Modified starch film solution, its preparation method and application in protecting fruits

Abstract

The present application relates to the field of agricultural biomaterials and plant protection technology, and provides a preparation method of modified starch film liquid, which comprises the following steps: (1) preparing an ethanol starch dispersion liquid; (2) preparing a hydroxypropyl distarch phosphate gelatinization liquid; (3) mixing a first plasticizer, a film-forming auxiliary agent and a film-forming enhancer with water to prepare a glue liquid; (4) adding the glue liquid into the hydroxypropyl distarch phosphate gelatinization liquid to obtain a mixed glue liquid; (5) adding the ethanol starch dispersion liquid into the mixed glue liquid; (6) adding an inorganic filler; (7) adding a second plasticizer, water-bathing and stirring to mix uniformly; and (8) adding water to constant volume, cooling to room temperature, and standing to obtain the modified starch film liquid.The present application further provides a modified starch film liquid and application thereof, and a method for protecting fruits.The modified starch film liquid prepared by the present application can be directly sprayed on the surface of fruits to form a protective film, so that the fruits are protected from the damage of diseases and pests.

Description

Technical Field

[0001] This invention relates to the fields of agricultural biomaterials and plant protection technology, and in particular to a modified starch film liquid, its preparation method, and its application in protecting fruits. Background Technology

[0002] In the production of fruits such as apples, pears, and peaches, fruit bagging is an important agronomic measure to prevent pests and diseases, reduce pesticide residues, and improve the smoothness of the fruit surface. Traditional bagging methods mainly use paper or plastic fruit bags, but these methods have problems such as high labor costs, environmental pollution from materials, poor air permeability of the fruit bags, and inability to control pathogens already present on the fruit surface. Starch, as a natural polymer material, has film-forming properties and can be made into liquid starch films. By spraying, a film layer is formed on the surface of the fruit to achieve the purpose of protecting the fruit. Starch has advantages such as wide availability, low price, good film-forming properties, and biodegradability.

[0003] Chinese patent CN 114916358 A discloses the application of edible corn starch film in fruit bagging, using high amylose corn starch, the main component of corn, as raw material to develop a corn starch-glycerol edible liquid agricultural film. However, the following problems exist: Firstly, films formed from natural pure starch or simply modified starch suffer from defects such as high brittleness, poor water resistance, and easy cracking. This patent uses high-amylose corn starch as the main film-forming component. Its molecules have a long-chain linear structure, and the molecules easily form a tight arrangement and crystallization through numerous hydrogen bonds. While this characteristic can form a high-strength rigid gel, it also results in a hard and brittle film with insufficient toughness. Although patent CN 114916358 A adds chitosan and glycerol to corn starch to modify the properties of the starch film, both chitosan and glycerol are hydrophilic substances that easily swell and leak when exposed to water, resulting in poor water resistance, easy cracking, and an inability to form a strong bond with fruit peels. During drying and shrinkage, the film is prone to peeling and has poor adhesion, exhibiting significant defects in adhering to the fruit surface and toughness. These defects mean that the film of this patent cannot meet the needs of long-term fruit protection under complex climatic conditions such as rainwater runoff and high-temperature sun exposure in the field, and there are currently no actual production application cases.

[0004] In addition, this patent uses corn starch as raw material, which requires gelatinization during the production process. This operation is not easy to control and is greatly affected by factors such as temperature, time, and stirring. It is very easy to have problems such as insufficient gelatinization (poor film-forming properties) or excessive gelatinization (starch denaturation), which affects film-forming properties, resulting in unstable final product quality. Moreover, the operation is cumbersome and not convenient for commercialization and promotion. Summary of the Invention

[0005] This invention aims to address the shortcomings of existing technologies by providing a modified starch film solution, its preparation method, and its application in fruit protection. The modified starch film solution prepared by this invention can be directly sprayed onto the fruit surface to form a protective film, protecting the fruit from pests and diseases.

[0006] To address the above problems, the present invention provides the following technical solution: A method for preparing a modified starch film solution includes the following steps: (1) Mix pregelatinized starch with ethanol to prepare an ethanol-starch dispersion; (2) Dissolve hydroxypropyl distarch phosphate in water, heat to gelatinize, cool to obtain hydroxypropyl distarch phosphate gelatinized solution; (3) Dissolve the first plasticizer, film-forming aid, and film-forming enhancer in water to prepare a glue solution; (4) Add the adhesive solution obtained in step (3) to the hydroxypropyl distarch phosphate gelatinized solution obtained in step (2) to obtain a mixed adhesive solution; (5) Add the ethanol starch dispersion obtained in step (1) to the mixed adhesive solution obtained in step (4) and stir to mix evenly; (6) Add inorganic filler and stir to mix evenly; (7) Add the second plasticizer, place in a water bath, and stir until well mixed; (8) Add water to make up the volume, stir evenly, cool to room temperature, and let stand to obtain modified starch film solution.

