Preparation method and application of intelligent modified atmosphere preservative film

Abstract

The invention relates to the technical field of food preservative films, and discloses a preparation method and application of an intelligent modified atmosphere preservative film, and the intelligent modified atmosphere preservative film is prepared by the following steps: adding polylactic acid into dichloromethane, stirring, adding modified Cu3 (BTC) 2, and stirring to obtain an outer layer film casting solution; adding gellan gum into deionized water, stirring, adding glycerol and an anthocyanin compound, and continuously stirring, so as to obtain an inner-layer membrane casting solution; coating a PET film with the outer-layer film casting solution, horizontally pushing and scraping the film, drying, and taking out to obtain an outer-layer PET film; and coating an outer PET film with the inner-layer film casting solution, horizontally pushing and scraping the film, drying, and stripping the PET film to obtain the modified atmosphere preservative film. The modified Cu3 (BTC) 2 is taken out and is used as an outer layer film together with polylactic acid, so that the modified Cu3 (BTC) 2 has relatively good mechanical properties and an air conditioning function; the anthocyanin compound and the gellan gum inner layer film have a fresh-keeping monitoring function and play a role in monitoring the freshness of food.

Description

Technical Field

[0001] The present invention relates to the technical field of food preservative films, and specifically to a preparation method and application of an intelligent modified atmosphere preservative film. Background Art

[0002] Fruits and vegetables are rich in nutrients and very beneficial to human health. However, they are vulnerable to microbial invasion or over-ripening and spoilage, resulting in a large amount of waste during post-harvest storage. By monitoring the changes in components such as moisture, temperature, gas, pH value, etc. inside the preservative film or in the storage environment to monitor the freshness of fruits and vegetables, and then converting the sensory changes into visual color changes to convey to consumers. Therefore, how to visually monitor the freshness of fruits and extend the shelf life is particularly important.

[0003] Cu3(BTC)2 (copper-based metal-organic framework) has a high porosity and forms a film material with a modified atmosphere function with polylactic acid, making the film material have gas permeability, which can extend the fresh-keeping period of fruits and vegetables and maintain the quality of fruits and vegetables. However, Cu3(BTC)2 is prone to shedding metal ions into fruits and vegetables, posing a potential food safety risk, and the product of Cu3(BTC)2 is not evenly dispersed in the polylactic acid matrix; anthocyanin is a natural extract and forms an inner layer film with pH responsiveness with gellan gum, which can effectively monitor the freshness of fruits and vegetables. However, anthocyanin is extremely unstable and is easily degraded when exposed to environments such as oxygen, light, pH, high temperature, enzymes, and other active compounds, thus limiting its application. Summary of the Invention

[0004] The present invention provides a preparation method and application of an intelligent modified atmosphere preservative film, which solves the problems that the uneven dispersion of Cu3(BTC)2 and the poor stability of anthocyanin affect the monitoring and fresh-keeping efficacy of the modified atmosphere preservative film.

[0005] The technical solution of the present invention:

[0006] A preparation method of an intelligent modified atmosphere preservative film includes the following preparation steps:

[0007] S1. Add polylactic acid to dichloromethane, stir at 300 - 400 r / min for 1 - 2 h, add modified Cu3(BTC)2, and stir at 30 - 40 °C for 0.5 - 1.5 h to obtain an outer layer casting solution;

[0008] S2. Add gellan gum to deionized water, stir at 300 - 400 r / min for 20 - 30 min, add glycerol and anthocyanin complex, and continue to stir for 20 - 30 min to obtain an inner layer casting solution;

[0009] S3. Coating the outer casting solution on the PET film, after flat scraping, drying at 45 - 55 °C for 2 - 5 min, taking it out to obtain the outer PET film;

[0010] S4. Coating the inner casting solution on the outer PET film, after flat scraping, drying at 45 - 55 °C for 1.5 - 2.5 h, peeling off the PET film to obtain the modified atmosphere - controlled fresh - keeping film;

[0011] The modified Cu3(BTC)2 is obtained by surface - modifying Cu3(BTC)2 with tannic acid and then mixing and reacting with tea polyphenols and nanocellulose;

[0012] The anthocyanin complex is obtained by mixing vitamin E, rosmarinic acid, procyanidins and anthocyanins.

[0013] Further, in step S1, the dosage ratio of polylactic acid, dichloromethane and modified Cu3(BTC)2 is (5 - 10) g:(80 - 100) mL:(0.15 - 0.25) g.

[0014] Further, in step S2, the dosage ratio of gellan gum, deionized water, glycerol and anthocyanin complex is (0.8 - 1.2) g:(90 - 110) mL:(0.8 - 1.2) mL:(0.06 - 0.1) g.

[0015] Further, in steps S3 and S4, the flat - scraping process: the distance between the flat plate and the scraping knife is 25 - 35 μm.

[0016] Further, the thickness of the PET film is 45 - 55 μm.

[0017] Further, the thickness of the outer PET film is 70 - 90 μm.

[0018] Further, the thickness of the modified atmosphere - controlled fresh - keeping film is 50 - 70 μm.

[0019] Further, the modified Cu3(BTC)2 is specifically prepared by the following steps:

[0020] A1. Adding trimesic acid to dimethylacetamide, stirring evenly to obtain solution a, adding copper nitrate trihydrate to dimethylacetamide, stirring evenly to obtain solution b, mixing solution a and solution b, stirring evenly, stirring and reacting at 75 - 85 °C for 10 - 14 h, cooling to room temperature, filtering, washing and drying to obtain Cu3(BTC)2;

[0021] A2. Adding Cu3(BTC)2 and tannic acid to deionized water, stirring evenly, stirring and reacting at 50 - 60 °C for 1 - 2 h, filtering, washing and drying to obtain tannic - acid - modified Cu3(BTC)2;

[0022] A3. Add nanocellulose to deionized water, stir evenly, add tea polyphenols, stir at 300 - 400 r / min for 1 - 2 h, add tannic acid - modified Cu3(BTC)2, stir until it becomes gel - like, wash the gel, and freeze - dry to obtain modified Cu3(BTC)2.

