A nano-network structured chitosan-based edible film, its preparation method and application

Abstract

This invention provides a chitosan-based edible film with a nano-network structure, its preparation method, and its application. The preparation method includes the following steps: 1) Adding zein to water, adjusting the pH, and then adding a phosphorylation reagent to obtain a mixed solution. After dialysis, centrifugation is performed, and the supernatant is freeze-dried to obtain phosphorylated zein nanoparticles; 2) Adding microcrystalline cellulose to FeCl3·6H2O solution, heating in a water bath, vacuum filtering, washing the precipitate until white, and freeze-drying the obtained precipitate to obtain cellulose nanocrystals; 3) Preparing a coating solution: Adding the phosphorylated zein nanoparticles and cellulose nanocrystals to a chitosan hydrochloride solution to obtain a coating solution; 4) Adding a plasticizer to the coating solution, ultrasonically degassing, and then pouring the mixture onto an acrylic plate to naturally form a film, thus obtaining an edible film. The chitosan-based edible film prepared by this invention can be used to reduce cherry cracking when exposed to rain. It is safe and non-toxic and has high market value.

Description

Technical Field

[0001] This invention belongs to the field of edible coatings and edible films, and particularly relates to a nano-network structure chitosan-based edible film, its preparation method, and its application. Background Technology

[0002] Cherry rain cracking is a global problem, especially when triggered by pre-harvest rainfall in sweet cherries, leading to a significant drop in cherry yields and substantial economic losses for farmers. The two main causes of cherry rain cracking are: 1) excessive water absorption by the fruit peel or through the tree's vascular system, resulting in cracks; and 2) water adhering to the surface, causing micro-cracks to expand into visible cracks. Currently, there are no highly effective methods to reduce cherry rain cracking. Breeding crack-resistant varieties is difficult, and physical barriers (such as rain covers) have limited effectiveness. Furthermore, researchers have explored methods such as applying calcium and growth regulators before harvest, but these are costly. Summary of the Invention

[0003] The purpose of this invention is to provide a nano-network structured chitosan-based edible film, its preparation method, and its applications. This edible film can effectively reduce cherry rain cracking, thereby protecting cherry quality and increasing its market value. Through research on the application of edible coatings, this invention has found it to be a feasible and effective method for reducing cherry rain cracking. The edible coating needs good wetting properties, surface hydrophobic properties, and excellent barrier properties, and should not inhibit or toxicize cherry fruit growth.

[0004] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0005] This invention provides a method for preparing a chitosan-based edible film with a nano-network structure, comprising the following steps:

[0006] 1) Preparation of phosphorylated zein nanoparticles: Zein was added to water at 10-50 °C and stirred for 10-60 min to dissolve. Then it was added to an alkaline solution, followed by a phosphorylation reagent. The mixture was stirred at 10-80 °C for 1-5 h to obtain a mixed solution. The mixed solution was dialyzed in water for 12-72 h to obtain a dialysate. The residue in the dialysate was centrifuged, and the supernatant was freeze-dried to obtain phosphorylated zein nanoparticles PZNP.

[0007] 2) Preparation of cellulose nanocrystals: Microcrystalline cellulose was added to FeCl3·6H2O solution, heated and stirred in a water bath for 4-8 h. After the reaction was completed, the precipitate was collected by vacuum filtration, washed with ultrapure water until white, and the obtained precipitate was freeze-dried to obtain cellulose nanocrystals CNC.

[0008] 3) Preparation of coating solutions: Chitosan hydrochloride solution CHC was used as the basic coating solution, and phosphorylated zein nanoparticles PZNP and cellulose nanocrystals CNC were added as reinforcing nanoparticles to obtain four coating solutions: CHC, CHC-PZNP, CHC-CNC, and CHC-PZNP-CNC.

[0009] 4) Preparation of edible film: Add plasticizer to the coating solution, degas it by ultrasound, pour it onto acrylic plates, and place it at 10-50℃ for 36-50h to form a film naturally, thus obtaining CHC, CHC-PZNP, CHC-CNC, and CHC-PZNP-CNC edible films.

[0010] Furthermore, the pH value of the alkaline solution in step 1) is 12-14.

[0011] Furthermore, in step 1), the stirring speed is 1000-3000 r / min, and the centrifugation speed is 2000-6000 r / min.

[0012] Furthermore, in step 1), the pH is adjusted using 1 mol / L NaOH, and the phosphorylation reagent is sodium tripolyphosphate.

