Algin composite film and preparation method and application thereof

Abstract

The invention discloses an algin composite membrane which is prepared by the following steps: adding 0.8-1.2 g of algin, 0.8-1.2 g of hydroxypropyl methyl cellulose, 0.8-1.2 mL of glycerol, 0.4-0.6 g of titanium dioxide and 0.8-1.2 g of polylysine into water, uniformly mixing, fixing the volume, putting into a container, drying, immersing into a CaCl2 solution for crosslinking treatment, and drying, thereby obtaining the algin composite membrane. The algin composite film can be used as or used for preparing a food preservative film. The algin composite film disclosed by the invention realizes good balance between strength and ductility, has good antioxidant capacity and remarkable antibacterial effect, and can effectively enhance the antioxidant defense capacity of the cape gooseberries, reduce oxidative damage, prolong the storage period and improve the fresh-keeping effect of the cape gooseberries. According to the method, the economic value of the sargassum is favorably improved, and powerful support is provided for sustainable development of the food industry.

Description

Technical Field

[0001] The invention relates to an alginate composite film and a preparation method and application thereof, belonging to the technical field of food preservation. Background Art

[0002] Traditional food wraps are made from plastic materials like polyethylene, which are difficult to degrade, pollute the environment, and are prone to bacterial growth, impacting food quality and safety. Therefore, the development of safe, environmentally friendly, non-toxic, and antibacterial food wraps is urgent. Seaweed is rich in various polysaccharides, which can form gels into thin films with good flexibility and biodegradability, making them promising candidates for food wraps.

[0003] Algin (Sodium alginate, SA), also known as sodium alginate, is a type of polysaccharide widely found in various brown algae. It has good biodegradability, biocompatibility and gel properties. Algin is widely found in Sargassum ( Sargassum ), in the cell walls of brown algae such as giant kelp, kelp, kombu, carrageenan, and fucus, but the traditional alginate extraction method has high energy consumption and easily causes environmental pollution. It is necessary to develop a green and efficient extraction method. Summary of the Invention

[0004] In view of the above-mentioned prior art, the present invention provides an alginate composite film, a preparation method thereof, and an application thereof as a food preservative film.

[0005] The present invention is achieved through the following technical solutions: A method for preparing an alginate composite film is as follows: adding 0.8-1.2 g of alginate, 0.8-1.2 g of hydroxypropyl methylcellulose (HPMC), 0.8-1.2 mL of glycerol, 0.4-0.6 g of titanium dioxide (TiO2), and 0.8-1.2 g of polylysine (ε-PL) to water, mixing the mixture, and adjusting the volume to 100 mL to obtain a film-forming solution; placing the film-forming solution in a container so that the liquid level is 4.3-4.7 mm, drying the solution, immersing the solution in a CaCl2 solution for cross-linking treatment, removing the solution, and drying the solution to obtain an alginate composite film having a thickness of approximately 0.03 mm.

[0006] Preferably, the dosage of each component is: 1 g of alginate, 1 g of hydroxypropyl methylcellulose, 1 mL of glycerol, 0.5 g of titanium dioxide, and 1 g of polylysine.

[0007] Furthermore, the titanium dioxide is added in the form of a solution.

[0008] Furthermore, the polylysine is added in the form of a solution.

[0009] Preferably, the liquid level height is 4.5 mm.

[0010] Furthermore, the concentration of the CaCl2 solution is 2% (w / v, g / mL), and the cross-linking treatment time is 15 min.

[0011] Furthermore, the alginate is extracted by the following method: (1) Take dried Sargassum and crush it to obtain Sargassum powder; (2) Adding Sargassum powder to citric acid-disodium hydrogen phosphate buffer, adding cellulase, pectinase and xylanase, and performing enzymolysis to obtain an enzymatic hydrolyzate; (3) Centrifuge the enzymatic hydrolyzate to obtain a supernatant and algal residue; add sodium chloride solution to the algal residue, digest it, and centrifuge it to obtain a supernatant; combine the two supernatants and adjust the pH to 6.0-7.0 to obtain an enzyme extract; (4) Add calcium chloride to the enzyme extract and let it stand to form calcium alginate gel; wash with deionized water; decalcify with hydrochloric acid; (5) Place the decalcified gel in a sodium hydroxide solution to obtain a sodium alginate solution; add anhydrous ethanol, let it stand to obtain a precipitate, and dry it to obtain alginate.

[0012] Furthermore, in step (2), the material-liquid ratio of Sargassum powder to citric acid-disodium hydrogen phosphate buffer is 1:20, g:mL.

[0013] Furthermore, in step (2), the amount of cellulase added is 4% (w / w), the amount of pectinase added is 3% (w / w), and the amount of xylanase added is 2% (w / w).

