A hydrogel coating for fruit preservation and its preparation method and application

Abstract

This invention discloses a hydrogel coating for fruit preservation, its preparation method, and its application. The invention introduces active aldehyde groups into chitosan to obtain oxidized chitosan, grafts phenylboronic acid compounds onto the oxidized chitosan molecular chain, introduces quercetin by forming borate ester bonds with the ortho-hydroxyl groups of quercetin using borate groups, and then uses quercetin to reduce sodium selenite, forming a stable and dispersed composite nanosystem. Finally, this composite system is compounded with a polyvinyl alcohol solution, utilizing the reversible borate ester bonds between the phenylboronic acid groups and the ortho-diol structures in the polyvinyl alcohol molecular chain to achieve rapid cross-linking, forming a three-dimensional hydrogel network structure in situ on the fruit surface to obtain the coating. The resulting coating exhibits excellent film-forming properties, adhesion, and structural stability, effectively reducing the rate of water evaporation and gas exchange on the fruit surface. Furthermore, through the synergistic effect of quercetin and nano-selenium, it inhibits oxidation reactions and microbial growth, thereby significantly delaying the aging and spoilage process of fruit.

Description

Technical Field

[0001] This invention belongs to the field of fruit preservation, specifically relating to a hydrogel coating for fruit preservation, its preparation method, and its application. Background Technology

[0002] Even after harvesting, fruits remain living organisms, continuing respiration, transpiration, and enzymatic reactions. This makes them highly susceptible to moisture loss, tissue softening, nutrient degradation, and microbial contamination, ultimately leading to spoilage. These changes significantly reduce the commercial value of fruits and cause substantial losses during storage and transportation. Therefore, developing efficient, safe, and sustainable fruit preservation technologies is of significant practical importance.

[0003] Currently, commonly used fruit preservation methods mainly include low-temperature refrigeration, controlled atmosphere storage, chemical preservative treatment, and edible coatings. While low-temperature storage effectively slows down respiratory metabolism, it relies heavily on cold chain transportation and is costly. Controlled atmosphere technology achieves preservation by adjusting gas composition, but the equipment is complex and its application is limited. Although chemical preservatives have some antibacterial effect, they pose potential safety hazards and residue problems, and are gradually being strictly restricted. Against this backdrop, edible coatings, due to their advantages of being environmentally friendly, safe, and non-toxic, have gradually become a research hotspot.

[0004] Traditional edible coating materials are mostly based on chitosan, polysaccharides, proteins, or lipids. They form a semi-permeable protective film on the fruit surface to regulate gas exchange and water migration, thereby delaying fruit senescence. However, existing coating materials generally suffer from poor mechanical properties, insufficient adhesion, and limited functionality, making it difficult to maintain stable effects in complex storage and transportation environments over long periods. Furthermore, these coatings typically rely on added active substances for antibacterial and antioxidant properties, and their release efficiency and stability still need improvement.

[0005] In recent years, hydrogels have shown promising applications in food preservation due to their unique three-dimensional network structure and excellent water retention properties. Hydrogels can adsorb and lock in large amounts of water while forming selective mass transfer barriers, thus creating a stable microenvironment on the surface of fruits and effectively slowing down water evaporation and gas exchange. Furthermore, hydrogels can also serve as carriers for active substances, enabling the sustained release of functional components such as antioxidants and antibacterial agents, thereby improving the durability of preservation effects.

[0006] In existing research, modifying chitosan and introducing functional groups to enhance its film-forming properties and active substance loading capacity has become an important direction for improving hydrogel performance. For example, oxidative modification of chitosan followed by grafting functional monomers can significantly improve its reactivity and structural tunability. Simultaneously, synergistically introducing natural antioxidants and inorganic elements into nanosystems can enhance their antioxidant and antibacterial properties to a certain extent, and improve their dispersibility in aqueous systems.

[0007] Furthermore, combining modified chitosan systems with polymers such as polyvinyl alcohol allows for the in-situ formation of hydrogel coatings on fruit surfaces by leveraging their rapid cross-linking properties. These methods typically achieve cross-linking and film formation quickly through simple spraying or dipping operations, offering advantages such as ease of processing and broad applicability. However, existing technologies still have some limitations, such as the sensitivity of the hydrogel formation process to conditions, the need to improve film uniformity, the unclear synergistic mechanism of functional components, and insufficient compatibility with different fruit surfaces.

[0008] Meanwhile, current research on hydrogel coatings largely focuses on optimizing single properties, lacking a systematic study of the relationship between material structure design, preparation methods, and preservation effects. Especially in practical applications, different fruits exhibit significant differences in skin structure, respiration rate, and storage environment, placing higher demands on the performance of coating materials. Therefore, developing a hydrogel coating system with tunable structure, simple preparation process, and multifunctional synergistic effects is of great significance for improving fruit preservation efficiency.