[0013] Furthermore, the freeze-drying process in step 1) is as follows: the supernatant is frozen in an ultra-low temperature freezer at -80 ℃ for 0.5-2 days, and finally freeze-dried in a freeze dryer for 2-5 days. After being taken out, it is stored in a dry container at 10-50 ℃.

[0014] Furthermore, the conditions for water bath heating and stirring in step 2) are 30-120 ℃ and 500-1500 r / min.

[0015] Furthermore, the preparation methods for the four coating solutions in step 3) are as follows:

[0016] CHC: Add the chitosan hydrochloride to water and stir until completely dissolved to obtain a CHC coating solution;

[0017] CHC-PZNP: Dissolve the phosphorylated zein nanoparticles in water, mix them with the CHC coating solution, and stir the mixture until completely dissolved to obtain the CHC-PZNP coating solution;

[0018] CHC-CNC: The cellulose nanocrystals are mixed with water until dispersed, then mixed with the CHC coating solution, and the mixture is stirred until completely dissolved to obtain the CHC-CNC coating solution;

[0019] CHC-PZNP-CNC: The phosphorylated zein nanoparticles and the cellulose nanocrystals are dispersed in water, added to the CHC coating solution, and stirred evenly to obtain the CHC-PZNP-CNC coating solution;

[0020] In the four methods for preparing coating solutions, the temperature is 10~50℃.

[0021] Furthermore, in step 4), the plasticizer is glycerol, and the mass ratio of glycerol to chitosan hydrochloride is 1:20, the ultrasonic frequency is 20-60 kHz, the temperature is 10-50 ℃, and the time is 10-60 min.

[0022] The present invention also provides a nano-network structured chitosan-based edible film prepared by the aforementioned method.

[0023] This invention also provides the application of the aforementioned nano-network structure chitosan-based edible film in reducing cherry cracking.

[0024] Furthermore, the edible film is applied to the surface of the cherry fruit using a sprayer to naturally form a thin film.

[0025] Furthermore, the nano-network structure chitosan-based edible film selected is CHC-PZNP-CNC edible film, which has the best effect on reducing the cracking rate.

[0026] Compared with the prior art, the advantages and positive effects of the present invention are:

[0027] This invention utilizes coating and spraying methods to prepare chitosan hydrochloride (CHC) coatings, synergistically incorporating phosphorylated zein nanoparticles (PZNP) and cellulose nanocrystals (CNC) to enhance their wetting and water-repellent properties. Adding PZNP to edible films significantly improves their wetting properties; adding CNC significantly enhances their water-barrier properties. When both are simultaneously incorporated, the CHC-PZNP-CNC edible film exhibits excellent surface hydrophobicity, wettability, and the strongest water-barrier performance.

[0028] This invention, through laboratory analysis (coating performance, hydrophobicity / hydrophobicity, barrier properties, mechanical properties, etc.) and field research (cracking coefficient and fruit cracking rate) of the edible film, found that the synergistic inclusion of PZNP and CNC can give the edible film better coating performance (surface tension: 51.16 mN / m), hydrophobicity (water contact angle: 106.4.6°), and water vapor barrier properties (water vapor transmission rate: 5.32 × 10⁻⁶). -10 gm -1 Pa -1 s -1It has better mechanical properties (elastic modulus: 482.2MPa; tensile strength: 37.5MPa; elongation at break: 7.76%) than other films, making it the best choice to reduce cherry fruit cracking when exposed to rain.

[0029] This invention, through field trials, demonstrated the efficacy of the CHC-PZNP-CNC edible film in reducing cherry rain cracking (cracking index of 19.7%) and its overall performance (cracking rate of 15.8%), proving that the edible film prepared by this invention is an effective and environmentally friendly solution for combating cherry rain cracking. With its enhanced wetting and barrier properties, the CHC-PZNP-CNC coating can provide long-lasting moisture protection, thereby protecting cherry quality and increasing market value. Attached Figure Description

[0030] Figure 1 Fourier transform infrared spectra of PZNP and CNC;

[0031] Figure 2 Atomic force microscopy images and particle size distribution diagrams of PZNP and CNC;

[0032] Figure 3 Fourier transform infrared spectra of four edible films (CHC, CHC-PZNP, CHC-CNC, CHC-PZNP-CNC);

[0033] Figure 4 XRD images of four edible films;

[0034] Figure 5 Atomic force microscopy images of four edible films (A: CHC, B: CHC-PZNP, C: CHC-CNC, D: CHC-PZNP-CNC; phase mode: top is phase 2D, middle is solid 3D, bottom is height 3D; Ra is arithmetic mean roughness, Rq is root mean square roughness).