[0014] Furthermore, in step (2), the enzymatic hydrolysis conditions are 50°C, 220 r / min water bath shaker, and the enzymatic hydrolysis time is 3 hours.

[0015] Furthermore, in step (3), the centrifugal conditions are: 8000 r / min, 15 min.

[0016] Furthermore, in step (3), the concentration of the sodium carbonate solution is 3% (w / w), and the amount of sodium carbonate solution added is 15 mL / 1 g of Sargassum powder.

[0017] Furthermore, in step (3), the digestion conditions are: 60°C, 3 h.

[0018] Furthermore, in step (4), the concentration of calcium chloride is 10% (w / w).

[0019] Furthermore, in step (4), the specific method of decalcification with hydrochloric acid is: first decalcify with 1 M hydrochloric acid for 2 to 4 hours, and then decalcify again with 0.5 M hydrochloric acid for 3 hours.

[0020] Furthermore, in step (5), the concentration of the sodium hydroxide solution is 0.4 M.

[0021] The alginate composite film prepared by the method is used as a food preservative film, in the preparation of food preservative film, and in food preservation.

[0022] Furthermore, the food is cape gooseberry.

[0023] The alginate composite film of the present invention is prepared using alginate, glycerin, hydroxypropyl methylcellulose, titanium dioxide and polylysine as raw materials. Experimental studies have shown that the alginate composite film achieves a good balance between strength and ductility, has a high oxygen permeability, has good antioxidant capacity, and has a significant antibacterial effect (against Staphylococcus aureus and Escherichia coli). It can effectively enhance the antioxidant defense ability of the cape gooseberry, reduce oxidative damage, extend the storage period and improve the preservation effect of the cape gooseberry.

[0024] The present invention prepares alginate films and optimizes their properties to promote their application in higher value-added fields. It also optimizes the process for extracting alginate from Sargassum, improving extraction efficiency and purity. This environmentally friendly process can reduce environmental issues caused by chemical reagents and alleviate environmental pressure. This invention not only helps increase the economic value of Sargassum, but also provides a new green alternative material for food packaging, effectively reducing environmental pollution caused by plastic packaging, and its application in food preservation can better maintain food quality and thus reduce food waste, providing strong support for the sustainable development of the food industry.

[0025] Various terms and phrases used herein have the general meanings that are well known to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 : Determination results of the rheological properties of alginate, where: a: Effect of shear rate on viscosity and shear stress, the black curve represents the apparent viscosity, and the red curve represents the shear stress; b: Effect of temperature on viscosity; c: Linear viscoelastic range; d: Changes in G' and G" under frequency changes.

[0027] Figure 2 : The test results of tensile strength and elongation at break of composite films, wherein the letter marking method is used to indicate the significance of the difference.

[0028] Figure 3: Oxygen transmission rate measurement results of composite films, wherein the letter marking method is used to indicate the significance of the difference.

[0029] Figure 4 : Infrared spectrum of the composite film.

[0030] Figure 5 : AFM images, three-dimensional valley morphology images and root mean square roughness values ​​of the composite films, where, from top to bottom, they are the results of SA film, SA+HPMC composite film, SA+HPMC+TiO2 composite film, and SA+HPMC+TiO2+ε-PL composite film; the left, middle and right columns are the AFM images, three-dimensional valley morphology images and root mean square roughness values, respectively.

[0031] Figure 6 : DPPH free radical scavenging activity test results of composite membranes, where letters are used to indicate the significance of the differences Figure 7 : Antibacterial photos of the composite membrane against Staphylococcus aureus and Escherichia coli, where the left picture is the antibacterial photo of Staphylococcus aureus and the right picture is the antibacterial photo of Escherichia coli.

[0032] Figure 8 : Determination results of weight loss rate of Cape gooseberry after composite film treatment.

[0033] Figure 9 : Determination results of vitamin C content in Cape gooseberry after composite film treatment. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the embodiments. However, the scope of the present invention is not limited to the following embodiments. Those skilled in the art will appreciate that various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention.

[0035] Unless otherwise specified, the instruments, reagents, and materials used in the following examples are all conventional instruments, reagents, and materials available in the prior art and can be obtained through regular commercial channels. The experimental methods and detection methods used in the following examples are all conventional experimental methods and detection methods available in the prior art, unless otherwise specified.

[0036] Example 1 Extraction of alginate from Sargassum Here are the steps: (1) Take dried Sargassum, crush it with a grinder, and sieve it through a 60-mesh sieve; place it in an oven and dry it at 60°C to obtain Sargassum powder for use in subsequent experiments.

