Curcumin nanoemulsion and Nisin-A loaded starch / pectin composite film as well as preparation method and application thereof

Abstract

The invention belongs to the field of food packaging, and discloses a curcumin nanoemulsion and Nisin-A loaded starch / pectin composite film as well as a preparation method and application thereof. The composite film is prepared from the following components in parts by mass: 10 to 100 parts of starch, 10 to 30 parts of pectin, 1 to 10 parts of plasticizer, 0.1 to 10 parts of Nisin-A, 0 to 20 parts of curcumin nano-emulsion and 20 to 80 parts of water, wherein the average particle size of the curcumin nano-emulsion is 100 to 180nm, and the dispersity of the curcumin nano-emulsion is 0.18 to 0.25. The preparation method comprises the following steps: uniformly mixing starch with pectin, a plasticizer and water, heating for full gelatinization, adding the curcumin nano-emulsion and Nisin-A, uniformly stirring, casting to form a film, and drying in vacuum to obtain the curcumin nano-film. The composite film disclosed by the invention has efficient antibacterial property and intelligent response performance, is suitable for food preservation and real-time monitoring of food freshness, is particularly applied to salmon packaging, and can prolong the preservation time of refrigerated salmons from 4 days to 8 days, prolong the shelf life and reduce waste.

Description

Technical Field

[0001] The invention belongs to the field of food packaging, and particularly relates to a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A, and a preparation method and application thereof. Background Art

[0002] Salmon is a high-protein food rich in Omega-3 fatty acids and is widely loved by consumers. However, salmon is easily affected by factors such as microbial contamination, oxidation and water loss during packaging, which leads to a rapid decrease in its freshness. In order to ensure the food safety and taste of salmon and extend its shelf life, it is urgent to adopt effective packaging and preservation technologies. Currently, traditional packaging materials such as polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC) plastic films have good mechanical strength and barrier properties, but most of them cannot be degraded and cause serious pollution to the environment. Therefore, it is of great practical significance to develop degradable packaging materials and apply them to the preservation of salmon.

[0003] Starch is a naturally abundant polysaccharide that has attracted much attention due to its wide availability, low cost, high biodegradability, and safety in contact with food. However, starch-based films face challenges such as insufficient mechanical strength and high hygroscopicity, which limit their practical applications. Pectin has excellent film-forming properties and high compatibility with starch, and has been widely used to enhance the mechanical strength of starch-based films. However, as consumers' requirements for food safety continue to increase, the functional requirements for traditional packaging materials are also getting higher and higher. Integrating active functions such as antibacterial properties and intelligent responsiveness into traditional films to achieve dynamic monitoring of food freshness has become a technical problem that needs to be urgently solved in this field.

[0004] Curcumin is a natural polyphenol compound extracted from turmeric, which has attracted much attention due to its broad-spectrum antibacterial, antioxidant, anti-inflammatory and photodynamic antibacterial properties. It is worth noting that its unique pH-responsive properties make it particularly suitable for smart food packaging applications. However, the inherent hydrophobicity of curcumin poses a challenge to its uniform dispersion and stability in the film matrix, which in turn affects its functional effects. Wang Yuemeng et al. (Wang Yuemeng, Liu Anjun, Li Xin, et al. Effect of ginger essential oil on gelatin CaCO 3Research on the Influence of Physicochemical and Antibacterial Properties of Edible Films [J]. Modern Food Science and Technology, 2015, 31(2): 57-62, 127. A packaging film was prepared by compounding gelatin and calcium carbonate, and turmeric essential oil was added to improve the antibacterial activity of the packaging film. The results showed that the packaging film had a certain fresh-keeping effect on chilled meat. However, due to the poor water solubility and stability of curcumin, its bioavailability was relatively low, which limited the application of curcumin in the fields of food and medicine. CN119529338A added a curcumin ethanol solution to a starch-based film to endow the film with pH-responsive function. However, the high crystallinity of curcumin in the film led to a decrease in the elongation at break of the film, affecting the mechanical properties of the film. Aiming at the disadvantages of traditional starch-based films, such as insufficient mechanical strength, lack of antibacterial ability and food freshness monitoring function, it is urgent to find a convenient, environmentally friendly and low-cost method for modifying starch-based films to provide a starch-based film with high antibacterial efficiency, fresh-keeping effect and the ability to real-time monitor the freshness of salmon, providing theoretical guidance and reference value for antibacterial modification and freshness monitoring modification in the film field, and further expanding the application scope and application value of starch-based films in the food packaging field. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the primary object of the present invention is to provide a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

[0006] Another object of the present invention is to provide a preparation method of a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

[0007] Another object of the present invention is to provide the application of a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

[0008] The object of the present invention is achieved by the following technical solutions:

[0009] A starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A, comprising the following components in parts by mass:

[0010]

[0011]

[0012] Preferably, the composite film comprises the following components in parts by mass:

[0013]

[0014] Preferably, the average particle size of the curcumin nanoemulsion is 100-180 nm, the dispersity is 0.18-0.25, and the zeta potential is -20 to -40 mV.

