Preparation method and application of composite tannin active membrane liquid

Abstract

The invention discloses a preparation method and application of a composite tannin active membrane solution. The method comprises the following steps: S1, pretreatment; s2, preparing an extraction solvent; s3, carrying out ultrasonic-microwave cooperative treatment; s4, purifying and concentrating; and S5, preparing a membrane solution. According to the preparation method of the compound tannin active membrane liquid, a high-activity tannin extract is prepared through a green and efficient technology, and the high-activity tannin extract is independently prepared or compounded with other components such as chitosan to be used for preparing a preservative product for preserving postharvest fruits such as apples and tomatoes, the technical problem that the natural active ingredient tannin is easy to migrate and inactivate in application is solved through triple combination cross-linking of tannin, chitosan and nanocellulose, so that the preservative film has lasting and stable antioxidant and antibacterial functions, the film material is biodegradable and environment-friendly, and the preparation process is clean.

Description

Technical Field

[0001] This invention relates to the field of plant-based preservative materials technology, specifically to a method for preparing and applying a composite tannin active film liquid. Background Technology

[0002] Tannin-based active film is a functional food packaging material based on plant tannins (such as tannic acid). By loading tannin components, the film acquires antibacterial, antioxidant, and gas regulation properties to extend the shelf life of food. The core function of tannin-based active film lies in its multiple preservation mechanisms: tannin components effectively inhibit microbial growth and oxidation reactions. For example, the tannic acid-chitosan microsphere hybrid film has significant antibacterial activity against putrefactive microorganisms and molds such as Aspergillus niger, Penicillium, and Candida albicans. Simultaneously, by regulating oxygen and carbon dioxide permeability, it maintains an ideal gaseous atmosphere within the packaging, slowing down the respiration and metabolism of fruit.

[0003] The tannin-activated membrane has been proven effective on a variety of perishable fruits, such as oranges, strawberries, and cherries, extending their shelf life to over 45 days at room temperature. The material is biocompatible, edible, reusable, simple to use, and environmentally friendly.

[0004] In existing technologies, many researchers have conducted studies on this topic. For example, patent publication number CN117510926A discloses a method for preparing a biodegradable, high-toughness carboxymethyl cellulose / polyvinyl alcohol-based antioxidant packaging film. The method involves mixing carboxymethyl cellulose and polyvinyl alcohol solutions in a 1:1 volume ratio, adding glutaraldehyde, and stirring thoroughly in an 85°C oil bath. Then, at room temperature, glycerol and tannic acid are added separately, stirred thoroughly, coated into a film, and dried to obtain the carboxymethyl cellulose / polyvinyl alcohol-based antioxidant film. This technology claims that the high-toughness carboxymethyl cellulose / polyvinyl alcohol-based antioxidant packaging film possesses excellent mechanical properties, exhibiting excellent toughness with an elongation of up to 751.12% and a tensile strength of up to 48.51 MPa; its antioxidant performance reaches 86.45%, showing potential application prospects in the field of biodegradable plastics. However, in this method, glutaraldehyde is first added as a strong crosslinking agent to crosslink with carboxymethyl cellulose, and then tannic acid is added. This results in tannic acid being unable to participate in effective crosslinking. At the same time, crosslinking increases strength, but toughness decreases. Therefore, this technology or similar technologies may still have defects. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for preparing and applying a composite tannin active film solution.

[0006] The technical solution of this invention is: A method for preparing a composite tannin active film solution includes the following steps: S1. Pretreatment: Take fresh pomegranate peel, and wash, dry and crush it in sequence to obtain pomegranate peel powder; S2. Preparation of extraction solvent: Mix choline chloride and glycerol in a molar ratio of 1:2 to 2.2, stir evenly in a water bath, and then add deionized water to obtain an extraction solvent with a water content of 30 to 40 wt%. S3. Ultrasonic-microwave synergistic treatment: Mix the pomegranate peel powder with the extraction solvent at a ratio of 1g:15~20mL, treat with ultrasonic pulse for 10~15min, and simultaneously perform microwave heating assisted treatment, then centrifuge and collect the supernatant. S4. Purification and concentration: The macroporous resin is soaked in anhydrous ethanol, and then the supernatant is filtered and loaded onto the sample. The tannin purified solution is then eluted with a 20-30% ethanol solution to obtain a tannin purified solution. The solution is concentrated at a low temperature of 36-40℃ to obtain a concentrated solution, which is then freeze-dried in a freeze dryer to obtain freeze-dried tannin powder. S5. Preparation of membrane solution: The freeze-dried tannin powder is prepared into an aqueous tannin solution. Chitosan solution is taken and the aqueous tannin solution is added dropwise. The mixture is stirred for 20-30 min, then glycerol and nanocellulose are added and the mixture is stirred for another 40-60 min. The pH is adjusted to 5-5.5, and the mixture is allowed to stand for vacuum degassing to obtain a composite tannin active membrane solution. In the composite tannin active membrane solution, the mass percentage of chitosan is 1-3 wt%, the mass percentage of tannin is 0.8-2.2%, the mass percentage of glycerol is 0.5-1.5%, and the mass percentage of nanocellulose is 0.2-1%.

