Gelatin film based on porous dialdehyde starch and tea polyphenol crosslinking modification and preparation method thereof

Abstract

The invention discloses a gelatin film based on porous dialdehyde starch and tea polyphenol crosslinking modification and a preparation method thereof. The preparation method comprises the following steps: dispersing starch in an acetic acid buffer solution, adding a compound enzyme for enzymolysis, stirring, and adding an acidic material to adjust the pH of a starch suspension so as to terminate the reaction; porous starch solids are obtained; under a water bath condition, mixing the porous starch solid with water to form a starch suspension, adding an oxidizing agent, and adjusting the pH value of the system to 3-4 by using an alkaline substance; a porous dialdehyde starch solid is obtained; dissolving gelatin in water, stirring, sequentially adding porous dialdehyde starch, tea polyphenol and glycerol, and sequentially stirring and carrying out ultrasonic treatment; and transferring the obtained composite solution to a film forming device, and drying. The porous dialdehyde starch prepared by the invention can be used as a natural chemical cross-linking agent to construct an interconnected cross-linked network with tea polyphenol in a gelatin-based film, so that the difficult problems of poor mechanical property, low ultraviolet barrier property, weak oxidation resistance and the like of the gelatin film are synergistically improved.

Description

Technical Field

[0001] The present invention relates to the technical field of gelatin-based films, and specifically to a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols and a preparation method thereof. Background Art

[0002] As an alternative to petrochemical packaging, natural biodegradable packaging materials such as polysaccharides, proteins, and phenolic compounds can be recovered from agricultural wastes, becoming a solution to alleviate environmental problems and meet the needs of sustainable packaging practices.

[0003] In a sodium alginate microcapsule of pine polyphenol with dialdehyde starch as the wall material and a preparation method thereof disclosed in Chinese invention patent CN118266585A, when the molar ratio of sodium periodate to glucose in starch is 1.0, the aldehyde group content is only about 80%. To solve the problem of the relatively low aldehyde group content of dialdehyde starch, researchers have tried to use two methods synergistically to increase its aldehyde group content. Taking the combination of chemical method and electrochemical method as an example, Chinese invention patent CN114807988B provides a cathode material for electrolyzing starch to synthesize dialdehyde starch, and dialdehyde starch is prepared by electroplating an Au-Ni-Sn composite coating on the surface of a graphite electrode. However, this method requires the use of pyridine solvent, and the waste liquid generated will pollute the environment and the residue of the solvent is also likely to cause food safety risks, so this method is only applicable to non-food fields. Summary of the Invention

[0004] The main purpose of the present invention is to provide a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols and a preparation method thereof, using the combination of enzymatic hydrolysis method and oxidation method to prepare porous dialdehyde starch, solving the problem of the relatively low aldehyde group content of dialdehyde starch, and at the same time using porous dialdehyde starch as a natural macromolecular crosslinking agent to synergistically improve the mechanical properties, ultraviolet barrier properties, and antioxidant properties of the gelatin-based film with tea polyphenols.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A preparation method of a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols, comprising the following steps:

[0007] (1) Enzymatic hydrolysis treatment: Starch is dispersed in an acetate buffer solution, a composite enzyme is added, and enzymatic hydrolysis is carried out at 40-55°C. After stirring, an acidic substance is added to adjust the pH of the starch suspension to terminate the reaction; the obtained reaction solution is washed with water and alcohol and then filtered by suction to obtain porous starch solid; the composite enzyme is a combination of glucoamylase and α-amylase;

[0008] (2) Oxidation treatment: Under water bath conditions, mix the porous starch solid with water to form a starch suspension, then add an oxidizing agent, and adjust the pH of the system to 3 - 4 with an alkaline substance; after washing the obtained reaction solution with water and alcohol and then filtering by suction, obtain a porous dialdehyde starch solid;

[0009] (3) Preparation of gelatin film: Dissolve gelatin in water and stir, sequentially add porous dialdehyde starch, tea polyphenols and glycerol and stir sequentially, then perform ultrasonic treatment; transfer the obtained composite solution to a film-forming device and dry to obtain a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols.

[0010] To further achieve the object of the present invention, preferably, in step (1):

[0011] The starch is corn starch, cassava starch or potato starch;

[0012] The acetic acid buffer solution is a mixed solution of acetic acid and sodium acetate, with a pH of 4.5 - 5.0, and the mass - volume ratio of sodium acetate to acetic acid is 1g:3mL - 1g:4mL;

[0013] The acidic substance is one or more of hydrochloric acid, acetic acid or sulfuric acid, and the pH of the adjusted system is 1.2 - 1.5;

[0014] The alcohol washing is carried out with 75% - 90% ethanol;

[0015] The rotation speed of the stirring is 400 - 600 rpm.

