A hydroxypropyl methylcellulose coated fruit and vegetable preservative film and a method for preparing the same

Abstract

This invention belongs to the field of food packaging technology and discloses a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film and its preparation method. The preservation film is composed of hydroxypropyl methylcellulose and phenolic-quaternary ammonium salt grafted chitosan, supplemented with food-grade additives such as glycerol, glyceryl monostearate, sodium citrate, talc, and potassium sorbate, forming a composite coating material with good antibacterial, moisture-barrier, and gas control capabilities. By uniformly spraying this film solution onto the surface of fruits and vegetables and drying it with low-temperature hot air, the shelf life of fruits and vegetables can be significantly extended. Experiments show that the film can effectively inhibit the growth of *Escherichia coli* and *Penicillium*, while exhibiting low water vapor and oxygen permeability, good storage stability of the film solution, and no stratification or flocculation, making it suitable for industrial applications in fruit and vegetable preservation. This invention has the advantages of safe raw materials, simple process, and stable performance, making it suitable for widespread application in the green packaging and transportation of fresh fruits and vegetables.

Description

Technical Field

[0001] This invention belongs to the field of food packaging technology, specifically relating to a hydroxypropyl methylcellulose coated fruit and vegetable preservation film and its preparation method. Background Technology

[0002] With the extended post-harvest storage, transportation, and sales cycle of fruits and vegetables, they are highly susceptible to moisture loss, oxidative deterioration, and microbial contamination at room temperature, leading to quality decline and spoilage. Therefore, coating fruits and vegetables with edible preservative films has become an important method for storage and preservation. Hydroxypropyl methylcellulose (HPMC), as a widely available, film-forming, non-toxic, and edible polymer material, is widely used in food packaging and surface protective films. HPMC coatings have good oxygen and water vapor barrier properties, but they themselves do not possess significant antibacterial activity and are prone to moisture absorption and expansion in humid environments, affecting the structural stability and preservation effect of the coating.

[0003] To enhance the functionality of edible coatings, existing technologies often incorporate natural antibacterial components such as chitosan, essential oils, and polyphenols in compound formulations. However, most studies rely solely on physical mixing methods, lacking effective structural synergistic mechanisms. This leads to uneven distribution or migration failure of antibacterial components within the membrane, and the membrane is prone to phase separation or precipitation, affecting the stability and functional continuity of practical applications. While some modified chitosan systems possess certain antibacterial properties, their poor water solubility and low compatibility with HPMC make it difficult to construct stable composite membrane structures.

[0004] Therefore, there is an urgent need to develop an HPMC-based fruit and vegetable coating preservation film that is structurally stable, functionally synergistic, and has high antibacterial and preservation effects, especially to achieve synergistic enhancement at the molecular level to improve the film-forming properties, antibacterial properties, and storage protection performance of the film, so as to meet the needs of modern fruit and vegetable processing and green preservation. Summary of the Invention

[0005] To address the shortcomings mentioned in the background art, the present invention aims to provide a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film and its preparation method. By grafting chitosan with phenolic-quaternary ammonium salt and HPMC, a functional coating film with good antibacterial, antioxidant and gas barrier properties is obtained, which can significantly extend the shelf life of fruits and vegetables. The process is simple and the film is safe and edible.

[0006] The objective of this invention can be achieved through the following technical solutions: A hydroxypropyl methylcellulose-coated fruit and vegetable preservation film comprises the following raw materials in parts by weight: 100-150 parts hydroxypropyl methylcellulose, 5-30 parts phenolic-quaternary ammonium salt grafted chitosan, 5-15 parts glycerol, 1-5 parts glyceryl monostearate, 0.5-3 parts sodium citrate, 0.5-3 parts talc, and 0.1-1.5 parts potassium sorbate.

