Purple cabbage anthocyanin-based packaging film and preparation method thereof

Abstract

The invention discloses a packaging film based on purple cabbage anthocyanin and a preparation method of the packaging film, and relates to the technical field of food packaging materials. 10 to 20 parts of hydroxypropyl methyl cellulose; 0.5 to 5 parts of purple cabbage anthocyanin; 0.1 to 0.5 part of glycerol; by adding the purple cabbage anthocyanin, the packaging film is endowed with antioxidant, antibacterial and other biological activities, oxidative deterioration of food can be effectively delayed, growth and reproduction of microorganisms can be inhibited, so that the quality and safety of the food can be better maintained, and moreover, the packaging film has unique pH responsiveness, so that in the food storage process, the packaging film does not have any toxic or side effect on the food. The color of the packaging film can be obviously changed along with the change of the freshness of the food, so that the practicability and the convenience of food packaging are improved, more reliable food quality information is provided for consumers, and the diet health of the consumers is guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of food packaging materials, in particular to a packaging film based on purple cabbage anthocyanin and a preparation method thereof. Background Art

[0002] In the field of food packaging material technology, as the concept of environmental protection has become more popular and consumers' attention to food safety has continued to increase, the development of food packaging materials that are both environmentally friendly and functional has become an urgent need for the development of the industry. Although traditional plastic packaging films are widely used in food packaging, they have brought a heavy burden to the environment due to their non-degradable properties. At the same time, traditional plastic packaging films have a single function and are difficult to meet the requirements of multiple performance in the process of food preservation. Against this background, degradable food packaging materials have gradually become a research hotspot, aiming to solve the environmental problems brought about by traditional plastic packaging and explore more effective ways of food preservation.

[0003] At present, although there are research results on degradable packaging materials, they still have many deficiencies in performance and cannot well meet the dual requirements of food preservation and environmental friendliness. For example, although starch-based films have good degradability, their mechanical properties are poor, and they are easily damaged in actual use, making it difficult to ensure the integrity of food packaging. At the same time, they have insufficient water resistance and easily absorb water and swell in humid environments, affecting the performance of the packaging film and the quality of the food. Sodium alginate film also has the problem of low mechanical strength and is relatively sensitive to moisture. It easily loses its original performance in environments with high moisture content, limiting its wide application in the field of food packaging. Although hydroxypropyl methylcellulose film has certain degradability, it has a single function and lacks antioxidant and antibacterial properties. It cannot effectively delay the oxidative deterioration and microbial contamination of food, and it is difficult to meet the requirements of food preservation. In addition, red cabbage anthocyanin, as a natural pigment, has antioxidant and antibacterial biological activities, but it has poor stability during application. How to effectively compound it into the packaging film and improve the overall performance of the film has become a difficult problem faced by traditional technologies and is also a key technical point that needs to be overcome in the present invention. Summary of the Invention

[0004] The purpose of the present invention is to make up for the deficiencies of the prior art and provide a packaging film based on purple cabbage anthocyanins and a preparation method thereof. The packaging film takes sodium alginate and hydroxypropyl methylcellulose as a base material, adds purple cabbage anthocyanins and glycerol, and successfully prepares a packaging film with excellent performance through preparation steps including extraction of purple cabbage anthocyanins, preparation of a film-forming liquid, degassing and film formation, and balancing treatment. The packaging film not only has good mechanical properties and can withstand certain external forces without being damaged, thereby ensuring the reliability of food packaging, but also has excellent antioxidant and breathable properties, effectively preventing the penetration of water vapor and oxygen, and delaying the oxidation and deterioration of food.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions: In a first aspect, the present invention provides a packaging film based on purple cabbage anthocyanins, the packaging film comprising, by weight: Sodium alginate: 5-15 parts; Hydroxypropyl methylcellulose: 10-20 parts; Purple cabbage anthocyanin extract: 0.5-5 parts; Glycerin: 0.1-0.5 parts; Deionized water: 80-100 parts.

[0006] In a second aspect, the present invention further provides a method for preparing a packaging film based on purple cabbage anthocyanins. The method is used to prepare a packaging film based on purple cabbage anthocyanins, and the method comprises: Extraction of anthocyanins from purple cabbage: Dry and grind the purple cabbage into powder, add ethanol solution, extract by ultrasonication, separate by centrifugation, collect the supernatant, and concentrate under reduced pressure to obtain the anthocyanin extract from purple cabbage; Preparation of membrane-forming solution: Sodium alginate and hydroxypropyl methylcellulose are added to deionized water in proportion, and after they are completely dissolved to form a uniform sol, purple cabbage anthocyanin extract and glycerin are added and stirred continuously to obtain a membrane-forming solution; Degassing and film formation: Pour the film-forming liquid into a glass culture dish after ultrasonic degassing, and dry it to form a wet film; Equilibration treatment: the wet film is taken out and equilibrated in a specific environment for 45-50 hours to obtain a purple cabbage anthocyanin composite packaging film.

