A degradable fresh-keeping film for keeping fruits fresh and preparation method thereof

Abstract

The present invention discloses a degradable cling film for fruit preservation and a preparation method thereof, and belongs to the technical field of cling film. The cling film comprises the following raw materials in parts by weight: 40-60 parts of polylactic acid, 15-25 parts of polyvinyl alcohol, 6-12 parts of cellulose, 10-20 parts of modified chitosan, and 3-9 parts of lubricant. The prepared cling film is based on polylactic acid, which gives the cling film high transparency, heat resistance and degradability; wherein the polyvinyl alcohol is a degradable material that can enhance the gas barrier property of the cling film; wherein the cellulose can improve the thermal stability and mechanical properties of the cling film; wherein the modified chitosan can enhance the antibacterial property, heat resistance, water barrier property and mechanical property of the cling film, and the performance is long-lasting and stable; therefore, the cling film prepared by the present invention has excellent mechanical properties, good barrier property, stable and efficient antibacterial and heat resistance, and is environmentally friendly and degradable, and has important application value in the technical field of cling film.

Description

Technical Field

[0001] The invention belongs to the technical field of fresh-keeping films, and in particular relates to a degradable fresh-keeping film for preserving fruits and a preparation method thereof. Background Art

[0002] With the improvement of people's living standards and the acceleration of the pace of life, fresh-cut fruits are becoming increasingly popular due to their convenience, health and nutrition. However, since fresh-cut fruits need to be peeled and cut into pieces, the integrity of the fruits is damaged and they are very prone to spoilage. Therefore, the preservation of fresh-cut fruits has become a hot research topic in recent years. Common methods for preserving fruits in daily life are to refrigerate the fruits or seal them with plastic wrap.

[0003] Cling film is a plastic packaging product typically made from ethylene through polymerization. It can be divided into three categories: polyethylene (PE); polyvinyl chloride (PVC); and polyvinylidene chloride (PVDC). It is widely used in household applications, supermarkets, hotels, restaurants, and industrial food packaging, including microwave heating, refrigerator food storage, and fresh and cooked food packaging. However, the commonly used plastic wrap on the market generally does not have antibacterial properties, and there are a large number of microorganisms in nature. These hundreds of millions of tiny organisms are distributed in the air, water, and soil, and can be said to be everywhere. They are easy to breed various bacteria on fruits, affecting their edibility, especially fruits such as lychees, whose flesh is rich in sugar and water and has high nutritional value, but is more prone to rot and deterioration. In addition, in hot seasons such as summer, the plastic wrap needs to have certain heat resistance to ensure the normal use of the plastic wrap. Moreover, most plastic wraps are disposable and are usually discarded as garbage after use. Due to the difficulty of recycling and the inability to degrade, they cause serious pollution to the environment. Therefore, it is urgent to solve the above problems and invent a degradable plastic wrap for fruit preservation to meet the higher demands in the field of plastic wrap technology. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a degradable preservative film for preserving fruits and a preparation method thereof.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A method for preparing a degradable fresh-keeping film for keeping fruits fresh comprises the following steps:

[0007] Polylactic acid, polyvinyl alcohol, cellulose, modified chitosan and lubricant are mixed in a mixer to obtain a mixture; the mixture is then added into a screw extruder for hot-melt extrusion and blow molding to form a film, thereby obtaining a degradable fresh-keeping film for preserving fruits.

[0008] Furthermore, the raw materials are calculated in parts by weight as follows: 40-60 parts of polylactic acid, 15-25 parts of polyvinyl alcohol, 6-12 parts of cellulose, 10-20 parts of modified chitosan, and 3-9 parts of lubricant.

[0009] Furthermore, the cellulose is one of wood cellulose, fruit cellulose and hemp cellulose.

[0010] Furthermore, the lubricant is one of epoxidized soybean oil and paraffin.

[0011] The prepared cling film is based on polylactic acid, a new type of biodegradable material that is not only highly transparent but also has good heat resistance. In addition, polyvinyl alcohol has a large number of hydroxyl groups in its chain, which easily form hydrogen bonds. The presence of a large number of hydrogen bonds gives the cling film good mechanical properties and high gas barrier properties, and polyvinyl alcohol is a degradable material. The addition of cellulose can further improve the thermal stability and mechanical properties of the cling film.

