Preparation method of modified polylactic acid packaging material for food

Abstract

The invention provides a preparation method of a modified polylactic acid packaging material for food and a preparation method thereof, and belongs to the technical field of packaging materials.The preparation method comprises the steps that 2-methylimidazole, methyl alcohol and vitamin C are mixed to prepare a ligand solution; the preparation method comprises the following steps: mixing zinc nitrate hexahydrate with N, N-dimethylformamide according to a ratio of (25-32) to prepare a metal salt solution, mixing the two solutions, reacting at room temperature, washing for three times, and drying to obtain ZIF-8 powder; and dissolving lycopene in dichloromethane, adding the ZIF-8 powder, then adding the dried polylactic acid particles, stirring, preparing a film by a spin-coating method, and carrying out vacuum drying to obtain the modified polylactic acid packaging material. The prepared modified polylactic acid packaging material for the food has excellent oxidation resistance and antibacterial ability, and the shelf life of the food can be effectively prolonged. By combining the natural antioxidant characteristic of lycopene and the porous adsorption structure of ZIF-8, the barrier property and stability of polylactic acid are remarkably improved, and meanwhile, the biodegradable polylactic acid material has good biodegradability and meets the environmental protection requirement.

Description

Technical Field

[0001] The invention relates to the technical field of packaging materials, and in particular to a method for preparing a modified polylactic acid packaging material for food. Background Art

[0002] In recent years, with the increasing concern about food safety and health issues, the development of biopolymer films has received increasing attention. Food packaging films need to protect food from external contaminants and prevent chemical, physical and biological changes during storage. Although traditional packaging plays an important role in blocking oxygen and moisture, its non-degradability has caused serious environmental problems.

[0003] To meet these challenges, the latest research in materials science and bioengineering is focused on developing a new packaging technology to improve food quality and safety. Active packaging technology has emerged, which reduces the need for preservatives by allowing active compounds to diffuse continuously into the food environment. The core of this technology lies in the controlled release mechanism, that is, through a specific packaging film substrate and controlled release technology, the on-demand release of active ingredients such as antioxidants is achieved.

[0004] In the past decade, with the development of controlled-release technology and the increasing demand for antioxidants in high-fat foods, antioxidant active food packaging based on controlled-release has become a hot topic in the field of active food packaging research. Researchers have conducted in-depth research on packaging film substrate materials, new controlled-release technologies and controlled-release mechanisms. These advances not only improve the freshness preservation performance of packaging, but also provide new ideas for the sustainable development of the food industry. For example, Professor Hu Bing's team at Nanjing Agricultural University (Shi Q, Nian Y, You Y, et al. Heterogeneity in Diameters of Protein Fibrils and Chitosan for a High CO2 / O2 Selectivity and Desired Mechanical Properties of Edible Bioplastic Films [J]. Small, 2405346.) developed a bioplastic film based on natural polymers, which has excellent gas barrier properties and mechanical properties, and has significant antibacterial activity.

[0005] In summary, the development of bio-based polymer films not only helps to solve the environmental problems of traditional packaging, but also improves the functionality of food packaging through controlled release technology, providing new solutions for food preservation and safety.

[0006] As a natural pigment with strong antioxidant and biological activity, lycopene has gradually attracted attention for its application in food packaging materials and its mechanism of action. As a natural antioxidant, lycopene can effectively delay the oxidative deterioration of food during storage. Its antioxidant ability comes from the conjugated double bonds in its molecular structure, which can quench active oxygen and free radicals, thereby protecting the nutrients in food. This property allows it to be used as an active ingredient in food packaging, which can be slowly released through controlled release technology to continuously protect food from oxidative damage. Lycopene can also enhance the overall performance of the packaging by interacting with other components in the packaging material. Through microencapsulation or compounding with other biopolymers, lycopene can be encapsulated in a specific carrier, so that it can be slowly released during storage and transportation. This controlled release technology not only improves the bioavailability of lycopene, but also prolongs its effective action time in food packaging. In addition, the antioxidant activity of lycopene can also extend the shelf life of food by inhibiting the growth of microorganisms.

