrostatic interaction, thereby improving the tensile strength and elongation at break of the film.

[0047] The antibacterial properties of the fresh-keeping films of Examples 1-3 of the present invention and the corresponding Comparative Examples 1-2 provided were tested: The test method is as follows: Evaluate the antibacterial properties of the film against Escherichia coli. The prepared samples were placed in a constant temperature and humidity chamber (20 °C, 50% RH), and samples were taken continuously for 5 days. Each time, 0.2 g of the film was weighed and immersed in 15 mL of LB culture medium (containing 105 CFU / mL of bacterial culture), and incubated at a constant temperature of 37 °C. Samples were taken at 12 h, and the optical density was measured at 560 nm.

[0048] The final measurement results are the antibacterial effects of the fresh-keeping film products of Examples 1-3 and Comparative Examples 1-2, as shown specifically in Figure 2 .

[0049] According to Figure 2 it can be seen that Comparative Example 2 is a single chitosan film, and its antibacterial properties are poor because when chitosan forms a film morphology, the active amino groups are hidden, thus reducing the antibacterial activity of chitosan itself. The other is consistent with the trend of the slow-release effect.

[0050] The slow-release test was carried out on the fresh-keeping films of Examples 1-3 of the present invention and the corresponding Comparative Example 1 provided: The test method is as follows: For Examples 1-3 and the comparative example, 3 parallel samples were selected for each fresh-keeping film product; Then the prepared samples were placed in a constant temperature and humidity chamber (20 °C, 50% RH), and samples were taken continuously for 5 days; specifically, each time when sampling, 0.5 g was taken for each parallel sample, and 3 samples were taken for each fresh-keeping film every day; Then the selected samples were subjected to gas chromatography analysis to determine the release amount of the main components of the essential oil.

[0051] The final measurement results are the slow-release effects of the fresh-keeping film products of Examples 1-3 and the comparative example, as shown specifically in Figure 3 .

[0052] According to Figure 3 it can be seen that from the trend, the slow-release effect of the fresh-keeping film product of Example 2 is the best, and the reasons are mainly in two aspects; firstly, the concentration of pullulan polysaccharide solution in Example 2 is high, and the thickness of the film made under the same conditions is thicker; secondly, there are more active amino functional groups that can react with the nanocellulose film, resulting in stronger adsorption of thyme essential oil.

[0053] From Figure 3 it can be seen that the slow-release effect of the fresh-keeping film product of Example 3 is also good, but because the concentration of nanocellulose is low, less essential oil is adsorbed, and a large amount of essential oil volatilizes during the drying process, so the initial content of the active ingredient is low.

[0054] In Comparative Example 1, since pullulan is a neutral polysaccharide itself and cannot be self-assembled layer by layer with the nanofibrillated cellulose membrane before modification, the sustained-release effect is poor.

[0055] The fresh-keeping film prepared by the present invention not only has stable mechanical properties and enhanced antibacterial properties, but also has sustained-release properties. Through the adsorption of essential oils by nanofibrillated cellulose and the multi-layer film structure, the release time of essential oils may be extended, and the fresh-keeping effect is more durable compared with single-layer films or directly mixed systems; the synergistic antibacterial effect of the chitosan outer layer and plant essential oils is used to significantly reduce the growth of microorganisms, and the experimental data shows that its antibacterial rate is better than that of existing single materials, enhancing the antibacterial properties; the control of the thickness of each layer (such as a total thickness of 150-300 μm) may optimize the flexibility and strength of the film, avoiding the problem of easy breakage of traditional fresh-keeping films and ensuring sufficient mechanical properties.

[0056] It is obvious to those skilled in the art that, based on the above teachings, certain modifications, combinations, and variations can also be made.

Claims

1. A preparation method of a nano-cellulose-based sustained-release fresh-keeping film, characterized in that: The preparation method includes the following steps: Step 1: First, modify pullulan polysaccharide to make it a positively charged polysaccharide; Step 2: Dissolve the positively charged pullulan polysaccharide obtained after the modification treatment in Step 1 in water to prepare an aqueous solution with a mass concentration of 1.0 wt% - 5.0 wt%. Then pour the aqueous solution into a film-forming tray and dry it to form a film at 30 - 45 °C; Step 3: Use a nanofibrillated cellulose mixture with a mass concentration of 1.0 wt% - 3.0 wt% to adsorb a plant essential oil with antibacterial function to obtain an antibacterial mixture; Step 4: Dry the antibacterial mixture obtained in Step 3 on the film formed in Step 2 at 30 - 45 °C to form a film, and form a composite film with the film in Step 2; Step 5: Dry a 1.0 wt% - 3.0 wt% chitosan solution on the composite film formed in Step 4 at 30 - 45 °C to form a film, and finally obtain a composite film with a multi-layer structure, which is the nanofibrillated cellulose-based slow-release fresh-keeping film.

2. The preparation method of a nano-cellulose-based sustained-release fresh-keeping film according to claim 1, characterized in that: In Step 1, the modification treatment of pullulan polysaccharide specifically includes the following steps: A) Activation of pullulan polysaccharide. Specifically, dissolve pullulan polysaccharide powder in dimethyl sulfoxide and stir evenly, then add carbonyldiimidazole under continuous stirring to prepare a mixed solution. The mass concentration of pullulan polysaccharide is 1.0 wt% - 5.0 t%, and the mass concentration of carbonyldiimidazole is 0.5 wt% - 2.5 wt%. Activate at 25 °C for 2 hours; B) In the product of Step A), add N,N-dimethyl-1,3-propanediamine and 3-dimethylamino-1-propanamine. After adding, the mass concentration of N,N-dimethyl-1,3-propanediamine is 0.03 wt% - 0.25 wt%, and then stir and react at room temperature for 48 hours; C) Pour the product obtained in Step B) into acetone and recover the precipitate by filtration; D) Dissolve the precipitate obtained in Step C) in distilled water, first dialyze with acidic water, then dialyze with alkaline water to remove residual N,N-dimethyl-1,3-propanediamine, and finally dialyze with distilled water until completely deionized; E) Recover the polymer by freeze-drying the product obtained in Step D), which is the modified positively charged pullulan polysaccharide.

3. The preparation method of a nano-cellulose-based sustained-release fresh-keeping film according to claim 1, characterized in that: In Step B), the molar ratio of 3-dimethylamino-1-propanamine to pullulan polysaccharide is 4.12:

1.

4. The preparation method of a nano-cellulose-based sustained-release fresh-keeping film according to claim 1, characterized in that: The thickness of the film formed after drying in Step 2 is 50 - 100 μm.

5. The preparation method of a nano-cellulose-based sustained-release fresh-keeping film according to claim 1, characterized in that: The thickness of the film formed after drying in Step 4 is 100 - 200 μm, which is the composite thickness of the two layers of films.

6. The preparation method of a nanocellulose-based sustained-release fresh-keeping film according to claim 1, wherein: The thickness of the film formed after drying in Step 5 is 150 - 300 μm, which is the thickness of the three layers of films.

7. The preparation method of a nano-cellulose-based sustained-release fresh-keeping film according to claim 1, characterized in that: The adsorption ratio of the plant essential oil in Step 3 is 1.0 wt%.

8. A nano-cellulose-based slow-release fresh-keeping film, characterized in that: The fresh-keeping film is prepared by the preparation method described in any one of claims 1 - 7 above.

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