[0007] In the preparation method of the modified starch film liquid as described above, in step (1), the pregelatinized starch is selected from any one of food-grade pregelatinized corn starch, pregelatinized potato starch, and pregelatinized sweet potato starch. Preferably, in step (1), the pregelatinized starch is selected from food-grade pregelatinized corn starch. Through extensive screening experiments, this invention, starting from improving the starch chain structure, uses pregelatinized starch as the main material to enhance the stability of film formation. Industrial standardized gelatinization makes the starch granules break down and the molecular chains extend more fully and uniformly, greatly improving the stability of film quality.

[0008] In the preparation method of the modified starch film solution described above, the volume fraction of ethanol is 90%–95%, and the volume ratio (mL / g) of ethanol to pregelatinized starch is (1–3):1. Ethanol, as an organic solvent, accelerates the dissolution of starch, reduces the viscosity of the finished modified starch film solution, prevents starch agglomeration, and facilitates full dispersion. Preferably, in step (1), the volume fraction of ethanol is 95%, and the volume ratio (mL / g) of ethanol to pregelatinized starch is 1.5:1. Under these conditions, the appropriate amount of ethanol can promote the full dissolution of starch, avoid the generation of undissolved particles, effectively regulate the solvent evaporation rate, improve the density and uniformity of the film, and achieve good film formation.

[0009] In the preparation method of the modified starch film liquid described above, the mass ratio of the pregelatinized starch to the hydroxypropyl distarch phosphate is (1:2) to (2:1). Preferably, the mass ratio of the pregelatinized starch to the hydroxypropyl distarch phosphate is 1:1. In this invention, considering the requirements for film formation and adhesion of the starch film, corn starch is used as the main starch component of the film agent. Starch film formation requires gelatinization, so pregelatinized corn starch is selected. After pregelatinization, the crystalline structure is destroyed, and it has cold water solubility. Modified starch hydroxypropyl distarch phosphate is selected as another starch component. Hydroxypropyl distarch phosphate has strong stability to temperature, acidity, and shear force, weak sedimentation, and good salt resistance. It can maintain stable performance in high-salt and acidic environments, while also having good water retention capacity and freeze-thaw resistance. By compounding the pregelatinized starch and the hydroxypropyl distarch phosphate at a mass ratio of 1:1, the film-forming properties and adhesion of the modified starch film are controlled. Experimental results show that pregelatinized corn starch provides rapid initial adhesion, while hydroxypropyl distarch phosphate continuously provides flexibility to buffer shrinkage stress during subsequent drying. The synergistic effect of the two significantly improves the adhesion stability between the film layer and the leaf surface, resulting in strong flexibility and no edge lifting of the film spot.

[0010] In the modified starch film preparation method described above, in step (2), the heating and gelatinization temperature is 75-85℃ and the time is 10-20 min. If the temperature is too low (<75℃), the heat is insufficient to completely melt the starch crystallization zone. Even if the heating time is extended, the particles cannot fully absorb water and expand, and the molecular chains cannot effectively extend, resulting in incomplete gelatinization. After film formation, the film layer is uneven, has poor adhesion, and is prone to a grainy feel or insufficient strength.

[0011] Too short a time (<10 min): Even if the gelatinization temperature reaches the required level, the starch granules will not have enough time to fully absorb water, swell, crystallize, and melt. The degree of gelatinization is low, and the remaining intact granule structure will cause the film layer to become less continuous, making the film brittle and prone to breakage.

[0012] Excessive temperature (>85℃): Starch granules expand excessively and rupture, a large amount of amylose dissolves, molecular chains are overextended or even thermally degraded, and aging (recrystallization) occurs rapidly upon cooling, resulting in brittle film, significantly reduced flexibility, and easy cracking.

[0013] Excessive heating time (>20 min): Prolonged heating exacerbates excessive gelatinization and molecular degradation, promotes excessive dissolution of amylose, and also strengthens the aging tendency, resulting in a significant increase in the brittleness of the final film layer and a severe reduction in flexibility and crack resistance.

[0014] Preferably, in step (2), the heating and gelatinization temperature is 80°C and the time is 15 min.

[0015] In the modified starch film preparation method described above, in step (3), the first plasticizer is selected from at least one of glycerol, propylene glycol, butylene glycol, PEG4000, and PEG6000; in step (7), the second plasticizer is selected from at least one of sorbitol, xylitol, maltitol, mannitol, isomaltitol, and lactitol; and the first plasticizer and the second plasticizer are different. Preferably, in step (3), the first plasticizer is selected from glycerol; in step (7), the second plasticizer is selected from sorbitol. More preferably, in step (3), the amount of glycerol added is 50% to 70% of the mass of the pregelatinized starch; in step (7), the amount of sorbitol added is 15% to 25% of the mass of the pregelatinized starch. Under these conditions, the film surface is smooth and flat, without cracks or particle agglomeration, and the film-forming effect is good.

[0016] In the preparation method of the modified starch film liquid as described above, in step (6), the inorganic filler is selected from at least one of kaolin, bentonite, montmorillonite, talc, and diatomaceous earth.

[0017] In the preparation method of the modified starch film solution as described above, in step (3), the film-forming aid is selected from at least one of sodium alginate, xanthan gum, guar gum, and gum arabic. Preferably, in step (3), the film-forming aid is selected from sodium alginate.

