Preparation method and application of packaging film with antibacterial and ultraviolet barrier properties

Abstract

The invention discloses a preparation method and application of a packaging film with antibacterial and ultraviolet barrier properties, the method comprises the following steps: step (1) lignin extraction: crushing, cleaning and drying hemp straws, and extracting lignin by using a deep eutectic solvent method; (2) preparing lignin nano-particles: preparing the lignin nano-particles by taking tetrahydrofuran as a solvent through a self-assembly method; and (3) dissolving the chitosan phenolic acid graft in acetic acid, adding gelatin as a membrane matrix and glycerol as a flexibilizer, adding lignin nanoparticles, and preparing the phenolic acid-chitosan-lignin-gelatin-glycerol composite membrane by a tape casting method. The preparation method is simple, the raw materials are easy to obtain and low in price, and the prepared composite film has an obvious inhibiting effect on escherichia coli and staphylococcus aureus, has a shielding effect on ultraviolet light and has basic properties of a food packaging film.

Description

Technical Field

[0001] The present invention belongs to the field of food packaging materials, and relates to a food packaging film, in particular to a preparation method and application of a degradable food packaging film with both antibacterial and ultraviolet barrier properties. Background Art

[0002] Hemp is a crop with extremely strong vitality and can grow almost all over the world except in polar regions and tropical rainforests. According to statistics, only 2% of lignin is applied with high value, such as being used in dispersants, adhesives, and surfactants, etc. Most of the remaining lignin is directly burned, causing a large amount of resource waste and environmental pollution. Nanonizing lignin is an important way to realize the high-value application of lignin. The reduction of the size of nano-lignin can enhance its antibacterial, antioxidant, and ultraviolet light absorption properties, and also enhance the surface effect and small-size effect of the particles. Applying nano-lignin to polymer matrix materials can achieve the effect of introducing functional groups to replace metal nano-materials. Nano-lignin can be used as a raw material for carriers, membranes, and hydrogels and applied in functional surface coatings, packaging materials, biomedicine, and other fields.

[0003] Adding nano-lignin as a filler to high molecular compounds is the key to preparing sustainable polymer composites. Among them, gelatin, as a degradable packaging material, is mainly used to prepare gelatin coatings and films with several common edible biopolymers and has been widely used in food preservation. Existing research shows that graft copolymerization of chitosan and phenolic acids can improve the antioxidant, antibacterial, film-forming properties, etc. of chitosan, expand the application range of chitosan, and greatly improve the functions such as antioxidant and antibacterial properties of the film.

[0004] Currently, the main methods for extracting lignin are acid extraction, alkaline separation, organic solvent extraction, ionic liquid treatment method, and deep eutectic solvent method. Among them, the acid method and the alkaline method have problems such as changing the lignin structure, high equipment maintenance cost, difficult subsequent treatment, and low yield. The organic solvent and ionic liquid treatment methods have problems such as too high cost and difficult recovery. The solvent used in the deep eutectic solvent method is a green solvent and is inexpensive, with the advantages of simple operation, non-toxicity, and high extraction efficiency. Therefore, the deep eutectic solvent method has become a hot method for extracting lignin. And currently, there are only studies on the performance and functions of packaging films with only lignin nanoparticles added alone and chitosan-phenolic acid grafts added alone, and there are few studies on packaging films with these two compounds mixed together as additives. Summary of the Invention

[0005] In order to improve the utilization rate of hemp straw, the present invention provides a preparation method and application of a packaging film with both antibacterial and ultraviolet barrier properties. Through the pretreatment of hemp straw, the extraction of lignin, the preparation of lignin nanoparticles, the preparation of chitosan-phenolic acid grafts, and the preparation of composite packaging films, and through structural characterization and performance determination, it is shown that the packaging film has certain antibacterial and ultraviolet shielding functions.

[0006] The object of the present invention is achieved by the following technical solutions:

[0007] A preparation method of a packaging film with both antibacterial and ultraviolet barrier properties, comprising the following steps:

[0008] Step (1) Extraction of lignin: After crushing, washing, and drying hemp straw, lignin is extracted by the deep eutectic solvent method, where:

[0009] The deep eutectic solvent includes choline chloride and lactic acid. Lactic acid is a hydrogen bond donor, and choline chloride is a hydrogen bond acceptor. The molar ratio of choline chloride to lactic acid is 1:4 - 12, preferably 1:8;

[0010] The mass ratio of the hemp straw powder to the deep eutectic solvent is 1:25;

[0011] The extraction temperature is 100 - 140 °C, preferably 120 °C, and the time is 4 - 8 h, preferably 6 h;

[0012] Step (2) Preparation of lignin nanoparticles: Using tetrahydrofuran as a solvent, lignin nanoparticles are prepared by the self-assembly method. The specific steps are as follows: Lignin is completely dissolved in tetrahydrofuran by ultrasonic treatment, deionized water is added and stirred continuously, so that lignin is decomposed into nanoscale, and after dialysis and freeze-drying, lignin nanoparticles are obtained, where:

