A fresh-keeping composite film for green leaf vegetables containing nicotinamide and a preparation method thereof

Abstract

The application 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, main raw materials of the composite film including a base material, citric acid, glacial acetic acid and nicotinamide; the base material is a mixture of chitosan and polyvinyl alcohol; the preparation method is that chitosan and polyvinyl alcohol are dissolved and mixed, then a nicotinamide solution is added, stirring and ultrasonic treatment are conducted to uniformly distribute the components, and finally a flow casting process is adopted to form a film. The composite film can effectively delay oxidation and corruption of green leaf vegetables, and significantly prolong the shelf life of the green leaf vegetables. The composite film has good mechanical properties and flexibility, and is suitable for various fresh-keeping applications. The preparation process is simple, easy for large-scale production, the material is environment-friendly and biodegradable, and the composite film has a wide application prospect.

Description

Technical Field

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

[0002] Leafy green vegetables play an important role in our daily diet, and are loved by consumers for their rich vitamins, minerals, and fiber. However, due to their high water content and vigorous metabolism, leafy green vegetables are highly susceptible to wilting, discoloration, and spoilage during storage and transportation. These problems not only shorten the shelf life of leafy green vegetables but also increase food waste and pose challenges to supply chain management. Therefore, extending the shelf life of leafy green vegetables has always been a pressing issue in the food packaging industry.

[0003] Existing preservation methods include cryogenic storage, modified atmosphere packaging, and the use of chemical preservatives. While these methods can extend the shelf life of leafy green vegetables to some extent, they also have many drawbacks. Cryogenic storage and modified atmosphere packaging typically require expensive equipment, are complex to operate, and are difficult to maintain stable conditions during long-distance transportation. Chemical preservatives, on the other hand, may have negative impacts on food safety, and their application is facing increasing restrictions as consumers' demand for healthy foods continues to rise.

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

[0005] This invention addresses the shortcomings of existing technologies in the preservation of leafy green vegetables by providing a composite film for leafy green vegetable preservation and its preparation method. The main raw materials include a substrate, citric acid, glacial acetic acid, and nicotinamide; wherein the substrate is a mixture of chitosan and polyvinyl alcohol; the preparation method involves dissolving and mixing chitosan and polyvinyl alcohol, adding a nicotinamide solution, and then stirring and ultrasonically treating the mixture to ensure uniform distribution of the components. Finally, a film is formed using a casting process. This composite film effectively delays the oxidation and spoilage of leafy green vegetables, significantly extending their shelf life. The composite film also possesses good mechanical properties and flexibility, making it suitable for various preservation applications. Its preparation process is simple, easy to mass-produce, and the materials are environmentally friendly and biodegradable, showing broad application prospects.

[0006] This invention proposes a nicotinamide-based composite film for preserving leafy green vegetables. Nicotinamide, as an important bioactive substance, has significant effects in inhibiting plant respiration, slowing metabolism, and exhibiting antioxidant and antibacterial properties. By introducing nicotinamide into the composite film, the shelf life of leafy green vegetables can be effectively extended while maintaining their nutritional components and appearance.

[0007] The composite membrane of this invention is prepared using chitosan and polyvinyl alcohol as substrates via a casting method. This invention is not limited to these materials; alternative substrates include gelatin, polylactic acid, hydroxypropyl methylcellulose, and xanthan gum. Furthermore, additional substrates, including one or more of nanocellulose, sodium alginate, and polyethylene glycol, can be added, which can also be used in the preparation process of this invention, thus providing a wider range of choices for different application needs.

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

[0009] In practical applications, nicotinamide has shown significant effects as an active ingredient in the preservation of leafy green vegetables, but there are also some challenges in its preparation:

[0010] 1. The amount of nicotinamide added needs to be precisely controlled. Excessive addition may affect the physical properties of the composite film, while insufficient addition may fail to achieve the desired preservation effect. Different substrates exhibit varying dispersibility and release effects of nicotinamide. Optimizing the substrate ratio to achieve the best preservation effect is a key research focus of this invention.

[0011] 2. Nicotinamide may decompose at higher temperatures, which requires controlling the temperature appropriately during the preparation process to ensure that the activity of nicotinamide is not affected.

[0012] To overcome these problems, the present invention has made technical improvements in the following aspects:

[0013] (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. This invention optimizes the dispersibility and release effect of nicotinamide by adjusting the ratio of chitosan and polyvinyl alcohol.

[0014] (2) During the preparation process, low-temperature stirring and ultrasonic treatment were used to ensure the uniform distribution of nicotinamide in the composite membrane and to prevent decomposition in a high-temperature environment.

[0015] After considering the above improvements, the specific technical solution of the present invention is as follows.

