Fruit and vegetable fresh-keeping agent based on modified chitosan and preparation process of fruit and vegetable fresh-keeping agent

Abstract

The invention belongs to the technical field of fruit and vegetable fresh-keeping, particularly relates to a fruit and vegetable fresh-keeping agent based on modified chitosan and a preparation process thereof, and aims at solving the problems that a traditional chitosan fresh-keeping agent depends on an acid environment in dissolution, is narrow in antibacterial spectrum, single in function, poor in mechanical property and the like, chitosan is subjected to three-step modification, phosphorylation, quaternization and methacrylic acid grafting, and the fruit and vegetable fresh-keeping agent based on modified chitosan is obtained. According to the composite fresh-keeping agent disclosed by the invention, the three-dimensional protection of bacteriostasis, respiration control, pesticide residue reduction and moisture retention is realized at normal temperature by combining the composite fresh-keeping agent with the nano-composite photocatalytic material Cu-Zn-TiO2, the limitation of single function of the traditional fresh-keeping agent is broken through, and a safe and efficient composite fresh-keeping system is constructed.

Description

Technical Field

[0001] The present invention relates to the technical field of fruit and vegetable preservation, and in particular to a fruit and vegetable preservative based on modified chitosan and a preparation process thereof. Background Art

[0002] The core reasons for high losses of fruits and vegetables caused by improper post-harvest handling are: first, microbial infection is the main cause. About 70% of post-harvest rot is caused by fungal diseases. Common pathogens include Botrytis, which causes gray mold, Geotrichum, which causes sour rot, and Penicillium, which produces penicillium. They reproduce rapidly through mechanical damage or changes in environmental temperature and humidity. Secondly, the intensification of physiological metabolism after harvest leads to quality deterioration. Respiration causes the weight loss rate of fruits and vegetables to reach 5%-10%, and ethylene release accelerates ripening and softening. Furthermore, pesticide residues create a double dilemma. The rate of excessive pesticide residues in fruits and vegetables in my country is about 8.1%, which not only exacerbates post-harvest losses but also poses food safety risks.

[0003] Current preservation technologies face significant limitations: Cold chain coverage is inadequate, nearly 80% of fruits and vegetables are still stored and transported at ambient temperatures, and existing preservatives suffer from functional deficiencies. Most commercially available preservatives utilize a single mechanism of action: approximately 65% are antioxidants, while less than 15% combine antibacterial, respiratory, and moisturizing properties. Furthermore, those capable of pesticide degradation are even scarcer. Furthermore, 38% of preservatives contain organic polymers. While these meet the GB2760-2021 standard, they exhibit a dose-toxicity inflection point, posing a risk of hepatotoxicity and renal toxicity with excessive use. Inspections conducted by the State Administration for Market Regulation from 2019 to 2022 revealed an average annual increase of 12.7% in cases of excessive preservative residues in fruits and vegetables, necessitating the development of new and safer preservation systems.

[0004] Chinese patent CN103598322A provides a chitosan fruit and vegetable preservative and its preparation method and use, comprising the following components in the following weight percentage ranges: chitosan 0.5-3%, citric acid 1-5%, nano zinc oxide 0.01-0.05%, nano silicon dioxide 0.01-0.05%, glycerol 0.1-0.5%, Tween 80 0.5-2%, sodium chloride 0.1-1%, water 88.4-97.78%. However, the mechanical properties of the chitosan film, such as tensile strength and water barrier, are poor, and it is easy to crack or dissolve due to humidity changes. It is only effective against some bacteria, and the inhibition rate of the main pathogenic fungi of fruits and vegetables (such as Penicillium and Botrytis) is insufficient. Moreover, the water vapor barrier rate of the unmodified chitosan film is low, making it difficult to adapt to the high humidity environment of tropical fruits. Therefore, it is necessary to break through the traditional formula design and build a multiple protection mechanism through material composite and nano-modification to develop a new safe preservative. Summary of the Invention

[0005] In response to the problems of existing fruit and vegetable preservatives such as single function, safety risks and low room-temperature preservation efficiency, the present invention proposes a triple functional synergistic system of "metal ion antibacterial-photocatalytic degradation of pesticide residues-film formation and respiration regulation". Through the triple synergistic mechanism of copper and zinc ion antibacterial, titanium dioxide photocatalytic degradation of pesticide residues, and modified chitosan film formation and respiration regulation, it breaks through the single-function limitation of traditional preservatives and avoids the toxicity risk of organic polymer compounds, providing an efficient and green solution for the storage and transportation of fruits and vegetables.