[0012] Furthermore, the modified chitosan is prepared by the following steps:

[0013] S1. In a three-necked flask equipped with a stirring reflux device, stearic acid, hydroquinone and toluene were mixed and stirred uniformly, and dicyclohexylcarbodiimide (DCC, dehydrating agent) and 4-dimethylaminopyridine (DMAP, catalyst) were added in sequence. The reaction temperature was controlled to 80°C, and the reflux reaction was carried out for 8 hours. After the reaction was completed, the mixture was filtered, and some solvent was removed by rotary evaporation. The mixture was washed with anhydrous ethanol 2-3 times and dried in vacuo to obtain intermediate 1; the ratio of stearic acid, hydroquinone, toluene, dicyclohexylcarbodiimide and 4-dimethylaminopyridine was 28.4 g:12.4 g:100 mL:20.6 g:0.2 g;

[0014] Under the action of DCC and DMAP, stearic acid and hydroquinone undergo an esterification reaction. By controlling the molar ratio of the two to be close to 1:1 and a slight excess of hydroquinone, only one hydroxyl group on the hydroquinone participates in the reaction, obtaining intermediate 1. The specific reaction process is shown below:

[0015]

[0016] S2. The intermediate 1 and toluene were mixed and added to a three-necked round-bottom flask with a thermometer, a magnetic stirring system and a spherical condenser. Under ice-water bath conditions, formaldehyde solution (mass fraction 28%) was added dropwise and stirred for 45 minutes. Then, diethylenetriamine was added and the reaction temperature was controlled to 60°C. The reaction was refluxed for 6 hours. After the reaction was completed, the mixture was cooled to room temperature, the solvent was removed by rotary evaporation, toluene and deionized water were added to separate the layers, and the upper organic phase was taken and washed with saturated sodium carbonate, saturated sodium bicarbonate and saturated brine in sequence. The upper organic phase was separated and dried to obtain intermediate 2; the ratio of the amount of intermediate 1, toluene, formaldehyde solution and diethylenetriamine was 37.6g:100mL:20mL:11.7g;

[0017] Intermediate 1 reacts with diethylenetriamine, and by controlling the molar ratio of the two to be close to 1:1 and a slight excess of diethylenetriamine, intermediate 2 is obtained. The specific reaction process is as follows:

[0018]

[0019] S3, in a three-necked flask equipped with a stirring apparatus, intermediate 2 and toluene are added, after stirring, glutaraldehyde and piperidine (condensing agent) are added, stirring mixes, and controlling the reaction temperature is 70 ℃, insulation reaction 5h, reaction is complete, and part of the solvent is removed by distillation under reduced pressure, then purified by column chromatography (eluent adopts a mixed solvent of petroleum ether / ethyl acetate, the volume ratio of the two is 5:1), the eluent is removed by rotary evaporation to obtain intermediate 3; the ratio of the amount of intermediate 2, toluene, glutaraldehyde, and piperidine is 50.3g:150mL:10.8g:15mL;

[0020] Under the action of a condensing agent, the aldehyde group on glutaraldehyde condenses with the amino group on intermediate 2 to form an imine group (C=N Schiff base structure). By controlling the molar ratio of the two to be close to 1:1 and a slight excess of glutaraldehyde, intermediate 3 is obtained. The specific reaction process is as follows:

[0021]

[0022] S4, in a three-necked flask equipped with a stirring reflux device, the intermediate 3 and toluene were mixed and stirred uniformly, and methyl iodide was added dropwise. The reaction temperature was controlled to 60 ° C. and reflux reaction was started. After the addition of methyl iodide was completed, the temperature was raised to 80 ° C. and the reflux reaction was continued for 8 hours. After the reaction was completed, the reaction was distilled under reduced pressure to obtain intermediate 4; the ratio of intermediate 3, methyl iodide and toluene was 58.5 g:42.6 g:150 mL;

[0023] Intermediate 3 undergoes alkylation reaction with methyl iodide to obtain intermediate 4 (quaternary ammonium product); the structure of intermediate 4 is shown below:

[0024]