[0016] This invention first provides a nicotinamide-containing composite film for preserving leafy green vegetables. The main raw materials of the composite film contain the following components calculated by mass percentage:

[0017] The substrate comprises 1.8-6 wt%, citric acid 0.3-1.0 wt%, glacial acetic acid 0.3-1.0 wt%, nicotinamide 0.05-0.5 wt%, and additional substrate of 0-0.5 wt%, with deionized water as the balance; wherein the substrate is a mixture of chitosan and polyvinyl alcohol.

[0018] In the composite film, chitosan provides excellent film-forming properties and mechanical strength, while polyvinyl alcohol enhances the film's flexibility and durability. The introduction of citric acid and glacial acetic acid improves the film's physical properties, enabling the composite film to effectively delay the aging and spoilage of vegetables during preservation. Meanwhile, nicotinamide, as a preservative, is evenly distributed within the composite film, significantly extending the shelf life of leafy green vegetables through its antioxidant and antibacterial effects.

[0019] Preferably, in the substrate, chitosan accounts for 0.3-1.5 wt% of the total mass of the composite film raw material, and polyvinyl alcohol accounts for 1.5-4.5 wt% of the total mass of the composite film raw material.

[0020] In addition to chitosan and polyvinyl alcohol, alternative substrates can include one or more of gelatin, polylactic acid, xanthan gum, and hydroxypropyl methylcellulose. These substrates each possess different physicochemical properties, and through proper selection and combination, the performance of the composite film can be further optimized to meet the preservation needs of various leafy green vegetables.

[0021] Preferably, the additional substrate includes one or more of nanocellulose, sodium alginate and polyethylene glycol, and its amount accounts for 0.3-0.5 wt% of the total composite membrane raw material.

[0022] The composite film can be modified by adding additional substrates to adjust its mechanical properties, strength and barrier properties, but the purpose of this invention can still be achieved without adding additional substrates, and a food preservation composite film with good mechanical properties, strength, barrier properties and biodegradability can still be prepared.

[0023] In practical applications, one or more substrates can be selected and used together with additional substrates according to specific needs. For example, a composite film can be prepared in which xanthan gum and polyvinyl alcohol are used together as substrates, while nanocellulose is added as an additional substrate. Such a combination can produce a food preservation film that has both good mechanical properties and biodegradability, as well as high strength and barrier properties.

[0024] Preferably, the thickness of the nicotinamide-containing leafy vegetable preservation composite film obtained by the present invention is 30-80 μm.

[0025] Preferably, the nicotinamide-containing leafy vegetable preservation composite film also contains other additives, including one or more of antioxidants and preservatives.

[0026] The antioxidants can be selected from one or more of vitamin E, vitamin C, gallic acid, tea polyphenols, and butylated hydroxyanisole, accounting for 0-1.0 wt% of the total composite film raw materials; the preservatives can be selected from one or more of potassium sorbate, sodium benzoate, parabens, sodium dehydroacetate, and nisin, accounting for 0-0.5 wt% of the total composite film raw materials. The addition of the above additives can further enhance the preservation effect of vegetables.

[0027] In practical implementation, antioxidants and preservatives can be used in combination according to the different preservation needs of leafy green vegetables. For example, vitamin C and potassium sorbate can be added to the preparation solution of the membrane, so that it contains 0.3 wt% vitamin C and 0.2 wt% 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 as needed. Even without adding antioxidants and preservatives, a preservation composite film with good mechanical properties, strength, barrier properties, and biodegradability can still be prepared, and the purpose of this invention can still be achieved.

[0028] The present invention also provides a method for preparing the nicotinamide-containing leafy vegetable preservation composite film, comprising the following steps:

[0029] S1. Preparation of 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℃ and stir for 2-3 hours to ensure that the chitosan is fully dissolved and forms a homogeneous solution;

[0030] S2. Preparation of Solution B: Dissolve polyvinyl alcohol in deionized water and stir in a water bath at 90-95℃ for 2-3 hours until completely dissolved and a homogeneous solution is formed. Mix solutions A and B in equal volumes, then slowly add the pre-prepared nicotinamide solution and stir in an oil bath at 45-50℃ for 5-7 hours to ensure that the nicotinamide is evenly dispersed in the mixture.

[0031] S3. The obtained mixture is placed in an ultrasonic device for 1-2 hours to eliminate air bubbles and improve the density of the composite film. The treated mixture is then slowly and evenly poured into a pre-treated acrylic mold to ensure a smooth surface. The mixture is then dried in an oven at 45-50°C for 8-10 hours to form a uniform and dense composite film.

[0032] The ultrasonic treatment can be performed under conventional conditions. In this invention, the preferred ultrasonic power is 360 W, the temperature is 25°C, and the ultrasonic time is 1 hour.

