Chitosan grafted tannic acid-iron composite coating solution based on mussel adhesion simulation and application of chitosan grafted tannic acid-iron composite coating solution in mango preservation

Abstract

The invention discloses a chitosan grafted tannic acid-iron composite coating solution based on mussel adhesion simulation and application of the chitosan grafted tannic acid-iron composite coating solution in mango preservation, and belongs to the technical field of food preservation. According to the method, tannic acid is grafted to chitosan through a free radical mediation method to form a compound, then ferric ions are added for crosslinking, and a composite coating film with mussel adhesion imitating characteristics is constructed. According to the method, the technical problems that a traditional chitosan coating film is poor in adhesion, low in polyphenol utilization rate and prone to failure in a high-humidity environment are solved, the shelf life of mangoes can be effectively prolonged, the postharvest loss is reduced, and the method has important industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of food preservation technology, specifically, it relates to a chitosan-grafted tannic acid-iron composite coating solution based on mussel-like adhesion and its application in mango preservation. Background Technology

[0002] Mangoes are typical climacteric fruits, meaning they age rapidly during post-harvest transportation, storage, and retail due to their vigorous respiratory metabolism. Furthermore, aged mangoes are highly susceptible to infection by pathogens such as anthracnose, leading to widespread rot and high post-harvest losses. Chitosan, a positively charged polysaccharide, has been widely used in the preparation of edible coatings and films due to its excellent biocompatibility, strong film-forming ability, and natural antibacterial properties.

[0003] Currently, post-harvest preservation of mangoes mainly relies on chitosan-based edible coating technology. However, single-component chitosan coatings have insufficient post-harvest preservation capabilities, exhibiting poor adhesion and weak antibacterial properties. Direct application of polyphenols is also inefficient: existing technologies directly add polyphenols such as tannins to chitosan coatings, resulting in low polyphenol utilization, high volatility, and poor stability. This directly limits the coating's long-lasting antioxidant capacity. The coating easily peels off the fruit surface, failing to effectively isolate oxygen and moisture exchange. Its inhibitory effect on pathogenic microorganisms such as anthracnose is limited, leading to a high mango rot rate. Importantly, uniform and complete adhesion of chitosan is a key factor in achieving optimal performance.

[0004] Although some studies have physically mixed polyphenols with chitosan, the problems of uncontrollable polyphenol release and unstable coating network structure remain unresolved, as does the adhesion defect. Few studies have focused on the impact of chitosan film adhesion on mango preservation, leading to coating failure in post-harvest high-humidity environments. The inherent flaws of existing technologies can be attributed to three factors: inefficient polyphenol utilization, insufficient coating structural stability, and a lack of adhesion mechanisms, all of which contribute to limited preservation efficacy.

[0005] Therefore, there is an urgent need to develop a chitosan-based composite coating with strong adhesion, high stability and good antibacterial properties to solve the practical problems in post-harvest preservation of mangoes. Summary of the Invention

[0006] The purpose of this invention is to provide a chitosan-grafted tannic acid-iron composite coating solution based on mussel-like adhesion and its application in mango preservation, so as to solve the problems of low polyphenol utilization efficiency, insufficient antibacterial properties, poor adhesion of coating materials to fruit peel, and rapid failure under high humidity conditions in the prior art.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: This invention provides a method for preparing a chitosan-grafted tannic acid-iron composite coating solution based on mussel-like adhesion, comprising the following steps: S1. Add 2 mol / L hydrogen peroxide and 0.108 g ascorbic acid to a 2% w / v chitosan solution and stir magnetically for 30 min to obtain a chitosan mixture solution. S2. Add the tannic acid solution to the above chitosan mixture solution and react in the dark for 24 h to obtain the chitosan-tannic acid mixture solution. S3. The obtained chitosan-tannic acid mixture solution was dialyzed using a dialysis membrane for 48 h. S4. Centrifuge the dialysis solution at 8000 rpm / min, collect the supernatant, pre-freeze it at -80℃, and freeze-dry it to obtain chitosan tannic acid powder. S5. Dissolve chitosan tannic acid powder in deionized water and homogenize at 200 rpm / min for 30 min to obtain a chitosan tannic acid solution with a concentration of 2% w / v. S6. Add ferric chloride powder to the chitosan tannic acid solution and homogenize at 200 rpm / min for 30 min to obtain a chitosan tannic acid ferric composite coating solution.