[0009] In summary, existing fruit preservation technologies still have certain limitations in terms of safety, functionality, and ease of application. Although hydrogel coatings show great promise in the field of preservation, their preparation methods and application performance still need further optimization or improvement. Therefore, it is necessary to provide a method for preparing hydrogel coatings for fruit preservation and its application to overcome the shortcomings of existing technologies and achieve efficient preservation of fruit quality. Summary of the Invention

[0010] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a method for preparing a hydrogel coating for fruit preservation. This invention modifies the structure of chitosan and introduces multifunctional active components to construct a composite hydrogel system with a dynamic cross-linking structure and antioxidant and antibacterial functions, thereby effectively improving the preservation effect of fruits.

[0011] This invention uses safe and non-toxic natural high-molecular-weight chitosan as raw material. Active aldehyde groups are introduced under the action of sodium periodate to obtain oxidized chitosan. Then, phenylboronic acid compounds are grafted onto the oxidized chitosan molecular chain to obtain boron-functionalized chitosan derivatives, enabling the material to form reversible borate ester bonds with polyhydroxy structures, providing a reaction basis for subsequent construction of a dynamic cross-linked network. Subsequently, the borate group of the boron-functionalized chitosan derivative forms a borate ester bond with the ortho-hydroxyl group of quercetin, which has hydrophobic properties, to solubilize quercetin and thus introduce quercetin into the material. Simultaneously, quercetin reduces sodium selenite to generate selenium nanoparticles in situ, forming a stable and dispersed composite nanosystem. The nano-selenium effectively inhibits aggregation under the coating and stabilizing effect of the chitosan derivative, and synergistically exerts antioxidant and antibacterial functions with quercetin. Finally, the above composite system is compounded with a polyvinyl alcohol solution. By utilizing the reversible borate ester bond between the phenylboronic acid groups and the vicinal diol structure in the polyvinyl alcohol molecular chain, rapid cross-linking is achieved, forming a three-dimensional hydrogel network structure in situ on the fruit surface, thereby obtaining a uniform and dense preservation coating.

[0012] Compared with existing technologies, this invention constructs a composite system of "chitosan-modified framework - dynamic borate ester crosslinking network - synergistic nano-functional components," resulting in a hydrogel coating that possesses excellent film-forming properties, adhesion, and structural stability. Simultaneously, this coating effectively reduces the rate of water evaporation and gas exchange on the fruit surface, and through the synergistic effect of quercetin and nano-selenium, inhibits oxidation reactions and microbial growth, thereby significantly delaying the aging and spoilage process of fruit.

[0013] Therefore, the hydrogel coating preparation method provided by the present invention is simple in process, rapid in film formation, and has diverse functions, and has good application prospects in the field of fruit preservation.

[0014] Another object of the present invention is to provide a hydrogel coating for fruit preservation prepared by the above preparation method.

[0015] Another object of the present invention is to provide the application of the above-mentioned hydrogel coating for fruit preservation in fruit preservation.

[0016] To achieve this objective, the present invention adopts the following technical solution:

[0017] In a first aspect, the present invention provides a method for preparing a hydrogel coating for fruit preservation, comprising the following steps:

[0018] (1) Add sodium periodate solution to chitosan acetic acid aqueous solution, react at room temperature in the dark, terminate the reaction, dialyze, dry to obtain oxidized chitosan (OCS).

[0019] (2) Dissolve the oxidized chitosan from step (1) in water, add phenylboronic acid compounds to react, then add sodium borohydride solution and carry out a reduction reaction under light-protected conditions, dialyze, and dry to obtain chitosan-aminophenylboronic acid graft.

[0020] (3) Dissolve the chitosan-aminophenylboronic acid graft from step (2) in water, add quercetin (Que) to obtain a mixed solution, add sodium selenite to the mixed solution, and react at room temperature in the dark to obtain a mixed solution of chitosan-aminophenylboronic acid graft / quercetin nano-selenium (OCS-PBA / Que / Se NPs);

[0021] (4) The chitosan-aminophenylboronic acid graft / quercetin nano-selenium (OCS-PBA / Que / SeNPs) mixed solution from step (3) is mixed with polyvinyl alcohol (PVA) solution and crosslinked to obtain a hydrogel coating for fruit preservation.

[0022] Preferably, in the chitosan-acetic acid aqueous solution in step (1), the ratio of chitosan to water is (4-8) g: 150 mL; and the volume ratio of acetic acid to water is (2-4): 150.

[0023] Preferably, in step (1), the mass ratio of chitosan to sodium periodate is (4-8):(6-12).

[0024] Preferably, the concentration of the sodium periodate solution in step (1) is 0.12 to 0.24 g / mL.

[0025] Preferably, the time for the room temperature light-protected reaction in step (1) is 6 to 12 hours.

[0026] Preferably, the termination reaction in step (1) is: adding ethylene glycol and continuing the reaction for 1-2 hours to terminate the reaction.

[0027] More preferably, the stirring speed for continuing the reaction after adding ethylene glycol is 800-1000 rpm.

[0028] More preferably, the mass ratio of chitosan to ethylene glycol is (4-8):(2-3).

[0029] Preferably, the dialysate used in step (1) is water, the dialysate duration is 3 to 4 days, and the water needs to be changed 2 to 3 times a day.

[0030] Preferably, the drying in step (1) is freeze drying; more preferably, it is freeze drying at -90±10℃ for 24 to 36 hours.

