Preparation method and application of pH response type controlled release-fresh keeping nanofiber membrane

Abstract

The invention belongs to the technical field of intelligent food packaging films, and particularly relates to a preparation method and application of a pH response type controlled release-fresh keeping nanofiber film. According to the invention, the essential oil and the phenol mixture are combined, and the antibacterial purpose is realized through multi-target and multi-mechanism cooperation; secondly, using lignin modified copper-based MOFs as a pH controlled release carrier; and finally, preparing the pH controlled release-fresh keeping type nanofiber membrane through an electrostatic spinning device. The film-forming property, the mechanical property and the flexibility are effectively improved by introducing TPU, the antibacterial property is effectively improved by using the essential oil and phenol mixture, and finally, a flexible film with a high specific surface area and a porous structure is formed through a proper solution ratio and an electrostatic spinning process, so that the mixed fresh-keeping material can be effectively coated and protected, and the antibacterial property of the fresh-keeping material is improved. Meanwhile, the fiber has excellent mechanical property and gas permeability, is suitable for various food packaging scenes, and is wide in application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of food intelligent packaging films, and in particular relates to a preparation method and application of a pH-responsive controlled-release-fresh-keeping nanofiber film. Background Art

[0002] Food processing, storage, and transportation are constantly evolving worldwide, and a wide variety of foods are now available for distribution worldwide. This requires that foods not only have a long shelf life but also maintain their freshness during storage and transportation. Consequently, a variety of active preservation materials and methods have emerged. While these materials and methods can extend the shelf life of foods, traditional preservation methods (such as chemical preservatives and low-temperature storage) pose potential health risks, high energy consumption, or limited effectiveness. Consequently, the development of new intelligent fresh-keeping packaging has become a research hotspot. Essential oils and phenols are both plant extracts. When mixed, they possess excellent broad-spectrum antimicrobial and strong antioxidant properties, making them widely used in food preservation. However, their volatility, poor stability, and uncontrollable release limit their effectiveness. The invention patent "A Cellulase and pH Dual-Responsive Meat Food Preservation Method (CN202311015251.2)" discloses a method for preparing a fiber membrane that can achieve both food freshness indication and preservation by using electrospinning technology to create pores in chitosan oligosaccharides with polyethylene glycol. This is then loaded with plant essential oils via physical adsorption to form a core layer. Carboxymethyl cellulose, chondroitin sulfate, and anthocyanins are then blended to form a responsive coating. However, the sustained release of cellulase can be unstable due to the influence of external pH and temperature, making precise controlled release difficult. Furthermore, a single antimicrobial indicator can only be effective against specific bacteria or microorganisms, and long-term use of a single antimicrobial component can easily lead to microbial resistance, resulting in a decrease in antimicrobial effectiveness. The invention patent "pH-responsive controlled-release preservative hydrogel membrane, its preparation method, and use (ZL202311370821.X)" discloses a method for preparing a pH-responsive controlled-release preservative hydrogel membrane suitable for preserving foods such as livestock and poultry meat. Despite the pH-responsive nature of hydrogel membranes, precise control of the release rate of active substances under varying pH conditions remains a challenge in practical applications. The pH values ​​of different foods can vary significantly, resulting in unstable controlled-release effects. Furthermore, low temperatures can cause hydrogels to lose certain physical properties, compromising their effectiveness during cold chain transportation and storage. Summary of the Invention

[0003] In response to the shortcomings of the existing technology, the present invention provides a method for preparing a pH-controlled release-fresh-keeping nanofiber membrane; it mainly utilizes the combination of essential oils and phenolic mixtures to achieve the antibacterial purpose through a multi-target, multi-mechanism synergistic process (physical destruction of cell membranes, biochemical inhibition of enzymes and DNA, etc.), secondly, lignin-modified copper-based MOFs are used as pH-controlled release carriers, and finally, the pH-controlled release-fresh-keeping nanofiber membrane prepared by an electrospinning device has the effects of large specific surface area, porosity, and easy absorption.

