Preservative film as well as preparation method and application thereof

Abstract

The invention relates to the field of food preservation, and provides a preservative film as well as a preparation method and application thereof. The preparation method comprises the following steps: loading the fruit and vegetable preservative on a gamma-cyclodextrin-metal organic framework to obtain a composite material; mixing the composite material, a hydrophobic substance and a first solvent to obtain a first solution; mixing a hydrophilic substance with water to obtain a second solution; mixing the first solution and the second solution to obtain a film forming solution, and preparing the preservative film by adopting the film forming solution. According to the invention, the composite material is dispersed into the hydrophobic film base liquid and then mixed with the water-based film base liquid, and the hydrophobic base material can rapidly wrap the composite material into a spherical shape due to hydrophilic and hydrophobic effects, so that a barrier layer is formed to isolate moisture, and the composite material is protected. According to the prepared preservative film, a complete skeleton structure of the gamma-cyclodextrin-metal organic skeleton in the water-based film base solution is kept, and the gamma-cyclodextrin-metal organic skeleton can be uniformly dispersed in the preservative film.

Description

Technical Field

[0001] The invention relates to the field of food preservation, and in particular to a fresh-keeping film and a preparation method and application thereof. Background Art

[0002] With the development of science and technology and economy, more and more vegetables and fruits can break the limitations of region and season and be sold all over the world. However, the shelf life of fruits and vegetables is often short, and they are very easy to rot and deteriorate after harvesting, resulting in huge waste and loss. Since harvesting, respiration and transpiration have become their main metabolic activities, which will consume their own nutrients and enter the aging process. At the same time, environmental changes can easily lead to physiological disorders of fruits and vegetables. Fruits and vegetables are also susceptible to mechanical damage during storage and transportation. These greatly increase the probability of fruits and vegetables being infected by microorganisms, resulting in rapid quality deterioration. Fruit and vegetable preservatives are widely available, non-toxic and harmless, and have strong antibacterial effects, but they are often unstable and volatile, which greatly limits their application in the food field.

[0003] In response to the demand for post-harvest preservation of fruits and vegetables, researchers have developed many new materials. As a research hotspot in the field of food preservation, nano-packaging materials can intervene in different spoilage factors through encapsulation, adsorption, barrier, contact, catalysis and other modes of action to achieve the purpose of extending the shelf life of food. Among them, metal organic frameworks (MOFs) are nanoporous substances composed of metal ions / clusters (linkers) and organic ligands (connectors). Metal ions / clusters and multi-legged organic ligands form secondary building units (SBUs). They have the characteristics of uniform structure, ultra-high porosity, adjustable composition and easy surface functionalization. They can adsorb ethylene, encapsulate slow-release food antimicrobial agents, and the complexes can also be composited with other materials for food preservation, which can effectively extend the shelf life of food. However, as far as MOF itself is concerned, such as ZIP-67 and Ag MOF's Zn 2+ With Ag + Metal centers are often biologically toxic, and most preparation methods are time-consuming, inefficient, dangerous, require high equipment, and are costly. In terms of loading rate, the specific surface area, porosity, pore size, and surface functionalization of MOF are the key factors that determine the loading rate. Among them, α-CD-MOF and β-CD-MOF have smaller pore sizes and low embedding efficiency. Compared with the above MOFs, γ-CD-MOF is based on K + It is a nanoporous material composed of a metal center and γ-cyclodextrin. It has a higher specific surface area and surface functionalization degree, is edible, biofriendly, and sustainable. The preparation method is simple and low-cost. It has a larger cavity pore size than α-CD-MOF and β-CD-MOF, and has a higher encapsulation efficiency.

[0004] However, in the aqueous membrane base liquid, γ-CD-MOF is easy to disintegrate, and it is difficult to obtain a cling film that maintains the integrity of the γ-CD-MOF skeleton. For this reason, researchers often use hydrophobic membrane base liquid to ensure the integrity of γ-CD-MOF. During preparation, since the hydrophobic substrate is insoluble in water, an organic solvent is required to dissolve it. However, since γ-CD-MOF has extremely high hydrophilicity, it often deposits in large quantities in the hydrophobic membrane base liquid due to the sedimentation effect, which will cause γ-CD-MOF to be unable to be evenly distributed in the prepared cling film. Summary of the invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defect in the prior art that γ-CD-MOF is difficult to be evenly distributed in the hydrophobic membrane base liquid, thereby providing a fresh-keeping film and a preparation method and application thereof.

