Green leaf vegetable fresh-keeping composite film containing nicotinamide and preparation method of green leaf vegetable fresh-keeping composite film

Abstract

The invention relates to the field of new packaging materials, and provides a composite film for fresh-keeping of green leaf vegetables and a preparation method thereof, the main raw materials of the composite film comprise a base material, citric acid, glacial acetic acid and nicotinamide; wherein the base material is a mixture of chitosan and polyvinyl alcohol; the preparation method comprises the following steps: dissolving and mixing the chitosan and the polyvinyl alcohol, adding a nicotinamide solution, stirring and carrying out ultrasonic treatment to uniformly distribute the components, and finally forming the film by adopting a tape casting process. The composite film can effectively delay oxidation and decay of the green vegetables and remarkably prolong the shelf life of the green vegetables. The composite film has good mechanical properties and flexibility, and is suitable for various fresh-keeping applications. The preparation process is simple, large-scale production is easy, and the material is environment-friendly and biodegradable and has a wide application prospect.

Description

Technical Field

[0001] The invention relates to the field of new food packaging materials, and in particular to a composite film for preserving green leafy vegetables and a preparation method thereof. The composite film contains nicotinamide and can effectively prolong the shelf life of the green leafy vegetables. Background Art

[0002] Green leafy vegetables play an important role in our daily diet. They are rich in vitamins, minerals and cellulose, and are very popular among consumers. However, due to their high water content and vigorous metabolic activity, green leafy vegetables are prone to wilting, discoloration, corruption and other problems during storage and transportation. These problems not only shorten the shelf life of green leafy vegetables, but also increase food waste and bring challenges to supply chain management. Therefore, how to extend the shelf life of green leafy vegetables has always been a difficult problem to be solved in the field of food packaging.

[0003] Existing preservation methods include low-temperature storage, modified atmosphere packaging, and the use of chemical preservatives. Although these methods can extend the shelf life of leafy greens to a certain extent, they also have many shortcomings. Low-temperature storage and modified atmosphere packaging usually require expensive equipment, are complicated to operate, and are difficult to maintain stable conditions during long-term transportation. Chemical preservatives may have a negative impact on food safety, and as consumers' demand for healthy food continues to increase, their application is increasingly restricted.

[0004] In recent years, with the improvement of environmental awareness, biodegradable materials have received extensive attention in the field of food packaging. Chitosan and polyvinyl alcohol are two commonly used biodegradable materials. They have good film-forming properties and biocompatibility and are considered to be ideal substrates for food cling film. However, when these materials are used alone, the preservation effect is limited and it is difficult to effectively inhibit the metabolism and aging of vegetables over a long period of time. Summary of the invention

[0005] The present invention aims at the deficiencies of the prior art in the field of green leafy vegetable preservation, and provides a composite film for green leafy vegetable preservation and a preparation method thereof. The main raw materials include substrate, citric acid, glacial acetic acid, and nicotinamide; wherein the substrate is a mixture of chitosan and polyvinyl alcohol; the preparation method is to dissolve and mix chitosan and polyvinyl alcohol, add nicotinamide solution, stir and ultrasonically treat to make the components evenly distributed, and finally form a film by a casting process. The composite film can effectively delay the oxidation and corruption of green leafy vegetables and significantly extend their shelf life. The composite film also has good mechanical properties and flexibility, and is suitable for a variety of preservation applications. The preparation process is simple, easy to mass produce, and the material is environmentally friendly and biodegradable, and has broad application prospects.

[0006] The present invention proposes a green leafy vegetable preservation composite film based on nicotinamide. As an important bioactive substance, nicotinamide has significant effects in inhibiting plant respiration, delaying metabolism, anti-oxidation and antibacterial. By introducing nicotinamide into the composite film, the shelf life of green leafy vegetables can be effectively extended and their nutritional components and appearance quality can be maintained.

[0007] The composite film of the present invention is prepared by a casting method using chitosan and polyvinyl alcohol as substrates. The present invention is not limited to these materials, and replaceable substrates include gelatin, polylactic acid, hydroxypropyl methylcellulose, and xanthan gum; in addition, additional substrates may be added, and the additional substrates include one or more of nanocellulose, sodium alginate, and polyethylene glycol, which may also be used in the preparation process of the present invention, thereby providing a wider range of choices for different application requirements.

[0008] The addition of nicotinamide not only enhances the preservation effect of the composite film, but also significantly delays the aging process of green leafy vegetables through its multiple biological activities. The preparation method of the present invention is simple, environmentally friendly, suitable for large-scale production, and can significantly extend the shelf life of green leafy vegetables while ensuring food safety, and has broad application prospects.

[0009] In actual application, nicotinamide as an active substance has shown significant effects in preserving green leafy vegetables, but there are also some challenges in the preparation process: 1. The amount of nicotinamide added needs to be precisely controlled. Too much addition may affect the physical properties of the composite film, while too little addition may not achieve the expected preservation effect. Different substrates have different dispersibility and release effects on nicotinamide. How to optimize the substrate ratio to achieve the best preservation effect is an important research content of the present invention.

[0010] 2. Nicotinamide may decompose at higher temperatures, which requires controlling the appropriate temperature during the preparation process to ensure that the activity of nicotinamide is not affected.

