Compounding method of plant pigment protective coating used before and after citrus picking

Abstract

The invention relates to the technical field of pre-harvest and post-harvest treatment of agricultural products, in particular to a compounding method of a plant pigment protective coating for preventing sunburn before harvest and preserving after harvest of oranges. The coating is mainly prepared from curcumin-loaded porous starch, sodium alginate, anthocyanin and chitosan. The compounding method comprises the following steps: firstly, loading curcumin on porous starch to form composite powder, and then forming a stable eggshell packaging structure on the surface of the powder through crosslinking of sodium alginate and calcium ions; then dispersing the functional powder and anthocyanin in an acid solution in proportion, and finally adding chitosan to prepare a uniform functional fresh-keeping coating. The whole components of the product are safe and environment-friendly, the coating is dual-purpose, and a full-chain and low-cost green protection solution from the field to storage is provided for the citrus industry.

Description

Technical Field

[0001] This technology belongs to the field of green preservation and protection materials for agricultural products, specifically involving the research and application of natural polymer-based functional coatings. Traditional fruit and vegetable preservation technologies mostly focus on the post-harvest stage and have limited functionality, making it difficult to address the complex problems of sunburn in the field and spoilage during storage and transportation. This invention, through the functional recombination and structural design of natural components, breaks through the technical limitations of single materials, realizing an integrated solution for pre-harvest protection and post-harvest preservation, providing new materials and technical support for reducing agricultural product losses and promoting the development of green agriculture. Background Technology

[0002] As the world's largest citrus producer, my country experiences post-harvest losses of 20%-30%, with sunburn and post-harvest rot being key issues hindering the industry's development. Currently, field protection relies mainly on highly reflective coatings or bagging, which have limitations such as poor weather resistance and impaired photosynthesis. Post-harvest preservation commonly employs chemical fungicide soaking and polyethylene film packaging. The former easily leads to pesticide residues and pathogen resistance, while the latter is non-degradable and has limited functionality, failing to effectively address fruit deterioration caused by the combined effects of ultraviolet radiation and high temperatures. Although bio-based preservation films such as chitosan show application potential, their photothermal shielding performance is insufficient (UV blocking rate <40%), and their active ingredients are easily degraded and inactivated. Therefore, developing multifunctional integrated protective materials that combine pre-harvest sun protection, post-harvest antibacterial properties, and environmental friendliness has become a key direction for overcoming existing technological bottlenecks.

[0003] The core innovation of this invention lies in the creative combination of curcumin (effectively blocking high-temperature heat radiation in the 435-610nm wavelength band) and anthocyanins (efficiently absorbing ultraviolet rays in the 200-400nm wavelength band) with complementary spectra. A unique "eggshell" structure enables the stable and controlled release of the active ingredients. For pre-harvest application, this coating can be sprayed onto the surface of citrus fruits, forming a highly efficient photothermal protective shield that significantly reduces the incidence of sunburn. For post-harvest application, this coating, applied as a film, delays fruit dehydration and senescence while utilizing anthocyanins for rapid antibacterial action and curcumin for long-lasting antibacterial effects, thereby achieving efficient preservation and extending shelf life. Summary of the Invention

[0004] To address the problems of existing citrus protection technologies, such as the separation of pre-harvest sunburn prevention and post-harvest preservation, chemical residues, and environmental pollution from non-degradable materials, this invention provides a multifunctional composite coating and its compounding method that combines photothermal dual barrier and antibacterial preservation.

[0005] Based on the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A multifunctional preservation material for citrus fruits: the material is obtained by reacting porous starch and curcumin; furthermore, sodium alginate and calcium chloride are added to the material to form an eggshell structure to achieve the stability of the active ingredients; the material, anthocyanins and chitosan can also be added to an aqueous acetic acid solution in different proportions.

[0007] Porous starch: As a carrier of active ingredients, its rich porous structure can load curcumin through physical adsorption, with a loading rate of ≥60%. At the same time, it has good biocompatibility and degradability, laying a green and environmentally friendly foundation for the material.

[0008] Curcumin: As the core antibacterial and antioxidant component, natural curcumin with a purity of ≥98% is selected. Its phenolic hydroxyl structure can scavenge free radicals and inhibit the synthesis of microbial cell membranes. It has an inhibition rate of ≥85% against pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, and Aspergillus niger commonly found in citrus preservation.