[0015] Preferably, the starch is one or more of corn starch, potato starch, wheat starch or pea starch;

[0016] The pectin is one or more of apple pectin, citrus pectin or grape pectin;

[0017] The plasticizer is one or more of glycerol, citric acid or isosorbide.

[0018] Preferably, the curcumin nanoemulsion is prepared by a self-assembly method.

[0019] Preferably, the preparation method of the curcumin nanoemulsion is as follows:

[0020] (1) Mix curcumin, absolute ethanol and medium-chain triglyceride (MCT), dissolve curcumin by ultrasonic wave, centrifuge and collect the supernatant to obtain the oil phase;

[0021] (2) Gradually add a surfactant to the oil phase, then add an appropriate amount of water, mix well, and perform ultrasonic homogenization to obtain a curcumin nanoemulsion.

[0022] Preferably, in step (1), the mass ratio of curcumin, absolute ethanol to medium-chain triglyceride (MCT) is 0.1-1:5:5; the rotation speed of the centrifugation is 6000-10000 rpm;

[0023] In step (2), the mass ratio of the oil phase to the surfactant is 0.5-2:1, preferably 1.35:1, the ultrasonic homogenization power is 300-500 W, and the ultrasonic homogenization time is 3-5 min.

[0024] Preferably, the surfactant in step (2) is a non-ionic surfactant, preferably Tween-80.

[0025] Preferably, the number of carbon atoms in the medium-chain triglyceride (MCT) in step (1) is 8-12.

[0026] A preparation method of a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A includes the following steps:

[0027] (1) Mix starch, pectin, plasticizer and water evenly, and stir to obtain a gelatinized mixed solution;

[0028] (2) Add Nisin-A and curcumin nanoemulsion to the gelatinized mixed solution, stir, remove bubbles, then cast into a film, and then perform vacuum drying to obtain a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

[0029] Preferably, the stirring in step (1) is constant temperature stirring at 80-95 °C for 2-4 h;

[0030] The temperature of the casting film formation in step (2) is 40-60°C, the vacuum drying temperature is 40-60°C, and the vacuum drying time is 36-72 h.

[0031] The application of the above-mentioned starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A in food preservation and film packaging.

[0032] The application of the above-mentioned starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A in fish preservation.

[0033] The application of the above-mentioned curcumin nanoemulsion and Nisin-A starch / pectin composite film in salmon preservation.

[0034] Preferably, it is applied in the packaging material for salmon preservation.

[0035] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0036] (1) The nanoemulsion in the present invention realizes a high-capacity loading of curcumin, and compared with the traditional method, it greatly improves the utilization rate of curcumin.

[0037] (2) The present invention provides a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A, which is prepared by the casting method. The process is simple, the cost is low, and it meets the requirements of green environmental protection. Casting film formation can reduce energy consumption and material waste during the production process while maintaining the structural stability of the film, which is an efficient and sustainable preparation method.

[0038] (3) The present invention provides a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A, which successfully overcomes the problem of uneven dispersion of hydrophobic materials in hydrophilic matrices in traditional methods. Through nanoemulsification technology, the hydrophobicity of curcumin is effectively controlled to ensure its uniform distribution in the film, thereby fully exerting the functional properties of curcumin. This film not only has significant ultraviolet shielding ability but also has a pH-responsive function, which can monitor the freshness and spoilage status of food in real time, providing a new solution for intelligent food packaging.

[0039] (4) The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A provided by the present invention has significant antibacterial ability. After being combined with blue light lamp treatment, the survival rates of Staphylococcus aureus and Salmonella can be reduced to 1.85% and 3.79% respectively within 2 hours. Curcumin nanoemulsion and Nisin-A have a synergistic antibacterial effect. This significant antibacterial effect makes the film perform excellently in extending the food preservation period. Especially in the preservation of perishable foods such as salmon, the present invention can extend the preservation period of refrigerated salmon from 4 days to 8 days, reducing the waste caused by food spoilage, and has broad application prospects in the field of food packaging, especially in the preservation and quality monitoring of perishable foods such as fish. Brief Description of the Drawings

[0040] Figure 1 : Schematic flow chart for the preparation of the starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A of the present invention.

[0041] Figure 2 : Average particle size, dispersity, Zeta potential and transmission electron microscope images of curcumin nanoemulsion in Examples 4, 5, 6, and 7. Figure 3 : Mechanical property test results of Examples 1, 2, 3, 4, and 5, labeled as Starch film, SP film, SP-Nfilm, SP-C film, and SP-NC film in sequence.

