A modified chitosan coating for enhancing fruit surface adhesion and resistance to fungal spores

Abstract

This application relates to a modified chitosan coating for enhancing adhesion to fruit surfaces and resisting fungal spores, and a method for preparing the same. The modified chitosan coating comprises a chitosan-tannic acid-stearic acid copolymer, a plasticizer, and water. The copolymer is modified by grafting tannic acid and stearic acid onto a chitosan backbone. The modified chitosan coating exhibits enhanced adhesion and hydrophobicity to hydrophobic fruit surfaces, improves wettability, reduces swelling under high humidity, enhances gas barrier properties, displays significant antioxidant activity, reduces enzymatic browning and scavenges free radicals, exhibits anti-adhesion against fungi and bacteria, and possesses excellent biocompatibility. It provides a promising, safe, and sustainable solution for extending the shelf life and quality of fresh agricultural products.

Description

Technical Field

[0001] This application relates to functional coating technology, specifically to a modified chitosan coating for enhancing the adhesion of fruit surfaces and resisting fungal spores, and a method for preparing the same. Background Technology

[0002] Fruits are highly perishable commodities due to their inherent biological characteristics. Despite their natural protective epidermis, fruits remain susceptible to rapid spoilage due to a variety of factors, including oxidation, dehydration, respiration and physiological aging, and fungal infection (Jung et al., 2020). These degradation mechanisms significantly limit the post-harvest shelf life of fruits and accelerate their decay. Current estimates suggest that approximately 50% of field-grown fruit is discarded annually due to spoilage (Springmann et al., 2018). Therefore, developing effective preservation technologies has become an urgent priority to alleviate these challenges, extend the shelf life of fruit products, and ultimately reduce global food waste.

[0003] Functional coating technologies, particularly chitosan-based coatings, have emerged as a promising preservation method due to their cost-effectiveness, ease of operation, and ability to maintain the physiological quality of agricultural products (Saberi-Riseh et al., 2023). The coating forms a semi-permeable layer on the fruit surface, acting as a protective barrier that regulates gas exchange and prevents microbial penetration (Kumar et al., 2022). To enhance functionality, bioactive compounds (such as polyphenols and essential oils) and nanoparticles have been successfully incorporated into chitosan (CS) matrices, endowing them with antibacterial and antioxidant properties (Arroyo et al., 2020; Haghighi et al., 2019; Hu, Q. et al., 2023). Despite these advantages, the practical application of chitosan-based coatings in postharvest preservation remains limited by interfacial challenges. Specifically, weak molecular adhesion between the chitosan-based coating and the fruit surface leads to rapid solution loss and incomplete film formation (Zhang, W. et al., 2025; Zhou, Chaomei et al., 2023a). Furthermore, the insufficient wettability of acidic chitosan solutions on the cuticle of hydrophobic fruits leads to uneven deposition and layer discontinuities (Feng, N. et al., 2025; Zhou, Q. et al., 2024). These combined defects ultimately impair the barrier properties of chitosan-based coatings, limiting their preservation effectiveness. Summary of the Invention

[0004] In view of the above-mentioned problems of the prior art, this application provides a multifunctional chitosan-based coating film with enhanced adhesion, antioxidant capacity and antifungal spore adhesion, which can effectively extend the shelf life of fresh farm products, especially fruits.

[0005] To achieve the above objectives, this application provides a modified chitosan coating for enhancing the adhesion of fruit surfaces and resisting fungal spores, comprising a chitosan-tannic acid-stearic acid copolymer, a plasticizer, and water, wherein the copolymer is modified by grafting tannic acid and stearic acid onto a chitosan backbone.

[0006] In one possible implementation of the modified chitosan coating of this application, the mass ratio of chitosan:tannic acid:stearic acid in the copolymer is 1:0.1:0.5 to 1.

[0007] In one possible implementation of the modified chitosan coating of this application, the FTIR spectrum of the chitosan-tannic acid-stearic acid copolymer is at 2920 cm⁻¹. -1 2850 cm -1 1703 cm -1 1638 cm -1 and 1537 cm -1 It has a peak.

[0008] In one possible implementation of the modified chitosan coating of this application, the chitosan-tannic acid-stearic acid copolymer... 1 The 1H NMR spectrum shows peaks at 6.98 ppm, 2.6 ppm, and 1.9 ppm.

[0009] In one possible implementation of the modified chitosan coating of this application, the XRD spectrum of the chitosan-tannic acid-stearic acid copolymer has peaks at 2θ of 6.8°, 18.4°, 21.5° and 24°.

[0010] Another objective of this application is to provide a method for preparing a modified chitosan coating for enhancing the adhesion of fruit surfaces and resisting fungal spores, comprising the following steps: dispersing chitosan in an acetic acid solution to obtain a chitosan solution; adding H2O2 and ascorbic acid to the chitosan solution, stirring continuously, and then adding an aqueous solution of tannic acid to obtain a mixed solution; adjusting the pH of the mixed solution to 7 to deposit a chitosan-tannic acid conjugate; dissolving the chitosan-tannic acid conjugate in an acetic acid solution to obtain a chitosan-tannic acid solution; dissolving stearic acid in anhydrous ethanol, and adding EDC and NHS to react and obtain a mixture; slowly adding the mixture to the chitosan-tannic acid solution, stirring at room temperature to obtain a chitosan-tannic acid-stearic acid copolymer; and dispersing the chitosan-tannic acid-stearic acid copolymer powder in water, adding a plasticizer, and stirring evenly to obtain a modified chitosan coating.

[0011] In one possible implementation of the preparation method of this application, the mass ratio of chitosan to tannic acid in the mixed solution is 1:0.1.

[0012] In one possible implementation of the preparation method of this application, the mass ratio of chitosan:tannic acid:stearic acid in the chitosan-tannic acid-stearic acid copolymer is 1:0.1:0.5 to 1.