[0007] Furthermore, in S1, the material is dried under hot air at 40~50℃, and then pulverized and passed through a 60-mesh sieve.

[0008] Note: By controlling the drying temperature and powder size, the specific surface area of ​​the raw material is increased while avoiding the loss of heat-sensitive components, which is beneficial for the efficient and uniform extraction of tannins in the subsequent process.

[0009] Furthermore, in S2, the water bath stirring temperature is 75~80℃, and the stirring time is 15~30min until a homogeneous and transparent liquid is obtained.

[0010] Note: By defining the optimal temperature and time for tannin preparation, a uniform and stable transparent liquid is formed, ensuring the consistency of the extraction solvent quality and thus guaranteeing a stable tannin extraction rate.

[0011] Furthermore, in S3, the ultrasonic pulse is activated for 10-15 seconds and paused for 10-15 seconds, the ultrasonic frequency is 40-60kHz, the ultrasonic power is 250-400W, the microwave power is 300-500W, the temperature is 60-70℃, and the centrifugal speed is 4000-5000rpm.

[0012] Explanation: By using a synergistic mode of pulsed ultrasound and microwaves, the cell walls can be fully broken through cavitation and mechanical force, while the target components are dissolved through microwave heating. This avoids the destruction of tannin structures caused by continuous high-energy input, achieving the best balance between efficiency and activity.

[0013] Furthermore, in S4, the macroporous resin is AB-8 resin, and a 0.45μm filter membrane is used for filtration. The sample loading flow rate is 0.5~1BV / h, and the elution flow rate is 2~3BV / h.

[0014] Note: By controlling the purification parameters, the adsorption saturation and elution selectivity of the purification process are ensured, which is the key process guarantee for high yield and high purity tannin products.

[0015] Further, in S5, the tannin aqueous solution is obtained by placing the freeze-dried tannin powder in deionized water to obtain a tannin aqueous solution with a mass concentration of 10-20%. The chitosan solution is obtained by adding chitosan powder to an acetic acid solution with a mass concentration of 1-2%, stirring magnetically for 1-2 hours until completely dissolved, and allowing it to stand to remove bubbles to obtain a chitosan solution with a mass concentration of 5-10%. The pH is adjusted by adding a NaOH solution with a molar concentration of 0.1-0.2 mol / L.

[0016] Note: By adjusting the concentration and pH of the tannin aqueous solution and chitosan solution, a uniform and stable compounding of the two was achieved, avoiding the easy flocculation of high-concentration tannin and chitosan under acidic conditions due to charge interaction.

[0017] Further, in S5, the nanocellulose undergoes citric acid esterification modification. The specific method is as follows: by weight, take 100 parts of nanocellulose suspension with a solid content of 1-2%, add 4-5 parts of citric acid powder and 0.2-0.5 parts of sodium hypophosphite, stir magnetically for 1-2 hours until completely dissolved, place in an oven and dry at 110-120℃ for 1-2 hours to obtain a solid block, take it out and add 150-200 parts of deionized water and stir to disperse, add NaOH solution with a molar concentration of 0.1-0.2 mol / L to adjust the pH to 6.5-7, centrifuge to remove the supernatant, repeat 3 times to obtain citric acid esterified modified nanocellulose.

[0018] Explanation: By citric acid esterification to modify nanocellulose, carboxyl groups are introduced on its surface, thereby improving compatibility and dispersibility, and enhancing interfacial bonding and crosslinking density.