[0006] In order to solve the above technical problems, the present invention provides a technical solution: a fruit and vegetable preservative based on modified chitosan, comprising the following components in the following weight percentage ranges: modified chitosan 5% to 8%, zinc nitrate 1.5% to 2.5%, copper nitrate 0.5% to 1.2%, nano-titanium dioxide 1% to 2%, glycerol 1% to 1.5%, tea polyphenols 0.2% to 0.3%, and distilled water as the balance.

[0007] Since chitosan has inherent defects such as dependence on acidic environment for dissolution, resulting in acidification of fruit and vegetable skin, narrow antibacterial spectrum, poor film-forming mechanical properties, and inability to effectively combine with photocatalytic materials, the present invention performs a dual modification treatment of carboxymethylation and quaternization on chitosan.

[0008] The preparation method of modified chitosan comprises the following steps:

[0009] (1) Phosphorylation: Disperse chitosan with a deacetylation degree of ≥90% in 85% phosphoric acid solution, add phosphorus pentoxide, and react at 75-85°C under a nitrogen flow of 0.5 L / min for 4-6 hours with stirring. Cool the reaction solution, dialyze it to a neutral pH, and freeze-dry to obtain phosphorylated chitosan.

[0010] (2) Quaternization: Phosphorylated chitosan was dissolved in deionized water to a concentration of 5% to 10%, and then glycidyl trimethylammonium chloride was added to adjust the pH to 8.5. The reaction was carried out at 60-70°C for 3-4 hours. After ethanol precipitation, centrifugation and washing were performed, and vacuum drying was performed at 50-60°C to obtain phosphoric acid-quaternary ammonium double-modified chitosan.

[0011] (3) Methacrylic acid grafting: Phosphate-quaternized chitosan and glycidyl methacrylate were mixed in a mass ratio of 1:0.5, potassium persulfate and ascorbic acid were added as initiators, and then continuously introduced with a nitrogen flow of 0.5-0.8 L / min at 70-80 °C for 2-4 h. After dialysis, the modified chitosan was spray-dried to obtain the result.

[0012] Furthermore, in step (1), the mass ratio of chitosan to phosphoric acid solution is 1:10, and the molar ratio of chitosan to phosphorus pentoxide is 1:1.5.

[0013] Furthermore, in step (2), the molar ratio of phosphorylated chitosan to glycidyl trimethylammonium chloride is 1:0.8.

[0014] Furthermore, in step (3), the mass of potassium persulfate is 0.6% of the total mass of phosphate-quaternized chitosan and glycidyl methacrylate, and the mass of ascorbic acid is 0.2% of the total mass.

[0015] The preparation process of the modified chitosan of the present invention strictly follows the three-step sequence of phosphorylation → quaternization → methacrylic acid grafting, which is determined by the reaction mechanism and functional synergy and cannot be changed. The phosphorylation step introduces a strong polar phosphate group (-PO43-) into the chitosan molecular chain, so that it is transformed from insoluble in neutral to alkaline environments to broad-spectrum water-soluble, which is a necessary prerequisite for subsequent aqueous phase reactions; if this step is skipped and quaternization is directly performed, the chitosan will have insufficient solubility in deionized water, resulting in serious inhomogeneity in the reaction system. Quaternization must be carried out after phosphorylation, because the negative charge of the phosphate group can effectively neutralize the quaternary ammonium group (-N + The positive charge accumulation after the introduction of (CH3)3) maintains the extended conformation of the molecular chain to ensure that the functional groups are fully exposed; if the order is reversed, the quaternized product will undergo irreversible hydrolysis and degradation in a strong acid phosphating environment. Methacrylic acid grafting is irreplaceable as the final step. If the hydrophobic cross-linked network it forms is constructed in advance, it will hinder the diffusion of the reagent (experiments have confirmed that the wrong order can reduce the quaternization efficiency by 40%), and this step requires the precise use of the remaining hydroxyl sites after quaternization to complete the cross-linking structure assembly. This gradient process lays the molecular foundation for triple functional synergy through the progressive design of solubility modification, directional anchoring of functional groups and cross-linking reinforcement.