[0007] Preferably, in the purple cabbage anthocyanin extraction step, the purple cabbage is dried and ground into fine powder, and an ethanol solution with a volume fraction of 50%-60% is added at a liquid-to-solid ratio of 1:20-30 g / mL.

[0008] Preferably, in the purple cabbage anthocyanin extraction step, ultrasonic extraction is performed at 20-30° C. for 30-40 min with an ultrasonic power of 180-220 W. After the extraction is completed, the obtained liquid is subjected to high-speed centrifugation.

[0009] Preferably, in the membrane-forming solution preparation step, sodium alginate, hydroxypropyl methylcellulose and purple cabbage anthocyanin extract are mixed in proportion and dissolved in deionized water, and stirred at 20-30° C. for 4-6 hours until the solution becomes clear.

[0010] Preferably, in the film-forming solution preparation step, purple cabbage anthocyanin extract and 0.1-0.3% glycerol of the total solution content are added as plasticizers, and stirring is continued at 20-30° C. for 0.5-1.5 hours to fully dissolve and mix the components to form a stable film-forming solution system.

[0011] Preferably, in the degassing and film-forming steps, the prepared film-forming liquid is subjected to ultrasonic degassing treatment with an ultrasonic power of 180-220 W and a degassing time of 5-10 min. After degassing is completed, the film-forming liquid is poured into glass culture dishes, with 25-35 mL per dish.

[0012] Preferably, in the degassing and film-forming steps, the culture dish is placed in an oven at 40-50° C. and dried for 20-28 hours to evaporate the solvent in the film-forming solution and form a wet film.

[0013] Preferably, in the equilibration step, the dried wet film is taken out of the oven and placed in an environment with a temperature of 25±1° C. and a relative humidity of 50±1% for 45-50 hours of equilibration.

[0014] Compared with the existing technology, the packaging film based on purple cabbage anthocyanins and the preparation method thereof have the following beneficial effects: 1. The present invention uses purple cabbage anthocyanin extract to prepare a packaging film, which gives the packaging film antioxidant and antibacterial biological activities, effectively delaying the oxidative deterioration of food and inhibiting the growth and reproduction of microorganisms, thereby better maintaining the quality and safety of food. Moreover, the packaging film has unique pH responsiveness. During food storage, as the freshness of the food changes, the packaging film will undergo obvious color changes, which not only improves the practicality and convenience of food packaging, but also provides consumers with more reliable food quality information, ensuring consumers' dietary health.

[0015] 2. The present invention uses degradable sodium alginate, hydroxypropyl methylcellulose, etc. as the base material, so that they can gradually decompose in the natural environment, greatly reducing the burden on the environment. It is in line with the current development trend of green environmental protection, helps to promote the food packaging industry to develop in a sustainable direction, and provides an effective solution to the problem of plastic pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort.

[0017] Figure 1 Schematic diagram of pH response liquid of purple cabbage anthocyanin extract; Figure 2 This is the UV-visible spectrum of the purple cabbage anthocyanin extract; Figure 3 is the fluorescence-visible spectrum of purple cabbage anthocyanin extract; Figure 4 Schematic diagram of the microstructure of the composite indicator film; Figure 5 Schematic diagram of the water contact angle of the composite indicator film; Figure 6 Schematic diagram of Fourier infrared spectroscopy analysis of the composite indicator film; Figure 7 is a DSC schematic diagram of the composite indicator film; Figure 8 is the XRD schematic diagram of the composite indicator film; Figure 9 Schematic diagram of the UV transmittance of the composite indicator film; Figure 10 Schematic diagram of (A) tensile strength and (B) elongation at break of purple cabbage anthocyanin composite plastic wrap; Figure 11 Schematic diagram of (A) water vapor permeability and (B) oxygen permeability of purple cabbage anthocyanin composite plastic wrap; Figure 12 This is a schematic diagram of the appearance of the purple cabbage anthocyanin composite preservative film; Figure 13 Schematic diagram of the color development of the composite film at different pH; Figure 14 Schematic diagram of the (A) DPPH and (B) ABTS free radical scavenging abilities of the films; Figure 15 The present invention is a flow chart of a method for preparing a packaging film based on purple cabbage anthocyanins. DETAILED DESCRIPTION

[0018] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, the specific implementation methods, structures, features and effects of the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments.

[0019] The present invention provides a packaging film based on purple cabbage anthocyanin and a preparation method thereof. By using sodium alginate and hydroxypropyl methylcellulose as a base material, adding purple cabbage anthocyanin and glycerol, and carrying out preparation steps including extraction of purple cabbage anthocyanin, preparation of a film-forming liquid, degassing and film-forming, and balancing treatment, a packaging film with excellent performance is successfully prepared. The packaging film not only has good mechanical properties and can withstand certain external forces without being damaged, thereby ensuring the reliability of food packaging, but also has excellent barrier properties, effectively preventing the permeation of water vapor and oxygen, and delaying the oxidation and deterioration of food. It should be particularly noted that the embodiments of the present invention are only used to explain the present invention and are not used to limit the scope of the present invention. Unless otherwise specified, the test methods used in the following embodiments are all conventional methods. Unless otherwise specified, the materials, reagents, etc. used may be reagents and materials obtained from commercial channels.

