Fresh-keeping composite film and preparation method thereof

Abstract

The invention discloses a fresh-keeping composite film and a preparation method thereof, and belongs to the field of food, the fresh-keeping composite film is used for coating food, and the fresh-keeping composite film comprises a catechol grafted hyaluronic acid polymer layer group and a catechol grafted chitosan polymer layer group, the catechol grafted hyaluronic acid polymer layer group and the catechol grafted chitosan polymer layer group take food as a center and cover the food. The fresh-keeping film is applied to fresh keeping of fresh vegetables and fruits, in particular to fresh keeping of vegetables and fruits easy to lose water, and is safe, environment-friendly, good in water-retaining property, high in film stability and low in raw material cost.

Description

Technical Field

[0001] The present invention relates to the field of food, and in particular to a fresh-keeping composite film and a preparation method thereof. Background Art

[0002] my country boasts a rich resource of various specialty fruits and vegetables, which are indispensable and valuable foods for the human population. However, the production of most agricultural products, including fruits and vegetables, is highly seasonal, and they maintain vigorous metabolic activity even after harvest. If not properly stored, their rich water and sugar content will cause them to rapidly dehydrate and rot during storage. According to surveys, due to insufficient storage and preservation technology, the annual post-harvest fruit loss rate in my country reaches 30%, and the vegetable loss rate reaches tens of millions of tons.

[0003] Post-harvest preservation technology for fruits and vegetables has become a key issue hindering the healthy development of the agricultural products industry. Extending the shelf life of agricultural products, improving their storage ratio, duration, and quality, thereby further increasing the economic income of fruit and vegetable farmers, providing technical support for the prepared food industry, further safeguarding the cold chain transportation of agricultural products, and reducing resource waste have become urgent issues that scientific and technological workers need to address.

[0004] In response to the serious waste of fruits and vegetables caused by loss, researchers have actively developed a series of fruit and vegetable preservation technologies. Currently, the fruit and vegetable preservation technologies used in the market mainly include physical preservation technology, chemical preservation technology, and biological preservation technology.

[0005] Physical methods include vacuum cooling, modified atmosphere packaging (MAP), reduced pressure storage, ultrahigh pressure (UHP), and irradiation. Vacuum cooling primarily removes heat through the rapid evaporation of moisture from the product, quickly eliminating field heat and improving cooling efficiency. However, it is costly and carries a relatively high rate of product quality loss. Modified atmosphere packaging (MAP) can suppress product respiration and slow metabolism, but MAP requires inert gases and specialized equipment, and the high space occupied by gas during packaging leads to high costs. Reduced pressure storage reduces the atmospheric pressure and oxygen concentration in the fruit and vegetable environment, thereby slowing the growth of fruit and vegetables. However, the reduced moisture content increases the rate of quality loss, which can lead to a loss of value for high-value fruits and vegetables. Ultrahigh pressure treatment (UHP) primarily uses high pressure to reduce microbial content and enzyme activity, but is currently limited by the low throughput of related equipment, making it unsuitable for industrial application. Irradiation, the second most popular food sterilization technology after pasteurization, applies ionizing radiation, such as gamma and electron beams, to food to extend its shelf life. However, the different radiation types and doses required for different types of fruits and vegetables require extensive validation.

[0006] Chemical methods include preservatives, electrolyzed water, and plasma-activated water. Chemical preservatives reduce microbial infection and extend shelf life by forming a barrier film. Gupta et al. used a 2.5% citric acid solution to preserve Agaricus bisporus after harvest and found that the activities of catalase, peroxidase, and polyphenol oxidase were effectively inhibited. Although the use of chemical fungicides is an effective method of preservation and storage, improper use of chemical fungicides may have adverse effects on human health, and their residues have potential hazards to nature and the environment. Electrolyzed water and plasma-activated water both inhibit the growth of microorganisms. From the perspective of reproduction, the shelf life of fruits and vegetables can be extended. Electrolyzed water releases a certain concentration of Cl2, which can inhibit Listeria monocytogenes and Escherichia coli. Plasma-activated water produces an acidic environment, which leads to changes in redox potential and conductivity, as well as the formation of reactive oxygen species (ROS) and nitrogen species (RNS). Therefore, plasma-activated water has a different chemical composition from water and can be used as an alternative method for microbial disinfection. However, the preparation of electrolyzed water and plasma-activated water relies on large instruments and is relatively expensive. In short, these preservation technologies are difficult to apply industrially due to problems such as safety, universality, and cost.

[0007] At present, although physical preservation technology and chemical preservation technology are more widely used, biological preservation technology is more popular considering issues such as preservation cost and the safety of preservation reagents.

[0008] CN202410142285.6 discloses a method for preparing a thymol-sea buckthorn oil-chitosan composite film, which is used to preserve strawberries. The sea buckthorn oil in this solution is extracted from sea buckthorn berries and seeds, which requires a large amount of sea buckthorn puree, is costly, and the preservation effect needs to be improved.

