Chitosan-pectin polyelectrolyte multilayer active packaging film, its preparation method and application

Abstract

The application discloses a chitosan-pectin polyelectrolyte bilayer active packaging film and a preparation method and application thereof, and the preparation method comprises the following steps: mixing an acetic acid solution of chitosan with natamycin and stirring under light-proof conditions to obtain a natamycin-chitosan acetic acid solution, mixing a water solution of pectin with epigallocatechin gallate and stirring to obtain an epigallocatechin gallate-pectin solution, mixing the epigallocatechin gallate-pectin solution with glycerol to obtain a film-forming solution, and after ultrasonic treatment, the chitosan-pectin polyelectrolyte bilayer active packaging film is prepared by using a layer-by-layer flow casting method. The bilayer active packaging film prepared by the application has excellent ultraviolet shielding performance, mechanical performance, gas barrier and antioxidant performance, can protect the natamycin in the inner layer and thus long-term provide efficient bacteriostatic effect, and can be applied to strawberry preservation.

Description

Technical Field

[0001] The present invention belongs to the technical field of food packaging, and relates to a chitosan-pectin polyelectrolyte double-layer active packaging film, a preparation method and application thereof, and particularly relates to a chitosan-pectin polyelectrolyte double-layer active packaging film loaded with natamycin and epigallocatechin gallate, a preparation method and application thereof. Background Art

[0002] As an important part of the food industry supply chain, food packaging acts as a protective layer or barrier layer for food, protecting it from potential damage and degradation during storage and transportation, thereby extending the shelf life of food. Most traditional packaging is derived from petroleum-based plastics, which are increasingly polluting. In recent years, studies have found that natural, biodegradable and non-toxic biomass polymers (such as polysaccharides, proteins, lipids or mixtures of these substances) can minimize the impact of microplastics on the environment. Among them, coatings and films developed based on polysaccharides have good gas barrier properties and excellent mechanical properties. However, biodegradable packaging materials made of a single substance usually have functional property limitations, so composite films that combine the beneficial properties of different substances will become a better choice. Among the many methods for preparing thin films, layer-by-layer electrostatic self-assembly is an effective means for preparing functional multilayer films due to its advantages, including a wide range of assembly polymer materials and templates, a simple operational process, environmental friendliness, excellent controllability and designability of material structure, and the ability to incorporate a variety of active substances into the film. Compared with single-layer films, multilayer films prepared by self-assembly exhibit lower water vapor permeability, better antibacterial effects, and better freshness stability. Polysaccharide-polyelectrolyte packaging materials such as chitosan and pectin exhibit excellent mechanical properties, gas barrier properties, and anti-fogging properties, but still have shortcomings in terms of UV shielding, antioxidant properties, and antimicrobial properties. Therefore, the preparation of chitosan-pectin-based multi-active packaging materials with active ingredient properties warrants further investigation. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a chitosan-pectin polyelectrolyte double-layer active packaging film that can protect natamycin in the inner layer to exert an antibacterial effect for a long time, has excellent ultraviolet shielding properties, mechanical properties, gas barrier and antioxidant properties, as well as a preparation method and application thereof.

[0004] In order to solve the above technical problems, the present invention adopts the following technical solutions.

[0005] A method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film comprises the following steps:

[0006] (1) mixing a chitosan acetic acid solution with natamycin, and stirring under a dark condition to obtain a natamycin-chitosan acetic acid solution;

[0007] (2) mixing the aqueous solution of pectin and epigallocatechin gallate and stirring to obtain an epigallocatechin gallate-pectin solution;

[0008] (3) mixing the epigallocatechin gallate-pectin solution with glycerol to obtain a film-forming solution;

[0009] (4) The film-forming solution is ultrasonically treated to remove bubbles, and the film-forming solution is first poured into a film-forming mold by a layer-by-layer casting method and dried once to form an epigallocatechin gallate-pectin layer. Then, natamycin-chitosan acetic acid solution is poured on the epigallocatechin gallate-pectin layer. After secondary drying, a chitosan-pectin polyelectrolyte double-layer active packaging film is obtained.

[0010] In the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (2), the mass of the epigallocatechin gallate is 1% to 15% of the mass of the pectin, and the mass fraction of the pectin in the pectin aqueous solution is 2%.

[0011] In the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (1), the mass of the natamycin is 25% of the mass of the chitosan, the mass fraction of chitosan in the chitosan acetic acid solution is 2%, the solvent is acetic acid solution, and the mass fraction of acetic acid in the acetic acid solution is 2%.

[0012] In the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (3), the mass of the glycerol is 25% of the mass of the pectin.

[0013] In the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (1), stirring is performed at 25° C. to 28° C. in the dark for 12 h to 15 h.

[0014] In the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (2), stirring is performed at 25° C. to 28° C. for 12 h to 15 h.

[0015] In the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (4), the power of the ultrasonic treatment is 200W to 220W, and the time of the ultrasonic treatment is 30min to 35min.

[0016] The above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film, preferably, in step (4), the primary drying temperature is 40°C to 45°C, the primary drying time is 12h to 16h, the secondary drying temperature is 40°C to 45°C, and the secondary drying time is 12h to 16h.

[0017] As a general technical concept, the present invention also provides a chitosan-pectin polyelectrolyte double-layer active packaging film prepared by the above-mentioned method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film.

[0018] As a general technical concept, the present invention also provides a use of the chitosan-pectin polyelectrolyte double-layer active packaging film in preserving strawberries.

[0019] In the present invention, the film-forming tool used in the tape casting method is a substrate, typically 10 cm x 10 cm in size. The film-forming process of the tape casting method can be performed by pouring the film-forming solution onto a flat disposable plastic dish (10 cm x 10 cm) and drying the film under a constant temperature and humidity environment, but is not limited thereto.

[0020] Natamycin (also known as natamycin), a natural polyene macrolide, is derived from the fermentation of Streptomyces natalis. It is a natural antifungal agent that significantly inhibits the growth of major postharvest pathogens (Geodispora cinerea and Penicillium) but has no inhibitory effect on bacteria or viruses. Currently, the World Health Organization, the European Food Safety Authority, and the Food and Drug Administration have comprehensively evaluated and designated natamycin as Generally Recognized as Safe. Therefore, natamycin is widely used in the food processing industry as a preservative in foods such as cheese, sausage, yogurt, juice, and wine. Adding natamycin to a chitosan-based active coating can significantly extend the shelf life of strawberries, indicating that natamycin has good application prospects in food packaging films. However, the applicant found that due to the large molecular weight and conjugated double bond structure of natamycin, the solubility of natamycin in water is poor (50 mg / L), and the unsaturated carbon chain of natamycin will undergo a large amount of degradation loss under ultraviolet irradiation in the 200nm to 380nm region, causing natamycin to be rapidly inactivated, thereby affecting its antibacterial efficiency. These problems seriously limit the further application of natamycin in packaging materials. Therefore, the applicant has conducted further research and development and exploration.