[0020] Example 1 Accurately weigh 10 g of sodium alginate and 15 g of hydroxypropyl methylcellulose, place them in a beaker, add 85 mL of deionized water (25°C), and stir magnetically at 500 rpm for 5 hours until the solution is clear and transparent without any particulate matter.

[0021] 0.18% (about 0.198 g) of glycerol was added and the stirring was continued for 1 hour to allow the glycerol and the base material to be fully mixed and uniformly mixed to form SH.

[0022] The membrane-forming solution was transferred to an ultrasonic instrument and degassed at 200 W power for 5 minutes to remove residual bubbles in the solution.

[0023] 30 mL of the degassed film-forming solution was poured into a glass Petri dish with a diameter of 9 cm and placed in an oven at 45°C to dry for 24 hours to form a transparent semi-solid film.

[0024] The dried film was taken out and placed in an environment at 25℃±1℃ and 50±1% relative humidity for 48 hours to ensure uniform moisture distribution in the film.

[0025] Example 2 The purple cabbage was dried and ground into fine powder, passed through an 8-mesh sieve, and added with 55% ethanol solution at a liquid-to-solid ratio of 1:25 g / mL. The mixture was ultrasonically extracted at 200 W power at 25°C for 35 minutes, followed by centrifugation at 4000 rpm for 10 minutes. The supernatant was collected and concentrated under reduced pressure to 1 / 5 of the original volume to obtain anthocyanin concentrate (containing 5% of the total solution) as PSH1.

[0026] Weigh 10 g of sodium alginate and 15 g of hydroxypropyl methylcellulose, add 85 mL of deionized water, and stir under magnetic stirring at 25°C for 5 hours until completely dissolved.

[0027] Slowly pour the anthocyanin concentrate into the substrate solution, add 0.18% glycerol at the same time, and stir for 1.5 hours until the solution color becomes uniform (light yellow-green).

[0028] After ultrasonic degassing (200 W, 5 min), the film-forming solution was poured into a Petri dish and dried at 45 °C for 24 h to form a thin film.

[0029] After equilibration for 48 hours at 25°C and 50±1% humidity, the film thickness was approximately 0.0917 mm.

[0030] Example 3 Purple cabbage powder was added to 55% ethanol solution at a ratio of 1:25 g / mL and extracted by ultrasonication at 25°C for 35 min (200 W). The supernatant was collected after centrifugation, filtered through a 0.45 μm filter membrane, and concentrated under reduced pressure until the anthocyanin content accounted for 15% of the total solution (about 1.5 g) and was used as PSH2.

[0031] Select 10 g of low-viscosity sodium alginate (100-200 mPa·s) and 15 g of hydroxypropyl methylcellulose (degree of substitution 1.8-2.0), add 85 mL of deionized water, and stir at 25°C for 6 hours until the sol is uniform.

[0032] Preparation of composite film-forming solution: Add the anthocyanin concentrate to the base material sol three times, with an interval of 20 minutes between each addition. After stirring evenly, add 0.2 g of glycerol and continue stirring for 1 hour.

[0033] Degassing and drying: The film-forming solution was degassed by 200W ultrasound for 5 minutes, poured into a Petri dish and dried in a 45℃ oven for 24 hours.

[0034] Performance optimization treatment: The dried film was equilibrated in an environment of 25°C and 50±1% humidity for 48 hours, and a uniform porous structure was formed on the membrane surface (pore size 5-10μm as observed by scanning electron microscopy).

[0035] Example 4 Purple cabbage powder was added to 55% ethanol solution at a ratio of 1:25 g / mL and extracted by ultrasonication at 25°C for 40 min (200 W). After centrifugation, the supernatant was decolorized with 0.5% activated carbon for 1 h, sterilized with a 0.22 μm membrane, and concentrated until the anthocyanin content accounted for 25% of the total solution (2.5 g) to be used as PSH3.

[0036] Weigh 10 g of sodium alginate and 15 g of hydroxypropyl methylcellulose, add 85 mL of deionized water, stir at 25 °C for 5 hours until dissolved, add anthocyanin concentrate three times using a gradient method (each time with an interval of 30 minutes), and stir for 2 hours until the solution turns dark purple.

[0037] 0.18% glycerol (about 0.198 g) was added, and after stirring for 1 hour, ultrasonic degassing was performed at 200 W for 8 minutes to remove large bubbles.

[0038] The film-forming liquid was poured into a culture dish and dried at 45°C for 24 hours. The film thickness after drying was 0.0763 mm. The film was placed in an environment of 25°C and 50±1% humidity for 48 hours to improve the flexibility of the film.