[0031] Preferably, the mass concentration of the oxidized chitosan in step (2) after dissolving in water is 2-4%.

[0032] Preferably, the phenylboronic acid compound in step (2) includes 3-aminophenylboronic acid (PBA).

[0033] Preferably, the mass ratio of oxidized chitosan to phenylboronic acid compounds in step (2) is (1-2) g: (400-800) mg.

[0034] Preferably, the mass ratio of the phenylboronic acid compound and sodium borohydride in step (2) is (40-80):(25-50).

[0035] Preferably, the reaction in step (2) involving the addition of phenylboronic acid compounds is carried out at room temperature for 15 ± 5 minutes.

[0036] Preferably, the concentration of the sodium borohydride solution in step (2) is 12.5 to 25 mg / mL, and the solvent is a mixture of ethanol and water with a volume ratio of 3 ± 0.5:1.

[0037] Preferably, the reduction reaction in step (2) is carried out at room temperature for 12 to 24 hours.

[0038] Preferably, the stirring speed of the reduction reaction in step (2) is 600-800 rpm.

[0039] Preferably, the dialysate used in step (2) is water, the dialysate duration is 3 to 4 days, and the water needs to be changed 2 to 3 times a day.

[0040] Preferably, the drying in step (2) is freeze drying; more preferably, it is freeze drying at -90±10℃ for 24 to 36 hours.

[0041] Preferably, the ratio of chitosan-aminophenylboronic acid graft material to water in step (3) is (0.04~0.08) g: 10 mL.

[0042] Preferably, the mass ratio of chitosan-aminophenylboronic acid graft to quercetin in step (3) is (40-80):(8-16).

[0043] Preferably, the mass ratio of quercetin to sodium selenite in step (3) is (8-16):(2.4-4.8).

[0044] Preferably, the room temperature light-protected reaction in step (3) lasts for 12 to 24 hours.

[0045] Preferably, the stirring speed of the room temperature light-protected reaction in step (3) is 600-800 rpm.

[0046] Preferably, the concentration of the polyvinyl alcohol (PVA) solution in step (4) is 0.03 to 0.05 g / mL.

[0047] Preferably, the volume ratio of the chitosan-aminophenylboronic acid graft / quercetin nano-selenium (OCS-PBA / Que / SeNPs) mixed solution and the polyvinyl alcohol (PVA) solution in step (4) is (1-2):(1-2).

[0048] Preferably, the crosslinking time in step (4) is 10 ± 5 s.

[0049] Secondly, the present invention provides a hydrogel coating for fruit preservation prepared by the above preparation method.

[0050] Thirdly, the present invention provides the application of the above-mentioned hydrogel coating for fruit preservation in fruit preservation.

[0051] Preferably, the specific method of application is as follows: a mixed solution of chitosan-aminophenylboronic acid graft / quercetin nano-selenium (OCS-PBA / Que / Se NPs) and a polyvinyl alcohol (PVA) solution are sprayed or soaked together on the surface of the fruit, and after cross-linking, a hydrogel coating is obtained that coats the surface of the fruit.

[0052] This invention constructs a polymeric framework structure with dynamic and reversible cross-linking capabilities by oxidatively modifying chitosan and introducing phenylboronic acid functional groups. Further, quercetin and nano-selenium functional components are introduced to form a composite hydrogel system integrating structural regulation and functional synergy. This system utilizes the reversible borate ester bond between the phenylboronic acid groups and the vicinal diol structure in the polyvinyl alcohol molecular chain to achieve rapid in-situ cross-linking and film formation of the hydrogel on the fruit surface, significantly improving film formation efficiency and simplifying the operation process. Compared to traditional chitosan coatings or single-functional coating materials, the hydrogel network structure formed by this invention is more stable and has a certain degree of self-adaptation, making it less prone to cracking or detachment during storage and transportation, and able to continuously provide protection. Simultaneously, this coating not only effectively reduces the rate of water evaporation and gas exchange on the fruit surface, inhibiting respiration, but also significantly enhances antioxidant and antibacterial properties through the synergistic effect of quercetin and nano-selenium, thus exhibiting a superior comprehensive preservation effect in delaying fruit senescence and reducing spoilage. Furthermore, by modifying chitosan to encapsulate and stabilize functional components, its dispersibility and utilization efficiency can be effectively improved, and the duration of activity can be extended. Overall, this invention has the advantages of simple preparation process, rapid film formation, stable structure, safety and environmental protection, and diverse functions. It overcomes the problems of limited preservation effect, insufficient stability, and limited application in existing technologies, and has good practical application value and promotion prospects.

[0053] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0054] (1) Faster film formation speed and simpler process

[0055] Compared to existing chitosan coatings or traditional hydrogel systems, which typically require longer film-forming times or more complex processing conditions, this invention utilizes the reversible borate ester bond between phenylboronic acid groups and polyvinyl alcohol to achieve rapid cross-linking. This allows for the in-situ formation of a hydrogel coating on the fruit surface in a short time, significantly improving film-forming efficiency, simplifying the operation process, and making it more suitable for practical production applications.