[0021] Compared with the prior art, the advantages of the present invention are:

[0022] (1) In view of the disadvantage that natamycin is easily decomposed under ultraviolet light, the present invention uses the spectrum superposition theory, that is, based on the principle of light stability, to add an active substance that absorbs a similar wavelength range to protect another light-unstable compound, thereby shielding ultraviolet radiation that has a destructive effect on natamycin, protecting natamycin in the material and achieving a long-term effective antibacterial effect. In the present invention, epigallocatechin gallate is selected as a protective additive to directly participate in the functional application of the film. Epigallocatechin gallate is the main polyphenol compound present in green tea and is considered to be a potential substitute for synthetic food additives. Epigallocatechin gallate is rich in phenolic hydroxyl groups and shows excellent antioxidant activity in food systems. It can also inhibit the growth of Gram-positive and Gram-negative bacteria that cause food spoilage. In addition, epigallocatechin gallate contains an aromatic hydrocarbon ring structure, which has strong ultraviolet blocking properties. The present invention can be used to shield harmful ultraviolet radiation and protect natamycin in packaging materials through the absorption ability of epigallocatechin gallate in the ultraviolet wavelength range.

[0023] (2) In view of the disadvantage of poor water solubility of natamycin, the present invention adopts a layer-by-layer self-assembly method to prepare a chitosan-pectin polyelectrolyte bilayer membrane loaded with natamycin and epigallocatechin gallate, wherein water-soluble epigallocatechin gallate is added to the pectin at the upper bottom of the bilayer membrane; natamycin is uniformly dissolved in the chitosan acetic acid solution and serves as the lower layer of the bilayer membrane, thereby solving the problem of its poor water solubility. At the same time, the epigallocatechin gallate in the upper layer of the pectin in the double-layer packaging significantly enhances the antioxidant and antibacterial properties of the active packaging. The chitosan-pectin polyelectrolyte bilayer membrane loaded with natamycin and epigallocatechin gallate prepared by the present invention has excellent ultraviolet shielding, mechanical, gas barrier and antioxidant properties, shows good natamycin protection under ultraviolet conditions at different times, and has stable antifungal activity in the inner layer. In addition, the chitosan-pectin polyelectrolyte bilayer membrane loaded with natamycin and epigallocatechin gallate prepared by the present invention has a significant improvement effect in delaying the growth of strawberry microorganisms and quality deterioration, confirming the application potential of this multi-active food packaging system in strawberry preservation.

[0024] (3) The preparation process of the present invention is simple, the process conditions are mild, easy to control and pollution-free.

[0025] (4) The chitosan-pectin polyelectrolyte bilayer membrane loaded with natamycin and epigallocatechin gallate prepared by the present invention has excellent UV shielding, mechanical, gas barrier, antioxidant, and antibacterial properties and can maintain the integrity and quality of food during storage, transportation, and sales. The chitosan-pectin polyelectrolyte bilayer membrane loaded with natamycin and epigallocatechin gallate is biodegradable and can be used as an environmentally friendly bio-based material to reduce the use of petroleum-based materials and the resulting environmental pollution.

[0026] (5) The experiment on preserving post-harvest strawberries showed that the double-layer active film can effectively maintain the hardness of strawberries during storage, reduce the weight loss rate, changes in soluble solids and total acid content, reduce the loss of vitamin C in strawberries, delay the accumulation of anthocyanins and slow down the reddening of strawberries, maintain the appearance quality of strawberries, and thus extend the shelf life of strawberries. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a process flow chart of the preparation method of the chitosan-pectin polyelectrolyte double-layer active packaging film of Examples 1-4 of the present invention.

[0028] Figure 2 This is a diagram showing the light barrier performance of the chitosan-pectin polyelectrolyte double-layer active packaging film prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer film prepared in Comparative Examples 1-2.

[0029] Figure 3 Graph showing the mechanical properties of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0030] Figure 4 These are scanning electron microscope images of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0031] Figure 5 These are atomic force microscope images of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0032] Figure 6 Graph showing the barrier properties of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0033] Figure 7 Graph showing the antioxidant activity of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0034] Figure 8 These are X-ray diffraction patterns of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0035] Figure 9 These are Fourier transform infrared spectra of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0036] Figure 10 These are thermogravimetric analysis graphs of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0037] Figure 11 Photostability performance graphs, UV-visible absorption spectra, HPLC graphs and natamycin quantitative analysis graphs of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0038] Figure 12 This is a graph showing the antibacterial performance of the chitosan-pectin polyelectrolyte double-layer active packaging films prepared in Examples 1-4 of the present invention and the chitosan-pectin polyelectrolyte double-layer films prepared in Comparative Examples 1-2.

[0039] Figure 13 The color difference change graphs of strawberries preserved by the chitosan-pectin polyelectrolyte double-layer active packaging film prepared in Example 4 of the present invention and the chitosan-pectin polyelectrolyte double-layer film prepared in Comparative Examples 1-2, as well as the color difference change graphs of strawberries preserved by the control group without any treatment and the commercial film group.

[0040] Figure 14 The graphs of weight loss rate and hardness change of fresh-keeping strawberries of chitosan-pectin polyelectrolyte double-layer active packaging film prepared in Example 4 of the present invention and chitosan-pectin polyelectrolyte double-layer film prepared in Comparative Examples 1-2, as well as the graphs of weight loss rate and hardness change of fresh-keeping strawberries of control group and commercial film group without any treatment.

[0041] Figure 15 Graphs showing changes in soluble solids and total acid content of strawberries preserved by the chitosan-pectin polyelectrolyte double-layer active packaging film prepared in Example 4 of the present invention and the chitosan-pectin polyelectrolyte double-layer film prepared in Comparative Examples 1-2, as well as graphs showing changes in soluble solids and total acid content of strawberries preserved by the untreated control group and the commercial film group.

[0042] Figure 16Graphs showing changes in vitamin C and anthocyanin content in strawberries preserved using the chitosan-pectin polyelectrolyte double-layer active packaging film prepared in Example 4 of the present invention and the chitosan-pectin polyelectrolyte double-layer film prepared in Comparative Examples 1-2, as well as graphs showing changes in vitamin C and anthocyanin content in strawberries preserved using the untreated control group and the commercial film group. DETAILED DESCRIPTION

[0043] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but the scope of protection of the present invention is not limited thereby. The materials and instruments used in the following examples are all commercially available.