[0016] The improved performance of modified chitosan is due to the synergistic effect of the phosphate group, quaternary ammonium group and hydrophilic ester group in the molecular chain. The phosphate group completely solves the defect of traditional chitosan that relies on acidic solvents, making it soluble in neutral environment and avoiding the risk of acidification of fruit and vegetable skin; the quaternary ammonium cation (-N + (CH3)3) works through a dual mechanism: firstly, it binds tightly to negatively charged sites on the surface of fruits and vegetables (such as pectin carboxyl groups) through electrostatic interaction, significantly enhancing the adhesion of the preservative film; secondly, it directly acts on the phospholipid bilayer of microbial cell membranes, disrupting their structural integrity and achieving broad-spectrum antimicrobial activity against fungi such as Penicillium and Botrytis. The grafted structure of glycidyl methacrylate creates a dense cross-linked network between molecular chains, significantly enhancing the film's tensile strength and moisture barrier properties through hydrogen bonding and van der Waals forces, effectively preventing swelling and rupture in high-humidity environments.

[0017] A method for preparing a fruit and vegetable preservative based on modified chitosan, comprising the following steps:

[0018] S1: Dissolve zinc nitrate and copper nitrate in water in proportion, add nano-TiO2, the amount of water can be 5 to 10 times the total mass of zinc nitrate, copper nitrate and nano-TiO2, and disperse under 300W ultrasonic dispersion until the nano-TiO2 is uniformly dispersed in the solution. Under continuous stirring, slowly add 20% NaOH solution by mass until the pH is 9 to 10, carry out hydrothermal reaction at 80 to 90°C for 2 to 3 hours, and calcine at 400 to 500°C to obtain Cu-Zn-TiO2 composite powder;

[0019] S2: Dissolve the modified chitosan in water to form a 5wt% to 8wt% solution, and stir at 50 to 60°C until completely dissolved. Mix glycerol and water evenly, and dissolve tea polyphenols in an appropriate amount of ethanol. After both are completely dissolved, add the Cu-Zn-TiO2 composite powder, the mixed glycerol solution, and the tea polyphenol ethanol solution to the modified chitosan solution in sequence, stir at 400 to 500 rpm for 1 to 2 hours, and after the components are fully mixed, spray dry to obtain a solid powder which is the preservative.

[0020] Furthermore, the mass ratio of glycerol to water is 1:1, and the mass ratio of ethanol to tea polyphenols is 10:1.

[0021] During the methacrylic acid grafting step, the abundant hydroxyl (-OH) and amino (-NH2) groups on the phosphate-quaternized chitosan chains react with the epoxy groups of glycidyl methacrylate (GMA) molecules under the initiator potassium persulfate / ascorbic acid and heating conditions to undergo a ring-opening reaction. The GMA molecules are grafted onto the chitosan backbone via covalent bonds (ether and ester bonds), while their terminal double bonds (C=C) polymerize within the reaction system. This process forms crosslinks between adjacent chitosan chains: on the one hand, a single GMA molecule acts as a "bridge" connecting two or more chitosan chains; on the other hand, the double bonds of multiple GMA molecules aggregate to form a network of polymethacrylate branches. These chemical crosslinks (covalent bonds) work synergistically with pre-existing hydrogen bonds between the molecular chains (provided by phosphate groups, quaternary ammonium groups, hydroxyl groups, etc.) and electrostatic interactions (negative charges on phosphate groups and positive charges on quaternary ammonium groups) to weave a highly interconnected, three-dimensional crosslinked structure throughout the system. When this preservative is mixed with water and applied to the surface of fruits and vegetables to form a film and dry, the network structure is fixed, thus giving the film excellent mechanical strength, density and stability. The three-dimensional network film constructed by modified chitosan constitutes a multifunctional synergistic micro-platform: First, the Cu-Zn-TiO2 composite powder uniformly distributed in the film exerts antibacterial and pesticide residue degradation functions through the synergistic effect of metal ions and semiconductors. Copper zinc ions (Cu 2+ 、Zn 2+) enters microbial cells through ion exchange, combines with DNA base pairs to inhibit replication, and at the same time, nano-TiO2 generates photogenerated electron-hole pairs under natural light or weak light in the storage environment. The holes oxidize water molecules to generate hydroxyl radicals (·OH), and the electrons reduce oxygen to generate superoxide anions (·O2 - ), two types of free radicals synergistically attack the CC and CO bonds of microbial cell walls and pesticide molecules, achieving broad-spectrum antibacterial and pesticide residue degradation; secondly, the dense structure of the film itself can regulate the respiration of fruits and vegetables, inhibiting their aerobic respiration by hindering oxygen diffusion, delaying ethylene synthesis and the activity of cell wall-degrading enzymes (such as pectinases), and reducing the weight loss rate; thirdly, glycerol molecules form hydrogen bonds with chitosan molecular chains through hydroxyl groups, filling the pores of the membrane structure, maintaining flexibility and reducing water loss; the phenolic hydroxyl groups of tea polyphenols are embedded in the chitosan molecular chains through π-π stacking, and their antioxidant properties complement the photocatalytic oxidation of TiO2, scavenging free radicals generated by fruit and vegetable metabolism and delaying cell membrane lipid peroxidation. In addition, the modified chitosan as a matrix can combine with the hydroxyl groups on the surface of nano-titanium dioxide through intermolecular hydrogen bonds to form a stable interface anchoring structure, solving the problem that traditional chitosan is difficult to load photocatalytic materials and providing a uniformly dispersed carrier for Cu-Zn-TiO2 composite powder. The multiple microscopic effects of "matrix film formation - ion antibacterial - photocatalytic degradation - antioxidant synergy" enable the preservative to achieve three-dimensional protection of antibacterial, respiration control, pesticide residue reduction and moisture retention at room temperature, breaking through the single function limitations of traditional preservatives and building a safe and efficient composite preservation system.

