A starch / pectin composite film loaded with curcumin nanoemulsion and nisin-a, and a preparation method and application thereof

Abstract

The application belongs to the field of food packaging, and discloses a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A as well as a preparation method and application thereof. The composite film comprises the following components in mass fractions: starch 10-100 parts, pectin 10-30 parts, plasticizer 1-10 parts, Nisin-A 0.1-10 parts, curcumin nanoemulsion 0-20 parts, and water 20-80 parts. The average particle size of the curcumin nanoemulsion is 100-180 nm, and the dispersity is 0.18-0.25. The starch is mixed with the pectin, the plasticizer and the water to be uniformly heated and gelatinized, the curcumin nanoemulsion and the Nisin-A are added, and then stirring and uniform mixing are performed, followed by casting into a film and vacuum drying to obtain the composite film. The composite film has efficient antibacterial property and intelligent response performance, is suitable for food preservation and real-time monitoring of food freshness, and is particularly suitable for application in salmon packaging. The preservation period of refrigerated salmon can be extended from 4 days to 8 days, the shelf life is extended, and waste is reduced.

Description

TECHNICAL FIELD

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

[0002] Salmon is a kind of food with high protein and rich in Omega-3 fatty acids, which is widely favored by consumers. However, salmon is easily affected by factors such as microbial contamination, oxidation and water loss during packaging, resulting in rapid decline in freshness. In order to ensure the food safety and taste of salmon and prolong its shelf life, it is urgent to take effective packaging preservation technology. The commonly used traditional packaging materials such as polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC) plastic film have good mechanical strength and barrier properties, but most of them cannot be degraded, causing 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, which 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 application. 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, with the increasing demand of consumers for food safety, the functional requirements for traditional packaging materials are also increasing. Integrating active functions such as antibacterial and intelligent response into traditional films to realize dynamic monitoring of food freshness has become a technical problem to be solved in the field.

[0004] Curcumin is a natural polyphenolic compound extracted from turmeric, which has attracted attention due to its broad-spectrum antibacterial, antioxidant, anti-inflammatory, and photodynamic antibacterial properties. Notably, its unique pH-responsive characteristics make it particularly suitable for intelligent food packaging applications. However, the inherent hydrophobicity of curcumin poses challenges for its uniform dispersion and stability in film matrices, which in turn affects its functional effectiveness. Wang et al. (Wang, Y., Liu, A., Li, X., et al. Effects of ginger essential oil on the physicochemical and antibacterial properties of gelatin-CaCO3 edible films [J]. Modern Food Science and Technology, 2015, 31(2): 57-62, 127.) used gelatin and calcium carbonate to form a packaging film, adding ginger essential oil to improve the antibacterial activity of the packaging film. The results showed that the packaging film had a certain preservation effect on chilled meat, but the low bioavailability of curcumin due to its poor water solubility and stability limited its application in the food and pharmaceutical industries. CN119529338A added an ethanol solution of curcumin to a starch-based film to impart pH-responsive functionality to the film, but the high crystallinity of curcumin in the film resulted in a decrease in the elongation at break of the film, affecting the mechanical properties of the film. To overcome the shortcomings of traditional starch-based films, such as insufficient mechanical strength, lack of antibacterial ability, and inability to monitor food freshness, there is an urgent need to find a convenient, environmentally friendly, and low-cost modification method for starch-based films to provide a starch-based film that is highly efficient in antibacterial activity, has a preservation effect, and can monitor the freshness of salmon in real time. This provides theoretical guidance and reference value for antibacterial modification and freshness monitoring modification in the field of films, thereby expanding the application range and value of starch-based films in the field of food packaging. SUMMARY

[0005] In view of the deficiencies of the prior art, the primary purpose of the present application is to provide a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

[0006] Another purpose of the present application is to provide a method for preparing a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

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

[0008] The purposes of the present application are achieved by the following technical solutions:

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

[0010]

[0011]

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

[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, anhydrous ethanol, and medium-chain triglyceride (MCT), ultrasonically dissolve the curcumin, centrifuge and collect the supernatant to obtain an oil phase;

[0021] (2) Gradually add a surfactant to the oil phase, then add an appropriate amount of water, mix thoroughly, and ultrasonically homogenize to obtain a curcumin nanoemulsion.

