Rice starch-based citral nano-emulsion asymmetric functional packaging film as well as preparation method and application thereof

Abstract

The invention discloses a rice starch-based citral nano-emulsion asymmetric functional packaging film and a preparation method and application thereof.The preparation method comprises the steps that sulfonated cellulose nanocrystals and pea protein isolate are added into water to prepare a suspension, soybean oil and citral are added into the suspension, citral nano-crude milk is obtained after homogenization, and the rice starch-based citral nano-emulsion asymmetric functional packaging film is obtained. The preparation method comprises the following steps: performing ultrasonic treatment to obtain citral nano-emulsion, adding water into rice starch, heating and stirring to obtain rice starch paste, adding glycerol and the citral nano-emulsion to obtain film-forming slurry, forming a film by a tape casting method, soaking in an ethanol solution of myristic acid, taking out, and drying to obtain the packaging film. The packaging film has an asymmetric function, one side of the film has hydrophobicity and can effectively prevent moisture loss, the other side of the film has antibacterial property and hydrophilicity, moisture and quality of substances in the film can be effectively kept, breeding and reproduction of microorganisms in the film can be inhibited, and the preparation method is simple, convenient, safe, green, environmentally friendly and wide in application prospect.

Description

Technical Field

[0001] This invention belongs to the field of agricultural product storage and preservation technology, and relates to an asymmetric functional packaging film of rice starch-based citral nanoemulsion, its preparation method and application. Background Technology

[0002] Fruits and vegetables, as an indispensable part of daily diets, are highly favored in global food cultures due to their rich nutrition, diverse tastes, and wide applicability. However, their tissues easily soften and rot when stored at room temperature after harvest, resulting in high post-harvest losses. To extend the shelf life and freshness of fruits and vegetables and achieve added value and premium pricing after harvest, research on post-harvest physiology and storage preservation technologies has become a key focus. Although traditional petroleum-based plastic food packaging is widely used, its overuse significantly increases global plastic waste, harming ecosystems and human health. Therefore, bio-based packaging materials based on natural substances are a promising strategy to meet the growing demand for environmentally friendly alternatives and have been extensively studied. Starch, an readily available and sustainable resource, possesses excellent film-forming properties, safety, biocompatibility, and renewability, making it a promising new packaging material. However, starch-based materials are still limited by poor performance and a lack of functional characteristics, hindering their practical application. A common approach to overcoming these drawbacks is to incorporate natural antibacterial agents as fillers to impart functional properties to starch films. Natural essential oils are widely used due to their effectiveness, usability, and biocompatibility; however, direct addition often results in low stability and poor compatibility with the starch matrix. Starch films with added nanomaterials also exhibit poor barrier properties and excessive hydrophilicity, limiting their feasibility as a plastic alternative. To address the inherent hydrophilicity of starch films, researchers have explored hydrophobic modification strategies, including chemical modification, mixing with hydrophobic materials, or coating modification. However, these methods often face challenges such as weak interfacial adhesion, inhomogeneous microstructure, and potential toxicity. Therefore, a simple and effective strategy is needed to improve the compatibility and stability between hydrophobic agents and the film matrix while maintaining the desired physical properties and sustained bioactivity of the film. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a rice starch-based citral nanoemulsion asymmetric functional packaging film with excellent hydrophobic, water-locking, antibacterial, hydrophilic and asymmetric protective properties, as well as its preparation method and application. The asymmetric functional packaging film can bidirectionally protect the quality and moisture stability of the contents and delay the dehydration, softening and microbial contamination of fruits and vegetables.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0005] A method for preparing an asymmetric functional packaging film based on rice starch and citral nanoemulsion includes the following steps: (1) Add sulfonated cellulose nanocrystals and pea protein isolate to water and sonicate until a suspension is formed; (2) Add soybean oil and citral to the suspension obtained in step (1), homogenize it to obtain citral nano emulsion, and sonicate the citral nano emulsion to obtain citral nano emulsion. (3) Add rice starch to water, heat to 60℃~85℃ and stir to disperse to obtain rice starch paste. Add glycerol to rice starch paste and stir. After cooling, add citral nanoemulsion and homogenize to obtain film-forming slurry. (4) The film-forming slurry obtained in step (3) is cast into a film by casting to obtain a rice starch-based citral nanoemulsion packaging film; (5) Place one side of the rice starch-based citral nanoemulsion packaging film obtained in step (4) on an ethanol solution of myristic acid for immersion, take it out and dry it to obtain a rice starch-based citral nanoemulsion asymmetric functional packaging film.

[0006] In the above-mentioned method for preparing asymmetric functional packaging film of rice starch-based citral nanoemulsion, preferably, in step (1), the ratio of sulfonated cellulose nanocrystals, pea protein isolate, and water is 0.1g~0.3g∶0.1g~0.3g∶10mL.

[0007] In the above-mentioned method for preparing asymmetric functional packaging film of rice starch-based citral nanoemulsion, preferably, in step (2), the ratio of soybean oil, citral, and suspension is 0.1g~0.3g∶0.1g~0.3g∶10mL, and the average particle size of the citral nanoemulsion particles is 100 nm~300 nm.

