Preparation method of active coating liquid loaded with zanthoxylum bungeanum essential oil and application thereof

Abstract

The application provides a preparation method of a zanthoxylum bungeanum essential oil active coating liquid and application thereof, and relates to the field of fruit and vegetable preservation technology. The preparation method of the zanthoxylum bungeanum essential oil active coating liquid is as follows: 3.5% zein nanoparticles are prepared; 3.5% citrus pectin solution is prepared; the 3.5% zein nanoparticles are dropped into the citrus pectin solution to obtain zein-citrus pectin Pickering emulsion; zanthoxylum bungeanum essential oil and 3.5% zein-citrus pectin Pickering emulsion are mixed to obtain zanthoxylum bungeanum essential oil Pickering emulsion; carboxymethyl cellulose sodium is dissolved in deionized water; glycerol is added; the zanthoxylum bungeanum essential oil Pickering emulsion is added under room temperature, and standing and defoaming are conducted, so that the zanthoxylum bungeanum essential oil active coating liquid is obtained. The application is favorable for inhibiting the spoilage bacteria of fresh red peppers, delaying the aging time of the peppers, and prolonging the preservation period of the peppers.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of fruit and vegetable preservation, and particularly relates to a preparation method of a coating liquid loaded with Zanthoxylum bungeanum essential oil and application thereof. BACKGROUND

[0002] Zanthoxylum bungeanum essential oil (ZBEO), also known as Zanthoxylum bungeanum essential oil, is an oily mixture extracted from the pericarp of Zanthoxylum bungeanum, has a strong fragrance and strong volatility, and is mostly yellow. ZBEO has many functional activities, such as broad-spectrum antibacterial activity, antioxidant activity, analgesic activity and insecticidal activity, and is commonly used in food processing, cosmetic development, pharmaceutical production and other fields. However, the poor stability and poor hydrophilicity of ZBEO also limit its application range when it is directly added to the preservation system. Therefore, how to improve the application defects of ZBEO, enhance the uniform distribution effect of ZBEO in a hydrophilic environment and broaden the application scenarios of ZBEO is the focus of the current application research of ZBEO. In this case, various hydrophilic embedding methods are gradually applied to the improvement of the application defects of ZBEO, and Pickering emulsion is one of the widely used hydrophilic embedding methods.

[0003] The natural green and biodegradable advantages of the food-grade Pickering solid particles also provide greater application safety advantages for the prepared emulsion. Zein is abundant in resources and has low cost. Zein nanoparticles (ZNPS) can be prepared by anti-solvent precipitation method and are commonly used in the preparation of Pickering emulsion. However, ZNPS itself is too hydrophobic and can easily aggregate and precipitate in solution, and the emulsion cannot be stable for a long time. Therefore, in actual application, ZNPS is not used alone, but is compounded with other biological macromolecules to prepare nanoparticles, which are then used in the preparation of Pickering emulsion, which not only significantly improves the stability of the emulsion, but also expands the functional characteristics of the emulsion. Citrus pectin (CP) is a negatively charged biological polysaccharide, and Zein is positively charged, so the two can be combined into composite nanoparticles through electrostatic adsorption and hydrophobic interaction to improve the hydrophilicity of Zein. In recent years, many studies on the use of pectin from different sources and ZNPS to stabilize Pickering emulsion have proved the ability of Zein-Citrus pectin composite nanoparticles (ZCPS) to stabilize Pickering emulsion and the great development potential.

[0004] Coating film preservation technology refers to a food preservation technology that uses natural biodegradable macromolecular substances as main components to prepare a coating film liquid, and uses brushing, dipping, spraying or layer-by-layer self-assembly to solidify the coating film liquid on the surface of food to form a dense film, so as to isolate the food from the external environment and inhibit the growth and reproduction of microorganisms, thereby protecting the food. The film formed by the coating film preservation technology not only serves as food packaging, but also serves as a carrier of biologically active ingredients such as nutrients, antioxidants and antibacterial agents, so as to improve the quality of food and prolong the shelf life. SUMMARY

[0005] In view of the above technical problems, the purpose of the present application is to provide a preparation method of a zanthoxylum bungeanum essential oil-loaded active coating film liquid and application thereof in pepper preservation. By preparing the zanthoxylum bungeanum essential oil-loaded active coating film liquid, the pepper is soaked in the coating film liquid and then taken out and air-dried, so as to delay the aging of the pepper, improve the preservation effect and maintain the storage quality of the pepper.

[0006] Technical scheme: A preparation method of a zanthoxylum bungeanum essential oil-loaded active coating film liquid, comprising the following steps:

[0007] S1: completely dissolving zein in 85% ethanol, removing precipitates by centrifugation to obtain supernatant, and adding the supernatant drop by drop into distilled water under magnetic stirring, and then performing ultrasonic treatment and concentrating by a rotary evaporator to obtain 3.5% zein nanoparticles;

[0008] S2: adding citrus pectin into distilled water, stirring overnight at 25°C until complete hydration to obtain a 3.5% citrus pectin solution;

[0009] S3: using electrostatic adsorption, adding the 3.5% zein nanoparticles into the citrus pectin solution under magnetic stirring to obtain a zein-citrus pectin Pickering emulsion;

[0010] S4: mixing zanthoxylum bungeanum essential oil and 3.5% zein-citrus pectin Pickering emulsion, and treating with a high-speed disperser at 14000 rpm for 4 min to obtain zanthoxylum bungeanum essential oil-loaded Pickering emulsion;

[0011] S5: adding carboxymethyl cellulose sodium into deionized water, and dissolving by constant-temperature water bath stirring to form a uniform and transparent film liquid; and adding glycerol and continuing to stir for 1 h;

[0012] S6: adding 30-50% zanthoxylum bungeanum essential oil-loaded Pickering emulsion under room temperature conditions, and stirring at a speed of 800 rpm for 1 h by magnetic stirring, and then standing to remove bubbles, to obtain a zanthoxylum bungeanum essential oil-loaded active coating film liquid.

[0013] Further, in the step S1, the centrifugation speed is 5000 g, and the centrifugation time is 10 min.

