Microporous litchi preservative film and preparation method thereof

Abstract

The invention belongs to the field of food fresh-keeping packaging, and discloses a microporous litchi fresh-keeping film and a preparation method thereof, the microporous litchi fresh-keeping film is formed by compounding an inner layer, a middle layer and an outer layer; the inner layer is a blend of 60-70 parts of polylactic acid, 25-35 parts of chitosan and 3-5 parts of a rosemary extract, and the rosemary extract is powder obtained after ethanol reflux extraction and freeze drying; the middle layer is a blend of 4-6 parts of nano titanium dioxide, 2-4 parts of microencapsulated sodium bicarbonate, 5-8 parts of montmorillonite and 100 parts of poly (butylene succinate), the core material is sodium bicarbonate, and the mass ratio of the core material to the capsule material is 3: 1; the outer layer is a blend of 100 parts of polybutylene adipate-terephthalate and 5-8 parts of beewax, and the thickness ratio of the inner layer to the middle layer to the outer layer is 3: 2: 3; the invention aims to provide the microporous litchi preservative film and the preparation method thereof so as to solve the problems that the litchi preservative film in the prior art is poor in hydrophobicity, insufficient in gas regulation capability, incapable of effectively inhibiting peel discoloration and the like.

Description

Technical Field

[0001] This invention relates to the field of food preservation and packaging technology, specifically to a microporous lychee preservation film and its preparation method. Background Technology

[0002] Lychee is a precious and characteristic fruit of southern my country. Its flesh is rich in vitamins, minerals, and other nutrients. However, lychee has a high respiration rate after harvest, making its peel prone to browning and extremely susceptible to rotting and spoilage at room temperature, severely impacting its commercial value and shelf life. Currently, lychee preservation mainly relies on a combination of low-temperature refrigeration and plastic wrap packaging, but existing plastic wrap has many shortcomings.

[0003] From a material perspective, while traditional polyethylene cling film is relatively inexpensive, its poor breathability makes it difficult to regulate the gas composition inside the packaging. This can easily lead to anaerobic respiration in lychees due to oxygen deficiency, resulting in the accumulation of harmful substances such as ethanol and accelerating fruit decay. Cling films made from natural polymers such as starch and cellulose, while possessing some breathability and biodegradability, lack sufficient mechanical strength, are easily damaged, and have poor moisture-proof properties, failing to effectively address the condensation problem that occurs during lychee preservation.

[0004] In terms of functionality, existing food preservation films generally lack targeted gas regulation mechanisms, making it difficult to stably maintain a suitable balance of carbon dioxide and oxygen concentrations within the packaging. Regarding the critical issue of browning in lychee peels, most food preservation films rely solely on physical barriers, without adding effective enzyme inhibitors, thus failing to fundamentally slow down the browning process. Furthermore, some food preservation films add chemical preservatives to enhance performance, posing food safety risks and contradicting the green and environmentally friendly principles of modern food packaging.

[0005] Meanwhile, the interlayer bonding strength of existing composite food preservation films is often insufficient, making them prone to delamination during use, which affects the preservation effect and service life. In addition, the manufacturing process of some food preservation films is complex and involves toxic and harmful solvents, which not only increases production costs but may also pollute the environment, hindering their industrial-scale promotion and application.

[0006] Therefore, a litchi preservation film with good hydrophobic properties, capable of generating carbon dioxide, maintaining suitable oxygen content, and inhibiting discoloration of the peel is proposed. Summary of the Invention

[0007] The present invention aims to provide a microporous litchi preservation film and its preparation method, so as to solve the problems of poor hydrophobicity, insufficient gas regulation capability, and inability to effectively inhibit fruit peel discoloration in existing litchi preservation films.

[0008] To achieve the above objectives, the present invention provides the following technical solution: The technical solution provided by this invention is: A lychee preservation film, composed of an inner layer, a middle layer and an outer layer; The inner layer is a blend of 60-70 parts polylactic acid, 25-35 parts chitosan and 3-5 parts rosemary extract, wherein the rosemary extract is a powder obtained by ethanol reflux extraction and freeze drying. The middle layer is a blend of 4-6 parts nano-titanium dioxide, 2-4 parts microencapsulated sodium bicarbonate, 5-8 parts montmorillonite, and 100 parts polybutylene succinate. The capsule material of the microencapsulated sodium bicarbonate is ethyl cellulose, and the core material is sodium bicarbonate. The mass ratio of the core material to the capsule material is 3:1. The outer layer is a blend of 100 parts polybutylene adipate-terephthalate and 5-8 parts beeswax, wherein the beeswax is food-grade beeswax that has been melt-filtered to remove impurities; The thickness ratio of the inner layer, middle layer and outer layer is 3:2:3.

[0009] Furthermore, the preparation method of the rosemary extract is as follows: after crushing the rosemary leaves, extract them twice with 70% ethanol solution at 60°C for 2 hours each time. After combining the filtrates, concentrate them under reduced pressure until the solid content is ≥30%. After freeze-drying at -40°C, crush them to 100-200 mesh.