[0004] In order to achieve the above purpose, the specific implementation steps of the present invention are as follows:

[0005] A method for preparing a pH controlled-release and fresh-keeping nanofiber membrane, the method being specifically implemented by electrospinning technology, comprises the following steps:

[0006] (1) dissolving m1 of thermoplastic polyurethane (TPU) in m2 of N,N-dimethylformamide (DMF) under light-shielding stirring to obtain a spinning base solution;

[0007] (2) Preparation of pH controlled release material (lignin-modified copper MOF): Dissolve mass m3 of lignin in deionized water, then add mass m4 of copper nitrate trihydrate and continue stirring to obtain solution A; then dissolve mass m5 of 1,3,5-benzenetricarboxylic acid (H3BTC) in anhydrous ethanol and continue stirring to obtain solution B; slowly add solution A to solution B at room temperature, ultrasonically treat, and continue stirring for a period of time. After stirring, centrifuge, wash, and dry to obtain lignin-modified copper MOF;

[0008] (3) mixing the essential oil of concentration C1 and the phenolic substance of concentration C2, stirring evenly, to obtain a fresh-keeping material; the essential oil comprises tea tree essential oil, cinnamon essential oil and oregano essential oil, and the phenolic substance comprises thymol, eugenol and carvacrol;

[0009] (4) adding the fresh-keeping material in step (3) to ethanol, and then adding the lignin-modified copper MOF obtained in step (2), stirring the reaction to obtain a mixed solution, centrifuging the obtained mixed solution to obtain a precipitate, and then washing and vacuum drying to obtain a lignin-modified copper MOF-coated active material loaded with the fresh-keeping material;

[0010] (5) mixing the spinning base solution obtained in step (1) with the lignin-modified copper MOF-coated active material in step (4), and stirring evenly in the dark to obtain a spinning solution;

[0011] (6) Based on the spinning solution obtained in step (5), electrospinning technology is used for spinning, and tin foil is used as a receiving substrate and collected by a drum to obtain a nanofiber membrane, which is recorded as TPU-lignin modified copper MOF-active material, that is, pH controlled release-freshness-preserving nanofiber membrane.

[0012] Preferably, the mass ratio of m1 to m2 in step (1) is 0.5-1.5:2.0-6.0; and the stirring time in the dark is 12 hours.

[0013] Preferably, the amount of lignin, deionized water, copper nitrate trihydrate and 1,3,5-benzenetricarboxylic acid in step (2) is 1.0-3.0 g:40 mL:0.1-0.5 g:1.0-3.0 g; the ultrasonic treatment time is 10-15 min, the continuous stirring time is 2 h, the centrifugation conditions are: 8000 rpm, 10 min, washing is performed using 50% anhydrous ethanol, and the drying conditions are 60°C, 10-12 h.

[0014] Preferably, in step (3), the essential oil C1 is 0.5-1.5 mg / mL, and the phenolic substance C2 is 2.0-6.0 mg / mL; and the volume ratio of the essential oil to the phenolic substance is 1-2:1-2.

[0015] Preferably, the amount of the preservative material, ethanol, and lignin-modified copper MOF in step (4) is 10-50 mg: 5-10 mL: 10-50 mg; the stirring reaction time is 2-4 h, the centrifugation conditions are: 8000 rpm, 10 min, washing is performed using 50% anhydrous ethanol, and the vacuum drying conditions are 60°C, 10-12 h.

[0016] Preferably, the mass concentration of the lignin-modified copper MOF-coated active material in the spinning solution in step (5) is 2-5%; and the time for stirring in the dark is 12 hours.

[0017] Preferably, the electrospinning technology conditions in step (6) are as follows: the voltage applied by the high-voltage power supply is 15-20 kV, the receiving distance is 10-20 cm, the solution propulsion rate is 0.1-0.4 ml / h, the spinning temperature is 25-45 ° C, and the relative humidity is 30%-60%;

[0018] The pH controlled release-fresh-keeping nanofiber membrane prepared by the method of the present invention is used for food fresh-keeping and antibacterial purposes.

[0019] Beneficial effects of the present invention

[0020] The application improves the antibacterial limitations of single antibacterial agent and the instability of substrate material affected by external environment, improves the antibacterial properties by introducing mixed antibacterial materials (a mixture of essential oils and phenolic substances), and better improves the substrate stability by using the high porosity, functional modification and environmental responsiveness of metal organic frameworks (MOFs). The use of modified MOF materials effectively realizes the encapsulation and on-demand release of fresh-keeping materials, greatly improves the food preservation effect; compared with the traditional film, it has a larger specific surface area, which can make the fresh-keeping materials uniformly dispersed in the fiber film, thereby improving the effective contact area of the fresh-keeping materials in the film and the food.