[0037] (2) Weigh 1.00 g of Sargassum powder and mix it with 20 mL of citric acid-sodium hydrogen phosphate buffer (pH 5.5) at a solid-liquid ratio of 1:20 (w / v, g / mL). Then, add cellulase, pectinase, and xylanase. The amount of cellulase added was 4% (w / w, referring to the mass ratio of enzyme to Sargassum powder, the same below), the amount of pectinase added was 3% (w / w), and the amount of xylanase added was 2% (w / w). The mixture was enzymolyzed in a shaking water bath at 50°C and 220 r / min for 3 h. The enzymes were then inactivated by boiling in a water bath for 15 min.

[0038] (3) The enzymatic hydrolysate was centrifuged at 8000 r / min for 15 min to obtain the supernatant and algal residue; 15 mL of 3% (w / v, g / mL) sodium carbonate solution was added to the algal residue, digested at 60°C for 3 h, and centrifuged at 8000 r / min for 15 min to obtain the supernatant; the two supernatants were combined, the volume was adjusted to 120 mL with water, and the pH was adjusted to 6.5 with hydrochloric acid to obtain the enzyme extract.

[0039] (4) Add calcium chloride to the enzyme extract to a concentration of 10% (w / v, g / mL) and let it stand to form a calcium alginate gel; wash the gel with deionized water to remove salt ions; decalcify with an equal volume of 1 M hydrochloric acid for 3 h, and then decalcify again with twice the volume of 0.5 M hydrochloric acid for 3 h; (5) Place the decalcified gel in a 0.4 M sodium hydroxide solution until the pH is equal to 7.0 to obtain a sodium alginate solution; add three times the volume of anhydrous ethanol, let it stand to obtain a precipitate, and dry it at 60°C to a constant weight to obtain alginate.

[0040] The physicochemical properties of the alginate prepared above were measured (commercially available alginate was used as a control). The total sugar content was determined using the sulfuric acid-phenol method, and the uronic acid content was determined using the carbazole sulfate method. The results of the physicochemical properties of the alginate are shown in Table 1. As shown in Table 1, the alginate extracted using the method of the present invention has a high uronic acid content, significantly superior to commercially available alginate. The gel strength reached 957.78 g, nearly nine times that of commercially available alginate. It also significantly surpassed commercially available alginate in terms of hardness, chewiness, and cohesiveness.

[0041]

[0042] The rheological properties of the alginate prepared above were measured by using a rotational rheometer with parallel plate fixtures (25 mm in diameter and 1 mm in gap) to measure the rheological properties of the alginate (dissolved in deionized water to a concentration of 2%). The rheological properties of the alginate were measured at 25°C and 0.01 to 1000 s -1 The changes of apparent viscosity and shear stress within the shear rate range are shown in Figure 2. At a fixed shear rate of 100 s -1The viscosity changes in the temperature range of 25-100°C and the linear viscoelastic range in the strain range of 0.01% to 100% were investigated. At the same time, the frequency dependence of the storage modulus (G') and loss modulus (G") was analyzed by frequency sweep.

[0043] The results of the rheological properties of alginate are as follows Figure 1 As shown. Figure 1 It can be seen from Figure 1 that the alginate extracted by the present invention exhibits typical shear thinning pseudoplastic fluid characteristics at higher shear rates, that is, the viscosity decreases significantly with increasing shear rate, and the shear stress shows a nonlinear increasing trend; this pseudoplastic behavior indicates that the molecular chain structure disentangles and flows under high shear force, resulting in the need for higher shear stress to maintain flow. Figure 1 As can be seen from Figure b, the viscosity of the alginate extracted by the present invention decreases significantly with increasing temperature, showing high heat sensitivity. This may be because the increase in temperature weakens the hydrogen bonds and van der Waals forces between molecules, enhancing the mobility of the polysaccharide chains and resulting in a decrease in viscosity. In the range of 50-100°C, the viscosity decreases faster, indicating that the stability of its molecular structure decreases under high temperature conditions. Figure 1 As shown in Figure 1c, the alginate extracted in the present invention is in the linear viscoelastic region within the strain range of 1% to 10%, and the storage modulus (G') and loss modulus (G") remain strain-independent. Based on this, a 2% strain was used in subsequent dynamic rheological experiments. A frequency sweep of 2% SA solution was performed in the range of 0 to 100 rad / s. The changes in G' and G" of the 2% SA solution under frequency variation are shown in Figure 1d. Under low-frequency conditions, the loss modulus (G") and storage modulus (G') of the SA solution gradually increase with increasing frequency, and G' is smaller than G", indicating that the solution exhibits fluid properties dominated by viscosity. As the frequency continues to increase, G' exceeds G", and the system gradually changes to an elasticity-dominated state, showing weak gel properties. The reason for this change is that at low frequencies, the molecular relaxation time is longer, the molecular deformation is slower, and the system energy is lower, so the solution exhibits obvious viscous behavior. As the frequency increases, the relaxation time shortens, and the molecules can quickly store energy, resulting in enhanced elasticity of the solution. When the angular frequency exceeds 80 rad / s, G' and G" tend to stabilize, indicating that within this frequency range, the viscoelastic properties of the material have reached a state of equilibrium.

