Preparation method of hydrogel composite coating and application of hydrogel composite coating in preservation of fruits and vegetables

Abstract

The invention relates to the technical field of food science, and discloses a preparation method of a hydrogel composite coating and application of the hydrogel composite coating in fresh keeping of fruits and vegetables. According to the hydrogel composite coating disclosed by the invention, chitosan is taken as a base material, chlorogenic acid derived from the honeysuckle flower extract and zinc oxide nanoparticles are combined, and the hydrogel composite coating has a multilayer three-dimensional porous structure with a clear structure, and is good in structural stability and excellent in biocompatibility; the fruit and vegetable fresh-keeping agent is used for fruit and vegetable fresh-keeping application, effectively relieves nutrition loss caused by oxidative decay, has a broad-spectrum inhibition effect on various food-borne pathogens, is especially suitable for perishable fruits and vegetables, conforms to the green packaging trend and is biodegradable.

Description

Technical Field

[0001] This invention belongs to the field of food science and technology, specifically relating to a method for preparing a hydrogel composite coating and its application in fruit and vegetable preservation. Background Technology

[0002] Currently, about one-third of the food wasted globally each year is due to spoilage and damage caused by microbial contamination or oxidation during storage, transportation, and distribution. This problem is particularly serious for perishable foods such as fruits and vegetables, which typically have a short shelf life and are easily deteriorated by external environmental factors such as temperature fluctuations and humidity changes.

[0003] To reduce food waste and spoilage, various preservatives and preservation technologies have been developed. Traditional preservatives typically include oxidants, reducing agents, acidifiers, and flavor enhancers. While these substances can slow down the spoilage process to some extent, they often have significant drawbacks: long-term use may lead to unpleasant changes in the taste of food, and in some cases, may even pose health risks to consumers.

[0004] In recent years, advances in food science have opened up new avenues for food preservation technology. Natural active substances are gaining attention due to their biocompatibility and safety, and are increasingly being explored as alternatives to synthetic preservatives.

[0005] Therefore, developing methods for preserving fruits and vegetables from natural sources of active substances is an important way to solve the current problems in fruit and vegetable preservation. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a hydrogel composite coating and its application in the preservation of fruits and vegetables.

[0007] Based on the above concept, the technical solution adopted by this invention is as follows: According to a first aspect of the present invention, a method for preparing a hydrogel composite coating is provided, comprising the following steps: S1: Preparation of zinc oxide nanoparticles: Using zinc(II) di(2,4-pentanedione) as the zinc source and polyethylene glycol aqueous solution with a molecular weight of 2000-10000 as the morphology modifier, the reaction mixture was prepared by mixing zinc(II) di(2,4-pentanedione) as zinc (II) and polyethylene glycol aqueous solution at a mass-to-volume ratio of 1:(10-15). The mixture was magnetically stirred for 30 min until a complete solution was obtained. Subsequently, the solution was heated under reflux, cooled, and centrifuged to obtain zinc oxide nanoparticles. S2: Preparation of chlorogenic acid extract: Using honeysuckle as raw material, chlorogenic acid extract powder is obtained by soaking in pure water, enzymatic hydrolysis with cellulase, hot water extraction, centrifugation and rotary evaporation; the enzymatic hydrolysis conditions are: enzymatic hydrolysis temperature 50~60℃, cellulase dosage 0.1% of the dry weight of honeysuckle, and enzymatic hydrolysis time 1~2h. S3: Preparation of hydrogel composite coating: Dissolve 30 mg chitosan in 0.1 M acetic acid aqueous solution. Add 50% (w / v) sodium β-glycerophosphate aqueous solution to chitosan solution at a volume ratio of 9:1. At the same time, add 100 μg of zinc oxide nanoparticles obtained in step S1 and chlorogenic acid extract powder obtained in step S2 containing 100 μg of chlorogenic acid. Incubate in a constant temperature shaker for 48 h to obtain hydrogel composite coating.

[0008] Optionally, in step S1, the molecular weight of the polyethylene glycol aqueous solution is 6000, and the heating reflux, cooling, and centrifugal purification includes: The reaction mixture was heated to 100°C, then raised to 180-200°C and held for 2 hours; Then raise the temperature to 250-260℃ at a rate of 10℃ / min and reflux for 1 hour; After removing the heat source, the product was cooled to 80°C and dispersed in a 1:1 volume ratio hexane / ethanol mixture. It was then purified by centrifugation at 8000-10000 r / min for 15 min-1 h.

[0009] Optionally, in step S1, the obtained zinc oxide nanoparticles are particles obtained by passing through a 100-mesh sieve.