[0056] (2) The stability and adaptability of the coating are significantly improved.

[0057] Existing coating materials generally suffer from problems such as easy cracking and poor adhesion. However, this invention constructs a dynamic and reversible cross-linking network, enabling the hydrogel coating to maintain structural integrity and good adhesion under changes in the external environment or mechanical disturbances, which is significantly better than the traditional static cross-linking coating system.

[0058] (3) The preservation effect is significantly better than that of single-function coatings.

[0059] Traditional coatings mainly rely on physical barriers, resulting in relatively limited functionality. The hydrogel coating constructed in this invention not only effectively reduces the rate of water evaporation and gas exchange but also synergistically regulates fruit respiration, thereby exhibiting superior overall preservation effects in delaying water loss, inhibiting softening, and slowing down aging.

[0060] (4) Significantly enhanced antioxidant and antibacterial properties

[0061] Compared to systems that only add a single antioxidant or antibacterial agent, this invention introduces quercetin and nano-selenium to construct a synergistic system, which can significantly improve free radical scavenging ability and antibacterial effect, thereby more effectively delaying oxidative damage and inhibiting microbial growth. The overall performance is superior to existing conventional preservation materials.

[0062] (5) The functional components have higher stability and utilization efficiency.

[0063] In existing technologies, active substances are prone to aggregation or deactivation, affecting their effectiveness. This invention utilizes modified chitosan to coat and stabilize nano-selenium and quercetin, ensuring their uniform dispersion and maintaining activity within the system, thereby significantly improving the stability and sustained action of the functional components.

[0064] (6) It is safer and in line with the development direction of green preservation.

[0065] Compared with traditional chemical preservatives, the chitosan and quercetin used in this invention have good biocompatibility and biodegradability. At the same time, the nano-selenium exists in a stable form in the system, reducing potential toxicity risks and making the overall safety higher, which is more in line with the current trend of green food preservation.

[0066] (7) It has wider applicability and good application prospects.

[0067] The system of this invention can achieve flexible performance adjustment by controlling the proportion of each component and the cross-linking conditions, making it suitable for the preservation needs of different kinds of fruits. It is also water-soluble and washable, and has a wider range of applications and promotional value compared with existing single coating systems that are more targeted. Attached Figure Description

[0068] Figure 1 The infrared spectrum of OCS-PBA obtained in Example 5 is shown.

[0069] Figure 2 The image shows the actual dissolution of quercetin obtained in Example 5 in OCS-PBA solution and deionized water.

[0070] Figure 3 This is a TEM image of the selenium nanoparticles in the OCS-PBA / Que / Se NPs solution obtained in Example 5.

[0071] Figure 4 The results of fruit preservation in Example 6.

[0072] Figure 5 The image shows the gelation time of Example 5, which illustrates that the OCS-PBA / Que / Se NPs solution and the PVA solution rapidly crosslink within 10 seconds to form a hydrogel, indicating a fast film formation rate.

[0073] Figure 6 The figures shown are of the amplitude and actual application of the hydrogel preservation coating in Example 5. They demonstrate that the obtained hydrogel has good stability and diverse application scenarios, and that the hydrogel can adaptively form a film.

[0074] Figure 7 The graphs for the antibacterial and antioxidant properties of the hydrogel preservation coating in Example 5 show the excellent antibacterial and antioxidant properties of the obtained hydrogel.

[0075] Figure 8 The figure for the OCS-PBA / Que / Se NPs composite solution in Example 5 shows that there was no significant change in the solution from day 1 to day 10, indicating that the hydrogel components were stable. Detailed Implementation

[0076] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the implementation of the present invention is not limited thereto.

[0077] Unless otherwise specified in the embodiments of this invention, the conditions shall be performed according to conventional conditions or conditions recommended by the manufacturer. All raw materials and reagents used, unless otherwise specified, are commercially available conventional products.

[0078] Example 1

[0079] (1) Preparation of OCS: 4 g of chitosan was dispersed in 150 mL of deionized water, followed by the addition of 2 mL of acetic acid. After stirring until the chitosan was completely dissolved, sodium periodate solution (6 g dissolved in 50 mL of water) was slowly added dropwise. The reaction was carried out at room temperature in the dark for 6 h, and then 2.66 g of ethylene glycol was added to terminate the reaction. The mixture was stirred for another hour. Subsequently, the reaction mixture was dialyzed with purified water for 3 days, with the water changed twice a day. The mixture was then freeze-dried at -90℃ for 24 h to obtain OCS.

[0080] (2) Preparation of OCS-PBA: Dissolve 1g of OCS obtained in step (1) in 50ml of purified water, add 400mg of PBA and react for 15 minutes, then add sodium borohydride ethanol solution (250mg sodium borohydride + 15ml ethanol + 5ml water), and stir for 12 hours in the dark. Finally, purify the reaction solution by dialysis with pure water for 3 days, changing the water twice a day, and then freeze-dry at -90℃ for 24 hours to finally obtain OCS-PBA.