[0007] In summary, the application of lycopene in food packaging materials can not only provide antioxidant protection and natural coloring functions, but also enhance the freshness preservation effect of packaging through controlled release technology. These characteristics make it a functional ingredient with great potential in the future food packaging field. Summary of the invention

[0008] The present invention aims to provide a method for preparing a modified polylactic acid packaging material for food. In the method, the combination of lycopene with ZIF-8 and polylactic acid shows a significant mechanism of action and application potential. ZIF-8, as a metal organic framework material (MOFs), has a highly ordered pore structure and excellent gas adsorption performance. During the preparation process, ZIF-8 is formed by the reaction of 2-methylimidazole and zinc nitrate hexahydrate, and its porous structure can efficiently load lycopene, thereby significantly enhancing the dispersibility and stability of lycopene in packaging materials. Lycopene, as a natural strong antioxidant, can effectively inhibit oxidation reactions in food and extend the shelf life of food. By dissolving it in dichloromethane and mixing it with ZIF-8, lycopene can be evenly dispersed in the pores of ZIF-8, further improving its stability in the polylactic acid matrix. Polylactic acid, as a biodegradable polymer material, has good mechanical properties and transparency, but its antioxidant properties are limited. By combining lycopene-loaded ZIF-8 with polylactic acid, the antioxidant properties of polylactic acid film can be significantly improved. This modified polylactic acid film can not only effectively block oxygen and moisture, but also inhibit oxidation reactions in food through the antioxidant effect of lycopene, thereby extending the shelf life of food. In addition, the porous structure of ZIF-8 can also absorb odorous substances in food, further improving the preservation effect of packaging. The application of this composite material in food packaging not only meets consumers' demand for natural and healthy packaging materials, but also provides new ideas for solving the environmental problems of traditional plastic packaging.

[0009] In food packaging, ZIF-8 (zeolitic imidazolate framework-8) has a variety of important functions and mechanisms of action. First, the porous structure and high specific surface area of ​​ZIF-8 enable it to efficiently load and slowly release lycopene, significantly enhancing the antioxidant properties of the packaging film. In addition, ZIF-8 also has a pH-responsive slow-release function, which can release the loaded active substances in an acidic environment, thereby achieving a long-lasting antibacterial effect. Secondly, the porous structure of ZIF-8 can adsorb and fix bacteria, and the released Zn2+ ions can destroy the bacterial cell membrane and inhibit the growth of microorganisms. For example, after compounding with polylactic acid, ZIF-8 can significantly improve the antibacterial properties of the packaging film and effectively inhibit spoilage bacteria and pathogens in food. ZIF-8 also has good ultraviolet blocking properties, which can protect food from damage by ultraviolet rays, thereby extending the shelf life of food. In addition, the addition of ZIF-8 can improve the mechanical properties and barrier properties of the packaging film, making it more suitable for food packaging.

[0010] In order to solve the above technical problems, the present invention provides a method for preparing a modified polylactic acid packaging material for food and a preparation method thereof.

[0011] To achieve the above object, the present invention provides the following technical solution: a method for preparing a modified polylactic acid packaging material for food, comprising the following steps:

[0012] Step 1, 2-methylimidazole: methanol: vitamin C are fully mixed at a mass ratio of 1:20-30:2-5 to obtain a ligand solution; zinc nitrate hexahydrate: N,N-dimethylformamide are fully mixed at a molar ratio of 1:25-32 to obtain a metal salt solution, the two solutions are fully mixed and stirred, transferred to a reaction bottle, reacted at room temperature for 45-60 hours, washed three times with methanol, and dried to obtain ZIF-8 powder;

[0013] Step 2: Dissolve lycopene in dichloromethane, add ZIF-8 powder and stir magnetically at room temperature for 6 to 15 hours, then add polylactic acid particles dried in an oven at 50°C to 60°C for 10 to 15 hours to prepare a polylactic acid solution with a mass fraction of 5% to 35%, and stir magnetically at room temperature for 4 hours. After complete dissolution, prepare a film by spin coating, and vacuum dry to remove excess solvent and moisture in the film to obtain a modified polylactic acid packaging material for food.

[0014] Furthermore, the mass ratio of zinc nitrate hexahydrate in the metal salt solution in step 1 to 2-methylimidazole in the ligand solution is 1:0.8-1.17.

[0015] Furthermore, the lycopene in step 2 is lycopene with a specification of 10% to 60%.

[0016] Furthermore, the mass ratio of lycopene to dichloromethane and ZIF-8 powder in step 2 is 1:25-100:0.5-2. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0018] The polylactic acid used in the following examples is item number 665596456;

[0019] The functionalized acrylic modified polysiloxane heavy-duty anticorrosive coating and its preparation method provided by the present invention are specifically described below through examples.

