Biodegradable chitosan-polylipoic acid composite film and its preparation method and application

Abstract

The present invention provides a biodegradable chitosan-polylipoic acid composite film, a preparation method, and applications, belonging to the technical field of food preservation materials. The chitosan-polylipoic acid composite film is obtained by electrospinning an emulsion containing a chitosan-polylipoic acid complex, a plasticizer, a spinning aid, and cinnamaldehyde. The chitosan-polylipoic acid composite film is biodegradable, has a uniform and controllable structure, and is multifunctional. It has a high encapsulation efficiency, a tight coordination, good antibacterial and antioxidant activity, excellent mechanical properties and gas barrier properties, and can control the release of cinnamaldehyde by responding to changes in pH and humidity in the environment. It has good application prospects in the field of fruit and vegetable preservation.

Description

Technical Field

[0001] The invention belongs to the technical field of food preservation materials, and particularly relates to a biodegradable chitosan-polylipoic acid composite film and a preparation method and application thereof. Background Art

[0002] In recent years, edible films and packaging technologies based on natural polymer materials have shown broad application prospects in the field of food preservation due to their excellent biodegradability, environmental friendliness and biocompatibility.

[0003] Chitosan, a natural cationic polysaccharide, has been widely studied for its excellent film-forming properties and antibacterial activity. However, pure chitosan films suffer from deficiencies such as insufficient mechanical strength and limited functionality, which limit their practical applications.

[0004] Cinnamaldehyde, the main active ingredient in cinnamon essential oil, has been shown to have broad-spectrum antibacterial and antioxidant properties, and has outstanding advantages in food preservation. However, the compound's volatility, poor water solubility, and strong odor directly affect its stability and applicability in preservation systems. Microcapsules, emulsions, nanofibers, films, or coatings can improve the stability of cinnamaldehyde and reduce its adverse sensory effects. However, these methods are difficult to solve the problem of controlled release of cinnamaldehyde and cannot achieve intelligent and precise response to the fruit corruption microenvironment. For example, when the fruit begins to rot and produces acidic metabolites or the ambient humidity increases, the dynamic covalent bonds in the film will not be selectively broken, and the targeted release of cinnamaldehyde cannot be achieved, thus failing to achieve a good preservation effect.

[0005] Therefore, it is urgent to provide a food preservative film with good cinnamaldehyde controlled-release function and good mechanical strength. Summary of the Invention

[0006] In order to solve all or part of the above technical problems, the present invention provides the following technical solutions:

[0007] A first aspect of the present invention provides a biodegradable chitosan-polylipoic acid composite film, which is obtained by electrospinning an emulsion containing a chitosan-polylipoic acid complex, a plasticizer, a spinning aid, and cinnamaldehyde. The chitosan and polylipoic acid in the chitosan-polylipoic acid complex are connected by an amide bond, the mass ratio of the chitosan-polylipoic acid complex, the plasticizer, and the spinning aid is (0.5-1.5):(1-3):(1-3), and the mass ratio of the chitosan-polylipoic acid complex to cinnamaldehyde is (0.5-1.5):(0.2-3).

[0008] The chitosan-polylipoic acid composite film provided by the present invention is biodegradable, has a high encapsulation rate, tight coordination, excellent antibacterial and antioxidant activities, and also has excellent mechanical properties and gas barrier properties. In particular, it can control the release of cinnamaldehyde by responding to changes in pH and humidity in the environment, so that it has good application prospects in the field of fruit and vegetable preservation.

[0009] In some embodiments, the chitosan-polylipoic acid composite film can release cinnamaldehyde based on changes in pH and / or humidity. Furthermore, the amount of cinnamaldehyde released from the chitosan-polylipoic acid composite film increases with increasing humidity; and within the pH range of 5-7, the amount of cinnamaldehyde released from the chitosan-polylipoic acid composite film increases with decreasing pH.

[0010] In some embodiments, the plasticizer includes one or more of glycerol, xylitol, sorbitol, polyethylene glycol, acetyl tributyl citrate, triethyl citrate, or tributyl citrate.

[0011] In some embodiments, the spinning aid includes one or a combination of polyethylene oxide, polyvinyl pyrrolidone, polyacrylic acid or sodium dodecylbenzene sulfonate.

[0012] In some embodiments, chitosan and polylipoic acid in the chitosan-polylipoic acid complex are linked via an amide bond.

[0013] In some embodiments, the weight-average molecular weight of polylipoic acid in the chitosan-polylipoic acid complex is 2,000-20,000 Da. If the molecular weight of polylipoic acid is too high, its solubility in water may decrease and it may form aggregates, affecting the uniformity of the chitosan-polylipoic acid complex. Furthermore, chain entanglement or folding of high-molecular-weight polylipoic acid may affect the exposure of functional groups, reduce active sites for binding to chitosan, and thus reduce crosslinking efficiency.

[0014] In some embodiments, the thickness of the chitosan-polylipoic acid composite film is 30-70 μm.

[0015] A second aspect of the present invention provides a method for preparing a biodegradable chitosan-polylipoic acid composite film, comprising:

[0016] causing chitosan and polylipoic acid to undergo amidation reaction to obtain a chitosan-polylipoic acid complex;

[0017] emulsifying a liquid mixture containing the chitosan-polylipoic acid complex, a plasticizer, a spinning aid and cinnamaldehyde to form an emulsion;

[0018] The electrostatic spinning technology is adopted to spin the emulsion into a film to obtain a chitosan-polylipoic acid composite film.

[0019] This invention uses electrospinning technology to create a chitosan-polylipoic acid composite film with a three-dimensional network structure. This 3D network structure is sensitive to environmental stimuli (such as changes in pH and humidity) and achieves precise, controlled release of the active ingredient, cinnamaldehyde, through dynamic chemical bond cleavage. Upon detecting a specific stimulus (such as organic acids produced by fruit spoilage or increased humidity), the chitosan-polylipoic acid composite film undergoes a controlled molecular transformation, triggering the targeted release of cinnamaldehyde. This allows the composite film to dynamically adjust the release kinetics of the functional ingredients based on food preservation needs, significantly improving food preservation and extending shelf life.

[0020] In addition, polylipoic acid has excellent biocompatibility and edible safety, and the disulfide bonds in its molecular structure also give it good antioxidant activity. The present invention prepares a chitosan-polylipoic acid complex through an amidation reaction. As a composite membrane matrix, it can not only improve the mechanical properties and thermal stability of the film, but also make the film have good antioxidant properties.

[0021] In some embodiments, in the mixture, in the liquid mixture, the mass volume ratio of the chitosan-polylipoic acid complex is 0.5-1.5%, the mass volume ratio of the plasticizer is 1-3%, the mass volume ratio of the spinning aid is 1-3%, and the mass volume ratio of cinnamaldehyde is 0.2-3%.