[0006] To this end, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a fresh-keeping film, comprising the following steps:

[0008] S1, loading a fruit and vegetable preservative on a γ-cyclodextrin-metal organic framework to obtain a composite material;

[0009] mixing the composite material, the hydrophobic substance and the first solvent to obtain a first solution;

[0010] mixing a hydrophilic substance and water to obtain a second solution;

[0011] S2, mixing the first solution and the second solution to obtain a film-forming solution, and using the film-forming solution to prepare the fresh-keeping film.

[0012] In the art, generally, the contact angle of a hydrophobic substance is >90°, and the contact angle of a hydrophilic substance is <90°.

[0013] In an optional embodiment, the fruit and vegetable preservative in step S1 is a synthetic or natural substance that has antibacterial, anti-ultraviolet, anti-aging, weakening life activities, regulating physiological states and other preservative effects on the application objects, and can extend the storage time of the application objects and increase the shelf life of the application objects.

[0014] Preferably, the fruit and vegetable preservative comprises one or more of oregano essential oil, carvacrol, thymol, curcumin and dioscorea lignin.

[0015] Furthermore, the γ-cyclodextrin-metal organic framework (γ-CD-MOF) in step S1 is a micrometer-scale γ-CD-MOF or a nanometer-scale γ-CD-MOF; the γ-CD-MOF is not subjected to any functional group modification process and is prepared by conventional technical means in the art.

[0016] Preferably, the preparation method of γ-CD-MOF includes gas phase diffusion method, hydrothermal method, solvothermal method, microwave radiation method, electrochemical synthesis method, mechanochemical synthesis method or sonochemical synthesis method.

[0017] In an optional embodiment, the method for loading the fruit and vegetable preservative on the γ-cyclodextrin-metal organic framework to obtain the composite material is selected from the group consisting of impregnation, infiltration, co-crystallization, microwave-assisted, ultrasonic or high-temperature adsorption; the impregnation, infiltration, co-crystallization, microwave-assisted, ultrasonic or high-temperature adsorption is carried out by conventional technical means in the art, and each condition in the impregnation, infiltration, co-crystallization, microwave-assisted, ultrasonic or high-temperature adsorption is determined by conventional technical means in the art.

[0018] In an optional embodiment, the impregnation method includes: preparing a fruit and vegetable preservative solution, mixing the γ-cyclodextrin-metal organic framework with the fruit and vegetable preservative solution, heating and shaking, then separating the solid from the liquid, and drying the solid to obtain a composite material.

[0019] Optionally, the solvent of the fruit and vegetable preservative solution is an organic solvent.

[0020] Optionally, the concentration of the fruit and vegetable preservative in the fruit and vegetable preservative solution is 245-250 g / L.

[0021] Optionally, in the step of mixing the γ-cyclodextrin-metal organic framework with the fruit and vegetable preservative solution, the ratio of the mass of the γ-cyclodextrin-metal organic framework to the volume of the fruit and vegetable preservative solution is (1-3) g: (0.02-0.03) L.

[0022] Optionally, the heating temperature is 35-45°C.

[0023] Optionally, the oscillation speed is 175-185 rpm.

[0024] Optionally, the solid-liquid separation is performed by centrifugal separation, and the centrifugal speed is 4000-5000 rpm and the time is 5-15 min.

[0025] The size of the fruit and vegetable preservative is smaller than the pore size of the γ-cyclodextrin-metal organic framework (γ-CD-MOF), ensuring that the fruit and vegetable preservative can enter the pores of the γ-CD-MOF; during the loading process, the intermolecular contact is increased by means of shaking or thermal motion. Since cyclodextrin itself has a hydrophilic and hydrophobic structure, during the continuous contact process, the fruit and vegetable preservative molecules will be loaded into the pores of the γ-CD-MOF through hydrophilic and hydrophobic effects, covalent bond interactions, electrostatic force interactions, etc., and are mainly fixed by hydrogen bonds.

[0026] In an alternative embodiment, the hydrophobic substance comprises prolamin.

[0027] Optionally, the alcohol-soluble protein includes one or more of zein and gliadin.

[0028] In an optional embodiment, the first solvent includes an organic solvent.

[0029] Preferably, the organic solvent comprises ethanol.

[0030] Preferably, the first solvent further comprises water, and the volume fraction of the organic solvent in the first solvent is 60 to 95%.

[0031] Preferably, the mass volume ratio of the composite material, the hydrophobic substance and the first solvent is (0.2-1.5) g: (2-6) g: (80-120) mL.

[0032] In an optional embodiment, the hydrophilic substance includes at least one of polysaccharide and protein.

[0033] Preferably, the hydrophilic substance includes one or more of pectin, sodium alginate, chitosan, gelatin, carboxymethyl cellulose and methyl cellulose.

[0034] In an optional embodiment, the ratio of the mass of the hydrophilic substance to the volume of water is (2-8) g: (150-250) mL.