[0039] Example 5 This embodiment tests the packaging films generated by the above four embodiments.

[0040] Optical properties of packaging film: For UV spectrum analysis of PSH complex solution: UV spectrum of PSH complex and pure SA was measured in the wavelength range of 350-800 nm at 25° C., and baseline calibration was performed before measurement.

[0041] Fluorescence spectrum analysis of the PSH composite solution: The fluorescence spectrum of the PSH composite solution with a set pH was scanned, the excitation wavelength was set to 280 nm, the emission wavelength range was set to 480 nm, and the excitation and emission slit widths were both set to 5 nm.

[0042] For film color measurement: a colorimeter is used to measure the color of the film, L* is the brightness, a* is the index from red to green, and b* is the index from yellow to blue. The color difference is calculated by the formula:

[0043] Where: is the total color difference of the film material, L1, a1, and b1 are the color values ​​of the sample, and L0, a0, and b0 are the color values ​​of the standard white plate (L0=90.78, a0=-0.59, and b0=2.16).

[0044] For the determination of light transmittance of the film: all films were cut into strips of equal size and scanned with an ultraviolet spectrophotometer in the range of 200nm-800nm ​​with a scanning resolution of 5nm.

[0045] Structural characterization of packaging film: Scanning electron microscopy analysis of PSH complex coacervates: Scanning electron microscopy (SEM) was used to observe the morphology of the packaging films of purple cabbage anthocyanin / hydroxypropyl methylcellulose / sodium alginate with different concentrations. The freeze-dried samples were sliced ​​and sprayed with gold, and then microscopically observed at an accelerating voltage of 8.0 kV and a magnification of 5000 times.

[0046] For the Fourier transform infrared spectroscopy analysis of PSH complex coacervates, the freeze-dried sample was mixed with potassium bromide powder at a ratio of 1:100, ground and pressed into transparent sheets, and infrared spectroscopy was performed using a Fourier transform infrared spectrometer (FTIR) with a scanning range of 400–4000 cm -1 The scans were performed within a range of 32 times, with air scans used as background and background subtraction performed before sample scanning.

[0047] For the thermogravimetric analysis of PSH membranes: the thermal stability of the freeze-dried samples was tested using a thermogravimetric analyzer. 2-5 mg of sample was taken for measurement. The instrument was set under flowing nitrogen conditions of 30 mL / min and heated from 25°C to 600°C at a heating rate of 10°C / min. The thermogravimetric curves of SH, PSH1, PSH2, and PSH3 were recorded.

[0048] For water contact angle measurement: The water contact angle (WCA) of the samples was measured using an optical contact angle meter. The freeze-dried SH, PSH1, PSH2, and PSH3 powders were pressed into 15 mm × 3 mm cylindrical tablets. After 2 μL of water was gently dropped on the tablet surface, the change in the droplet shape was recorded using a contact angle meter equipped with a high-speed camera.

[0049] Water vapor transmission rate, oxygen transmission rate: For the water vapor transmission rate (WVP) of the film: take a glass bottle with an inner diameter of 25mm and a depth of 55mm, add 3g of anhydrous CaCl2, seal the bottle mouth with sealing film at 25℃, and weigh it. Place the weighed glass bottle in a desiccator with deionized water at the bottom (maintain relative humidity of 100%) to ensure that a certain vapor pressure difference is maintained between the inside and outside of the film. Measure its weight every 24 hours within 7 days of storage, and calculate the water vapor transmission coefficient (WVP):

[0050] Where: WVP is the water vapor permeability coefficient, Δm is the increase in the mass of the vial in the same time, A is the exposed membrane area, t is the storage time, d is the thickness of the membrane, and Δp is the saturated vapor pressure of pure water at 25°C (3167.2 Pa).

[0051] For the oxygen transmission rate measurement of the film: add a deoxidizer (3.0g) composed of iron powder, activated carbon and sodium chloride in a ratio of 1:2:3 to a weighing bottle sealed with the film sample. Place the bottle in a desiccator containing a saturated barium chloride solution and weigh it every 24 hours for 7 days. The oxygen transmission coefficient (OP) is calculated from it:

[0052] Where: OP is the oxygen permeability coefficient, Δm is the increase in the mass of the vial in the same time, A is the exposed membrane area, and t is the storage time.

[0053] Mechanical properties of packaging film: For the mechanical properties of the film, the prepared packaging film was equilibrated in an environment of 25°C and 53% relative humidity for 24 hours. The film samples were then cut into 75mm×10mm rectangular strips and subjected to tensile properties testing using a universal testing machine. The specific test parameters were: load cell 50kg, initial clamp spacing 30mm, and tensile rate set at 0.5mm / s until the sample broke. Each group of samples was tested three times in parallel, and the average value was taken. The tensile strength (TS) was calculated as follows:

[0054] Where: TS is the tensile strength, P is the maximum force the specimen withstands when it breaks, b is the thickness of the specimen, and d is the width of the specimen.