[0022] In some embodiments, the plasticizer includes one or a combination of glycerol, xylitol, sorbitol, polyethylene glycol, acetyl tributyl citrate, triethyl citrate, or tributyl citrate.

[0023] In some embodiments, the spinning aid includes one or a combination of polyethylene oxide, polyvinyl pyrrolidone, polyacrylic acid or sodium dodecylbenzene sulfonate.

[0024] In some embodiments, the process parameters of the electrospinning technology include: using a 10-20 ml syringe, a needle model of 22-24G, a distance between the needle and the electrode of 10-15 cm, a reciprocating distance of 5-10 cm, an applied electric field of 15-25 kV, and a feed rate of 0.5-1 mL / h.

[0025] For example, the chitosan-polylipoic acid composite film prepared by electrospinning was collected on a grounded aluminum foil, and the temperature was maintained at 25-35° C. and the humidity was 30-50%.

[0026] In some embodiments, the preparation method of polylipoic acid includes: allowing a mixed reaction system containing lipoic acid, trifluoromethanesulfonic acid and a solvent to undergo polymerization reaction at a temperature of 20-30° C. to obtain the polylipoic acid.

[0027] Furthermore, in the mixed reaction system, the mass volume ratio of lipoic acid to solvent is 1:10-1:15, and the mass volume ratio of lipoic acid to trifluoromethanesulfonic acid is 1:0.0025-1:0.005.

[0028] Furthermore, the solvent includes dichloromethane.

[0029] Furthermore, the polymerization reaction time is 1 to 3 hours.

[0030] In some typical embodiments, the preparation method of polylipoic acid may include the following steps: dissolving lipoic acid powder in dichloromethane to form a solution, and maintaining the reaction temperature at 20-30° C.; adding trifluoromethanesulfonic acid to the solution, and performing ring-opening polymerization of lipoic acid under stirring; after sufficient polymerization, adding 1-propanethiol to terminate the reaction.

[0031] For example, stirring is maintained during the reaction, and the stirring speed may be 400-600 rpm.

[0032] Exemplarily, the mass volume ratio of lipoic acid to 1-propanethiol is 1:0.1-1:0.2.

[0033] For example, after the reaction is completed, the crude product can be obtained by rotary evaporation of the solvent, washed with petroleum ether, and then freeze-dried to obtain the polylipoic acid product. The rotary evaporation temperature can be 40-50° C., the petroleum ether washing times can be 2-3 times, and the freeze-drying time can be 36-48 hours.

[0034] In some embodiments, the amidation reaction comprises:

[0035] Providing a mixed solution containing chitosan, polylipoic acid, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS), adjusting the pH value of the mixed solution to a weak acidity, and reacting at a temperature of 0-4°C to obtain a chitosan-polylipoic acid complex;

[0036] Furthermore, the mass ratio of chitosan to polylipoic acid in the mixed solution is 1:0.1-1:0.5.

[0037] Furthermore, the pH value of the mixed solution is adjusted to 5-6.

[0038] Furthermore, in the mixed solution, the mass ratio of polylipoic acid to EDC is 1:1-1:1.5, and the mass ratio of EDC to NHS is 1:1.

[0039] In some typical embodiments, the amidation reaction comprises the following steps:

[0040] dissolving chitosan in an acetic acid aqueous solution to obtain a chitosan solution; dissolving polylipoic acid in water to obtain a polylipoic acid solution;

[0041] The chitosan solution and the polylipoic acid solution are mixed, and 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide are added. The pH value of the mixed solution is adjusted to 5-6, and the mixture is reacted at a temperature of 0-4° C. to obtain a chitosan-polylipoic acid complex.

[0042] Illustratively, the mass volume ratio of chitosan to acetic acid aqueous solution in the chitosan solution is 1:40-1:60, and the mass volume ratio of polylipoic acid to water in the polylipoic acid solution is 1:100-1:500; the chitosan solution and the polylipoic acid solution are mixed in a volume ratio of 1:1-1:2.

[0043] Exemplarily, the concentration of the acetic acid aqueous solution is 1-2%.

[0044] Exemplarily, the chitosan solution and the polylipoic acid solution are mixed, and the temperature is controlled at 0-4°C and stirred for 0.5-1h, then the 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride is added and stirred for 0.5-1h, and then the N-hydroxysuccinimide is added and stirred for 2-4h, thereby reacting to obtain the chitosan-polylipoic acid complex.

[0045] Exemplarily, after the amidation reaction is completed, the reaction product is dialyzed using a dialysis bag to remove residual EDC and NHS; the dialysate is freeze-dried to obtain a chitosan-polylipoic acid complex. The dialysate is distilled water, the molecular weight cut-off of the dialysis bag can be 3.5-8.0 kDa, the dialysis time can be 36-48 hours, the dialysate is changed every 12 hours, and the drying time can be 36-48 hours.

[0046] In some embodiments, the preparation method of the emulsion comprises:

[0047] Dissolving the chitosan-polylipoic acid complex in a first solvent, adding the plasticizer and the spinning aid, and fully dispersing the mixture to form a first solution;

[0048] dissolving cinnamaldehyde in a second solvent, adding a surfactant, and performing ultrasonic emulsification to obtain a second emulsion;

[0049] The second emulsion is added dropwise to the first solution, stirred in the dark to fully mix, and then ultrasonically degassed to obtain the emulsion.

[0050] Furthermore, the mass-to-volume ratio of the chitosan-polylipoic acid complex to the first solvent can be 1:100-1:120. The mass-to-volume ratio of the cinnamaldehyde to the second solvent can be 1:5-1:10. The combined volume of the first solvent and the second emulsion is calculated based on the required amount of cinnamaldehyde. For example, the first solution and the second emulsion are mixed at a volume ratio of 100:1-100:30.

[0051] Furthermore, the first solvent includes acetic acid and water, wherein the volume proportion of acetic acid is 1-2%.

[0052] Furthermore, the second solvent includes anhydrous ethanol.

[0053] Furthermore, the mass ratio of the cinnamaldehyde to the surfactant is 2:1-1:1.

[0054] Furthermore, the surfactant includes one or a combination of multiple of Tween 80, Tween 20, Tween 60, sucrose ester, and monoglyceride.

[0055] Furthermore, the ultrasonic emulsification time is 10-15 minutes.

[0056] Furthermore, the light-proof stirring time is 2-4 hours, and the ultrasonic degassing time is 1-2 hours.

[0057] Furthermore, the added amount of the plasticizer and the spinning aid is in a mass volume ratio of 1% to 3% of the mixture of the first solution and the second emulsion.