[0081] (3) Preparation of OCS-PBA / Que / Se NPs: 0.04 g of OCS-PBA obtained in step (2) was directly dissolved in 10 mL of deionized water to obtain an OCS-PBA solution (0.4% w / v). 8 mg of hydrophobic quercetin (Que) was added under stirring and magnetic stirring was used to dissolve it to obtain an OCS-PBA / Que solution. Subsequently, 2.4 mg of sodium selenite was added to the above solution and stirred at room temperature in the dark for 12 h to obtain an OCS-PBA / Que / Se NPs solution.

[0082] (4) Preparation of hydrogel preservation coating: 0.5g PVA was dispersed in 10 mL of deionized water and heated to 90℃ to dissolve. Then, the PVA solution was cooled to room temperature to obtain a PVA solution. An equal volume of PVA solution and the OCS-PBA / Que / Se NPs solution obtained in step (3) were sprayed simultaneously or immersed in the surface of the fruit. The hydrogel preservation coating could be cross-linked within 10 seconds.

[0083] Example 2

[0084] (1) Preparation of OCS: 8 g of chitosan was dispersed in 150 mL of deionized water, followed by the addition of 4 mL of acetic acid. The mixture was stirred until the chitosan was completely dissolved, and then sodium periodate solution (12 g dissolved in 50 mL of water) was slowly added dropwise. The mixture was reacted at room temperature in the dark for 12 h, and then 2.66 g of ethylene glycol was added to terminate the reaction. The mixture was stirred for another hour. Subsequently, the reaction mixture was dialyzed with purified water for 3 days, with the water changed twice a day. The mixture was then freeze-dried at -90℃ for 24 h to obtain OCS.

[0085] (2) Preparation of OCS-PBA: Dissolve 2g of OCS obtained in step (1) in 50ml of purified water, add 800mg of PBA and react for 15 minutes, then add sodium borohydride ethanol solution (500mg sodium borohydride + 15ml ethanol + 5ml water), and stir in the dark for 24 hours. Finally, purify the reaction solution by dialysis with pure water for 3 days, changing the water twice a day, and then freeze-dry at -90℃ for 24 hours to finally obtain OCS-PBA.

[0086] (3) Preparation of OCS-PBA / Que / Se NPs: 0.08 g of OCS-PBA obtained in step (2) was directly dissolved in 10 mL of deionized water to obtain an OCS-PBA solution (0.8% w / v). 16 mg of hydrophobic quercetin (Que) was added under stirring and magnetic stirring was used to dissolve it to obtain an OCS-PBA / Que solution. Subsequently, 4.8 mg of sodium selenite was added to the above solution and stirred at room temperature in the dark for 24 h to obtain an OCS-PBA / Que / Se NPs solution.

[0087] (4) Preparation of hydrogel preservation coating: 0.5g PVA was dispersed in 10 mL of deionized water and heated to 90℃ to dissolve. Then, the PVA solution was cooled to room temperature to obtain a PVA solution. An equal volume of PVA solution and the OCS-PBA / Que / Se NPs solution obtained in step (3) were sprayed simultaneously or immersed in the surface of the fruit. The hydrogel preservation coating could be cross-linked within 10 seconds.

[0088] Example 3

[0089] (1) Preparation of OCS: 4 g of chitosan was dispersed in 150 mL of deionized water, followed by the addition of 4 mL of acetic acid. After stirring until the chitosan was completely dissolved, sodium periodate solution (6 g dissolved in 50 mL of water) was slowly added dropwise. The reaction was carried out at room temperature in the dark for 12 h, and then 2.66 g of ethylene glycol was added to terminate the reaction. The mixture was stirred for another hour. Subsequently, the reaction mixture was dialyzed with purified water for 3 days, with the water changed twice a day. The mixture was then freeze-dried at -90℃ for 24 h to obtain OCS.

[0090] (2) Preparation of OCS-PBA: Dissolve 1g of OCS obtained in step (1) in 50ml of purified water, add 800mg of PBA and react for 15 minutes, then add sodium borohydride ethanol solution (250mg sodium borohydride + 15ml ethanol + 5ml water), and stir in the dark for 24 hours. Finally, purify the reaction solution by dialysis with pure water for 3 days, changing the water twice a day, and then freeze-dry at -90℃ for 24 hours to finally obtain OCS-PBA.

[0091] (3) Preparation of OCS-PBA / Que / Se NPs: 0.04 g of OCS-PBA obtained in step (2) was directly dissolved in 10 mL of deionized water to obtain an OCS-PBA solution (0.4% w / v). 16 mg of hydrophobic quercetin (Que) was added under stirring and magnetic stirring was used to dissolve it to obtain an OCS-PBA / Que solution. Subsequently, 2.4 mg of sodium selenite was added to the above solution and stirred at room temperature in the dark for 24 h to obtain an OCS-PBA / Que / Se NPs solution.

[0092] (4) Preparation of hydrogel preservation coating: 0.3g PVA was dispersed in 10 mL of deionized water and heated to 90℃ to dissolve. Then, the PVA solution was cooled to room temperature to obtain a PVA solution. An equal volume of PVA solution and the OCS-PBA / Que / Se NPs solution obtained in step (3) were sprayed simultaneously or immersed in the fruit surface before and after. The hydrogel preservation coating could be cross-linked within 10 seconds.

