Biodegradable antioxidant preservative film with high oxidative stability, and preparation method therefor

Abstract

Disclosed is a biodegradable antioxidant preservative film with a high oxidative stability, and a preparation method therefor. The method comprises: taking crude soy protein as a raw material, performing alkali dissolution, acid precipitation and freeze drying to obtain soy protein isolate (SPI); then subjecting same to oxidation in an alkaline solution, dialysis and freeze-drying to prepare an SPI-EGCG conjugate; and finally formulating a film-forming base solution, performing casting, drying, demolding and cutting, so as to obtain a biodegradable antioxidant preservative film with high oxidative stability. In the present invention, by means of a conjugation treatment, the small-molecule antioxidant EGCG is immobilized within the macromolecular structure of the SPI polymer; and the antioxidant properties of the SPI-EGCG and the film-forming characteristics of the SPI and polyvinyl alcohol are utilized to prepare an EGCG-loaded SPI / polyvinyl alcohol composite preservative film, which can not only effectively retain the biodegradability of the SPI film, but can also effectively improve the antioxidant effect of the composite preservative film and enhance the stability and retention rate of EGCG in the film during storage.

Description

Biodegradable antioxidant food preservation film with high oxidative stability and its preparation method Technical Field

[0001] This invention relates to the field of food packaging materials, specifically to a biodegradable antioxidant preservation film with high oxidation stability and its preparation method. Background Technology

[0002] In recent years, soy protein isolate (SPI) has attracted widespread attention due to its excellent nutritional and functional properties, and its applications are diverse, including emulsions, gels, and films. SPI has a complex structure containing various amino acids and functional groups, resulting in strong intermolecular interactions, making it an ideal material for protein film preparation. Furthermore, SPI is derived from soybeans, a renewable plant and one of the richest renewable resources among various soy products. Due to its low cost, renewability, non-toxicity, and biodegradability, it has been used in food packaging materials. However, the compact molecular structure of natural soy protein isolate, with hydrophobic groups shielded by a hydrophilic shell, results in weak antioxidant properties. Moreover, films prepared from a single natural soy protein isolate are highly susceptible to external environmental influences, limiting its application in food preservation packaging.

[0003] Tea polyphenols are the main active components in tea, possessing various biological activities such as antioxidation, anti-inflammation, and anti-cancer properties. Among them, epigallocatechin gallate (EGCG) is one of the most active components of tea polyphenols, exhibiting strong antioxidant capabilities and preventive effects against various diseases. Introducing tea polyphenols or EGCG into food preservation packaging films can effectively enhance the film's antioxidant properties. However, EGCG is a small molecule; if directly introduced into food preservation film, the free EGCG molecules in the film are easily oxidized and dissipated during storage, thus affecting the antioxidant effect and shelf life of the film. Crosslinking EGCG onto SPI molecules to form SPI-EGCG conjugates can effectively solve the above problems. SPI-EGCG conjugates combine SPI and EGCG through certain chemical or physical methods. This crosslinking not only retains the advantages of both SPI and EGCG, but their interaction can also generate new functional properties. The published invention patent "Chitosan-Pectin Polyelectrolyte Bilayer Active Packaging Film and Its Preparation Method and Application" (application number: CN2023114508047) directly adds EGCG to the film-forming base solution to enhance the antioxidant and antibacterial properties of the packaging film. The published invention patent "An Antioxidant Food Preservation Film and Its Preparation Method" (application number: CN202110360192.7) introduces EGCG into a composite film of sodium lignosulfonate, polyethylene oxide and polyvinyl alcohol to enhance the antioxidant effect of the preservation film. However, the above inventions do not perform conjugated pretreatment on EGCG, and their EGCG mainly exists in the form of small molecule free state. Although the antioxidant properties of such packaging films are improved, there is a problem that EGCG is easy to transfer and dissipate, which leads to a rapid decline in the antioxidant properties of the packaging film during storage. Moreover, the processing technology of the above patents is fundamentally different from that of this patent. Furthermore, the published invention patent "A whey protein isolate-epigallocatechin gallate covalent complex-based lemon essential oil nanoemulsion coating preservative and its preparation method and application" (application number: CN202311492413.1) uses whey protein isolate-EGCG conjugate as the main component to prepare an emulsion coating preservative. However, this invention does not mention the film-forming properties of the conjugate, and the processing technology of this patent is fundamentally different from that of this patent. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a biodegradable antioxidant preservation film with high oxidation stability and a method for preparing the same.