[0010] Furthermore, the nano-titanium dioxide is anatase type, prepared by the sol-gel method, with a particle size of 10-30 nm and a specific surface area of ​​50-80 m². 2 / g, used after ultrasonic dispersion treatment.

[0011] Furthermore, the montmorillonite is sodium-based montmorillonite modified with a silane coupling agent. The modification method is as follows: montmorillonite is dispersed in an ethanol aqueous solution, and a silane coupling agent accounting for 10% of the mass of montmorillonite is added. The mixture is stirred and reacted at 80°C for 4 hours, filtered and dried to obtain the product. The interlayer spacing is 2-3 nm. The silane coupling agent is selected from γ-methacryloyloxypropyltrimethoxysilane.

[0012] Furthermore, the preparation method of the microencapsulated sodium bicarbonate is as follows: sodium bicarbonate powder is dispersed in an ethanol solution of ethyl cellulose, and granulation is carried out by spray drying with an inlet air temperature of 120°C and an outlet air temperature of 60°C to obtain microcapsule particles with a particle size of 5-10 μm.

[0013] A method for preparing a lychee preservation film includes the following steps: S1: Material pretreatment: S1-1: Polylactic acid is vacuum dried at 85-90℃ for 5-6 hours, chitosan is vacuum dried at 65-70℃ for 6-8 hours, and rosemary extract is vacuum dried at 40-45℃ for 2-3 hours. The three are then added to a twin-screw mixer in proportion and premixed at 120-130℃ at 300r / min for 10 minutes. S1-2: Nano titanium dioxide is vacuum dried at 120-130℃ for 2-3 hours, microencapsulated sodium bicarbonate is vacuum dried at 50-55℃ for 1-2 hours, montmorillonite is vacuum dried at 100-110℃ for 3-4 hours, and polybutylene succinate is vacuum dried at 75-80℃ for 5-6 hours. S1-3: Polybutylene adipate-terephthalate was vacuum dried at 70-75℃ for 6-7 hours, and beeswax was melted at 50-55℃ and filtered through a 100-mesh filter, and then degassed under vacuum for 30-40 minutes. S2: Co-extrusion composite: S2-1: Inner layer material: Add the premixed inner layer material to the first extruder, with a screw speed of 60-80 r / min and an extrusion temperature of 160-170℃ (the three sections of the barrel are 160℃, 165℃, and 170℃ respectively). S2-2: Middle layer material: Nano titanium dioxide, microencapsulated sodium bicarbonate, montmorillonite and polybutylene succinate are added to a high-speed mixer in proportion and mixed at 1500-2000 r / min for 8-10 min. Then it is added to the second extruder with a screw speed of 50-70 r / min and an extrusion temperature of 150-160℃ (the three sections of the barrel are 150℃, 155℃ and 160℃ respectively). S2-3: Outer layer material: Polybutylene adipate-terephthalate is added to the third extruder to melt (temperature 170-180℃), and molten beeswax is added by spraying through the side feed port. The spraying pressure is 0.3-0.5MPa and the screw speed is 70-90r / min. S2-4: Three materials are compounded by a co-extrusion die head with a die head pressure of 7-10MPa and a compounding temperature of 165-175℃. After being shaped by a cooling roller (temperature 25-30℃), a three-layer composite film is obtained, which is the lychee preservation film.

[0014] Furthermore, in S2, the surface roughness Ra of the cooling roller is ≤0.8μm, and the bonding pressure with the composite film is 0.1-0.2MPa.

[0015] The beneficial effects of this technical solution are: (1) The outer layer of the lychee preservation film of the present invention is a blend of polybutylene adipate-terephthalate and beeswax. The beeswax molecule contains a large number of hydrophobic groups, which can form a continuous hydrophobic layer on the film surface, reduce the adhesion and penetration of water on the film surface, reduce the possibility of condensation accumulation, and provide a dry preservation environment for lychees.

[0016] (2) Microencapsulated sodium bicarbonate is added to the middle layer. The ethyl cellulose of the capsule material has good film-forming and permeability, which can control the slow release of carbon dioxide from sodium bicarbonate. At the same time, the composite structure formed by polybutylene succinate and montmorillonite in the middle layer has suitable gas permeability. Combined with the permeability of the inner and outer layers, it can maintain the dynamic balance of gas in the packaging and provide a suitable gas environment for the preservation of lychees.

[0017] (3) The rosemary extract in the inner layer contains active ingredients such as sarsaparilla acid, which can interact with enzymes such as polyphenol oxidase that participate in the browning of the peel, inhibit their catalytic activity, thereby delaying the discoloration process of the litchi peel and maintaining the color quality of the peel.

[0018] (4) The inner layer of polylactic acid and chitosan has good compatibility and can provide good mechanical support and flexibility for the food preservation film; the middle layer of montmorillonite is tightly bonded to polybutylene succinate after modification, which can enhance the mechanical properties and barrier properties of the film; the outer layer of polybutylene adipate-terephthalate is blended with beeswax, which retains the mechanical strength of the polymer and also has the hydrophobicity of beeswax. The materials of each layer are co-extruded to form a stable whole, so that the food preservation film has comprehensive performance that meets the packaging requirements.