[0035] Figure 6 Water contact angle and surface tension diagrams for four edible films (CHC(A,E), CHC-PZNP(B,F), CHC-CNC(C,G) and CHC-PZNP-CNC(D,H)).

[0036] Figure 7 The mechanical properties and WVP diagrams for four types of edible films are shown (TS is tensile strength, EAB is elongation at break).

[0037] Figure 8 The film formation process diagram and film diagram of four edible films are shown ((1): CHC, (2): CHC-PZNP, (3): CHC-CNC, (4): CHC-PZNP-CNC).

[0038] Figure 9 The rheological properties of four coating solutions are shown in the graphs. (A represents the relationship between viscosity and shear rate, B represents the relationship between storage modulus and angular frequency, and C represents the relationship between loss modulus and angular frequency).

[0039] Figure 10 The test method and data graph for cherry crack index (CI) in field trials;

[0040] Figure 11 A schematic diagram of cherry blossom crack types;

[0041] Figure 12 The image shows the quality of cherry fruit (including weight (g), size (mm), and color after picking (L*, brightness of cherry, + represents white; a*, redness / greenness of cherry, + represents red) and cracking rate (CR)). Detailed Implementation

[0042] The present invention will now be described in detail with reference to specific embodiments. However, the embodiments are only used to illustrate the present invention, and the scope of protection of the present invention is not limited thereto.

[0043] Example 1: Preparation method of chitosan-based edible film with nano-network structure

[0044] The reagents required for this invention are: chitosan hydrochloride (CHC, degree of deacetylation: 85%), purchased from Jinan Haidebai Marine Biotechnology Co., Ltd. (Jinan, China); and sodium tripolyphosphate (STP, Na5P3O4). 10 98% zeolite and microcrystalline cellulose (MCC, CAS: 9004-34-6, size: 25μm) were obtained from Shanghai Maclean Biochemical Co., Ltd. (Shanghai, China). Zeolite (CAS: 9010-66-6, molecular weight: 2.5×10⁻⁶) was obtained from Shanghai Maclean Biochemical Co., Ltd. (Shanghai, China). 4 Up to 4×10 4 The following reagents were purchased from Sigma-Aldrich Co. Ltd. (St. Louis, Missouri, USA). Glycerin and NaOH (all analytical grade) were purchased from Kangde Chemical Reagent Co., Ltd. (Yantai, China). Ferric chloride hexahydrate (FeCl3·6H2O, analytical grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

[0045] The preparation method of the nano-network structure chitosan-based edible film of the present invention includes the following steps:

[0046] 1) Preparation of phosphorylated zein nanoparticles: Zein (1.00 g) was dissolved and stirred in 50 mL of ultrapure water (1600 r / min, 30 min) at 30 °C, and then dissolved in 50 mL of pH=13 solution, adjusted with 1 mol / L NaOH. STP (1.00 g) was added as a phosphorylation reagent, and the resulting mixture was stirred vigorously at 40 °C (1600 r / min) for 3 h. Afterwards, the mixture was dialyzed in water for 48 h, and the residue in the dialysate was centrifuged at 4000 r / min for 30 min. Finally, the supernatant was frozen at -80 °C for 1 day and freeze-dried in a freeze dryer for 3 days. The PZNPs were then removed and stored in a desiccator at 25 °C.

[0047] 2) Preparation of cellulose nanocrystals: MCC (2.00 g) was added to 100 mL of FeCl3·6H2O solution (1 mol / L) to obtain a fixative solution. The solution was heated and stirred in a water bath (800 r / min, 80 °C) for 6 hours. After the reaction was complete, the supernatant was collected by vacuum filtration, and the precipitate was washed with ultrapure water until it turned white. Finally, the obtained precipitate was frozen in an ultra-low temperature freezer at -80 °C for 1 day, and then freeze-dried in a freeze dryer for 3 days to obtain CNC, which was then stored in a dry container at 25 °C.

[0048] 3) Preparation of coating solutions: Chitosan hydrochloride solution (CHC) was used as the basic coating solution, and phosphorylated zein nanoparticles (PZNP) and cellulose nanocrystals (CNC) were added as reinforcing nanoparticles to obtain four coating solutions: CHC, CHC-PZNP, CHC-CNC, and CHC-PZNP-CNC. The composition of the four film-forming coating solutions is shown in Table 1.

[0049] 4) Preparation of edible films: Add 5% (by weight of CHC) of glycerol as a plasticizer to the coating solution, and degas the solution by sonication at 40 kHz and 25 ℃ for 30 min. After being placed at 25 ℃ for 36-50 h, a film naturally forms, yielding CHC, CHC-PZNP, CHC-CNC, and CHC-PZNP-CNC edible films.