[0009] The above background technology is for facilitating understanding of the present invention and is not a known technology disclosed to the general public before the application of the present invention. Summary of the Invention

[0010] In view of the above-mentioned defects, the present invention provides a fresh-keeping composite film, which aims to improve at least one of the problems mentioned in the background art.

[0011] The technical solution is: a fresh-keeping composite film, including a catechol-grafted hyaluronic acid polymer layer group and a catechol-grafted chitosan polymer layer group, which are centered on food and coated on the outside of the food.

[0012] Furthermore, the catechol-grafted hyaluronic acid polymer layer group includes at least one catechol-grafted hyaluronic acid polymer layer, and the catechol-grafted chitosan polymer layer group includes at least one catechol-grafted chitosan polymer layer.

[0013] Furthermore, the catechol-grafted hyaluronic acid polymer layer group includes a first catechol-grafted hyaluronic acid polymer layer, a second catechol-grafted hyaluronic acid polymer layer and a third catechol-grafted hyaluronic acid polymer layer; the catechol-grafted chitosan polymer layer group includes a first catechol-grafted chitosan polymer layer, a second catechol-grafted chitosan polymer layer and a third catechol-grafted chitosan polymer layer, wherein the first catechol-grafted hyaluronic acid polymer layer, the first catechol-grafted chitosan polymer layer, the second catechol-grafted hyaluronic acid polymer layer, the second catechol-grafted chitosan polymer layer, the third catechol-grafted hyaluronic acid polymer layer and the third catechol-grafted chitosan polymer layer are arranged in sequence, the first catechol-grafted hyaluronic acid polymer layer is in contact with food, and the third catechol-grafted chitosan polymer layer is in contact with air.

[0014] Furthermore, the fresh-keeping composite film is prepared by the following steps: S1, preparing a dipping solution: taking a catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted hyaluronic acid polymer solution; taking a catechol-grafted chitosan polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted chitosan polymer solution; S2, dipping: Soak the food in a catechol grafted hyaluronic acid polymer solution for 1 to 10 minutes, take it out and let it stand for 0.5 to 5 minutes, then soak it in a catechol grafted chitosan polymer solution for 1 to 10 minutes, forming a cycle. Repeat the cycle at least once to obtain the fresh-keeping composite film.

[0015] Furthermore, the preparation method of catechol-grafted hyaluronic acid polymer comprises the following steps: M1, slowly dissolving hyaluronic acid in an aqueous solution of 2-morpholineethanesulfonic acid and adjusting the pH to 4-6 to form a hyaluronic acid solution; M2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; M3, add the carbodiimide solution in M2 to the hyaluronic acid solution in M1, stirring, and then immediately add N-hydroxysuccinimide and continue stirring until evenly mixed; M4, slowly add dopamine, adjust the pH to 4-6, continue stirring, and after the reaction, remove the liquid for dialysis; M5, taking out the dialysate and freeze-drying it to obtain catechol-grafted hyaluronic acid polymer; The preparation method of catechol grafted chitosan polymer comprises the following steps: N1, chitosan was added to deionized water, and concentrated hydrochloric acid was slowly added while stirring to dissolve it, and the pH was adjusted to 4-6 to form a chitosan solution; N2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; N3, slowly add the aqueous solution of dihydroxyphenylpropionic acid to the chitosan solution in N1 while stirring, then add the carbodiimide solution in N2 and adjust the pH to 4-6; continue stirring, and after the reaction, remove the liquid for dialysis; N4, the dialysate was taken out and freeze-dried to obtain catechol-grafted chitosan polymer.

[0016] Furthermore, the food is fresh fruits and vegetables.

[0017] The invention also provides a method for preparing the fresh-keeping composite film.

[0018] The technical solution is: a method for preparing a fresh-keeping composite film, comprising the following steps: S1, preparing a dipping solution: taking a catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted hyaluronic acid polymer solution; taking a catechol-grafted chitosan polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted chitosan polymer solution; S2, dipping: soaking the food in a catechol grafted hyaluronic acid polymer solution for 1 to 10 minutes, taking it out and letting it stand for 0.5 to 5 minutes, and then soaking it in a catechol grafted chitosan polymer solution for 1 to 10 minutes, forming a cycle, and repeating the cycle at least once to obtain the fresh-keeping composite film.

[0019] Furthermore, the number of repeated cycles is 2 times.

[0020] Furthermore, the preparation method of catechol-grafted hyaluronic acid polymer comprises the following steps: M1, slowly dissolving hyaluronic acid in an aqueous solution of 2-morpholineethanesulfonic acid and adjusting the pH to 4-6 to form a hyaluronic acid solution; M2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; M3, add the carbodiimide solution in M2 to the hyaluronic acid solution in M1, stirring, and then immediately add N-hydroxysuccinimide and continue stirring until evenly mixed; M4, slowly add dopamine, adjust the pH to 4-6, continue stirring, and after the reaction, remove the liquid for dialysis; M5, taking out the dialysate and freeze-drying it to obtain catechol-grafted hyaluronic acid polymer; The preparation method of catechol grafted chitosan polymer comprises the following steps: N1, chitosan was added to deionized water, and concentrated hydrochloric acid was slowly added while stirring to dissolve it, and the pH was adjusted to 4-6 to form a chitosan solution; N2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; N3, slowly add the aqueous solution of dihydroxyphenylpropionic acid to the chitosan solution in N1 while stirring, then add the carbodiimide solution in N2 and adjust the pH to 4-6; continue stirring, and after the reaction, remove the liquid for dialysis; N4, the dialysate was taken out and freeze-dried to obtain catechol-grafted chitosan polymer.