[0011] In order to overcome these problems, the present invention has made technical improvements in the following aspects: (1) Different substrates have different adsorption and release characteristics for nicotinamide. Therefore, selecting a suitable substrate combination and ratio is crucial to achieving the best preservation effect. The present invention optimizes the dispersibility and release effect of nicotinamide by adjusting the ratio of chitosan and polyvinyl alcohol.

[0012] (2) During the preparation process, low-temperature stirring and ultrasonic treatment were used to ensure the uniform distribution of nicotinamide in the composite film and prevent decomposition in a high-temperature environment.

[0013] After summarizing the above improvements, the specific technical solutions of the present invention are as follows.

[0014] The present invention first provides a nicotinamide-containing green leafy vegetable fresh-keeping composite film, wherein the main raw materials of the composite film include the following components calculated by mass percentage: 1.8-6 wt% of substrate, 0.3-1.0 wt% of citric acid, 0.3-1.0 wt% of glacial acetic acid, 0.05-0.5 wt% of nicotinamide, 0-0.5 wt% of additional substrate, and the balance is deionized water; wherein the substrate is a mixture of chitosan and polyvinyl alcohol.

[0015] In the composite film, chitosan provides good film-forming properties and mechanical strength, while polyvinyl alcohol enhances the flexibility and durability of the film. The introduction of citric acid and glacial acetic acid improves the physical properties of the film, enabling the composite film to effectively delay the aging and deterioration of vegetables during the preservation process. At the same time, nicotinamide, as a preservative, is evenly distributed in the composite film, and through its antioxidant and antibacterial effects, it significantly extends the shelf life of green leafy vegetables.

[0016] Preferably, in the substrate, the mass percentage of chitosan in the entire composite membrane raw material is 0.3-1.5 wt%, and the mass percentage of polyvinyl alcohol in the entire composite membrane raw material is 1.5-4.5 wt%.

[0017] Alternatively, in addition to chitosan and polyvinyl alcohol, the substrate may also use one or more of gelatin, polylactic acid, xanthan gum, and hydroxypropyl methylcellulose. These substrates each have different physical and chemical properties. Through reasonable selection and matching, the performance of the composite film can be further optimized to meet the preservation requirements of different green leafy vegetables.

[0018] Preferably, the additional substrate includes one or more of nanocellulose, sodium alginate and polyethylene glycol, and the amount thereof accounts for 0.3-0.5 wt % of the total composite membrane raw material.

[0019] The composite film can be modified by adding additional substrates to adjust its mechanical properties, strength and barrier properties, but the purpose of the present invention can still be achieved without adding additional substrates, and a fresh-keeping composite film with good mechanical properties, strength, barrier properties and biodegradability can still be prepared.

[0020] In practical applications, one or more substrates can be selected and used together with the additional substrate according to specific needs. For example, a composite film is prepared in which xanthan gum and polyvinyl alcohol are used together as substrates, and nanocellulose is added as an additional substrate. Such a combination can obtain a fresh-keeping film having both good mechanical properties and biodegradability, and high strength and barrier properties.

[0021] Preferably, the thickness of the green leafy vegetable fresh-keeping composite film containing nicotinamide finally obtained by the present invention is 30-80 μm.

[0022] Preferably, the nicotinamide-containing green leafy vegetable fresh-keeping composite film further comprises other additives, wherein the additives include one or more of antioxidants and preservatives.

[0023] The antioxidant can be selected from one or more of vitamin E, vitamin C, gallic acid, tea polyphenols, butylated hydroxyanisole, and the proportion of the whole composite film raw material is 0-1.0 wt%; the preservative can be selected from one or more of potassium sorbate, sodium benzoate, parabens, sodium dehydroacetate, nisin, and the proportion of the whole composite film raw material is 0-0.5 wt%. The addition of the above additives can further enhance the preservation effect of vegetables.

[0024] In the specific implementation process, antioxidants and preservatives can be used in combination according to the different preservation requirements of green leafy vegetables. For example, vitamin C and potassium sorbate are added to the preparation solution of the film to contain 0.3 wt% of vitamin C and 0.2 wt% of potassium sorbate. The antioxidant prevents oxidative browning and the preservative inhibits microbial growth, thereby achieving the best preservation effect. Antioxidants and preservatives can be added according to actual needs. Without adding antioxidants and preservatives, a fresh-keeping composite film with good mechanical properties, strength, barrier properties and biodegradability can still be prepared, and the purpose of the present invention can still be achieved.

[0025] The present invention also provides a method for preparing the green leafy vegetable fresh-keeping composite film containing nicotinamide, comprising the following steps: S1. Prepare solution A: dissolve chitosan in deionized water and add glacial acetic acid and citric acid. Place the solution in an oil bath at 40-60°C and stir for 2-3 hours to ensure that the chitosan is fully dissolved and forms a uniform solution; S2. Prepare liquid B: dissolve polyvinyl alcohol in deionized water, place in a 90-95°C water bath and stir for 2-3 hours until it is completely dissolved and forms a uniform solution. Mix liquid A and liquid B in equal volumes, slowly add the pre-prepared nicotinamide solution, stir in a 45-50°C oil bath for 5-7 hours, and evenly disperse the nicotinamide in the mixture; S3. The obtained mixed solution is placed in an ultrasonic device for treatment for 1-2 hours to eliminate bubbles in the mixed solution and improve the density of the composite film. The treated mixed solution is slowly and evenly poured into the pretreated acrylic mold to ensure that the liquid surface is flat, and dried in an oven at 45-50°C for 8-10 hours to form a uniform and dense composite film.