[0013] The concentration of lignin in tetrahydrofuran is 1.5 mg / mL;

[0014] The power of the ultrasonic treatment is 120 W, and the time is 30 min;

[0015] The volume ratio of deionized water to tetrahydrofuran is 3:1;

[0016] Step (3) Preparation of chitosan phenolic acid graft copolymer: Chitosan-protocatechuic acid graft copolymer was prepared by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) / N-hydroxysuccinimide (NHS) coupling reaction. The specific steps are as follows: (1) Add the EDC solution to the protocatechuic acid ethanol solution for reaction to obtain intermediate compound 1; (2) Add NHS to intermediate compound 1 and continue the reaction in an ice bath to obtain intermediate compound 2; (3) Add intermediate compound 2 to the chitosan solution. After the reaction, transfer the reaction solution to a dialysis bag for dialysis; (4) Centrifuge the dialyzed liquid to remove residual protocatechuic acid, and freeze-dry the supernatant to obtain chitosan-protocatechuic acid graft copolymer. The molar ratio of chitosan, protocatechuic acid, EDC, and NHS is 1:1:1:1;

[0017] Step (4) Dissolve the chitosan phenolic acid graft copolymer in acetic acid, add gelatin as the film matrix, glycerol as the toughening agent, and add lignin nanoparticles. Prepare the phenolic acid-chitosan-lignin-gelatin-glycerol composite film by the casting method, where:

[0018] The mass ratio of the graft copolymer, acetic acid, gelatin, and glycerol is 1:0.49:1.15:0.3;

[0019] The addition amount of the lignin nanoparticles is 0.25-3% of the mass of the gelatin;

[0020] The composite film can be used as a food packaging film.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] The preparation method of the present invention is simple, the raw materials are easy to obtain and inexpensive. The prepared composite film has obvious inhibitory effects on Escherichia coli and Staphylococcus aureus, has a shielding effect on ultraviolet light, and has the basic properties of a food packaging film. Description of the Drawings

[0023] Figure 1 is the Fourier transform infrared spectrum of the composite packaging film;

[0024] Figure 2 is the ultraviolet absorption spectrum of the composite packaging film;

[0025] Figure 3 is the water vapor transmission rate of the composite packaging film;

[0026] Figure 4 is the tensile strength of the composite packaging film;

[0027] Figure 5 is the elongation at break of the composite packaging film;

[0028] Figure 6For the antibacterial performance of the composite packaging film;

[0029] Figure 7 It is the actual picture of the antibacterial zone of the composite packaging film. Specific implementation mode

[0030] The technical solution of the present invention will be further described below in conjunction with the accompanying drawings, but it is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention shall be covered by the protection scope of the present invention.

[0031] Experimental materials and instruments: Hemp straw was taken from Harbin City, Heilongjiang Province, and stored at room temperature after sampling. Choline chloride, lactic acid, tetrahydrofuran, absolute ethanol, chitosan (MW = 150000), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), 2-morpholinoethanesulfonic acid (MES) (pH = 5.5), acetic acid were purchased from Shanghai Macklin Biochemical Co., Ltd., protocatechuic acid was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., glycerol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd., gelatin was purchased from Beijing Solarbio Science & Technology Co., Ltd., Nicoleti S50 Fourier transform infrared spectrometer, TU191 ultraviolet-visible spectrophotometer, Texture Pro CTV 1.9 Build 35 texture analyzer.

[0032] Example 1

[0033] This example provides a method for preparing a packaging film with both antibacterial and ultraviolet barrier properties, and the method includes the following steps:

[0034] Step 1, extraction of lignin: Grind the hemp straw, boil it in water at 60 °C for 30 min, and obtain clean hemp straw powder after drying. Mix choline chloride and lactic acid according to a molar ratio of 1:8, and stir to a clear and transparent liquid at 60 °C. Mix the crushed hemp straw with the deep eutectic solvent according to a mass ratio of 1:25, and extract at a high temperature of 120 °C for 6 h. After the extraction is completed, cool naturally, add 95% ethanol, and filter by suction. The filtrate is rotary evaporated to remove ethanol and the remaining deep eutectic solvent, the filtrate is mixed with water in a ratio of 1:9, allowed to stand and precipitate for 12 h, the precipitate is collected by centrifugation, and the lignin sample is obtained by freeze-drying.

[0035] Step 2, preparation of lignin nanoparticles: lignin is completely dissolved in tetrahydrofuran by ultrasound, deionized water is added and stirred continuously to decompose the lignin into nanometer scale, wherein the concentration of lignin in tetrahydrofuran is 1.5 mg / mL, the ultrasonic condition is 120W, 30min, the volume of deionized water added is three times the volume of tetrahydrofuran, the addition speed is 4mL / min, the stirring condition is 600r / min, 4h, and then dialyzed for 72h using a dialysis bag with a retention capacity of 1000Da, and then freeze-dried to obtain lignin nanoparticles.