[0018] In the preparation method of the modified starch film liquid as described above, in step (3), the film-forming enhancer is selected from at least one of hydroxypropyl methylcellulose (in this invention, or simply referred to as "HPMC"), hydroxypropyl cellulose, methylcellulose, hydroxyethyl methylcellulose, and hydroxyethyl cellulose.

[0019] Preferably, in step (6), the inorganic filler is selected from kaolin; in step (3), the film-forming enhancer is selected from HPMC. Kaolin, as a layered silicate mineral filler, can play a dual role after being uniformly dispersed in the polymer matrix: firstly, it extends the permeation path of water molecules in the membrane layer, improving barrier performance; secondly, it fills the micropores inside the membrane layer, improving membrane density. HPMC molecular chains contain abundant hydroxypropyl and methyl groups, which can form hydrogen bonds and physical entanglements with the hydroxyl groups on the starch molecular chains, constructing a three-dimensional cross-linked network structure, thereby effectively improving the mechanical strength and flexibility of the membrane layer. When the mass ratio of kaolin to HPMC is 2:3, the tensile strength and elongation at break of the prepared modified starch membrane are much higher than those of the group with only kaolin, the group with only HPMC, and the group without either HPMC or kaolin. This indicates that HPMC and kaolin have a synergistic effect in improving the mechanical properties and weather resistance of the membrane layer.

[0020] This invention uses pregelatinized starch and hydroxypropyl distarch phosphate in a certain proportion as the starch film skeleton. Glycerin and sorbitol plasticizers are added to improve the film-forming effect. Hydroxypropyl methylcellulose (HPMC) and sodium alginate are added to reshape the starch film-forming structure and enhance the flexibility of the film. Kaolin is added as a filler to improve water erosion resistance.

[0021] This invention utilizes a 1:1 mass ratio of hydroxypropyl distarch phosphate to pregelatinized starch to improve the tensile strength and water resistance of starch films. Hydroxypropyl distarch phosphate is a product of corn starch undergoing dual chemical modification through hydroxypropylation and phosphate crosslinking. The phosphate crosslinks in its molecular structure provide shear stability and hydrophobicity, while the hydroxypropyl groups weaken excessive hydrogen bonding between starch molecular chains through steric hindrance, enhancing film flexibility and anti-aging capabilities. Pregelatinized starch, when fully dispersed in an ethanol system, rapidly forms a film and provides excellent initial adhesion. When used alone, the film edges begin to peel and crack after 3 days; hydroxypropyl distarch phosphate alone tends to peel and bubble after 2 days. However, when the two are blended in an equal proportion, they remain tightly adhered without peeling even after 7 days, demonstrating significantly better film-forming performance than single components. To further enhance the adhesion and rain erosion resistance of the starch film, HPMC and kaolin are added as functional fillers. Layered ultrafine kaolin, synergistically with HPMC and a double starch network, constructs a dense water-blocking layer, effectively enhancing the starch film's resistance to rain erosion. When HPMC is added alone, the tensile strength is 2.18 MPa and the elongation at break is 35.40%; when kaolin is added alone, the tensile strength is 1.25 MPa and the elongation at break is 23.60%. The combined effect of the two results in a tensile strength of 3.65 MPa and an elongation at break of 52.80%, demonstrating a significant synergistic improvement. Furthermore, the adsorption properties of kaolin particles provide an attachment medium for biocontrol bacteria or adsorb pesticides for slow-release purposes.

[0022] Based on the same inventive concept, this invention provides a modified starch film liquid, prepared by the method described above. The modified starch film liquid provided by this invention can be sprayed onto the surface of fruits to protect them.

[0023] Based on the same inventive concept, this invention provides a method for protecting fruit by mixing a modified starch film solution prepared as described above, or a modified starch film solution prepared as described above, with a biocontrol fungal spore suspension, and spraying the mixture onto the fruit surface. In this invention, the biocontrol fungal spores are Trichoderma spores. During use, the modified starch film solution is diluted with water to a final starch concentration of 2%, and the Trichoderma spore suspension is mixed evenly with the diluted film solution at a 1:1 volume ratio. Experimental results show that the modified starch film solution has no inhibitory effect on the growth of Trichoderma biocontrol fungi. Biocontrol fungi can be added during use to improve the survival rate and field colonization rate of the biocontrol fungi.

[0024] Based on the same inventive concept, the present invention provides a modified starch film prepared by the method described above, or the application of the modified starch film as described above in protecting fruits.

[0025] Compared with the prior art, the present invention has the following beneficial effects: 1. Green and Environmentally Friendly: This invention modifies and optimizes the formed membrane from the perspective of starch chain structure, making it superior to using natural starch directly. Multiple embodiments have verified the membrane's stability, safety, and practicality, allowing for direct application in green fruit production. All raw materials are food-grade or of natural origin, and are biodegradable after use, posing no environmental pollution problems.

[0026] 2. Functional and flexible formulation: The modified starch film has a certain degree of air permeability, which can regulate the fruit microenvironment. Chemical agents or biocontrol agents can be added to the film solution to achieve synergistic effects of physical protection and disease control.

