Gallic acid type (CBEI) and ellagic acid type (CBEA) Chinese chestnut bud polyphenol composition and preparation method of fresh-keeping active film thereof

Abstract

According to the invention, the gallic acid type (CBEI) and ellagic acid type (CBEA) polyphenol composition is extracted from Chinese chestnut bracts by innovatively adopting an efficient and environment-friendly method, and the polyphenol content of the composition can be up to 80% at most. The active packaging material is successfully developed by adding CBEI and CBEA with different contents (1%, 5% and 10%) into a chitosan / polyvinyl alcohol composite film and carrying out the steps of raw material screening, low-temperature ultrasonic graded extraction, extraction and purification, film preparation and the like. The thermal stability of the composite film is obviously improved along with the increase of the concentration of CBEI / CBEA, the tensile strength is up to 9.29 MPa, and the elongation at break is up to 293.76%. Fourier transform infrared spectroscopy, scanning electron microscope and X-ray diffraction analysis show that the extract is integrated with a film substrate through hydrogen bonds to form a dual-network cross-linked structure. The composite film shows huge application potential in the field of food preservation.

Description

Technical Field

[0001] This invention relates to the technology for the resource utilization of chestnut agricultural and forestry waste and the field of polymers, and in particular to a green, environmentally friendly, efficient, low-temperature graded extraction method for chestnut bud polyphenols and chitosan / polyvinyl alcohol preservation film, belonging to the fields of plant natural extract processing and polymer materials. Background Technology

[0002] Traditional film materials are primarily derived from non-renewable resources, such as petroleum-based plastics like polyethylene, polypropylene, and polystyrene. Statistics show that over 800 million tons of plastic products are manufactured globally each year, with less than 10% being recycled. Their unique chemical properties make them difficult to degrade, posing a significant environmental challenge. In recent years, bio-based polymers have emerged as alternatives to petroleum-based polymers across various industries. Currently, bio-based materials developed in the packaging industry include chitosan, starch, cellulose, and biodegradable synthetic polymers (polycaprolactone and polylactic acid). These bio-based materials, due to their non-toxic, harmless, environmentally friendly, and biocompatible properties, have become a focus of international attention and research.

[0003] Chitosan, due to its inherent properties of being non-toxic, biocompatible, and possessing excellent film-forming ability, is currently widely used in food preservation, processing, packaging, and as an additive. However, chitosan films typically exhibit poor mechanical and barrier properties, limiting their practical applications. To overcome these limitations, researchers have explored combining chitosan with other materials to enhance its performance. Among various materials, polyvinyl alcohol (PVA), as a non-toxic, water-soluble polymer, stands out. It also possesses excellent chemical stability, environmental friendliness, and water solubility, leading to its increasingly widespread application in food packaging. Notably, PVA exhibits good miscibility and strong intermolecular hydrogen bonding with chitosan, significantly enhancing the performance of chitosan-based materials. Currently, PVA has been proven to improve the tensile strength and elasticity of composite films. Although chitosan / PVA hybrid films possess relatively good performance, they still suffer from drawbacks such as lack of flexibility, poor water resistance, and low thermal stability, which greatly limits their potential applications in the food and pharmaceutical industries. Therefore, there is considerable interest in improving the performance of chitosan / PVA films through modification methods to expand their application range.

[0004] Natural extracts, due to their safety, non-toxicity, biocompatibility, and sustainable production, have become widely used additives in biopolymer materials. Among them, natural polyphenols or flavonoids are the main contributors to their antioxidant and antibacterial activities. Currently, many researchers use them as additives in chitosan / polyvinyl alcohol hybrid films to inhibit microbial growth and extend the shelf life of food. These natural extracts have also been shown to improve various properties and preservation capabilities of the films.