[0016] Preferably, in step (1), the enzyme activity of the glucoamylase is 20000 - 25000 U / mL, and the enzyme activity of the α - amylase is 3000 - 4000 U / mL.

[0017] Preferably, in step (1), the mass ratio of the glucoamylase to the α - amylase is 1:1 - 3:1, the enzymolysis time is 1 - 3 h, and the concentration of the composite enzyme is 1 wt% - 3 wt%.

[0018] Preferably, in step (2), the water bath temperature is 30 - 40 °C; the mass ratio of the porous starch to water is 1:3 - 1:4.

[0019] Preferably, in step (2), the oxidizing agent is one or more of sodium periodate and periodic acid; the molar ratio of the oxidizing agent to the glucose unit in the starch is 0.8:1 - 1.2:1; the oxidation reaction time for adding the oxidizing agent is 3 - 5 h.

[0020] Preferably, in step (2), the alkaline substance is one or more of sodium hydroxide and sodium bicarbonate; the alcohol washing is carried out with 75% - 90% ethanol.

[0021] Preferably, in step (3):

[0022] Taking g and mL as units respectively, the material ratio of gelatin to water is 1:30 to 1:40;

[0023] The porous dialdehyde starch solid is 5% to 15% of the mass of gelatin, tea polyphenols are 1% to 5% of the mass of gelatin, and glycerol is 9% to 15% of the mass of gelatin.

[0024] Preferably, in step (3):

[0025] The temperature of stirring after adding gelatin is 70 to 80 °C, the stirring time is 5 to 15 min, and the stirring speed is 400 to 600 rpm;

[0026] The stirring temperature after adding porous dialdehyde starch is 70 to 80 °C, and the stirring time is 30 to 50 min;

[0027] The stirring temperature after adding tea polyphenols is 70 to 80 °C, and the stirring time is 10 to 20 min;

[0028] The stirring temperature after adding glycerol is 70 to 80 °C, and the stirring time is 12 to 16 min.

[0029] The stirring speeds for stirring after adding porous dialdehyde starch, stirring after adding tea polyphenols, and stirring after adding glycerol are all 400 to 600 rpm;

[0030] The ultrasonic treatment is to remove bubbles; the ultrasonic treatment time is 1 to 3 min;

[0031] The film-forming device includes a plastic petri dish, a glass petri dish or a feeding trough;

[0032] The drying is carried out in an oven or a dryer, the temperature is 35 to 45 °C, and the time is 24 to 48 h.

[0033] A gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols is prepared by the above preparation method.

[0034] Compared with the prior art, the present invention has the following advantages and beneficial effects.

[0035] 1) The present invention prepares dialdehyde starch through enzymatic hydrolysis treatment and oxidation treatment. The high specific surface area of porous starch increases its contact with sodium periodate, solving the problem of low aldehyde group content of dialdehyde starch.

[0036] 2) The porous dialdehyde starch and natural tea polyphenols prepared by the present invention used in the gelatin-based film significantly improve the mechanical properties, ultraviolet barrier properties and antioxidant properties of the film.

[0037] 3) The raw materials used in the present invention are all biodegradable, non-toxic and harmless, and can be used in the field of food packaging, which is very friendly to the environment.

[0038] 4) The process for preparing the thin film of the present invention is energy-saving, clean and environmentally friendly, and is an efficient and green processing method. Description of the Drawings

[0039] Figure 1 It is a comparison chart of the test results of the mechanical properties of the thin films of the examples and comparative examples of the present invention.

[0040] Figure 2 It is a diagram showing the test situation of the ultraviolet barrier performance of the thin films of the examples and comparative examples of the present invention.

[0041] Figure 3 It is a diagram showing the test situation of the antioxidant performance of the thin films of the examples and comparative examples of the present invention. Detailed Embodiments

[0042] To better understand the present invention, the present invention will be further described below in conjunction with the drawings and specific embodiments, but the embodiments of the present invention are not limited thereto. The described embodiments are some embodiments of the present invention, not all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts shall fall within the scope of protection of the present invention.

[0043] Although gelatin is a soluble protein compound obtained by partial hydrolysis of collagen, it has advantages such as high biocompatibility and effective film formation. However, gelatin-based thin films have poor mechanical properties, low ultraviolet barrier performance, and weak antioxidant ability. Due to these limitations, it is difficult for such thin films to be widely used in the food packaging industry.

[0044] In view of the above situation, the present invention finds that porous dialdehyde starch with a high aldehyde group content can be obtained by the synergistic action of enzymatic hydrolysis and oxidation; because dialdehyde starch, as a natural macromolecular cross-linking agent, can be used to develop gelatin-based thin films, and its aldehyde group content is positively correlated with the mechanical strength, ultraviolet barrier performance and antioxidant properties of gelatin-based thin films. Natural starch is hydrolyzed by amylase to obtain porous starch, and α-amylase and glucoamylase can randomly cut the α-1,4 glycosidic bonds inside the starch to produce dextrin, maltose and glucose with shorter molecular chains. During this hydrolysis process, many uniform pores gradually form on the surface of the porous starch; the porous starch is oxidized by sodium periodate or periodic acid to obtain porous dialdehyde starch. Due to the large specific surface area of the porous starch, more reaction sites with sodium periodate are provided, so the use of the same molar amount of sodium periodate or periodic acid can produce more reactive aldehyde groups.