[0021] At the same time, as a renewable biological resource, lignin has excellent biocompatibility and pH responsiveness, and by modifying copper-based MOFs (such as HKUST-1), the material can be endowed with pH-sensitive drug controlled release capability. In the food spoilage process, microbial metabolism will cause the pH value of the microenvironment to change (such as the increase of pH during the spoilage of meat, fruits and vegetables), and the pH-responsive MOFs can trigger the targeted release of essential oil and phenolic mixture accordingly, realizing "on-demand preservation".

[0022] In addition, the above-mentioned composite material can be prepared into a nanofiber film by electrospinning technology, which can form a flexible film with high specific surface area and porous structure, which can effectively encapsulate and protect the mixed fresh-keeping materials, and also realize the slow release function by adjusting the fiber morphology, and has excellent mechanical properties and gas permeability, which is suitable for various food packaging scenes.

[0023] TPU (Thermoplastic Polyurethane, thermoplastic polyurethane) is a high-performance elastomer material, the application combines the use of thermoplastic polyurethane (TPU), which improves the mechanical properties and flexibility of the spun fiber film, and its own porous structure and high specific surface area also effectively realize the slow release of fresh-keeping materials. On the other hand, the addition of modified MOF materials in the application can effectively realize the purpose of pH-controlled release of fresh-keeping materials, and the copper ions in the MOF material can also effectively play an antibacterial role, which can reduce food spoilage and deterioration. Finally, the addition of essential oil and phenolic mixture in the application effectively realizes the purpose of food preservation, and the antibacterial property is reflected in: destroying the integrity of the cell membrane, inhibiting enzyme activity and metabolic pathways, and inhibiting biofilm formation.

[0024] Therefore, the application effectively realizes the purpose of pH-controlled release of fresh-keeping materials by introducing modified MOF materials, effectively improves the film-forming property, mechanical property and flexibility by introducing thermoplastic polyurethane (TPU), effectively improves the antibacterial property by using essential oil and phenolic mixture, and finally a food pH-controlled release-preservation type fiber film can be prepared by electrospinning process and appropriate solution ratio. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The figure is the whole experimental flow chart of the method of the present invention.

[0026] Figure 2 Application of TPU-lignin modified copper MOF-active material (2.5%) fiber membrane in beef. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to the accompanying drawings, but the present invention is not limited to these embodiments. It should be noted that those skilled in the art may make various modifications and improvements without departing from the scope of the present invention. Such modifications and improvements are all within the scope of protection of the present invention.

[0028] Example 1:

[0029] 1. Screening antibacterial materials by the inhibition zone method;

[0030] First, a filter membrane with a pore size of 6 mm was prepared using a punch to obtain a pretreated filter membrane. Then, five samples were prepared: A: 0.5 mg / mL tea tree essential oil; B: 2.0 mg / mL carvacrol; C: a solution of 2.0 mg / mL carvacrol and 0.5 mg / mL tea tree essential oil mixed in a volume ratio of 1:1; D: a solution of 2.0 mg / mL carvacrol and 0.5 mg / mL tea tree essential oil mixed in a volume ratio of 1:2; and E: a solution of 2.0 mg / mL carvacrol and 0.5 mg / mL tea tree essential oil mixed in a volume ratio of 2:1.

[0031] Take the pre-treated filter membrane and soak it in 5 samples of AE respectively; then add 100 μL 10 6 Escherichia coli, Staphylococcus aureus, Salmonella, Pseudomonas aeruginosa, and Bacillus subtilis were evenly dispersed on agar plates. Sterilized filter paper with a 6 mm pore size, soaked in antimicrobial material, was then placed on the nutrient agar plates containing the bacterial suspension. The plates were then incubated in a 37°C incubator for 24 hours. The diameter of the inhibition zone was measured using a vernier caliper. Each experiment was repeated three times.

[0032] The final results are shown in Table 1. Both antibacterial materials and antibacterial materials in different proportions after mixing have certain inhibitory effects on bacteria. In comparison, the antibacterial effect of carvacrol-tea tree oil at the ratio of 2:1 is more significant, with the inhibition zone diameters for Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella and Bacillus subtilis being 10.33±0.47, 12.10±1.25, 13.33±0.47, 13.67±1.15 and 11.0±0.54, respectively. This is mainly attributed to the synergistic effect between the two. Carvacrol mainly exerts its effect by destroying the bacterial cell membrane and inhibiting its growth, while the main components of tea tree oil (such as terpenes) achieve antibacterial effects through different mechanisms, such as interfering with the bacterial metabolic process and changing the permeability of the cell membrane.