[0005] Chestnut husks are an agricultural and food industry byproduct obtained from chestnuts. According to the latest data from the Food and Agriculture Organization of the United Nations (FAO), the international annual chestnut production reaches as high as 2.3 million tons. The chestnut peeling process generates a large amount of chestnut husk waste (accounting for approximately 20% of the total fruit weight). Due to its high content of phenolic compounds, flavonoids, and tannins, chestnut husks are considered an excellent source for producing high-value natural products. The abundant polyphenols endow chestnut husks with strong antioxidant and antibacterial activities, and this strong biological activity also gives chestnut husk extracts great potential in the field of active packaging films. Summary of the Invention

[0006] To achieve the above objectives, this invention innovatively employs a highly efficient, environmentally friendly, and sustainable method to extract chestnut buds, obtaining gallic acid-type (CBEI) and ellagic acid-type polyphenol compositions (CBEA). Different contents (1%, 5%, 10%) of CBEI and CBEA are added to a chitosan / polyvinyl alcohol preservation composite film. The main steps include raw material screening, low-temperature ultrasonic fractionation extraction, extraction purification, preparation of the chitosan / polyvinyl alcohol composite film, and characterization and performance analysis of the composite film. The extracted chestnut bud extract can achieve a polyphenol content as high as 80%. CBEI and CBEA exhibit good compatibility with chitosan / polyvinyl alcohol, enhancing the thermal stability and mechanical properties of the composite film, and endowing it with strong antioxidant and antibacterial properties. This reveals the function of gallic acid and ellagic acid in the chestnut bud extract in utilizing the polyphenol hydroxyl structure characteristics.

[0007] The specific steps are as follows: 1. Raw material screening: Select dried chestnut burrs with a light yellow appearance and spiky texture, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is low-temperature water at 0~10 ℃, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration are performed to obtain the low-temperature water extract; the filter residue is ultrasonically treated with alkaline water at 0~10 ℃, the solid-liquid ratio is 1:10~30, ultrasonic extraction is performed for 10~30 min, extraction is performed 1~2 times, centrifugation and filtration are performed, and the two filtrates are combined to obtain the alkaline water extract; 3. Preparation of chestnut bud extract: The low-temperature aqueous extract and the alkaline aqueous extract were extracted 3-5 times with equal amounts of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain two types of chestnut bud extract: gallic acid type (CBEI) and ellagic acid type (CBEA), which were light brown and dark brown powders, respectively, with a polyphenol content greater than 80%. 4. Preparation of chitosan / polyvinyl alcohol composite film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol, and react for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain the chitosan / polyvinyl alcohol composite film, which is named CP; 5. Preparation of chestnut bud polyphenol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, and add 1%, 5%, and 10% of CBEI and CBEA to the matrix respectively. Stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, named CPCI (1, 2, 3) and CPCA (1, 2, 3) respectively. 6. Characterization and properties of chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film: The structural characteristics of the film were determined by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD), and the thermal properties and mechanical strength of the composite film were also determined. 7. Preservation performance of chestnut bud polyphenol composite film: The antioxidant capacity of the composite film was determined by DPPH and ABTS free radical scavenging methods, and the antibacterial performance of the composite film was determined by inhibition zone (Escherichia coli and Staphylococcus aureus) method.

[0008] Furthermore, the chestnut buds selected in step 1 above have a moisture content of 4% to 8% and a total tannin content of 12% to 18%. Furthermore, the pH of the alkaline water used in the alkaline water treatment described in step 2 above is 9-12, and alkaline solutions such as ammonia water, quicklime water, or sodium carbonate are used. Furthermore, in step 4 above, the volume ratio of acetic acid to water is 1:99, resulting in a 1% acetic acid aqueous solution. The drying temperature in the oven is 40℃-60℃.