[0022] The beneficial effects of the present invention are:

[0023] The present invention forms a fruit and vegetable preservation solution with significant advantages through the directional modification of chitosan and the coordinated design of multiple components: first, a triple functional synergistic system of "metal ion antibacterial-photocatalytic degradation of pesticide residues-film formation and respiration regulation" is constructed. Copper and zinc ions inhibit fungal growth by destroying microbial cell membranes and DNA, nano-titanium dioxide produces free radicals under light to degrade pesticide molecules, and the dense film formed by carboxymethyl chitosan blocks oxygen and delays the respiratory metabolism of fruits and vegetables. The combination of the three breaks through the limitation of the single action mechanism of traditional preservatives and realizes multi-dimensional protection of antibacterial, residue reduction and consumption control; secondly, the modified chitosan solves the problem that traditional chitosan relies on acid dissolution through phosphorylation, quaternization and grafting modification. , narrow antibacterial spectrum, and poor film-forming performance. The positively charged groups in its molecular structure form electrostatic adsorption with the surface of fruits and vegetables, which improves the adhesion of the film. The cross-linked network structure improves the tensile strength and enhances the water barrier. It adapts to the normal temperature and high humidity environment, controls the weight loss rate of fruits and vegetables, and extends the shelf life. Furthermore, the formula uses natural and degradable chitosan derivatives and inorganic nanomaterials to avoid the risk of dose toxicity. At the same time, the introduction of tea polyphenols synergistically enhances the antioxidant capacity, forming a safe and controllable green preservation system, so that the preservative exhibits excellent antibacterial activity, pesticide residue degradation rate and film-forming uniformity during storage and transportation at room temperature, effectively responding to the industry pain points of high post-harvest losses of fruits and vegetables and high pesticide residue risks in my country, and has both technological innovation and practical application value. DETAILED DESCRIPTION

[0024] The present invention will be further described below with reference to the following examples, but the present invention is not limited thereto. The experimental methods in the following examples are all conventional methods unless otherwise specified.

[0025] Example 1

[0026] A fruit and vegetable preservative based on modified chitosan comprises the following components in the following weight percentage ranges: 5% modified chitosan, 1.5% zinc nitrate, 0.5% copper nitrate, 1% nano titanium dioxide, 1% glycerol, 0.2% tea polyphenols, and the balance distilled water.

[0027] The preparation method of modified chitosan comprises the following steps:

[0028] (1) Phosphorylation: Chitosan with a deacetylation degree of ≥90% was dispersed in 85% phosphoric acid solution at a chitosan to phosphoric acid solution mass ratio of 1:10. Phosphorus pentoxide was added at a chitosan to phosphorus pentoxide molar ratio of 1:1.5. The mixture was stirred at 75°C under a nitrogen flow of 0.5 L / min for 4 h. The reaction solution was cooled, dialyzed to a neutral pH, and freeze-dried to obtain phosphorylated chitosan.

[0029] (2) Quaternization: Phosphorylated chitosan was dissolved in deionized water to prepare a 5% solution, and then glycidyl trimethyl ammonium chloride was added. The molar ratio of phosphorylated chitosan to glycidyl trimethyl ammonium chloride was 1:0.8. The pH was adjusted to 8.5, and the reaction was carried out at 60°C for 3 h. After ethanol precipitation, centrifugation and washing, the solution was dried in vacuo at 50°C to obtain phosphoric acid-quaternary ammonium double-modified chitosan.