[0021] Compared with the prior art, the beneficial effects are: This invention uses phosphate buffer as a solvent and EDC as a coupling agent. Chitosan and dihydrocaffeic acid react, followed by freeze-drying to produce a catechol-grafted chitosan polymer, which enhances chitosan's adhesion and solubility. The invention also uses water as a solvent, N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) as coupling agents, hyaluronic acid and dopamine react, followed by freeze-drying to produce a catechol-grafted hyaluronic acid polymer, which enhances hyaluronic acid's adhesion.

[0022] The present invention utilizes the only natural polycationic polysaccharide chitosan and the polyanionic polysaccharide hyaluronic acid as raw materials, and imparts solubility and adhesion to chitosan and adhesion to hyaluronic acid through modification. The surfaces of fruits and vegetables (cherries) are then impregnated (i.e., self-assembled and mixed) with the solutions formed by the two modifications to form a stable fresh-keeping film, thereby extending the shelf life of the fruits and vegetables. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the infrared spectrum of the catechol-grafted hyaluronic acid polymer of the present invention; Figure 2 This is the nuclear magnetic resonance characterization spectrum of the catechol-grafted hyaluronic acid polymer of the present invention; Figure 3 This is the infrared spectrum of the catechol-grafted chitosan polymer of the present invention; Figure 4 This is the nuclear magnetic resonance characterization spectrum of the catechol-grafted chitosan polymer of the present invention; Figure 5 is a schematic cross-sectional view of the first combined structure; Figure 6 is a schematic cross-sectional view of a second combined structure; Figure 7 is a schematic cross-sectional view of the third combined structure; Figure 8 is a schematic cross-sectional view of the first combined structure for comparison; Figure 9 is a schematic cross-sectional view of a second combined structure for comparison; Figure 10is a schematic cross-sectional view of the third combined structure for comparison; Figure 11 is a schematic cross-sectional view of the fourth combined structure for comparison; Figure 12 is a schematic cross-sectional view of the fifth combined structure for comparison; In the figure, 1, the preserved object, 2, the first catechol grafted hyaluronic acid polymer layer, 3, the first catechol grafted chitosan polymer layer, 4, the second catechol grafted hyaluronic acid polymer layer, 5, the second catechol grafted chitosan polymer layer, 6, the third catechol grafted hyaluronic acid polymer layer, 7, the third catechol grafted chitosan polymer layer, 8, the fourth catechol grafted chitosan polymer layer, 9, the fourth catechol grafted hyaluronic acid polymer layer, 10, the fifth catechol grafted hyaluronic acid polymer layer, Compound layer, 11. Sixth catechol grafted hyaluronic acid polymer layer, 12. Fifth catechol grafted chitosan polymer layer, 13. Sixth catechol grafted chitosan polymer layer, 20. First hyaluronic acid layer, 21. Second hyaluronic acid layer, 22. Third hyaluronic acid layer, 23. Fourth hyaluronic acid layer, 24. Fifth hyaluronic acid layer, 25. Sixth hyaluronic acid layer, 26. First sea buckthorn oil layer, 27. Second sea buckthorn oil, 28. Third sea buckthorn oil layer, 29. Fourth sea buckthorn oil layer, 30. Fifth sea buckthorn oil layer, 31. Sixth sea buckthorn oil layer. DETAILED DESCRIPTION

[0024] As used herein: "Prepared from" is synonymous with "comprising." As used herein, the terms "comprising," "including," "having," "containing," or any other variations thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0025] The conjunction "consisting of" excludes any unspecified element, step, or component. If used in a claim, this phrase renders the claim closed, excluding materials other than those described, except for conventional impurities associated therewith. When the phrase "consisting of" appears in a clause of the body of a claim, rather than immediately following the subject matter, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0026] When an amount, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper preferred values and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pairing of any range upper limit or preferred value with any range lower limit or preferred value, regardless of whether the range is disclosed alone. For example, when a range of "1 to 5" is disclosed, the described range should be interpreted as including the range "1 to 4", "1 to 3", "1 to 2", "1 to 2 and 4 to 5", "1 to 3 and 5", etc. When a numerical range is described herein, unless otherwise stated, the range is intended to include its end values and all integers and fractions within the range.

[0027] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0028] Those skilled in the art will appreciate that the following examples are intended to illustrate the present invention only and should not be construed as limiting the scope of the present invention. In the examples, where specific conditions are not specified, conventional conditions or manufacturer-recommended conditions were used. Reagents or instruments used where the manufacturer is not specified are conventional products that can be purchased commercially.