[0020] Embodiment 1:

[0021] Step 1, 2-methylimidazole: methanol: vitamin C are fully mixed in a mass ratio of 1:25:3.5 to obtain a ligand solution; zinc nitrate hexahydrate: N,N-dimethylformamide are fully mixed in a molar ratio of 1:27 to obtain a metal salt solution, the two solutions are fully mixed and stirred (the mass ratio of zinc nitrate hexahydrate in the metal salt solution and 2-methylimidazole in the ligand solution is 1:0.98), transferred to a reaction bottle, reacted at room temperature for 52 hours, washed three times with methanol, and dried to obtain ZIF-8 powder;

[0022] Step 2: Dissolve 20% lycopene in dichloromethane, add ZIF-8 powder (the mass ratio of lycopene to dichloromethane and ZIF-8 powder is 1:48:0.8), stir magnetically at room temperature for 10 hours, add polylactic acid particles dried in a 50°C oven for 12 hours, and prepare a polylactic acid solution with a mass fraction of 8%, and stir magnetically at room temperature for 4 hours. After complete dissolution, prepare a film by spin coating, and vacuum dry to remove excess solvent and moisture in the film to obtain a modified polylactic acid packaging material for food.

[0023] Embodiment 2:

[0024] Step 1, 2-methylimidazole: methanol: vitamin C are fully mixed at a mass ratio of 1:27:4 to obtain a ligand solution; zinc nitrate hexahydrate: N,N-dimethylformamide are fully mixed at a molar ratio of 1:27 to obtain a metal salt solution, the two solutions are fully mixed and stirred (the mass ratio of zinc nitrate hexahydrate in the metal salt solution and 2-methylimidazole in the ligand solution is 1:1.02), transferred to a reaction bottle, reacted at room temperature for 50 hours, washed three times with methanol, and dried to obtain ZIF-8 powder;

[0025] Step 2: Dissolve 20% lycopene in dichloromethane, add ZIF-8 powder (the mass ratio of lycopene to dichloromethane and ZIF-8 powder is 1:55:1.35), stir magnetically at room temperature for 10 hours, add polylactic acid particles dried in a 50°C oven for 12 hours, and prepare a polylactic acid solution with a mass fraction of 8%, and stir magnetically at room temperature for 4 hours. After complete dissolution, prepare a film by spin coating, and vacuum dry to remove excess solvent and moisture in the film to obtain a modified polylactic acid packaging material for food.

[0026] Embodiment 3:

[0027] Step 1, 2-methylimidazole: methanol: vitamin C are fully mixed at a mass ratio of 1:25:4.2 to obtain a ligand solution; zinc nitrate hexahydrate: N,N-dimethylformamide are fully mixed at a molar ratio of 1:27 to obtain a metal salt solution, the two solutions are fully mixed and stirred (the mass ratio of zinc nitrate hexahydrate in the metal salt solution and 2-methylimidazole in the ligand solution is 1:0.98), transferred to a reaction bottle, reacted at room temperature for 52 hours, washed three times with methanol, and dried to obtain ZIF-8 powder;

[0028] Step 2: Dissolve 20% lycopene in dichloromethane, add ZIF-8 powder (the mass ratio of lycopene to dichloromethane and ZIF-8 powder is 1:65:1.8), stir magnetically at room temperature for 10 hours, add polylactic acid particles dried in a 50°C oven for 12 hours, and prepare a polylactic acid solution with a mass fraction of 8%, and stir magnetically at room temperature for 4 hours. After complete dissolution, prepare a film by spin coating, and vacuum dry to remove excess solvent and moisture in the film to obtain a modified polylactic acid packaging material for food.

[0029] Comparative Example 1:

[0030] The polylactic acid particles dried in a 50°C oven for 12 hours were dissolved in dichloromethane to prepare a 5% polylactic acid solution, and magnetically stirred for 4 hours at room temperature. After complete dissolution, a film was prepared by spin coating, and the excess solvent and moisture in the film were removed by vacuum drying to obtain pure polylactic acid packaging material.

[0031] Comparative Example 2:

[0032] Step 1, 2-methylimidazole: methanol: fully mixed at a mass ratio of 1:25 to obtain a ligand solution; zinc nitrate hexahydrate: N,N-dimethylformamide were fully mixed at a molar ratio of 1:25 to obtain a metal salt solution, the two solutions were fully mixed and stirred (the mass ratio of zinc nitrate hexahydrate in the metal salt solution and 2-methylimidazole in the ligand solution was 1:0.98), transferred to a reaction bottle, reacted at room temperature for 52 hours, washed three times with methanol, and dried to obtain ZIF-8 powder;

[0033] Step 2: Dissolve ZIF-8 powder in dichloromethane (the mass ratio of dichloromethane to ZIF-8 powder is 50:1) and stir magnetically at room temperature for 10 hours, then add polylactic acid particles dried in a 50°C oven for 12 hours to prepare a polylactic acid solution with a mass fraction of 5%, and stir magnetically at room temperature for 4 hours. After complete dissolution, prepare a film by spin coating, and vacuum dry to remove excess solvent and moisture in the film to obtain a modified polylactic acid packaging material for food.