[0044] Example 2 Preparation of SA membrane Here are the steps: (1) Add 1 g of alginate and 1 mL of glycerol to an appropriate amount of deionized water, mix well, and dilute to 100 mL to prepare a 1% (w / v, g / mL) alginate solution; stir with a magnetic stirrer at room temperature for 0.5 h to obtain a membrane-forming solution; take 20 mL of the membrane-forming solution and pour it into a Petri dish with a side length of 10 cm so that the liquid level is 4.5 mm. Place it in an oven and dry it at 50°C to obtain a SA film (thickness is about 0.03 mm).

[0045] (2) The SA membrane was immersed in a 2% (w / v, g / mL) CaCl2 solution and cross-linked for 15 min; the membrane was dried at room temperature; the dried membrane was placed in a constant temperature and humidity chamber at 25°C and a relative humidity of 60% for 48 h and then used for the following experiments.

[0046] Example 3 Preparation of SA+HPMC composite membrane Here are the steps: (1) Add 1 g of alginate, 1 g of hydroxypropyl methylcellulose, and 1 mL of glycerol to an appropriate amount of deionized water, mix well, and dilute to 100 mL. Stir with a magnetic stirrer at room temperature for 0.5 h to obtain a membrane-forming solution. Pour 20 mL of the membrane-forming solution into a 10 cm Petri dish so that the liquid level is 4.6 mm. Place the dish in an oven and dry at 50°C to obtain a SA+HPMC composite membrane (approximately 0.03 mm thick).

[0047] (2) Immerse the SA+HPMC composite membrane in a 2% CaCl2 solution and cross-link for 15 min; dry at room temperature; and place the dried membrane in a constant temperature and humidity chamber at 25°C and a relative humidity of 60% for 48 h for standby use.

[0048] Example 4 Preparation of SA+HPMC+TiO2 composite film Here are the steps: (1) Disperse 1.00 g of nano-titanium dioxide (TiO2) in 9 mL of deionized water, add 1 mL of anhydrous ethanol to reduce the surface tension, treat with ultrasound (40 kHz, 200 W) for 30 min, and let it stand for degassing to obtain a TiO2 solution with a concentration of 10% (w / v, g / mL).

[0049] (2) Add 1 g of alginate, 1 g of hydroxypropyl methylcellulose, 1 mL of glycerol, and 5 mL of 10% TiO2 solution (containing 0.5 g of TiO2) to an appropriate amount of deionized water, mix well, and dilute to 100 mL. Stir with a magnetic stirrer at room temperature for 0.5 h to obtain a membrane-forming solution. Pour 20 mL of the membrane-forming solution into a 10 cm Petri dish so that the liquid level is 4.6 mm. Place the dish in an oven and dry at 50°C to obtain a SA+HPMC+TiO2 composite membrane (approximately 0.03 mm thick).

[0050] (3) Immerse the SA+HPMC+TiO2 composite membrane in a 2% CaCl2 solution and cross-link for 15 min; dry at room temperature; and place the dried membrane in a constant temperature and humidity chamber at 25°C and a relative humidity of 60% for 48 h for standby use.

[0051] Example 5 Preparation of SA+HPMC+TiO2+ε-PL composite film Here are the steps: (1) Disperse 1.00 g of ε-PL in 10 mL of deionized water and adjust the pH to 7.0 with sodium hydroxide to obtain a 10% (w / v, g / mL) ε-PL solution.

[0052] (2) Add 1 g of alginate, 1 g of hydroxypropyl methylcellulose, 1 mL of glycerol, 5 mL of 10% TiO2 solution (containing 0.5 g of titanium dioxide), and 10 mL of 10% ε-PL solution (containing 1 g of ε-PL) to an appropriate amount of deionized water, mix well, and dilute to 100 mL with deionized water. Stir with a magnetic stirrer at room temperature for 0.5 h to obtain a membrane-forming solution. Pour 20 mL of the membrane-forming solution into a 10 cm Petri dish so that the liquid level is 4.7 mm. Place the dish in an oven and dry at 50°C to obtain a SA+HPMC+TiO2+ε-PL composite membrane (approximately 0.03 mm thick).

[0053] (3) Immerse the SA+HPMC+TiO2+ε-PL composite membrane in a 2% CaCl2 solution and cross-link for 15 min; dry at room temperature; and place the dried membrane in a constant temperature and humidity chamber at 25°C and a relative humidity of 60% for 48 h for standby use.

[0054] Experiment 1 Performance testing of composite membranes The properties of the composite films prepared in Examples 2, 3, 4 and 5 were tested, as shown below.

[0055] (1) Mechanical property determination: tensile strength and elongation at break were tested. The test method was carried out in accordance with GB13022-91.