[0023] Further, in the above - mentioned reaction process of A1, trimesic acid is used as the organic ligand, and copper nitrate trihydrate provides copper ions, which react with the organic ligand to synthesize the three - dimensional porous material Cu3(BTC)2.

[0024] Further, in the above - mentioned reaction process of A2, tannic acid contains a large number of phenolic hydroxyl structures, which can combine with the hydroxyl groups on the surface of Cu3(BTC)2 through chemical bonds, enabling tannic acid to graft onto the surface of Cu3(BTC)2 to obtain tannic acid - modified Cu3(BTC)2.

[0025] Further, in the above - mentioned reaction process of A3, nanocellulose serves as the support framework, tea polyphenols can combine with the oxygen - containing functional groups in nanocellulose through chemical bonds to form a three - dimensional network aerogel structure, and the hydroxyl and carboxyl groups on the surface of tannic acid - modified Cu3(BTC)2 can also react with tea polyphenols and nanocellulose, enabling tannic acid - modified Cu3(BTC)2 to be embedded in the porous aerogel to obtain an aerogel loaded with Cu3(BTC)2, that is, modified Cu3(BTC)2.

[0026] Further, in step A1, the dosage ratio of trimesic acid to dimethylacetamide is (2 - 3) g:(45 - 55) mL.

[0027] Further, in step A1, the dosage ratio of copper nitrate trihydrate to dimethylacetamide is (4 - 6) g:(70 - 80) mL.

[0028] Further, in step A2, the dosage ratio of Cu3(BTC)2, tannic acid, and deionized water is (1 - 3) g:(5 - 7) g:(180 - 220) mL.

[0029] Further, in step A3, the dosage ratio of nanocellulose, deionized water, tea polyphenols, and tannic acid - modified Cu3(BTC)2 is (3 - 5) g:(90 - 110) mL:(2 - 3) g:(1 - 1.4) g.

[0030] Further, the anthocyanin complex is specifically prepared by the following steps:

[0031] Add vitamin E, rosmarinic acid, procyanidins, and anthocyanins to ethanol, stir at 400 - 500 r / min for 10 - 15 min, and obtain the anthocyanin complex through freeze - drying.

[0032] Furthermore, the dosage ratio of vitamin E, rosmarinic acid, proanthocyanidins, anthocyanins and ethanol is (0.1 - 0.2) g : (0.1 - 0.12) g : (0.11 - 0.13) g : (0.2 - 0.3) g : (18 - 22) mL.

[0033] The present invention has the following beneficial effects:

[0034] (1) In the technical solution of the present invention, the copper - based metal - organic framework Cu3(BTC)2 has a high specific surface area and porosity. When mixed with polylactic acid as the outer - layer membrane material, it can increase the internal diffusion channels of the membrane material, has adsorption selectivity and gas - permeation performance, is beneficial to the adsorption and diffusion of carbon dioxide and oxygen, and enables the membrane material to have a modified - atmosphere function. Grafting tannic acid on the surface of Cu3(BTC)2, on the one hand, the grafting of tannic acid on the surface of Cu3(BTC)2 makes Cu3(BTC)2 have higher activity, can be better dispersed in polylactic acid, and avoids the decline of the modified - atmosphere function of the modified - atmosphere fresh - keeping film caused by the agglomeration of Cu3(BTC)2. Moreover, the agglomeration of Cu3(BTC)2 will affect the mechanical properties of the modified - atmosphere fresh - keeping film. On the other hand, the phenolic - hydroxyl structure contained in tannic acid has excellent complexing ability with metal particles, can adsorb and fix the metal ions shed by Cu3(BTC)2 to avoid potential food - safety risks.

[0035] (2) In the technical solution of the present invention, reacting tannic - acid - modified Cu3(BTC)2, tea polyphenols and nanocellulose forms a three - dimensional network aerogel structure, namely modified Cu3(BTC)2. On the one hand, embedding tannic - acid - modified Cu3(BTC)2 into the porous aerogel increases the cross - linking density of the aerogel, has excellent mechanical strength, and the nanocellulose aerogel can absorb and weaken external stress, avoiding the collapse of pores caused by the external force acting on Cu3(BTC)2, which affects the mechanical properties and modified - atmosphere function of the modified - atmosphere fresh - keeping film. On the other hand, the tea polyphenols contained in the aerogel loaded with Cu3(BTC)2 can form coordination bonds with metal ions, have a synergistic effect with tannic acid in adsorbing and fixing the metal ions shed by Cu3(BTC)2 to avoid potential food - safety risks, and tea polyphenols can form coordination bonds with metal ions, increasing the cross - linking density of the aerogel and significantly increasing the mechanical properties and modified - atmosphere function of the modified - atmosphere fresh - keeping film.

[0036] (3) In the technical solution of the present invention, anthocyanin is a natural extract. The spoilage of food is accompanied by the generation of volatile nitrogen-containing compounds such as amines and ammonia, which causes a change in the pH environment. According to the characteristic that the color of anthocyanin changes with the change of the environmental pH value, as a gas-sensitive material for food preservation film, it reflects the freshness of food; vitamin E, rosmarinic acid, and proanthocyanidins are used as natural antioxidants, which have strong antioxidant activity and can effectively eliminate superoxide anion radicals and hydroxyl radicals. When compounded with anthocyanin, it can effectively improve the antioxidant activity of anthocyanin, avoid the easy degradation and oxidation of anthocyanin when exposed to oxygen, light, pH, high temperature, enzymes, and other active compound environments, and endow the modified atmosphere preservation film with a freshness monitoring function. In addition, vitamin E, rosmarinic acid, and proanthocyanidins can interact with anthocyanin through hydrogen bonding or π-π stacking to improve the stability of anthocyanin.