[0033] Furthermore, when carvacrol and tea tree oil act together, tea tree oil enhances the permeability of carvacrol, allowing it to more easily enter bacterial cells, thereby improving its antibacterial effect. In summary, the antibacterial activity of the carvacrol and tea tree oil mixture is superior to that of either alone, primarily due to their synergistic effect, enhanced permeability and penetration, and broad-spectrum antibacterial activity.

[0034] Table 1 Inhibition zone diameters (mm) of essential oils, phenols, and essential oil-phenol mixtures

[0035]

[0036]

[0037] 2. pH release rate of copper MOF before and after modification:

[0038] (1) Preparation of solution:

[0039] 1 g of lignin was dissolved in 30 mL of deionized water, followed by the addition of 0.1 g of copper nitrate trihydrate with continuous stirring to prepare solution A;

[0040] Dissolve 1 g of H3BTC in 30 mL of deionized water with continuous stirring to prepare solution B.

[0041] Dissolve 0.1 g of copper nitrate trihydrate in 60 mL of deionized water and continue stirring to obtain solution C.

[0042] (2) Preparation of copper MOF: Solution C was slowly added to solution B at room temperature, followed by the addition of 0.2 mL of triethylamine. The mixture was ultrasonicated for 10 min and stirred continuously for 1 h. After stirring, the precipitate was collected by centrifugation at 8000 rpm for 10 min and washed three times with 50% anhydrous ethanol. The washed precipitate was vacuum dried at 60°C overnight to obtain copper MOF.

[0043] (3) Preparation of lignin-modified copper MOF: Solution A was slowly added to solution B, ultrasonicated for 10 min, and stirred for 2 h. After stirring, the precipitate was collected by centrifugation at 8000 rpm for 10 min and washed three times with 50% anhydrous ethanol. The washed precipitate was vacuum-dried at 60°C overnight to obtain lignin-modified copper MOF.

[0044] (4) Preparation of copper MOF-coated active material: tea tree essential oil and carvacrol were mixed in a volume ratio of 1:2 to obtain a fresh-keeping material; 50 mg of the fresh-keeping material was then dissolved in 5 mL of anhydrous ethanol, and then 50 mg of copper MOF was added. The mixture was stirred at room temperature for 2 h to obtain a mixed solution. The obtained mixed solution was centrifuged at 8000 rpm for 10 min to collect the precipitate, which was then washed three times with 50% anhydrous ethanol. The washed precipitate was vacuum-dried at 60°C overnight to obtain a copper MOF-coated active material.

[0045] (5) Preparation of lignin-modified copper MOF-coated active material: The operation is the same as step (4), except that the copper MOF is replaced by the modified copper MOF of step (3), and finally the lignin-modified copper MOF-coated active material is obtained.

[0046] In order to better verify the pH-controlled release ability of copper MOF before and after modification, the release rates of active materials of copper MOF-coated active materials and lignin-modified copper MOF-coated active materials at different pH (pH 4.5, pH 6.0, pH 7.5) were measured respectively;

[0047] 30 mg of copper MOF-coated active material and lignin-modified copper MOF-coated active material were immersed in 30 mL of release medium containing different pH values ​​(PBS buffer solutions at pH 4.5, pH 6.0, and pH 7.5) and shaken at 150 rpm for release studies. 1 mL of sample was taken at intervals (5 min, 10 min, 15 min, 20 min, 15 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 90 min, and 120 min), and the absorbance of the supernatant was measured after centrifugation. At the same time, 1 mL of new buffer solution was added to maintain a constant volume. The specific release rate was finally calculated using the formula.

[0048] The specific results are shown in Table 2. The modified copper MOF has improved stability under acidic conditions, with the release rate reduced from 90% at an initial pH of 4.5 to 70% at present. This greatly improves the stability of the substance under acidic conditions and reduces the problems of premature release and excessive release of the coating. This is due to the lignin modification. The phenolic and alcohol groups rich in lignin can chemically interact with the MOF surface or metal ions, improving its stability under different pH environments. In addition, the addition of lignin may change the pore structure of the copper MOF and the binding mode of the drug, making the drug release process more controllable and slow.