[0050] Table 1. Preparation methods of CHC, CHC-PZNP, CHC-CNC and CHC-PZNP-CNC coating solutions

[0051]

[0052] Example 2: Characterization of Nanoparticles

[0053] Figure 1The FTIR spectra of PZNP and CNC prepared for this invention show that PZNP is at 1122 cm⁻¹. -1 There is a P=O vibration peak at 1013 cm⁻¹. -1 There is an asymmetric vibration at approximately 898 cm, attributed to the tensile vibration of PO. -1 The peak value at 1735 cm⁻¹ was also assigned to the PO tensile vibration, demonstrating that PZNP was successfully grafted onto zein by STP. CNC at 1735 cm⁻¹ -1 The presence of a weak peak at the carboxyl group indicates that MCC is oxidized by Fe. 3+ By disrupting its structure, it can be reduced to the nanoscale.

[0054] As shown in Table 2, the PZNP and CNC prepared in this invention have dimensions of 85.2 nm and 430.4 nm, respectively. Figure 2 AFM images showed that PZNP (72.6 nm) and CNC (417.0 nm) had spherical and rod-like morphologies, respectively, with dimensions consistent with those in Table 2. Zeta potential measurements revealed negative zeta potentials in both PZNP and CNC solutions, with all absolute zeta potentials exceeding 20 mV, indicating excellent stability. Furthermore, the PDI values ​​for PZNP and CNC were 0.382 and 0.348, respectively, indicating good dispersion of both nanoparticles.

[0055] Table 2. Dimensions, Zeta Potential, and PDI of PZNP and CNC

[0056]

[0057] Example 3: Characterization of edible films

[0058] 1. FTIR and XRD analysis

[0059] This invention, through the Figure 3 FTIR analysis of four types of edible films revealed that in pure CHC edible films, the concentration at 3370 cm⁻¹ was [missing information]. -1 The peak at 1518 cm⁻¹ represents the stretching vibration of the hydroxyl group. -1 The absorption peak at that point corresponds to NH3. + A shift to lower wavenumbers is generally considered to indicate richer hydrogen bonding. In edible CHC-PZNP membranes, intermolecular hydrogen bonds are significantly increased (the stretching peak of the hydroxyl group increases from 3370 cm⁻¹). -1 up to 3280 cm -1 ), while the COC peak (1122 cm) -1 and 1013 cm -1 The decrease is due to structural rearrangement caused by excessive hydrogen bonding and NH3. +The displacement (from 1518 cm) -1 up to 1522 cm -1 This leads to electrostatic interactions. In the CHC-CNC edible membrane, hydrogen bonds (from 3370 cm⁻¹) are present. -1 up to 3329 cm -1 A slight increase was observed, with a new weak peak appearing (1715 cm⁻¹). -1 This is considered to be NH3. + The new peak generated by combining with the carboxyl peak in CNC indicates successful inclusion of CHC, followed by NH3. + Displacement (from 1518 cm) -1 up to 1532 cm -1 This proves that electrostatic interaction occurred.

[0060] In the CHC-PZNP-CNC edible membrane, hydrogen bonding is reduced (3288 cm⁻¹). -1 ), compared to CHC-PZNP edible film NH3 + Displacement (from 1518 cm) -1 up to 1526 cm -1 The characteristic CNC peak appears at 1735 cm⁻¹. -1 And a new peak of 1715cm appeared. -1 These changes are due to the fact that both PZNP (-58.5 mV) and CNC (-25.4 mV) have negative surface charges, which repel each other on the positively charged CHC surface, reducing the chance of PZNP and CNC binding to the CHC matrix. Due to the high crystallinity of CNC, the intensity of the COC peak increases compared to the CHC-PZNP edible film, and the doping of CNC enhances the orderliness.

[0061] In summary, FTIR indicates that hydrogen bonding and electrostatic interactions are the main forces between CHC and PZNP and CNC.

[0062] Depend on Figure 4 X-ray diffraction (XRD) analysis showed that the characteristic peak of CHC was located at 2. θ At 11.5°, 18°, and 23°, typical chitosan characteristics are observed. In the CHC-CNC edible film, enhanced crystallinity peaks become apparent at 11.5° and 18°, indicating that the introduction of CNC enhances the overall crystallinity of the composite material; a new peak (16.7°) appears, corresponding to the characteristic peak of CNC. Conversely, in the CHC-PZNP edible film, the peak mainly persists at 2θ=23°, indicating that the zein-chitosan interaction (hydrogen bonding) reduces the crystallinity of chitosan hydrochloride, especially at lower zein concentrations.