[0008] In the above-mentioned method for preparing asymmetric functional packaging film of rice starch-based citral nanoemulsion, preferably, in step (3), the mass-volume ratio of rice starch to water is 3.5g~5g∶100mL, and the mass ratio of rice starch, glycerol, and citral nanoemulsion is 1∶0.01~0.05∶0.01~0.1; In step (5), the mass concentration of myristic acid in the ethanol solution of myristic acid is 1g / 100mL to 3g / 100mL.

[0009] In the above-mentioned method for preparing asymmetric functional packaging film of rice starch-based citral nanoemulsion, preferably, in step (1), the ultrasonic treatment time is 10 min to 20 min, and the ultrasonic treatment power is 250 W to 450 W.

[0010] In the above-mentioned method for preparing asymmetric functional packaging film of rice starch-based citral nanoemulsion, preferably, in step (2), the rotation speed of the homogenization treatment is 10000 rpm to 15000 rpm, the homogenization treatment time is 3 min to 5 min, the ultrasonic treatment power is 300 W to 400 W, and the ultrasonic treatment time is 15 min to 20 min.

[0011] In the preferred method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film, in step (3), the stirring and dispersion speed is 800 rpm to 1000 rpm, the stirring and dispersion time is 20 min to 30 min, the stirring time after adding glycerol to the rice starch paste is 10 min to 20 min, and the homogenization speed after adding citral nanoemulsion is 2800 rpm to 4000 rpm, and the homogenization time is 1 min to 5 min.

[0012] In the above-mentioned method for preparing asymmetric functional packaging film of rice starch-based citral nanoemulsion, preferably, in step (4), the casting method includes pouring the film-forming slurry onto the film-forming tool and drying it to form a film, wherein the drying temperature is 30℃~35℃ and the drying time is 24h~30h; in step (5), the soaking time is 10s~40s and the drying is the natural evaporation of the solvent.

[0013] As a general technical concept, the present invention also provides a method for preparing the above-mentioned rice starch-based citral nanoemulsion asymmetric functional packaging film, resulting in a rice starch-based citral nanoemulsion asymmetric functional packaging film.

[0014] As a general technical concept, the present invention also provides an application of the above-mentioned rice starch-based citral nanoemulsion asymmetric functional packaging film in the preservation of fruits and vegetables.

[0015] In this invention, the citral nanoemulsion contains nanoparticles (mainly citral nanoparticles) and belongs to a dispersion system.

[0016] In this invention, Janus refers to a membrane with asymmetric structure or properties, and a novel membrane material with different functions on both sides, such as one side of the membrane being hydrophilic and the other side being hydrophobic.

[0017] In this invention, the film-forming tool used in the casting method is a substrate, which is usually 10 cm × 10 cm in size, but is not limited to this.

[0018] Compared with the prior art, the advantages of the present invention are: (1) This invention provides a method for preparing an asymmetric functional packaging film based on rice starch and citral nanoemulsion. The method uses rice starch as the substrate, citral nanoemulsion as the reinforcing filler, and mixes it with glycerol to prepare a film-forming solution. After film formation, a functional packaging film is obtained. In this preparation method, citral nanoemulsion can promote the uniform distribution of myristic acid. Through the rapid self-assembly of myristic acid, the rice starch film is transformed into a moisture barrier with self-cleaning ability. Myristic acid can form a dense structure on its surface, which not only improves the strength but also stabilizes the interface between the starch matrix and the citral nanoemulsion, ensuring that the packaging film maintains its flexibility without affecting its enhanced strength. At the same time, the synergy of rice starch, citral nanoemulsion and myristic acid can also make the packaging film have antibacterial, ultraviolet shielding and barrier properties. In addition, this multi-component synergistic effect can also make the advantages of each component fully play their role, effectively improving the comprehensive performance of the packaging film. Compared with single-component packaging films, it is more comprehensive in performance.

[0019] (2) The rice starch-based citral nanoemulsion asymmetric functional packaging film of the present invention has excellent antibacterial and UV shielding properties, effectively inhibiting softening and browning of fruits during storage. It can be applied to the preservation of fruits and vegetables. The good barrier properties of the packaging film can significantly reduce the evaporation loss of fruits and vegetables and reduce the consumption of nutrients caused by respiration, thereby effectively maintaining the texture and appearance quality of fruits and vegetables, and significantly extending the storage period of fruits and vegetables. It is suitable for the storage and circulation of perishable foods such as fruits and vegetables. As a natural, biodegradable and non-toxic biomass polymer material, the functional packaging film of the present invention can minimize the pollution of petroleum-based materials to the environment. It has a significant improvement effect in delaying the dehydration, shrinkage, softening and browning of fruits and vegetables, thus confirming the application potential of this multifunctional active food packaging system in the preservation of fruits and vegetables. (3) The rice starch-based citral nanoemulsion asymmetric functional packaging film of the present invention adopts an asymmetric structural design, that is, introducing heterogeneous structures on different surfaces of the film material, so that the two sides of the packaging film (i.e., the inner and outer layers in application) have different functional characteristics: the inner layer of the film contains citral nanoemulsion, which is used to play an antibacterial and freshness-preserving function; the outer layer of the film has enhanced mechanical strength, UV resistance and hydrophobicity, improving the overall durability of the packaging film. This asymmetric structural design gives the packaging film multiple technical advantages, enhances the overall performance of the packaging film, such as simultaneously improving moisture resistance, antibacterial properties and light resistance, reducing the volatilization and migration of active ingredients, improving the sustainability of the freshness preservation effect, the inner layer can directly contact food, and the outer layer acts as an external protective barrier.