[0010] Optionally, in step S2, honeysuckle is used as raw material, and chlorogenic acid extract powder is obtained by soaking in pure water, enzymatic hydrolysis with cellulase, hot water extraction, centrifugation, and rotary evaporation. Specifically, this includes: Honeysuckle was dried in a 50°C oven and cooled to room temperature. It was then pulverized using a multi-functional pulverizer to obtain honeysuckle powder. The honeysuckle powder was added to pure water at a ratio of 1:30 g / mL and soaked for 1 hour. Cellulase of 0.1% of the dry weight of honeysuckle was added and enzymatically hydrolyzed at 50-60℃ for 1-2 hours. After enzymatic hydrolysis, the enzymes were inactivated by boiling water bath for 10 minutes, and honeysuckle was extracted using hot water extraction method. The extraction temperature and extraction time were set to 80-90 ℃ and 1.5-2.0 h, respectively. Centrifuge the extract at 4,000 r / min for 15 min, collect the supernatant, and dry it by rotary evaporation.

[0011] Optionally, in step S2, the chlorogenic acid extract powder contains 80%-90% chlorogenic acid, the honeysuckle powder is granules obtained by passing through a 40-mesh sieve, and the cellulase activity is 100,000 U / g.

[0012] Optionally, in step S3, 30 mg of chitosan is dissolved in a 0.1 M aqueous acetic acid solution, specifically including: 30 mg of chitosan was gradually added to 3 mL of acetic acid aqueous solution and continuously vortexed to obtain a homogeneous suspension; the suspension was then transferred to a 5 mL centrifuge tube, sealed, and allowed to stand overnight at 4 °C to ensure complete dissolution of the chitosan.

[0013] Optionally, in step S3, the sodium β-glycerophosphate aqueous solution is added dropwise, and after adding zinc oxide nanoparticles and chlorogenic acid extract powder, it is gently vortexed to ensure uniform dispersion.

[0014] Optionally, in step S3, The parameters for the spin coating method are: rotation speed 2000 r / min, film formation time 30 s; The incubation conditions for the constant temperature shaker are: temperature 37℃ and rotation speed 100r / min.

[0015] Secondly, this application also provides an application of the hydrogel composite coating prepared by the method described in the first aspect in the preservation of fruits and vegetables, comprising the following steps: (1) Wash fruits and vegetables with sterile water or disinfect them with sodium hypochlorite at a volume ratio of 2%; (2) Place the washed or disinfected fruits and vegetables in a well-ventilated room temperature for 1-2 hours to air dry naturally and remove surface moisture; (3) Immerse the dried fruits and vegetables in the hydrogel composite coating for 1-5 minutes to ensure that the surface is completely covered by the adhesive; take them out and let them air dry at room temperature for 10-20 minutes to form a uniform coating on the surface of the fruits and vegetables, and store them at 25±2℃.

[0016] Thirdly, this application also provides a method for preserving fruits and vegetables, using a hydrogel composite coating obtained by the preparation method described in the first aspect.

[0017] The beneficial effects of this invention are as follows: The hydrogel composite coating of this invention uses chitosan as a base material and combines chlorogenic acid derived from honeysuckle extract and zinc oxide (ZnO) nanoparticles. The hydrogel composite coating has a clear multilayer three-dimensional porous structure with good structural stability and excellent biocompatibility. When used for fruit and vegetable preservation, it effectively reduces nutrient loss caused by oxidative spoilage, has a broad-spectrum inhibitory effect on a variety of foodborne pathogens, and is especially suitable for perishable fruits and vegetables. It conforms to the trend of green packaging and is biodegradable. Attached Figure Description

[0018] Figure 1 This is a scanning electron microscope image of the hydrogel composite coating of the present invention; Figure 2 These are hemolysis experimental images and relative hemolysis rates of the chitosan used in step S3 of this invention; Figure 3These are hemolysis test images and relative hemolysis rates of the chlorogenic acid extract powder obtained in step S2 of this invention; Figure 4 These are hemolysis experimental images and relative hemolysis rates of the zinc oxide nanoparticles prepared in step S1 of this invention; Figure 5 These are hemolysis experimental images and relative hemolysis rates of the hydrogel composite coating of the present invention; Figure 6 This is an experimental result diagram of the application of the hydrogel composite coating of the present invention in tomato preservation; Figure 7 This is an experimental result diagram of the application of the hydrogel composite coating of the present invention in the preservation of citrus fruits; Figure 8 This is a schematic diagram of the preparation process of the hydrogel composite coating of the present invention. Detailed Implementation