[0019] (5) The polylactic acid, polybutylene succinate, polybutylene adipate-terephthalate and other selected materials are all biodegradable polymer materials. Combined with chitosan, rosemary extract and beeswax from natural sources, the plastic wrap can be naturally degraded after completing its preservation function, reducing pollution to the environment and conforming to the green and environmentally friendly development concept. Attached Figure Description

[0020] Figure 1 This is a table showing the component differences of a microporous litchi preservation film and its preparation method proposed in this invention; Figure 2 Table showing the differences in the preparation of a microporous litchi preservation film and its preparation method proposed in this invention; Figure 3 This is an experimental data table showing the microporous litchi preservation film and its preparation method proposed in this invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] The specific implementation process is as follows: The raw materials are prepared as follows: Rosemary extract: Take 1000g of fresh rosemary leaves, rinse them with clean water, dry them in an oven at 60℃ until constant weight, and then pulverize the dried leaves to 40 mesh using a plant pulverizer to obtain rosemary powder. Add 70% ethanol solution to the dried rosemary powder at a solid-liquid ratio of 1:10 (g / mL), stir well, transfer to a three-necked flask, and reflux extract twice under a constant temperature water bath at 60℃. The first extraction lasts for 2 hours. Filter to obtain filtrate A and residue. Add 70% ethanol solution to the filter residue again according to the above solid-liquid ratio, and perform a second extraction for 2 hours under the same conditions. Filter to obtain filtrate B. Combine filtrate A and filtrate B, and concentrate them under reduced pressure using a rotary evaporator at 50°C and a vacuum of 0.08 MPa until the solid content is 30%, to obtain a concentrated solution; The concentrate was placed in a freeze dryer and freeze-dried at -40°C and 10Pa vacuum for 24 hours. The dried solid was then removed and pulverized to 100-200 mesh using a universal pulverizer to obtain rosemary extract powder. Nano titanium dioxide: Measure 50 mL of tetrabutyl titanate and slowly add it to 100 mL of anhydrous ethanol. Stir magnetically for 30 minutes to form a homogeneous titanium alcohol solution A. Take another 50 mL of deionized water, add 5 mL of glacial acetic acid as an inhibitor, stir well and slowly add it dropwise to titanium alcohol solution A at a rate of 1 drop / second, stirring continuously during the dropwise process. After the dropwise addition is completed, continue stirring for 2 hours to obtain a pale yellow sol. The sol was left to stand at room temperature for 24 hours to form a gel, and then dried in an oven at 80°C for 12 hours to obtain a dry gel. The dry gel was placed in a muffle furnace and heated to 500°C at a rate of 5°C / min. It was then kept at that temperature for 2 hours for calcination. After naturally cooling to room temperature, it was taken out and ground into powder using an agate mortar. Take 10g of the above powder and add it to 100mL of deionized water. Use an ultrasonic cleaner with a power of 300W to ultrasonically disperse it for 30 minutes to obtain a nano titanium dioxide dispersion. Montmorillonite: Take 50g of sodium-based montmorillonite and add it to 500mL of 50% ethanol aqueous solution. Disperse it at 80℃ and stirring speed of 300r / min for 1 hour to obtain montmorillonite dispersion. Weigh 5g of γ-methacryloxypropyltrimethoxysilane (10% of the mass of montmorillonite), add it to 50mL of anhydrous ethanol and stir to dissolve. Then slowly add it dropwise to the montmorillonite dispersion and react for 4 hours at 80℃ and stirring speed of 300r / min. After the reaction was completed, the mixture was filtered, the filter cake was washed three times with anhydrous ethanol, and then dried in an oven at 100°C for 8 hours to obtain modified montmorillonite. Microencapsulated sodium bicarbonate: Weigh 10g of ethyl cellulose, add it to 100mL of anhydrous ethanol, and stir until completely dissolved in a 60℃ water bath to obtain the encapsulation material solution. Weigh 30g of sodium bicarbonate powder (the mass ratio of core material to capsule material is 3:1), add it to the above capsule material solution, and disperse it for 30 minutes at 3000r / min using a high-speed disperser to obtain a uniform suspension. The suspension was added to the feed tank of the spray dryer, the inlet air temperature was set to 120℃, the outlet air temperature to 60℃, and the feed rate to 5mL / min. Spray drying and granulation were carried out, and microencapsulated sodium bicarbonate particles were collected. Beeswax: Take 100g of food-grade beeswax, place it in a beaker, melt it in a 60℃ water bath, and then filter it while hot through a 100-mesh filter to remove impurities; The filtered beeswax melt was placed in a vacuum drying oven and degassed for 30 minutes at 50°C and a vacuum of 0.08 MPa to obtain the processed beeswax. Polylactic acid, chitosan, polybutylene succinate, and polybutylene adipate-terephthalate are all commercially available food-grade raw materials.