[0049] Table 2 Release rate of active materials before and after modification of copper MOF under different pH conditions

[0050]

[0051] 3. Effects of different contents of lignin-modified copper MOF coated active materials on the mechanical properties of the film;

[0052] The fiber membranes were prepared using an electrospinning machine under the following conditions: a high-voltage power supply with a voltage of 18 kV, a receiving distance of 15 cm, a solution propulsion rate of 0.2 ml / h, a spinning temperature of 30 °C, and a relative humidity of 40%.

[0053] (1) TPU fiber membrane: 1 g TPU was dissolved in 4 g DMF and stirred in the dark for 12 h to obtain a mixed solution; the obtained mixed solution was used as the spinning solution and the TPU fiber membrane was prepared by an electrospinning machine.

[0054] (2) TPU-lignin-modified copper MOF-active material: 1 g TPU was dissolved in 4 g DMF and stirred in the dark for 12 h to obtain solution D. Then, lignin-modified copper MOF-coated active material was added and stirred for 12 h to obtain spinning solution. The mass concentration of lignin-modified copper MOF-coated active material in the spinning solution was adjusted to 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%, respectively.

[0055] TPU-lignin modified copper MOF-active material can be prepared based on the spinning solution through an electrospinning machine, which is correspondingly recorded as TPU-lignin modified copper MOF-active material (1.0%), TPU-lignin modified copper MOF-active material (1.5%), TPU-lignin modified copper MOF-active material (2.0%), TPU-lignin modified copper MOF-active material (2.5%), and TPU-lignin modified copper MOF-active material (3.0%).

[0056] The mechanical properties of film materials directly influence their performance and applicability in various applications. To evaluate the effects of varying concentrations of modified copper MOF-coated active material on the film's mechanical properties, tensile strength (TS) and elongation at break (%) were measured. The results, shown in Table 3, show that varying concentrations of lignin-modified copper MOF-coated active material have a certain impact on the film's mechanical properties, with the optimal ratio being TPU-lignin-modified copper MOF-active material (2.5%). This is because increasing the concentration of lignin-modified copper MOF enhances the film's wear resistance and hardness. The introduction of copper MOF may improve the film's wear resistance, making it suitable for applications requiring higher wear resistance. Low concentrations (0.5-2%) can improve the film's surface properties and stability, with minimal impact on the film's tensile properties, making it suitable for applications requiring slight hardness enhancement and surface modification. Medium concentrations (2-5%) of copper MOF significantly enhance the TPU film's overall mechanical properties, including increased tensile strength and wear resistance, while maintaining a certain degree of elongation. This makes it suitable for applications requiring both strength and toughness. High concentrations (>5%) of copper MOF can lead to brittle films with poor ductility and low tensile strength, making them suitable for applications requiring only high hardness and not ductility. Therefore, this study selected TPU-lignin-modified copper MOF-active material (2.5%) for film preparation.

[0057] Table 3 Mechanical properties of fiber membrane

[0058]

[0059] Example 2:

[0060] (1) 1.5 g of TPU was dissolved in 4.5 g of DMF and stirred in the dark for 12 h to obtain a spinning base solution;

[0061] (2) 2 g of lignin was dissolved in 30 mL of deionized water, and then 0.3 g of copper nitrate trihydrate was added and stirred continuously to prepare solution A; 3 g of H3BTC was dissolved in 30 mL of deionized water and stirred continuously to prepare solution B; then solution A was slowly added to solution B, ultrasonicated for 10 min and stirred for 2 h, the resulting mixed solution was centrifuged at 8000 rpm for 10 min, and the precipitate was collected and washed three times with 50% anhydrous ethanol. The washed precipitate was vacuum dried at 60 °C overnight to obtain lignin-modified copper MOF;

[0062] (3) 0.5 mg / mL tea tree essential oil and 2.0 mg / mL carvacrol were mixed in a volume ratio of 2:1 and stirred to obtain a fresh-keeping material;

[0063] (4) 50 mg of the fresh-keeping material was added to 5 ml of ethanol, and then 50 mg of the lignin-modified copper MOF obtained in step (2) was added. After stirring at room temperature for 2 h, the precipitate was collected by centrifugation at 8000 rpm for 10 min, and washed three times with 50% anhydrous ethanol. The washed precipitate was vacuum-dried at 60 ° C overnight to obtain the lignin-modified copper MOF-coated active material.

