Preparation method and application of wood nanocellulose ternary composite preservative film

Abstract

The invention discloses a preparation method and application of a wood nanocellulose ternary composite preservative film, and belongs to the technical field of biomass nanocellulose functional materials. Lignin and nanocellulose components are separated from peanut shells by using a ternary eutectic solvent, the novel green wood nanocellulose ternary composite fresh-keeping liquid is prepared by blending chitosan natural macromolecules, a wood nanocellulose ternary composite film can be formed on the surfaces of fruits and vegetables, and a good fresh-keeping effect is achieved. Meanwhile, the preparation method provided by the invention is simple, and the prepared film has excellent mechanical properties and stability, and can effectively delay ripening, aging and water loss of fruits, so that the luster and freshness of the fruits are maintained, the storage capacity of fresh fruits and vegetables is improved, and the film has a wide application prospect. In addition, the used deep eutectic solvent is very environment-friendly, meets the standard of green chemical production, and has good practical application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of biomass nanocellulose functional materials, and particularly relates to a preparation method and application of a ternary composite preservative film of lignocellulose nanocellulose. Background Art

[0002] Consumers always hope to improve the safety, maximize the edible quality, and extend the shelf life of food during production, transportation, and storage. Therefore, postharvest preservation is generally considered the main means to maintain the freshness of fruits and vegetables and extend their circulation cycle. Using natural polymers to make coating preservation materials for fresh-keeping packaging of fruits and vegetables can reduce the contact between fruits and vegetables and the outside world, prevent the loss of moisture in fruits and vegetables, delay the oxidation of fruits and vegetables, and inhibit the reproduction of microorganisms, which can effectively extend the shelf life of fruits and vegetables. Natural polymer coating materials have the advantages of edibility, degradability, biocompatibility, low cost, non-toxicity, and environmental friendliness. Therefore, natural polymer coating materials have broad development prospects.

[0003] Chitosan is the second most abundant natural polymer linear polysaccharide in nature. It is obtained by deacetylating chitin with (1,4)-linked 2-amino-deoxy-β-d-glucan and is one of the most promising edible active films. Chitosan has advantages such as good biocompatibility, antibacterial property, film-forming property, safety, and non-toxicity, and has advantages over other biomolecule-based films in the preservation packaging of various foods. However, the mechanical strength of a single chitosan film is low and its water resistance is poor, and its practical application is restricted to a certain extent. Therefore, modifying the chitosan film by certain methods to improve its solubility, mechanical properties, barrier properties, etc. is the current development direction for expanding the application of chitosan preservative films in fruit and vegetable packaging.

[0004] As a new type of environmentally friendly nanomaterial, nanocellulose is prepared from natural cellulose by physical or chemical means. Peanut shells are rich in cellulose, and nanoscale cellulose can be obtained by physical or chemical means. Compared with cellulose, nanocellulose has a larger specific surface area, better dispersibility, and better control ability of physical and chemical properties. In the food field, nanocellulose can be used in food additives, food packaging, and functional foods. Lignin is a sustainable natural polymer compound and together with cellulose and hemicellulose constitutes the skeletal structure of the plant cell wall. In nature, lignin is the second most abundant biomass polymer after cellulose in terms of source. Due to its complex structure, lignin has many unique properties, such as resistance to decay and aging and microorganisms, ultraviolet absorption, high stiffness, and the ability to delay and inhibit oxidation reactions. Applying lignin nanoparticles to composite films and endowing the composite films with new functions is a research hotspot.

[0005] In addition, China is a major peanut-producing country in the world, with an annual output of tens of millions of tons, accounting for 42% of the world's total output. Every year, nearly 5 million tons of peanut shells are produced. As the residue of peanut processing, except for a small amount of peanut shells being processed into feed, adsorption materials, and extracting the contained functional substances, most of them are discarded or burned, resulting in a great waste of resources. The abundant cellulose in peanut shells is a biological resource to be developed and utilized. Developing a preparation method for peanut shell nano-lignocellulose composite membranes can not only realize the high-value utilization of crop waste resources and broaden the utilization fields of peanut shells, but also has great theoretical and practical significance for reducing the production cost of biological composite membranes and synthesizing new composite membranes in the future. Summary of the Invention

[0006] Aiming at the problems of poor solubility, high water permeability, low mechanical strength, easy cracking, and easy fragmentation of single chitosan, the purpose of the present invention is to provide a method for preparing a ternary composite preservative film of lignin nano-cellulose and its application. The method of the present invention separates nano-cellulose and lignin through a deep eutectic solvent, and then adds the nano-cellulose and lignin to a chitosan solution to form a ternary composite film. Compared with a single chitosan film and a chitosan / lignin binary composite film, the performance of the ternary composite film is significantly improved while it can be used for fruit and vegetable preservation to extend the shelf life of fruits and vegetables.

[0007] In order to achieve the above purpose, the present invention adopts the following scheme:

[0008] The present invention first provides a method for preparing a ternary composite preservative solution of lignin nano-cellulose, and the specific steps are as follows:

[0009] (1) Synthesis of ternary deep eutectic solvent:

[0010] Mix choline chloride, lactic acid, and aluminum trichloride (AlCl 3 ·6H 2 O) in a certain molar ratio, stir under certain temperature conditions until a uniform and transparent liquid is formed, and then the ternary deep eutectic solvent is obtained;

[0011] (2) Preparation of peanut shell lignin:

[0012] Weigh peanut shells and add them to the deep eutectic solvent in step (1), react at a certain temperature, cool to room temperature after the reaction, centrifuge the reaction solution to obtain the supernatant, add an ethanol solution to the remaining precipitate, wash the precipitate with the ethanol solution by suction filtration, and collect the filtrate; then, combine the filtrate and the supernatant, collect the concentrated solution after rotary evaporation, add deionized water to the concentrated solution for precipitation, and finally centrifuge to separate the precipitate, which is lignin;

[0013] (3) Preparation of peanut shell nano-cellulose:

[0014] Add the lignin obtained in step (2) to water, put it into a dialysis bag, and dialyze it in deionized water (to remove the deep eutectic solvent) until the pH of the solution becomes neutral; then take out the solution, homogenize and centrifuge it to collect the turbid supernatant (the turbid supernatant indicates the presence of nanocellulose), and add water to the remaining precipitate again for homogenization and centrifugation to collect the turbid supernatant; repeat the operation until the supernatant collected by centrifugation becomes transparent; finally, mix the collected turbid supernatants and freeze-dry them to obtain peanut shell nanocellulose;

[0015] (4) Preparation of the ternary composite preservative solution of lignin nanocellulose:

[0016] Add chitosan powder to the acetic acid aqueous solution and stir to form a homogeneous and transparent liquid to obtain a chitosan solution; add the lignin obtained in step (2) to water and stir evenly to obtain a lignin solution; then add the peanut shell nanocellulose prepared in step (3) to water and stir evenly to obtain a nanocellulose solution;

[0017] Then mix the chitosan solution, lignin solution, and nanocellulose solution to obtain a mixed solution; add glycerol as a plasticizer to the mixed solution and continuously stir until the mixed solution becomes homogeneous and there is no precipitate at the bottom to obtain a chitosan / nanocellulose / lignin film-forming solution, which is the ternary composite preservative solution of lignin nanocellulose.