[0036] Step 3, preparation of chitosan phenolic acid graft: chitosan-protocatechuic acid graft was prepared by 1-(3-dimethylaminopropyl) 3-ethylcarbodiimide hydrochloride (EDC) / N-hydroxysuccinimide (NHS) coupling reaction, the specific steps are as follows: (1) chitosan (Mw = 150000, 1.22 g, 7.5 mmol) was dissolved in 25 mL of acetic acid solution (0.5%, v / v%) and stirred at room temperature overnight; (2) protocatechuic acid (7.5 mmol) was dissolved in 5 mL of ethanol; (3) the mediator 1-(3-dimethylaminopropyl) 3-ethylcarbodiimide hydrochloride (EDC) (1.44 g, 7.5 mmol) was dissolved in 5 mL of ethanol; (4) adding N-hydroxysuccinimide (NHS) (0.86 g, 7.5 mmol) to the intermediate compound 1 and continuing the reaction in an ice bath for 1 h to obtain the intermediate compound 2; (5) gradually adding the intermediate compound 2 to the pre-prepared chitosan solution and leaving it to react at room temperature in the dark for 12 h; (6) dialyzing with distilled water for 72 h, the dialysis bag cut-off flow rate was 14 kDa, centrifuging (10000 x g, 4 ° C) for 30 min to remove the residual protocatechuic acid, and freeze-drying the supernatant to obtain the chitosan phenolic acid graft.

[0037] Step 4, preparation of composite film: Take 0.27g graft copolymer and dissolve it in 13.3mL 1% (v / v) acetic acid solution, add 0.31g gelatin, heat and stir at 60℃ to dissolve, then add 0.08g glycerol and 3.1mg lignin nanoparticles to prepare a mixed film, continue stirring for 1h, and the solvent of the blank film is 13.3mL 1% (v / v) acetic acid solution. Ultrasonic degassing for 30min, cast the film liquid on a polycarbonate plate (10cm×10cm), and dry at room temperature for 48h. After removing the film, place it in a constant temperature and humidity box at 25℃ and 50%RH for 24h.

[0038] Example 2

[0039] This embodiment provides a method for preparing a packaging film having both antibacterial and ultraviolet blocking properties, the method comprising the following steps:

[0040] Step 1. Extraction of lignin: Grind hemp straw, boil it in water at 60 °C for 30 min, and obtain clean hemp straw powder after drying. Mix choline chloride and lactic acid in a molar ratio of 1:8, and stir at 60 °C until it becomes a clear and transparent liquid. Mix the crushed hemp straw with the deep eutectic solvent in a mass ratio of 1:25, and extract at a high temperature of 120 °C for 6 h. After extraction, cool it naturally, add 95% ethanol, and perform suction filtration. The filtrate is rotary evaporated to remove ethanol and the remaining deep eutectic solvent. The filtrate is mixed with water in a ratio of 1:9, allowed to stand and precipitate for 12 h, the precipitate is collected by centrifugation, and the lignin sample is obtained by freeze-drying.

[0041] Step 2. Preparation of lignin nanoparticles: Dissolve lignin in tetrahydrofuran, with the concentration of lignin in tetrahydrofuran being 1.5 mg / mL, and ultrasonically treat it at 120 W for 30 min at room temperature. Add deionized water to the tetrahydrofuran mixed solution at a rate of 4 mL / min, with the volume of deionized water being three times that of tetrahydrofuran, and perform magnetic stirring at 600 r / min for 4 h. After stirring, transfer the solution to a 1000 Da dialysis bag and dialyze it in excess deionized water for 72 h to remove the residual tetrahydrofuran. After dialysis, it is freeze-dried to obtain lignin nanoparticles.

[0042] Step 3. Preparation of chitosan phenolic acid graft copolymer: Prepare chitosan-protocatechuic acid graft copolymer through 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) / N-hydroxysuccinimide (NHS) coupling reaction. The specific steps are as follows: (1) Dissolve chitosan (Mw = 150000, 1.22 g, 7.5 mmol) in 25 mL of acetic acid solution (0.5%, v / v%) and stir overnight at room temperature; (2) Dissolve protocatechuic acid (7.5 mmol) in 5 mL of ethanol; (3) Dissolve the mediator 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) (1.44 g, 7.5 mmol) in 20 mL of 2-(N-morpholino)ethanesulfonic acid (MES) buffer solution (pH = 5.5), add the protocatechuic acid solution to obtain intermediate compound 1; (4) Add N-hydroxysuccinimide (NHS) (0.86 g, 7.5 mmol) to intermediate compound 1 and continue to react in an ice bath for 1 h to obtain intermediate compound 2; (5) Gradually add intermediate compound 2 to the pre-prepared chitosan solution and react in the dark at room temperature for 12 h; (6) Dialyze with distilled water for 72 h, the cut-off molecular weight of the dialysis bag is 14 kDa, centrifuge (10000 x g, 4 °C) for 30 min to remove the residual protocatechuic acid, and freeze-dry the supernatant to obtain the chitosan phenolic acid graft copolymer.