[0029] In these examples, parts and percentages are by mass unless otherwise indicated.

[0030] "Parts by mass" refers to the basic unit of measurement used to express the mass ratio of multiple components. One part can represent any unit of mass, such as 1g or 2.689g. If we say that the mass of component A is a parts and the mass of component B is b parts, this means the ratio of the mass of component A to the mass of component B is a:b. Alternatively, we could say that the mass of component A is aK and the mass of component B is bK (K is an arbitrary number representing a multiplication factor). It's important to note that, unlike parts by mass, the sum of the mass of all components is not limited to 100 parts.

[0031] "And / or" is used to indicate that one or both of the stated situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0032] The preparation of catechol-grafted hyaluronic acid polymer comprises the following steps: M1, slowly dissolve 1 g of hyaluronic acid with a molecular weight of 1.2 million Da in an aqueous solution of 2-morpholineethanesulfonic acid (MES) (0.05 mol MES, 80 ml water), and adjust its pH to 4-6 to form a hyaluronic acid solution.

[0033] M2, 388 mg of carbodiimide (EDC) was dissolved in a mixed solvent (the mixed solvent was formed by 10 ml of water and 10 ml of ethanol) to form a carbodiimide solution.

[0034] M3: Add the carbodiimide solution in M2 to the hyaluronic acid solution in M1 while stirring. Immediately add 288mg of N-hydroxysuccinimide (NHS) and continue stirring until combined. The purpose of adding the carbodiimide solution in M2 to the hyaluronic acid solution in M1 is to catalyze the formation of lipids from the carboxyl groups, while the purpose of adding hydroxysuccinimide (NHS) is to prevent lipid hydrolysis.

[0035] M4: Slowly add 471 mg of dopamine and adjust the pH to 4-6. Stir continuously. After 4 hours of reaction, remove the liquid and dialyze for 48 hours. The pH is adjusted to 4-6 to prevent oxidation of the catechol on the dopamine.

[0036] M5, the dialysate was taken out and freeze-dried to obtain catechol-grafted hyaluronic acid polymer, and the catechol-grafted hyaluronic acid polymer was characterized by infrared and nuclear magnetic resonance to verify the successful grafting. Figure 1-Figure 2 .

[0037] Preparation of catechol grafted chitosan polymer: N1, add 0.5g chitosan to 25ml deionized water, slowly add 100μL concentrated hydrochloric acid (37.5wt%) while stirring to dissolve it. If it is not completely dissolved, add 10μL concentrated hydrochloric acid at a time until the chitosan is completely dissolved. Adjust the pH to 4-6 to form a chitosan solution.

[0038] N2, 0.623 g of carbodiimide (EDC) was dissolved in a mixed solvent (the mixed solvent was formed by 10 ml of water and 10 ml of ethanol) to form a carbodiimide solution.

[0039] N3: Slowly add 0.59 g of dihydroxyphenylpropionic acid (HCA) to the chitosan solution in N1 while stirring. Then add the carbodiimide solution in N2 and adjust the pH to 4-6. Continue stirring for 4 h, then remove the liquid and dialyze it for 48 h.

[0040] N4, the dialysate was taken out and freeze-dried to obtain catechol grafted chitosan polymer. The product was characterized by infrared and nuclear magnetic resonance to verify the success of grafting. Figure 3-Figure 4 .

[0041] Example 1 S1, preparing dipping solution: taking catechol grafted hyaluronic acid polymer, adding deionized water to prepare 1 mg / mL catechol grafted hyaluronic acid polymer solution; taking catechol grafted chitosan polymer, adding deionized water to prepare 1 mg / mL catechol grafted chitosan polymer solution.

[0042] S2, Dip Coating: Soak fresh cherries (fresh cherries are the preserved objects) in a catechol grafted hyaluronic acid polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in a catechol grafted chitosan polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then put them in a catechol grafted hyaluronic acid polymer solution again, and repeat the above operation process until a first combined structure of three layers of catechol grafted hyaluronic acid polymer / catechol grafted chitosan polymer is obtained (cross-sectional view as shown in FIG. Figure 5 Schematic diagram shown).

[0043] Figure 5 In the embodiment, the first combined structure includes an object to be preserved 1, and the outer surface of the object to be preserved 1 is covered from the inside to the outside with a first catechol-grafted hyaluronic acid polymer layer 2, a first catechol-grafted chitosan polymer layer 3, a second catechol-grafted hyaluronic acid polymer layer 4, a second catechol-grafted chitosan polymer layer 5, a third catechol-grafted hyaluronic acid polymer layer 6 and a third catechol-grafted chitosan polymer layer 7.

[0044] Example 2 S1, preparing dipping solution: taking catechol grafted hyaluronic acid polymer, adding deionized water to prepare 1 mg / mL catechol grafted hyaluronic acid polymer solution; taking catechol grafted chitosan polymer, adding deionized water to prepare 1 mg / mL catechol grafted chitosan polymer solution.