[0008] Further, in step S1, the chitosan solution is prepared by dissolving chitosan powder in an acetic acid solution with a concentration of 1% v / v.

[0009] Furthermore, in step S2, the solubility of the tannic acid solution is 40% of the solubility of the chitosan solution.

[0010] Furthermore, in step S3, the water used for dialysis treatment needs to be replaced every 6 hours.

[0011] Further, in step S5, the amount of ferric chloride powder added is 0.1% of the amount of chitosan tannic acid powder added.

[0012] This invention also provides an application of the chitosan-grafted tannic acid-iron composite coating solution prepared by the method in mango preservation, comprising the following steps: (1) Disinfect the mango surface with 0.05% sodium hypochlorite solution for 5 min, then rinse with tap water; (2) Allow the disinfected mangoes to air dry for 1 hour; (3) Immerse the dried mango completely in the chitosan-tannic acid-ferric composite coating solution according to any one of claims 1-5 for 1-2 seconds, take it out and air dry it to obtain the coated mango.

[0013] Furthermore, the mangoes are Taiwanese mangoes that are uniform in size, color, and maturity, and are free from mechanical damage and pest infestation.

[0014] The present invention also provides a method for preserving mangoes, wherein the mangoes are coated with a chitosan-grafted tannic acid-iron composite coating solution prepared by the method to extend the shelf life of the mangoes. Beneficial effects

[0015] This invention employs a combination of mussel-inspired chemical modification and metal ion crosslinking technology. Through covalent grafting of tannic acid and chitosan, and coordination crosslinking of ferric ions and catechol groups, a stable network structure is constructed, significantly improving the adhesion, barrier properties, and functional stability of the chitosan-based coating solution. This effectively solves the technical bottlenecks of traditional physically blended coatings, such as easy release and detachment of polyphenols and poor stability. The chitosan-tannic acid-ferric composite coating solution provided by this invention forms a stable adhesive layer on mango peel with a significantly reduced contact angle. Furthermore, its residue rate after washing with deionized water, sodium chloride, and urea is significantly higher than that of a single chitosan coating. This coating can effectively reduce mango weight loss, significantly inhibit anthrax growth, and extend the shelf life of mangoes, demonstrating significant industrial application value. Attached Figure Description

[0016] Figure 1 Images of mango fruits treated with different concentrations of chitosan, tannic acid, and ferric iron coatings, and untreated mango fruits.

[0017] Figure 2 Figures showing changes in appearance (A), weight loss (B), and decay rate (C) of treated and untreated mango fruits.

[0018] Figure 3 The graph shows the changes in respiration rate (A) and flesh firmness (B) of treated and untreated mango fruits.

[0019] Figure 4 The appearance of CS, CS / TA, and CS / TA / Fe solutions (A), the appearance of the mixture of the three solutions with brilliant blue on a glass slide (B), the release of brilliant blue in the three mixtures before and after 120 minutes (C), the change in absorbance within 120 minutes (D), the release of brilliant blue in the three mixture coatings on green grape skin before and after 60 minutes (E) and the change in absorbance within 60 minutes (F), and the release of brilliant blue in the three mixture coatings on mango before and after 60 minutes (G) and the change in absorbance within 60 minutes (H).

[0020] Figure 5 The results show the contact angles between CS, CS / TA, and CS / TA / Fe solutions and the fruit epidermis.

[0021] Figure 6The adhesion of different solutions to brilliant blue on mango and passion fruit peels (A), the residue rate of different solutions on mango peels (B), and the schematic diagram of the interaction force analysis between CS / TA / Fe and mango peels (C).

[0022] Figure 7 CS and CS / TA / Fe and C for molecular dynamics simulations 31 H 64 Diagram showing interfacial adhesion force (A), root mean square deviation (B), and radius of gyration (C) between the sheets.

[0023] Figure 8 CS and CS / TA / Fe and C for molecular dynamics simulations 31 H 64 The temporal variation distance (A) between layers, CS and C 31 H 64 Van der Waals forces and electrostatic interactions between the layers (B) and CS / TA / Fe with C 31 H 64 Figure (C) shows the van der Waals forces and electrostatic interactions between the layers.