[0009] Sodium alginate: The carboxyl groups on its molecular chain can undergo cross-linking reactions with calcium ions to form a stable "eggshell-like" structure, which encapsulates the porous starch-curcumin composite material and reduces the loss of active ingredients.

[0010] Calcium chloride: as a cross-linking agent, provides Ca 2+ It forms ionic bonds with sodium alginate, preferably with a calcium chloride solution of 2-5 g / L, and the cross-linking time is controlled at 30-40 min to ensure that the formed structure is dense and has a certain degree of air permeability, so as to avoid the rotting of citrus fruits due to lack of oxygen.

[0011] Anthocyanins: Anthocyanins are selected as the anti-ultraviolet component. Their benzopyran ring structure can absorb ultraviolet rays of 200-400nm. When the amount added accounts for 0.5%-1.5% of the total mass of the material, the anti-ultraviolet transmittance is ≤35%, and it also has a certain antioxidant activity (DPPH free radical scavenging rate ≥70%).

[0012] Chitosan: Chitosan is selected as an antibacterial enhancer and membrane structure stabilizer. Its amino groups can form hydrogen bonds with the carboxyl groups of sodium alginate, which can improve the barrier properties of the coating. At the same time, its own inhibition rate against microorganisms is ≥75%. It can further enhance the antibacterial effect by working synergistically with curcumin.

[0013] Acetic acid aqueous solution: As a solvent for chitosan, the concentration is controlled at 1%-2% (v / v) to ensure that the chitosan is fully dissolved and to avoid damage to the citrus peel caused by high concentration of acetic acid.

[0014] The compounding process employing "stepwise loading-crosslinking to form a coating" involves the following steps, with optimization and control points for each step's process parameters as follows:

[0015] S1: Preparation of curcumin loaded on porous starch (active ingredient loading)

[0016] Pretreatment: Place the porous starch in an oven at 50-60℃ and dry for 24-48 hours to remove moisture (moisture content ≤5%) to ensure the adsorption activity of the porous structure;

[0017] Loading reaction: The dried porous starch was soaked in a curcumin / methanol solution with a concentration of 5-12 g / L at a solid-liquid ratio of 1:100 (g / mL), and placed in a constant temperature water bath shaker. The temperature was controlled at 25-28℃ and the shaking rate was 150-200 r / min. The shaking was continued for 24-48 h to allow curcumin to be fully adsorbed into the pores of the porous starch.

[0018] Post-processing: The reaction solution was centrifuged (5000 r / min, 8-10 min), the precipitate was collected, washed 2-3 times with anhydrous ethanol (to remove unadsorbed free curcumin on the surface), and then dried in a vacuum drying oven at 40-50℃ for 12-24 h. After grinding, porous starch-loaded curcumin powder was obtained.

[0019] S2: Preparation of porous starch-supported curcumin / sodium alginate complex (cross-linked stable)

[0020] Preparation of sodium alginate solution: Weigh an appropriate amount of sodium alginate, add deionized water, stir and dissolve at 25-28℃ for 30-40 minutes to prepare a sodium alginate solution with a concentration of 2-5 g / L for later use.

[0021] Dispersion and mixing: Porous starch-loaded curcumin powder was added to deionized water at a mass-to-volume ratio of 1:200 (g / mL), and ultrasonically dispersed for 20-30 min (power 200W, frequency 40kHz) to form a uniform dispersion; then sodium alginate solution with a concentration of 2-5 g / L was added, and magnetic stirring was performed for 30-40 min (speed 300 r / min) to ensure thorough mixing.

[0022] Cross-linking reaction: Slowly add a calcium chloride solution with a concentration of 2-5 g / L to the above mixture at a volume ratio of 40:7, stirring while adding. After the addition is complete, continue the reaction for 30-40 minutes.

[0023] Post-processing: After the reaction is complete, centrifuge (4000 r / min, 10-15 min), collect the precipitate, wash it 2-3 times with deionized water, dry it in a forced-air dryer at 50-60℃ for 8-12 h, grind it to obtain porous starch-loaded curcumin / sodium alginate powder.