[0019] This invention also provides an application of a composite tannin active film liquid, which is prepared according to the above-mentioned preparation method of a composite tannin active film liquid. The composite tannin active film liquid is used for the preservation of apples or tomatoes. The specific method is as follows: fresh apples or tomatoes are washed and air-dried naturally. The composite tannin active film liquid is evenly coated onto the surface of the fresh apples or tomatoes, sprayed evenly 2-3 times, with an interval of 5-10 minutes between each coat. After drying, it is placed in an environment with a room temperature of 25-28℃ and a relative humidity of 50-60% for 1-2 hours to solidify and form a composite tannin active film.

[0020] Note: By providing specific coating process parameters for apples and tomatoes, we ensure that the active film forms a uniform, firmly adhered, and breathable / moisture-permeable protective layer on the fruit surface, giving the preparation method clear guidance value for industrial application.

[0021] The beneficial effects of this invention are: (1) The preparation method of the composite tannin active film liquid of the present invention prepares a highly active tannin extract through a green and efficient process, and combines it alone or with other components such as chitosan to prepare a fresh-keeping agent product for the post-harvest preservation of fruits such as apples and tomatoes. The triple cross-linking of tannin, chitosan and nanocellulose solves the technical problem of easy migration and inactivation of natural active ingredient tannin in application, so that the preservation film has a long-lasting and stable antioxidant and antibacterial function. The film material is biodegradable, environmentally friendly and clean in preparation process.

[0022] (2) The preparation method of the composite tannin active film liquid of the present invention selects pomegranate peel as the only plant raw material for tannin extraction. The tannin type is mainly punicin, and the tannin content and biological activity are significantly higher than those of common fruit peel raw materials such as grape peel and apple peel. This provides a source guarantee for the preparation of high-activity preservation film. Secondly, tannins in pomegranate peel coexist naturally with flavonoids, organic acids and other substances. In the green extraction process, they may form a composite extract with synergistic effect, and its preservation function is better than that of single tannin components. As a by-product of bulk agricultural product processing, the utilization of pomegranate peel is in line with the concept of green and sustainable development. The hydrogen bonding mechanism of the choline chloride / glycerol-based eutectic solvent extraction system is highly matched with the polyphenolic hydroxyl structure of ellagic tannin in pomegranate peel, realizing the efficient and selective extraction of high-activity components, achieving mutual matching of raw materials and processes, and significantly improving the overall preservation effect.

[0023] (3) In the preparation method of the composite tannin active membrane liquid of the present invention, the nanocellulose is modified by citric acid esterification, and carboxyl groups are introduced on its surface to improve compatibility and dispersibility. The modified nanocellulose is more uniformly dispersed in the chitosan-tannin system, avoiding membrane defects caused by agglomeration. At the same time, it enhances the interfacial bonding and cross-linking density. The carboxyl groups on the surface form stronger hydrogen bonds and possible ionic bond networks with the amino groups of chitosan and the phenolic hydroxyl groups of tannin, which significantly improves the mechanical strength and toughness, water resistance and structural density of the membrane. It also imparts additional functions. Citric acid residues may provide a certain ability to chelate metal ions and assist in anti-oxidation. Attached Figure Description

[0024] Figure 1 This is a schematic diagram showing the change in weightlessness rate with the number of days in each case in Experiment Example 3 of this invention; Figure 2 This is a schematic diagram showing the change in hardness retention rate over days in each case in Experimental Example 3 of this invention. Detailed Implementation