[0035] In an optional embodiment, the mass ratio of the first solution to the second solution is 1:(1-5).

[0036] In an optional embodiment, mixing the first solution and the second solution includes: adding the first solution dropwise into the second solution; the present invention does not specifically limit the rate of adding dropwise, and it can be determined by conventional technical means in the art, as long as the droplets can be evenly dispersed during the adding dropwise, and the hydrophobic substance will not evaporate excessively and precipitate during the entire adding dropwise process.

[0037] In an optional embodiment, the first solution is added dropwise to the second solution under stirring, and the hydrophobic substrate self-assembles when in contact with water, wraps the composite material, and forms microspheres, which are then evenly distributed under the rotational force generated by stirring.

[0038] In an optional embodiment, the second solution is divided into two parts, and the first solution is firstly added dropwise to the first part of the second solution under stirring, and then the remaining second solution is added under stirring to obtain a membrane-forming solution.

[0039] The hydrophobic substance and the hydrophilic substance do not participate in the sustained-release process during the application of the plastic wrap, and will not have any adverse effects on the objects to which the film is applied.

[0040] In the prepared membrane-forming liquid, the composite material is wrapped by the hydrophobic substrate in the form of spherical droplets, the structure of the composite material is complete, and the spherical droplets are evenly distributed in the aqueous membrane base liquid.

[0041] In an optional embodiment, the cling film is prepared using the film-forming liquid using conventional techniques in the art.

[0042] Preferably, the method for preparing the fresh-keeping film using the film-forming liquid is selected from casting, extrusion, blow molding, electrospinning, dip coating, spin coating, screen printing, inkjet printing or spray pyrolysis.

[0043] The invention also provides a fresh-keeping film prepared by the method for preparing the fresh-keeping film.

[0044] The invention also provides application of the fresh-keeping film in food preservation.

[0045] In an optional embodiment, the application comprises the following steps: using plastic wrap to wrap the opening of the container for storing food or directly using plastic wrap to wrap the food.

[0046] Preferably, the food objects used are fruits and vegetables.

[0047] Furthermore, fruits and vegetables should have obvious aerobic respiration during the harvesting and storage process. During the storage period, the water vapor produced can cause structural changes in γ-CD-MOF.

[0048] The plastic wrap changes the structural collapse speed of γ-CD-MOF according to the respiration of the fruits and vegetables used, thereby changing the release rate of the fruit and vegetable preservative. Specifically, the respiration of fruits and vegetables will bring about changes in the environmental humidity, and the water will slowly penetrate into the hydrophobic substrate of the film. The higher the humidity, the more water molecules there are and the faster the penetration. The contact between water and the γ-CD-MOF in the hydrophobic substrate will destroy its structure, thereby releasing the fruit and vegetable preservative.

[0049] The technical solution of the present invention has the following advantages:

[0050] 1. The present invention provides a method for preparing a fresh-keeping film, comprising the following steps: loading a fruit and vegetable preservative on a γ-cyclodextrin-metal organic framework to obtain a composite material; mixing the composite material, a hydrophobic substance and a first solvent to obtain a first solution; mixing a hydrophilic substance and water to obtain a second solution; mixing the first solution and the second solution to obtain a film-forming liquid, and using the film-forming liquid to prepare the fresh-keeping film.

[0051] The present invention disperses the composite material into a hydrophobic film base liquid, and then mixes it with an aqueous film base liquid. The hydrophobic substrate can quickly wrap the composite material into a spherical shape due to the hydrophilic-hydrophobic effect, forming a barrier layer to isolate moisture and protect the composite material. The present invention enables γ-CD-MOF to exist stably in a hydrophilic substrate by means of microspheres. The prepared cling film maintains a complete skeleton structure of the γ-cyclodextrin-metal organic framework in the aqueous film base liquid, and it can be evenly dispersed in the cling film. The present invention expands the application scope of γ-CD-MOF in the field of food preservation packaging.

[0052] The present invention selects a γ-cyclodextrin metal organic framework loaded with a fruit and vegetable preservative, and the complete skeleton structure of γ-CD-MOF in the preservative film can ensure that the fruit and vegetable preservative is embedded therein. The main physiological metabolic activity of fruits and vegetables after picking, respiration, produces a large amount of water, causing humidity changes. The composite material structure gradually decomposes when it encounters water, releasing the fruit and vegetable preservative to play a preservative and antibacterial role. The preservative film has a preservative sustained release function, which greatly improves the preservation effect of fruits and vegetables. The preservative film prepared by the present invention can be used as a packaging film, a liner, a preservative sheet, etc. for fruit and vegetable preservation. At the same time, the packaging film can be used as a physical barrier to isolate the influence of external factors, play a protective role to prevent mechanical damage from occurring, and effectively extend the shelf life of fruits and vegetables.