[0033] 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. A staged addition process is adopted during mixing, combined with low-temperature stirring and ultrasonic treatment, and finally, a composite film is prepared by casting. During the mixing process, because the amount of nicotinamide added is relatively small, and its physical properties make it difficult to disperse in the matrix, this invention first mixes a portion of chitosan with glacial acetic acid and citric acid to ensure the formation of an initial solution with certain viscosity and stability. Then, a polyvinyl alcohol solution is added, and through stirring and temperature control, the viscosity of the solution is made moderate, which helps to uniformly disperse nicotinamide. A pre-prepared nicotinamide solution is slowly added to the above mixture, and low-temperature stirring technology is used to ensure that nicotinamide is uniformly dispersed in the matrix material, avoiding its decomposition or deactivation at higher temperatures. Ultrasonic treatment further eliminates air bubbles in the solution, increases the uniformity and stability of the solution, and ensures the effective distribution of nicotinamide during film formation.

[0034] The concentration of the pre-prepared nicotinamide solution mentioned above is 0.5-3.0 wt%. Using nicotinamide solutions within this concentration range can effectively improve the shelf life of leafy green vegetables. If the concentration is less than 0.5 wt%, it may affect the preservation performance of the preservation composite film, resulting in a weak preservation effect, failing to significantly delay the spoilage of leafy green vegetables, and failing to achieve the goal 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 improvement in effect is not significant, resulting in the phenomenon of "diminishing marginal returns." In addition, high concentrations of nicotinamide may precipitate, causing changes in other physical properties of the film, such as transparency, flexibility, and water vapor permeability, affecting the performance of the preservation composite film.

[0035] The pretreatment involves cleaning and drying the acrylic mold before use.

[0036] In the specific application and implementation of this invention, the nicotinamide-containing leafy vegetable preservation composite film can not only be used alone, but also 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.

[0037] Specifically, the combined use includes, but is not limited to, the following two optimal methods:

[0038] 1. Hot pressing method

[0039] Hot pressing softens the materials at the contact surfaces through heating, forming a strong adhesive. It is commonly used to laminate nicotinamide-containing food wraps with other materials (such as polyethylene and polypropylene). This method offers the following advantages:

[0040] Advantages: Ensures the airtightness and strength of the packaging, suitable for efficient, large-scale production.

[0041] Application scenarios: Commonly used in the food packaging industry, it provides good barrier properties and mechanical strength, and can effectively extend the shelf life of leafy green vegetables.

[0042] 2. Co-extrusion method

[0043] Co-extrusion is a method that uses nicotinamide-containing food preservation film to simultaneously form multilayer composite films with other packaging materials (such as polyethylene, polypropylene, and polylactic acid). The advantages of this method include:

[0044] Advantages: It can achieve good bonding between different materials and improve the overall performance of the membrane, such as strength, barrier properties and transparency.

[0045] Application scenarios: Suitable for high-end food packaging that requires high barrier properties and comprehensive performance, especially suitable for mass production.

[0046] The nicotinamide-containing leafy vegetable preservation composite film of the present invention is suitable for the preservation of leafy vegetables, including but not limited to rapeseed, spinach, and bok choy.

[0047] This invention allows for flexible control of the physical properties and preservation effect of the composite film by adjusting the ratio of chitosan to polyvinyl alcohol and the amounts of citric acid and glacial acetic acid, according to different preservation needs. Furthermore, this invention can further optimize the functionality and applicability of the film by selecting different substrate combinations, such as gelatin and sodium alginate, to meet the preservation needs of different types of leafy green vegetables.

[0048] Compared with the prior art, the advantages of the present invention are:

[0049] 1) The composite film with added nicotinamide not only shows a significant advantage in preservation effect, but also has obvious improvements in many physical performance indicators of the film. Specifically, the addition of nicotinamide significantly improves the performance of the film in terms of gas permeability, tensile strength, elongation at break, haze, oxygen permeability, carbon dioxide permeability and water vapor permeability. From the perspective of multiple performance indicators of the composite film, the preservation performance of the composite film is greatly improved. These improvements are crucial for improving the preservation performance of leafy green vegetables at room temperature.

[0050] 2) The composite film with added nicotinamide showed significant optimization in oxygen and carbon dioxide permeability. By reducing oxygen permeation, the composite film can effectively inhibit the respiration rate of leafy green vegetables, slow down their metabolic processes, and delay the oxidation and spoilage of vegetables. Meanwhile, the moderate carbon dioxide permeability helps maintain the gas balance inside the packaging and prevents the accumulation of internal gas from accelerating spoilage. Compared with the composite film without added nicotinamide, this regulation of gas permeability plays a key role in extending the shelf life of vegetables.

[0051] 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, and can better adapt to various operating conditions during vegetable packaging and storage, reducing the possibility of damage and thus better protecting the vegetables inside the packaging.