[0020] In summary, this invention, by constructing a functional packaging film of rice starch-based citral nanoemulsion with an asymmetric structure, not only demonstrates significant effects in delaying water loss, softening, and browning of fruits and vegetables, but also exhibits significant advantages in material environmental friendliness, functional durability, and structural controllability, showcasing the broad application prospects of this multifunctional active packaging system in the field of fruit and vegetable product storage and preservation. Attached Figure Description

[0021] Figure 1 The images are scanning electron microscope (SEM) images of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3. Among them, (1) is a scanning electron microscope image of the film surface and cross section with a magnification of 500x, and (2) is a scanning electron microscope image of the cross section of Example 1 with a magnification of 50000x.

[0022] Figure 2 Fourier transform infrared spectra of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of this invention and the packaging films prepared in Comparative Examples 1-3.

[0023] Figure 3 X-ray diffraction patterns of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3.

[0024] Figure 4 The mechanical properties of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3 are shown in the figure. (1) is a comparison figure of tensile strength and (2) is a comparison figure of elongation at break.

[0025] Figure 5 The diagrams show the barrier properties of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3. Among them, (1) is a comparison diagram of water vapor transmission rate, (2) is a comparison diagram of CO2 transmission rate, and (3) is a comparison diagram of O2 transmission rate.

[0026] Figure 6 The thermal stability diagrams are shown for the packaging films prepared by comparative examples 1-3 of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention, where (1) is a thermogravimetric comparison diagram and (2) is a microentropy thermogravimetric comparison diagram.

[0027] Figure 7 The figures show the hydrophobic properties of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3. In the figures, (1) is a comparison of water contact angles and (2) is a comparison of the changes in film morphology after adding distilled water.

[0028] Figure 8 The light blocking properties of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of this invention and the packaging films prepared in Comparative Examples 1-3 are shown in the diagram.

[0029] Figure 9 The antioxidant analysis diagrams are of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3. Among them, (1) is a comparison diagram of ABTS free radical scavenging ability and (2) is a comparison diagram of DPPH free radical scavenging ability.

[0030] Figure 10 The antibacterial properties of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of the present invention and the packaging films prepared in Comparative Examples 1-3 are shown in the following diagrams: (1) is a comparison diagram of antibacterial performance, (2) is a comparison diagram of relative antibacterial rate, and (3) is a comparison diagram of antibacterial zone diameter.

[0031] Figure 11 Safety evaluation diagrams of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of this invention and the packaging films and water treatment groups prepared in Comparative Examples 1-3.

[0032] Figure 12 The images show the appearance of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of this invention, and the fresh-cut kiwifruit preserved in Comparative Example 1, the untreated group, and the PE group.

[0033] Figure 13 The graph shows the weight loss rate of fresh-cut kiwifruit preserved in the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of this invention, and in Comparative Example 1, the untreated group, and the PE group.

[0034] Figure 14 The graph shows the changes in hardness of fresh-cut kiwifruit preserved in the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in Example 1 of this invention, and in Comparative Example 1, the untreated group, and the PE group. Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. All materials and instruments used in the following embodiments are commercially available. Citral was purchased from Jiangxi Yisenyuan Plant Fragrance Co., Ltd. (Jiangxi, China); soybean oil and myristic acid were purchased from Maclean Biochemical Technology Co., Ltd. (Shanghai, China); pea protein isolate was purchased from Shanghai Yuyan Biotechnology Co., Ltd. (Shanghai, China); sulfonated cellulose nanocrystals were purchased from Guilin Qihong Technology Co., Ltd. (Guilin, China); and rice starch was commercially available or prepared according to literature. Room temperature is typically between 15°C and 35°C, preferably between 24°C and 28°C.

[0036] The relevant detection and characterization methods in the examples are as follows: (1) Scanning electron microscopy: The surface and cross-sectional structure of the thin film were examined using a scanning electron microscope at an accelerating voltage of 10 kV. To analyze the cross-section, the sample was fractured in liquid nitrogen and coated with a thin layer of gold. Microstructural details were captured at 500x magnification.

[0037] (2) Fourier transform infrared (FTIR): Take about 2 mg of sample and about 100 mg of potassium bromide, grind them evenly, compress them together to 20 MPa, and then perform infrared testing directly. The wavenumber range is 400-4000 cm⁻¹. -1 The spectrometer has a resolution of 4 cm⁻¹. -1 The signal-to-noise ratio is 50,000:1, and the scan is performed 32 times.

[0038] (3) X-ray diffraction pattern: The X-ray diffraction pattern of the composite film was scanned in the range of 5° to 50° (2θ) using an X-ray diffractometer. The test voltage was 40 kV and the current was 30 mA.

