A degradable packaging film containing mulberry leaf extract and a preparation method thereof

Abstract

This invention relates to the field of biodegradable packaging film technology, and in particular to a biodegradable packaging film containing mulberry leaf extract and its preparation method. The film is prepared from the following raw materials in parts by weight: 40-60 parts chitosan, 20-30 parts sodium alginate, 10-20 parts gelatin, 1-3 parts nano zinc oxide, 3-8 parts mulberry leaf extract, 0.5-1.5 parts crosslinking agent, 3-6 parts plasticizer, 0.1-0.5 parts nano titanium dioxide, and 0.5-1 parts cellulose nanocrystals. This invention uses chitosan, sodium alginate, and gelatin as biodegradable matrices, and combines them with mulberry leaf extract, nano zinc oxide, nano titanium dioxide, and cellulose nanocrystals. These are crosslinked with a food-grade crosslinking agent to form a dense three-dimensional network. The resulting packaging film possesses biodegradability, high strength and high barrier properties, water resistance, broad-spectrum antibacterial properties, strong antioxidant properties, and highly efficient photocatalytic ethylene removal capabilities. The entire preparation process is green, leaves no organic solvent residue, and is simple to operate.

Description

Technical Field

[0001] This invention relates to the field of biodegradable packaging film technology, and in particular to a biodegradable packaging film containing mulberry leaf extract and its preparation method. Background Technology

[0002] With the widespread use of plastic packaging, the white pollution problem caused by petroleum-based non-degradable plastics is becoming increasingly serious. Traditional packaging films are difficult to degrade naturally and easily produce microplastic residues, seriously threatening the ecological environment and food safety. Biodegradable packaging films have become an important direction for replacing traditional plastics. Among them, packaging films based on natural polymers such as chitosan, sodium alginate, and gelatin have received widespread attention due to their advantages such as wide availability, good biocompatibility, and complete degradability.

[0003] However, existing single biodegradable films generally have defects such as poor mechanical properties, insufficient barrier properties, weak water resistance, and single function. They can only meet basic packaging needs and lack key functions required for food preservation, such as antibacterial, antioxidant, ethylene removal, and enzyme inhibition preservation. They are difficult to adapt to long-term preservation packaging for perishable foods such as fruits, vegetables, and cooked food.

[0004] Meanwhile, natural functional additives have low utilization efficiency and complex extraction processes, with some processes using organic solvents, posing a risk of residue and failing to meet the green and safe requirements for food packaging. Furthermore, most biodegradable films have not achieved functional synergy and structural reinforcement, making them prone to damage, moisture absorption and swelling, and poor preservation in practical applications, thus limiting their large-scale application in the high-end food packaging sector. Summary of the Invention

[0005] The purpose of this invention is to provide a biodegradable packaging film containing mulberry leaf extract and its preparation method. Using chitosan, sodium alginate, and gelatin as biodegradable matrices, mulberry leaf extract, nano-zinc oxide, nano-titanium dioxide, and cellulose nanocrystals are compounded and cross-linked with a food-grade cross-linking agent to form a dense three-dimensional network. The resulting packaging film possesses biodegradability, high strength and high barrier properties, water resistance, broad-spectrum antibacterial activity, strong antioxidant properties, and highly efficient photocatalytic ethylene removal capabilities. The entire preparation process is green, leaves no organic solvent residue, and is simple to operate.

[0006] To achieve the above objectives, the present invention provides a biodegradable packaging film containing mulberry leaf extract, which is prepared from the following raw materials in parts by weight: 40-60 parts chitosan, 20-30 parts sodium alginate, 10-20 parts gelatin, 1-3 parts nano zinc oxide, 3-8 parts mulberry leaf extract, 0.5-1.5 parts crosslinking agent, 3-6 parts plasticizer, 0.1-0.5 parts nano titanium dioxide, and 0.5-1 parts cellulose nanocrystals.

[0007] Preferably, the crosslinking agent includes one of citric acid, malic acid, and tartaric acid.

[0008] Preferably, the plasticizer includes one of glycerin, sorbitol, and polyethylene glycol.

[0009] This invention also provides a method for preparing a biodegradable packaging film containing mulberry leaf extract, comprising the following steps: S1. Preparation of mulberry leaf extract: S1.1. After pretreatment, mulberry leaves are subjected to ultrasonic-assisted water extraction, and solid-liquid separation is performed to obtain crude mulberry leaf extract. S1.2 The crude extract of mulberry leaves is subjected to first-stage ultrafiltration, second-stage ultrafiltration and nanofiltration in sequence. The retentate of the first-stage ultrafiltration is collected as mulberry leaf protein concentrate, and the retentate of the nanofiltration is collected as 1-deoxynojirimycin-flavonoid concentrate. S1.3. Mix the mulberry leaf protein concentrate and the 1-deoxynojirimycin-flavonoid concentrate and freeze-dry to obtain mulberry leaf extract; S2. Prepare an acidic sol containing chitosan and a mixed sol containing sodium alginate and gelatin respectively. After mixing the acidic sol and the mixed sol, add a crosslinking agent to carry out a crosslinking reaction to obtain a composite matrix sol. S3. Add the mulberry leaf extract, plasticizer, nano zinc oxide, nano titanium dioxide and cellulose nanocrystals prepared in S1 to the composite matrix sol of S2, mix evenly and degas to obtain the coating liquid. S4. Cast the coating liquid from S3 into a film, dry it, demold it, and then heat-treat it to obtain a biodegradable packaging film containing mulberry leaf extract.