[0053] The fresh-keeping film prepared by the present invention improves the uniformity of the distribution of γ-CD-MOF in the film, ensures that the film has good integrity and mechanical strength, ensures the safety of the film in food application, and also ensures that the fruit and vegetable preservative has a suitable release rate, combines respiration with humidity changes, and dynamically preserves fruits and vegetables, thereby meeting the application requirements of sustained-release, convenient, green, sustainable, low-cost, and high-efficiency fruit and vegetable preservation.

[0054] 2. The present application uses ethanol as an organic solvent to dissolve the hydrophobic substrate, and ethanol is easily volatilized and removed; and compared with the solution of using only a hydrophobic film base liquid to prepare the cling film, the amount of organic solvent used in the present invention is significantly less, and has higher safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0056] Figure 1 is an appearance diagram of γ-CD-MOF in Example 1 of the present invention;

[0057] Figure 2is a scanning electron microscope image of γ-CD-MOF in Example 1 of the present invention;

[0058] Figure 3 is a scanning electron microscope image of Thymol-γ-CD-MOF in Example 1 of the present invention;

[0059] Figure 4 is a release curve diagram of the cling film in Example 1 of the present invention at different humidity levels;

[0060] Figure 5 is a scanning electron microscope image of the cling film obtained in Comparative Example 1 and Comparative Example 4 of the present invention;

[0061] Figure 6 This is a surface scanning electron microscope image of the cling film obtained in Comparative Example 2 of the present invention at a magnification of 400 times;

[0062] Figure 7 is a scanning electron microscope image of a cross section of the cling film obtained in Comparative Example 2 of the present invention at a magnification of 600 times;

[0063] Figure 8 is a surface scanning electron microscope image of the cling film obtained in Example 1 of the present invention at a magnification of 800 times;

[0064] Fig. 9 is a scanning electron microscope image of a cross section of the cling film obtained in Example 1 of the present invention at a magnification of 5000 times;

[0065] Fig.10 is a scanning electron microscope image of a cross section of the cling film obtained in Example 1 of the present invention at a magnification of 20,000 times;

[0066] Fig.11 It is a schematic diagram of the preservation condition of the cling film obtained by using Example 1 of the present invention and Comparative Example 3. DETAILED DESCRIPTION

[0067] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0068] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in the field can be used. If no manufacturer is specified for the reagents or instruments used, they are all conventional reagent products that can be obtained commercially.

[0069] Example 1

[0070] This embodiment provides a method for preparing a fresh-keeping film, comprising the following steps:

[0071] (1) Preparation of γ-cyclodextrin-metal organic framework (γ-CD-MOF) by gas phase diffusion method:

[0072] 810 mg γ-CD and 280 mg KOH were weighed into a 50 mL centrifuge tube, dissolved in 25 mL deionized water, and ultrasonically mixed. The mixture was filtered into a beaker through a disposable mixed cellulose esters (MCE) filter with a pore size of 0.45 μm. The beaker was placed in a sealed container with methanol at the bottom, sealed and reacted at 40 ° C for 6 h. After the reaction, the beaker in the sealed container was taken out and centrifuged to remove the formed micron-sized crystals. 200 mg hexadecyltrimethylammonium bromide (CTAB) and 25 mL methanol were added to the centrifuge successively. Crystallization was carried out at room temperature overnight. The precipitated crystals were collected by centrifugation and washed with 100 mL isopropanol. The washing steps were repeated for a total of 3 times to remove excess CTAB. Finally, the mixture was dried at 40 ° C for 12 h in a vacuum drying oven to obtain nano-sized γ-CD-MOF.

[0073] (2) The fruit and vegetable preservative is loaded on the γ-cyclodextrin-metal organic framework to obtain a composite material (Thymol-γ-CD-MOF) by an impregnation method:

[0074] Take 0.025L of 247.5g / L thymol solution (solvent is ethanol) in a 50mL conical flask, add 2g of γ-CD-MOF, mix well, wrap the conical flask with aluminum foil to form a dark environment, and place it in a shaker at 40°C and 180rpm for 24h. After the reaction is completed, centrifuge the mixture at 25°C and 4500rpm for 10min, wash with 20mL of ethanol, repeat the washing step for a total of 3 times to remove unencapsulated thymol. Finally, dry the sample in vacuum at 40°C for 12h to obtain Thymol-γ-CD-MOF.