[0019] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise defined, the technical or scientific terms used in this application should be understood in their ordinary sense by one of ordinary skill in the art to which this invention pertains. The singular forms “a,” “the,” and “the” used in this specification and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0020] The technical concept of this invention includes the following: Although certain natural active substances have proven effective in inhibiting microbial growth and slowing food spoilage, there is still a lack of food-grade antibacterial and preservative coatings on the market that can effectively combat bacteria without significantly affecting food quality. While chitosan and alginate coatings have been successful in fruits such as apples, citrus fruits, and berries, their mechanical stability and water resistance remain limited, often requiring modification or combination with other bioactive ingredients to ensure stability under humid storage conditions. Current preservation technologies often target only a single aspect of spoilage (such as oxidation or microbial contamination), making it difficult to address multiple degradation mechanisms simultaneously. Therefore, it is crucial to develop food-grade antibacterial coatings that can both inhibit microbial proliferation and delay nutrient oxidation—coatings that not only improve food freshness and extend shelf life but also provide a more comprehensive approach to food preservation. Recent research has focused on nanocomposite materials that integrate the beneficial properties of natural polymers and nanomaterials, aiming to develop multifunctional coatings with enhanced antibacterial and antioxidant capabilities.

[0021] This invention successfully developed a food-grade hydrogel composite antibacterial and preservative coating. The coating uses chitosan as a base material, combined with chlorogenic acid derived from honeysuckle extract and zinc oxide (ZnO) nanoparticles, for fruit and vegetable preservation applications. Chitosan is a natural polysaccharide derived from chitin, widely studied for its antibacterial and film-forming properties; its ability to form a semi-permeable barrier makes it an ideal candidate material for food preservation and packaging applications. The incorporation of zinc oxide nanoparticles further enhances the system's antibacterial efficacy, thanks to its broad-spectrum activity, high surface area, and ability to generate reactive oxygen species (ROS). Furthermore, chlorogenic acid, a potent polyphenolic antioxidant found in honeysuckle, provides oxidative stability to the coating, thereby protecting food from degradation.

[0022] Firstly, this application provides a method for preparing a hydrogel composite coating, such as... Figure 8 As shown, it includes the following steps: S1: Preparation of zinc oxide nanoparticles: Using bis(2,4-pentanedione)zinc(II) as the zinc source and a 20% (w / v) polyethylene glycol aqueous solution with a molecular weight of 6000 as the morphology modifier, the zinc(II) and polyethylene glycol aqueous solution were mixed at a mass-to-volume ratio of 1:(20-30) g / mL to obtain a reaction mixture. The mixture was magnetically stirred for 30 min under a nitrogen atmosphere until completely dissolved. The reaction mixture was then heated to 100℃ and kept at that temperature for 30 min to remove free water from the system. The temperature was then raised to 180-200℃ and held for 2 hours. The temperature was then raised to 250-260℃ at a rate of 10℃ / min and refluxed for 1 hour. After removing the heat source, the mixture was cooled to 80℃. The product was dispersed in a 1:1 volume ratio hexane / ethanol mixture and centrifuged at 5000-60000 r / min for 15 min-1 h. The lower precipitate was collected and purified with a 1:1 volume ratio hexane / ethanol mixture. Wash 2-3 times with a hexane / ethanol mixture, dry under vacuum at 60℃ for 6-8 hours, grind, and pass through a 100-mesh sieve to obtain zinc oxide nanoparticles for later use.

[0023] S2: Preparation of chlorogenic acid extract powder: Honeysuckle was dried in a 50 ℃ oven and cooled to room temperature. After cooling, it was pulverized using a multi-functional pulverizer and passed through a 40-mesh sieve to obtain honeysuckle powder. The honeysuckle powder was added to pure water at a material-to-liquid ratio of 1:30 g / mL and soaked for 1 h. Cellulase with an activity of 100,000 U / g and 0.1% of the dry weight of honeysuckle was added and enzymatically hydrolyzed at 50-60℃ for 1-2 h. After enzymatic hydrolysis, the temperature was raised to 80℃ for 10 min to inactivate the enzyme. Then, hot water extraction was performed at 80-90℃, maintaining a material-to-liquid ratio of 1:30 g / mL. The extraction temperature and extraction time were set to 80-90℃ and 1.5-2.0 h, respectively. The extract was centrifuged at 4000 r / min for 15 min. The supernatant was collected, and the extract was dried by rotary evaporation to obtain chlorogenic acid extract powder. The powder was stored in the dark for later use.