[0045] The pore structure of the porous dialdehyde starch of the present invention significantly increases its specific surface area. On the basis of retaining the uniform pores of the porous starch, sodium periodate or periodic acid is used to oxidize the porous starch, so that more hydroxyl groups are oxidized to aldehyde groups, making it easier for the aldehyde groups to undergo Schiff base reaction with the amino groups of gelatin, forming a denser and more uniform cross-linked network. At the same time, the interaction between tea polyphenols and amino, carboxyl and hydroxyl groups in the film matrix promotes the formation of hydrogen bonds, further strengthening the cross-linked network and improving the tensile strength of the examples. On the other hand, the more imine groups generated by the aldehyde groups of the porous dialdehyde starch and the amino groups of gelatin, the better the UV barrier performance. In particular, using porous dialdehyde starch as a cross-linking agent not only forms a cross-linked network of imine covalent bonds and hydrogen bonds, but also makes the structure of gelatin loaded with tea polyphenols denser. This compact structure helps the slow release of tea polyphenols over time, further enhancing the antioxidant capacity of the film.

[0046] Therefore, the present invention solves the problems of the prior art by synergistically using enzymatic hydrolysis and oxidation methods to obtain porous dialdehyde starch with a high aldehyde group content, and at the same time, combining the porous dialdehyde starch with gelatin and tea polyphenols to form a novel natural cross-linking agent and gelatin and tea polyphenols to form a bio-composite film, making the composite film have excellent mechanical properties, UV barrier properties and antioxidant properties, and the comprehensive performance is very excellent, especially suitable for applications in the fields of food, medicine, instruments, etc. Specifically, the bio-based multifunctional film material can be used for packaging foods such as fresh fruits and vegetables, meats, and aquatic products to extend their shelf life; it can be used as a drug sustained-release carrier to achieve the slow release of drugs, extend the action time of drugs, and improve the efficacy of drugs; it can protect electronic devices from being eroded by water vapor, oxygen and dust, and improve the stability and service life of electronic devices.

[0047] For this reason, the present invention provides a preparation method of a gelatin film cross-linked and modified based on porous dialdehyde starch and tea polyphenols, comprising the following steps:

[0048] (1) Enzymatic hydrolysis treatment: Starch is dispersed in an acetate buffer solution, and a composite enzyme is added for enzymatic hydrolysis at 40-55 °C. After stirring, an acidic substance is added to adjust the pH of the starch suspension to terminate the reaction; the obtained reaction solution is washed with water and alcohol and then filtered by suction to obtain a porous starch solid; the composite enzyme is a combination of glucoamylase and α-amylase;

[0049] (2) Oxidation treatment: Under water bath conditions, the porous starch solid is mixed with water to form a starch suspension, and then an oxidant is added, and the pH of the system is adjusted to 3-4 with an alkaline substance; the obtained reaction solution is washed with water and alcohol and then filtered by suction to obtain a porous dialdehyde starch solid;

[0050] (3) Preparation of film: Dissolve gelatin in water and stir, then successively add porous dialdehyde starch, tea polyphenols and glycerol and stir them in turn, followed by ultrasonic treatment; transfer the obtained composite solution to a film-forming device and dry it to obtain a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols.

[0051] In this field, enzymatic hydrolysis treatment of starch, and even oxidation treatment, are existing technical measures for reference. The feature of the present invention is to prepare porous dialdehyde starch by combining enzymatic hydrolysis treatment and oxidation treatment of starch; on the basis of the obtained porous dialdehyde starch, it is compounded with gelatin, tea polyphenols and glycerol, and then a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols with excellent comprehensive properties is prepared. As for the specific enzymatic hydrolysis process, the present invention preferably uses an acetate buffer solution as a mixed solution of acetic acid and sodium acetate, and the mass-volume ratio of sodium acetate to acetic acid is 1 g: 3 mL - 1 g: 4 mL; the starch can preferably be corn starch, tapioca starch or potato starch; in fact, other starches can also achieve the purpose of the invention, but based on cost, the above starches are preferably selected as raw material sources. The pH is preferably controlled at 4.5 - 5.0 during enzymatic hydrolysis.