[0075] (3) 4 g of zein was dissolved in 100 mL of 80% ethanol aqueous solution, dispersed at 10,000 rpm for 1 min, and then 0.8 g of Thymol-γ-CD-MOF was added to obtain a first solution;

[0076] Pectin was dispersed in deionized water at 70°C (the mass volume ratio of pectin to water was 2 g:100 mL) under magnetic stirring at 800 rpm for 3 h, and then centrifuged at 4000 rpm for 20 min to remove insoluble components to obtain a pectin solution;

[0077] Sodium alginate was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of sodium alginate to water was 3 g:100 mL) under the condition of magnetic stirring at 800 rpm, and after stirring for 3 h, a sodium alginate solution was obtained;

[0078] Under the condition of 70°C and 800rpm magnetic stirring, the first solution is added dropwise to the pectin solution using a syringe, and then the sodium alginate solution (the mass ratio of the first solution, the pectin solution, and the sodium alginate solution is 1:1:2) is added under the condition of 70°C and 800rpm magnetic stirring, and the mixture is evenly mixed to obtain a film-forming liquid. After the addition is completed, stirring is continued for 20 minutes to evaporate the ethanol, and the mixture is poured into a disposable culture dish and dried in a drying oven at 37°C to form a film (casting method), thereby obtaining the preservative film.

[0079] Example 2

[0080] This embodiment provides a method for preparing a fresh-keeping film, comprising the following steps:

[0081] (1) The same method as in Example 1 was used to prepare nano-sized γ-CD-MOF.

[0082] (2) Loading the fruit and vegetable preservative on the γ-cyclodextrin-metal organic framework to obtain a composite material by an impregnation method:

[0083] Take 0.02L of 245g / L oregano essential oil solution (solvent is ethanol) in a 50mL conical flask, add 3g of γ-CD-MOF, mix well, wrap the conical flask with aluminum foil to form a dark environment, and place it in a shaker at 45°C and 185rpm for 22h. After the reaction is completed, centrifuge the mixture at 25°C and 4000rpm for 13min, wash with 20mL of ethanol, repeat the washing step for a total of 3 times to remove the unencapsulated oregano essential oil. Finally, dry the sample in vacuum at 40°C for 12h to obtain a composite material.

[0084] (3) dissolving 5 g of gliadin in 90 mL of 85% ethanol aqueous solution, dispersing at 10,000 rpm for 1 min, and then adding 0.6 g of the composite material to obtain a first solution;

[0085] Pectin was dispersed in deionized water at 70°C (the mass volume ratio of pectin to water was 1 g:100 mL) under magnetic stirring at 800 rpm for 3 h, and then centrifuged at 4000 rpm for 20 min to remove insoluble components to obtain a pectin solution;

[0086] Methyl cellulose was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of methyl cellulose to water was 3 g:100 mL) under the condition of magnetic stirring at 800 rpm, and after stirring for 3 h, a methyl cellulose solution was obtained;

[0087] Under the condition of 70°C and 800rpm magnetic stirring, the first solution is added dropwise to the pectin solution using a syringe, and then the methyl cellulose solution is added (the mass ratio of the first solution, the pectin solution, and the methyl cellulose solution is 1:0.8:1.8) under the condition of 70°C and 800rpm magnetic stirring, and the mixture is evenly mixed to obtain a film-forming liquid, and the mixture is stirred for 20 minutes to evaporate the ethanol, and the mixture is poured into a disposable culture dish and dried in a drying oven at 37°C to form a film (casting method), thereby obtaining the preservative film.

[0088] Example 3

[0089] This embodiment provides a method for preparing a fresh-keeping film, comprising the following steps:

[0090] (1) The same method as in Example 1 was used to prepare nano-sized γ-CD-MOF.

[0091] (2) Loading the fruit and vegetable preservative on the γ-cyclodextrin-metal organic framework to obtain a composite material by an impregnation method:

[0092] Take 0.03L of 250g / L carvacrol solution (solvent is ethanol) in a 50mL conical flask, add 1g of γ-CD-MOF, mix well, wrap the conical flask with aluminum foil to form a dark environment, and place it in a shaker at 35°C and 175rpm for 26h. After the reaction is completed, the mixture is centrifuged at 25°C and 5000rpm for 5min, washed with 20mL of ethanol, and repeated the washing steps for a total of 3 times to remove unencapsulated carvacrol. Finally, the sample is vacuum dried at 40°C for 12h to obtain a composite material.

