Slow-release antibacterial fruit and vegetable fresh-keeping film as well as preparation method and application thereof

Abstract

The invention provides a slow-release bacteriostatic fruit and vegetable fresh-keeping film as well as a preparation method and application thereof, and the preparation method comprises the following steps: adding gelatin and sodium carboxymethyl cellulose into deionized water to obtain a wall material solution, and adding curcumin into the wall material solution to obtain a prepolymer solution; stirring and emulsifying the prepolymer solution at a high speed, dropwise adding an acetic acid solution into the prepolymer solution to adjust the pH value of the prepolymer solution, and cooling in an ice-water bath to obtain a prepolymer suspension; dispersing carboxymethyl chitosan in an acetic acid solution to obtain a chitosan solution, adding the chitosan solution into the prepolymer suspension to obtain a microcapsule solution, adding tannic acid into the microcapsule solution to obtain a microcapsule suspension, and performing spray drying to obtain double-layer microcapsule powder; uniformly mixing polylactic acid, the double-layer microcapsule powder, a plasticizer, a lubricant and an antioxidant to obtain a mixed material, and sequentially performing melt extrusion, cooling, pelletizing and film blowing on the mixed material to obtain the slow-release antibacterial fruit and vegetable fresh-keeping film.

Description

Technical Field

[0001] The invention belongs to the technical field of fruit and vegetable preservation, and relates to a slow-release antibacterial fruit and vegetable preservation film, a preparation method and application thereof. Background Art

[0002] As people's living standards improve, the demand for fresh fruits and vegetables is growing, requiring not only a rich variety but also strict requirements for their quality and freshness. After being picked, fruits and vegetables are still living organisms, and they will continue to carry out physiological activities such as respiration and transpiration. At the same time, they are also exposed to the invasion of microorganisms. These factors will lead to the decline of the quality of fruits and vegetables, the loss of nutrients, and even rot and deterioration, causing huge economic losses.

[0003] Traditional methods of preserving fruits and vegetables include low-temperature refrigeration, controlled atmosphere preservation, and chemical preservative treatment. Low-temperature refrigeration can reduce the respiration intensity and enzyme activity of fruits and vegetables and slow down their metabolic rate, but it consumes a lot of energy, and for some fruits and vegetables that are sensitive to low temperatures, chilling damage is prone to occur, affecting quality. Controlled atmosphere preservation inhibits the respiration and microbial growth of fruits and vegetables by adjusting the gas composition in the storage environment (such as reducing the oxygen content and increasing the carbon dioxide content), but the equipment cost is high and the operation is complicated, making it difficult to popularize in ordinary households and small commercial places. Although chemical preservative treatment has the advantages of good preservation effect and low cost, some chemical preservatives may have residue problems, causing potential harm to human health and the environment. With the increasing awareness of food safety and environmental protection, the use of chemical preservatives has been increasingly restricted.

[0004] In this context, fruit and vegetable fresh-keeping film has emerged as a new type of fresh-keeping technology. Fresh-keeping film can form a physical barrier on the surface of fruits and vegetables, reduce water evaporation, and reduce the exchange rate of oxygen and carbon dioxide, thereby delaying the aging and deterioration of fruits and vegetables. At present, common fruit and vegetable fresh-keeping films mainly include plastic films such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC). Although these films have certain barrier properties, most of them are non-degradable polymer materials. Large-scale use will cause serious "white pollution" and does not meet the requirements of sustainable development.

[0005] In order to solve the environmental problems of traditional cling film, degradable bio-based cling film has gradually become a research hotspot. Bio-based cling film is usually made of natural biomaterials such as polysaccharides, proteins, lipids, etc., with good biocompatibility and degradability, and is environmentally friendly. However, cling film made of a single bio-based material often has disadvantages such as poor mechanical properties and unsatisfactory barrier properties, which limits its practical application. In order to improve the performance of bio-based cling film, a composite modification method is usually adopted to compound different biomaterials, or add additives such as nanoparticles and plasticizers.

[0006] In addition, in order to give the fresh-keeping film an antibacterial function and inhibit the growth and reproduction of microorganisms on the surface of fruits and vegetables, antibacterial agents are usually added to the film. Commonly used antibacterial agents include natural antibacterial agents (such as plant essential oils, chitosan, etc.) and chemical antibacterial agents (such as benzoic acid, potassium sorbate, etc.). However, the traditional method of adding antibacterial agents has problems such as too fast release speed and short duration of antibacterial agents, making it difficult to achieve long-term antibacterial effect. Therefore, the development of fruit and vegetable fresh-keeping films with slow-release antibacterial functions, so that antibacterial agents can be released slowly and continuously to extend the shelf life, is one of the current research focuses in the field of fruit and vegetable preservation. Summary of the invention

[0007] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a sustained-release antibacterial fruit and vegetable fresh-keeping film, a preparation method and use thereof. The fruit and vegetable fresh-keeping film prepared by the present invention has excellent antibacterial properties and mechanical properties. Using it for fruit and vegetable packaging can significantly extend the shelf life of fruits and vegetables and reduce food waste and loss.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, the preparation method comprising:

[0010] (I) adding gelatin and sodium carboxymethyl cellulose to deionized water, mixing, stirring and heating to obtain a wall material solution, adding curcumin to the wall material solution to obtain a prepolymer solution; subjecting the prepolymer solution to high-speed stirring and emulsification, then dropping an acetic acid solution into the prepolymer solution under stirring and water bath heating conditions to adjust the pH value thereof, and cooling in an ice water bath to obtain a prepolymer suspension;

[0011] (II) dispersing carboxymethyl chitosan in an acetic acid solution to obtain a chitosan solution, adding the chitosan solution to the prepolymer suspension, mixing uniformly to obtain a microcapsule solution, adding an alkali solution to the microcapsule solution to adjust the pH value thereof, then adding tannic acid to the microcapsule solution, mixing uniformly to obtain a microcapsule suspension, and spray drying the microcapsule suspension to obtain a double-layer microcapsule powder;

[0012] (III) Polylactic acid, the double-layer microcapsule powder obtained in step (II), a plasticizer, a lubricant and an antioxidant are uniformly mixed to obtain a mixed material, and the mixed material is sequentially melt-extruded, cooled, pelletized and film-blown to obtain the sustained-release antibacterial fruit and vegetable fresh-keeping film.

[0013] The present invention firstly uses curcumin as a core material, gelatin and sodium carboxymethyl cellulose as wall materials to prepare a single-layer microcapsule with a core-shell structure, then uses carboxymethyl chitosan to perform secondary coating reinforcement on the single-layer microcapsule, and uses electrostatic action and intermolecular force to form a carboxymethyl chitosan protective layer on the surface of the single-layer microcapsule, which can increase the film thickness of the curcumin surface to better resist external adverse factors, reduce the environmental sensitivity of the double-layer microcapsule, and improve its thermal stability. The present invention adds double-layer microcapsule powder to a polylactic acid matrix, obtains a fruit and vegetable fresh-keeping film after melt extrusion, cooling, pelletizing and film blowing, and significantly improves the antibacterial and mechanical properties of the fruit and vegetable fresh-keeping film.

[0014] Gelatin is a protein obtained by hydrolyzing collagen in connective tissues such as animal skin and bones. It has good film-forming properties, biodegradability and biocompatibility. The isoelectric point of gelatin is around pH = 5, which means that when the pH value is lower than 5, gelatin is positively charged; when the pH value is higher than 5, gelatin is negatively charged. Sodium carboxymethyl cellulose is a water-soluble anionic polysaccharide obtained by chemical modification of cellulose. It contains negatively charged carboxyl functional groups, so it always carries a negative charge in aqueous solution. The present invention adjusts the pH value of the reaction system to below 5 so that gelatin is positively charged, and utilizes the positively charged gelatin and the negatively charged sodium carboxymethyl cellulose to neutralize charges under a specific pH value environment, so that the solubility is reduced and phase separation is formed, which promotes them to aggregate and deposit around curcumin to form a complex coacervate coated around the curcumin, and finally obtains a single-layer microcapsule with a stable core-shell structure, wherein curcumin as a core material is wrapped inside the complex coacervate formed by gelatin and sodium carboxymethyl cellulose, and the complex coacervate as an outer shell has a certain thickness and mechanical strength, and can protect the curcumin from the influence of the external environment.

[0015] In order to further improve the environmental stability, thermal stability and sustained release effect of the microcapsules, the present invention further coats the surface of the single-layer microcapsules with a carboxymethyl chitosan protective layer, which has the following functions:

[0016] On the one hand, the carboxymethyl chitosan protective layer significantly improves the environmental stability of the double-layer microcapsules and reduces the volatilization and degradation of curcumin during storage and transportation. This is because, as a chitosan derivative, the charge properties of carboxymethyl chitosan depend on its chemical structure and environmental pH. The carboxymethyl chitosan molecule contains negatively charged carboxyl groups (-COOH) and positively charged amine groups (-NH 2). Under acidic conditions, the amine groups of carboxymethyl chitosan will be protonated and positively charged; under alkaline conditions, the carboxyl groups will be ionized and negatively charged. Therefore, carboxymethyl chitosan can present different charge states in the solution, depending on the pH value of the solution. The present invention adjusts the pH value of the reaction system to weak acidity (pH value is 5-6) so that the carboxymethyl chitosan is positively charged, and the positively charged carboxymethyl chitosan and the negatively charged complex coacervate are tightly combined through electrostatic interaction to form a stable double-layer shell structure. This electrostatic combination not only enhances the density of the wall material, but also effectively isolates the adverse effects of the external environment on curcumin, such as pH changes, ionic strength fluctuations and temperature changes, thereby significantly improving the environmental stability of the double-layer microcapsules.

[0017] On the other hand, the carboxymethyl chitosan protective layer significantly improves the thermal stability of the double-layer microcapsules. This is because the carboxymethyl chitosan has a high melting point and is not easy to decompose at high temperatures. In the double-layer microcapsules, the introduction of the carboxymethyl chitosan protective layer effectively improves the overall thermal stability of the double-layer microcapsules. When the double-layer microcapsules are in a high-temperature environment, the carboxymethyl chitosan protective layer can be tightly wrapped on the surface of the complex coacervate layer to prevent the rapid release of curcumin due to heat during the subsequent melt extrusion process. At the same time, the thermal shrinkage of the carboxymethyl chitosan protective layer enables it to form a denser barrier in the high-temperature environment of melt extrusion, further enhancing the thermal stability of the double-layer microcapsules.

[0018] On the other hand, the presence of the carboxymethyl chitosan protective layer significantly slows down the release rate of curcumin, achieving stable delivery and sustained release of curcumin. This is because the carboxymethyl chitosan protective layer has a certain permeability and can control the speed at which curcumin molecules pass through, thereby achieving a sustained release effect. By adjusting the thickness and molecular structure of the carboxymethyl chitosan protective layer, precise control of the release rate of curcumin can be achieved.