[0052] After enzymatic hydrolysis, the reaction is terminated by adding an acidic substance to adjust the pH of the starch suspension, and the pH here is controlled at 1.2 - 1.5;

[0053] The acidic substance is hydrochloric acid, acetic acid or sulfuric acid, and the pH of the adjusted system is 1.2 - 1.5;

[0054] The enzyme activity of glucoamylase in the complex enzyme is preferably 20000 - 25000 U / mL, and the enzyme activity of α - amylase is preferably 3000 - 4000 U / mL. The mass ratio of glucoamylase to α - amylase is 1:1 - 3:1, the enzymatic hydrolysis time is 1 - 3 h, and the concentration of the complex enzyme is 1 wt% - 3 wt%.

[0055] The oxidation treatment of starch also has reference in the prior art. In the present invention, the oxidant is preferably sodium periodate and / or periodic acid, and the molar ratio of the oxidant to the glucose unit in the starch is preferably controlled at 0.8:1 - 1.2:1; the oxidation reaction time for adding the oxidant is preferably controlled at 3 - 5 h. The water bath conditions of the present invention are preferably controlled at a temperature of 30 - 40 °C, and the mass ratio of porous starch to water is 1:3 - 1:4; the alkaline substance is preferably sodium hydroxide and / or sodium bicarbonate; alcohol washing is preferably carried out with 75% - 90% ethanol.

[0056] Step (3) of the technical measures of the present invention is a specific embodiment of film forming, which reflects the raw material requirements of gelatin, porous dialdehyde starch, tea polyphenols and glycerol of the present invention, and requires ultrasonic cooperation treatment. As for the proportional relationship of the raw material dosage, it can be determined by testing in combination with the purpose of the present invention. Preferably, the solid of porous dialdehyde starch is 5-15% of the mass of gelatin, tea polyphenols is 1-5% of the mass of gelatin, and glycerol is 9%-15% of the mass of gelatin. Gelatin should be dissolved in water first, in units of g and mL respectively. Preferably, the material ratio of gelatin to water is controlled to be 1:30-1:40. The feeding method of this step requires adding porous dialdehyde starch, tea polyphenols and glycerol in sequence and stirring them in sequence before ultrasonic treatment; preferably, the temperature of stirring after adding gelatin is controlled to be 70-80°C, the stirring time is 5-15 min, and the stirring speed is 400-600 rpm; the stirring temperature after adding porous dialdehyde starch is 70-80°C, and the stirring time is 30-50 min; the stirring temperature after adding tea polyphenols is 70-80°C, and the stirring time is 10-20 min; the stirring temperature after adding glycerol is 70-80°C, and the stirring time is 12-16 min. The stirring speeds of stirring after adding porous dialdehyde starch, stirring after adding tea polyphenols and stirring after adding glycerol are all 400-600 rpm.

[0057] Before the film analysis and testing, it must be balanced in a dryer with a relative humidity of 58% for 48 h. The testing methods are as follows:

[0058] (1) The methods for measuring the aldehyde group content of porous dialdehyde starch and dialdehyde starch in the examples and comparative examples of the present invention are as follows (CN104087688-A):

[0059] Dissolve 0.2 g of porous dialdehyde starch or dialdehyde starch in 10 mL of sodium hydroxide solution (0.25 mol / L). Place the mixture in a 70°C water bath for 2 min, and then cool the mixture with cold water. The resulting solution is yellowish-brown. Add 15 mL of sulfuric acid solution (0.125 mol / L), and shake evenly until the solution turns light yellow. Then add 4 drops of phenolphthalein to the solution and titrate it with sodium hydroxide solution (0.1 mol / L). When the solution turns light pink and does not fade within 30 s, the titration reaches the end point. The aldehyde group content is calculated by the following formula:

[0060] -CHO% = (C1V1 + C2V2 - 2xC3V3) × 161 / m / 1000 × 100%

[0061] In the formula, -CHO% represents the aldehyde group content of the porous dialdehyde starch or dialdehyde starch to be measured, the value of C1 is 0.25 mol / L, the value of V1 is 10 mL, the value of C2 is 0.1 mol / L, V2 is the volume of the NaOH solution used in the titration process, the value of C3 is 0.125 mol / L, the value of V3 is 15 mL, m is the mass of the porous dialdehyde starch or dialdehyde starch, and 161 is the average relative molecular mass of the starch glucose unit.

[0062] (2) The method for measuring the tensile strength of the films in the examples and comparative examples of the present invention is as follows (IN202541011414-A):

[0063] Cut the film into a 70 mm × 10 mm rectangle, and use a tensile testing machine to conduct tensile tests on the film. The tensile speed is set at 2 mm / s, and the tensile strength, elongation at break, and Young's modulus of the film are measured. Each sample is tested three times, and the average value is taken.

[0064] (3) The method for measuring the ultraviolet barrier performance of the films in the examples and comparative examples of the present invention is as follows (CN118620402-A):

[0065] Cut the film into a 1 cm × 7 cm rectangle, place it on one side of a cuvette, and use an ultraviolet-visible spectrophotometer to measure the transmittance of the sample in the wavelength range of 200 - 800 nm.