[0014] Furthermore, in step S1, the ultrasonic power is 600 W and the time is 10 min.

[0015] Furthermore, in step S3, the volume ratio of 3.5% zein nanoparticles to 3.5% citrus pectin solution is 1:(2-4).

[0016] Furthermore, in step S4, the volume ratio of Sichuan pepper essential oil to 3.5% zein-citrus pectin pickering emulsion is 1:(8-10).

[0017] Furthermore, in step S5, the mass-to-volume ratio of sodium carboxymethyl cellulose / deionized water and glycerol is 6g:400mL:3g.

[0018] Application of the ZCZPE / CMC composite coating liquid prepared by any of the above preparation methods in the preservation of fresh red peppers. Beneficial effects

[0019] The ZCZPE / CMC composite coating liquid prepared by this invention has a stable system and good antibacterial effect. When applied to the preservation of chili peppers, it can reduce the quality loss of fresh red chili peppers during storage, inhibit the growth of spoilage bacteria, delay the post-harvest aging of chili peppers, maintain the appearance and hardness of chili peppers, extend the shelf life of chili peppers, and improve their storage resistance. Attached Figure Description

[0020] Figure 1 Figure 1 shows the physicochemical properties of the ZCZPE / CMC composite coating solutions prepared in Examples 1-3 and Comparative Examples 1-2. Figure A shows the pH value of the ZCZPE / CMC composite coating solution; Figure B shows the particle size distribution of the ZCZPE / CMC composite coating solution; Figure C shows the zeta potential of the ZCZPE / CMC composite coating solution. Different letters (ac) in the figures indicate significant differences between different treatment groups. p<0.05 );

[0021] Figure 2 The figure shows the retention rate of Sichuan pepper essential oil in the ZCZPE / CMC composite coating solutions prepared in Examples 1-3 and Comparative Examples 1-2; different letters (AE) in the figure indicate significant differences between different treatment groups. p<0.05 );

[0022] Figure 3 Figure 1 shows the rheological properties of the ZCZPE / CMC composite coating solutions prepared in Examples 1-3 and Comparative Examples 1-2. Figure A shows the apparent viscosity of the ZCZPE / CMC composite coating solution; Figure B shows the loss tangent of the ZCZPE / CMC composite coating solution; Figure C shows the storage modulus of the ZCZPE / CMC composite coating solution; and Figure D shows the loss modulus of the ZCZPE / CMC composite coating solution.

[0023] Figure 4 DPPH· and ABTS of ZCZPE / CMC composite coating liquid prepared for Examples 1-3 and Comparative Examples 1-2 + · clearance rate; different letters (a-d, A-E) in the figure represent significant differences between different treatment groups p<0.05

[0024] Figure 5 Bacteriostatic rate of ZCZPE / CMC composite coating liquid prepared for Examples 1-3 and Comparative Examples 1-2, wherein Figure A is the inhibition rate of CZPE / CMC composite coating liquid on Escherichia coli; Figure B is the inhibition rate of CZPE / CMC composite coating liquid on Staphylococcus aureus; different letters (a-e) in the figure represent significant differences between different treatment groups p<0.05

[0025] Figure 6 Figure for the change of weight loss rate of fresh red pepper during storage after preservation in different ways;

[0026] Figure 7 Figure for the change of rot index of fresh red pepper during storage after preservation in different ways;

[0027] Figure 8 Figure for the change of appearance of fresh red pepper during storage after preservation in different ways;

[0028] Figure 9 Figure for the change of color of fresh red pepper during storage after preservation in different ways, wherein Figure A is the change figure of L* value of fresh red pepper during storage after preservation in different ways; Figure B is the a* value of fresh red pepper during storage after preservation in different ways; Figure C is the b* value of fresh red pepper during storage after preservation in different ways; Figure D is the ΔE value of fresh red pepper during storage after preservation in different ways;

[0029] Figure 10 Figure for the change of hardness of fresh red pepper during storage after preservation in different ways;

[0030] Figure 11 Figure for the change of soluble solid content of fresh red pepper during storage after preservation in different ways;

[0031] Figure 12 Figure for the change of malondialdehyde content of fresh red pepper during storage after preservation in different ways;

[0032] Figure 13 ​​Figure of the change of polyphenol oxidase activity after the fresh red pepper is preserved in different ways during storage;

[0033] Figure 14 Figure of the change of peroxidase activity after the fresh red pepper is preserved in different ways during storage. DETAILED DESCRIPTION

[0034] The application will be further described in conjunction with the accompanying drawings and examples. The following examples are an explanation of the application and the application is not limited to the following examples: Example 1

[0035] A preparation method of a ZCZPE / CMC composite coating solution, comprising the following steps:

[0036] S1: completely dissolve 1.75 g of zein in 40 mL of 85% ethanol, centrifuge at 5000 g for 10 min to remove the precipitate to obtain a supernatant; add the supernatant dropwise to 120 mL of distilled water under magnetic stirring, concentrate to 50 mL after ultrasonic treatment at 600 W for 10 min using a rotary evaporator, and obtain 3.5% zein nanoparticles;

[0037] S2: add 3.5 g of citrus pectin to 100 mL of distilled water, and stir overnight at 25 ℃ until completely hydrated to obtain a 3.5% citrus pectin solution;

[0038] S3: use an electrostatic adsorption method, and add 3.5% zein nanoparticles to the citrus pectin solution in a volume ratio of 1:3 under magnetic stirring to obtain a 3.5% zein-citrus pectin Pickering emulsion;

[0039] S4: mix zanthoxylum oil and 3.5% zein-citrus pectin Pickering emulsion in a volume ratio of 1:9, and treat using a high-speed disperser at 14000 rpm for 4 min to obtain ZCZPE;

[0040] S5: take 1.5 g of CMC, add 100 mL of deionized water, and stir to dissolve in a constant-temperature water bath to form a uniform and transparent film solution; add 0.75 g of glycerol and continue to stir for 1 h;