[0042] Figure 4 : Light transmittance effect test results of Examples 1, 2, 3, 4, and 5. a) is the optical image of the five examples, b) is the ultraviolet-visible light spectrum of the five examples, and c) is the light transmittance percentage and maximum light transmittance of the five examples under ultraviolet and visible light.

[0043] Figure 5 : Barrier property test results of Examples 1, 2, 3, 4, and 5. a) is the water contact angle image of the five examples, b) is the water vapor transmission rate of the five examples, and c) is the oxygen transmission rate of the five examples.

[0044] Figure 6 : pH response test results of Example 5. a) is the appearance of Example 5 after being treated with different pH buffer solutions, b) is the color value of Example 5 after being treated with different pH buffer solutions, c) is the change of L*, a*, and b* values of Example 5 after being treated with different pH buffer solutions, and d) is the change of ΔE value of Example 5 after being treated with different pH buffer solutions.

[0045] Figure 7: Antibacterial function test results of Example 5. a) is a blue light lamp device, b) is a simplified diagram of the photodynamic bactericidal mechanism of curcumin nanoemulsion, c) is a photo of Staphylococcus aureus on a solid medium after being treated with a thin film and a blue light lamp, d) is a scanning electron microscope image of Staphylococcus aureus after being treated with a thin film and a blue light lamp, e) is a photo of Salmonella on a solid medium after being treated with a thin film and a blue light lamp, f) is a scanning electron microscope image of Salmonella after being treated with a thin film and a blue light lamp, g) is the survival rate of Staphylococcus aureus, h) is the survival rate of Salmonella.

[0046] Figure 8 : Monitoring and preservation test results of the freshness of salmon by Example 2, 5 and PE film. Detailed implementation mode

[0047] The present invention will be further described in detail with specific examples below, but the implementation mode of the present invention is not limited thereto. For process parameters not specifically noted, conventional techniques can be referred to.

[0048] In the embodiments of the present invention, starch (corn starch, potato starch, wheat starch, and pea starch, Shanghai Macklin Biochemical Technology Co., Ltd.), pectin (apple pectin, citrus pectin, and grape pectin, Shanghai Macklin Biochemical Technology Co., Ltd.), Nisin-A (nisin, Shanghai Macklin Biochemical Technology Co., Ltd.), curcumin (Shanghai Yuanye Bio-Technology Co., Ltd.), plasticizer (glycerol, citric acid, isosorbide, Shanghai Macklin Biochemical Technology Co., Ltd.).

[0049] Example 1

[0050] (1) Preparation of pure starch film: Set the magnetic stirring temperature to 90 °C and the rotation speed to 1000 rpm. Add 50 parts of water, 10 parts of corn starch, and 3 parts of glycerol to the stirring pot in sequence, stir at a constant temperature for 6 h, cool to room temperature, set the ultrasonic power to 100 W, and treat for 30 min under ultrasonic conditions to remove bubbles. After casting into a film, place it in a vacuum drying oven at 45 °C for 48 h to obtain a uniform and transparent pure starch film.

[0051] Example 2

[0052] (1) Preparation of starch pectin film (SP film): Set the magnetic stirring temperature to 90 °C and the rotation speed to 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol to the stirring pot in sequence, stir at a constant temperature for 6 h, cool to room temperature, set the ultrasonic power to 100 W, and treat for 30 min under ultrasonic conditions to remove bubbles. After casting into a film, place it in a vacuum drying oven at 45 °C for 48 h to obtain a uniform and transparent starch pectin film.

[0053] Example 3

[0054] (1) Preparation of starch pectin antibacterial film (SP-N film): Set the magnetic stirring temperature at 90 °C and the rotation speed at 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol to the stirring pot in sequence, and stir at a constant temperature for 2 h to obtain a gelatinized mixture. Then add 0.1 part of Nisin-A and stir at a constant temperature for 4 h. Cool to room temperature, set the ultrasonic power at 100 W, and treat for 30 min under ultrasonic conditions to remove air bubbles. After casting into a film, place it in a vacuum drying oven at 45 °C and dry for 48 h to obtain a uniform and transparent SP-N film.

[0055] Example 4

[0056] (1) Preparation of curcumin nanoemulsion (CNE): Add 1 part of curcumin, 50 parts of absolute ethanol, and 50 parts of medium-chain triglyceride (MCT) to a centrifuge tube in sequence. Dissolve curcumin with ultrasonic waves, centrifuge at 8000 rpm for 10 min, and collect the supernatant to obtain the oil phase. Gradually add 75 parts of Tween-80 to the oil phase at room temperature, and then add an appropriate amount of ultrapure water. Mix the mixture thoroughly and stir evenly with a magnetic stirrer to obtain a coarse emulsion, and then use a cell disruptor for ultrasonic homogenization, set the power at 300 W, and treat for 4 min to obtain a uniform curcumin nanoemulsion. The average particle size of the obtained curcumin nanoemulsion is 124.8 nm, the polydispersity index (PDI) is 0.213, and the zeta potential is -23.3 mV.