[0025] Example 1 A method for preparing a composite tannin active film solution includes the following steps: S1. Pretreatment: Take fresh pomegranate peel, wash, dry and crush it to obtain pomegranate peel powder. Dry it in hot air at 46℃ and crush it to pass through a 60-mesh sieve. S2. Preparation of extraction solvent: Choline chloride and glycerol were mixed in a molar ratio of 1:2.1, stirred evenly in a water bath, and then deionized water was added to obtain an extraction solvent with a water content of 35wt%. The water bath stirring temperature was 77℃ and the stirring time was 20min until a homogeneous and transparent liquid was obtained. S3. Ultrasonic-microwave synergistic treatment: Pomegranate peel powder and extraction solvent were mixed at a ratio of 1g:17mL, ultrasonic pulse treatment was performed for 12min, and microwave heating was performed simultaneously. Then, the supernatant was collected by centrifugation. The ultrasonic pulse was 12s on and 12s off, the ultrasonic frequency was 50kHz, the ultrasonic power was 300W, the microwave power was 400W, the temperature was 65℃, and the centrifugation speed was 4500rpm. S4. Purification and Concentration: The macroporous resin was soaked in anhydrous ethanol, and the supernatant was filtered and loaded onto the sample. The sample was then eluted with a 25% ethanol solution to obtain a purified tannin solution. The solution was concentrated at 38°C to obtain a concentrated solution. The macroporous resin used was AB-8 resin. The sample was filtered using a 0.45μm filter membrane. The loading flow rate was 0.8 BV / h, and the elution flow rate was 2.5 BV / h. The sample was then freeze-dried to obtain freeze-dried tannin powder. S5. Membrane Solution Preparation: A 15% (w / w) tannin aqueous solution was prepared by placing the lyophilized tannin powder in deionized water. A 7% (w / w) chitosan solution was obtained by adding chitosan powder to a 1.5% (w / w) acetic acid solution and magnetically stirring for 1.5 hours until completely dissolved. After standing and degassing, a 7% (w / w) chitosan solution was obtained. The tannin aqueous solution was added dropwise, and the mixture was stirred for 25 minutes. Then, glycerol and nanocellulose were added, and stirring continued for 50 minutes. The pH was adjusted to 5.2 by adding a 0.15 mol / L NaOH solution. After standing and vacuum degassing, a composite tannin active membrane solution was obtained. In the composite tannin active membrane solution, the mass percentages of chitosan, tannin, glycerol, and nanocellulose were 2.5 wt%, 1%, and 0.5%, respectively. Nanocellulose was modified by citric acid esterification. The specific method is as follows: 100 parts by weight of nanocellulose suspension with a solid content of 1.5% were taken, and 4.5 parts of citric acid powder and 0.3 parts of sodium hypophosphite were added sequentially. The mixture was magnetically stirred for 1.5 h until completely dissolved. The solid block was dried in an oven at 115℃ for 1.5 h to obtain a solid block. After removal, 180 parts of deionized water were added and stirred to disperse the block. The pH was adjusted to 6.8 by adding NaOH solution with a molar concentration of 0.15 mol / L. The supernatant was removed by centrifugation. The process was repeated 3 times to obtain citric acid esterified modified nanocellulose.

[0026] Example 2 The difference between this embodiment and Embodiment 1 is that: S2. Preparation of extraction solvent: Choline chloride and glycerol are mixed in a molar ratio of 1:2, stirred evenly in a water bath, and then deionized water is added to obtain an extraction solvent with a water content of 30wt%. The water bath stirring temperature is 75℃ and the stirring time is 30min until a homogeneous and transparent liquid is obtained.

[0027] Example 3 The difference between this embodiment and Embodiment 1 is that: S2. Preparation of extraction solvent: Choline chloride and glycerol are mixed in a molar ratio of 1:2.2, stirred evenly in a water bath, and then deionized water is added to obtain an extraction solvent with a water content of 40wt%. The water bath stirring temperature is 80℃ and the stirring time is 15min until a homogeneous and transparent liquid is obtained.

[0028] Explanation: By employing choline chloride and glycerol in an appropriate molar ratio to form a eutectic solvent backbone, a stable and moderate hydrogen bond network is ensured between the chloride ions provided by choline chloride and the hydroxyl groups provided by glycerol. This provides an ideal basic solvent environment for the selective binding and dissolution of tannin polyphenols in pomegranate peel. Furthermore, by rationally controlling the water content, the solvent viscosity can be effectively reduced, significantly increasing its penetration and mass transfer rate into the pomegranate peel powder cells, thereby improving extraction efficiency. Simultaneously, this water content is sufficient to optimize solvent properties while avoiding the disruption of the solvent backbone's specific dissolution ability for tannins due to excessive moisture, ensuring high purity and bioactivity of the extract. Therefore, the synergistic limitation of the molar ratio of choline chloride and glycerol and the water content is one of the key points for achieving green, efficient, and highly active extraction in this method. Within the scope of this invention, the adjustments described in Examples 1-3 all achieve good results.