[0023] Beeswax: Food grade, melt-filtered to remove impurities, acid value 15mgKOH / g, melting point 63℃.

[0024] Polylactic acid, chitosan, polybutylene succinate, and polybutylene adipate-terephthalate are all commercially available food-grade raw materials.

[0025] Example 1: Please see Figure 1-3 The present invention provides a technical solution: a microporous lychee preservation film and its preparation method, comprising the following steps: S1: Material pretreatment: S1-1: Take 180g of polylactic acid and vacuum dry it at 85℃ for 5h, take 105g of chitosan and vacuum dry it at 65℃ for 6h, take 9g of rosemary extract and vacuum dry it at 40℃ for 2h, then add the three to a twin-screw mixer and premix them at 120℃ and 300r / min for 10min. S1-2: Take 12g of nano titanium dioxide and dry it under vacuum at 120℃ for 2h, take 6g of microencapsulated sodium bicarbonate and dry it under vacuum at 50℃ for 1h, take 15g of montmorillonite and dry it under vacuum at 100℃ for 3h, and take 300g of polybutylene succinate and dry it under vacuum at 75℃ for 5h. S1-3: Take 300g of polybutylene adipate-terephthalate and dry it under vacuum at 70℃ for 6h. Melt 15g of beeswax at 50℃ and filter it through a 100-mesh filter. Degas it under vacuum for 30min. S2: Co-extrusion composite: S2-1: Inner layer material: Add the premixed inner layer material to the first extruder, with a screw speed of 60 r / min and an extrusion temperature of 160℃ (the three sections of the barrel have temperatures of 160℃, 165℃, and 170℃ respectively). S2-2: Middle layer material: Nano titanium dioxide, microencapsulated sodium bicarbonate, montmorillonite and polybutylene succinate are added to a high-speed mixer and mixed at 1500 r / min for 8 min. Then it is added to the second extruder with a screw speed of 50 r / min and an extrusion temperature of 150℃ (the three sections of the barrel are 150℃, 155℃ and 160℃ respectively). S2-3: Outer layer material: Polybutylene adipate-terephthalate is added to the third extruder to melt (temperature 170℃), and molten beeswax is added by spraying through the side feed port at a spray pressure of 0.3MPa and a screw speed of 70r / min; S2-4: Three materials are compounded by a co-extrusion die with a die pressure of 7MPa and a compounding temperature of 165℃. After being shaped by a cooling roller (temperature 25℃, surface roughness Ra=0.6μm, bonding pressure 0.1MPa), a three-layer composite film is obtained. The thickness ratio of the inner, middle and outer layers is 3:2:3, which is the lychee preservation film.

[0026] The litchi preservation film prepared in this embodiment has an outer beeswax layer that forms a hydrophobic layer to reduce moisture adhesion; a middle layer of microencapsulated sodium bicarbonate that slowly releases carbon dioxide, combined with the material's breathability, provides a suitable gaseous environment for the litchi; and an inner layer of rosemary extract that inhibits polyphenol oxidase activity and delays browning of the peel. The preservation film has certain tensile strength and elongation at break, meeting basic packaging requirements.

[0027] Example 2: Please see Figure 1-3 The present invention provides a technical solution: a microporous lychee preservation film and its preparation method, comprising the following steps: S1: Material pretreatment: S1-1: Take 195g of polylactic acid and vacuum dry it at 87℃ for 5.5h, take 90g of chitosan and vacuum dry it at 67℃ for 7h, take 12g of rosemary extract and vacuum dry it at 42℃ for 2.5h, then add the three to a twin-screw mixer and premix them at 125℃ and 300r / min for 10min. S1-2: Take 15g of nano titanium dioxide and vacuum dry it at 125℃ for 2.5h, microencapsulated sodium bicarbonate 9g and vacuum dry it at 52℃ for 1.5h, montmorillonite 18g and vacuum dry it at 105℃ for 3.5h, and polybutylene succinate 300g and vacuum dry it at 77℃ for 5.5h. S1-3: Take 300g of polybutylene adipate-terephthalate and dry it under vacuum at 72℃ for 6.5h. Melt 18g of beeswax at 52℃ and filter it through a 100-mesh filter. Degas it under vacuum for 35min. S2: Co-extrusion composite: S2-1: Inner layer material: Add the premixed inner layer material to the first extruder, with a screw speed of 70 r / min and an extrusion temperature of 165℃ (the three sections of the barrel are 160℃, 165℃, and 170℃ respectively). S2-2: Middle layer material: Nano titanium dioxide, microencapsulated sodium bicarbonate, montmorillonite and polybutylene succinate are added to a high-speed mixer and mixed at 1700 r / min for 9 min. Then it is added to the second extruder with a screw speed of 60 r / min and an extrusion temperature of 155℃ (the three sections of the barrel are 150℃, 155℃ and 160℃ respectively). S2-3: Outer layer material: Polybutylene adipate-terephthalate is added to the third extruder to melt (temperature 175℃), and molten beeswax is added by spraying through the side feed port. The spraying pressure is 0.4MPa and the screw speed is 80r / min. S2-4: Three materials are compounded by a co-extrusion die with a die pressure of 8MPa and a compounding temperature of 170℃. After being shaped by a cooling roller (temperature 27℃, surface roughness Ra=0.7μm, bonding pressure 0.15MPa), a three-layer composite film is obtained. The thickness ratio of the inner, middle and outer layers is 3:2:3, which is the lychee preservation film.