[0057] (2) Preparation of starch pectin pH-responsive film (SP-C film): Set the magnetic stirring temperature at 90 °C and the rotation speed at 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol to the stirring pot in sequence, and stir at a constant temperature for 2 h to obtain a gelatinized mixture. Then add 4 parts of curcumin nanoemulsion and stir at a constant temperature for 4 h. Cool to room temperature, set the ultrasonic power at 100 W, and treat for 30 min under ultrasonic conditions to remove air bubbles. After casting into a film, place it in a vacuum drying oven at 45 °C and dry for 48 h to obtain a uniform and transparent SP-C film.

[0058] Example 5

[0059] (1) Preparation of curcumin nanoemulsion: Add 1 part of curcumin, 50 parts of absolute ethanol, and 50 parts of medium-chain triglyceride (MCT) to a centrifuge tube in sequence. Dissolve curcumin with ultrasonic waves, centrifuge at 8000 rpm for 10 min, and collect the supernatant to obtain the oil phase. At room temperature, gradually add 75 parts of Tween-80 to the oil phase, and then add an appropriate amount of ultrapure water. Mix the mixture thoroughly and stir evenly with a magnetic stirrer to obtain a coarse emulsion, and then use a cell disruptor for ultrasonic homogenization, set the power at 300 W, and treat for 4 min to obtain a uniform curcumin nanoemulsion.

[0060] (2) Preparation of starch pectin pH-responsive antibacterial film (SP-NC film): Set the magnetic stirring temperature at 90 °C and the rotation speed at 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol to the stirring pot in sequence, and stir at a constant temperature for 2 h to obtain a gelatinized mixture. Then add 0.1 part of Nisin-A and 4 parts of curcumin nanoemulsion, and stir at a constant temperature for 4 h. Cool to room temperature, set the ultrasonic power at 100 W, and treat for 30 min under ultrasonic conditions to remove bubbles. After casting into a film, place it in a vacuum drying oven at 45 °C and dry for 48 h to obtain a uniform and transparent SP-NC film.

[0061] Example 6

[0062] Different from Example 5, 2 parts of curcumin nanoemulsion were added in step (2).

[0063] Example 7

[0064] Different from Example 5, 12 parts of curcumin nanoemulsion were added in step (2).

[0065] Comparative Example 1

[0066] Different from Example 5, 4 parts of curcumin alcohol solution were added in step (2).

[0067] Test Example

[0068] (1) Test method for the microstructure of CNE:

[0069] The average particle size, dispersity, and Zeta potential of CNE were measured three times using a nanoparticle size analyzer (Zetasizer Nano ZS, UK). The microstructure of CNE was observed by a transmission electron microscope (JEM 1400Plus type, Japan).

[0070] Figure 2 It shows that the average particle size of CNE is 124.8 nm, indicating that its particle size is small. The particle size distribution curve is unimodal, indicating that the particle size distribution range is narrow and uniform. The dispersity is 0.213, confirming its uniform and stable dispersion, which is suitable for film preparation. The Zeta potential is -23.3 mV, indicating its strong stability and reducing the possibility of instability over time. The morphology of CNE can be directly observed by a transmission electron microscope. The nanoemulsion presents a uniform spherical shape, proving the successful preparation of CNE.

[0071] (2) Test method for the mechanical properties of starch-based films:

[0072] The tensile tear resistance of the starch-based films was evaluated using a universal testing machine (HZ-1007E, China). The films were cut into dumbbell shapes with a central width of 2 mm and a length of 35 mm. The tensile strength and elongation at break were measured according to ASTM D638-08 standard at a constant strain rate of 5 mm / min. To ensure reproducibility, five film samples were tested for each condition. Figure 3 The tensile strength and elongation at break of the starch-based films are shown. The pure starch film had the lowest tensile strength and elongation at break, which were 2.83 ± 0.14 MPa and 35.36 ± 0.87%, respectively. After adding pectin, the mechanical properties were significantly improved, and the tensile strength and elongation at break increased to 3.74 ± 0.16 MPa and 39.78 ± 1.03%, respectively. Compared with the SP film, the tensile strength of the SP-N film was significantly increased after adding Nisin-A. The tensile strength and elongation at break of the SP-C film were also significantly improved. These results indicate that the nanoscale properties of Nisin-A and CNE contribute to the improvement of the tensile strength of the starch-based films. In addition, CNE may change the internal structure of the starch-based films, thus increasing the elongation at break. It is worth noting that the SP-NC film showed more superior mechanical properties, with the tensile strength and elongation at break reaching 4.51 ± 0.10 MPa and 43.58 ± 0.73%, respectively. This improvement may be due to the formation of hydrogen bonds and intermolecular interactions among starch, pectin, Nisin-A, and CNE. In addition, in Example 7, due to the addition of more CNE, the film texture was softer, and the tensile strength and elongation at break decreased significantly; in Comparative Example 1, due to the addition of curcumin alcohol solution, curcumin crystals precipitated in the film, which also led to a decrease in the tensile strength and elongation at break of the film.