[0029] Example 4 The difference between this embodiment and Embodiment 1 is that: S3. Ultrasonic-microwave synergistic treatment: Pomegranate peel powder and extraction solvent were mixed at a ratio of 1g:15mL, ultrasonic pulse treatment was performed for 15min, and microwave heating was performed simultaneously. Then, the supernatant was collected by centrifugation. The ultrasonic pulse was 10s on and 10s off, the ultrasonic frequency was 40kHz, the ultrasonic power was 250W, the microwave power was 300W, the temperature was 60℃, and the centrifugation speed was 4000rpm.

[0030] Example 5 The difference between this embodiment and Embodiment 1 is that: S3. Ultrasonic-microwave synergistic treatment: Pomegranate peel powder and extraction solvent were mixed at a ratio of 1g:20mL, ultrasonic pulse treatment was performed for 10min, and microwave heating was performed simultaneously. Then, the supernatant was collected by centrifugation. The ultrasonic pulse was 15s on and 15s off, the ultrasonic frequency was 60kHz, the ultrasonic power was 400W, the microwave power was 500W, the temperature was 70℃, and the centrifugation speed was 5000rpm.

[0031] Note: The ultrasonic-microwave synergistic processing parameter system used in this invention needs to be properly adjusted. Microwaves, as a highly efficient and uniform primary heat source, operate at 300-500W and are interlocked with the temperature feedback system to ensure the extraction process remains within a mild window of 60-70℃, minimizing the degradation of heat-sensitive tannins. The synergistic ultrasonic processing employs a low-to-medium frequency of 40-60kHz and a moderate power of 250-400W, applied in 10-15s / 10-15s pulse patterns. This setup achieves a triple synergistic effect: First, the bulk heating of the microwaves effectively reduces the viscosity of the extraction solvent, significantly enhancing the cavitation effect and material cell disruption efficiency generated by the subsequent ultrasonic pulses. Second, the strong mechanical disturbances and microjets generated by the ultrasonic pulses continuously renew the solid-liquid interface, greatly improving the uniformity of microwave heating and mass transfer rate, preventing localized overheating. Third, the pulsed energy input method allows the system to achieve a dynamic balance between high-intensity instantaneous action and a generally mild thermal environment, thus preserving the natural antioxidant activity of tannins while achieving high-efficiency extraction. The specific combination and synergistic effect of the above parameters is one of the key points that distinguishes this method from conventional sequential processing or simple superposition processing. Within the scope of this invention, the adjustment methods in Examples 1, 4 and 5 can all achieve good results.

[0032] Example 6 The difference between this embodiment and Embodiment 1 is that: S1. Pretreatment: Take fresh pomegranate peel, wash, dry and crush it to obtain pomegranate peel powder. Dry it in hot air at 40℃ and crush it to pass through a 60-mesh sieve.

[0033] Example 7 The difference between this embodiment and Embodiment 1 is that: S1. Pretreatment: Take fresh pomegranate peel, wash, dry and crush it to obtain pomegranate peel powder. Dry it in hot air at 50℃ and crush it to pass through a 60-mesh sieve.

[0034] Example 8 The difference between this embodiment and Embodiment 1 is that: S4. Purification and Concentration: The macroporous resin was soaked in anhydrous ethanol, and the supernatant was filtered and loaded onto the sample. The sample was then eluted with a 20% ethanol solution to obtain a purified tannin solution. The solution was concentrated at 36°C to obtain a concentrated solution. The macroporous resin used was AB-8 resin. The sample was filtered using a 0.45μm filter membrane. The loading flow rate was 0.5 BV / h, the elution flow rate was 2 BV / h, and the sample was lyophilized in a freeze dryer to obtain lyophilized tannin powder. S5. Preparation of membrane solution: The lyophilized tannin powder was prepared into a tannin aqueous solution by placing the lyophilized tannin powder in deionized water to obtain a 10% (w / w) tannin aqueous solution. Chitosan solution was prepared by adding chitosan powder to a 1% (w / w) acetic acid solution and stirring magnetically for 1 hour until completely dissolved. After standing and degassing, a 5% (w / w) chitosan solution was obtained. The tannin aqueous solution was added dropwise to the solution and stirred for 20 minutes. Then, glycerol and nanocellulose were added and stirring was continued for 40 minutes. The pH was adjusted to 5 by adding a 0.1 mol / L NaOH solution. After standing and vacuum degassing, the composite tannin active membrane solution was obtained.