[0027] 3. Good adhesion and flexibility: In view of the problems of high brittleness and easy peeling of starch films in existing technologies, the present invention uses a 1:1 compound of pregelatinized starch and hydroxypropyl distarch phosphate. The pregelatinized starch provides rapid initial adhesion, while the hydroxypropyl distarch phosphate buffers the drying shrinkage stress. The synergistic effect of the two makes the film layer adhere tightly to the fruit surface for 7 days without peeling, which significantly improves the adhesion stability between the film layer and the fruit peel surface.

[0028] 4. Excellent mechanical properties: Addressing the issue of insufficient flexibility in existing starch films, this invention utilizes the synergistic enhancement of the film layer through HPMC and kaolin. Experimental results show that after HPMC and kaolin are combined, the tensile strength of the film reaches 3.65 MPa, and the elongation at break reaches 52.80%, significantly enhancing the film's crack resistance and flexibility.

[0029] 5. Excellent water erosion resistance: Addressing the problem of starch films easily swelling and losing moisture upon contact with water in existing technologies, this invention synergistically constructs a dense water-blocking layer through the network cross-linking of HPMC and the layered barrier properties of kaolin. Simultaneously, the plasticizer effect of glycerol and sorbitol ensures a uniform and dense film layer free of micro-cracks, preventing water penetration along defects and further enhancing water erosion resistance. Experimental results show that after three sprayings, the film layer plasticized with glycerol and sorbitol only lost 5.57% of its moisture, demonstrating excellent water erosion resistance.

[0030] 6. Good storage stability: After standing for 30 days, the modified starch film solution prepared by this invention maintains a uniform and stable state, without any layering, precipitation or gelation, and the fluidity is not significantly changed, showing good storage stability.

[0031] 7. Saves labor and effort: Mechanized spraying replaces manual bagging and bag removal, significantly reducing orchard management costs and conforming to the trend of modern agriculture towards simplification.

[0032] 8. High biosafety: The membrane has no adverse effects on leaf and fruit growth. Attached Figure Description

[0033] Figure 1 The results are observed under a microscope at 200x magnification of the modified starch film solution. Among them, (A) the film formation when the amount of 95% ethanol aqueous solution added accounts for 5% of the total volume of the starch film; (B) the film formation when the amount of 95% ethanol aqueous solution added accounts for 7.5% of the total volume of the starch film. Figure 2 The effect of spraying modified starch film solution on apples is shown in the following figures: (A) front view of apple; (B) side view of apple. Figure 3 The results of Trichoderma conidia germination include (A) 2% modified starch film solution and (B) sterile water. Figure 4 The growth status of Chinese cabbage 5 days after spraying the treatment agent is shown. Among them, (A) 2% modified starch film solution; (B) 7.5% 95% ethanol solution; (C) water control. Figure 5 To investigate the effects of different treatments on the control of apple anthracnose, the following treatments were used: (A) 2% modified starch film solution; (B) chitosan apple fruit protective film; and (C) water control. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. The term "and / or" as used in this specification includes any and all combinations of one or more of the associated listed items.

[0036] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0037] Example 1: Preparation method of modified starch film solution 1. Experimental materials: 100 g of pregelatinized corn starch (purchased from Henan Wanbang Chemical Technology Co., Ltd.), 100 g of hydroxypropyl distarch phosphate (purchased from Henan Wanbang Chemical Technology Co., Ltd.), 60 g of glycerol, 25 g of sorbitol, 25 g of sodium alginate, 30 g of HPMC, 20 g of kaolin, and 150 mL of 95% ethanol aqueous solution.

[0038] 2. Preparation steps of modified starch film solution: (1) Measure 150 mL of 95% ethanol, add 100 g of pregelatinized corn starch, stir with a glass rod for 15 min to mix, and prepare ethanol starch dispersion; (2) Take another beaker, add 200 mL of distilled water, add 100 g of hydroxypropyl distarch phosphate, stir with a glass rod for 5-15 min until dissolved; heat to 80℃, stir with a glass rod for 15 min to gelatinize, cool to 50℃ for later use, and obtain hydroxypropyl distarch phosphate gelatinized liquid. (3) Take 60 g of glycerin, add 25 g of sodium alginate, stir with a glass rod to moisten; add 30 g of HPMC and stir well, add 100 mL of 50℃ first grade water, stir with a glass rod until dissolved, and make a gel solution; (4) Add the adhesive solution obtained in step (3) to the hydroxypropyl distarch phosphate gelatinized solution obtained in step (2), and stir evenly with a glass rod to obtain a mixed adhesive solution; (5) Slowly add the ethanol starch dispersion obtained in step (1) to the mixed gel obtained in step (4), and stir with a glass rod for 10 min to mix evenly. (6) Add 20 g of kaolin and stir with a glass rod for 10 min to mix evenly; (7) Add 25 g of sorbitol and stir well. Place in a 50°C water bath and stir with a glass rod for 20 min to mix evenly. (8) Add water to a final volume of 2000 mL, stir with a glass rod until homogeneous, cool to room temperature, and let stand for 30 min to obtain the modified starch film solution.