[0024] Figure 9 The graph shows the changes in appearance (A), weight loss (B), and plaque diameter (C) of treated and untreated mango fruits.

[0025] Figure 10 A graph showing the changes in porosity of mango peels treated with different coatings during storage. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solutions of this invention, the present application will be further described in detail below with reference to embodiments.

[0027] Example 1: Preparation of chitosan-grafted tannic acid-iron composite coating solution 1. Dissolve 2 g of chitosan (CS) powder in 100 mL of 1% v / v acetic acid solution and homogenize at 200 rpm for 30 min to obtain a 2% w / v CS solution.

[0028] 2. Add 2 mol / L hydrogen peroxide and 0.108 g ascorbic acid to a 2% w / v CS solution and stir magnetically for 30 min to obtain a CS mixture solution.

[0029] 3. Add 40% (w / w, relative to CS) tannic acid (TA) solution to the CS mixture solution and react in the dark for 24 h to obtain chitosan-tannic acid mixture (CS-TA) solution.

[0030] 4. Dialyze the obtained CS-TA solution using a dialysis membrane, changing the water every 6 hours, for a total dialysis time of 48 hours.

[0031] 5. After dialysis, the solution is centrifuged at 8000 rpm / min, the supernatant is collected, pre-frozen at -80°C, and finally lyophilized to obtain CS / TA powder.

[0032] 6. Dissolve 2 g of CS / TA powder in 100 mL of deionized water and homogenize at 200 rpm / min for 30 min to obtain a 2% w / v CS / TA solution.

[0033] 7. Add 0.1% (w / w, relative to CS / TA) FeCl3 powder to a 2% w / v CS / TA solution, and homogenize at 200 rpm / min for 30 min to obtain a chitosan-tannic acid-trivalent iron composite coating solution (CS / TA / Fe solution).

[0034] Example 2: Application of coating solution in mango preservation Select Taiwanese mangoes that are uniform in size, color, and maturity, and free from mechanical damage and pest infestation. Mangifera indica The fruit (Linn) was surface-sterilized with 0.05% NaClO solution for 5 min, then rinsed with tap water. After air drying for 1 h, the fruit was randomly divided into eight batches. The mangoes were completely immersed in CS solution, CS / TA solution, and CS / TA / Fe solutions with concentrations of 0.5%, 1.0%, 1.5%, and 2.0% for 1-2 seconds, then removed and air-dried. Fruit treated with distilled water served as a control.

[0035] All fruits were evenly packed into plastic baskets (50 cm × 20 cm × 15 cm) and stored at a temperature of 25 ± 2℃ and a relative humidity of 60-70%. Within 18 days, the weight loss and firmness of the fruits were measured, and the decay index was assessed.

[0036] Weight loss: Fruit weight loss was determined using gravimetric analysis.

[0037] Rot rate: The rot rate of each group of 3 mangoes was assessed every two days.

[0038] Respiration rate: Two mangoes were placed in a sealed container (15 cm in diameter) for 30 minutes daily, and the respiration rate was measured. CO2 production (mg CO2 kg) was measured using a portable infrared gas analyzer (GXH-3010E). −1 h −1 ), and calculate based on the weight of the fruit.

[0039] Hardness parameter measurement: Pulp hardness was measured using a texture analyzer (TA-XT plus 40762, SMS UK) equipped with a P / 2 probe (2 mm in diameter).

[0040] Visual observations showed that mangoes treated with 1%, 1.5%, and 2% CS / TA / Fe solutions failed to turn yellow normally during storage. Untreated mangoes (control group, CK) and those treated with 0.5% CS / TA / Fe solution turned yellow normally, but the untreated mangoes rotted rapidly during storage. Figure 1 Therefore, a 0.5% CS / TA / Fe solution was subsequently chosen to treat the mangoes.