[0052] 4) The introduction of nicotinamide also affects the water vapor transmission rate and haze of the composite film. Lower water vapor transmission rate helps prevent vegetables from wilting due to excessive water loss during storage, and also avoids the problem of microbial growth caused by excessive moisture 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 mitigate the impact of light on the quality of vegetables to a certain extent, thereby prolonging the preservation effect.

[0053] 5) In practical applications, the composite film of this invention exhibits significant advantages in terms of preservation effect, environmental friendliness, and production cost. Experiments show that leafy greens packaged using the composite film of this invention have a shelf life extended by 2 to 3 times compared to traditional methods, effectively maintaining the nutritional value and freshness of the vegetables. Furthermore, the preparation process of this invention is simple and easy to implement, suitable for industrial production, and has low production costs, meeting the needs of large-scale market applications. At the same time, all components of the composite film are environmentally friendly materials that can naturally degrade after use, meeting the requirements of sustainable development.

[0054] In summary, nicotinamide not only enhances the preservation effect of composite films through its own active substance properties, but also significantly improves the overall performance of composite films by improving various performance indicators. These improvements enable nicotinamide-containing composite films to better preserve leafy green vegetables at room temperature, extending their shelf life and maintaining excellent appearance and quality. These performance enhancements provide strong support for the development and application of novel packaging materials for leafy green vegetables, offering an environmentally friendly and economical preservation solution with broad application prospects and market value. Attached Figure Description

[0055] Figure 1 This is a comparison of the morphology of the rapeseed blank group and the soaking experimental group in Experiment Example 1 during the preservation experiment;

[0056] Figure 2 This is a comparison of the morphology of the rapeseed blank group and the experimental groups with composite membranes of different nicotinamide concentrations in Experiment Example 2 during the preservation experiment;

[0057] Figure 3 The image shows a comparison of the morphology of the rapeseed soaking experimental group and the 0.17 wt% nicotinamide composite membrane experimental group in Experiment Example 3 during the preservation experiment.

[0058] Figure 4 The image shows a comparison of the morphology of the rapeseed blank group and the experimental group with a composite film at a concentration of 0.17 wt% nicotinamide in Experiment Example 4 during the preservation experiment.

[0059] Figure 5 This is a comparison of respiration intensity between the rapeseed blank group and the experimental group with a composite membrane containing 0.17 wt% nicotinamide in Experiment Example 4.

[0060] Figure 6 This is a comparison of the weight loss rates between the rapeseed blank group and the composite membrane experimental group with a concentration of 0.17 wt% nicotinamide in Experiment Example 4.

[0061] Figure 7 This is a comparison of the leaf yellowing rate between the rapeseed blank group and the experimental group with a composite film containing 0.17 wt% nicotinamide in Experiment Example 4.

[0062] Figure 8 This is a comparison of the morphology of the rapeseed control group and the experimental group in Experiment Example 5 during the preservation experiment;

[0063] Figure 9 This is a comparison of the morphology of the spinach blank group and the experimental group with a composite membrane at a concentration of 0.17 wt% nicotinamide in the preservation experiment of the application example;

[0064] Figure 10 The image shows a comparison of the morphology of the Shanghai bok choy blank group and the experimental group with a composite membrane containing 0.17 wt% nicotinamide in the preservation experiment of the application example.

[0065] Figure 11 This is a comparison of the morphology of the cabbage blank group and the experimental group with a composite membrane at a concentration of 0.17 wt% nicotinamide in the preservation experiment of the application example. Detailed Implementation

[0066] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are only for illustrating the present invention and are not intended to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims. Unless otherwise specified, the technologies used are all prior art.

[0067] The methods for measuring weight loss rate, leaf chlorosis rate, and respiration intensity are as follows:

[0068] Weight loss rate: Using the same precision balance, the weight of the same vegetable was measured at 24-hour intervals. Weight loss rate = (initial weight - final weight) / initial weight × 100%.

[0069] Leaf yellowing rate: Grade 0 is defined as no yellowing of leaves, Grade 1 as 0-1 / 4 yellowing, Grade 2 as 1 / 4-2 / 4 yellowing, Grade 3 as 2 / 4-3 / 4 yellowing, and Grade 4 as 3 / 4 to completely yellow. The formula for calculating the yellowing rate is: Yellowing rate = ΣX i Y / 4A. Where: X i = The yellowing level of the leaves; i = 0, 1, 2, 3, 4; Y = the number of leaves with different yellowing levels; A = the total number of leaves per treatment.

[0070] Respiration intensity: The respiration intensity of leafy green vegetables was measured using a SY-1022 fruit and vegetable respiration meter. It is expressed as the cumulative CO2 released per kilogram of leafy green vegetables per hour, in mg / (kg·h).

[0071] Example 1: A nicotinamide-containing preservative composite film with a nicotinamide content of 0.08 wt% in the raw materials was prepared using the following method.

[0072] (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%.