[0035] Example 9 The difference between this embodiment and Embodiment 1 is that: S4. Purification and Concentration: The macroporous resin was soaked in anhydrous ethanol, and the supernatant was filtered and loaded onto the sample. The sample was then eluted with a 30% ethanol solution to obtain a purified tannin solution. The solution was concentrated at 40°C to obtain a concentrated solution. The macroporous resin was AB-8 resin. A 0.45μm filter membrane was used for filtration. The loading flow rate was 1 BV / h, the elution flow rate was 3 BV / h, and the solution was freeze-dried in a freeze dryer to obtain freeze-dried tannin powder. S5. Preparation of membrane solution: The lyophilized tannin powder was prepared into a tannin aqueous solution by placing the lyophilized tannin powder in deionized water to obtain a 20% (w / w) tannin aqueous solution. Chitosan solution was prepared by adding chitosan powder to a 2% (w / w) acetic acid solution and stirring magnetically for 2 hours until completely dissolved. After standing and degassing, a 10% (w / w) chitosan solution was obtained. The tannin aqueous solution was added dropwise to the solution and stirred for 30 minutes. Then, glycerol and nanocellulose were added and stirring was continued for 60 minutes. The pH was adjusted to 5.5 by adding a 0.2 mol / L NaOH solution. After standing and vacuum degassing, the composite tannin active membrane solution was obtained.

[0036] Example 10 The difference between this embodiment and Embodiment 1 is that: In the composite tannin active film solution, the mass percentage of chitosan is 1 wt%, the mass percentage of tannin is 1.8%, the mass percentage of glycerol is 0.5%, and the mass percentage of nanocellulose is 0.2%.

[0037] Example 11 The difference between this embodiment and Embodiment 1 is that: In the composite tannin active film solution, chitosan accounts for 2 wt%, tannin accounts for 0.8% wt%, glycerol accounts for 1% wt%, and nanocellulose accounts for 1% wt%.

[0038] Example 12 The difference between this embodiment and Embodiment 1 is that: In the composite tannin active film solution, chitosan accounts for 3 wt%, tannin accounts for 2.2% wt%, glycerol accounts for 0.6% wt%, and nanocellulose accounts for 0.6% wt%.

[0039] Example 13 The difference between this embodiment and Embodiment 1 is that: Nanocellulose was modified by citric acid esterification. The specific method is as follows: 100 parts by weight of nanocellulose suspension with a solid content of 1% were taken, and 4 parts of citric acid powder and 0.2 parts of sodium hypophosphite were added sequentially. The mixture was magnetically stirred for 1 hour until completely dissolved. The solid block was dried in an oven at 110℃ for 1 hour. After removal, 150 parts of deionized water were added and stirred to disperse. The pH was adjusted to 6.5 by adding 0.1 mol / L NaOH solution. The supernatant was removed by centrifugation. The process was repeated 3 times to obtain citric acid esterified modified nanocellulose.

[0040] Example 14 The difference between this embodiment and Embodiment 1 is that: Nanocellulose was modified by citric acid esterification. The specific method is as follows: 100 parts by weight of nanocellulose suspension with a solid content of 2% were taken, and 5 parts of citric acid powder and 0.5 parts of sodium hypophosphite were added sequentially. The mixture was magnetically stirred for 2 hours until completely dissolved. The solid block was dried in an oven at 120°C for 2 hours. After that, 200 parts of deionized water were added and stirred to disperse the solid block. NaOH solution with a molar concentration of 0.2 mol / L was added to adjust the pH to 7. The supernatant was removed by centrifugation. The process was repeated 3 times to obtain citric acid esterified modified nanocellulose.

[0041] Example 15 This embodiment provides an application of a composite tannin active film liquid. The composite tannin active film liquid is prepared according to a preparation method of a composite tannin active film liquid in Example 1. The composite tannin active film liquid is used for the preservation of apples. The specific method is as follows: fresh apples are washed and air-dried naturally. The composite tannin active film liquid is evenly coated onto the surface of the fresh apples. It is evenly sprayed 3 times, with a 6-minute interval between each coat. After drying, it is placed in an environment with a room temperature of 26°C and a relative humidity of 55% for 1.5 hours to solidify and form a composite tannin active film.