[0028] The outer layer of the cling film in this embodiment exhibits good hydrophobic properties, effectively reducing condensation; the middle layer provides stable gas regulation, maintaining a suitable gas balance within the packaging; and the inner layer, with its rosemary extract, demonstrates significant enzyme inhibition, effectively preserving the fruit peel's color. The cling film also possesses well-balanced mechanical properties, making it suitable for most lychee packaging scenarios.

[0029] Example 3: Please see Figure 1-3 The present invention provides a technical solution: a microporous lychee preservation film and its preparation method, comprising the following steps: S1: Material pretreatment: S1-1: Take 210g of polylactic acid and dry it under vacuum at 90℃ for 6h, take 75g of chitosan and dry it under vacuum at 70℃ for 8h, take 15g of rosemary extract and dry it under vacuum at 45℃ for 3h, then add the three to a twin-screw mixer and premix them at 130℃ and 300r / min for 10min. S1-2: Take 18g of nano titanium dioxide and dry it in vacuum at 130℃ for 3h, 12g of microencapsulated sodium bicarbonate and dry it in vacuum at 55℃ for 2h, 24g of montmorillonite and dry it in vacuum at 110℃ for 4h, and 300g of polybutylene succinate and dry it in vacuum at 80℃ for 6h. S1-3: Take 300g of polybutylene adipate-terephthalate and dry it under vacuum at 75℃ for 7h. Melt 24g of beeswax at 55℃ and filter it through a 100-mesh filter. Degas it under vacuum for 40min. S2: Co-extrusion composite: S2-1: Inner layer material: Add the premixed inner layer material to the first extruder, with a screw speed of 80 r / min and an extrusion temperature of 170℃ (the three sections of the barrel have temperatures of 160℃, 165℃, and 170℃ respectively). S2-2: Middle layer material: Nano titanium dioxide, microencapsulated sodium bicarbonate, montmorillonite and polybutylene succinate are added to a high-speed mixer and mixed at 2000 r / min for 10 min. Then it is added to the second extruder with a screw speed of 70 r / min and an extrusion temperature of 160℃ (the three sections of the barrel are 150℃, 155℃ and 160℃ respectively). S2-3: Outer layer material: Polybutylene adipate-terephthalate is added to the third extruder to melt (temperature 180℃), and molten beeswax is added by spraying through the side feed port at a spray pressure of 0.5MPa and a screw speed of 90r / min; S2-4: Three materials are compounded by a co-extrusion die with a die pressure of 10MPa and a compounding temperature of 175℃. After being shaped by a cooling roller (temperature 30℃, surface roughness Ra=0.8μm, bonding pressure 0.2MPa), a three-layer composite film is obtained. The thickness ratio of the inner, middle and outer layers is 3:2:3, which is the lychee preservation film.

[0030] In this embodiment, the rosemary extract content is high, resulting in strong enzyme inhibition and prolonged maintenance of the fruit peel color. The increased beeswax content enhances the hydrophobic effect of the outer layer. The middle layer provides ample carbon dioxide release, and combined with the material's breathability, it meets the gas requirements for lychee preservation. The preservation film exhibits good tensile strength, making it suitable for packaging applications with high mechanical performance requirements.

[0031] Example 4: Please see Figure 1-3 The present invention provides a technical solution: a microporous lychee preservation film and its preparation method, comprising the following steps: S1: Material pretreatment: S1-1: Take 189g of polylactic acid and vacuum dry it at 88℃ for 5.5h, take 96g of chitosan and vacuum dry it at 68℃ for 7h, take 10.5g of rosemary extract and vacuum dry it at 43℃ for 2.5h, then add the three to a twin-screw mixer and premix them at 125℃ and 300r / min for 10min. S1-2: Take 13.5g of nano titanium dioxide and vacuum dry it at 125℃ for 2.5h, microencapsulated sodium bicarbonate 7.5g and vacuum dry it at 53℃ for 1.5h, montmorillonite 21g and vacuum dry it at 105℃ for 3.5h, and polybutylene succinate 300g and vacuum dry it at 78℃ for 5.5h. S1-3: Take 300g of polybutylene adipate-terephthalate and dry it under vacuum at 73℃ for 6.5h. Melt 21g of beeswax at 53℃ and filter it through a 100-mesh filter. Degas it under vacuum for 35min. S2: Co-extrusion composite: S2-1: Inner layer material: Add the premixed inner layer material to the first extruder, screw speed 75 r / min, extrusion temperature 168℃ (the three sections of the barrel are 160℃, 165℃ and 170℃ respectively). S2-2: Middle layer material: Nano titanium dioxide, microencapsulated sodium bicarbonate, montmorillonite and polybutylene succinate are added to a high-speed mixer and mixed at 1800 r / min for 9 min. Then it is added to the second extruder with a screw speed of 65 r / min and an extrusion temperature of 158℃ (the three sections of the barrel are 150℃, 155℃ and 160℃ respectively). S2-3: Outer layer material: Polybutylene adipate-terephthalate is added to the third extruder to melt (temperature 178℃), and molten beeswax is added by spraying through the side feed port. The spraying pressure is 0.45MPa and the screw speed is 85r / min. S2-4: Three materials are compounded by a co-extrusion die with a die pressure of 9MPa and a compounding temperature of 172℃. After being shaped by a cooling roller (temperature 28℃, surface roughness Ra=0.7μm, bonding pressure 0.18MPa), a three-layer composite film is obtained. The thickness ratio of the inner, middle and outer layers is 3:2:3, which is the lychee preservation film.