[0063] A peak at 16.7° was observed in the CHC-PZNP-CNC edible film, indicating the effective incorporation and retention of the CNC crystalline structure. The addition of PZNP made the edible film more stretchable, while the addition of CNC enhanced the crystallinity of the edible film.

[0064] 2. Wetting properties and surface roughness analysis

[0065] Water contact angle (WCA), surface tension (ST), water content (MC), and swelling degree (SD) are effective methods for measuring the surface wettability of edible films. Water solubility (WS) is a key factor for films suitable for food packaging.

[0066] Figure 5 The results show that the surface of the CHC-PZNP-CNC edible film is rougher (Ra=4.64 nm, Rq=6.16 nm) after CNC doping compared to the CHC-PZNP edible film. This is because the addition of two negatively charged nanoparticles (PZNP and CNC) with positive surface charges to CHC leads to mutual repulsion, resulting in a reduction of hydrogen bonds in the Fourier transform infrared spectrum. Compared to CHC-PZNP, this reduces the surface uniformity of CHC-PZNP-CNC, thus increasing the roughness.

[0067] Depend on Figure 6 It can be seen that adding PZNP to the edible film reduces the surface tension (CHC: 55.3 mN / m, CHC-PZNP: 48.9 mN / m) and enhances hydrophilicity (CHC: 115.7°, CHC-PZNP: 81.7°); adding CNC to the edible film enhances its hydrophobicity (117.4°). Compared with CHC edible film, CHC-PZNP-CNC edible film has better coating adhesion (51.2 mN / m) and good surface hydrophobicity (106.4°), providing better protection in humid environments.

[0068] Due to the hydrogen bonds and electrostatic interactions between the nanofiller and CHC molecules, the movement of CHC molecular chains is restricted, thus preventing the CHC membrane from dissolving in water. Water content (MC) and swelling degree (SD) show that the addition of PZNP to the edible film reduces water absorption, but not significantly, due to its good extensibility and hydrophilic surface. The addition of CNC to the edible film reduces its hydrophobic surface, water content, and swelling degree, as well as its water solubility (WS). This ensures the stability and integrity of the edible film under humid conditions, providing better protection for cherry fruit against cracking after rain.

[0069] Table 3. Wetting properties of the coating

[0070]

[0071] 3. Mechanical properties and barrier properties

[0072] Elastic modulus (YM), tensile strength (TS), and elongation at break (EAB) are important mechanical properties for evaluating the strength and flexibility of films. Water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) are important parameters for evaluating the barrier properties of films.

[0073] Depend on Figure 7 It can be seen that the synergistic effect of PZNP and CNC improves the mechanical and barrier properties of edible films: compared with CHC edible film (25.3 MPa, 2.9%), the introduction of PZNP and CNC improves the mechanical properties of CHC-PZNP-CNC edible film by 67% and 167%, respectively, with TS at 37.5 MPa and EAB at 7.76%. Compared with CHC edible film, the WVP of CHC-PZNP-CNC edible film (from 6.75 × 10⁻⁶ MPa) is significantly improved. -10 gm -1 Pa -1 s -1 Increased to 5.42 × 10 -10 gm -1 Pa -1 s -1 ) and OTR (from 346.83 cm 3 / m 2 The day length increased to 243.32 cm. 3 / m 2 The daily rate also improved.

[0074] As shown in Table 4, the CHC edible film exhibited the lowest stress transfer (TS) (25.32 MPa) and elastic modulus (EAB) (2.9%). The addition of PZNP and CNC improved the mechanical properties of the CHC film (p<0.05). The addition of PZNP improved ductility, significantly increased EAB (from 2.9% to 10.0%), and reduced surface roughness (CHC: Ra = 5.98 nm, Rq = 7.33 nm vs. CHC-PZNP: Ra = 1.89 nm, Rq = 2.09 nm), which is beneficial for stress transfer and thus improves TS (from 25.3 MPa to 30.8 MPa). Because PZNP is uniformly dispersed in the CHC matrix, the elastic modulus (decreased from 873.8 MPa to 307.9 MPa) is significantly reduced, which is beneficial for ductility.

[0075] Compared with CHC edible film, the addition of CNC slightly enhanced the effect of CHC-CNC edible film on TS (22%) and EAB (29%). The lattice structure of CHC was not affected due to the weak interaction observed in FTIR. This is due to the charge repulsion effect of the ionic bond between CHC and CNC in low concentrations of CHC, as well as the charge enhancement effect of CNC on TS and EAB.