[0025] S5. Chitosan was added to a mixed solution of acetic acid and toluene (the mass fraction of acetic acid was 28%), and the mixture was stirred until completely dissolved. Intermediate 4 and piperidine were then added, and the mixture was stirred at room temperature for 45 min. Sodium hydroxide solution was added dropwise to adjust the pH to 5.5, and the reaction was continued for 2 h. After the reaction was completed, the mixture was allowed to settle, filtered, washed with ethanol and water in sequence, and dried in an oven to obtain modified chitosan; the ratio of the mixed solution of acetic acid and toluene, chitosan, intermediate 4, and piperidine was 100 mL:1 g:10.5 g:15 mL;

[0026] Under the action of the condensing agent, the aldehyde group on the intermediate 4 condenses with the amino group on the chitosan to form an imine group (C=N Schiff base structure) to obtain modified chitosan;

[0027] Chitosan is a natural biodegradable material with good stability and antibacterial properties. It is also a high-molecular compound with good film-forming properties, which can enhance the mechanical strength of the matrix. By modifying chitosan and connecting it with the organic molecular chain through C=N, the hydrogen bonding effect of the chitosan amino group is weakened, and the biological activity and antibacterial properties of chitosan are improved. The surface hydrophobicity of the modified chitosan is improved, which enhances the compatibility with the matrix and promotes the dispersion of the modified chitosan, so that the performance of the modified chitosan is fully exerted. In addition, the modified chitosan molecule also contains The structure consists of a quaternary ammonium salt, a long carbon chain, a benzene ring, and a benzoxazine. The quaternary ammonium salt, as an excellent antibacterial structure, has good permeability, low toxicity, and stable performance. By penetrating the cell wall, it causes the leakage of intracellular substances, leading to bacterial death. It works synergistically with Schiff bases and chitosan to significantly enhance the antibacterial properties of the matrix. The introduced long carbon chain is a methylene chain segment, which is a flexible chain segment. It not only enhances the flexibility of the matrix, but also has hydrophobicity like the benzene ring, which can enhance the waterproofness of the matrix and improve its water barrier performance. Finally, the introduced benzoxazine can enhance the heat resistance of the matrix.

[0028] It should be added that chitosan connects organic molecular chains, which can effectively protect small organic molecules, improve their resistance to migration and exudation, and maintain the long-term stability of the modified chitosan performance.

[0029] Beneficial effects of the present invention:

[0030] 1. The cling film prepared by the present invention uses polylactic acid as a matrix, which gives the cling film high transparency, heat resistance and degradability;

[0031] 2. Adding polyvinyl alcohol not only makes it a degradable material, but also enhances the mechanical properties and gas barrier properties of the cling film;

[0032] 3. Adding cellulose can improve the thermal stability and mechanical properties of the cling film to a certain extent;

[0033] 4. By modifying chitosan, compared with ordinary chitosan, it has better compatibility with the matrix, and can also greatly enhance the antibacterial property, heat resistance, water barrier property and mechanical properties of the cling film to a certain extent, and the performance is long-lasting and stable;

[0034] Therefore, the cling film prepared by the present invention has excellent mechanical properties, high barrier properties to water and gas, stable and efficient antibacterial and heat resistance, and is environmentally friendly and degradable, and has important application value in the field of cling film technology. DETAILED DESCRIPTION

[0035] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] Example 1

[0037] Preparation of modified chitosan:

[0038] S1. In a three-necked flask equipped with a stirring reflux apparatus, 28.4 g of stearic acid, 12.4 g of hydroquinone, and 100 mL of toluene were mixed and stirred uniformly, and 20.6 g of dicyclohexylcarbodiimide and 0.2 g of 4-dimethylaminopyridine were added in sequence. The reaction temperature was controlled at 80°C and refluxed for 8 h. After the reaction was completed, the mixture was filtered, and some of the solvent was removed by rotary evaporation. The mixture was washed with anhydrous ethanol 2-3 times and dried in vacuo to obtain intermediate 1.