[0043] Step 4: Preparation of composite film: Dissolve 0.27 g of graft copolymer in 13.3 mL of 1% (v / v) acetic acid solution, add 0.31 g of gelatin, heat and stir to dissolve at 60 °C, then add 0.08 g of glycerol and 6.2 mg of lignin nanoparticles to prepare a mixed film, continue stirring for 1 h. The solvent for the blank film is 13.3 mL of 1% (v / v) acetic acid solution, and continue stirring for 1 h. Degas by ultrasound for 30 min, cast the film solution on a polycarbonate plate (10 cm × 10 cm), and dry at room temperature for 48 h. After peeling off the film, place it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h.

[0044] Example 3

[0045] This example provides a method for preparing a packaging film with both antibacterial and ultraviolet barrier properties, and the method includes the following steps:

[0046] Step 1: Extraction of lignin: Grind hemp straw, boil it in water at 60 °C for 30 min, and obtain clean hemp straw powder after drying. Mix choline chloride and lactic acid in a molar ratio of 1:8, and stir at 60 °C until it becomes a clear and transparent liquid. Mix the crushed hemp straw with the deep eutectic solvent in a mass ratio of 1:25, and extract at 120 °C for 6 h. After extraction, cool naturally, add 95% ethanol, and filter by suction. The filtrate is rotary evaporated to remove ethanol and residual deep eutectic solvent, the filtrate is mixed with water in a ratio of 1:9, allowed to stand and precipitate for 12 h, the precipitate is collected by centrifugation, and lignin sample is obtained by freeze-drying.

[0047] Step 2: Preparation of lignin nanoparticles: Dissolve lignin in tetrahydrofuran, the concentration of lignin in tetrahydrofuran is 1.5 mg / mL, and ultrasonically treat at 120 W for 30 min at room temperature. Add deionized water to the tetrahydrofuran mixed solution at a rate of 4 mL / min, the volume of deionized water is three times the volume of tetrahydrofuran, and stir magnetically at 600 r / min for 4 h. After stirring, transfer the solution to a 1000 Da dialysis bag and dialyze in excess deionized water for 72 h to remove residual tetrahydrofuran, and freeze-dry after dialysis to obtain lignin nanoparticles.

[0048] Step 3. Preparation of chitosan phenolic acid graft copolymer: The chitosan-protocatechuic acid graft copolymer was prepared by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) / N-hydroxysuccinimide (NHS) coupling reaction. The specific steps were as follows: (1) Chitosan (Mw = 150,000, 1.22 g, 7.5 mmol) was dissolved in 25 mL of acetic acid solution (0.5%, v / v%) and stirred overnight at room temperature; (2) Protocatechuic acid (7.5 mmol) was dissolved in 5 mL of ethanol; (3) The mediator 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) (1.44 g, 7.5 mmol) was dissolved in 20 mL of 2-morpholinoethanesulfonic acid (MES) buffer (pH = 5.5), and the protocatechuic acid solution was added to obtain intermediate compound 1; (4) N-hydroxysuccinimide (NHS) (0.86 g, 7.5 mmol) was added to intermediate compound 1 and the reaction was continued for 1 h in an ice bath to obtain intermediate compound 2; (5) Intermediate compound 2 was gradually added to the pre-prepared chitosan solution and the reaction was carried out at room temperature in the dark for 12 h; (6) Dialysis was carried out with distilled water for 72 h, the cut-off molecular weight of the dialysis bag was 14 kDa, and centrifugation (10,000 xg, 4 °C) was carried out for 30 min to remove the residual protocatechuic acid. The supernatant was freeze-dried to obtain the chitosan phenolic acid graft copolymer.

[0049] Step 4. Preparation of the composite film: 0.27 g of the graft copolymer was dissolved in 13.3 mL of 1% (v / v) acetic acid solution, 0.31 g of gelatin was added, and the mixture was heated and stirred at 60 °C until dissolved. Then 0.08 g of glycerol and 9.3 mg of lignin nanoparticles were added to prepare a mixed film, and stirring was continued for 1 h. The solvent for the blank film was 13.3 mL of 1% (v / v) acetic acid solution. Degassing was carried out by ultrasound for 30 min. The film solution was cast on a polycarbonate plate (10 cm × 10 cm) and dried at room temperature for 48 h. After the film was peeled off, it was placed in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h.