[0019] The invention adds double-layer microcapsule powder to the polymer matrix, which can not only give the fruit and vegetable fresh-keeping film excellent sustained-release antibacterial ability, but also improve the mechanical properties of the fruit and vegetable fresh-keeping film. Its functions are:

[0020] On the one hand, the double-layer microcapsule powder is evenly dispersed in the fruit and vegetable fresh-keeping film system, building a solid support network at the microscopic level. In terms of size, the particle size of the double-layer microcapsule powder matches the microstructural scale of the fruit and vegetable fresh-keeping film, and can effectively fill the molecular gaps of the polylactic acid matrix. When the fruit and vegetable fresh-keeping film is subjected to tensile external force, these double-layer microcapsule powders are like tiny "rivets", limiting the relative sliding of the molecular chains of the polylactic acid matrix, thereby dispersing the external force to the entire system. In pure polylactic acid film without the addition of double-layer microcapsule powder, the interaction between the molecular chains is relatively weak, and the molecular chains are prone to slip when stretched, resulting in rapid breakage of the film. After adding the double-layer microcapsule powder, the double-layer microcapsule powder forms a close bond with the polylactic acid matrix, and the rigid shell of the carboxymethyl chitosan of the double-layer microcapsule powder can withstand part of the tensile stress, avoiding stress concentration on a few molecular chains, thereby improving the tensile strength of the fruit and vegetable fresh-keeping film. At the same time, the presence of the double-layer microcapsule powder also provides a certain space for the slip of the molecular chain, so that the fruit and vegetable fresh-keeping film can undergo a certain degree of deformation during the stretching process, thereby increasing the elongation at break of the fruit and vegetable fresh-keeping film.

[0021] On the other hand, there are abundant interfacial interactions between the wall material components of the double-layer microcapsule powder, such as gelatin and carboxymethyl chitosan, and the polylactic acid matrix. The polar functional groups such as hydroxyl and amino groups in the gelatin molecules can form hydrogen bonds with the carbonyl groups on the polylactic acid molecular chain. This hydrogen bonding not only enhances the binding force between the double-layer microcapsule powder and the polylactic acid matrix, but also enables the double-layer microcapsule powder to better transfer stress. When the fruit and vegetable fresh-keeping film is stretched, the stress can be transferred from the polylactic acid matrix to the double-layer microcapsule powder through hydrogen bonds, so that the entire system can synergistically resist external forces. At the same time, the van der Waals force at the interface also plays an important role, further enhancing the adhesion between the double-layer microcapsule powder and the polylactic acid matrix, and preventing the double-layer microcapsule powder from debonding from the polylactic acid matrix. This strong interfacial interaction enables the double-layer microcapsule powder and the polylactic acid matrix to deform together during the stretching process of the fruit and vegetable fresh-keeping film, avoiding damage at the interface, thereby effectively improving the tensile strength and elongation at break of the fruit and vegetable fresh-keeping film.

[0022] On the other hand, the addition of double-layer microcapsule powder has a positive regulatory effect on the movement of polylactic acid molecular chains. The presence of double-layer microcapsule powder limits the free movement range of polylactic acid molecular chains, making the polylactic acid molecular chains more orderly arranged during the stretching process. This orderly arrangement increases the interaction between polylactic acid molecular chains and improves the tensile strength of fruit and vegetable fresh-keeping film. At the same time, the sodium carboxymethyl cellulose in the double-layer microcapsule powder can reduce the glass transition temperature between polylactic acid molecular chains to a certain extent, so that the polylactic acid molecular chains can move under a lower external force. When the fruit and vegetable fresh-keeping film is stretched, the polylactic acid molecular chains can stretch and orient more freely, thereby increasing the elongation at break of the fruit and vegetable fresh-keeping film. This dual regulatory effect on the movement of polylactic acid molecular chains has significantly improved the tensile strength and elongation at break of the fruit and vegetable fresh-keeping film.

[0023] As a preferred technical solution of the present invention, in step (I), the concentration of gelatin and sodium carboxymethyl cellulose in the wall material solution is 15-20 g / L, for example, it can be 15 g / L, 15.5 g / L, 16 g / L, 16.5 g / L, 17 g / L, 17.5 g / L, 18 g / L, 18.5 g / L, 19 g / L, 19.5 g / L or 20 g / L, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0024] In some optional examples, the mass ratio of gelatin to sodium carboxymethyl cellulose is (6-7):1, for example, it can be 6.0:1, 6.1:1, 6.2:1, 6.3:1, 6.4:1, 6.5:1, 6.6:1, 6.7:1, 6.8:1, 6.9:1 or 7.0:1, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0025] In the process of preparing single-layer microcapsules, the mass ratio of gelatin to sodium carboxymethyl cellulose has a crucial influence on the thermal stability of single-layer microcapsules, the sustained release effect of curcumin, and the embedding rate. The collagen chains in gelatin hydrogen bond with the hydroxyl and carboxymethyl groups in sodium carboxymethyl cellulose, which enhances the cohesion of the capsule wall material and forms a tight and uniform capsule wall structure. This tight capsule wall structure can effectively prevent the rapid transfer of heat, so that the microcapsules can maintain structural integrity when heated, thereby delaying the decomposition or leakage of internal curcumin.

[0026] The present invention further defines the mass ratio of gelatin to sodium carboxymethyl cellulose as (6-7):1. When the mass ratio of gelatin to sodium carboxymethyl cellulose is controlled within this mass range, the two can work synergistically to form a capsule wall structure with ideal sustained-release performance. The water-swelling property of gelatin enables the capsule wall to gradually soften and swell in water, forming a dense gel layer, thereby slowing down the release rate of curcumin. The viscosity of sodium carboxymethyl cellulose can increase the viscosity inside the capsule wall, further extending the release time. This synergistic effect enables the microcapsule to maintain a stable release rate for a long time, meeting the sustained-release requirement for curcumin.

[0027] When the amount of gelatin used exceeds the upper limit of the range defined in the present invention, the proportion of gelatin in the capsule wall increases, which will cause the capsule wall material to become too soft and easy to deform, reducing the thermal stability of the microcapsule. Under heating conditions, the capsule wall is more likely to change shape or even rupture, causing the curcumin wrapped inside to leak. In addition, when the amount of gelatin used is too high, the swelling property of the capsule wall is too strong, causing a large amount of curcumin to be released in a short period of time, thereby losing the sustained-release effect; at the same time, too much gelatin will also increase the permeability of the capsule wall, making it easier for water to enter the capsule, accelerating the dissolution and release of curcumin. In addition, when the amount of gelatin used is too high, the capsule wall material will become too viscous and difficult to disperse, making it difficult for curcumin to be evenly wrapped inside the capsule wall, thereby reducing the embedding rate of curcumin.

[0028] On the contrary, when the amount of gelatin is lower than the lower limit of the range defined in the present invention, the proportion of sodium carboxymethyl cellulose in the capsule wall is relatively too high. Although the rigidity and stability of the capsule wall can be improved, the capsule wall material will become too hard and brittle, and it is easy to be mechanically damaged during processing or storage, which not only affects the thermal stability of the microcapsules, but also causes curcumin to leak from the capsule wall, thereby reducing the embedding rate of curcumin. In addition, when the amount of gelatin is too low, the swelling performance of the capsule wall is insufficient, and an effective gel layer cannot be formed to control the release rate of curcumin, resulting in curcumin being released into the external environment at a faster rate, and the ideal sustained release effect cannot be achieved.

[0029] In some optional examples, the mixing and stirring time is 1 to 2 hours, for example, it can be 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0030] In some optional embodiments, the heating temperature during mixing and stirring is 40-50°C, for example, it can be 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C or 50°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] In some optional examples, the ratio of the total mass of gelatin and sodium carboxymethyl cellulose in the wall material solution to the mass of the curcumin is (4-5):1, for example, 4.0:1, 4.1:1, 4.2:1, 4.3:1, 4.4:1, 4.5:1, 4.6:1, 4.7:1, 4.8:1, 4.9:1 or 5.0:1, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0032] In some optional examples, the stirring speed of the high-speed stirring emulsification of the prepolymer solution is 8000~10000rpm, for example, it can be 8000rpm, 8200rpm, 8400rpm, 8600rpm, 8800rpm, 9000rpm, 9200rpm, 9400rpm, 9600rpm, 9800rpm or 10000rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0033] In some optional examples, the high-speed stirring emulsification time of the prepolymer solution is 3 to 5 minutes, for example, it can be 3.0 minutes, 3.2 minutes, 3.4 minutes, 3.6 minutes, 3.8 minutes, 4.0 minutes, 4.2 minutes, 4.4 minutes, 4.6 minutes, 4.8 minutes or 5.0 minutes, but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] In some optional embodiments, the stirring speed during the process of dripping the acetic acid solution is 300~400rpm, for example, it can be 300rpm, 310rpm, 320rpm, 330rpm, 340rpm, 350rpm, 360rpm, 370rpm, 380rpm, 390rpm or 400rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0035] In some optional embodiments, the temperature of the water bath heating during the process of dripping the acetic acid solution is 40-50°C, for example, it can be 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, 46°C, 47°C, 48°C, 49°C or 50°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0036] In some optional examples, the acetic acid solution is added dropwise to the prepolymer solution to adjust its pH value to 4-5, for example, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9 or 5.0, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0037] The present invention specifically limits the addition of acetic acid solution to the prepolymer solution to adjust its pH value to 4-5. As the pH value of the prepolymer solution decreases, the electrostatic repulsion between gelatin and sodium carboxymethyl cellulose decreases, and the particle size of the formed single-layer microcapsules gradually increases. When the pH value of the prepolymer solution drops below 4, the particle size of the microcapsules is too large and can no longer maintain a good spherical dispersion state, and they begin to adhere to each other. However, when the pH value of the prepolymer solution exceeds 5, although microcapsules with a good spherical dispersion state can be obtained, the particle size of the obtained microcapsules is too small, resulting in too low an embedding amount of curcumin, which affects the sustained-release antibacterial effect of curcumin. Therefore, in order to take into account both the spherical dispersion state and the large particle size of the microcapsules, the present invention specifically limits the pH value of the prepolymer solution to 4-5.

[0038] In some optional embodiments, the temperature of the ice water bath is 5-15°C, for example, it can be 5°C, 6°C, 7°C, 8°C, 9°C, 10°C, 11°C, 12°C, 13°C, 14°C or 15°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0039] In some optional examples, the cooling in the ice water bath is 1 to 2 hours, for example, 1.0 h, 1.1 h, 1.2 h, 1.3 h, 1.4 h, 1.5 h, 1.6 h, 1.7 h, 1.8 h, 1.9 h or 2.0 h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0040] In some optional examples, the mixing and stirring is continued for 20 to 30 minutes after cooling to obtain the prepolymer suspension, for example, it can be 20 minutes, 21 minutes, 22 minutes, 23 minutes, 24 minutes, 25 minutes, 26 minutes, 27 minutes, 28 minutes, 29 minutes or 30 minutes, but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0041] As a preferred technical solution of the present invention, in step (II), the carboxymethyl chitosan is prepared by the following method:

[0042] Chitosan powder is mixed with sodium hydroxide and then ball-milled once to obtain a mixture, sodium chloroacetate and sodium iodide are added to the mixture, and then ball-milled twice to obtain a solid product, the solid product is uniformly mixed with deionized water to obtain a product solution, and a hydrochloric acid solution is added to the product solution to adjust the pH value of the product solution to 7; then, the product solution is filtered, washed and dried to obtain the carboxymethyl chitosan.

[0043] Chitosan is mainly composed of random copolymerization of N-acetylglucosamine and glucosamine units. The particularity of this structure gives chitosan a series of significant functional properties, such as excellent biocompatibility, non-toxicity, good biodegradability and antibacterial activity, but unmodified chitosan is only water-soluble and antibacterial under acidic conditions, which limits its scope of application. The present invention uses sodium chloroacetate to carboxymethylate chitosan, aiming to introduce hydrophilic carboxymethyl functional groups, thereby improving the water solubility and reactivity of chitosan. Compared with chitosan, carboxymethyl chitosan not only retains the original biocompatibility and degradability of chitosan, but also gives it certain antibacterial and reactivity.