[0024] S3: Compound formulation of food preservation coatings (forming and functional integration)

[0025] Preparation of mixed solution: Weigh out porous starch-loaded curcumin / sodium alginate powder and anthocyanin according to the ratio, add them to 2% (v / v) acetic acid aqueous solution, ultrasonically disperse for 15-20 min (power 150W), then place on a magnetic stirrer and stir at room temperature for 50-60 min to form a uniform dispersion.

[0026] Chitosan addition and dissolution: Add an appropriate amount of chitosan to the above dispersion and continue stirring at room temperature for 90-120 minutes (250 r / min) to completely dissolve the chitosan and fully blend it with other components to form a viscous coating liquid.

[0027] Based on the material's multifunctional properties, it can be widely used for pre-harvest protection and post-harvest preservation of citrus fruits. Specific applications and effects are as follows:

[0028] 1. The material according to any one of claims 2-4 is used for pre-harvest heat insulation and UV protection of citrus fruits.

[0029] Application method: The composite coating is applied to the surface of citrus fruits to directly block high temperatures and ultraviolet rays;

[0030] Results: Tests showed that under high summer temperatures (35-38℃), the surface temperature of the citrus peels treated with the covering was reduced by ≥35% compared to the untreated group, effectively preventing peel burns caused by high temperatures. At the same time, compared to the untreated group, the material's transmittance to ultraviolet rays was ≤35%, reducing the damage of ultraviolet rays to the peel pigments and maintaining the bright color of the citrus.

[0031] 2. Application of the material according to any one of claims 2-4 in postharvest antibacterial and antioxidant effects in citrus.

[0032] Application method:

[0033] After harvesting, the citrus fruits are soaked in the coating solution to cover them evenly, then taken out, dried, and stored.

[0034] Results: After soaking, the citrus fruits stored at room temperature for 15-20 days showed an inhibition rate of ≥90% against Escherichia coli and Staphylococcus aureus, and an inhibition rate of ≥85% against Aspergillus niger, effectively reducing fruit rot. At the same time, the antioxidant activity of the material can reduce the browning rate of the peel to ≤40% and the weight loss rate of the pulp to ≤30%, thus maintaining the flavor and nutrition of the fruit.

[0035] 3. Application of the material according to any one of claims 2-4 in postharvest comprehensive preservation of citrus.

[0036] Application scheme: The treatment method of composite coating soaking is adopted. After the citrus is picked, it is first washed and dried, then soaked in composite preservation coating, then taken out and dried, and stored in a room temperature environment.

[0037] Preservation effect: Citrus fruits treated with this method can have their shelf life extended to 15-17 days, which is 1-2 times longer than the untreated group. The fruit rot rate at the end of storage is ≤5%, the good fruit rate is ≥90%, and the quality indicators such as peel color, pulp firmness, and soluble solids content are all maintained in excellent condition.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] This invention innovatively achieves synergistic spectral protection: anthocyanins (200-400nm) and curcumin (435-610nm) form complementary absorption bands, achieving a UV blocking rate of ≥70% and an infrared shielding rate of ≥70%. This invention possesses dual pre-harvest and post-harvest functions: pre-harvest spraying can reduce the incidence of sunburn by more than 30%, while post-harvest spraying can reduce citrus weight loss by ≤30% and extend shelf life by at least 10-15 days. This invention not only solves the white pollution problem of traditional plastic films but also endows the composite coating with multiple functions such as heat insulation, UV resistance, antibacterial properties, and antioxidant properties, showing broad application prospects. Furthermore, this invention has the advantages of low energy consumption, simple equipment, convenient operation, and suitability for industrial production. Attached Figure Description

[0040] Figure 1 This is a summary figure;

[0041] Figure 2 Infrared, XRD, and electron microscopy characterization images of the powder materials in Examples 1 and 2;

[0042] Figure 3 Infrared, XRD, XPS, and electron micrographs of the CS coating, Cur-PS / ALG / CS coating, and Cur-PS / ALG / ACNs / CS coating;

[0043] Figure 4 Figures showing the heat insulation and UV protection of CS coating, Cur-PS / ALG / CS coating, and Cur-PS / ALG / ACNs / CS coating;

[0044] Figure 5 Antibacterial and antioxidant properties of CS coating, Cur-PS / ALG / CS coating, and Cur-PS / ALG / ACNs / CS coating;

[0045] Figure 6 Images of citrus preservation using CS coating, Cur-PS / ALG / CS coating, and Cur-PS / ALG / ACNs / CS coating. Detailed Implementation

[0046] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0047] The reagents and solvents used in the embodiments of this invention were purchased from commercial reagent companies such as Shanghai Maclean Biochemical Co., Ltd., Sinopharm Reagent Co., Ltd., and Yuanye Biotechnology Co., Ltd.

