Method for adjusting gel strength of fish skin gelatin

Abstract

The invention discloses a method for adjusting the gel strength of fish skin gelatin, which comprises the following steps: (1) dissolving a phenolic compound in an ethanol solution to obtain a phenolic aldehyde-ethanol solution; (2) dissolving the fish skin gelatin protein isolate in ultrapure water, adjusting the pH value to 9.0, and stirring to obtain a protein dispersion liquid; (3) adding the phenolic aldehyde-ethanol solution into the protein dispersion liquid, adjusting the pH value and the reaction temperature, and carrying out a cross-linking reaction; and (4) dialyzing the product after the cross-linking reaction, and freeze-drying to obtain powder. The fish skin gelatin protein isolate-phenol conjugate with a multi-layer cross-linked structure can be obtained through the method, the gel strength and stability of the fish skin gelatin are remarkably improved, the maximum gel strength can reach 75.2 kPa, and the shelf life of the product is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of food and pharmaceutical product processing, and particularly relates to a method for adjusting the gel strength of fish skin gelatin. Background Art

[0002] As a hydrolysis product of collagen, fish skin gelatin is widely used in the fields of food, medicine, etc. due to its high biocompatibility and good gelling property. However, its gel strength is limited by the destruction of the natural collagen structure, and traditional cross-linking methods (such as formaldehyde, glutaraldehyde) have the risk of toxic residue. In addition, during the long-term storage of traditional fish skin gelatin, the gel structure is prone to degradation, affecting the shelf life of the product.

[0003] Phenolic compounds include aromatic rings with one or more hydroxyl substituents. They can exist as simple phenolic molecules, polycyclic aromatic hydrocarbon molecules, and highly polymerized compounds. The interaction between proteins and phenolic compounds can be non-covalent or covalent. Covalent bonds can be formed by deprotonating the hydroxyl groups on phenolic compounds under alkaline conditions to form quinones. Once these quinones are formed, they will undergo covalent reactions with other phenolic compounds, resulting in dark-colored dimers and larger polyphenolic compounds. In the presence of proteins, quinones react covalently with amino acid side chains, especially the free amino group of lysine and the sulfhydryl group of cysteine, forming dark brown, yellow, or green colors. The products formed by covalent protein-phenolic interactions are called conjugates.

[0004] The interaction between phenolic compounds and proteins is generally considered unfavorable because it changes the expected properties of proteins. For example, covalent binding usually reduces the solubility of proteins and the bioavailability of essential amino acids, and changes the color of protein solutions or dispersions. However, controlling the change of protein properties through phenolic conjugation is a favorable targeted application method. Under controllable conditions (such as selecting the optimal modification degree of proteins), the functionality of proteins is enhanced, such as emulsifying property, foaming property, and gelling property, etc. It is speculated that the formation of protein-phenolic conjugates can increase the gel strength of protein gels because (1) intermolecular covalent bonds are formed between proteins, which can act as cross-linking agents; (2) the newly formed conjugates generate additional hydrogen bonds and hydrophobic interactions, which contribute to increasing the total density of covalent and non-covalent bonds in the gel structure.

[0005] The Chinese patent document (application number: CN202011453931.9) discloses a method for enhancing the gel strength of fish skin gelatin, which uses modified tannic acid combined with transglutaminase to jointly enhance the gel strength of fish skin gelatin. By optimizing conditions such as the modification method of tannic acid, the addition concentrations of tannic acid and transglutaminase, and the treatment time, the optimal method and parameters for enhancing the gel strength of fish skin gelatin are obtained. Compared with the initial fish skin gelatin, its final gel strength increases by 60.6%. However, this technology has problems such as excessive cross-linking leading to increased brittleness, and the low cross-linking efficiency of single phenols, which easily causes uneven structure and is not conducive to the long-term preservation of products. Summary of the Invention

[0006] The object of the present invention is to provide a method for regulating the gel strength of fish skin gelatin, which can form a more uniform and stable multi-level network structure by adopting stepwise cross-linking, thereby significantly improving the gel strength and stability of fish skin gelatin and extending the shelf life of fish skin gelatin.

[0007] In order to achieve the above object, the present invention adopts the following technical effects:

[0008] A method for regulating the gel strength of fish skin gelatin, comprising the following steps:

[0009] (1) Dissolve a phenolic compound in an ethanol solution to obtain a phenolic aldehyde-ethanol solution;

[0010] (2) Dissolve fish skin gelatin isolate in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion;

[0011] (3) Add the phenolic aldehyde-ethanol solution to the protein dispersion, adjust the pH and reaction temperature, and carry out a cross-linking reaction;

[0012] (4) Dialyze the product after the cross-linking reaction, and then freeze-dry it into a powder.