[0056] The results of the tensile strength and elongation at break of the composite film are shown in Figure 2. Figure 2 As shown. The SA membrane has the highest tensile strength of 44.37±2.22 MPa, but the elongation at break is low, at 7.00%±1.35%, showing high rigidity and low ductility. After adding HPMC, the tensile strength decreased to 17.71±1.89 MPa, and the elongation at break increased to 13.52%±0.68%. The tensile strength decreased by 60.09%, and the elongation at break increased by 93.14%, showing good flexibility. Further addition of TiO2 can enhance the rigidity of the membrane, resulting in a slight increase in the tensile strength of the membrane, but there is no significant difference ( P >0.05), while the elongation at break decreased significantly ( P <0.05), indicating that TiO2 can serve as a reinforcing agent for polysaccharide materials. Studies have shown that the introduction of an appropriate amount of particles can optimize the cross-linked network structure of the film, similar to the reinforcing effect of nanofillers, thereby improving the tensile strength of the material. The addition of ε-PL slightly increases the tensile strength and elongation at break of the film. This is likely due to the cross-linking reaction between ε-PL and SA. The amino groups in ε-PL react with the carboxyl groups in SA to form hydrogen bonds and covalent bonds. This cross-linking enhances the mechanical strength of the film. In summary, the different composite components can affect the mechanical properties of the film. The SA+HPMC+TiO2+ε-PL composite film achieves a good balance between strength and ductility.

[0057] (2) Oxygen transmission rate measurement: Place 3 g of deoxidizer (reduced iron powder, sodium chloride, and activated carbon in a weight ratio of 0.5:1.5:1) at the bottom of a 100 mL conical flask and cover the flask mouth with a composite film. After recording the initial mass of the conical flask, place it in an incubator at 25°C and 90% relative humidity. After 48 hours, weigh it and calculate the oxygen transmission rate using the following formula.

[0058] ; Where M f is the final mass of the deoxidizer after 48 h (g); M i is the initial mass of the deoxidizer (g); s is the effective area of ​​the film (m 2 ); t is time (h).

[0059] The oxygen transmission rate of the composite film is measured as follows: Figure 3 The oxygen transmission rates of the SA film and SA+HPMC film were higher, which were 59.91±1.99 g·m -2 ·h -1 and 56.45±1.82 g·m -2 ·h -1 , the addition of HPMC can slightly reduce the oxygen permeability of the membrane, but there is no significant difference ( P>0.05). It is possible that HPMC and SA molecular chains form a dense network structure through hydrogen bonding or electrostatic interaction. The increased film density can effectively block oxygen penetration, thereby reducing the oxygen transmission rate of the film. After adding TiO2, the oxygen transmission rate was significantly reduced to 47.35±3.37 g·m -2 ·h -1 (P < 0.05), it is possible that TiO2 as an inorganic filler can further enhance the density of the film. After adding ε-PL, the oxygen permeability slightly increased, but there was no significant difference ( P >0.05), which may be due to the introduction of microscopic pores by ε-PL, affecting the barrier effect.

[0060] (3) Determination of UV-visible light transmittance: The composite film was cut into 4 cm × 1 cm rectangular samples and fixed on the surface of a 10 mm quartz cuvette. A blank cuvette was used as a reference and a UV-visible spectrophotometer was used to perform full-band scanning in the wavelength range of 200 to 800 nm.

[0061] The infrared spectrum of the composite film is shown in Figure 4 The main absorption peak positions and intensities of the four composite films are similar, indicating that the addition of HPMC, TiO2 and ε-PL did not significantly change the chemical structure of the SA matrix. -1 Around 2924 cm, the broad and strong absorption peak corresponds to the -OH stretching vibration, indicating that the surface of the membrane material is rich in hydroxyl groups. -1 The absorption peaks correspond to the antisymmetric stretching vibrations of the CH bonds of methyl (CH3) and methylene (CH2). 1597 cm -1 and 1409 cm -1 The absorption peak at corresponds to the -COO - After adding TiO2 and ε-PL, no chemical bond reorganization or new functional group generation occurred between the components of the composite film, which was specifically manifested in that the characteristic absorption peak did not shift and no new absorption peak appeared. At the same time, after adding ε-PL, the peak at 1597 cm -1 The intensity of the characteristic peak of the carboxylic acid group is slightly weakened at 1025 cm-1, which is speculated to be due to the electrostatic complexation between the carboxyl group of SA and the amino group of ε-PL, which leads to a decrease in the proportion of free carboxyl groups. -1 The absorption peaks correspond to the stretching vibrations of COC and C-OH. The four membranes remained essentially identical, with no new absorption peaks appearing, indicating that chemical reactions had not occurred. The interactions between the composite membrane components were primarily due to electrostatic interactions and hydrogen bonding. Overall, the similar FTIR spectra of the four membranes suggest good compatibility between the composite components, with no significant impact on the chemical structure.