[0026] The ultrasonic treatment can be carried out under conventional conditions. In the present invention, the preferred ultrasonic power is 360 W, the temperature is 25° C., and the ultrasonic time is 1 hour.

[0027] In the above preparation method, chitosan and polyvinyl alcohol are selected as film-forming matrices, citric acid and glacial acetic acid are used as modifiers, and nicotinamide is used as a preservative active substance. In the mixing process, a segmented addition process is adopted, combined with low-temperature stirring and ultrasonic treatment, and finally a composite film is prepared by a casting method. In the mixing process, since the amount of nicotinamide added is relatively small, and its physical properties make it difficult to disperse in the matrix, the present invention first mixes part of the chitosan with glacial acetic acid and citric acid to ensure that it forms an initial solution with certain viscosity and stability. Subsequently, the polyvinyl alcohol solution is added thereto, and the viscosity of the solution is moderate through stirring and temperature control operations, which helps the uniform dispersion of nicotinamide. The pre-prepared nicotinamide solution is slowly added to the above-mentioned mixed solution, and the low-temperature stirring technology is used to ensure that nicotinamide is evenly dispersed in the matrix material to avoid its decomposition or inactivation at higher temperatures. Through ultrasonic treatment, the bubbles in the solution are further eliminated, the uniformity and stability of the solution are increased, and the effective distribution of nicotinamide in the film forming process is ensured.

[0028] The concentration of the pre-prepared nicotinamide solution is 0.5-3.0 wt%. The use of nicotinamide solution within this concentration range can effectively play a role in increasing the shelf life of green leafy vegetables. If the concentration is less than 0.5 wt%, it may affect the preservation performance of the fresh-keeping composite film, resulting in a weak preservation effect, unable to significantly delay the decay of green leafy vegetables, and unable to achieve the purpose of extending the preservation time; if the concentration is greater than 3.0 wt%, it will lead to a significant increase in material costs, while the effect is not significantly improved, resulting in a "diminishing marginal effect" phenomenon, and high concentrations of nicotinamide may precipitate, resulting in changes in other physical properties of the film, such as transparency, flexibility, water vapor permeability, etc., affecting the performance of the fresh-keeping composite film.

[0029] The pretreatment is to clean and dry the acrylic mold before use.

[0030] In the specific application implementation process of the present invention, the niacinamide-containing green leafy vegetable fresh-keeping composite film can not only be used alone, but also can be used in combination with other types of packaging materials to further enhance the functionality and applicability of the packaging; the other types of packaging materials include but are not limited to one or more of polyethylene, polypropylene, polyethylene terephthalate, polylactic acid, aluminum foil, paper-based materials, and nylon.

[0031] Specifically, the combined use method includes but is not limited to the following two optimal methods: 1. Hot Pressing The hot pressing method softens the materials on the contact surface by heating and forms a strong bond. It is often used to laminate nicotinamide-containing cling film with other materials (such as polyethylene, polypropylene, etc.). This method has the following advantages: Advantages: It can ensure the sealing and strength of the packaging and is suitable for efficient and large-scale production.

[0032] Application scenarios: Commonly used in the field of food packaging, it provides good barrier properties and mechanical strength, and can effectively extend the shelf life of green leafy vegetables.

[0033] 2. Co-extrusion The co-extrusion method uses co-extrusion technology to simultaneously form a multi-layer composite film with nicotinamide-containing cling film and other packaging materials (such as polyethylene, polypropylene, polylactic acid, etc.). The advantages of this method include: Advantages: It can achieve good combination between different materials and improve the overall performance of the film, such as strength, barrier properties and transparency.

[0034] Application scenarios: Suitable for high-end food packaging that requires higher barrier properties and comprehensive performance, especially suitable for mass production.

[0035] The green leafy vegetable fresh-keeping composite film containing nicotinamide in the present invention is suitable for preserving green leafy vegetables, and the green leafy vegetables include but are not limited to rapeseed, spinach, and cabbage.

[0036] The present invention can flexibly adjust the physical properties and preservation effect of the composite film according to different preservation requirements by adjusting the ratio of chitosan to polyvinyl alcohol and the addition amount of citric acid and glacial acetic acid. In addition, the present invention can further optimize the functionality and applicability of the film by selecting different base material combinations, such as gelatin, sodium alginate, etc., to meet the preservation requirements of different types of green leafy vegetables.

[0037] Compared with the prior art, the present invention is beneficial in that: 1) The composite film with added nicotinamide not only shows significant advantages in preservation effect, but also has obvious improvements in many physical performance indicators of the film. Specifically, the addition of nicotinamide significantly improves the gas permeability, tensile strength, elongation at break, haze, oxygen permeability, carbon dioxide permeability and water vapor permeability of the film. The preservation performance of the composite film is greatly improved based on the various performance indicators of the composite film. These improvements are crucial to improving the preservation performance of green leafy vegetables at room temperature.