[0045] S2, Dip Coating: Soak fresh cherries (fresh cherries are the preserved objects) in a catechol grafted chitosan polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in a catechol grafted hyaluronic acid polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then put them in a catechol grafted chitosan polymer solution again, and repeat the above operation process until a second combined structure of three layers of catechol grafted chitosan polymer / catechol grafted hyaluronic acid polymer is obtained (cross-sectional view as shown in FIG. Figure 6 Schematic diagram shown).

[0046] Figure 6 In the embodiment, the second composite structure includes an object to be preserved 1, and the outer surface of the object to be preserved 1 is covered from the inside to the outside with a first catechol grafted chitosan polymer layer 3, a first catechol grafted hyaluronic acid polymer layer 2, a second catechol grafted chitosan polymer layer 5, a second catechol grafted hyaluronic acid polymer layer 4, a third catechol grafted chitosan polymer layer 7 and a third catechol grafted hyaluronic acid polymer layer 6.

[0047] Example 3 S1, preparing dipping solution: taking catechol grafted hyaluronic acid polymer, adding deionized water to prepare 1 mg / mL catechol grafted hyaluronic acid polymer solution; taking catechol grafted chitosan polymer, adding deionized water to prepare 1 mg / mL catechol grafted chitosan polymer solution.

[0048] S2, Dipping: Soak fresh cherries (fresh cherries are preserved) in a catechol grafted hyaluronic acid polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in a catechol grafted chitosan polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in a catechol grafted chitosan polymer solution for 5 minutes again, take them out and let them stand for 2 minutes, then soak them in a catechol grafted chitosan polymer solution for 5 minutes again, take them out and let them stand for 2 minutes, then soak them in a catechol grafted hyaluronic acid polymer solution for 5 minutes again, take them out and let them stand for 2 minutes, then soak them in a catechol grafted chitosan polymer solution for 5 minutes again, take them out and let them stand for 2 minutes, to form a third combined structure (cross-sectional view as shown in FIG. Figure 7 Schematic diagram shown).

[0049] Figure 7 In the embodiment, the third composite structure includes an object to be preserved 1, the epidermis of which is covered from the inside to the outside with a first catechol grafted hyaluronic acid polymer layer 2, a first catechol grafted chitosan polymer layer 3, a second catechol grafted chitosan polymer layer 5, a third catechol grafted chitosan polymer layer 7, a second catechol grafted hyaluronic acid polymer layer 4 and a fourth catechol grafted chitosan polymer layer 8.

[0050] Comparative Example 1 S1, preparing a dipping solution: taking a catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 1 mg / mL catechol-grafted hyaluronic acid polymer solution.

[0051] S2, Dip Coating: Soak fresh cherries (fresh cherries are preserved) in the catechol grafted hyaluronic acid polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in the catechol grafted hyaluronic acid polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then repeat until the first combined structure is formed (cross-section as shown in the figure). Figure 8 and observe its microscopic morphology.

[0052] Figure 8In the embodiment, the first combined structure includes the preserved object 1, the epidermis of which is covered from the inside to the outside with a first catechol-grafted hyaluronic acid polymer layer 2, a second catechol-grafted hyaluronic acid polymer layer 4, a third catechol-grafted hyaluronic acid polymer layer 6, a fourth catechol-grafted hyaluronic acid polymer layer 9, a fifth catechol-grafted hyaluronic acid polymer layer 10 and a sixth catechol-grafted hyaluronic acid polymer layer 11.

[0053] Comparative Example 2 S1, preparing a dipping solution: taking a catechol-grafted chitosan polymer, adding deionized water to prepare a 1 mg / mL catechol-grafted chitosan polymer solution.

[0054] S2, Dip Coating: Soak fresh cherries (fresh cherries are preserved) in the catechol grafted chitosan polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in the catechol grafted chitosan polymer solution again for 5 minutes, take them out and let them stand for 2 minutes, then repeat until the second composite structure is formed (cross-section diagram as shown). Figure 9 and observe its microscopic morphology.

[0055] Figure 9 In the embodiment, the second composite structure includes the preserved object 1, the outer surface of which is covered from the inside to the outside with a first catechol grafted chitosan polymer layer 3, a second catechol grafted chitosan polymer layer 5, a third catechol grafted chitosan polymer layer 7, a fourth catechol grafted chitosan polymer layer 8, a fifth catechol grafted chitosan polymer layer 12 and a sixth catechol grafted chitosan polymer layer 13.

[0056] Comparative Example 3 The same amount of hyaluronic acid, chitosan, and dopamine as used in the preparation of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer was mixed, and deionized water was added to prepare a solution with a hyaluronic acid + chitosan content of 1 mg / mL. This solution had a flocculent precipitate due to the insolubility of chitosan, and therefore could not be used to coat fresh cherries.

[0057] Comparative Example 4 S1, preparing the dipping solution: taking the same amount of hyaluronic acid as that used in the preparation of the catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 1 mg / mL hyaluronic acid solution; taking the catechol-grafted chitosan polymer, adding deionized water to prepare a 1 mg / mL catechol-grafted chitosan polymer solution.