[0024] S3: Preparation of hydrogel composite coating: 30 mg of chitosan with a degree of deacetylation ≥ 85% was gradually added to 3 mL of 0.1 M acetic acid aqueous solution, and continuously vortexed to obtain a homogeneous suspension; transferred to a 5 mL centrifuge tube, sealed and allowed to stand overnight at 4 °C to ensure complete dissolution of chitosan; 50% (w / v) of β-glycerophosphate sodium aqueous solution was slowly added to the chitosan solution at a volume ratio of 9:1, along with 100 μg of zinc oxide nanoparticles obtained in step S1 and chlorogenic acid extract powder obtained in step S2 containing 100 μg of chlorogenic acid (chlorogenic acid content 80%-90%), and gently vortexed to ensure uniform dispersion; the mixture was incubated in a constant temperature shaker at 37 °C and 100 rpm for 48 hours, and the coating surface was gently rinsed twice with deionized water to remove uncrosslinked free components, resulting in a homogeneous hydrogel composite coating.

[0025] Performance Characterization 1 The morphology of the prepared hydrogel composite coating was observed using a scanning electron microscope (SEM, SU8600, Japan), such as... Figure 1 The image shown is a scanning electron microscope image of the hydrogel composite coating (CTS-ZnO&CA hydrogel).

[0026] Figure 1 Image A in the image is a scanning electron microscope (SEM) image of the cross-section of the hydrogel composite coating. Figure 1 Image B is a scanning electron microscope image of its longitudinal section. Figure 1 C in the image represents a cross-sectional scanning electron microscope (SEM) image of the hydrogel composite coating. Figure 1 The Chinese Academy of Sciences (CAS) demonstrated the dense porous structure of the hydrogel composite coating, with interconnected pores that retain the breathability of the hydrogel while improving structural stability through densification.

[0027] Figure 1 Image D is a scanning electron microscope image showing the surface distribution of oxygen (O) in the hydrogel composite coating. The O element is uniformly distributed throughout the coating, reflecting the uniform dispersion of oxygen-containing groups of chitosan (-OH, -NH2), ZnO (O²⁻) and chlorogenic acid (phenolic hydroxyl, carboxyl groups) in the matrix, providing a reliable structural basis for the broad-spectrum antibacterial properties of the coating (ZnO).

[0028] Figure 1 Image E shows the scanning electron microscope image of the zinc (Zn) element distribution in the hydrogel composite coating. The Zn element shows no obvious agglomeration and is uniformly distributed, which directly proves that the ZnO nanoparticles are well dispersed in the hydrogel matrix, providing structural protection for the antibacterial properties of the coating.

[0029] Therefore, through Figure 1It is known that the hydrogel composite coating retains an interconnected porous structure, which facilitates the permeability of water and oxygen (meeting the breathing needs of fruit and vegetable preservation) and allows for the continuous release of antibacterial / antioxidant components. At the same time, the uniform microstructure can prevent local failure of the coating.

[0030] Performance Characterization 2 To verify the chitosan used in step S3, the chlorogenic acid extract powder obtained in step S2, and the chitosan used in step S1 of this invention, The biosafety of the prepared zinc oxide nanoparticles and the final hydrogel composite coating was evaluated using a erythrocyte hemolysis test: 4% (v / v) fresh rat erythrocyte (RBC) suspension (washed 3 times with PBS) was mixed with different concentrations (12.5, 25, 50, 100, 200, 500 μg / mL) of four different hydrogels after freeze-drying: chitosan hydrogel, chitosan and chlorogenic acid composite hydrogel, zinc oxide nanoparticle and chitosan composite hydrogel, and chitosan-chlorogenic acid-zinc oxide composite hydrogel. The mixtures were incubated at 37°C for 1 hour; the supernatant was collected by centrifugation (5000 rpm, 5 minutes), and the absorbance was measured at 570 nm. PBS and 0.1% (v / v) Triton X-100 were used as negative (labeled "-") and positive (labeled "+") controls, respectively. Hemolysis was observed, and the relative hemolysis rate was quantitatively calculated. The results are shown below. Figure 2-5 As shown.

[0031] like Figure 2 As shown, Figure 2 Image A shows the hemolysis experiment of chitosan hydrogel. The solution color of each concentration experimental group is highly consistent with the negative control, and there is no obvious hemolysis phenomenon. Figure 2 E represents the relative hemolysis rate of chitosan hydrogel. It can be seen that the relative hemolysis rate of chitosan is <2% in the full concentration range of 12.5~500μg / mL. As the main matrix material of hydrogel coating, the hemolysis rate of chitosan is <2% in the full concentration range, which proves that it has no erythrocyte toxicity and will not damage the cell membrane structure.