[0041] Weight loss tests showed that mango fruit weight loss gradually increased with prolonged storage time. By day 18, the weight loss of the control group (CK) fruit reached 17.48% ± 1.57. The weight loss of fruit coated with the CS coating was consistently lower than that of the control group. This reduction in weight loss was attributed to the edible coating effectively slowing down the respiration and transpiration of the fruit. Compared to the control group, the weight loss of fruit coated with CS and CS / TA coatings decreased by 16.76% and 22.39%, respectively. The weight loss of fruit coated with the CS / TA / Fe coating decreased during storage, reaching only 12.27% ± 1.56 by day 18, indicating that TA and Fe... 3+ The addition of [a specific ingredient] further suppressed the weight loss of the mango. This effect may be due to the coating altering the fruit's environment, limiting permeability and gas exchange. Figure 2 -B).

[0042] The decay test results showed that the decay index of mango fruit increased with prolonged storage time, but CS coating treatment significantly reduced the decay index (p < 0.05). Furthermore, the incorporation of TA significantly reduced the decay index compared to the CS group (p < 0.05). Notably, from day 12 and 18 of storage, the decay index of fruit coated with CS / TA / Fe was significantly lower than that of other groups (p < 0.05). Figure 2 -C).

[0043] The control group (CK) and CS-coated fruits showed obvious rotting on day 12, while the CS / TA and CS / TA / Fe-coated fruits maintained a good appearance. Figure 2 -A). Studies have shown that TA, as an antibacterial and cross-linking agent, can enhance the barrier and antibacterial properties of edible coatings. Fe 3+ The incorporation of [a specific ingredient] also enhances the antibacterial effect of the CS / TA coating on the fruit, thereby better maintaining the quality of the post-harvest fruit. Furthermore, it is related to the enhanced adhesion of the CS / TA / Fe composite coating.

[0044] Respiration rate is a key factor affecting fruit senescence. Respiration rate tests show that the respiration rate of mango fruit increases linearly throughout the storage period. Figure 3 -A). Except for day 15, the respiration rates of fruits treated with CS, CS / TA, and CS / TA / Fe coatings were consistently lower than those of the control group (CK). Edible coatings act as a barrier to gas exchange, limiting the oxygen required for plant tissue respiration and thus reducing carbon dioxide emissions. Therefore, applying edible coatings to minimally processed fruits creates a altered atmosphere around each fruit, reducing respiration rates and slowing metabolic processes.

[0045] Fruit texture (firmness) is an important factor for consumers in evaluating fruit quality. Firmness test results show that the firmness of mangoes decreases linearly during storage. Figure 3 -B). Compared to the control group (CK), CS, CS / TA, and CS / TA / Fe coatings better maintained fruit firmness. The CS / TA / Fe coating significantly enhanced these properties, which is likely mainly attributed to changes in respiratory metabolic pathways caused by the barrier properties of the coating. Consistent with this trend, the nano-CS coating significantly delayed the softening process and helped maintain fruit firmness. Under normal circumstances, the firmness of mango fruit decreases linearly over time, but on the last day, mango fruit coated with CS, CS / TA, and CS / TA / Fe had higher firmness than the control group. Similarly, the firmness of citrus fruits treated with water-soluble dialdehyde cellulose (DAC) / CS coating was increased, indicating that the coating has a positive effect on the preservation of citrus fruits. Studies have shown that immersion in 0.5 mM salicylic acid (SA) can effectively maintain fruit firmness. Therefore, the CS / TA / Fe coating helps maintain higher fruit firmness by delaying metabolic processes by reducing fruit respiration.

[0046] Example 3: Coating solution adhesion performance test Fresh passion fruit, mango, banana, melon, apple, citrus, and cucumber were sampled from relatively flat areas of their skin and placed on an interfacial tensiometer (OSA100C, Ningbo Xinjie Scientific Instruments Co., Ltd., Ningbo, China). The same volume of CS, CS / TA, and CS / TA / Fe solutions were added dropwise using a 10 mL dropper. Images were taken at 0 s and 30 s, and the contact angle between the solution and the fruit skin was calculated.

[0047] CS, CS / TA, and CS / TA / Fe solutions were mixed with 1% w / v Brilliant Blue solution at a volume ratio of 1 / 2. Equal volumes of each mixture were then evenly spread onto a glass slide. After air-drying at 25°C for 12 h, the slides were placed in an equal volume of deionized water. The absorbance (initial OD) of the deionized water containing Brilliant Blue was measured at 610 nm at 10, 20, 40, 60, 80, and 120 min. Furthermore, the release of Brilliant Blue from the skins of grapes and mangoes was also determined using the three Brilliant Blue mixtures.