[0037] (4) In the technical solution of the present invention, the oxygen-containing functional groups contained in the anthocyanin complex can be combined with the hydroxyl groups in gellan gum through chemical bonds, so that the anthocyanin complex and gellan gum are mixed to form a stable inner membrane structure. Moreover, vitamin E, rosmarinic acid, and proanthocyanidins in the anthocyanin complex can also be combined with gellan gum through chemical bonds, enhancing the mechanical properties of the inner membrane material. And with the change of the environmental pH, the color change of the membrane material is obvious, playing a role in monitoring the freshness of food. Specific embodiments

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0039] The raw materials used in the embodiments of the present invention are as follows, and all the reagents used are of analytical grade.

[0040] Among them, the PET film has a thickness of 50 μm and is purchased from Tianjin Deli Film Co., Ltd.

[0041] The polylactic acid has a molecular weight of 7000 and is purchased from Hubei Xinghengye Technology Co., Ltd., and gellan gum is purchased from Shaanxi Ranakang Biotechnology Co., Ltd.

[0042] Nanocellulose is purchased from Huzhou Shansi New Materials Technology Co., Ltd.

[0043] Vitamin E is purchased from Xi'an Haoyu Kangze Biotechnology Co., Ltd., rosmarinic acid is purchased from Baoji Fangsheng Biological Development Co., Ltd., and proanthocyanidins are purchased from Shanghai Ruichu Biotechnology Co., Ltd.

[0044] Anthocyanin is specifically prepared by the following steps:

[0045] After black rice was crushed in a pulverizer and passed through a 100-mesh sieve, a powdery raw material was obtained. 5 g of the powdery raw material was added to a mixture of 50 mL of an acetic acid aqueous solution with a mass fraction of 2% and 50 mL of ethanol, and leached at 50 °C for 2 h. After suction filtration, it was subjected to vacuum rotary evaporation at 50 °C to obtain a concentrated solution, and the concentrated solution was freeze-dried at -20 °C for 24 h to obtain anthocyanins.

[0046] Example 1

[0047] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0048] S1. Add polylactic acid to dichloromethane, stir at 300 r / min for 1 h, add modified Cu3(BTC)2, and stir at 30 °C for 0.5 h to obtain an outer casting solution;

[0049] S2. Add gellan gum to deionized water, stir at 300 r / min for 20 min, add glycerol and an anthocyanin complex, and continue to stir for 20 min to obtain an inner casting solution;

[0050] S3. Coat 15 g of the outer casting solution on a PET film, scrape the film by flat pushing, the distance between the flat plate and the film scraping knife is 30 μm, dry at 45 °C for 2 min, and take out to obtain an outer PET film;

[0051] S4. Coat 15 g of the inner casting solution on the outer PET film, scrape the film by flat pushing, the distance between the flat plate and the film scraping knife is 30 μm, dry at 45 °C for 1.5 h, and peel off the PET film to obtain a modified atmosphere fresh-keeping film;

[0052] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane and modified Cu3(BTC)2 is 5 g: 80 mL: 0.15 g;

[0053] In step S2, the dosage ratio of gellan gum, deionized water, glycerol and the anthocyanin complex is 0.8 g: 90 mL: 0.8 mL: 0.06 g;

[0054] The thickness of the modified atmosphere fresh-keeping film is 60 μm.

[0055] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0056] A1. Add 2 g of benzene-1,3,5-tricarboxylic acid to 45 mL of dimethylacetamide, stir evenly to obtain solution a. Add 4 g of copper nitrate trihydrate to 70 mL of dimethylacetamide, stir evenly to obtain solution b. Mix solution a and solution b, stir at 25 °C for 15 min, place it in an oil bath at 75 °C, stir and react for 10 - 14 h, cool to room temperature, filter to collect the product, wash the product 5 times with dimethylacetamide, and dry it in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0057] A2. Add 1 g of Cu3(BTC)2 and 5 g of tannic acid to 180 mL of deionized water, stir evenly, stir and react at 50 °C for 1 h, filter, wash 3 times with deionized water, and dry in an oven at 70 °C for 10 min to obtain tannic acid-modified Cu3(BTC)2;

[0058] A3. Add 3 g of nanocellulose to 90 mL of deionized water, stir evenly, add 2 g of tea polyphenols, stir at 300 r / min for 1 h, add 1 g of tannic acid-modified Cu3(BTC)2, stir until it becomes gel-like, wash the gel 3 times with deionized water, and freeze-dry at -30 °C for 24 h to obtain modified Cu3(BTC)2.

[0059] The anthocyanin complex is specifically prepared by the following steps:

[0060] Add 0.1 g of vitamin E, 0.1 g of rosmarinic acid, 0.11 g of procyanidins and 0.2 g of anthocyanins to 18 mL of ethanol, stir at 400 r / min for 10 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0061] Example 2

[0062] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0063] S1. Add polylactic acid to dichloromethane, stir at 350 r / min for 1.5 h, add modified Cu3(BTC)2, and stir at 35 °C for 1 h to obtain an outer casting solution;

[0064] S2. Add gellan gum to deionized water, stir at 350 r / min for 25 min, add glycerol and the anthocyanin complex, and continue to stir for 25 min to obtain an inner casting solution;

[0065] S3. Coat 15 g of the outer casting solution on a PET film, perform flat scraping, with the distance between the flat plate and the scraping knife being 30 μm, dry at 50 °C for 3 min, and take it out to obtain an outer PET film;

[0066] S4. Coat 15 g of the inner casting solution on the outer PET film, and after flat scraping, the distance between the flat plate and the scraping knife is 30 μm. Dry it at 50 °C for 2 h, and then peel off the PET film to obtain the modified atmosphere - adjusting fresh - keeping film;

[0067] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane, and modified Cu3(BTC)2 is 8 g:90 mL:0.2 g;

[0068] In step S2, the dosage ratio of gellan gum, deionized water, glycerol, and anthocyanin complex is 1.2 g:110 mL:1.2 mL:0.1 g;

[0069] The thickness of the modified atmosphere - adjusting fresh - keeping film is 60 μm.

