A method for preparing pectin-based films based on chestnut bud polyphenol nanoemulsion

Abstract

This invention belongs to the field of food packaging materials, specifically providing a method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion. The method mainly includes the following steps: extracting polyphenols from chestnut buds, obtaining a nanoemulsion by stirring and high-pressure microfluidic spraying a mixture of polyphenols and cinnamaldehyde, mixing the nanoemulsion with a pectin aqueous solution, adding glycerol, homogenizing under high pressure via microfluidic spraying, and then degassing, spreading, and drying or air-drying to obtain a composite film. The resulting composite film has high mechanical properties, gas barrier properties, antioxidant and antibacterial properties, and other multifunctionalities. It is a bio-based plastic with great potential and can replace petrochemical plastics in food packaging.

Description

Technical Field

[0001] This invention relates to the field of food packaging materials, specifically to a method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion. Background Technology

[0002] Fruits are widely known for their unique flavors and rich bioactive compounds; however, they are highly susceptible to pathogen contamination during harvest, often resulting in significant post-harvest losses. Plastic films are widely used in food packaging to enhance freshness, shelf life, and safety during production, storage, transportation, and distribution. However, non-degradable plastic films cause serious pollution problems, resulting in substantial damage to the environment and ecosystems. Therefore, biodegradable natural polymers have received widespread attention.

[0003] As an environmentally friendly material, pectin-based films suffer from a core problem stemming from their strong hydrophilicity. This directly results in poor mechanical strength and a brittle texture, making them prone to softening and absorbing moisture, especially in humid environments. Furthermore, their ability to block water vapor is extremely weak, and their oxygen barrier properties are unstable under humidity, easily swelling and breaking upon contact with water, severely limiting their application range. In addition, the films face challenges such as low thermal stability, processing difficulties, limited functionality, poor economic viability due to raw material costs and modification processes, and challenges in large-scale production. Their safety standards and regulatory approval for use in food packaging also need improvement. These factors collectively restrict their transition from laboratory applications to large-scale commercial use.

[0004] Chestnut byproducts are considered a rich source of polyphenols, which possess antioxidant and antibacterial properties. However, current research primarily focuses on the value of leaves, bark, and shells, while chestnut husks (chestnut bristle shells) remain underutilized. Currently, over 90% of chestnut husks are either piled up in fields to rot, promoting pest reproduction, or burned, releasing carbon dioxide, resulting in both resource waste and environmental pollution.

[0005] Besides lignocellulose and other components, chestnut buds, shells, and skins are rich in natural polyphenols and polyphenolic compounds such as brown pigments, tannins, and proanthocyanidins. The resource utilization of chestnut buds can provide sustainable resources such as polyphenols while reducing carbon dioxide emissions, representing a solid step towards a circular economy and carbon neutrality. Patent CN105669633A discloses a method for co-producing polyphenols, pigments, tannins, proanthocyanidins, and wood flour from chestnut buds. The method involves pulverizing chestnut buds to 30-80 mesh, placing them in a tank with alkaline deionized water, and extracting them three times with microwave radiation and stirring. The aqueous extract is then separated through multiple membranes, concentrated, and dried to obtain dry powders of polyphenols, pigments, tannins, and proanthocyanidins. The filter residue is dried into wood flour. Patent CN106867408A discloses a method for extracting tannin from chestnut buds and preparing composite materials. The method involves crushing, soaking, removing impurities, evaporating under reduced pressure, removing impurities by adsorption, filtering, and then concentrating and spray drying the filtrate to obtain the tannin product. This method is relatively complicated and requires further reaction with epoxy resin to produce epoxy resin foam plastic. This composite material is used in pipe insulation materials, lightweight cement sandwich materials, lightweight wood sandwich materials, and rock wool sandwich materials.