[0010] Preferably, in S1.1, the ultrasonic-assisted water extraction is performed by adding deionized water to mulberry leaves at a liquid-to-material ratio of 25-35 mL / g, and extracting for 30-40 min under the conditions of ultrasonic power of 350-450 W, water bath temperature of 40-45℃, and stirring speed of 250-350 r / min.

[0011] Preferably, in step S1.2, the primary ultrafiltration uses a polyacrylonitrile ultrafiltration membrane with a molecular weight cutoff of 5~50kDa and an operating pressure of 0.5~0.7MPa; the secondary ultrafiltration uses a polyphenylsulfone ultrafiltration membrane with a molecular weight cutoff of 0.5~2kDa and an operating pressure of 0.5~0.7MPa; and the nanofiltration uses a nanofiltration membrane with a molecular weight cutoff of 100~500Da and an operating pressure of 0.5~0.7MPa.

[0012] Preferably, in S1.3, the volume ratio of mulberry leaf protein concentrate and 1-deoxynojirimycin-flavonoid concentrate is 1:1, and the freeze-drying is carried out at -50°C to -40°C for 36 to 60 hours.

[0013] Preferably, in S2, the amount of crosslinking agent added accounts for 5-10 wt% of the total mass of the acidic sol and the mixed sol, and the crosslinking reaction is carried out by stirring at room temperature for 20-50 min.

[0014] Preferably, in S3, degassing is performed by ultrasonic dispersion for 15-35 minutes at a temperature of 20-30℃ and a power of 350-450W.

[0015] Preferably, in step S4, the product is dried at 25°C and 50% relative humidity for 36-60 hours and then heat-treated at 40-50°C for 1.5-3 hours.

[0016] Therefore, the present invention employs the above-mentioned biodegradable packaging film containing mulberry leaf extract and its preparation method, which has the following beneficial effects: (1) In this invention, chitosan, sodium alginate, and gelatin form a stable and biodegradable matrix, nano zinc oxide enhances strength and antibacterial properties, mulberry leaf extract provides natural antibacterial, antioxidant, enzyme-inhibiting and anti-aging effects, nano titanium dioxide achieves photosensitive antibacterial and efficient photocatalytic ethylene removal, and cellulose nanocrystals enhance mechanical and barrier properties. The multiple functions are integrated, so that the biodegradable packaging film containing mulberry leaf extract has multiple functions such as complete biodegradability, efficient antibacterial, antioxidant, enzyme-inhibiting preservation, ethylene removal, high barrier, and excellent mechanical properties. (2) This invention uses natural biomass materials as the main base material, combined with food-grade components such as mulberry leaf extract, which is completely biodegradable and can be rapidly degraded in the natural environment after use. It does not produce microplastics and can replace traditional petroleum-based plastics, with significant environmental benefits and social value. (3) This invention uses a combination of ultrasonic-assisted water extraction and multi-stage membrane separation technology to simultaneously extract mulberry leaf protein, 1-deoxynojirimycin and flavonoids, achieving efficient utilization of all components of mulberry leaves. The process is green, with no organic solvents or acid and alkali residues, greatly increasing the added value of agricultural and forestry waste, and is suitable for industrial promotion. (4) The present invention uses food-grade crosslinking agents such as citric acid to form a dense three-dimensional network through mild crosslinking via esterification and amidation, which significantly improves the tensile strength, water resistance, dimensional stability and barrier properties of the membrane, while maintaining good flexibility. (5) The present invention adopts solution blending and casting film formation throughout the process. The process is stable, easy to operate, universal equipment, and low energy consumption. It does not require high temperature, high pressure and complex equipment, and is suitable for large-scale production. All raw materials are food contact grade materials, with no odor, no migration and no toxic side effects.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 These are schematic diagrams illustrating the mechanical properties of the packaging films in Embodiments 1-3 and Comparative Examples 1-2 of the present invention; Figure 2 These are schematic diagrams illustrating the barrier properties of the packaging films in Examples 1-3 and Comparative Examples 1-2 of the present invention. Figure 2In the diagram, 'a' represents the oxygen permeability of the packaging films in Examples 1-3 and Comparative Examples 1-2. Figure 2 In the diagram, b represents the water vapor transmission rate of the packaging film in Examples 1-3 and Comparative Examples 1-2. Detailed Implementation

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise defined, the technical or scientific terms used in this invention should be understood in their ordinary sense by those skilled in the art. The features mentioned above or in the specific examples mentioned in this invention can be combined arbitrarily, and these specific embodiments are only used to illustrate the invention and are not intended to limit the scope of the invention.

[0020] This invention provides a biodegradable packaging film containing mulberry leaf extract, which is prepared from the following raw materials by weight: 40-60 parts chitosan, 20-30 parts sodium alginate, 10-20 parts gelatin, 1-3 parts nano zinc oxide, 3-8 parts mulberry leaf extract, 0.5-1.5 parts crosslinking agent, 3-6 parts plasticizer, 0.1-0.5 parts nano titanium dioxide, and 0.5-1 parts cellulose nanocrystals.