[0039] Example 2: Screening and optimization of modified starch film-forming agent components To successfully prepare modified starch films with good film-forming effect, strong adhesion, and good water resistance, the effects of different components on the performance of starch films were studied, and the following screening and optimization experiments were conducted: 1. Screening of starch as the main component of modified starch film (1) Experimental method: To clarify the film-forming properties and adhesion to plant surfaces of the two modified starches, pregelatinized corn starch and hydroxypropyl distarch phosphate, the following modified starch ratio was established: Treatment group 1: Pregelatinized corn starch; Treatment group 2: Hydroxypropyl distarch phosphate; Treatment Group 3: Pregelatinized corn starch and hydroxypropyl distarch phosphate were mixed in a mass ratio of 1:1, i.e. Example 1.

[0040] The preparation process of the modified starch film solution is the same as that in Example 1, but the treatment group 1 omits step (2) of hydroxypropyl distarch phosphate treatment, and deletes the operations related to hydroxypropyl distarch phosphate in steps (3) to (8) accordingly. Finally, water is added to make up to 2000 mL. The treatment group 2 omits step (1) of pregelatinized corn starch treatment, and deletes the operations related to pregelatinized corn starch in steps (3) to (8) accordingly. Finally, water is added to make up to 2000 mL. The treatment group 3 is the same as in Example 1.

[0041] Three modified starch film solutions were diluted to a final starch concentration of 5% and sprayed onto the surface of pakchoi leaves and glass slides, respectively. The adhesion between the film and the carrier was observed after film formation. In the leaf experiment, each group treated 5 pakchoi seedlings, and 3 leaves of each seedling were sprayed. The results were observed for 7 consecutive days. In the slide experiment, 3 samples were prepared for each group, dried at room temperature, and the state of the film was observed under a microscope. The results are shown in Table 1.

[0042] (2) Experimental results and analysis: Table 1. Test results of the performance of three modified starch films

[0043] As shown in Table 1, the modified starch film with pregelatinized starch as the component in treatment group 1 has a fast film-forming speed but slightly poor flexibility, and is prone to internal stress during drying shrinkage, leading to edge curling; the modified starch film with hydroxypropyl distarch phosphate as the component in treatment group 2 has good flexibility but insufficient adhesion to the leaf surface; while in treatment group 3, where the two are combined in a 1:1 ratio, the pregelatinized starch provides rapid initial adhesion, and the hydroxypropyl distarch phosphate continuously provides flexibility during the subsequent drying process to buffer shrinkage stress. The synergistic effect of the two significantly improves the adhesion stability between the film layer and the leaf surface, with strong flexibility and no edge curling of the film spot.

[0044] The combination of pregelatinized starch and hydroxypropyl distarch phosphate exhibits significantly better film-forming properties and adhesion than either component alone. When the mass ratio of the two components is in the range of 1:2 to 2:1, the film layers show good adhesion and the film edges do not curl up. Among these, a mass ratio of 1:1 results in the best film uniformity and overall performance, and is therefore the preferred option.

[0045] 2. Optimization of the ethanol ratio in modified starch films (1) Experimental method: To clarify the effect of the ethanol ratio on film-forming properties and membrane characteristics, following the preparation steps of Example 1, and keeping other components constant, four treatments were set up with the percentage of 95% ethanol aqueous solution added to the total volume of the starch film being 0%, 5%, 7.5%, and 15%, respectively. This corresponds to the addition amounts of 0, 100 mL, 150 mL, and 300 mL of 95% ethanol by volume. The prepared starch film was uniformly sprayed onto a clean glass slide and allowed to air dry at room temperature. The film morphology and structural integrity were then observed under an optical microscope (magnification 200×). The results are as follows: Figure 1 As shown.

[0046] (2) Experimental results and analysis: When the amount of 95% ethanol aqueous solution added accounts for 0% of the total volume of the starch film, i.e., without adding ethanol, the starch is not completely dissolved when dissolved in water. The film surface is rough and uneven, with a large number of undissolved particles agglomerated, resulting in poor local light transmittance. The starch dissolves slowly in water, and it is difficult for the starch to dissolve in a short time. When the amount of 95% ethanol aqueous solution added accounts for 5% of the total volume of the starch film, although there are no obvious cracks on the film surface, slight unevenness can be seen in some areas, and the degree of starch dissolution is still insufficient. When the amount of 95% ethanol aqueous solution added accounts for 7.5% of the total volume of the starch film, the film-forming effect is better. Under a microscope, the film surface is uniform and smooth, without obvious particles or cracks, exhibiting good continuity and a dense structure. At this dosage, the starch is fully dissolved, which is conducive to the formation of a homogeneous and stable film layer. Figure 1 As shown, when the addition of 95% ethanol aqueous solution accounts for 15% of the total volume of the starch film, obvious irregular cracks appear on the dried film surface, with localized network cracking, and the film layer becomes more brittle. Excessive dosage and rapid evaporation cause internal stress concentration in the film layer during drying, leading to cracking and affecting the integrity and mechanical strength of the film.