[0030] (3) Methacrylic acid grafting: Phosphate-quaternized chitosan and glycidyl methacrylate were mixed in a mass ratio of 1:0.5, and 0.6% of potassium persulfate and 0.2% of ascorbic acid were added as initiators. Then, a nitrogen flow of 0.5 L / min was continuously introduced at 70°C for 2 h. After dialysis, the mixture was spray-dried to obtain the modified chitosan.

[0031] A method for preparing a fruit and vegetable preservative based on modified chitosan, comprising the following steps:

[0032] S1: Dissolve zinc nitrate and copper nitrate in water in proportion, add nano-TiO2, the amount of water can be 5 times the total mass of zinc nitrate, copper nitrate and nano-TiO2, and disperse under 300W ultrasonic dispersion until nano-TiO2 is uniformly dispersed in the solution. Under continuous stirring, slowly add 20% by mass fraction of NaOH solution until the pH reaches 9. After hydrothermal reaction at 80℃ for 2h, calcinate at 400℃ to obtain Cu-Zn-TiO2 composite powder;

[0033] S2: Dissolve the modified chitosan in water to form a 5wt% solution, and stir at 50°C until completely dissolved. Mix glycerol and water in a mass ratio of 1:1, and dissolve tea polyphenols in an appropriate amount of ethanol. The mass ratio of ethanol to tea polyphenols is 10:1. After both are completely dissolved, add the Cu-Zn-TiO2 composite powder, the mixed glycerol solution, and the tea polyphenol ethanol solution to the modified chitosan solution in turn. Stir at 400rpm for 1h to fully mix the components. The solid powder obtained by spray drying is the preservative.

[0034] Example 2

[0035] A fruit and vegetable preservative based on modified chitosan comprises the following components in the following weight percentage ranges: 8% modified chitosan, 2.5% zinc nitrate, 1.2% copper nitrate, 2% nano titanium dioxide, 1.5% glycerol, 0.3% tea polyphenols, and the balance distilled water.

[0036] The preparation method of modified chitosan comprises the following steps:

[0037] (1) Phosphorylation: Chitosan with a deacetylation degree of ≥90% was dispersed in 85% phosphoric acid solution at a chitosan to phosphoric acid solution mass ratio of 1:10. Phosphorus pentoxide was added at a chitosan to phosphorus pentoxide molar ratio of 1:1.5. The mixture was stirred at 85°C under a nitrogen flow of 0.5 L / min for 6 h. The reaction solution was cooled, dialyzed to a neutral pH, and freeze-dried to obtain phosphorylated chitosan.

[0038] (2) Quaternization: Phosphorylated chitosan was dissolved in deionized water to prepare a 10% solution, and then glycidyl trimethyl ammonium chloride was added. The molar ratio of phosphorylated chitosan to glycidyl trimethyl ammonium chloride was 1:0.8. The pH was adjusted to 8.5, and the reaction was carried out at 70°C for 4 h. After ethanol precipitation, centrifugation and washing, the solution was dried in vacuo at 60°C to obtain phosphoric acid-quaternary ammonium double-modified chitosan.

[0039] (3) Methacrylic acid grafting: Phosphate-quaternized chitosan and glycidyl methacrylate were mixed in a mass ratio of 1:0.5, and 0.6% of potassium persulfate and 0.2% of ascorbic acid were added as initiators. Then, a nitrogen flow of 0.8 L / min was continuously introduced at 80°C for 4 h. After dialysis, the mixture was spray-dried to obtain the modified chitosan.

[0040] A method for preparing a fruit and vegetable preservative based on modified chitosan, comprising the following steps:

[0041] S1: Dissolve zinc nitrate and copper nitrate in water in proportion, add nano-TiO2, the amount of water can be 10 times the total mass of zinc nitrate, copper nitrate and nano-TiO2, and disperse under 300W ultrasonic dispersion until nano-TiO2 is uniformly dispersed in the solution. Under continuous stirring, slowly add 20% NaOH solution by mass until the pH is 9-10. After hydrothermal reaction at 90℃ for 3h, calcinate at 500℃ to obtain Cu-Zn-TiO2 composite powder;

[0042] S2: Dissolve the modified chitosan in water to form an 8wt% solution, and stir at 60°C until completely dissolved. Mix glycerol and water in a mass ratio of 1:1, and dissolve tea polyphenols in an appropriate amount of ethanol. The mass ratio of ethanol to tea polyphenols is 10:1. After both are completely dissolved, add the Cu-Zn-TiO2 composite powder, the mixed glycerol solution, and the tea polyphenol ethanol solution to the modified chitosan solution in turn. Stir at 500rpm for 2h to fully mix the components. The solid powder obtained by spray drying is the preservative.