[0093] (3) dissolving 3 g of zein in 110 mL of 75% ethanol aqueous solution, dispersing at 10,000 rpm for 1 min, and then adding 1.0 g of the composite material to obtain a first solution;

[0094] Pectin was dispersed in deionized water at 70°C (the mass volume ratio of pectin to water was 3 g:100 mL) under magnetic stirring at 800 rpm for 3 h, and then centrifuged at 4000 rpm for 20 min to remove insoluble components to obtain a pectin solution;

[0095] Sodium alginate was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of sodium alginate to water was 4 g:100 mL) under the condition of magnetic stirring at 800 rpm, and after stirring for 3 h, a sodium alginate solution was obtained;

[0096] Under the condition of 70°C and 800rpm magnetic stirring, the first solution is added dropwise to the pectin solution using a syringe, and then the sodium alginate solution is added (the mass ratio of the first solution, the pectin solution, and the sodium alginate solution is 1:1.2:2.5) under the condition of 70°C and 800rpm magnetic stirring, and the mixture is evenly mixed to obtain a film-forming liquid, and the mixture is stirred for 20 minutes to evaporate the ethanol, and the mixture is poured into a disposable culture dish and dried in a drying oven at 37°C to form a film (casting method), thereby obtaining the preservative film.

[0097] Comparative Example 1

[0098] This comparative example provides a method for preparing a fresh-keeping film. The preparation method of Thymol-γ-CD-MOF is the same as that of Example 1, except that:

[0099] Pectin was dispersed in deionized water at 70°C (the mass volume ratio of pectin to water was 2 g:100 mL) under magnetic stirring at 800 rpm for 3 h, and then centrifuged at 4000 rpm for 20 min to remove insoluble components to obtain a pectin solution;

[0100] Sodium alginate was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of sodium alginate to water was 3 g:100 mL) under the condition of magnetic stirring at 800 rpm, and after stirring for 3 h, a sodium alginate solution was obtained;

[0101] Under the conditions of 70°C and 800 rpm magnetic stirring, 0.8 g of Thymol-γ-CD-MOF was weighed and added to 25 mL of pectin solution. Then, 50 mL of sodium alginate solution was added under the conditions of 70°C and 800 rpm magnetic stirring. The mixture was evenly mixed to obtain a film-forming liquid. The film-forming liquid was poured into a disposable culture dish and dried in a drying oven at 37°C to form a film to obtain a plastic wrap.

[0102] Comparative Example 2

[0103] This comparative example provides a method for preparing a fresh-keeping film. The preparation method of Thymol-γ-CD-MOF is the same as that of Example 1, except that:

[0104] Weigh zein, dissolve it with 80% ethanol-water solution to prepare 100 mL of zein solution with a mass concentration of 10.00 g / 100.00 mL, stir magnetically for 15 min until the zein is completely dissolved, add 2 g of Thymol-γ-CD-MOF, stir at 800 rpm and 45 °C for 30 min to obtain a film-forming solution, pour the film-forming solution into a disposable culture dish, dry it in a drying oven at 40 °C to form a film, and obtain a fresh-keeping film.

[0105] Comparative Example 3

[0106] This comparative example provides a method for preparing a fresh-keeping film, comprising the following steps:

[0107] 4 g of zein was dissolved in 100 mL of 80% ethanol aqueous solution and dispersed at 10000 rpm for 1 min to obtain a first solution; pectin was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of pectin to water was 2 g:100 mL) under the condition of 800 rpm magnetic stirring for 3 h, and then centrifuged at 4000 rpm for 20 min to remove insoluble components to obtain a pectin solution; sodium alginate was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of pectin to water was 2 g:100 mL) under the condition of 800 rpm magnetic stirring; The mass volume ratio of sodium alginate and water is 3g:100mL), and after stirring for 3h, a sodium alginate solution is obtained; under the condition of 70℃ and 800rpm magnetic stirring, the first solution is added dropwise to the pectin solution using a syringe, and then the sodium alginate solution is added (the mass ratio of the first solution, pectin solution, and sodium alginate solution is 1:1:2) under the condition of 70℃ and 800rpm magnetic stirring to obtain a film-forming solution. After the addition is completed, stirring is continued for 20min to evaporate the ethanol, and the solution is poured into a disposable culture dish and dried in a drying oven at 37℃ to form a film, thereby obtaining a fresh-keeping film.

[0108] Comparative Example 4

[0109] Pectin was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of pectin to water was 2g:100mL) under the condition of 800rpm magnetic stirring, and stirred for 3h, and then centrifuged at 4000rpm for 20min to remove insoluble components to obtain a pectin solution; sodium alginate was dispersed in deionized water at a temperature of 70°C (the mass volume ratio of sodium alginate to water was 3g:100mL) under the condition of 800rpm magnetic stirring, and stirred for 3h to obtain a sodium alginate solution; under the conditions of 70°C and 800rpm magnetic stirring, the pectin solution and the sodium alginate solution were mixed (the mass ratio of the pectin solution to the sodium alginate solution was 1:2) to obtain a film-forming solution, which was poured into a disposable culture dish and dried in a drying oven at 37°C to form a film, thereby obtaining a fresh-keeping film.