[0006] Therefore, how to utilize waste chestnut husks to improve pectin-based films is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion. Polyphenols are extracted from chestnut buds and mixed with cinnamaldehyde. A polyphenol-cinnamaldehyde nanoemulsion is formed using a high-pressure microfluidic method, which is then used to modify the pectin-based film to enhance its mechanical and antibacterial properties. This invention comprehensively studies the effects of the polyphenol-cinnamaldehyde nanoemulsion on the key properties of the pectin-based film, including microstructure, mechanical strength, hydrophobicity, UV resistance, and antioxidant / antibacterial activity. Fruit packaging experiments successfully demonstrated the preservation efficacy of the composite film, confirming that the pectin composite film prepared using chestnut polyphenol nanoemulsion can be applied in the field of active food packaging.

[0008] The specific technical solution of the present invention is as follows: A method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion, the specific steps of which are as follows: 1) Wash, dry, crush, and sift the chestnut buds to obtain chestnut bud powder for later use; 2) Place the chestnut powder obtained in step 1) into a reaction vessel, add ethanol, heat under reflux and stir to obtain a mixture; 3) The mixture obtained in step 2) was vacuum filtered, the filtrate was rotary evaporated to remove ethanol, and then freeze-dried to obtain a brown powder, which is the polyphenol extract; 4) Disperse the powder obtained in step 3) in water, then add it to a mixture of cinnamaldehyde and anhydrous ethanol, mix and stir, and then obtain a stable nanoemulsion through high-pressure microfluidic treatment. 5) Mix the nanoemulsion obtained in step 4) with pectin solution, then add glycerol, and degas after high-pressure microfluidic treatment to obtain film-forming solution; 6) Spread the film-forming liquid obtained in step 5) onto the substrate, dry it or let it air dry, and then peel off the film to obtain the composite film.

[0009] Furthermore, in step 1), the chestnut powder needs to be sieved through a 250-mesh sieve to increase the surface area of ​​the reaction, which can improve the extraction yield and shorten the reaction time.

[0010] Further, in step 2), the solid-liquid ratio of chestnut powder to ethanol is 1g:(8-12)mL, the volume concentration of ethanol is 70%; the temperature is heated to 80℃-85℃, the reflux time is 2.5-3.5 h, and the stirring rate is 150-500 rpm.

[0011] Step 2) aims to separate the polyphenols contained in chestnut bud powder. The total phenol content of chestnut bud extract (CBE) is quantitatively analyzed by using the Folin-Ciocalteu method with gallic acid equivalent (GAE). This method achieves the determination of the relative content of phenolic substances through the gallic acid standard curve, and the total phenol content of CBE is measured to be 40-200 mg GAE / g.

[0012] Furthermore, in step 3), the vacuum filtration time is 2-3 hours, the filter paper pore size is 10-20 micrometers, and the rotary evaporation temperature is 40-45℃; the freeze drying time is 60-80 hours, and the temperature is -40 to -45℃.

[0013] The purpose of step 3) is to separate the polyphenol-containing solution from the precipitate by filtration to obtain a polyphenol extract, and to remove ethanol by rotary evaporation in preparation for subsequent freeze drying.

[0014] Further, in step 4), the concentration of the polyphenol extract aqueous solution obtained by dispersing the brown powder in water is 1-3 mg / mL, the solid-liquid ratio of cinnamaldehyde to anhydrous ethanol in the cinnamaldehyde-anhydrous ethanol mixture is 1 g: (4-6) mL, the volume ratio of the polyphenol extract aqueous solution to the cinnamaldehyde-anhydrous ethanol mixture is 20-30:1, the stirring speed is 150-500 rpm, the stirring time is 25-35 minutes, the microjet pressure is 12000-18000 psi, and the number of times is 15-25.

[0015] This step involves stirring the mixture to initially create a heterogeneous and unstable emulsion. Then, the instantaneous high-energy collision and high-frequency shear force of the high-pressure microjet are used to transform the originally unstable and heterogeneous emulsion into a stable and homogeneous polyphenol-cinnamaldehyde nanoemulsion.