[0044] The present invention uses sodium chloroacetate as a carboxymethyl modification agent and sodium iodide as a catalyst, and adopts a mechanochemical method to induce a chemical reaction, thereby introducing a carboxymethyl functional group on the chitosan molecular chain, and the introduction of the carboxymethyl functional group greatly improves the solubility of chitosan. The present invention adopts mechanical ball milling to promote the alkalization reaction and the carboxymethylation reaction. During the ball milling process, the ball milling medium and the reaction material are subjected to the combined force of rotation and revolution in the ball mill, so that the centrifugal acceleration of the ball milling medium can reach 10-20g; at the same time, the pressure between the ball milling medium and the ball mill can reach 5-6 times of the weight of the ball milling medium itself. When the chitosan molecule is subjected to a strong mechanical force of the ball milling medium, the impact force and shear force of the chitosan particles will destroy the crystal structure in the molecule. During the ball milling alkalization process, sodium hydroxide can better enter the interior of the chitosan molecule, thereby achieving the effect of alkalization expansion. At the same time, during the carboxymethylation reaction, as the hydrogen bonds within and between chitosan molecules are broken by external mechanical force, the hydroxyl and amino groups within the chitosan molecules are exposed to the outside, thereby providing an attack target for the carboxymethyl groups in the system and promoting the occurrence of the carboxymethylation reaction.

[0045] As a preferred technical solution of the present invention, the mass ratio of the chitosan powder to sodium hydroxide is 1:(3.5-4.5), for example, it can be 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9, 1:4.0, 1:4.1, 1:4.2, 1:4.3, 1:4.4 or 1:4.5, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0046] In some optional examples, the rotation speed of the primary ball milling is 500~600rpm, for example, it can be 500rpm, 510rpm, 520rpm, 530rpm, 540rpm, 550rpm, 560rpm, 570rpm, 580rpm, 590rpm or 600rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0047] In some optional examples, the time for the first ball milling is 4 to 5 hours, for example, it can be 4.0 hours, 4.1 hours, 4.2 hours, 4.3 hours, 4.4 hours, 4.5 hours, 4.6 hours, 4.7 hours, 4.8 hours, 4.9 hours or 5.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0048] In some optional examples, the mass ratio of the chitosan powder to sodium chloroacetate is 1:(3~4), for example, it can be 1:3.0, 1:3.1, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6, 1:3.7, 1:3.8, 1:3.9 or 1:4.0, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0049] The reaction of sodium chloroacetate and chitosan mainly occurs on the amino group on the chitosan molecular chain. Under alkaline conditions, sodium chloroacetate ionizes chloroacetate ions, which have nucleophilicity and can attack the amino group on the chitosan molecular chain to form the carbon anion of the intermediate. Subsequently, the intermediate undergoes intramolecular or intermolecular rearrangement to generate carboxymethyl chitosan and release chloride ions. Chitosan itself is a polymer material with poor hydrophilicity. Through carboxymethylation modification, hydrophilic carboxymethyl functional groups can be introduced to improve its hydrophilicity. The amount of sodium chloroacetate has a significant effect on the hydrophilicity of carboxymethyl chitosan. The present invention particularly limits the mass ratio of chitosan powder and sodium chloroacetate to 1: (3-4). When the amount of sodium chloroacetate is within the numerical range defined by the present invention, the carboxymethylation reaction of chitosan is carried out more completely, and the number of carboxymethyl functional groups introduced into the chitosan molecule is moderate, so that the carboxymethyl chitosan not only maintains the original excellent properties of chitosan, but also has good hydrophilicity. When the amount of sodium chloroacetate is lower than the lower limit of the range defined in the present invention, the carboxymethylation reaction is not completely carried out, and the number of carboxymethyl functional groups introduced into the chitosan molecules is insufficient, so that the hydrophilicity of the carboxymethyl chitosan is limited, and it is difficult to meet the requirements of the hydrophilicity of the fruit and vegetable fresh-keeping film. When the amount of sodium chloroacetate exceeds the upper limit of the range defined in the present invention, it will cause excessive carboxymethylation reaction, and too many carboxymethyl functional groups will be introduced into the chitosan molecules, which will not only destroy the original molecular structure of chitosan and reduce its mechanical strength, but also make the hydrophilicity of the carboxymethyl chitosan too strong, causing the fruit and vegetable fresh-keeping film to easily absorb water and deform during use, affecting the fresh-keeping effect.

[0050] In some optional examples, the mass ratio of sodium chloroacetate to sodium iodide is 1:(0.5~0.7), for example, it can be 1:0.5, 1:0.52, 1:0.54, 1:0.56, 1:0.58, 1:0.6, 1:0.62, 1:0.64, 1:0.66, 1:0.68 or 1:0.7, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0051] In some optional examples, the rotation speed of the secondary ball milling is 500~600rpm, for example, it can be 500rpm, 510rpm, 520rpm, 530rpm, 540rpm, 550rpm, 560rpm, 570rpm, 580rpm, 590rpm or 600rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0052] The invention particularly limits the rotation speed of the secondary ball milling to 500-600 rpm, and as the rotation speed of the ball milling increases, the substitution degree of the carboxymethyl functional group of the product shows a trend of first increasing and then decreasing. When the rotation speed of the ball mill is within the range of 500-600 rpm, accelerating the rotation speed of the ball milling can increase the collision probability and the number of collisions of the chitosan powder, sodium hydroxide, sodium chloroacetate and sodium iodide, and when the mechanical force is large enough, the stress generated instantly by the chitosan powder far exceeds its mechanical strength, and then the powder particles are broken, the specific surface area is increased, the contact of the chitosan powder with the sodium hydroxide, sodium chloroacetate and sodium iodide is promoted, and the reaction rate of the carboxymethyl reaction is increased, so the substitution degree of the carboxymethyl functional group of the product is gradually increased. When the ball mill speed exceeds 600rpm, the excessively fast speed causes the contact time between the reaction materials and the ball milling medium to be too short. At the same time, the particle size of the chitosan powder cannot be further reduced after reaching the limit, and the tiny particles will agglomerate into secondary particles under the action of surface energy, van der Waals force and electrostatic force. When the ball mill speed exceeds 600rpm, the agglomeration rate of the tiny particles will exceed the generation rate of the tiny particles. At this time, a large number of secondary particles will be generated in the reaction system, resulting in a sharp decrease in the specific surface area of ​​the material and a rapid decrease in the reaction rate, which ultimately leads to a decrease in the degree of substitution of the carboxymethyl functional group of the product.

[0053] In some optional embodiments, the secondary ball milling time is 5 to 6 hours, for example, it can be 5.0h, 5.1h, 5.2h, 5.3h, 5.4h, 5.5h, 5.6h, 5.7h, 5.8h, 5.9h or 6.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0054] In some optional instances, the mass fraction of the hydrochloric acid solution is 8~10wt%, for example, it can be 8.0wt%, 8.2wt%, 8.4wt%, 8.6wt%, 8.8wt%, 9.0wt%, 9.2wt%, 9.4wt%, 9.6wt%, 9.8wt% or 10.0wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0055] As a preferred technical solution of the present invention, in step (II), the mass fraction of the carboxymethyl chitosan in the chitosan solution is 1-2wt%, for example, it can be 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt% or 2.0wt%, but it is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0056] In some optional instances, the mass of the carboxymethyl chitosan is 25-30wt% of the total mass of the gelatin and the sodium carboxymethyl cellulose, for example, it can be 25wt%, 25.5wt%, 26wt%, 26.5wt%, 27wt%, 27.5wt%, 28wt%, 28.5wt%, 29wt%, 29.5wt% or 30wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0057] In the process of preparing double-layer microcapsule powder, carboxymethyl chitosan is introduced to strengthen the single-layer microcapsules in the prepolymer suspension, which not only optimizes the structural stability of the double-layer microcapsules, but also significantly improves its protection and sustained-release ability for curcumin. Among them, the amount of carboxymethyl chitosan has an important influence on the thermal stability of the double-layer microcapsule powder and the sustained-release effect of curcumin. The present invention can optimize the structural stability and sustained-release performance of the double-layer microcapsules by accurately controlling the amount of carboxymethyl chitosan, providing strong support for the protection and utilization of curcumin.

[0058] The present invention specifically limits the mass of carboxymethyl chitosan to 25-30wt% of the total mass of gelatin and sodium carboxymethyl cellulose. When the amount of carboxymethyl chitosan is controlled within this numerical range, the functional groups such as hydroxyl, amino and carboxymethyl on its molecular chain can form a good hydrogen bond network with gelatin and sodium carboxymethyl cellulose molecules, thereby enhancing the intermolecular force of the capsule wall material. This enhancement effect not only improves the mechanical strength of the capsule wall, but also significantly improves the thermal stability of the double-layer microcapsule powder. During the melt extrusion process, the dense capsule wall structure can effectively block the rapid transfer of heat, slow down the thermal decomposition rate of the curcumin wrapped in the capsule, thereby extending the thermal stability time of the double-layer microcapsule. In addition, an appropriate amount of carboxymethyl chitosan can not only strengthen the capsule wall structure, but also adjust the release rate of curcumin through its unique gel properties. There is an interaction force between the functional groups on the carboxymethyl chitosan molecular chain and the curcumin molecules, and this interaction can slow down the speed of curcumin diffusion from the capsule to the outside, thereby achieving a sustained release effect.

[0059] When the dosage of carboxymethyl chitosan is lower than 25wt%, the cross-linking density of the capsule wall material is insufficient, and the capsule wall structure is relatively loose. This loose structure is prone to deformation and rupture during melt extrusion, causing the curcumin wrapped in the capsule to be exposed to the high temperature environment in advance, accelerating the thermal decomposition process. Therefore, too low a dosage of carboxymethyl chitosan will significantly reduce the thermal stability of the double-layer microcapsules. In addition, when the dosage of carboxymethyl chitosan is too low, the gel structure inside the capsule wall is not dense enough to effectively block the rapid diffusion of curcumin molecules. Curcumin will be released in large quantities in a short period of time, resulting in poor sustained release effect; at the same time, the loose capsule wall structure will also make it easier for harmful substances such as moisture and oxygen in the external environment to penetrate into the capsule, accelerating the degradation of curcumin.

[0060] On the contrary, when the amount of carboxymethyl chitosan exceeds 30wt%, although the cross-linking density of the capsule wall material is further increased, the excessive cross-linking degree will cause the capsule wall to become too rigid and brittle. This change not only reduces the flexibility of the capsule wall, but also increases the risk of cracking of the capsule wall when heated. In addition, excessive carboxymethyl chitosan may block the microporous structure inside the capsule wall, affecting the uniform distribution and loss of heat, and reducing the overall thermal stability of the double-layer microcapsules. In addition, when the amount of carboxymethyl chitosan is too high, the capsule wall structure becomes too dense, making it difficult for curcumin molecules to diffuse outward through the capsule wall. This slow release rate cannot meet the demand for rapid release of curcumin in practical applications. At the same time, excessive carboxymethyl chitosan may also cause an imbalance in the osmotic pressure inside and outside the capsule wall, causing the capsule wall to swell or even rupture, affecting the stable release of curcumin.