[0048] The interaction types of solutions were analyzed by washing naturally air-dried CS, CS / TA, and CS / TA / Fe coatings on mango peels with deionized water, NaCl, and urea solutions. Specifically, equal volumes of the three mixtures were evenly applied to the mango peels after being mixed with equal proportions of brilliant blue. After the peels were air-dried at 25°C for 12 h, they were placed in 10 mL of deionized water, 1% w / v NaCl, and 1% w / v urea, and shaken at 200 rpm for 5 min at room temperature. The supernatant was then measured after centrifugation. 610 (OD elution). The residual rate is calculated using the following formula: Residual rate (%) = (1 - OD elution / OD initial) × 100%.

[0049] The adhesion of the three coated grape and mango skins was characterized by the release of brilliant blue. The appearance of the CS, CS / TA, and CS / TA / Fe solutions, and their mixtures with brilliant blue, on glass slides are shown in [reference needed]. Figure 4 -A and Figure 4 -B, the release appearance of Brilliant Blue in the three coatings at 120 min is shown in [the image / description]. Figure 4 -C. Over time, more and more of the three coatings containing brilliant blue are released into them. Figure 4 -D), but the addition of TA significantly reduced the release of brilliant blue in the CS coating (p < 0.05), and the incorporation of Fe significantly reduced the release of brilliant blue in the CS / TA coating (p < 0.05). Similar trends were also observed in the skins of grapes and mangoes ( Figure 4 -E、 Figure 4 -F、 Figure 4 -G、 Figure 4 -H). This may be because of Fe. 3+ The formation of a coordination cross-linking network with TA inhibited the release of brilliant blue to some extent.

[0050] The test results of the three solutions on the epidermis of passion fruit, mango, banana, melon, apple, citrus, and cucumber showed that with the addition of TA, the contact angle of CS solution on the fruit epidermis decreased significantly (p < 0.05). Figure 5Studies have found that pure CS and CS / gelatin films do not exhibit wet adhesion, while the addition of a certain amount of TA shows significant adhesion. Specifically, the contact angles of CS, CS / TA, and CS / TA / Fe solutions on fresh mango peel were 119.33°±6.75, 106.70°±3.18, and 90.97°±5.52, respectively. After 30 s, the contact angles decreased to 109.77°±2.22, 98.87°±7.16, and 83.77°±2.81, respectively. Significant differences were observed in the contact angles of the three solutions on the fruit peel surface at 0 s and 30 s (p < 0.05). Similarly, the same trend was observed in the contact angles of passion fruit, apple, melon, and tangerine peels. Notably, only the CS / TA / Fe group showed a significant difference from the CS group on banana and cucumber peels (p < 0.05). This suggests that this may be related to the different adhesive work on different fruit surfaces.

[0051] Adhesion test results showed that CS, CS / TA, and CS / TA / Fe, when mixed with brilliant blue, exhibited different adhesion characteristics on mango and passion fruit peels. Figure 6 -A). The mixture of CS and Brilliant Blue exhibited droplet-like aggregation on mango and passion fruit peels. However, the addition of TA reduced this aggregation. Among the three mixtures, the CS / TA / Fe mixture with Brilliant Blue showed the most significant improvement in adhesion to the peel. After washing with deionized water, 1% w / v NaCl, and 1% w / v urea solution, the residue rates of the CS / TA and CS / TA / Fe groups were higher than those of the CS mixture. Figure 6 -B). This is likely because the phenolic hydroxyl groups of TA form multiple hydrogen bonds and hydrophobic interactions with the waxy layer of the pericarp, significantly enhancing the basic adhesion. It is noteworthy that the residual rate of the CS / TA / Fe group is significantly higher than that of the CS / TA group. This is likely because Fe³⁺ forms a coordination cross-linking network with TA, enhancing resistance through two mechanisms: 1. Physical shielding: The rigid network structure protects hydrogen bonds from urea damage. 2. Charge neutralization: Reduces the ionization dependence of TA and decreases the damage to electrostatic bonds caused by NaCl.