[0066] Example 1

[0067] Disperse 12 g of corn starch in 40 mL of acetic acid buffer solution (pH = 4.5), mix well, add 2.9 wt% of a composite enzyme, in which the mass ratio of glucoamylase (enzyme activity of 21000 U / mL) to α-amylase (enzyme activity of 3200 U / mL) is 2:1. After stirring in a water bath at 50 °C for 2.5 h, add hydrochloric acid to adjust the pH of the starch suspension to 1.5 to terminate the reaction. Wash the reaction solution with deionized water and ethanol (75%) and filter by suction to obtain a porous starch solid.

[0068] Under the condition of a 35 °C water bath, take 8 g of porous starch and mix it with 25 mL of distilled water to prepare a starch suspension, and add an aqueous sodium periodate solution (the molar ratio of sodium periodate to glucose units in starch is 1:1). Adjust the pH of the system to 3.5 with sodium hydroxide solution and react for 4 h. Wash the reaction solution with deionized water and ethanol (75%) and filter by suction to obtain a porous dialdehyde starch solid, in which the aldehyde group content of the porous dialdehyde starch is 98.56%.

[0069] After dissolving 3 g of gelatin in 100 mL of distilled water, it was placed in a 75 °C water bath and stirred for 10 min. Subsequently, 0.30 g of porous dialdehyde starch solid was added, and stirring continued at this temperature for 40 min. Then, 0.06 g of tea polyphenols was mixed into the gelatin / starch solution, and stirring continued for 15 min. Finally, 0.30 g of glycerol was added to the mixture, and stirring continued for 15 min. The composite solution was ultrasonically treated for 2 min to remove air bubbles, and then transferred to a plastic Petri dish and placed in an oven at 40 °C for drying for 36 h, obtaining the gelatin / porous dialdehyde starch / tea polyphenol film. The tensile strength of the film was 29.97 MPa, the ultraviolet shielding ability was 99.07%, and the DPPH and ABTS radical scavenging rates were 79.49% and 86.07% respectively.

[0070] Example 2

[0071] After dispersing 12 g of tapioca starch in 36 mL of acetic acid buffer solution (pH = 5) and mixing well, 1.0 wt% of a composite enzyme was added, where the mass ratio of glucoamylase (enzyme activity of 20000 U / mL) to α-amylase (enzyme activity of 3000 U / mL) was 1:1. After stirring in a 40 °C water bath for 1 h, acetic acid was added to adjust the pH of the starch suspension to 1.2 to terminate the reaction. The reaction solution was washed with deionized water and ethanol (85%) and filtered by suction to obtain the porous starch solid.

[0072] Under the condition of a 30 °C water bath, 8 g of porous starch was mixed with 24 mL of distilled water to prepare a starch suspension, and an aqueous solution of periodic acid (the molar ratio of periodic acid to glucose units in starch was 0.8:1) was added. The pH of the system was adjusted to 3.0 with sodium bicarbonate solution and reacted for 3 h. The reaction solution was washed with deionized water and ethanol (85%) and filtered by suction to obtain the porous dialdehyde starch solid, and the aldehyde group content of the porous dialdehyde starch was 96.68%.

[0073] After dissolving 3 g of gelatin in 90 mL of distilled water, it was placed in a 70 °C water bath and stirred for 5 min. Subsequently, 0.15 g of porous dialdehyde starch solid was added, and stirring continued at this temperature for 30 min. Then, 0.03 g of tea polyphenols was mixed into the gelatin / starch solution, and stirring continued for 10 min. Finally, 0.27 g of glycerol was added to the mixture, and stirring continued for 12 min. The composite solution was ultrasonically treated for 2 min to remove air bubbles, and then transferred to a feeding tank and placed in a dryer at 35 °C for drying for 48 h, obtaining the gelatin / porous dialdehyde starch / tea polyphenol film. The tensile strength of the film was 28.96 MPa, the ultraviolet shielding ability was 96.93%, and the DPPH and ABTS radical scavenging rates were 72.55% and 80.90% respectively.

[0074] Example 3

[0075] After dispersing 12 g of potato starch in 48 mL of acetate buffer solution (pH = 4.7) and mixing well, 3.0 wt% of a composite enzyme was added, where the mass ratio of glucoamylase (enzyme activity of 25000 U / mL) to α-amylase (enzyme activity of 4000 U / mL) was 3:1. After stirring continuously in a water bath at 55 °C for 3 h, sulfuric acid was added to adjust the pH of the starch suspension to 1.4 to terminate the reaction. The reaction solution was washed with deionized water and ethanol (90%) and then filtered by suction to obtain porous starch solids.