[0016] Further, in step 5), the solid-liquid ratio of pectin to water in the pectin solution is 1g:(40-60)mL. Using the pectin solution as the solution, the volume concentration of the nanoemulsion in the nanoemulsion-pectin solution is 5-40%. The amount of glycerol added is 30-50% of the pectin mass. Glycerol, as a plasticizer, can improve processing performance and enhance the flexibility and extensibility of the pectin-based film. The pressure of the microjets in this step is 12000-18000 psi, and the number of times is 4-6, so that the pectin solution and the nanoemulsion are mixed evenly, and the polyphenol-cinnamaldehyde nanoemulsion is evenly dispersed in the pectin matrix.

[0017] Further, in step 6), the film-forming liquid is spread evenly on the substrate, the substrate size is 25 cm × 15 cm, and the thickness of the film-forming liquid is 0.3-0.5 cm; the drying conditions are drying at 25-45℃ or natural drying until the film can be easily peeled off.

[0018] This invention extracts polyphenols from chestnut buds and then prepares a polyphenol-cinnamaldehyde nanoemulsion via high-pressure microfluidization, which is then used to modify pectin films. The polyphenol-cinnamaldehyde nanoemulsion exhibits good compatibility with the pectin matrix and promotes stable film formation through hydrogen bonding. Compared to pure pectin films, the pectin composite film modified with the polyphenol-cinnamaldehyde nanoemulsion possesses significant advantages, including higher tensile strength and elongation at break, lower hydrophilicity, and lower water vapor permeability and oxygen permeability. Furthermore, the polyphenol-cinnamaldehyde nanoemulsion imparts excellent UV resistance to the film. The polyphenols and cinnamaldehyde in the polyphenol-cinnamaldehyde nanoemulsion exhibit outstanding antioxidant activity (DPPH inhibition rate is 5.08 times that of pure pectin films; T-AOC is 38.95 times that of pure pectin films) and antibacterial activity. In food packaging, the pectin composite film modified with the polyphenol-cinnamaldehyde nanoemulsion can effectively delay the ripening, browning, and decay of fruits such as strawberries, thereby extending shelf life. Therefore, polyphenol-cinnamaldehyde nanoemulsion can improve the shortcomings of pectin film to meet the requirements of food packaging applications.

[0019] The present invention has achieved the following beneficial effects: (1) This invention extracts polyphenols from chestnut buds, an agricultural waste, and introduces them into a pectin film, giving the film antioxidant and antibacterial properties. This allows it to be used in food packaging, realizing the resource utilization of waste, thereby increasing the income of local farmers, helping to establish a circular economy, and also providing a practical solution for preventing and controlling plastic pollution. (2) The pectin-based film prepared by the method provided in this invention is biodegradable and has great advantages such as higher tensile strength and elongation at break, lower hydrophilicity, lower water vapor permeability and oxygen permeability. (3) The polyphenols and cinnamaldehyde in the polyphenol-cinnamaldehyde nanoemulsion have excellent antioxidant (DPPH inhibition rate is 5.08 times that of pure pectin film, and T-AOC is 38.95 times that of pure pectin film) and antibacterial activity. In addition, the polyphenol-cinnamaldehyde nanoemulsion gives the film excellent UV resistance, which can significantly extend the shelf life and maintain the quality of the fruit when used for packaging. Attached Figure Description