[0021] The biodegradable packaging film containing mulberry leaf extract of this invention possesses excellent mechanical properties, good barrier properties, broad-spectrum antibacterial activity, high-efficiency antioxidant properties, and complete biodegradability. Chitosan, as a natural film-forming substrate, provides good film-forming properties, cationic antibacterial activity, and biodegradability, and forms strong hydrogen bonds with sodium alginate, gelatin, and mulberry leaf extract. Sodium alginate enhances the film's density, oxygen barrier properties, and water vapor barrier capabilities, and improves the film's surface uniformity and formability. Gelatin improves the film's flexibility, elongation at break, and processing adaptability, and synergistically constructs a stable three-dimensional network structure with chitosan and sodium alginate. Nano-zinc oxide further enhances the film's mechanical strength and hardness, strengthens the broad-spectrum antibacterial effect, and improves UV barrier performance. Mulberry leaf extract, rich in 1-deoxynojirimycin (DNJ), flavonoids, and mulberry leaf protein, provides natural antioxidant, antibacterial, enzyme-inhibiting, and starch-retarding functions. Nano-titanium dioxide has excellent photocatalytic activity and can be used in visible light / Ultraviolet light degrades ethylene gas inside the packaging, delaying the aging of fruits and vegetables; at the same time, it works synergistically with nano zinc oxide and mulberry leaf extract to achieve highly efficient and broad-spectrum antibacterial effects; cellulose nanocrystals, as a nano-reinforcing phase, significantly improve the tensile strength, heat resistance, and dimensional stability of the film, while further enhancing its barrier properties. The synergistic effect of each component enables the packaging film to simultaneously achieve multiple functions such as complete biodegradability, efficient preservation, antibacterial and anti-mildew properties, antioxidant properties, enzyme inhibition and anti-aging properties, and ethylene removal.

[0022] Preferably, the crosslinking agent includes one of citric acid, malic acid, and tartaric acid. The crosslinking agent selected in this invention is a food-grade natural polyhydroxycarboxylic acid, which can undergo esterification and amidation crosslinking with chitosan and gelatin, significantly improving the mechanical strength, water resistance, and structural stability of the packaging film, and is safe, non-toxic, and environmentally friendly.

[0023] Preferably, the plasticizer includes one of glycerol, sorbitol, and polyethylene glycol. The plasticizer selected in this invention can insert into the gaps between polymer chains to weaken intermolecular forces, significantly improving the elongation at break and processability of the packaging film, while not reducing the barrier and preservation properties of the packaging film. It also exhibits good compatibility, no migration, and no odor.

[0024] This invention also provides a method for preparing a biodegradable packaging film containing mulberry leaf extract, comprising the following steps: S1. Preparation of mulberry leaf extract: S1.1. After pretreatment, mulberry leaves are subjected to ultrasonic-assisted water extraction, and solid-liquid separation is performed to obtain crude mulberry leaf extract. S1.2 The crude extract of mulberry leaves is subjected to first-stage ultrafiltration, second-stage ultrafiltration and nanofiltration in sequence. The retentate of the first-stage ultrafiltration is collected as mulberry leaf protein concentrate, and the retentate of the nanofiltration is collected as 1-deoxynojirimycin-flavonoid concentrate. S1.3. Mix the mulberry leaf protein concentrate and the 1-deoxynojirimycin-flavonoid concentrate and freeze-dry to obtain mulberry leaf extract; S2. Prepare an acidic sol containing chitosan and a mixed sol containing sodium alginate and gelatin respectively. After mixing the acidic sol and the mixed sol, add a crosslinking agent to carry out a crosslinking reaction to obtain a composite matrix sol. S3. Add the mulberry leaf extract, plasticizer, nano zinc oxide, nano titanium dioxide and cellulose nanocrystals prepared in S1 to the composite matrix sol of S2, mix evenly and degas to obtain the coating liquid. S4. Cast the coating liquid from S3 into a film, dry it, demold it, and then heat-treat it to obtain a biodegradable packaging film containing mulberry leaf extract.

[0025] Preferably, in S1.1, the ultrasound-assisted water extraction is performed by adding deionized water to mulberry leaves at a liquid-to-material ratio of 25-35 mL / g, and extracting for 30-40 minutes under the conditions of ultrasonic power of 350-450 W, water bath temperature of 40-45℃, and stirring speed of 250-350 r / min. In this invention, ultrasound-assisted water extraction utilizes cavitation effect to efficiently break down the cell walls of mulberry leaves, allowing for the full dissolution of water-soluble active ingredients such as mulberry leaf protein, 1-deoxynojirimycin, and flavonoids. This method offers high extraction efficiency, mild conditions, and does not damage the active structure.

[0026] In a further preferred embodiment, in S1.1, the mulberry leaf pretreatment is as follows: fresh mulberry leaves are washed, dried with hot air at 50℃ until the moisture content is <8%, and then crushed and passed through a 60-mesh sieve.

[0027] In a further preferred embodiment, in S1.1, the solid-liquid separation is performed by centrifuging at a speed of 4000~6000 r / min for 8~12 min and taking the supernatant.

[0028] Preferably, in step S1.2, the first-stage ultrafiltration uses a polyacrylonitrile ultrafiltration membrane with a molecular weight cutoff of 5-50 kDa and an operating pressure of 0.5-0.7 MPa; the second-stage ultrafiltration uses a polyphenylsulfone ultrafiltration membrane with a molecular weight cutoff of 0.5-2 kDa and an operating pressure of 0.5-0.7 MPa to remove impurities such as polysaccharides and peptides; and the nanofiltration uses a nanofiltration membrane with a molecular weight cutoff of 100-500 Da and an operating pressure of 0.5-0.7 MPa. This invention uses a three-stage membrane separation process for step-by-step purification, which can efficiently retain mulberry leaf proteins and precisely enrich 1-deoxynojirimycin and flavonoids, while removing polysaccharides, pigments, and macromolecular impurities, thus achieving high-value utilization of all active components of mulberry leaves.