[0039] (4) Mechanical properties: The tensile strength of the rice starch-based citral nanoemulsion asymmetric functional packaging film was determined by an electronic tensile testing machine at a tensile rate of 50 mm / min.

[0040] (5) Barrier performance: Weigh 5 g of anhydrous calcium chloride, deoxidizing agent, or potassium hydroxide into a test tube, seal the test tube with a film, and then place it under constant temperature (25 ℃) and constant humidity (relative humidity: 75%) for 48 h, and record the weight of the test tube before and after placement. Calculate the water vapor transmission rate, oxygen transmission rate, and carbon dioxide transmission rate of the film using the formulas below: Water vapor transmission rate = ( W × d ) / ( A × t ×Δ P )Formula (1) Oxygen transmission rate / Carbon dioxide transmission rate = ( W ×d ) / ( A × t )Formula (2) in, W Adding weight (kg) to the test container, d The film thickness is in meters (m). A The effective area of ​​the thin film (m²) 2 ), t For the equilibrium time (s), ΔP The vapor pressure difference is expressed in Pa.

[0041] (6) Thermal stability: The thermal properties and stability of the thin film were evaluated using a thermogravimetric analyzer under a protective atmosphere of continuous high-purity nitrogen. The heating rate was 10 °C / min, and the mass change data of the sample from 25 °C to 800 °C were recorded. The thermal decomposition behavior curve of the sample was obtained simultaneously by thermogravimetric analysis. Combined with the first derivative curve of temperature-mass loss rate, the thermal stability characteristics and thermal degradation kinetic parameters of the thin film material were systematically evaluated.

[0042] (7) Water contact angle: 10 μL of distilled water was dropped onto the sample surface to measure the water contact angle and observe the wetting situation. The water contact angle of the film was measured using a contact angle analyzer.

[0043] (8) Transmittance: The transmittance of the film was evaluated using a spectrophotometer in the wavelength range of 200-800 nm.

[0044] (9) Antioxidant performance: 20, 40, 60, 80, and 100 mg of the membrane were dissolved in 1 mL of distilled water for 3 h to obtain colloidal solutions. 50 μL of each colloidal solution was mixed thoroughly with 200 μL of DPPH reaction solution and 200 μL of ABTS reaction solution, respectively, and allowed to stand for 30 min. After the reaction, the mixture was filtered through a 0.45 μm filter. The absorbance of the filtrate was measured at 517 nm and 734 nm using a UV-Vis spectrophotometer. The scavenging capacity of the membrane for DPPH and ABTS free radicals was calculated using the following formula: Formula (3) In the formula, A0 and A1 are the absorbance of the blank control and the sample, respectively.

[0045] (10) Antibacterial properties: The antibacterial properties of the membrane were characterized by its resistance to Escherichia coli and Staphylococcus aureus. A 20 mm × 20 mm membrane was mixed with 0.5 mL of an active bacterial suspension (10... 6 Mix (CFU / mL) and shake well for 1 h. Then, evenly spread 150 μL of the treated bacterial solution onto a culture dish containing LB agar. After incubation at 37℃ for 16 h, calculate the relative inhibition rate using the following formula: Formula (4) In the formula, N0 is the number of colonies in the control group and N1 is the number of colonies in the treatment group.

[0046] The antifungal properties of the film were characterized by assessing its resistance to Botrytis cinerea. The film was mixed with PDA medium, and 20 mL of the mixture was poured into a petri dish. Subsequently, 7 mm mycelium was placed in the center of the dish and incubated at 28 °C and 90% RH (humidity) for 5 days. The diameter of the inhibition zone was measured using calipers.

[0047] (11) Safety evaluation: The safety of the membrane was evaluated using a hemolysis test. 0.5 mL of erythrocyte suspension (2%, v / v) was incubated with equal volumes of water, membrane solution, and 0.9% NaCl solution at 37 °C for 4 h. After incubation, the membrane was centrifuged at 2000 rpm for 5 min, and the absorbance of the supernatant was measured at 492 nm. The hemolysis rate was calculated using the formula: Formula (5) In the formula, A0, A2, and A1 represent the negative group, positive group, and experimental group, respectively.

[0048] (12) Appearance quality: Take photos of the appearance and cross-section of the kiwifruit with a mobile phone to observe its appearance quality.

[0049] (13) Weight loss rate: Weigh the fresh-cut kiwifruit using an electronic balance and calculate the weight loss rate of the fresh-cut kiwifruit according to formula (6).

[0050] Formula (6) In the formula, W 0 represents the initial weight (g) of the kiwifruit. W 1 represents the weight (g) of the kiwifruit at the time of sampling.

[0051] (14) Hardness: The hardness of fresh-cut kiwifruit during storage was determined by a texture analyzer. The equatorial portion of the kiwifruit was uniformly selected for puncture experiments. The specific parameters are as follows: TA-39 probe was used, puncture speed was 0.5 mm / s, puncture depth was 7 mm, and the results are represented by N.