[0039] S2, 37.6g of intermediate 1 and 100mL of toluene were mixed and added to a three-necked round-bottom flask equipped with a thermometer, a magnetic stirring system and a spherical condenser. Under ice-water bath conditions, 20mL of formaldehyde solution (mass fraction 28%) was added dropwise and stirred for 45min. Then, 11.7g of diethylenetriamine was added and the reaction temperature was controlled to 60°C. The reaction was refluxed for 6h. After the reaction was completed, the mixture was cooled to room temperature, the solvent was removed by rotary evaporation, toluene and deionized water were added to separate the layers, the upper organic phase was taken, and washed with saturated sodium carbonate, saturated sodium bicarbonate and saturated brine in sequence. The upper organic phase was separated and dried to obtain intermediate 2;

[0040] S3, in a three-necked flask equipped with a stirring device, 50.3g of intermediate 2 and 150mL of toluene were added, and after stirring, 10.8g of glutaraldehyde and 15mL of piperidine were added, and the mixture was stirred and mixed. The reaction temperature was controlled to be 70°C, and the reaction was kept warm for 5h. After the reaction was completed, part of the solvent was removed by distillation under reduced pressure, and then purified by column chromatography (eluent was a mixed solvent of petroleum ether / ethyl acetate in a volume ratio of 5:1), and the eluent was removed by rotary evaporation to obtain intermediate 3;

[0041] S4, in a three-necked flask equipped with a stirring reflux device, 58.5g of intermediate 3 and 150mL of toluene were mixed and stirred uniformly, and then 42.6g of iodomethane was added dropwise. The reaction temperature was controlled at 60°C, and a reflux reaction was started. After the addition of iodomethane was completed, the temperature was raised to 80°C, and the reflux reaction was continued for 8h. After the reaction was completed, the reaction was distilled under reduced pressure to obtain intermediate 4;

[0042] S5. Add 1 g of chitosan to 100 mL of a mixed solution of acetic acid and toluene (the mass fraction of acetic acid is 28%), stir until completely dissolved, then add 10.5 g of intermediate 4 and 15 mL of piperidine, stir and react at room temperature for 45 minutes, add sodium hydroxide solution dropwise to adjust the pH to 5.5, continue the reaction for 2 hours, and after the reaction is complete, let it stand and precipitate, filter, wash with ethanol and water in sequence, and dry in an oven to obtain modified chitosan.

[0043] Example 2

[0044] Preparation of modified chitosan:

[0045] S1. In a three-necked flask equipped with a stirring reflux apparatus, 56.8 g of stearic acid, 24.8 g of hydroquinone, and 200 mL of toluene were mixed and stirred uniformly, and 41.2 g of dicyclohexylcarbodiimide and 0.4 g of 4-dimethylaminopyridine were added in sequence. The reaction temperature was controlled to 80°C and refluxed for 8 h. After the reaction was completed, the mixture was filtered, and some of the solvent was removed by rotary evaporation. The mixture was washed twice with anhydrous ethanol and dried in vacuo to obtain intermediate 1.

[0046] S2, 75.2g of intermediate 1 and 200mL of toluene were mixed and added to a three-necked round-bottom flask equipped with a thermometer, a magnetic stirring system and a spherical condenser. Under ice-water bath conditions, 40mL of formaldehyde solution (mass fraction 28%) was added dropwise and stirred for 45min. Then, 23.4g of diethylenetriamine was added and the reaction temperature was controlled to 60°C. The reaction was refluxed for 6h. After the reaction was completed, the mixture was cooled to room temperature, the solvent was removed by rotary evaporation, toluene and deionized water were added to separate the layers, the upper organic phase was taken, and washed with saturated sodium carbonate, saturated sodium bicarbonate and saturated brine in sequence. The upper organic phase was separated and dried to obtain intermediate 2;

[0047] S3, in a three-necked flask equipped with a stirring device, 100.6g of intermediate 2 and 300mL of toluene were added, and after stirring, 21.6g of glutaraldehyde and 30mL of piperidine were added, and the mixture was stirred and mixed. The reaction temperature was controlled to 70°C, and the reaction was kept warm for 5h. After the reaction was completed, part of the solvent was removed by distillation under reduced pressure, and then purified by column chromatography (eluent was a mixed solvent of petroleum ether / ethyl acetate in a volume ratio of 5:1), and the eluent was removed by rotary evaporation to obtain intermediate 3;

[0048] S4. In a three-necked flask equipped with a stirring reflux device, 117 g of intermediate 3 and 300 mL of toluene were mixed and stirred uniformly, and then 85.2 g of iodomethane was added dropwise. The reaction temperature was controlled at 60° C. and a reflux reaction was started. After the addition of iodomethane was completed, the temperature was raised to 80° C. and the reflux reaction was continued for 8 h. After the reaction was completed, the mixture was distilled under reduced pressure to obtain intermediate 4.