[0029] Preferably, in step S1.3, the volume ratio of mulberry leaf protein concentrate to 1-deoxynojirimycin-flavonoid concentrate is 1:1, and the freeze-drying is performed at -50°C to -40°C for 36-60 hours. The mulberry leaf extract of this invention integrates mulberry leaf protein, 1-deoxynojirimycin, flavonoids, and polysaccharides, possessing multiple functions including antibacterial, antioxidant, enzyme-inhibiting, and preservation properties. It exhibits high activity, high purity, and good stability.

[0030] Preferably, in step S2, the amount of crosslinking agent added accounts for 5-10 wt% of the total mass of the acidic sol and the mixed sol, and the crosslinking reaction is carried out by stirring at room temperature for 20-50 min. The crosslinking reaction of the present invention is mild and controllable, forming a stable three-dimensional network structure, significantly improving the tensile strength, water resistance and thermal stability of the packaging film, and avoiding component loss.

[0031] In a further preferred embodiment, in S2, chitosan is added to an aqueous solution of acetic acid with a volume fraction of 1% at a solid-liquid ratio of 1:66~100 (g / mL), stirred at room temperature until completely dissolved, and allowed to stand to remove bubbles, to obtain an acidic sol containing chitosan with a mass fraction of 1.0~1.5wt%; sodium alginate and gelatin are mixed at a mass ratio of (3~2):(2~1), and then 40~60 times the total mass of sodium alginate and gelatin is added to deionized water, stirred in a water bath at 55~65℃ until completely dissolved, and cooled to room temperature to obtain a mixed sol containing sodium alginate and gelatin; Preferably, in S3, degassing is performed by ultrasonic dispersion for 15-35 minutes at a temperature of 20-30℃ and a power of 350-450W.

[0032] In a further preferred embodiment, in S3, the mixing is achieved by high-speed stirring at a stirring speed of 8000~12000r / min for 10~20min.

[0033] Preferably, in step S4, the film is dried at 25°C and 50% relative humidity for 36-60 hours, and then heat-treated at 40-50°C for 1.5-3 hours. This invention further promotes the cross-linking reaction through heat treatment, improving the film's density, hydrophobicity, and mechanical stability, reducing film shrinkage, and ensuring a smooth and flat surface.

[0034] Mechanism of the invention: This invention uses citric acid and other polyhydroxycarboxylic acids as crosslinking agents. Under mild conditions, these agents undergo esterification and amidation crosslinking with chitosan and gelatin to form a stable covalent three-dimensional network structure. On one hand, the carboxyl groups (-COOH) in the crosslinking agent undergo amidation with the amino groups (-NH2) on the chitosan molecular chain to form amide bonds (-CONH-). On the other hand, the carboxyl groups undergo esterification with the hydroxyl groups (-OH) and amino groups in the gelatin molecules, and combine with hydrogen bonds, allowing the originally linear polymer chains to be interconnected through chemical bonds, forming a dense, stable, and water-insoluble spatial network structure. This crosslinking effect can significantly reduce the water absorption and swelling of the packaging film, greatly improve tensile strength, thermal stability, and barrier properties, while stably anchoring mulberry leaf extract, nano-zinc oxide, nano-titanium dioxide, and cellulose nanocrystals in the network, achieving uniform dispersion, slow release, and long-lasting freshness preservation. Ultimately, a biodegradable packaging film with excellent mechanical properties, water resistance, high barrier properties, and multifunctional synergy is obtained.

[0035] Example 1 This invention provides a biodegradable packaging film containing mulberry leaf extract. By weight, the raw materials are: 50 parts chitosan, 25 parts sodium alginate, 15 parts gelatin, 2 parts nano zinc oxide, 5 parts mulberry leaf extract, 1 part citric acid, 4 parts glycerin, 0.3 parts nano titanium dioxide, and 0.8 parts cellulose nanocrystals.

[0036] The preparation method of the above-mentioned biodegradable packaging film containing mulberry leaf extract is as follows: S1. Preparation of mulberry leaf extract: S1.1 Wash fresh mulberry leaves, dry them with hot air at 50℃ until the moisture content is 7%, and crush them through a 60-mesh sieve; add deionized water to the crushed mulberry leaves at a liquid-to-material ratio of 30mL / g, extract for 35min under the conditions of ultrasonic power of 400W, water bath temperature of 42℃, and stirring speed of 300r / min, centrifuge at 5000r / min for 10min, and take the supernatant to obtain the crude mulberry leaf extract.

[0037] S1.2. The crude mulberry leaf extract was subjected to primary ultrafiltration, secondary ultrafiltration, and nanofiltration sequentially. Primary ultrafiltration used a 30 kDa molecular weight cutoff polyacrylonitrile ultrafiltration membrane at an operating pressure of 0.6 MPa. The crude mulberry leaf extract was diluted twice before ultrafiltration, with a concentration ratio of 4:1. The retentate from primary ultrafiltration was collected as a concentrated mulberry leaf protein solution. Secondary ultrafiltration used a 1 kDa molecular weight cutoff polyphenylsulfone ultrafiltration membrane at an operating pressure of 0.6 MPa. The permeate from primary ultrafiltration was passed through the 1 kDa polyphenylsulfone ultrafiltration membrane to remove impurities such as polysaccharides and peptides, yielding the secondary ultrafiltration permeate. Nanofiltration used a 300 Da molecular weight cutoff nanofiltration membrane at an operating pressure of 0.6 MPa. The permeate from secondary ultrafiltration was passed through the 300 Da nanofiltration membrane, and the retentate from nanofiltration was collected as a 1-deoxynojirimycin-flavonoid concentrate.