[0062] (4) Atomic force microscopy observation: The composite membrane was cut into 1 cm × 1 cm pieces and fixed on a glass slide with the dry side facing up. The membrane surface was scanned using an atomic force microscope (AFM) in tapping mode with a scanning rate of 0.997 Hz, 256 × 256 pixels, and a scanning area of ​​25 μm. 2 , the height difference and root mean square roughness were analyzed from the two-dimensional surface and three-dimensional valley morphology images of the film using AFM software.

[0063] The AFM images, three-dimensional valley morphology and root mean square roughness values ​​of the composite films are shown in Figure 2. Figure 5 As shown in the figure. The SA membrane surface exhibits a relatively uniform hill-and-valley structure. With the addition of HPMC, TiO2, and ε-PL, the film surface structure gradually becomes uneven. When TiO2 or ε-PL is further added, nanoparticles of varying sizes and quantities are visible on the membrane surface. The entire membrane surface is not smooth, which may be related to the molecular structure of the polymer, resulting in differences in roughness. Studies have shown that it may be the aggregates of ε-PL, or the aggregation of ε-PL with SA, HPMC, and TiO2, that cause the increase in tiny protrusions on the membrane surface. The roughness of the four composite membrane surfaces provides further numerical evidence. Their root mean square roughness (Rq) is 10.7 nm, 12.6 nm, 17.7 nm, and 33.0 nm, respectively, with the roughness value gradually increasing.

[0064] (5) Determination of antioxidant properties: The antioxidant activity of SA membrane, SA+HPMC composite membrane, SA+HPMC+TiO2 composite membrane, and SA+HPMC+TiO2+ε-PL composite membrane was evaluated using the 2,2-diphenyl-1-picrylhydrazide (DPPH) free radical scavenging method. 0.024 g of DPPH was weighed and dissolved in 100 mL of ethanol to obtain a 0.24 mM DPPH solution, which was stored in the dark. 100 mg of the composite membrane was cut into pieces and soaked in 20 mL of deionized water for 24 h. 2 mL of the extract was mixed with an equal volume of DPPH solution by vortexing, and the mixture was reacted in the dark for 30 min. The absorbance was measured at a wavelength of 517 nm. The blank control was 2 mL of deionized water + 2 mL of DPPH solution. The DPPH free radical scavenging rate was calculated according to the following formula.

[0065] ; Where A0 and A S are the absorbance of blank sample and measured sample, respectively.

[0066] The results of DPPH free radical scavenging activity test of composite membrane are as follows: Figure 6 The free radical scavenging rate of SA membrane was the lowest, only 17.40% ± 1.87%, which was significantly lower than that of other composite membrane groups ( P<0.05), indicating that its antioxidant activity was poor. The addition of HPMC increased the scavenging rate to 27.93%±1.39%, indicating that the introduction of HPMC enhanced the antioxidant capacity of the membrane. After further addition of TiO2 and ε-PL (SA+HPMC+TiO2 membrane) and (SA+HPMC+TiO2+ε-PL membrane), the DPPH free radical scavenging rates increased to 29.15%±0.75% and 28.14%±1.41%, respectively, which were not significantly different from those of the SA+HPMC group ( P >0.05), indicating that in this research system, the addition of TiO2 and ε-PL did not significantly improve the antioxidant capacity of the composite film.

[0067] (6) Antibacterial activity determination: The antibacterial properties of the four composite films were evaluated using the agar diffusion method. Staphylococcus aureus ( Staphylococcus aureus ) and Escherichia coli ( Escherichia coli ) were used as representative Gram-positive and Gram-negative bacterial species to evaluate the antibacterial properties of the membrane. The cultured bacterial solution was diluted to 1×10 6 ~1×10 7 CFU / mL for later use. Evenly spread 1 mL of the diluted bacterial solution on the surface of an LB agar plate and let it sit for 10 minutes. Aseptically transfer a UV-sterilized composite membrane disc (8 mm diameter) to the plate containing bacteria. Incubate the plate upside down in a 37°C incubator for 24 hours. Finally, observe the size of the inhibition zone around the membrane to assess the antibacterial effect.

[0068] The antibacterial photos of the composite membrane against Staphylococcus aureus and Escherichia coli are as follows Figure 7 Only the SA+HPMC+TiO2+ε-PL composite membrane exhibited a clear zone of inhibition in the culture medium containing Staphylococcus aureus and Escherichia coli, while the composite membrane without ε-PL exhibited no antibacterial effect. Studies have shown that ε-PL has significantly lower inhibitory activity against Gram-positive bacteria than Gram-negative bacteria. This phenomenon may be due to electrostatic interactions between ε-PL and Gram-negative bacteria. However, the results of this experiment showed that ε-PL had a significantly stronger antibacterial effect against Staphylococcus aureus than against Escherichia coli. This may be due to the influence of bacterial cell wall structure on the antibacterial effect. Gram-positive bacteria lack an outer membrane, making it easier for antibacterial substances such as ε-PL to penetrate their cell walls. Gram-negative bacteria, on the other hand, possess an outer membrane with a three-layer structure, making it more difficult for antibacterial compounds to penetrate, resulting in a less effective antibacterial effect.