[0064] (5) The lignin-modified copper MOF-coated active material obtained in step (4) was mixed with the spinning base solution in step (1), and stirred for 12 h in the dark to obtain a spinning solution. The mass concentration of the lignin-modified copper MOF-coated active material in the spinning solution was adjusted to 2.5%;

[0065] (6) Preparation of nanofiber membrane: The electrospinning technology was used to inject the above spinning solution into a syringe. The corresponding condition parameters were: the voltage applied by the high-voltage power supply was 18KV, the receiving distance was 15cm, the solution propulsion rate was 0.2ml / h, the spinning temperature was 30℃, and the relative humidity was 40%. The nanofiber membrane obtained was collected by a drum using tin foil as the receiving substrate, which was a pH-controlled release-preservation nanofiber membrane.

[0066] Application: The pH controlled release-fresh-keeping nanofiber membrane obtained in Example 2 was used in beef preservation experiments:

[0067] Under a sterile environment, beef with uniform appearance and good texture was selected and evenly cut into small pieces of approximately 40g. Then, 40g of fresh beef was selected from the cuts and placed in a food-grade polyethylene (PE) ziplock bag as a blank control group. 40g of beef was wrapped with 0.5g of pH-controlled release-preservation nanofiber membrane and placed in a PE ziplock bag as an experimental group.

[0068] The beef from the control group and the experimental group were refrigerated at 4°C, and the pH value, total bacterial count (TVC), TVB-N and other physical and chemical indicators of the beef samples were measured within one week.

[0069] (1) pH Determination: Under sterile conditions, 10 g of uniform beef chunks were placed in a homogenizing bag (sterile) containing 90 mL of 0.85% sterile saline. The bag was then struck with a hammer to thoroughly mix the contents. The pH was then measured using a handheld pH meter. The results were expressed as the average of three measurements.

[0070] (2) Determination of TVC value: 10 g of beef with uniform texture was placed in a homogenization bag (sterile) containing 90 mL of 0.85% physiological saline, and then homogenate was prepared by knocking the contents of the homogenization bag. The resulting homogenate was serially diluted to the appropriate concentration by 10-fold dilution method. First, 200 μL of the diluent was spread on the NA plate, and then it was incubated in a 37°C constant temperature incubator for 48 hours. The entire experiment was carried out under sterile conditions. The average of three experiments was recorded, and the data were recorded in lg(CFU / mL) units (GB 4789.2-2016).

[0071] (3) Determination of TVB-N value: 10 g of beef was placed in a beaker containing 75 mL of distilled water, and homogenized with a homogenizer, and then the homogenate was placed in a centrifuge tube and centrifuged at 5000 rpm for 10 min. 1 g of magnesium oxide was added to the supernatant obtained by centrifugation, and then the Kjeldahl nitrogen analyzer was used to distill the solution with 2% boric acid as the receiving solution. After sufficient distillation, the distillate was titrated with an automatic potentiometric titrator (titrant was 0.1 mol / L hydrochloric acid standard solution). The above experiment was repeated three times, and calculated according to the following formula:

[0072]

[0073] In the formula, X (mg / 100 g) is the TVB-N content of beef; V1 (mL) is the volume of standard solution consumed by the sample; V2 (mL) is the volume of solution consumed by the sample (blank); m (g) is the weight of the sample; C (mol / L) is the concentration of hydrochloric acid standard solution, 0.1 mol / L.

[0074] The specific results are shown in Table 1. Figure 2 The control group began to spoil on the fourth day, with a TVB-N value of 18.14 mg / 100 g, a pH value of 6.15, and a TVC value of 7.32 (far exceeding the threshold value of fresh beef specified in the national standard, with a pH value not exceeding 6, a TVB-N value not exceeding 15 mg / 100 g, and a TVC value not exceeding 7.0).

[0075] After the beef was wrapped with TPU-lignin modified copper MOF active material (2.5%), the TVB-N value on the fourth day was 11.21 mg / 100 g, the pH value was 5.75, and the TVC value was 6.54, which was far below the threshold value specified in the national standard, indicating that the beef was in a fresh state at that moment. In summary, the addition of TPU-modified MOF active material (2.5%) fiber membrane effectively reduced the spoilage rate of beef and greatly extended the shelf life of food.