[0005] To achieve the above objectives, the first technical solution adopted by the present invention is as follows:

[0006] A highly oxidatively stable biodegradable antioxidant food preservation film, made from the following raw materials by weight percentage:

[0007] SPI-EGCG conjugate 5.0-8.0 wt%;

[0008] Polyvinyl alcohol 3.0-5.0 wt%;

[0009] Glycerin 3.0-5.0 wt%;

[0010] The crosslinking agent is 1.0-2.0 wt%, which is prepared by mixing N,N-methylenebisacrylamide and citric acid, and the mass ratio of N,N-methylenebisacrylamide to citric acid is N,N-methylenebisacrylamide:citric acid = 1:19;

[0011] The remainder is water, and the sum of the weight percentages of all components is 100%.

[0012] Furthermore, the cling film is prepared by casting: the raw materials of each component are mixed evenly to obtain a film base liquid, wherein the SPI-EGCG conjugate and polyvinyl alcohol solution are heated in a water bath at 70-90℃ for 1-2 hours. After the film base liquid is cooled to room temperature, it is cast into a polytetrafluoroethylene film forming device and dried at a constant temperature of 30-40℃ for 4-7 hours. After drying, the film is removed, cut and shaped to obtain the cling film.

[0013] Furthermore, the SPI-EGCG conjugate is prepared by the following method:

[0014] A 5.0 wt% SPI dispersion was prepared using ultrapure water and stirred with a magnetic stirrer at 4 °C for 24 h. Then, EGCG was added at a mass ratio of SPI:EGCG = 1:0.1-0.3. The pH was adjusted to 9.0 with 6 M / L sodium hydroxide and stirred with a magnetic stirrer at 4 °C for 24 h to obtain an SPI-EGCG conjugate solution.

[0015] The pH of the SPI-EGCG conjugate solution was adjusted to 7.0 with 6M / L hydrochloric acid, and then placed in a 3000Da dialysis bag for dialysis for 48 hours, with 8 water changes.

[0016] The dialysis residue containing the SPI-EGCG conjugate was pre-frozen and then freeze-dried for 48 hours to obtain the SPI-EGCG conjugate.

[0017] Furthermore, the SPI is prepared by the following method:

[0018] A 5.0 wt% crude soybean protein powder dispersion was prepared using ultrapure water. The dispersion was stirred evenly with a magnetic stirrer, and the pH was adjusted to 9.0 with 6 M / L sodium hydroxide. After stirring with a magnetic stirrer at room temperature for 2 h, the supernatant was collected by centrifugation. The pH of the supernatant was adjusted to 4.5 with 6 M / L hydrochloric acid, and the dispersion was allowed to stand at room temperature for 2 h before centrifugation to collect the precipitate. The precipitate was dissolved in ultrapure water, and the pH was adjusted to 7.0 with 6 M / L sodium hydroxide. After pre-freezing, the dispersion was freeze-dried for 48 h to obtain SPI.

[0019] Furthermore, the centrifugation parameters are set as follows: 8000 r / min, 30 min, 4℃.

[0020] To achieve the above objectives, the second technical solution adopted by the present invention is as follows:

[0021] A method for preparing a biodegradable antioxidant food preservation film with high oxidative stability includes the following steps:

[0022] Step S1: Preparation of SPI-EGCG conjugate solution: Prepare a 5.0 wt% SPI dispersion with ultrapure water, stir with a magnetic stirrer at 4℃ for 24 h, then add EGCG at a mass ratio of SPI:EGCG = 1:0.1-0.3, adjust the pH to 9.0 with 6M / L sodium hydroxide, and stir with a magnetic stirrer at 4℃ for 24 h to obtain the SPI-EGCG conjugate solution;

[0023] Step S2, Dialysis: Adjust the pH of the SPI-EGCG conjugate solution to 7.0 with 6M / L hydrochloric acid, then place it in a 3000Da dialysis bag and dialyze for 48 hours, changing the water 8 times;