[0061] In some optional examples, the mixing time of the prepolymer suspension and the chitosan solution is 30 to 40 min, for example, 30 min, 31 min, 32 min, 33 min, 34 min, 35 min, 36 min, 37 min, 38 min, 39 min or 40 min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0062] In some optional examples, the mass fraction of the alkali solution is 10~20wt%, for example, it can be 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt%, but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0063] In some optional embodiments, alkaline solution is added to the microcapsule solution to adjust its pH value to 5-6, for example, it can be 5.0, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9 or 6.0, but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0064] In some optional instances, the mass of the tannic acid is 35-40wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, for example, it can be 35wt%, 35.5wt%, 36wt%, 36.5wt%, 37wt%, 37.5wt%, 38wt%, 38.5wt%, 39wt%, 39.5wt% or 40wt%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0065] Tannic acid is a polyphenolic compound with multiple phenolic hydroxyl groups in its molecular structure. These phenolic hydroxyl groups can form hydrogen bonds and covalent bonds with amino and carboxyl functional groups in carboxymethyl chitosan molecules, thereby achieving cross-linking and curing. Through the cross-linking and curing of tannic acid, a tighter network structure is formed between carboxymethyl chitosan molecules, which not only enhances the mechanical strength of the capsule wall, but also improves its thermal stability.

[0066] The present invention specifically limits the mass of tannic acid to 35-40wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan. When the amount of tannic acid is controlled within this numerical range, its cross-linking and curing effect is most significant. An appropriate amount of tannic acid can ensure that a uniform and dense cross-linking network is formed between carboxymethyl chitosan molecules. This network structure can effectively resist the transfer of heat and the thermal motion of molecules when heated, thereby slowing down the thermal decomposition rate of curcumin wrapped in the capsule. Therefore, an appropriate amount of tannic acid can significantly improve the thermal stability of the double-layer microcapsule powder. In addition, an appropriate amount of tannic acid can not only enhance the capsule wall structure, but also adjust the release rate of curcumin through its cross-linking and curing effect. The cross-linking network between the tannic acid molecules and the carboxymethyl chitosan molecules can form a dense barrier to effectively block the rapid diffusion of curcumin molecules. At the same time, the phenolic hydroxyl groups on the tannic acid molecules can also interact with the curcumin molecules to further slow down their release rate. Therefore, an appropriate amount of tannic acid can optimize the sustained release effect of the double-layer microcapsules on curcumin.

[0067] When the amount of tannic acid is less than 35wt%, its cross-linking and curing effect is insufficient, and a sufficiently dense capsule wall structure cannot be formed. The loose capsule wall is prone to deformation and collapse when heated, resulting in leakage of curcumin wrapped in the capsule, which accelerates the thermal decomposition process of curcumin. In addition, the loose capsule wall cannot effectively prevent the rapid diffusion of curcumin molecules, resulting in a too fast release rate of curcumin, which not only shortens the release time of curcumin, but also makes the local concentration too high, leading to adverse reactions or side effects. On the contrary, when the amount of tannic acid exceeds 40wt%, its cross-linking and curing effect is too strong, making the capsule wall structure too rigid and brittle, and more prone to cracking and breaking when heated, thereby reducing the overall thermal stability of the double-layer microcapsules; at the same time, excessive tannic acid may also block the microporous structure of the capsule wall, affecting the uniform distribution and loss of heat, further exacerbating the reduction of thermal stability. In addition, when the dosage of tannic acid is too high, the cross-linked network it forms is too dense, making it difficult for curcumin molecules to diffuse outward through the capsule wall, which in turn causes a significant decrease in the release rate of curcumin, and may even lead to a situation where the curcumin cannot be released.

[0068] In some optional examples, the mixing time of the microcapsule solution and tannic acid is 3 to 4 hours, for example, 3.0 hours, 3.1 hours, 3.2 hours, 3.3 hours, 3.4 hours, 3.5 hours, 3.6 hours, 3.7 hours, 3.8 hours, 3.9 hours or 4.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0069] In some optional embodiments, the inlet air temperature of the spray drying is 130-150°C, for example, it can be 130°C, 132°C, 134°C, 136°C, 138°C, 140°C, 142°C, 144°C, 146°C, 148°C or 150°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0070] In some optional embodiments, the outlet air temperature of the spray drying is 70-80°C, for example, it can be 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C or 80°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0071] In some optional examples, the air flow pressure of the spray drying is 0.6~0.7MPa, for example, it can be 0.6MPa, 0.61MPa, 0.62MPa, 0.63MPa, 0.64MPa, 0.65MPa, 0.66MPa, 0.67MPa, 0.68MPa, 0.69MPa or 0.7MPa, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0072] As a preferred technical solution of the present invention, in step (III), the weight proportions of the components in the mixed material are as follows:

[0073] Polylactic acid 85-90 parts;

[0074] 8-12 parts of double-layer microcapsule powder;

[0075] 1~2 parts of plasticizer;

[0076] Lubricant 0.5~1.5 parts;

[0077] Antioxidant 0.1~0.5 parts.

[0078] Among them, the weight parts of polylactic acid can be 85 parts, 85.5 parts, 86 parts, 86.5 parts, 87 parts, 87.5 parts, 88 parts, 88.5 parts, 89 parts, 89.5 parts or 90 parts; the weight parts of double-layer microcapsule powder can be 8 parts, 8.5 parts, 9 parts, 9.5 parts, 10 parts, 10.5 parts, 11 parts, 11.5 parts or 12 parts; the weight parts of plasticizer can be 1.0 parts, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, 1.5 parts, 1.6 parts, 1.7 parts, The weight parts of the lubricant can be 0.5 parts, 0.6 parts, 0.7 parts, 0.8 parts, 0.9 parts, 1.0 parts, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts or 1.5 parts; the weight parts of the antioxidant can be 0.1 parts, 0.15 parts, 0.2 parts, 0.25 parts, 0.3 parts, 0.35 parts, 0.4 parts, 0.45 parts or 0.5 parts, but are not limited to the listed values, and other unlisted values ​​within the numerical range are equally applicable.

[0079] In some optional examples, the plasticizer includes any one of triacetin, tributyl citrate, dibutyl phthalate, ethylene glycol, propylene glycol, sorbitol, pentaerythritol, acetyl tributyl citrate, epoxy soybean oil, propylene glycol adipate polyester, dioctyl maleate, polyethylene glycol or polyvinyl alcohol, or a combination of at least two thereof.

[0080] In some optional examples, the lubricant includes any one of paraffin, zinc stearate, calcium stearate, polyethylene wax, glyceryl monostearate or glyceryl distearate, or a combination of at least two thereof.

[0081] In some optional examples, the antioxidant includes any one of antioxidant 1010, antioxidant 1076, antioxidant 300, antioxidant 1790, antioxidant DSTP, antioxidant 1098, antioxidant 168, and antioxidant 691, or a combination of at least two thereof.

[0082] As a preferred technical solution of the present invention, in step (III), the melt extrusion process is carried out in a screw extruder, and the barrel of the screw extruder is divided into a first zone, a second zone, a third zone, a fourth zone, a fifth zone, a sixth zone, a seventh zone and an eighth zone with different temperatures along the material flow direction.

[0083] In some optional instances, the temperature of the first zone is 150-160°C, for example, it may be 150°C, 151°C, 152°C, 153°C, 154°C, 155°C, 156°C, 157°C, 158°C, 159°C or 160°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0084] In some optional instances, the temperature of the second zone is 160-170°C, for example, it may be 160°C, 161°C, 162°C, 163°C, 164°C, 165°C, 166°C, 167°C, 168°C, 169°C or 170°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0085] In some optional instances, the temperature of the third zone is 170-180°C, for example, it may be 170°C, 171°C, 172°C, 173°C, 174°C, 175°C, 176°C, 177°C, 178°C, 179°C or 180°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0086] In some optional instances, the temperature of the fourth zone is 175-185°C, for example, it can be 175°C, 176°C, 177°C, 178°C, 179°C, 180°C, 181°C, 182°C, 183°C, 184°C or 185°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0087] In some optional examples, the temperature of the fifth zone is 185-190°C, for example, it can be 185°C, 186°C, 187°C, 188°C, 189°C or 190°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0088] In some optional examples, the temperature of the sixth zone is 190-195°C, for example, it can be 190°C, 191°C, 192°C, 193°C, 194°C or 195°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0089] In some optional examples, the temperature of the seventh zone is 200-205°C, for example, it can be 200°C, 201°C, 202°C, 203°C, 204°C or 205°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0090] In some optional examples, the temperature of the eighth zone is 195-200°C, for example, it can be 195°C, 196°C, 197°C, 198°C, 199°C or 200°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0091] In some optional instances, the head temperature of the screw extruder is 180-190°C, for example, it can be 180°C, 181°C, 182°C, 183°C, 184°C, 185°C, 186°C, 187°C, 188°C, 189°C or 190°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0092] In some optional examples, the screw speed of the screw extruder is 200~300rpm, for example, it can be 200rpm, 210rpm, 220rpm, 230rpm, 240rpm, 250rpm, 260rpm, 270rpm, 280rpm, 290rpm or 300rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0093] As a preferred technical solution of the present invention, in step (III), the film blowing process is carried out in a film blowing machine, and the interior of the barrel of the film blowing machine is divided into a ninth zone, a tenth zone, an eleventh zone, a twelfth zone and a thirteenth zone with different temperatures along the material flow direction.

[0094] In some optional instances, the temperature of the ninth zone is 175-185°C, for example, it can be 175°C, 176°C, 177°C, 178°C, 179°C, 180°C, 181°C, 182°C, 183°C, 184°C or 185°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0095] In some optional instances, the temperature of the tenth zone is 180-190°C, for example, it can be 180°C, 181°C, 182°C, 183°C, 184°C, 185°C, 186°C, 187°C, 188°C, 189°C or 190°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0096] In some optional instances, the temperature of the eleventh zone is 185-195°C, for example, it can be 185°C, 186°C, 187°C, 188°C, 189°C, 190°C, 191°C, 192°C, 193°C, 194°C or 195°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0097] In some optional instances, the temperature of the twelfth zone is 195-205°C, for example, it can be 195°C, 196°C, 197°C, 198°C, 199°C, 200°C, 201°C, 202°C, 203°C, 204°C or 205°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0098] In some optional instances, the temperature of the thirteenth zone is 190-200°C, for example, it can be 190°C, 191°C, 192°C, 193°C, 194°C, 195°C, 196°C, 197°C, 198°C, 199°C or 200°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0099] In some optional examples, the screw speed of the film blowing machine is 100~200rpm, for example, it can be 100rpm, 110rpm, 120rpm, 130rpm, 140rpm, 150rpm, 160rpm, 170rpm, 180rpm, 190rpm or 200rpm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0100] In some optional examples, the traction speed of the film blowing machine is 5~10m / min, for example, it can be 5.0m / min, 5.5m / min, 6.0m / min, 6.5m / min, 7.0m / min, 7.5m / min, 8.0m / min, 8.5m / min, 9.0m / min, 9.5m / min or 10.0m / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0101] In some optional examples, the blowing ratio of the film blowing machine is 2 to 5, for example, 2, 3, 4 or 5, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0102] In a second aspect, the present invention provides a sustained-release antibacterial fruit and vegetable fresh-keeping film prepared by the preparation method described in the first aspect, wherein the sustained-release antibacterial fruit and vegetable fresh-keeping film comprises polylactic acid, double-layer microcapsule powder, a plasticizer, a lubricant and an antioxidant.

[0103] In a third aspect, the present invention provides a use of the sustained-release antibacterial fruit and vegetable fresh-keeping film according to the second aspect, wherein the sustained-release antibacterial fruit and vegetable fresh-keeping film is used for fruit and vegetable packaging.