[0048] Example 1

[0049] This embodiment uses porous starch to load curcumin powder, and its preparation method includes the following steps:

[0050] Dissolve 3g of curcumin (Cur) in 400mL of methanol, add 1g of porous starch (PS), and then shake in a constant temperature water bath at room temperature (avoid light) for 2 days. Then wash twice with methanol to remove free curcumin from the surface of the porous starch, centrifuge (the centrifugation temperature should not exceed 28℃), and vacuum dry at 50℃ for 12 hours.

[0051] Example 2

[0052] In this embodiment, sodium alginate powder is coated with curcumin using porous starch. The preparation method includes the following steps:

[0053] Step 1: Disperse 1g of Cur-loaded PS and (0.365g of Cur) in 200mL of water, mechanically stir for 5min, and sonicate for 15min. Step 2: Dissolve 0.2g of sodium alginate (ALG) in 100mL of water until completely dissolved. Step 3: Add the solution from Step 2 dropwise to the solution from Step 1 (while stirring), and continue the reaction for 30min after the addition is complete. Step 4: Add 35mg of CaCl2 to 17.5mL of water until completely dissolved. Step 5: Add the solution from Step 4 dropwise to the solution from Step 3 (while stirring), and continue the reaction for 30min after the addition is complete. Step 6: Centrifuge and wash the resulting solution three times, then vacuum dry at 60℃ for 24h.

[0054] Example 3

[0055] This embodiment uses a porous starch-loaded curcumin / sodium alginate / anthocyanin / chitosan coating, and its preparation method includes the following steps:

[0056] The first step is to disperse Cur-loaded PS / ALG in 100 mL of aqueous solution (0.5% acetic acid) at different ratios (10%, 20%, 30%, 40%, 50%) and mechanically stir for 30 min. The second step is to add anthocyanins (ACNs) to the above solution at different ratios (10%, 20%, 30%, 40%, 50%) and stir until homogeneous. The third step is to slowly add chitosan (CS) powder while stirring, allowing it to dissolve completely to form a coating solution.

[0057] Performance testing

[0058] 1. Morphological test

[0059] The successful preparation of Cur-PS / ALG / ACNs / CS was verified using characterization techniques such as infrared spectroscopy, XPS, XRD, and SEM. Figure 2 As shown in (a), Cur exhibits a phenolic hydroxyl stretch at 3503 cm⁻¹, a C=C double bond at 1508 cm⁻¹, a CO group vibration at 1282 cm⁻¹, and a COC stretching band at 1026 cm⁻¹. PS shows a hydroxyl stretch at 3354 cm⁻¹, a methylene stretching vibration at 2932 cm⁻¹, a COC stretching at 1158 cm⁻¹, and a C=O band at 999 cm⁻¹. The presence of a hydroxyl peak at 3503 cm⁻¹ in Cur-PS, while the peak at 2932 cm⁻¹ in PS shifts to 2941 cm⁻¹, and the broad peak at 3354 cm⁻¹ in PS disappears. This indicates that the hydroxyl groups in Cur and PS combine to form hydrogen bonds. Furthermore, the shift in characteristic peaks of Cur in Cur-PS is attributed to the high loading rate of Cur on PS (62.9%). In ALG, the hydroxyl peak is at 3392 cm⁻¹, the carboxyl peak is at 1616 cm⁻¹, and the absorption band at 1030 cm⁻¹ is related to CO stretching. After the addition of ALG, the CO peak of Cur-PS / ALG shifts to 1026 cm⁻¹, and the main infrared peaks of Cur-PS / ALG are basically consistent with those of Cur. This indicates the successful loading of curcumin onto porous starch and the successful coating of sodium alginate. Figure 2 As shown in (b), the characteristic peak of ACNs at 3362 cm⁻¹ is caused by the stretching vibration of the OH bond. Furthermore, the sharp absorption band at 1616 cm⁻¹ is due to the stretching vibration of the CO bond in ACNs. CS exhibits a hydroxyl peak at 3192 cm⁻¹ and an amino peak at 1539 cm⁻¹. With the addition of Cur-PS / ALG and ACNs, the hydroxyl and amino peaks shift to higher wavenumbers, indicating the successful preparation of Cur-PS / ALG / ACNs / CS.