[0038] S1.3 Mulberry leaf protein concentrate and 1-deoxynojirimycin-flavonoid concentrate were mixed at a volume ratio of 1:1 and then freeze-dried at -45℃ for 48 hours to obtain mulberry leaf extract.

[0039] S2. Add 1g of chitosan to 100mL of 1% (v / v) acetic acid aqueous solution, stir magnetically at room temperature for 12h until completely dissolved, and allow to stand for 60min to remove bubbles, obtaining an acidic sol containing 1.0wt% chitosan. Mix 3.0g of sodium alginate and 2.0g of gelatin, add 100mL of deionized water, stir in a 60℃ water bath for 60min until completely dissolved, and cool to room temperature to obtain a mixed sol containing sodium alginate and gelatin. Slowly pour the acidic sol containing chitosan into the mixed sol containing sodium alginate and gelatin, and stir for 30min to mix evenly. Add 8wt% (v / v) citric acid of the total mass of the acidic sol containing chitosan and the mixed sol containing sodium alginate and gelatin, and stir at room temperature for 35min to obtain a composite matrix sol.

[0040] S3. Add the mulberry leaf extract, glycerol, nano zinc oxide, nano titanium dioxide, and cellulose nanocrystals prepared in S1 to the composite matrix sol. After stirring at 10000r / min for 15min, ultrasonically disperse at 25℃ and 400W for 25min to remove bubbles, and obtain the coating solution.

[0041] S4. The coating liquid is cast into a film, and the wet film thickness is controlled at 600μm. The film is dried at 25℃ and 50% relative humidity for 48h. After demolding, it is heat-treated at 45℃ for 2h to obtain a biodegradable packaging film containing mulberry leaf extract.

[0042] Example 2 This invention provides a biodegradable packaging film containing mulberry leaf extract. By weight, the raw materials are: 40 parts chitosan, 20 parts sodium alginate, 10 parts gelatin, 1 part nano zinc oxide, 3 parts mulberry leaf extract, 0.5 parts malic acid, 3 parts sorbitol, 0.1 parts nano titanium dioxide, and 0.5 parts cellulose nanocrystals.

[0043] The preparation method of the above-mentioned biodegradable packaging film containing mulberry leaf extract is as follows: S1. Preparation of mulberry leaf extract: S1.1 Wash fresh mulberry leaves, dry them with hot air at 50℃ until the moisture content is 6%, and crush them through a 60-mesh sieve; add deionized water to the crushed mulberry leaves at a liquid-to-material ratio of 25mL / g, extract for 30min under the conditions of ultrasonic power of 350W, water bath temperature of 40℃, and stirring speed of 250r / min, centrifuge at 4000r / min for 8min, and take the supernatant to obtain the crude mulberry leaf extract.

[0044] S1.2. The crude mulberry leaf extract was subjected to primary ultrafiltration, secondary ultrafiltration, and nanofiltration sequentially. Primary ultrafiltration used a 5 kDa polyacrylonitrile ultrafiltration membrane at an operating pressure of 0.5 MPa. The crude mulberry leaf extract was diluted twice before ultrafiltration, with a concentration ratio of 4:1. The retentate from primary ultrafiltration was collected as a concentrated mulberry leaf protein solution. Secondary ultrafiltration used a 0.5 kDa polyphenylsulfone ultrafiltration membrane at an operating pressure of 0.5 MPa. The permeate from primary ultrafiltration was passed through a 1 kDa polyphenylsulfone ultrafiltration membrane to remove impurities such as polysaccharides and peptides, yielding the secondary ultrafiltration permeate. Nanofiltration used a 100 Da nanofiltration membrane at an operating pressure of 0.5 MPa. The permeate from secondary ultrafiltration was passed through a 100 Da nanofiltration membrane. The retentate from nanofiltration was collected as a 1-deoxynojirimycin-flavonoid concentrate.

[0045] S1.3 Mulberry leaf protein concentrate and 1-deoxynojirimycin-flavonoid concentrate were mixed at a volume ratio of 1:1 and then freeze-dried at -50℃ for 36 hours to obtain mulberry leaf extract.

[0046] S2. Add 1g of chitosan to 100mL of 1% (v / v) acetic acid aqueous solution, stir magnetically at room temperature for 12h until completely dissolved, and allow to stand for 60min to remove bubbles, obtaining an acidic sol containing 1.0wt% chitosan. Mix 3.0g of sodium alginate and 2.0g of gelatin, add 100mL of deionized water, stir in a 60℃ water bath for 60min until completely dissolved, and cool to room temperature to obtain a mixed sol containing sodium alginate and gelatin. Slowly pour the acidic sol containing chitosan into the mixed sol containing sodium alginate and gelatin, and stir for 30min to mix evenly. Add malic acid at 5wt% of the total mass of the acidic sol containing chitosan and the mixed sol containing sodium alginate and gelatin, and stir at room temperature for 20min to obtain a composite matrix sol.

[0047] S3. Add the mulberry leaf extract, sorbitol, nano zinc oxide, nano titanium dioxide, and cellulose nanocrystals prepared in S1 to the composite matrix sol. After stirring at 8000 r / min for 10 min, ultrasonically disperse for 15 min at 20℃ and 350 W to remove bubbles, and obtain the coating solution.

[0048] S4. The coating liquid is cast into a film, and the wet film thickness is controlled at 600 μm. The film is dried at 25℃ and 50% relative humidity for 36 h. After demolding, it is heat-treated at 40℃ for 1.5 h to obtain a biodegradable packaging film containing mulberry leaf extract.