[0076] Under the condition of a 40 °C water bath, 8 g of porous starch was mixed with 32 mL of distilled water to prepare a starch suspension, and an aqueous sodium periodate solution (the molar ratio of sodium periodate to glucose units in starch was 1.2:1) was added. The pH of the system was adjusted to 4.0 with sodium hydroxide solution and reacted for 5 h. The reaction solution was washed with deionized water and ethanol (90%) and then filtered by suction to obtain porous dialdehyde starch solids, and the aldehyde group content of the porous dialdehyde starch was 93.81%.

[0077] After dissolving 3 g of gelatin in 120 mL of distilled water, it was placed in a water bath at 80 °C and stirred for 15 min. Subsequently, 0.45 g of porous dialdehyde starch solids was added, and stirring was continued at this temperature for 50 min. Then, 0.15 g of tea polyphenols was mixed into the gelatin / starch solution, and stirring was continued for 20 min. Finally, 0.45 g of glycerol was added to the mixture, and stirring was continued for 16 min. The composite solution was ultrasonically treated for 3 min to remove air bubbles, and then transferred to a glass petri dish and placed in an oven at 45 °C for drying for 24 h to obtain a gelatin / porous dialdehyde starch / tea polyphenol film, where the tensile strength of the film was 27.51 MPa, the ultraviolet shielding ability was 94.74%, and the DPPH and ABTS radical scavenging rates were 69.77% and 76.92% respectively.

[0078] Comparative Example 1

[0079] After dissolving 3 g of gelatin in 100 mL of distilled water, it was placed in a water bath at 75 °C and stirred for 20 min. Subsequently, 0.30 g of glycerol was added to the mixture, and stirring was continued for 15 min. The composite solution was ultrasonically treated for 2 min to remove air bubbles, and then transferred to a plastic petri dish and placed in an oven at 40 °C for drying for 48 h to obtain a gelatin film, where the tensile strength of the film was 6.68 MPa, the ultraviolet shielding ability was 11.93%, and the DPPH and ABTS radical scavenging rates were 14.12% and 24.55% respectively.

[0080] Comparative Example 2

[0081] Under the condition of a 35 °C water bath, 8 g of corn starch was taken and mixed with 25 mL of distilled water to prepare a starch suspension, and an aqueous sodium periodate solution (the molar ratio of sodium periodate to glucose units in starch was 1:1) was added. The pH of the system was adjusted to 3.5 with sodium hydroxide solution and reacted for 4 h. The reaction solution was washed with deionized water and ethanol (90%) and filtered by suction to obtain solid dialdehyde starch, and the aldehyde group content of the dialdehyde starch was 80.77%.

[0082] After dissolving 3 g of gelatin in 100 mL of distilled water, it was placed in a 75 °C water bath and stirred for 10 min. Subsequently, 0.30 g of solid dialdehyde starch was added, and stirring was continued at this temperature for 55 min. Finally, 0.30 g of glycerol was added to the mixed solution, and stirring was continued for 15 min. The composite solution was ultrasonically treated for 2 min to remove bubbles, and then transferred to a plastic petri dish and placed in an oven at 40 °C for drying for 48 h to obtain a gelatin / dialdehyde starch film. The tensile strength of the film was 17.83 MPa, the ultraviolet shielding ability was 76.35%, and the DPPH and ABTS radical scavenging rates were 44.25% and 48.89% respectively.

[0083] Comparative Example 3

[0084] After dispersing 12 g of corn starch in 40 mL of acetate buffer solution (pH = 4.5) and mixing well, 3.0 wt% of a composite enzyme was added, in which the mass ratio of glucoamylase (enzyme activity of 20000 U / mL) to α-amylase (enzyme activity of 3000 U / mL) was 2:1. After stirring in a 50 °C water bath for 2.5 h, hydrochloric acid was added to adjust the pH of the starch suspension to 1.5 to terminate the reaction. The reaction solution was washed with deionized water and ethanol (90%) and filtered by suction to obtain solid porous starch.

[0085] Under the condition of a 35 °C water bath, 8 g of porous starch was taken and mixed with 25 mL of distilled water to prepare a starch suspension, and an aqueous sodium periodate solution (the molar ratio of sodium periodate to glucose units in starch was 1:1) was added. The pH of the system was adjusted to 3.5 with sodium hydroxide solution and reacted for 4 h. The reaction solution was washed with deionized water and ethanol (90%) and filtered by suction to obtain solid porous dialdehyde starch, and the aldehyde group content of the porous dialdehyde starch was 98.56%.

[0086] After dissolving 3 g of gelatin in 100 mL of distilled water, it was placed at 75 °C and stirred for 10 min. Subsequently, 0.30 g of porous dialdehyde starch solid was added, and stirring was continued at this temperature for 40 min. Finally, 0.30 g of glycerol was added to the mixture, and stirring was continued for 15 min. The mixed solution was ultrasonically treated for 3 min to remove bubbles, and then transferred to a plastic petri dish and dried in an oven at 40 °C for 36 h to obtain a gelatin / porous dialdehyde starch film. The tensile strength of the film was 25.37 MPa, the ultraviolet shielding ability was 91.45%, and the DPPH and ABTS radical scavenging rates were 48.70% and 55.10% respectively.