[0007] More preferably, the preparation method of phenolic-quaternary ammonium salt grafted chitosan specifically includes the following steps: S101. Add gallic acid to the reaction vessel, add sodium carbonate to adjust the pH to alkaline, stir to dissolve, then add hexamethylenetrimethylamine and continue stirring to react, to obtain a phenolic-quaternary ammonium salt intermediate; S102. Dissolve chitosan in dilute acetic acid solution and stir. Add the phenolic-quaternary ammonium salt intermediate obtained in step S101 to the chitosan solution and continue the reaction under stirring to obtain the grafting product. S103. Transfer the reaction solution into a dialysis bag and dialyze it in deionized water to remove unreacted small molecules. Freeze-dry the dialysate to obtain phenolic-quaternary ammonium salt grafted chitosan.

[0008] More preferably, the hydroxypropyl methylcellulose is food-grade hydroxypropyl methylcellulose with a viscosity of 8000~12000 mPa·s.

[0009] More preferably, the talc is food-grade ultrafine talc with a particle size of less than 10 μm.

[0010] More preferably, the components in the formula are mixed evenly by magnetic stirring before use, and the plastic wrap is applied to the surface of fruits and vegetables by spraying and dried by hot air at a temperature of 35℃~50℃.

[0011] More preferably, glycerol and sodium citrate work together as a plasticizing buffer composite system to control the stability and flexibility of the coating film in an environment of pH 4.5–5.5.

[0012] A method for preparing a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film includes the following steps: S1. Dissolve hydroxypropyl methylcellulose in deionized water and stir to form a homogeneous solution; S2. Add phenolic-quaternary ammonium salt grafted chitosan, glycerol, glyceryl monostearate, sodium citrate, talc and potassium sorbate in sequence, and continue to stir and mix to obtain the coating solution; S3. Apply the obtained coating solution to the surface of fruits and vegetables and dry it to form a film.

[0013] More preferably, the coating solution can be stably stored for no less than 48 hours under sealed refrigeration conditions at 4–10°C, and preferably for up to 5 days, during which no obvious stratification, flocculation or precipitation occurs.

[0014] More preferably, the coating solution is filtered through a 200-400 mesh filter before use to remove insoluble impurities and improve the uniformity of the coating.

[0015] More preferably, the coating liquid is sprayed using a high-atomization nozzle with a nozzle diameter of no more than 0.5 mm, so as to control the film thickness between 10 and 30 μm.

[0016] The beneficial effects of this invention are: This invention constructs a composite system of phenolic-quaternary ammonium salt-grafted chitosan and hydroxypropyl methylcellulose to prepare a fruit and vegetable surface coating preservation film with film-forming, antibacterial, and preservation functions, overcoming the problems of poor antibacterial performance, unstable film layer, and loss of active ingredients in traditional edible films. The introduction of phenolic-quaternary ammonium salt-grafted chitosan in this invention not only provides a stable cationic antibacterial structure but also enhances the network structure stability and interfacial bonding ability of the composite film through covalent grafting and synergistic distribution with polyphenol groups. The formed composite film has good flexibility, transparency, and continuity, forming a uniform, dense, and non-shedding protective film layer on the surface of fruits and vegetables. While blocking oxygen and moisture, it significantly inhibits microbial growth, thereby effectively delaying the respiration metabolism and spoilage of fruits and vegetables. Furthermore, the composite modification strategy employed in this invention endows the coating system with excellent storage stability and processability. The food-grade excipients introduced into the formulation, such as glycerol, talc, and sodium citrate, further regulate the rheological properties and pH stability of the film solution, making the coating process more controllable. Experimental results show that this preservation film has a highly effective inhibitory effect on Escherichia coli and Penicillium, and can extend the shelf life of fruits and vegetables to 10-14 days at room temperature (25℃), demonstrating a significantly better preservation effect than existing single natural polymer coating materials. Attached Figure Description

[0017] The invention will now be further described with reference to the accompanying drawings.