[0073] (2) Preparation of solution 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℃ and stir for 2 hours to ensure that the chitosan is completely dissolved and form a uniform solution A.

[0074] (3) Preparation of solution B: Dissolve 4g of polyvinyl alcohol in 100 mL of deionized water and stir in a 95℃ water bath for 2 hours until the polyvinyl alcohol is completely dissolved to form a uniform solution B.

[0075] (4) After mixing solution A and solution B, add 25 mL of nicotinamide solution with a mass concentration of 0.79 wt% obtained in step (1) to make a solution with a nicotinamide weight percentage of 0.08 wt%. Stir the above solution continuously for 5 hours in an oil bath at 50°C to ensure that nicotinamide is evenly dispersed in the solution to obtain a mixed solution.

[0076] (5) The mixed solution was subjected to ultrasonic treatment at 25°C for 1 hour with an ultrasonic power of 360 W to eliminate bubbles and improve the homogeneity of the solution.

[0077] (6) Slowly pour the treated solution onto the pretreated acrylic plate to ensure that the solution is spread evenly. Then, place the acrylic plate in an oven at 50°C and dry for 8 hours to form a composite film with uniform thickness and a smooth surface.

[0078] (7) The dried composite film is peeled off from the acrylic plate and placed at room temperature for 24 hours to fully set and stabilize. The composite film has good mechanical properties and flexibility.

[0079] Example 2: A nicotinamide-containing preservative composite film with a nicotinamide content of 0.17 wt% was prepared using the same method as in Example 1.

[0080] 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 mixing liquid A and liquid B in equal volumes, 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 nicotinamide weight percentage of 0.17 wt%. The above solution is continuously stirred in an oil bath at 50°C for 5 hours to ensure that the nicotinamide is uniformly dispersed in the solution to obtain a mixed solution.

[0081] The remaining steps are the same as in Example 1.

[0082] Example 3: A nicotinamide-containing preservative composite film with a nicotinamide content of 0.25 wt% in the raw materials was prepared using the same method as in Example 1.

[0083] 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 obtain a nicotinamide solution with a mass concentration of 2.34 wt%; in step (4) of this embodiment, after mixing liquid A and liquid B in equal volumes, 25 mL of the nicotinamide solution with a mass concentration of 2.34 wt% obtained in step (1) is added to make a solution with a nicotinamide weight percentage of 0.25 wt%. The above solution is continuously stirred in an oil bath at 50°C for 5 hours to ensure that the nicotinamide is uniformly dispersed in the solution to obtain a mixed solution.

[0084] The remaining steps are the same as in Example 1.

[0085] Example 4: A nicotinamide-containing preservation composite film was prepared using the same method as in Example 2.

[0086] The difference is that chitosan was not added in this embodiment, and xanthan gum was used instead of chitosan; in addition, after mixing solution A and solution B in step 4), vitamin E and potassium sorbate were added to the mixture of solution A and solution B, so that the mixture contained 0.3 wt% vitamin E and 0.1 wt% potassium sorbate.

[0087] Example 5

[0088] The composite film prepared by the above method can be used in combination with other types of packaging materials to further enhance the functionality and applicability of the packaging. Specifically, the food preservation composite film of the present invention can be used in combination with other packaging materials using hot pressing and co-extrusion methods.

[0089] 1. Hot pressing method

[0090] Hot pressing softens the materials at the contact surfaces through heating, forming a strong adhesive. It is commonly used to laminate nicotinamide-containing food wraps with other materials (such as polyethylene and polypropylene). This method offers the following advantages:

[0091] Advantages: Ensures the airtightness and strength of the packaging, suitable for efficient, large-scale production.

[0092] Application scenarios: Commonly used in the food packaging industry, it provides good barrier properties and mechanical strength, and can effectively extend the shelf life of leafy green vegetables.

[0093] 2. Co-extrusion method

[0094] Co-extrusion is a method that uses nicotinamide-containing food preservation film to simultaneously form multilayer composite films with other packaging materials (such as polyethylene, polypropylene, and polylactic acid). The advantages of this method include:

[0095] Advantages: It can achieve good bonding between different materials and improve the overall performance of the membrane, such as strength, barrier properties and transparency.

[0096] Application scenarios: Suitable for high-end food packaging that requires high barrier properties and comprehensive performance, especially suitable for mass production.

[0097] 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.

[0098] The hot pressing and co-extrusion methods described above can be performed using conventional methods and processes, and will not be elaborated further here.

[0099] The following is a comparative method for preparing a food preservation composite film without nicotinamide.

[0100] (1) Prepare solution A and solution B in the same manner as in Example 1.

[0101] (2) After mixing solution A and solution B, add 25 mL of deionized water to prepare a solution without nicotinamide. Stir the solution continuously for 5 hours in an oil bath at 50°C to obtain a mixed solution.