[0020] Figure 1 These are microscopic images of the polyphenol-cinnamaldehyde nanoemulsion before and after microfluidization in step 4) of Example 1, where A is before microfluidization and B is after microfluidization. Figure 2 This is a particle size distribution diagram of the polyphenol-cinnamaldehyde nanoemulsion before microfluidization in step 4) of Example 1; Figure 3 Fourier transform infrared spectra of Comparative Example 1 and Examples 1-4; Figure 4 The XRD patterns are for Comparative Example 1 and Examples 1-4; Figure 5 TGA curves for Comparative Example 1 and Examples 1-4; Figure 6 The DTG curves for Comparative Example 1 and Examples 1-4 are shown. Figure 7 The water contact angles for Comparative Example 1 and Examples 1-4 are given, where a is Comparative Example 1, b is Example 1, c is Example 2, d is Example 3, and e is Example 4. Figure 8 Mechanical properties of Comparative Example 1 and Examples 1-4; Figure 9 The water / oxygen permeability of Comparative Example 1 and Examples 1-4; Figure 10 Optical images of Comparative Example 1 and Examples 1-4; Figure 11 The UV-Vis spectra of Comparative Example 1 and Examples 1-4 are shown. Figure 12 The total antioxidant capacity of Comparative Example 1 and Examples 1-4; Figure 13 To demonstrate the antibacterial properties of Comparative Example 1 and Examples 1-4; Figure 14 Figures showing the changes in appearance of fruits before and after preservation tests in Comparative Example 1 and Examples 1-4; Figure 15The weight loss trend of fruits before and after the preservation test in Comparative Example 1 and Examples 1-4; Figure 16 The graph shows the total soluble solids (TSS) content of fruits before and after preservation tests in Comparative Example 1 and Examples 1-4. Figure 17 The graph shows the change in firmness of the fruits before and after the preservation test for Comparative Example 1 and Examples 1-4. Detailed Implementation

[0021] Other materials used in this invention, unless otherwise stated, are commercially available. Other terms used in this invention, unless otherwise specified, generally have the meanings commonly understood by those skilled in the art. The invention is further described in detail below with reference to specific embodiments and data. The following embodiments are merely illustrative and not intended to limit the scope of the invention in any way.

[0022] Example 1: A method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion, the specific steps of which are as follows: 1) Wash, dry, and crush the chestnut buds, then pass them through a 250-mesh sieve to obtain chestnut bud powder for later use; 2) Place the chestnut powder obtained in step 1) into a round-bottom flask, add 70% ethanol at a solid-liquid ratio of 1g:10mL, heat to 80℃ and reflux for 3h on an electric furnace, and stir at 500 rpm to obtain a mixture. 3) The mixture obtained in step 2) was vacuum filtered for 3 h with a filter paper pore size of 15 micrometers. After removing the ethanol by rotary evaporation at 40°C, the filtrate was freeze-dried at -40°C for 72 h to obtain a brown powder. 4) Disperse the powder obtained in step 3) in water to prepare a concentration of 2 mg / mL, and add it to a mixture of cinnamaldehyde and anhydrous ethanol. The volume ratio of the polyphenol extract aqueous solution to the cinnamaldehyde-anhydrous ethanol mixture is 25:1, and the solid-liquid ratio of cinnamaldehyde to ethanol in the cinnamaldehyde-anhydrous ethanol mixture is 1 g: 5 mL. Mix and stir at 300 rpm for 30 minutes. After that, after being treated with microfluidic jet at a pressure of 15000 psi 20 times, a stable polyphenol-cinnamaldehyde nanoemulsion is obtained. 5) Mix the nanoemulsion obtained in step 4) with pectin solution (2%, w / v). The volume concentration of the nanoemulsion in the mixture is 5%, i.e., 5 mL of nanoemulsion and 95 mL of pectin solution. Then add glycerol accounting for 40% of the pectin mass. After microfluidic treatment at a pressure of 15000 psi for 5 times and degassing, the film-forming solution is obtained. 6) Spread the film-forming liquid obtained in step 5) onto a glass plate. The size of the substrate is 25 cm × 15 cm and the thickness of the slurry is 0.4 cm. After drying at 40°C, peel off the film to obtain the composite film.

[0023] Example 2: A method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion, the specific steps of which are as follows: The difference from Example 1 is that the volume concentration of the nanoemulsion and pectin solution after mixing in step 5) is 10%, while the rest is the same as in Example 1.

[0024] Example 3: A method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion, the specific steps of which are as follows: The difference from Example 1 is that the volume concentration of the nanoemulsion and pectin solution after mixing in step 5) is 20%, while all other aspects are the same as in Example 1.

[0025] Example 4: A method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion, the specific steps of which are as follows: The difference from Example 1 is that the volume concentration of the nanoemulsion and pectin solution after mixing in step 5) is 40%, while the rest is the same as in Example 1.