[0013] In one possible implementation of the preparation method of this application, before preparing the chitosan-tannic acid solution, the chitosan-tannic acid conjugate is dialyzed with a 7000-14000 Da molecular weight cutoff membrane to remove unreacted raw materials and dehydroascorbic acid byproducts.

[0014] In one possible implementation of the preparation method of this application, before preparing the coating liquid, the chitosan-tannic acid-stearic acid copolymer is dialyzed with a 7000-14000 Da molecular weight cutoff membrane to remove unreacted raw materials and urea byproducts.

[0015] This application discloses a multifunctional chitosan-based coating to enhance fruit preservation by grafting chitosan (CS) with tannic acid (TA) and stearic acid (SA). A chitosan-tannic acid-stearic acid (CS-TA-SA) conjugate was synthesized via a free radical and carbodiimide coupling method. This coating exhibits enhanced adhesion and hydrophobicity to hydrophobic fruit surfaces. The chitosan-tannic acid-stearic acid copolymer, after being mixed with water and a plasticizer and dried, forms a uniform and stable film, improving wettability, reducing swelling under high humidity, and enhancing gas barrier properties. The coating of this application exhibits significant antioxidant activity, reducing enzymatic browning and scavenging free radicals. Importantly, it exhibits anti-adhesion against fungal spores and bacteria, common postharvest pathogens, thereby reducing fungal and bacterial colonization and decay in cherry tomatoes and blueberries. Biodegradability tests confirmed complete degradation in soil within 21 days, and cytotoxicity tests demonstrated excellent biocompatibility. Overall, the CS-TA-SA coating of this application combines enhanced adhesion, antioxidant capacity, and antifungal spore adhesion, providing a promising, safe, and sustainable solution for extending the shelf life and quality of fresh produce.

[0016] The above content of this application will be explained in more detail and in a more understandable way through multiple embodiments and accompanying drawings. Attached Figure Description

[0017] The following are accompanying drawings of this application. These drawings are provided only to illustrate the application in a more intuitive form. They are exemplary and are not intended to limit the scope of this application.

[0018] Figure 1 A schematic diagram of the synthesis of the chitosan-tannic acid-stearic acid copolymer of this application is shown.

[0019] Figure 2The Fourier transform infrared (FTIR) spectra of a polysaccharide-tannic acid-stearic acid copolymer prepared according to an embodiment of this application are shown.

[0020] Figure 3 The following is a 1H NMR spectrum (also known as the nuclear magnetic resonance spectrum) of a polysaccharide-tannic acid-stearic acid copolymer prepared according to an embodiment of this application. 1 H NMR spectrum).

[0021] Figure 4 The X-ray diffraction (XRD) spectrum of a polysaccharide-tannic acid-stearic acid copolymer prepared according to an embodiment of this application is shown.

[0022] Figure 5 Scanning electron microscope images of the surface morphology of the fruit peel with and without a modified chitosan coating example of this application are shown, wherein Figure 5 (a) The surface morphology of orange peel without coating and with modified chitosan coating 2. Figure 5 (b) shows the cross-sectional morphology of blueberries without coating and with chitosan (CS) and modified chitosan coating 2, respectively.

[0023] Figure 6 Optical photographs of a chitosan coating and a modified chitosan coating example of this application are shown.

[0024] Figure 7 This illustration shows a schematic diagram of the process of applying a modified chitosan coating of this application to a blueberry surface.

[0025] Figure 8 The transmittance of the chitosan coating and the modified chitosan coating of this application are shown.

[0026] Figure 9 Photographs showing the contact angles of the chitosan coating and the modified chitosan coating of this application are presented.

[0027] Figure 10 The images show the effects of chitosan coating and the modified chitosan coating of this application on the surface of cherry tomatoes and blueberries.

[0028] Figure 11 The contact angles of water and the modified chitosan coating of this application are shown on different fruit surfaces.

[0029] Figure 12 The erosion resistance of chitosan coating and modified chitosan coating of this application on cherry tomato surface is shown.

[0030] Figure 13 The adhesion of the modified chitosan coating of this application is shown.

[0031] Figure 14 Photographs showing the swelling properties of the chitosan coating and the modified chitosan coating of this application are displayed.

[0032] Figure 15 The washability of chitosan coating and modified chitosan coating of this application on cherry tomato surface is shown.

[0033] Figure 16 The oxygen permeability of the chitosan coating and the modified chitosan coating of this application is shown.

[0034] Figure 17 The carbon dioxide permeability of the chitosan coating and the modified chitosan coating of this application is shown.

[0035] Figure 18 The antifungal spore adhesion effect of chitosan coating and the modified chitosan coating of this application is shown.

[0036] Figure 19 The antifungal spore adhesion rates of the chitosan coating and the modified chitosan coating of this application are shown.

[0037] Figure 20 The antibacterial adhesion effect of chitosan coating and modified chitosan film coating of this application is shown.

[0038] Figure 21 The antibacterial adhesion rates of the chitosan coating and the modified chitosan coating of this application are shown.

[0039] Figure 22 A photograph of a potato coated with a chitosan coating and a chitosan film coating modified according to this application is shown.

[0040] Figure 23 The DPPH and ABTS scavenging activities of the chitosan coating and the modified chitosan coating of this application are shown.

[0041] Figure 24 The images show photographs of the soil degradation of the chitosan film and the modified chitosan film of this application.

[0042] Figure 25 The soil sowing situation after degradation of the chitosan film modified in this application is shown.

[0043] Figure 26 The viability of L929 cells cultured using chitosan coating and the chitosan coating modified in this application is shown.

[0044] Figure 27 The photos show the preservation effect of cherry tomatoes in the control group, the chitosan-coated group, and the group coated with the modified chitosan film of this application during three days of storage.

[0045] Figure 28 The photos show the preservation effect of blueberries in the control group, the chitosan-coated group, and the group coated with the modified chitosan film of this application during 5 days of storage. Detailed Implementation

[0046] To make this application easier to understand, specific embodiments are described below to further illustrate this application. Unless otherwise specified, the experimental methods described in this application are conventional methods; the materials described, unless otherwise specified, are all commercially available. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. In case of any inconsistency, the meaning as described in this specification or derived from the content described in this specification shall prevail. Furthermore, the terminology used herein is for the purpose of describing embodiments of this application only and is not intended to limit this application.

