Preparation method and application of modified chitosan coating liquid for fruit preservation

Abstract

The invention discloses a preparation method of a modified chitosan coating liquid for fruit preservation, which comprises the following steps: grafting gallic acid to chitosan by using a biological coupling method to obtain gallic acid modified chitosan; and dissolving anhydrous calcium chloride powder in the gallic acid modified chitosan to obtain the chitosan-gallic acid / Ca < 2 + > coating liquid material. The invention also discloses an application of the preservative in fruit preservation. The modified chitosan coating liquid prepared by the invention forms a high-density spatial network structure through a biological coupling method and a metal coordination reaction, and has good antibacterial and antioxidant activity, mechanical properties, water vapor barrier property and peel adhesion. The prepared modified chitosan coating liquid can form a uniform film on the surface of the fruit, weaken respiration, prevent and inhibit postharvest diseases, reduce the weight loss rate and rotting rate, maintain the brightness of the fruit and prolong the shelf life of the fruit.

Description

Technical Field

[0001] The present invention relates to the technical field of fruit preservation, and specifically relates to a preparation method and application of a modified chitosan coating solution for fruit preservation. Background Art

[0002] As typical regional agricultural products, fruits have extremely high requirements for the logistics and transportation system due to their characteristics of separated production and sales. However, during the post-harvest circulation process, due to factors such as strong respiratory metabolism and pathogen infection, the decay rates of perishable varieties such as berries (such as blueberries) and tropical fruits (such as mangoes) can reach 30%-50%.

[0003] As a natural polysaccharide material, chitosan is regarded as an ideal green preservation matrix due to its excellent film-forming property, broad-spectrum antibacterial property and biodegradability. However, it is found in practical applications that the rigid structure of its molecular chain leads to insufficient bioadhesion, and the film layer is prone to peeling when the wetting angle on the fruit surface > 90°; moreover, the degree of protonation of chitosan amino groups is significantly affected by environmental humidity. When the relative humidity > 85%, the water absorption and swelling rate of the film material can reach 380%, resulting in rapid loss of antibacterial components. These defects make it difficult for single chitosan coating to meet the long-term preservation needs of surface-wet fruits such as blueberries and mangoes, and become the key bottleneck restricting its industrial application. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method and application of a modified chitosan coating solution for fruit preservation to solve the deficiencies of the prior art.

[0005] The purpose of the present invention is achieved through the following technical solutions: A preparation method of a modified chitosan coating solution for fruit preservation, comprising the following steps: (1) Prepare an ethanol aqueous solution, add gallic acid to it, and after it is completely dissolved, successively add 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide, mix evenly, and the mixed solution is activated and reacted under ice bath conditions. After the activation reaction ends, a gallic acid grafted activator is obtained; (2) Prepare a chitosan-acetic acid aqueous solution, add the gallic acid grafted activator described in step (1) to it, mix evenly, and the mixed solution is allowed to stand and react in the dark. After the reaction ends, the reaction solution is dialyzed, and then the dialyzed reaction solution is freeze-dried to obtain a gallic acid-modified chitosan freeze-dried sample; (3) Prepare an acetic acid aqueous solution, add the gallic acid-modified chitosan freeze-dried sample described in step (2) to it, and after it is completely dissolved, add anhydrous calcium chloride powder to the mixed solution, and allow it to stand and react to obtain a chitosan-gallic acid / Ca 2+ coating solution.

[0006] Further, in step (1), the aqueous ethanol solution is a 50 - 70 v / v% aqueous ethanol solution; the final molar concentration of gallic acid added is 6.0 - 7.0 mmol / L, the final molar concentration of 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride added is 6.0 - 7.0 mmol / L, and the final molar concentration of N-hydroxysuccinimide added is 6.0 - 7.0 mmol / L; the activation reaction time is 1 - 3 h.

[0007] Furthermore, in step (1), the aqueous ethanol solution is a 60 v / v% aqueous ethanol solution; the final molar concentrations of gallic acid, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride, and N-hydroxysuccinimide added are all 6.2 mmol / L; the activation reaction time is 2 h.