[0044] In the following examples, relevant detection and characterization are as follows:

[0045] (1) Color and opacity: The three color indices of the film, L, a, and b, were determined using a colorimeter. A white standard (L* = 91.84, a* = -0.84, and b* = 2.06) was used. The total color difference (ΔE) and whiteness index (WI) of the film were calculated according to the following formula:

[0046]

[0047]

[0048] Measure the absorbance of the film at 600 nm and calculate the opacity according to the following formula:

[0049]

[0050] Among them, A 600 is the absorbance at 600 nm, and d is the film thickness (mm).

[0051] (2) Thickness and Mechanical Properties: The precise thickness of the film was obtained by calculating the average thickness at five random locations on the film using a digital microscope. The tensile strength and elongation at break of the film were measured using an electronic tensile testing machine. The film was cut into 80 × 10 mm strips and measured in tensile mode with a spacing of 50 mm and a tensile rate of 10.0 mm / min.

[0052] (3) Film microstructure: The surface and cross-sectional morphological characteristics of the film were obtained by scanning electron microscopy. The film sample was fractured in liquid nitrogen and plated with gold for sputtering at an accelerating voltage of 10 kV to observe the surface and cross-section. The micromorphology was obtained at a magnification of 1000 times to enable observation of the surface and cross-section. The surface roughness of the film was characterized by atomic force microscopy. The surface average roughness (R a ) and root mean square roughness (R q ) was calculated using Nanoscope analysis software.

[0053] (4) Barrier properties: The film was sealed in a test container containing 5 g of anhydrous calcium chloride, a deoxidizer, or potassium hydroxide and then placed at a constant temperature (25°C) and humidity (relative humidity: 75%) for 48 hours. The weight changes in the film's water vapor permeability, oxygen permeability, and carbon dioxide permeability before and after weighing were evaluated. The calculation formula is as follows:

[0054]

[0055]

[0056] Where, ΔM is the added weight of the test container (kg), d is the film thickness (m), and A is the effective area of ​​the film (m 2 ), t is the equilibrium time (s), and ΔP is the penetration area of ​​the vapor pressure difference (Pa).

[0057] (5) Film antioxidant capacity: DPPH radical scavenging activity test. Approximately 4, 8, 12, 16, or 20 mg of the composite film was dissolved in 1 mL of deionized water and then mixed with 3 mL of DPPH (1 mM) methanol solution. The mixture was then reacted in a water bath at 37°C. The mixture was centrifuged at 6076 × g for 20 min at room temperature. The absorbance of the sample was obtained at 517 nm using a microplate reader. The DPPH radical scavenging activity of the film was determined using equation (6):

[0058] DPPH clearance rate (%) = [(A0-A1) / A0] × 100% (6)

[0059] Wherein, A0 and A1 are the DPPH absorbance of the control (without membrane) and the membrane, respectively.

[0060] (6) Experimental method for free radical scavenging activity of ABTS. Accurately weigh 7 mM ABTS and 2.45 mM potassium persulfate solution were mixed in a ratio of 1:1 (v / v) for 12 h, and then the solution was diluted with ethanol until an absorbance of 0.70 ± 0.02 was reached at 734 nm. Approximately 4, 8, 12, 16, or 20 mg of the composite film was dissolved in 1 mL of deionized water and then mixed with 3 mL of ABTS solution. The mixture was then reacted in the dark for 30 min, and the mixture was centrifuged at 6076 × g for 20 min at room temperature. The absorbance value of the film was obtained at 734 nm. The ABTS free radical scavenging activity of the film was determined by equation (7):

[0061] ABTS clearance rate (%) = [(A0-A1) / A0] × 100% (7)

[0062] where A0 and A1 are the absorbances of ABTS of the control (without membrane) and membrane, respectively.

[0063] (7) Thermogravimetric analysis, X-ray diffraction, Fourier transform infrared spectroscopy: Thermogravimetric testing was performed using a thermogravimetric analyzer. The sample was heated from 25°C to 600°C under a nitrogen atmosphere. The heating rate was 10°C / min. Derivatized thermogravimetric analysis was performed on different films. The X-ray diffraction of the film samples was analyzed using an X-ray diffractometer at 40 kV and 30 mA voltage and current in the 2θ range of 5° to 45°. The Fourier transform infrared spectrum of the film samples was measured using a spectrometer. Fourier transform infrared spectroscopy was performed at 400 to 4000 cm -1 within the scope.

[0064] (8) Protection ability of the film system for natamycin: Weigh 0.05 g of the film and place it under ultraviolet light for irradiation, ensuring that it is placed on the same horizontal line and parallel to the ultraviolet lamp (253.7 nm, 15 W) at a straight-line distance of 30 cm. Samples are taken after 0, 1, 2, and 3 h of irradiation.

[0065] UV photometer method: A UV-visible spectrophotometer was used to perform full-band scanning in the range of 200-800 nm to characterize the UV-visible spectrum of the chitosan-pectin based active film.

[0066] Liquid chromatography: Add 2 mL of 2% acetic acid solution to a 10 mL centrifuge tube, add 6 mL of methanol, homogenize for 20 minutes, centrifuge at 6076 × g for 15 minutes, aspirate with a 1 mL needle, and filter through a 0.22 μm organic filter membrane. The filtrate is collected and analyzed on a C18 column (4.6 mm × 250 mm, 5 μm). Methanol is used as mobile phase A, and 5% acetic acid aqueous solution is used as mobile phase B. The flow rate is 1.0 mL / min, the detection wavelength is 305 nm, the column temperature is 30°C, and the injection volume is 10 μL. Gradient elution is used. A series of natamycin standard working solutions are injected into the liquid chromatograph, and the corresponding peak areas are measured. A standard curve is plotted with the natamycin concentration in the series of standard working solutions as the horizontal axis and the natamycin peak area as the vertical axis. The pretreated test solution is injected into the liquid chromatograph to obtain the peak area of ​​natamycin in the test solution. The concentration of natamycin in the test solution is then determined based on the standard curve.