[0041] S6: add 30% ZCZPE under room temperature conditions, and stir at a speed of 800 rpm for 1 h using magnetic stirring, and then stand to remove bubbles to obtain a ZCZPE / CMC composite coating solution. Example 2

[0042] A preparation method of a ZCZPE / CMC composite coating solution, comprising the following steps:

[0043] S1: 1.75 g of zein was completely dissolved in 40 mL of 85% ethanol, and the precipitate was removed by centrifugation at 5000 g for 10 min to obtain a supernatant; the supernatant was added dropwise to 120 mL of distilled water under magnetic stirring, and after ultrasonic treatment at 600 W for 10 min, it was concentrated to 50 mL by rotary evaporation to obtain 3.5% zein nanoparticles;

[0044] S2: 3.5 g of citrus pectin was added to 100 mL of distilled water, and stirred at 25°C overnight to completely hydrate to obtain a 3.5% citrus pectin solution;

[0045] S3: Using the electrostatic adsorption method, 3.5% zein nanoparticles were added dropwise into the citrus pectin solution at a volume ratio of 1:3 under magnetic stirring to obtain a zein-citrus pectin Pickering emulsion;

[0046] S4: ZCZPE was obtained by mixing 3.5% zein-citrus pectin Pickering emulsion and Zanthoxylum bungeanum essential oil at a volume ratio of 1:9, and treating with a high-speed disperser at 14000 rpm for 4 min;

[0047] S5: 1.5 g of CMC was added to 100 mL of deionized water, and stirred to dissolve in a constant temperature water bath to form a uniform and transparent film solution; 0.75 g of glycerol was added and stirred for 1 h;

[0048] S6: 40% ZCZPE was added under room temperature conditions, and stirred at a speed of 800 rpm for 1 h, and then left to stand to remove bubbles, to obtain a ZCZPE / CMC composite coating solution.

[0049] Example 3

[0050] A preparation method of a ZCZPE / CMC composite coating solution, comprising the following steps:

[0051] S1: 1.75 g of zein was completely dissolved in 40 mL of 85% ethanol, and the precipitate was removed by centrifugation at 5000 g for 10 min to obtain a supernatant; the supernatant was added dropwise to 120 mL of distilled water under magnetic stirring, and after ultrasonic treatment at 600 W for 10 min, it was concentrated to 50 mL by rotary evaporation to obtain 3.5% zein nanoparticles;

[0052] S2: 3.5 g of citrus pectin was added to 100 mL of distilled water, and stirred at 25°C overnight to completely hydrate to obtain a 3.5% citrus pectin solution;

[0053] S3: 3.5% zein nanoparticles were added dropwise into the citrus pectin solution at a volume ratio of 1:3 under magnetic stirring by using electrostatic adsorption method to obtain a zein-citrus pectin Pickering emulsion;

[0054] S4: Zanthoxylum bungeanum oil and 3.5% zein-citrus pectin Pickering emulsion were mixed at a volume ratio of 1:9, and treated at 14000 rpm for 4 min by using a high-speed disperser to obtain ZCZPE;

[0055] S5: 1.5 g of CMC was added to 100 mL of deionized water, and dissolved by constant temperature water bath stirring to form a uniform and transparent film solution; 0.75 g of glycerol was added and continuously stirred for 1 h;

[0056] S6: 50% ZCZPE was added at room temperature, and stirred at a speed of 800 rpm for 1 h by magnetic stirring, and then defoamed by standing to obtain a ZCZPE / CMC composite coating solution. Comparative Example 1

[0057] A preparation method of a ZCZPE / CMC composite coating solution, comprising the following steps:

[0058] S1: 1.75 g of zein was completely dissolved in 40 mL of 85% ethanol, and the precipitate was removed by centrifugation at 5000 g for 10 min to obtain a supernatant; the supernatant was added dropwise to 120 mL of distilled water under magnetic stirring, and then concentrated to 50 mL by using a rotary evaporator after ultrasonic treatment at 600 W for 10 min to obtain 3.5% zein nanoparticles;

[0059] S2: 3.5 g of citrus pectin was added to 100 mL of distilled water, and stirred at 25°C overnight until completely hydrated to obtain a 3.5% citrus pectin solution;

[0060] S3: 3.5% zein nanoparticles were added dropwise into the citrus pectin solution at a volume ratio of 1:3 under magnetic stirring by using electrostatic adsorption method to obtain a zein-citrus pectin Pickering emulsion;

[0061] S4: Zanthoxylum bungeanum oil and 3.5% zein-citrus pectin Pickering emulsion were mixed at a volume ratio of 1:9, and treated at 14000 rpm for 4 min by using a high-speed disperser to obtain ZCZPE;

[0062] S5: 1.5 g of CMC was added to 100 mL of deionized water, and dissolved by constant temperature water bath stirring to form a uniform and transparent film solution; 0.75 g of glycerol was added and continuously stirred for 1 h;

[0063] S6: 20% ZCZPE was added under room temperature, and magnetic stirring was carried out at a speed of 800 rpm for 1 h, and then the ZCZPE / CMC composite coating solution was obtained after standing and defoaming. Comparative Example 2

[0064] A preparation method of a ZCZPE / CMC composite coating solution, comprising the following steps:

[0065] S1: 1.75 g of zein was completely dissolved in 40 mL of 85% ethanol, and the supernatant was obtained by centrifugation at 5000 g for 10 min to remove the precipitate; the supernatant was added dropwise to 120 mL of distilled water under magnetic stirring, and then concentrated to 50 mL by a rotary evaporator after ultrasonic treatment at 600 W for 10 min, to obtain 3.5% zein nanoparticles;

[0066] S2: 3.5 g of citrus pectin was added to 100 mL of distilled water, and stirred at 25°C overnight to obtain a 3.5% citrus pectin solution;

[0067] S3: The 3.5% zein nanoparticles were added dropwise into the citrus pectin solution at a volume ratio of 1:3 under magnetic stirring by using the electrostatic adsorption method to obtain a zein-citrus pectin Pickering emulsion;

[0068] S4: ZCZPE was obtained by mixing the 3.5% zein-citrus pectin Pickering emulsion and the pepper essential oil at a volume ratio of 1:9, and then treating with a high-speed disperser at 14000 rpm for 4 min;

[0069] S5: 1.5 g of CMC was added to 100 mL of deionized water, and then dissolved by constant temperature water bath stirring to form a uniform and transparent film solution; 0.75 g of glycerol was added and stirred for 1 h;

[0070] S6: 20% ZCZPE was added under room temperature, and magnetic stirring was carried out at a speed of 800 rpm for 1 h, and then the ZCZPE / CMC composite coating solution was obtained after standing and defoaming.