[0073] (3) Test method for the light transmittance effect of starch-based films:

[0074] The ultraviolet-visible spectrum of the starch-based films was tested using an ultraviolet-visible spectrophotometer (Cary60, Agilent Technologies, USA). The starch-based films were cut into a size of 2 cm × 4 cm. The ultraviolet light analysis range was 200 to 380 nm, and the visible light analysis range was 380 to 780 nm.

[0075] Figure 4 a) shows the optical images of the five films. Obviously, the starch film, SP film, and SP-N film are colorless and transparent, while the SP-C film and SP-NC film remain transparent but show yellow due to the addition of CNE. This indicates that the films have good light transmittance, which is beneficial for displaying the appearance of food in packaging. To further evaluate the optical properties of the films, the ultraviolet (200 - 380 nm) and visible light (380 - 780 nm) transmittance tests were carried out on the films. As Figure 4As shown in (b), the starch film has poor UV-blocking performance. However, after adding pectin and Nisin-A, the UV-blocking ability of the starch-based film is significantly enhanced. After adding curcumin nanoemulsion, the starch-based film almost completely blocks UV. It is worth noting that the SP-C and SP-NC films have obvious blocking effects on visible light near 425 nm, which is exactly the excitation wavelength for curcumin to activate its photodynamic bactericidal activity. As Figure 4 As shown in (c), the UV transmittance of starch, SP, SP-N, SP-C, and SP-NC films are 52.91 ± 1.01%, 16.79 ± 0.86%, 15.77 ± 1.08%, 5.49 ± 1.24%, and 3.69 ± 0.65%, respectively. These results indicate that the addition of CNE significantly enhances the UV-blocking ability of the starch-based film. Specifically, at a wavelength of 400 - 780 nm, the visible light transmittance of starch film, SP film, SP-N film, SP-C film, and SP-NC film are 86.47 ± 0.68%, 78.47 ± 1.31%, 66.62 ± 1.76%, 45.31 ± 1.89%, and 44.70 ± 1.15%, respectively. This shows that the SP-NC composite film has excellent optical properties and film-forming ability. In addition, the maximum light transmittance of the five films are 87.97 ± 2.48%, 85.30 ± 2.04%, 72.53 ± 1.19%, 63.92 ± 2.01%, and 61.66 ± 0.88%, respectively. Among all the films, the starch film has the highest transmittance, indicating its uniform internal structure. In contrast, the SP-NC film also shows a good maximum light transmittance. These findings highlight the excellent UV-blocking and light-transmitting characteristics of the SP-NC film, making it very suitable for packaging applications.

[0076] (4) Test method for the contact angle of starch-based film:

[0077] The surface wettability of the starch-based film was evaluated by measuring the contact angle (CA). After the film was equilibrated at 57% relative humidity for 10 hours, the CA was measured at room temperature using a contact angle system (OCA40, DataPhysics, Germany). 10 μL of water droplets were deposited on the film surface and photographed after 5 seconds. Measurements were made at five random positions on each sample to ensure measurement accuracy.

[0078] The CA value of the starch film was the lowest, at 42.5 ± 0.4°, indicating its hydrophilicity. After adding pectin, the CA value increased to 58.6 ± 0.8°, enhancing the hydrophobicity of the film. This increase can be attributed to the promotion of intermolecular hydrogen bonds between starch and pectin, thus improving the hydrophobicity of the starch-based film. Compared with the SP film, the CA value of the SP-N film decreased to 43.1 ± 0.3°, which may be attributed to the hydrophilicity of Nisin-A. Notably, the CA value of the SP-C film increased significantly to 68.7 ± 0.6°, indicating enhanced hydrophobicity. The SP-NC film maintained a relatively high CA value (64.3 ± 0.6°), indicating that CNE played a dominant role in enhancing hydrophobicity. This effect is not only attributed to the inherent hydrophobicity of curcumin but also to the rough surface structure formed after its addition to the film. In addition, in Example 7, due to the excessive addition of CNE, the water contact angle decreased significantly to 34.1 ± 0.4°, making it unsuitable for food packaging.