[0067] (9) Antibacterial properties of films: Staphylococcus aureus and Escherichia coli represent Gram-positive and Gram-negative bacteria, respectively. The bacterial strains were incubated in nutrient broth for 24 h, and then 0.1 mL of inoculum containing pathogens (1×10 6CFU / mL) are evenly distributed on the nutrient broth agar medium. The prepared film disc (10 mm in diameter) is placed in a culture dish filled with nutrient broth agar medium and cultured at 37 ° C for 24 h. The inhibition diameter is measured using a vernier caliper. The antibacterial performance is characterized using the inhibition zone membrane method. Prepare potato dextrose agar solid culture medium, wait for the culture medium to cool to about 45-50 ° C, use a needle to transfer a small amount of Penicillium expansum and Botrytis cinerea suspension to the surface of the culture medium, and then seal the culture dish containing the inoculated culture medium with a sealing film and incubate at temperature (28 ° C) and humidity (80%). The incubated Penicillium expansum and Botrytis cinerea are eluted and taken out, and the spore suspension (1×10 6 CFU / mL). The obtained spore suspension was added to a potato dextrose agar solid medium and evenly coated for later use. The prepared film was cut into uniform-sized film discs using a 10 mm hole punch and placed in the culture medium (at the intersection of the crosshairs) using sterile tweezers. The control was the comparative example 2 film. After the film disc was tightly fitted to the potato dextrose agar solid medium, the culture medium was cultured at 28 ° C for 1 week. After the culture was completed, the diameter of the inhibition zone was measured using the cross-cross method.

[0068] Based on the evaluation results of the physicochemical and functional properties of chitosan-pectin polyelectrolyte bilayer membranes loaded with natamycin and epigallocatechin gallate, a composite membrane with a mass ratio of epigallocatechin gallate to pectin of 15% in epigallocatechin gallate-pectin solution was selected to carry out a strawberry preservation experiment.

[0069] (10) Color difference measurement: Using a fully automatic colorimeter, the equatorial region of the strawberry was uniformly selected for color measurement experiments. Before measurement, calibration was performed using a standard white plate (L* = 93.48, a* = -0.65, b* = 1.91). The color values ​​of L (brightness), a (red / green), and b (yellow / blue) were used to calculate the fruit color index (CI), hue angle (h), and color saturation (C) using the following equations:

[0070] Color index (CI) = (180-H) / (L+C) (8)

[0071]

[0072] (11) Determination of weight loss rate

[0073] The weight loss of strawberry fruit during storage was determined using the gravimetric method. The weight loss rate was calculated using the following formula:

[0074] Weight loss rate (%) = [(W0-W1) / W0] × 100% (10)

[0075] Where W0 and W1 are the initial weight of strawberry fruit and the weight of strawberry fruit at different storage times, respectively.

[0076] (12) Hardness test: The hardness of strawberry fruit was measured using a texture analyzer. The equatorial region of the strawberry was uniformly selected for puncture test. The probe used was TA-39, the probe penetration depth was 7 mm, and the probe test speed was 0.5 mm / s. The test results were expressed in Newtons (N).

[0077] (13) Determination of soluble solids and total acid content: A handheld refractometer was used to measure the contents. Strawberry juice was squeezed from strawberries in different treatment groups in batches, transferred into 10 ml centrifuge tubes, and centrifuged at 6076 × g and 4°C for 15 min. 20 μl of the supernatant was accurately aspirated for measurement. The soluble solids content was expressed in Brix, and the total acid content was expressed in percentage (%).

[0078] (14) Determination of Vitamin C content: Weigh 0.50 g of strawberry powder into a 10 mL brown volumetric flask, dilute to volume with 20 g / mL metaphosphoric acid solution, shake well, filter with filter paper first, and then filter the extract with a 0.45 μm filter membrane. Mark it and immediately measure it on the instrument. Accurately weigh 10 mg of vitamin C into a 10 mL brown volumetric flask, dilute it with metaphosphoric acid solution to vitamin C mixed standard solutions with mass concentrations of 0.05, 0.10, 0.50, 1.00, 10.00, and 50.0 μg / mL, and filter it with a 0.45 μm filter membrane. Selection of detection wavelength: Inject the standard solution into the liquid chromatograph with a detection wavelength of 245 nm. The mobile phase is 0.05 mol / L phosphoric acid solution: methanol, with a volume ratio of 98:2. The flow rate is 0.70 mL / min., the column temperature is 25 °C, and the injection volume is 10 μL.

[0079] (15) Determination of anthocyanin content: Liquid chromatography was used to determine the anthocyanin (pelargonidin-3-O-glucoside) content in strawberry samples. Weigh 0.50 g of strawberry powder sample into a 10 mL centrifuge tube, add 4.00 mL of methanol-hydrochloric acid extract, mix well, shake on a shaker for 30 minutes, and then centrifuge at 6076 × g for 20 minutes. Pour the supernatant into a 10 mL centrifuge tube with a lid, add 4.00 mL of methanol-hydrochloric acid extract to the centrifuge tube for a second extraction, mix well, shake for 30 minutes, and then centrifuge at 6076 × g for 10 minutes. Pour the supernatant of the second extraction into the same centrifuge tube, combine the two extracts, mix well, and filter through a 0.45 μm organic phase filter for testing.

[0080] Pelargonidin-3-O-glucoside Single Standard Stock Solution: Accurately weigh 0.01 g of pelargonidin-3-O-glucoside standard (accurate to 0.00001 g) and dilute to 100 mL with the extract to obtain a 100 mg / L pelargonidin-3-O-glucoside single standard stock solution. Accurately pipette 0.02 mL, 0.05 mL, 0.20 mL, 0.50 mL, 2.00 mL, and 5.00 mL of the pelargonidin-3-O-glucoside single standard stock solution into six 10 mL volumetric flasks and dilute to volume to obtain pelargonidin-3-O-glucoside standard solutions with concentrations of 0.20 mg / L, 0.50 mg / L, 2.00 mg / L, 5.00 mg / L, 20.00 mg / L, and 50 mg / L, respectively.

[0081] Chromatographic conditions: mobile phase A is an aqueous solution containing 0.5% formic acid, mobile phase B is a methanol solution containing 0.5% formic acid; detection wavelength: 520 nm; column temperature: 30° C.; injection volume: 10 μL.

[0082] Example 1

[0083] A method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film of the present invention, wherein the double-layer active packaging film is specifically a chitosan-pectin polyelectrolyte double-layer active packaging film loaded with natamycin and epigallocatechin gallate, such as Figure 1 As shown, the following steps are included:

[0084] (1) A 2 wt% chitosan acetic acid solution was mixed with natamycin, and the mixture was stirred at 25°C for 12 h in the dark to obtain a natamycin-chitosan acetic acid solution, wherein the mass of natamycin was 25% of the mass of chitosan, and the solvent in the chitosan acetic acid solution was acetic acid solution, and the mass fraction of acetic acid in the acetic acid solution was 2%;

[0085] (2) A 2 wt% aqueous solution of pectin was mixed with epigallocatechin gallate and stirred at 25°C for 12 h to obtain an epigallocatechin gallate-pectin solution, wherein the mass of epigallocatechin gallate was 1% of the mass of pectin;

[0086] (3) mixing the epigallocatechin gallate-pectin solution with glycerol to obtain a film-forming solution, wherein the mass of glycerol is 25% of the mass of pectin;

[0087] (4) The film-forming solution was ultrasonically treated at 200W for 30 minutes to remove bubbles, and a layer-by-layer casting method was used to form a film. The specific process was as follows: the film-forming solution was poured into a film-forming mold and dried at a drying temperature of 45°C for 16 hours until a firm and adherent surface, i.e., an epigallocatechin gallate-pectin layer, was obtained. Then, the natamycin-chitosan acetic acid solution was poured onto the epigallocatechin gallate-pectin layer, and the double-layer film was dried on the mold at a drying temperature of 45°C for 16 hours to obtain a chitosan-pectin polyelectrolyte double-layer membrane loaded with natamycin and epigallocatechin gallate, i.e., a chitosan-pectin polyelectrolyte double-layer active packaging film.