[0032] like Figure 3 As shown, Figure 3 Image B shows the hemolysis experiment of chitosan and chlorogenic acid composite hydrogel. The solution color of each concentration experimental group is highly consistent with the negative control, and there is no obvious hemolysis phenomenon. Figure 3 F represents the relative hemolysis rate of the chitosan and chlorogenic acid composite hydrogel. It can be seen that within the full concentration range of 12.5~500μg / mL, the relative hemolysis rate of the chitosan and chlorogenic acid composite hydrogel is <1%, proving that it has no erythrocyte toxicity, does not damage the cell membrane structure, and fully meets the safety requirements of fruit and vegetable preservation coatings.

[0033] like Figure 4 As shown, Figure 4Image C shows the hemolysis experiment of zinc oxide nanoparticles and chitosan composite hydrogel. The solution color of each concentration experimental group is highly consistent with the negative control, and there is no obvious hemolysis phenomenon. Figure 4 G represents the relative hemolysis rate of the zinc oxide nanoparticles and chitosan composite hydrogel. The ZnO nanoparticles of this invention are the key antibacterial component of the hydrogel composite coating. Their low hemolysis rate directly proves that the composite coating will not damage the epidermal cells of fruits and vegetables in the application of fruit and vegetable preservation, nor will it release toxic substances during food contact, thus providing a safety basis for the practical application of the coating.

[0034] like Figure 5 As shown, Figure 5 Image D shows the hemolysis test image of the final hydrogel composite coating. The solution color of each concentration experimental group is highly consistent with the negative control, and there is no obvious hemolysis phenomenon. Figure 5 H represents the relative hemolysis rate of the hydrogel composite coating. The hemolysis rate of the hydrogel composite coating is <2% across the entire concentration range, proving that the three components (chitosan + ZnO + chlorogenic acid) do not produce synergistic toxicity after being combined, maintaining the low toxicity characteristics of each raw material. This provides crucial safety support for the practical application of fruit and vegetable preservation. Even at the highest test concentration (500 μg / mL), the hemolysis induced by the hydrogel coating is negligible, indicating that the hydrogel coating has excellent biocompatibility.

[0035] Secondly, the present invention provides an application of the hydrogel composite coating prepared by the above method in the preservation of fruits and vegetables, comprising the following steps: (1) Wash fruits and vegetables with sterile water or disinfect them with sodium hypochlorite at a volume ratio of 2%; (2) Place the washed or disinfected fruits and vegetables in a well-ventilated room temperature for 1-2 hours to air dry naturally and remove surface moisture; (3) Immerse the dried fruits and vegetables in the hydrogel composite coating for 1-5 minutes to ensure that the surface is completely covered by the adhesive; take them out and let them air dry at room temperature for 10-20 minutes to form a uniform coating on the surface of the fruits and vegetables, and store them at 25±2℃.

[0036] In this embodiment, the fruits and vegetables include tomatoes, citrus fruits, apples, and radishes, with citrus fruits and tomatoes being the preferred varieties. Example

[0037] Application of hydrogel composite coating in tomato preservation.

[0038] I. Experimental Materials Fruit and vegetable samples: Select fresh tomatoes that are uniform in size, consistent in maturity, and free from mechanical damage and pests; Coating materials: The single chitosan (CTS) hydrogel, the chlorogenic acid and chitosan mixed hydrogel (CTS-CA), the zinc oxide and chitosan mixed hydrogel (CTS-ZnO) prepared by this invention, and the final chitosan-chlorogenic acid-zinc oxide composite hydrogel coating (CTS-ZnO&CA). Control sample: Fresh tomatoes without any coating treatment.

[0039] II. Experimental Procedure 1. Sample pretreatment: Rinse the tomatoes with sterile water and let them air dry naturally at room temperature and in a ventilated place for 1-2 hours to remove free surface moisture; 2. Coating treatment: The dried tomatoes were treated separately using an immersion method: Control group: No coating treatment; CTS group: Immersed in single chitosan (CTS) hydrogel for 2 min; ZnO&CA group: Immersed in a chlorogenic acid and chitosan mixed hydrogel (CTS-CA) for 2 min; CTS-ZnO&CA group: Immersed in the chitosan-chlorogenic acid-zinc oxide composite hydrogel coating (CTS-ZnO&CA) prepared in this invention for 2 min; 3. Film formation and storage: Remove the tomatoes and allow them to air dry at room temperature for 15 minutes to allow the coating to form a film naturally. Then, place all samples in a constant temperature and humidity chamber at 25±2℃ and 60%-70% relative humidity for 14 days. 4. Performance Evaluation: Appearance observation: Record the changes in the appearance of tomatoes daily (degree of wrinkling, surface integrity, and color); Weight loss rate determination: Weigh the tomatoes regularly and calculate the weight loss rate according to the formula: Weight loss rate (%) = (W0 - W) t ) / W0×100%, where W0 is the initial weight of the tomato, W t The weight after storage for t days; Antioxidant activity evaluation: The free radical scavenging rate of each coating at different concentrations (25, 50, 100, 200, 500 μg / mL) was determined by DPPH free radical scavenging experiment.