[0022] Preferably, in step (1), the mass ratio of curcumin, anhydrous ethanol, and medium-chain triglyceride (MCT) is 0.1-1:5:5; and the centrifugation speed 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, in step (2), the surfactant is a non-ionic surfactant, preferably Tween-80.

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

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

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

[0028] (2) In the gelatinization mixed solution, Nisin-A and curcumin nanoemulsion are added, stirred, and after removing bubbles, a film is cast and dried in vacuum 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 DEG C for 2-4 h.

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

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

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

[0033] The above-mentioned starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A is applied in salmon preservation.

[0034] Preferably, it is applied in salmon preservation packaging materials.

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

[0036] (1) The nanoemulsion in the present application realizes high-capacity loading of curcumin, greatly improving the utilization of curcumin compared with traditional methods.

[0037] (2) The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A provided by the present application is prepared by a casting method, which is simple, low in cost and meets the green environmental protection requirements. The casting film can maintain the stability of the film structure while reducing energy consumption and material waste in the production process, and is a high-efficiency and sustainable preparation method.

[0038] (3) The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A provided by the present application successfully overcomes the problem of uneven dispersion of hydrophobic materials in a hydrophilic matrix in traditional methods. Through nanoemulsification technology, the hydrophobicity of curcumin is effectively controlled, ensuring its uniform distribution in the film, so as to fully exert the functional properties of curcumin. The film not only has significant ultraviolet shielding ability, but also has pH response function, which can monitor the freshness and corruption state 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 application has significant antibacterial capacity, and after being treated by a blue light lamp, the survival rates of Staphylococcus aureus and Salmonella can be reduced to 1.85% and 3.79% respectively within 2h, and the curcumin nanoemulsion and Nisin-A have synergistic antibacterial effect. The significant antibacterial effect makes the film perform well in prolonging the food preservation period, especially in the preservation of salmon and other perishable foods, and the application can prolong the preservation period of refrigerated salmon from 4 days to 8 days, reduces the waste caused by food spoilage, and has a wide application prospect in the field of food packaging, especially in the preservation and quality monitoring of fish and other perishable foods. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 : The simple flow chart of preparation of the starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A provided by the application.

[0041] Figure 2 : The average particle size, dispersity, Zeta potential and transmission electron microscope image of the curcumin nanoemulsion in Examples 4, 5, 6 and 7. Figure 3 : The mechanical property test results of Examples 1, 2, 3, 4 and 5 are marked as Starch film, SP film, SP-N film, SP-C film and SP-NC film respectively.

[0042] Figure 4 : The light transmission 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 transmission rate percentage and maximum light transmission rate of the five examples under ultraviolet light and visible light.

[0043] Figure 5 : The 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 : The pH response test results of Example 5, a) is the appearance of Example 5 after being treated by different pH buffers, b) is the color value of Example 5 after being treated by different pH buffers, c) is the L*, a* and b* value changes of Example 5 after being treated by different pH buffers, and d) is the ΔE value changes of Example 5 after being treated by different pH buffers.

[0045] Figure 7: Example 5 antibacterial function test effect, a) is blue light lamp equipment, b) is curcumin nano emulsion photodynamic sterilization mechanism simple diagram, c) is staphylococcus aureus after film and blue light lamp irradiation treatment in solid culture medium, d) is staphylococcus aureus after film and blue light lamp irradiation treatment scanning electron microscope picture, e) is salmonella after film and blue light lamp irradiation treatment in solid culture medium, f) is staphylococcus aureus after film and blue light lamp irradiation treatment scanning electron microscope picture, g) is staphylococcus aureus survival rate, h) is salmonella survival rate.

[0046] Figure 8 : Example 2, 5 and PE film salmon freshness monitoring and preservation test results. DETAILED DESCRIPTION

[0047] The application will be further described in detail below in conjunction with specific examples, but the embodiments of the application are not limited thereto, and for the process parameters not specifically indicated, conventional techniques can be referred to.