[0047] Adding an appropriate amount of ethanol can promote the complete dissolution of starch, avoid the formation of undissolved particles, and effectively regulate the solvent evaporation rate, improving the density and uniformity of the film. Considering both the film-forming effect and the microstructure, the percentage of 95% ethanol aqueous solution added to the total volume of the starch film is determined to be 5-15%, with 7.5% being the preferred option.

[0048] 3. Screening of composite plasticizers in modified starch films (1) Experimental method: Based on the characteristics of corn starch, glycerol, sorbitol, and a combination of glycerol and sorbitol were screened as candidate plasticizers for testing. To screen for appropriate addition amounts of plasticizers, four groups of plasticizers with different contents were compared, with the mass ratio being the ratio relative to the mass of pregelatinized starch: Treatment group 1: 60% glycerol by mass; other components and amounts are the same as in Example 1; Treatment group 2: 25% sorbitol and 60% glycerol by mass; the remaining components and amounts are the same as in Example 1; Treatment group 3: 25% ethylene glycol and 60% glycerol by mass; the remaining components and amounts are the same as in Example 1; Treatment group 4: 25% sorbitol by mass. The remaining components and amounts are the same as in Example 1; Following the preparation method of Example 1, modified starch film solutions with different plasticizers were prepared. The modified starch film solutions with different plasticizers were uniformly sprayed onto the surface of glass slides and allowed to dry at room temperature to form films. The water erosion resistance of the starch films was tested: the mass of each glass slide with the added starch film was weighed on an electronic balance. Then, water was sprayed onto the side of the slide with the starch film using a spray bottle to simulate natural rainfall. 200 mL of water was sprayed evenly each time, for a total of 3 times, with an interval of 20 min between each spray. After spraying, the slides were dried again at room temperature, weighed on an electronic balance, and the mass reduction was calculated to evaluate the water erosion resistance of the starch film. Each group was repeated 3 times, and the average value was taken. In addition, the film-forming properties and structure were evaluated under an optical microscope. The results are shown in Table 2.

[0049] (2) Experimental results and analysis: Table 2. Water resistance of starch films after adding different plasticizers

[0050] The morphology and structure of each group of membranes were observed under a microscope. The membrane surface of treatment group 1 showed signs of wetting and slight swelling in some areas; the membrane surface of treatment group 2 was smooth and flat, without cracks or particle agglomeration; the membrane surface of treatment group 3 was rough and had small needle-like pores; the membrane surface of treatment group 4 was brittle and had fine cracks.

[0051] In summary, the combination of glycerol and sorbitol exhibits good plasticizing effect and water resistance. The preferred plasticizer combination of this invention is 25% sorbitol and 60% glycerol by mass.

[0052] 4. Screening of polysaccharides and inorganic fillers (1) Experimental method: To verify the effects of hydroxypropyl methylcellulose (HPMC) and kaolin on improving the extensibility and water resistance of starch films, the following four treatment groups were tested: Treatment group 1: Add 20 g of kaolin, without HPMC, and the remaining components and dosages are the same as in Example 1; Treatment group 2: 30 g HPMC was added, but no kaolin was added. The remaining components and dosages were the same as in Example 1. Treatment group 3: No HPMC or kaolin were added, and the remaining components and dosages were the same as in Example 1; Treatment group 4: The components and dosages are the same as in Example 1.

[0053] The four groups of modified starch film solutions were prepared according to the preparation method described in Example 1. The film solutions were poured into polytetrafluoroethylene molds, dried at room temperature to form a film, peeled off, and cut into dumbbell-shaped samples. The width, thickness, and original gauge length of the samples were measured and recorded using vernier calipers. A simple weight-type vertical tensile device was used. One end of the sample was fixed above a support, and the lower end was connected to a weight loading tray via a clamp. Weights were added uniformly at an equivalent rate of approximately 50 mm / min. The total mass of the weights and the elongation of the gauge length at the time of sample breakage were recorded. Each group of samples was tested three times. The tensile strength (σ) and elongation at break (E) were calculated and compared. Note: The calculation method used is as follows: σ = (m×g) / ( b×d) E = (L1-L0) / L0×100% σ: Tensile strength, m: Total mass of the weights, g: Gravitational acceleration (9.8 N / kg), b: Specimen width, d: Specimen thickness, E: Elongation at break, L0: Original specimen length, L1: Length of specimen after fracture (2) Experimental results and analysis Table 3 Tensile strength and elongation at break of modified starch films after adding different fillers

[0054] As shown in Table 3, the tensile strength and elongation at break of treatment group 4 were significantly improved compared with treatment groups 1 and 3. HPMC molecular chains contain abundant hydroxypropyl and methyl groups, which can form hydrogen bonds and physical entanglements with the hydroxyl groups on the starch molecular chains, constructing a three-dimensional cross-linked network structure, thereby effectively improving the mechanical strength and flexibility of the film.

[0055] Compared with treatment groups 2 and 3, treatment group 4 showed significantly improved water resistance and a markedly reduced mass reduction rate. Kaolin, as a layered silicate mineral filler, can play a dual role when uniformly dispersed in the polymer matrix: firstly, it extends the permeation path of water molecules in the membrane layer, improving barrier performance; secondly, it fills the micropores inside the membrane layer, improving membrane density.