[0026] Comparative Example 1: A method for preparing a pectin-based antibacterial and preservative coating, the specific steps of which are as follows: Pectin powder was dissolved in deionized water to prepare a pectin aqueous solution (2%, w / v). The solution was stirred overnight to obtain a film-forming solution and degassed. The film-forming solution was poured onto a glass plate with a substrate size of 25 cm × 15 cm and a slurry thickness of 0.4 cm. The film was then naturally dried at room temperature and peeled off to obtain the composite film.

[0027] Experimental Example Microscopic and particle size distribution observations were performed on the polyphenol-cinnamaldehyde nanoemulsions before and after microfluidization in step 4) of Example 1. The microscopic images were taken with a Zeiss 360mm microscope, and the particle size was measured using a Zetasizer-Nano-ZS laser nanometer. The microscopic images of the emulsions before and after microfluidization are shown below. Figure 1 As shown in the image, where A represents the state before microfluidization and B represents the state after microfluidization, the image clearly shows that microfluidization homogenizes the emulsion; the particle size distribution of the emulsion before and after microfluidization is shown in the figure. Figure 2 As shown, the size distribution indicates that the emulsion size distribution is narrower after microfluidization, with the average size decreasing from 4550 nm to 190 nm.

[0028] The interaction between polyphenol-cinnamaldehyde nanoemulsion and pectin matrix was studied using ATR-FTIR spectroscopy, and the composite films prepared in Examples 1-4 and Comparative Example 1 were analyzed. Figure 3As shown, the FTIR spectra of Comparative Example 1 are very similar to those of Examples 1-4, indicating that no new chemical bonds are formed between the polyphenol-cinnamaldehyde nanoemulsion and pectin. Since the amount of polyphenol-cinnamaldehyde nanoemulsion added in the examples is relatively small, and most of the cinnamaldehyde is encapsulated by pectin, there is no significant difference in the infrared spectra. All spectra are in the range of 3600-3000 cm⁻¹. -1 The region exhibits broad absorption bands, attributable to the OH stretching vibrations of intermolecular and intramolecular hydrogen bonds. This is particularly evident in the 2900–2800 cm⁻¹ region. -1 The band seen between these points represents the CH stretching vibrations of sp³ hybridized carbon atoms. (1740 cm⁻¹) -1 and 1600 cm -1 The characteristic absorptions at these locations are the stretching vibrations of esterified carbonyl groups (C=O) and free carboxylates (COO-), both typical of pectin. Furthermore, at 1150 cm⁻¹... -1 The band at 1020 cm⁻¹ represents the asymmetric valence vibration of the COO- group, while the band at 1020 cm⁻¹ represents the asymmetric valence vibration of the COO- group. -1 The sharp and intense absorption at that point is caused by the CO valence vibration.

[0029] The X-ray diffraction (XRD) spectra of Comparative Example 1 and Examples 1-4 are as follows: Figure 4 As shown, the pure pectin film exhibits diffraction peaks at 14.76° and 19.34°, indicating a semi-crystalline structure. The small diffraction peak at 24.29° corresponds to the small crystalline peak of pectin. After incorporating polyphenol-cinnamaldehyde nanoemulsion, the position of the main diffraction peak of the film did not change significantly, indicating that the semi-crystalline structure of the pectin matrix remained intact.

[0030] Thermal stability testing was conducted under a nitrogen atmosphere, with thermogravimetric analysis used to evaluate the thermal degradation behavior of the films from 30°C to 800°C. The corresponding profiles of Comparative Example 1 and Examples 1-4 are shown below. Figure 5 and Figure 6 As shown, the degradation of the film has three similar stages, and the residual mass values ​​of all films are similar, indicating that the incorporation of polyphenol-cinnamaldehyde nanoemulsion does not significantly change the thermal degradation curve of the pectin matrix.