[0060] like Figure 3As shown in (c), the binding energies of C=O, CO / C-OH, and -CH2- / C-NH2 in the C1s spectrum of CS are 287.1 eV, 285.5 eV, and 283.9 eV, respectively. With the addition of Cur-PS / ALG, the bond energy of C=O changes from 287.1 eV to 287.2 eV. After the addition of ACNs, the bond energies of C=O, CO / C-OH, and -CH2- / C-NH2 change from 287.2 eV, 285.5 eV, and 283.9 eV to 287.3 eV, 285.6 eV, and 284.0 eV, respectively. Furthermore, in the O1s spectrum, the bond energy of C-OH / COC changes from 531.8 eV to 531.9 eV and finally to 532 eV with the addition of Cur-PS / ALG and ACNs, indicating that hydrogen bonds are formed between the hydroxyl and amino groups of CS and the hydroxyl groups in Cur-PS / ALG and ACNs.

[0061] like Figure 2 As shown in Figure 1, the SEM images reveal that the PS surface has a smooth and regular morphology. In contrast, after ALG coating, significant adhesion occurs between particles, and the surface becomes rough. Untreated CS is used as a blank control. Figure 3 In images d and e, electron microscopy images clearly show that the surface and cross-section of CS are smooth and uniform, while dispersed particles are visible on the surface of Cur-PS / ALG / CS. Cross-sections reveal micron-sized pores at the interface between Cur-PS / ALG and the CS matrix, which may accelerate the burst release of the active ingredient. Figure 3 f and g). Figure 3 As shown in h and i, after the addition of ACNs, the particles of Cur-PS / ALG / ACNs / CS are more uniformly dispersed, and the surface exhibits a micron-scale honeycomb structure. The introduction of cross-sectional ACNs makes the interlayer bonding tighter and the porosity significantly reduced. ACNs form hydrogen bonds with -NH2 and -COO- in ALG and CS through phenolic hydroxyl groups, which improves interfacial compatibility.

[0062] The structures of PS, Cur, and other powders, as well as CS, Cur-PS / ALG / CS, and Cur-PS / ALG / ACNs / CS coatings, were further investigated using X-ray diffraction (XRD). Figure 2As shown in (b), the XRD pattern of PS shows diffraction peaks at 2θ = 14.95°, 16.87°, 18.07°, and 22.84°, which is typical for type A starch. Furthermore, the peaks of Cur-PS at 2θ = 8.87°, 12.23°, 14.52°, 17.19°, and 24.74° coincide with the peaks of Cur, while the main peak of PS disappears. This may be due to hydrogen bonding between Cur and PS, disrupting the crystal structure of PS. The ALG crystal structure shows diffraction peaks between 14.60° and 20°-30°. The peaks of Cur-PS / ALG also largely coincide with those of Cur, indicating successful synthesis of the material. Figure 3 As shown in (b), the XRD pattern of ACNs shows a single broad peak near 20.52°, indicating that ACNs are in an amorphous state. CS decreases at 10.98° and 17.82° with the addition of Cur-PS / ALG and ACNs, indicating that the hydrogen bonding interaction between Cur-PS / ALG and CS disrupts the crystal structure of CS. SEM, FTIR, and XRD results show that Cur-PS / ALG / ACNs / CS coatings were successfully prepared in Examples 1–3.

[0063] 2. Thermal insulation performance test

[0064] Fresh oranges were selected and coated with PE film and Cur-PS / ALG / ACNs / CS coatings (10%, 20%, 30%, 40%, and 50%) respectively. After 4 hours of sunlight exposure, the temperature change of the orange peel before and after sunlight exposure was measured using a DLX-HC2501C handheld thermometer. The thermal insulation performance of the coatings was studied by measuring the temperature difference of the orange peel. Oranges without coatings were used as a control group.