[0093] Example 4

[0094] (1) Preparation of OCS: 8g of chitosan was dispersed in 150 mL of deionized water, followed by the addition of 2 mL of acetic acid. After stirring until the chitosan was completely dissolved, sodium periodate solution (12g dissolved in 50 mL of water) was slowly added dropwise. The reaction was carried out at room temperature in the dark for 6 hours, and then 2.66g of ethylene glycol was added to terminate the reaction. The mixture was stirred for another hour. Subsequently, the reaction mixture was dialyzed with purified water for 3 days, with the water changed twice a day. The mixture was then freeze-dried at -90℃ for 24 hours to obtain OCS.

[0095] (2) Preparation of OCS-PBA: Dissolve 2g of OCS obtained in step (1) in 50ml of purified water, add 400mg of PBA and react for 15 minutes, then add sodium borohydride ethanol solution (500mg sodium borohydride + 15ml ethanol + 5ml water), and stir for 12 hours in the dark. Finally, purify the reaction solution by dialysis with pure water for 3 days, changing the water twice a day, and then freeze-dry at -90℃ for 24 hours to finally obtain OCS-PBA.

[0096] (3) Preparation of OCS-PBA / Que / Se NPs: 0.08 g of OCS-PBA obtained in step (2) was directly dissolved in 10 mL of deionized water to obtain an OCS-PBA solution (0.8% w / v). 8 mg of hydrophobic quercetin (Que) was added under stirring and magnetic stirring was used to dissolve it to obtain an OCS-PBA / Que solution. Subsequently, 4.8 mg of sodium selenite was added to the above solution and stirred at room temperature in the dark for 12 h to obtain an OCS-PBA / Que / Se NPs solution.

[0097] (4) Preparation of hydrogel preservation coating: 0.5g PVA was dispersed in 10 mL of deionized water and heated to 90℃ to dissolve. Then, the PVA solution was cooled to room temperature to obtain a PVA solution. An equal volume of PVA solution and the OCS-PBA / Que / Se NPs solution obtained in step (3) were sprayed simultaneously or immersed in the surface of the fruit. The hydrogel preservation coating could be cross-linked within 10 seconds.

[0098] Example 5

[0099] (1) Preparation of OCS: 6 g of chitosan was dispersed in 150 mL of deionized water, followed by the addition of 3 mL of acetic acid. After stirring until the chitosan was completely dissolved, sodium periodate solution (8 g dissolved in 50 mL of water) was slowly added dropwise. The reaction was carried out at room temperature in the dark for 8 h, and then 2.66 g of ethylene glycol was added to terminate the reaction. The mixture was stirred for another hour. Subsequently, the reaction mixture was dialyzed with purified water for 3 days, with the water changed twice a day. The mixture was then freeze-dried at -90℃ for 24 h to obtain OCS.

[0100] (2) Preparation of OCS-PBA: Dissolve 1.5g of OCS obtained in step (1) in 50ml of purified water, add 600mg of PBA and react for 15 minutes, then add sodium borohydride ethanol solution (375mg sodium borohydride + 15ml ethanol + 5ml water), and stir for 16 hours in the dark. Finally, purify the reaction solution by dialysis with pure water for 3 days, changing the water twice a day, and then freeze-dry at -90℃ for 24 hours to finally obtain OCS-PBA.

[0101] (3) Preparation of OCS-PBA / Que / Se NPs: 0.06 g of OCS-PBA obtained in step (2) was directly dissolved in 10 mL of deionized water to obtain an OCS-PBA solution (0.6% w / v). 12 mg of hydrophobic quercetin (Que) was added under stirring and magnetic stirring was used to dissolve it to obtain an OCS-PBA / Que solution. Subsequently, 3.6 mg of sodium selenite was added to the above solution and incubated at room temperature for 16 h to obtain an OCS-PBA / Que / Se NPs solution.

[0102] (4) Preparation of hydrogel preservation coating: 0.4g PVA was dispersed in 10 mL of deionized water and heated to 90℃ to dissolve. Then, the PVA solution was cooled to room temperature to obtain a PVA solution. An equal volume of PVA solution and the OCS-PBA / Que / Se NPs solution obtained in step (3) were sprayed simultaneously or immersed in the surface of the fruit. The hydrogel preservation coating could be cross-linked within 10 seconds.

[0103] Example 6: Experimental study on the application of hydrogel coating in fruit preservation

[0104] Fresh bananas were washed with deionized water and divided into two groups (experimental group and control group). The fruit in the experimental group was sprayed with a hydrogel coating (Example 5), while the control group received no treatment. Both groups of fruit were stored at room temperature (25°C). The fruit was placed on plastic trays with adequate spacing between each fruit to ensure air circulation and reduce crush damage. The appearance and weight of the fruit were recorded at predetermined time intervals to monitor changes over time. In addition, weight loss was measured and photographs were taken to record morphological changes. At the beginning and end of the experiment, the hardness of the fruit was measured using a cylindrical flat-bottomed fruit hardness tester. The tester was inserted into three different areas of each fruit to a depth of 10 mm. Three fruits were measured each time.