[0008] Further, in step (2), the chitosan - acetic acid aqueous solution is a 1 - 2 m / v% chitosan - 0.5 - 1 v / v% acetic acid aqueous solution; the volume ratio of the chitosan - acetic acid aqueous solution to the gallic acid grafting activator is 1:1 - 2; the static reaction is carried out for 18 - 30 h.

[0009] Furthermore, in step (2), the chitosan - acetic acid aqueous solution is a 2 m / v% chitosan - 1 v / v% acetic acid aqueous solution; the volume ratio of the chitosan - acetic acid aqueous solution to the gallic acid grafting activator is 1:1; the static reaction is carried out for 24 h.

[0010] Furthermore, in step (2), the dialysis is specifically operated as follows: The reaction solution is filled into a 14 kDa dialysis bag and dialyzed for 72 h. The dialysis solution in the first 48 h is acidic pure water with a pH of 4.7 ± 0.5, and the dialysis solution in the last 24 h is pure water with a pH of 7.0 ± 0.5. The water is changed every 8 h.

[0011] Further, in step (3), the acetic acid aqueous solution is a 0.2 - 0.5 v / v% acetic acid aqueous solution; the addition ratio of the gallic acid-modified chitosan freeze-dried sample is 0.5 - 2 m / v%; the addition ratio of the anhydrous calcium chloride powder is 0.05 - 0.2 m / v%; the static reaction time is 6 - 12 h.

[0012] Furthermore, in step (3), the acetic acid aqueous solution is a 0.5 v / v% acetic acid aqueous solution; the addition ratio of the gallic acid-modified chitosan freeze-dried sample is 1 m / v%; the addition ratio of the anhydrous calcium chloride powder is 0.1 m / v%; the static reaction time is 6 h.

[0013] Application of the above-mentioned modified chitosan coating solution for fruit preservation in fruit preservation.

[0014] Further, during application, soak the fruits in the modified chitosan coating solution for 10 - 30 s, take them out, and air-dry or naturally dry them; or evenly spray the modified chitosan coating solution on the surface of the fruits and then air-dry or naturally dry them.

[0015] Advantages of the present invention: When preparing the modified chitosan coating solution of the present invention, gallic acid is grafted onto chitosan by a bioconjugation method to obtain gallic acid-modified chitosan; then anhydrous calcium chloride powder is dissolved in the gallic acid-modified chitosan to obtain chitosan-gallic acid / Ca 2+ coating solution material. Under the action of EDC / NHS, gallic acid forms an NHS ester intermediate; subsequently, this intermediate reacts with the amino group of the chitosan backbone to form a chitosan-gallic acid copolymer through an amide bond; finally, Ca 2+ can promote the oxidation of the phenolic hydroxyl group of gallic acid in the copolymer, form a complex bond with the adjacent hydroxyl group, and form a phenolic acid-metal ion three-dimensional network structure, further enhancing the cross-linking degree of the copolymer. The modified chitosan coating solution prepared by the present invention forms a high-density spatial network structure through a bioconjugation method and a metal coordination reaction, and has good antibacterial, antioxidant activity, mechanical properties, water vapor barrier property, and peel adhesion. The modified chitosan coating solution prepared by the present invention can form a uniform film on the surface of the fruit, weaken the respiration, prevent and inhibit postharvest diseases, reduce the weight loss rate and decay rate, maintain the fruit brightness, and extend the shelf life of the fruit.

[0016] The preparation and application of the modified chitosan coating solution of the present invention are both simple and easy to operate, and are suitable for popularization. Description of the Drawings

[0017] Figure 1 Appearance diagrams of modified chitosan films with different Ca 2+ addition amounts (Example 1).

[0018] Figure 2 Fourier transform infrared spectra diagrams of modified chitosan films with different Ca 2+ addition amounts (Example 2).

[0019] Figure 3 Elongation at break diagrams and tensile strength diagrams of modified chitosan films (Example 3).

[0020] Figure 4 Water vapor transmission rate diagrams of modified chitosan films (Example 4).

[0021] Figure 5 Light transmittance test diagrams of modified chitosan films (Example 5).