[0104] Compared with the prior art, the present invention has the following beneficial effects:

[0105] The present invention firstly uses curcumin as a core material, gelatin and sodium carboxymethyl cellulose as wall materials to prepare a single-layer microcapsule with a core-shell structure, then uses carboxymethyl chitosan to perform secondary coating reinforcement on the single-layer microcapsule, and uses electrostatic action and intermolecular force to form a carboxymethyl chitosan protective layer on the surface of the single-layer microcapsule, which can increase the film thickness of the curcumin surface to better resist external adverse factors, reduce the environmental sensitivity of the double-layer microcapsule, and improve its thermal stability. The present invention adds double-layer microcapsule powder to a polylactic acid matrix, obtains a fruit and vegetable fresh-keeping film after melt extrusion, cooling, pelletizing and film blowing, and significantly improves the antibacterial and mechanical properties of the fruit and vegetable fresh-keeping film. BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Figure 1 A flow chart of the preparation process of the sustained-release antibacterial fruit and vegetable fresh-keeping film provided in Examples 1-5 of the present invention;

[0107] Figure 2 The infrared spectra of gelatin, sodium carboxymethyl cellulose, carboxymethyl chitosan prepared in Example 1 of the present invention, prepolymer suspension and double-layer microcapsule powder;

[0108] Figure 3 This is an optical micrograph of the prepolymer suspension prepared in Example 1 of the present invention;

[0109] Figure 4 This is an optical micrograph of the double-layer microcapsule powder prepared in Example 1 of the present invention;

[0110] Figure 5 This is a scanning electron microscope image of the prepolymer suspension prepared in Example 1 of the present invention;

[0111] Figure 6 This is a scanning electron microscope image of the double-layer microcapsule powder prepared in Example 1 of the present invention;

[0112] Figure 7 This is a laser confocal micrograph of the prepolymer suspension prepared in Example 1 of the present invention;

[0113] Figure 8 This is a laser confocal micrograph of the double-layer microcapsule powder prepared in Example 1 of the present invention;

[0114] Fig. 9 This is the curcumin sustained-release curve of the double-layer microcapsule powder prepared in Example 1 of the present invention;

[0115] Fig.10 This is a colony diagram obtained in the antibacterial performance test of Escherichia coli for the fruit and vegetable fresh-keeping film prepared in the comparative example of the present invention;

[0116] Fig.11This is a colony diagram obtained in the antibacterial performance test of Escherichia coli of the fruit and vegetable fresh-keeping film prepared in Example 1 of the present invention;

[0117] Fig.12 This is a colony diagram obtained in a test of the antibacterial performance of Staphylococcus aureus in the fruit and vegetable fresh-keeping film prepared in the comparative example of the present invention;

[0118] Fig.13 This is a colony diagram obtained in a test of the antibacterial performance of the fruit and vegetable fresh-keeping film prepared in Example 1 of the present invention against Staphylococcus aureus;

[0119] Fig.14 The photos are taken of the fruit and vegetable fresh-keeping films prepared in Example 1 of the present invention and the comparative example in the rapeseed fresh-keeping experiment;

[0120] Fig.15 The following are actual photos of the fruit and vegetable fresh-keeping films prepared in Example 1 of the present invention and the comparative example taken in a banana fresh-keeping experiment. DETAILED DESCRIPTION

[0121] The technical solution of the present invention is described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments recorded herein are specific embodiments of the present invention, which are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be construed as limitations on the embodiments of the present invention and the scope of protection of the present invention. In addition to the embodiments recorded herein, those skilled in the art can also adopt other obvious technical solutions based on the contents disclosed in the claims of this application and its specification, including technical solutions that adopt any obvious replacements and modifications to the embodiments recorded herein.

[0122] Example 1

[0123] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, such as Figure 1 As shown, the preparation method specifically comprises the following steps:

[0124] (1) Add gelatin and sodium carboxymethyl cellulose to deionized water in a mass ratio of 6:1, mix and stir at 40°C for 2 hours to obtain a wall material solution, wherein the concentrations of gelatin and sodium carboxymethyl cellulose in the wall material solution are 15 g / L, add curcumin to the wall material solution to obtain a prepolymer solution, wherein the mass ratio of the total mass of gelatin and sodium carboxymethyl cellulose to the mass of curcumin is 4:1; emulsify the prepolymer solution at a speed of 8000 rpm for 5 minutes, and after emulsification, add acetic acid dropwise to the prepolymer solution at a stirring speed of 300 rpm and a water bath heating condition of 40°C to adjust the pH value thereof to 4, and then cool in an ice water bath at 5°C for 1 hour, and continue to mix and stir for 20 minutes after cooling to obtain a prepolymer suspension;

[0125] (2) chitosan powder and sodium hydroxide were mixed in a mass ratio of 1:3.5, and ball-milled at a speed of 500 rpm for 5 h to obtain a mixture; then, sodium chloroacetate and sodium iodide were added to the mixture, the mass ratio of chitosan powder to sodium chloroacetate was 1:3, and the mass ratio of sodium chloroacetate to sodium iodide was 1:0.5, and ball-milled for a second time at a speed of 500 rpm for 6 h to obtain a solid product; the solid product was mixed evenly with deionized water to obtain a product solution, and a hydrochloric acid solution with a mass fraction of 8 wt % was added to the product solution to adjust the pH value of the product solution to 7, and then the product solution was filtered, washed and dried to obtain carboxymethyl chitosan;

[0126] Dispersing carboxymethyl chitosan in acetic acid solution, wherein the mass fraction of acetic acid in the acetic acid solution is 5wt%, and mixing evenly to obtain a chitosan solution, wherein the mass fraction of carboxymethyl chitosan in the chitosan solution is 1wt%, adding the chitosan solution to the prepolymer suspension, wherein the mass of carboxymethyl chitosan is 25wt% of the total mass of gelatin and sodium carboxymethyl cellulose, and mixing and stirring for 30min to obtain a microcapsule solution, adding a sodium hydroxide solution with a mass fraction of 10wt% to the microcapsule solution to adjust its pH value to 5, and then adding tannic acid to the microcapsule solution, wherein the mass of tannic acid is 35wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and mixing and stirring for 3h to obtain a microcapsule suspension, spray drying the microcapsule suspension, wherein the inlet air temperature of the spray drying is 130°C, the outlet air temperature is 70°C, and the air flow pressure is 0.6MPa, and a double-layer microcapsule powder is obtained after spray drying;

[0127] (3) 85 parts of polylactic acid, 11 parts of the double-layer microcapsule powder obtained in step (2), 2 parts of ethylene glycol, 1.5 parts of calcium fatty acid and 0.5 parts of antioxidant 1010 are mixed uniformly to obtain a mixture, and the mixture is fed into a screw extruder for melt extrusion. The temperature of the first zone in the barrel of the screw extruder is 150°C, the temperature of the second zone is 160°C, the temperature of the third zone is 170°C, the temperature of the fourth zone is 175°C, the temperature of the fifth zone is 185°C, the temperature of the sixth zone is 190°C, the temperature of the seventh zone is 200°C, the temperature of the eighth zone is 195°C, the temperature of the die head is 180°C, and the screw speed is 200 rpm;

[0128] After screw extrusion, cooling and pelletizing, a composite masterbatch is obtained, and the composite masterbatch is sent to a film blowing machine for film blowing. The temperature of the ninth zone in the barrel of the film blowing machine is 175° C., the temperature of the tenth zone is 180° C., the temperature of the eleventh zone is 185° C., the temperature of the twelfth zone is 195° C., and the temperature of the thirteenth zone is 190° C. The screw speed of the film blowing machine is 100 rpm, the traction speed of the film blowing machine is 5 m / min, and the blowing ratio of the film blowing machine is 2. After film blowing, the sustained-release and antibacterial fruit and vegetable fresh-keeping film is obtained.

[0129] Figure 2 The infrared spectra of gelatin, sodium carboxymethyl cellulose, carboxymethyl chitosan, prepolymer suspension and double-layer microcapsule powder are shown in Figure 1. In the infrared curve of gelatin, 1553 cm -1 Corresponding to -NH 3 + Stretching vibration absorption peak. In the infrared curve of sodium carboxymethyl cellulose, 3430cm -1 The stretching vibration absorption peak corresponding to the hydrogen bond in the sodium carboxymethyl cellulose molecule is 2925cm -1 The corresponding CH stretching vibration absorption peak is 1600 cm -1 and 1411cm -1 The symmetric and asymmetric stretching vibration absorption peaks of -COO- correspond to the absorption peaks at 1600 cm-1 and 1600 cm-2, respectively. -1 The absorption peak at 1553 cm -1 The absorption peaks at 3 + The -COO- of sodium carboxymethylcellulose has an electrostatic interaction. In addition, gelatin has a -1 The absorption peak at 3346 cm -1 , indicating that hydrogen bonds are involved in the formation of the complex coacervate.

[0130] In the infrared curve of carboxymethyl chitosan, 3374 cm -1 The corresponding NH stretching vibration absorption peak is 2877cm -1 The corresponding CH stretching vibration absorption peak is 1079 cm -1 The corresponding stretching vibration absorption peak of CO is at 3346 cm. By comparing the infrared curves of carboxymethyl chitosan, prepolymer suspension and double-layer microcapsule powder, it can be seen that after adding carboxymethyl chitosan, the amide I band (3346 cm -1 )、Amide II band (1531cm -1 ) and amide III band (1195 cm -1 ) were shifted to 3406cm of the double-layer microcapsule powder. -1、1558cm -1 and 1199cm -1 , indicating that there is a hydrogen bonding interaction between carboxymethyl chitosan and the complex coacervate. In addition, the prepolymer suspension has a -1 With 1028cm -1 The absorption peaks at 2932 cm -1 With 1034cm -1 This is due to the electrostatic interaction between the amino group and the carbonyl group, which indicates that there is also an electrostatic attraction between carboxymethyl chitosan and the complex coacervate.

[0131] Figure 3 and Figure 4 The optical micrographs of the prepolymer suspension and the double-layer microcapsule powder prepared in this embodiment are shown respectively. It can be seen from the figures that both the single-layer microcapsule powder and the double-layer microcapsule powder maintain a good spherical dispersion state, but the particle size of the double-layer microcapsule powder is significantly larger than that of the single-layer microcapsule powder, and it can be clearly seen that a new coating layer is formed on the surface of the single-layer microcapsule powder, which is the carboxymethyl chitosan protective layer.

[0132] Figure 5 and Figure 6 The scanning electron microscope images of the prepolymer suspension and the double-layer microcapsule powder prepared in this example are shown respectively. It can be seen from the figure that the surface of the single-layer microcapsule powder in the prepolymer suspension is smooth and flat, while the surface of the double-layer microcapsule powder is rough, and a carboxymethyl chitosan protective layer is formed on the surface of the single-layer microcapsule powder.

[0133] Figure 7 and Figure 8 They are laser confocal micrographs of the prepolymer suspension and the double-layer microcapsule powder prepared in this embodiment, respectively. The laser confocal microscope can detect the distribution of gelatin, sodium carboxymethyl cellulose and chitosan on the microcapsule wall. By comparison, it can be clearly seen that the outer layer of the double-layer microcapsule is covered with a coating, namely, the carboxymethyl chitosan protective layer.

[0134] The sustained release performance of the double-layer microcapsule powder prepared in this example was tested, and the specific test steps were as follows:

[0135] (1) Accurately weigh 5 mg of curcumin, dissolve it in 10 mL of anhydrous ethanol, and then dilute it with PBS solution to make up to 50 mL. Use a pipette to transfer 0.25 mL, 1 mL, 2 mL, 3 mL, 4 mL, and 5 mL of the solution, respectively, dilute it with 20% ethanol-PBS solution and make up to 50 mL. Measure its absorbance at the maximum absorption wavelength of 425 nm and draw a standard curve of curcumin solution.