[0065] High temperatures are also a major factor contributing to sunburn disease in fruits. For example... Figure 4 As shown in (a), the initial surface temperature of the orange peel was 26.3℃. With increasing light exposure time, the surface temperature did not rise. After 2 hours of irradiation, the temperature of the control group increased to 44.8℃, while the temperature of the sample group was 38.2℃. After 4 hours of irradiation, the temperature difference reached a maximum of approximately 7℃ before stabilizing. Compared to the control group, the surface temperature of the sample group was reduced by 35%, indicating that Cur-PS / ALG / ACNs / CS possesses certain heat insulation properties and good stability.

[0066] 3. UV resistance test

[0067] 4 mg of avermectin was dissolved in 20 mL of methanol. The solutions were then coated with PE membranes and Cur-PS / ALG / ACNs / CS membranes (10%, 20%, 30%, 40%, and 50%). After 3 days of UV irradiation at a distance of 10 cm, the remaining avermectin content was measured at 245 nm using a ZF-1 three-way UV analyzer. The UV resistance of the membranes was studied by determining the avermectin concentration. The uncoated solution served as a control group.

[0068] The photodegradation resistance of avermectin solutions with and without a PE membrane, and with a Cur-PS / ALG / ACNs / CS membrane, was investigated. The photodegradation resistance is reflected in the residual curves of avermectin over UV irradiation time, as shown in the figure. Figure 4 As shown in (b), the residual amount of avermectin decreased significantly with prolonged irradiation time. The photodegradation rates were as follows: uncoated solution > PE-coated solution > Cur-PS / ALG / ACNs / CS (40%) membrane-coated solution; the degradation rates of avermectin within 72 hours were 60%, 34%, and 25%, respectively. The UV resistance of the solution coated with Cur-PS / ALG / ACNs / CS (40%) membrane was 35% higher than that of the uncoated solution, indicating that the coating layer has a positive protective effect on avermectin, preventing it from being exposed to ultraviolet radiation.

[0069] 4. Antibacterial performance test

[0070] Eighty strains of *Escherichia coli*, *Staphylococcus aureus*, and mold were used in the experiment. The antibacterial properties of CS, Cur-PS / ALG / CS, and Cur-PS / ALG / ACNs / CS were evaluated using the inhibition zone test. First, a culture medium was prepared by mixing 5g beef extract, 10g peptone, 5g NaCl, and 1000mL deionized water. Then, 150μL of *E. coli* and *Staphylococcus aureus* suspensions were evenly spread onto the medium. CS, Cur-PS / ALG / CS, and Cur-PS / ALG / ACNs / CS were placed into the wells (d = 7mm). Finally, the samples were incubated at 37℃ for 24 hours and photographed. Similarly, mold was inoculated onto potato dextrose agar, and CS and Cur-PS / CS were added to the wells. The mixture was incubated at 28℃ for 6 days, and the mold growth was recorded photographically.

[0071] Here, *Escherichia coli*, *Staphylococcus aureus*, and mold were used as model bacteria to evaluate the antibacterial activity of the Cur-PS / ALG / ACNs / CS coating. Figure 5As shown in (a), the inhibition zone radius increased continuously with the addition of Cur-PS / ALG and ACNs, reaching 5.64 mm, 4.41 mm, and 3.48 mm, respectively. The optimal antibacterial effect of Cur-PS / ALG / ACNs / CS was also attributed to the synergistic antibacterial action of CS, Cur, and ACNs. The results indicate that Cur-PS / ALG / ACNs / CS has good antibacterial effects against Escherichia coli, Staphylococcus aureus, and molds.