[0048] In the embodiments of the application, starch (corn starch, potato starch, wheat starch and pea starch, Shanghai Maikelin Biotechnology Co., Ltd.), pectin (apple pectin, citrus pectin and grape pectin, Shanghai Maikelin Biotechnology Co., Ltd.), Nisin-A (lactic acid streptococcus, Shanghai Maikelin Biotechnology Co., Ltd.), curcumin (Shanghai Yuan Ye Biological Technology Co., Ltd.), plasticizer (glycerol, citric acid, isosorbide, Shanghai Maikelin Biotechnology Co., Ltd.).

[0049] Example 1

[0050] (1) Preparation of pure starch film: set the magnetic stirring temperature to 90 DEG C, and the rotating speed to 1000 rpm; add 50 parts of water, 10 parts of corn starch and 3 parts of glycerol into a stirring pot in sequence; constant temperature stirring for 6 hours; cool to room temperature; set the ultrasonic power to 100 W; treat under ultrasonic condition for 30 minutes to remove bubbles; after film casting, place in a 45 DEG C vacuum drying oven for drying for 48 hours 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 DEG C, and the rotating speed to 1000 rpm; add 50 parts of water, 8 parts of corn starch, 2 parts of pectin and 3 parts of glycerol into a stirring pot in sequence; constant temperature stirring for 6 hours; cool to room temperature; set the ultrasonic power to 100 W; treat under ultrasonic condition for 30 minutes to remove bubbles; after film casting, place in a 45 DEG C vacuum drying oven for drying for 48 hours 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 to 90°C and the stirring speed to 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol into a stirring pot in sequence. Stir at constant temperature for 2 h to obtain a gelatinized mixed solution. Then add 0.1 part of Nisin-A and stir at constant temperature for 4 h. Cool to room temperature. Set the ultrasonic power to 100 W and treat for 30 min under ultrasonic conditions to remove bubbles. Cast into a film and dry in a vacuum drying oven at 45°C 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 anhydrous ethanol, and 50 parts of medium-chain triglyceride (MCT) into a centrifuge tube in sequence. Dissolve the curcumin by ultrasonic wave. Centrifuge at 8000 rpm for 10 min and collect the supernatant to obtain an oil phase. Then gradually add 75 parts of Tween-80 into the oil phase, and then add an appropriate amount of ultrapure water. Mix the mixture well and stir it uniformly by a magnetic stirrer to obtain a coarse emulsion. Then use a cell disrupter to homogenize the coarse emulsion by ultrasonic wave. Set the power to 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 to 90°C and the stirring speed to 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol into a stirring pot in sequence. Stir at constant temperature for 2 h to obtain a gelatinized mixed solution. Then add 4 parts of curcumin nanoemulsion and stir at constant temperature for 4 h. Cool to room temperature. Set the ultrasonic power to 100 W and treat for 30 min under ultrasonic conditions to remove bubbles. Cast into a film and dry in a vacuum drying oven at 45°C 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 anhydrous ethanol, and 50 parts of medium-chain triglyceride (MCT) into a centrifuge tube in sequence. Dissolve the curcumin by ultrasonic wave. Centrifuge at 8000 rpm for 10 min and collect the supernatant to obtain an oil phase. Then gradually add 75 parts of Tween-80 into the oil phase at room temperature, and then add an appropriate amount of ultrapure water. Mix the mixture well and stir it uniformly by a magnetic stirrer to obtain a coarse emulsion. Then use a cell disrupter to homogenize the coarse emulsion by ultrasonic wave. Set the power to 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 to 90°C and the stirring speed to 1000 rpm. Add 50 parts of water, 8 parts of corn starch, 2 parts of pectin, and 3 parts of glycerol into a stirring pot in sequence. Stir at constant temperature for 2 h to obtain a gelatinized mixed solution. Then add 0.1 parts of Nisin-A and 4 parts of curcumin nanoemulsion. Stir at constant temperature for 4 h. Cool to room temperature. Set the ultrasonic power to 100 W. Treat for 30 min under ultrasonic conditions to remove bubbles. Cast into a film and dry in a vacuum drying oven at 45°C 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 determined three times using a nanoparticle size analyzer (Zetasizer Nano ZS, UK). The microstructure of CNE was observed by transmission electron microscopy (JEM 1400Plus, Japan).