[0047] In order to accurately describe the technical content of this application and to accurately understand the present invention, the following explanations or definitions of the words and terms used in this specification are given before describing the specific embodiments.

[0048] The terms "one embodiment" or "implementation" as used in this specification mean that a particular feature, step, or characteristic described in conjunction with that embodiment is included in at least one embodiment of the invention. Therefore, the terms "in one embodiment" or "in an embodiment" appearing throughout this specification do not necessarily refer to the same embodiment, but may refer to the same embodiment. Furthermore, in one or more embodiments, the particular features, steps, or characteristics can be combined in any suitable manner, as will be apparent to those skilled in the art from this application.

[0049] refer to Figure 1The chitosan-tannic acid-stearic acid copolymer contained in the modified chitosan coating of this application is modified by grafting tannic acid (TA) and stearic acid (SA) onto the chitosan (CS) backbone. First, tannic acid is grafted onto the chitosan backbone via free radial grafting to obtain a CS-TA coupling (i.e., a chitosan-tannic acid conjugate). In this reaction system, ascorbic acid interacts with hydrogen peroxide to generate ascorbic acid free radicals, which then react with CS to form large CS free radicals. Then, TA monomers are grafted onto CS by forming ether or carbon-nitrogen (CN) bonds. The inventors found that the phenolic hydroxyl groups present in TA may reduce the hydrophobicity of the coating; therefore, the amount of TA relative to CS is reduced to a mass ratio of 0.1:1. Subsequently, stearic acid (TA) is grafted onto the CS-TA coupling via carbodiimide-mediated coupling to obtain a CS-TA-SA conjugate (i.e., a chitosan-tannic acid-stearic acid copolymer). In this reaction system, SA initially reacts with EDC and NHS to form an amine-reactive NHS ester intermediate; then this intermediate reacts with CS-TA to form a CS-TA-SA conjugate via an amide bond.

[0050] For ease of implementation, specific examples are provided below.

[0051] <Example 1>

[0052] Chitosan (CS)-tannic acid (TA) conjugates were prepared using a free radical grafting process. First, 2.001 g of chitosan was dispersed in 100 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved, yielding a 2% (w / v) CS solution. 0.203 g of TA was dissolved in 1 mL of deionized water. Then, 2 mL of 1.0 mol / L H₂O₂ and 0.108 g of ascorbic acid were dissolved in the above CS solution, and the mixture was stirred continuously for 30 minutes. Then, 1 mL of TA aqueous solution was added dropwise to the CS solution, resulting in a CS:TA mass ratio of 0.1:1. After storing at room temperature for 24 hours, the pH of the solution was adjusted to 7.0 with NaOH (2 mol / L) to deposit CS-TA. The solution was then dialyzed for 48 hours using a 7000-14000 Da molecular weight cutoff membrane (with 8 water changes) to remove unreacted raw materials, free TA, and dehydroascorbic acid byproducts. The product was collected and pre-cooled at -20°C for 12 hours, followed by freeze-drying for 72 hours to obtain a CS-TA conjugate with a mass ratio of 1:0.1. The CS-TA-grafted stearic acid (SA) copolymer CS-TA-SA was synthesized via an EDC / NHS coupling reaction. 2.001 g of CS-TA was dispersed in 100 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved. 1 g of 3.52 mmol SA was dissolved in 30 mL of anhydrous ethanol, followed by the addition of EDC (1.5 mol / mol SA) to activate the carboxyl groups, and then NHS (1.5 mol / mol SA). The mixture was placed in an ice bath and reacted in the dark for 1 hour. This mixture was then slowly added to the CS-TA solution and stirred overnight at room temperature. Subsequently, the mixture was dialyzed for 48 hours in a 7000–14000 Da dialysis bag (with 8 water changes) to remove excess small molecule starting materials and urea byproducts. After freeze-drying, CS-TA-SA0.5 with a mass ratio of 1:0.1:0.5 was obtained. 1.010 g of CS-TA-SA0.5 powder was dispersed in 100 mL of deionized water and stirred for 3 hours until completely dispersed, preparing a 1% (w / v) suspension. Then, 0.3 g of glycerol was added as a plasticizer, and the mixture was sonicated for 30 minutes to ensure uniform dispersion of all components. This yielded CS-TA-SA0.5 coating 1.

[0053] <Example 2>

[0054] Chitosan (CS)-tannic acid (TA) conjugates were prepared using a free radical grafting process. First, 2.001 g of chitosan was dispersed in 100 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved, yielding a 2% (w / v) CS solution. 0.203 g of TA was dissolved in 1 mL of deionized water. Then, 2 mL of 1.0 mol / L H₂O₂ and 0.108 g of ascorbic acid were dissolved in the above CS solution, and the mixture was stirred continuously for 30 minutes. Then, 1 mL of TA aqueous solution was added dropwise to the CS solution, resulting in a CS:TA mass ratio of 0.1:1. After storing at room temperature for 24 hours, the pH of the solution was adjusted to 7.0 with NaOH (2 mol / L) to deposit CS-TA. The solution was then dialyzed for 48 hours using a 7000-14000 Da molecular weight cutoff membrane (with 8 water changes) to remove unreacted raw materials, free TA, and dehydroascorbic acid byproducts. The product was collected and pre-cooled at -20°C for 12 hours, followed by freeze-drying for 72 hours to obtain a CS-TA conjugate with a mass ratio of 1:0.1. The CS-TA-grafted stearic acid (SA) copolymer CS-TA-SA was synthesized via an EDC / NHS coupling reaction. 2.001 g of CS-TA was dispersed in 100 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved. 1.5 g of 5.29 mmol of SA was dissolved in 45 mL of anhydrous ethanol, followed by the addition of EDC (1.5 mol / mol SA) to activate the carboxyl groups, and then NHS (1.5 mol / mol SA). The mixture was placed in an ice bath and reacted in the dark for 1 hour. This mixture was then slowly added to the CS-TA solution and stirred overnight at room temperature. Subsequently, the mixture was dialyzed for 48 hours (with 8 water changes) in a 7000–14000 Da dialysis bag to remove excess small molecule starting materials and urea byproducts. After freeze-drying, CS-TA-SA0.75 with a mass ratio of 1:0.1:0.75 was obtained. 1.010 g of CS-TA-SA0.75 powder was dispersed in 100 mL of deionized water and stirred for 3 hours until completely dispersed, preparing a 1% (w / v) suspension. Then, 0.3 g of glycerol was added as a plasticizer, and the mixture was sonicated for 30 minutes to ensure uniform dispersion of all components. This yielded the CS-TA-SA0.75 coating film 2.