[0076] The improvements in mechanical properties from CHC edible films to CHC-PZNP-CNC edible films (elastic modulus, CHC: 873.8 MPa vs. CHC-PZNP-CNC: 482.2 MPa; TS, CHC: 25.3 MPa vs. CHC-PZNP-CNC: 37.5 MPa; EAB, CHC: 2.9% vs. CHC-PZNP-CNC: 7.76%) can be attributed to the use of low concentrations of PZNP and CNC with the CHC matrix through hydrogen bonding and electrostatic interactions, and the doping of CNC as a nanoparticle filling effect, leading to increased strength of the CHC-PZNP edible films (as shown in Table 4). XRD shows that CNC enhances crystallinity, achieving a relatively compact CHC-PZNP-CNC film structure.

[0077] Table 4. Mechanical properties of the coating

[0078]

[0079] Table 5 illustrates the role of CNC and PZNP in reducing WVP in CHC-based films. The synergistic interaction between these nanoparticles enhances the barrier properties within the film. Simultaneously, the increase in thickness and density confirms densification. This enhancement can be understood through two distinct mechanisms. First, the filling effect introduced by the nanoparticles promotes greater interactions between polymer chains. Fourier transform infrared spectroscopy highlights the formation of hydrogen bonds and electrostatic interactions between the nanoparticles (PZNP and CNC) and the CHC matrix. These interactions limit the affinity of the CHC-based film for water molecules, effectively hindering their ability to penetrate the material. This formation of hydrogen bonds and electrostatic interactions enhances the overall structural integrity of the film. Second, the incorporation of nanoparticles leads to a reduction in the free volume present in the polymer matrix. The decrease in molecular mobility restricts chain movement, ultimately contributing to improved barrier quality.

[0080] This collective synergy between thin-film interaction mechanisms and densification has proven to be a comprehensive approach to enhancing the barrier properties of coatings, thereby reducing cherry rain cracking.

[0081] Table 5. Barrier properties, thickness, and density of the coating

[0082]

[0083] 4. Rheological behavior of the coating

[0084] Rheological behavior reflects the spreading ability, uniformity, and film-forming properties of fluid coatings. For example... Figure 9 As shown, the viscosity of all film-forming solutions decreases with increasing shear rate, indicating that these solutions are pseudoplastic fluids exhibiting shear thinning and typical non-Newtonian behavior. Furthermore, this invention also measured the dynamic rheological behavior of the film-forming solutions through oscillation experiments. The G′ and G′′ values ​​of the CHC-based film-forming solutions increased with increasing rotation frequency, while the G′ value of the film-forming solutions was lower than G′′, indicating that the CHC-based solutions belong to a sol system and possess good film-forming properties.

[0085] The viscosity of CHC-PZNP and CHC-CNC solutions is higher than that of pure CHC solutions. This is because PZNP and CNC, as nanofillers, are effectively dispersed in the CHC matrix, enhancing the network structure of CHC and improving the viscoelasticity of the film-forming solution. The viscosity of CHC-PZNP-CNC solutions falls between that of CHC-PZNP and CHC-PZNP-CNC film-forming solutions. This is due to the weakening of the interactions (hydrogen bonds and electrostatic interactions) between PZNP and CNC in the CHC-PZNP-CNC solutions formed by their addition.

[0086] Therefore, CHC-PZNP-CNC exhibits excellent film-forming properties due to its rheological behavior.

[0087] Example 4: Field Trial

[0088] A field trial was conducted in 2022-2023 in a commercial orchard in Baishahe Street, Pingdu City, Qingdao (36°79′N, 120°08′E, altitude 64 meters). During the cherry growth process, Tieton Fruit trees (7 years old) and Cerasus pseudocerasus The efficiency of CHC-based coatings in reducing cherry splitting was tested on 5-year-old fruit trees. 150 trees ( 'Tieton' 75 trees; selected from those evenly distributed (row spacing: 5.00 m, row spacing: 3.00 m). 'Cerasus pseudocerasus' 75) As experimental trees in greenhouses and open fields. All experimental trees were cultivated and managed in accordance with local commercial orchard requirements prior to field trials.