[0024] Step S3, freeze drying: The dialysis residue containing SPI-EGCG conjugate was pre-frozen and then freeze-dried for 48 hours to obtain SPI-EGCG conjugate;

[0025] Step S4: Preparation of membrane base solution: Mix 5.0-8.0 wt% SPI-EGCG conjugate, 3.0-5.0 wt% polyvinyl alcohol, 3.0-5.0 wt% glycerol, 1.0-2.0 wt% crosslinking agent, and the balance being water, with the sum of the weight percentages of all components being 100%, and mix them evenly; wherein the crosslinking agent is prepared by mixing N,N-methylenebisacrylamide and citric acid, and the mass ratio of N,N-methylenebisacrylamide to citric acid is N,N-methylenebisacrylamide:citric acid = 1:19; heat the SPI-EGCG conjugate and polyvinyl alcohol solution in a water bath at 70-90℃ for 1-2 hours;

[0026] Step S5, film formation: After cooling the film base solution to room temperature, cast it into a polytetrafluoroethylene film forming apparatus and place it in a 30-40℃ forced-air drying oven to dry for 4-7 hours;

[0027] Step S6, Demolding: After drying, demold and cut into shape to obtain cling film.

[0028] Furthermore, prior to step S1, the following steps are also included:

[0029] Step S0: Preparation of SPI: Prepare a 5.0 wt% crude soybean protein powder dispersion using ultrapure water. Stir evenly with a magnetic stirrer, adjust the pH to 9.0 with 6 M / L sodium hydroxide, stir at room temperature for 2 h, centrifuge to collect the supernatant, adjust the pH of the supernatant to 4.5 with 6 M / L hydrochloric acid, let stand at room temperature for 2 h, centrifuge to collect the precipitate, dissolve the precipitate in ultrapure water, adjust the pH to 7.0 with 6 M / L sodium hydroxide, pre-freeze and freeze-dry for 48 h to obtain SPI.

[0030] Furthermore, the centrifugation parameters are set as follows: 8000 r / min, 30 min, 4℃.

[0031] To achieve the above objectives, the third technical solution adopted by the present invention is as follows:

[0032] A biodegradable antioxidant preservation film with high oxidation stability prepared by the preparation method described in technical solution 2 above.

[0033] The key concept of this invention is to introduce polyvinyl alcohol, a compound with certain biodegradability, into the membrane base liquid, and form a SPI-EGCG and polyvinyl alcohol double network membrane matrix structure under the action of a crosslinking agent (N,N-methylenebisacrylamide:citric acid = 1:19). This enhances the mechanical properties and storage stability of the membrane without affecting its antioxidant and biodegradability properties.

[0034] The advantages of this invention, which differ from existing technologies, are as follows:

[0035] (1) This invention replaces the traditional soy protein isolate and free EGCG molecules with a soy protein isolate and EGCG conjugate (SPI-EGCG) to produce a biodegradable antioxidant preservation film with high oxidative stability. The SPI-EGCG conjugate can improve the antioxidant performance and oxidative stability of the SPI film during storage, while also being biodegradable.

[0036] (2) The biodegradable antioxidant preservation film provided by the present invention has a film-forming matrix of SPI-EGCG conjugate and polyvinyl alcohol complex. Under the action of N,N-methylenebisacrylamide and citric acid (1:19) as crosslinking agents, a double network structure of the film matrix is ​​formed, which can significantly improve the mechanical properties and storage stability of the film and has good tensile strength (TS) and elongation at break (EAB).

[0037] (3) The EGCG-containing biodegradable antioxidant preservation film provided by the present invention not only has high oxidation stability, but the oxidative browning products of EGCG also make the preservation film have excellent ultraviolet and visible light blocking properties. Detailed Implementation