[0070] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0071] A1. Add 2.5 g of trimesic acid to 50 mL of dimethylacetamide, stir evenly to obtain solution a. Add 5 g of copper nitrate trihydrate to 75 mL of dimethylacetamide, stir evenly to obtain solution b. Mix solution a and solution b, stir at 25 °C for 15 min, place it in an 80 °C oil - bath pot, stir and react for 12 h, cool to room temperature, filter and collect the product. Wash the product 5 times with dimethylacetamide, and dry it in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0072] A2. Add 2 g of Cu3(BTC)2 and 6 g of tannic acid to 200 mL of deionized water, stir evenly, stir and react at 55 °C for 1.5 h, filter, wash with deionized water 3 times, and dry in an oven at 70 °C for 10 min to obtain tannic - acid - modified Cu3(BTC)2;

[0073] A3. Add 4 g of nanocellulose to 100 mL of deionized water, stir evenly, add 2.5 g of tea polyphenols, stir at 350 r / min for 1.5 h, add 1.2 g of tannic - acid - modified Cu3(BTC)2, stir until it becomes gel - like, wash the gel 3 times with deionized water, and freeze - dry at - 30 °C for 24 h to obtain modified Cu3(BTC)2.

[0074] The anthocyanin complex is specifically prepared by the following steps:

[0075] Add 0.15 g of vitamin E, 0.11 g of rosmarinic acid, 0.12 g of procyanidins, and 0.25 g of anthocyanins to 20 mL of ethanol, stir at 450 r / min for 10 - 15 min, and freeze - dry at - 20 °C for 12 h to obtain the anthocyanin complex.

[0076] Example 3

[0077] A preparation method of an intelligent modified atmosphere fresh-keeping film, comprising the following preparation steps:

[0078] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain an outer casting solution;

[0079] S2. Add gellan gum to deionized water, stir at 400 r / min for 30 min, add glycerol and anthocyanin complex, and continue to stir for 30 min to obtain an inner casting solution;

[0080] S3. Coat 15 g of the outer casting solution on a PET film, perform flat-pushing film scraping, with a distance of 30 μm between the flat plate and the film scraping knife, dry at 55 °C for 5 min, take out to obtain an outer PET film;

[0081] S4. Coat 15 g of the inner casting solution on the outer PET film, perform flat-pushing film scraping, with a distance of 30 μm between the flat plate and the film scraping knife, dry at 55 °C for 2.5 h, and peel off the PET film to obtain a modified atmosphere fresh-keeping film;

[0082] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane and modified Cu3(BTC)2 is 10 g:100 mL:0.25 g;

[0083] In step S2, the dosage ratio of gellan gum, deionized water, glycerol and anthocyanin complex is 1.2 g:110 mL:1.2 mL:0.1 g;

[0084] The thickness of the modified atmosphere fresh-keeping film is 60 μm.

[0085] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0086] A1. Add 3 g of trimesic acid to 55 mL of dimethylacetamide, stir evenly to obtain solution a, add 6 g of copper nitrate trihydrate to 80 mL of dimethylacetamide, stir evenly to obtain solution b, mix solution a and solution b, stir at 25 °C for 15 min, place in an 85 °C oil bath, stir and react for 14 h, cool to room temperature, filter and collect the product, wash the product 5 times with dimethylacetamide, and dry in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0087] A2. Add 3 g of Cu3(BTC)2 and 7 g of tannic acid to 180 mL of deionized water, stir evenly, stir and react at 60 °C for 2 h, filter, wash 3 times with deionized water, and dry in an oven at 70 °C for 10 min to obtain tannic acid-modified Cu3(BTC)2;

[0088] A3. Add 5 g of nanocellulose to 110 mL of deionized water, stir evenly, add 3 g of tea polyphenols, stir at 400 r / min for 2 h, add 1.4 g of tannic acid-modified Cu3(BTC)2, stir until it becomes gel-like, wash the gel 3 times with deionized water, and freeze-dry at -30 °C for 24 h to obtain modified Cu3(BTC)2.

[0089] The anthocyanin complex is specifically prepared by the following steps:

[0090] Add 0.2 g of vitamin E, 0.12 g of rosmarinic acid, 0.13 g of procyanidins, and 0.3 g of anthocyanins to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0091] Comparative Example 1

[0092] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0093] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain an outer casting solution;

[0094] S2. Add gellan gum to deionized water, stir at 400 r / min for 30 min, add glycerol and the anthocyanin complex, and continue to stir for 30 min to obtain an inner casting solution;

[0095] S3. Coat 15 g of the outer casting solution on a PET film, perform flat-pushing film scraping, with the distance between the flat plate and the film scraping knife being 30 μm, dry at 55 °C for 5 min, take it out, and obtain an outer PET film;

[0096] S4. Coat 15 g of the inner casting solution on the outer PET film, perform flat-pushing film scraping, with the distance between the flat plate and the film scraping knife being 30 μm, dry at 55 °C for 2.5 h, and peel off the PET film to obtain the modified atmosphere fresh-keeping film;

[0097] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane, and modified Cu3(BTC)2 is 10 g:100 mL:0.25 g;

[0098] In step S2, the dosage ratio of gellan gum, deionized water, glycerol, and the anthocyanin complex is 1.2 g:110 mL:1.2 mL:0.1 g;

[0099] The thickness of the modified atmosphere fresh-keeping film is 60 μm.