[0105] Figure 4 As shown in Figure A, the appearance quality of bananas in the control group significantly declined over time during storage. Brown spots gradually appeared on the peel starting from day 2, intensified after day 4, and resulted in widespread browning accompanied by significant softening from day 6 to day 8. In contrast, bananas in the experimental group, under the same storage conditions, maintained a relatively uniform yellow color overall, with only slight spots appearing in the later stages. The degree of browning was significantly lower than that of the control group, indicating that this treatment can effectively delay the ripening and senescence process of bananas.

[0106] like Figure 4 As shown in Figure B, the weight loss rate of bananas in both groups increased with prolonged storage time, but the increase in the control group was significantly higher than that in the experimental group. By day 8, the weight loss rate of the control group reached a relatively high level, while the weight loss rate of the experimental group was significantly lower, indicating that the experimental treatment could effectively inhibit moisture loss and slow down quality deterioration.

[0107] like Figure 4 As shown in Figure C, the firmness of bananas decreased continuously during storage, but the trends of change between the two groups were significantly different. The firmness of the control group decreased more rapidly, and was significantly lower than that of the experimental group in the later stages; while the experimental group maintained a higher firmness level throughout the storage period, indicating that this treatment can delay the softening process of the fruit and maintain a better tissue structure.

[0108] In summary, this hydrogel preservation coating can significantly delay the browning of bananas, reduce weight loss, and maintain high firmness, thereby effectively extending the storage period of bananas and maintaining their quality.

[0109] Example 5: The antibacterial performance of the hydrogel preservation coating was tested as follows:

[0110] 1. Preparation of liquid and solid culture media

[0111] Liquid culture medium: Add 20g LB broth to 1L ultrapure water and stir well;

[0112] Solid culture medium: Add 20g LB broth to 1L ultrapure water, stir well, then add 15g agar and stir well.

[0113] Both liquid and solid culture media were autoclaved at 120°C for 3 hours, then inverted and placed in a clean bench for later use.

[0114] 2. Shaking

[0115] Take 10 μl of Staphylococcus aureus stock solution and Escherichia coli stock solution respectively and add them to 10 ml of the liquid culture medium obtained from step 1 after high pressure. Incubate at 37°C and 120 rpm for 12 h on a shaker.

[0116] 3. Measure bacterial OD value

[0117] Take 4 ml of the liquid culture medium obtained from step 1 (autoclaved), and set the absorbance to 0 at 600 nm using a UV spectrophotometer. Take the Staphylococcus aureus and Escherichia coli bacterial suspensions prepared in step 2, and add them to the liquid culture medium obtained from step 1 (autoclaved) at a ratio of bacterial suspension to liquid culture medium of 1:1. Measure the absorbance at 600 nm. Continue this process, maintaining the bacterial suspension: liquid culture medium ratio of 1:1, until the absorbance reaches 1.

[0118] 4. Dilution

[0119] Take 1 μl of the bacterial solution with an absorbance of 1 in step 3 and add it to 100 ml of the liquid culture medium after high pressure in step 1. Mix well and set aside.

[0120] 5. Co-cultivation

[0121] Take 2 mg of lyophilized gel (Example 5), add 15 ml of diluted bacterial solution with absorbance of 1 (Step 4), and incubate at 37°C and 120 rpm for 12 h on a shaker.

[0122] 6. Dilute again and coat the plate.

[0123] Take 100 μl of the co-cultured bacterial solution (step 5) and add it to 900 μl of the liquid culture medium after high autoclaving in step 1. Vortex mix well, dilute seven times in a gradient, and take 30 μl to plate, that is, plate it on the solid culture medium after high autoclaving in step 1.

[0124] 7. Spread the plating and dry it, then invert it and incubate it at 37℃ for 12 hours. Observe and record the results.

[0125] 8. Calculation of antibacterial rate

[0126] The antibacterial effect of the gel was evaluated using the plate count method, and the antibacterial rate was calculated according to the formula:

[0127]

[0128] In the formula, w eThe colony count for the control group is calculated as follows: step 5 is performed without adding the lyophilized gel, and all other procedures are the same as for the experimental group. o This represents the number of colonies in the experimental group.

[0129] In addition, the blank group experiment is the same as steps 1 to 8 above, except that the experimental group gel is replaced with the blank group gel, and the blank group gel is not loaded with nano-selenium and quercetin. All other aspects are the same.

[0130] Example 5: The antioxidant properties of the hydrogel preservation coating were tested as follows:

[0131] 1. After cutting and grinding the lyophilized gel (Example 5) into powder, take 5mg and add it to 5ml of ultrapure water for later use.

[0132] 2. Prepare 20 ml of 80% (v / v) methanol solution, add 5 ml of the prepared methanol solution to the gel liquid in step 1, mix well and sonicate for 30 min to obtain the sample.

[0133] 3. Turn on the microplate reader and preheat for 30 minutes. Set the corresponding blank and control groups (blank group: 150 μl methanol + 150 μl DPPH; control group: 150 μl sample from step 2 + 150 μl methanol; assay group: 150 μl sample from step 2 + 150 μl DPPH).