[0055] <Example 3>

[0056] Chitosan (CS)-tannic acid (TA) conjugates were prepared using a free radical grafting process. First, 2.001 g of chitosan was dispersed in 100 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved, yielding a 2% (w / v) CS solution. 0.203 g of TA was dissolved in 1 mL of deionized water. Then, 2 mL of 1.0 mol / L H₂O₂ and 0.108 g of ascorbic acid were dissolved in the above CS solution, and the mixture was stirred continuously for 30 minutes. Then, 1 mL of TA aqueous solution was added dropwise to the CS solution, resulting in a CS:TA mass ratio of 0.1:1. After storing at room temperature for 24 hours, the pH of the solution was adjusted to 7.0 with NaOH (2 mol / L) to deposit CS-TA. The solution was then dialyzed for 48 hours using a 7000-14000 Da molecular weight cutoff membrane (with 8 water changes) to remove unreacted raw materials, free TA, and dehydroascorbic acid byproducts. The product was collected and pre-cooled at -20°C for 12 hours, followed by freeze-drying for 72 hours to obtain a CS-TA conjugate with a mass ratio of 1:0.1. The CS-TA-grafted stearic acid (SA) copolymer CS-TA-SA was synthesized via an EDC / NHS coupling reaction. 2.001 g of CS-TA was dispersed in 100 mL of 1% (v / v) acetic acid solution and stirred until completely dissolved. 2 g of 7.04 mmol of SA was dissolved in 60 mL of anhydrous ethanol, followed by the addition of EDC (1.5 mol / mol SA) to activate the carboxyl groups, and then NHS (1.5 mol / mol SA). The mixture was placed in an ice bath and reacted in the dark for 1 hour. This mixture was then slowly added to the CS-TA solution and stirred overnight at room temperature. Subsequently, the mixture was dialyzed for 48 hours (with 8 water changes) in a 7000–14000 Da dialysis bag to remove excess small molecule starting materials and urea byproducts. After freeze-drying, CS-TA-SA1 with a mass ratio of 1:0.1:1 was obtained. 1.010 g of CS-TA-SA1 powder was dispersed in 100 mL of deionized water and stirred for 3 hours until completely dispersed, preparing a 1% (w / v) suspension. Then, 0.3 g of glycerol was added as a plasticizer, and the mixture was sonicated for 30 minutes to ensure uniform dispersion of all components. This yielded the CS-TA-SA1 coating film.

[0057] Characterization of chitosan-tannic acid-stearic acid copolymer

[0058] The representative chitosan-tannic acid-stearic acid copolymer CS-TA-SA0.75 of this application was characterized.

[0059] See Figure 2 The FTIR spectra of CS and CS-TA-SA conjugates were examined to determine the structural modifications. The FTIR spectra of CS were at 1645, 1590, and 1320 cm⁻¹.-1 Characteristic absorption bands are observed at 1703 and 1537 cm⁻¹, corresponding to C=O stretching vibration, NH bending vibration, and CN stretching vibration, respectively. After grafting, the CS-TA-SA conjugate exhibits absorption bands at 1703 and 1537 cm⁻¹. -1 A new absorption peak was observed at 2920, 2850, and 1638 cm⁻¹, attributed to the C=O and C=C stretching vibrations of TA, thus confirming the successful grafting of TA onto CS. Furthermore, the CS-TA-SA conjugate showed absorption peaks at 2920, 2850, and 1638 cm⁻¹. -1 A new peak appeared at 1590 cm⁻¹, which is characteristic of the C=O stretching vibration within the SA alkyl chain and N-stearoyl moiety, while the CS-TA-SA conjugate has a peak at 1590 cm⁻¹. -1 The NH bending peak of the primary amide disappeared. These spectral changes indicate that the SA in the CS-TA-SA conjugate undergoes conjugation and amide bond formation through the conversion of primary amine to secondary amine.

[0060] See Figure 3 ,use 1 ¹H NMR further investigated the synthesis of the CS-TA-SA conjugate. CS... 1 ¹H NMR spectroscopy revealed characteristic resonances in the range of 1.5 to 5.5 ppm, corresponding to intrinsic protons in the CS framework. After modification with TA and SA, the CS-TA-SA conjugate... 1 The 1H NMR spectrum showed an enhanced signal at 1.9 ppm, corresponding to the methyl proton (-CH3)acetamido (-NHCOCH3). This signal enhancement is likely a result of the formation of an amide bond between SA and CS, which disrupts the intermolecular hydrogen bond network within CS. Furthermore, new resonances appeared at 2.6 and 6.98 ppm, attributed to the aromatic protons of α-methylene (-CO-CH2–)SA and TA, respectively. These spectral changes further confirm the successful synthesis of CS-TA-SA.

[0061] See Figure 4 The crystal structure of the CS-TA-SA conjugate was characterized by XRD. The XRD pattern of CS showed two prominent diffraction peaks at 2θ≈10.4° and 19.5°. In contrast, the XRD spectrum of the CS-TA-SA conjugate showed the disappearance of these peaks, with new peaks appearing near 2θ≈6.8°, 21.5°, and 24°, corresponding to the characteristic diffraction peaks of SA. The results indicate that the grafting reaction primarily disrupted the original crystal structure of CS and promoted the formation of dispersed domains in SA.