[0031] The water contact angle was determined using a video-based optical contact angle analyzer, where the droplet profile was fitted according to the Laplace-Young equation, and WCA θ=65° is a common demarcation between "hydrophilic" (θ<65°) and "hydrophobic" (θ>65°) thin film surfaces. Figure 7As shown, the WCA of the pure pectin film is 63.25°, indicating slight hydrophilicity. After adding the polyphenol-cinnamaldehyde nanoemulsion, the WCA of the film increased to 68.35°-76.48°, demonstrating that the polyphenol-cinnamaldehyde nanoemulsion significantly reduced the hydrophilicity of the film. Although the WCA of Examples 1-4 decreased to below 65° after 300 seconds of water droplet contact, it remained higher than that of the pure pectin film throughout the measurement period. This consistently high contact angle further confirms that the polyphenol-cinnamaldehyde nanoemulsion imparts enhanced water resistance to the composite film. This is because the polyphenols in the polyphenol-cinnamaldehyde nanoemulsion bind to the hydrophilic functional groups of the pectin, thereby reducing the availability of hydroxyl groups and thus decreasing the hydrophilicity of the film.

[0032] The tensile strength (TS, MPa) and elongation at break (EAB, %) of the films were determined using an automated tensile testing machine. Films prepared in Comparative Example 1 and Examples 1-4 were cut into 100 mm × 50 mm rectangles, and their tensile strength was determined using an automated tensile testing machine with an initial clamping distance of 50 mm and a speed of 1 mm / s. The TS and EAB values ​​for Comparative Example 1 and Examples 1-4 are shown below. Figure 8 As shown, the TS (12 MPa) of Comparative Example 1 indicates poor tensile strength of pectin. In contrast, the TS values ​​of Examples 1-4 are higher (20-40 MPa), indicating that the polyphenol-cinnamaldehyde nanoemulsion improves the mechanical strength of the film by 2-4 times. This is because the polyphenol-cinnamaldehyde nanoemulsion particles are uniformly dispersed in the pectin matrix, and through hydrogen bonding, they increase intermolecular interactions, thereby improving tensile strength. In Example 4, the polyphenol-cinnamaldehyde nanoemulsion content was further increased, and the TS value of the film decreased, but it was still higher than that of Comparative Example 1. The excess polyphenol-cinnamaldehyde nanoemulsion damaged the internal structure of the film, resulting in a decrease in the mechanical strength of the film.

[0033] The water vapor permeability (WVP) of the membrane was measured using a water vapor permeability tester. The WVP measurement was performed at a relative humidity of 53% and a temperature of 23°C, with a preheating time of 1 hour and a weight recording interval of 5 minutes. The oxygen permeability (OP) was determined using a differential pressure gas permeameter. Figure 9 The WVP and OP of Comparative Example 1 and Examples 1-4 are shown. Comparative Example 1 has the highest WVP (1.31 × 10⁻⁶). -12 The highest g / cm·s·Pa values ​​and OP values ​​(7.41×10) were achieved. -12 cm -2 ·s -1 ·Pa -1The values ​​of WVP in Examples 1-4 are low and dose-dependent with the content of polyphenol-cinnamaldehyde nanoemulsion, indicating that polyphenol-cinnamaldehyde nanoemulsion significantly enhances the water vapor barrier properties of the film. The interaction between polyphenol-cinnamaldehyde nanoemulsion and the pectin matrix forms a dense network, resulting in longer and narrower pathways for water molecules and reduced water vapor permeability of the film. Example 3 has the lowest OP value, indicating good oxygen barrier properties at this concentration. With further increases in polyphenol-cinnamaldehyde nanoemulsion content, the oxygen permeability (OP) of the film increases, but remains lower than that of Comparative Example 1. This phenomenon can be attributed to the excessive polyphenol-cinnamaldehyde nanoemulsion disrupting the internal structure of the film matrix, creating additional gas diffusion pathways and thus impairing oxygen barrier properties.