[0087] Comparative Example 4

[0088] After dissolving 3 g of gelatin in 100 mL of distilled water, it was placed at 75 °C and stirred for 10 min. Subsequently, 0.36 g of tea polyphenols was added and mixed into the gelatin solution, and stirring was continued for 15 min. Finally, 0.30 g of glycerol was added to the mixture, and stirring was continued for 15 min. The composite solution was ultrasonically treated for 2 min to remove bubbles, and then transferred to a plastic petri dish and dried in an oven at 40 °C for 48 h to obtain a gelatin / tea polyphenol film. The tensile strength of the film was 11.31 MPa, the ultraviolet shielding ability was 16.34%, and the DPPH and ABTS radical scavenging rates were 55.05% and 65.51% respectively.

[0089] Comparative Example 5

[0090] Under the condition of a 45 °C water bath, 8 g of corn starch was mixed with 25 mL of distilled water to prepare a starch suspension, and an aqueous sodium periodate solution (the molar ratio of sodium periodate to glucose units in starch was 1.5:1) was added. The pH of the system was adjusted to 3.5 with sodium hydroxide solution and reacted for 4 h. The reaction solution was washed with deionized water and ethanol (90%) and filtered by suction to obtain solid dialdehyde starch, and the aldehyde group content of the dialdehyde starch was 80.77%.

[0091] After dissolving 3 g of gelatin in 100 mL of distilled water, it was placed at 75 °C and stirred for 10 min. Subsequently, 0.30 g of dialdehyde starch solid was added, and stirring was continued at this temperature for 40 min. Then, 0.06 g of tea polyphenols was mixed into the gelatin / starch solution, and stirring was continued for 15 min. Finally, 0.30 g of glycerol was added to the mixture, and stirring was continued for 15 min. The composite solution was ultrasonically treated for 2 min to remove bubbles, and then transferred to a plastic petri dish and dried in an oven at 40 °C for 36 h to obtain a gelatin / dialdehyde starch / tea polyphenol film. The tensile strength of the film was 21.01 MPa, the ultraviolet shielding ability was 87.42%, and the DPPH and ABTS radical scavenging rates were 66.64% and 72.44% respectively.

[0092] Figure 1Show the comparison of the mechanical property test results of the films obtained in the examples and comparative examples. The tensile strengths of Examples 1, 2, and 3 are 29.97 MPa, 28.96 MPa, and 27.51 MPa, respectively. Compared with Comparative Examples 4 and 5, this enhancement in mechanical properties can be attributed to the significantly increased specific surface area of the porous dialdehyde starch due to its pore structure, making its aldehyde groups more likely to undergo Schiff base reactions with the amino groups of gelatin, forming a denser and more uniform cross-linked network. Compared with Comparative Examples 1, 2, and 3, the interaction between tea polyphenols and the amino, carboxyl, and hydroxyl groups within the film matrix promotes the formation of hydrogen bonds, further strengthening the cross-linked network and increasing the tensile strength of the examples.

[0093] Figure 2 Show the UV barrier property test results of the films obtained in the examples and comparative examples. Comparative Example 1 film has the worst UV barrier property, probably because there are no groups in the film that can absorb ultraviolet light. Due to the presence of hydroxyl groups and conjugated double bonds in tea polyphenols in Comparative Example 4, it has a certain ability to absorb ultraviolet light in the UVB (280 - 320 nm) region, but it has almost no UV shielding ability in the UVA (320 - 400 nm) region. The UV barrier properties of Examples 1, 2, and 3 in the UVA (320 - 400 nm) region are 99.07%, 96.93%, and 94.74%, respectively. Compared with Comparative Example 5, this indicates that when the film thickness is relatively consistent, the more imine groups formed by the aldehyde groups of the porous dialdehyde starch and the amino groups of gelatin, the better the UV barrier property.

[0094] Figure 3 Show the antioxidant property test results of the films obtained in the examples and comparative examples. Comparative Examples 2 and 3 show weak DPPH and ABTS radical scavenging activities. This is mainly because these aldehyde groups can react with free radicals to stabilize and terminate the free radical chain reaction. In Comparative Examples 4 and 5, the phenolic hydroxyl groups of tea polyphenols can reduce free radicals by providing hydrogen atoms, but due to the weak binding between tea polyphenols and the carrier, it is easy to cause rapid release of a large amount of antioxidants, resulting in limited antioxidant properties. In the examples of the present invention, using porous dialdehyde starch as a cross-linking agent not only forms a cross-linked network of imine covalent bonds and hydrogen bonds, but also makes the structure of gelatin carrying tea polyphenols denser. This compact structure helps the slow release of tea polyphenols over time, further enhancing the antioxidant ability of the film.