[0038] Biodegradable films are environmentally friendly materials primarily made from natural macromolecules such as cellulose, protein, and starch, and have broad application prospects. These biomolecules possess excellent biocompatibility and degradability, making them ideal materials for preparing biodegradable films. Packaging films prepared using them as a matrix can not only be degraded by microorganisms in the natural environment, but the degradation products are also harmless to the environment, meeting the requirements of green environmental protection. Soy protein isolate (SPI) has good biodegradability, which strongly supports its application in the fields of sustainable development and environmental protection. However, food preservation packaging films made from SPI alone have weaknesses such as weak mechanical strength and high hygroscopicity, which limits their effective application. This invention introduces polyvinyl alcohol (PVA), a compound with certain biodegradability, into the film base liquid, forming a SPI-EGCG and PVA dual-network membrane matrix structure under the action of a crosslinking agent. This enhances the mechanical properties and storage stability of the film without affecting its antioxidant and biodegradability properties. Therefore, this invention utilizes the antioxidant properties of SPI-EGCG and the film-forming characteristics of SPI and polyvinyl alcohol to prepare SPI / polyvinyl alcohol composite preservation film loaded with EGCG. This not only effectively preserves the biodegradability of the SPI film, but also effectively improves the antioxidant effect of the mixed preservation film, as well as enhances the stability and retention rate of EGCG in the film during storage.

[0039] To help those skilled in the art better understand the technical content of this invention and its technical contributions compared to the prior art, the invention will be further described below with reference to specific embodiments:

[0040] Example 1

[0041] The method for producing the highly oxidatively stable biodegradable antioxidant food preservation film specifically includes the following steps:

[0042] 1) Preparation of SPI: A 5.0 wt% crude soybean protein powder dispersion was prepared by mixing crude soybean protein powder with ultrapure water. The dispersion was stirred evenly using a magnetic stirrer. The pH was adjusted to 9.0 with 6 M / L sodium hydroxide. After stirring at room temperature for 2 h, the mixture was centrifuged (8000 r / min, 30 min, 4℃) to collect the supernatant. The pH of the supernatant was adjusted to 4.5 with 6 M / L hydrochloric acid. After standing at room temperature for 2 h, the mixture was centrifuged (8000 r / min, 30 min, 4℃) to collect the precipitate. The precipitate was dissolved in ultrapure water, and the pH was adjusted to 7.0 with 6 M / L sodium hydroxide. After pre-freezing, the mixture was freeze-dried for 48 h to obtain SPI, which was stored at 4℃ for later use.

[0043] 2) SPI-EGCG conjugation: The prepared SPI was used to prepare a 5.0 wt% SPI dispersion with ultrapure water, and stirred with a magnetic stirrer at 4℃ for 24 h. EGCG was added at a mass ratio of SPI:EGCG = 1:0.1, and the pH was adjusted to 9.0 with 6M / L sodium hydroxide. The mixture was stirred with a magnetic stirrer at 4℃ for 24 h, maintaining the pH at 9.0, to obtain the SPI-EGCG conjugated solution.

[0044] 3) Dialysis: The pH of the prepared SPI-EGCG conjugate solution was adjusted to 7.0 with 6M / L hydrochloric acid, and then placed in a 3000Da dialysis bag for dialysis for 48 hours, with water changed 8 times;

[0045] 4) Freeze-drying: The dialysis residue containing SPI-EGCG conjugate was pre-frozen and then freeze-dried for 48 h to obtain SPI-EGCG conjugate, which was stored at 4℃ for later use.

[0046] 5) Preparation of membrane base solution: 6.0 wt% of the prepared SPI-EGCG conjugate, 3.0 wt% of polyvinyl alcohol, 3.0 wt% of glycerol, 1.0 wt% of crosslinking agent (N,N-methylenebisacrylamide:citric acid = 1:19), and the balance being water, with the sum of the weight percentages of all components being 100%, are mixed thoroughly. The SPI-EGCG conjugate and polyvinyl alcohol solution need to be heated in a 70°C water bath for 2 hours.

[0047] 6) Film formation: After the film base solution is cooled to room temperature, the film base solution is cast into a polytetrafluoroethylene film forming apparatus and placed in a 30°C forced-air drying oven for 6.5 hours;

[0048] 7) Demolding: After drying, the film is removed, cut into shape, and finally obtained as plastic wrap.