[0018] Figure 1 Bar charts showing the antibacterial rates of samples from Examples 1-3 and Comparative Examples 1-2; Figure 2 The bar charts show the water vapor transmission rate and oxygen transmission rate of the samples from Examples 1-3 and Comparative Examples 1-2; Figure 3 Line graphs showing the moisture loss rate of coated fruits and vegetables in Examples 1-3 and Comparative Examples 1-2. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Example 1 I. Preparation of phenolic-quaternary ammonium salt grafted chitosan Add 50 mL of deionized water to a 250 mL three-necked flask, add 3.0 g of gallic acid, and stir until completely dissolved. Then add 1.5 g of sodium carbonate to adjust the pH of the reaction system to 9.0–9.5. Under constant temperature conditions of 40°C in a water bath, slowly add 2.5 g of hexamethylenetrimethylamine dropwise, and continue stirring for 4 hours. The reaction solution gradually changes from light yellow to light brown, yielding a phenolic-quaternary ammonium salt intermediate solution, which is then cooled and set aside. Weigh 2.0 g of chitosan and add it to 100 mL of 1.0% acetic acid aqueous solution. Dissolve the chitosan solution by magnetic stirring to form a clear and homogeneous chitosan solution.

[0021] The intermediate solution was slowly added dropwise to the chitosan solution while stirring. The pH of the reaction solution was adjusted to 6.0–6.5, and the reaction was continued at 45°C for 6 hours with stirring. During the reaction, the system gradually became more viscous and darker in color, indicating that the grafting reaction was underway. The resulting mixture was transferred to a dialysis bag with a molecular weight cutoff of 8000–14000 Da and dialyzed in deionized water for 72 hours, with the water changed every 12 hours to remove unreacted small molecules. The dialysis solution was then freeze-dried to obtain a light brown phenolic-quaternary ammonium salt grafted chitosan.

[0022] II. Preparation of Hydroxypropyl Methylcellulose-Coated Fruit and Vegetable Preservative Film The hydroxypropyl methylcellulose coated fruit and vegetable preservation film comprises the following raw materials in parts by weight: 100 parts hydroxypropyl methylcellulose, 5 parts phenolic-quaternary ammonium salt grafted chitosan, 5 parts glycerol, 1 part glyceryl monostearate, 0.5 parts sodium citrate, 0.5 parts talc, and 0.1 parts potassium sorbate.

[0023] The preparation steps are as follows: Take 500 mL of deionized water into a 1000 mL beaker, add 10.0 g of hydroxypropyl methylcellulose, and stir at room temperature for 30 min to form a homogeneous HPMC colloidal solution. Then heat the solution to 60℃ and continue stirring for 30 min until completely dissolved. Cool to 40℃, and while continuously stirring, add 0.50 g of phenolic-quaternary ammonium salt grafted chitosan (pre-dissolved in 10 mL of warm water), 0.50 g of glycerol, 0.10 g of glyceryl monostearate, 0.05 g of sodium citrate, 0.05 g of talc, and 0.01 g of potassium sorbate sequentially. Stir for 10 minutes after each component is added to ensure thorough and uniform dispersion. Make up the volume to 1000 mL with deionized water, stir evenly, filter through a 300-mesh stainless steel filter to remove impurities, and treat with 8000 rpm for 3 minutes to improve particle uniformity and rheological properties. The pH of the system was measured using a pH meter, and the pH was adjusted to 5.0 ± 0.1 using 0.1 mol / L citric acid solution or dilute sodium hydroxide solution. The mixture was allowed to stand for 30 minutes to remove bubbles. Fresh tomato samples were selected, washed, and air-dried. The film-forming solution was then evenly sprayed onto the tomato surface using a pneumatic sprayer. The samples were placed in a constant temperature forced-air drying oven and dried at 45℃ for 30 minutes to form a uniform, transparent, and crack-free preservation film, which is the hydroxypropyl methylcellulose-coated fruit and vegetable preservation film.

[0024] Example 2 The preparation method of phenolic-quaternary ammonium salt grafted chitosan is the same as in Example 1.