[0056] HPMC and kaolin have a synergistic effect in improving the mechanical properties and weather resistance of the film. Therefore, in this embodiment, HPMC and kaolin are the optimal choices for the modified starch film components of treatment group 4 (i.e., Example 1).

[0057] Example 3: Effect of modified starch film solution on biocontrol bacteria activity (1) Experimental method: Adding biocontrol bacteria to modified starch film solution can expand its effect and enhance its control over fruit diseases. To verify the effect of modified starch film solution on the activity of biocontrol bacteria, Trichoderma spores (Trichoderma spores were selected from Trichoderma echinosporum TAZ61, patent number CN 202411538511.9) were used as the experimental subject. The activated Trichoderma strain was inoculated onto PDA plates and cultured at 25℃ until a large number of spores were produced. The spores were washed away with sterile water, filtered through sterile gauze, and then mixed with water and LB medium to prepare a 1×10⁻⁶ solution. 6 A spore suspension of *Trichoderma* spores per mL was prepared, with LB medium comprising 10% of the total spore suspension. Modified starch film solution was diluted with water to a final starch concentration of 2%. The *Trichoderma* spore suspension and the diluted film solution were mixed thoroughly at a 1:1 volume ratio. A control group was prepared by mixing sterile water with the spore suspension at a 1:1 volume ratio. Spore germination rate was tested on glass slides. Using a pipette, 20 μL of the above mixture was evenly dropped onto the surface of clean glass slides, with three slides per group and one drop of the mixture at each end of each slide. The slides were placed in a sealed container lined with absorbent filter paper, and sufficient water was added to maintain a 100% humidity environment. The container was incubated at 25°C. After 48 hours, the spore germination status was observed under a microscope, and the germination rate of *Trichoderma* spores was recorded.

[0058] (2) Experimental results and analysis Microscopic observation revealed that after adding the modified starch film solution, the spore germination rate of *Trichoderma* reached approximately 90%, with no significant difference compared to the control group (P>0.05); spore germination and germ tube growth were normal. Figure 3 As shown, the modified starch film solution does not inhibit the growth of Trichoderma biocontrol bacteria. When using it, biocontrol bacteria can be added to improve their survival rate and field colonization rate.

[0059] Example 4: Storage stability test of modified starch film solution (1) Experimental method: To verify the storage resistance of the modified starch film solution, the modified starch film solution prepared in Example 1 was placed into a 50 mL centrifuge tube, sealed, and placed at a constant temperature of 10 °C for 30 days. The appearance and fluidity of the film solution were then observed.

[0060] (2) Experimental results and analysis: After standing for 30 days, the modified starch film solution remained uniform and stable, with no stratification, precipitation, or gelation observed, and no significant change in fluidity. This indicates that the modified starch film solution prepared according to Example 1 has good storage stability.

[0061] Example 5: Experiment on the safety of modified starch film solution to plants (1) Experimental method: To verify the safety of modified starch film for crops, Chinese cabbage seedlings were used as test subjects. Modified starch film and diluted ethanol were sprayed onto the leaf surface, and the leaf growth was observed. Three treatment groups were set up in the experiment: Treatment Group 1: The film solution prepared in Example 1 was sprayed and diluted to a total starch concentration of 2%; Treatment Group 2: Prepare a 7.5% ethanol solution (95% ethanol diluted with water to 7.5%) and spray directly. Treatment group 3: Blank control group, sprayed with water; After spraying, each treatment group was placed in a greenhouse under the same environmental conditions for normal growth. Five days later, the leaf color, morphology, wilting and necrosis of the Chinese cabbage were observed.

[0062] (2) Experimental data and analysis: Compared with the blank control group, none of the pakchoi seedlings in the spraying treatment groups showed common symptoms of pesticide damage or stress, such as yellowing, wilting, curling, or necrosis of leaves, throughout the observation period. Figure 4 As shown, the leaves of all groups of plants were uniformly bright green, with naturally extended leaf surfaces, and no significant differences were observed in plant height or overall growth. Neither the modified starch film nor the added ethanol caused any harm to the plants within the experimental concentration range, indicating that the film agent of this invention is safe for plants.

[0063] Example 6: Film-forming properties test of modified starch film solution on apple fruit surface (1) Experimental method: Apple fruit was selected as the experimental material to verify the film formation and actual adhesion effect of the modified starch film on the fruit surface. The modified starch film solution prepared in Example 1 was diluted with water to a working concentration of 2% of the total starch concentration. The starch film solution was sprayed onto the surface of the apple peel using a handheld spray bottle. After spraying, the apple was placed indoors to dry, and the film formation on the apple surface was observed with the naked eye and a magnifying glass.

[0064] (2) Experimental results and analysis: The results showed that a uniform, colorless, and transparent starch film formed on the surface of the apple fruit, almost invisible to the naked eye. The film surface was smooth and continuous, without any curling or whitening, and had no impact on the original appearance of the fruit. Figure 2 As shown, the modified starch film exhibits good film-forming properties on the fruit surface, and the film is colorless and transparent, not affecting light penetration.