[0102] (3) The mixed solution was subjected to ultrasonic treatment at 25°C for 1 hour with an ultrasonic power of 360 W to eliminate bubbles and improve the homogeneity of the solution.

[0103] (4) Slowly pour the treated solution onto the pretreated acrylic plate to ensure that the solution is spread evenly. Then, place the acrylic plate in an oven at 50°C and dry for 8 hours to form a composite film with uniform thickness and a smooth surface.

[0104] (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.

[0105] Experiment Example 1: Preparation of Nicotinamide Solution and its Preservation Effect on Rapeseed:

[0106] (1) Preliminary experiments were conducted. Specifically, the same batch of freshly harvested rapeseed was selected, and rapeseed of similar size and without mechanical damage was randomly grouped. The blank group was soaked in water for 5-10 seconds, while the experimental groups were soaked in nicotinamide solutions of different concentrations for the same time as the blank group. Afterward, a preservation experiment was conducted at room temperature. Based on the results of the preliminary experiments, it was found that 30 mmol / L nicotinamide solution achieved the best preservation effect for leafy green vegetables. 0.366 g of nicotinamide was weighed and dissolved in 100 mL of deionized water. The solution was stirred until the nicotinamide was completely dissolved to obtain a nicotinamide solution with a concentration of 30 mmol / L.

[0107] (2) Select fresh rapeseed, wash and drain excess water. Then, immerse the rapeseed completely in the prepared nicotinamide solution for 5-10 seconds.

[0108] (3) The treated rapeseed and the untreated rapeseed (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 to compare the preservation effect of the nicotinamide-treated group and the blank group during storage.

[0109] On the fourth day of storage, the rapeseed leaves in the nicotinamide-treated group remained bright green, firm, and showed no obvious yellowing or wilting. In contrast, the untreated control group showed obvious yellowing and wilting, indicating a significantly worse preservation effect compared to the nicotinamide-treated group (e.g., ...). Figure 1 (As shown in the image). Experimental results show that soaking in nicotinamide solution can effectively extend the shelf life of rapeseed and delay the aging process of vegetables.

[0110] Experimental Example 2: The preservation performance of the composite films of Examples 1-3 and the comparative examples was compared using the following method.

[0111] (1) Select fresh rapeseed, wash and drain excess water. Pack the rapeseed with the nicotinamide composite film prepared in Examples 1, 2 and 3 above, and set up a comparative composite film packaging group as a blank group.

[0112] (2) Store the packaged rapeseed separately at room temperature of 25°C, and regularly observe and record the appearance, color, texture and spoilage of the vegetables during storage.

[0113] (3) Test the tensile strength, elongation at break and water vapor permeability of composite membranes with different nicotinamide contents.

[0114] The preservation effects of different concentrations of nicotinamide composite films on leafy green vegetables were compared (e.g., Figure 2 As shown in Table 1, the experimental groups with added nicotinamide showed better preservation effects than the control group. Among them, the composite film of Example 2 showed the best effect. Specifically, vegetables packaged with the composite film of Example 2 maintained good appearance and texture on the 4th day of storage, with bright green leaves and no obvious wilting; while the vegetables packaged with the nicotinamide composite films of Examples 1 and 3 showed slightly worse preservation effects, and yellowing occurred on the 4th day. In addition, the nicotinamide composite film of Example 2 showed excellent performance in tensile strength, elongation at break, and water vapor transmission coefficient, which were significantly improved compared with the composite film without added nicotinamide in the control group, indicating that it has obvious advantages in preservation effect and physical properties of the film (as shown in Table 1).

[0115] The thickness was determined in accordance with GB / T 6673-2001 "Mechanical Measurement Method for Determination of Thickness of Plastic Films and Sheets";

[0116] The determination of tensile strength and elongation at break shall be in accordance with GB-T 13022-1991 "Test Method for Tensile Properties of Plastic Films";

[0117] The determination of haze shall be in accordance with GB / T 2410-2008 "Determination of light transmittance and haze of transparent plastics";

[0118] The oxygen permeability was determined according to ASTM D3985 2556 (coulometric method) at a temperature of 23°C.

[0119] The carbon dioxide transmission rate was determined according to ISSO 2556 (pressure method) at a temperature of 23°C.

[0120] Water vapor transmission rate was determined according to ISO 2528 (gravimetric method), at a temperature of 38°C and a relative humidity of 90%.

[0121] Table 1. Comparison of characterization between composite membranes without nicotinamide and composite membranes with a concentration of 0.17 wt% nicotinamide.

[0122]

[0123] The experimental results shown in Table 1 revealed that the composite film with added nicotinamide not only exhibited significant advantages in preservation but also showed marked improvements in several physical properties. 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 for enhancing the preservation performance of leafy green vegetables at room temperature.