[0058] The third aspect of the present invention provides a chitosan-polylipoic acid composite film, which is prepared by the preparation method described in any one of the above technical solutions.

[0059] A fourth aspect of the present invention provides use of the chitosan-polylipoic acid composite film described in any one of the technical solutions in food packaging or food preservation.

[0060] In some embodiments, the food includes fruits and vegetables. The chitosan-polylipoic acid composite film of the present invention can release cinnamaldehyde based on both humidity and pH response, which is highly compatible with the slightly acidic environment and high humidity conditions of fruit and vegetable storage, making it particularly suitable for packaging and preserving fruits and vegetables.

[0061] Compared with the prior art, the present invention has at least the following beneficial effects:

[0062] (1) The chitosan-polylipoic acid composite film provided by the present invention is biodegradable, has a high encapsulation efficiency, tight coordination, excellent antibacterial and antioxidant activities, and also has excellent mechanical properties and gas barrier properties.

[0063] (2) The chitosan-polylipoic acid composite film provided by the present invention can control the release of cinnamaldehyde by responding to changes in pH and humidity in the environment, so that it has good application prospects in the field of fruit and vegetable preservation.

[0064] (3) The preparation method provided by the present invention has low energy consumption, mild conditions, simple operation, low cost, and the obtained composite film structure is uniform and controllable. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in this application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0066] Figure 1a 、 Figure 1b SEM images of the films prepared in Comparative Example 1 and Example 1 at different magnifications;

[0067] Figure 2a 、 Figure 2b 、 Figure 2c 3D laser confocal image of the film prepared in Comparative Example 1;

[0068] Figure 3a 、 Figure 3b 、 Figure 3c 3D laser confocal image of the film prepared in Example 1;

[0069] Figure 4a 、 Figure 4b They are water contact angle diagrams of the films in Comparative Example 1 and Example 1, respectively;

[0070] Figure 5 FTIR spectra of the films prepared in Comparative Example 1 and Examples 1-5;

[0071] Figure 6 is the thickness of the films prepared in Comparative Example 1 and Examples 1-5;

[0072] Figure 7 is the tensile strength of the films prepared in Comparative Example 1 and Examples 1-5;

[0073] Figure 8 is the elongation at break of the films prepared in Comparative Example 1 and Examples 1-5;

[0074] Figure 9a 、 Figure 9b and Figure 9cThe water vapor transmission rate comparison chart, oxygen transmission rate comparison chart, and carbon dioxide transmission rate comparison chart of the films prepared in Comparative Example 1 and Examples 1-5 are respectively;

[0075] Figure 10 The antibacterial properties of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5 are shown;

[0076] Figure 11a 、 Figure 11b and Figure 11c The figures are comparative diagrams of the antibacterial rates of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5 against Staphylococcus aureus, Escherichia coli and Botrytis cinerea;

[0077] Figure 12a 、 Figure 12b The scavenging rates of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5 for DPPH (2,2-diphenyl-1-picrylhydrazyl free radical) and ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) are respectively; Figure 12c is the hemolysis rate of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5;

[0078] Figure 13 is the encapsulation efficiency of the cinnamaldehyde / chitosan-polylipoic acid film prepared in Examples 1-5;

[0079] Figure 14a The cinnamaldehyde response release performance of the cinnamaldehyde / chitosan-polylipoic acid film prepared in Example 1 under 30% and 90% humidity conditions; Figure 14b The cinnamaldehyde response release performance of the cinnamaldehyde / chitosan-polylipoic acid film prepared in Example 1 under pH 5, 6, and 7 conditions;

[0080] Figure 15 This is a comparison chart of the biodegradability of the cinnamaldehyde / chitosan-polylipoic acid film and the polyethylene film prepared in Example 1;

[0081] Figure 16a When the films in Comparative Example 1 and Example 1 are used to preserve cherries, Value (color brightness) changes with storage days; Figure 16b This is a graph showing the change in fruit hardness over storage days when the films in Comparative Example 1 and Example 1 are used to preserve cherries; Figure 16c This is a graph showing the change in weight loss over storage days when the films in Comparative Example 1 and Example 1 are used to preserve cherries; Figure 16d This is a graph showing the change in total microbial count over storage days when the films in Comparative Example 1 and Example 1 are used to preserve cherries.

[0082] in, Figure 6-8 、 Figures 9a-9c 、 Figures 11a-11c 、 Figures 12a-12c 、 Figure 13 、 Figures 14a-14b 、 Figures 16a-16d The letters involved (such as a, b, c, d, e, f, g) are grouping marks for statistically significant differences. DETAILED DESCRIPTION

[0083] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments so that those skilled in the art can better understand and implement the technical solutions of the present invention. The specific functional details disclosed herein should not be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching those skilled in the art to employ the present invention in various ways in virtually any appropriate detailed embodiment.

[0084] In addition, unless otherwise specified, the various raw materials used in the following examples can be purchased from the market and other channels, the various production and testing equipment used are also equipment known in the art, and the testing methods used are also methods known in the art.

[0085] Example 1

[0086] This embodiment provides a biodegradable chitosan-polylipoic acid composite film and a preparation method thereof, which specifically comprises the following steps:

[0087] (1) Dissolve 2 g of lipoic acid powder in 20 mL of dichloromethane and maintain the reaction temperature at 25°C. After complete dissolution, add 5 μL of trifluoromethanesulfonic acid to the solution and perform ring-opening polymerization of lipoic acid under stirring (500 rpm). After 2 h of complete polymerization, terminate the reaction with 200 μL of 1-propanethiol. Obtain the crude product by rotary evaporation of the solvent (45°C), wash it three times with petroleum ether, and freeze-dry it for 48 h to obtain the polylipoic acid product.

[0088] (2) Dissolve 2 g of chitosan powder in 100 mL of 1% acetic acid aqueous solution, then take 0.6 g of the above-mentioned polylipoic acid powder and dissolve it in 100 mL of distilled water; after sufficient dissolution, mix the two in a 1:1 ratio, adjust the pH to 5.0, and maintain the temperature at 4°C; stir the mixed solution for 1 hour, add 0.6 g of EDC powder, and stir it for another hour to activate the carboxyl group, then add 0.6 g of NHS powder and stir it for another 3 hours; dialyze the solution for 48 hours using a dialysis bag (molecular weight cutoff of 3.5–8.0 kDa) to remove residual EDC and NHS. Finally, freeze-dry the dialyzate for 48 hours to obtain a chitosan-polylipoic acid complex.