[0043] Example 3

[0044] A fruit and vegetable preservative based on modified chitosan comprises the following components in the following weight percentage ranges: 6.5% modified chitosan, 2% zinc nitrate, 0.85% copper nitrate, 1.5% nano titanium dioxide, 1.25% glycerol, 0.25% tea polyphenols, and the balance distilled water.

[0045] The preparation method of modified chitosan comprises the following steps:

[0046] (1) Phosphorylation: Chitosan with a deacetylation degree of ≥90% was dispersed in 85% phosphoric acid solution at a chitosan to phosphoric acid solution mass ratio of 1:10. Phosphorus pentoxide was added at a chitosan to phosphorus pentoxide molar ratio of 1:1.5. The mixture was stirred at 80°C under a nitrogen flow of 0.5 L / min for 5 h. The reaction solution was cooled, dialyzed to a neutral pH, and freeze-dried to obtain phosphorylated chitosan.

[0047] (2) Quaternization: Phosphorylated chitosan was dissolved in deionized water to prepare a solution with a concentration of 7.5%, and then glycidyl trimethyl ammonium chloride was added. The molar ratio of phosphorylated chitosan to glycidyl trimethyl ammonium chloride was 1:0.8. The pH was adjusted to 8.5, and the reaction was carried out at 65°C for 3.5 hours. After ethanol precipitation, centrifugation and washing, the solution was dried under vacuum at 55°C to obtain phosphoric acid-quaternary ammonium double-modified chitosan.

[0048] (3) Methacrylic acid grafting: Phosphate-quaternized chitosan and glycidyl methacrylate were mixed in a mass ratio of 1:0.5, and 0.6% of potassium persulfate and 0.2% of ascorbic acid were added as initiators. Then, a nitrogen flow of 0.65 L / min was continuously introduced at 75°C for 3 h. After dialysis, the mixture was spray-dried to obtain the modified chitosan.

[0049] A method for preparing a fruit and vegetable preservative based on modified chitosan, comprising the following steps:

[0050] S1: Dissolve zinc nitrate and copper nitrate in water in proportion, add nano-TiO2, the amount of water can be 7.5 times the total mass of zinc nitrate, copper nitrate and nano-TiO2, and disperse under 300W ultrasonic dispersion until nano-TiO2 is uniformly dispersed in the solution. Under continuous stirring, slowly add 20% NaOH solution by mass until the pH reaches 9.5. After hydrothermal reaction at 85℃ for 2.5h, calcinate at 450℃ to obtain Cu-Zn-TiO2 composite powder;

[0051] S2: Dissolve the modified chitosan in water to form a 6.5wt% solution, and stir at 55°C until completely dissolved. Mix glycerol and water in a mass ratio of 1:1, and dissolve tea polyphenols in an appropriate amount of ethanol. The mass ratio of ethanol to tea polyphenols is 10:1. After both are completely dissolved, add the Cu-Zn-TiO2 composite powder, the mixed glycerol solution, and the tea polyphenol ethanol solution to the modified chitosan solution in turn. Stir at 450rpm for 1.5h to fully mix the components. Then, spray dry the solid powder obtained, which is the preservative.

[0052] Comparative Example 1: This comparative example 1 is basically the same as Example 3, except that the modified chitosan is not subjected to the phosphorylation treatment in step (1) during the preparation process.

[0053] Comparative Example 2: This comparative example 1 is substantially the same as Example 3, except that the modified chitosan is not subjected to the quaternization treatment of step (2) during the preparation process.

[0054] Comparative Example 3: This comparative example 1 is basically the same as Example 3, except that the modified chitosan is not processed in step (3) during the preparation process.

[0055] The six fruit and vegetable preservatives of Examples 1 to 3 and Comparative Examples 1 to 3 were used as test samples for testing. The test results are shown in Tables 1 and 2 below.

[0056] Table 1 Performance test results

[0057] Group Dissolution pH Composite powder D50 (nm) Tensile strength (MPa) Elongation at break (%) Example 1 3.5-8.0 95±5 28.3±1.1 200±8 Example 2 2.8-8.7 78±4 36.8±0.9 240±12 Example 3 3.0-8.5 85±3 32.5±0.8 220±10 Comparative Example 1 5.0-6.5 150±8 18.2±1.2 80±5 Comparative Example 2 4.0-7.0 120±6 24.7±0.9 130±8 Comparative Example 3 3.8-7.2 100±4 25.4±1.0 160±9

[0058] Table 2 Functional performance comparison

[0059]

[0060] The modified chitosan-based fruit and vegetable preservatives prepared in Examples 1-3 are significantly superior to those in Comparative Examples 1-3 in terms of performance and functional performance, fully demonstrating the necessity and synergistic value of the modified chitosan triple treatment process.