[0050] Example 4

[0051] This example provides a method for preparing a packaging film with both antibacterial and ultraviolet barrier properties. The method includes the following steps:

[0052] Step 1. Extraction of lignin: The hemp straw was ground, boiled in water at 60 °C for 30 min, and dried to obtain clean hemp straw powder. Choline chloride and lactic acid were mixed at a molar ratio of 1:8 and stirred at 60 °C until a clear and transparent liquid was obtained. The crushed hemp straw was mixed with the deep eutectic solvent at a mass ratio of 1:25 and extracted at a high temperature of 120 °C for 6 h. After extraction, it was naturally cooled, 95% ethanol was added, and filtration was carried out by suction. The filtrate was rotary evaporated to remove ethanol and the remaining deep eutectic solvent. The filtrate was mixed with water at a ratio of 1:9, allowed to stand and precipitate for 12 h, the precipitate was collected by centrifugation, and the lignin sample was obtained by freeze-drying.

[0053] Step 2. Preparation of lignin nanoparticles: The lignin was dissolved in tetrahydrofuran, and the concentration of lignin in tetrahydrofuran was 1.5 mg / mL. It was ultrasonically treated at 120 W for 30 min at room temperature. Deionized water was added to the tetrahydrofuran mixed solution at a rate of 4 mL / min, and the volume of deionized water was three times that of tetrahydrofuran. It was magnetically stirred at 600 r / min for 4 h. After stirring, the solution was transferred to a 1000 Da dialysis bag and dialyzed in excess deionized water for 72 h to remove the residual tetrahydrofuran. After dialysis, it was freeze-dried to obtain lignin nanoparticles.

[0054] Step 3. Preparation of chitosan phenolic acid graft: The chitosan-protocatechuic acid graft was prepared by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) / N-hydroxysuccinimide (NHS) coupling reaction. The specific steps are as follows: (1) Chitosan (Mw = 150000, 1.22 g, 7.5 mmol) was dissolved in 25 mL of acetic acid solution (0.5%, v / v%) and stirred overnight at room temperature; (2) Protocatechuic acid (7.5 mmol) was dissolved in 5 mL of ethanol; (3) The mediator 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) (1.44 g, 7.5 mmol) was dissolved in 20 mL of 2-morpholinoethanesulfonic acid (MES) buffer (pH = 5.5), and the protocatechuic acid solution was added to obtain intermediate compound 1; (4) N-hydroxysuccinimide (NHS) (0.86 g, 7.5 mmol) was added to intermediate compound 1 and the reaction was continued in an ice bath for 1 h to obtain intermediate compound 2; (5) Intermediate compound 2 was gradually added to the pre-prepared chitosan solution and reacted in the dark at room temperature for 12 h; (6) It was dialyzed with distilled water for 72 h, the cut-off flow rate of the dialysis bag was 14 kDa, and centrifuged (10000 xg, 4 °C) for 30 min to remove the residual protocatechuic acid. The supernatant was freeze-dried to obtain the chitosan phenolic acid graft.

[0055] Step 4: Preparation of the composite film: Dissolve 0.27 g of the graft copolymer in 13.3 mL of 1% (v / v) acetic acid solution, add 0.31 g of gelatin, heat and stir to dissolve at 60 °C, then add 0.08 g of glycerol and 12.4 mg of lignin nanoparticles to prepare a mixed film. Continue stirring for 1 h. The solvent for the blank film is 13.3 mL of 1% (v / v) acetic acid solution, and continue stirring for 1 h. Degas by ultrasound for 30 min, cast the film solution on a polycarbonate plate (10 cm × 10 cm), and dry at room temperature for 48 h. After peeling off the film, place it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h.

[0056] Example 5

[0057] This example provides a method for preparing a packaging film with both antibacterial and ultraviolet barrier properties. The method includes the following steps:

[0058] Step 1: Extraction of lignin: Grind hemp straw, boil it in water at 60 °C for 30 min, and obtain clean hemp straw powder after drying. Mix choline chloride and lactic acid in a molar ratio of 1:8, and stir at 60 °C until it becomes a clear and transparent liquid. Mix the crushed hemp straw with the deep eutectic solvent in a mass ratio of 1:25, and extract at 120 °C for 6 h. After extraction, cool naturally, add 95% ethanol, and filter by suction. The filtrate is rotary evaporated to remove ethanol and the remaining deep eutectic solvent. The filtrate is mixed with water in a ratio of 1:9, allowed to stand and precipitate for 12 h, the precipitate is collected by centrifugation, and the lignin sample is obtained by freeze-drying.

[0059] Step 2: Preparation of lignin nanoparticles: Dissolve lignin in tetrahydrofuran, and the concentration of lignin in tetrahydrofuran is 1.5 mg / mL. Ultrasonically treat at 120 W for 30 min at room temperature. Add deionized water to the tetrahydrofuran mixed solution at a rate of 4 mL / min, and the volume of deionized water is three times that of tetrahydrofuran. Stir magnetically at 600 r / min for 4 h. After stirring, transfer the solution to a 1000 Da dialysis bag and dialyze in excess deionized water for 72 h to remove the residual tetrahydrofuran. After dialysis, freeze-dry to obtain lignin nanoparticles.