[0076] Note: The examples described above are intended only to illustrate the present invention and are not intended to limit the present invention. Furthermore, the present invention is not limited to the examples described above. Any changes, additions, deletions, or substitutions made by a person skilled in the art within the spirit and scope of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for preparing a pH controlled release-preservation nanofiber membrane, characterized in that: The following steps are involved: (1) dissolving a mass m1 of thermoplastic polyurethane in a mass m2 of N,N-dimethylformamide, stirring in the dark, to obtain a spinning base solution; (2) Dissolve mass m3 of lignin in deionized water, then add mass m4 of copper nitrate trihydrate and continue stirring to obtain solution A; then dissolve mass m5 of 1,3,5-benzenetricarboxylic acid in anhydrous ethanol and continue stirring to obtain solution B; under room temperature, slowly add solution A to solution B, ultrasonically treat, and continue stirring for a period of time. After stirring, centrifuge, wash, and dry to obtain lignin-modified copper MOF; (3) mixing the essential oil of concentration C1 and the phenolic substance of concentration C2, stirring evenly, to obtain a fresh-keeping material; the essential oil comprises tea tree essential oil, cinnamon essential oil and oregano essential oil, and the phenolic substance comprises thymol, eugenol and carvacrol; (4) adding the fresh-keeping material in step (3) to ethanol, and then adding the lignin-modified copper MOF obtained in step (2), stirring the reaction to obtain a mixed solution, centrifuging the obtained mixed solution to obtain a precipitate, and then washing and vacuum drying to obtain a lignin-modified copper MOF-coated active material loaded with the fresh-keeping material; (5) mixing the spinning base solution obtained in step (1) with the lignin-modified copper MOF-coated active material in step (4), and stirring evenly in the dark to obtain a spinning solution; (6) Based on the spinning solution obtained in step (5), electrospinning technology is used for spinning, and tin foil is used as a receiving substrate and collected by a drum to obtain a nanofiber membrane, which is recorded as TPU-lignin modified copper MOF-active material, that is, pH controlled release-freshness-preserving nanofiber membrane.

2. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: The mass ratio of m1 to m2 in step (1) is 0.5-1.5:2.0-6.0; and the stirring time in the dark is 12 hours.

3. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: The amount of lignin, deionized water, copper nitrate trihydrate and 1,3,5-benzenetricarboxylic acid in step (2) is in the following relationship: 1.0-3.0 g: 40 mL: 0.1-0.5 g: 1.0-3.0 g.

4. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: The ultrasonic treatment time in step (2) is 10-15 minutes, the continuous stirring time is 2 hours, the centrifugal conditions are: 8000 rpm, 10 minutes, washing is performed using 50% anhydrous ethanol, and the drying conditions are 60°C, 10-12 hours.

5. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: In step (3), the essential oil C1 is 0.5-1.5 mg / mL, and the phenolic substance C2 is 2.0-6.0 mg / mL; the volume ratio of the essential oil to the phenolic substance is 1-2:1-2.

6. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: The dosage of the preservative material, ethanol, and lignin-modified copper MOF in step (4) is 10-50 mg: 5-10 mL: 10-50 mg.

7. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: The stirring reaction time in step (4) is 2-4 hours, the centrifugal conditions are: 8000 rpm, 10 minutes, washing is performed using 50% anhydrous ethanol, and the vacuum drying conditions are 60° C., 10-12 hours.

8. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: In step (5), the mass concentration of the lignin-modified copper MOF-coated active material in the spinning solution is 2-5%; and the time for stirring in the dark is 12 hours.

9. The method for preparing a pH controlled release-fresh-keeping nanofiber membrane according to claim 1, characterized in that: The conditions and parameters of the electrospinning technology described in step (6) are as follows: the voltage applied by the high-voltage power supply is 15-20 kV, the receiving distance is 10-20 cm, the solution propulsion rate is 0.1-0.4 ml / h, the spinning temperature is 25-45 ° C, and the relative humidity is 30%-60%.

10. Use of the pH controlled release-fresh-keeping nanofiber membrane prepared according to the method of any one of claims 1 to 9 for food preservation and antibacterial treatment.

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