[0070] Figure 2 The average particle size of CNE was 124.8 nm, indicating that the particle size was small. The particle size distribution curve was unimodal, indicating that the particle size distribution range was narrow and uniform. The dispersity was 0.213, confirming that the dispersion was uniform and stable, which was suitable for film preparation. The Zeta potential was -23.3 mV, indicating that the stability was strong, reducing the possibility of instability over time. The morphology of CNE was directly observed by transmission electron microscopy. The nanoemulsion showed uniform spherical shape, proving that CNE was successfully prepared.

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

[0072] A universal testing machine (HZ-1007E, China) was used to evaluate the tensile tear resistance of starch-based films. The films were cut into dumbbell shapes with a 2 mm central width and a 35 mm length. The tensile strength and elongation at break were measured according to the ASTM D638-08 standard with 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 starch-based films are shown. The tensile strength and elongation at break of pure starch films were the lowest, 2.83 ± 0.14 MPa and 35.36 ± 0.87%, respectively. After adding pectin, the mechanical properties improved significantly, with the tensile strength and elongation at break increasing to 3.74 ± 0.16 MPa and 39.78 ± 1.03%, respectively. Compared with SP films, the tensile strength of SP-N films increased significantly after the addition of Nisin-A. The tensile strength and elongation at break of SP-C films also improved significantly. These results indicate that the nanoscale properties of Nisin-A and CNE contribute to the improvement of the tensile strength of starch-based films. In addition, CNE can change the internal structure of starch-based films, thereby improving the elongation at break. Notably, SP-NC films exhibited superior mechanical properties, with a tensile strength of 4.51 ± 0.10 MPa and an elongation at break of 43.58 ± 0.73%. This improvement may be due to the formation of hydrogen bonds and intermolecular interactions between starch, pectin, Nisin-A, and CNE. In addition, the film in Example 7 was softer due to the addition of more CNE, resulting in a significant decrease in tensile strength and elongation at break; in Comparative Example 1, the addition of curcumin alcohol solution led to the precipitation of curcumin crystals in the film, also resulting in a decrease in tensile strength and elongation at break.

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

[0074] A UV-Vis spectrophotometer (Cary 60, Agilent Technologies, USA) was used to test the UV-Vis spectrum of starch-based films. The starch-based films were cut to a size of 2 cm x 4 cm, and the UV light analysis range was 200 to 380 nm, and the visible light analysis range was 380 to 780 nm.

[0075] Figure 4 a) Optical images of the five films are shown. It is clear that 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 exhibit a yellow color due to the addition of CNE. This indicates that the films have good light transmission, which is beneficial for displaying the appearance of food in packaging. To further evaluate the optical properties of the films, the films were tested for UV (200-380 nm) and visible light (380-780 nm) transmission. As shown in Figure 4b) shown, the UV barrier property of starch film was poor. However, the UV barrier ability of starch-based films was significantly enhanced after the addition of pectin and Nisin-A. After the addition of curcumin nanoemulsion, the starch-based film almost completely blocked UV light. Notably, the SP-C and SP-NC films had a significant barrier effect on visible light near 425 nm, which is the excitation wavelength for the activation of the photodynamic bactericidal activity of curcumin. As Figure 4 c) shown, the UV transmittance of starch, SP, SP-N, SP-C and SP-NC films were 52.91 ± 1.01%, 16.79 ± 0.86%, 15.77 ± 1.08%, 5.49 ± 1.24% and 3.69 ± 0.65%, respectively. These results showed that the addition of CNE significantly enhanced the UV barrier ability of starch-based films. Specifically, the visible light transmittance of starch film, SP film, SP-N film, SP-C film and SP-NC film was 86.47 ± 0.68%, 78.47 ± 1.31%, 66.62 ± 1.76%, 45.31 ± 1.89% and 44.70 ± 1.15%, respectively, at a wavelength of 400-780 nm. This indicated that the SP-NC composite film had excellent optical properties and film-forming ability. In addition, the maximum light transmittance of the five films was 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 had the highest transmittance, indicating that its internal structure was uniform. In contrast, the SP-NC film also exhibited good maximum light transmittance. These findings highlighted the excellent UV barrier and light transmittance properties of the SP-NC film, making it very suitable for packaging applications.