[0136] (2) Take a certain mass of double-layer microcapsule powder m0 (g), the absorbance was measured at 425 nm, and the mass m of curcumin was calculated according to the standard curve of the solution. 1 (g), the calculation formula of drug loading RDL (%) is as follows:

[0137]

[0138] (3) Take a certain mass of double-layer microcapsule powder (w 0 ) was placed in a ground-mouth conical flask, and a certain amount of PBS (pH = 7.4) solution was added to the conical flask. The absorbance was measured at 425 nm at regular intervals at room temperature, and the mass w of curcumin was calculated based on the solution standard curve. n , the cumulative release rate of curcumin R SF The calculation formula of (%) is as follows:

[0139]

[0140] The cumulative release rate of curcumin at different times was summarized and plotted, as shown in the following figure. Fig. 9 The curcumin sustained-release curve of the double-layer microcapsule powder is shown. It can be seen from the figure that the double-layer microcapsule powder is continuously released within 24 hours and has an excellent antibacterial sustained-release effect.

[0141] Example 2

[0142] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, such as Figure 1 As shown, the preparation method specifically comprises the following steps:

[0143] (1) Add gelatin and sodium carboxymethyl cellulose to deionized water in a mass ratio of 6.2:1, mix and stir at 42°C for 1.8h to obtain a wall material solution, in which the concentrations of gelatin and sodium carboxymethyl cellulose in the wall material solution are 16g / L, add curcumin to the wall material solution to obtain a prepolymer solution, in which the mass ratio of the total mass of gelatin and sodium carboxymethyl cellulose to the mass of curcumin is 4.2:1; emulsify the prepolymer solution at a speed of 8500rpm for 4.5min, and after emulsification, add acetic acid dropwise to the prepolymer solution at a stirring speed of 320rpm and a water bath heating condition of 42°C to adjust the pH value thereof to 4.2, and then cool in an ice water bath at 8°C for 1.2h, and continue to mix and stir for 22min after cooling to obtain a prepolymer suspension;

[0144] (2) chitosan powder and sodium hydroxide were mixed in a mass ratio of 1:3.8, and ball-milled for 4.8 hours at a rotation speed of 520 rpm to obtain a mixture; then, sodium chloroacetate and sodium iodide were added to the mixture, the mass ratio of chitosan powder to sodium chloroacetate was 1:3.2, and the mass ratio of sodium chloroacetate to sodium iodide was 1:0.55, and ball-milled for a second time at a rotation speed of 520 rpm for 5.8 hours to obtain a solid product; the solid product was mixed evenly with deionized water to obtain a product solution, and a hydrochloric acid solution with a mass fraction of 8.5 wt % was added to the product solution to adjust the pH value of the product solution to 7, and then the product solution was filtered, washed and dried to obtain carboxymethyl chitosan;

[0145] Disperse carboxymethyl chitosan in acetic acid solution, wherein the mass fraction of acetic acid in the acetic acid solution is 5wt%, and mix well to obtain chitosan solution, wherein the mass fraction of carboxymethyl chitosan in the chitosan solution is 1.2wt%, add chitosan solution to prepolymer suspension, wherein the mass of carboxymethyl chitosan is 26wt% of the total mass of gelatin and sodium carboxymethyl cellulose, and mix and stir for 32min to obtain microcapsule solution, add sodium hydroxide solution with a mass fraction of 12wt% to the microcapsule solution to adjust its pH value to 5.2, then add tannic acid to the microcapsule solution, wherein the mass of tannic acid is 36wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and mix and stir for 3.2h to obtain microcapsule suspension, spray dry the microcapsule suspension, wherein the inlet air temperature of the spray drying is 135°C, the outlet air temperature is 72°C, and the air flow pressure is 0.62MPa, and double-layer microcapsule powder is obtained after spray drying;

[0146] (3) 86 parts of polylactic acid, 12 parts of the double-layer microcapsule powder obtained in step (2), 1.4 parts of epoxidized soybean oil, 0.5 parts of paraffin wax and 0.1 parts of antioxidant 1076 were mixed uniformly to obtain a mixture, and the mixture was fed into a screw extruder for melt extrusion. The temperature of the first zone in the barrel of the screw extruder was 152° C., the temperature of the second zone was 162° C., the temperature of the third zone was 172° C., the temperature of the fourth zone was 178° C., the temperature of the fifth zone was 186° C., the temperature of the sixth zone was 191° C., the temperature of the seventh zone was 201° C., the temperature of the eighth zone was 196° C., the head temperature was 182° C., and the screw speed was 220 rpm;

[0147] After screw extrusion, cooling and pelletizing, the composite masterbatch is obtained, and the composite masterbatch is sent to a film blowing machine for film blowing. The temperature of the ninth zone in the barrel of the film blowing machine is 178° C., the temperature of the tenth zone is 182° C., the temperature of the eleventh zone is 188° C., the temperature of the twelfth zone is 198° C., and the temperature of the thirteenth zone is 192° C. The screw speed of the film blowing machine is 120 rpm, the traction speed of the film blowing machine is 6 m / min, and the blowing ratio of the film blowing machine is 3. After film blowing, the sustained-release and antibacterial fruit and vegetable fresh-keeping film is obtained.

[0148] Example 3

[0149] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, such as Figure 1 As shown, the preparation method specifically comprises the following steps:

[0150] (1) Add gelatin and sodium carboxymethyl cellulose to deionized water in a mass ratio of 6.5:1, mix and stir at 45°C for 1.5 hours to obtain a wall material solution, in which the concentrations of gelatin and sodium carboxymethyl cellulose in the wall material solution are 17 g / L, add curcumin to the wall material solution to obtain a prepolymer solution, in which the mass ratio of the total mass of gelatin and sodium carboxymethyl cellulose to the mass of curcumin is 4.5:1; emulsify the prepolymer solution at a speed of 9000 rpm for 4 minutes, and after emulsification, drip acetic acid into the prepolymer solution at a stirring speed of 350 rpm and a water bath heating condition of 45°C to adjust the pH value to 4.5, and then cool in an ice water bath at 10°C for 1.5 hours, and continue to mix and stir for 25 minutes after cooling to obtain a prepolymer suspension;

[0151] (2) chitosan powder and sodium hydroxide were mixed in a mass ratio of 1:4, and ball-milled for 4.5 hours at a rotation speed of 550 rpm to obtain a mixture; then, sodium chloroacetate and sodium iodide were added to the mixture, the mass ratio of chitosan powder to sodium chloroacetate was 1:3.5, and the mass ratio of sodium chloroacetate to sodium iodide was 1:0.6, and ball-milled for a second time at a rotation speed of 550 rpm for 5.5 hours to obtain a solid product; the solid product was mixed evenly with deionized water to obtain a product solution, and a hydrochloric acid solution with a mass fraction of 9 wt % was added to the product solution to adjust the pH value of the product solution to 7, and then the product solution was filtered, washed and dried to obtain carboxymethyl chitosan;

[0152] Dispersing carboxymethyl chitosan in acetic acid solution, wherein the mass fraction of acetic acid in the acetic acid solution is 5wt%, and mixing evenly to obtain a chitosan solution, wherein the mass fraction of carboxymethyl chitosan in the chitosan solution is 1.5wt%, adding the chitosan solution to the prepolymer suspension, wherein the mass of carboxymethyl chitosan is 27wt% of the total mass of gelatin and sodium carboxymethyl cellulose, and mixing and stirring for 35min to obtain a microcapsule solution, adding a sodium hydroxide solution with a mass fraction of 15wt% to the microcapsule solution to adjust its pH value to 5.5, and then adding tannic acid to the microcapsule solution, wherein the mass of tannic acid is 37wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and mixing and stirring for 3.5h to obtain a microcapsule suspension, spray drying the microcapsule suspension, wherein the inlet air temperature of the spray drying is 140°C, the outlet air temperature is 75°C, and the air flow pressure is 0.65MPa, and a double-layer microcapsule powder is obtained after spray drying;

[0153] (3) 88 parts of polylactic acid, 10 parts of the double-layer microcapsule powder obtained in step (2), 1 part of pentaerythritol, 0.7 parts of glyceryl monostearate and 0.3 parts of antioxidant DSTP are mixed uniformly to obtain a mixture, and the mixture is fed into a screw extruder for melt extrusion, wherein the temperature of the first zone in the barrel of the screw extruder is 155° C., the temperature of the second zone is 165° C., the temperature of the third zone is 175° C., the temperature of the fourth zone is 180° C., the temperature of the fifth zone is 187° C., the temperature of the sixth zone is 192° C., the temperature of the seventh zone is 202° C., the temperature of the eighth zone is 197° C., the temperature of the die head is 185° C., and the screw speed is 250 rpm;

[0154] After screw extrusion, cooling and pelletizing, the composite masterbatch is obtained, and the composite masterbatch is sent to a film blowing machine for film blowing. The temperature of the ninth zone in the barrel of the film blowing machine is 180° C., the temperature of the tenth zone is 185° C., the temperature of the eleventh zone is 190° C., the temperature of the twelfth zone is 200° C., and the temperature of the thirteenth zone is 195° C. The screw speed of the film blowing machine is 150 rpm, the traction speed of the film blowing machine is 7 m / min, and the blowing ratio of the film blowing machine is 3. After film blowing, the sustained-release antibacterial fruit and vegetable fresh-keeping film is obtained.

[0155] Example 4

[0156] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, such as Figure 1 As shown, the preparation method specifically comprises the following steps:

[0157] (1) Add gelatin and sodium carboxymethyl cellulose to deionized water in a mass ratio of 6.8:1, mix and stir at 48°C for 1.2h to obtain a wall material solution, in which the concentrations of gelatin and sodium carboxymethyl cellulose in the wall material solution are 18g / L, add curcumin to the wall material solution to obtain a prepolymer solution, in which the mass ratio of the total mass of gelatin and sodium carboxymethyl cellulose to the mass of curcumin is 4.8:1; emulsify the prepolymer solution at a speed of 9500rpm for 3.5min, and after emulsification, add acetic acid to the prepolymer solution at a stirring speed of 380rpm and a water bath heating condition of 48°C to adjust the pH value to 4.8, and then cool in an ice water bath at 12°C for 1.8h, and continue to mix and stir for 28min after cooling to obtain a prepolymer suspension;

[0158] (2) chitosan powder and sodium hydroxide were mixed in a mass ratio of 1:4.2, and ball-milled for 4.2 h at a rotation speed of 580 rpm to obtain a mixture; then, sodium chloroacetate and sodium iodide were added to the mixture, the mass ratio of chitosan powder to sodium chloroacetate was 1:3.8, and the mass ratio of sodium chloroacetate to sodium iodide was 1:0.65, and ball-milled for a second time at a rotation speed of 580 rpm for 5.2 h to obtain a solid product; the solid product was mixed evenly with deionized water to obtain a product solution, and a hydrochloric acid solution with a mass fraction of 9.5 wt % was added to the product solution to adjust the pH value of the product solution to 7, and then the product solution was filtered, washed and dried to obtain carboxymethyl chitosan;

[0159] Dispersing carboxymethyl chitosan in acetic acid solution, wherein the mass fraction of acetic acid in the acetic acid solution is 5wt%, and mixing evenly to obtain a chitosan solution, wherein the mass fraction of carboxymethyl chitosan in the chitosan solution is 1.8wt%, adding the chitosan solution to the prepolymer suspension, wherein the mass of carboxymethyl chitosan is 28wt% of the total mass of gelatin and sodium carboxymethyl cellulose, and mixing and stirring for 38min to obtain a microcapsule solution, adding a sodium hydroxide solution with a mass fraction of 18wt% to the microcapsule solution to adjust its pH value to 5.8, and then adding tannic acid to the microcapsule solution, wherein the mass of tannic acid is 38wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and mixing and stirring for 3.8h to obtain a microcapsule suspension, spray drying the microcapsule suspension, wherein the inlet air temperature of the spray drying is 145°C, the outlet air temperature is 78°C, and the air flow pressure is 0.68MPa, and a double-layer microcapsule powder is obtained after spray drying;

[0160] (3) 89 parts of polylactic acid, 9 parts of the double-layer microcapsule powder obtained in step (2), 1.2 parts of polyvinyl alcohol, 0.5 parts of glyceryl distearate and 0.3 parts of antioxidant 168 are mixed uniformly to obtain a mixture, and the mixture is fed into a screw extruder for melt extrusion, wherein the temperature of the first zone in the barrel of the screw extruder is 158° C., the temperature of the second zone is 168° C., the temperature of the third zone is 178° C., the temperature of the fourth zone is 182° C., the temperature of the fifth zone is 188° C., the temperature of the sixth zone is 193° C., the temperature of the seventh zone is 203° C., the temperature of the eighth zone is 198° C., the temperature of the die head is 188° C., and the screw speed is 280 rpm;

[0161] After screw extrusion, cooling and pelletizing, the composite masterbatch is obtained, and the composite masterbatch is sent to a film blowing machine for film blowing. The temperature of the ninth zone in the barrel of the film blowing machine is 182° C., the temperature of the tenth zone is 188° C., the temperature of the eleventh zone is 192° C., the temperature of the twelfth zone is 202° C., and the temperature of the thirteenth zone is 198° C. The screw speed of the film blowing machine is 180 rpm, the traction speed of the film blowing machine is 8 m / min, and the blowing ratio of the film blowing machine is 4. After film blowing, the sustained-release and antibacterial fruit and vegetable fresh-keeping film is obtained.