[0049] Example 2

[0050] The method for producing the highly oxidatively stable biodegradable antioxidant food preservation film specifically includes the following steps:

[0051] 1) Preparation of SPI: A 5.0 wt% crude soybean protein powder dispersion was prepared by mixing crude soybean protein powder with ultrapure water. The dispersion was stirred evenly using a magnetic stirrer. The pH was adjusted to 9.0 with 6 M / L sodium hydroxide. After stirring at room temperature for 2 h, the mixture was centrifuged (8000 r / min, 30 min, 4℃) to collect the supernatant. The pH of the supernatant was adjusted to 4.5 with 6 M / L hydrochloric acid. After standing at room temperature for 2 h, the mixture was centrifuged (8000 r / min, 30 min, 4℃) to collect the precipitate. The precipitate was dissolved in ultrapure water, and the pH was adjusted to 7.0 with 6 M / L sodium hydroxide. After pre-freezing, the mixture was freeze-dried for 48 h to obtain SPI, which was stored at 4℃ for later use.

[0052] 2) SPI-EGCG conjugation: The prepared SPI was used to prepare a 5.0% SPI dispersion with ultrapure water, and stirred with a magnetic stirrer at 4℃ for 24 h. EGCG was added at a mass ratio of SPI:EGCG = 1:0.2, and the pH was adjusted to 9.0 with 6M / L sodium hydroxide. The mixture was stirred with a magnetic stirrer at 4℃ for 24 h, maintaining the pH at 9.0, to obtain the SPI-EGCG conjugated solution.

[0053] 3) Dialysis: The pH of the prepared SPI-EGCG conjugate solution was adjusted to 7.0 with 6M / L hydrochloric acid, and then placed in a 3000Da dialysis bag for dialysis for 48 hours, with water changed 8 times;

[0054] 4) Freeze-drying: The dialysis residue containing SPI-EGCG conjugate was pre-frozen and then freeze-dried for 48 h to obtain SPI-EGCG conjugate, which was stored at 4℃ for later use.

[0055] 5) Preparation of membrane base solution: 6.0 wt% of the prepared SPI-EGCG conjugate, 5.0 wt% of polyvinyl alcohol, 5.0 wt% of glycerol, 2.0 wt% of crosslinking agent (N,N-methylenebisacrylamide:citric acid = 1:19), and the balance being water, with the sum of the weight percentages of all components being 100%, are mixed thoroughly. The SPI-EGCG conjugate and polyvinyl alcohol solution need to be heated in a water bath at 70-90℃ for 1-2 hours.

[0056] 6) Film formation: After the film base solution is cooled to room temperature, the film base solution is cast into a polytetrafluoroethylene film forming apparatus and placed in a 30°C forced-air drying oven for 6.5 hours;

[0057] 7) Demolding: After drying, the film is removed, cut into shape, and finally obtained as plastic wrap.

[0058] Example 3

[0059] The method for producing the highly oxidatively stable biodegradable antioxidant food preservation film specifically includes the following steps:

[0060] 1) Preparation of SPI: A 5.0% crude soybean protein powder dispersion was prepared by mixing crude soybean protein powder with ultrapure water. The dispersion was stirred evenly using a magnetic stirrer. The pH was adjusted to 9.0 with 6M / L sodium hydroxide. After stirring at room temperature for 2 hours, the mixture was centrifuged (8000 rpm, 30 min, 4℃) to collect the supernatant. The pH of the supernatant was adjusted to 4.5 with 6M / L hydrochloric acid. After standing at room temperature for 2 hours, the mixture was centrifuged (8000 rpm, 30 min, 4℃) to collect the precipitate. The precipitate was dissolved in ultrapure water, and the pH was adjusted to 7.0 with 6M / L sodium hydroxide. After pre-freezing, the mixture was freeze-dried for 48 hours to obtain SPI, which was stored at 4℃ for later use.

[0061] 2) SPI-EGCG conjugation: The prepared SPI was used to prepare a 5.0 wt% SPI dispersion with ultrapure water, and stirred with a magnetic stirrer at 4℃ for 24 h. EGCG was added at a mass ratio of SPI:EGCG = 1:0.3, and the pH was adjusted to 9.0 with 6M / L sodium hydroxide. The mixture was stirred with a magnetic stirrer at 4℃ for 24 h, maintaining the pH at 9.0, to obtain the SPI-EGCG conjugated solution.