[0009] Furthermore, the FTIR, SEM, and XRD results in step 6 above confirmed that the chestnut bud extract is mainly integrated into the film matrix through hydrogen bonding, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The mechanical properties of the composite film were improved, with tensile strength and elongation at break increasing from 6.38 MPa / 165.54% to 7.16 MPa / 202.77% (CPCI-1), 9.29 MPa / 252.70% (CPCI-2), 8.11 MPa / 186.64% (CPCI-3), 6.45 MPa / 252.04% (CPCA-1), 8.84 MPa / 294.08% (CPCA-2), and 7.46 MPa / 191.34% (CPCA-3), respectively. Adding a high concentration of chestnut bud extract significantly improved the thermal stability of the film, with the residual mass of the film increasing from 14.28% to 14.57%. % (CPCI-1), 14.97 % (CPCI-2), 15.94 % (CPCI-3), 14.96 % (CPCA-1), 14.98 % (CPCA-2), 16.40 % (CPCA-3); Furthermore, the scavenging rates of DPPH and ABTS free radicals by the chestnut husk composite film in step 7 above increased from 8.06% / 5.29% to 30.53% / 23.53% (CPCI-1), 72.87% / 61.42% (CPCI-2), 90.62% / 89.69% (CPCI-3), 18.53% / 17.36% (CPCA-1), 53.01% / 54.97% (CPCA-2), and 89.89% / 89.86% (CPCA-3). The size of the inhibition zone against Escherichia coli and Staphylococcus aureus increased from 10.40 mm / 11.26 mm to 10.73 mm / 11.62 mm (CPCI-1), 11.95 mm / 12.62 mm (CPCI-2), 13.39 mm / 13.40 mm (CPCI-3), 11.46 mm / 11.78 mm (CPCA-1), 12.08 mm / 12.82 mm (CPCA-2), and 13.58 mm / 13.89 mm (CPCA-3).

[0010] The present invention has the following advantages: 1. A green and efficient fractional extraction and purification method is adopted to extract and purify chestnut buds, which allows different types of polyphenols (gallic acid and ellagic acid) to dissolve sequentially, thereby enriching the two types of polyphenols. This method is characterized by its rapid and efficient extraction. The polyphenol content of chestnut bud extract was found to be up to 80%, which greatly increases the polyphenol content of chestnut buds.

[0011] 2. The innovative addition of chestnut bud extract to chitosan / polyvinyl alcohol active film not only improves the thermal and mechanical properties of the film, but also endows it with strong antioxidant and antibacterial activities. Furthermore, it reveals the function of gallic acid and ellagic acid in chestnut bud extract in exerting the structural characteristics of polyphenolic hydroxyl groups. Attached Figure Description

[0012] Figure 1 High performance liquid chromatograms of CBEI and CBEA Figure 2 Scanning electron microscope image of the thin film Figure 3 Infrared spectrum of the thin film Figure 4 X-ray diffraction pattern of the thin film Figure 5 Thermogravimetric analysis diagram of the thin film Figure 6 Comparison of the mechanical properties of thin films Figure 7 Figure showing the antioxidant properties of the thin film Figure 8 Diagram showing the antibacterial properties of the film. Detailed Implementation

[0013] To make the content of this invention easier to understand, the technical solution of this invention will be further described below with reference to specific embodiments, but this invention is not limited thereto. Example

[0014] 1. Raw material screening: Select dried chestnut burrs with a light yellow and spiky appearance, with a moisture content of 4%~8% and a total tannin content of 12%~18%, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is low-temperature water at 0~10 ℃, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration are performed to obtain low-temperature water extract; 3. Extraction and purification: The low-temperature water extract was extracted 3-5 times with an equal amount of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain gallic acid type (CBEI) chestnut bud extract, which was a light brown powder with a polyphenol content of 796.40 mg. 4. Preparation of chitosan / polyvinyl alcohol film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol. React for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain a chitosan / polyvinyl alcohol composite film, which is named CP. 5. Preparation of chestnut bud / chitosan / polyvinyl alcohol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, add CBEI equivalent to 1% of the matrix, stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish, and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, which is named CPCI-1; 6. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. FTIR, SEM, and XRD confirmed that chestnut bud extract (CBEI) is mainly integrated into the film matrix through hydrogen bonding, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The tensile strength of the film reached 7.16 MPa, the elongation at break reached 202.77%, and the residual mass of the film was 14.57%. 7. Preservation performance of chitosan / polyvinyl alcohol composite film: The scavenging rates of DPPH and ABTS free radicals were 30.53% and 23.53%, respectively; the diameters of the inhibition zones of Escherichia coli and Staphylococcus aureus were 10.73 mm and 11.62 mm, respectively. Example