[0022] Figure 6 DPPH free radical scavenging rate diagrams of modified chitosan coating solution (Example 6).

[0023] Figure 7 It is the bacteriostatic rate graph of the modified chitosan coating solution (Example 7).

[0024] Figure 8 It is the peel adhesion graph of the modified chitosan coating solution (Example 8).

[0025] Figure 9 It is the appearance change graph of blueberry fruits during normal temperature storage after being treated with the modified chitosan coating solution (Example 9).

[0026] Figure 10 It is the weight loss rate graph of blueberry fruits after being treated with the modified chitosan coating solution (Example 9).

[0027] Figure 11 It is the shelf life graph of blueberry fruits after being treated with the modified chitosan coating solution (Example 9).

[0028] Figure 12 It is the appearance change graph of mango fruits during normal temperature storage after being treated with the modified chitosan coating solution (Example 10). Detailed implementation manners

[0029] The technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments and the accompanying drawings, but the embodiments and the protection scope of the present invention are not limited thereto. Unless otherwise specified, the raw materials in the embodiments of the present invention are all purchased through commercial channels.

[0030] The chitosan involved in the following examples and comparative examples was purchased from Aladdin Biochemical Technology Co., Ltd., with a degree of deacetylation ≥ 95% and a viscosity of 100 - 200 mPa·s.

[0031] The diameter of the disposable plastic petri dishes involved in the following examples and comparative examples is 90 mm.

[0032] Example 1

[0033] (1) Prepare a 60 v / v% ethanol aqueous solution, add gallic acid thereto (final molar concentration is 6.2 mmol / L), and after it is completely dissolved, sequentially add 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC hydrochloride, final molar concentration is 6.2 mmol / L) and N-hydroxysuccinimide (NHS, final molar concentration is 6.2 mmol / L), mix evenly, and carry out an activation reaction on the mixed solution under ice bath conditions for 2 h. After the activation reaction is completed, a gallic acid grafting activator is obtained.

[0034] (2)Prepare a 2 m / v% chitosan - 1 v / v% acetic acid aqueous solution, add an equal volume of the gallic acid grafting activator described in step (1) thereto, mix evenly, let the mixture stand and react for 24 h under dark conditions. After the reaction, load the reaction solution into a 14 kDa dialysis bag and dialyze for 72 h (the dialysis solution in the first 48 h is acidic pure water with a pH of 4.7, and the dialysis solution in the last 24 h is neutral pure water with a pH of 7.0), change the water every 8 h, and then lyophilize the dialyzed reaction solution to obtain a freeze - dried sample of gallic acid - modified chitosan.

[0035] (3)Prepare a 0.5 v / v% acetic acid aqueous solution, add 1 m / v% of the freeze - dried sample of gallic acid - modified chitosan described in step (2) thereto. After complete dissolution, add anhydrous calcium chloride powder with final concentrations of 0.05 m / v%, 0.1 m / v%, 0.3 m / v%, and 0.5 m / v% to the mixture respectively, and let it stand and react for 6 h to obtain chitosan - gallic acid / Ca 2+ coating solutions with different Ca 2+ concentrations. And set the treatment without adding anhydrous calcium chloride powder as the control group to obtain chitosan - gallic acid coating solution.

[0036] Take 10 mL of chitosan - gallic acid / Ca 2+ coating solutions with different Ca 2+ concentrations and chitosan - gallic acid coating solution respectively, pour them into disposable plastic petri dishes, and dry them at 35 °C for 10 h to obtain modified chitosan films (denoted as CSGA + 0.05%Ca 2+ , CSGA + 0.1%Ca 2+ , CSGA + 0.3%Ca 2+ , CSGA + 0.5%Ca 2+ , CSGA respectively). Cut the obtained films into squares and place them on white paper to take pictures. The results are as Figure 1 shown. Ca 2+ can promote the network cross - linking degree of the film (copolymer), but excessive Ca 2+ causes excessive oxidation of phenolic hydroxyl groups to form quinone compounds, which instead reduces the cross - linking degree and deepens the appearance color of the film. Therefore, Ca 2+ concentrations of 0.3 m / v% and above will affect the film appearance and are not suitable for fruit coating and fresh - keeping applications.