[0076] (4) Starch-based film contact angle test method:

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

[0078] The CA value of starch film was the lowest, 42.5 ± 0.4°, indicating its hydrophilic nature. Upon addition of 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 bonding between starch and pectin, thus improving the hydrophobicity of the starch-based film. Compared to SP film, the CA value of SP-N film decreased to 43.1 ± 0.3°, which can be attributed to the hydrophilic nature of Nisin-A. Notably, the CA value of SP-C film significantly increased to 68.7 ± 0.6°, indicating enhanced hydrophobicity. 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 can be attributed not only to the inherent hydrophobicity of curcumin but also to the rough surface structure formed upon its addition to the film. Furthermore, Example 7, due to the excessive addition of CNE, the water contact angle decreased significantly to 34.1 ± 0.4°, which is not suitable for food packaging.

[0079] (5) Starch-based film water vapor permeability test method:

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

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

[0082] Where ΔW represents the weight gain (g), T is the film thickness (mm), A is the cup opening area (mm 2 ), Δt is the equilibrium time (s), and ΔP is the vapor pressure difference on the film at 25°C.

[0083] The WVP value of starch film was (10.69 ± 0.24) x 10 -10 g / (mm·s·Pa). The addition of pectin to the starch film reduced the WVP value, which can be attributed to the formation of intermolecular hydrogen bonds between starch and pectin, enhancing the structural compactness of the film and reducing water vapor permeability. Compared to SP film, the WVP of SP-N film decreased to (5.12 ± 0.20) x 10 -10 g / (mm·s·Pa), indicating that the addition of Nisin-A improved the compactness of the starch-based film. The WVP of SP-C film was (8.42 ± 0.16) x 10 - 10g / (mm-s-Pa), higher than the SP-N film, which can be attributed to the formation of microporous structure within the film by CNE, thus facilitating the water vapor transmission. The WVP value of the SP-NC film was (6.82 ± 0.18) x 10 -10 g / (mm-s-Pa), which showed an improvement in water vapor barrier properties compared to the SP film.

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

[0085] The film was placed on the mouth of a bottle containing 10 grams of desiccant (a mixture of reduced iron powder, activated carbon, and sodium chloride in a ratio of 1:2:1). The bottle was then stored in a desiccator, maintaining a temperature of 25°C and a relative humidity of 75%. Each sample was measured in triplicate. The weight of the bottle was monitored over 48 hours, and the OP value of the film was calculated using the following formula:

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

[0087] where AW 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 were (7.81 ± 0.10) x 10 -9 g-mm-m - 2-s - 1 and (6.16 ± 0.16) x 10 -9 g-mm-m - 2-s - 1, respectively, indicating relatively good gas permeability. In contrast, the OP value of the SP-N film was lower at (4.21 ± 0.14) x 10 -9 g-mm-m - 2-s - 1, which can be attributed to the addition of Nisin-A enhancing the compactness of the starch-based film. Notably, the OP value of the SP-NC film was (6.10 ± 0.20) x 10 -9 g-mm-m - 2-s - 1, which showed no significant difference compared to the SP film. This indicates that the simultaneous addition of Nisin-A and CNE had no significant effect on the oxygen permeability of the starch-based film.

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

[0090] SP-NC films were immersed in buffer solutions of different pH values, air-dried and photographed. The colour of the film samples was recorded using a hand-held colourimeter (CR30) and the L*, a* and b* values were recorded. The colour difference (DE) was calculated using the following equation:

[0091]

[0092] where L0*, a0* and b0* are the colour parameters of the initial dry film and L*, a* and b* are the colour parameters at different pH values.

[0093] The colour change of SP-NC films under different pH conditions is shown in Figure 5 a). The solution was light yellow at pH 3-7, and gradually turned into reddish brown as the pH value increased from 8 to 11. This indicates that the CNE in the starch-based film retains its sensitive pH response function. Figure 5 b) demonstrates the colour change of SP-NC films captured using a hand-held colourimeter, showing the detailed changes in L*, a* and b* values. The specific trends are shown in Figure 5 c). The L*, a* and b* values ranged from 4.69, 3.72 and 9.53, respectively, at pH 3-7, and no significant changes were observed within this range. At pH 8, there was a sudden change in L* and a* values. Specifically, the L* value decreased by 14.00 and the a value increased by 20.99, with DE increasing 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 meat is approaching or has already gone bad. Within the pH range of 7-11, the L*, a* and b* values of the SP-NC film continued to change significantly, with DE eventually increasing to 74.68 ± 0.34. The sensitive colour change ability of the SP-NC film demonstrates its potential as an intelligent packaging material.