[0060] Step 3. Preparation of chitosan phenolic acid graft copolymer: Chitosan-protocatechuic acid graft copolymer was prepared by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) / N-hydroxysuccinimide (NHS) coupling reaction. The specific steps were as follows: (1) Dissolve chitosan (Mw = 150000, 1.22 g, 7.5 mmol) in 25 mL of acetic acid solution (0.5%, v / v%) and stir overnight at room temperature; (2) Dissolve protocatechuic acid (7.5 mmol) in 5 mL of ethanol; (3) Dissolve the mediator 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) (1.44 g, 7.5 mmol) in 20 mL of 2-morpholinoethanesulfonic acid (MES) buffer (pH = 5.5), add the protocatechuic acid solution to obtain intermediate compound 1; (4) Add N-hydroxysuccinimide (NHS) (0.86 g, 7.5 mmol) to intermediate compound 1 and continue to react in an ice bath for 1 h to obtain intermediate compound 2; (5) Gradually add intermediate compound 2 to the pre-prepared chitosan solution and place it in the dark at room temperature for 12 h; (6) Dialyze with distilled water for 72 h, the cut-off molecular weight of the dialysis bag is 14 kDa, centrifuge (10000 xg, 4 °C) for 30 min to remove residual protocatechuic acid, and freeze-dry the supernatant to obtain chitosan phenolic acid graft copolymer.

[0061] Step 4. Preparation of composite film: Dissolve 0.27 g of graft copolymer in 13.3 mL of 1% (v / v) acetic acid solution, add 0.31 g of gelatin, heat and stir to dissolve at 60 °C, then add 0.08 g of glycerol and 15.5 mg of lignin nanoparticles to prepare a mixed film, continue to stir for 1 h. The solvent of the blank film is 13.3 mL of 1% (v / v) acetic acid solution, and continue to stir for 1 h. Degas by ultrasound for 30 min, cast the film solution on a polycarbonate plate (10 cm × 10 cm), and dry at room temperature for 48 h. After peeling off the film, place it in a constant temperature and humidity chamber at 25 °C and 50% RH for 24 h.

[0062] Example 6

[0063] In this example, the structure of the packaging film was determined by Fourier transform infrared spectroscopy, the antibacterial property of the packaging film was determined by the antibacterial zone method, the ultraviolet blocking ability of the packaging film was determined by ultraviolet absorption spectroscopy, the mechanical properties of the packaging film were characterized by tensile strength and elongation at break, and the moisture retention function of the packaging film was characterized by water vapor transmission rate.

[0064] 1. Structure analysis of the packaging film:

[0065] Fourier transform infrared spectra were recorded by a NicoletiS50 FT-IR spectrometer, and the scanning range was from 4000 to 400 cm -1 , and scanned 32 times. The resolution of the spectrometer was 4 cm-1 The structure of the film was tested using ATR. First, the ATR accessory was placed in the optical path of the infrared spectrometer, and the air background was scanned. Then, the surface of the sample to be measured was closely attached to the infrared-transmitting crystal surface of the ATR accessory, and the infrared spectrum of the surface of the sample to be measured was scanned.

[0066] The results are as Figure 1 shown. The characteristic peaks of the packaging film mainly appear at 3320, 2930, 1630, 1538 and 1240, 1033, 920 cm -1 -1. They correspond to N-H / OH stretching vibration, C-H group stretching vibration, C=O stretching vibration, N-H bending vibration, C-N / N-H stretching vibration, C-O stretching vibration of C-O in the C-O-C group, and characteristic peak of amide (CO-NH), respectively. After adding lignin nanoparticles, peaks appear at 3320 cm -1 -1 due to the O-H stretching of aliphatic and phenolic groups of lignin nanoparticles, and peaks appear at 2930 cm -1 -1 due to the stretching vibration of C-H bond in lignin. Moreover, new peaks appear around 1600 cm -1 -1, which is due to the C-C stretching of the aromatic skeleton of lignin nanoparticles. The diffraction peaks of the gelatin film added with lignin nanoparticles in the wavelength range of 400 cm -1 -1 to 2000 cm -1 -1 are all enhanced, indicating that there are interactions between the hydroxyl and carbonyl groups in lignin nanoparticles and the hydroxyl groups, C=O, N-H in chitosan and the carboxyl groups in acids.

[0067] 2. UV absorption spectrum of the packaging film:

[0068] The UV absorption spectrum was measured by a TU191 UV-visible spectrophotometer with air as the substrate, and the scanning wavelength was 200-600 nm.