[0089] 1. Cracking index (CI)

[0090] Approximately 150 ripe, unopened cherries (each fruit weighing ≥12.0 g, with a transverse diameter ≥3.00 cm, a longitudinal diameter ≥2.50 cm, and a color difference L*≤26 and a*≤15) were randomly selected from normally growing cherry trees (approximately 75 trees in a greenhouse and 75 trees in the open field). Each cherry was treated with four layers of coating (see Table 1). A control group (untreated) was also selected. Therefore, each group contained 30 fruits, and the experiment was repeated three times.

[0091] A motorized sprayer (pressure: 1.6-2.0 Bar, capacity: 2.0 L) was used to apply the spray to the surface of the cherry fruit, ensuring uniform treatment at a fixed spraying time of 10 seconds. After spraying, a naturally formed film was formed on the cherry surface. The cherries were then picked off with their stems attached and brought back to the laboratory, where they were immediately immersed in ultrapure water at 20 °C for 6 hours. Cracked cherries were counted every hour, and the CI was calculated using the following formula:

[0092]

[0093] Note: a, b, c, d, e, and f represent the number of cherry cracks per hour; MPV is the theoretical maximum crack value (30 fruits × 6 h = 180).

[0094] Based on CI testing results, the CHC-PZNP-CNC coating was selected for application in greenhouse and open-field cultivation in Pingdu, Qingdao, China. Tieton Cherries and Cerasus pseudocerasus cherry. Figure 10 , 11 The study demonstrates the adhesion of the coating on the fruit surface and comprehensively describes the occurrence of rain-induced cracking in cherries, including the stem tip, cheek, and surrounding areas (stem cavity area, cheek area, and style scar area). Notably, cherry rain-induced cracking occurs during ripening, frequently in humid weather, after rain or fog in greenhouses, while in open-air environments, cracking coincides with rain or fog. In greenhouses, the cheek area of ​​the cherry experiences the primary cracking phenomenon, while in open-air conditions, due to water accumulation in this area, cracking is mainly observed around the cherry stem cavity, indicating excessive water absorption by the peel.

[0095] 0 as Figure 12As shown, in the CI test, the CHC-PZNP-CNC coating treatment produced the most favorable data (control group: 42.3% vs. CHC; 35.3% vs. CHC-PZNP; 25.7% vs. CHC-CNC; 35.0% vs. CHC-PZNP-CNC; 19.7%). The CHC coating provided some protection for cherries in the CI test (control group: 42.3% vs. CHC; 35.3%). The CI test results for CHC-PZNP were lower compared to both CHC and CHC-CNC. This effect can be attributed to enhanced wettability (ST: CHC: 55.32 mN / m vs. CHC-PZNP: 48.91 mN / m vs. CHC-CNC: 58.65 mN / m), which facilitates greater adhesion and diffusion of the coating on the cherry fruit surface. With the addition of CNC, CHC-PZNP-CNC achieves enhanced surface hydrophobicity (WCA, CHC-PZNP: 81.7°, while CHC-PZNP-CNC: 106.4°) and a denser coating (density: CHC-PZNP: 1.11 g / cm³). 3 , vs. CHC-PZNP-CNC: 1.35 g / cm 3 The results showed that the CI test was significantly reduced using the CHC-PZNP-CNC coating (from 42.3% to 19.7%).

[0096] 2. Cracking ratio (CR)

[0097] Four coatings, CHC-PZNP, CHC-CNC, and CHC-PZNP-CNC, were tested respectively. Each group consisted of 100 non-adjacent fruit trees. Tieton ': 50 trees;' Cerasus pseudocerasus' Fifty trees were selected, each protected by surrounding buffer trees. Three to five clusters of cherry trees (mid-to-late expansion) around each branch were taken as experimental samples. Each selected cluster contained 15 to 30 cherries that were able to receive sufficient sunlight and rain and were easy to apply the coating solution to.

[0098] The spraying procedures and details were as described above. After application, the cherries were allowed to continue growing normally on the tree, in the greenhouse, and in the open air, respectively. The first phase of the experiment was conducted from April 2 to April 17, 2023, and the second phase was conducted from May 25 to June 11, 2023.

[0099] Weather data from the China Meteorological Information Sharing System (CIMISS, 2023) shows that during the experimental period from April 2nd to April 17th, 2023, there were two rainfall events (total rainfall: 52.9 mm) and three rainfall events (total rainfall: 22.5 mm) on May 25th, 2023. From April 3rd to June 11th, 2023, rainfall occurred on May 28th, 2023 (20.0 mm), May 29th, 2023 (2.0 mm), and June 10th, 2023 (0.5 mm). At the end of the field trial, cherries were harvested and CR (chromatographic response) was evaluated using the following formula:

[0100]

[0101] Note: C represents the number of cherries that split open in each group (control and CHC-PZNP-CNC, respectively), and T represents the total number of cherries in each group (control group and CHC-PZNP-CNC, 500-600 fruits, respectively).