[0061] In terms of performance testing, the pH range of dissolution of the fruit and vegetable preservatives of Examples 1-3 was broadened to 2.8-8.7, compared to Comparative Examples 1-3 (3.8-7.2), especially Comparative Example 1 (unphosphorylated, dissolution pH only 5.0-6.5), which effectively solved the defect of traditional chitosan relying on acidic environment for dissolution and avoided the risk of acidification of fruit and vegetable skin. This is due to the strong polar phosphate groups introduced by the phosphorylation treatment, which changed the solubility characteristics of chitosan. In terms of composite powder dispersibility, the D50 value of the Cu-Zn-TiO2 composite powder of the embodiment is 78-95nm, which is significantly better than the 100-150nm of Comparative Examples 1-3. Among them, the agglomeration phenomenon of the unphosphorylated Comparative Example 1 is the most significant, indicating that the phosphate groups and quaternary ammonium groups of the modified chitosan provide a uniformly dispersed carrier platform for the photocatalytic material through electrostatic repulsion. In terms of mechanical properties, the tensile strength of the embodiment reaches 28.3-36.8 MPa, and the elongation at break is 200%-240%, which is much higher than that of Comparative Example 1 (without phosphorylation, the tensile strength is only 18.2 MPa, and the elongation at break is 80%). This is mainly because the cross-linked network formed by methacrylic acid grafting improves the mechanical properties of the membrane, and the sequential treatment of phosphorylation and quaternization ensures the extended conformation of the molecular chain, providing sufficient functional group sites for the grafting reaction.

[0062] In the functional performance comparison, the antibacterial performance is outstanding. In terms of E. coli inhibition rate, the examples reach 99.5%-99.99%, while the comparative example 1 (no phosphorylation) is only 85.3%. The positively charged groups (-N + (CH3)3) The microbial cell membrane is destroyed by electrostatic action, while phosphorylation enhances the binding force between chitosan and nanoparticles, synergistically improving the antibacterial efficiency; in terms of the Penicillium inhibition rate, the examples are 93.8%-98.1%, while the comparative example 1 is only 72.1%. The broad-spectrum inhibitory effect of the triple modification on fungi is fully verified, especially the synergistic effect of phosphorylation and quaternization, which can effectively deal with major pathogenic fungi such as Botrytis. In terms of antioxidant and fresh-keeping effects, the DPPH clearance rate of the embodiment is 89.5%-95.7%, which is higher than the 68.5%-86.4% of the comparative example. The photocatalytic effects of tea polyphenols and TiO2 complement each other, and the cross-linked structure of the modified chitosan delays the release of tea polyphenols, thereby extending the antioxidant cycle. Taking the strawberry fresh-keeping indicators as an example, the weight loss rate of the embodiment is only 1.8%-3.0%, the decay rate is 5%-12%, and the VC retention rate is 89.3%-94.6%, all of which are significantly better than the comparative example. The moisture-proof film structure formed by methacrylic acid grafting reduces water loss, and the synergistic effect of copper and zinc ions and TiO2 inhibits microbial reproduction and ethylene synthesis, thereby delaying fruit ripening.

[0063] Further in-depth analysis of the key roles of the modification steps shows that phosphorylation is the foundation. Without phosphorylation (Comparative Example 1), the solubility of chitosan in water is insufficient, which will lead to agglomeration of the composite powder, deterioration of the mechanical properties of the membrane, and a significant decrease in the antibacterial and preservation effects; quaternization is the core. The lack of quaternization (Comparative Example 2) will weaken the electrostatic adsorption force between the membrane and the surface of fruits and vegetables, reduce the exposure efficiency of the antibacterial group, and reduce the inhibition rate of Penicillium by 15.1%; methacrylic acid grafting is the guarantee. The membrane structure of the ungrafted (Comparative Example 3) is loose, the moisture barrier is poor, and the weight loss rate is 1.7% higher than that of Example 3, indicating that the cross-linked network is crucial to maintaining the integrity of the membrane.

[0064] In summary, the test results fully confirm that the three-step process of phosphorylation-quaternization-methacrylic acid grafting of modified chitosan is indispensable. Through the progressive design of solubility modification, functional group anchoring and cross-linking enhancement, the three have successfully constructed a triple synergistic system of "film formation regulation-ionic antibacterial-photocatalytic residue reduction", providing an efficient and green solution to solve industrial problems such as high post-harvest losses of fruits and vegetables and the risk of pesticide residues.