[0038] 2) The composite film with added niacinamide showed significant optimization in oxygen permeability and carbon dioxide permeability. By reducing the penetration of oxygen, the composite film can effectively inhibit the respiration rate of green leafy vegetables, slow down their metabolic process, and delay the oxidation and corruption of vegetables. The moderate carbon dioxide permeability helps maintain the gas balance in the package and prevents the acceleration of corruption caused by internal gas accumulation. Compared with the composite film without niacinamide, the regulation of gas permeability plays a key role in extending the shelf life of vegetables.

[0039] 3) The tensile strength and elongation at break of the composite film are also significantly improved after the addition of nicotinamide. These improvements mean that the composite film is more durable and flexible during use, can better adapt to various operating conditions during vegetable packaging and storage, reduce the possibility of breakage, and thus better protect the vegetables in the package.

[0040] 4) The introduction of nicotinamide also affects the water vapor permeability and haze of the composite film. A lower water vapor permeability helps prevent vegetables from wilting due to excessive water loss during storage, and also avoids the problem of microbial reproduction caused by excessive water in the film. Although the increase in haze means that the transparency of the film is reduced, it also indicates that the light scattering performance of the film is enhanced, which can slow down the impact of light on the quality of vegetables to a certain extent, thereby prolonging the preservation effect.

[0041] 5) In practical applications, the composite film of the present invention shows significant advantages in terms of preservation effect, environmental protection and production cost. Experiments show that the shelf life of green leafy vegetables packaged using the composite film of the present invention is extended by 2 to 3 times compared with the traditional method, and can effectively maintain the nutritional value and freshness of vegetables. In addition, the preparation process of the present invention is simple and easy, suitable for industrial production, has a low production cost, and can meet the large-scale application needs of the market. At the same time, all components in the composite film are environmentally friendly materials, which can be naturally degraded after use, meeting the requirements of sustainable development.

[0042] In summary, nicotinamide not only improves the preservation effect of the composite film through its own active substance characteristics, but also significantly enhances the overall performance of the composite film by improving multiple performance indicators of the composite film. These improvements enable the composite film containing nicotinamide to better preserve green leafy vegetables at room temperature, extend their shelf life, and maintain excellent appearance and quality. These performance improvements provide strong support for the development and application of new green leafy vegetable packaging materials, providing an environmentally friendly and economical preservation solution with broad application prospects and market value. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a comparison of the morphology of the rapeseed blank group and the soaking experiment group in Experimental Example 1; Figure 2 This is a comparison of the morphology of the rapeseed blank group and the composite film experimental groups with different nicotinamide concentrations in the preservation experiment in Experimental Example 2; Figure 3 This is a comparison of the morphology of the preservation experiment of the rapeseed soaking experiment group and the 0.17 wt% nicotinamide concentration composite film experiment group in Experimental Example 3; Figure 4 This is a comparison of the morphology of the rapeseed blank group and the 0.17 wt% nicotinamide concentration composite film experimental group in Experimental Example 4; Figure 5 This is a comparison chart of the respiration intensity between the rapeseed blank group and the 0.17 wt% nicotinamide concentration composite membrane experimental group in Experimental Example 4; Figure 6 This is a comparison chart of the weight loss rates of the rapeseed blank group and the 0.17 wt% nicotinamide concentration composite membrane experimental group in Experimental Example 4; Figure 7 This is a comparison chart of the yellowing rate of rapeseed blank group and 0.17 wt% nicotinamide concentration composite film experimental group in Experimental Example 4; Figure 8 This is a comparison of the morphology of the rapeseed control group and the experimental group in the preservation experiment of Experimental Example 5; Fig. 9 This is a comparison of the morphology of the spinach blank group and the 0.17 wt% nicotinamide concentration composite film experimental group in the application example; Fig.10 This is a comparison of the morphology of the Shanghai Qing blank group and the 0.17 wt% nicotinamide concentration composite film experimental group in the application example; Fig.11 This is a comparison chart of the preservation experiment morphology of the blank group of the vegetable and the composite film experimental group with a concentration of 0.17 wt% niacinamide in the application example. DETAILED DESCRIPTION

[0044] The present invention is further illustrated below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent modifications of the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application. Unless otherwise specified, the technologies used are all prior arts.

[0045] Among them, the measurement methods of weight loss rate, leaf yellowing rate and respiratory intensity are: Weight loss rate: Use the same precise balance to measure the mass of the same vegetable every 24 hours. Weight loss rate = (initial mass - final mass) / initial mass × 100%.

[0046] Yellowing rate: No yellowing of leaves is 0, 0-1 / 4 of the area is 1, 1 / 4-2 / 4 of the area is 2, 2 / 4-3 / 4 of the area is 3, and 3 / 4-all yellow is 4. The calculation formula for yellowing rate is: Yellowing rate = ΣX i Y / 4A. Where: X i = the degree of leaf yellowing; i = 0, 1, 2, 3, 4; Y = the number of leaves with different yellowing degrees; A = the total number of leaves per treatment.

[0047] Respiration intensity: The SY-1022 fruit and vegetable respirometer was used to measure the respiration intensity of green leafy vegetables, which is expressed as the mass of CO2 released per kilogram of green leafy vegetables per hour, in mg / (kg·h).

[0048] Example 1 A nicotinamide-containing fresh-keeping composite film in which the mass percentage of nicotinamide in the raw material is 0.08 wt % is prepared by the following method.

[0049] (1) Weigh 0.2 g of nicotinamide and dissolve it in 25 mL of deionized water to prepare a nicotinamide solution with a mass concentration of 0.79 wt%.