[0079] (5) Test method for water vapor permeability of starch-based films:

[0080] First, cut the film sample and place it at the opening of the permeation cup containing 10 mL of distilled water. Then, place the assembly in a desiccator, maintain a relative humidity of 75%, and standardize it with a saturated potassium nitrate solution. To ensure accuracy and reliability, each film sample was measured at least five times, and the measurement results were averaged to minimize potential errors. The WVP value was calculated according to the following formula:

[0081] WVP (g·mm -1 ·s -1 ·Pa -1 ) = ΔW × T / A × Δt × ΔP (1)

[0082] In the formula, ΔW represents the weight increase (g), T is the film thickness (mm), A is the cup mouth area (mm 2 ), Δt is the equilibrium time (s), and ΔP is the vapor pressure difference across the film at 25°C.

[0083] The WVP value of the starch film was (10.69 ± 0.24) × 10 -10 g / (mm·s·Pa). The addition of pectin to the starch film decreased the WVP value, which may be due to the formation of intermolecular hydrogen bonds between starch and pectin, enhancing the structural compactness of the film and reducing water vapor permeability. Compared with the SP film, the WVP of the SP-N film decreased to (5.12 ± 0.20) × 10 -10 g / (mm·s·Pa), indicating that the addition of Nisin-A improved the compactness of the starch-based film. The WVP of the SP-C film was (8.42 ± 0.16) × 10 - 10g / (mm·s·Pa), higher than that of the SP-N film, which may be due to the formation of microporous structures by CNE in the film, thus promoting the transport of water vapor. The WVP value of the SP-NC film is (6.82 ± 0.18) × 10 -10 g / (mm·s·Pa), showing improved water vapor barrier performance compared to the SP film.

[0084] (6) Test method for oxygen permeability of starch-based films:

[0085] Place the film on the mouth of a bottle containing 10 g of desiccant (a mixture of reduced iron powder, activated carbon, and sodium chloride in a ratio of 1:2:1). Then keep the bottle in a desiccator at a temperature of 25 °C and a relative humidity of 75%. Each sample is measured in triplicate. Monitor the weight of the bottle within 48 hours and calculate the OP value of the film using the following formula:

[0086] OP (g·mm·mm -2 ·s -1 ) = (ΔW × d) / (A × t) (2)

[0087] Where ΔW represents the weight change of the film (g), d is the film thickness (mm), A is the area of the bottle (mm 2 ), and t is the time interval (s).

[0088] The OP values of the starch film and the SP film are (7.81 ± 0.10) × 10 -9 g·mm·m - 2·s - -1 and (6.16 ± 0.16) × 10 -9 g·mm·m - 2·s - -1, indicating relatively good gas permeability. In contrast, the OP value of the SP-N film is lower, at (4.21 ± 0.14) × 10 -9 g·mm·m - 2·s - -1, which may be due to the addition of Nisin-A enhancing the denseness of the starch-based film. It should be noted that the OP value of the SP-NC film is (6.10 ± 0.20) × 10 -9 g·mm·m - 2·s - -1, showing no significant difference compared to the SP film. This indicates that the simultaneous addition of Nisin-A and CNE has no obvious effect on the oxygen permeability of the starch-based film.

[0089] (7) Test method for pH response effect of starch-based films:

[0090] The SP-NC film was immersed in buffer solutions with different pH values, air-dried and photographed. A handheld colorimeter (CR30) was used to record the color of the film samples, and the L*, a* and b* values were recorded. The color difference (ΔE) was calculated using the following formula:

[0091]

[0092] where L 0 *, a 0 *, and b 0 * are the color parameters of the initial dry film, and L*, a* and b* are the color parameters at different pH values.

[0093] The color changes of the SP-NC film under different pH conditions are shown as Figure 5 a). When the pH value is 3 - 7, the solution is light yellow. As the pH value rises from 8 to 11, the solution gradually turns reddish-brown. This indicates that the CNE in the starch-based film maintains its sensitive pH response function. Figure 5 b) shows the color changes of the SP-NC film captured by a handheld colorimeter, showing the detailed changes in the L*, a* and b* values. The specific trend is shown in Figure 5 c). When the pH value is 3 - 7, the ranges of the L*, a* and b* values are 4.69, 3.72 and 9.53 respectively, and no obvious changes are observed within this range. When the pH value is 8, sudden changes occur in the L* and a* values. Specifically, the L* value drops by 14.00, the a value rises by 20.99, and the ΔE rises from 7.02 ± 0.58 to 31.89 ± 0.64. These results indicate that the SP-NC film can provide an accurate response to alert retailers and consumers when the meat is approaching or has already deteriorated. In the pH range of 7 - 11, the L*, a* and b* values of the SP-NC film continue to change significantly, and the ΔE finally increases to 74.68 ± 0.34. The sensitive color-changing response ability of the SP-NC film proves its potential as an intelligent packaging material.