[0034] Optical images of thin films, such as Figure 10 As shown, the addition of polyphenol-cinnamaldehyde nanoemulsion had no effect on the visual transparency of the pectin film. The film was cut into rectangular specimens (60 mm × 15 mm), placed in a quartz dish, and the ultraviolet-visible transmission spectrum in the wavelength range of 200-800 nm was recorded using a spectrophotometer. Figure 11 UV-Vis spectroscopy revealed that Comparative Example 1 exhibited the highest transparency but limited UV resistance. In contrast, Examples 1-4 showed significantly reduced UV transmittance, indicating a markedly enhanced UV protection capability, which increased in a dose-dependent manner with increasing polyphenol-cinnamaldehyde nanoemulsion concentration. This is primarily due to the effective absorption of UV light by the benzene rings and phenolic groups in the polyphenol structure, which play a crucial role in the UV resistance of the film.

[0035] The antioxidant activity of the membrane was evaluated using the following two test kits: Total Antioxidant Capacity Test Kit and DPPH Radical Scavenging Capacity Test Kit.

[0036] The composite films prepared in Comparative Example 1 and Examples 1-4 were mixed with reagents, and the absorbance was measured at 593 nm (T-AOC) and 515 nm (DPPH). The results are as follows. Figure 12 As shown, Comparative Example 1 exhibits the lowest T-AOC and DPPH inhibition rates, indicating poor antioxidant capacity. Meanwhile, the DPPH inhibition rate and total antioxidant capacity of T-AOC in Examples 1-4 significantly increased with increasing polyphenol-cinnamaldehyde nanoemulsion content. This enhancement is attributed to the antioxidant activity imparted by the polyphenol-cinnamaldehyde nanoemulsion, where the phenolic hydroxyl groups provide hydrogen atoms to neutralize lipid and oxygen free radicals. This reaction forms stable phenoxy and acyl free radicals, thereby interrupting the chain reaction of the oxidation process.

[0037] The antibacterial activity of the membrane against *Escherichia coli* and *Staphylococcus aureus* was tested by immersing the composite membranes prepared in Comparative Example 1 and Examples 1-4 in a bacterial suspension after UV sterilization at 37°C for 1 hour. Then, 100 μL of the diluted suspension was spread on an agar plate and incubated at 37°C for 12 hours. A blank control without a membrane was added under the same conditions. Figure 13 As shown, Comparative Example 1 showed no antibacterial activity against Staphylococcus aureus and Escherichia coli; in contrast, the composite film with a high polyphenol-cinnamaldehyde nanoemulsion loading exhibited strong photocatalytic bactericidal activity.