[0049] S5. Add 2 g of chitosan to a mixed solution of 200 mL of acetic acid and toluene (the mass fraction of acetic acid is 28%), stir until completely dissolved, then add 21 g of intermediate 4 and 30 mL of piperidine, stir and react at room temperature for 45 min, add sodium hydroxide solution dropwise to adjust the pH to 5.5, continue the reaction for 2 h, and after the reaction is complete, let it stand and precipitate, filter, wash with ethanol and water in sequence, and dry in an oven to obtain modified chitosan.

[0050] Example 3

[0051] 40 g of polylactic acid, 15 g of polyvinyl alcohol, 6 g of lignocellulose, 10 g of the modified chitosan prepared in Example 1, and 3 g of paraffin were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder and subjected to hot-melt extrusion blow molding at 160° C. to obtain a degradable fresh-keeping film for preserving fruits.

[0052] Example 4

[0053] 50 g of polylactic acid, 20 g of polyvinyl alcohol, 9 g of fruit cellulose, 15 g of the modified chitosan prepared in Example 2, and 6 g of epoxidized soybean oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder and subjected to hot-melt extrusion blow molding at 170° C. to obtain a degradable fresh-keeping film for preserving fruits.

[0054] Example 5

[0055] 60 g of polylactic acid, 25 g of polyvinyl alcohol, 12 g of fruit cellulose, 20 g of the modified chitosan prepared in Example 2, and 9 g of epoxidized soybean oil were mixed in a mixer to obtain a mixture; the mixture was then added to a screw extruder and subjected to hot-melt extrusion blow molding at 180° C. to obtain a degradable fresh-keeping film for preserving fruits.

[0056] Comparative Example 1

[0057] Ordinary chitosan of the same mass was used to replace the modified chitosan in Example 5, and the remaining steps were the same as in Example 5 to prepare a fresh-keeping film.

[0058] Comparative Example 2

[0059] Commercially available polylactic acid biodegradable food wrap was used.

[0060] Examples 3-5 and Comparative Examples 1-2 were made into corresponding shapes according to different test standards and subjected to the following performance tests:

[0061] The tensile strength was measured according to the national standard GB / T 1040-2006, "Determination of Tensile Properties of Plastics." The specimen was then heat-treated at 150°C for 12 hours, and the tensile strength was measured. The tensile strength retention was then calculated. Tensile strength retention = tensile strength after test / tensile strength before test × 100%.

[0062] The biodegradation rate was determined using the national standard GB / T 19811-2005;

[0063] The antibacterial rate of the sample was measured by QB / T 2591-2003 "Antibacterial Plastics - Test Methods for Antibacterial Performance and Antibacterial Effect" after the sample was placed at 20℃±5℃ and humidity 10%±5%RH for 30 days.

[0064] The oxygen permeation rate is determined using the national standard GB / T 1038.1-2022 "Plastic film and sheeting gas permeability test method Part 1: Pressure difference method";

[0065] The water vapor transmission rate was determined using the national standard GB / T 1037-2010 "Plastic film and sheeting test method for water vapor permeability - Cup method";

[0066] The measurement results are shown in Table 1:

[0067] Table 1

[0068]

[0069]

[0070] 500 lychees of uniform size, relatively uniform maturity (8-9 ripe), free of rot, pests and diseases, and inorganic damage were selected and divided into 5 groups, each with 100 lychees. The lychees in each group were packaged with the plastic wrap of Examples 3-5 and Comparative Examples 1-2 respectively. The 5 groups of fruits were then stored in a climate chamber at 25° C., and the good fruit rate of each group of lychees was tested on the 1st, 3rd, 5th, 8th and 10th day.