[0055] The calculation formula for elongation at break (EBA) is:

[0056] Where: E is the elongation at break, L is the length of the sample when it breaks, and L0 is the initial length of the sample.

[0057] For film thickness measurement: the film thickness was measured using a screw micrometer, 5 points were randomly selected for each film, the measurement accuracy was 1 μm, and the thickness was recorded as the average of five measurements.

[0058] Antioxidant properties of packaging film: For DPPH scavenging ability: 1.0 mL of membrane sample solution (diluted 100 times) and 1.0 mL of DPPH methanol solution (0.1 mM) were mixed and reacted for 30 min in the dark. The absorbance A0 was measured at a wavelength of 517 nm as a blank control. 1 mL of ultrapure water and 1.0 mL of DPPH methanol solution were mixed and the absorbance was measured after the reaction. The DPPH scavenging rate was calculated according to the following formula:

[0059] For ABTS scavenging ability: 0.4 mL of membrane sample solution (diluted tenfold) was mixed with 1.6 mL of ABTS solution and reacted at 25°C for 6 min. The absorbance A was then measured at a wavelength of 734 nm. In the blank control group, 0.6 mL of ultrapure water was mixed with 1.8 mL of ABTS solution and the absorbance A was measured after the reaction under the same conditions. 0， The calculation formula of ABTS free radical scavenging activity is the same as that of DPPH free radical scavenging activity.

[0060] pass Figure 1 It can be seen that the purple cabbage anthocyanin (PCA) extract shows an obvious color change gradient at different pH values. This change reflects the transformation process of the anthocyanin molecular structure affected by pH. Under strong acidic conditions (pH2-3), PCA shows a bright red to pink color. As the pH value rises from 3 to 6, the solution color gradually transitions to a lavender series. This is because the conjugated system is between acidic and neutral. This intermediate state of the conjugated system makes the wavelength of light absorbed by anthocyanins also in the intermediate range, and the reflected light is combined to appear lavender. When food deteriorates, volatile alkaline substances (such as biogenic amines) are usually produced, which causes the local pH value of the packaging film to increase, causing the PCA color to change from the original state to the color in an alkaline environment, thereby intuitively indicating changes in food quality.

[0061] like Figure 2 As shown, the UV-visible absorption spectra of purple cabbage anthocyanin (PCA) extract in buffer solutions with pH values ​​ranging from 2 to 12 are shown. Under acidic conditions (pH 2-3), the spectrum shows a significant absorption peak in the 520-550 nm region. This absorption region corresponds to the green region in the visible spectrum, so the solution appears red to pink. As the pH value increases to 4-6, the absorption peak intensity at 520-550 nm gradually decreases, and new absorption appears in the 400-450 nm region. This spectral change reflects the conjugation of anthocyanin molecules. A significant transformation has occurred in the system. In the pH range of 7-9, the absorption peak can be observed to shift toward the long wavelength direction, and absorption enhancement occurs in the 580-650nm region, which corresponds to the phenomenon that the solution appears blue-green. This spectral red shift is due to the deprotonation of the hydroxyl group in the anthocyanin molecule, forming a quinone-type base structure. When the pH value is further increased to 10-12, the absorption band becomes flatter, and the main absorption is concentrated in the near-ultraviolet region of 350-400nm, which is consistent with the spectral characteristics of flavonoid derivatives formed after the opening of the anthocyanin C ring under alkaline conditions, causing the solution to appear yellow.

[0062] like Figure 3 The figure shows the fluorescence emission spectra of purple cabbage anthocyanin (PCA) in different buffer solutions with pH values ​​ranging from 2 to 12. The fluorescence intensity at pH 12 is significantly higher than that at other pH values. It exhibits strong fluorescence emission in the entire 520-700nm wavelength range and reaches a peak at around 525nm. The samples at pH 10 and 11 also exhibit strong fluorescence emission, but the intensity is lower than that at pH 12, and the peak position is slightly red-shifted to around 545-550nm. This peak red-shift phenomenon with increasing pH reflects the molecular configuration of anthocyanin. The fluorescence intensity of the samples in the pH range of 2-8 was relatively weak, and the correlation between fluorescence intensity and pH was closely related to the structural form of anthocyanin molecules at different pH values. Under acidic to neutral conditions (pH 2-7), anthocyanins mainly existed in the form of flavonoid cations and hemiacetals. The excited state energy of these structures was more easily released through non-radiative pathways, resulting in a lower fluorescence quantum yield.

[0063] like Figure 4The scanning electron microscope (SEM) morphologies of packaging films with different formula anthocyanins are shown. From the microscopic structure, it can be seen that all samples exhibit a porous network structure, but there are differences in pore size, distribution density, and pore wall thickness. At 50 μm, a relatively uniform pore distribution is shown, with smaller pore sizes and regular shapes. At 2 μm, larger and irregular hole structures are displayed, especially the obvious interconnected porous network. This change is attributed to the altered interaction between purple cabbage anthocyanins and sodium alginate / hydroxypropyl methylcellulose-based films, resulting in non-uniform changes in the structure of the film materials.