[0062] 3) Dialysis: The pH of the prepared SPI-EGCG conjugate solution was adjusted to 7.0 with 6M / L hydrochloric acid, and then placed in a 3000Da dialysis bag for dialysis for 48 hours, with water changed 8 times;

[0063] 4) Freeze-drying: The dialysis residue containing SPI-EGCG conjugate was pre-frozen and then freeze-dried for 48 h to obtain SPI-EGCG conjugate, which was stored at 4℃ for later use.

[0064] 5) Preparation of membrane base solution: 6.0 wt% of the prepared SPI-EGCG conjugate, 5.0 wt% of polyvinyl alcohol, 3.0 wt% of glycerol, 2.0 wt% of crosslinking agent (N,N-methylenebisacrylamide:citric acid = 1:19), and the balance being water, with the sum of the weight percentages of all components being 100%, are mixed thoroughly. The SPI-EGCG conjugate and polyvinyl alcohol solution need to be heated in a 90℃ water bath for 1 hour.

[0065] 6) Film formation: After the film base solution is cooled to room temperature, the film base solution is cast into a polytetrafluoroethylene film forming apparatus and placed in a 40°C forced-air drying oven for 4 hours;

[0066] 7) Demolding: After drying, the film is removed, cut into shape, and finally obtained as plastic wrap.

[0067] Comparative Example 1

[0068] Example 2 is repeated, except that the SPI-EGCG conjugate of step (5) is replaced with SPI obtained by freeze drying, and steps (2)-(4) are not required.

[0069] Comparative Example 2

[0070] Example 2 is repeated, except that the SPI-EGCG conjugate in step (5) is replaced with a mixture of SPI and EGCG (SPI∶EGCG=1∶0.2), and steps (2)-(4) are not required.

[0071] The plastic wraps prepared in Examples 1-3 and Comparative Examples 1-2 were tested, and the test results are as follows:

[0072] Table 1. Color parameters (L*, a*, and b*) and light transmittance (ultraviolet and visible light) of different component cling films

[0073] As shown in Table 1, the form in which EGCG exists has a significant impact on the film's color and light transmittance. For the food preservation films containing SPI-EGCG conjugates (Examples 1-3), the UV and visible light transmittance decreased with increasing EGCG conjugation, while the yellowness (b*) increased significantly. In contrast, the food preservation films without SPI-EGCG conjugates exhibited a brighter color and higher UV and visible light transmittance, indicating that SPI-EGCG conjugates can impart excellent UV and visible light blocking properties to the food preservation films. Furthermore, adding EGCG in a free form to the food preservation film also gives it excellent UV and visible light blocking properties, possibly because free EGCG is more prone to oxidative browning during film formation.

[0074] Table 2. Mechanical properties (tensile strength and elongation at break) of different components of food preservation film

[0075] As shown in Table 2, the TS and EAB of the plastic wrap containing SPI-EGCG conjugation (Examples 1-3) increased with increasing EGCG conjugation. However, compared to the plastic wrap without EGCG (Comparative Example 1), the addition of SPI-EGCG conjugation significantly reduced the EAB, indicating that EGCG conjugation reduces the cross-linking degree between SPI molecules, thereby increasing the membrane's brittleness. Furthermore, compared to the composite membrane without EGCG, even when EGCG was added in a free form (Comparative Example 2), the EAB of the membrane was still reduced because EGCG, as a small molecule, hinders the cross-linking between macromolecules.

[0076] Table 3. Antioxidant properties of different components of food preservation film

[0077] As shown in Table 3, the plastic wrap without added EGCG did not exhibit antioxidant properties (Comparative Example 1). In contrast, the plastic wrap containing SPI-EGCG conjugates (Examples 1-3) showed a gradual increase in antioxidant activity as the amount of EGCG conjugation increased, indicating that conjugated EGCG endowed the plastic wrap with certain antioxidant properties. Furthermore, compared to plastic wrap containing added EGCG conjugates, the plastic wrap containing free EGCG molecules exhibited a greater DPPH free radical scavenging rate (Comparative Example 2 and Example 2). This may be because some phenolic hydroxyl groups in the EGCG molecules were oxidized during the conjugated crosslinking process, leading to a weakening of its antioxidant properties. Alternatively, it may be because free EGCG molecules are more easily released and then bind to the substrate.