[0134] 4. Number the 96-well plates as blank group, control group (each sample was measured once), and assay group (each sample was measured three times).

[0135] 5. Number the 0.5 mL centrifuge tubes to match the numbers on the 96-well plate. Take the sample solution after sonication (150 μl methanol + 150 μl DPPH for the blank group; 150 μl sample from step 2 + 150 μl methanol for the control group; 150 μl sample from step 2 + 150 μl DPPH for the assay group) and add it to the centrifuge tube. React at room temperature in the dark for 30 min.

[0136] 6. Take 200 μl of the solution from step 5 into each 96-well plate and measure the absorbance at 517 nm. Calculate the scavenging rate based on the degree of reduction in DPPH radical absorbance by the sample using the following formula:

[0137]

[0138] In the formula, A 测 To measure the absorbance of the group, A 对 A represents the absorbance of the control group. 空 The absorbance of the blank group is shown.

[0139] in addition, Figure 7The test process for the DPPH removal rate of the blank coating group is the same as steps 1 to 6 above, except that the gel of the test group is replaced with the gel of the blank group, and the gel of the blank group is not loaded with nano selenium and quercetin. All other aspects are the same.

[0140] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a hydrogel coating for fruit preservation, characterized in that, Includes the following steps: (1) Add sodium periodate solution to chitosan acetic acid aqueous solution, react at room temperature in the dark, terminate the reaction, dialyze, dry, and obtain oxidized chitosan; (2) Dissolve the oxidized chitosan from step (1) in water, add phenylboronic acid compounds to react, then add sodium borohydride solution and carry out a reduction reaction under light-protected conditions, dialyze, and dry to obtain chitosan-aminophenylboronic acid graft. (3) Dissolve the chitosan-aminophenylboronic acid graft from step (2) in water, add quercetin to obtain a mixed solution, add sodium selenite to the mixed solution, and react at room temperature in the dark to obtain a chitosan-aminophenylboronic acid graft / quercetin nano-selenium mixed solution. (4) The chitosan-aminophenylboronic acid graft / quercetin nano-selenium mixed solution from step (3) is mixed with a polyvinyl alcohol solution and crosslinked to obtain a hydrogel coating for fruit preservation.

2. The preparation method according to claim 1, characterized in that, The phenylboronic acid compounds mentioned in step (2) include 3-aminophenylboronic acid; And / or, the mass ratio of oxidized chitosan to phenylboronic acid compounds in step (2) is (1-2) g: (400-800) mg; And / or, the mass ratio of the phenylboronic acid compound and sodium borohydride in step (2) is (40-80):(25-50).

3. The preparation method according to claim 1, characterized in that, In step (3), the mass ratio of chitosan-aminophenylboronic acid graft to quercetin is (40-80):(8-16). And / or, the mass ratio of quercetin to sodium selenite in step (3) is (8-16):(2.4-4.8).

4. The preparation method according to claim 1, 2, or 3, characterized in that, The concentration of the polyvinyl alcohol solution in step (4) is 0.03–0.05 g / mL; And / or, in step (4), the volume ratio of the chitosan-aminophenylboronic acid graft / quercetin nanoselenium mixed solution and the polyvinyl alcohol solution is (1-2):(1-2).

5. The preparation method according to claim 1, 2, or 3, characterized in that, In step (1), the mass ratio of chitosan to sodium periodate is (4-8):(6-12). And / or, the time for the room temperature light-protected reaction in step (1) is 6 to 12 hours.

6. The preparation method according to claim 1, 2, or 3, characterized in that, The mass concentration of the oxidized chitosan in step (2) after dissolving in water is 2-4%; And / or, the reaction of adding phenylboronic acid compounds in step (2) is carried out at room temperature for 15 ± 5 minutes; And / or, the reduction reaction in step (2) is carried out at room temperature for 12 to 24 hours; And / or, the concentration of the sodium borohydride solution in step (2) is 12.5 to 25 mg / mL, and the solvent is a mixture of ethanol and water with a volume ratio of 3 ± 0.5:

1.

7. The preparation method according to claim 1, 2, or 3, characterized in that, In step (3), the ratio of chitosan-aminophenylboronic acid graft to water is (0.04-0.08) g: 10 mL; And / or, the room temperature light-protected reaction described in step (3) for 12–24 h; And / or, the crosslinking time in step (4) is 10 ± 5 s.

8. The preparation method according to claim 1, 2, or 3, characterized in that, In step (1), the ratio of chitosan to water in the chitosan-acetic acid aqueous solution is (4-8) g: 150 mL; the volume ratio of acetic acid to water is (2-4):

150. And / or, the concentration of the sodium periodate solution in step (1) is 0.12–0.24 g / mL; And / or, the termination reaction in step (1) is: adding ethylene glycol and continuing the reaction for 1-2 hours to terminate the reaction; the mass ratio of chitosan to ethylene glycol is (4-8):(2-3).

9. A hydrogel coating for fruit preservation prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the hydrogel coating for fruit preservation as described in claim 9 in fruit preservation.

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