[0088] The chitosan-pectin polyelectrolyte double-layer active packaging film prepared in this example can be used for preserving strawberries.

[0089] Example 2

[0090] A method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film of the present invention, wherein the preparation process is substantially the same as that of Example 1, except that in step (2), the mass of epigallocatechin gallate is 5% of the mass of pectin.

[0091] Example 3

[0092] A method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film of the present invention, wherein the preparation process is substantially the same as that of Example 1, except that in step (2), the mass of epigallocatechin gallate is 10% of the mass of pectin.

[0093] Example 4

[0094] A method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film of the present invention is substantially the same as that of Example 1, except that in step (2), the mass of epigallocatechin gallate is 15% of the mass of pectin.

[0095] Comparative Example 1

[0096] A method for preparing a chitosan-pectin polyelectrolyte double-layer membrane, the preparation process is basically the same as that of Example 1, except that: in step (2), epigallocatechin gallate is not added.

[0097] Comparative Example 2

[0098] A method for preparing a chitosan-pectin polyelectrolyte double-layer membrane, wherein the preparation process is substantially the same as that of Example 1, except that: in step (1), natamycin is not added; and in step (2), epigallocatechin gallate is not added, thereby obtaining a pure chitosan-pectin double-layer membrane.

[0099] Data Analysis:

[0100] (1) Film color and light blocking properties

[0101] As can be seen from Table 1, the composite film is generally smooth and uniform. Compared with the composite film of the comparative example, after the addition of epigallocatechin gallate, the L value of the active film decreases, while the a value and b value gradually increase. The decrease in the L value indicates that the brightness of the active film decreases with increasing epigallocatechin gallate concentration, the change in the a value indicates that the film color tends to be red, and the change in the b value indicates that the film gradually changes from colorless to yellow. This may be due to the film-forming solution of epigallocatechin gallate exhibiting a light yellow color, and the film color gradually darkens when the concentration gradually increases. The opacity of the film was characterized by measuring the absorbance at 600nm of the film. The opacity value of the film of the example increased significantly (p < 0.05), and the visual appearance of the film tended to be blurred. As the concentration of epigallocatechin gallate increased, the opacity of the film of the example gradually increased, and the opacity of the epigallocatechin gallate film-forming solution itself increased, showing a change from light yellow to yellowish brown. In addition, the interaction between epigallocatechin gallate and pectin molecules in the example film is enhanced, resulting in closer contact between the polymers and limiting light transmission.

[0102] Table 1. Color parameters (L, a, b, ΔE), opacity and appearance characteristics of the double-layer films of Examples 1-4 and Comparative Examples 1-2

[0103]

[0104] Note: Different lowercase letters indicate significant differences among different film samples (p<0.05).

[0105] (2) UV shielding performance (light blocking performance)

[0106] Ultraviolet radiation is one of the common factors that cause food oxidative deterioration. As a food packaging material, it is necessary to evaluate the effectiveness of chitosan-pectin polyelectrolyte double-layer active packaging film in blocking ultraviolet radiation. Figure 2It can be seen that compared to the comparative film without the two active substances, the double-layer film of the embodiment has a stronger UV shielding ability. The addition of epigallocatechin gallate significantly reduces the UV transmittance of the active film, particularly within the 200-350nm UV range. The UV-visible transmittance of the films of Examples 2-4 is less than 1%, indicating that the chitosan-pectin polyelectrolyte double-layer active packaging film loaded with epigallocatechin gallate can effectively block UV rays. Natamycin is a powerful polyene macrolide antibiotic, but it is also very sensitive to UV rays and is extremely susceptible to photodegradation. Epigallocatechin gallate is the main polyphenol compound in green tea and exhibits strong UV absorption properties due to the presence of UV-visible light absorbing groups (phenolic hydroxyl groups) in its structure. The present invention uses epigallocatechin gallate as a biodefense substance that absorbs UV-visible light to control the photodegradation process of natamycin and explores the feasibility of developing UV protection properties for active packaging films based on spectral superposition theory.

[0107] (3) Film mechanical properties

[0108] from Figure 3 (1) and Figure 3 (2) It can be seen that the tensile strength of the film of Comparative Example 2 is 44.61 MPa and the elongation at break is 3.21%. After the addition of natamycin, the mechanical properties of the film did not change significantly. This may be because the natamycin is evenly dispersed in the film liquid of the film of Comparative Example 1, thereby forming a uniform and stable internal structure. After the addition of epigallocatechin gallate, the mechanical properties of the film of the examples increased from 49.94 MPa to 71.64 MPa, and the elongation at break varied within 3.12%-3.71%. Among them, the tensile strength of the composite films of Examples 2-4 was significantly enhanced (p < 0.05), while the elongation at break remained unchanged (p > 0.05). Compared with the composite film of Comparative Example 2, the tensile strength of the film of Example 3 increased by 60.59%, and the tensile strength of the film of Example 4 increased by 44.66%. This improvement in mechanical properties is attributed to the polyhydroxy compound characteristics of epigallocatechin gallate. As the concentration of epigallocatechin gallate increases, new intermolecular hydrogen bonds are formed between it and pectin and chitosan, and the interaction force in the matrix polysaccharide chain is enhanced, thereby enhancing the tensile strength of the film. At the same time, the viscosity of the film solution increases with the increase of the concentration of epigallocatechin gallate. The polyphenol compound and the components in the film produce a synergistic effect, which can also improve the density and mechanical properties of the film.