[0018] Preferably, the molar ratio of choline chloride, lactic acid, and aluminum trichloride (AlCl 3 ·6H 2 O) in step (1) is 1:1:0.1; the certain temperature condition is 70 °C.

[0019] Preferably, the mass ratio of peanut shell to deep eutectic solvent in step (2) is 1:10 (wt / wt), the reaction temperature is 100 °C, and the reaction time is 3 h.

[0020] Preferably, the centrifugation conditions in step (2) are all: 8000 rpm, 10 min;

[0021] Preferably, the volume concentration of the ethanol solution in step (2) is 95%; the rotary evaporation temperature is 60 °C, and it is rotary evaporated to 1 / 3 of the original solution volume; the volume ratio of the concentrated solution to deionized water is 1:6; the time for adding deionized water for precipitation is 12 h.

[0022] Preferably, the cut-off molecular weight of the dialysis bag in step (3) is 8000 - 14000 Da; the homogenization conditions are all: homogenize at 10000 rpm for 30 min; the centrifugation conditions are all 4000 rpm, 5 min.

[0023] Preferably, in step (3), lignin is added to water, and the dosage relationship between the two is 0.5-1 g: 10 mL; the dosage relationship between the precipitate and water is 0.5-1 g: 10 mL;

[0024] Preferably, in step (4), the mass concentrations of the lignin solution and the nanocellulose solution are both 2.5%, the mass concentration of the chitosan solution is 2.5%, and the concentration of the acetic acid aqueous solution is 2% (v / v); in the mixed solution, the mass percentage of the lignin solution is 1-5%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 92%-96%, and the addition amount of glycerol is 0.1-0.2% of the mass of the mixed solution.

[0025] The use of the lignin-based nanocellulose ternary composite preservative solution prepared according to the present invention for fruit and vegetable preservation is as follows:

[0026] Select fruits and vegetables with uniform size, similar maturity, no pests, diseases and mechanical damage, wash them with water and dry them at room temperature; then immerse the fruits and vegetables in the lignin-based nanocellulose ternary composite preservative solution, dry them at room temperature after coating, and form a protective film on the surface of the fruits and vegetables, which is the lignin-based nanocellulose ternary composite preservative film, and the use for fruit and vegetable preservation can be realized.

[0027] Preferably, the fruits and vegetables include red grapes and blueberries; the immersion time in the lignin-based nanocellulose ternary composite preservative solution is 30-60 s.

[0028] The beneficial effects of the present invention:

[0029] (1) The ternary eutectic solvent pretreatment system used in the present invention is simple to operate and has good pretreatment effect, which can efficiently separate lignin and realize the efficient dissociation of the peanut shell structure; at the same time, the preparation conditions are mild, the operation is simple, the cost is low and the pollution is small. The used eutectic solvent can be recovered by methods such as rotary evaporation, so that it can be recycled, is environmentally friendly, meets the production standards of green chemistry, has good practical application value, and is suitable for industrial production.

[0030] (2) The present invention adds nanocellulose and lignin nanoparticles to chitosan. Their unique physical and chemical properties enable them to effectively improve the functional properties of the chitosan film through physical filling effect or interfacial interaction. The prepared lignin-based nanocellulose composite film has excellent mechanical properties and barrier properties. The light barrier property of the composite film gradually increases with the increase of NLC, but still can achieve a relatively high light transmittance.

[0031] (3) The lignin-based nanocellulose composite film prepared by the present invention has strong degradation ability, and is basically completely degraded after 10 days buried in the soil and completely degraded after 18 days.

[0032] (4) The lignocellulose nanofiber composite film prepared by the present invention has excellent fresh-keeping ability for fruits and vegetables, significantly reducing the quality loss of Red Globe grapes and blueberries in the later stage of storage, improving the hardness of fruits and vegetables, reducing the spoilage rate, delaying the ripening, senescence and water loss of fruits, thereby maintaining the luster and freshness of fruits and improving the storage capacity of fresh fruits and vegetables. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a graph showing the test results of the mechanical properties of the lignocellulose nanofiber composite films in Examples 1, 2, 3, 4, and 5.

[0034] Figure 2 It is a graph showing the test results of the water contact angle (A) of the lignocellulose nanofiber composite films in Comparative Example 1 and Example 3 and their contact results with other liquid components (B).

[0035] Figure 3 It is a graph showing the light transmittance test of the lignocellulose nanofiber composite films in Examples 1, 2, 3, 4, and 5.

[0036] Figure 4 It is the ultraviolet-visible transmission curve of the lignocellulose nanofiber composite films in Examples 1, 2, 3, 4, and 5.

[0037] Figure 5 It is a graph showing the fresh-keeping effect of the lignocellulose nanofiber composite films in Examples 1, 2, 3, 4, and 5 on blueberries.

[0038] Figure 6 It is a graph showing the degradation results of the lignocellulose nanofiber composite films in Examples 1, 2, 3, 4, and 5 after being buried in soil for 10 days.

[0039] Figure 7 It is a graph showing the fresh-keeping effect of the lignocellulose nanofiber composite films in Examples 1, 2, 3, 4, and 5 on Red Globe grapes. DETAILED DESCRIPTION OF THE INVENTION

[0040] The various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and embodiments of the present invention.

[0041] It should be understood that the terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0042] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of the present invention are merely exemplary.

[0043] Regarding the use of "comprising", "including", "having", "containing", etc. in this article, they are all open-ended terms, meaning including but not limited to.