[0058] S2, Dip coating: Soak fresh cherries (fresh cherries are preserved) in hyaluronic acid solution for 5 minutes, take them out and let them stand for 2 minutes, then soak them in catechol grafted chitosan polymer solution for 5 minutes, take them out and let them stand for 2 minutes, then put them in hyaluronic acid solution again, and repeat the process until the third composite structure is formed (cross-section diagram as shown). Figure 10 Schematic diagram shown).

[0059] Figure 10 In the embodiment, the third combined structure includes the preserved object 1, the outer skin of which is covered from the inside to the outside with a first acid layer 20, a first catechol grafted chitosan polymer layer 3, a second acid layer 21, a second catechol grafted chitosan polymer layer 5, a third acid layer 2218 and a third catechol grafted chitosan polymer layer.

[0060] Comparative Example 5 Chitosan was added to deionized water to prepare a 1 mg / mL solution. However, the solution could not be used to coat fresh cherries because chitosan could not be dissolved and the mixed solution had flocculent precipitation.

[0061] Comparative Example 6 S1, prepare dipping solution: add hyaluronic acid to deionized water to prepare a 1 mg / mL solution.

[0062] S2, Dip Coating: Place fresh cherries (fresh cherries are preserved) in the solution of S1 and soak for 5 minutes, take them out and let them stand for 2 minutes, then place them in the solution of S1 and soak for 5 minutes again, take them out and let them stand for 2 minutes, then repeat until the fourth combination structure (cross-section diagram as shown) is formed. Figure 11 Schematic diagram shown).

[0063] Figure 11 In the embodiment, the fourth combination structure includes the preserved object 1, the outer surface of which is covered from the inside to the outside with a first acid layer 20, a second acid layer 21, a third acid layer 22, a fourth acid layer 23, a fifth acid layer 24 and a sixth acid layer 25.

[0064] Comparative Example 7 S1, preparing dipping solution: chitosan was added to deionized water to prepare a first solution of 1 mg / mL. Hyaluronic acid was added to deionized water to prepare a second solution of 1 mg / mL.

[0065] S2, because chitosan could not be dissolved in the first solution, the mixed solution had flocculent precipitation, so it was impossible to coat the fresh cherries, resulting in the ultimate coating failure.

[0066] Comparative Example 8 S1. Prepare the dipping solution: Add the same amount of hyaluronic acid and dopamine as in the preparation of the catechol-grafted hyaluronic acid polymer to deionized water to prepare a first solution of 1 mg / mL. Add the same amount of chitosan and dihydroxyphenylpropionic acid (HCA) as in the preparation of the catechol-grafted chitosan polymer to deionized water to prepare a second solution of 1 mg / mL.

[0067] S2, the second solution cannot be coated on fresh cherries because chitosan cannot be dissolved and the mixed solution has flocculent precipitation, resulting in ultimate coating failure.

[0068] Comparative Example 9 S1, preparing the dipping solution: taking the catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 1 mg / mL catechol-grafted hyaluronic acid polymer solution; taking the same amount of chitosan as that in the preparation of the catechol-grafted chitosan polymer, adding deionized water to prepare a 1 mg / mL chitosan solution.

[0069] S2, because chitosan in the chitosan solution cannot be dissolved, resulting in flocculent precipitation in the solution, it is impossible to coat the fresh cherries, resulting in the ultimate coating failure.

[0070] Comparative Example 10 S1, prepare dipping solution: prepare the solution according to the method of Example 1 in CN202410142285.6.

[0071] S2, Dip Coating: Place fresh cherries (fresh cherries are preserved) in the solution of S1 and soak for 5 minutes, take them out and let them stand for 2 minutes, then place them in the solution of S1 and soak for 5 minutes again, take them out and let them stand for 2 minutes, then repeat again until the fifth combination structure (cross-section diagram as shown) is formed. Figure 12 Schematic diagram shown).

[0072] Figure 12 In the comparison, the fifth combined structure includes the preserved object 1, the outer skin of the preserved object 1 is covered with the first sea buckthorn oil layer 26, the second sea buckthorn oil 27, the third sea buckthorn oil layer 28, the fourth sea buckthorn oil layer 29, the fifth sea buckthorn oil layer 30 and the sixth sea buckthorn oil layer 31 from the inside to the outside.

[0073] In the above Examples 1 to 3 and Comparative Examples 1 to 10, there was no difference in the quality and size of the preserved object 1 before dipping. In the following examples, there was also no difference in the quality and size of the fresh cherries used as the blank control group and the preserved object 1.

[0074] Example 4 Fresh cherries were taken as a blank control group, and weight loss rate tests were carried out on the first combination structure (Example 1), the second combination structure (Example 2), the third combination structure (Example 3), the comparative first combination structure (Comparative Example 1), the comparative second combination structure (Comparative Example 2), the comparative third combination structure (Comparative Example 4), the comparative fourth combination structure (Comparative Example 6), and the comparative fifth combination structure (Comparative Example 10). The results are shown in Table 1 below.