[0037] Example 2

[0038] (1) Prepare a 60 v / v% ethanol aqueous solution, add gallic acid to it (final molar concentration is 6.2 mmol / L), and after it is completely dissolved, add 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC hydrochloride, final molar concentration is 6.2 mmol / L) and N-hydroxysuccinimide (NHS, final molar concentration is 6.2 mmol / L) in sequence, mix evenly, and activate the reaction of the mixed solution under ice bath conditions for 2 h. After the activation reaction ends, a gallic acid grafted activator is obtained.

[0039] (2) Prepare a 2 m / v% chitosan - 1 v / v% acetic acid aqueous solution, add an equal volume of the gallic acid grafted activator described in step (1) to it, mix evenly, and let the mixed solution stand and react for 24 h under light - shielding conditions. After the reaction ends, load the reaction solution into a 14 kDa dialysis bag and dialyze for 72 h (the dialysis solution in the first 48 h is acidic pure water with a pH of 4.7, and the dialysis solution in the last 24 h is neutral pure water with a pH of 7.0), change the water every 8 h, and then freeze - dry the dialyzed reaction solution to obtain a freeze - dried sample of gallic acid - modified chitosan.

[0040] (3) Prepare a 0.5 v / v% acetic acid aqueous solution, add 1 m / v% of the freeze - dried sample of gallic acid - modified chitosan described in step (2) to it. After it is completely dissolved, add anhydrous calcium chloride powder with final concentrations of 0.05 m / v%, 0.1 m / v%, 0.3 m / v%, and 0.5 m / v% to the mixed solution respectively, and let it stand and react for 6 h to obtain chitosan - gallic acid / Ca 2+ coating solutions with different Ca 2+ concentrations. And set the treatment without adding anhydrous calcium chloride powder as the control group to obtain a chitosan - gallic acid coating solution.

[0041] Take 10 mL of chitosan - gallic acid / Ca 2+ coating solutions with different Ca 2+ concentrations and the chitosan - gallic acid coating solution, pour them into disposable plastic petri dishes respectively, and dry them at 35 °C for 10 h to obtain modified chitosan films (denoted as CSGA + 0.05%Ca 2+ , CSGA + 0.1%Ca 2+ [[ID=2l]], CSGA + 0.3%Ca 2+ , CSGA + 0.5%Ca 2+ , CSGA respectively). And take 10 mL of 1 m / v% chitosan - 0.5 v / v acetic acid aqueous solution and pour it into a disposable plastic petri dish, dry it at 35 °C for 10 h to obtain a chitosan film (CS). Use a Fourier transform infrared spectrometer (FT - IR) to measure the infrared spectra of each film by the attenuated total reflection (ATR) method, and the scanning range is 400 cm -1To 4000 cm -1 . The results are as Figure 2 shown. The results indicate that under the action of the bioconjugate, gallic acid was successfully grafted onto the chitosan carbon chain, and Ca 2+ can form coordination bonds with the phenolic hydroxyl groups of gallic acid, forming more hydrogen bonds within the molecule and strengthening the tightness of the molecular network structure. As the concentration of Ca 2+ increases, the peak near 3300 cm -1 of the film shifts towards a higher waveband, indicating that more hydrogen bonds are formed inside the film. At the same time, as the concentration of Ca 2+ increases, the peak intensity of the film at 1652 cm -1 also increases, indicating that there are more carbonyl groups inside the film. This is because the phenolic hydroxyl groups of gallic acid in the film are oxidized to form quinone compounds. Finally, 0.1 m / v% Ca 2+ is selected as the optimal Ca 2+ addition amount.

[0042] Example 3

[0043] (1) Prepare a 60 v / v% ethanol aqueous solution, add gallic acid (final molar concentration of 6.2 mmol / L) to it. After it is completely dissolved, sequentially add 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC hydrochloride, final molar concentration of 6.2 mmol / L) and N-hydroxysuccinimide (NHS, final molar concentration of 6.2 mmol / L), mix evenly, and activate the reaction for 2 h under ice bath conditions. After the activation reaction ends, a gallic acid grafted activator is obtained.