[0071] Performance test

[0072] pH value determination of ZCZPE / CMC composite coating solution

[0073] The pH values of the ZCZPE / CMC composite coating solutions prepared in the above examples and comparative examples were determined at room temperature, and the average value of three determinations was taken as the experimental result.

[0074] As Figure 1(A) It can be found that the pH value of ZCZPE / CMC composite coating solution decreases with the increasing of ZCZPE addition ratio. When the ZCZPE addition ratio reaches 50%, the pH value of ZCZPE / CMC composite coating solution decreases to 4.45.

[0075] Particle size and zeta potential of ZCZPE / CMC composite coating solution

[0076] The ZCZPE / CMC composite coating solution prepared in the above examples and comparative examples was diluted to different concentrations at room temperature according to the requirements of instrumental analysis, the average particle size was determined by using a laser particle size analyzer, the pump speed was set to 60%, and 1 min was waited before each determination.

[0077] The zeta potential of the ZCZPE / CMC composite coating solution prepared in the above examples and comparative examples was determined after the emulsion was diluted to an appropriate concentration (attenuation value = 7) with deionized water.

[0078] By Figure 1 (B) It can be found that the particle size of ZCZPE / CMC composite coating solution with different ZCZPE addition ratios generally presents a trend of first increasing and then decreasing with the increasing of ZCZPE addition ratio. When the ZCZPE addition ratio is 10%, the pH value of ZCZPE / CMC composite coating solution is the highest, at this time the ZCZPE system is unstable, but the ZCZPE / CMC composite coating solution exhibits a smaller particle size, the reason may be that the high concentration of CMC matrix solution plays a role in stability in the whole ZCZPE / CMC composite coating solution system, providing strong steric hindrance effect for ZCZPE, limiting the movement of ZCZPE droplets in the ZCZPE / CMC composite coating solution system, so that the particle size of the whole system is smaller. With the increasing of ZCZPE addition ratio, the proportion of corresponding CMC matrix solution in the composite coating solution system decreases, and the steric hindrance effect provided by it decreases, while the pH value of ZCZPE / CMC composite coating solution still greatly destroys the stability of ZCZPE, so when the ZCZPE addition concentration increases to 20%, the particle size of ZCZPE / CMC composite coating solution system increases sharply. With the continuous increase of ZCZPE addition ratio, the pH value of ZCZPE / CMC composite coating solution gradually decreases, which continuously improves the stability of ZCZPE in the ZCZPE / CMC composite coating solution system, and the particle size of ZCZPE / CMC composite coating solution gradually decreases. When the ZCZPE addition ratio increases to 50%, the pH value of ZCZPE / CMC composite coating solution reaches the minimum, which is most conducive to the stability of ZCZPE in the ZCZPE / CMC composite coating solution system, and the particle size of ZCZPE / CMC composite coating solution reaches the minimum of 3.3 μm.

[0079] From Figure 1(C) It can be found that with the increase of the ZCZPE addition ratio and the decrease of the CMC coating matrix ratio in the ZCZPE / CMC composite coating liquid system, the absolute value of the zeta potential of the composite coating liquid is continuously reduced, but it is always greater than 30 mV. When the absolute value of the zeta potential of the system is greater than 30 mV, the whole system can be considered as a stable system.

[0080] ZCZPE / CMC composite coating liquid

[0081] First, 10 mL of the ZCZPE / CMC composite coating liquid prepared in the above examples and comparative examples was injected into a disposable plastic culture dish (d = 90 mm), and was air-dried at room temperature for 12 h to obtain a film. One of the different films was placed in a 100 mL conical flask, 30 mL of n-hexane was added, and a magnetic stirrer was used to stir vigorously at room temperature overnight. Then the solution was centrifuged at 11000 rpm for 20 min by a high-speed refrigerated centrifuge, the supernatant was diluted to an appropriate concentration, and the absorbance value was measured at 268 nm by a UV spectrophotometer to calculate the content of the ZCZPE in the film. Similarly, the content of the ZCZPE in the ZCZPE / CMC composite coating liquid with different ZCZPE addition ratios was measured by a UV spectrophotometer. The retention rate of the ZCZPE in the ZCZPE / CMC composite coating liquid prepared in the above examples and comparative examples was the ratio of the content of the ZCZPE in the film to the content of the ZCZPE in the composite coating liquid, and each sample was repeated three times.

[0082] Figure 2The results show that with the increase of the ZCZPE addition ratio, the retention rate of the pepper essential oil in the film presents a trend of first decreasing and then increasing. The retention rate of the pepper essential oil in the film is changed by the comprehensive regulation of the pH value, particle size and zeta potential in the ZCZPE / CMC composite coating film liquid system. When the ZCZPE addition ratio is small, the pH value of the ZCZPE / CMC composite coating film liquid system is not conducive to the stability of the ZCZPE, but the high concentration of the CMC coating matrix provides a higher steric hindrance, so that the ZCZPE remains basically stable, and therefore the retention rate of the pepper essential oil is high. When the ZCZPE addition ratio is further increased to 20%, the stability of the system has decreased significantly, but the ZCZPE addition ratio is low at this time. Therefore, although the retention rate of the pepper essential oil in the film decreases, it still maintains a high value. With the further increase of the ZCZPE retention rate, the high proportion of unstable ZCZPE causes the further decrease of the retention rate of the pepper essential oil in the film. However, when the ZCZPE addition ratio increases to 50%, the retention rate of the pepper essential oil in the film actually increases, which is caused by the fact that the pH value of the ZCZPE / CMC composite coating film liquid system is closer to the pH interval in which the ZCZPE can remain stable.