[0109] (4) Film microstructure

[0110] from Figure 4Scanning electron microscope images of the cross-section of the polyelectrolyte bilayer membrane reveal a dense, continuous, and unbroken structure, with the upper layer being a pectin layer and the lower layer being a chitosan layer, forming a dense and uniform bilayer structure between the two substrates. Compared to the film of Comparative Example 2, the chitosan layer of the film of Comparative Example 1 contained a small amount of granular material, which is attributed to the low solubility of natamycin. With increasing amounts of epigallocatechin gallate added, the pectin layer gradually developed a uniform, dense, striated structure, which was most pronounced in the film of Example 4. This is related to the interaction between epigallocatechin gallate and the pectin matrix. Furthermore, hydrogen bonds exist between the epigallocatechin gallate in the pectin film-forming solution and the chitosan layer, forming an ordered matrix that makes the distribution of interaction points between the layers more uniform, thereby making the arrangement structure between the film molecules more dense.

[0111] from Figure 5 It can be seen that the R a The values ​​are 4.84, 6.57nm, R q The values ​​are 6.19 and 8.51 nm, while the R values ​​of the pectin side of the membranes of Examples 1-4 are a The value is 8.79-10.70nm, R q The values ​​are 11.30-17.10 nm, which indicates that the incorporated epigallocatechin gallate increases the surface roughness of the film. In addition, with the increase of epigallocatechin gallate concentration, the film surface tends to be smoother, which basically meets the stability properties required for food packaging industry applications.

[0112] (5) Film water vapor permeability, oxygen permeability and carbon dioxide permeability

[0113] from Figure 6 (1) It can be seen that compared with the comparative example, the water vapor permeability of the composite film of Example 2-4 is from 0.74×10 -13 kg·m -1 ·s -1 ·Pa -1 Significantly reduced to 0.69×10 -13 kg·m -1 ·s -1 ·Pa -1 (p < 0.05), indicating that high concentrations of epigallocatechin gallate enhance the water vapor barrier properties of the active film. Water vapor permeability depends on the hydrophilic and hydrophobic properties of the film and the added bioactive compound. Intermolecular forces are a key factor influencing the water vapor permeability of the film. Epigallocatechin gallate contains abundant polar groups (-OH), which interact with the film's hydrophilic matrix, causing a competitive binding effect and increasing the mass transfer resistance of water molecules in the matrix. Consequently, the example film has a lower water vapor permeability.

[0114] from Figure 6 (2) and Figure 6 (3) It can be seen that the oxygen permeability and carbon dioxide permeability of the film of comparative example 1 increased, showing weak oxygen and carbon dioxide barrier properties, which is due to the insolubility and easy aggregation of natamycin particles. The oxygen permeability and carbon dioxide permeability values ​​of the example films decreased significantly (p < 0.05), among which Example 4 had the lowest carbon dioxide permeability and Example 2 had the lowest oxygen permeability. This may be that epigallocatechin gallate forms intermolecular bonds at the interface between the inner layers of the film, filling the free space between the film matrix, and the more tortuous transmission path hinders the flow of carbon dioxide; secondly, epigallocatechin gallate provides rich polar groups inside the film, making it more difficult for non-polar carbon dioxide and oxygen molecules to diffuse through the matrix, thereby giving the film lower carbon dioxide and oxygen permeability; in addition, from Figure 6 (4) It can be seen that the example film has a higher carbon dioxide gas exchange rate, which is beneficial to inhibiting the respiration of fruits and vegetables.

[0115] (6) Membrane antioxidant capacity

[0116] from Figure 7 (1) and Figure 7 (2) It can be seen that the DPPH free radical scavenging rates of the 5 mg / L films of Comparative Example 1 and Comparative Example 2 were 7.33% and 7.21%, respectively, which is attributed to the -OH and -NH2 present on the polysaccharide chains in the chitosan and pectin structures. The antioxidant activity of the films of the examples showed a dependence on the amount of epigallocatechin gallate additive, and the free radical scavenging rate increased accordingly with the increase of the film equivalent. Among them, when the content of the film of Example 4 was only 2.0 mg / mL, the DPPH free radical scavenging rate was as high as 87.29%; when the content of the film of Example 4 was only 3.0 mg / mL, the ABTS free radical scavenging rate was as high as 92.77%, showing high antioxidant activity. Epigallocatechin gallate contains a large amount of free -OH, which can act as an excellent electron donor to participate in free radical reactions and thus effectively scavenge free radicals. Therefore, chitosan-pectin active films containing epigallocatechin gallate are expected to be used to delay the oxidative deterioration of food.

[0117] (7) Thin film X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis

[0118] from Figure 8It can be seen that the films all have a broad amorphous peak near 2θ≈20°, with a broad amorphous region, exhibiting a typical semi-crystalline structure. The pectin layer does not have an independent crystalline structure, while the chitosan layer structure exhibits a semi-crystalline state. Both Comparative Example 1 and the Example films exhibit three new diffraction peaks of relatively low intensity at 7°-23°, which is related to the production of high crystals with their own crystal morphology by natamycin. Compared to the Comparative Example 1 film, the Example film exhibits a slight shift in the diffraction peak near 20°, but no new diffraction peaks were detected, indicating good mixed solubility and molecular interaction between epigallocatechin gallate and the film-forming substrate.

[0119] from Figure 9 It can be seen that between 3200 and 3500 cm -1 The peaks observed between 2934 cm and 2964 cm are the stretching vibrations of the NH bonds in chitosan and the OH bonds in the polymer matrix, which are related to the polar interactions between the components. -1 The band at 1738 cm corresponds to the CH stretching vibration; -1 The characteristic band at 1633 cm is related to the C=O vibration of esterified carboxyl (-COOR) and free carboxyl (-COOH) in pectin; -1 The bands appearing at 1150-900 cm are the stretching vibrations of the C=O bond in chitosan and the asymmetric stretching vibrations of COO- in pectin; -1 The absorption peak at 3334 cm is the COC stretching vibration of chitosan and pectin on the glycosidic bond. Compared with the comparative film, there are differences in the position of the absorption peaks between the active film bands of the example, but no new peaks appear, indicating that the addition of epigallocatechin gallate does not change the structure of the film matrix. However, the center of the OH and NH vibration bands has shifted from 3334 cm to 3334 cm. -1 To 3323cm -1 The low-wavelength band shifts (red-shifts) and the absorption peak flattens. This is likely due to intermolecular hydrogen bonding between the -OH and -NH2 groups in the film-forming matrix and the -OH groups in epigallocatechin gallate, resulting in the loss of stretching vibrations of the OH and NH groups in the film. Therefore, these changes in energy bands indicate that epigallocatechin gallate is embedded in the chitosan-pectin matrix through intermolecular hydrogen bonding interactions.