[0095] It should be noted that: the above examples are only used to illustrate the technical solutions of the present invention, and are not intended to limit it; although the present invention has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the examples of the present invention.

Claims

1. A preparation method of a gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols, characterized in that It includes the following steps: (1) Enzymatic hydrolysis treatment: The starch is dispersed in an acetate buffer solution, a composite enzyme is added, and enzymatic hydrolysis is carried out at 40 - 55 °C. After stirring, an acidic substance is added to adjust the pH of the starch suspension to terminate the reaction; the obtained reaction solution is filtered by suction after being washed with water and alcohol to obtain a porous starch solid; the composite enzyme is a combination of glucoamylase and α - amylase; (2) Oxidation treatment: Under the condition of a water bath, the porous starch solid is mixed with water to form a starch suspension, then an oxidant is added, and the pH of the system is adjusted to 3 - 4 with a basic substance; the obtained reaction solution is filtered by suction after being washed with water and alcohol to obtain a porous dialdehyde starch solid; (3) Preparation of gelatin film: Gelatin is dissolved in water and stirred, and porous dialdehyde starch, tea polyphenols, and glycerol are added in sequence and stirred in sequence, followed by ultrasonic treatment; the obtained composite solution is transferred to a film - forming device and dried to obtain a gelatin film cross - linked and modified with porous dialdehyde starch and tea polyphenols.

2. According to the preparation method described in claim 1, in step (1): The starch is corn starch, cassava starch, or potato starch; The acetate buffer solution is a mixed solution of acetic acid and sodium acetate, with a pH of 4.5 - 5.0, and the mass - to - volume ratio of sodium acetate to acetic acid is 1 g:3 mL - 1 g:4 mL; The acidic substance is one or more of hydrochloric acid, acetic acid, or sulfuric acid, and the pH of the adjusted system is 1.2 - 1.5; The alcohol washing is carried out with 75% - 90% ethanol; The rotation speed of the stirring is 400 - 600 rpm.

3. The preparation method according to claim 1, characterized in that, In step (1), the enzyme activity of the glucoamylase is 20000 - 25000 U / mL, and the enzyme activity of the α - amylase is 3000 - 4000 U / mL.

4. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of the glucoamylase to the α - amylase is 1:1 - 3:1, the enzymatic hydrolysis time is 1 - 3 h, and the concentration of the composite enzyme is 1 wt% - 3 wt%.

5. The preparation method according to claim 1, characterized in that, In step (2), the water bath temperature is 30 - 40 °C; the mass ratio of the porous starch to water is 1:3 - 1:

4.

6. The preparation method according to claim 1, wherein, In step (2), the oxidant is one or more of sodium periodate and periodic acid; the molar ratio of the oxidant to the glucose unit in the starch is 0.8:1 - 1.2:1; the oxidation reaction time for adding the oxidant is 3 - 5 h.

7. The preparation method according to claim 1, wherein In step (2), the basic substance is one or more of sodium hydroxide and sodium bicarbonate; the alcohol washing is carried out with 75% - 90% ethanol.

8. The preparation method according to claim 1, characterized in that, In step (3): Taking g and mL as units respectively, the material ratio of the gelatin to water is 1:30 - 1:40; The porous dialdehyde starch solid is 5% - 15% of the mass of the gelatin, the tea polyphenols are 1% - 5% of the mass of the gelatin, and the glycerol is 9% - 15% of the mass of the gelatin.

9. The preparation method according to claim 1, wherein, In step (3): The temperature for stirring after adding gelatin is 70 - 80 °C, the stirring time is 5 - 15 min, and the rotation speed of the stirring is 400 - 600 rpm; The temperature for stirring after adding the porous dialdehyde starch is 70 - 80 °C, and the stirring time is 30 - 50 min; The temperature for stirring after adding the tea polyphenols is 70 - 80 °C, and the stirring time is 10 - 20 min; The temperature for stirring after adding the glycerol is 70 - 80 °C, and the stirring time is 12 - 16 min. The stirring speeds for stirring after adding porous dialdehyde starch, stirring after adding tea polyphenols, and stirring after adding glycerol are all 400 - 600 rpm; The ultrasonic treatment is to remove bubbles; the time for ultrasonic treatment is 1 - 3 min; The film-forming device includes a plastic petri dish, a glass petri dish, or a feeding tank; The drying is carried out in an oven or a dryer at a temperature of 35 - 45 °C for a time of 24 - 48 h.

10. A gelatin film crosslinked and modified based on porous dialdehyde starch and tea polyphenols, characterized in that, It is prepared by the preparation method according to any one of claims 1 - 9.

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