[0083] Rheological property determination of ZCZPE / CMC composite coating film liquid

[0084] The static and dynamic rheological properties of the emulsions prepared in the above examples and comparative examples were determined using a 40 mm aluminum-plate-parallel-jig fixture. The shear rate was adjusted to 0.01-100 s -1 , and the apparent viscosity of the ZCZPE / CMC composite coating film liquid containing different ZCZPE addition ratios was recorded.

[0085] The angular frequency was adjusted to 0.1-100 rad / s at a fixed strain of 0.1%, and the changes of the storage modulus (G') and the loss modulus (G'') of the emulsions were recorded, and the storage modulus, the loss modulus and the loss tangent angle of the ZCZPE / CMC composite coating film liquid prepared in the above examples and comparative examples were determined.

[0086] From Figure 3 (A), it can be seen that with the increase of the shear rate, the apparent viscosities of the CMC and all the composite coating film liquids continuously decrease, showing obvious shear thinning behavior. It can also be found that with the increase of the ZCZPE addition ratio, the apparent viscosities of the composite coating film liquids also continuously decrease, indicating that the addition of the ZCZPE has destroyed the internal structure of the CMC coating matrix, and the composite coating film liquid is continuously thinned, but the composite coating film liquid is still a pseudoplastic fluid. From Figure 3(C) and (D) can be seen that with the increase of the angular frequency, the G' and G" of the CMC coating matrix and all the composite coating liquids increase continuously. With the increase of the proportion of ZCZPE, the G' and G" of the composite coating liquid basically follow the trend of continuous decrease, which also proves the destruction of the internal network of the CMC coating matrix by the addition of ZCZPE. From Figure 3 (B) It is found that with the increase of the angular frequency, the loss tangent angle continuously decreases and eventually is less than 1, indicating that the internal structure of the ZCZPE / CMC composite coating liquid changes with the increase of the angular frequency.

[0087] Evaluation of the antioxidant capacity of the ZCZPE / CMC composite coating liquid

[0088] The DPPH· clearance rate of the ZCZPE / CMC composite coating liquid prepared in the above examples and comparative examples was determined. The concentration of the used DPPH methanol solution was 100 μmol / L, 400 μL of the sample was mixed with 4 mL of the reaction solution, and the absorbance value was determined at 517 nm after 30 min of reaction at 25 ℃, and the whole process was carried out in the dark. The DPPH· clearance rate of the ZCZPE / CMC composite coating liquid prepared in the above examples and comparative examples was calculated according to the following formula:

[0089] D=(A0-A1) / A0×100%

[0090] In the formula, D(%) is the DPPH· clearance rate; A0 is the absorbance value of the blank group; and A1 is the absorbance value of the experimental group.

[0091] The ABTS + clearance rate of the ZCZPE / CMC composite coating liquid prepared in the above examples and comparative examples was determined. A 7 mmol / L ABTS + solution and a 2.45 mmol / L K2S2O8 solution were prepared, and after mixing, they were reacted in the dark at 25 ℃ for 12 h to obtain an ABTS + mother solution. The ABTS + working solution was obtained by diluting the mother solution until an absorbance value of 0.700±0.020 was obtained at 734 nm. 100 μL of the sample was mixed with 6.0 mL of the ABTS + diluted solution, and the absorbance value was determined at 734 nm after 15 min of reaction at 30 ℃. The ABTS + clearance rate of the ZCZPE / CMC composite coating liquid prepared in the above examples and comparative examples was calculated according to the following formula:

[0092] A=(A0-A1) / A0×100%

[0093] Wherein A (%) is the ABTS+· scavenging rate; A0 is the absorbance value of the blank group; and A1 is the absorbance value of the experimental group.

[0094] From Figure 4 It can be seen that, with the increase of the ZCZPE addition ratio, the ZCZPE / CMC composite coating solution has a significant increase in the scavenging ability of DPPH free radicals and ABTS + free radicals, and there is a positive correlation between them. This is mainly because, with the increase of the ZCZPE addition ratio, the content of the Zanthoxylum bungeanum essential oil with strong free radical scavenging ability in the composite coating solution system is also increasing. When the ZCZPE addition ratio is 50%, the ZCZPE / CMC composite coating solution prepared has a DPPH free radical scavenging rate of 47.38% and an ABTS + free radical scavenging rate of 49.24%, which is 15.7 times and 1.36 times higher than the scavenging ability of the two free radicals of the coating with a ZCZPE addition ratio of 10%, respectively. Therefore, the DPPH free radical is more sensitive to the increase of the ZCZPE addition ratio, that is, the change of the concentration of the Zanthoxylum bungeanum essential oil in the system.

[0095] Evaluation of the antibacterial ability of the ZCZPE / CMC composite coating solution

[0096] The antibacterial circle of the ZCZPE / CMC composite coating solution prepared in the above examples and comparative examples was determined. 200 μL of E. coli or S. aureus diluent was uniformly coated on an LB agar plate. A sterile filter paper sheet with a diameter of 6 mm was placed on the LB agar medium plate, and 20 μL of sample was added to each filter paper sheet. After the plate was cultured at 37 ℃ for 24 hours, the size of the antibacterial circle was observed and the diameter of the antibacterial circle was measured, and the average value of three measurement results was taken as the result.

[0097] The inhibition rate of the ZCZPE / CMC composite coating solution prepared in the above examples and comparative examples was detected by the plate colony counting method. After 2 mL of diluted bacterial solution was mixed with 30 μL of sample and shaken and cultured at 37 ℃ for 2 hours, the obtained bacterial solution was diluted to an appropriate concentration, and 200 μL of it was uniformly coated on an LB agar medium plate. After the plate was cultured at 37 ℃ for 24 hours, the colony count was performed. The inhibition rate of the ZCZPE / CMC composite coating solution prepared in the above examples and comparative examples was calculated according to the following formula:

[0098] I=(A-B) / B×100

[0099] In the formula, I (%) is the inhibition rate; A is the number of colonies in the deionized water control; and B is the number of colonies in the sample.