[0050] (2) Preparation of Liquid A: Dissolve 1 g of chitosan in 100 mL of deionized water, add 1 mL of glacial acetic acid (analytical grade, purity ≥99.8%, mass of glacial acetic acid is about 1.049 g) and 1 g of citric acid, place the solution in an oil bath at 60 °C and stir for 2 h to ensure that the chitosan is completely dissolved to form a uniform Liquid A.

[0051] (3) Preparation of Solution B: Dissolve 4 g of polyvinyl alcohol in 100 mL of deionized water. Place the mixture in a 95°C water bath and stir for 2 h until the polyvinyl alcohol is completely dissolved to form a uniform Solution B.

[0052] (4) After mixing liquid A and liquid B, 25 mL of the 0.79 wt % nicotinamide solution obtained in step (1) was added to prepare a solution with a 0.08 wt % nicotinamide weight percentage. The solution was stirred continuously in a 50° C. oil bath for 5 hours to ensure that nicotinamide was evenly dispersed in the solution to obtain a mixed solution.

[0053] (5) The mixed solution was subjected to ultrasonic treatment at 25 °C for 1 h with an ultrasonic power of 360 W to eliminate bubbles and improve the homogeneity of the solution.

[0054] (6) The treated solution is slowly poured onto the pre-treated acrylic plate to ensure that the solution is evenly spread. Afterwards, the acrylic plate is placed in an oven at 50°C and dried for 8 hours to form a composite film with uniform thickness and smooth surface.

[0055] (7) The dried composite film was peeled off from the acrylic plate and placed at room temperature for 24 hours to completely set and stabilize. The composite film had good mechanical properties and flexibility.

[0056] Example 2 A nicotinamide-containing fresh-keeping composite film in which the mass percentage of nicotinamide in the raw material is 0.17 wt % is prepared by the same method as in Example 1.

[0057] The difference is that in step (1) of this embodiment, 0.4 g of nicotinamide is weighed and dissolved in 25 mL of deionized water to prepare a nicotinamide solution with a mass concentration of 1.57 wt%; in step (4) of this embodiment, after equal volumes of liquid A and liquid B are mixed, 25 mL of the nicotinamide solution with a mass concentration of 1.57 wt% obtained in step (1) is added to prepare a solution with a weight percentage of nicotinamide of 0.17 wt%, and the above solution is continuously stirred in an oil bath at 50°C for 5 hours to ensure that nicotinamide is evenly dispersed in the solution to obtain a mixed solution.

[0058] The remaining steps are the same as those in Example 1.

[0059] Example 3 A nicotinamide-containing fresh-keeping composite film in which the mass percentage of nicotinamide in the raw material is 0.25 wt % is prepared by the same method as in Example 1.

[0060] The difference is that in step (1) of this embodiment, 0.6 g of nicotinamide is weighed and dissolved in 25 mL of deionized water to prepare a nicotinamide solution with a mass concentration of 2.34 wt%; in step (4) of this embodiment, after equal volumes of liquid A and liquid B are mixed, 25 mL of the nicotinamide solution with a mass concentration of 2.34 wt% obtained in step (1) is added to prepare a solution with a weight percentage of nicotinamide of 0.25 wt%, and the above solution is continuously stirred in a 50° C. oil bath for 5 hours to ensure that nicotinamide is evenly dispersed in the solution to obtain a mixed solution.

[0061] The remaining steps are the same as those in Example 1.

[0062] Example 4 The same method as in Example 2 was used to prepare a fresh-keeping composite film containing nicotinamide.

[0063] The difference is that chitosan is not added in this embodiment, and xanthan gum is used instead of chitosan; in addition, after liquid A and liquid B are mixed in step 4), vitamin E and potassium sorbate are added to the mixture of liquid A and liquid B, so that the mixture contains 0.3 wt% of vitamin E and 0.1 wt% of potassium sorbate.

[0064] Example 5 The composite film prepared by the above scheme is used in combination with other types of packaging materials to further enhance the functionality and applicability of the packaging. Specifically, the fresh-keeping composite film of the present invention can be used in combination with other packaging materials by using a hot pressing method and a co-extrusion method.

[0065] 1. Hot Pressing The hot pressing method softens the materials on the contact surface by heating and forms a strong bond. It is often used to laminate nicotinamide-containing cling film with other materials (such as polyethylene, polypropylene, etc.). This method has the following advantages: Advantages: It can ensure the sealing and strength of the packaging and is suitable for efficient and large-scale production.

[0066] Application scenarios: Commonly used in the field of food packaging, it provides good barrier properties and mechanical strength, and can effectively extend the shelf life of green leafy vegetables.

[0067] 2. Co-extrusion The co-extrusion method uses co-extrusion technology to simultaneously form a multi-layer composite film with nicotinamide-containing cling film and other packaging materials (such as polyethylene, polypropylene, polylactic acid, etc.). The advantages of this method include: Advantages: It can achieve good combination between different materials and improve the overall performance of the film, such as strength, barrier properties and transparency.

[0068] Application scenarios: Suitable for high-end food packaging that requires higher barrier properties and comprehensive performance, especially suitable for mass production.

[0069] The other types of packaging materials include, but are not limited to, one or more of polyethylene, polypropylene, polyethylene terephthalate, polylactic acid, aluminum foil, paper-based materials, and nylon.