[0032] In this embodiment, the proportions of the components and the process parameters are well-matched. The outer layer of the preservation film exhibits good synergy in its hydrophobic properties, the middle layer's gas regulation capability, and the inner layer's enzyme inhibition effect, providing a suitable preservation environment for lychees. The preservation film also possesses balanced mechanical properties, meeting the needs of various packaging scenarios.

[0033] Comparative Example 1: Please see Figure 1-3 The present invention provides a comparative scheme: Compared to Example 2, the inner layer does not contain rosemary extract, but the other raw materials and preparation steps are the same.

[0034] The plastic wrap in this comparative study lacked rosemary extract, which failed to inhibit polyphenol oxidase activity, and the litchi peel was prone to browning. This indicates that rosemary extract plays a key role in inhibiting peel discoloration.

[0035] Comparative Example 2: Please see Figure 1-3 The present invention provides a comparative scheme: Compared to Example 2, the middle layer does not contain microencapsulated sodium bicarbonate, but the other raw materials and preparation steps are the same.

[0036] Since the middle layer lacks microencapsulated sodium bicarbonate, it cannot release carbon dioxide, making it difficult to meet the preservation requirements of lychees. As a result, the respiration of lychees is not effectively inhibited, indicating that microencapsulated sodium bicarbonate is crucial for regulating gas composition.

[0037] Comparative Example 3: Please see Figure 1-3 The present invention provides a comparative scheme: Compared to Example 2, the outer layer does not contain beeswax, but the other raw materials and preparation steps are the same.

[0038] The outer layer does not contain beeswax and has insufficient hydrophobic properties, making it easy for condensation to accumulate inside the packaging, which can cause lychees to mold. This indicates that beeswax can effectively improve the hydrophobic properties of the plastic wrap.

[0039] Please see Figure 1-3 : Through systematic analysis of Examples 1-4 and Comparative Examples 1-3, it can be clearly seen that the core advantages of the litchi preservation film proposed in this invention lie in its breakthrough solution to the bottlenecks of existing preservation technologies and the synergistic optimization of multi-dimensional performance.

[0040] Examples 1-4 all strictly adhere to a three-layer composite structure design. Through the material ratio and functional synergy of the inner, middle, and outer layers, a complete system is constructed to specifically address the pain points of lychee preservation. The inner layer uses polylactic acid and chitosan as base materials, combined with rosemary extract. The compatibility of polylactic acid and chitosan provides basic mechanical support for the membrane material, while the arugula acid in the rosemary extract, as a key active ingredient, fundamentally blocks the browning pathway of lychee peel by inhibiting the activity of browning-related enzymes such as polyphenol oxidase. This design directly addresses the technical shortcomings of existing preservation films that rely solely on physical barriers and cannot inhibit enzymatic reactions. The middle layer uses polybutylene succinate as the matrix, combined with nano-titanium dioxide, microencapsulated sodium bicarbonate, and modified montmorillonite. The microencapsulated sodium bicarbonate, through the controlled release of ethyl cellulose encapsulation, achieves the slow release of carbon dioxide. Combined with the ordered structure formed by montmorillonite and the polymer, this precisely regulates gas permeability, creating a stable dynamic equilibrium system. This solves the problems of insufficient gas regulation capacity in traditional food preservation films, which easily lead to anaerobic or excessive respiration in lychees. The outer layer is a blend of polybutylene adipate-terephthalate and beeswax. The hydrophobic groups of beeswax form a continuous barrier on the film surface, significantly reducing the probability of condensation adhesion. The biodegradable polymer substrate ensures the mechanical strength and environmental performance of the film material, overcoming the dual defects of poor moisture resistance in existing natural polymer food preservation films and the difficulty in degradation of synthetic food preservation films.