[0100] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0101] A1. Add 3 g of benzene-1,3,5-tricarboxylic acid to 55 mL of dimethylacetamide, stir evenly to obtain solution a. Add 6 g of copper(II) nitrate trihydrate to 80 mL of dimethylacetamide, stir evenly to obtain solution b. Mix solution a and solution b, stir at 25 °C for 15 min, place in an 85 °C oil bath, stir and react for 14 h, cool to room temperature, filter to collect the product, wash the product 5 times with dimethylacetamide, and dry in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0102] A2. Add 5 g of nanocellulose to 110 mL of deionized water, stir evenly, add 3 g of tea polyphenols, stir at 400 r / min for 2 h, add 1.4 g of Cu3(BTC)2, stir until it becomes gel-like, wash the gel 3 times with deionized water, and freeze-dry at -30 °C for 24 h to obtain modified Cu3(BTC)2.

[0103] The anthocyanin complex is specifically prepared by the following steps:

[0104] Add 0.2 g of vitamin E, 0.12 g of rosmarinic acid, 0.13 g of procyanidins, and 0.3 g of anthocyanins to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0105] Comparative Example 2

[0106] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0107] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain the outer casting solution;

[0108] S2. Add gellan gum to deionized water, stir at 400 r / min for 30 min, add glycerol and the anthocyanin complex, and continue to stir for 30 min to obtain the inner casting solution;

[0109] S3. Coat 15 g of the outer casting solution on a PET film, perform flat scraping, with a distance of 30 μm between the flat plate and the scraping knife, dry at 55 °C for 5 min, take out to obtain the outer PET film;

[0110] S4. Coat 15 g of the inner casting solution on the outer PET film, perform flat scraping, with a distance of 30 μm between the flat plate and the scraping knife, dry at 55 °C for 2.5 h, and peel off the PET film to obtain the modified atmosphere fresh-keeping film;

[0111] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane, and modified Cu3(BTC)2 is 10 g:100 mL:0.25 g;

[0112] In step S2, the dosage ratio of gellan gum, deionized water, glycerol and anthocyanin complex is 1.2 g: 110 mL: 1.2 mL: 0.1 g;

[0113] The thickness of the modified atmosphere fresh-keeping film is 60 μm.

[0114] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0115] A1. Add 3 g of trimesic acid to 55 mL of dimethylacetamide, stir evenly to obtain solution a, add 6 g of copper nitrate trihydrate to 80 mL of dimethylacetamide, stir evenly to obtain solution b, mix solution a and solution b, stir at 25 °C for 15 min, place it in an 85 °C oil bath, stir and react for 14 h, cool to room temperature, filter and collect the product, wash the product with dimethylacetamide 5 times, and dry it in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0116] A2. Add 3 g of Cu3(BTC)2 and 7 g of tannic acid to 180 mL of deionized water, stir evenly, stir and react at 60 °C for 2 h, filter, wash with deionized water 3 times, and dry in an oven at 70 °C for 10 min to obtain tannic acid-modified Cu3(BTC)2;

[0117] A3. Add 5 g of nanocellulose to 110 mL of deionized water, stir at 400 r / min for 2 h, add 1.4 g of tannic acid-modified Cu3(BTC)2, stir until it becomes gel-like, wash the gel with deionized water 3 times, and freeze-dry at -30 °C for 24 h to obtain the modified Cu3(BTC)2.

[0118] The anthocyanin complex is specifically prepared by the following steps:

[0119] Add 0.2 g of vitamin E, 0.12 g of rosmarinic acid, 0.13 g of procyanidins and 0.3 g of anthocyanins to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0120] Comparative Example 3

[0121] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0122] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add the modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain the outer layer casting solution;

[0123] S2. Add gellan gum into deionized water, stir at 400 r / min for 30 min, add glycerol and anthocyanin complex, and continue to stir for 30 min to obtain the inner casting solution.

[0124] S3. Coat 15 g of the outer casting solution on the PET film, perform flat-push scraping with a distance of 30 μm between the flat plate and the scraping knife, dry at 55 °C for 5 min, take out to obtain the outer PET film.

[0125] S4. Coat 15 g of the inner casting solution on the outer PET film, perform flat-push scraping with a distance of 30 μm between the flat plate and the scraping knife, dry at 55 °C for 2.5 h, peel off the PET film to obtain the modified atmosphere preservation film.

[0126] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane and modified Cu3(BTC)2 is 10 g: 100 mL: 0.25 g.

[0127] In step S2, the dosage ratio of gellan gum, deionized water, glycerol and anthocyanin complex is 1.2 g: 110 mL: 1.2 mL: 0.1 g.

[0128] The thickness of the modified atmosphere preservation film is 60 μm.

[0129] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0130] A1. Add 3 g of trimesic acid into 55 mL of dimethylacetamide, stir evenly to obtain solution a, add 6 g of copper nitrate trihydrate into 80 mL of dimethylacetamide, stir evenly to obtain solution b, mix solution a and solution b, stir at 25 °C for 15 min, place in an 85 °C oil bath, stir and react for 14 h, cool to room temperature, filter and collect the product, wash the product with dimethylacetamide 5 times, and dry in an oven at 100 °C for 24 h to obtain Cu3(BTC)2.