[0110] The appearance of γ-CD-MOF in Example 1 is as follows: Figure 1 As shown. Figure 1 It can be found that the γ-CD-MOF prepared in Example 1 is a white powdery solid.

[0111] The scanning electron microscopy image of γ-CD-MOF in Example 1 is as follows Figure 2 As shown; the scanning electron microscope image of Thymol-γ-CD-MOF in Example 1, as shown Figure 3 shown. Figure 2 and 3 It was shown that both γ-CD-MOF and Thymol-γ-CD-MOF were cubic in shape, indicating that γ-CD-MOF could still maintain the integrity of its skeleton structure after loading thymol.

[0112] The specific surface areas of γ-CD-MOF and Thymol-γ-CD-MOF prepared in Example 1 were analyzed, and the specific surface area of ​​γ-CD-MOF was 1,162.979 m 2 / g. At the same time, the BET analysis of Thymol-γ-CD-MOF showed that its specific surface area was reduced to 32.510m after loading with Thymol. 2 / g, which is considered to be caused by Thymol occupying its pores, indicating that a high loading effect has been achieved.

[0113] The release rate of the cling film prepared in Example 1 was tested under different humidity environments. Specifically, the cling film was subjected to a sustained release experiment under different humidity conditions (25°C, 23% RH; 25°C, 58% RH and 25°C, 85% RH). Each sample was taken out at a specific time interval, dissolved in a PBS buffer solution, and shaken at 37°C and 100 rpm for 24 hours. Finally, the absorbance of thymol was measured at 275 nm using a UV-visible spectrometer. The test was repeated three times to obtain the release curve of the cling film in Example 1 under different humidity conditions, as shown in FIG. Figure 4 As shown. Figure 4 It can be seen that the release rate of the preservative in the cling film increases with the increase of humidity. Under 85% RH, the release rate of the cling film reaches 50% after 238 hours; under 23% RH, the release rate is only 15.42% after 336 hours. Figure 4 It can also be seen that the fresh-keeping film prepared in Example 1 has a good sustained-release effect under the condition of humidity 23% RH ~ 85% RH, and can achieve a long-lasting sustained-release of the fruit and vegetable preservative, thereby extending the preservation time of fruits and vegetables.

[0114] The fresh-keeping films obtained in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 4 were observed under a scanning electron microscope. Figure 5-Figure 10 Among them, the scanning electron microscope images of the cling film obtained in Comparative Example 1 and Comparative Example 4 are as shown in FIG. Figure 5 As shown; Figure 5 The yellow arrows in the figure point to the damaged and irregular MOF structures. Figure 5 It can be seen that the morphological characterization images of the cling film obtained in Comparative Example 1 and Comparative Example 4 are not much different. No complete cubic Thymol-γ-CD-MOF was found in the cross section and surface of the cling film obtained in Comparative Example 1, which indicates that the Thymol-γ-CD-MOF in Comparative Example 1 disintegrated in the water environment. The surface scanning electron microscopy image of the cling film obtained in Comparative Example 2 at a magnification of 400 times is shown in FIG. Figure 6 As shown; Comparative Example 2 obtained plastic wrap cross-sectional scanning electron microscope image at a magnification of 600 times, as Figure 7 As shown. Figure 6-7 It can be seen that a large number of Thymol-γ-CD-MOF crystals are found on the surface and cross section of the cling film obtained in Comparative Example 2, but the crystals are piled up in large quantities, unevenly distributed and mostly deposited at the bottom, and there are many holes on the surface of the film; the surface scanning electron microscopy image of the cling film obtained in Example 1 at a magnification of 800 times, as shown in Figure 8 As shown; the cross-sectional scanning electron microscope image of the cling film obtained in Example 1 at a magnification of 5000 times, as shown Fig. 9 As shown; the cross-sectional scanning electron microscope image of the cling film obtained in Example 1 at a magnification of 20,000 times, as shown Fig.10 shown. Figure 8-Figure 10 In the figure, a large number of Thymol-γ-CD-MOF crystals can be seen on the surface and cross-section of the plastic wrap, indicating that the crystal structure is complete and evenly distributed, and the surface of the film is smoother and denser, which can successfully protect Thymol-γ-CD-MOF from disintegration in an aqueous environment and enable it to be used in more aqueous environments.