[0042] Example 16 The difference between this embodiment and embodiment 15 is that: Wash fresh tomatoes and air dry them naturally. Apply the composite tannin active film liquid evenly to the surface of the fresh tomatoes, spraying it evenly twice, with a 5-minute interval between each application. After drying, place it in an environment with a room temperature of 25℃ and a relative humidity of 50% for 1 hour to cure, forming a composite tannin active film.

[0043] Example 17 The difference between this embodiment and embodiment 15 is that: After washing and air-drying fresh apples, apply the composite tannin active film liquid evenly to the surface of the fresh apples. Spray evenly three times, with a 10-minute interval between each coat. After drying, place the apples in an environment with a room temperature of 28°C and a relative humidity of 60% for 2 hours to cure, forming a composite tannin active film.

[0044] Experimental Example 1 To demonstrate that the eutectic solvent combined with ultrasound-microwave synergistic extraction method of the present invention has advantages over traditional methods in terms of extraction efficiency and tannin activity protection, comparative experiments were conducted. Comparative Example 1 used hot water extraction, with the same raw materials as the present invention, a material-to-liquid ratio of 1:20, at 90°C for 2 hours; Comparative Example 2 used an organic solvent method, with 60% ethanol as the organic solvent, the same raw materials as the present invention, a material-to-liquid ratio of 1:15, at 70°C for 2 hours. The results are shown in Table 1.

[0045] Table 1. Extraction efficiency and tannin activity protection under different preparation methods

[0046] It can be seen that the method in Example 1 of the present invention has better yield and antioxidant activity, and a lower IC50. 50 The extraction efficiency and total phenol content are significantly better than traditional methods, demonstrating that it can extract and retain the bioactivity of pomegranate peel tannins more efficiently and completely.

[0047] Experimental Example 2 Subsequently, to demonstrate the synergistic enhancing effect of the citric acid esterified modified nanocellulose of the present invention on the mechanical and barrier properties of the composite membrane, comparative experiments were conducted. Comparative Example 3 did not add any nanocellulose, while Comparative Example 4 added 0.5% unmodified nanocellulose. All other steps were the same as in Example 1. The results are shown in Table 2.

[0048] Table 2 Mechanical and barrier properties of composite membranes prepared by different methods

[0049] It can be seen that the introduction of modified nanocellulose significantly improves the strength and toughness of the membrane while significantly reducing its permeability to water vapor and oxygen. This confirms that the modified nanocellulose achieves excellent strengthening, toughening, and densification effects through enhanced interfacial interactions.

[0050] Experimental Example 3 Below, we will verify the preservation effect of the composite tannin active film of this invention on apples / tomatoes under real storage conditions through comparative experiments. Comparative Example 5 was conducted without any active film, while Comparative Example 6 involved spraying with a commercially available chitosan fruit preservative. Apples of uniform freshness were selected for testing, and the results are as follows: Figure 1 and Figure 2 As shown.

[0051] It can be seen that the weight loss rate increases linearly with time, but the slope is the lowest in Example 1. After 15 days of storage, the composite tannin active film of the present invention reduced the weight loss rate by 64.5% compared with Comparative Example 5 and by 45.7% compared with Comparative Example 6, proving that the film of the present invention has the best water barrier performance and effectively reduces the transpiration of the fruit.

[0052] Fruit firmness decreased with storage time, with Example 1 showing the slowest decrease. On day 15, the composite tannin active film of the present invention increased the firmness retention rate by 79.5% compared to Comparative Example 5 and by 21.7% compared to Comparative Example 6. This demonstrates that the film of the present invention more effectively maintains the fruit cell structure and firmness by inhibiting fruit respiration and delaying the activity of cell wall degrading enzymes.