[0052] Example 1 A method for preparing an asymmetric functional packaging film of rice starch-based citral nanoemulsion according to the present invention includes the following steps: (1) Add 0.1 g of sulfonated cellulose nanocrystals and 0.3 g of pea protein isolate to 10 mL of water and sonicate continuously at room temperature for 10 min with an ultrasonic power of 300 W to obtain a homogeneous suspension.

[0053] (2) Add 0.2 g soybean oil and 0.2 g citral to the suspension obtained in step (1), and homogenize at 10,000 rpm for 3 min to obtain citral nanoemulsion; sonicate the citral nanoemulsion at 350 W for 15 min to obtain citral nanoemulsion, the average particle size of the citral nanoemulsion being 254 nm. The antibacterial properties of citral enable the membrane to have certain food preservation functions, and the nanoparticles in the citral nanoemulsion can act as molecular bridges to connect the hydrophobic layer in the subsequent step (5).

[0054] (3) Add 3.5 g of rice starch to 100 mL of distilled water, heat to 85 ℃ and stir for 30 min to obtain rice starch paste. Add 0.07 g of glycerol to the rice starch paste, stir for 10 min to remove air bubbles, cool to room temperature, add 0.35 g of citral nanoemulsion, and homogenize at 2800 rpm for 1 min to obtain film-forming slurry. The mass ratio of rice starch, glycerol and citral nanoemulsion is 3.5:0.07:0.35.

[0055] (4) Pour the film-forming slurry obtained in step (3) onto the film-forming tool and dry it to form a film. The drying temperature is 30 ℃ and the drying time is 24 h to obtain rice starch-based citral nanoemulsion packaging film.

[0056] (5) Dissolve 1 g of myristic acid in 100 mL of ethanol to obtain an ethanol solution of myristic acid. Place one side of the rice starch-based citral nanoemulsion packaging film (the side that serves as the outer surface of the packaging, i.e., the outer layer of the film) on the surface of the ethanol solution of myristic acid for 20 s. After removal, the ethanol on the film evaporates naturally at room temperature, forming a myristic acid layer on the outer layer of the film. The nanoparticles in the citral nanoemulsion act as synergistic molecular bridges, enhancing the adsorption energy barrier of the myristic acid layer, allowing it to be uniformly deposited through hydrophobic chain arrangement and hydrogen bonding to form a dense hydrophobic layer, forming a Janus film, and finally obtaining an asymmetric functional packaging film of rice starch-based citral nanoemulsion. Overall, the inner layer of the film has the functional properties of food preservation due to citral, while the outer layer has hydrophobicity due to the myristic acid layer.

[0057] An application of the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in this embodiment in the preservation of fruits and vegetables, specifically kiwifruit, includes the following steps: Fruits and vegetables were selected according to the following criteria: uniform ripeness (similar firmness), moderate size (similar fruit quality), and no disease spots or mechanical damage on the surface. The selected fruits and vegetables were randomly grouped and placed in preservation boxes measuring 8 cm × 8 cm × 15 cm. The preservation boxes were packaged and sealed using the rice starch-based citral nanoemulsion asymmetric functional packaging film prepared in this embodiment, and stored at room temperature for preservation.

[0058] Comparative Example 1 A method for preparing an active packaging film made of rice starch involves mixing a 3.5 wt% rice starch solution with glycerol to obtain a film-forming slurry, wherein the glycerol is 10% of the mass of the rice starch. The film-forming slurry is then cast into a film using a casting method to obtain the active packaging film made of starch.

[0059] Comparative Example 2 A method for preparing an active hydrophobic packaging film of rice starch is basically the same as that of Comparative Example 1, except that: 1 g of myristic acid is dissolved in 100 mL of ethanol to obtain an ethanol solution of myristic acid; the outer layer of the active packaging film of rice starch is immersed in the ethanol solution of myristic acid for 20 s; after removal, the ethanol on the film evaporates naturally at room temperature; the adsorbed myristic acid layer makes the outer layer of the film hydrophobic, thus obtaining an active hydrophobic packaging film of rice starch.

[0060] Comparative Example 3 A method for preparing an active packaging film of rice starch is basically the same as that in Example 1, except that step (5) is not performed.

[0061] Characterization analysis: (1) Microstructure from Figure 1 (1) It can be seen that the film of Comparative Example 1 exhibits a smooth and uniform surface, while the film of Comparative Example 2 shows irregularly distributed particle aggregates. Due to the interfacial tension between nanoparticles in the rice starch and citral nanoemulsion, the film of Comparative Example 3 shows a loose and continuous interfacial structure. In contrast, the asymmetric functional packaging film of the rice starch-based citral nanoemulsion in Example 1 exhibits a uniform and tight continuous interfacial structure, indicating that the citral nanoemulsion may mediate myristic acid deposition through enhanced molecular interactions, forming a continuous and tight myristic acid layer. Similarly, the cross-section of the film of Comparative Example 2 shows discontinuous myristic acid embedded in the starch matrix. Figure 1 (2) It can be seen that Example 1 shows that starch and myristic acid form a dense interface, indicating that the interfacial compatibility is improved.