[0052] Example 4: Molecular simulation verification The molecular simulation system was set in a closed environment at 25°C, pH 5.5, and 1 bar. A periodic boundary was used with the system's center of mass as the center, and the minimum distance from the box edge was set to 10 Å. The receptor structure topology file was converted to a GROMACS-recognizable file using the pdb2gmx tool, with a CHARMM32 field parameter. After initial system construction, the steepest descent algorithm was applied to all atoms to minimize system energy. Constant number of particles, pressure, and temperature (NPT) equilibrium simulations were performed for 1000 ps under protein location constraints. After NPT equilibrium, a kinetic simulation was performed for 100 ns, with simulations every 2 fs. Covalent bond lengths were constrained using a linear constraint algorithm, and long-distance electrostatic interactions were handled using the Ewald method for ion networks. The cutoff radius for nearest neighbor search and non-bonded interactions was set to 12 Å. All simulation results were visualized using the open-source software PyMOL 2.04 (DeLano, 2002).

[0053] The results showed that molecules in the CS system exhibited stable binding after 40 ns, while stable binding was observed in the CS / TA / Fe composite system after 15 ns, indicating that the CS / TA / Fe composite system and the C31H64 sheets were more likely to bind. Figure 7 -A). To quantify the differences between the two systems, the root mean square deviation (RMSD) was calculated to reflect the stability of the bonding during the simulation. A smaller RMSD value indicates more stable bonding, while a larger RMSD indicates less stable bonding. Data shows that the CS / TA / Fe composite system has a smaller RMSD, stabilizing at around 3.5 Å, while the CS-only system has a larger RMSD, stabilizing at around 5 Å. This indicates that the composite system has better bonding stability, while the CS system has worse stability. Figure 7 -B). The trend of the radius of gyration (Rg) was used to analyze the binding compactness between the molecule and the C31H64 sheets. A lower Rg value indicates a more compact binding, while a higher value indicates a looser binding. Data shows that the Rg value of the CS / TA / Fe composite system is lower, while the Rg value of the CS system is higher, indicating that the composite system has a high binding compactness and good stability. Figure 7 -C).

[0054] Furthermore, during the simulation, the centroid distance of the CS / TA / Fe complex decreased in the later stages, indicating a tighter bond between molecules. Conversely, the centroid of the CS system was larger, suggesting a weaker bond between molecules. This also indicates that the CS / TA / Fe complex molecules are more closely bound to C. 31 H 64 Better integration ability ( Figure 8 -A). During the simulation, their interaction was primarily dominated by van der Waals interactions, followed by coulomb interactions. Data shows that at the end of the simulation, the van der Waals energy of the CS system was -50.165 kcal / mol and the coulomb energy was -22.964 kcal / mol, while the van der Waals energy of the CS / TA / Fe system was -68.572 kcal / mol and the coulomb energy was -38.993 kcal / mol, indicating that the CS / TA / Fe system interacts with C... 31 H 64 The bonding energy of the lamellar structure is stronger, while that of the CS system is weaker than that of the CS / TA / Fe system. Figure 8 -B、 Figure 8 -C).

[0055] Example 5: Antibacterial Performance Test of Coating Solution Mango fruits treated with water, CS, CS / TA, and CS / TA / Fe were pipetted along the equator with 10 μL of anthracnose suspension (1×10⁻⁶). 6 Inoculate each mango with an artificial wound 5 mm deep and 2 mm wide near the equator using CFU / mL. Pack all fruits evenly into plastic baskets (50 cm × 20 cm × 15 cm) and store at 25 ± 2℃ and 60–70% relative humidity. Record appearance every two days for 7 days after infection, while measuring lesion diameter and weight.