[0041] Comparing the experimental results of Comparative Examples 1-3, it is evident that the key technical features of this invention are indispensable, and their synergistic effect far exceeds the simple summation of individual components. Comparative Example 1, lacking rosemary extract, resulted in significant browning of the lychee peel within 3 days, demonstrating the irreplaceable role of rosemary extract as an enzyme inhibitor. It competitively inhibits enzymatic reactions by binding to the active site of polyphenol oxidase, a mechanism not yet applied in existing lychee preservation technologies. In Comparative Example 2, after removing the middle layer of microencapsulated sodium bicarbonate, the oxygen concentration inside the packaging rose to 8%, causing uncontrolled lychee respiration, leading to fruit softening and flavor deterioration within 4 days. This fully verifies the precise control value of microencapsulation controlled-release technology for the gas environment. Furthermore, the 3:1 ratio of ethyl cellulose encapsulation material to sodium bicarbonate is the optimal solution derived through extensive experimental optimization. This ensures that the carbon dioxide release rate matches the lychee's respiration consumption while avoiding localized high-concentration damage caused by instantaneous carbon release. The construction of this controlled-release system reflects a deep integration of material properties and fruit physiology. Comparative Example 3, lacking beeswax in its outer layer, had a water contact angle reduced to 75°, resulting in mold growth caused by condensation accumulation within 5 days. This highlights the crucial regulatory role of the hydrophobic layer formed by the blending of beeswax and polybutylene adipate-terephthalate on the microenvironment humidity. By altering the surface properties of the membrane, it achieves anti-fogging and moisture-proof performance, exhibiting better durability and compatibility than traditional coating-type hydrophobic technologies.

[0042] Examples 1-4 achieve a balanced optimization of overall performance while maintaining the advantages of each individual function. Example 1, as the basic formulation, has demonstrated good overall performance: the hydrophobic layer effectively reduces moisture adhesion, the controlled-release system maintains a suitable gas environment, the enzyme inhibitor delays browning, and the mechanical properties meet basic packaging requirements. Example 2, by fine-tuning the component ratio (4 parts rosemary extract, 6 parts beeswax, etc.), increased the hydrophobic angle to 112°, stabilized the oxygen concentration at 3.0%, and extended the browning inhibition period to 7 days, demonstrating the performance gain from formulation optimization. Example 3, by increasing the content of rosemary extract and beeswax, further enhanced the enzyme inhibition and hydrophobic effects, and the degree of fruit peel browning remained low within 9 days, making it suitable for long-distance transportation. Example 4, through precise formulation (63 parts polylactic acid, 7 parts montmorillonite, etc.) and process parameter optimization (screw speed 75 r / min, composite temperature 172℃), achieved the optimal balance of hydrophobic performance (113°), gas regulation (oxygen concentration 3.0%), browning inhibition (8-day shelf life), and mechanical properties (tensile strength 21 MPa), verifying the adjustability and stability of this technical solution in different application scenarios. This synergistic improvement of multiple performance parameters breaks the inherent perception in existing technologies that "hydrophobicity and breathability are mutually exclusive" and "preservation and environmental protection are contradictory." Through the rationality of material selection and structural design, and the 3:2:3 thickness ratio of the three layers, a breakthrough in performance dimensions has been achieved.

[0043] The negative results of Comparative Examples 1-3, from the opposite perspective, confirm the necessity of the technical features of this invention: the lack of rosemary extract leads to accelerated browning, proving the irreplaceable role of natural enzyme inhibitors in lychee preservation; the removal of microencapsulated sodium bicarbonate causes gas balance to become uncontrolled, highlighting the core value of the controlled-release gas regulation system; the omission of beeswax causes a sharp drop in hydrophobic properties, verifying the unique advantage of the synergistic effect of natural waxes and synthetic polymers. These comparative results clearly demonstrate that the technical solution of this invention is not a simple superposition of existing technologies, but rather a qualitative leap in lychee preservation performance achieved through systematic innovation in key materials, structural design, and process parameters.

[0044] The experimental method is as follows: Hydrophobicity test: Cut the sample and use a contact angle meter to measure the water contact angle using the seat drop method. Take the average value. The larger the contact angle, the better the hydrophobicity.

[0045] Gas regulation capability test: The lychees were sealed in plastic wrap and stored at constant temperature and humidity. The oxygen and carbon dioxide concentrations inside the packaging were measured periodically using a gas chromatograph to observe whether the oxygen concentration was stable at around 3%.

[0046] Test to inhibit peel discoloration: After packaging lychees and storing them, the color change of the peel (ΔE) was measured with a colorimeter, and the activity of polyphenol oxidase (PPO) was measured with a spectrophotometer. The smaller the ΔE and the lower the PPO activity, the better the effect.

[0047] Mechanical property testing: Cut the sample and test the tensile strength and elongation at break using a universal testing machine according to national standards. The higher the value, the better the performance.

[0048] Comprehensive test of preservation effect: After packaging lychees, they are stored in cold storage, and the mold rate, weight loss rate, fruit firmness and soluble solids content are measured regularly. The better the indicators, the better the preservation effect.

[0049] Note: Figure 3 middle The data are test results under storage conditions at 25℃, and the average value is taken from 3 parallel tests in each group; 2. TSS represents the soluble solids content; 3. The ΔE value represents the total change in color; the smaller the value, the less browning. 4. PPO represents polyphenol oxidase activity; the lower the value, the better the inhibition effect.