[0070] The above-mentioned hot pressing method and co-extrusion method can adopt conventional methods and processes, which will not be described in detail here.

[0071] Comparative Example A fresh-keeping composite film without nicotinamide was prepared, and the specific method was as follows.

[0072] (1) Prepare Solution A and Solution B in the same manner as in Example 1.

[0073] (2) After mixing solution A and solution B, 25 mL of deionized water was added to prepare a solution free of niacinamide. The solution was continuously stirred in an oil bath at 50°C for 5 hours to obtain a mixed solution.

[0074] (3) The mixed solution was subjected to ultrasonic treatment at 25 °C for 1 h with an ultrasonic power of 360 W to eliminate bubbles and improve the homogeneity of the solution.

[0075] (4) The treated solution is slowly poured onto the pre-treated acrylic plate to ensure that the solution is evenly spread. Afterwards, the acrylic plate is placed in an oven at 50°C and dried for 8 hours to form a composite film with uniform thickness and smooth surface.

[0076] (5) The dried composite film is peeled off from the acrylic plate and left at room temperature for 24 hours to fully set and stabilize.

[0077] Experimental Example 1 Preparation of nicotinamide solution and its effect on the preservation of rapeseed: (1) Conduct preliminary experiments. The specific method is to take the same batch of freshly picked rapeseed, select rapeseed of similar size and without mechanical damage and randomly group them. The blank group is soaked in water for 5-10 seconds, and the experimental group is soaked in nicotinamide solutions of different concentrations for the same time as the blank group. Then, a preservation experiment is carried out at room temperature. According to the results of the preliminary experiments, 30 mmol / L is the best preservation effect for soaking green leafy vegetables in nicotinamide solution. Weigh 0.366 g nicotinamide, dissolve it in 100 mL deionized water, and stir until the nicotinamide is completely dissolved to prepare a nicotinamide solution with a concentration of 30 mmol / L.

[0078] (2) Select fresh rapeseed, wash and drain excess water. Then, completely soak the rapeseed in the above-prepared nicotinamide solution for 5-10 seconds.

[0079] (3) The treated rapeseed and the rapeseed not treated with niacinamide (blank group) were stored at room temperature of 25°C. The appearance, texture and other indicators of the rapeseed were observed and recorded every day, and the preservation effects of the niacinamide-treated group and the blank group during storage were compared.

[0080] On the fourth day of storage, the rapeseed leaves in the nicotinamide-treated group still maintained a bright green color, with a relatively firm texture and no obvious yellowing or wilting. In contrast, the rapeseed leaves in the untreated blank group showed obvious yellowing and wilting, and the preservation effect was significantly worse than that of the nicotinamide-treated group (such as Figure 1 The experimental results show that soaking in nicotinamide solution can effectively extend the shelf life of rapeseed and delay the aging process of vegetables.

[0081] Experimental Example 2 The following method was used to compare the freshness-keeping performance of the composite films of Examples 1-3 and the comparative example.

[0082] (1) Select fresh rapeseed, wash and drain excess water. Package the rapeseed with the nicotinamide composite films prepared in the above-mentioned Examples 1, 2 and 3, respectively, and set up a composite film packaging group of the comparative example as a blank group.

[0083] (2) Store the packaged rapeseed at room temperature of 25°C and regularly observe and record the appearance, color, texture and spoilage of the vegetables during storage.

[0084] (3) The performance parameters such as tensile strength, elongation at break and water vapor permeability coefficient of composite films with different niacinamide contents were tested respectively.

[0085] By comparing the preservation effects of different concentrations of nicotinamide composite films on green leafy vegetables (such as Figure 2As shown in Table 1), it was found that the preservation effect of the experimental group with added nicotinamide was better than that of the blank group. Among them, the composite film of Example 2 showed the best effect. Specifically, the vegetables packaged with the composite film of Example 2 still maintained a good appearance and texture on the 4th day of storage, with bright green leaves and no obvious wilting; while the vegetables preserved by the nicotinamide composite films of Examples 1 and 3 were slightly inferior, and yellowing occurred on the 4th day. In addition, the nicotinamide composite film of Example 2 performed extremely well in tensile strength, elongation at break and water vapor permeability coefficient, which was significantly improved compared with the composite film of the comparative example without nicotinamide, indicating that it has obvious advantages in preservation effect and physical properties of the film (as shown in Table 1).

[0086] The thickness is measured according to GB / T 6673-2001 "Mechanical measurement method for thickness of plastic film and sheet"; The determination of tensile strength and elongation at break shall refer to GB-T 13022-1991 "Test method for tensile properties of plastic films"; The haze measurement refers to GB / T 2410-2008 "Determination of light transmittance and haze of transparent plastics"; The oxygen transmission rate was determined according to ASTM D3985 2556 (coulometric method) at a temperature of 23°C. The carbon dioxide transmission rate is determined according to ISSO 2556 (pressure measurement method) at a temperature of 23°C. The water vapor transmission rate was measured in accordance with ISO 2528 (gravimetric method) at a temperature of 38°C and a relative humidity of 90%.