[0049] Example 3 This invention provides a biodegradable packaging film containing mulberry leaf extract. By weight, the raw materials are: 60 parts chitosan, 30 parts sodium alginate, 20 parts gelatin, 3 parts nano zinc oxide, 8 parts mulberry leaf extract, 1.5 parts tartaric acid, 6 parts polyethylene glycol, 0.5 parts nano titanium dioxide, and 1 part cellulose nanocrystals.

[0050] The preparation method of the above-mentioned biodegradable packaging film containing mulberry leaf extract is as follows: S1. Preparation of mulberry leaf extract: S1.1 Wash fresh mulberry leaves, dry them with hot air at 50℃ until the moisture content is 6%, and crush them through a 60-mesh sieve; add deionized water to the crushed mulberry leaves at a liquid-to-material ratio of 35mL / g, extract for 40min under the conditions of ultrasonic power of 450W, water bath temperature of 45℃, and stirring speed of 350r / min, centrifuge at 6000r / min for 12min, and take the supernatant to obtain the crude mulberry leaf extract.

[0051] S1.2. The crude mulberry leaf extract was subjected to primary ultrafiltration, secondary ultrafiltration, and nanofiltration sequentially. Primary ultrafiltration used a 50 kDa polyacrylonitrile ultrafiltration membrane at an operating pressure of 0.7 MPa. The crude mulberry leaf extract was diluted twice before ultrafiltration, with a concentration ratio of 4:1. The retentate from primary ultrafiltration was collected as a concentrated mulberry leaf protein solution. Secondary ultrafiltration used a 2 kDa polyphenylsulfone ultrafiltration membrane at an operating pressure of 0.7 MPa. The permeate from primary ultrafiltration was passed through the 2 kDa polyphenylsulfone ultrafiltration membrane to remove impurities such as polysaccharides and peptides, yielding the secondary ultrafiltration permeate. Nanofiltration used a 500 Da nanofiltration membrane at an operating pressure of 0.7 MPa. The permeate from secondary ultrafiltration was passed through the 500 Da nanofiltration membrane. The retentate from nanofiltration was collected as a 1-deoxynojirimycin-flavonoid concentrate.

[0052] S1.3 Mulberry leaf protein concentrate and 1-deoxynojirimycin-flavonoid concentrate were mixed at a volume ratio of 1:1 and then freeze-dried at -40℃ for 60h to obtain mulberry leaf extract.

[0053] S2. Add 1g of chitosan to 100mL of 1% (v / v) acetic acid aqueous solution, stir magnetically at room temperature for 12h until completely dissolved, and allow to stand for 60min to remove bubbles, obtaining an acidic sol containing 1.0wt% chitosan. Mix 3.0g of sodium alginate and 2.0g of gelatin, add 100mL of deionized water, stir in a 60℃ water bath for 60min until completely dissolved, and cool to room temperature to obtain a mixed sol containing sodium alginate and gelatin. Slowly pour the acidic sol containing chitosan into the mixed sol containing sodium alginate and gelatin, and stir for 30min to mix evenly. Add tartaric acid at 10wt% of the total mass of the acidic sol containing chitosan and the mixed sol containing sodium alginate and gelatin, and stir at room temperature for 50min to obtain a composite matrix sol.

[0054] S3. Add the mulberry leaf extract, polyethylene glycol, nano zinc oxide, nano titanium dioxide, and cellulose nanocrystals prepared in S1 to the composite matrix sol. After stirring at 12000 r / min for 20 min, ultrasonically disperse at 30℃ and 450 W for 35 min to remove bubbles, and obtain the coating solution.

[0055] S4. The coating liquid is cast into a film, and the wet film thickness is controlled at 600μm. The film is dried at 25℃ and 50% relative humidity for 60h. After demolding, it is heat-treated at 50℃ for 3.0h to obtain a biodegradable packaging film containing mulberry leaf extract.

[0056] Comparative Example 1 Compared with Example 1, no mulberry leaf extract was added to the raw materials, and the preparation method did not include S1; all other aspects were the same as in Example 1.

[0057] Comparative Example 2 Compared with Example 1, no nano-titanium dioxide and cellulose nanocrystals were added to the raw materials, but all other aspects were the same as in Example 1.

[0058] Performance testing The packaging films prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to performance testing. The test methods and standards are as follows: Mechanical properties: Tensile strength and elongation at break were tested according to standard GB / T 1040.3-2006 "Tension Properties Test". Results are shown in [Table missing]. Figure 1 .

[0059] Barrier performance: Water vapor transmission rate was tested according to GB / T 1037-1988 "Test Method for Water Vapor Permeability of Plastic Films and Sheets"; oxygen transmission rate was tested according to GB / T 1038-2000 "Test Method for Gas Permeability of Plastic Films and Sheets - Differential Pressure Method". Results are shown below. Figure 2 .

[0060] Antibacterial properties: The antibacterial properties of antibacterial coatings (films) against Escherichia coli and Staphylococcus aureus were tested according to GB / T 21866-2008 "Determination of antibacterial properties and antibacterial effect by film application method". The results are shown in Table 1.

[0061] Antioxidant performance: The packaging films prepared in Examples 1-3 and Comparative Examples 1-2 were made into uniform samples. The antioxidant performance was determined by the DPPH free radical scavenging method. The absorbance at 517 nm was measured by ultraviolet spectrophotometer, and the free radical scavenging rate was calculated. The results are shown in Table 2.