[0089] (3) Take 1g of the above chitosan-polylipoic acid complex and add it to 100mL of 1% acetic acid aqueous solution. Ultrasonic treatment is carried out for 2h to promote its complete dissolution. Glycerol and polyethylene oxide (both 2% by mass volume) are added to the chitosan-polylipoic acid solution as plasticizer and spinning aid, respectively. Ultrasonic treatment is carried out for 2h to promote its uniform dispersion. 4g of cinnamaldehyde is pre-dissolved in 20mL of anhydrous ethanol, 4g of Tween 80 is added, and ultrasonic emulsification is carried out for 10min. Then, stir in the dark for 3h to fully mix, and ultrasonic degassing is carried out for 1h to obtain an emulsion.

[0090] (4) At room temperature, the emulsion was spun into a film using an electrospinning machine to obtain a chitosan-polylipoic acid composite film, which was recorded as CA / CS-PLA-2 film. The specific conditions for electrospinning were as follows: a 20 ml syringe was used, the needle model was 22 G, the distance between the needle and the electrode was maintained at 12 cm, and the reciprocating distance was 10 cm; a 15 kV electric field was applied to all solutions, and the feed rate was 1 ml / h; the resulting electrospun film was collected on a grounded aluminum foil, the temperature was maintained at 25°C, and the humidity was 30%.

[0091] Example 2

[0092] Example 2 is substantially the same as Example 1, except that, in step (3) of Example 2, 0.25 g of pre-dissolved cinnamaldehyde is slowly added dropwise to the chitosan-polylipoic acid solution to obtain a chitosan-polylipoic acid composite film, designated as CA / CS-PLA-0.25 film. The remaining steps are the same as in Example 1 and are not described in detail here.

[0093] Example 3

[0094] Example 3 is substantially the same as Example 1, except that, in step (3) of Example 3, 0.5 g of pre-dissolved cinnamaldehyde is slowly added dropwise to the chitosan-polylipoic acid solution to obtain a chitosan-polylipoic acid composite film, designated as CA / CS-PLA-0.5 film. The remaining steps are the same as in Example 1 and are not described in detail here.

[0095] Example 4

[0096] Example 4 is essentially the same as Example 1, except that, in step (3) of Example 4, 1 g of pre-dissolved cinnamaldehyde is slowly added dropwise to the chitosan-polylipoic acid solution to obtain a chitosan-polylipoic acid composite film, designated as CA / CS-PLA-1 film. The remaining steps are the same as in Example 1 and are not described in detail here.

[0097] Example 5

[0098] Example 5 is essentially the same as Example 1, except that, in step (3) of Example 5, 3 g of pre-dissolved cinnamaldehyde is slowly added dropwise to the chitosan-polylipoic acid solution to obtain a chitosan-polylipoic acid composite film, designated as CA / CS-PLA-3 film. The remaining steps are the same as in Example 1 and are not described in detail here.

[0099] Example 6

[0100] This embodiment provides a biodegradable chitosan-polylipoic acid composite film and a preparation method thereof, which specifically comprises the following steps:

[0101] (1) Dissolve 2 g of lipoic acid powder in 20 mL of dichloromethane and maintain the reaction temperature at 20°C. After complete dissolution, add 5 μL of trifluoromethanesulfonic acid to the solution and perform ring-opening polymerization of lipoic acid under stirring (500 rpm). After full polymerization for 1 h, terminate the reaction with 200 μL of 1-propanethiol. Obtain the crude product by rotary evaporation of the solvent (45°C), wash it three times with petroleum ether, and freeze-dry it for 48 h to obtain the polylipoic acid product.

[0102] (2) Dissolve 2 g of chitosan powder in 100 mL of 1% acetic acid aqueous solution, then take 0.2 g of the above-mentioned polylipoic acid powder and dissolve it in 100 mL of distilled water; after sufficient dissolution, mix the two in a 1:1 ratio, adjust the pH to 6.0, and maintain the temperature at 4°C; stir the mixed solution for 1 hour, add 0.3 g of EDC powder, and stir it for another hour to activate the carboxyl group, then add 0.3 g of NHS powder and stir it for another 3 hours; dialyze the solution for 48 hours using a dialysis bag (molecular weight cutoff of 3.5–8.0 kDa) to remove residual EDC and NHS. Finally, freeze-dry the dialyzate for 48 hours to obtain a chitosan-polylipoic acid complex.

[0103] (3) Chitosan-polylipoic acid complex was added to 1% acetic acid aqueous solution and ultrasonically treated for 2 h to promote its complete dissolution. Glycerol and polyethylene oxide were added to the chitosan-polylipoic acid solution and ultrasonically treated for 2 h to promote its uniform dispersion. Cinnamaldehyde was pre-dissolved in anhydrous ethanol, Tween 80 was added (the mass ratio of Tween 80 to cinnamaldehyde was 1:1), and ultrasonic emulsification was performed for 10 min. Then, cinnamaldehyde solution was slowly added dropwise to the chitosan-polylipoic acid solution to form a liquid mixture. In the liquid mixture, the mass volume ratios of chitosan-polylipoic acid complex, glycerol, polyethylene oxide, and cinnamaldehyde were 0.5%, 1%, 1%, and 0.2%, respectively. The liquid mixture was stirred in the dark for 3 h to fully mix it, and ultrasonic degassed for 1 h to obtain an emulsion.

[0104] (4) At room temperature, the emulsion was spun into a film using an electrospinning machine to obtain a chitosan-polylipoic acid composite film. The specific conditions for electrospinning were as follows: a 10 ml syringe was used, the needle model was 24 G, the distance between the needle and the electrode was maintained at 10 cm, and the reciprocating distance was 5 cm; a 25 kV electric field was applied to all solutions, and the feed rate was 0.5 ml / h; the resulting electrospun film was collected on a grounded aluminum foil, the temperature was maintained at 25°C, and the humidity was 30%.

[0105] Example 7

[0106] This embodiment provides a biodegradable chitosan-polylipoic acid composite film and a preparation method thereof, which specifically comprises the following steps:

[0107] (1) Dissolve 2 g of lipoic acid powder in 20 mL of dichloromethane and maintain the reaction temperature at 30°C. After complete dissolution, add 5 μL of trifluoromethanesulfonic acid to the solution and perform ring-opening polymerization of lipoic acid under stirring (500 rpm). After complete polymerization for 3 h, terminate the reaction with 200 μL of 1-propanethiol. Obtain the crude product by rotary evaporation of the solvent (45°C), wash it three times with petroleum ether, and freeze-dry it for 48 h to obtain the polylipoic acid product.