Claims

1. A method for preparing a composite tannin active film solution, characterized in that, Includes the following steps: S1. Pretreatment: Take fresh pomegranate peel, and wash, dry and crush it in sequence to obtain pomegranate peel powder; S2. Preparation of extraction solvent: Mix choline chloride and glycerol in a molar ratio of 1:2 to 2.2, stir evenly in a water bath, and then add deionized water to obtain an extraction solvent with a water content of 30 to 40 wt%. S3. Ultrasonic-microwave synergistic treatment: Mix the pomegranate peel powder with the extraction solvent at a ratio of 1g:15~20mL, treat with ultrasonic pulse for 10~15min, and simultaneously perform microwave heating assisted treatment, then centrifuge and collect the supernatant. S4. Purification and concentration: The macroporous resin is soaked in anhydrous ethanol, and then the supernatant is filtered and loaded onto the sample. The tannin purified solution is then eluted with a 20-30% ethanol solution to obtain a tannin purified solution. The solution is concentrated at a low temperature of 36-40℃ to obtain a concentrated solution, which is then freeze-dried in a freeze dryer to obtain freeze-dried tannin powder. S5. Preparation of membrane solution: The freeze-dried tannin powder is prepared into an aqueous tannin solution. Chitosan solution is taken and the aqueous tannin solution is added dropwise. The mixture is stirred for 20-30 min, then glycerol and nanocellulose are added and the mixture is stirred for another 40-60 min. The pH is adjusted to 5-5.5, and the mixture is allowed to stand for vacuum degassing to obtain a composite tannin active membrane solution. In the composite tannin active membrane solution, the mass percentage of chitosan is 1-3 wt%, the mass percentage of tannin is 0.8-2.2%, the mass percentage of glycerol is 0.5-1.5%, and the mass percentage of nanocellulose is 0.2-1%.

2. The method for preparing a composite tannin active film liquid according to claim 1, characterized in that, In S1, the product is dried under hot air at 40~50℃, and then pulverized and passed through a 60-mesh sieve.

3. The method for preparing a composite tannin active film liquid according to claim 1, characterized in that, In S2, the water bath stirring temperature is 75~80℃, and the stirring time is 15~30min until a homogeneous and transparent liquid is obtained.

4. The method for preparing a composite tannin active film liquid according to claim 1, characterized in that, In S3, the ultrasonic pulse is activated for 10-15 seconds and paused for 10-15 seconds, the ultrasonic frequency is 40-60kHz, the ultrasonic power is 250-400W, the microwave power is 300-500W, the temperature is 60-70℃, and the centrifugal speed is 4000-5000rpm.

5. The method for preparing a composite tannin active film liquid according to claim 1, characterized in that, In S4, the macroporous resin is AB-8 resin, and a 0.45μm filter membrane is used for filtration. The loading flow rate is 0.5~1BV / h, and the elution flow rate is 2~3BV / h.

6. The method for preparing a composite tannin active film liquid according to claim 1, characterized in that, In S5, the tannin aqueous solution is obtained by placing the freeze-dried tannin powder in deionized water to obtain a tannin aqueous solution with a mass concentration of 10-20%. The chitosan solution is obtained by adding chitosan powder to an acetic acid solution with a mass concentration of 1-2%, stirring magnetically for 1-2 hours until completely dissolved, and allowing it to stand to remove bubbles to obtain a chitosan solution with a mass concentration of 5-10%. The pH is adjusted by adding a NaOH solution with a molar concentration of 0.1-0.2 mol / L.

7. The method for preparing a composite tannin active film liquid according to claim 1, characterized in that, In S5, the nanocellulose undergoes citric acid esterification modification. The specific method is as follows: by weight, take 100 parts of nanocellulose suspension with a solid content of 1-2%, add 4-5 parts of citric acid powder and 0.2-0.5 parts of sodium hypophosphite, stir magnetically for 1-2 hours until completely dissolved, place in an oven and dry at 110-120℃ for 1-2 hours to obtain solid blocks, take them out and add 150-200 parts of deionized water and stir to disperse, add NaOH solution with a molar concentration of 0.1-0.2 mol / L to adjust the pH to 6.5-7, centrifuge to remove the supernatant, repeat 3 times to obtain citric acid esterified modified nanocellulose.

8. An application of a composite tannin active film solution, wherein the composite tannin active film solution is prepared according to any one of claims 1 to 7 by a method for preparing a composite tannin active film solution, characterized in that, The composite tannin active film liquid is used for the preservation of apples or tomatoes. The specific method is as follows: after washing fresh apples or tomatoes, let them air dry naturally. Then, evenly coat the surface of the fresh apples or tomatoes with the composite tannin active film liquid, spraying it evenly 2-3 times, with an interval of 5-10 minutes between each coat. After drying, place it in an environment with a room temperature of 25-28℃ and a relative humidity of 50-60% for 1-2 hours to solidify and form a composite tannin active film.

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