[0062] Ethylene removal rate: The packaging films prepared in Examples 1-3 and Comparative Examples 1-2 were placed in a sealed container, a certain amount of ethylene gas was introduced, and the container was placed at 25°C in the dark for 24 hours. The amount of ethylene residue in the container was determined by gas chromatography, and the ethylene removal rate was calculated. The results are shown in Table 3.

[0063] Soil biodegradation rate: According to GB / T19277.1-2011 "Determination of final aerobic biodegradation capacity of materials under controlled composting conditions by means of determination of carbon dioxide released - Part 1: General method", the degradation rate of soil buried for 30 days and 60 days was tested, and the results are shown in Table 4.

[0064] Water resistance: The packaging films prepared in Examples 1-3 and Comparative Examples 1-2 were cut into samples of fixed size, weighed, and then soaked in deionized water at 25°C for 24 hours. After being taken out and dried, they were weighed again, and the water absorption rate and swelling degree were calculated. The results are shown in Table 5.

[0065] like Figure 1 As shown, the tensile strengths of Examples 1-3 were 40.2 MPa, 35.6 MPa, and 42.8 MPa, respectively, and the elongation at break were 49.7%, 46.1%, and 52.4%, respectively, all significantly higher than those of Comparative Example 1 (28.3 MPa, 36.5%) and Comparative Example 2 (22.5 MPa, 31.8%). Example 3 exhibited the best mechanical properties. Compared to Comparative Example 1 without mulberry leaf extract, Example 1 showed a 42.0% increase in tensile strength and a 36.2% increase in elongation at break; compared to Comparative Example 2 without nano-titanium dioxide and cellulose nanocrystals, Example 1 showed a 78.7% increase in tensile strength and a 56.3% increase in elongation at break. This indicates that the synergistic effect of mulberry leaf extract, cellulose nanocrystals, and the matrix material can significantly improve the tensile strength and toughness of the packaging film.

[0066] Depend on Figure 2 It can be seen that the oxygen permeability of Examples 1-3 is 12.1-14.6 cm. 3 / (m 2 (24h·0.1MPa), water vapor transmission rate is 22.9~25.7g / (m²). 2(24h), significantly lower than Comparative Example 1 and Comparative Example 2. Example 3 showed the best oxygen and water barrier performance, indicating that the three-dimensional network structure formed by cross-linking, combined with cellulose nanocrystals and nano zinc oxide, can effectively reduce molecular diffusion channels and significantly improve the barrier performance of the membrane.

[0067] Table 1. Antibacterial properties of Examples 1-3 and Comparative Examples 1-2

[0068] As shown in Table 1, Examples 1-3 exhibited superior inhibition rates against *Escherichia coli* and *Staphylococcus aureus* compared to Comparative Examples 1-2, demonstrating excellent broad-spectrum antibacterial effects. Comparative Example 1, lacking mulberry leaf extract, and Comparative Example 2, lacking nano-titanium dioxide and cellulose nanocrystals, showed significantly lower inhibition rates than the Examples. This demonstrates that mulberry leaf extract, nano-zinc oxide, and nano-titanium dioxide produce a synergistic antibacterial effect, resulting in a substantial improvement in antibacterial performance.

[0069] Table 2 Antioxidant properties of Examples 1-3 and Comparative Examples 1-2

[0070] As shown in Table 2, the DPPH free radical scavenging rates of Examples 1-3 were all superior to those of Comparative Examples 1-2, demonstrating outstanding antioxidant performance. The scavenging rate of Comparative Example 1 was only 41.2%, while that of Comparative Example 2 was 53.6%, which was higher than that of Comparative Example 1. This indicates that the active components such as flavonoids and 1-deoxynojirimycin in mulberry leaf extract are key to enhancing the antioxidant capacity of the membrane.

[0071] Table 3. 24h ethylene removal rates of Examples 1-3 and Comparative Examples 1-2

[0072] As shown in Table 3, the ethylene removal rates of Examples 1-3 over 24 hours were all higher than those of Comparative Example 2. Comparative Example 2, which did not add nano-titanium dioxide, had an ethylene removal rate of only 38.2%, while Comparative Example 1 showed no ethylene removal effect. This indicates that nano-titanium dioxide can efficiently catalyze the degradation of ethylene.

[0073] Table 4 Soil biodegradation rates of Examples 1-3 and Comparative Examples 1-2

[0074] As shown in Table 4, the degradation rates of Examples 1-3 after 30 days and 60 days of soil burial are similar to those of the comparative examples. This indicates that the addition of functional components does not affect the degradability of the matrix material, and the packaging film of this invention can rapidly degrade in the natural environment after use, posing no risk of environmental pollution.

[0075] Table 5. Water resistance of Examples 1-3 and Comparative Examples 1-2

[0076] Table 5 shows that Examples 1-3 used a chitosan, sodium alginate, and gelatin composite matrix, with the addition of food-grade crosslinking agents such as citric acid, malic acid, and tartaric acid. Through esterification and amidation reactions of carboxyl, amino, and hydroxyl groups, a dense covalent three-dimensional network was formed, significantly reducing water molecule penetration and membrane swelling. Simultaneously, cellulose nanocrystals and nano-zinc oxide filled the gaps between polymers, further blocking water molecule diffusion channels, resulting in lower water absorption and swelling. Comparative Example 1, without the addition of mulberry leaf extract, showed decreased internal hydrogen bonding and network density, with water absorption increasing to 35.7% and swelling to 48.6%, indicating significantly worse water resistance. Comparative Example 2, without the addition of nano-titanium dioxide and cellulose nanocrystals, had a loose network structure with increased defects, allowing water molecules to penetrate more easily, resulting in a water absorption rate of 42.5% and a swelling to 55.3%, exhibiting the worst water resistance.