[0069] Experiment 2 Application of composite film in the preservation of cape gooseberry The composite films prepared in Examples 2, 3, 4 and 5 are used for Physalis peruvianaThe specific method for preserving cape berries (L.) is as follows: uniformly sized, free of mechanical damage, disease, and uniformly mature cape berries were selected and disinfected by soaking them in a 100 mg / L sodium hypochlorite solution at room temperature for 8 minutes. The berries were then rinsed with deionized water and air-dried. The cape berries were randomly divided into five groups, each containing 20. Experimental Group 1 was preserved using a commercial PE film, while Experimental Groups 2, 3, 4, and 5 were preserved using the composite films prepared in Examples 2, 3, 4, and 5, respectively. All samples were stored at a constant temperature (25 ± 1°C) and relative humidity (80 ± 5%) for 15 days. Samples were collected every two days to measure weight loss and vitamin C content. The results are shown below.

[0070] (1) Determination of weight loss rate of lantern fruit: weigh the initial weight and weight after storage of the sample, and calculate the weight loss rate (%) according to the following formula.

[0071] The weight loss rate (%) was calculated according to the following formula.

[0072] ; Where W0 is the initial weight of the sample (g); W is the weight of the sample after storage (g).

[0073] The results of the weight loss rate of lantern fruit after composite film treatment are as follows: Figure 8 As shown. With the extension of storage time, the weight loss rate of the Chinese lantern fruit in each treatment group gradually increased. This is mainly because the weight loss of the fruit during storage is mainly caused by water loss caused by respiration and transpiration. Compared with the control group (commercial PE film), the weight loss rate of the Chinese lantern fruit in the SA+HPMC composite group was higher during the 12-day storage period. This may be related to the addition of HPMC changing the microstructure of the membrane. HPMC is a hydrophilic polymer. After being blended with SA, it may form a more complex water diffusion path, thereby reducing the membrane's barrier ability to water. At the same time, the water absorption properties of HPMC may increase the swelling of the membrane, making it easier to absorb and release water during storage, promoting the evaporation loss of water in the Chinese lantern fruit. The weight loss rates of the other SA composite films for the gooseberry fruit were all low. After 14 days of storage, the SA+HPMC+TiO2+ε-PL composite film group exhibited the lowest weight loss rate, at 12.75%±0.89%. This was approximately 19.60% lower than the commercial PE film group (weight loss rate of 15.85%±0.95%). This indicates that the SA+HPMC+TiO2+ε-PL composite film effectively mitigates water evaporation and dehydration in the gooseberry fruit. This is likely due to the synergistic effect of HPMC, TiO2, and ε-PL, which imparts enhanced barrier properties to the composite film, thereby effectively inhibiting water loss and maintaining the fruit's moisture content.

[0074] (2) Determination of vitamin C content in kiwano: The vitamin C content in kiwano was determined by 2,6-dichloroindophenol titration method, referring to GB 5009.86-2016. About 20 g of kiwano sample was selected and ground into a uniform slurry in a mortar. About 15 g of the homogenized sample was weighed and quantitatively transferred to a 100 mL volumetric flask. 2% (w / v, g / mL) oxalic acid solution was added to the volume to the mark. 10 mL of the extract was transferred to a 50 mL conical flask and titrated with a pre-calibrated 2,6-dichloroindophenol standard solution until a pink color appeared and did not fade for 15 s. A blank control of oxalic acid solution was set up at the same time. The experiment was repeated three times. The vitamin C content was calculated according to the following formula.

[0075] ; Where V is the volume of 2,6-dichloroindophenol solution consumed in the titration of the sample (mL); V0 is the volume of 2,6-dichloroindophenol solution consumed in the titration of the blank (mL); T is the titer of the 2,6-dichloroindophenol solution, that is, the number of milligrams of ascorbic acid per milliliter of 2,6-dichloroindophenol solution (mg / mL); A is the dilution factor; and M is the mass of the sample (g).