[0062] (2) FTIR spectrum from Figure 2 It can be seen that the film of Comparative Example 1 at 3260 cm⁻¹ -1 There is a broad OH stretching peak at 2930 cm⁻¹. -1 There is a CH bend at 1650 cm. -1Tensile vibrations were observed in the C=O band of the amide. The incorporation of citral nanoemulsion did not produce new peaks, indicating good compatibility between the citral nanoemulsion and the starch matrix. After modification with myristic acid, the packaging film of Example 1 exhibited distinct myristic acid-specific characteristics: at 1705 cm⁻¹... -1 Intense C=O tensile vibrations occur at the (carboxylic acid group) at 2850-2920 cm⁻¹. -1 The asymmetric stretching of the CH chain during alkyl chain formation directly confirms the successful incorporation of myristic acid. Furthermore, the OH peak of the film from Example 1 shifted to 3250 cm⁻¹. -1 This indicates that hydrogen bonds are formed between the starch hydroxyl groups and the myristic acid carboxyl groups, stabilizing the interfacial layer.

[0063] (3) X-ray diffraction pattern from Figure 3 As can be seen, the film of Comparative Example 1 has a type A crystal structure of starch, with peaks located at 15°, 17°, and 23°. The introduction of citral nanoemulsion slightly reduced the crystallinity (Comparative Example 3), which is because the citral nanoemulsion disrupted the original crystal structure of the starch film. In addition, the film of Comparative Example 2 has a weak diffraction peak at 8.41°, corresponding to the characteristic peak of myristic acid. Compared with the film of Example 1, the film of Comparative Example 2 does not have a significant characteristic peak of myristic acid, indicating that the relative amount of myristic acid adsorbed on its surface is lower.

[0064] (4) Mechanical properties from Figure 4 As can be seen from (1) and (2), the tensile strength of the film in Comparative Example 1 is 9.27 MPa and the elongation at break is 9.24%, reflecting its inherent brittleness. This is because the starch molecules are tightly connected by a rigid hydrogen bond network, which restricts its tensile strength and makes it prone to cracking under stress. The incorporation of citral nanoemulsion increased the tensile strength of the film in Comparative Example 3 to 18.95 MPa and the elongation at break to 17.98%, significantly improving the mechanical properties of the film. In addition, compared with the film in Comparative Example 3, the myristic acid coating significantly improved the tensile strength of the Janus film, while also giving it greater flexibility. Compared with Comparative Example 2, the film in Example 1 is the most advantageous, providing an excellent balance between strength and flexibility. Therefore, the myristic acid coating not only improves the strength but also stabilizes the interface between the starch matrix and the citral nanoemulsion, ensuring that the film maintains its flexibility without affecting its enhanced strength.

[0065] (5) Barrier performance from Figure 5 As can be seen from (1), (2), and (3), the film of Comparative Example 1 has a high water vapor transmission rate (2.34 × 10⁻⁶). -13 kg·m -¹·s - ¹·Pa - ¹) and oxygen permeability (3.69×10 -10 kg·m - ¹·s - ¹), high water vapor transmission and oxygen transmission rates can lead to excessive moisture loss and microbial growth in food. The addition of citral nanoemulsion significantly improved barrier performance; the water vapor transmission, O2 transmission, and CO2 transmission rates of the film in Comparative Example 3 decreased by 50%, 33%, and 29%, respectively. Furthermore, the film in Comparative Example 2 showed a significant decrease in water vapor transmission, O2 transmission, and CO2 transmission rates, indicating the hydrophobicity of myristic acid. In particular, the film in Example 1 had water vapor transmission, O2 transmission, and CO2 transmission rates of 0.88 × 10⁻⁶. -13 kg·m - ¹·s - ¹·Pa - ¹、2.02×10 -10 kg·m - ¹·s - ¹、1.57×10 -10 kg·m - ¹·s - ¹. Therefore, the starch matrix densification and myristic acid surface impermeability of the film of Example 1 overcome the limitations of starch in humid and aerobic environments.

[0066] (6) Thermal stability from Figure 6 As can be seen from (1) and (2), the film of Comparative Example 1 exhibits a two-stage degradation process: initial moisture loss (50-150 °C) and starch degradation and decomposition (250-400 °C). The incorporation of citral nanoemulsion reduces the weight loss of the starch film, which is attributed to the plasticizing effect of citral, thereby delaying the decomposition of the polymer chains. The films of Comparative Example 2 and Example 1 exhibit similar thermal decomposition processes, with a slight increase in the decomposition peak, which is due to the hydrophobic long alkyl chain of myristic acid, reducing moisture absorption and forming a protective barrier against heat transfer. The film of Example 1 has a higher residual mass at 800 °C than that of Comparative Example 1, confirming that myristic acid can form a dense structure on its surface.