[0108] (2) Dissolve 2 g of chitosan powder in 100 mL of 1% acetic acid aqueous solution, then take 1 g of the above-mentioned polylipoic acid powder and dissolve it in 100 mL of distilled water; after sufficient dissolution, mix the two in a 1:1 ratio, adjust the pH to 5.0, and maintain the temperature at 4°C; stir the mixed solution for 1 hour, then add 1 g of EDC powder, stir it for another hour to activate the carboxyl group, and then add 1 g of NHS powder and stir it for another 3 hours; dialyze the solution for 48 hours using a dialysis bag (molecular weight cutoff of 3.5–8.0 kDa) to remove residual EDC and NHS. Finally, freeze-dry the dialyzate for 48 hours to obtain a chitosan-polylipoic acid complex.

[0109] (3) Chitosan-polylipoic acid complex was added to 1% acetic acid aqueous solution and ultrasonically treated for 2 h to promote its complete dissolution. Glycerol and polyethylene oxide were added to the chitosan-polylipoic acid solution and ultrasonically treated for 2 h to promote its uniform dispersion. Cinnamaldehyde was pre-dissolved in anhydrous ethanol, Tween 80 was added (the mass ratio of Tween 80 to cinnamaldehyde was 1:1), and ultrasonic emulsification was performed for 10 min. Then, cinnamaldehyde solution was slowly added dropwise to the chitosan-polylipoic acid solution to form a liquid mixture. In the liquid mixture, the mass volume ratios of chitosan-polylipoic acid complex, glycerol, polyethylene oxide, and cinnamaldehyde were 1.5%, 3%, 3%, and 3%, respectively. The liquid mixture was stirred in the dark for 3 h to fully mix it, and ultrasonic degassed for 1 h to obtain an emulsion.

[0110] (4) At room temperature, the emulsion was spun into a film using an electrospinning machine to obtain a chitosan-polylipoic acid composite film. The specific conditions for electrospinning were as follows: a 20 ml syringe was used, the needle model was 22 G, the distance between the needle and the electrode was maintained at 15 cm, and the reciprocating distance was 8 cm; a 20 kV electric field was applied to all solutions, and the feed rate was 0.8 ml / h; the resulting electrospun film was collected on a grounded aluminum foil, the temperature was maintained at 25°C, and the humidity was 30%.

[0111] Comparative Example 1

[0112] The difference between Comparative Example 1 and Example 1 is that the chitosan-polylipoic acid composite film prepared in Comparative Example 1 does not contain cinnamaldehyde, and specifically comprises the following steps:

[0113] (1) Dissolve 2 g of lipoic acid powder in 20 mL of dichloromethane and maintain the reaction temperature at 25°C. After full dissolution, add 5 μL of trifluoromethanesulfonic acid to the solution and perform ring-opening polymerization of lipoic acid under stirring (500 rpm). After full polymerization for 2 h, terminate the reaction with 200 μL of 1-propanethiol. Obtain the crude product by rotary evaporation of the solvent (45°C), wash it three times with petroleum ether, and freeze-dry it for 48 h to obtain the polylipoic acid product.

[0114] (2) Dissolve 2 g of chitosan powder in 100 mL of 1% acetic acid aqueous solution, and then take 0.6 g of the above-mentioned polylipoic acid powder and dissolve it in 100 mL of distilled water. After sufficient dissolution, mix the two in a 1:1 ratio, adjust the pH to 5.0, and maintain the temperature at 4°C. After stirring the mixed solution for 1 hour, add 0.6 g of EDC powder, stir it for another hour to activate the carboxyl group, and then add 0.6 g of NHS powder and stir it for another 3 hours. Use a dialysis bag (molecular weight cutoff of 3.5–8.0 kDa) to dialyze the solution for 48 hours to remove residual EDC and NHS. Finally, the dialyzate was freeze-dried for 48 hours to obtain the chitosan-polylipoic acid complex.

[0115] (3) 1 g of the chitosan-polylipoic acid complex was added to 100 mL of 1% acetic acid aqueous solution and sonicated for 2 h to promote its complete dissolution. Glycerol and polyethylene oxide (2% by mass volume) were added to the chitosan-polylipoic acid solution as a plasticizer and a spinning aid, respectively, and sonicated for 2 h to promote their uniform dispersion.

[0116] (4) At room temperature, the solution prepared above was spun into a film using an electrospinning machine to obtain a chitosan-polylipoic acid composite film, which was recorded as CA / CS-PLA-0 film. The specific conditions of electrospinning were as follows: a 20 ml syringe was used, the needle model was 22 G, the distance between the needle and the electrode was maintained at 12 cm, and the reciprocating distance was 10 cm. A 15 kV electric field was applied to all solutions, and the feed rate was 1 ml / h. The resulting electrospun film was collected on a grounded aluminum foil, the temperature was maintained at 25°C, and the humidity was 30%.

[0117] Figure 1a 、 Figure 1b The following are SEM images of the films prepared in Comparative Example 1 and Example 1 at different magnifications. The SEM images of Examples 2-4 are similar to those of Example 1. According to the SEM images, all film samples exhibited a typical fibrous network structure with no phase separation, indicating good compatibility between cinnamaldehyde and the chitosan-polylipoic acid complex. However, with increasing amounts of cinnamaldehyde added, there were some differences in fiber diameter and smoothness between the films. Compared to the film in Comparative Example 1, the addition of cinnamaldehyde resulted in increases in fiber diameter and roughness. For the films in Examples 2 and 3, the fiber diameter and roughness did not change significantly, but the overall distribution was uneven. For the films in Examples 1 and 4, the fiber diameter and roughness increased further, but the overall distribution was relatively uniform and dense. However, for the film in Example 5, the fiber diameter increased significantly, the surface became rougher, and some beaded structures and fiber adhesion appeared.

[0118] Figure 2a 、 Figure 2b 、 Figure 2c is a 3D laser confocal image of the film prepared in Comparative Example 1, Figure 3a 、 Figure 3b 、 Figure 3c This is a 3D laser confocal image of the film prepared in Example 1. The 3D surface image also directly demonstrates that the addition of cinnamaldehyde increases the film's line roughness and surface roughness. Notably, the film in Example 1 exhibits optimal surface uniformity while maintaining high roughness. This unique microstructure may enhance the film's mechanical interlocking effect.

[0119] Figure 4a 、 Figure 4bThe following graphs show the water contact angles of the films from Comparative Example 1 and Example 1, respectively. In Examples 1-5, the water contact angles of the films gradually decreased with increasing amounts of cinnamaldehyde. Compared to the film from Comparative Example 1, the addition of cinnamaldehyde significantly reduced the water contact angles of the films from Examples 1-5, indicating increased cohesion and adhesion between water and the film surface. This phenomenon can be attributed to the Schiff base reaction between chitosan and cinnamaldehyde, which may alter the molecular chain arrangement, exposing more hydrophilic groups (such as -OH and -NH2) on the surface.