[0162] Example 5

[0163] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, such as Figure 1 As shown, the preparation method specifically comprises the following steps:

[0164] (1) Add gelatin and sodium carboxymethyl cellulose to deionized water in a mass ratio of 7:1, mix and stir at 50°C for 1 hour to obtain a wall material solution, wherein the concentration of gelatin and sodium carboxymethyl cellulose in the wall material solution is 20 g / L, add curcumin to the wall material solution to obtain a prepolymer solution, wherein the mass ratio of the total mass of gelatin and sodium carboxymethyl cellulose to the mass of curcumin is 5:1; emulsify the prepolymer solution at a speed of 10000 rpm for 3 minutes, after emulsification, add acetic acid dropwise to the prepolymer solution at a stirring speed of 400 rpm and a water bath heating condition of 50°C to adjust the pH value thereof to 5, then cool in an ice water bath at 15°C for 2 hours, and continue to mix and stir for 30 minutes after cooling to obtain a prepolymer suspension;

[0165] (2) chitosan powder and sodium hydroxide were mixed in a mass ratio of 1:4.5, and ball-milled at a speed of 600 rpm for 4 hours to obtain a mixture; then, sodium chloroacetate and sodium iodide were added to the mixture, the mass ratio of chitosan powder to sodium chloroacetate was 1:4, and the mass ratio of sodium chloroacetate to sodium iodide was 1:0.7, and ball-milled for a second time at a speed of 600 rpm for 5 hours to obtain a solid product; the solid product was mixed evenly with deionized water to obtain a product solution, and a hydrochloric acid solution with a mass fraction of 10 wt% was added to the product solution to adjust the pH value of the product solution to 7, and then the product solution was filtered, washed and dried to obtain carboxymethyl chitosan;

[0166] Dispersing carboxymethyl chitosan in acetic acid solution, wherein the mass fraction of acetic acid in the acetic acid solution is 5wt%, and mixing evenly to obtain a chitosan solution, wherein the mass fraction of carboxymethyl chitosan in the chitosan solution is 2wt%, adding the chitosan solution to the prepolymer suspension, wherein the mass of carboxymethyl chitosan is 30wt% of the total mass of gelatin and sodium carboxymethyl cellulose, and mixing and stirring for 40min to obtain a microcapsule solution, adding a sodium hydroxide solution with a mass fraction of 20wt% to the microcapsule solution to adjust its pH value to 6, and then adding tannic acid to the microcapsule solution, wherein the mass of tannic acid is 40wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and mixing and stirring for 4h to obtain a microcapsule suspension, spray drying the microcapsule suspension, wherein the inlet air temperature of the spray drying is 150°C, the outlet air temperature is 80°C, and the air flow pressure is 0.7MPa, and a double-layer microcapsule powder is obtained after spray drying;

[0167] (3) 90 parts of polylactic acid, 8 parts of the double-layer microcapsule powder obtained in step (2), 1 part of triacetin, 0.5 parts of zinc stearate and 0.5 parts of antioxidant 691 are mixed uniformly to obtain a mixture, and the mixture is fed into a screw extruder for melt extrusion. The temperature of the first zone in the barrel of the screw extruder is 160° C., the temperature of the second zone is 170° C., the temperature of the third zone is 180° C., the temperature of the fourth zone is 185° C., the temperature of the fifth zone is 190° C., the temperature of the sixth zone is 195° C., the temperature of the seventh zone is 205° C., the temperature of the eighth zone is 200° C., the temperature of the die head is 190° C., and the screw speed is 300 rpm;

[0168] After screw extrusion, cooling and pelletizing, a composite masterbatch is obtained, and the composite masterbatch is sent to a film blowing machine for film blowing. The temperature of the ninth zone in the barrel of the film blowing machine is 185° C., the temperature of the tenth zone is 190° C., the temperature of the eleventh zone is 195° C., the temperature of the twelfth zone is 205° C., and the temperature of the thirteenth zone is 200° C. The screw speed of the film blowing machine is 200 rpm, the traction speed of the film blowing machine is 10 m / min, and the blowing ratio of the film blowing machine is 5. After film blowing, the sustained-release and antibacterial fruit and vegetable fresh-keeping film is obtained.

[0169] Example 6

[0170] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (1), the mass ratio of gelatin to sodium carboxymethyl cellulose is adjusted to 5:1, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0171] Example 7

[0172] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (1), the mass ratio of gelatin to sodium carboxymethyl cellulose is adjusted to 8:1, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0173] Example 8

[0174] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (2), the mass ratio of chitosan powder to sodium chloroacetate is adjusted to 1:2, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0175] Example 9

[0176] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (2), the mass ratio of chitosan powder to sodium chloroacetate is adjusted to 1:5, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0177] Example 10

[0178] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (2), the mass of carboxymethyl chitosan is 20wt% of the total mass of gelatin and sodium carboxymethyl cellulose, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0179] Embodiment 11

[0180] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (2), the mass of carboxymethyl chitosan is 35 wt % of the total mass of gelatin and sodium carboxymethyl cellulose, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0181] Example 12

[0182] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (2), the mass of tannic acid is 30 wt % of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0183] Example 13

[0184] This embodiment provides a method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, which differs from Embodiment 1 in that, in step (2), the mass of tannic acid is 45 wt % of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan, and the other process parameters and operation steps are exactly the same as those in Embodiment 1.

[0185] Comparative Example

[0186] This comparative example provides a method for preparing a polylactic acid film. The difference from Example 1 is that steps (1) to (2) are omitted, double-layer microcapsule powder is not added to the mixed material, and the weight proportions of the other components are as follows:

[0187] Polylactic acid 97 parts;

[0188] 1 part of ethylene glycol;

[0189] 1.5 parts of calcium stearate;

[0190] Antioxidant 10100.5 parts;

[0191] Other process parameters and operation steps are exactly the same as those in Example 1.

[0192] Application Examples

[0193] The fresh-keeping film for fruits and vegetables provided in Example 1 and the comparative example was used to carry out a fresh-keeping storage experiment on rapeseed. Two identical cartons were prepared, and the fruit and vegetable fresh-keeping film provided in Example 1 was pasted on the inner wall of one of the cartons, and the fruit and vegetable fresh-keeping film provided in the comparative example was pasted on the inner wall of the other cartons. Two bunches of rapeseed with similar freshness and the same weight were placed in the two cartons respectively and sealed, and stored at 5°C. The state of the rapeseed was observed and photographed every two days. Fig.14 As shown, by comparison, it can be seen that the use of the fruit and vegetable fresh-keeping film provided in Example 1 of the present invention to preserve and store rapeseed can keep the rapeseed at a very high level of freshness on the 8th day of storage, effectively inhibits the rapeseed from losing water, maintains the color of the rapeseed, maintains its quality during storage, and extends its shelf life.

[0194] The fruit and vegetable fresh-keeping films provided in Example 1 and the comparative example were used to carry out a fresh-keeping storage experiment on bananas. Two identical cartons were prepared, and the fruit and vegetable fresh-keeping film provided in Example 1 was pasted on the inner wall of one of the cartons, and the fruit and vegetable fresh-keeping film provided in the comparative example was pasted on the inner wall of the other cartons. Two bananas of similar freshness and same weight were placed in the two cartons respectively and sealed, and stored at 5°C. The status of the bananas was observed and photographed every two days, as shown in FIG. Fig.15 As shown, by comparison, it can be seen that the use of the fruit and vegetable fresh-keeping film provided in Example 1 of the present invention to preserve and store bananas can keep the bananas very fresh on the 10th day of storage, effectively inhibit the bananas from rotting and deteriorating, maintain their quality during storage, and extend their shelf life.

[0195] The antibacterial properties and mechanical properties of the fruit and vegetable fresh-keeping films prepared in Examples 1-13 and the comparative example were tested, and the testing method was as follows:

[0196] (1) Mechanical properties: The mechanical properties of the fruit and vegetable fresh-keeping films were characterized by tensile strength and elongation at break. The tensile strength and elongation at break of the fruit and vegetable fresh-keeping films provided in Examples 1 to 13 and the comparative example were measured using a tensile testing machine, and the tensile rate was set to 50 mm / min.

[0197] (2) Antibacterial properties:

[0198] The antibacterial properties of the fruit and vegetable fresh-keeping films provided in Example 1 and the comparative example were analyzed by the antibacterial zone method. The fruit and vegetable fresh-keeping films provided in Example 1 and the comparative example were first cut into discs with a diameter of 6 mm and uniform size. Then 100 μL of a 10 6 CFU / mL of Escherichia coli suspension (model D1125B, purchased from Shanghai Roadway Technology Co., Ltd., China) and Staphylococcus aureus suspension (model E2528B, purchased from Shanghai Roadway Technology Co., Ltd., China) were evenly spread on the surface of the nutrient agar medium plate. The cut film samples were placed on the surface of the nutrient agar medium plate coated with the bacterial solution, and then placed in a 37℃ constant temperature box for 24 hours. The size of the antibacterial zone around the film sample was observed and photographed and recorded, and the following was obtained: Fig.10 , Fig.11 , Fig.12 and Fig.13 Colony images are shown.

[0199] Fig.10 and Fig.11The colony images of the fruit and vegetable fresh-keeping films prepared in the comparative example and Example 1 respectively obtained in the antibacterial performance test of Escherichia coli. By comparison, it can be seen that there is almost no antibacterial zone around the fruit and vegetable fresh-keeping film prepared in the comparative example, and there is an obvious antibacterial zone around the fruit and vegetable fresh-keeping film prepared in Example 1, which indicates that the fruit and vegetable fresh-keeping film prepared by the present invention has excellent antibacterial ability against Escherichia coli.

[0200] Fig.12 and Fig.13 The colony images are obtained in the antibacterial performance test of Staphylococcus aureus of the fruit and vegetable fresh-keeping films prepared in the comparative example and in Example 1, respectively. By comparison, it can be seen that there is almost no antibacterial zone around the periphery of the fruit and vegetable fresh-keeping film prepared in the comparative example, and there is an obvious antibacterial zone around the periphery of the fruit and vegetable fresh-keeping film prepared in Example 1, which indicates that the fruit and vegetable fresh-keeping film prepared by the present invention has excellent antibacterial ability against Staphylococcus aureus.