[0100] The inhibition zone demonstrates the antibacterial effect of the composite coating solution, and the diameter of the inhibition zone directly reflects the antibacterial strength of the composite coating solution. Table 1 shows that the control group CMC coating matrix had an inhibition zone diameter of 0 against *Escherichia coli* and *Staphylococcus aureus*, indicating that CMC itself does not possess antibacterial activity. However, when ZCZPE was added, the diameter of the inhibition zone of the composite coating solution was significantly increased, reaching 8.5 mm for *Escherichia coli* and 6.42 mm for *Staphylococcus aureus*. The main substance contributing to the antibacterial effect was Sichuan pepper essential oil, which also shows that the composite coating solution has different inhibitory effects on the two pathogenic bacteria, with stronger inhibition against *Escherichia coli*. As the proportion of ZCZPE added to the composite coating solution increased, the diameter of the inhibition zone against the two pathogenic bacteria also increased significantly. When the ZCZPE addition ratio reached a maximum of 50%, the diameter of the inhibition zone against both bacteria reached its maximum, at 11.77 mm for Escherichia coli and 9.18 mm for Staphylococcus aureus. To obtain more information on the antibacterial effect of the composite coating solution, further inhibition rate experiments were conducted.

[0101] Table 1. Diameter of inhibition zones of ZCZPE / CMC composite coatings with different ZCZPE addition ratios against Escherichia coli and Staphylococcus aureus.

[0102]

[0103] Note: Different letters (af) indicate significant differences between different treatment groups. p <0.05)

[0104] from Figure 5 It can be seen that as the proportion of ZCZPE added increases, the inhibition rate of the composite coating solution against Escherichia coli and Staphylococcus aureus also increases. It is noteworthy that the inhibition rate of the composite coating solution against Staphylococcus aureus increases significantly with increasing ZCZPE addition ratio. p<0.05 The inhibition rate of the composite coating solution against E. coli reached 90.8%. However, the improvement in the inhibition rate of E. coli ceased to be significant after the ZCZPE addition ratio reached 30%. This is because the composite coating solution has a stronger inhibitory effect on E. coli; when the ZCZPE addition ratio reached 30%, the inhibition rate of the composite coating solution against E. coli reached 95.83%, consistent with the conclusions of the inhibition zone experiment. This strong antibacterial effect will also enable the composite coating solution to achieve better preservation results in practical applications.

[0105] Application of the ZCZPE / CMC composite coating liquid of this invention in the preservation of fresh red peppers:

[0106] A preservation test was conducted using the ZCZPE / CMC composite coating solution prepared in Example 3:

[0107] Select fresh red chilies with uniform color, ripeness, and no surface damage. Wash the chilies in deionized water for 2 minutes to remove dust and impurities from the fruit surface. Then rinse again with deionized water and air dry at room temperature for 1 hour to maintain dryness. Immerse the fresh red chilies in ZCZPE / CMC composite coating solution for 1 minute, then remove and air dry. Pack the fresh red chilies into polypropylene storage boxes and seal with polyethylene plastic wrap to maintain relative humidity. Store the boxes at 15-20°C and 75% humidity. On days 0, 5, 10, 15, and 20, test the following physiological and biochemical indicators of the fresh red chilies: weight loss rate, decay index, color, hardness, soluble solids content, malondialdehyde content, polyphenol oxidase activity, and peroxidase activity. At least three chilies are used for each indicator.

[0108] The blank control group received no treatment and was treated with sterile water.

[0109] Soak fresh red chili peppers in sterile water for 1 minute, then remove the fresh red chili peppers and air dry them.

[0110] The control group (CMC group) used CMC coating solution.

[0111] The preparation method of CMC coating solution includes the following steps: take 1.5g CMC, add 100mL deionized water, stir and dissolve in a constant temperature water bath to form a uniform and transparent film solution; add 0.75g glycerol and continue stirring for 1 h;

[0112] Soak fresh red chili peppers in CMC coating solution for 1 minute, then remove the fresh red chili peppers and air dry them.

[0113] Performance testing:

[0114] (1) Changes in the weight loss rate of red peppers

[0115] The weight of fresh red pepper fruits in each group was weighed on days 0, 5, 10, 15 and 20 respectively, and the weight loss rate was calculated according to the following formula.

[0116] Weight loss rate (%) = (W0 - W) n ) / W0

[0117] In the formula, W0 (g) is the initial weight of fresh red pepper fruit, W n (g) represents the weight of fresh red peppers on day n.

[0118] Figure 6The effects of different treatments on the weight loss rate of fresh red peppers on days 0, 5, 10, 15, and 20 are shown in the figure. As the storage time increases, the weight loss rate of all components continuously increases, which is due to the water loss caused by the transpiration and respiration of the fresh red peppers themselves. For the coating treatment group, the evaporation of water from the coating during storage also has a certain impact on the weight loss rate data. From day 10 onwards, the weight loss rate of the composite coating group was significantly lower than that of the blank control group. The weight loss rate of the CMC coating group was lower than that of the blank coating group, but not significantly. This may be because, in addition to the inhibition of water loss by transpiration in fresh red peppers by the coating structure, the Sichuan pepper essential oil contained in the composite coating group may also indirectly reduce water loss by affecting the physiological activities of the fresh red pepper fruit. Therefore, the ZCZPE / CMC composite coating liquid can reduce the increase in weight loss rate of fresh red peppers during storage, maintaining the storage quality of fresh red peppers.

[0119] (2) Changes in the decay index of red peppers

[0120] Based on the degree of rot on the surface of fresh red chili peppers, they are divided into five rot levels, each corresponding to a different rot score. The grading details and scores are shown in Table 2.