[0050] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific technical solutions or characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A lychee preservation film, characterized in that, It is composed of an inner layer, a middle layer, and an outer layer; The inner layer is a blend of 60-70 parts polylactic acid, 25-35 parts chitosan and 3-5 parts rosemary extract, wherein the rosemary extract is a powder obtained by ethanol reflux extraction and freeze drying. The middle layer is a blend of 4-6 parts nano-titanium dioxide, 2-4 parts microencapsulated sodium bicarbonate, 5-8 parts montmorillonite, and 100 parts polybutylene succinate. The capsule material of the microencapsulated sodium bicarbonate is ethyl cellulose, and the core material is sodium bicarbonate. The mass ratio of the core material to the capsule material is 3:

1. The outer layer is a blend of 100 parts polybutylene adipate-terephthalate and 5-8 parts beeswax, wherein the beeswax is food-grade beeswax that has been melt-filtered to remove impurities; The thickness ratio of the inner layer, middle layer and outer layer is 3:2:

3.

2. The lychee preservation film according to claim 1, characterized in that, The preparation method of the rosemary extract is as follows: after crushing the rosemary leaves, extract them twice with 70% ethanol solution at 60°C for 2 hours each time. After combining the filtrates, concentrate them under reduced pressure until the solid content is ≥30%. After freeze-drying at -40°C, crush them to 100-200 mesh.

3. The lychee preservation film according to claim 1, characterized in that, The nano-titanium dioxide is anatase type, prepared by the sol-gel method, with a particle size of 10-30 nm and a specific surface area of ​​50-80 m². 2 / g, used after ultrasonic dispersion treatment.

4. The lychee preservation film according to claim 1, characterized in that, The montmorillonite is sodium-based montmorillonite modified with a silane coupling agent. The modification method is as follows: montmorillonite is dispersed in an ethanol aqueous solution, and a silane coupling agent accounting for 10% of the mass of montmorillonite is added. The mixture is stirred and reacted at 80°C for 4 hours, filtered and dried to obtain the product. The interlayer spacing is 2-3 nm. The silane coupling agent is selected from γ-methacryloyloxypropyltrimethoxysilane.

5. The lychee preservation film according to claim 1, characterized in that, The method for preparing the microencapsulated sodium bicarbonate is as follows: sodium bicarbonate powder is dispersed in an ethanol solution of ethyl cellulose, and granulation is carried out by spray drying with an inlet air temperature of 120°C and an outlet air temperature of 60°C to obtain microcapsule particles with a particle size of 5-10 μm.

6. A method for preparing a litchi preservation film as described in any one of claims 1-5, characterized in that, Includes the following steps: S1: Material pretreatment: S1-1: Polylactic acid is vacuum dried at 85-90℃ for 5-6 hours, chitosan is vacuum dried at 65-70℃ for 6-8 hours, and rosemary extract is vacuum dried at 40-45℃ for 2-3 hours. The three are then added to a twin-screw mixer in proportion and premixed at 120-130℃ at 300r / min for 10 minutes. S1-2: Nano titanium dioxide is vacuum dried at 120-130℃ for 2-3 hours, microencapsulated sodium bicarbonate is vacuum dried at 50-55℃ for 1-2 hours, montmorillonite is vacuum dried at 100-110℃ for 3-4 hours, and polybutylene succinate is vacuum dried at 75-80℃ for 5-6 hours. S1-3: Polybutylene adipate-terephthalate was vacuum dried at 70-75℃ for 6-7 hours, and beeswax was melted at 50-55℃ and filtered through a 100-mesh filter, and then degassed under vacuum for 30-40 minutes. S2: Co-extrusion composite: S2-1: Inner layer material: Add the premixed inner layer material to the first extruder, with a screw speed of 60-80 r / min and an extrusion temperature of 160-170℃ (the three sections of the barrel are 160℃, 165℃, and 170℃ respectively). S2-2: Middle layer material: Nano titanium dioxide, microencapsulated sodium bicarbonate, montmorillonite and polybutylene succinate are added to a high-speed mixer in proportion and mixed at 1500-2000 r / min for 8-10 min. Then it is added to the second extruder with a screw speed of 50-70 r / min and an extrusion temperature of 150-160℃ (the three sections of the barrel are 150℃, 155℃ and 160℃ respectively). S2-3: Outer layer material: Polybutylene adipate-terephthalate is added to the third extruder to melt (temperature 170-180℃), and molten beeswax is added by spraying through the side feed port. The spraying pressure is 0.3-0.5MPa and the screw speed is 70-90r / min. S2-4: Three materials are compounded by a co-extrusion die head with a die head pressure of 7-10MPa and a compounding temperature of 165-175℃. After being shaped by a cooling roller (temperature 25-30℃), a three-layer composite film is obtained, which is the lychee preservation film.

7. The preparation method according to claim 6, characterized in that, The surface roughness Ra of the cooling roller in S2 is ≤0.8μm, and the bonding pressure with the composite film is 0.1-0.2MPa.

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