[0094] (8) Test method for the antibacterial effect of starch-based films:

[0095] The antibacterial effects against Staphylococcus aureus and Salmonella were evaluated. In the preliminary experiment, it was found that the antibacterial effect of Example 6 was poor, so Example 5 was selected for the antibacterial effect determination: Small pieces of the SP-NC film were placed in a conical flask containing 1 mL of a bacterial suspension with a concentration of 10 5 CFU / mL. The film was in contact with the bacterial suspension for 1 hour and was irradiated with blue light for 20 min during this period. After the bacterial suspension was spread on a solid medium and incubated for 24 hours, optical photos were taken. The colonies were counted and the bacterial survival rate was calculated to evaluate the antibacterial effect of the film. In addition, images of Salmonella and Staphylococcus aureus were taken using a scanning electron microscope.

[0096] The image of Staphylococcus aureus shows that the bacterial cell wall of the control group remained smooth and intact. After 20 min of blue light irradiation, there was no obvious change in Staphylococcus aureus, indicating that blue light itself does not have bactericidal effect. After treatment with the film, obvious shrinkage and rupture of the bacteria occurred, which was due to Nisin-A damaging the bacterial cell wall and causing the leakage of intracellular contents. When the film was combined with blue light treatment, Staphylococcus aureus was almost completely killed, and the scanning electron microscope image showed a large number of bacteria shrinking and rupturing. This is because under blue light irradiation, intramolecular charge transfer occurs in curcumin, generating singlet oxygen 1O 2 interacts with various components in the cell, leading to the death of microbial cells. The image of Salmonella shows that in the film treatment group, a slight decrease in the number of bacteria was observed, indicating that the inhibitory effect of Nisin-A on it is limited. After treatment with the film and blue light lamp, the number of Salmonella decreased significantly, and the cells shrank and ruptured over a large area. The bacterial survival rate on the solid medium was further calculated. After the combined treatment of the film and blue light, Staphylococcus aureus and Salmonella were almost completely eradicated, and the survival rates were only 1.85±0.62% and 3.79±1.57% respectively. In conclusion, the synergistic effect of Nisin-A and CNE enhanced the antibacterial effect of the film against Gram-positive and Gram-negative bacteria, demonstrating the great potential of SP-NC film in antibacterial packaging applications.

[0097] (9) Test method for monitoring and preserving the freshness of salmon with starch-based film:

[0098] Total volatile basic nitrogen (TVB-N):

[0099] Weigh 5 g of salmon wrapped with PE film, SP film and SP-NC film, mix it with 25 mL of water, filter it after soaking for 30 minutes. Add an appropriate amount of the filtrate to the Kjeldahl distillation device, and then add 5 mL of magnesium chloride suspension. The distillate is collected in a 2% (w / v) boric acid solution, and titrated with 0.01 mol / L hydrochloric acid to calculate TVB-N, using methyl red and bromocresol green as a mixed indicator. The samples are detected every two days during storage.

[0100] Total viable count (TVC):

[0101] Transfer 5 g of salmon samples to 45 mL of sterile physiological saline, homogenize and filter. Then dilute the filtrate by 10-fold serial dilution method. Aliquot 100 μL of the diluted samples onto the counting agar plates, spread them evenly with a disposable spreader, and culture them overnight at 28 °C. The blank control group consists of counting agar plates added with physiological saline. The colony numbers of each sample at different dilution degrees are recorded on the plates.

[0102] Thiobarbituric acid value TBARS:

[0103] Homogenize 5 g of salmon samples in 7.50% (w / v) trichloroacetic acid solution, and then shake in a thermostatic shaker for 30 minutes. Filter the mixture, add an equal volume of 0.02 mol / L thiobarbituric acid solution, and then react in a 90 °C water bath for 30 minutes. Measure the absorbance of the cooled solution at a wavelength of 532 nm.

[0104] pH value:

[0105] Transfer 5 g of salmon samples to 45 mL of deionized water, homogenize, filter, and measure the pH value with a pH meter.

[0106] To minimize experimental errors, five replicates were set for each group of samples above.

[0107] The appearance of salmon slices packaged with SP-NC, PE, and SP films during storage at 4 °C is as Figure 6 shown in a). The appearance of salmon packaged with PE film did not change significantly within 12 days, so it was difficult to evaluate freshness solely based on appearance. On the 10th day, slight oil leakage occurred in the salmon packaged with SP film, indicating that the salmon was no longer suitable for consumption. In contrast, on the 8th day, the SP-NC film in contact with the salmon turned orange. This color change was the result of the decomposition of salmon protein, and the ammonia and amine compounds released after decomposition reacted with curcumin in the composite film, indicating a decrease in the freshness of the salmon. By the 12th day, the SP-NC film turned orange-red, indicating that the salmon had completely deteriorated. The results of visual observation showed that the SP-NC film had a sensitive freshness indicating function.