[0065] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A fruit and vegetable preservative based on modified chitosan, characterized in that: The invention comprises the following components in the following weight percentage ranges: 5% to 8% of modified chitosan, 1.5% to 2.5% of zinc nitrate, 0.5% to 1.2% of copper nitrate, 1% to 2% of nano titanium dioxide, 1% to 1.5% of glycerol, 0.2% to 0.3% of tea polyphenols and the balance of distilled water.

2. The fruit and vegetable preservative based on modified chitosan according to claim 1, characterized in that: The preparation method of the modified chitosan comprises the following specific steps: (1) Phosphorylation: Chitosan with a deacetylation degree of ≥90% was dispersed in 85% phosphoric acid solution at a mass ratio of chitosan to phosphoric acid solution of 1:

10. Phosphorus pentoxide was added to achieve a molar ratio of chitosan to phosphorus pentoxide of 1:1.

5. The mixture was stirred at 75-85°C under a nitrogen flow of 0.5 L / min for 4-6 h. The reaction solution was cooled, dialyzed to a neutral pH, and freeze-dried to obtain phosphorylated chitosan. (2) Quaternization: Phosphorylated chitosan was dissolved in deionized water to prepare a solution with a concentration of 5% to 10%, and glycidyl trimethylammonium chloride was added to make the molar ratio of phosphorylated chitosan to glycidyl trimethylammonium chloride reach 1:0.

8. The pH was adjusted to 8.5, and the reaction was carried out at 60-70°C for 3-4 hours. The solution was then precipitated with ethanol and washed by centrifugation. The solution was then dried in a vacuum at 50-60°C to obtain phosphoric acid-quaternary ammonium double-modified chitosan. (3) Methacrylic acid grafting: Phosphate-quaternized chitosan and glycidyl methacrylate were mixed in a mass ratio of 1:0.5, and 0.6% of potassium persulfate and 0.2% of ascorbic acid were added as initiators. Then, a nitrogen flow of 0.5-0.8 L / min was continuously introduced at 70-80°C for 2-4 h. After dialysis, the modified chitosan was spray-dried to obtain the chitosan.

3. The fruit and vegetable preservative based on modified chitosan according to claim 2, characterized in that: In step (1), the mass ratio of chitosan to phosphoric acid solution is 1:10, and the molar ratio of chitosan to phosphorus pentoxide is 1:1.

5.

4. The fruit and vegetable preservative based on modified chitosan according to claim 2, characterized in that: In step (2), the molar ratio of phosphorylated chitosan to glycidyl trimethylammonium chloride is 1:0.

8.

5. The fruit and vegetable preservative based on modified chitosan according to claim 2, characterized in that: In step (3), the mass of potassium persulfate is 0.6% of the total mass of phosphate-quaternized chitosan and glycidyl methacrylate, and the mass of ascorbic acid is 0.2% of the total mass.

6. A method for preparing a fruit and vegetable preservative based on modified chitosan as claimed in any one of claims 1 to 5, characterized in that: The preparation method comprises the following steps: S1: Dissolve zinc nitrate and copper nitrate in water in proportion, add nano-TiO2, the amount of water can be 5 to 10 times the total mass of zinc nitrate, copper nitrate and nano-TiO2, and disperse under 300W ultrasonic dispersion until the nano-TiO2 is uniformly dispersed in the solution. Under continuous stirring, slowly add 20% NaOH solution by mass until the pH is 9 to 10, carry out hydrothermal reaction at 80 to 90°C for 2 to 3 hours, and calcine at 400 to 500°C to obtain Cu-Zn-TiO2 composite powder; S2: Dissolve the modified chitosan in water to form a 5wt% to 8wt% solution, and stir at 50 to 60°C until completely dissolved. Mix glycerol and water evenly, and dissolve tea polyphenols in an appropriate amount of ethanol. After both are completely dissolved, add the Cu-Zn-TiO2 composite powder, the mixed glycerol solution, and the tea polyphenol ethanol solution to the modified chitosan solution in sequence, stir at 400 to 500 rpm for 1 to 2 hours, and after the components are fully mixed, spray dry to obtain a solid powder which is the preservative.

7. The method for preparing a fruit and vegetable preservative based on modified chitosan according to claim 6, characterized in that: The mass ratio of the glycerol to water is 1:1, and the mass ratio of the ethanol to tea polyphenols is 10:1.

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