[0040] III. Experimental Results 1. Appearance preservation effect like Figure 6As shown in Figure B, the tomatoes in the control group showed obvious wrinkling and wilting of the skin on the 4th day of storage, and the surface integrity was severely damaged on the 10th day; the deterioration of tomatoes in the CTS group and ZnO&CA group was delayed until the 7th-8th day; while the hydrogel composite coating (CTS-ZnO&CA) group prepared in this invention maintained the best appearance throughout the 14-day storage period, with the least degree of wrinkling, excellent surface integrity and no obvious color deterioration.

[0041] 2. Quantitative analysis of weight loss rate like Figure 6 As shown in Figure C, the weight loss rate of tomatoes in the control group was over 23% after 14 days of storage; the weight loss rate of the CTS group was 20% after 14 days; the weight loss rate of the ZnO&CA group decreased to 15%; and the weight loss rate of the CTS-ZnO&CA group decreased to 12%, which was significantly lower than that of other groups. This demonstrates that the hydrogel composite coating can effectively regulate the water migration and respiration intensity of fruits and vegetables, reduce water loss, and thus prolong the freshness of the fruit.

[0042] 3. Evaluation of antioxidant activity like Figure 6 As shown in Figure D, all concentrations of the hydrogel composite coating (CTS-ZnO&CA) exhibited concentration-dependent DPPH free radical scavenging activity, which approached saturation with increasing concentration. The enhanced antioxidant capacity is attributed to the synergistic effect of the electron-donating functional groups of chitosan, the phenolic hydroxyl reducing ability of chlorogenic acid, and the catalytic surface properties of ZnO nanoparticles, which can efficiently neutralize free radicals and delay the oxidative aging of fruits and vegetables.

[0043] In summary, the hydrogel composite coating prepared by this invention can significantly improve the postharvest quality of tomatoes, reduce weight loss, provide strong antioxidant protection, and effectively extend the shelf life of fruits and vegetables. It has important practical application potential in postharvest preservation and active food packaging systems. Example

[0044] Application of hydrogel composite coating in citrus preservation.

[0045] I. Experimental Materials 1. Fruit and vegetable samples: Select fresh citrus fruits that are uniform in size, consistent in maturity, and free from mechanical damage and pests; 2. Coating materials: The single chitosan (CTS) hydrogel, the chlorogenic acid and chitosan mixed hydrogel (CTS-CA), the zinc oxide and chitosan mixed hydrogel (CTS-ZnO) prepared by this invention, and the final chitosan-chlorogenic acid-zinc oxide composite hydrogel coating (CTS-ZnO&CA). 3. Bacterial strain and reagents: Pseudomonas aeruginosa (concentration 1×10⁻⁶) 6 CFU / mL), 2% (v / v) sodium hypochlorite solution, LB medium; 3. Control sample: Infected citrus fruits that have not undergone any coating treatment.

[0046] II. Experimental Procedure 1. Sample pretreatment: The citrus fruits were disinfected by soaking in 2% (v / v) sodium hypochlorite solution for 2 min, rinsed with sterile water and then placed on a sterile operating table to air dry; a uniform 3×5 mm wound was made in the equatorial region of the citrus fruit with a sterile scalpel, and each fruit was inoculated with 50 μL of Pseudomonas aeruginosa suspension and allowed to stand at room temperature for 30 min to allow the bacterial suspension to be fully adsorbed.