[0034] Experiments have shown that the modified polylactic acid packaging materials for food obtained in Examples 1 to 3 of the present invention have good freshness-keeping performance, antibacterial performance, and antioxidant performance.

[0035] The specific data of the performance tests of Examples 1 to 3 and Comparative Examples 1-2 are shown in Table 1.

[0036] Fresh-keeping performance: All fruits were sorted and selected for the same size, color, maturity, no pests, no mechanical damage and other conditions. The strawberries were divided into 5 groups on average. These small berries were placed in matching beakers that had been washed and dried. The beakers were then sealed with films of different compositions, and the blank beakers without films were recorded as KB. During the test, samples were taken once a day and evaluated using two indicators: sensory indicators (the time when mold spots began to appear) and weight loss rate (7 days). Each evaluation was repeated 3 times and the average value was taken.

[0037]

[0038] Wherein, W is weight loss (%); m1 is the mass of strawberries before storage, and m2 is the mass of strawberries after storage.

[0039] ABTS free radical scavenging activity assay: In order to dissolve lycopene and polylactic acid, 0.1 mL of dimethyl sulfoxide and dichloromethane were added. The solution was diluted with water to a solvent concentration of 1 mg / mL in the solution. Sample solutions of different concentrations were mixed with 4 mL of ABTS (7 mol / L). After 6 min of reaction, the absorbance was recorded using a Spectrum Max M5 microplate reader (Sunnyvale, CA, USA). All tests were performed in triplicate. The antioxidant activity of each sample was expressed as a 50% inhibition concentration (half inhibition concentration), indicating that the concentration of the sample had a 50% ABTS free radical scavenging effect.

[0040] Antibacterial performance determination: The antibacterial performance was evaluated by agar diffusion method. The synthesized composite materials were tested for antibacterial activity using E. coli as raw material. The ability of different samples to destroy E. coli was demonstrated based on the reduction in the number of colonies observed on agar. Calculate the fiber membrane inhibition rate R:

[0041] R is the inhibition rate (%), A is the average colony count in the control group and B is the average colony count in the test sample.

[0042] Table 1 Performance test of Examples 1-3 and Comparative Examples 1-2

[0043]

[0044]

[0045] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should all be covered by the scope of the claims of the present invention.

Claims

1. A method for preparing a modified polylactic acid packaging material for food, characterized in that: The following steps are involved: Step 1, 2-methylimidazole: Methanol: vitamin C are fully mixed at a mass ratio of 1:20-30:2-5 to obtain a ligand solution; zinc nitrate hexahydrate: N,N-dimethylformamide are fully mixed at a molar ratio of 1:25-32 to obtain a metal salt solution, the two solutions are fully mixed and stirred, transferred to a reaction bottle, reacted at room temperature for 45-60 hours, washed three times with methanol, and dried to obtain ZIF-8 powder; Step 2: Dissolve lycopene in dichloromethane, add ZIF-8 powder and stir magnetically at room temperature for 6 to 15 hours, then add polylactic acid particles dried in an oven at 50°C to 60°C for 10 to 15 hours to prepare a polylactic acid solution with a mass fraction of 5% to 35%, and stir magnetically at room temperature for 4 hours. After complete dissolution, prepare a film by spin coating, and vacuum dry to remove excess solvent and moisture in the film to obtain a modified polylactic acid packaging material for food.

2. The method for preparing a modified polylactic acid packaging material for food according to claim 1, characterized in that: The mass ratio of zinc nitrate hexahydrate in the metal salt solution and 2-methylimidazole in the ligand solution in step 1 is 1:0.8-1.

17.

3. The method for preparing a modified polylactic acid packaging material for food according to claim 1, characterized in that: The lycopene in step 2 has a specification of 10% to 60%.

4. The method for preparing a modified polylactic acid packaging material for food according to claim 1, characterized in that: The mass ratio of lycopene to dichloromethane and ZIF-8 powder in step 2 is 1:25~100:0.5~2.

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