[0025] The preparation method of hydroxypropyl methylcellulose coated fruit and vegetable preservation film is as follows: The hydroxypropyl methylcellulose coated fruit and vegetable preservation film comprises the following raw materials in parts by weight: 150 parts hydroxypropyl methylcellulose, 30 parts phenolic-quaternary ammonium salt grafted chitosan, 15 parts glycerol, 5 parts glyceryl monostearate, 3 parts sodium citrate, 3 parts talc, and 1.5 parts potassium sorbate.

[0026] The preparation steps for hydroxypropyl methylcellulose-coated fruit and vegetable preservation film are the same as in Example 1.

[0027] Example 3 The preparation method of phenolic-quaternary ammonium salt grafted chitosan is the same as in Example 1.

[0028] The preparation method of hydroxypropyl methylcellulose coated fruit and vegetable preservation film is as follows: The hydroxypropyl methylcellulose coated fruit and vegetable preservation film comprises the following raw materials in parts by weight: 125 parts hydroxypropyl methylcellulose, 17.5 parts phenolic-quaternary ammonium salt grafted chitosan, 10 parts glycerol, 3 parts glyceryl monostearate, 1.75 parts sodium citrate, 1.75 parts talc, and 0.8 parts potassium sorbate.

[0029] The preparation steps for hydroxypropyl methylcellulose-coated fruit and vegetable preservation film are the same as in Example 1.

[0030] Comparative Example 1 The preparation method of hydroxypropyl methylcellulose coated fruit and vegetable preservation film is as follows: The hydroxypropyl methylcellulose coated fruit and vegetable preservation film comprises the following raw materials in parts by weight: 125 parts hydroxypropyl methylcellulose, 10 parts glycerin, 3 parts glyceryl monostearate, 1.75 parts sodium citrate, 1.75 parts talc, and 0.8 parts potassium sorbate.

[0031] The preparation steps are as follows: Take 500 mL of deionized water into a 1000 mL beaker, add 12.5 g of hydroxypropyl methylcellulose, and magnetically stir for 30 min to form a colloidal dispersion; heat to 60℃ and stir continuously for 30 min until completely dissolved. Cool to 40℃, and sequentially add 1.0 g of glycerol, 0.30 g of glyceryl monostearate, 0.175 g of sodium citrate, 0.175 g of talc, and 0.08 g of potassium sorbate, stirring for 10 minutes after each addition to ensure uniform dispersion. Make up the volume to 1000 mL with deionized water, filter through a 300-mesh filter to remove impurities, and shear at 8000 rpm for 3 minutes to improve homogeneity and stability. Measure the pH of the system using a pH meter, add a small amount of citric acid or NaOH solution to adjust to 5.0 ± 0.1, and let stand for 30 minutes to remove bubbles. The film liquid is evenly sprayed onto the surface of the washed and dried fresh tomatoes, and then dried in a 45°C hot air drying oven for 30 minutes to form a preservation film without grafted antibacterial components.

[0032] Comparative Example 2 The preparation method of phenolic-quaternary ammonium salt grafted chitosan is the same as in Example 1.

[0033] The preparation method of hydroxypropyl methylcellulose coated fruit and vegetable preservation film is as follows: The hydroxypropyl methylcellulose coated fruit and vegetable preservation film comprises the following raw materials in parts by weight: 125 parts hydroxypropyl methylcellulose, 17.5 parts phenolic-quaternary ammonium salt grafted chitosan, 10 parts glycerol, 3 parts glyceryl monostearate, 1.75 parts talc, and 0.8 parts potassium sorbate.