[0062] Characterization of Modified Chitosan Coatings

[0063] The representative chitosan-tannic acid-stearic acid copolymer CS-TA-SA0.75 of this application was characterized.

[0064] The surface morphology of the fruit peel with and without the modified chitosan coating of this application was observed using scanning electron microscopy. Figure 5 As shown in (a), compared with the control group, the orange peel coated with modified chitosan coating 2 showed a significant coverage phenomenon, indicating that modified chitosan coating 2 can be applied to the fruit surface for coating. Furthermore, the coatings on the grape fruit surface were analyzed using an optical microscope. Both chitosan (CS) and modified chitosan coating 2 were clearly visible under a 10x optical microscope, and there was a clear difference between the coated grapes and the uncoated grapes. Measurements showed that modified chitosan coating 2, like the CS solution, can form a relatively uniform coating layer on the fruit surface.

[0065] <Optical Properties of Modified Chitosan Coatings>

[0066] Optical properties are an important indicator for evaluating coatings. Figure 5 Optical photographs of CS and modified chitosan coating 2 are shown. After grafting TA and SA, the color of modified chitosan coating 2 is significantly darker, changing from the original pale yellow of CS to light brown, but the pattern covered at the bottom of modified chitosan coating 2 is still clearly visible.

[0067] Figure 6 This demonstrates the process of coating the surface of blueberries with a modified chitosan coating 2 via dip-coating. Experimental results show that the modified chitosan coating 2 covering the fruit surface does not affect the color of the fruit.

[0068] Ultraviolet (UV) blocking performance is also a key factor to consider in coating materials. A certain degree of UV blocking can delay the oxidation of lipids, pigments, proteins, and vitamins in fruits and vegetables. In this disclosure, a CS-TA-SA0.7.5 coating was poured into a 10cm diameter polytetrafluoroethylene mold and dried at 40°C for 6 hours to form a modified chitosan coating layer. The transmittance of the chitosan coating and the modified chitosan coating of this application in the UV (280nm) and visible light regions (660nm) was measured using a UV-Vis spectrophotometer. See [link to relevant documentation]. Figure 8See Table 1. Compared to the CS coating, the transmittance of the modified chitosan coating 2 decreased slightly, but its visible light transmittance at 660 nm remained as high as 87.9% (Table 1), maintaining good transparency. Furthermore, the UV transmittance of the modified chitosan coating 2 showed a significant decrease in the 220-400 nm range, indicating that the UV blocking performance of CS was improved after grafting TA and SA. The reduced UV transmittance of the modified chitosan coating is attributed to the presence of unsaturated carbon bonds on TA that can absorb UV light. These results demonstrate that the modified chitosan coating 2 can maintain excellent transmittance while blocking UV light to a certain extent, possessing the potential to slow down oxidation and maintain the sensory and nutritional value of coated fruits and vegetables.

[0069] Table 1. Light transmittance of chitosan coating and modified chitosan coating of this application <![CDATA[T 280 (%)]]> <![CDATA[T 660 (%)]]> CS 84.1 96.9 <![CDATA[CS-TA-SA 0.5 ]]> 52.8 87.2 <![CDATA[CS-TA-SA 0.75 ]]> 55.5 87.9 <![CDATA[CS-TA-SA1]]> 59.9 89.1

[0070] <Hydrophobicity of Modified Chitosan Coating>

[0071] Water contact angle (WCA) is an important indicator used to evaluate the hydrophobicity or hydrophilicity of a coating surface

[33] . The hydrophobicity of the coating film enables it to inhibit the adhesion of pathogens to the fruit surface. By introducing hydrophobic groups and reducing the surface free energy of the material, the adhesion of pathogens (Candida albicans) can be reduced. Since CS molecules contain abundant hydroxyl and amino groups, they have superhydrophilicity and swelling properties. The hydrophobic mechanism of CS-TA-SA coating is different from that of pure CS film rich in hydrophilic groups. CS-TA-SA grafted with SA has more long-chain fatty acid hydrophobic groups on its chitosan backbone. Therefore, CS-TA-SA coating exhibits better hydrophobic effect. The water contact angles of CS, CS-TA-SA0.5, CS-TA-SA0.75, and CS-TA-SA1 films were measured. Figure 9 As shown, the initial contact angle of CS is 71°, and swelling occurs within 2 seconds of a water droplet contacting CS. Within seconds, the water droplet on the surface is completely absorbed by CS. The contact angle of the grafted CS-TA-SA surface is even larger, with CS-TA-SA0.75 reaching a water contact angle of 109°, and the coating no longer absorbs water and swells, exhibiting a certain degree of stability. In conclusion, the hydrophobicity of the CS-TA-SA0.75 coating helps protect fruits and vegetables from pathogen invasion.

[0072] <Wetting and Adhesion of Modified Chitosan Coatings>

[0073] The wettability and adhesion of the coating are key to its ability to remain on the fruit surface. Studies have found that the catechol or gallic acid (trihydroxyphenyl) side groups in the polymer exhibit a good effect on improving adhesion. This is because the phenolic hydroxyl groups and the long-chain hydrophobic groups of SA have more hydrogen bonding and hydrophobic interactions with the waxy substance of the fruit peel. Figure 10 As shown in the upper part, a more uniform liquid film was observed on the surface of cherry tomatoes sprayed with CS-TA-SA0.75, which contrasts sharply with the scattered liquid droplets on the surface of cherry tomatoes sprayed with CS solution. Meanwhile, when blueberries soaked in CS-TA-SA0.75 were removed and placed on a transparent plate, less liquid accumulation was observed at the bottom, a phenomenon that contrasts sharply with the obvious CS solution accumulation at the bottom of blueberries soaked in CS solution. Figure 10 The lower half is shown. This coating phenomenon can be attributed to the suitable contact angle of the fruit surface. Studies have found similar phenomena when amyloid protein coatings form films on the surfaces of fruits such as bananas, strawberries, and kumquats. The wetting properties of the CS-TA-SA0.75 coating solution on hydrophobic fruit surfaces were evaluated using contact angle measurements. The ability of the coating solution to form a uniform surface coating is crucial to its effectiveness. See also Figure 11 The initial contact angle of cherry tomato surfaces was approximately 69.5°, decreasing to 55.2° within 300 seconds. Similar trends were observed on blueberry and citrus surfaces.