[0044] Example 1:

[0045] (1) Synthesis of ternary eutectic solvent: Choline chloride, lactic acid and aluminum trichloride (AlCl 3 ·6H 2 O) were mixed in a molar ratio of 1:1:0.1 and stirred at 70 °C for 2 h until a homogeneous and transparent liquid was formed;

[0046] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shells, and then weigh 10 g of the ternary eutectic solvent. Place the two in a glass reaction test tube, react at 100 °C for 3 h and then cool to room temperature. The reaction solution was centrifuged at 8000 rpm for 10 min using a high-speed centrifuge. After pouring out the supernatant, the precipitate in the centrifuge tube was washed with 90% ethanol solution by suction filtration. The filtrate and the supernatant after centrifugation were combined, and the eutectic solvent and ethanol were removed by rotary evaporation at 60 °C. After rotary evaporation, the concentrated solution (1 / 3 of the original solution volume) was collected, and deionized water was added to the concentrated solution in a volume ratio of 1:6 to precipitate for 12 h to obtain lignin, and the lignin was separated by centrifugation;

[0047] (3) Preparation of peanut shell nanocellulose: Add the lignin in step (2) to water at a dosage of 1 g:10 mL, put it into a dialysis bag with a molecular weight cut-off of 8000 - 14000 Da, and dialyze it in deionized water to remove the deep eutectic solvent until the solution pH becomes neutral; then take out the solution, homogenize it with a high-speed homogenizer at 10000 rpm for 30 min, and then collect the turbid supernatant by centrifugation (the turbid supernatant indicates the presence of nanocellulose); and add water to the remaining precipitate again at a dosage of 1 g:10 mL, homogenize it at 10000 rpm for 30 min, and collect the turbid supernatant by centrifugation; repeat the operation like this until the finally centrifuged supernatant becomes transparent. The centrifugation conditions are 4000 rpm and 5 min; finally, mix all the collected turbid supernatants and conduct freeze-drying to obtain peanut shell nanocellulose;

[0048] (4) Preparation of peanut shell lignin nanocellulose composite film:

[0049] Add chitosan powder to 2.5% (v / v) acetic acid aqueous solution, and continuously stir with a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%, and place it in a 4°C refrigerator for later use. Add the lignin in step (2) to water to prepare a lignin solution with a mass concentration of 2.5%; add the peanut shell nanocellulose in step (3) to water to prepare a nanocellulose solution with a mass concentration of 2.5%.

[0050] Take the lignin solution, nanocellulose solution and chitosan solution and mix them to obtain a mixed solution; among them, the mass percentage of the lignin solution is 1%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 96%; and add glycerol as a plasticizer to the mixed solution, and the addition amount is 0.1% of the mass of the mixed solution; continuously stir until the mixed solution becomes uniform and no precipitate can be observed to obtain a film-forming solution; after the film-forming solution is ultrasonically treated for 30 min to remove bubbles, take out 5 g and pour it into a polyvinyl alcohol petri dish (diameter 50 mm), and dry it in an oven at 40°C for 18 h. After film formation, place it in a desiccator with saturated potassium carbonate solution to adjust for 48 h to obtain a film denoted as chitosan / nanocellulose - 3% / lignin - 1%.

[0051] Example 2:

[0052] (1) Synthesis of ternary deep eutectic solvent: Choline chloride, lactic acid and aluminum chloride (AlCl 3 ·6H 2 O) are mixed in a molar ratio of 1:1:0.1, and stirred at 70°C for 2 h until a homogeneous and transparent liquid is formed, that is, the ternary deep eutectic solvent is obtained;

[0053] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shells and then weigh 10 g of the ternary eutectic solvent. Place both in a glass reaction test tube and react at 100 °C for 3 h, then cool to room temperature. Centrifuge the resulting reaction solution at 8000 rpm for 10 min. After pouring out the supernatant, wash the precipitate in the centrifuge tube by suction filtration with a 90% ethanol solution. Combine the filtrate and the centrifuged supernatant, and remove the eutectic solvent and ethanol by rotary evaporation at 60 °C. After rotary evaporation, collect the concentrated solution (1 / 3 of the original solution volume), and add deionized water to the concentrated solution at a volume ratio of 1:6 to precipitate for 12 h. Finally, separate the lignin by centrifugation (centrifuge at 8000 rpm for 10 min);

[0054] (3) Preparation of peanut shell nanocellulose: Add the lignin in step (2) to water at a dosage of 1 g:10 mL, place it in a dialysis bag with a molecular weight cut-off of 8000 - 14000 Da, and dialyze in deionized water to remove the eutectic solvent until the solution pH becomes neutral; then take out the solution, homogenize it with a high-speed homogenizer at 10000 rpm for 30 min, and then collect the turbid supernatant by centrifugation (the turbid supernatant indicates the presence of nanocellulose); and add water to the remaining precipitate again at a dosage of 1 g:10 mL, homogenize at 10000 rpm for 30 min, and centrifuge to collect the turbid supernatant; repeat this operation until the finally centrifuged supernatant becomes transparent. The centrifugation conditions are 4000 rpm and 5 min each time; finally, mix all the collected turbid supernatants and perform freeze-drying to obtain peanut shell nanocellulose;

[0055] (4) Preparation of peanut shell lignin-nanocellulose composite film:

[0056] Add chitosan powder to a 2.5% (v / v) acetic acid aqueous solution, and continuously stir with a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%, and store it in a 4 °C refrigerator for later use. Add the lignin in step (2) to water to prepare a lignin solution with a mass concentration of 2.5%; add the peanut shell nanocellulose in step (3) to water to prepare a nanocellulose solution with a mass concentration of 2.5%.

[0057] Take the lignin solution, nanocellulose solution, and chitosan solution and mix them to obtain a mixed solution, where the mass percentage of the lignin solution is 2%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 95%; and add glycerol as a plasticizer to the mixed solution, with the addition amount being 0.1% of the mass of the mixed solution; continuously stir until the mixed solution becomes uniform and no precipitate can be observed to obtain a film-forming solution;

[0058] After the film-forming solution was sonicated for 30 min to remove air bubbles, 5 g was taken out and poured into a polyvinyl alcohol petri dish (diameter 50 mm), dried in an oven at 40 °C for 18 h, and then placed in a desiccator with saturated potassium carbonate solution for 48 h to adjust. The obtained film was denoted as chitosan / nanocellulose-3% / lignin-2%.