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

[0095] The antibacterial effect of Staphylococcus aureus and Salmonella was evaluated. Pre-experiments found that the antibacterial effect of Example 6 was not satisfactory, so Example 5 was selected for antibacterial effect determination: small pieces of SP-NC film were placed in a conical flask containing 1 mL of bacterial suspension with a concentration of 10 5 CFU / mL. The film was in contact with the bacterial suspension for 1 hour, during which it received 20 min of blue light irradiation. After incubating the bacterial suspension on solid culture medium for 24 hours, optical photographs 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 images of S. aureus showed that the bacterial cell wall of the control group remained smooth and intact. After 20 min of blue light irradiation, S. aureus did not change significantly, indicating that blue light itself did not have a bactericidal effect. After treatment with the film, the bacteria showed obvious shrinkage and rupture, which was due to the destruction of the bacterial cell wall by Nisin-A, leading to the leakage of intracellular contents. When the film was combined with blue light treatment, S. aureus was almost completely killed, and the scanning electron microscope images showed that the bacteria were largely shrunk and ruptured. This was due to the intramolecular charge transfer of curcumin under blue light irradiation, producing singlet oxygen 102 that interacted with various components inside the cell, leading to microbial cell death. The images of Salmonella showed that in the film treatment group, a slight decrease in the number of bacteria was observed, indicating that Nisin-A had limited inhibitory effect on it. After treatment with the film and blue light, the number of Salmonella was significantly reduced, and the cells were largely shriveled and ruptured. The survival rate of bacteria on solid medium was further calculated, and after the combined treatment of the film and blue light, S. aureus and Salmonella were almost completely eradicated, with survival rates of only 1.85 ± 0.62% and 3.79 ± 1.57%, respectively. In summary, the synergistic effect of Nisin-A and CNE enhanced the antibacterial effect of the film on gram-positive and gram-negative bacteria, demonstrating the great potential of SP-NC film in antibacterial packaging applications.

[0097] (9) Test method for freshness monitoring and preservation of salmon using starch-based film:

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

[0099] Five grams of salmon wrapped with PE film, SP film and SP-NC film were mixed with 25 ml of water, soaked for 30 minutes and then filtered. An appropriate amount of filtrate was added to a Kjeldahl distillation device, and then 5 ml of magnesium chloride suspension was added. The distillate was collected in a 2% (w / v) boric acid solution, and the TVB-N was calculated by titration with 0.01 mol / L hydrochloric acid, using methyl red and bromocresol green as mixed indicators. The samples were tested every two days during storage.

[0100] Total viable count (TVC):

[0101] Five grams of salmon sample were transferred to 45 ml of sterile physiological saline, homogenized and filtered. Then the filtrate was diluted by 10-fold serial dilution method. 100 μL of diluted sample was aliquoted onto count agar plates, evenly spread with a disposable spreader, and incubated at 28°C overnight. The blank control group consisted of count agar plates with added physiological saline. The number of colonies for each sample at different dilutions was recorded on the plates.

[0102] Thiobarbituric acid value (TBARS):

[0103] A 5 g salmon sample was homogenized in 7.50% (w / v) trichloroacetic acid solution and then shaken in a thermostatic shaker for 30 minutes. The mixture was filtered, an equal volume of 0.02 mol / L thiobarbituric acid solution was added, and then reacted in a 90°C water bath for 30 minutes. The absorbance of the cooled solution was measured at a wavelength of 532 nm.

[0104] pH value:

[0105] A 5 g salmon sample was transferred to 45 ml of deionized water, homogenized, filtered, and the pH value was measured with a pH meter.