[0074] In addition, water erosion resistance is another manifestation of the adhesion of the CS-TA-SA0.75 coating. When a cherry tomato coated with CS is placed under running water, the CS coating on the surface of the cherry tomato will completely peel off within 10 seconds. However, after another 10 seconds of rinsing with running water, the CS-TA-SA0.75 coating remains on the surface of the cherry tomato, as if... Figure 12 As shown, this indicates that the modified CS-TA-SA has better adhesion. The adhesion strength of the coating was determined using a simulated fruit peel adhesion experiment. Tensile testing in the vertical direction using an electronic universal testing machine showed that the adhesive strength of CS-TA-SA A0.75 was almost 1.8 times that of pure CS. Figure 13 As shown.

[0075] <Swelling properties of modified chitosan coating>

[0076] Chitosan (CS) molecules contain numerous hydroxyl and amino groups. These functional groups can form hydrogen bonds with water molecules, giving CS a strong affinity for water. Hydrogen ions from water molecules combine with the amino groups of CS, filling the CS molecular chain with positively charged groups (-NH3+), thus generating repulsive forces and causing swelling. Studies have mentioned that the swelling efficiency of CS biopolymers increases further when immersed. The modified chitosan coating of this application reduces the swelling property of the coating by grafting SA. On one hand, through an EDC / NHS-mediated coupling reaction, the carboxyl groups of SA undergo a condensation reaction with the hydrophilic groups (-NH2) of CS, reducing the ability of CS to bind with water molecules. Simultaneously, stable amide bonds are formed, grafting the highly hydrophobic long-chain fatty acid SA onto the CS molecular chain, thereby significantly reducing the hydrophilic swelling property of CS. Figure 14 As shown, when a glass plate coated with CS is placed in deionized water, obvious swelling is observed within just 10 seconds, and the CS coating almost completely detaches from the substrate within 60 seconds. In contrast, the CS-TA-SA0.75 coating only shows slight swelling and deformation at the bottom within 60 seconds before detaching from the glass plate surface; some peeling is only observed after 5 minutes, and the CS-TA-SA0.75 coating retains its original shape. Removing the glass plates coated with CS and CS-TA-SA0.75 from the beakers respectively, the swelling of the CS coating can be clearly observed.

[0077] Although the components of the CS-TA-SA coating are all edible and safe for human use, in practical applications, the coating material is usually removed during the final cleaning process. Therefore, while enhancing adhesion and reducing swelling, washability is also essential. Although the CS-TA-SA 0.75 coating remained on the surface of a completely dried cherry tomato after 10 seconds of rinsing with running water, a light hand wash demonstrated the same surface-cleaning ability as the CS coating. Figure 15 As shown. In summary, the modified CS-TA-SA0.75 coating improves adhesion and hydrophobicity, while reducing swelling and maintaining good washability.

[0078] <Gas permeability of modified chitosan coating>

[0079] Fruits and vegetables continue to respire after harvest, and their freshness is affected by their respiration rate and the gas transport rate of the coating. The coating on the surface of fruits and vegetables should effectively remove carbon dioxide produced by respiration while ensuring a certain amount of oxygen contact, thereby reducing the respiration rate and preventing accelerated spoilage caused by anaerobic respiration. Figure 16-18As shown, the oxygen permeability of the CS-TA-SA0.75 coating is only 36.59% of that of the pure CS coating, while the carbon dioxide permeability is 76.33% of that of the pure CS coating. After introducing SA and TA groups, the polymer coating exhibits greater oxygen barrier properties than carbon dioxide barrier properties. This is because the molecular chains of the modified CS-TA-SA0.75 coating are relatively tightly packed, thus providing effective oxygen barrier properties. Furthermore, under certain humidity conditions, the residual amino groups on CS may undergo reversible interactions with carbon dioxide (such as the formation of bicarbonate), thereby promoting carbon dioxide transport within the membrane. With the introduction of TA, the strong interactions formed by the cross-linked network within the CS film significantly reduce the oxygen permeability of the CS film. Studies have also shown that the oxygen permeability of CS-based oxide films significantly decreases with the addition of the long-chain fatty acid oleic acid. Figure 16-18 As shown, the carbon dioxide permeability of the CS-TA-SA0.75 coating is only slightly reduced compared to the CS coating. This may be because, on the one hand, carbon dioxide, as a polarizable molecule, has a stronger affinity for the polar groups (-OH) of TA, making it easier for the film to adsorb and resulting in poorer carbon dioxide barrier properties. On the other hand, the surface layer with long-chain hydrophobic groups significantly enhances the film's barrier properties against small molecule gases. Similar phenomena have been observed in studies on the improvement of hydrophobicity of cellulose nanofiber films by SA and modified calcium carbonate, where the water vapor and oxygen permeability of SA-modified films both decreased.

[0080] <Antibacterial Adhesion of Modified Chitosan Coating>

[0081] Since *Botrytis cinerea*, *Aspergillus niger*, and *Penicillium expansum* are common pathogenic fungi causing postharvest rot of fresh fruits and vegetables, this application used the disc diffusion method to study the anti-adhesion performance of the CS-TA-SA0.75 coating against spores of these three fungi. After culturing in PDA medium for 5 days, as... Figure 18-20 As shown, the CS-TA-SA0.75 coating effectively reduces the adhesion of *Botrytis cinerea*, *Aspergillus niger*, and *Penicillium expansum* spores. A significant amount of fungal spore suspension remains after passing over the CS coating; therefore, the hyphal growth in the PDA medium is similar to that in the blank control group. Slower hyphal growth indicates fewer spores remaining on the film surface. This inhibitory effect on hyphal growth mainly depends on the grafting properties of SA. SA contributes more long-chain fatty acid hydrophobic groups to the CS backbone, thus imparting hydrophobicity to the coating.