[0044] (2) Prepare a 2 m / v% chitosan - 1 v / v% acetic acid aqueous solution, add an equal volume of the gallic acid grafted activator described in step (1) to it, mix evenly, and let the mixture stand and react for 24 h in the dark. After the reaction ends, put the mixture into a 14 kDa dialysis bag and dialyze for 72 h (the dialysis solution in the first 48 h is acidic pure water with a pH of 4.7, and the dialysis solution in the last 24 h is neutral pure water with a pH of 7.0), change the water every 8 h, and then freeze-dry the dialyzed reaction solution to obtain a freeze-dried sample of gallic acid modified chitosan.

[0045] (3) Prepare a 0.5 v / v% acetic acid aqueous solution, add 1 m / v% of the freeze-dried sample of gallic acid modified chitosan described in step (2) to it. After it is completely dissolved, add anhydrous calcium chloride powder with a final concentration of 0.1 m / v% to the mixture, let it stand and react for 6 h, and obtain a chitosan-gallic acid / Ca 2+ coating solution. And set the treatment without adding anhydrous calcium chloride powder as the control group to obtain a chitosan-gallic acid coating solution.

[0046] Take 10 mL of chitosan-gallic acid / Ca 2+ coating solution and chitosan-gallic acid coating solution and pour them into disposable plastic petri dishes respectively. Dry them at 35 °C for 10 h to obtain modified chitosan films (denoted as CSGACa and CSGA respectively). Also take 10 mL of 1 m / v% chitosan-0.5 v / v acetic acid aqueous solution and pour it into a disposable plastic petri dish. Dry it at 35 °C for 10 h to obtain a chitosan film (CS). Analyze the mechanical properties of each film using a physical property analyzer. First, equilibrate the film samples at 23 °C for 48 h, then cut the film samples into rectangles of 60 mm×10 mm. Fix the rectangular film samples flatly on the fixture with an initial distance of 40 mm, and the stretching rate is 50 mm·min -1 , and calculate the elongation at break and tensile strength of the film based on the data, and conduct three parallel determinations. The results are as Figure 3 shown. The elongation at break and tensile strength of the CSGACa film are both significantly greater than those of the CSGA film, indicating that the introduction of Ca 2+ can improve the mechanical properties of the film and reduce the mechanical damage suffered by the coated fruits during logistics transportation.

[0047] Example 4

[0048] Chitosan-gallic acid / Ca 2+ The preparation of the coating solution of chitosan-gallic acid / Ca and the chitosan-gallic acid coating solution is the same as that in Example 3.

[0049] Take 10 mL of chitosan-gallic acid / Ca 2+ coating solution and chitosan-gallic acid coating solution and pour them into disposable plastic petri dishes respectively. Dry them at 35 °C for 10 h to obtain modified chitosan films (denoted as CSGACa and CSGA respectively). Also take 10 mL of 1 m / v% chitosan-0.5 v / v acetic acid aqueous solution and pour it into a disposable plastic petri dish. Dry it at 35 °C for 10 h to obtain a chitosan film (CS). Place the test cup in a drying oven at 102 °C and dry it for 1 h. Wait until the temperature drops to 65 °C and take it out, then put it into a desiccator. Under the environment of 25 °C, put 10 g of anhydrous calcium chloride into the test cup. Select a film with uniform texture, no holes and no wrinkles, cover the film on the cup mouth of the test cup, and then seal it with vaseline and weigh it. Subsequently, put the weighed test cup into a constant temperature and humidity chamber, and let it stand for 24 h under the conditions of a temperature of 25 °C and a humidity of 100%. Measure the weight gain of the test cup within a certain period of time. This test is repeated twice, and three parallel tests are carried out each time. The formula for the water vapor transmission rate is as follows:

[0050] The results are as Figure 4As shown, the water vapor transmission rate of the CSGACa film is significantly lower than that of the CS film and the CSGA film, indicating that Ca 2+ can enhance the degree of network crosslinking of the film, form a denser structure, and reduce the water vapor transmission rate inside and outside the film. This result also shows that the CSGACa film can effectively reduce the water loss of the coated fruits.