[0059] Example 3:

[0060] (1) Synthesis of ternary deep eutectic solvent: Choline chloride, lactic acid and aluminum chloride (AlCl 3 ·6H 2 O) were mixed in a molar ratio of 1:1:0.1 and stirred at 70 °C for 2 h until a homogeneous transparent liquid was formed;

[0061] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shells and then weigh 10 g of the ternary deep eutectic solvent. Place the two in a glass reaction test tube, react at 100 °C for 3 h and then cool to room temperature. The reaction solution was centrifuged at 8000 rpm for 10 min using a high-speed centrifuge. After pouring out the supernatant, the precipitate in the centrifuge tube was washed with 90% ethanol solution by suction filtration. The filtrate and the centrifuged supernatant were combined, and the deep eutectic solvent and ethanol were removed by rotary evaporation at 60 °C. After rotary evaporation, the concentrated solution (1 / 3 of the original solution volume) was collected, and deionized water was added to the concentrated solution in a volume ratio of 1:6 to precipitate for 12 h to obtain lignin, and the lignin was separated by centrifugation;

[0062] (3) Preparation of peanut shell nanocellulose: The lignin in step (2) was added to water at a dosage of 1 g:10 mL, filled into a dialysis bag with a molecular weight cut-off of 8000 - 14000 Da, and dialyzed in deionized water to remove the deep eutectic solvent until the solution pH became neutral; then the solution was taken out, homogenized at 10000 rpm for 30 min using a high-speed homogenizer, and then the turbid supernatant was collected by centrifugation. The turbid supernatant indicated the presence of nanocellulose; and water was added to the remaining precipitate again at a dosage of 1 g:10 mL, homogenized at 10000 rpm for 30 min, and the turbid supernatant was collected by centrifugation; this operation was repeated until the finally centrifuged supernatant became transparent. The centrifugation conditions were 4000 rpm and 5 min; finally, all the collected turbid supernatants were mixed and freeze-dried to obtain peanut shell nanocellulose;

[0063] (4) Preparation of peanut shell lignin-nanocellulose composite film:

[0064] Chitosan powder was added to an aqueous acetic acid solution of 2.5% (v / v), and continuously stirred using a magnetic stirrer until a homogeneous and transparent liquid was formed to obtain a chitosan solution with a mass concentration of 2.5%, which was placed in a refrigerator at 4 °C for later use. Lignin in step (2) was added to water to prepare a lignin solution with a mass concentration of 2.5%; peanut shell nanocellulose in step (3) was added to water to prepare a nanocellulose solution with a mass concentration of 2.5%.

[0065] Take the lignin solution, nanocellulose solution and chitosan solution and mix them to obtain a mixed solution, where the mass percentage of the lignin solution is 3%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 94%; and glycerol was added as a plasticizer to the mixed solution, and the addition amount was 0.1% of the mass of the mixed solution; continuously stirred until the mixed solution became uniform and no precipitation was observed to obtain a film-forming solution; after the film-forming solution was ultrasonicated for 30 min to remove air bubbles, 5 g was taken out and poured into a polyvinyl alcohol petri dish (diameter 50 mm), and dried in an oven at 40 °C for 18 h. After film formation, it was placed in a desiccator with saturated potassium carbonate solution for 48 h to adjust, and the obtained film was denoted as chitosan / nanocellulose-3% / lignin-3%.

[0066] Example 4:

[0067] (1) Synthesis of ternary deep eutectic solvent: Choline chloride, lactic acid and aluminum chloride (AlCl 3 ·6H 2 O) were mixed in a molar ratio of 1:1:0.1 and stirred at 70 °C for 2 h until a homogeneous and transparent liquid was formed;

[0068] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shells, and then weigh 10 g of the ternary deep eutectic solvent. Place the two in a glass reaction test tube, react at 100 °C for 3 h and then cool to room temperature. The reaction solution was centrifuged at 8000 rpm for 10 min using a high-speed centrifuge. After pouring out the supernatant, the precipitate in the centrifuge tube was washed with 90% ethanol solution by suction filtration. The filtrate and the supernatant after centrifugation were combined, and the deep eutectic solvent and ethanol were removed by rotary evaporation at 60 °C. After rotary evaporation, the concentrated solution was collected (1 / 3 of the original solution volume), and deionized water was added to the concentrated solution in a volume ratio of 1:6 to precipitate for 12 h to obtain lignin, and the lignin was separated by centrifugation;

[0069] (3) Preparation of peanut shell nanocellulose: Add the lignin in step (2) to water at a dosage of 1 g:10 mL, place it in a dialysis bag with a molecular weight cut-off of 8000 - 14000 Da, and dialyze it in deionized water to remove the deep eutectic solvent until the solution pH becomes neutral; then take out the solution, homogenize it with a high-speed homogenizer at 10000 rpm for 30 min, and then collect the turbid supernatant by centrifugation (the turbid supernatant indicates the presence of nanocellulose); and add water to the remaining precipitate again at a dosage of 1 g:10 mL, homogenize it at 10000 rpm for 30 min, and collect the turbid supernatant by centrifugation; repeat the operation until the finally centrifuged supernatant becomes transparent, and the centrifugation conditions are 4000 rpm for 5 min; finally, mix all the collected turbid supernatants and freeze-dry them to obtain peanut shell nanocellulose;

[0070] (4) Preparation of peanut shell lignin nanocellulose composite film:

[0071] Add chitosan powder to 2.5% (v / v) acetic acid aqueous solution, and continuously stir with a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%, and place it in a refrigerator at 4°C for later use. Add the lignin in step (2) to water to prepare a lignin solution with a mass concentration of 2.5%; add the peanut shell nanocellulose in step (3) to water to prepare a nanocellulose solution with a mass concentration of 2.5%.

[0072] Take the lignin solution, nanocellulose solution and chitosan solution and mix them to obtain a mixed solution, where the mass percentage of the lignin solution is 4%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 93%; and add glycerol as a plasticizer to the mixed solution, and the addition amount is 0.1% of the mass of the mixed solution; continuously stir until the mixed solution becomes uniform and no precipitation is observed to obtain a film-forming solution; after removing the bubbles from the film-forming solution by ultrasonic treatment for 30 min, take out 5 g and pour it into a polyvinyl alcohol petri dish (diameter 50 mm), dry it in an oven at 40°C for 18 h, and after forming the film, place it in a desiccator with saturated potassium carbonate solution to adjust for 48 h to obtain a film denoted as chitosan / nanocellulose - 3% / lignin - 4%.