[0069] Figure 2 Shown is the light transmittance of the gelatin film in the range of 200-800 nm. Compared with the blank film, adding grafted substances to the film matrix weakened the transmittance of the gelatin film. In other words, the gelatin film added with protocatechuic acid-chitosan has better UV resistance than the blank gelatin film, which is attributed to the aromatic groups in phenolic acids that can absorb UV / visible radiation. With the addition of lignin nanoparticles, the transmittance in the UV region decreased significantly, indicating that the addition of nanoparticles enhanced the UV barrier performance of the film. The presence of lignin nanoparticles in the film matrix affected the occurrence of light scattering and reflection or interfered with the light trajectory. This change is mainly due to the interaction between chitosan / acid and lignin in the internal structure, thus changing the light transmittance. In addition, the uniform distribution of lignin nanoparticles is beneficial to form a uniform protection for the food contact surface.

[0070] 3. Water vapor transmission rate of the packaging film:

[0071] The water vapor transmission rate of the composite film was measured by the quasi-cup method. The composite film was cut into a square of 3 cm × 3 cm. Then, the discolored silica gel was placed in a test tube with a diameter of 15.7 mm, and there was a 1 cm spacing between the bottle mouth and the silica gel. A represents the circular area (cm 2 ) of the test tube mouth. Then, the composite film was covered on the weighing bottle mouth, and the film at the cup mouth was tightly tied with a rubber band and weighed. Then, it was placed in a glass desiccator with deionized water at the bottom and put into a constant temperature oven at 27°C. The weighing bottle was weighed every 2 h. t represents the time interval (h), and the mass difference M (g) was recorded. The total duration was 24 h. The film thickness was denoted as D (mm), and ΔP represents the vapor pressure difference inside and outside the film (the vapor pressure difference at 27°C is 3567 Pa). The result is expressed as g mm / h cm 2 Pa.

[0072]

[0073] The results are as Figure 3 shown. The experiment shows that when the grafted substance is added, the water vapor transmission rate of the gelatin film decreases because when the grafted substance is added, the molecular forces between phenolic acids, chitosan, and glycerol reduce the free volume of the film, weakening the interaction between water molecules in the film, thereby improving the barrier performance. The chitosan protocatechuic acid grafted substance fills the voids and gaps in the gelatin film, thus inhibiting the diffusion of water in the film and improving the film barrier performance. The gelatin film with 0.25 wt% lignin nanoparticles has the lowest water vapor transmission rate. Compared with the blank gelatin film, after adding lignin nanoparticles, the water vapor transmission rate decreases significantly, especially when adding 0.25 wt%, and the water vapor transmission rate decreases by 36%. This may be due to the insertion of an appropriate content of LNPs into the voids between adjacent colloids, resulting in an enhanced tortuosity of water vapor passage. In addition, as the content of lignin nanoparticles increases, the water vapor transmission rate of the composite film increases because further increasing the LNPs content may cause agglomeration, affecting the uniform distribution of nanoparticles in the matrix, thereby reducing the film density and water interception ability.

[0074] 4. Mechanical properties of the packaging film:

[0075] The film sample was cut into a regular uniform size (100 mm × 15 mm). The film thickness was denoted as d (mm), and the film width was denoted as h (mm). The tensile strength and elongation at break of the film were tested using a texture analyzer in tension mode. The set conditions were as follows: the tensile rate was 0.5 mm / s, and the initial clamp distance was 50 mm. The maximum tensile force F (N) of the film, the displacement L (mm) at film break, and the initial length L0 (mm) of the film were recorded.

[0076]

[0077] The results are as Figure 4 and Figure 5 shown. The tensile strength and elongation at break of the blank film are 6.33 ± 0.45 MPa and 5.99 ± 0.7%, respectively. The mechanical properties of the film added with the grafted substance show no difference compared with those of the blank film, but there is also a certain enhancement. Moreover, the incorporation of the grafted substance has different degrees of influence on the tensile properties and elongation at break of the gelatin film. This is due to the intermolecular hydrogen bond action of the chitosan-protocatechuic acid graft copolymer, resulting in a decrease in the molecular freedom degree and the covalent connection effect between the phenolic acid and the internal network of chitosan. With the increase in the content of LNPs, both the tensile strength and elongation at break of the hybrid film show an upward trend. The lignin nanoparticles nearly double the tensile strength of the film, and both mechanical properties are enhanced with the increase in the concentration of lignin nanoparticles. The rigid hybrid lignin nanoparticles as reinforcing fillers can improve the mechanical properties of polymer films, which is also the potential advantage of nanocomposites applied in materials. The lignin nanoparticles can be filled into the gaps of the gelatin film, restricting the movement of chitosan molecular chains through hydrogen bonds and hydrophobic interactions, making the film more compact, and thus can improve the mechanical properties of the film.

[0078] 5. Antibacterial property of the packaging film:

[0079] The antibacterial activities of the gelatin film against Escherichia coli and Staphylococcus aureus were determined by the inhibition zone method. The bacterial suspensions (concentration of 10 6 CFU / mL, 0.2 mL) of the two activated foodborne pathogenic bacteria were respectively inoculated onto the solidified agar medium and spread evenly by the coating method. Then, circular filter papers (diameter 0.6 cm) containing the film solution, as well as filter papers soaked in water (blank control) and the same content of double antibiotics (positive control), were gently placed on the culture dish. Finally, the culture dish was inverted in a constant temperature incubator (set temperature at 37 °C) and cultured for one day. Finally, the diameter of the inhibition zone observable on the culture dish was measured with a vernier caliper. The antibacterial activity of the gelatin film was evaluated by the filter paper method.