[0087] Table 1 Comparison of the characterization of composite membranes without nicotinamide and composite membranes with 0.17 wt% nicotinamide concentration Through the experimental study shown in Table 1, it was found that the composite film with nicotinamide added not only showed significant advantages in preservation effect, but also had obvious improvements in many physical performance indicators of the film. Specifically, the addition of nicotinamide significantly improved the film's gas permeability, tensile strength, elongation at break, haze, oxygen permeability, carbon dioxide permeability and water vapor permeability. These improvements are crucial to improving the preservation performance of green leafy vegetables at room temperature.

[0088] First, the composite film containing 0.17 wt% nicotinamide in the raw materials of Example 2 showed significant optimization in oxygen permeability and carbon dioxide permeability. By reducing the penetration of oxygen, the composite film can effectively inhibit the respiration rate of green leafy vegetables, slow down their metabolic process, and delay the oxidation and corruption of vegetables. The moderate carbon dioxide permeability helps maintain the gas balance in the package and prevents the acceleration of corruption caused by internal gas accumulation. Compared with the composite film without nicotinamide, the regulation of gas permeability plays a key role in extending the shelf life of vegetables.

[0089] Secondly, after adding nicotinamide, the tensile strength and elongation at break of the composite film are also significantly improved. These improvements mean that the composite film is more durable and flexible during use, can better adapt to various operating conditions during vegetable packaging and storage, reduce the possibility of breakage, and thus better protect the vegetables in the package.

[0090] In addition, the introduction of nicotinamide also affects the water vapor permeability and haze of the composite film. A lower water vapor permeability helps prevent vegetables from wilting due to excessive water loss during storage, and also avoids the problem of microbial growth caused by excessive water in the film. Although the increase in haze means that the transparency of the film is reduced, it also indicates that the light scattering performance of the film is enhanced, which can slow down the impact of light on the quality of vegetables to a certain extent, thereby prolonging the preservation effect.

[0091] Experimental Example 3 Comparison of preservation effects between nicotinamide immersion treatment and composite film treatment: (1) Weigh 0.366 g of nicotinamide and dissolve it in 100 mL of deionized water to prepare a nicotinamide solution with a concentration of 30 mmol / L, which is the optimal concentration for immersion treatment determined by preliminary experiments.

[0092] (2) Select fresh rapeseed, wash and drain excess water. Soak the rapeseed completely in the above nicotinamide solution for 5-10 seconds. After soaking, take out the rapeseed, gently shake off excess water, and place it in a ventilated place to air dry.

[0093] (3) The rapeseed was divided into two groups: one group was the rapeseed treated by soaking in the nicotinamide solution in step (2), and the other group was the untreated rapeseed packaged only with the nicotinamide-containing composite film prepared in Example 2.

[0094] (4) The two groups of rapeseed were stored at room temperature of 25°C. The appearance, color, texture and spoilage of the rapeseed were regularly observed and recorded during storage.

[0095] By comparison, it was found that (such as Figure 3As shown in the figure, the vegetables packaged with composite film are significantly better than those directly soaked in nicotinamide solution in terms of preservation effect. The vegetables in the composite film packaging group still maintained a good appearance and texture on the fourth day of storage, with bright green leaves and no obvious wilting; while the vegetables in the soaking treatment group began to turn yellow and slightly wilt on the fourth day. The experimental results show that the composite film is not only superior in preservation effect, but also can provide a longer shelf life.

[0096] Experimental Example 4 Comparison of preservation effects between nicotinamide composite film packaging and blank group: (1) The composite membrane prepared in Example 2 containing 0.17 wt% nicotinamide was used as the experimental group, and the composite membrane without nicotinamide was used as the blank group.

[0097] (2) Select fresh rapeseed, wash and drain excess water. Package the rapeseed with the composite films of the experimental group and the blank group respectively.

[0098] (3) The packaged rapeseed was stored at room temperature of 25°C for preservation experiments.

[0099] (4) During storage, regularly measure and record the respiration intensity, weight loss rate, and leaf yellowing rate.

[0100] The experimental results show that (such as Figure 4-7 As shown in the figure, on the 4th day of the preservation experiment, the respiration rate of the rapeseed packaged with the nicotinamide composite film in the experimental group was about 20% lower than that in the blank group, indicating that its metabolism slowed down and the preservation effect was significant. The weight loss rate of the rapeseed packaged with the nicotinamide composite film in the experimental group was 17% during storage, while the weight loss rate of the blank group reached 24%, indicating that the nicotinamide composite film in the experimental group effectively reduced water loss. On the 4th day of storage, the yellowing rate of the rapeseed packaged with the nicotinamide composite film in the experimental group was 15%, while the yellowing rate of the blank group was as high as 27%. The results show that the nicotinamide composite film in the experimental group is significantly better than the blank group in inhibiting leaf yellowing.

[0101] Experimental Example 5 The nicotinamide-containing fresh-keeping composite film of Example 4 was used as the experimental group, and the composite film without nicotinamide added in the comparative example was used as the control group. The rapeseed was packaged and stored at room temperature of 25°C using the same method as in Experimental Example 2. The experimental results were observed and recorded regularly. Figure 8 As shown, the fresh-keeping effect of the nicotinamide-containing fresh-keeping composite film of this embodiment is significantly better than that of the control group. No obvious wilting occurs after 4 days, while the control group shows obvious yellowing and wilting.

[0102] Application Example: Spinach, Shanghai green and cabbage are treated with the nicotinamide composite film of Example 2: (1) A 0.17 wt% nicotinamide composite film was prepared in the same manner as in Example 2.