[0102] The CHC-PZNP-CNC coating significantly reduced the incidence of cherry blossom cracking from 35.5% to 15.8%. Figure 7 Furthermore, quality tests on cherry fruit showed no statistically significant differences in weight, size, and color. Field trials ultimately verified that the CHC-PZNP-CNC coating possesses excellent water resistance (WVP = 5.42 × 10⁻⁶). -10 gm -1 Pa -1 s -1 The CHC-PZNP-CNC coating exhibits excellent wettability (ST=51.16 mN / m) and surface hydrophobicity (WCA=106.4°), effectively reducing cherry rain-induced cracking. Field studies have proven this to be a viable method for reducing cherry rain-induced cracking. Furthermore, it is noteworthy that the development of this systematic study has resulted in a validated waterproof CHC-PZNP-CNC coating with significantly improved wettability and barrier properties, achieving a major breakthrough in reducing cherry rain cracking while maintaining optimal cherry growth and quality.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions of the above embodiments or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A method for preparing a nano-network structured chitosan-based edible film, characterized in that: The preparation method includes the following steps: 1) Preparation of phosphorylated zein nanoparticles PZNP: Zein was added to water at 10-50 °C, dissolved and stirred, then added to an alkaline solution, and a phosphorylation reagent was added. The mixture was stirred at 10-80 °C for 1-5 h to obtain a mixed solution. The mixed solution was dialyzed in water for 12-72 h to obtain a dialysate. The residue in the dialysate was centrifuged, and the supernatant was freeze-dried to obtain phosphorylated zein nanoparticles PZNP. 2) Preparation of cellulose nanocrystals (CNC): Microcrystalline cellulose was added to FeCl3·6H2O solution and heated and stirred in a water bath for 4-8 hours. After the reaction was completed, the precipitate was collected by vacuum filtration, washed with ultrapure water until white, and the obtained precipitate was freeze-dried to obtain cellulose nanocrystals (CNC). 3) Preparation of coating solution: Chitosan hydrochloride solution CHC is used as the basic coating solution, and phosphorylated zein nanoparticles PZNP and cellulose nanocrystals CNC are added as reinforcing nanoparticles to obtain CHC-PZNP-CNC coating solution. The specific preparation method of the CHC-PZNP-CNC coating solution is as follows: Chitosan hydrochloride was added to water and stirred until completely dissolved to obtain a chitosan hydrochloride solution (CHC). Phosphorylated zein nanoparticles (PZNP) and cellulose nanocrystals (CNC) were separately dispersed in water and added to the chitosan hydrochloride solution (CHC). After stirring until homogeneous, a CHC-PZNP-CNC coating solution was obtained. The mass concentrations of chitosan hydrochloride, phosphorylated zein nanoparticles (PZNP), and cellulose nanocrystals (CNC) in the CHC-PZNP-CNC coating solution were 20 mg / mL, 0.4 mg / mL, and 0.4 mg / mL, respectively. In the above method for preparing the CHC-PZNP-CNC coating solution, the temperature is 10~50℃; 4) Preparation of edible film: Add plasticizer to the CHC-PZNP-CNC coating solution, degas it by ultrasound, pour it onto an acrylic plate, and place it at 10-50℃ for 36-50h to form a film naturally, thus obtaining CHC-PZNP-CNC edible film.

2. The method for preparing the nano-network structure chitosan-based edible film according to claim 1, characterized in that: The freeze-drying process in step 1) is as follows: the supernatant is frozen in an ultra-low temperature freezer at -80 ℃ for 0.5-2 days, and then freeze-dried in a freeze dryer for 2-5 days. After being taken out, it is stored in a dry container at 10-50 ℃. The pH value of the alkaline solution is 12-14, and the phosphorylation reagent is sodium tripolyphosphate. The water bath heating and stirring conditions in step 2) are 80 ℃ and 500-1500 r / min.

3. The method for preparing the nano-network structure chitosan-based edible film according to claim 1, characterized in that: In step 4), the plasticizer is glycerol, and the mass ratio of glycerol to chitosan hydrochloride is 1:15-30. The ultrasonic frequency is 20-60 kHz, the temperature is 10-50℃, and the time is 10-60 min.

4. The nano-network structure chitosan-based edible film prepared by any of the preparation methods described in claims 1-3.

5. The application of the nano-network structure chitosan-based edible film according to claim 4 in reducing cherry cracking.

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