[0034] The preparation steps are as follows: 12.5 g of hydroxypropyl methylcellulose was added to 500 mL of deionized water and pre-dissolved by magnetic stirring at room temperature for 30 minutes. Then, the solution was heated to 60°C and stirred for another 30 minutes until completely dissolved. After the system cooled to 40°C, 1.75 g of phenolic-quaternary ammonium salt grafted chitosan (pre-dissolved in 10 mL of warm water), 1.0 g of glycerol, 0.30 g of glyceryl monostearate, 0.175 g of talc, and 0.08 g of potassium sorbate were added sequentially. The mixture was stirred for 10 minutes after each addition. Deionized water was added to bring the total volume to 1000 mL. The solution was filtered through a 300-mesh stainless steel filter to remove impurities and treated at 8000 rpm for 3 minutes to improve dispersion uniformity. No sodium citrate or other buffers were added to maintain the original reaction pH (approximately 6.4–6.6). The resulting membrane solution was sprayed onto the surface of washed and dried tomatoes and dried in a 45°C oven for 30 minutes to obtain the final membrane layer.

[0035] Performance testing 1. Antibacterial rate determination Escherichia coli and Penicillium were inoculated onto nutrient agar plates and cultured to form microbial films. Films with a diameter of 2 cm prepared in Examples 1-3 and Comparative Examples 1-2 were cut and affixed to the center of the inoculated agar plates, respectively, and incubated at 37°C (bacteria) and 28°C (fungi) for 24 hours. The films were removed, and the area beneath the film was scraped off with a sterile cotton swab, diluted, and spread onto plates. Colony forming units (CFU) were then cultured and counted. A blank film was used as a control group, and the antimicrobial inhibition rate of the sample films was calculated. Where N0 is the control colony count and N1 is the sample colony count, the results are shown in Table 1 below.

[0036] Table 1. Results of antibacterial rate As shown in Table 1, the antibacterial rates of the preservation films in Examples 1–3 against *Escherichia coli* and *Penicillium* were significantly higher than those in the comparative examples, especially in Example 2, where the antibacterial rates reached 95.1% and 91.3%, respectively. This indicates that the antibacterial performance of the film was significantly enhanced with the increase of the content of phenolic-quaternary ammonium salt grafted chitosan. This demonstrates the key role of the introduced grafted antibacterial structure in the inhibition of microorganisms. In contrast, Comparative Example 1, without the addition of this grafting material, had antibacterial rates of only 21.6% and 18.3%, showing almost no antibacterial function. Furthermore, Comparative Example 2, without the addition of sodium citrate, although containing the grafted component, still had a significantly lower antibacterial rate than the examples, indicating that the buffering and regulating components also play an important role in stabilizing the pH of the membrane solution and maintaining the activity of the functional components.

[0037] 2. Film thickness and uniformity test A film-forming solution was evenly sprayed onto the surface of a standard tomato, and dried with hot air at 45℃ for 30 minutes to form a dry film. Using a micrometer or laser thickness gauge, five measuring points were selected at the top, two sides, and bottom of each fruit and vegetable sample to record the film thickness (μm), and the average thickness and maximum deviation were calculated. The thickness difference rate was calculated using the formula: The results are shown in Table 2 below.

[0038] Table 2 Results of film thickness and uniformity As shown in Table 2, the average film thickness of Examples 1–3 is higher than that of the comparative examples, and the thickness difference rate is controlled within ±10%, indicating that the film liquid system of the present invention has good fluidity and spreadability, and can form a uniform, dense and continuous coating on the surface of fruits and vegetables. In particular, Example 2 has the thickest film layer and the lowest thickness difference rate, indicating that the film-forming stability and structural density of the film liquid system are the strongest under the condition of high proportion of phenolic-quaternary ammonium salt grafted chitosan. In contrast, the film layers of Comparative Examples 1 and 2 are relatively thin, and the difference rate is greater than 10%, showing obvious unevenness, indicating that the lack of grafting structure or pH buffer system will lead to decreased film liquid stability and increased differences in film distribution.