[0078] Table 4. Oxidative stability of different components of food preservation film during storage

[0079] As shown in Table 4, the antioxidant properties of the plastic wrap containing free EGCG molecules decreased by approximately 87% after 4 weeks of storage (Comparative Example 2). This is because free EGCG molecules are highly susceptible to factors such as temperature, light, and oxygen, and are easily degraded during storage, leading to a reduction in the antioxidant properties of the plastic wrap. In contrast, the plastic wrap containing SPI-EGCG conjugates exhibited better oxidative stability (Examples 1-3), maintaining good antioxidant properties even after 4 weeks of storage. This is because the conjugated EGCG molecules are encapsulated within SPI molecules, providing a certain protective effect on the EGCG molecules.

[0080] In summary, the present invention has the following significant advantages:

[0081] (1) This invention replaces the traditional soy protein isolate and free EGCG molecules with a soy protein isolate and EGCG conjugate (SPI-EGCG) to produce a biodegradable antioxidant preservation film with high oxidative stability. The SPI-EGCG conjugate can improve the antioxidant performance and oxidative stability of the SPI film during storage, while also being biodegradable.

[0082] (2) The biodegradable antioxidant preservation film provided by the present invention has a film-forming matrix of SPI-EGCG conjugate and polyvinyl alcohol complex. Under the action of N,N-methylenebisacrylamide and citric acid (1:19) as crosslinking agents, a double network structure of the film matrix is ​​formed, which can significantly improve the mechanical properties and storage stability of the film and has good tensile strength (TS) and elongation at break (EAB).

[0083] (3) The EGCG-containing biodegradable antioxidant preservation film provided by the present invention not only has high oxidation stability, but the oxidative browning products of EGCG also make the preservation film have excellent ultraviolet and visible light blocking properties.

[0084] The present invention has been described by the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. It must be noted that the disclosed embodiments do not limit the scope of the present invention. Conversely, modifications and equivalents included within the spirit and scope of the claims are all included within the scope of the present invention.

Claims

1. Biodegradable antioxidant preservative film with high oxidative stability, characterized in that, The preservative film is made of raw materials in the following weight percentages: 5.0-8.0 wt% of SPI-EGCG conjugate; 3.0-5.0 wt% of polyvinyl alcohol; 3.0-5.0 wt% of glycerol; 1.0-2.0 wt% of crosslinking agent, which is prepared by mixing N,N-methylene bisacrylamide and citric acid at a mass ratio of N,N-methylene bisacrylamide: citric acid = 1:19; the balance being water, and the sum of the weight percentages of the components being 100%.

2. The biodegradable antioxidant fresh-keeping film with high oxidative stability according to claim 1, characterized in that, The preservative film is prepared by casting: the raw materials are mixed uniformly to obtain a film base solution, wherein the SPI-EGCG conjugate and the polyvinyl alcohol solution are heated in a water bath at 70-90°C for 1-2 hours, the film base solution is cooled to room temperature, and then cast in a polytetrafluoroethylene film former, dried at a constant temperature of 30-40°C for 4-7 hours, removed from the mold after drying, cut into shape, and the preservative film is obtained.

3. The biodegradable antioxidant fresh-keeping film with high oxidative stability according to claim 1, characterized in that: The SPI-EGCG conjugate is prepared by the following method: 5.0 wt% SPI dispersion solution is prepared using ultrapure water, stirred using a magnetic stirrer at 4°C for 24 hours, then EGCG is added according to a mass ratio of SPI: EGCG = 1:0.1-0.3, the pH is adjusted to 9.0 using 6M / L sodium hydroxide, and the solution is stirred using a magnetic stirrer at 4°C for 24 hours to obtain an SPI-EGCG conjugate solution; The SPI-EGCG conjugate solution is adjusted to pH 7.0 using 6M / L hydrochloric acid, then placed in a 3000Da dialysis bag and dialyzed for 48 hours, with water changed 8 times; The dialysis residue containing the SPI-EGCG conjugate is pre-frozen and freeze-dried for 48 hours to obtain the SPI-EGCG conjugate.