[0072] 5. Antioxidant performance test

[0073] The antioxidant properties of CS, Cur-PS / ALG / CS, and Cur-PS / ALG / ACNs / CS coatings were measured using the DPPH radical scavenging method. Briefly, a 0.1 mM fresh DPPH / methanol solution was prepared. Then, samples of different concentrations (0.25, 0.50, 1.00, 2.00, 4.00 mg / mL) were mixed with 5 mL of DPPH / methanol solution and reacted at 28 °C in the dark for 30 min. The absorbance change of the samples was measured at 517 nm using a UV spectrophotometer. The DPPH radical scavenging activity was calculated according to the formula:

[0074]

[0075] Abs control is the absorbance of the DPPH methanol solution without any sample, while Abs sample is the absorbance of the DPPH methanol solution with a sample.

[0076] Oxidation is a significant factor affecting the perishability of fruits, and biofilms can significantly extend the shelf life of foods by enhancing their antioxidant capacity. For example... Figure 5 As shown in (b), CS and Cur-PS / ALG / CS did not exhibit significant antioxidant activity in the range of 0.25–4.00 mg / mL, while the Cur-PS / ALG / ACNs / CS coating showed high antioxidant activity (75.62%) at 4.00 mg / mL. This is likely due to the high antioxidant activity of anthocyanins. Therefore, the Cur-PS / ALG / ACNs / CS coating exhibits high antioxidant activity, effectively inhibiting peel browning and extending fruit shelf life, thus demonstrating good preservation effects.

[0077] 6. Postharvest preservation experiment of Cur-PS / ALG / ACNs / CS coating

[0078] Fresh citrus fruits were selected, washed with deionized water, and dried at room temperature. The peels were then coated with CS, Cur-PS / ALG / CS, and Cur-PS / ALG / ACNs / CS coatings, respectively, and stored after drying. A control group (CK) received no treatment. These citrus fruits were stored at 28-32℃ and RH = 65-70%, and changes in weight loss, firmness, color, and titratable acid (TA) were recorded at 0, 1, 5, 9, 13, and 17 days.

[0079] like Figure 6 As shown in (a), no treatment was administered on day 13. Figure 6 The surface of the citrus fruit with CS coating showed obvious putrefactive bacteria. Figure 6 Citrus fruits covered with aII) turned black. During the same storage time, CS, Cur-PS / ALG / CS ( Figure 6 aIII) and Cur-PS / ALG / ACNs / CS group ( Figure 6 The degree of citrus decay in the Cur-PS / ALG / CS-coated group was lower than that in the control group. Citrus treated with the Cur-PS / ALG / CS coating showed blackening on day 13, later than the CS group. However, by day 17, the blackening in the CS group was more severe than in the Cur-PS / ALG / CS group. The comparison between the CS and Cur-PS / ALG / CS groups shows that although CS effectively isolates microorganisms, once food is contaminated, it spoils rapidly due to its poor antibacterial ability. Citrus treated with Cur-PS / ALG / ACNs / CS maintained good appearance and color, indicating that Cur-PS / ALG / ACNs / CS effectively inhibits microbial growth, thus delaying citrus spoilage by at least 12 days.

[0080] During storage, quality changes in the citrus fruits were also assessed, including weight loss, firmness, titratable acid (TA), and color. Figure 6 As shown in (b), all groups of citrus fruits inevitably experienced weight loss due to material loss and water evaporation. On day 17, the weight losses of citrus fruits in the control group, CS group, Cur-PS / ALG / CS group, and Cur-PS / ALG / ACNs / CS group were 48.5%, 41.1%, 38.1%, and 34.4%, respectively. The decrease in protopectin concentration led to a decrease in fruit firmness during storage. The change in firmness showed a decreasing trend, consistent with the change in weight loss. Figure 6 c) That is, the greater the weight loss, the more significant the decrease in firmness. Therefore, Cur-PS / ALG / ACNs / CS can effectively slow down the metabolic activity of citrus fruits and inhibit microbial spoilage, thereby reducing weight loss and firmness loss. During storage, the TA content also decreases ( Figure 6d). CS and Cur-PS / ALG / CS packaging can reduce TA loss to some extent. The Cur-PS / ALG / ACNs / CS group showed the smallest decrease in TA (from 0.24% to 0.12%). Therefore, Cur-PS / ALG / ACNs / CS can effectively maintain the freshness of citrus and ensure its nutritional value.