[0015] As shown in Example 1, the difference lies in the amount of chestnut bud extract (CBEI) added. 1. Raw material screening: Select dried chestnut burrs with a light yellow and spiky appearance, with a moisture content of 4%~8% and a total tannin content of 12%~18%, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is 0~10 ℃ low-temperature water, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration are performed to obtain low-temperature water extract; 3. Extraction and purification: The low-temperature water extract was extracted 3-5 times with an equal amount of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain gallic acid type (CBEI) chestnut bud extract, which was a light brown powder with a polyphenol content of 796.40 mg. 4. Preparation of chitosan / polyvinyl alcohol film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol. React for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain a chitosan / polyvinyl alcohol composite film, which is named CP. 5. Preparation of chestnut bud / chitosan / polyvinyl alcohol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, add 5% CBEI to the matrix, stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, which is named CPCI-2; 6. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. FTIR, SEM, and XRD confirmed that chestnut bud extract (CBEI) is mainly integrated into the film matrix through hydrogen bonding, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The tensile strength of the film reached 9.29 MPa, the elongation at break reached 252.70%, and the residual mass of the film was 14.97%. 7. Preservation performance of chitosan / polyvinyl alcohol composite film: The scavenging rates of DPPH and ABTS free radicals were 72.87% and 61.42%, respectively; the diameters of the inhibition zones of Escherichia coli and Staphylococcus aureus were 11.95 mm and 12.62 mm, respectively. Example

[0016] As shown in Example 1, the difference lies in the amount of chestnut bud extract (CBEI) added. 1. Raw material screening: Select dried chestnut burrs with a light yellow and spiky appearance, with a moisture content of 4%~8% and a total tannin content of 12%~18%, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is low-temperature water at 0~10 ℃, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration are performed to obtain low-temperature water extract; 3. Extraction and purification: The low-temperature water extract was extracted 3-5 times with equal amounts of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain gallic acid type (CBEI) chestnut bud extract, which was a light brown powder with a polyphenol content of 796.40 mg. 4. Preparation of chitosan / polyvinyl alcohol film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol. React for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain a chitosan / polyvinyl alcohol composite film, which is named CP. 5. Preparation of chestnut bud / chitosan / polyvinyl alcohol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, add 10% CBEI to the matrix, stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, which is named CPCI-3; 6. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. FTIR, SEM, and XRD confirmed that chestnut bud extract (CBEI) is mainly integrated into the film matrix through hydrogen bonds, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The tensile strength of the film reached 8.11 MPa, the elongation at break reached 186.64%, and the residual mass of the film was 15.94%. 7. Preservation performance of chitosan / polyvinyl alcohol composite film: The scavenging rates of DPPH and ABTS free radicals were 90.62% and 89.69%, respectively, and the diameters of the inhibition zones of Escherichia coli and Staphylococcus aureus were 13.39 mm and 13.40 mm, respectively. Example