[0051] Example 5

[0052] Chitosan-gallic acid / Ca 2+ The preparation of the chitosan-gallic acid / Ca coating solution and the chitosan-gallic acid coating solution is the same as that in Example 3.

[0053] Take 10 mL of the chitosan-gallic acid / Ca 2+ coating solution and the chitosan-gallic acid coating solution and pour them into disposable plastic petri dishes. Dry them at 35 °C for 10 h to obtain modified chitosan films (denoted as CSGACa and CSGA respectively). Also, take 10 mL of 1 m / v% chitosan-0.5 v / v acetic acid aqueous solution and pour it into a disposable plastic petri dish. Dry it at 35 °C for 10 h to obtain a chitosan film (CS). Use a UV-visible spectrophotometer to measure the transmittance of each film. The measurement wavelength range is 200 - 800 nm, with air as the background, and three samples are measured for each treatment. The results are as Figure 5 shown. The CSGA film and the CSGACa film have almost no transmittance in the wavelength range of 200 - 320 nm, indicating that the modified chitosan film has good barrier properties to ultraviolet light. In addition, in the wavelength range of 600 - 800 nm, the transmittance of the CSGA and CSGACa films is greater than 80%, indicating that the modified chitosan film has a high transparency and does not affect the appearance of the coated fruits.

[0054] Example 6

[0055] Chitosan-gallic acid / Ca 2+ The preparation of the chitosan-gallic acid / Ca coating solution and the chitosan-gallic acid coating solution is the same as that in Example 3 (denoted as CSGACa and CSGA respectively, with a concentration of 10 mg / mL). Take 10 mL of 1 m / v% chitosan-0.5 v / v acetic acid aqueous solution (denoted as CS, with a concentration of 10 mg / mL).

[0056] Add 50 μL of different concentration coating solutions (0, 0.5, 1, 2, 4 mg / mL; diluted to the corresponding concentration with 0.5 v / v acetic acid aqueous solution) into a 96-well plate, and then add 200 μL of 0.5 mmol / L DPPH methanol solution. After mixing the two, let it stand for 30 min in the dark at 30 °C, and measure its absorbance at 517 nm using a microplate reader. The calculation formula for the DPPH radical scavenging rate is as follows:

[0057] Wherein: A0 represents the absorbance value measured in the control group experiment (i.e., using neutral pure water with a pH of 7.0 to replace the sample solution); A1 represents the absorbance value measured in the experimental group; A2 represents the absorbance value of the sample itself interfering with the experimental group (i.e., using methanol to replace the DPPH methanol solution).

[0058] The results are as Figure 6 shown. The DPPH radical scavenging rates of both the CSGA coating solution and the CSGACa coating solution are significantly higher than that of the CS coating solution, indicating that the modified chitosan coating solution has better antioxidant activity and can reduce the oxidative browning of fruits.

[0059] Example 7

[0060] Chitosan-gallic acid / Ca 2+ The preparation of the coating solutions of chitosan-gallic acid / Ca and chitosan-gallic acid was the same as in Example 3 (denoted as CSGACa and CSGA respectively, with a concentration of 10 mg / mL). Take 10 mL of 1 m / v% chitosan-0.5 v / v% acetic acid aqueous solution (denoted as CS, with a concentration of 10 mg / mL).