[0108] TVB-N, TVC, and TBARS values are important traditional indicators for evaluating the freshness of aquatic products. As Figure 6 shown in b), the TVB-N value of salmon slices increased during storage. According to previous reports, a TVB-N value ≤ 15 mg / 100 g indicates high-quality fresh seafood, while a value between 15 and 30 mg / 100 g is considered acceptable. On the 6th day, the TVB-N values of salmon wrapped with PE, SP, and SP-NC films were 31.4 ± 0.7 mg N / 100 g, 34.7 ± 1.2 mg N / 100 g, and 24.5 ± 1.1 mg N / 100 g, respectively. The salmon wrapped with PE and SP films was no longer edible, while the SP-NC film had a better preservation effect. By the 8th day, the TVB-N value of the salmon wrapped with the SP-NC film increased to 30.6 ± 0.6 mg N / 100 g and was not suitable for consumption, which was consistent with the color change observed in the film. The International Commission on Microbiological Specifications for Foods (ICMSF) stipulates that the TVC of fresh fish should not exceed 1×10 6 CFU / g. As Figure 6As shown in c), on the 4th day, the TVC values of salmon wrapped with PE, SP, and SP-NC films were 6.81 ± 0.32 log10 CFU / g , 7.10 ± 0.33 log10 CFU / g , and 5.22 ± 0.28 log10 CFU / g . Therefore, salmon wrapped with SP film is not suitable for consumption. Compared with the commercial PE film, the SP film slightly promoted the growth of microorganisms during storage, resulting in more serious spoilage of salmon. However, the addition of Nisin-A and CNE to the SP film matrix could effectively inhibit the growth of microorganisms in salmon fillets. Figure 6 The changing trend of TBARS in d) was the same as that of TVB-N and TVC. Compared with the PE film, the SP film slightly increased the TBARS value, while the SP-NC film significantly inhibited the increase in the TBARS value, highlighting the preservation effect of the SP-NC film. Figure 6 ) shows the pH value change of salmon, and all three groups showed a situation of first decreasing and then increasing. The reason for the initial decrease in pH value was that fish respiration stopped, and glycogen and ATP were decomposed into lactic acid and phosphoric acid. Then, the reason for the subsequent increase in pH value was that microbial metabolism decomposed proteins, peptides, and amino acids into alkaline substances. The SP-NC film could significantly inhibit the activities of microorganisms and effectively relieve the increase in pH value during storage. Generally speaking, salmon wrapped with PE, SP, and SP-NC films became unfit for consumption on the 6th day, 4th day, and 8th day respectively. The SP-NC film greatly delayed the spoilage process and extended the shelf life of refrigerated salmon from 4 days to 8 days.

[0109] The above embodiments are the preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A, characterized in that: The composition includes the following components in parts by weight:

2. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to claim 1, characterized in that: The composite film comprises the following components in parts by weight:

3. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to claim 1, characterized in that: The average particle size of the curcumin nanoemulsion is 100-180 nm, and the dispersion degree is 0.18-0.

25.

4. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to claim 1, characterized in that: The starch is one or more of corn starch, potato starch, wheat starch or pea starch; The pectin is one or more of apple pectin, citrus pectin or grape pectin; The plasticizer is one or more of glycerol, citric acid or isosorbide.

5. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to claim 1, characterized in that: The preparation method of the curcumin nanoemulsion is as follows: (1) mixing curcumin, anhydrous ethanol and medium chain triglycerides, dissolving the curcumin by ultrasonication, centrifuging and collecting the supernatant to obtain an oil phase; (2) gradually adding a surfactant to the oil phase, then adding an appropriate amount of water, mixing thoroughly, and ultrasonically homogenizing to obtain a curcumin nanoemulsion.

6. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to claim 5, characterized in that: In step (1), the mass ratio of curcumin, anhydrous ethanol and medium chain triglycerides is 0.1 to 1:5:5; the centrifugal speed is 6000 to 10000 rpm; In step (2), the mass ratio of the oil phase to the surfactant is 0.5 to 2:1, the ultrasonic homogenization power is 300 to 500 W, and the ultrasonic homogenization time is 3 to 5 min; The surfactant is a nonionic surfactant.

7. A method for preparing a thin film according to any one of claims 1 to 6, characterized in that: The following steps are involved: (1) Mix starch, pectin, plasticizer and water evenly and stir to obtain a gelatinized mixture; (2) Nisin-A and curcumin nanoemulsion are added to the gelatinized mixture, stirred, and cast into a film after removing bubbles, and then vacuum dried to obtain a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

8. The method for preparing a thin film according to claim 7, characterized in that: The stirring in step (1) is performed at a constant temperature of 80 to 95° C. for 2 to 4 hours; The temperature of the film-casting in step (2) is 40-60° C., the vacuum drying temperature is 40-60° C., and the vacuum drying time is 36-72 hours.

9. Use of the starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A as claimed in any one of claims 1 to 8 in food preservation and film packaging.

10. Use of the starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to any one of claims 1 to 8 in fish preservation.

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