[0115] The practical application of the cling film provided in Test Example 1 and Comparative Example 3 in the preservation of Agaricus bisporus includes the following steps: select fresh Agaricus bisporus without mechanical damage, pests and diseases, with similar maturity and uniform size, randomly divide them into two groups, and place them in a container for standby; fix the cling film obtained in Example 1 to the opening of the container storing Agaricus bisporus and seal it, and fix the cling film obtained in Comparative Example 3 to the opening of the container storing Agaricus bisporus and seal it. After the treatment, put them into a 25°C constant temperature and humidity incubator for storage, observe and record the condition of Agaricus bisporus every 2 days, and obtain a schematic diagram of the preservation condition of the cling film obtained in Example 1 and Comparative Example 3, as shown in FIG. Fig.11 As shown. Fig.11It can be seen that the respiration of Agaricus bisporus is strong and produces a lot of water. The Agaricus bisporus in the blank control group began to turn brown, lose water, open the cap, and elongate the stem on the second day, and was corrupted on the sixth day, while the Thymol-γ-CD-MOF film group was still in good condition on the sixth day, indicating that the plastic wrap prepared in Example 1 has a good preservation effect on Agaricus bisporus and can extend the shelf life.

[0116] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A method for preparing a fresh-keeping film, characterized in that: The following steps are involved: S1, loading a fruit and vegetable preservative on a γ-cyclodextrin-metal organic framework to obtain a composite material; mixing the composite material, the hydrophobic substance and the first solvent to obtain a first solution; mixing a hydrophilic substance and water to obtain a second solution; S2, mixing the first solution and the second solution to obtain a film-forming solution, and using the film-forming solution to prepare the fresh-keeping film.

2. The method for preparing a fresh-keeping film according to claim 1, wherein The fruit and vegetable preservative comprises one or more of oregano essential oil, carvacrol, thymol, curcumin and diosmin.

3. The method for preparing the cling film according to claim 1, characterized in that: The method of loading the fruit and vegetable preservative on the γ-cyclodextrin-metal organic framework to obtain the composite material is selected from an impregnation method, an infiltration method, a co-crystallization method, a microwave-assisted method, an ultrasound method or a high-temperature adsorption method; Preferably, the impregnation method comprises: preparing a fruit and vegetable preservative solution, mixing the γ-cyclodextrin-metal organic framework with the fruit and vegetable preservative solution, heating and shaking, then separating the solid from the liquid, and drying the solid to obtain a composite material; Optionally, the solvent of the fruit and vegetable preservative solution is an organic solvent; Optionally, the concentration of the fruit and vegetable preservative in the fruit and vegetable preservative solution is 245-250 g / L; Optionally, in the step of mixing the γ-cyclodextrin-metal organic framework with the fruit and vegetable preservative solution, the ratio of the mass of the γ-cyclodextrin-metal organic framework to the volume of the fruit and vegetable preservative solution is (1-3) g: (0.02-0.03) L; Optionally, the heating temperature is 35-45°C; Optionally, the oscillation speed is 175-185 rpm; Optionally, the solid-liquid separation is performed by centrifugal separation, and the centrifugal speed is 4000-5000 rpm and the time is 5-15 min.

4. The method for preparing the cling film according to claim 1, characterized in that: The hydrophobic substance includes prolamin; Optionally, the alcohol-soluble protein includes one or more of zein and gliadin.

5. The method for preparing the cling film according to claim 1, characterized in that: The first solvent comprises an organic solvent; Preferably, the organic solvent comprises ethanol; Preferably, the first solvent further comprises water, and the volume fraction of the organic solvent in the first solvent is 60 to 95%; Preferably, the mass volume ratio of the composite material, the hydrophobic substance and the first solvent is (0.2-1.5) g: (2-6) g: (80-120) mL.

6. The method for preparing a fresh-keeping film according to claim 1, characterized in that: The hydrophilic substance includes at least one of a polysaccharide and a protein; Preferably, the hydrophilic substance includes one or more of pectin, sodium alginate, chitosan, gelatin, carboxymethyl cellulose and methyl cellulose.

7. The method for preparing a fresh-keeping film according to claim 1, characterized in that: The ratio of the mass of the hydrophilic substance to the volume of water is (2-8) g: (150-250) mL.

8. The method for preparing a fresh-keeping film according to claim 1, characterized in that: The mass ratio of the first solution to the second solution is 1:(1-5); Optionally, the method for preparing the fresh-keeping film using the film-forming liquid is selected from casting, extrusion, blow molding, electrospinning, dip coating, spin coating, screen printing, inkjet printing or spray pyrolysis.

9. The fresh-keeping film prepared by the method for preparing fresh-keeping film according to any one of claims 1 to 8.

10. Use of the fresh-keeping film according to claim 9 in food preservation.

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