[0076] The results of the determination of vitamin C content in lantern fruit after composite film treatment are as follows Figure 9As shown in Figure 2 . During storage, the vitamin C content of all treatments showed an overall downward trend, primarily due to vitamin C's susceptibility to factors such as oxygen, light, and temperature, leading to oxidative decomposition during storage. Initially, the vitamin C content of all treatments decreased rapidly, particularly on the sixth day. The vitamin C content of the kiwano fruit covered with the commercial PE film group dropped to 15.27 ± 0.86 mg / 100 g. The SA+HPMC composite film group showed a similar downward trend in vitamin C content as the commercial PE film group, but lower than the SA film group. This suggests that the addition of HPMC may have reduced the barrier properties of the SA film, thereby exacerbating vitamin C loss. In contrast, the SA+HPMC+TiO2+ε-PL composite film group showed the highest vitamin C content on the sixth day, reaching 18.92 ± 1.29 mg / 100 g. By the 14th day, the vitamin C content of the kiwano fruit in this group remained high, exceeding that of the other composite film groups and the commercial PE film group, indicating that the composite film effectively slowed the degradation of vitamin C, thereby preventing the deterioration of kiwano fruit quality. This may be due to the light-shielding effect of TiO2, which reduces the risk of photooxidative degradation of vitamin C. The antimicrobial properties of ε-PL effectively reduce microbial infestation, thereby reducing the consumption of vitamin C by microbial metabolism. TiO2 produces reactive oxygen species (ROS) under visible light irradiation, and its broad-spectrum antimicrobial properties help inhibit microbial growth, thereby reducing microbial consumption of vitamin C. This mechanism is similar to the photocatalytic properties of TiO2 and the antimicrobial effect of ε-PL in this study, further demonstrating that the SA+HPMC+TiO2+ε-PL composite film can synergistically improve the storage stability of cape gooseberry, delay vitamin C degradation, and thus extend its shelf life.

[0077] The above examples are provided to those skilled in the art to fully disclose and describe how to implement and use the claimed embodiments, rather than to limit the scope of the disclosure herein. Modifications that are obvious to those skilled in the art will fall within the scope of the appended claims.

Claims

1. A method for preparing an alginate composite film, characterized in that: Add 0.8-1.2 g of alginate, 0.8-1.2 g of hydroxypropyl methylcellulose, 0.8-1.2 mL of glycerol, 0.4-0.6 g of titanium dioxide, and 0.8-1.2 g of polylysine to water, mix well, and dilute to 100 mL to obtain a film-forming solution; place the film-forming solution in a container so that the liquid level is 4.3-4.7 mm, dry it, immerse it in a CaCl2 solution for cross-linking treatment, take it out, and dry it to obtain an alginate composite film.

2. The method for preparing the alginate composite film according to claim 1, wherein The dosage of each component is: 1 g of alginate, 1 g of hydroxypropyl methylcellulose, 1 mL of glycerol, 0.5 g of titanium dioxide, and 1 g of polylysine.

3. The method for preparing the alginate composite film according to claim 1, wherein: The titanium dioxide is added in the form of a solution; the polylysine is added in the form of a solution.

4. The method for preparing the alginate composite film according to claim 1, wherein: The concentration of the CaCl2 solution is 2%, and the cross-linking treatment time is 15 min.

5. The method for preparing the alginate composite film according to claim 1, wherein The alginate is extracted by the following method: (1) Take dried Sargassum and crush it to obtain Sargassum powder; (2) Adding Sargassum powder to citric acid-disodium hydrogen phosphate buffer, adding cellulase, pectinase and xylanase, and performing enzymatic hydrolysis to obtain an enzymatic hydrolyzate; (3) Centrifuge the enzymatic hydrolyzate to obtain a supernatant and algal residue; add sodium chloride solution to the algal residue, digest it, and centrifuge it to obtain a supernatant; combine the two supernatants and adjust the pH to 6.0-7.0 to obtain an enzyme extract; (4) Add calcium chloride to the enzyme extract and let it stand to form calcium alginate gel; then wash with deionized water; hydrochloric acid decalcification; (5) Place the decalcified gel in a sodium hydroxide solution to obtain a sodium alginate solution; add anhydrous ethanol, let it stand to obtain a precipitate, and dry it to obtain alginate.

6. The method for preparing the alginate composite film according to claim 5, wherein: In step (2), the amount of cellulase added is 4%, the amount of pectinase added is 3%, and the amount of xylanase added is 2%; The enzymatic hydrolysis conditions were 50°C, 220 r / min water bath shaker, and the enzymatic hydrolysis time was 3 h.

7. The method for preparing the alginate composite film according to claim 5, wherein: In step (4), the concentration of calcium chloride is 10%; The specific method of hydrochloric acid decalcification is: first decalcify with 1 M hydrochloric acid for 2 to 4 hours, and then decalcify again with 0.5 M hydrochloric acid for 3 hours.

8. An alginate composite film prepared by the method for preparing an alginate composite film according to any one of claims 1 to 7.

9. Use of the alginate composite film according to claim 8 as a food preservative film, or in the preparation of a food preservative film, or in food preservation.

10. The use according to claim 9, characterized in that: The food is cape gooseberry.

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