[0120] Figure 5 The Fourier transform infrared spectra of the films prepared in Comparative Example 1 and Examples 1-5 are shown in FIG. Figure 5 As shown, compared with the film in comparative example 1, the infrared spectra of the films in examples 1-5 retain the original -C=O and -NH characteristic peaks, and two new characteristic peaks appear: The characteristic peak corresponding to -C=N appeared at 1732 The characteristic peak corresponding to -CHO appeared. This change in characteristic peak indicates that the chitosan molecular chain The group covalently bonds with the -CHO group at the end of cinnamaldehyde, forming a new Schiff base imine bond through the Schiff base reaction.

[0121] Figure 6 is the thickness of the films prepared in Comparative Example 1 and Examples 1-5. Figure 7 is the tensile strength of the films prepared in Comparative Example 1 and Examples 1-5. Figure 8 is the elongation at break of the films prepared in Comparative Example 1 and Examples 1-5. Figure 6-8 As shown. As the amount of cinnamaldehyde added increases, the thickness of the film also increases, which is attributed to the increase in the density of the film-forming solution. At the same time, the tensile strength of the film in Comparative Example 1 is 16.78 MPa, while the tensile strength of the films of Examples 1-5 significantly increases to 17.59-29.09 MPa. The tensile strength of the composite film increases with the increase of cinnamaldehyde content. When the cinnamaldehyde content reaches 2g and 3g, the tensile strength of the film increases by 61.79% and 73.32%. This shows that after the incorporation of cinnamaldehyde, the maximum tensile stress that the film can withstand is enhanced, especially for the films in Examples 1 and 5. At the same time, the elongation at break of the film in Comparative Example 1 is 90.87%, while the incorporation of cinnamaldehyde reduces the elongation at break, indicating that the brittleness of the film increases. However, even at high cinnamaldehyde content, the films in Examples 1 and 5 still reach 83.05% and 82.26%, respectively, showing their relatively high elasticity.

[0122] Figure 9a 、 Figure 9b and Figure 9cThe following are the comparison charts of water vapor permeability, oxygen permeability and carbon dioxide permeability of the films prepared in Comparative Example 1 and Examples 1-5 respectively. Packaging materials with low water vapor permeability are crucial to maintaining the moisture balance of food. Figure 9a As shown, the film in Comparative Example 1 exhibits the highest water vapor transmission rate ( ), and with the introduction of cinnamaldehyde, the water vapor permeability of the composite film showed a significant downward trend. When the amount of cinnamaldehyde added was increased to 2 g and 3 g, the water vapor permeability of the films in Example 1 and Example 5 decreased by 39.90% and 44.61% respectively compared with the control group, indicating that the introduction of cinnamaldehyde significantly enhanced the moisture barrier properties of the film. It is worth noting that although the decrease in water vapor permeability when the cinnamaldehyde content increased from 2 g to 3 g did not show a statistical difference, this phenomenon may indicate that cinnamaldehyde is close to forming an optimal cross-linked network structure at 2 g, and further increasing the cinnamaldehyde content has limited effect on the improvement of the film density. In addition to moisture barrier properties, the carbon dioxide permeability and oxygen permeability of packaging materials also have an important influence on regulating the respiratory metabolism of fruits and vegetables. The experimental results show that the carbon dioxide permeability and oxygen permeability of the film show similar changes: with the increase of cinnamaldehyde content, the film's resistance to gas ( and ) has significantly enhanced barrier properties. Compared with the film in comparative example 1, the carbon dioxide permeability of the films in Example 1 and Example 5 decreased by 27.93% and 29.60%, respectively, and the oxygen permeability also showed a synchronous downward trend. In addition, there was no significant difference in the carbon dioxide permeability and oxygen permeability of the films in Example 1 and Example 5, which further supports that cinnamaldehyde may form an efficient gas barrier structure at 2 g. Based on the above analysis, the introduction of cinnamaldehyde constructs a dense cross-linked network through the Schiff base reaction, which significantly improves the composite film's resistance to water vapor, and This property makes the film in Example 1 suitable as a preservation material for fruits with high respiration intensity (such as strawberries, blueberries or cherries).

[0123] Figure 10 The antibacterial properties of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5 are shown. Figure 11a 、 Figure 11b and Figure 11c The antibacterial rates of chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5 against Staphylococcus aureus, Escherichia coli and Botrytis cinerea are shown. Antibacterial performance is a key indicator of food packaging materials. Figure 10 、 Figure 11a 、 Figure 11b and Figure 11cAs shown, by testing the inhibitory effects of the films against Staphylococcus aureus, Escherichia coli, and Botrytis cinerea, the present invention found that the cinnamaldehyde / chitosan-polylipoic acid films (particularly the films in Examples 1 and 5 with high cinnamaldehyde additions) exhibited significant broad-spectrum antimicrobial activity, with inhibition rates exceeding 99%, far exceeding those of chitosan or polylipoic acid alone. The antimicrobial mechanism may involve the intrinsic antimicrobial properties of cinnamaldehyde and its synergistic effect with the chitosan-polylipoic acid complex, with the dynamic disulfide bond structure of polylipoic acid also contributing to the disruption of bacterial cell membranes.

[0124] Figure 12a 、 Figure 12b The scavenging rates of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5 for DPPH (2,2-diphenyl-1-picrylhydrazyl free radical) and ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) are shown, respectively. Figure 12c is the hemolysis rate of the chitosan-polylipoic acid films prepared in Comparative Example 1 and Examples 1-5. Figure 12a and Figure 12b The DPPH (2,2-diphenyl-1-picrylhydrazyl radical) scavenging rates of the films in Examples 1 and 5 reached 80.64% and 82.48% respectively, while the ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) scavenging rates were increased to 87.15% and 88.18%, which were significantly higher than those of the film in Comparative Example 1. In addition, Figure 12c Biocompatibility experiments showed that the hemolysis rate of cinnamaldehyde / chitosan-polylipoic acid film (0.26%-0.77%) was far below the safety threshold (5%), confirming its excellent blood compatibility and providing safety guarantees for its application in food packaging.

[0125] Figure 13 is the encapsulation efficiency of the cinnamaldehyde / chitosan-polylipoic acid film prepared in Examples 1-5. The encapsulation efficiency of cinnamaldehyde in the composite film directly affects its functional performance in fruit preservation applications. Figure 13As shown, the encapsulation efficiencies of the films in Examples 1-5 were 21.45%, 30.29%, 41.05%, 56.76%, and 59.58%, respectively, demonstrating an increasing trend with increasing cinnamaldehyde content. Notably, the difference between the films in Example 1 and Example 5 was not significant, likely due to the near-saturation loading capacity of the chitosan-polylipoic acid complex for cinnamaldehyde. Based on the above characterization results, the film in Example 1 exhibited the best overall performance across multiple key performance indicators: it not only possessed ideal surface uniformity and hydrophilicity, but also maintained excellent mechanical properties. More importantly, the film exhibited excellent barrier properties, broad-spectrum antimicrobial activity, and significant antioxidant capacity. These properties, combined with the high cinnamaldehyde encapsulation efficiency, provide the scientific basis for its application as a fruit preservation material.