[0073] Example 5:

[0074] (1) Synthesis of ternary deep eutectic solvent: Choline chloride, lactic acid and aluminum chloride (AlCl 3 ·6H 2 O) are mixed in a molar ratio of 1:1:0.1, and stirred at 70°C for 2 h until a homogeneous and transparent liquid is formed;

[0075] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shells and then weigh 10 g of ternary deep eutectic solvent. Place both in a glass reaction test tube and react at 100 °C for 3 h, then cool to room temperature. The reaction solution is centrifuged at 8000 rpm for 10 min using a high-speed centrifuge. After pouring out the supernatant, the precipitate in the centrifuge tube is washed with 90% ethanol solution by suction filtration. The filtrate and the centrifuged supernatant are combined, and the deep eutectic solvent and ethanol are removed by rotary evaporation at 60 °C. After rotary evaporation, the concentrated solution (1 / 3 of the original solution volume) is collected, and deionized water is added to the concentrated solution at a volume ratio of 1:6 to precipitate for 12 h to obtain lignin, and the lignin is separated by centrifugation;

[0076] (3) Preparation of peanut shell nanocellulose: Add the lignin in step (2) to water at a dosage of 1 g:10 mL, place it in a dialysis bag with a molecular weight cut-off of 8000 - 14000 Da, and dialyze in deionized water to remove the deep eutectic solvent until the solution pH becomes neutral; then take out the solution, homogenize it at 10000 rpm for 30 min using a high-speed homogenizer, and then collect the turbid supernatant by centrifugation (the turbid supernatant indicates the presence of nanocellulose); and add water to the remaining precipitate again at a dosage of 1 g:10 mL, homogenize it at 10000 rpm for 30 min, and collect the turbid supernatant by centrifugation; repeat this operation until the finally centrifuged supernatant becomes transparent. The centrifugation conditions are 4000 rpm and 5 min; finally, all the collected turbid supernatants are mixed and freeze-dried to obtain peanut shell nanocellulose;

[0077] (4) Preparation of peanut shell lignin-nanocellulose composite film:

[0078] Add chitosan powder to 2.5% (v / v) acetic acid aqueous solution and continuously stir using a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%, and place it in a 4 °C refrigerator for later use. Add the lignin in step (2) to water to prepare a lignin solution with a mass concentration of 2.5%; add the peanut shell nanocellulose in step (3) to water to prepare a nanocellulose solution with a mass concentration of 2.5%.

[0079] Take lignin solution, nanocellulose solution and chitosan solution and mix them to obtain a mixed solution, where the mass percentage of the lignin solution is 5%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 92%; and add glycerol as a plasticizer to the mixed solution, with the addition amount being 0.1% of the mass of the mixed solution; continuously stir until the mixed solution becomes uniform and no precipitation can be observed to obtain a film-forming solution; after removing bubbles from the film-forming solution by ultrasonic treatment for 30 min, take out 5 g and pour it into a polyvinyl alcohol petri dish (diameter 50 mm), and dry it in an oven at 40 °C for 18 h. After film formation, place it in a desiccator with saturated potassium carbonate solution and condition it for 48 h. The obtained film is denoted as chitosan / nanocellulose-3% / lignin-5%.

[0080] Comparative Example 1:

[0081] (1) Preparation of chitosan film:

[0082] Add chitosan powder to 2.5% (v / v) acetic acid aqueous solution, and continuously stir using a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%. Place it in a refrigerator at 4 °C for standby. Take 5 g of the chitosan solution with a mass concentration of 2.5%, remove bubbles by ultrasonic treatment for 30 min, then pour it into a polyvinyl alcohol petri dish (diameter 50 mm), and dry it in an oven at 40 °C for 18 h. After film formation, place it in a desiccator with saturated potassium carbonate solution and condition it for 48 h. The obtained film is denoted as chitosan film.

[0083] Comparative Example 2:

[0084] (1) Synthesis of ternary deep eutectic solvent: Mix choline chloride, lactic acid and aluminum trichloride (AlCl 3 ·6H 2 O) in a molar ratio of 1:1:0.1, and stir at 70 °C for 2 h until a homogeneous and transparent liquid is formed;

[0085] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shell, and then weigh 10 g of the ternary deep eutectic solvent. Place the two in a glass reaction test tube, react at 100 °C for 3 h, and then cool to room temperature. Centrifuge the reaction solution at 8000 rpm for 10 min using a high-speed centrifuge. After pouring out the supernatant, wash the precipitate in the centrifuge tube with 90% ethanol solution by suction filtration. Combine the filtrate and the centrifuged supernatant, and remove the deep eutectic solvent and ethanol by rotary evaporation at 60 °C. After rotary evaporation, collect the concentrated solution (1 / 3 of the original solution volume), and add deionized water to the concentrated solution in a volume ratio of 1:6 to precipitate for 12 h to obtain lignin, and centrifuge to separate the lignin;

[0086] (3) Preparation of peanut shell nanocellulose: Add the lignin in step (2) to water at a dosage of 1 g:10 mL, place it in a dialysis bag with a molecular weight cut-off of 8000 - 14000 Da, and dialyze it in deionized water to remove the deep eutectic solvent until the solution pH becomes neutral; then take out the solution, homogenize it with a high-speed homogenizer at 10000 rpm for 30 min, and then collect the turbid supernatant by centrifugation (the turbid supernatant indicates the presence of nanocellulose); and add water again to the remaining precipitate at a dosage of 1 g:10 mL, homogenize it at 10000 rpm for 30 min, and collect the turbid supernatant by centrifugation; repeat the operation until the finally centrifuged supernatant becomes transparent, and the centrifugation conditions are 4000 rpm and 5 min; finally, mix all the collected turbid supernatants and perform freeze-drying to obtain peanut shell nanocellulose;

[0087] (4) Preparation of chitosan / nanocellulose - 3% composite film:

[0088] Add chitosan powder to a 2.5% (v / v) acetic acid aqueous solution, and continuously stir it with a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%, and place it in a refrigerator at 4°C for later use. Add the peanut shell nanocellulose in step (3) to water to make a nanocellulose solution with a mass concentration of 2.5%.

[0089] Take the nanocellulose solution and the chitosan solution and mix them to obtain a mixed solution, where the mass percentage of the nanocellulose solution is 3% and the mass percentage of the chitosan solution is 97%; and add glycerol as a plasticizer to the mixed solution, and the addition amount is 0.1% of the mass of the mixed solution; continuously stir until the mixed solution becomes uniform and no precipitate can be observed to obtain a film-forming solution; take the film-forming solution, remove the bubbles by ultrasonic treatment for 30 min, and then pour it into a polyvinyl alcohol petri dish (diameter 50 mm), dry it in an oven at 40°C for 18 h, and after forming the film, place it in a desiccator with a saturated potassium carbonate solution to condition it for 48 h to obtain a film denoted as chitosan / nanocellulose - 3%.