[0013] Preferably, the phenolic compound is one or more of gallic acid, epigallocatechin gallate, and tannic acid.

[0014] Preferably, the addition amount of the phenolic compound is 2-4% of the mass of fish skin gelatin isolate.

[0015] Preferably, the molecular weight of gallic acid is 0.17 kDa, the molecular weight of epigallocatechin gallate is 0.458 kDa, and the molecular weight of tannic acid is 1.71 kDa.

[0016] Preferably, in step (3), a stepwise crosslinking method is adopted for the reaction. First, a gallic acid-ethanol solution is added to the protein dispersion for preliminary crosslinking, then an epigallocatechin gallate-ethanol solution is added for secondary crosslinking, and finally a tannic acid-ethanol solution is added for surface encapsulation.

[0017] Preferably, the preliminary crosslinking is carried out at pH 8.3 - 8.5 and 25 °C for 10 - 12 h; the secondary crosslinking is carried out at pH 9.0 - 9.2 and 30 °C for 18 - 20 h; the surface encapsulation is carried out at pH 9.5 - 9.7 and 35 °C for 24 - 28 h.

[0018] Preferably, an antioxidant is added in the preliminary crosslinking stage, and the addition amount of the antioxidant is 0.05 - 0.1% of the mass of the fish skin gelatin isolate. The antioxidant is vitamin E or tea polyphenols.

[0019] Preferably, a metal chelator is added in the secondary crosslinking stage, and the addition amount of the metal chelator is 0.05 - 0.1% of the mass of the fish skin gelatin isolate. The metal chelator is EDTA or citric acid or phytic acid.

[0020] Preferably, in step (4), after dialysis of the product after the crosslinking reaction, ultrasonic treatment is carried out first, and then freeze-drying; the power of the ultrasonic treatment is 50 - 100 W, and the ultrasonic time is 5 - 10 min.

[0021] Preferably, the dialysis is carried out at 1 - 5 °C for 12 - 28 h, and the freeze-drying is carried out at -20 - -15 °C for freezing.

[0022] Phenolic compounds form quinones under alkaline conditions and undergo covalent reactions with amino acid side chains in proteins (such as the amino group of lysine and the sulfhydryl group of cysteine) to form crosslinks. At the same time, the hydroxyl groups and aromatic rings of phenolic compounds can also form hydrogen bonds and hydrophobic interactions with proteins, further enhancing the gel network structure. The following phenolic compounds are the gallic acid (GA, 170 Da), epigallocatechin gallate (EGCG, 458 Da), and tannic acid (TA, 1700 Da) selected by the present invention from left to right in sequence.

[0023]

[0024] In summary, the present invention adopts the above technical solutions, and the present invention has the following technical effects:

[0025] 1. The method for adjusting the gel strength of fish skin gelatin in the present invention adopts step-by-step crosslinking. First, gallic acid with a small molecular weight is used for preliminary crosslinking to form a basic network structure; then, epigallocatechin gallate with a medium molecular weight is added for secondary crosslinking to enhance the network strength; finally, tannic acid with a large molecular weight is used for surface encapsulation to form a protective layer. Through the method of the present invention, a fish skin gelatin isolate-phenol conjugate with a multi-level crosslinked structure can be obtained, which can significantly improve the gel strength and stability of fish skin gelatin and ensure the shelf life of the product.

[0026] 2. The method for adjusting the gel strength of fish skin gelatin in the present invention adds an antioxidant and a metal chelating agent during the crosslinking process to prevent the over-oxidation of phenolic compounds during the reaction and simultaneously inhibit the degradation reaction catalyzed by metal ions. After the crosslinking is completed, the product is dialyzed to remove free phenolic compounds; then ultrasonic treatment is carried out to break up large aggregates that may form and make the gel network more uniform; finally, slow freeze-drying is carried out to maintain the integrity of the network structure. These post-treatment steps further optimize the microstructure of the gel. Through the step-by-step crosslinking method and the post-treatment steps of dialysis, ultrasonic treatment, and freeze-drying, it can be ensured that the fish skin gelatin isolate-phenol conjugate forms a more uniform and stable network structure, improving the gel strength of fish skin gelatin. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a scanning electron micrograph of the conjugated fish skin gelatin in Example 11.

[0028] Figure 2 It is a scanning electron micrograph of unmodified fish skin gelatin.