[0103] (2) Select fresh spinach, Shanghai green and cabbage, wash them and drain excess water. The vegetables are divided into two groups, one group is packaged with the 0.17 wt% nicotinamide composite film prepared in step (1) as the experimental group, and the other group is not treated and serves as the blank group.

[0104] (3) Store the two groups of vegetables at room temperature of 25°C for 5 days, and measure and record the indicators regularly.

[0105] Packaging results show (such as Figure 9-11 As shown in the figure, the leaves of the vegetables in the blank group wilted and turned yellow and began to mold due to their own transpiration and respiration in the environment; while the vegetables in the experimental group packaged with the 0.17 wt% nicotinamide composite film had basically no significant changes on the fourth day of storage compared with before storage, the leaves remained fresh green, the wilting degree was light, and no mold appeared. The results show that the experimental group, i.e., the 0.17 wt% nicotinamide composite film of the present invention, has a significant fresh-keeping effect, can effectively maintain the quality of green leafy vegetables, and shows excellent practical application value.

[0106] In summary, the nicotinamide used in the present invention not only improves the fresh-keeping effect of the composite film through its own active substance characteristics, but also significantly enhances the overall performance of the composite film through the improvement of various physical properties. These improvements enable the composite film containing nicotinamide to better preserve green leafy vegetables at room temperature, extend their shelf life, and maintain excellent appearance and quality. These performance improvements provide strong support for the development and application of new green leafy vegetable packaging materials.

[0107] The above embodiments enable those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A nicotinamide-containing green leafy vegetable fresh-keeping composite film, characterized in that: The main raw materials of the composite film include the following components by mass percentage: 1.8-6 wt% of substrate, 0.3-1.0 wt% of citric acid, 0.3-1.0 wt% of glacial acetic acid, 0.05-0.5 wt% of nicotinamide, 0-0.5 wt% of additional substrate, and the balance is deionized water; the substrate is a mixture of chitosan and polyvinyl alcohol.

2. The green leafy vegetable fresh-keeping composite film containing nicotinamide according to claim 1, characterized in that: In the substrate, the mass percentage of chitosan in the entire composite membrane raw material is 0.3-1.5 wt%, and the mass percentage of polyvinyl alcohol in the entire composite membrane raw material is 1.5-4.5 wt%.

3. The green leafy vegetable fresh-keeping composite film containing nicotinamide according to claim 1, characterized in that: The additional substrate includes one or more of nanocellulose, sodium alginate and polyethylene glycol, and the amount thereof accounts for 0.3-0.5wt% of the entire composite membrane raw material.

4. The green leafy vegetable fresh-keeping composite film containing nicotinamide according to any one of claims 1 to 3, characterized in that: The thickness of the composite film is 30-80 μm.

5. The green leafy vegetable fresh-keeping composite film containing nicotinamide according to claim 1, characterized in that: The composite film further comprises other additives, wherein the additives include one or more of an antioxidant and a preservative.

6. The green leafy vegetable fresh-keeping composite film containing nicotinamide according to claim 5, characterized in that: The antioxidant is selected from one or more of vitamin E, vitamin C, gallic acid, tea polyphenols, butylated hydroxyanisole, and accounts for 0-1.0 wt% of the entire composite film raw material; the preservative is selected from one or more of potassium sorbate, sodium benzoate, parabens, sodium dehydroacetate, and nisin, and accounts for 0-0.5 wt% of the entire composite film raw material.

7. The method for preparing the green leafy vegetable fresh-keeping composite film containing nicotinamide according to claim 1, characterized in that: The following steps are involved: S1. Prepare solution A: dissolve chitosan in deionized water, add glacial acetic acid and citric acid, place the solution in an oil bath at 40-60° C., and stir for 2-3 hours to ensure that the chitosan is fully dissolved and forms a uniform solution; S2. Prepare liquid B: dissolve polyvinyl alcohol in deionized water, place in a water bath at 90-95°C and stir for 2-3 hours until it is completely dissolved and forms a uniform solution, mix liquid A and liquid B in equal volumes, slowly add the pre-prepared nicotinamide solution, stir in an oil bath at 45-50°C for 5-7 hours, and evenly disperse the nicotinamide in the mixed solution; S3. Place the obtained mixed solution in an ultrasonic device for treatment for 1-2 hours, slowly and evenly pour the treated mixed solution into a clean and dry acrylic mold, ensure that the liquid surface is flat, and dry it in an oven at 45-50°C for 8-10 hours to form a uniform and dense composite film.

8. The preparation method according to claim 7, characterized in that: The concentration of the nicotinamide solution in S2 is 0.5-3.0 wt %.

9. The use of the nicotinamide-containing green leafy vegetable fresh-keeping composite film according to claim 1, characterized in that: The composite film is suitable for preserving green leafy vegetables, which include but are not limited to rapeseed, spinach, and cabbage.

10. The use of the nicotinamide-containing green leafy vegetable fresh-keeping composite film according to claim 9, characterized in that: The green leafy vegetable fresh-keeping composite film containing nicotinamide is composited with other types of packaging materials, wherein the other types of packaging materials are selected from one or more of polyethylene, polypropylene, polyethylene terephthalate, polylactic acid, aluminum foil, paper-based materials, and nylon.

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