[0124] First, the composite film containing 0.17 wt% nicotinamide in the raw materials of Example 2 showed significant optimization in terms of oxygen and carbon dioxide permeability. By reducing oxygen permeation, the composite film effectively inhibited the respiration rate of leafy green vegetables, slowed down their metabolic processes, and delayed oxidation and spoilage. Meanwhile, the moderate carbon dioxide permeability helped maintain gas balance within the packaging, preventing accelerated spoilage caused by internal gas accumulation. Compared to the composite film without nicotinamide, this regulation of gas permeability played a crucial role in extending the shelf life of vegetables.

[0125] Secondly, the addition of nicotinamide significantly improves the tensile strength and elongation at break of the composite film. These improvements mean that the composite film is more durable and flexible during use, better adapting to various operating conditions during vegetable packaging and storage, reducing the possibility of breakage, and thus better protecting the vegetables inside the packaging.

[0126] Furthermore, the introduction of nicotinamide also affected the water vapor transmission rate and haze of the composite membrane. Lower water vapor transmission rate helps prevent vegetables from wilting due to excessive moisture loss during storage, while also avoiding microbial growth problems caused by excessive moisture inside the membrane. Although increased haze means a decrease in membrane transparency, it also indicates enhanced light scattering properties, which can mitigate the impact of light on vegetable quality to some extent, thereby prolonging the preservation effect.

[0127] Experiment Example 3: Comparison of preservation effects between nicotinamide immersion treatment and composite film treatment:

[0128] (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. This concentration is the optimal concentration for soaking treatment determined by the preliminary experiment.

[0129] (2) Select fresh rapeseed, wash and drain excess water. Immerse the rapeseed completely in the above nicotinamide solution for 5-10 seconds. After soaking, remove the rapeseed, gently shake off excess water, and place it in a ventilated place to air dry.

[0130] (3) Divide the rapeseed into two groups: one group is rapeseed treated with nicotinamide solution in step (2), and the other group is untreated rapeseed packaged with nicotinamide composite film prepared in Example 2.

[0131] (4) The two groups of rapeseed were stored at room temperature of 25°C. The appearance, color, texture and spoilage of the rapeseed were observed and recorded regularly during storage.

[0132] Through comparison, it was found that (e.g.) Figure 3 As shown in the figure, vegetables packaged with composite film exhibited significantly better preservation effects than vegetables directly immersed in nicotinamide solution. On the fourth day of storage, the vegetables in the composite film-packaged group maintained good appearance and texture, with bright green leaves and no obvious wilting; while the vegetables in the immersion treatment group began to show yellowing and slight wilting on the fourth day. The experimental results indicate that composite film not only provides superior preservation effects but also offers a longer shelf life.

[0133] Experiment Example 4: Comparison of preservation effects between nicotinamide composite film packaging and the control group:

[0134] (1) The composite membrane containing 0.17 wt% nicotinamide prepared in Example 2 was used as the experimental group, and the composite membrane without nicotinamide was used as the blank group.

[0135] (2) Select fresh rapeseed, wash and drain excess water. Pack the rapeseed separately using the experimental group composite film and the blank group composite film.

[0136] (3) The packaged rapeseed was stored at room temperature of 25°C for preservation experiments.

[0137] (4) During storage, respiration intensity, weight loss rate and leaf yellowing rate should be measured and recorded regularly.

[0138] Experimental results show (e.g.) Figure 4-7 As shown in the figure, on the fourth day of the preservation experiment, the respiration rate of rapeseed packaged with nicotinamide composite film in the experimental group was reduced by approximately 20% compared to the control group, indicating a slowdown in metabolism and a significant preservation effect. The weight loss rate of rapeseed packaged with nicotinamide composite film in the experimental group was 17% during storage, while the weight loss rate in the control group reached 24%, indicating that the nicotinamide composite film in the experimental group effectively reduced moisture loss. On the fourth day of storage, the yellowing rate of rapeseed leaves packaged with nicotinamide composite film in the experimental group was 15%, while the yellowing rate in the control group was as high as 27%. The results show that the nicotinamide composite film in the experimental group was significantly more effective than the control group in inhibiting leaf yellowing.

[0139] Experimental Example 5

[0140] The nicotinamide-containing preservative composite film of Example 4 was used as the experimental group, and the composite film without added nicotinamide was used as the control group. Using the same method as in Example 2, the rapeseed was packaged and stored at room temperature (25°C). Observations and records were made periodically. The experimental results are as follows: Figure 8 As shown, the preservation effect of the nicotinamide-containing preservation composite film in this embodiment is significantly better than that of the control group. It did not show obvious wilting after 4 days, while the control group showed obvious yellowing and wilting.

[0141] Application example: Spinach, bok choy, and tartary spinach were treated using the nicotinamide composite membrane from Example 2.