[0201] (4) Antibacterial properties:

[0202] The antibacterial activity of the fruit and vegetable fresh-keeping films provided in Examples 1-13 was analyzed by colony counting method. The fruit and vegetable fresh-keeping films provided in Examples 1-13 and the comparative example were cut into 3 mm×3 mm square film samples, and 100 μL of a 10 6 CFU / mL of Escherichia coli suspension (model D1125B, purchased from Shanghai Roadway Technology Co., Ltd., China) and Staphylococcus aureus suspension (model E2528B, purchased from Shanghai Roadway Technology Co., Ltd., China) were dispersed on the surface of the film sample and cultured at 37°C for 12 hours to obtain the sample; then 1500μL of PBS solution was used to wash the bacteria from the sample surface, and 20μL of PBS solution was taken and dispersed on the nutrient agar plate, and cultured at 37°C for 12 hours, after which the surviving colonies were counted. The antibacterial rate (%) was calculated using the following formula:

[0203]

[0204] Wherein, X is the number of colonies on the fruit and vegetable fresh-keeping film samples provided in the comparative example, and Y is the number of colonies on the fruit and vegetable fresh-keeping film samples provided in Examples 1-13.

[0205] The test data is shown in Table 1.

[0206] Table 1

[0207] Tensile strength MPa Elongation at break % Escherichia coli antibacterial rate% Antibacterial rate of Staphylococcus aureus % Example 1 53.7 201.3 95.8 93.4 Example 2 54.2 198.5 96.2 93.9 Example 3 55.6 195.6 96.5 94.6 Example 4 57.3 190.8 97.7 95.2 Example 5 56.5 193.7 98.2 95.5 Example 6 47.2 152.4 68.3 67.8 Example 7 48.4 148.6 72.5 71.1 Example 8 49.5 137.2 75.7 73.9 Example 9 50.9 135.9 78.2 76.2 Example 10 45.8 159.5 70.6 68.3 Embodiment 11 46.1 156.0 66.7 63.5 Example 12 43.5 170.8 63.5 59.8 Example 13 44.3 167.1 60.8 56.4 Comparative Example 32.3 78.2 — —

[0208] It can be seen from the test data provided in Table 1 that the fruit and vegetable fresh-keeping films provided in Examples 1-5 of the present invention have excellent antibacterial properties and mechanical properties.

[0209] It can be seen from the test data of Example 1, Example 6 and Example 7 that too high or too low a dosage of gelatin will directly affect the structural stability of the double-layer microcapsule powder, thereby affecting the sustained-release effect of the double-layer microcapsule powder on curcumin, and ultimately leading to a decrease in the antibacterial and mechanical properties of the fruit and vegetable preservation film.

[0210] It can be seen from the test data of Example 1, Example 8 and Example 9 that too high or too low an amount of sodium chloroacetate will directly affect the carboxymethylation modification effect of the chitosan powder, thereby affecting the various properties of the double-layer microcapsule powder, and ultimately leading to a decrease in the antibacterial and mechanical properties of the fruit and vegetable fresh-keeping film.

[0211] It can be seen from the test data of Example 1, Example 10 and Example 11 that too high or too low an amount of carboxymethyl chitosan will directly affect the structural stability of the double-layer microcapsule powder, thereby affecting the sustained-release effect of the double-layer microcapsule powder on curcumin, and ultimately leading to a decrease in the antibacterial and mechanical properties of the fruit and vegetable preservation film.

[0212] It can be seen from the test data of Example 1, Example 12 and Example 13 that too high or too low an amount of tannic acid will directly affect the structural stability of the double-layer microcapsule powder, thereby affecting the sustained-release effect of the double-layer microcapsule powder on curcumin, and ultimately leading to a decrease in the antibacterial and mechanical properties of the fruit and vegetable preservation film.

[0213] It can be seen from the test data of Example 1 and the comparative example that the present invention can significantly improve the antibacterial properties and mechanical properties of the fruit and vegetable fresh-keeping film by adding double-layer microcapsule powder into the polymer matrix.

[0214] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a sustained-release antibacterial fruit and vegetable fresh-keeping film, characterized in that: The preparation method comprises: (I) adding gelatin and sodium carboxymethyl cellulose to deionized water, mixing, stirring and heating to obtain a wall material solution, adding curcumin to the wall material solution to obtain a prepolymer solution; subjecting the prepolymer solution to high-speed stirring and emulsification, then dropping an acetic acid solution into the prepolymer solution under stirring and water bath heating conditions to adjust the pH value thereof, and cooling in an ice water bath to obtain a prepolymer suspension; (II) dispersing carboxymethyl chitosan in an acetic acid solution to obtain a chitosan solution, adding the chitosan solution to the prepolymer suspension, mixing uniformly to obtain a microcapsule solution, adding an alkali solution to the microcapsule solution to adjust the pH value thereof, then adding tannic acid to the microcapsule solution, mixing uniformly to obtain a microcapsule suspension, and spray drying the microcapsule suspension to obtain a double-layer microcapsule powder; (III) Polylactic acid, the double-layer microcapsule powder obtained in step (II), a plasticizer, a lubricant and an antioxidant are uniformly mixed to obtain a mixed material, and the mixed material is sequentially melt-extruded, cooled, pelletized and film-blown to obtain the sustained-release antibacterial fruit and vegetable fresh-keeping film.

2. The preparation method according to claim 1, characterized in that: In step (I), the concentration of gelatin and sodium carboxymethyl cellulose in the wall material solution is 15-20 g / L; The mass ratio of gelatin to sodium carboxymethyl cellulose is (6-7):1; The mixing time is 1 to 2 hours; The heating temperature during the mixing and stirring is 40-50°C; The ratio of the total mass of gelatin and sodium carboxymethyl cellulose in the wall material solution to the mass of curcumin is (4-5):1; The stirring speed of the high-speed stirring emulsification of the prepolymer solution is 8000-10000 rpm; The high-speed stirring emulsification time of the prepolymer solution is 3 to 5 minutes; The stirring speed during the process of dripping the acetic acid solution is 300~400rpm; The water bath heating temperature during the process of dripping the acetic acid solution is 40~50℃; Adding the acetic acid solution dropwise into the prepolymer solution to adjust the pH value thereof to 4-5; The temperature of the ice water bath is 5-15°C; Cool in the ice water bath for 1 to 2 hours; After cooling, the mixture is mixed and stirred for 20 to 30 minutes to obtain the prepolymer suspension.

3. The preparation method according to claim 1, characterized in that: In step (II), the carboxymethyl chitosan is prepared by the following method: Mixing chitosan powder and sodium hydroxide and then ball-milling once to obtain a mixture, adding sodium chloroacetate and sodium iodide to the mixture, mixing and then ball-milling twice to obtain a solid product, mixing the solid product with deionized water to obtain a product solution, and adding hydrochloric acid solution to the product solution to adjust the pH value of the product solution to 7; Subsequently, the product solution is filtered, washed and dried to obtain the carboxymethyl chitosan.

4. The preparation method according to claim 3, characterized in that: The mass ratio of the chitosan powder to sodium hydroxide is 1:(3.5-4.5); The rotation speed of the first ball mill is 500-600 rpm; The time of the first ball milling is 4 to 5 hours; The mass ratio of the chitosan powder to sodium chloroacetate is 1:(3-4); The mass ratio of the sodium chloroacetate to the sodium iodide is 1:(0.5-0.7); The rotation speed of the secondary ball milling is 500-600 rpm; The secondary ball milling time is 5 to 6 hours; The mass fraction of the hydrochloric acid solution is 8-10wt%.

5. The preparation method according to claim 1, characterized in that: In step (II), the mass fraction of the carboxymethyl chitosan in the chitosan solution is 1-2 wt %; The mass of the carboxymethyl chitosan is 25-30wt% of the total mass of the gelatin and the sodium carboxymethyl cellulose; The mixing time of the prepolymer suspension and the chitosan solution is 30 to 40 minutes; The mass fraction of the alkali solution is 10-20wt%; Adding alkali solution to the microcapsule solution to adjust its pH value to 5-6; The mass of the tannic acid is 35-40wt% of the total mass of gelatin, sodium carboxymethyl cellulose and carboxymethyl chitosan; The mixing time of the microcapsule solution and tannic acid is 3 to 4 hours; The inlet air temperature of the spray drying is 130-150°C; The outlet air temperature of the spray drying is 70-80°C; The air flow pressure of the spray drying is 0.6-0.7 MPa.

6. The preparation method according to claim 1, characterized in that: In step (III), the weight proportions of the components in the mixed material are as follows: Polylactic acid 85-90 parts; 8-12 parts of double-layer microcapsule powder; 1~2 parts of plasticizer; 0.5~1.5 parts of lubricant; Antioxidant 0.1~0.5 parts; The plasticizer includes any one of triacetin, tributyl citrate, dibutyl phthalate, ethylene glycol, propylene glycol, sorbitol, pentaerythritol, acetyl tributyl citrate, epoxy soybean oil, propylene glycol adipate polyester, dioctyl maleate, polyethylene glycol or polyvinyl alcohol, or a combination of at least two thereof; The lubricant includes any one of paraffin, zinc stearate, calcium stearate, polyethylene wax, glyceryl monostearate or glyceryl distearate, or a combination of at least two thereof; The antioxidant includes any one of antioxidant 1010, antioxidant 1076, antioxidant 300, antioxidant 1790, antioxidant DSTP, antioxidant 1098, antioxidant 168, and antioxidant 691, or a combination of at least two thereof.

7. The preparation method according to claim 1, characterized in that: In step (III), the melt extrusion process is carried out in a screw extruder, and the barrel of the screw extruder is divided into a first zone, a second zone, a third zone, a fourth zone, a fifth zone, a sixth zone, a seventh zone and an eighth zone with different temperatures along the material flow direction; The temperature of the first zone is 150-160°C; The temperature of the second zone is 160-170°C; The temperature of the third zone is 170-180°C; The temperature of the fourth zone is 175-185°C; The temperature of the fifth zone is 185-190°C; The temperature of the sixth zone is 190-195°C; The temperature of the seventh zone is 200-205°C; The temperature of the eighth zone is 195-200°C; The head temperature of the screw extruder is 180-190°C; The screw speed of the screw extruder is 200-300 rpm.

8. The preparation method according to claim 1, characterized in that: In step (III), the film blowing process is carried out in a film blowing machine, and the inside of the barrel of the film blowing machine is divided into a ninth zone, a tenth zone, an eleventh zone, a twelfth zone and a thirteenth zone with different temperatures along the material flow direction; The temperature of the ninth zone is 175-185°C; The temperature of the tenth zone is 180-190°C; The temperature of the eleventh zone is 185-195°C; The temperature of the twelfth zone is 195-205°C; The temperature of the thirteenth zone is 190-200°C; The screw speed of the film blowing machine is 100-200 rpm; The traction speed of the film blowing machine is 5-10 m / min; The blowing ratio of the film blowing machine is 2-5.

9. A sustained-release antibacterial fruit and vegetable fresh-keeping film prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The sustained-release antibacterial fruit and vegetable fresh-keeping film comprises polylactic acid, double-layer microcapsule powder, plasticizer, lubricant and antioxidant.

10. Use of the sustained-release antibacterial film for fruits and vegetables according to claim 9, characterized in that: The slow-release antibacterial fruit and vegetable fresh-keeping film is used for fruit and vegetable packaging.

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