[0017] As shown in Example 1, the difference lies in the type of chestnut bud extract (CBEA) added. 1. Raw material screening: Select dried chestnut burrs with a light yellow and spiky appearance, with a moisture content of 4%~8% and a total tannin content of 12%~18%, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is water at 0~10 ℃, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration, the filter residue is ultrasonically treated with alkaline water at 0~10 ℃, the solid-liquid ratio is 1:10~30, ultrasonic extraction for 10~30 min, extraction 1~2 times, centrifugation and filtration, the two filtrates are combined to obtain alkaline water extract; 3. Extraction and purification: The alkaline water ultrasonic extract was extracted 3-5 times with equal amounts of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain ellagic acid type (CBEA) chestnut bud extract, which was a dark brown powder with a polyphenol content of 637.03 mg. 4. Preparation of chitosan / polyvinyl alcohol composite film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol, and react for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain the chitosan / polyvinyl alcohol composite film, which is named CP; 5. Preparation of chestnut bud / chitosan / polyvinyl alcohol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, add CBEA equivalent to 1% of the matrix, stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish, and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, which is named CPCA-1; 6. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. FTIR, SEM, and XRD confirmed that the chestnut bud extract CBEA is mainly integrated into the film matrix through hydrogen bonds, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The tensile strength of the film reached 6.45 MPa, the elongation at break reached 252.04%, and the residual mass of the film was 14.96%. 7. Preservation performance of chitosan / polyvinyl alcohol composite film: DPPH and ABTS free radical scavenging rates were 18.53% and 17.36%, respectively; the inhibition zone diameters of Escherichia coli and Staphylococcus aureus were 11.46 mm and 11.78 mm, respectively. Example

[0018] As shown in Example 1, the difference lies in the type of chestnut bud added (CBEA). 1. Raw material screening: Select dried chestnut burrs with a light yellow and spiky appearance, with a moisture content of 4%~8% and a total tannin content of 12%~18%, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is water at 0~10 ℃, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration, the filter residue is ultrasonically treated with alkaline water at 0~10 ℃, the solid-liquid ratio is 1:10~30, ultrasonic extraction for 10~30 min, extraction 1~2 times, centrifugation and filtration, the two filtrates are combined to obtain alkaline water extract; 3. Extraction and purification: The alkaline water ultrasonic extract was extracted 3-5 times with equal amounts of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain ellagic acid type (CBEA) chestnut bud extract, which was a dark brown powder with a polyphenol content of 637.03 mg. 4. Preparation of chitosan / polyvinyl alcohol composite film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol, and react for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain the chitosan / polyvinyl alcohol composite film, which is named CP; 5. Preparation of chestnut bud / chitosan / polyvinyl alcohol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, add 5% of CBEA to the matrix, stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, which is named CPCA-2. 6. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. FTIR, SEM, and XRD confirmed that the chestnut bud extract CBEA is mainly integrated into the film matrix through hydrogen bonding, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The tensile strength of the film reached 8.85 MPa, the elongation at break reached 294.08%, and the residual mass of the film was 14.98%. 7. Preservation performance of chitosan / polyvinyl alcohol composite film: The scavenging rates of DPPH and ABTS free radicals were 53.01% and 54.97%, respectively; the diameters of the inhibition zones of Escherichia coli and Staphylococcus aureus were 12.08 mm and 12.82 mm, respectively. Example

[0019] As shown in Example 1, the difference lies in the type of chestnut bud added (CBEA). 1. Raw material screening: Select dried chestnut burrs with a light yellow and spiky appearance, with a moisture content of 4%~8% and a total tannin content of 12%~18%, and grind them to 5-50 mesh; 2. Low-temperature ultrasonic fractionation extraction: The solvent is water at 0~10 ℃, the solid-liquid ratio is 1:15~40, static soaking for 5~20 min, centrifugation and filtration, the filter residue is ultrasonically treated with alkaline water at 0~10 ℃, the solid-liquid ratio is 1:10~30, ultrasonic extraction for 10~30 min, extraction 1~2 times, centrifugation and filtration, the two filtrates are combined to obtain alkaline water extract; 3. Extraction and purification: The alkaline water ultrasonic extract was extracted 3-5 times with equal amounts of ethyl acetate. The ethyl acetate layers were combined, the solvent was recovered by vacuum concentration, and the concentrate was freeze-dried to obtain ellagic acid type (CBEA) chestnut bud extract, which was a dark brown powder with a polyphenol content of 637.03 mg. 4. Preparation of chitosan / polyvinyl alcohol composite film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol, and react for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain the chitosan / polyvinyl alcohol composite film, which is named CP; 5. Preparation of chestnut bud / chitosan / polyvinyl alcohol composite film: Take 15-30 ml of the chitosan / polyvinyl alcohol solution from step 4 as the matrix, add CBEA equivalent to 10% of the matrix, stir at 40 ℃ for 15-25 min, then slowly pour into a petri dish, and dry in an oven for 20-30 h to obtain chestnut bud polyphenol / chitosan / polyvinyl alcohol composite film, which is named CPCA-3; 6. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. FTIR, SEM, and XRD confirmed that chestnut bud extract is mainly integrated into the film matrix through hydrogen bonding, forming a double-network cross-linked structure with the chitosan / polyvinyl alcohol matrix. The tensile strength of the film reached 7.46 MPa, and the elongation at break reached 191.34%. The residual mass of the film was 16.40%. 7. Preservation performance of chitosan / polyvinyl alcohol composite film: The scavenging rates of DPPH and ABTS free radicals are 89.89% and 89.86%, respectively; the diameters of the inhibition zones of Escherichia coli and Staphylococcus aureus are 13.58 mm and 13.89 mm, respectively. Comparative Example 1 As shown in Example 1, the difference is that chestnut bud extract is not added.