[0121] Table 2 Scoring Criteria for Fruit Rot of Fresh Red Chili Peppers

[0122] Grade score Rating criteria 0 No rot 1 Slight rot, rot area less than 10% 2 Epidermal rot area between 10% and 25% 3 Epidermal rot area between 25% and 50% 4 Epidermal rot area more than 50%

[0123] Fresh red peppers were graded and scored according to the evaluation criteria in Table 2. The formula for calculating the rot index is as follows:

[0124] Rot Index = ∑ (Number of Fruits × Fruit Grade Score) / (Highest Grade Score × Total Number of Fruits)

[0125] Depend on Figure 7 It can be seen that the decay index increases continuously with the extension of storage time. Both the blank control group and the CMC group showed signs of decay on the 5th day of storage. However, during subsequent storage, the decay of fresh red peppers in the CMC group was lower than that in the blank control group. This indicates that the dense structure of the coating on the surface of the fresh red pepper fruit effectively prevented microbial contamination and reduced the decay index. The ZCZPE / CMC composite coating group showed no decay in the early stages of storage, only showing slight decay on the 15th day, and maintained a low decay index throughout the subsequent storage period. The ZCZPE / CMC composite coating group maintained the lowest decay index throughout the entire storage period. This is attributed not only to the dense structure formed on the fruit surface by the composite coating, but also to the important role played by the Sichuan pepper essential oil in the composite coating in inhibiting the bacteria that cause the spoilage of fresh red peppers.

[0126] (3) Color changes of red peppers

[0127] The appearance of red pepper was photographed and recorded on day 0, 5, 10, 15, 20, respectively, and the L*, a*, b* values of the surface of fresh red pepper fruits in each group were measured and recorded by a colorimeter. Multiple points were selected on the surface of the fruits for detection, and the number of points was not less than 3. The total color difference was calculated using the following formula Delta E:

[0128] ΔE=

[0129] Figure 8 To observe the changes in the appearance of fresh red peppers in each treatment group during storage, it can be seen from the figure that on day 0, the surface of fresh red peppers was bright red, with full color and high brightness, and the quality was excellent. After 5 days of storage, the color of the red peppers in the blank group and the CMC group became dull, the tail end was obviously dehydrated, and black spots appeared. By day 10, the overall color of the fruits gradually changed from bright red to dark red, and the longer the storage time, the more obvious the color change. At the same time, during the storage period, the color of the red peppers in the ZCZPE / CMC composite coating liquid group was more full and the brightness was higher than that in the other two groups, and the longer the storage time, the more obvious the difference.

[0130] From Figure 9 (A), it can be seen that during the entire storage process, the three treatments were ranked from high to low in terms of L* value, which was ZCZPE / CMC > CMC > blank control group. The brightness reduction of fresh red peppers in the ZCZPE / CMC composite coating liquid group was significantly smaller than that in the other two groups, which maintained a relatively high L* value.

[0131] From Figure 9 (B) and (C), it can be seen that as the storage time increased, the a* and b* values of the fresh red peppers decreased, and there was no significant difference between the CMC group and the blank control group, while the a* and b* values of the ZCZPE / CMC composite coating liquid group were significantly higher than those of the other two groups, with the smallest change range.

[0132] Delta E value integrates the changes of L*, a* and b* values, and through Delta E value, the overall color change of the surface of fresh red peppers can be more specifically observed. From Figure 9 (D), it can be seen that as the storage time increased, the Delta E value of fresh red peppers increased, while the Delta E value of fresh red peppers in the ZCZPE / CMC composite coating liquid group remained at a relatively low value, which was significantly smaller than that in the other two groups. In summary, the ZCZPE / CMC composite coating liquid significantly reduced the color deterioration of fresh red peppers during storage and maintained the appearance quality.

[0133] (4) Change in hardness of red pepper

[0134] The firmness of fresh red pepper fruits in each group was measured and recorded using a texture analyzer on days 0, 5, 10, 15, and 20. Multiple points were selected on the fruit surface for testing, with no fewer than three points.

[0135] from Figure 10 It was found that the firmness of fresh red peppers decreased continuously with prolonged storage time. The firmness of the fresh red peppers in the CMC group was significantly higher than that in the blank control group, proving the effect of the coating on delaying the decrease in firmness of fresh red peppers. This is because the coating inhibits the respiration rate of the fresh red peppers, thus slowing down the rate of fruit senescence and decay. The Sichuan pepper essential oil contained in the ZCZPE / CMC composite coating liquid not only provides the function of isolating the fruit from external factors, but also inhibits the senescence of fresh red peppers. Figure 10 This also indicates that the ZCZPE / CMC composite coating liquid group had the highest hardness in multiple measurements, and the hardness decreased the least among the three groups during the storage period, which has a significant effect on maintaining the hardness of fresh red peppers and delaying their aging and spoilage.

[0136] (5) Changes in the soluble solids content of red peppers

[0137] At room temperature, the soluble solids content of fresh red pepper fruits in each group was determined using a handheld refractometer on days 0, 5, 10, 15, and 20.

[0138] Depend on Figure 11 It can be seen that the soluble solids content of fresh red peppers in each treatment group showed a trend of first decreasing and then increasing. In the early stage, the fresh red peppers consumed sugars for normal physiological metabolism, thus the soluble solids content decreased. In the later stage of storage, the macromolecules in the fresh red pepper fruit were converted into soluble substances such as sugars. During this period, the accumulation of soluble substances exceeded the consumption of physiological metabolism, thus the soluble solids content increased. In addition, it can be seen from the figure that, regardless of whether it was in the early stage of decreasing soluble solids content or the later stage of increasing content, the change in soluble solids content in the fresh red pepper fruit of the ZCZPE / CMC composite coating liquid group was always smaller than that of the control group. This indicates that the composite coating liquid significantly inhibited the intensity of physiological metabolism of fresh red pepper fruit during postharvest storage, delayed its senescence process, and improved its storage resistance.

[0139] (6) Changes in malondialdehyde content in red chili peppers

[0140] The malondialdehyde (MDA) content of fresh red chili pepper samples was determined at days 0, 5, 10, 15, and 20. First, the extract was prepared according to the experimental procedure and stored at 4 °C. Then, experimental data were obtained and the MDA content was calculated according to the method. The final result is expressed as the MDA content per gram of fresh red chili pepper fruit.