[0039] 3. Water vapor transmission rate and oxygen transmission rate test The dried and molded membrane samples prepared in Examples 1-3 and Comparative Examples 1-2 were used for barrier performance testing after constant temperature and humidity pretreatment. Water vapor transmission rate was tested according to GB / T 1037-1988 standard, measuring the water vapor transmission rate per unit time at 38℃ and 90% relative humidity. Oxygen transmission rate was tested according to GB / T 1038 method, recording the oxygen permeation volume per unit time at 23℃ and a pressure difference of 0.1 MPa. The results are shown in Table 3 below.

[0040] Table 3 Water vapor and oxygen transmission rates As shown in Table 3, the water vapor permeability and oxygen permeability of Examples 1–3 were significantly lower than those of the comparative examples, indicating that the preservation film prepared by this invention has significant advantages in terms of moisture and gas barrier performance, and can effectively slow down the water loss and oxidation process of fruits and vegetables. Among them, Example 2 performed best, indicating that with the increase of the amount of phenolic-quaternary ammonium salt grafted chitosan, the membrane structure became denser and the network stability was stronger, further inhibiting water evaporation and oxygen permeation. Comparative Example 1 did not add this grafting component and had the worst barrier performance, verifying its core role in regulating membrane density and molecular permeability; although Comparative Example 2 contained the grafting component, it did not add sodium citrate, resulting in unstable membrane solution pH, incomplete membrane formation, and decreased barrier performance.

[0041] 4. Shelf life simulation test Tomato samples of uniform size and without mechanical damage were selected and coated with the preservation films prepared in Examples 1–3 and Comparative Examples 1–2, respectively. After natural drying, they were stored together with the untreated control group at 25°C and 60% relative humidity. The weight change, visible decay, and sensory scores of the samples were recorded daily. Three samples were parallel in each group, and records were taken every two days until the decay rate exceeded 50%. Moisture loss rate was calculated as a percentage of weight loss, and decay rate was calculated as the proportion of samples showing obvious decay. The actual preservation effect of the preservation film was evaluated based on the number of days of storage, and the results are shown in Table 4 below.

[0042] Table 4. Evaluation Results of Preservation Effect As shown in Table 4, the samples from Examples 1–3 all exhibited good preservation effects under normal temperature storage conditions, extending the shelf life to 11–14 days, significantly better than Comparative Examples 1 and 2. In particular, Example 2 showed a decay rate of only 5.6% and a moisture loss rate controlled at 6.5%, indicating that the high content of phenolic-quaternary ammonium salt grafted chitosan effectively inhibits microbial growth and enhances membrane density, thereby slowing down water loss and spoilage of fruits and vegetables. Comparative Example 1, lacking the grafted antibacterial structure, had a decay rate as high as 66.4%, demonstrating the crucial role of this functional component in extending shelf life. Although Comparative Example 2 contained grafted chitosan, the lack of sodium citrate buffering led to pH imbalance and unstable membrane structure, resulting in a decreased preservation effect.

[0043] 5. Storage stability test The coating solutions prepared in Examples 1–3 and Comparative Examples 1–2 were separately dispensed into sealed containers and stored at 4°C. Samples were taken on days 1, 3, 5, and 7 to observe whether the coating solution exhibited stratification or flocculation. The pH value and viscosity changes at 25°C were recorded. pH change was expressed as the difference from the initial value (ΔpH); viscosity change rate = (final viscosity) / (final viscosity) * ... (Initial viscosity) / (Initial viscosity × 100%). If obvious stratification or precipitation occurs, the product is considered unstable. The results are shown in Table 5 below.

[0044] Table 5. Results of the storage stability of the coating solution As shown in Table 5, Examples 1–3 did not exhibit stratification or flocculation after 7 days of storage at 4°C. pH changes were controlled within +0.3, and viscosity changes did not exceed -5%, demonstrating good storage stability. Example 2 showed the lowest ΔpH and the slightest viscosity change, indicating that the coating solution system synergistically constructed with phenolic-quaternary ammonium salt grafted chitosan and sodium citrate possesses excellent pH buffering capacity and rheological stability. Comparative Example 1, although containing buffer components, lacked the grafted structure and still showed a significant decrease in viscosity and slight flocculation. Comparative Example 2, having completely removed sodium citrate, experienced a significant pH fluctuation to +1.0, a viscosity decrease of 18.6%, and obvious stratification, verifying the crucial role of the buffer system in preventing incompatibility of membrane solution components and colloid decomposition.