[0082] In addition, the antimicrobial adhesion properties of the CS coating and the CS-TA-SA0.75 coating film against Escherichia coli and Staphylococcus aureus were evaluated using the colony counting method. Figure 20-22As shown, the hydrophilic groups on the CS molecule adsorb water from the bacterial solution, and the repulsive force between the positively charged groups generated when the amino groups on the CS molecule come into contact with water causes swelling. This hydrophilicity and swelling property of the CS coating results in a significant amount of bacterial solution residue after it passes over the coating. However, after grafting SA and TA, the hydrophobicity of the CS-TA-SA coating is enhanced, and its antibacterial adhesion ability is significantly improved. In conclusion, the modified CS-TA-SA coating exhibits good anti-adhesion effects against both fungal spores and bacteria.

[0083] <Oxidation resistance of modified chitosan coating>

[0084] Oxidation, as a key factor causing food spoilage and nutrient loss, has made its inhibition ability a core performance indicator for preservation materials. Potato browning experiments were conducted using chitosan coatings and the modified chitosan coating of this application. Figure 22 As shown, the potato pieces in the CS group turned white, possibly because the acidic environment of the CS solution damaged the structure of the potato cells. The potato pieces in the control group turned black due to oxidation, while those coated with the CS-TA-SA0.75 film maintained good color. The experimental results indicate that the coating has a certain antioxidant effect. Subsequently, the antioxidant properties of each coating were further evaluated using the DPPH and ABTS free radical scavenging methods. Figure 23 As shown, in this experiment, the purple DPPH radicals were reduced to the yellow diphenylpyridinium hydrazine compound. After grafting TA and SA, the DPPH scavenging activity of the CS coating was significantly enhanced, with the level positively correlated with the sample concentration. The CS-TA-SA0.75 coating at a concentration of 1 mg / mL exhibited the highest DPPH radical scavenging activity, reaching 98.5%. The antioxidant potential of the CS-TA-SA0.75 coating was further confirmed by scavenging ABTS radicals. Similarly, compared to the CS coating, the CS-TA-SA0.75 coating showed stronger ABTS radical scavenging ability, reaching 64.5% at a concentration of 1 mg / mL. The enhanced antioxidant capacity may be attributed to the grafting of TA, as the galloyl group of TA may result in strong hydrogen and electron donation capabilities. Therefore, the CS-TA-SA0.75 coating exhibits stronger DPPH and ABTS radical scavenging abilities. In summary, the modified CS-TA-SA0.75 coating film exhibits significantly improved antioxidant properties, reducing enzymatic browning and scavenging free radicals.

[0085] <Degradability and Cytotoxicity of Modified Chitosan Coatings>

[0086] The biodegradability of the CS-TA-SA0.75 film was studied using the soil burial method, with pure CS film as a control. Figure 24As shown, pure CS film still had small fragments remaining after 21 days of burial in soil, while CS-TA-SA0.75 film showed wrinkling, shrinkage, and significant size reduction within 7 days of burial. With the passage of time, the degradation of CS-TA-SA0.75 film gradually intensified, and it was completely degraded in the soil after 21 days. This phenomenon is attributed to the enhanced hydrophilicity of the grafted film, which facilitates the adsorption and diffusion of degrading bacteria and enzymes in the soil on the material surface. Moderate grafting of SA can form micropores, potentially promoting contact between the degrading substances and the substrate film. On the other hand, the introduction of TA and SA provides more carbon sources for soil microorganisms, attracting the aggregation of degrading microorganisms. Experimental results indicate that CS-TA-SA0.75 film has better biodegradability than pure CS film. Figure 25 As shown, the seeds germinated successfully in the soil after the sample was degraded, indicating that the material does not have an impact on the environment and has good safety.

[0087] Since the coating material comes into direct contact with food during use, good biocompatibility is crucial. In subsequent biocompatibility experiments, cell viability was measured using the MTT assay. No significant cytotoxicity was observed in cells treated with high concentrations of CS coating. Grafting TA and SA did not alter the biocompatibility of CS. In fact, cells cultured with the CS-TA-SA0.75 coating showed higher survival rates at concentrations of 312.5-1250 μg / mL, indicating that the CS-TA-SA0.75 coating possesses good biocompatibility and the ability to promote cell growth. Figure 26 As shown in the figures, these results indicate that grafting with phenolic acids such as gallic acid and protocatechuic acid can effectively promote cell proliferation and improve the compatibility of CS and its derivatives. Since SA is a major component of fats, it exhibits biocompatibility and low toxicity. SA grafted with carboxymethyl chitosan is completely safe and biodegradable as a drug carrier. In conclusion, the CS-TA-SA0.75 coating is non-toxic, does not pollute the environment, and has good application potential in the field of fruit and vegetable preservation.