[0126] Figure 14a This is the cinnamaldehyde response release performance of the cinnamaldehyde / chitosan-polylipoic acid film prepared in Example 1 under 30% and 90% humidity conditions. Figure 14b The cinnamaldehyde response release performance of the cinnamaldehyde / chitosan-polylipoic acid film prepared in Example 1 at pH 5, 6, and 7. Figure 14a 、 14b As shown, the film in Example 1 exhibits excellent dual humidity- and pH-responsive release properties, highly compatible with the slightly acidic (pH 5-6) and high humidity (>85% RH) environments of postharvest fruit storage. Results show that increasing humidity from 30% to 90% significantly increases the cumulative cinnamaldehyde release over 48 hours from 19.64% to 74.83%, attributed to the hydrophilic swelling effect of the chitosan-polylipoic acid complex under high humidity. Furthermore, the film exhibits significant acid-stimulated release, with cinnamaldehyde release at pH 5 (33.12 μg) exceeding three times that of a neutral environment (10.80 μg). This is primarily due to polymer network expansion and dynamic covalent bond cleavage caused by protonation of chitosan amino groups. This intelligent release mechanism achieves dual regulation: high humidity ensures basal release, while a slightly acidic environment triggers accelerated release, extending the duration of the active ingredient's action while preventing premature depletion, providing a precise and controllable active ingredient delivery system for fruit preservation.

[0127] Figure 15 The figure shows the biodegradability comparison of the cinnamaldehyde / chitosan-polylipoic acid film and the polyethylene film prepared in Example 1. Biodegradability is very important for composite films. Good biodegradability helps reduce environmental pollution, alleviate the "white pollution" problem, and promote sustainable development. Figure 15As shown, there is a huge gap between the polyethylene film and the composite film of Example 1. The polyethylene film did not degrade within 20 days, but the composite film of Example 1 was almost completely degraded within 20 days. This sharp contrast confirms that the composite film of Example 1 has excellent biodegradability.

[0128] The present invention also employed the films prepared in Example 1 and Comparative Example 1 to conduct fruit preservation tests. The specific method was as follows: Fresh cherries were harvested and carefully screened from an orchard to ensure that the selected fruits were similar in shape, size, color, and maturity, and free of obvious visual defects and diseases. After harvesting, the cherries were promptly transported to a laboratory. In the laboratory, they were thoroughly washed with distilled water and allowed to air dry. Plastic boxes of uniform size and specifications were then placed on a sponge mat. The composite films from Example 1 and Comparative Example 1 were then spread flat on the mat. A control group was also used, with no composite film applied. The cherries were then divided into three groups and sealed in three different types of plastic boxes. Each group contained 20 boxes, each containing eight cherries. After treatment, the boxes were stored in an incubator at 25°C and 75% relative humidity. Samples were collected regularly on days 0, 2, 4, 6, and 8. Various parameters, such as browning index (BI), flesh firmness, weight loss, decay rate, total soluble solids (TSS) content, and microbiological parameters, were measured, and optical photographs were taken. There were three replicates in each group, and all operations were performed at room temperature.

[0129] Figure 16a When the chitosan-polylipoic acid film and cinnamaldehyde / chitosan-polylipoic acid film prepared in Comparative Example 1 and Example 1 are used to preserve cherries, Changes in value (color brightness) with storage days; Figure 16b The changes of fruit firmness with storage days when the film is used to preserve cherries; Figure 16c The weight loss of the film used for cherry preservation changes with storage days; Figure 16d The total number of microorganisms in the film used to preserve cherries changes with storage days. Figure 16a 、 Figure 16b 、 Figure 16c and Figure 16d It can be seen that the composite film has shown significant effects in the application of cherry preservation. The results show that compared with the control group (the control group is cherries not coated with film) and the film in comparative example 1, the film in example 1 can effectively maintain the quality indicators of cherries during storage: maintaining higher The film improves color value (color vividness), delays firmness loss (inhibits cell wall degradation), reduces weight loss (reduces transpiration and respiration), and significantly inhibits microbial growth. These advantages stem from the film's excellent barrier properties, antimicrobial and antioxidant activity, and humidity and pH-responsive release properties. These advantages effectively protect cherries from microbial attack during an 8-day storage period, maintaining both their appearance and inherent quality, demonstrating its practical application as a multifunctional composite packaging film.

[0130] Comparative Example 2

[0131] The only difference between Comparative Example 2 and Example 1 is that polylipoic acid is not used in Comparative Example 2. That is, during the preparation process, a mixed solution containing chitosan, glycerol, polyethylene oxide, and cinnamaldehyde is directly emulsified and electrospun to form a film. The rest is the same as in Example 1 and will not be repeated here.

[0132] Compared to the chitosan-polylipoic acid composite film in Example 1, the film in Comparative Example 2 exhibited a looser, rougher, and more uneven microstructure, and its surface hydrophilicity was also reduced. In its microstructure, the film in Comparative Example 2 lacked the Schiff base imine bonds that are highly sensitive to pH changes. Mechanically, while maintaining the same film thickness, the tensile strength and elongation at break of the film in Comparative Example 2 were significantly lower. Gas barrier properties for water vapor, oxygen, and carbon dioxide were significantly inferior to those of the film in Example 1. While the film in Comparative Example 2 demonstrated some antioxidant and antibacterial capabilities, these were significantly lower than those of the film in Example 1. The encapsulation efficiency of the film in Comparative Example 2 was relatively low due to its limited pore structure and binding sites. While the film exhibited some pH- and humidity-responsive release, the response rate and level were significantly lower than those of the film in Example 1. Finally, in cherry preservation experiments, the film in Comparative Example 2 was effective in maintaining cherry quality during storage. But compared with the film of Example 1, the cherry The value and hardness are lower, and the weight loss and microbial growth are more serious.

[0133] In addition, the inventors of this case also referred to the aforementioned embodiments and conducted experiments using other raw materials, process operations, and process conditions described in this specification, and obtained relatively ideal results.

[0134] The various aspects, embodiments, features and examples of the present invention should be considered as illustrative and not intended to limit the present invention, the scope of which is defined solely by the claims. Other embodiments, modifications and uses will be apparent to those skilled in the art without departing from the spirit and scope of the present invention.