[0120] from Figure 10(1) It can be seen that the temperature-weight loss rate curve and its first-order derivative curve results both show multi-step decomposition of the film, which has undergone three mass loss stages: the mass loss in stage I (50℃-150℃) is related to the continuous evaporation of acetic acid and water, which may also be accompanied by the destruction of intermolecular hydrogen bonds; the mass loss in stage II (150℃-250℃) corresponds to the evaporation of glycerol and tightly bound water; stage III (250℃-450℃) is related to the decomposition of the film matrix polymer chain. Within the test temperature range, the temperature-weight loss rate curve and its first-order derivative curve of the film of Example 4 are almost higher than those of all other films, which may be related to the interaction between epigallocatechin gallate and the film matrix. From Figure 10 (2) It can be seen that the film has the maximum mass loss rate in the temperature range of 226℃ to 228℃, and the film of Example 4 has the lowest loss rate, indicating that the addition of epigallocatechin gallate enhances the thermal stability of the active film. This may be due to the intermolecular interaction increasing the steric hindrance between the polymer chains, thereby limiting the decomposition movement of the biopolymer chains. Overall, the thermal stability of the film of the example is improved, and it has good application potential in the field of food packaging in hot environments.

[0121] (8) Photostability of Natamycin in Films

[0122] from Figure 11 (1) It can be seen that natamycin is photosensitivity and is easily decomposed under ultraviolet light conditions. The film of Comparative Example 1 has absorption peaks at 291.4, 304.4 and 319.4 nm, which correspond to the three characteristic absorption peaks of natamycin. As the ultraviolet light irradiation time increases, the peak height of the characteristic peak of natamycin gradually decreases, which indicates that a large amount of natamycin in the film is lost through the photodegradation process. Figure 11 (2) It can be seen that in the film of Example 4 prepared based on the spectrum superposition theory, natamycin showed a better retention rate. The films after different illumination times all showed characteristic absorption peaks near 319nm. Compared with 0h of irradiation under ultraviolet light, the peak intensity of the characteristic peak of the film of Example 4 after 3h did not change significantly, which indicates that the natamycin in the film did not enter the photodegradation process. In addition, Figure 11 (3) and Figure 11(4) It can be seen that under the same chromatographic conditions, the corresponding characteristic absorption peaks appeared in the films of Comparative Example 1 and Example 4 at an elution time of 8.0 min, while the absorption peak of the photodegradation product of natamycin appeared at an elution time of 6.1 min. In the film of Comparative Example 1, the characteristic absorption peak of natamycin showed obvious attenuation after 3 hours of ultraviolet light irradiation, and the absorption peak of the photodegradation product also showed corresponding accumulation. During the photodegradation process, natamycin can produce products containing tetraene chromophore structures. This irregular absorption peak may be a mixture of several different degradation products. In the film of Example 4, the peak time and peak area of ​​natamycin were not affected, and the absorption peak of the photodegradation product did not show obvious accumulation, indicating that natamycin did not undergo a photodecomposition process and did not generate photodegradation products. Figure 11 (5) It can be seen that the content of natamycin in the film in its original state is 5.00-5.21 mg / g. After 1 hour of ultraviolet light irradiation, the loss rate of natamycin in the films of Comparative Example 1 and Example 4 was 44.00% and 0.57%, respectively. After 3 hours of exposure to light, the retention rate of natamycin in the films of Comparative Example 1 and Example 4 was 22.00% and 95.78%, respectively, indicating that the ultraviolet light stability of natamycin in the film has been significantly improved. The epigallocatechin gallate in the pectin layer has strong ultraviolet absorption due to its polyphenolic compound containing an aromatic hydrocarbon ring structure. Therefore, the outer layer of the double-layer film designed by spectral superposition theory has barrier properties, thereby preventing the photodegradation of natamycin.

[0123] (9) Film antibacterial properties

[0124] from Figure 12 (1) It can be seen that the film of Comparative Example 2 showed weak antibacterial activity against the two bacteria; the diameter of the inhibition zone of the film of Comparative Example 1 did not change significantly (p>0.05), and no clear area could be formed around the film, indicating that natamycin had no antibacterial effect on the test bacteria. However, the film of Example 2 had a significant inhibitory effect on Staphylococcus aureus and Escherichia coli, and a clear inhibition zone was formed around the film. As the concentration of epigallocatechin gallate increased, the diameter of the inhibition zone gradually increased. Figure 12 (2) and Figure 12 (3) It can be seen that the inhibition zone of the film of Example 4 against Staphylococcus aureus and Escherichia coli was significantly higher than that of the film of the comparative example (p < 0.05). This is because the -OH group in epigallocatechin gallate interferes with the protein or peptide structure in the bacterial cell wall, thereby hindering the synthesis of the cell wall and cell membrane. Therefore, the chitosan-pectin double-layer active packaging film containing epigallocatechin gallate has good application prospects as a new antibacterial packaging material.

[0125] from Figure 12(4) It can be seen that after different periods of ultraviolet irradiation, the film of Comparative Example 1 showed no antifungal activity, while the films of Comparative Example 1 and Example 4 showed excellent antifungal activity. Figure 12 (5) It can be seen that in the initial state without irradiation, the inhibition diameter of the film of Example 4 against Penicillium expansum is 25.57 mm, and the inhibition diameter against Botrytis cinerea is 24.29 mm, and the antibacterial efficiency is significantly higher than that of the film of Comparative Example 2. The antibacterial effect of natamycin is due to the specific binding of its polyene structure to fungal ergosterol, thereby changing the permeability of the fungal plasma membrane and causing fungal inactivation. After irradiation with ultraviolet light for different lengths, the range of the inhibition zone of the film of Example 4 was reduced to a certain extent. However, after irradiation for 3 hours, the diameter of the inhibition zone of the film against Penicillium expansum and Botrytis cinerea was only reduced by 5.76% and 11.82%, respectively. This shows that although natamycin is partially degraded, the natamycin in the film of Example 4 has better photostability and can exert antibacterial activity under ultraviolet radiation conditions, which is consistent with the research results of ultraviolet-visible absorption spectrum and liquid chromatography. Therefore, the film prepared by the present invention uses spectral superposition theory to design a barrier in the outer layer, protecting natamycin in the inner layer, thereby providing a long-term and efficient antibacterial effect.

[0126] (10) Appearance and color changes of strawberries during storage

[0127] from Figure 13 (1) It can be seen that the strawberry samples showed a certain degree of rot and browning as the storage time increased. The control group strawberries that were not packaged showed rot on the 8th day of storage. Although the strawberries treated with commercial films did not show rot, their maturity was significantly higher than that of other components on the 2nd day of storage. The strawberries treated with the films of Comparative Example 1 and Example 4 remained fresh after 8 days of storage, without fungal infection, and the moisture was able to be exchanged with the external environment in a timely manner, and the internal microenvironment humidity was low. Figure 13 (2) Figure 13 (3) It can be seen that the lightness and saturation of the strawberries showed a gradual downward trend, while the color index of the strawberry samples in the untreated control group and the commercial film group decreased rapidly (p < 0.05). The unpackaged control group strawberry samples lost a lot of water due to direct contact with the outside world, while the strawberry samples treated with the commercial film showed a higher degree of rot and browning. This is because the commercial film is impermeable to water and gas, resulting in the inability of water to diffuse, and the internal environment humidity is high, resulting in corruption caused by fungi. The membrane material of Example 4 has good water and gas permeability, and is not affected by fungal infection, maintaining the good appearance and color state of the strawberry samples.