[0067] (7) Hydrophobicity from Figure 7 (1) It can be seen that the rapid self-assembly of myristic acid aims to overcome the inherent hydrophilicity of starch films. The water contact angles of the films in Comparative Example 1 and Comparative Example 3 were 71.58° and 82.08°, respectively, indicating their hydrophilicity. In contrast, the film of Example 1 exhibited a water contact angle as high as 118.22°, demonstrating excellent hydrophobicity. Figure 7(2) It can be seen that the self-cleaning experiment further verified the special hydrophobicity of the film in Example 1. When contaminated by particulate matter, water droplets quickly roll off the surface of the Janus film, carrying away the contaminants, while residual water adheres to the modified starch film, exacerbating scaling. This highlights the practical advantages of the Janus design in real-world applications. This durability is attributed to the citral-mediated stability of the myristic acid layer, which resists mechanical damage. The improved hydrophobicity of Example 1 stems from the citral nanoemulsion promoting the uniform distribution of myristic acid. Through the rapid self-assembly of myristic acid, the starch film is transformed into a moisture barrier with self-cleaning capabilities. On the one hand, the nanoparticles in the citral nanoemulsion homogenize the starch network, reducing pore size and crystallinity, thereby limiting the diffusion of gases and vapors. On the other hand, the dense myristic acid layer blocks polar (water) and non-polar (oxygen / carbon dioxide) molecules, forming a favorable gas barrier. Therefore, the Janus film of Example 1 significantly improves the inherent hydrophilicity of starch, addressing a key challenge in sustainable packaging.

[0068] (8) Light transmittance from Figure 8 It can be seen that the ultraviolet (UV) shielding capability of the film in Example 1 is significantly enhanced. The film in Comparative Example 1 exhibits high transmittance across the entire UV range (200-400 nm), indicating that its UV blocking capability is negligible. By incorporating citral nanoemulsion into the starch film, the UV shielding performance of the film in Comparative Example 3 is significantly enhanced, attributed to the effective UV absorption of the conjugated double bonds in citral. Furthermore, by rapidly self-assembling the film in Example 1 with myristic acid, near-complete UV shielding (0% transmittance) is achieved; the dense hydrophobic layer formed by myristic acid directly blocks UV penetration.

[0069] (9) Antioxidant properties from Figure 9 As can be seen from (1) and (2), the film of Comparative Example 1 exhibits negligible antioxidant activity. In contrast, the film of Comparative Example 3 shows a concentration-dependent increase in free radical scavenging activity. When the sample concentration is 100 mg / mL, the DPPH and ABTS scavenging rates of the film of Example 1 are 46.99% and 82.79%, respectively. The monoterpene components of citral neutralize free radicals by providing hydrogen atoms and electrons. In addition, the nanoemulsion improves the accessibility of citral, ensuring sustained antioxidant activity. Notably, the self-assembly of the myristic acid coating does not impair the stability of citral, as evidenced by the consistent antioxidant performance between the films of Comparative Example 2 and Example 1. Therefore, this rapid, low-energy strategy has significant advantages, as it does not involve prolonged heating or solvent exposure, thus better preserving the active substances and exerting sustained and efficient antioxidant effects.

[0070] (10) Antibacterial properties from Figure 10 As can be seen from (1), (2), and (3), compared with the film of Comparative Example 1, the film of Comparative Example 3 exhibits superior bacterial inhibition, with inhibition rates of 98.93% and 90.23% against Staphylococcus aureus and Escherichia coli, respectively. The film of Example 1 exhibits inhibition rates of 98.63% and 91.56% against Staphylococcus aureus and Escherichia coli, respectively. For Botrytis cinerea, the film of Example 1 reduced the hyphal growth diameter by 42.53%, and the special antibacterial effect of this film is attributed to citral. In addition, the antibacterial efficacy of citral was further amplified by increasing the solubility of citral and its contact with microbial cells. In particular, the myristic acid hydrophobic surface in Example 1 provides a secondary barrier, reducing microbial adhesion and avoiding thermal degradation of citral, resulting in fewer colonies. Therefore, the rice starch-based citral nanoemulsion asymmetric functional packaging film of Example 1 is a multifunctional active packaging material. The citral nanoemulsion provides antioxidant and antibacterial effects, while the myristic acid coating enhances physical barrier properties. Together, they solve the oxidation and microbial spoilage pathways, providing a comprehensive solution for the preservation of perishable foods.

[0071] (11) Safety evaluation from Figure 11 As can be seen, the supernatants of the water-treated group and the Example 1 treatment group remained pale yellow and transparent, indicating minimal interaction with red blood cells. In stark contrast, the water-treated control group exhibited complete hemolysis, characterized by a bright red, turbid liquid due to red blood cell rupture. Quantitative analysis showed that the hemolysis rate of the film-treated group in Example 1 was 2.18%, significantly lower than the permissible safety threshold of 5%. The low hemolysis rate confirms the excellent biocompatibility of the film, meeting international standards for food contact materials. The results clearly verify the safety of the rice starch-based citral nanoemulsion asymmetric functional packaging film of the present invention in fruit and vegetable preservation applications, ensuring its suitability for direct contact with perishable foods without affecting human health.