[0142] (1) Prepare a 0.17 wt% nicotinamide composite membrane according to the same method as in Example 2.

[0143] (2) Select fresh spinach, bok choy and tatsoi, wash them thoroughly 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 in any way as the blank group.

[0144] (3) Store the two groups of vegetables at room temperature of 25°C for 5 days, and measure and record the indicators regularly.

[0145] Packaging results show (e.g.) Figure 9-11 As shown in the figure, the vegetables in the control group exhibited significant wilting and yellowing of leaves and began to mold due to their own transpiration and respiration in the environment. In contrast, the vegetables in the experimental group packaged with 0.17 wt% nicotinamide composite film showed virtually no change after four days of storage compared to before storage; the leaves remained bright green, the wilting was minimal, and no mold was observed. The results indicate that the 0.17 wt% nicotinamide composite film of this invention has a significant preservation effect, effectively maintaining the quality of leafy green vegetables and demonstrating superior practical application value.

[0146] In summary, the nicotinamide used in this invention not only improves the preservation effect of the composite film through its own active substance properties, but also significantly enhances the overall performance of the composite film through improvements in various physical properties. These improvements enable the nicotinamide-containing composite film to better preserve leafy green vegetables at room temperature, extending their shelf life and maintaining excellent appearance and quality. These performance enhancements provide strong support for the development and application of novel packaging materials for leafy green vegetables.

[0147] The above embodiments enable those skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily 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 invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A nicotinamide-containing composite film for preserving leafy green vegetables, characterized in that, The main raw materials of the composite membrane contain the following components by mass percentage: The substrate comprises 1.8-6 wt%, citric acid 0.3-1.0 wt%, glacial acetic acid 0.3-1.0 wt%, nicotinamide 0.05-0.5 wt%, and additional substrate of 0-0.5 wt%, with deionized water as the balance; the substrate is a mixture of chitosan and polyvinyl alcohol. The thickness of the composite membrane is 30-80 μm.

2. The nicotinamide-containing leafy vegetable preservation composite film according to claim 1, characterized in that, The additional substrate is 0 wt%.

3. The nicotinamide-containing leafy vegetable preservation composite film according to claim 1, characterized in that, In the substrate, chitosan accounts for 0.3-1.5 wt% of the total mass of the composite film raw material, and polyvinyl alcohol accounts for 1.5-4.5 wt% of the total mass of the composite film raw material.

4. The nicotinamide-containing leafy vegetable preservation composite film according to claim 1, characterized in that, The additional substrate includes one or more of nanocellulose, sodium alginate and polyethylene glycol, and its amount accounts for 0.3-0.5 wt% of the total composite membrane raw material.

5. The nicotinamide-containing leafy vegetable preservation composite film according to claim 1, characterized in that, The composite membrane also contains other additives, including one or more of antioxidants and preservatives.

6. The nicotinamide-containing leafy vegetable preservation composite film according to claim 5, characterized in that, The antioxidant is selected from one or more of vitamin E, vitamin C, gallic acid, tea polyphenols, and butylated hydroxyanisole, accounting for 0-1.0 wt% of the total composite membrane raw materials; the preservative is selected from one or more of potassium sorbate, sodium benzoate, para-hydroxybenzoate, sodium dehydroacetate, and nisin, accounting for 0-0.5 wt% of the total composite membrane raw materials.

7. The method for preparing the nicotinamide-containing leafy vegetable preservation composite film according to claim 2, characterized in that, Includes the following steps: S1. Preparation of solution A: Dissolve chitosan in deionized water, add glacial acetic acid and citric acid, place the solution in an oil bath at 40-60℃, and stir for 2-3 hours to ensure that the chitosan is fully dissolved and forms a homogeneous solution; S2. Preparation of solution B: Dissolve polyvinyl alcohol in deionized water and stir in a water bath at 90-95℃ for 2-3 hours until completely dissolved and a homogeneous solution is formed. Mix solution A and solution B in equal volumes, and slowly add the pre-prepared nicotinamide solution. Stir in an oil bath at 45-50℃ for 5-7 hours to ensure that nicotinamide is evenly dispersed in the mixture. S3. Place the obtained mixture in an ultrasonic device for 1-2 hours. After treatment, slowly and evenly pour the mixture into a clean and dry acrylic mold, ensuring that the liquid surface is flat. Dry it in an oven at 45-50℃ 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 nicotinamide solution in S2 is 0.5-3.0 wt%.

9. The use of the nicotinamide-containing leafy vegetable preservation composite film according to claim 1, characterized in that, The composite film is suitable for preserving leafy green vegetables, including rapeseed, spinach, and bok choy.

10. The use of the nicotinamide-containing leafy vegetable preservation composite film according to claim 9, characterized in that, The nicotinamide-containing leafy vegetable preservation composite film is combined 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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