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

[0107] The appearance of salmon fillets packaged with SP-NC, PE and SP films stored at 4°C was observed as shown in Figure 6 a). The salmon packaged with PE film did not show obvious changes in appearance within 12 days, so it was difficult to evaluate the freshness only by appearance. On the 10th day, the salmon packaged with SP film showed slight oil bleeding, indicating that the salmon was no longer suitable for consumption. On the contrary, 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 proteins, which released ammonia and amine compounds that 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 indication function.

[0108] TVB-N, TVC and TBARS values are important traditional indicators for evaluating the freshness of aquatic products. As shown in Figure 6 b), the TVB-N value of salmon fillets increased during storage. According to previous reports, a TVB-N value of ≤15 mg / 100 g indicates excellent quality of 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 packaged 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. Salmon packaged with PE and SP films were no longer edible, while the preservation effect of SP-NC film was better. By the 8th day, the TVB-N value of salmon packaged with SP-NC film increased to 30.6 ± 0.6 mg N / 100 g, which was not suitable for consumption, 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 x 10 6 CFU / g. As shown in Figure 6c) As shown, 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 respectively on the 4th day. Therefore, salmon wrapped with SP film was not fit for consumption. Compared with commercial PE film, SP film slightly promoted the growth of microorganisms during storage, leading to more severe deterioration of salmon. However, the addition of Nisin-A and CNE in SP film matrix effectively inhibited the growth of microorganisms in salmon fillets. Figure 6 d) The TBARS variation trend was the same as TVB-N and TVC. Compared with PE film, SP film slightly increased the TBARS value, while SP-NC film significantly inhibited the increase of TBARS value, highlighting the preservation effect of SP-NC film. Figure 6 ) shows the pH variation of salmon, all three groups showed a first decrease and then an increase. The initial decrease in pH value was due to the cessation of fish respiration, the decomposition of glycogen and ATP into lactic acid and phosphoric acid, while the later increase in pH value was due to the decomposition of proteins, peptides and amino acids into alkaline substances by microbial metabolism. SP-NC film could significantly inhibit the activity of microorganisms and effectively alleviate the increase of pH value during storage. In general, salmon wrapped with PE, SP and SP-NC films became unfit for consumption on the 6th day, 4th day and 8th day, respectively. SP-NC film greatly delayed the spoilage process, extending the shelf life of chilled salmon from 4 days to 8 days.

[0109] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A, characterized in that, The composite film comprises the following components by mass fraction: Starch 30-60 parts Pectin 10-20 parts Plasticizer 3-6 parts Nisin-A 0.1-5 parts Curcumin nanoemulsion 2-12 parts Water 30-60 parts. The preparation method of the curcumin nanoemulsion is as follows: (1) Mix curcumin, anhydrous ethanol and medium-chain triglyceride, ultrasonically dissolve the curcumin, centrifuge and collect the supernatant to obtain an oil phase; (2) Gradually add a surfactant to the oil phase, then add an appropriate amount of water, mix thoroughly, and ultrasonically homogenize to obtain a curcumin nanoemulsion.

2. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A as claimed in claim 1, wherein, The average particle size of the curcumin nanoemulsion is 100-180 nm, and the dispersity is 0.18-0.

25.

3. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A as claimed in claim 1, wherein, The starch is one or two or more of corn starch, potato starch, wheat starch or pea starch; The pectin is one or two or more of apple pectin, citrus pectin or grape pectin; The plasticizer is one or two or more of glycerol, citric acid or isosorbide.

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

5. A method of producing the film according to any one of claims 1 to 4, characterized by, The method comprises the following steps: (1) Mix the starch, pectin, plasticizer and water uniformly, and stir to obtain a gelatinized mixed solution; (2) Add Nisin-A and curcumin nanoemulsion to the gelatinized mixed solution, stir, remove the bubbles, flow into a film, and then perform vacuum drying to obtain a starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A.

6. The method of claim 5, wherein the film is prepared by a method comprising: In step (1), the stirring is constant temperature stirring at 80-95℃ for 2-4h; In step (2), the temperature for flow into a film is 40-60℃, the vacuum drying temperature is 40-60℃, and the vacuum drying time is 36-72h.

7. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to any one of claims 1-4 is used in food preservation and film packaging.

8. The starch / pectin composite film loaded with curcumin nanoemulsion and Nisin-A according to any one of claims 1-4 is used in fish preservation.

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