[0135] Although the present invention has been described with reference to illustrative embodiments, it will be understood by those skilled in the art that various other changes, omissions, and / or additions may be made without departing from the spirit and scope of the present invention, and that elements in the described embodiments may be substituted with substantial equivalents. Additionally, many modifications may be made without departing from the scope of the present invention to adapt specific circumstances or materials to the teachings of the present invention. Therefore, it is not intended herein to limit the present invention to the specific embodiments disclosed, but rather to include all embodiments within the scope of the appended claims. Furthermore, unless specifically stated, any use of the terms first, second, etc. does not indicate any order or importance, but rather uses the terms first, second, etc. to distinguish one element from another.

Claims

1. A biodegradable chitosan-polylipoic acid composite film, characterized in that: The chitosan-polylipoic acid composite film is obtained by electrospinning an emulsion containing a chitosan-polylipoic acid complex, a plasticizer, a spinning aid, and cinnamaldehyde; wherein the chitosan and polylipoic acid in the chitosan-polylipoic acid complex are connected by an amide bond, the mass ratio of the chitosan-polylipoic acid complex, the plasticizer, and the spinning aid is (0.5-1.5):(1-3):(1-3), and the mass ratio of the chitosan-polylipoic acid complex to cinnamaldehyde is (0.5-1.5):(0.2-3); The chitosan-polylipoic acid composite film can release cinnamaldehyde based on changes in pH value and / or humidity. The amount of cinnamaldehyde released from the chitosan-polylipoic acid composite film increases with increasing humidity. When the pH value is within the range of 5-7, the amount of cinnamaldehyde released from the chitosan-polylipoic acid composite film increases with decreasing pH value.

2. The biodegradable chitosan-polylipoic acid composite film according to claim 1, characterized in that: The weight average molecular weight of the polylipoic acid is 2000-20000 Da.

3. The biodegradable chitosan-polylipoic acid composite film according to claim 1, characterized in that: The thickness of the chitosan-polylipoic acid composite film is 30-70 μm.

4. The biodegradable chitosan-polylipoic acid composite film according to claim 1, characterized in that: The plasticizer includes one or a combination of glycerol, xylitol, sorbitol, polyethylene glycol, acetyl tributyl citrate, triethyl citrate or tributyl citrate.

5. The biodegradable chitosan-polylipoic acid composite film according to claim 1, characterized in that: The spinning aid comprises one or a combination of multiple of polyethylene oxide, polyvinyl pyrrolidone, polyacrylic acid or sodium dodecylbenzene sulfonate.

6. The method for preparing the biodegradable chitosan-polylipoic acid composite film according to any one of claims 1 to 5, characterized in that: include: causing chitosan and polylipoic acid to undergo amidation reaction to obtain a chitosan-polylipoic acid complex; emulsifying a liquid mixture containing the chitosan-polylipoic acid complex, a plasticizer, a spinning aid and cinnamaldehyde to form an emulsion; The electrostatic spinning technology is adopted to spin the emulsion into a film to obtain a chitosan-polylipoic acid composite film.

7. The preparation method according to claim 6, characterized in that: In the liquid mixture, the mass volume ratio of the chitosan-polylipoic acid complex is 0.5-1.5%, the mass volume ratio of the plasticizer is 1-3%, the mass volume ratio of the spinning aid is 1-3%, and the mass volume ratio of cinnamaldehyde is 0.2-3%.

8. The preparation method according to claim 6, characterized in that: The process parameters of the electrospinning technology include: using a 10-20 ml syringe, a needle model of 22-24G, a distance between the needle and the electrode of 10-15 cm, a reciprocating distance of 5-10 cm, an applied electric field of 15-25 kV, and a feed rate of 0.5-1 mL / h.

9. The preparation method according to claim 6, characterized in that The preparation method of the polylipoic acid comprises: causing a mixed reaction system containing lipoic acid, trifluoromethanesulfonic acid and a solvent to undergo polymerization reaction at a temperature of 20-30° C. to obtain the polylipoic acid.

10. The preparation method according to claim 9, characterized in that: The mass volume ratio of lipoic acid to solvent in the mixed reaction system is 1:10-1:15, and the mass volume ratio of lipoic acid to trifluoromethanesulfonic acid is 1:0.0025-1:0.

005.

11. The preparation method according to claim 9, characterized in that: The solvent in the mixed reaction system includes dichloromethane.

12. The preparation method according to claim 9, characterized in that: The polymerization reaction time is 1 to 3 hours.

13. The preparation method according to claim 6, characterized in that The amidation reaction comprises: providing a mixed solution containing chitosan, polylipoic acid, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride and N-hydroxysuccinimide, adjusting the pH value of the mixed solution to 5-6, and reacting at a temperature of 0-4°C to obtain a chitosan-polylipoic acid complex.

14. The preparation method according to claim 13, characterized in that: The mass ratio of chitosan to polylipoic acid in the mixed solution is 1:0.1-1:0.

5.

15. The preparation method according to claim 6, characterized in that The preparation method of the emulsion comprises: Dissolving the chitosan-polylipoic acid complex in a first solvent, adding the plasticizer and the spinning aid, and fully dispersing the mixture to form a first solution; dissolving cinnamaldehyde in a second solvent, adding a surfactant, and performing ultrasonic emulsification to obtain a second emulsion; The second emulsion is added dropwise to the first solution, stirred in the dark to fully mix, and then ultrasonically degassed to obtain the emulsion.

16. The preparation method according to claim 15, characterized in that: The mass volume ratio of the chitosan-polylipoic acid complex to the first solvent is 1:100-1:

120.

17. The preparation method according to claim 15, characterized in that: The mass volume ratio of the cinnamaldehyde to the second solvent is 1:5-1:

10.

18. The preparation method according to claim 15, characterized in that: The first solvent includes acetic acid and water, wherein the volume proportion of acetic acid is 1-2%; and / or the second solvent includes anhydrous ethanol.

19. The preparation method according to claim 15, characterized in that: The mass ratio of the cinnamaldehyde to the surfactant is 2:1-1:

1.

20. The preparation method according to claim 15, characterized in that: The surfactant includes one or a combination of multiple of Tween 80, Tween 20, Tween 60, sucrose ester, and monoglyceride.

21. The preparation method according to claim 15, characterized in that: The ultrasonic emulsification time is 10-15 minutes; and / or, the light-proof stirring time is 2-4 hours, and the ultrasonic degassing time is 1-2 hours.

22. Use of the biodegradable chitosan-polylipoic acid composite film according to any one of claims 1 to 5 in food packaging or food preservation, wherein the food comprises fruits and vegetables.

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