4. The biodegradable antioxidant fresh-keeping film with high oxidative stability according to claim 3, characterized in that, The SPI is prepared by the following method: 5.0 wt% crude soybean protein powder dispersion solution is prepared using ultrapure water, stirred uniformly using a magnetic stirrer, the pH is adjusted to 9.0 using 6M / L sodium hydroxide, the solution is stirred at room temperature using a magnetic stirrer for 2 hours, then centrifuged to obtain the supernatant, the supernatant is adjusted to pH 4.5 using 6M / L hydrochloric acid, then left to stand at room temperature for 2 hours, centrifuged to obtain the precipitate, the precipitate is dissolved using ultrapure water, the pH is adjusted to 7.0 using 6M / L sodium hydroxide, pre-frozen, and freeze-dried for 48 hours to obtain the SPI.

5. The biodegradable antioxidant fresh-keeping film with high oxidative stability according to claim 4, characterized in that, The centrifugation parameters are set as follows: 8000r / min, 30min, 4°C.

6. A process for the preparation of biodegradable antioxidant preservative film with high oxidative stability characterized by, The method comprises the following steps: Step S1, preparing an SPI-EGCG conjugate solution: 5.0 wt% SPI dispersion solution is prepared using ultrapure water, stirred using a magnet stirrer at 4°C for 24 hours, then EGCG is added according to a ratio of SPI: EGCG = 1:0.1-0.3, the solution is adjusted to pH 9.0 using 6M / L sodium hydroxide, and the solution is stirred at 4°C using a magnetic stirrer for 24 hours to obtain an SPI-EGCG conjugate solution; Step S2, dialysis: the SPI-EGCG conjugate solution is adjusted to pH 7.0 using 6M / L hydrochloric acid, then placed in a 3000Da dialysis bag and dialyze for 48 hours, with water changed 8 times; Step S3, obtaining the SPI-EGCG conjugate: the dialysis residue containing the SPI-EGCG conjugate is pre-frozen and freeze-dried to obtain the SPI-EGCG conjugate. Step S3, freeze-drying: freeze-drying the dialysis residual liquid containing the SPI-EGCG conjugate after pre-freezing for 48 h to obtain the SPI-EGCG conjugate; Step S4, preparing a film base solution: uniformly mixing 5.0-8.0 wt% of the SPI-EGCG conjugate, 3.0-5.0 wt% of polyvinyl alcohol, 3.0-5.0 wt% of glycerol, 1.0-2.0 wt% of a crosslinking agent, and the balance of water, wherein the sum of the weight percentages of the components is 100%; wherein the crosslinking agent is prepared by mixing N,N-methylene bisacrylamide and citric acid, and the mass ratio of N,N-methylene bisacrylamide to citric acid is 1:19; and heating the SPI-EGCG conjugate and the polyvinyl alcohol solution in a 70-90°C water bath for 1-2 h; Step S5, film forming: pouring the film base solution into a polytetrafluoroethylene film former after cooling to room temperature, and drying in a 30-40°C air-drying oven for 4-7 h; Step S6, film stripping: stripping after drying, cutting and forming to obtain the preservative film.

7. The method for preparing a biodegradable antioxidant preservation film with high oxidative stability according to claim 6, characterized in that, Before step S1, further comprising: Step S0, preparing SPI: preparing a 5.0 wt% crude soybean protein powder dispersion using ultrapure water, uniformly stirring using a magnetic stirrer, adjusting the pH to 9.0 using 6M / L sodium hydroxide, stirring at room temperature for 2 h using a magnetic stirrer, centrifuging to obtain the supernatant, adjusting the pH of the supernatant to 4.5 using 6M / L hydrochloric acid, standing at room temperature for 2 h, centrifuging to obtain the precipitate, dissolving the precipitate using ultrapure water, adjusting the pH to 7.0 using 6M / L sodium hydroxide, pre-freezing, and freeze-drying for 48 h to obtain the SPI.

8. The method for preparing a biodegradable antioxidant preservation film with high oxidative stability according to claim 7, characterized in that, The centrifugation parameters are set as: 8000 r / min, 30 min, 4°C.

9. The biodegradable antioxidant preservative film with high oxidation stability prepared by the preparation method of any one of claims 6-8.

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