[0090] Comparative Example 3:

[0091] (1) Synthesis of ternary deep eutectic solvent: Choline chloride, lactic acid and aluminum chloride (AlCl 3 ·6H 2 O) are mixed in a molar ratio of 1:1:0.1, and stirred at 70°C for 2 h until a homogeneous and transparent liquid is formed;

[0092] (2) Preparation of peanut shell lignin: Weigh 1 g of peanut shells and then weigh 10 g of ternary eutectic solvent. Place both in a glass reaction test tube. After reacting at 100 °C for 3 h, cool to room temperature. Centrifuge the reaction solution at 8000 rpm for 10 min using a high-speed centrifuge. After pouring out the supernatant, wash the precipitate in the centrifuge tube with 90% ethanol solution by suction filtration. Combine the filtrate and the centrifuged supernatant, and remove the eutectic solvent and ethanol by rotary evaporation at 60 °C. After rotary evaporation, collect the concentrated solution (1 / 3 of the original solution volume), and add deionized water to the concentrated solution at a volume ratio of 1:6 to precipitate for 12 h to obtain lignin, and then centrifuge to separate the lignin;

[0093] (3) Preparation of chitosan / lignin-3% composite film:

[0094] Add chitosan powder to 2.5% (v / v) acetic acid aqueous solution, and continuously stir using a magnetic stirrer until a homogeneous and transparent liquid is formed to obtain a chitosan solution with a mass concentration of 2.5%, and place it in a 4 °C refrigerator for later use. Add the lignin in step (2) to water to prepare a lignin solution with a mass concentration of 2.5%.

[0095] Take the lignin solution and the chitosan solution and mix them to obtain a mixed solution, where the mass percentage of the lignin solution is 3% and the mass percentage of the chitosan solution is 97%; and add glycerol as a plasticizer, with an addition amount of 0.1% of the mass of the mixed solution; continuously stir until the mixed solution becomes uniform and no precipitate can be observed to obtain a film-forming solution; take 5 g of the film-forming solution after removing bubbles by ultrasonic treatment for 30 min and pour it into a polyvinyl alcohol petri dish (diameter 50 mm), and dry it in an oven at 40 °C for 18 h. After forming the film, place it in a desiccator with saturated potassium carbonate solution to adjust for 48 h to obtain the film denoted as chitosan / lignin-3%.

[0096] S1. Test the mechanical properties of the composite film:

[0097] Table 1 Mechanical property tests of lignin nanofiber composite films in Comparative Examples 1-3 and Examples 1-5

[0098]

[0099]

[0100] As shown in Table 1 and Figure 1As shown; compared with the chitosan / lignin-3% composite film (Comparative Example 3), the elastic modulus, elongation at break, tensile fracture stress, and tensile strength of the chitosan / nanocellulose-3% / lignin-1% composite film (Example 1) increased by 2.4%, 23.1%, 145%, and 33.6% respectively. This shows that the addition of nanocellulose to the composite film can significantly improve the mechanical properties. The elastic modulus, tensile strength, and maximum force of the chitosan / nanocellulose-3% / lignin-2% composite film (Example 2) were significantly improved. Compared with the chitosan film (Comparative Example 1), the elastic modulus, tensile strength, and maximum force increased by 24.67%, 20.27%, and 17.55% respectively. Compared with the chitosan / nanocellulose-3% composite film (Comparative Example 2), the tensile strength and maximum force increased by 5.33% and 35.25% respectively. This shows that the addition of lignin to the composite film can significantly improve the mechanical properties. Compared with the chitosan film (Comparative Example 1), the elastic modulus and tensile strength of the chitosan / nanocellulose-3% / lignin-3% composite film (Example 3) increased by 50.7% and 44.2% respectively. This shows that the addition of nanocellulose and lignin can significantly improve the mechanical properties of the chitosan film. In addition, by comparing the ternary chitosan / nanocellulose / lignin composite films, it was found that as the lignin content increased, the elastic modulus, elongation at break, and tensile strength of the composite films all showed an upward trend. When the lignin addition amount was 3%, the mechanical properties of the composite film reached the highest. When the lignin content reached 4% (Example 4) and 5% (Example 5), the mechanical properties of the composite film gradually decreased. In the ternary composite film, nanocellulose has a certain strengthening and toughening ability, while the presence of lignin can reduce the hydrogen bonds of cellulose, enhance its dispersibility, and play a synergistic role. Lignin acts as an adhesive in the composite film to make the nanocellulose links closer, which helps stress transfer; however, lignin is not the more the better, and its dosage needs to be strictly controlled. For example, the properties of chitosan / nanocellulose-3% / lignin-4% (Example 4) and chitosan / nanocellulose-3% / lignin-5% (Example 5) decreased instead; therefore, the selection of the lignin dosage in the present invention is also creative.

[0101] S2. Test the hydrophobic properties of the composite film:

[0102] Figure 2 In Figure A, it is the test result of the water contact angle of the lignin nanocellulose composite film in Comparative Example 1 and Example 3. Among them, the water contact angle of the chitosan film (Comparative Example 1) is 108°, and the water contact angle of the composite film (Example 3) after adding 3% lignin and 3% nanocellulose is 110°, indicating that the surface hydrophobicity of the composite film after adding nanocellulose and lignin is enhanced.

[0103] Figure 2Figure B in the present invention shows the contact results of the wood-based nanocellulose composite film in Example 3 with edible oil, coffee, tea, soy sauce, vinegar, milk, Sudan red solution and methylene blue solution. The results show that the composite film of chitosan / nanocellulose-3% / lignin-3% also has certain barrier properties to solutions such as edible oil, coffee, tea, soy sauce, vinegar, milk, etc., which is beneficial to the preservation of fresh fruits and vegetables by the composite film.

[0104] S3. Ultraviolet light transmittance performance test:

[0105] The ultraviolet light transmittance performance of the composite film was tested. High transmittance is very important for the film used in fruit and vegetable packaging.

[0106] Figure 3 It is the light transmittance performance test diagram of the wood-based nanocellulose composite film in Comparative Examples 1-3 and Examples 1-5; Figure 4 It is the ultraviolet-visible transmission curve of the wood-based nanocellulose composite film in Comparative Examples 1-3 and Examples 1-5.