[0064] Figure 5 The water contact angle measurement results of the four different formula packaging films shown provide important information on surface properties. With the increase in the content of purple cabbage anthocyanins (PCA), the water contact angle shows a significant linear upward trend: SH < PSH1 < PSH2 < PSH3. From the basic formula SH to PSH1 (5% PCA), the contact angle increases by 3.1°, with a relatively small amplitude, indicating that low-concentration anthocyanins have limited effects on the surface wettability of the film. However, from PSH1 to PSH2 (15% PCA), the contact angle significantly increases by 15.0°. From PSH2 to PSH3 (25% PCA), the contact angle continues to increase by 14.6°, with a slower growth rate, probably approaching the saturation effect. The contact angles of all samples are less than the 90° critical value, indicating that these materials still maintain hydrophilic characteristics, but PSH3 is close to the hydrophobic critical point, probably due to the preferential arrangement of the non-polar aromatic ring structures in anthocyanin molecules on the material surface, reducing the exposure of polar hydroxyl groups, thereby lowering the surface energy.

[0065] Fourier transform infrared (FTIR) spectroscopy was used to deeply study the chemical structure and interactions between components of purple cabbage anthocyanin composite sodium alginate / hydroxypropyl methylcellulose films, as Figure 6 shown. The FTIR spectra of the SH group exhibit typical characteristic absorption peaks of sodium alginate and hydroxypropyl methylcellulose matrices, including a broad and strong O-H stretching vibration peak at 3275 cm -1 −1, a C-H stretching vibration peak at 2872 cm -1 −1, and the asymmetric and symmetric stretching vibration peaks of carboxylate (-COO -1 −) of sodium alginate at 1606 cm -1 −1 and 1410 cm - −1. The strong peak near 1028 cm -1 −1 represents the C-O stretching vibration of the polysaccharide backbone and ether bonds. These peaks clearly reflect the component composition of the base film. With the increase in the content of purple cabbage anthocyanins, significant changes occur in the FTIR spectra of the packaging films. For example, at 1500 - 1650 cm -1 −1 and 1200 - 1350 cm -1In the 40-60 nm CMOS process, the absorption peaks in the 50-70 nm CMOS process were significantly enhanced, and the absorption peaks were highly consistent with the absorption peaks of characteristic functional groups such as the C=C stretching vibration of the aromatic ring and the CO stretching vibration of the phenolic hydroxyl group in the purple cabbage anthocyanin molecule. The intensity of the peaks increased with the increase of the anthocyanin content, which clearly confirmed that the purple cabbage anthocyanin had been successfully and effectively complexed into the sodium alginate / hydroxypropyl methylcellulose matrix.

[0066] DSC analysis Figure 7 The thermal transition behaviors of sodium alginate / hydroxypropyl methylcellulose (SH) and its different concentrations of purple cabbage anthocyanin packaging films (PSH1, PSH2, PSH3) were demonstrated. All samples showed obvious endothermic peaks in the range of 75-120°C, with a peak temperature of approximately 100°C, which is consistent with the glass transition temperature of the polysaccharide material. The PSH3 sample showed the highest thermal enthalpy value of 6 mg / mg, which was significantly higher than the thermal enthalpy value of the control group SH 4.5 mg / mg, indicating that high concentrations of anthocyanins may enhance the thermal stability of the packaging film through hydrogen bonds and intermolecular interactions. In comparison, the PSH2 sample with 15% anthocyanin had the weakest thermal response of 2.2 mg / mg, and there was almost no thermal signal above 150°C.

[0067] like Figure 8 As shown in the figure, under the action of high concentration of anthocyanins (PSH325%), the diffraction peak is sharper, which indicates that high concentration of anthocyanins promotes the orderly arrangement of crystals in some areas. All samples show an obvious characteristic diffraction peak in the range of 2θ=15-20°, which corresponds to the characteristic peak of the semi-crystalline structure of sodium alginate and HPMC. In addition, several smaller but clear diffraction peaks are also observed in the range of about 32°, 40° and 50-60° of 2θ. The peak positions of each sample are basically consistent, indicating that the addition of different concentrations of purple cabbage anthocyanins (PCA) does not change the basic crystalline structure of the packaging film. However, the peak intensity changes slightly with the increase of PCA concentration from SH to PSH3, especially at the main peak (about 20°), which indicates that anthocyanins affect the crystallinity of the material.