[0088] <Experiment on Fruit Preservation with Modified Chitosan Coating>

[0089] Botrytis cinerea, Aspergillus niger, and Penicillium expansum are common pathogenic fungi that cause post-harvest rot in various fresh fruits and vegetables. Blueberries are highly susceptible to infection by Botrytis cinerea and Aspergillus species. Cherry tomatoes are also easily invaded by pathogenic microorganisms, accelerating spoilage, such as Escherichia coli, Staphylococcus aureus, and Botrytis cinerea. Therefore, blueberries and cherry tomatoes were selected as subjects for the preservation experiment. In this study, the CS-TA-SA0.75 coating was chosen as the coating for the fruit preservation experiment due to its excellent hydrophobicity, gas barrier properties, and antioxidant properties. Figure 27As shown, in the cherry tomato preservation experiment, cherry tomatoes that were not coated with any solution developed lesions and mycelial growth within one day after being coated with a suspension of Botrytis cinerea spores. Within two days, the fruit rotted and spoiled, accompanied by sap oozing. Cherry tomatoes coated with CS showed mild lesions within two days. However, no lesions were found in cherry tomatoes coated with a CS-TA-SA0.75 film. Figure 28 As shown, in the blueberry preservation experiment, small blueberries that were not coated with any solution developed lesions within three days after being coated with a suspension of *Botrytis cinerea* spores, with mycelium growing from the bottom of the blueberries. Within five days, obvious wrinkles and depressions appeared on the fruit surface. Blueberries coated with CS showed varying degrees of spoilage within three days. In contrast, blueberries coated with CS-TA-SA0.75 showed no spoilage during the five-day storage period. At the end of the storage period, most control tomatoes and blueberries were affected by fungal contamination, while the treated fruits showed no obvious disease symptoms.

[0090] The inhibitory effect of CS-TA-SA0.75 coating on postharvest rot may be due to its ability to block the adhesion of fungal spores. The rot rate of five groups of cherry tomatoes was statistically analyzed. The rot rate of the control group reached 95% after three days, while the rot rate of cherry tomatoes treated with CS-TA-SA0.75 was only 37.5%. Besides spoilage caused by microbial infection, weight loss, softening, and a decrease in soluble solids (SSC) content were also major reasons for the decline in cherry tomato fruit quality. According to the statistical results, the control group of cherry tomatoes showed severe water loss, while this phenomenon was alleviated in the fruit treated with CS-TA-SA0.75 coating. After three days of storage, the firmness of the CS-TA-SA0.75 group of cherry tomatoes was more than twice that of the control samples. In addition, the SSC content of the control group of cherry tomatoes also decreased more rapidly.

[0091] In the blueberry preservation experiment, the fruit coated with CS-TA-SA0.75 showed the best preservation effect. Meanwhile, the rot rate, weight loss, and firmness changes of the five groups of blueberries were recorded. According to the statistical results, the rot rate of the control group blueberries reached 75% after three days, while the rot rate of the blueberries coated with CS-TA-SA0.75 was only 12.5%. During the five-day storage period, the control blueberries also showed severe water loss, a phenomenon that was alleviated in the fruit coated with CS-TA-SA0.75. Furthermore, the blueberries in the CS-TA-SA0.75 coated group showed a slower overall trend of firmness decline. In conclusion, the results indicate that CS-TA-SA0.75 coating can effectively maintain the quality of fruit during storage.

[0092] The above description is merely a preferred embodiment and the technical principles employed in this application. Those skilled in the art will understand that this application is not limited to the specific embodiments described herein, and various obvious changes, readjustments, and substitutions can be made without departing from the scope of protection of this application. Therefore, although this application has been described in detail through the above embodiments, this application is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of this application, all of which are protected by this application.

Claims

1. A modified chitosan coating for enhancing the adhesion of fruit surfaces and resisting fungal spores, characterized in that, It includes a chitosan-tannic acid-stearic acid copolymer, a plasticizer, and water, wherein the copolymer is modified by grafting tannic acid and stearic acid onto a chitosan backbone.

2. The modified chitosan coating according to claim 1, characterized in that, In the copolymer, the mass ratio of chitosan:tannic acid:stearic acid is 1:0.1:0.5 to 1.

3. The modified chitosan coating according to claim 1, characterized in that, The FTIR spectrum of the chitosan-tannic acid-stearic acid copolymer is at 2920 cm⁻¹. -1 2850 cm -1 1703 cm -1 1638 cm -1 and 1537 cm -1 It has a peak.

4. The modified chitosan coating according to claim 1, characterized in that, The chitosan-tannic acid-stearic acid copolymer 1 The 1H NMR spectrum shows peaks at 6.98 ppm, 2.6 ppm, and 1.9 ppm.

5. The modified chitosan coating according to claim 1, characterized in that, The XRD spectrum of the chitosan-tannic acid-stearic acid copolymer has peaks at 2θ of 6.8°, 18.4°, 21.5° and 24°.

6. A method for preparing a modified chitosan coating for enhancing the adhesion of fruit surfaces and resisting fungal spores, characterized in that, Includes the following steps: Chitosan was dispersed in an acetic acid solution to obtain a chitosan solution; H2O2 and ascorbic acid were added to the chitosan solution, and after continuous stirring, an aqueous solution of tannic acid was added to obtain a mixed solution. The pH of the mixed solution was adjusted to 7 to obtain a chitosan-tannic acid conjugate by deposition. Chitosan-tannic acid conjugate was dissolved in acetic acid solution to prepare chitosan-tannic acid solution; Stearic acid was dissolved in anhydrous ethanol, and EDC and NHS were added to react and a mixture was prepared. The mixture was slowly added to the chitosan-tannic acid solution, and the mixture was stirred at room temperature to prepare a chitosan-tannic acid-stearic acid copolymer; and Chitosan-tannic acid-stearic acid copolymer powder was dispersed in water, plasticizer was added, and the mixture was stirred evenly to obtain a modified chitosan coating.

7. The preparation method according to claim 6, characterized in that, The mass ratio of chitosan to tannic acid in the mixed solution is 1:0.

1.

8. The preparation method according to claim 6, characterized in that, The mass ratio of chitosan:tannic acid:stearic acid in the chitosan-tannic acid-stearic acid copolymer is 1:0.1:0.5~1.

9. The preparation method according to claim 6, characterized in that, Before preparing the chitosan-tannic acid solution, the chitosan-tannic acid conjugate is dialyzed with a 7000-14000 Da molecular weight cutoff membrane to remove unreacted raw materials and dehydroascorbic acid byproducts.

10. The preparation method according to claim 6, characterized in that, Before preparing the coating solution, the chitosan-tannic acid-stearic acid copolymer is dialyzed with a 7000-14000 Da molecular weight cutoff membrane to remove unreacted raw materials and urea byproducts.

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