[0141] As shown in Figure 12 , with the extension of storage time, the MDA content in each group of fresh red pepper fruits was accumulated and increased. In the first 10 days of storage, the MDA content in the CMC group and the ZCZPE / CMC composite coating liquid group was significantly lower than that in the blank control group p<0.05 . In 15 and 20 days of storage, the MDA content in the CMC group increased faster than in the first 10 days. The MDA content in the ZCZPE / CMC composite coating liquid group continued to increase, but the growth rate was less than that in other groups, and the MDA content was significantly lower than that in other groups p<0.05 during the whole storage process. The change of MDA content proved that the coating inhibited the membrane lipid peroxidation of fresh red pepper fruits and delayed the aging and decay process, and also proved that the addition of ZCZPE endowed the CMC coating matrix with functional characteristics and improved the preservation ability. The ZCZPE / CMC composite coating liquid group combined the inhibition of coating and ZCZPE on the physiological metabolism and membrane lipid peroxidation of fresh red pepper fruits, further delaying the aging of the fruits.

[0142] (7) Changes in polyphenol oxidase activity of red pepper

[0143] The polyphenol oxidase (PPO) activity of fresh red pepper samples was determined on days 0, 5, 10, 15, and 20, respectively. First, the PPO enzyme extract was prepared according to the experimental operation requirements and stored at 4 ℃. Then the experimental data were obtained and the PPO activity was calculated according to the method, and the final result unit was ΔOD 420 / (min·g), where an increase of 1 in absorbance per minute per gram of fresh red pepper fruit represented 1 unit of activity.

[0144] As shown in Figure 13 , with the extension of storage time, the PPO activity in each treatment group of fresh red pepper fruits was continuously rising. From the 5th day, the PPO activity of the ZCZPE / CMC composite coating liquid group, the CMC coating group and the blank control group showed significant difference (p<0.05), and the PPO activity of the ZCZPE / CMC composite coating liquid group was significantly lower than that of the other groups (p<0.05) throughout the whole storage process. The PPO activity of the CMC coating group was significantly lower than that of the blank control group (p<0.05) except for the 10th day. The experimental results showed that the ZCZPE / CMC composite coating liquid group had the best preservation effect on fresh red pepper fruits among the treatment groups, could significantly inhibit the increase of PPO activity of fresh red pepper fruits during storage, reduce the degree of browning, and prolong the shelf life.

[0145] (8) Changes in peroxidase activity of red pepper

[0146] The peroxidase (POD) activity of fresh red pepper samples was determined on day 0, 5, 10, 15, and 20, respectively. First, the POD enzyme extract was prepared according to the experimental operation requirements, and was prepared at 4 ℃. Then, the experimental data was obtained according to the method, and the POD activity was calculated. The final result unit is ΔOD 470 (min·g), wherein the increase of 1 in the absorbance change value per gram of fresh red pepper per minute represents 1 unit of activity.

[0147] From Figure 14 It can be seen that the POD activity in fresh red pepper fruits continuously rises during storage. The ranking of POD activity in each treatment group remains the same during the entire storage period: blank control group > CMC coating group > ZCZPE / CMC composite coating liquid group, and there are significant differences between each group (P < 0.05). p <0.05). The same conclusion is obtained from the change of PPO activity in fresh red pepper fruits. The ZCZPE / CMC composite coating liquid group significantly inhibits the increase of POD activity and delays the aging process of the fruits during storage, and has the best preservation effect among the treatment groups.

[0148] The above is only the preferred embodiment of the present application, and does not limit the present application in any form. Any person skilled in the art can make many possible changes and modifications to the technical solutions of the present application, or modify equivalent embodiments, without departing from the spirit and technical solutions of the present application. Therefore, any simple modification, equivalent replacement, equivalent change and modification of the above embodiments according to the technical essence of the present application, which does not depart from the technical solutions of the present application, still belongs to the protection scope of the technical solutions of the present application.

Claims

1. A method for preparing an active coating liquid loaded with Sichuan pepper essential oil, characterized in that, The preparation method is as follows: S1: Dissolve zein completely in 85% ethanol, centrifuge to remove precipitate and obtain supernatant; add supernatant dropwise to distilled water under magnetic stirring, treat with ultrasound and concentrate by rotary evaporator to obtain 3.5% zein nanoparticles. S2: Add citrus pectin to distilled water and stir overnight at 25°C until fully hydrated to obtain a 3.5% citrus pectin solution; S3: Using electrostatic adsorption, 3.5% zein nanoparticles were added dropwise into a citrus pectin solution under magnetic stirring to obtain a zein-citrus pectin Pickering emulsion. S4: Mix Sichuan pepper essential oil with 3.5% zein-citrus pectin Pickering emulsion and treat with a high-speed disperser at 14,000 rpm for 4 min to obtain Sichuan pepper essential oil-loaded Pickering emulsion. S5: Take sodium carboxymethyl cellulose, add deionized water, stir and dissolve in a constant temperature water bath to form a uniform and transparent film solution; add glycerol and continue stirring for 1 h; S6: Add 30-50% of Sichuan pepper essential oil-loaded Pickering emulsion at room temperature, stir magnetically at 800 rpm for 1 hour, let stand to remove bubbles, and you will get the active coating liquid loaded with Sichuan pepper essential oil. In step S3, the volume ratio of 3.5% zein nanoparticles to 3.5% citrus pectin solution is 1:(2-4). In step S4, the volume ratio of Sichuan pepper essential oil to 3.5% zein-citrus pectin Picklein emulsion is 1:(8-10). In step S5, the mass-to-volume ratio of sodium carboxymethyl cellulose / deionized water and glycerol is 6g:400mL:3g.

2. The method for preparing an active coating liquid loaded with Sichuan pepper essential oil according to claim 1, characterized in that, In step S1, the centrifugation speed is 5000 g and the centrifugation time is 10 min.

3. The method for preparing an active coating liquid loaded with Sichuan pepper essential oil according to claim 1, characterized in that, In step S1, the ultrasonic power is 600 W and the time is 10 min.

4. The application of the active coating liquid loaded with Sichuan pepper essential oil prepared by the preparation method according to any one of claims 1-3 in the preservation of fresh red peppers.

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