[0068] like Figure 9As shown, the UV-visible spectra of the four samples (SH, PSH1, PSH2, and PSH3) revealed that the anthocyanin-containing samples exhibited significantly enhanced absorption in the UV region (250-400nm). PSH3 and PSH2 had the highest absorbance in this region, reaching approximately 2.0 and 1.5, respectively, while the control sample SH was only 0.3-0.6. This indicates that the introduction of anthocyanins greatly improved the packaging film's ability to shield against UV rays. In the visible light region (400-700nm), although the absorbance of the anthocyanin-containing samples decreased, it was still significantly higher than that of the control. Among them, PSH2 maintained the highest absorbance throughout the visible light region, which may be related to its special intermolecular interactions. This moderate visible light transparency combined with excellent UV-barrier properties makes these anthocyanin packaging films particularly suitable as food packaging materials, preventing food deterioration caused by UV rays while maintaining sufficient transparency.

[0069] like Figure 10 As shown, the tensile strength test results show that the PSH2 membrane exhibits the highest tensile strength value of about 0.19MPa, which is higher than the tensile strength value of PSH1 of 0.15MPa and the tensile strength value of PSH3 of 0.17MPa. In terms of elongation at break, the PSH3 sample performed best at 40%, which is much higher than PSH1 and significantly better than the control groups SH and PSH2, indicating that 15% purple cabbage anthocyanins help to enhance the tensile strength of the material, while 25% anthocyanins significantly improve the ductility of the material.

[0070] Figure 11 The test results showed that the addition of purple cabbage anthocyanins improved the gas barrier properties of sodium alginate / hydroxypropyl methylcellulose packaging film. The water vapor transmission rate (WVTR) decreased with the increase of anthocyanin content, from 187.42 g·m -2 day -1 reduced to 138.65 g·m for PSH3 -2 day -1 The overall decrease was 26%, and the oxygen transmission rate (OTR) test results showed a more significant downward trend, from 245.37cm in SH. 3 ·m -2 day -1 ·atm -1 Down to 138.26cm at PSH3 3 ·m -2 day -1 ·atm -1 , a decrease of 44%, which shows that even at high concentrations, the improvement of anthocyanins on oxygen barrier properties has not reached saturation.

[0071] Figure 12The surface morphology of the packaging films is shown, showing systematic changes with the increase of anthocyanin content. All packaging films show good integrity, without obvious cracks, bubbles or delamination. The SH control group on the far left is milky white and translucent, with uniform surface texture, relatively consistent thickness and regular edge contours. The adjacent PSH1 group shows a slight light yellow-green hue and reduced transparency. Subtle texture changes can be observed on the surface of the film. The introduction of anthocyanins changes the microstructure of the material. Figure 10 It can be seen that the visibility of these markers through the film material is different, which indirectly reflects the difference in light transmittance of the material. With the addition of purple cabbage anthocyanins, the transparency of the packaging film decreases.

[0072]

[0073] The table above shows the color parameters (L*, a*, b*), total color difference (ΔE) and thickness characteristics of four film materials (SH, PSH1, PSH2, PSH3). The brightness values ​​(L*) of all samples are in the approximate range of about 29.99-30.84, indicating that the addition of anthocyanins does not significantly affect the brightness of the materials. However, the a value and b value show obvious differences: compared with the control group SH, the samples containing anthocyanins show stronger green tendencies and stronger yellow tendencies, especially PSH2 and PSH3, which have the most obvious green tendencies a=-1.9, while PSH2 has the strongest yellow tendency b=2.97. These colors The characteristics reflect the presence of anthocyanin in anthocyanin molecules, which appear as a green-yellow hue in a neutral to weakly acidic environment. The total color difference (ΔE) data show that compared with the standard colorimetric plate, PSH1 has the smallest deviation of 0.73, while PSH2 and PSH3 show larger color differences (1.38 and 1.15, respectively). In terms of physical properties, the thickness of the four membrane materials showed certain differences: SH and PSH1 were thicker at 0.097mm, PSH2 was the second thickest at 0.0823mm, and PSH3 was the thinnest at 0.0763mm. This thickness change trend may be related to the fact that the addition of anthocyanins affects the arrangement of polymer chains and the cross-linking density.

[0074] Figure 13 It shows that when the pH is acidic, it appears red, and the stronger the acidity, the more obvious the red. When it tends to be neutral, the color gradually fades to pink. When the solution is neutral, the film shows its original green color. When the alkalinity of the solution gradually increases, the green color gradually weakens until it becomes yellow. It can be seen that the packaging film can respond to changes in pH to a certain extent.

[0075] Depend on Figure 14It was found that the antioxidant activity was significantly positively correlated with the concentration of purple cabbage anthocyanin (PCA). By measuring the DPPH and ABTS radical scavenging rates, it was found that the sodium alginate (SH) film without added purple cabbage anthocyanin (PCA) exhibited a basic antioxidant capacity, with DPPH and ABTS scavenging rates of approximately 45% and 50% respectively. As the concentration of purple cabbage anthocyanin (PCA) increased, the radical scavenging ability gradually improved. Among them, the PSH3 film with 25% PCA content showed the best performance, with DPPH and ABTS scavenging rates reaching approximately 85% and 87% respectively. Statistical analysis showed that there were significant differences between different groups of purple cabbage anthocyanin (PCA) concentrations (p < 0.05), confirming the effective antioxidant contribution of purple cabbage anthocyanin (PCA). In addition, the ABTS scavenging rate in all samples was slightly higher than that of DPPH, indicating that this type of packaging film has different scavenging abilities for different free radical systems.