[0107] As Figure 3 and Figure 4 shown, it can be seen from Figure 3 that each group of films has good light transmittance in the visible light range. By comparing the colored icons, it can be seen that the icon behind the chitosan film is clear and has the highest light transmittance. When 3% of lignin or nanocellulose is added to chitosan, the light transmittance gradually decreases and the film color deepens. The light transmittance of the chitosan / nanocellulose-3% / lignin-1% composite film is lower than that of the chitosan film, chitosan / lignin-3% and chitosan / nanocellulose-3% composite films. In the ternary composite film, with the increase of the lignin addition amount, a small amount of granular substances appear in the composite film, the film thickness increases, and the light transmittance performance also shows a downward trend. When the film is placed on the icon, the icon gradually begins to blur.

[0108] From Figure 4It can be seen that the chitosan film has the highest light transmittance. After adding nanocellulose, the light transmittance of the chitosan / nanocellulose-3% composite film (Comparative Example 2) decreases, but a relatively high transmittance can still be achieved. This is because nanocellulose will agglomerate itself after being fully swollen in water, and there is a strong hydrogen bond or chemical interaction between nanocellulose and chitosan, which can crosslink to form a dense network structure. The light transmittance of the chitosan / lignin-3% composite film (Comparative Example 3) decreases compared with the chitosan film. When lignin is added to the chitosan / nanocellulose composite film, the light transmittance of the film further decreases. However, through comparison, it is found that the light transmittance of the chitosan / nanocellulose-3% / lignin-4% composite film and the chitosan / nanocellulose-3% / lignin-5% composite film in the wavelength range of 400-800 nm is higher than that of the chitosan / nanocellulose-3% composite film, and the chitosan / nanocellulose-3% / lignin-3% composite film is not much different from it. Lignin has conjugated carbonyl groups that can absorb ultraviolet and visible light. Therefore, the content of lignin can affect the light absorption ability of the composite film, thereby affecting the light transmittance. The chitosan / nanocellulose / lignin composite films prepared in Examples 3-5 all exhibit excellent ultraviolet protection ability. This remarkable ultraviolet barrier performance is attributed to the ultraviolet-absorbing functional groups in lignin in the composite film, including phenolic units, ketones and other chromophores, making it a natural broad-spectrum blocker.

[0109] S4. Test the degradation performance of the composite film:

[0110] Figure 6 It is the degradation result diagram of the lignin nanocellulose composite films in Comparative Examples 1-3 and Examples 1-5 after being buried in soil for 10 days. The results show that after burying chitosan and its composite films in soil for 10 days, it can be found that the chitosan film has some wrinkles and is intact overall, while chitosan / lignin-3%, chitosan / nanocellulose-3% / lignin-2%, chitosan / nanocellulose-3% / lignin-3%, chitosan / nanocellulose-3% / lignin-5% are almost completely degraded; after being buried in soil for 18 days, the chitosan / nanocellulose / lignin ternary composite films prepared in Examples 1-5 are completely degraded. This shows that the addition of nanocellulose and lignin makes the composite film more environmentally friendly. The use of the composite film can not only effectively achieve the preservation of fruits and vegetables, but also has a fast degradation rate, and is green and safe.

[0111] Application 1 (blueberries):

[0112] Taking the product prepared in Example 3 as an example, a preservation experiment was carried out; chitosan / nanocellulose-3% and uncoated blueberries were used as controls; each group had about 90 fruits, and the blueberries in each group were stored at 4°C, and 3 fruits were taken out every 48 h for index determination.

[0113] Experimental group (chitosan / nanocellulose - 3% / lignin - 3%): Select fresh blueberries with uniform size, similar maturity, no pests, diseases and mechanical damage, wash them with deionized water, and place them in a fume hood to dry at room temperature. Immerse the blueberries in the film-forming solution in Example 3 (where the mass percentage of the lignin solution is 3%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 94%) for 30 s for film coating, and then place them in a fume hood to dry at room temperature after film coating.

[0114] Control group 1 (uncoated): Select fresh blueberries with uniform size, similar maturity, no pests, diseases and mechanical damage, wash them with deionized water, and place them in a fume hood to dry at room temperature.

[0115] Control group 2 (chitosan / nanocellulose - 3%): The operation is the same as that of the experimental group, the difference is that the film-forming solution is replaced with the film-forming solution in Comparative Example 2 (the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 97%).

[0116] Changes in the appearance of blueberries during the preservation process with the composite film coating are as Figure 5 shown. It can be seen from Figure 5 that at the initial stage of storage (1 - 8 d), both the coated and uncoated blueberries showed good gloss. As the storage time prolonged, the color of the blueberries gradually darkened. After 16 days of storage, wrinkles appeared on the surface of the uncoated blueberries, and the hardness gradually decreased, while no wrinkles appeared on the surface of the coated blueberries, and the hardness was higher than that of the uncoated group. After 24 days of storage, the blueberries coated with the chitosan / nanocellulose - 3% composite film showed shriveling, while the blueberries coated with the chitosan / nanocellulose - 3% / lignin - 3% composite film still maintained a good appearance. This indicates that the chitosan / nanocellulose - 3% / lignin - 3% composite coating can better delay the ripening, senescence and water loss of fruits, and maintain the gloss and freshness of fruits. Compared with the chitosan / nanocellulose - 3% coating, the chitosan / nanocellulose - 3% / lignin - 3% coating has higher stability.

[0117] Application 2 (red globe grapes):

[0118] Taking the product prepared in Example 3 as an example, a preservation experiment was carried out; red globe grapes with chitosan / nanocellulose - 3% and uncoated ones were used as controls; there were about 90 fruits in each group, and the blueberries in each group were stored at 4 °C, and 3 fruits were taken out every 48 h for index determination.

[0119] Experimental group (chitosan / nanocellulose - 3% / lignin - 3%): Select fresh red grapes with uniform size, similar maturity, no pests, diseases and mechanical damage, wash them with deionized water, and air-dry them at room temperature in a fume hood. Immerse the red grapes in the film-forming solution in Example 3 (where the mass percentage of the lignin solution is 3%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 94%) for 30 s for film coating, and then air-dry them at room temperature in a fume hood after film coating.

[0120] Control group 1 (uncoated): Select fresh red grapes with uniform size, similar maturity, no pests, diseases and mechanical damage, wash them with deionized water, and air-dry them at room temperature in a fume hood.

[0121] Control group 2 (chitosan / nanocellulose - 3%): The operation is the same as that of the experimental group, the difference is that the film-forming solution is replaced with the film-forming solution in Comparative Example 2 (the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 97%).