[0075] Table 1 Weight loss test results Comparison between Example 1 and Example 2 shows that the catechol-grafted hyaluronic acid polymer is the innermost layer, in contact with the preserved object; the catechol-grafted chitosan polymer is the outermost layer, in contact with the air, and has a better water retention effect. Comparison between Example 1 and Example 3 shows that the alternating arrangement of the catechol-grafted hyaluronic acid polymer layer and the catechol-grafted chitosan polymer layer has a better water retention effect. Comparison between Example 1 and Comparative Example 1 shows that the film prepared by alternating the two materials of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer has better water retention than the film prepared by a single catechol-grafted hyaluronic acid polymer. Comparison between Example 1 and Comparative Example 2 shows that the film prepared by alternating the two materials of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer has better water retention than the film prepared by a single catechol-grafted chitosan polymer. Comparison of Example 1 with Comparative Example 6 shows that the film prepared by alternating the two materials of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer has better water retention than the film prepared with only hyaluronic acid. Comparison of Example 1 with Comparative Example 4 shows that the film prepared by alternating the two materials of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer has better water retention than the film prepared by alternating the two materials of catechol-grafted chitosan polymer and hyaluronic acid. Comparison of Example 3 with Comparative Example 10 shows that the film prepared by alternating the two materials of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer has better water retention than the film prepared with six layers of sea buckthorn oil.

[0076] Example 5 Fresh cherries were taken as a blank control group, and the Vc content was tested with the first combination structure (Example 1), the second combination structure (Example 2), the third combination structure (Example 3), the comparative first combination structure (Comparative Example 1), the comparative second combination structure (Comparative Example 2), the comparative third combination structure (Comparative Example 4), the comparative fourth combination structure (Comparative Example 6), and the comparative fifth combination structure (Comparative Example 10). The results are shown in Table 2 below.

[0077] Table 2 Vc content test results It can be seen from the comparison of Example 1 and Example 2 that the catechol-grafted hyaluronic acid polymer is in contact with the preserved material as the innermost layer; the catechol-grafted chitosan polymer is in contact with the air as the outermost layer. The Vc content slowly increases from the 1st to the 10th day, reaches a maximum value on the 10th to the 15th day, and then begins to decline. The Vc content in the preserved material is maintained for a longer time, and the time of Vc loss is later. It can be seen from the comparison of Example 1 and Example 3 that the catechol-grafted hyaluronic acid polymer layer and the catechol-grafted chitosan polymer layer are arranged alternately, and the Vc content in the preserved material is maintained for a longer time, and the time of Vc loss is later. It can be seen from the comparison of Example 1 and Comparative Example 1 that the film prepared by alternating the two materials of catechol-grafted hyaluronic acid polymer and catechol-grafted chitosan polymer is more effective than the film prepared by a single catechol-grafted hyaluronic acid polymer in the preserved material. The Vc content is maintained for a longer time, and the time of Vc loss is later. Comparison of Example 1 with Comparative Example 2 shows that the film prepared by alternating the two materials of catechol grafted hyaluronic acid polymer and catechol grafted chitosan polymer maintains the Vc content longer and the time of Vc loss is later than that of the fresh-keeping product applied by the film prepared by a single catechol grafted chitosan polymer. Comparison of Example 1 with Comparative Example 6 shows that the film prepared by alternating the two materials of catechol grafted hyaluronic acid and catechol grafted chitosan maintains the Vc content longer and the time of Vc loss is later than that of the fresh-keeping product applied by the film prepared by a single hyaluronic acid. Comparison of Example 1 with Comparative Example 4 shows that the film prepared by alternating the two materials of catechol grafted hyaluronic acid polymer and catechol grafted chitosan polymer maintains the Vc content longer and the time of Vc loss is later than that of the fresh-keeping product applied by the film prepared by alternating the catechol grafted chitosan polymer and hyaluronic acid. Comparison of Example 3 with Comparative Example 10 shows that the films prepared from catechol grafted hyaluronic acid polymer and catechol grafted chitosan polymer retain Vc content longer and lose Vc later than the films prepared from six-layer sea buckthorn oil.

[0078] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A fresh-keeping composite film for coating food, characterized in that: The invention comprises a catechol grafted hyaluronic acid polymer layer group and a catechol grafted chitosan polymer layer group. The catechol grafted hyaluronic acid polymer layer group and the catechol grafted chitosan polymer layer group are centered on the food and coated on the outside of the food.

2. The fresh-keeping composite film according to claim 1, characterized in that: The catechol-grafted hyaluronic acid polymer layer group includes at least one catechol-grafted hyaluronic acid polymer layer, and the catechol-grafted chitosan polymer layer group includes at least one catechol-grafted chitosan polymer layer.

3. The fresh-keeping composite film according to claim 1, characterized in that: The catechol-grafted hyaluronic acid polymer layer group includes a first catechol-grafted hyaluronic acid polymer layer, a second catechol-grafted hyaluronic acid polymer layer and a third catechol-grafted hyaluronic acid polymer layer; the catechol-grafted chitosan polymer layer group includes a first catechol-grafted chitosan polymer layer, a second catechol-grafted chitosan polymer layer and a third catechol-grafted chitosan polymer layer, wherein the first catechol-grafted hyaluronic acid polymer layer, the first catechol-grafted chitosan polymer layer, the second catechol-grafted hyaluronic acid polymer layer, the second catechol-grafted chitosan polymer layer, the third catechol-grafted hyaluronic acid polymer layer and the third catechol-grafted chitosan polymer layer are arranged in sequence, the first catechol-grafted hyaluronic acid polymer layer is in contact with food, and the third catechol-grafted chitosan polymer layer is in contact with air.