[0056] Antibacterial test results showed that the diameter of lesions and weight loss of all inoculated mangoes gradually increased over time. Figure 9Specifically, visible lesions (4.60 ± 0.36 mm) appeared on uncoated mangoes by day 3. The lesion diameter in the CS-coated group was 4.17 ± 0.76 mm, indicating that CS partially inhibited lesion expansion compared to the uncoated control group, possibly due to its inherent antibacterial properties. Notably, the lesion diameters in the CS / TA and CS / TA / Fe coated groups were significantly smaller than those in the uncoated and CS-coated groups (p < 0.05). Furthermore, the inhibitory effects of CS / TA and CS / TA / Fe coatings on lesion expansion were significantly stronger than those of CS alone. The antibacterial activity of TA has been well-established in previous studies. On the last day of storage, the lesion diameters of uncoated, CS, CS / TA, and CS / TA / Fe treated mangoes were 37.67 ± 1.53, 26.33 ± 1.53, 13.67 ± 1.53, and 10.33 ± 1.53 mm, respectively. Figure 9 -A、 Figure 9 These results indicate that the metal-phenol network formed by the introduction of Fe³⁺ significantly enhances the antibacterial activity of mango, which is consistent with numerous studies confirming that Fe³⁺ has an antibacterial enhancing effect.

[0057] Furthermore, over time, the pores on the coated mango skin gradually become exposed. Figure 10 For the CS group, after 9 days of storage, the CS coating gradually loosened on the fruit skin, showing partial peeling and exposing the stomata. For the CS / TA group, the stomata were relatively tightly covered on the first day, compared to the CS group. At a magnification of 500x, compared to the other two groups, the stomata on the mango skin of the CS / TA / Fe group were still relatively tightly covered by the coating on the ninth day.

[0058] In summary, the chitosan-grafted tannic acid-iron composite coating of the present invention significantly improves the adhesion performance between the coating and the mango peel through a mussel-like adhesion mechanism, effectively extending the shelf life of the mango and reducing post-harvest losses, thus possessing significant industrial application value.

Claims

1. A method for preparing a chitosan-grafted tannic acid-iron complex coating solution based on mimic mussel adhesion, characterized by, The method comprises the following steps: S1, adding 2 mol / L hydrogen peroxide and 0.108 g ascorbic acid into a 2% w / v chitosan solution, and stirring magnetically for 30 min to obtain a chitosan mixture solution; S2, adding a tannic acid solution into the chitosan mixture solution, and reacting in a dark environment for 24 h to obtain a chitosan-tannic acid mixture solution; S3, performing dialysis treatment on the chitosan-tannic acid mixture solution using a dialysis membrane for 48 h; S4, centrifuging the solution after the dialysis treatment at a speed of 8000 rpm / min, collecting the supernatant, pre-freezing at -80℃, and freeze-drying to obtain a chitosan-tannic acid powder; S5, dissolving the chitosan-tannic acid powder in deionized water, and homogenizing at a speed of 200 rpm / min for 30 min to obtain a 2% w / v chitosan-tannic acid solution; S6, adding iron trichloride powder into the chitosan-tannic acid solution, and homogenizing at a speed of 200 rpm / min for 30 min to obtain a chitosan-tannic acid-iron trichloride complex coating solution.

2. The production method according to claim 1, characterized by, In step S1, the chitosan solution is prepared by dissolving chitosan powder in a 1% v / v acetic acid solution.

3. The preparation method according to claim 1, characterized in that, In step S2, the solubility of the tannic acid solution is 40% of the solubility of the chitosan solution.

4. The production method according to claim 1, characterized by, In step S3, the dialysis treatment water needs to be replaced every 6 h.

5. The method of claim 1, wherein, In step S5, the amount of the iron trichloride powder added is 0.1% of the amount of the chitosan-tannic acid powder added.

6. Use of the chitosan grafted tannic acid-iron complex coating solution prepared by the method according to any one of claims 1-5 for preserving mangoes, characterized in that, The method comprises the following steps: (1) disinfecting the mangoes with 0.05% sodium hypochlorite solution for 5 min, and then rinsing with tap water; (2) naturally drying the disinfected mangoes for 1 h; (3) completely immersing the dried mangoes in the chitosan-tannic acid-iron trichloride complex coating solution according to any one of claims 1-5 for 1-2 s, taking out and air-drying to obtain the mangoes after the coating treatment.

7. Use according to claim 6, characterized in that, The mangoes are Taizhong mangoes with uniform size, consistent color and maturity, and no mechanical damage and pest infection.

8. A method for preserving mangoes, characterized by, The chitosan-grafted tannic acid-iron complex coating solution prepared by the method according to any one of claims 1-5 is used for coating treatment of the mangoes to prolong the shelf life of the mangoes.

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