[0122] The changes in the appearance of red grapes during the preservation process with the composite film coating are as Figure 7 shown. Appearance is the most intuitive and convenient way to judge the freshness of fruits and whether they are spoiled. During the process of fruit ripening and senescence, the color of the appearance will also change. As Figure 7 shown, at the initial stage of storage (1 - 8 d), both the coated and uncoated red grapes showed good gloss and bright and vivid colors. As the storage time extended to 15 days, wrinkles and black spots began to appear on the surface of the uncoated red grapes, while the red grapes in the nanocellulose / chitosan / lignin coating group did not show the above situation until the end of storage. It is speculated that chitosan and lignin have certain antibacterial effects, and the coating made of them has the ability to resist the invasion of external pathogenic bacteria. Therefore, the nanocellulose / chitosan / lignin coating can significantly reduce the respiration rate of red grapes, reduce water evaporation, and extend the storage period of red grapes.

[0123] Combined with the application cases, at the initial stage of storage (1 - 12 d), compared with the uncoated group, the blueberries and red grapes in the chitosan / nanocellulose coating group were smoother and plumper on the outside. In the later stage of storage (13 - 24 d), wrinkles began to appear on the surface of blueberries and red grapes, but the overall hardness was higher than that of the uncoated group. The above research phenomena indicate that the good modified atmosphere ability of the chitosan / nanocellulose coating can better delay the ripening and senescence of fruits and water loss, thereby maintaining the gloss and freshness of fruits and improving the storage capacity of fresh fruits and vegetables.

[0124] Note: The above embodiments are only used to illustrate the present invention rather than to limit the technical solutions described in the present invention; therefore, although the present specification has described the present invention in detail with reference to the above embodiments, those of ordinary skill in the art should understand that the present invention can still be modified or equivalently replaced; and all technical solutions and their improvements that do not depart from the spirit and scope of the present invention should be covered by the scope of the claims of the present invention.

Claims

1. A method for preparing a wood nanocellulose ternary composite fresh-keeping liquid, characterized in that: The following steps are involved: (1) Synthesis of ternary deep eutectic solvent: Choline chloride, lactic acid and aluminum chloride are mixed in a certain molar ratio and stirred at a certain temperature until a uniform transparent liquid is formed, thereby obtaining a ternary deep eutectic solvent; (2) Preparation of peanut shell lignin: Weigh peanut shells and add them to the low eutectic solvent of step (1), react at a certain temperature, cool to room temperature after the reaction is completed, centrifuge the obtained reaction solution to obtain a supernatant, add an ethanol solution to the remaining precipitate, wash the precipitate with the ethanol solution by suction filtration, and collect the filtrate; then, combine the filtrate and the supernatant, collect the concentrated solution after rotary evaporation, add deionized water to the concentrated solution for precipitation, and finally separate the precipitate by centrifugation, which is lignin; (3) Preparation of peanut shell nanocellulose: The lignin obtained in step (2) is added to water and placed in a dialysis bag, and dialyzed in deionized water until the pH of the solution becomes neutral; Then, the solution is taken out, homogenized, and centrifuged to obtain a turbid supernatant, and water is added to the remaining precipitate again to homogenize and centrifuge to collect the turbid supernatant; the operation is repeated until the supernatant collected by centrifugation becomes transparent; finally, the collected turbid supernatant is mixed and freeze-dried to obtain peanut shell nanocellulose; (4) Preparation of wood nanocellulose ternary composite preservative solution: Adding chitosan powder to an acetic acid aqueous solution and stirring to form a uniform transparent liquid to obtain a chitosan solution; adding the lignin obtained in step (2) to water and stirring to obtain a lignin solution; then adding the peanut shell nanocellulose in step (3) to water and stirring to obtain a nanocellulose solution; The chitosan solution, lignin solution and nanocellulose solution are then mixed, and glycerol is added as a plasticizer, and stirred continuously until the mixed solution becomes uniform and there is no precipitation at the bottom to obtain a chitosan / nanocellulose / lignin film-forming solution, which is a wood nanocellulose ternary composite preservative solution.

2. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: The molar ratio of choline chloride, lactic acid and aluminum chloride in step (1) is 1:1:0.1; and the certain temperature condition is 70°C.

3. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: The mass ratio of peanut shells to the low eutectic solvent in step (2) is 1:10, the reaction temperature is 100° C., and the reaction time is 3 h.

4. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: The centrifugal conditions in step (2) are: 8000 rpm, 10 min.

5. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: The volume concentration of the ethanol solution in step (2) is 95%; the temperature of the rotary evaporation is 60° C., and the rotary evaporation is performed to 1 / 3 of the volume of the original solution; the volume ratio of the concentrated solution to deionized water is 1:6; and the time for adding deionized water for precipitation is 12 hours.

6. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: The molecular weight cutoff of the dialysis bag in step (3) is 8000-14000Da; the homogenization conditions are: homogenization at 10000rpm for 30min; the centrifugation conditions are: 4000rpm for 5min.

7. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: In step (3), the lignin is added to water in a ratio of 0.5-1 g:10 mL; the precipitate and water are added in a ratio of 0.5-1 g:10 mL.

8. The method for preparing a wood nanocellulose ternary composite fresh-keeping liquid according to claim 1, characterized in that: In step (4), the mass concentrations of the lignin solution and the nanocellulose solution are both 2.5%, the mass concentration of the chitosan solution is 2.5%, and the concentration of the acetic acid aqueous solution is 2% (v / v); in the mixed solution, the mass percentage of the lignin solution is 1-5%, the mass percentage of the nanocellulose solution is 3%, and the mass percentage of the chitosan solution is 92%-96%; and the amount of glycerol added is 0.1-0.2% of the mass of the mixed solution.

9. Use of the wood nanocellulose ternary composite fresh-keeping liquid prepared according to any one of claims 1 to 8 for preserving fruits and vegetables, characterized in that: Here are the steps: Select fruits and vegetables of uniform size, similar maturity, free from pests and diseases and mechanical damage, wash them with water and dry them at room temperature; then immerse the fruits and vegetables in a wood nano-cellulose ternary composite fresh-keeping liquid, dry them at room temperature after coating, and form a protective film on the surface of the fruits and vegetables, which is a wood nano-cellulose ternary composite fresh-keeping film, which can achieve the purpose of preserving fruits and vegetables.

10. The use according to claim 9, characterized in that The fruits and vegetables include red grapes and blueberries; the time for immersing in the wood nano-cellulose ternary composite fresh-keeping liquid is 30-60 seconds.

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