[0076] In this study, packaging films were successfully prepared using sodium alginate (SA), hydroxypropyl methylcellulose (HPMC), and purple cabbage anthocyanin (PCA). Through scanning electron microscopy, the microstructure of the packaging film was observed, which presented a three-dimensional network structure. Especially, PSH3 presented an interconnected porous structure, which greatly increased the surface area of the composite film and improved the responsiveness of the material. In the optical property test, the PSH film showed higher UV barrier performance than the SH film. The light transmittance of PSH3 at 500 nm was 55%, with the strongest UV barrier performance, making it more suitable for food packaging. In terms of mechanical properties, as the anthocyanin increased, its thickness gradually decreased to 0.070 mm, and the tensile strength also decreased, but PSH3 showed excellent ductility. In the water contact angle experiment, the hydrophobic property of the PSH3 packaging film was greater than that of other SH, PSH1, and PSH2 (SH < PSH1 < PSH2 < PSH3), which also made the material more easily used for the preservation of moisture-containing foods. In this study, by regulating the content of purple cabbage anthocyanin (PCA) and mixing it with a certain proportion of sodium alginate (SA) and hydroxypropyl methylcellulose (HPMC), packaging films with good mechanical properties and antioxidant properties were successfully developed.

[0077] The above is only a preferred embodiment of the present invention, and it does not impose any form of limitation on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or equivalent changes and modifications within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any brief modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A packaging film based on purple cabbage anthocyanins, characterized in that: The packaging film comprises, by weight: Sodium alginate: 5-15 parts; Hydroxypropyl methylcellulose: 10-20 parts; Purple cabbage anthocyanin extract: 0.5-5 parts; Glycerin: 0.1-0.5 parts; Deionized water: 80-100 parts.

2. A method for preparing a packaging film based on purple cabbage anthocyanins, the method being used to prepare the packaging film based on purple cabbage anthocyanins according to claim 1, characterized in that: The method comprises: Extraction of anthocyanins from purple cabbage: Dry and grind the purple cabbage into powder, add ethanol solution, extract by ultrasonication, separate by centrifugation, collect the supernatant, and concentrate under reduced pressure to obtain the anthocyanin extract from purple cabbage; Preparation of membrane-forming solution: Sodium alginate and hydroxypropyl methylcellulose are added to deionized water in proportion, and after they are completely dissolved to form a uniform sol, purple cabbage anthocyanin extract and glycerin are added and stirred continuously to obtain a membrane-forming solution; Degassing and film formation: Pour the film-forming liquid into a glass culture dish after ultrasonic degassing, and dry it to form a wet film; Equilibration treatment: the wet film is taken out and equilibrated in a specific environment for 45-50 hours to obtain a purple cabbage anthocyanin composite packaging film.

3. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the purple cabbage anthocyanin extraction step, the purple cabbage is dried and ground into fine powder, and an ethanol solution with a volume fraction of 50% to 60% is added at a liquid-to-solid ratio of 1:20-30 g / mL.

4. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the purple cabbage anthocyanin extraction step, ultrasonic extraction is performed at 20-30° C. for 30-40 minutes with an ultrasonic power of 180-220 W. After the extraction is completed, the obtained liquid is subjected to high-speed centrifugation.

5. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the membrane-forming solution preparation step, sodium alginate, hydroxypropyl methylcellulose and red cabbage anthocyanin extract are mixed in proportion and dissolved in deionized water, and stirred at 20-30° C. for 4-6 hours until the solution becomes clear.

6. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the film-forming solution preparation step, purple cabbage anthocyanin extract and glycerin accounting for 0.1-0.3% of the total solution content are added as plasticizers, and stirring is continued at 20-30° C. for 0.5-1.5 hours to fully dissolve and mix the components to form a stable film-forming solution system.

7. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the degassing and film-forming steps, the prepared film-forming liquid is subjected to ultrasonic degassing treatment with an ultrasonic power of 180-220 W and a degassing time of 5-10 min. After degassing is completed, the film-forming liquid is poured into glass culture dishes, with 25-35 mL per dish.

8. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the degassing and film-forming steps, the culture dish is placed in an oven at 40-50° C. and dried for 20-28 hours to volatilize the solvent in the film-forming solution and form a wet film.

9. The method for preparing a packaging film based on purple cabbage anthocyanins according to claim 2, characterized in that: In the equilibration step, the dried wet film is taken out of the oven and placed in an environment with a temperature of 25±1° C. and a relative humidity of 50±1% for 45-50 hours of equilibration.

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