[0020] 1. Preparation of chitosan / polyvinyl alcohol composite film: Prepare a 0.5%~1.5% chitosan-acetic acid aqueous solution and stir it in a water bath at room temperature until completely dissolved. Take 100 mL of the solution and add 1~3 g of polyvinyl alcohol and glycerol, respectively. Heat to 90 ℃, then add citric acid equivalent to 2% of the mass of polyvinyl alcohol. React for 4~8 h to allow for full cross-linking. Take 15~30 mL of the chitosan / polyvinyl alcohol solution, pour it onto a petri dish, and dry it in an oven for 20~30 h to obtain the chitosan / polyvinyl alcohol composite film, which is named CP; 2. Characterization and Properties of Chitosan / Polyvinyl Alcohol Composite Films: The structural characteristics of the films were determined using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The mechanical, thermal, antioxidant, and antibacterial properties of the films were also measured. The tensile strength of the film reached 6.38 MPa, the elongation at break reached 165.54%, the residual mass of the film was 14.28%, the DPPH and ABTS free radical scavenging rates were 8.06% and 5.29%, respectively, and the inhibition zone diameters for *Escherichia coli* and *Staphylococcus aureus* were 10.40 mm and 11.26 mm, respectively. The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art will readily understand that the above embodiments do not limit the invention in any way, and any modifications, equivalent substitutions, and improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for preparing a chestnut burr polyphenol composition of gallic acid type (CBEI) and ellagic acid type (CBEA) and a fresh-keeping active film thereof, comprising the following steps: Step 1: raw material selection: selecting dry chestnut burrs with a light yellow appearance and thorns, and crushing them to 5-50 mesh; Step 2: low-temperature ultrasonic fractionation extraction: using 0-10 ℃ low-temperature water as the solvent, a solid-liquid ratio of 1:15-40, static soaking for 5-20 min, centrifugal filtration, obtaining a low-temperature water extract; using 0-10 ℃ alkali water to ultrasonically treat the residue, a solid-liquid ratio of 1:10-30, ultrasonic extraction for 10-30 min, extracting 1-2 times, centrifugal filtration, and combining the filtrates of the two times to obtain an alkali water extract; Step 3: preparation of chestnut burr extract: using equal amounts of ethyl acetate to extract the low-temperature water extract and the alkali water extract 3-5 times, respectively, combining the ethyl acetate layers, recovering the solvent by vacuum concentration, freeze-drying the concentrate, and obtaining two kinds of chestnut burr extracts of gallic acid type (CBEI) and ellagic acid type (CBEA), which are light brown and dark brown powders, respectively, and the polyphenol content is greater than 80 %; Step 4: preparation of chitosan / polyvinyl alcohol composite film: preparing a 0.5 %-1.5 % chitosan acetic acid aqueous solution, and completely dissolving it in a water bath at room temperature. Taking 100 mL, respectively adding 1-3 g of polyvinyl alcohol and glycerol, heating to 90 ℃, then adding 2 % of the mass of the polyvinyl alcohol of citric acid, and reacting for 4-8 h to make it fully crosslinked. Taking 15-30 ml of the chitosan / polyvinyl alcohol solution, pouring it into a culture dish, and drying it in an oven for 20-30 h to obtain a chitosan / polyvinyl alcohol composite film (named CP); Step 5: preparation of chestnut burr polyphenol composite film: taking 15-30 ml of the chitosan / polyvinyl alcohol solution of Step 4 as the matrix, respectively adding 1 %-10 % of CBEI and CBEA to the matrix, stirring for 15-25 min at a temperature of 40 ℃, then slowly pouring it into a culture dish, and drying it in an oven for 20-30 h to obtain chestnut burr polyphenol / chitosan / polyvinyl alcohol composite films, which are named CPCI and CPCA, respectively; Step 6: characterization and performance of chestnut burr polyphenol / chitosan / polyvinyl alcohol composite film: using a scanning electron microscope (SEM), a Fourier transform infrared spectrometer (FTIR), and an X-ray diffractometer (XRD) to respectively determine the structural characteristics of the film, and respectively determining the thermal performance and mechanical strength of the composite film; Step 7: fresh-keeping performance of chestnut burr polyphenol composite film: using the DPPH and ABTS free radical scavenging method to determine the antioxidant capacity of the composite film, and using the inhibition zone (Escherichia coli and Staphylococcus aureus) method to determine the antibacterial fresh-keeping performance of the composite film. The selected chestnut burrs have a moisture content of 4 %-8 % and a total tannin content of 12 %-18 %. The pH of the alkali water during alkali water treatment is 9-12, and ammonia water or lime water or sodium carbonate or other alkali solution is used. The volume ratio of acetic acid to water is 1:99, i.e. 1 % acetic acid aqueous solution is obtained, and the temperature for oven drying is 40 ℃-60 ℃. ​ ​ ​ ​ 2. The first step of claim 1, wherein ​ 3. The second step of claim 1, wherein ​ 4. The fourth step of claim 1, wherein ​ 5. The fifth step of claim 1 wherein CPCI (1, 2, 3) and CPCA (1, 2, 3) were prepared by adding 1%~10% of the chestnut burr polyphenol complex film to the base material, preferably three additive amounts of 1%, 5% and 10%.