[0131] A2. Add 3 g of Cu3(BTC)2 and 7 g of tannic acid into 180 mL of deionized water, stir evenly, stir and react at 60 °C for 2 h, filter, wash with deionized water 3 times, and dry in an oven at 70 °C for 10 min to obtain tannic acid-modified Cu3(BTC)2.

[0132] A3. Add 110 mL of deionized water and 3 g of tea polyphenols, stir at 400 r / min for 2 h, add 1.4 g of tannic acid-modified Cu3(BTC)2, and stir evenly to obtain the modified Cu3(BTC)2.

[0133] The anthocyanin complex is specifically prepared by the following steps:

[0134] Add 0.2 g of vitamin E, 0.12 g of rosmarinic acid, 0.13 g of procyanidins, and 0.3 g of anthocyanins to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0135] In Comparative Example 4, vitamin E was not added.

[0136] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0137] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain the outer casting solution.

[0138] S2. Add gellan gum to deionized water, stir at 400 r / min for 30 min, add glycerol and the anthocyanin complex, and continue to stir for 30 min to obtain the inner casting solution.

[0139] S3. Coat 15 g of the outer casting solution on a PET film, perform flat scraping, with the distance between the flat plate and the scraping knife being 30 μm, dry at 55 °C for 5 min, take out, and obtain the outer PET film.

[0140] S4. Coat 15 g of the inner casting solution on the outer PET film, perform flat scraping, with the distance between the flat plate and the scraping knife being 30 μm, dry at 55 °C for 2.5 h, peel off the PET film, and obtain the modified atmosphere fresh-keeping film.

[0141] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane, and modified Cu3(BTC)2 is 10 g: 100 mL: 0.25 g;

[0142] In step S2, the dosage ratio of gellan gum, deionized water, glycerol, and the anthocyanin complex is 1.2 g: 110 mL: 1.2 mL: 0.1 g;

[0143] The thickness of the modified atmosphere fresh-keeping film is 60 μm.

[0144] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0145] A1. Add 3 g of trimesic acid to 55 mL of dimethylacetamide, stir evenly to obtain solution a, add 6 g of copper nitrate trihydrate to 80 mL of dimethylacetamide, stir evenly to obtain solution b, mix solution a and solution b, stir at 25 °C for 15 min, place in an 85 °C oil bath, stir and react for 14 h, cool to room temperature, filter and collect the product, wash the product 5 times with dimethylacetamide, and dry in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0146] A2. Add 3 g of Cu3(BTC)2 and 7 g of tannic acid to 180 mL of deionized water, stir evenly, stir and react at 60 °C for 2 h, filter, wash with deionized water 3 times, and dry in an oven at 70 °C for 10 min to obtain tannic acid-modified Cu3(BTC)2;

[0147] A3. Add 5 g of nanocellulose to 110 mL of deionized water, stir evenly, add 3 g of tea polyphenols, stir at 400 r / min for 2 h, add 1.4 g of tannic acid-modified Cu3(BTC)2, stir until it becomes gel-like, wash the gel with deionized water 3 times, and freeze-dry at -30 °C for 24 h to obtain modified Cu3(BTC)2.

[0148] The anthocyanin complex is specifically prepared by the following steps:

[0149] Add 0.12 g of rosmarinic acid, 0.13 g of procyanidin, and 0.3 g of anthocyanin to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0150] Comparative Example 5

[0151] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0152] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain an outer casting solution;

[0153] S2. Add gellan gum to deionized water, stir at 400 r / min for 30 min, add glycerol and the anthocyanin complex, and continue to stir for 30 min to obtain an inner casting solution;

[0154] S3. Coat 15 g of the outer casting solution on a PET film, perform flat scraping, with a distance of 30 μm between the flat plate and the scraping knife, dry at 55 °C for 5 min, take out to obtain an outer PET film;

[0155] S4. Coat 15 g of the inner casting solution on the outer PET film, perform flat scraping, with a distance of 30 μm between the flat plate and the scraping knife, dry at 55 °C for 2.5 h, and peel off the PET film to obtain the modified atmosphere fresh-keeping film;

[0156] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane, and modified Cu3(BTC)2 is 10 g:100 mL:0.25 g;

[0157] In step S2, the dosage ratio of gellan gum, deionized water, glycerol, and the anthocyanin complex is 1.2 g:110 mL:1.2 mL:0.1 g;

[0158] The thickness of the modified atmosphere fresh-keeping film is 60 μm.

[0159] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0160] A1. Add 3 g of trimesic acid to 55 mL of dimethylacetamide, stir evenly to obtain solution a. Add 6 g of copper nitrate trihydrate to 80 mL of dimethylacetamide, stir evenly to obtain solution b. Mix solution a and solution b, stir at 25 °C for 15 min, place it in an 85 °C oil bath, stir and react for 14 h, cool to room temperature, filter to collect the product, wash the product 5 times with dimethylacetamide, and dry it in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0161] A2. Add 3 g of Cu3(BTC)2 and 7 g of tannic acid to 180 mL of deionized water, stir evenly, stir and react at 60 °C for 2 h, filter, wash 3 times with deionized water, and dry in an oven at 70 °C for 10 min to obtain tannic acid-modified Cu3(BTC)2;

[0162] A3. Add 5 g of nanocellulose to 110 mL of deionized water, stir evenly, add 3 g of tea polyphenols, stir at 400 r / min for 2 h, add 1.4 g of tannic acid-modified Cu3(BTC)2, stir until it becomes gel-like, wash the gel 3 times with deionized water, and freeze-dry at -30 °C for 24 h to obtain the modified Cu3(BTC)2.