[0061] The antibacterial activity of the coating solution was evaluated using Escherichia coli and Staphylococcus aureus as target pathogenic bacteria. First, a single colony of the pathogenic bacteria was picked with an inoculation loop and inoculated into LB medium (10 g / L peptone, 5 g / L yeast extract, 10 g / L NaCl, pH 7.0), and cultured at 37 °C and 180 rpm for 12 h to obtain a pathogenic bacteria suspension. Subsequently, the pathogenic bacteria suspension was diluted with sterile water to a bacterial concentration of 1×10 6 CFU / mL. Take several clean 150 mL conical flasks, and add 50 mL of the pathogenic bacteria suspension to each. The experimental group was added with 1 mL of the coating solution with a concentration of 10 mg / mL, and the control group was added with an equal volume of 0.5 v / v% acetic acid aqueous solution. All samples were incubated at 37 °C and 180 rpm for 12 h. Finally, the bacterial concentration after treatment with different coating solutions was measured by the dilution plating method, and the results were expressed in CFU / mL. The inhibition rate formula for the pathogenic bacteria is as follows:

[0062] In the formula, A0 represents the bacterial concentration of the control group, and A1 represents the bacterial concentration of the experimental group.

[0063] The results are as Figure 7 shown. The inhibition rates of both the CSGA coating solution and the CSGACa coating solution against Escherichia coli and Staphylococcus aureus are greater than 99%, which are significantly higher than that of the CS coating solution, indicating that the modified chitosan coating solution has good antibacterial activity.

[0064] Example 8

[0065] Chitosan-gallic acid / Ca 2+ The preparation of the coating solution and the chitosan-gallic acid coating solution was the same as that in Example 3 (denoted as CSGACa and CSGA respectively).

[0066] The fresh fruits (blueberries) were soaked in the coating solution for 10 s, and a film was formed on the fruit surface by self-assembly adhesion. Then they were fished out and air-dried naturally.

[0067] The coated blueberries (17.95 ± 0.25 g before coating) were added to 100 mL of neutral pure water with a pH of 7.0 and soaked for a period of time to obtain a soaking solution. The absorbance of the soaking solution at 760 nm was measured by the Folin-Ciocalteu method (i.e., the gallic acid content was expressed by the absorbance at 760 nm), and the uncoated blueberries were used as a blank control (CK). The results are as Figure 8 shown. For the blueberries coated with CSGA, the gallic acid content in the soaking solution reached the maximum value after soaking in pure water for 30 min. In contrast, for the blueberries coated with CSGACa, the gallic acid content in the soaking solution reached the maximum value after soaking in pure water for 90 min, and was higher than the maximum value of the gallic acid content in the soaking solution of the CSGA-coated fruits. This result indicates that the CSGACa coating solution adhered a higher content of gallic acid on the fruit surface and required more time to be completely eluted into pure water, suggesting that the CSGACa coating solution has higher adhesion to blueberry fruits.

[0068] Example 9

[0069] Chitosan-gallic acid / Ca 2+ The preparation of the coating solution was the same as that in Example 3 (denoted as CSGACa). Take 10 mL of 1 m / v% chitosan-0.5 v / v acetic acid aqueous solution (denoted as CS).

[0070] The fresh fruits (blueberries) were soaked in the coating solution for 10 s, and a film was formed on the fruit surface by self-assembly adhesion. Then they were fished out and air-dried naturally, and stored at 25°C and 75% RH. The appearance of the coated blueberries was photographed every 2 d using a smartphone, and the fruit weight loss rate and shelf life were recorded. The uncoated blueberries were used as a blank control (CK). The results are as Figures 9 - 11 shown. Compared with the CK treatment and the CS coating solution treatment, the shrinkage of the blueberry fruits treated with the CSGACa coating solution was slower and the fruit fullness was better. In addition, the CSGACa coating solution treatment could reduce the fruit weight loss rate and extend the shelf life of the fruits. This indicates that the CSGACa coating solution treatment can reduce fruit water loss and shrinkage and oxidative deterioration.

[0071] Example 10

[0072] Chitosan-gallic acid / Ca 2+The preparation of the coating solution was the same as that in Example 3 (denoted as CSGACa). 10 mL of 1 m / v% chitosan-0.5 v / v acetic acid aqueous solution (denoted as CS) was taken.

[0073] Fresh fruits (mangoes) were soaked in the coating solution for 10 s, and a film was formed on the fruit surface by self-assembly adhesion. Then they were taken out, air-dried naturally, and stored at 25 °C and 90% RH. The appearance of the coated mangoes was photographed every 2 days using a smartphone. Uncoated mangoes were used as the blank control (CK). The results are as Figure 12 shown. Treatment with the CSGACa coating solution can reduce the incidence of lesions on mango fruits, keep the pulp plump, and reduce the degree of deterioration of the pulp texture.