[0128] (11) Weight loss and firmness changes of strawberries during storage

[0129] from Figure 14 (1) It can be seen that the strawberry samples lose weight continuously as the storage time increases. The unpackaged control group strawberry samples show the greatest weight loss (p<0.05). The weight loss of the strawberry samples treated with commercial films has always been at a low level, significantly lower than that of the other treatment groups (p<0.05). This is related to the characteristics of commercial films, which are not permeable to water vapor and gas, and moisture cannot diffuse outward, thereby inhibiting the evaporation and respiration of strawberries. However, the film of the embodiment significantly delays the weight loss of strawberries. In addition, Figure 14 (2) It can be seen that the hardness values ​​of the samples gradually decreased. Among them, on the second and fourth days of storage, the hardness of the strawberry samples treated with the film of Example 4 was always significantly higher than that of the other treatment groups.

[0130] (12) Changes in soluble solids and total acid content of strawberries during storage

[0131] from Figure 15 (1) It can be seen that as the storage time increases, the soluble solids values ​​of the strawberry samples in the control group show an increasing trend, and the nutrients in the fruit gradually decompose into monosaccharides and disaccharides; in the later stage of storage time, the soluble solids of the strawberries in the control group are significantly higher than those in the other groups. This is because the strawberries have a large amount of water loss, which leads to a continuous increase in the soluble solids value. The soluble solids values ​​of the samples in the membrane-treated group of Example 4 are lower than those of the samples in the other groups and are more stable. The active membrane reduces the respiration rate of the strawberries by limiting the exchange of gas and oxygen, thereby preventing drastic changes in soluble solids. Figure 15 (2) It can be seen that the total acid content of strawberries in the control group and the commercial film group increased significantly from the 4th to the 8th day of storage. The control group produced a large amount of organic acid due to stress, while the strawberries in the commercial film group produced a large amount of ethylene in the closed environment and could not penetrate. The high concentration of ethylene promoted the energy metabolism activities such as respiration of the strawberry samples, accelerated the decomposition of nutrients to produce monosaccharides and disaccharides. In contrast, the total acid content of the film of Example 4 changed less and remained at a low total acid content in the later stage of storage. This is because the active film has excellent barrier to external stress and good gas permeability, which facilitates the faster release of ripening agents such as ethylene into the microenvironment, thereby inhibiting the loss of nutrients in the strawberries.

[0132] (13) Changes in Vitamin C and Anthocyanin Content in Strawberries During Storage

[0133] from Figure 16As can be seen from (1), during the storage period, the vitamin C content of each group of strawberry samples showed a downward trend. Compared with the strawberry samples of the control group and the commercial film group, the vitamin C content of the strawberries after being treated with the film of Example 4 showed a slowly decreasing trend. The film of Example 4 effectively blocked the external stress and had good gas permeability. Therefore, the vitamin C content in the strawberries showed a slowly decreasing trend. Figure 16 As can be seen in (2), the anthocyanin content shows a continuous upward trend with the extension of storage time. After storage for 6 days, the vitamin C value of the strawberry samples in the control group and the commercial film group dropped sharply, and it was unable to effectively control the synthesis, accumulation and decomposition of anthocyanins. However, the film of Example 4 can maintain a steady increase in anthocyanin content, indicating that the use of polysaccharide active packaging can slow down the respiration and ripening process in strawberries.

[0134] The above description is only a preferred embodiment of the present invention and does not constitute any formal limitation to the present invention. Although the present invention has been disclosed as above in terms of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can, without departing from the spirit and technical solution of the present invention, use the methods and technical contents disclosed above to make many possible changes and modifications to the technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention, still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film, characterized in that: The following steps are involved: (1) mixing a chitosan acetic acid solution with natamycin, and stirring under a dark condition to obtain a natamycin-chitosan acetic acid solution; (2) mixing the aqueous solution of pectin and epigallocatechin gallate and stirring to obtain an epigallocatechin gallate-pectin solution; (3) mixing the epigallocatechin gallate-pectin solution with glycerol to obtain a film-forming solution; (4) The film-forming solution is ultrasonically treated to remove bubbles, and the film-forming solution is first poured into a film-forming mold by a layer-by-layer casting method and dried once to form an epigallocatechin gallate-pectin layer. Then, natamycin-chitosan acetic acid solution is poured on the epigallocatechin gallate-pectin layer. After secondary drying, a chitosan-pectin polyelectrolyte double-layer active packaging film is obtained.

2. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to claim 1, characterized in that: In step (2), the mass of the epigallocatechin gallate is 1% to 15% of the mass of the pectin, and the mass fraction of the pectin in the pectin aqueous solution is 2%.

3. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to claim 1, characterized in that: In step (1), the mass of the natamycin is 25% of the mass of the chitosan, the mass fraction of the chitosan in the acetic acid solution of the chitosan is 2%, the solvent is acetic acid solution, and the mass fraction of acetic acid in the acetic acid solution is 2%.

4. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to any one of claims 1 to 3, characterized in that: In step (3), the mass of the glycerol is 25% of the mass of the pectin.

5. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to any one of claims 1 to 3, characterized in that: In step (1), the mixture is stirred at 25° C. to 28° C. in the dark for 12 h to 15 h.

6. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to any one of claims 1 to 3, characterized in that: In step (2), the mixture is stirred at 25°C to 28°C for 12 to 15 hours.

7. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to any one of claims 1 to 3, characterized in that: In step (4), the power of the ultrasonic treatment is 200W to 220W, and the time of the ultrasonic treatment is 30min to 35min.

8. The method for preparing the chitosan-pectin polyelectrolyte double-layer active packaging film according to any one of claims 1 to 3, characterized in that: In step (4), the temperature of the primary drying is 40°C to 45°C, the time of the primary drying is 12h to 16h, the temperature of the secondary drying is 40°C to 45°C, and the time of the secondary drying is 12h to 16h.

9. A chitosan-pectin polyelectrolyte double-layer active packaging film prepared by the method for preparing a chitosan-pectin polyelectrolyte double-layer active packaging film according to any one of claims 1 to 8.

10. Use of the chitosan-pectin polyelectrolyte double-layer active packaging film according to claim 9 in preserving strawberries.

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