[0163] The anthocyanin complex is specifically prepared by the following steps:

[0164] Add 0.2 g of vitamin E, 0.13 g of procyanidins and 0.3 g of anthocyanins to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0165] Comparative Example 6

[0166] A preparation method of an intelligent modified atmosphere fresh-keeping film includes the following preparation steps:

[0167] S1. Add polylactic acid to dichloromethane, stir at 400 r / min for 2 h, add the modified Cu3(BTC)2, and stir at 40 °C for 1.5 h to obtain the outer layer casting solution;

[0168] S2. Add gellan gum to deionized water, stir at 400 r / min for 30 min, add glycerol and the anthocyanin complex, and continue to stir for 30 min to obtain the inner layer casting solution;

[0169] S3. Coat 15 g of the outer casting solution on the PET film, and perform flat scraping. The distance between the flat plate and the scraping knife is 30 μm. Dry it at 55 °C for 5 min, take it out, and obtain the outer PET film;

[0170] S4. Coat 15 g of the inner casting solution on the outer PET film, and perform flat scraping. The distance between the flat plate and the scraping knife is 30 μm. Dry it at 55 °C for 2.5 h, peel off the PET film, and obtain the modified atmosphere - controlled fresh - keeping film;

[0171] Among them, in step S1, the dosage ratio of polylactic acid, dichloromethane, and modified Cu3(BTC)2 is 10 g:100 mL:0.25 g;

[0172] In step S2, the dosage ratio of gellan gum, deionized water, glycerol, and anthocyanin complex is 1.2 g:110 mL:1.2 mL:0.1 g;

[0173] The thickness of the modified atmosphere - controlled fresh - keeping film is 60 μm.

[0174] The modified Cu3(BTC)2 is specifically prepared by the following steps:

[0175] A1. Add 3 g of trimesic acid to 55 mL of dimethylacetamide, stir evenly to obtain solution a. Add 6 g of copper nitrate trihydrate to 80 mL of dimethylacetamide, stir evenly to obtain solution b. Mix solution a and solution b, stir at 25 °C for 15 min, place it in an 85 °C oil - bath, stir and react for 14 h, cool to room temperature, filter and collect the product. Wash the product with dimethylacetamide 5 times, and dry it in an oven at 100 °C for 24 h to obtain Cu3(BTC)2;

[0176] A2. Add 3 g of Cu3(BTC)2 and 7 g of tannic acid to 180 mL of deionized water, stir evenly, stir and react at 60 °C for 2 h, filter, wash with deionized water 3 times, and dry in an oven at 70 °C for 10 min to obtain tannic - acid - modified Cu3(BTC)2;

[0177] A3. Add 5 g of nanocellulose to 110 mL of deionized water, stir evenly, add 3 g of tea polyphenols, stir at 400 r / min for 2 h, add 1.4 g of tannic - acid - modified Cu3(BTC)2, stir until it becomes gel - like. Wash the gel with deionized water 3 times, and freeze - dry at - 30 °C for 24 h to obtain the modified Cu3(BTC)2.

[0178] The anthocyanin complex is specifically prepared by the following steps:

[0179] Add 0.2 g of vitamin E, 0.12 g of rosmarinic acid, and 0.3 g of anthocyanin to 22 mL of ethanol, stir at 500 r / min for 15 min, and freeze-dry at -20 °C for 12 h to obtain the anthocyanin complex.

[0180] Now, perform performance tests on the modified atmosphere preservation films prepared in Examples 1-3 and Comparative Examples 1-6.

[0181] Cut the above-prepared modified atmosphere preservation film into a style of 20 mm × 20 mm.

[0182] Compressive strength test: Use a universal testing machine to test the tensile strength and elongation at break of the above-prepared modified atmosphere preservation film, and the test speed is 200 mm / min.

[0183] Gas permeability performance detection: Determine the CO2 permeation rate (cm 3 / m 2 ·24 h·0.1 MPa), O2 permeation rate (cm 3 / m 2 ·24 h·0.1 MPa), and gas permeability coefficient (cm 3 ·cm / cm 2 ·s·Pa) of the modified atmosphere preservation film according to the GB / T 1038.1-2022 standard, with the temperature at 25 °C and the relative humidity at 50%.

[0184] Enclose fruits and vegetables (mangoes, mushrooms) with the modified atmosphere preservation film for storage for 12 d at a temperature of 30 °C, and observe whether there are color changes in the modified atmosphere preservation film, pink (fresh), yellowish-green (sub-fresh), yellow (spoiled).

[0185] The test results are shown in Table 1 below.

[0186] Table 1 Performance detection of the modified atmosphere preservation films prepared in Examples 1-3 and Comparative Examples 1-6

[0187]

[0188] Table 2 Performance detection of the modified atmosphere preservation films prepared in Examples 1-3 and Comparative Examples 1-6

[0189]

[0190] It can be seen from the data in Table 1 that the modified atmosphere preservation films prepared in Examples 1-3 have good mechanical properties and modified atmosphere functions, and can endow the modified atmosphere preservation film with a freshness monitoring function according to the color change of the modified atmosphere preservation film.

[0191] In Comparative Example 1, the modified Cu3(BTC)2 prepared by replacing tannic acid-modified Cu3(BTC)2 with Cu3(BTC)2 was added to the modified atmosphere packaging film. The mechanical properties and gas permeability decreased, which proved that tannic acid was grafted onto the surface of Cu3(BTC)2, making Cu3(BTC)2 have higher activity and be able to disperse better in polylactic acid, avoiding the decline of the modified atmosphere function of the modified atmosphere packaging film caused by the aggregation of Cu3(BTC)2, and the aggregation of Cu3(BTC)2 would affect the mechanical properties of the modified atmosphere packaging film.

[0192] In Comparative Example 2, the modified Cu3(BTC)2 prepared without adding tea polyphenols was added to the modified atmosphere packaging film. The mechanical properties and gas permeability decreased, which proved that tea polyphenols and nanocellulose reacted to form a three-dimensional network aerogel structure with high mechanical properties, and tea polyphenols could form coordination bonds with metal ions to increase the cross-linking density of the aerogel, significantly increasing the mechanical properties and modified atmosphere function of the modified atmosphere packaging film.