[0071] The measured results are shown in Table 2:

[0072] Table 2

[0073]

[0074] It can be seen from the above two tables that the mechanical properties, heat resistance, antibacterial properties and barrier properties of the cling film prepared in the embodiment of the present invention are higher than those of the comparative example, and the antibacterial properties are long-lasting and stable, environmentally friendly and degradable, and have a good preservation effect on litchi, and have important application value in the field of cling film technology.

[0075] Throughout the specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0076] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in similar ways. As long as they do not deviate from the invention or exceed the scope defined by the claims, they should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a degradable preservative film for preserving fruits, characterized in that: The following steps are involved: Polylactic acid, polyvinyl alcohol, cellulose, modified chitosan and lubricant are mixed in a mixer to obtain a mixture; the mixture is then added to a screw extruder, and then hot-melt extrusion blow molding is performed to form a film to obtain a degradable fresh-keeping film for preserving fruits; Wherein, the modified chitosan is prepared by the following steps: S1. Stearic acid, hydroquinone, and toluene were mixed and stirred uniformly, and dicyclohexylcarbodiimide and 4-dimethylaminopyridine were added in sequence. The mixture was refluxed at 80°C for 8 h. After the reaction was completed, the mixture was filtered, rotary evaporated, washed, and vacuum dried to obtain intermediate 1. S2. Mix the intermediate 1 and toluene, add them to a round-bottom flask, add formaldehyde solution dropwise under ice-water bath, and continue stirring for 45 minutes, then add diethylenetriamine, reflux at 60°C for 6 hours. After the reaction is complete, cool, rotary evaporate, add toluene and deionized water to separate the layers, take the upper organic phase, wash, separate the upper organic phase, and dry to obtain the intermediate 2; S3, after the intermediate 2 and toluene are stirred evenly, glutaraldehyde and piperidine are added, and the mixture is stirred evenly. The mixture is reacted at 70°C for 5 hours. After the reaction is completed, the mixture is distilled under reduced pressure, purified by column chromatography, and rotary evaporated to obtain the intermediate 3; S4, after mixing intermediate 3 and toluene, add iodomethane dropwise, and reflux at 60°C until the addition of iodomethane is complete, raise the temperature to 80°C, and continue reflux reaction for 8 hours. After the reaction is complete, distill under reduced pressure to obtain intermediate 4; S5. Add chitosan to the mixed solution of acetic acid and toluene, stir until completely dissolved, then add intermediate 4 and piperidine, stir and react at room temperature for 45 minutes, adjust the pH to 5.5, continue the reaction for 2 hours, and after the reaction is complete, let it stand and precipitate, filter, wash, and dry to obtain modified chitosan; Among them, the ratio of stearic acid, hydroquinone, toluene, dicyclohexylcarbodiimide, and 4-dimethylaminopyridine in step S1 is 28.4g:12.4g:100mL:20.6g:0.2g; the ratio of intermediate 1, toluene, formaldehyde solution, and diethylenetriamine in step S2 is 37.6g:100mL:20mL:11.7g; and the ratio of intermediate 2, toluene, glutaraldehyde, and piperidine in step S3 is 50.3g:150mL:10.8g:15mL.

2. The method for preparing a degradable preservative film for preserving fruits according to claim 1, wherein: In step S4, the ratio of the amount of intermediate 3, methyl iodide, and toluene is 58.5 g:42.6 g:150 mL.

3. The method for preparing a degradable preservative film for preserving fruits according to claim 1, wherein: In step S5, the ratio of the mixed solution of acetic acid and toluene, chitosan, intermediate 4, and piperidine is 100 mL: 1 g: 10.5 g: 15 mL.

4. The method for preparing a degradable preservative film for preserving fruits according to claim 1, wherein: The raw materials are calculated in parts by weight as follows: 40-60 parts of polylactic acid, 15-25 parts of polyvinyl alcohol, 6-12 parts of cellulose, 10-20 parts of modified chitosan, and 3-9 parts of lubricant.

5. The method for preparing a degradable preservative film for preserving fruits according to claim 1, wherein: The cellulose is one of wood cellulose, fruit cellulose and hemp cellulose.

6. A degradable fresh-keeping film for preserving fruits, characterized in that: Prepared according to the method according to any one of claims 1 to 5.

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