[0077] Therefore, this invention employs the aforementioned biodegradable packaging film containing mulberry leaf extract and its preparation method. Using chitosan, sodium alginate, and gelatin as biodegradable matrices, it combines mulberry leaf extract, nano-zinc oxide, nano-titanium dioxide, and cellulose nanocrystals. A stable three-dimensional network is formed through gentle cross-linking with a food-grade cross-linking agent. The resulting packaging film possesses excellent mechanical properties, high barrier properties, broad-spectrum antibacterial properties, strong antioxidant properties, efficient ethylene scavenging ability, excellent water resistance, and complete biodegradability. The synergistic effect of each component is significant. Furthermore, the preparation process is green and mild, easy to operate, uses universal equipment, and is suitable for large-scale production.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A biodegradable packaging film containing mulberry leaf extract, characterized in that: It is prepared from the following raw materials in parts by weight, including 40-60 parts chitosan, 20-30 parts sodium alginate, 10-20 parts gelatin, 1-3 parts nano zinc oxide, 3-8 parts mulberry leaf extract, 0.5-1.5 parts crosslinking agent, 3-6 parts plasticizer, 0.1-0.5 parts nano titanium dioxide, and 0.5-1 parts cellulose nanocrystals.

2. The biodegradable packaging film containing mulberry leaf extract according to claim 1, characterized in that: Crosslinking agents include one of citric acid, malic acid, and tartaric acid.

3. The biodegradable packaging film containing mulberry leaf extract according to claim 1, characterized in that: Plasticizers include one of glycerin, sorbitol, and polyethylene glycol.

4. A method for preparing a biodegradable packaging film containing mulberry leaf extract as described in any one of claims 1-3, characterized in that: Includes the following steps: S1. Preparation of mulberry leaf extract: S1.

1. After pretreatment, mulberry leaves are subjected to ultrasonic-assisted water extraction, and solid-liquid separation is performed to obtain crude mulberry leaf extract. S1.2 The crude extract of mulberry leaves is subjected to first-stage ultrafiltration, second-stage ultrafiltration and nanofiltration in sequence. The retentate of the first-stage ultrafiltration is collected as mulberry leaf protein concentrate, and the retentate of the nanofiltration is collected as 1-deoxynojirimycin-flavonoid concentrate. S1.

3. Mix the mulberry leaf protein concentrate and the 1-deoxynojirimycin-flavonoid concentrate and freeze-dry to obtain mulberry leaf extract; S2. Prepare an acidic sol containing chitosan and a mixed sol containing sodium alginate and gelatin respectively. After mixing the acidic sol and the mixed sol, add a crosslinking agent to carry out a crosslinking reaction to obtain a composite matrix sol. S3. Add the mulberry leaf extract, plasticizer, nano zinc oxide, nano titanium dioxide and cellulose nanocrystals prepared in S1 to the composite matrix sol of S2, mix evenly and degas to obtain the coating liquid. S4. Cast the coating liquid from S3 into a film, dry it, demold it, and then heat-treat it to obtain a biodegradable packaging film containing mulberry leaf extract.

5. The method for preparing a biodegradable packaging film containing mulberry leaf extract according to claim 4, characterized in that: In S1.1, ultrasonic-assisted water extraction is performed as follows: deionized water is added to mulberry leaves at a liquid-to-material ratio of 25-35 mL / g, and extraction is carried out for 30-40 min under the conditions of ultrasonic power of 350-450 W, water bath temperature of 40-45℃, and stirring speed of 250-350 r / min.

6. The method for preparing a biodegradable packaging film containing mulberry leaf extract according to claim 4, characterized in that: In S1.2, the primary ultrafiltration uses a polyacrylonitrile ultrafiltration membrane with a molecular weight cutoff of 5~50kDa and an operating pressure of 0.5~0.7MPa; the secondary ultrafiltration uses a polyphenylsulfone ultrafiltration membrane with a molecular weight cutoff of 0.5~2kDa and an operating pressure of 0.5~0.7MPa; and the nanofiltration uses a nanofiltration membrane with a molecular weight cutoff of 100~500Da and an operating pressure of 0.5~0.7MPa.

7. The method for preparing a biodegradable packaging film containing mulberry leaf extract according to claim 4, characterized in that: In S1.3, the volume ratio of mulberry leaf protein concentrate and 1-deoxynojirimycin-flavonoid concentrate is 1:1, and the freeze-drying is carried out at -50℃ to -40℃ for 36~60h.

8. The method for preparing a biodegradable packaging film containing mulberry leaf extract according to claim 4, characterized in that: In S2, the amount of crosslinking agent added accounts for 5~10wt% of the total mass of acidic sol and mixed sol, and the crosslinking reaction is carried out by stirring at room temperature for 20~50min.

9. The method for preparing a biodegradable packaging film containing mulberry leaf extract according to claim 4, characterized in that: In S3, degassing is performed by ultrasonic dispersion for 15-35 minutes at a temperature of 20-30℃ and a power of 350-450W.

10. The method for preparing a biodegradable packaging film containing mulberry leaf extract according to claim 4, characterized in that: In S4, the product is dried at 25°C and 50% relative humidity for 36-60 hours, and then heat-treated at 40-50°C for 1.5-3 hours.

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