[0080] The results are as Figure 6 and Figure 7As shown, the blank gelatin film has no antibacterial effect. Although a small amount of acetic acid is added to it, it does not play an antibacterial role. The rest of the gelatin films all show obvious inhibition zones, indicating that the prepared films all have antibacterial effects, but there are differences in antibacterial effects. The antibacterial activity of the gelatin film against Staphylococcus aureus is better than that against Escherichia coli. This is because Gram-positive bacteria lack an outer membrane compared to Gram-negative bacteria and are more easily damaged. The gelatin film added with the graft shows inhibitory effects on both bacteria. The antibacterial activity of chitosan is due to the fact that, on the one hand, the positively charged amino groups on chitosan can interact with the negatively charged microbial cell components, resulting in an increase in the permeability of bacterial cells; on the other hand, chitosan can penetrate into the cell interior and interact with the components inside the cell, thereby inhibiting the growth and reproduction of microorganisms. As a natural antioxidant, protocatechuic acid has strong antibacterial activity. Therefore, grafting protocatechuic acid onto chitosan can effectively enhance the antibacterial activity of chitosan. In addition, the aromatic structure of lignin nanoparticles is more sensitive to Gram-positive bacteria than to Gram-negative bacteria. When lignin nanoparticles are added to the film matrix, the diameter of the inhibition zone of the mixed film against both strains is larger than that of the film only added with the graft.

Claims

1. A method for preparing a packaging film having both antibacterial and ultraviolet blocking properties, characterized in that The method comprises the following steps: Step (1) Extraction of lignin: After the hemp straw is crushed, washed and dried, lignin is extracted using a low eutectic solvent method; Step (2) Preparation of lignin nanoparticles: using tetrahydrofuran as solvent, preparing lignin nanoparticles by a self-assembly method; Step (3) dissolving chitosan phenolic acid grafts in acetic acid, adding gelatin as a membrane matrix, glycerol as a toughening agent, and adding lignin nanoparticles, and preparing a phenolic acid-chitosan-lignin-gelatin-glycerol composite membrane by a casting method, wherein: the mass ratio of the graft, acetic acid, gelatin and glycerol is 1:0.49:1.15:0.3; and the amount of the lignin nanoparticles added is 0.25-3% of the mass of the gelatin.

2. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 1, characterized in that In the step (1), the mass ratio of hemp straw powder to low eutectic solvent is 1:25, the low eutectic solvent comprises choline chloride and lactic acid, and the molar ratio of choline chloride to lactic acid is 1:4-12.

3. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 2, characterized in that The molar ratio of choline chloride to lactic acid is 1:

8.

4. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 1, characterized in that In the step (1), the extraction temperature is 100 to 140° C. and the extraction time is 4 to 8 hours.

5. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 4, characterized in that The extraction temperature is 120° C. and the extraction time is 6 hours.

6. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 1, characterized in that The specific steps of step (2) are as follows: lignin is completely dissolved in tetrahydrofuran by ultrasound, deionized water is added and stirred continuously to decompose the lignin into nanometer scale, and lignin nanoparticles are obtained by dialysis and freeze-drying.

7. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 6, characterized in that The concentration of the lignin in tetrahydrofuran is 1.5 mg / mL; the power of the ultrasound is 120 W, and the time is 30 min; the volume ratio of the deionized water to tetrahydrofuran is 3:

1.

8. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 1, characterized in that The chitosan phenolic acid graft is a chitosan-protocatechuic acid graft, which is prepared by 1-(3-dimethylaminopropyl) 3-ethylcarbodiimide hydrochloride / N-hydroxysuccinimide coupling reaction.

9. The method for preparing a packaging film having both antibacterial and ultraviolet blocking properties according to claim 8, characterized in that The specific steps of the chitosan-protocatechuic acid graft are as follows: (1) adding EDC solution to protocatechuic acid ethanol solution to react to obtain intermediate compound 1; (2) adding NHS to intermediate compound 1 and continuing the reaction in an ice bath to obtain intermediate compound 2; (3) adding the intermediate compound 2 to the chitosan solution, and after the reaction is completed, transferring the reaction solution to a dialysis bag for dialysis; (4) centrifuging the dialyzed liquid to remove residual protocatechuic acid, and freeze-drying the supernatant to obtain a chitosan-protocatechuic acid graft product, wherein the molar ratio of chitosan, protocatechuic acid, EDC, and NHS is 1:1:1:

1.

10. Use of the packaging film prepared by the method according to any one of claims 1 to 9 in food packaging film.

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