[0081] The color of fruit directly affects the appearance of processed products and consumer acceptance. During the storage process of citrus fruits, their color changes from yellow to brown. Figure 6 ).like Figure 6 As shown in (e.g.), the L*, a*, and b* values ​​of the control group, the CS group, and the Cur-PS / ALG / CS treated group decreased rapidly. In contrast, in citrus treated with Cur-PS / ALG / ACNs / CS, the L*, a*, and b* values ​​decreased from 62.6±0.5 to 59.3±0.2, from 34.0±0.28 to 31.2±2.3, and from 60.2±0.14 to 57.2±0.14, respectively. These values ​​decreased more slowly in the Cur-PS / ALG / ACNs / CS group than in the other groups. Therefore, Cur-PS / ALG / ACNs / CS treatment is the most effective method to slow down the decrease in L*, a*, and b* values. Furthermore, as... Figure 6 As shown in (h), during storage, the ΔE* value of citrus treated with Cur-PS / ALG / ACNs / CS was significantly lower than that of other treatment groups, including the control group, the group treated with CS and Cur-PS / ALG / CS, which confirms that Cur-PS / ALG / ACNs / CS can effectively slow down color changes.

Claims

1. A safe, non-toxic, and edible plant extract for pre-harvest sunburn prevention and post-harvest preservation of citrus fruits, with an optimized compound formulation ratio, wherein the plant extract is a compound of two or three pigments selected from lycopene, marigold pigment, curcumin, geranium pigment, beet red pigment, and anthocyanin, and the compound has a specific ultraviolet spectral absorption (200-400nm) and effective blocking of high-temperature radiation (435-610nm) and can be sprayed as a sunscreen product.

2. The powder material for blocking high temperatures according to claim 1, characterized in that, The powder material is obtained by reacting porous starch and curcumin.

3. The high-temperature barrier powder material according to claim 2, characterized in that, The material needs to be incorporating sodium alginate and calcium chloride to form an eggshell structure, thereby stabilizing the active ingredients.

4. The stable high-temperature blocking powder material and the ultraviolet blocking material according to claim 1, characterized in that, The materials, anthocyanins (which block ultraviolet rays), and chitosan were added to an aqueous acetic acid solution in different proportions.

5. A method for compounding the materials according to any one of claims 2 to 4, characterized in that, Includes the following steps: S1: Soak porous starch in curcumin / methanol solution and shake in a water bath at room temperature to form porous starch loaded with curcumin powder; S2: Sodium alginate solution and calcium chloride solution are added dropwise to a porous starch-loaded curcumin-dispersed aqueous solution in a certain proportion to react and form porous starch-loaded curcumin / sodium alginate powder. S3: Add porous starch-loaded curcumin / sodium alginate powder and anthocyanins to an acetic acid aqueous solution in different proportions, stir until uniform, and then add chitosan to form a porous starch-loaded curcumin / sodium alginate / anthocyanins / chitosan coating.

6. The compounding method according to claim 5, characterized in that, In step S1, the concentration of curcumin / methanol solution is 5-12 g / L.

7. The compounding method according to claim 5, characterized in that, In step S2, curcumin needs to be added to increase the loading rate; the volume ratio of sodium alginate solution and calcium chloride solution in step S2 is 40:7; the concentration of sodium alginate solution is 2-5 g / L; the concentration of calcium chloride solution is 2-5 g / L.

8. The application of the material according to any one of claims 2 to 4 in pre-harvest heat insulation and UV protection of citrus, characterized in that... Compared with the untreated group, the citrus fruits treated with the porous starch-loaded curcumin / sodium alginate / anthocyanin / chitosan coating showed a ≥35% reduction in peel surface temperature and a ≤35% UV transmittance.

9. The application of the material according to any one of claims 2 to 4 in postharvest antibacterial and antioxidant effects on citrus, characterized in that... The porous starch-loaded curcumin / sodium alginate / anthocyanin / chitosan coating has an inhibitory effect on Escherichia coli, Staphylococcus aureus, and Aspergillus niger; it has high antioxidant activity, can effectively inhibit browning of the fruit peel, and prolong the shelf life of the fruit.

10. The application of the material according to any one of claims 2 to 4 in postharvest preservation of citrus fruits, characterized in that... Porous starch-loaded curcumin / sodium alginate / anthocyanin / chitosan coating has good preservation properties and extends the shelf life of citrus fruits.

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