6. The sixth step of claim 1 wherein The tensile strength and elongation at break of CPCI (1, 2, 3) and CPCA (1, 2, 3) were 7.16 MPa / 202.77% (CPCI-1), 9.29 MPa / 252.70% (CPCI-2), 8.11 MPa / 186.64% (CPCI-3), 6.45 MPa / 252.04% (CPCA-1), 8.84 MPa / 294.08% (CPCA-2), 7.46 MPa / 191.34% (CPCA-3), respectively; the residual mass of the film changed from 14.28% to 14.57% (CPCI-1), 14.97% (CPCI-2), 15.94% (CPCI-3), 14.96% (CPCA-1), 14.98% (CPCA-2), 16.40% (CPCA-3), respectively.

7. The seventh step of claim 1 wherein The chestnut burr extract can endow the film with strong antioxidant and antibacterial properties and preservation capacity; the scavenging rate of DPPH and ABTS free radicals increased from 8.06% / 5.29% to 30.53% / 23.53% (CPCI-1), 72.87% / 61.42% (CPCI-2), 90.62% / 89.69% (CPCI-3), 18.53% / 17.36% (CPCA-1), 53.01% / 54.97% (CPCA-2), 89.89% / 89.86% (CPCA-3); the inhibition zone size of E. coli and S. aureus increased from 10.40 mm / 11.26 mm to 10.73 mm / 11.62 mm (CPCI-1), 11.95 mm / 12.62 mm (CPCI-2), 13.39 mm / 13.40 mm (CPCI-3), 11.46 mm / 11.78 mm (CPCA-1), 12.08 mm / 12.82 mm (CPCA-2), 13.58 mm / 13.89 mm (CPCA-3).

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