[0038] To evaluate the suitability of the composite film prepared in this invention for food packaging, the same batch of strawberries was wrapped with commercially available PVC cling film, Comparative Example 1, and the composite films of Examples 1-4. The samples were stored at room temperature for 6 days, and their visual changes were recorded by photograph over time. Several quality indicators, including weight, hardness, and total soluble solids (TSS), were analyzed. Hardness was tested using a texture analyzer, and total soluble solids (TSS) were measured using a handheld refractometer. Comparative analysis revealed significant differences in the preservation effects of the various materials. Figure 14 As shown, strawberries wrapped with plastic wrap and the film prepared in Comparative Example 1 exhibited significant rotting and spoilage after storage at room temperature, while strawberries wrapped with the composite film of Examples 1-4 maintained their original freshness, with only slight softening observed at the lowest polyphenol-cinnamaldehyde nanoemulsion concentration. This indicates that the composite film prepared in this invention can extend the shelf life of strawberries, thanks to its combined antioxidant and antibacterial properties, which effectively prevent surface browning. Further studies on preservation performance included monitoring total soluble solids (TSS) content and weight loss dynamics. Figure 15 It can be seen that, compared with Comparative Example 1, Examples 1-4 exhibited more stable weight retention, which is attributed to the optimized water vapor permeability. Although the plastic wrap resulted in minimal weight loss, it led to impaired fruit quality, manifested as surface softening, inconsistent firmness, and reduced TSS content. This quality decline was due to the limited water permeability of the plastic wrap, resulting in excessive moisture retention and creating an unfavorable microenvironment for strawberry preservation. Figure 16 , 17 The changes in total soluble solids (TSS) and firmness of strawberries before and after storage are shown. TSS is an indicator of fruit ripeness. The TSS of strawberries from Example 4 to Example 1 and then to Comparative Example 1 increased sequentially, indicating that the addition of nanoemulsion delayed the ripening of strawberries. Strawberries stored with plastic wrap ripened too early and began to rot, so their TSS values ​​were lower than the initial values. The strawberries stored with plastic wrap and the composite film of Comparative Example 1 showed a significant decrease in firmness after 6 days, which can be attributed to gradual ripening, softening, and quality decline. In contrast, the composite film of Examples 1-4 greatly reduced the loss of firmness, indicating that the film has the effect of delaying the ripening and softening process of fruit.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any other way. Any person skilled in the art can utilize the above technical content to make changes or modifications to create equivalent embodiments. Any simple modifications, equivalent changes, and modifications made to the above embodiments without departing from the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a pectin-based thin film based on chestnut bud polyphenol nanoemulsion, characterized in that, The specific steps are as follows: 1) Wash, dry, crush, and sift the chestnut buds to obtain chestnut bud powder for later use; 2) Place the chestnut powder obtained in step 1) into a reaction vessel, add ethanol, heat under reflux and stir to obtain a mixture; 3) The mixture obtained in step 2) was vacuum filtered, the filtrate was rotary evaporated to remove ethanol, and then freeze-dried to obtain a brown powder, which is the polyphenol extract; 4) Disperse the powder obtained in step 3) in water, then add it to a mixture of cinnamaldehyde and anhydrous ethanol, mix and stir, and then obtain a stable nanoemulsion through high-pressure microfluidic treatment. 5) Mix the nanoemulsion obtained in step 4) with pectin solution, then add glycerol, and degas after high-pressure microfluidic treatment to obtain film-forming solution; 6) Spread the film-forming liquid obtained in step 5) onto the substrate, dry it or let it air dry, and then peel off the film to obtain the composite film.

2. The method for preparing pectin-based films based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 1), the chestnut powder is passed through a 250-mesh sieve.

3. The method for preparing pectin-based films based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 2), the solid-liquid ratio of chestnut powder to ethanol is 1g:8-12mL, and the volume concentration of ethanol is 70%; the temperature is heated to 80℃-85℃, the reflux time is 2.5-3.5 h, and the stirring speed is 150-500 rpm.

4. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 3), the vacuum filtration time is 2-3 hours, the filter paper pore size is 10-20 micrometers, the rotary evaporation temperature is 40-45℃, and the freeze-drying time is 60-80 hours with a temperature of -40 to -45℃.

5. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 4), the concentration of the polyphenol extract aqueous solution obtained by dispersing the brown powder in water is 1-3 mg / mL. The solid-liquid ratio of cinnamaldehyde to anhydrous ethanol in the cinnamaldehyde-anhydrous ethanol mixture is 1g:4-6mL. The volume ratio of the polyphenol extract aqueous solution to the cinnamaldehyde-anhydrous ethanol mixture is 20-30:

1. The stirring speed is 150-500 rpm and the stirring time is 25-35 minutes.

6. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1 or 5, characterized in that, In step 4), the pressure of the microjet is 12,000-18,000 psi, and the number of times is 15-25.

7. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 5), the solid-liquid ratio of pectin to water is 1g:40-60mL, and the pectin solution is used as the solution to obtain a nanoemulsion volume concentration of 5-40% in the nanoemulsion-pectin solution.

8. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 5), the amount of glycerol added is 30-50% of the pectin mass.

9. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 5), the pressure of the microjet treatment is 12,000-18,000 psi, and the number of treatments is 4-6.

10. The method for preparing a pectin-based film based on chestnut bud polyphenol nanoemulsion according to claim 1, characterized in that, In step 6), the slurry is spread evenly on the substrate, the substrate size is 25 cm × 15 cm, and the thickness of the slurry is 0.3-0.5 cm; the drying conditions are drying at 25-45℃ or natural drying until the film can be easily peeled off.

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