[0072] (12) Apparent quality from Figure 12It can be seen that the untreated fresh-cut kiwifruit and the fresh-cut kiwifruit packaged with the packaging film in Comparative Example 1 both suffered severe water loss and shrinkage during storage. Furthermore, the cross-sectional view on day 7 shows that the untreated kiwifruit underwent severe oxidative browning, while the kiwifruit treated with the PE film experienced very severe softening and browning, exhibiting the worst edibility. This is attributed to the low moisture and gas permeability of polyethylene, leading to a higher ethylene concentration and respiration rate within the packaged kiwifruit. In contrast, the kiwifruit treated with the rice starch-based citral nanoemulsion asymmetric functional packaging film of Example 1 showed only slight softening and browning, thanks to the excellent antioxidant and barrier properties of the rice starch-based citral nanoemulsion asymmetric functional packaging film. Compared to the other three groups of kiwifruit, the kiwifruit treated with the packaging film of Example 1 not only maintained a relatively plump appearance throughout the storage period but also retained a fresh flesh color on day 7. The above results indicate that the rice starch-based citral nanoemulsion asymmetric functional packaging film of Example 1 has a good preservation effect on kiwifruit.

[0073] (13) Weight loss rate from Figure 13 It can be seen that the ultra-low permeability of PE film packaging causes a large amount of water generated by fruit respiration to remain inside the packaging, accelerating the spoilage of the fruit. In Example 1, the weight reduction of kiwifruit treated with the rice starch-based citral nanoemulsion asymmetric functional packaging film was limited to 6.27%, lower than the other three groups, indicating less loss of moisture and nutrients.

[0074] (14) Hardness from Figure 14 It can be seen that the firmness of all fresh-cut kiwifruit samples decreased continuously with storage time during the storage period. The kiwifruit treated with the film in Example 1 showed lower weight loss, indicating less loss of moisture and nutrients, which helps maintain the high firmness of the kiwifruit. Therefore, the film treatment in Example 1 can effectively maintain the textural quality of kiwifruit during storage. These results further confirm that the rice starch-based citral nanoemulsion asymmetric functional packaging film of the present invention has a dense structure, which can reduce water vapor transmission rate, oxygen transmission rate, and carbon dioxide transmission rate, preventing moisture loss and thus maintaining the firmness of the kiwifruit.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for preparing an asymmetric functional packaging film based on rice starch-citral nanoemulsion, characterized in that, Includes the following steps: (1) Add sulfonated cellulose nanocrystals and pea protein isolate to water and sonicate until a suspension is formed; (2) Add soybean oil and citral to the suspension obtained in step (1), homogenize it to obtain citral nano emulsion, and sonicate the citral nano emulsion to obtain citral nano emulsion. (3) Add rice starch to water, heat to 60℃~85℃ and stir to disperse to obtain rice starch paste. Add glycerol to rice starch paste and stir. After cooling, add citral nanoemulsion and homogenize to obtain film-forming slurry. (4) The film-forming slurry obtained in step (3) is cast into a film by casting to obtain a rice starch-based citral nanoemulsion packaging film; (5) Place one side of the rice starch-based citral nanoemulsion packaging film obtained in step (4) on an ethanol solution of myristic acid for immersion, take it out and dry it to obtain a rice starch-based citral nanoemulsion asymmetric functional packaging film.

2. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to claim 1, characterized in that, In step (1), the ratio of sulfonated cellulose nanocrystals, pea protein isolate, and water is 0.1g~0.3g∶0.1g~0.3g∶10mL.

3. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to claim 2, characterized in that, In step (2), the ratio of soybean oil, citral, and suspension is 0.1g~0.3g∶0.1g~0.3g∶10mL, and the average particle size of the citral nanoemulsion is 100 nm~300 nm.

4. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to claim 3, characterized in that, In step (3), the mass-to-volume ratio of rice starch to water is 3.5g~5g∶100mL, and the mass ratio of rice starch, glycerol, and citral nanoemulsion is 1∶0.01~0.05∶0.01~0.1; In step (5), the mass concentration of myristic acid in the ethanol solution of myristic acid is 1g / 100mL to 3g / 100mL.

5. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to any one of claims 1 to 4, characterized in that, In step (1), the ultrasonic treatment time is 10 min to 20 min, and the ultrasonic treatment power is 250 W to 450 W.

6. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to any one of claims 1 to 4, characterized in that, In step (2), the rotation speed of the homogenization process is 10,000 rpm to 15,000 rpm, the homogenization time is 3 min to 5 min, the ultrasonic power is 300 W to 400 W, and the ultrasonic time is 15 min to 20 min.

7. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to any one of claims 1 to 4, characterized in that, In step (3), the stirring speed is 800 rpm to 1000 rpm, the stirring time is 20 min to 30 min, after adding glycerol to the rice starch paste, the stirring time is 10 min to 20 min, after adding citral nanoemulsion, the homogenization speed is 2800 rpm to 4000 rpm, and the homogenization time is 1 min to 5 min.

8. The method for preparing the rice starch-based citral nanoemulsion asymmetric functional packaging film according to any one of claims 1 to 4, characterized in that, In step (4), the casting method includes pouring the film-forming slurry onto the film-forming tool and drying it to form a film, wherein the drying temperature is 30℃~35℃ and the drying time is 24h~30h; in step (5), the soaking time is 10s~40s and the drying is the natural evaporation of the solvent.

9. A rice starch-based citral nanoemulsion asymmetric functional packaging film prepared by the method described in any one of claims 1 to 8.

10. The application of the rice starch-based citral nanoemulsion asymmetric functional packaging film as described in claim 9 in the preservation of fruits and vegetables.

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