Claims

1. A preparation method of a modified chitosan coating solution for fruit preservation, characterized in that, It includes the following steps: (1) Prepare an ethanol aqueous solution, add gallic acid to it, and after it is completely dissolved, successively add 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide, mix evenly, and carry out an activation reaction on the mixed solution under ice bath conditions. After the activation reaction is completed, a gallic acid grafted activator is obtained; (2) Prepare a chitosan-acetic acid aqueous solution, add the gallic acid grafted activator described in step (1) to it, mix evenly, allow the mixed solution to stand and react in the dark. After the reaction is completed, dialyze the reaction solution, and then freeze-dry the dialyzed reaction solution to obtain a freeze-dried sample of gallic acid-modified chitosan; (3) Prepare an aqueous acetic acid solution, add the freeze-dried sample of gallic acid-modified chitosan described in step (2) thereto, and after complete dissolution, add anhydrous calcium chloride powder to the mixture, and let it stand for reaction to obtain chitosan-gallic acid / Ca 2+ coating solution.

2. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 1, characterized in that, In step (1), the ethanol aqueous solution is a 50-70 v / v% ethanol aqueous solution; the final molar concentration of the added gallic acid is 6.0-7.0 mmol / L, the final molar concentration of the added 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride is 6.0-7.0 mmol / L, and the final molar concentration of the added N-hydroxysuccinimide is 6.0-7.0 mmol / L; the activation reaction time is 1-3 h.

3. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 2, characterized in that, In step (1), the ethanol aqueous solution is a 60 v / v% ethanol aqueous solution; the final molar concentrations of the added gallic acid, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride, and N-hydroxysuccinimide are all 6.2 mmol / L; the activation reaction time is 2 h.

4. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 1, characterized in that, In step (2), the chitosan-acetic acid aqueous solution is a 1-2 m / v% chitosan - 0.5-1 v / v% acetic acid aqueous solution; the volume ratio of the chitosan-acetic acid aqueous solution to the gallic acid grafted activator is 1:1-2; the standing reaction time is 18-30 h.

5. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 4, characterized in that, In step (2), the chitosan-acetic acid aqueous solution is a 2 m / v% chitosan - 1 v / v% acetic acid aqueous solution; the volume ratio of the chitosan-acetic acid aqueous solution to the gallic acid grafted activator is 1:1; the standing reaction time is 24 h.

6. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 4 or 5, characterized in that, In step (2), the specific dialysis operation is as follows: load the reaction solution into a 14 kDa dialysis bag and dialyze for 72 h. The dialysis solution in the first 48 h is acidic pure water with a pH of 4.7±0.5, and the dialysis solution in the last 24 h is pure water with a pH of 7.0±0.

5. Change the water every 8 h.

7. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 1, characterized in that, In step (3), the acetic acid aqueous solution is a 0.2-0.5 v / v% acetic acid aqueous solution; the addition ratio of the freeze-dried sample of gallic acid-modified chitosan is 0.5-2 m / v%; the addition ratio of the anhydrous calcium chloride powder is 0.05-0.2 m / v%; the standing reaction time is 6-12 h.

8. The preparation method of a modified chitosan coating solution for fruit preservation according to claim 7, characterized in that, In step (3), the acetic acid aqueous solution is a 0.5 v / v% acetic acid aqueous solution; the addition ratio of the freeze-dried sample of gallic acid-modified chitosan is 1 m / v%; the addition ratio of the anhydrous calcium chloride powder is 0.1 m / v%; the standing reaction time is 6 h.

9. Application of the modified chitosan coating solution for fruit preservation prepared according to any one of claims 1-8 in fruit preservation.

10. The application according to claim 9, wherein When in application, soak the fruits in the modified chitosan coating solution for 10 - 30 s, take them out, and air-dry or naturally dry them; or evenly spray the modified chitosan coating solution on the surfaces of the fruits and then air-dry or naturally dry them.

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