4. The fresh-keeping composite film according to claim 1, characterized in that: The fresh-keeping composite film is prepared by the following steps: S1, preparing a dipping solution: taking a catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted hyaluronic acid polymer solution; taking a catechol-grafted chitosan polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted chitosan polymer solution; S2, dipping: Soak the food in a catechol grafted hyaluronic acid polymer solution for 1 to 10 minutes, take it out and let it stand for 0.5 to 5 minutes, then soak it in a catechol grafted chitosan polymer solution for 1 to 10 minutes, take it out and let it stand for 0.5 to 5 minutes, then put it in the catechol grafted hyaluronic acid polymer solution again to form a cycle. Repeat the cycle at least once to obtain the fresh-keeping composite film.

5. The fresh-keeping composite film according to any one of claims 1 to 4, characterized in that: The preparation method of catechol-grafted hyaluronic acid polymer comprises the following steps: M1, slowly dissolving hyaluronic acid in an aqueous solution of 2-morpholineethanesulfonic acid and adjusting the pH to 4-6 to form a hyaluronic acid solution; M2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; M3, add the carbodiimide solution in M2 to the hyaluronic acid solution in M1, stirring, and then immediately add N-hydroxysuccinimide and continue stirring until evenly mixed; M4, slowly add dopamine, adjust the pH to 4-6, continue stirring, and after the reaction, remove the liquid for dialysis; M5, taking out the dialysate and freeze-drying it to obtain catechol-grafted hyaluronic acid polymer; The preparation method of catechol grafted chitosan polymer comprises the following steps: N1, chitosan was added to deionized water, and concentrated hydrochloric acid was slowly added while stirring to dissolve it, and the pH was adjusted to 4-6 to form a chitosan solution; N2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; N3, slowly add the aqueous solution of dihydroxyphenylpropionic acid to the chitosan solution in N1 while stirring, then add the carbodiimide solution in N2 and adjust the pH to 4-6; continue stirring, and after the reaction, remove the liquid for dialysis; N4, the dialysate was taken out and freeze-dried to obtain catechol-grafted chitosan polymer.

6. The fresh-keeping composite film according to claim 1, characterized in that: The food is fresh fruits and vegetables.

7. A method for preparing a fresh-keeping composite film, characterized in that: The following steps are involved: S1, preparing a dipping solution: taking a catechol-grafted hyaluronic acid polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted hyaluronic acid polymer solution; taking a catechol-grafted chitosan polymer, adding deionized water to prepare a 0.5-3 mg / mL catechol-grafted chitosan polymer solution; S2, dipping: soaking the food in a catechol grafted hyaluronic acid polymer solution for 1 to 10 minutes, taking it out and letting it stand for 0.5 to 5 minutes, and then soaking it in a catechol grafted chitosan polymer solution for 1 to 10 minutes, forming a cycle, and repeating the cycle at least once to obtain the fresh-keeping composite film.

8. The method for preparing the fresh-keeping composite film according to claim 7, characterized in that: The cycle was repeated 2 times.

9. The method for preparing the fresh-keeping composite film according to claim 7, characterized in that: The preparation method of catechol-grafted hyaluronic acid polymer comprises the following steps: M1, slowly dissolving hyaluronic acid in an aqueous solution of 2-morpholineethanesulfonic acid and adjusting the pH to 4-6 to form a hyaluronic acid solution; M2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; M3, add the carbodiimide solution in M2 to the hyaluronic acid solution in M1, stirring, and then immediately add N-hydroxysuccinimide and continue stirring until evenly mixed; M4, slowly add dopamine, adjust the pH to 4-6, continue stirring, and after the reaction, remove the liquid for dialysis; M5, taking out the dialysate and freeze-drying it to obtain catechol-grafted hyaluronic acid polymer; The preparation method of catechol grafted chitosan polymer comprises the following steps: N1, chitosan was added to deionized water, and concentrated hydrochloric acid was slowly added while stirring to dissolve it, and the pH was adjusted to 4-6 to form a chitosan solution; N2, dissolving carbodiimide in a mixed solution of water and ethanol to form a carbodiimide solution; N3, slowly add the aqueous solution of dihydroxyphenylpropionic acid to the chitosan solution in N1 while stirring, then add the carbodiimide solution in N2 and adjust the pH to 4-6; continue stirring, and after the reaction, remove the liquid for dialysis; N4, the dialysate was taken out and freeze-dried to obtain catechol-grafted chitosan polymer.

10. The method for preparing the fresh-keeping composite film according to claim 7, characterized in that: The food is fresh fruits and vegetables.

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