[0193] In Comparative Example 3, the modified Cu3(BTC)2 prepared without adding nanocellulose was added to the modified atmosphere packaging film. The mechanical properties and gas permeability decreased, which proved that nanocellulose, as the aerogel skeleton, could absorb and weaken external stress, avoiding the collapse of pores caused by Cu3(BTC)2 under external force and affecting the mechanical properties and modified atmosphere function of the modified atmosphere packaging film.

[0194] Comparative Example 4, Comparative Example 5, and Comparative Example 6 were modified atmosphere packaging films prepared without adding vitamin E, rosmarinic acid, and procyanidin, respectively. The color change was not obvious, which proved that vitamin E, rosmarinic acid, and procyanidin, as natural antioxidants, could effectively improve the antioxidant activity of anthocyanins, avoiding the easy degradation and oxidation of anthocyanins when exposed to environments such as oxygen, light, pH, high temperature, enzymes, and other active compounds, making the color of the modified atmosphere packaging film change significantly with the change of environmental pH and playing a role in monitoring the freshness of food.

[0195] In the description of the specification, the description referring to terms such as "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0196] The above content is only an example and illustration of the present invention. Those skilled in the art to which this technology pertains may make various modifications, supplements, or use similar methods of substitution to the described specific embodiments. As long as they do not deviate from the invention or exceed the scope defined by this claims, they shall fall within the protection scope of the present invention.

Claims

1. A preparation method of an intelligent modified atmosphere fresh-keeping film, characterized in that, It includes the following preparation steps: S1. Add polylactic acid into dichloromethane, stir at 300 - 400 r / min for 1 - 2 h, add modified Cu3(BTC)2, and stir at 30 - 40 °C for 0.5 - 1.5 h to obtain the outer layer casting solution; S2. Add gellan gum into deionized water, stir at 300 - 400 r / min for 20 - 30 min, add glycerol and anthocyanin complex, and continue to stir for 20 - 30 min to obtain the inner layer casting solution; S3. Coat the outer layer casting solution on the PET film, after flat - push scraping, dry at 45 - 55 °C for 2 - 5 min, take out to obtain the outer layer PET film; S4. Coat the inner layer casting solution on the outer layer PET film, after flat - push scraping, dry at 45 - 55 °C for 1.5 - 2.5 h, peel off the PET film to obtain the modified atmosphere - regulating fresh - keeping film; The modified Cu3(BTC)2 is obtained by surface - modifying Cu3(BTC)2 with tannic acid and then mixing and reacting with tea polyphenols and nanocellulose; The anthocyanin complex is obtained by mixing vitamin E, rosmarinic acid, procyanidins and anthocyanins; 2. The preparation method of an intelligent modified atmosphere fresh-keeping film according to claim 1, characterized in that The modified Cu3(BTC)2 is specifically prepared by the following steps: A1. Add trimesic acid into dimethylacetamide, stir evenly to obtain solution a, add copper nitrate trihydrate into dimethylacetamide, stir evenly to obtain solution b, mix solution a and solution b, stir evenly, stir and react at 75 - 85 °C for 10 - 14 h, cool to room temperature, filter, wash and dry to obtain Cu3(BTC)2; A2. Add Cu3(BTC)2 and tannic acid into deionized water, stir evenly, stir and react at 50 - 60 °C for 1 - 2 h, filter, wash and dry to obtain tannic acid - modified Cu3(BTC)2; A3. Add nanocellulose into deionized water, stir evenly, add tea polyphenols, stir at 300 - 400 r / min for 1 - 2 h, add tannic acid - modified Cu3(BTC)2, stir until it becomes gel - like, wash the gel, and freeze - dry to obtain modified Cu3(BTC)2.

3. The preparation method of an intelligent modified atmosphere fresh-keeping film according to claim 2, wherein, In step A1, the dosage ratio of trimesic acid to dimethylacetamide is (2 - 3) g:(45 - 55) mL; The dosage ratio of copper nitrate trihydrate to dimethylacetamide is (4 - 6) g:(70 - 80) mL.

4. The preparation method of an intelligent modified atmosphere fresh-keeping film according to claim 2, wherein, In step A2, the dosage ratio of Cu3(BTC)2, tannic acid and deionized water is (1 - 3) g:(5 - 7) g:(180 - 220) mL.

5. The preparation method of an intelligent modified atmosphere fresh-keeping film according to claim 2, wherein, In step A3, the dosage ratio of nanocellulose, deionized water, tea polyphenols and tannic acid - modified Cu3(BTC)2 is (3 - 5) g:(90 - 110) mL:(2 - 3) g:(1 - 1.4) g.

6. The preparation method of an intelligent modified atmosphere fresh-keeping film according to claim 1, characterized in that, The anthocyanin complex is specifically prepared by the following steps: Add vitamin E, rosmarinic acid, procyanidins and anthocyanins into ethanol, stir at 400 - 500 r / min for 10 - 15 min, and freeze - dry to obtain the anthocyanin complex.

7. The preparation method of an intelligent modified atmosphere fresh-keeping film according to claim 6, characterized in that, The dosage ratio of the vitamin E, rosmarinic acid, procyanidin, anthocyanin and ethanol is (0.1-0.2) g: (0.1-0.12) g: (0.11-0.13) g: (0.2-0.3) g: (18-22) mL.

8. Application of a modified atmosphere fresh-keeping film prepared by the preparation method of the intelligent modified atmosphere fresh-keeping film according to any one of claims 1-7 in monitoring and preserving fruits and vegetables.

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