[0045] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0046] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A hydroxypropyl methylcellulose-coated fruit and vegetable preservation film, characterized in that, It contains the following raw materials in parts by weight: 100-150 parts hydroxypropyl methylcellulose, 5-30 parts phenolic-quaternary ammonium salt grafted chitosan, 5-15 parts glycerol, 1-5 parts glyceryl monostearate, 0.5-3 parts sodium citrate, 0.5-3 parts talc, and 0.1-1.5 parts potassium sorbate.

2. The hydroxypropyl methylcellulose coated fruit and vegetable preservation film according to claim 1, characterized in that, The preparation method of the phenolic-quaternary ammonium salt grafted chitosan specifically includes the following steps: S101. Add gallic acid to the reaction vessel, add sodium carbonate to adjust the pH to alkaline, stir to dissolve, then add hexamethylenetrimethylamine and continue stirring to react, to obtain a phenolic-quaternary ammonium salt intermediate; S102. Dissolve chitosan in dilute acetic acid solution and stir. Add the phenolic-quaternary ammonium salt intermediate obtained in step S101 to the chitosan solution and continue the reaction under stirring to obtain the grafting product. S103. Transfer the reaction solution into a dialysis bag and dialyze it in deionized water to remove unreacted small molecules. Freeze-dry the dialysate to obtain phenolic-quaternary ammonium salt grafted chitosan.

3. The hydroxypropyl methylcellulose-coated fruit and vegetable preservation film according to claim 1, characterized in that, The hydroxypropyl methylcellulose is a food-grade hydroxypropyl methylcellulose with a viscosity of 8000~12000 mPa·s.

4. The fruit and vegetable preservation film according to claim 1, characterized in that, The talc powder is food-grade ultrafine talc powder with a particle size of less than 10 μm.

5. The hydroxypropyl methylcellulose coated fruit and vegetable preservation film according to claim 1, characterized in that, Before use, the components of the formula are mixed evenly by magnetic stirring. The plastic wrap is applied to the surface of fruits and vegetables by spraying and then dried by hot air at a temperature of 35℃~50℃.

6. The hydroxypropyl methylcellulose coated fruit and vegetable preservation film according to claim 1, characterized in that, The glycerol and sodium citrate work synergistically as a plasticizing buffer composite system to control the stability and flexibility of the coating film in an environment of pH 4.5–5.

5.

7. A method for preparing a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film, wherein the hydroxypropyl methylcellulose-coated fruit and vegetable preservation film is as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Dissolve hydroxypropyl methylcellulose in deionized water and stir until homogeneous; S2. Add phenolic-quaternary ammonium salt grafted chitosan, glycerol, glyceryl monostearate, sodium citrate, talc and potassium sorbate in sequence, and continue stirring to obtain the coating solution; S3. Apply the obtained coating solution to the surface of fruits and vegetables and dry it to form a film.

8. The method for preparing a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film according to claim 7, characterized in that, The coating liquid can be stably stored for no less than 48 hours under sealed refrigeration conditions at 4-10℃, with a preferred storage period of up to 5 days, during which no obvious stratification, flocculation, or precipitation occurs.

9. The method for preparing a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film according to claim 7, characterized in that, The coating solution is filtered through a 200-400 mesh filter before use to remove insoluble impurities and improve coating uniformity.

10. The method for preparing a hydroxypropyl methylcellulose-coated fruit and vegetable preservation film according to claim 7, characterized in that, The coating liquid is sprayed using a high-atomization nozzle with a nozzle diameter of no more than 0.5 mm to control the film thickness between 10 and 30 μm.

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