[0047] 2. Coating treatment: The inoculated citrus fruits were divided into 4 groups, and each group was treated with an immersion method: Control group: No coating treatment; CTS group: Immersed in single chitosan (CTS) hydrogel for 2 min; ZnO&CA group: Immersed in a chlorogenic acid and chitosan mixed hydrogel (CTS-CA) for 2 min; CTS-ZnO&CA group: Immersed in the chitosan-chlorogenic acid-zinc oxide composite hydrogel coating (CTS-ZnO&CA) prepared in this invention for 2 min; 3. Incubation and Monitoring: Remove the citrus fruits and allow them to air dry at room temperature for 15 minutes to allow the coating to form a film. Then, place all samples in a constant temperature and humidity chamber at 37℃ and 80% relative humidity for 15 days. Record the following indicators on days 1, 3, 5, 7, 9, 11, 13, and 15: Appearance and lesion area: Observe the symptoms of citrus rot and use ImageJ software to quantify the lesion area; Bacterial biomass detection: An equal amount of tissue (0.5 g) was aseptically excised from the lesion, homogenized, and suspended in 5 mL LB medium. After incubation at 37°C for 2 h, the optical density (OD) at 600 nm was measured. 600 ), calculate the relative bacterial biomass.

[0048] III. Experimental Results 1. Appearance and corrosion resistance: such as Figure 7 As shown in Figure B, the control group citrus showed obvious rot lesions on the 3rd day of incubation, and by the 10th day the lesions had expanded to large areas of softening of the peel and tissue deterioration; the rot symptoms of the CTS group appeared late to the 5th day, but the lesions still expanded later; the ZnO&CA group further inhibited the progress of rot, but moderate lesions were still visible on the 12th day; while the CTS-ZnO&CA group only showed small lesions during the entire 15-day incubation period, the peel morphology remained relatively intact, and there was no obvious tissue softening or deterioration, demonstrating excellent in vivo preservation performance.

[0049] 2. Quantitative analysis of lesion area: As shown in Figure 7C, the lesion area in the control group continued to increase over time, reaching its maximum value (approximately 45 mm²) on day 15; the lesion area in the CTS group decreased to 32 mm², and in the ZnO&CA group it decreased to 22 mm²; while the lesion area in the CTS-ZnO&CA group remained the smallest, reaching only 8 mm² on day 15, significantly inhibiting bacterial invasion and tissue degradation. The similar lesion area trends observed in the citrus and apple models indicate that the protective effect of the composite coating is not fruit-specific, but is widely applicable to different fruit substrates.

[0050] 3. Results of bacterial biomass quantification: such as Figure 7 As shown in Figure D, the OD of the control group 600 The highest value (approximately 0.9) corresponds to the highest bacterial load; CTS group OD 600 The value decreased to 0.35, and the ZnO&CA group decreased to 0.12; while the OD of the CTS-ZnO&CA group decreased to 0.12. 600 With a value of only 0.03 and an antibacterial rate of 96.7%, it almost completely inhibited the proliferation of Pseudomonas aeruginosa. Its synergistic antibacterial mechanism stems from the combined effects of the physical barrier and antibacterial properties of chitosan, the strong bactericidal activity of ZnO nanoparticles, and the ability of chlorogenic acid to disrupt bacterial metabolism.

[0051] In summary, the hydrogel composite coating prepared in this invention can significantly inhibit the proliferation of postharvest pathogens and the spread of lesions in citrus fruits. The hydrogel composite coating exhibits synergistic antiseptic and antibacterial effects in vivo. Its enhanced performance can be attributed to the combined effects of the physical barrier and antibacterial properties of chitosan, the strong bactericidal activity of ZnO nanoparticles, and the ability of citric acid to disrupt bacterial metabolism and create an unfavorable microenvironment. This synergistic mechanism effectively limits bacterial proliferation, inhibits lesion spread, and significantly extends the shelf life of fruits. These findings highlight the great potential of hydrogel composite antibacterial and preservative coatings as a multifunctional and efficient strategy in postharvest preservation and antibacterial food packaging applications.

[0052] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. The invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the following claims.

[0053] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for preparing a hydrogel composite coating, characterized in that, Includes the following steps: S1: Preparation of zinc oxide nanoparticles: Using zinc(II) di(2,4-pentanedione) as the zinc source and polyethylene glycol aqueous solution with a molecular weight of 2000-10000 as the morphology modifier, the reaction mixture was prepared by mixing zinc(II) di(2,4-pentanedione) as zinc (II) and polyethylene glycol aqueous solution at a mass-to-volume ratio of 1:(10-15). The mixture was magnetically stirred for 30 min until completely dissolved, followed by heating under reflux, cooling and centrifugation purification to obtain zinc oxide nanoparticles. S2: Preparation of chlorogenic acid extract powder: Using honeysuckle as raw material, chlorogenic acid extract powder is obtained by soaking in pure water, enzymatic hydrolysis with cellulase, hot water extraction, centrifugation and rotary evaporation; the enzymatic hydrolysis conditions are: enzymatic hydrolysis temperature 50~60℃, cellulase dosage 0.1% of the dry weight of honeysuckle, and enzymatic hydrolysis time 1~2h. S3: Preparation of hydrogel composite coating: 30 mg of chitosan was dissolved in 0.1 M acetic acid aqueous solution. β-glycerophosphate sodium aqueous solution with a mass-volume ratio of 50% was added dropwise to the chitosan solution at a volume ratio of 9:

1. At the same time, 100 μg of zinc oxide nanoparticles obtained in step S1 and chlorogenic acid extract powder obtained in step S2 containing 100 μg of chlorogenic acid were added. The mixture was incubated in a constant temperature shaker for 48 h to obtain the hydrogel composite coating.

2. The preparation method according to claim 1, characterized in that, In step S1, the molecular weight of the polyethylene glycol aqueous solution is 6000, and the heating reflux, cooling, and centrifugal purification include: The reaction mixture was heated to 100°C, then kept at that temperature for 30 minutes to remove free water from the system, and then heated to 180-200°C and kept there for 2 hours. Then raise the temperature to 250-260℃ at a rate of 10℃ / min and reflux for 1 hour; After removing the heat source, the product was cooled to 80°C and dispersed in a 1:1 volume ratio hexane / ethanol mixture. It was then purified by centrifugation at 8000-10000 r / min for 15 min-1 h.

3. The preparation method according to claim 1, characterized in that, In step S1, the obtained zinc oxide nanoparticles are particles obtained by passing through a 100-mesh sieve.

4. The preparation method according to claim 1, characterized in that, In step S2, honeysuckle is used as raw material, and chlorogenic acid extract powder is obtained by soaking in pure water, enzymatic hydrolysis with cellulase, hot water extraction, centrifugation, and rotary evaporation. Specifically, the process includes: Honeysuckle was dried in a 50°C oven and cooled to room temperature. It was then pulverized using a multi-functional pulverizer to obtain honeysuckle powder. The honeysuckle powder was added to pure water at a ratio of 1:30 g / mL and soaked for 1 hour. Cellulase at 0.1% of the dry weight of honeysuckle was added and enzymatically hydrolyzed at 50-60℃ for 1-2 hours. After enzymatic hydrolysis, honeysuckle was extracted using hot water extraction, with the extraction temperature and extraction time set at 80-90 ℃ and 1.5-2.0 h, respectively. Centrifuge the extract at 4,000 r / min for 15 min, collect the supernatant, and dry it by rotary evaporation.

5. The preparation method according to claim 4, characterized in that, In step S2, the chlorogenic acid extract powder contains 80%-90% chlorogenic acid, the honeysuckle powder is granules obtained by passing through a 40-mesh sieve, and the cellulase activity is 100,000 U / g.

6. The preparation method according to claim 1, characterized in that, In step S3, 30 mg of chitosan is dissolved in a 0.1 M acetic acid aqueous solution, specifically including: 30 mg of chitosan was gradually added to 3 mL of acetic acid aqueous solution and continuously vortexed to obtain a homogeneous suspension; the suspension was then transferred to a 5 mL centrifuge tube, sealed, and allowed to stand overnight at 4 °C to ensure complete dissolution of the chitosan.

7. The preparation method according to claim 1, characterized in that, In step S3, the sodium β-glycerophosphate aqueous solution is added dropwise, and after adding zinc oxide nanoparticles and chlorogenic acid extract powder, it is gently vortexed to ensure uniform dispersion.

8. The preparation method according to claim 1, characterized in that, In step S3, The incubation conditions for the constant temperature shaker are: temperature 37℃ and rotation speed 100r / min.

9. The application of a hydrogel composite coating prepared by the method according to any one of claims 1-8 in the preservation of fruits and vegetables, characterized in that, Includes the following steps: (1) Wash fruits and vegetables with sterile water or disinfect them with sodium hypochlorite at a volume ratio of 2%; (2) Place the washed or disinfected fruits and vegetables in a well-ventilated room temperature for 1-2 hours to air dry naturally and remove surface moisture; (3) Immerse the dried fruits and vegetables in the hydrogel composite coating for 1-5 minutes to ensure that the surface is completely covered by the adhesive; take them out and let them air dry at room temperature for 10-20 minutes to form a uniform coating on the surface of the fruits and vegetables, and store them at 25±2℃.

10. A method for preserving fruits and vegetables, characterized in that, Hydrogel composite coatings obtained by any one of the preparation methods described in claims 1-8.

Images