[0065] Example 7: Experiment on the protective effect of modified starch film solution on apple fruit (1) Experimental method: To verify whether modified starch film can protect fruit from pathogen infection, this experiment was conducted using apple fruit as an example. Forty puncture wounds, approximately 1.0–1.5 mm deep, were evenly made on the surface of the apples using a sterile insect needle. Treatment group 1 was sprayed with a modified starch film solution diluted to 2% of the total starch concentration. Treatment group 2 used a commercially available chitosan apple fruit protective film (Hebei Jiahe Biotechnology Co., Ltd.), sprayed at the optimal concentration according to its instructions. The control group (CK) was sprayed with water, and after drying for 30 min, an inoculation concentration of 1×10⁻⁶ was applied. 6 Apple anthracnose bacteria ( cells / mL) Colletotrichum gloeosporioides Conidial suspension. After inoculation, all fruits were placed in sealed, humidified containers with moistened filter paper at the bottom, maintaining 100% relative humidity, and incubated at 28°C. The disease status of the apple fruits was observed 3 days after inoculation. The appearance of brown, water-soaked lesions at the edges of wounds or the production of conidial clusters were used as the criteria for disease diagnosis. The number of infected wounds was counted, and the incidence rate was calculated.

[0066] (2) Experimental results and analysis: Three days after inoculation, the average disease incidence rate of fruits in treatment group 1 was 42.12%, that in treatment group 2 was 53.11%, and that in the control group was 96.12%. The treatment groups were significantly lower than the control group, and the disease incidence rate in treatment group 1 was lower than that in treatment group 2. The lesions in the control group were larger, followed by the lesions in the commercially available chitosan film treatment group, while the lesions in the modified starch film treatment group of this invention were significantly reduced. Figure 5 As shown, the modified starch film of the present invention can effectively prevent pathogen infection and delay the development of diseases. Its protective effect is better than that of commercially available chitosan film, and it has better prospects for field application and promotion value.

[0067] It should be noted that the specific embodiments are merely representative examples of the present invention, and the technical solution of the present invention is obviously not limited to the above embodiments, and there can be many variations. Those skilled in the art who obtain the present invention based on its explicit disclosure or without objection from the written description should consider it to be within the scope of protection of this patent.

Claims

1. A method for preparing a modified starch film solution, characterized in that, Includes the following steps: (1) Mix pregelatinized starch with ethanol to prepare an ethanol-starch dispersion; (2) Dissolve hydroxypropyl distarch phosphate in water, heat to gelatinize, cool to obtain hydroxypropyl distarch phosphate gelatinized solution; (3) Dissolve the first plasticizer, film-forming aid, and film-forming enhancer in water to prepare a glue solution; (4) Add the adhesive solution obtained in step (3) to the hydroxypropyl distarch phosphate gelatinized solution obtained in step (2) to obtain a mixed adhesive solution; (5) Add the ethanol starch dispersion obtained in step (1) to the mixed adhesive solution obtained in step (4) and stir to mix evenly; (6) Add inorganic filler and stir to mix evenly; (7) Add the second plasticizer, place in a water bath, and stir until well mixed; (8) Add water to make up the volume, stir evenly, cool to room temperature, and let stand to obtain modified starch film solution.

2. The method for preparing the modified starch film solution according to claim 1, characterized in that, In step (1), the pregelatinized starch is selected from any one of food-grade pregelatinized corn starch, pregelatinized potato starch, and pregelatinized sweet potato starch.

3. The method for preparing the modified starch film solution according to claim 1, characterized in that, In step (1), the volume fraction of ethanol is 90% to 95%, and the volume ratio (mL / g) of the amount of ethanol used to the volume of pregelatinized starch is (1 to 3):

1.

4. The method for preparing the modified starch film solution according to claim 1, characterized in that, The mass ratio of the pregelatinized starch to the hydroxypropyl distarch phosphate is (1:2) to (2:1).

5. The method for preparing the modified starch film solution according to claim 1, characterized in that, In step (2), the heating and gelatinization temperature is 75-85℃ and the time is 10-20 min.

6. The method for preparing the modified starch film solution according to claim 1, characterized in that, In step (3), the first plasticizer is selected from at least one of glycerol, propylene glycol, butylene glycol, PEG4000, and PEG6000; in step (7), the second plasticizer is selected from at least one of sorbitol, xylitol, maltitol, mannitol, isomaltitol, and lactitol; and the first plasticizer and the second plasticizer are different.

7. The method for preparing the modified starch film solution according to claim 1, characterized in that, In step (6), the inorganic filler is selected from at least one of kaolin, bentonite, montmorillonite, talc, and diatomaceous earth.

8. A modified starch film solution, characterized in that, It is prepared by the method described in claims 1-7 for preparing modified starch film liquid.

9. A method for protecting fruit, characterized in that, The modified starch film liquid prepared by any one of the preparation methods described in claims 1-7 or the modified starch film liquid described in claim 8 is mixed with the biocontrol fungal spore suspension and sprayed onto the surface of the fruit.

10. The modified starch film liquid prepared by the method of any one of claims 1-7, or the modified starch film liquid as described in claim 8, is used in the protection of fruit.

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