[0029] Figure 3 It is a freeze-dried powder diagram of the conjugated fish skin gelatin, where a is unmodified with phenols, b is modified with gallic acid, c is modified with epigallocatechin gallate, d is modified with tannic acid, and e is obtained after step-by-step crosslinking modification in Example 11. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following preferred embodiments are cited with reference to the accompanying drawings for further detailed description of the present invention. However, it should be noted that many details listed in the specification are only for enabling the reader to have a thorough understanding of one or more aspects of the present invention, and these aspects of the present invention can be realized even without these specific details.

[0031] Example 1

[0032] A method for adjusting the gel strength of fish skin gelatin includes the following steps:

[0033] (1) Dissolve gallic acid with a molecular weight of 0.17 kDa in a 96% ethanol solution to obtain a phenolic aldehyde-ethanol solution;

[0034] (2) Take fish skin gelatin isolate and dissolve it in ultrapure water. After adjusting the pH to 9.0, stir to obtain a protein dispersion;

[0035] (3) Add the phenolic aldehyde-ethanol solution to the protein dispersion to obtain a protein-phenol mixture. Adjust the pH to 9.0 and react at 30 °C for 24 hours to carry out a cross-linking reaction; among them, the addition amount of gallic acid is 3% of the mass of the fish skin gelatin isolate;

[0036] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C and lyophilize it into a powder.

[0037] Example 2

[0038] A method for regulating the gel strength of fish skin gelatin, comprising the following steps:

[0039] (1) Dissolve epigallocatechin gallate with a molecular weight of 0.458 kDa in a 96% ethanol solution to obtain a phenolic aldehyde-ethanol solution;

[0040] (2) Take fish skin gelatin isolate and dissolve it in ultrapure water. After adjusting the pH to 9.0, stir to obtain a protein dispersion;

[0041] (3) Add the phenolic aldehyde-ethanol solution to the protein dispersion to obtain a protein-phenol mixture. Adjust the pH to 9.0 and react at 30 °C for 24 hours to carry out a cross-linking reaction; among them, the addition amount of epigallocatechin gallate is 3% of the mass of the fish skin gelatin isolate;

[0042] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C and lyophilize it into a powder.

[0043] Example 3

[0044] A method for regulating the gel strength of fish skin gelatin, comprising the following steps:

[0045] (1) Dissolve tannic acid with a molecular weight of 1.71 kDa in a 96% ethanol solution to obtain a phenolic aldehyde-ethanol solution;

[0046] (2) Take fish skin gelatin isolate and dissolve it in ultrapure water. After adjusting the pH to 9.0, stir to obtain a protein dispersion;

[0047] (3) Add the phenolic aldehyde-ethanol solution to the protein dispersion to obtain a protein-phenol mixture, adjust the pH to 9.0, and react at 30 °C for 24 hours to carry out the cross-linking reaction; wherein, the addition amount of tannic acid is 3% of the mass of the fish skin gelatin isolate.

[0048] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C, and lyophilize it into a powder.

[0049] Example 4

[0050] A method for adjusting the gel strength of fish skin gelatin, comprising the following steps:

[0051] (1) Dissolve gallic acid and epigallocatechin gallate in 96% ethanol solution respectively to obtain the corresponding phenolic aldehyde-ethanol solutions.

[0052] (2) Take the fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion.

[0053] (3) First, add the gallic acid-ethanol solution to the protein dispersion and react at pH 8.5 and 25 °C for 10 h for preliminary cross-linking, then add the epigallocatechin gallate-ethanol solution and react at pH 9.0 and 30 °C for 18 h for secondary cross-linking. Among them, the addition amount of phenolic compounds is 3% of the mass of the fish skin gelatin isolate, and the mass ratio of gallic acid to epigallocatechin gallate is 1:1.

[0054] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C, and lyophilize it into a powder.

[0055] Example 5

[0056] A method for adjusting the gel strength of fish skin gelatin, comprising the following steps:

[0057] (1) Dissolve gallic acid and tannic acid in 96% ethanol solution respectively to obtain the corresponding phenolic aldehyde-ethanol solutions.

[0058] (2) Take the fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion.

[0059] (3) First, add the gallic acid-ethanol solution to the protein dispersion and react at pH 8.5 and 25 °C for 10 h for preliminary cross-linking, then add the tannic acid-ethanol solution and react at pH 9.5 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds is 3% of the mass of the fish skin gelatin isolate, and the mass ratio of epigallocatechin gallate to tannic acid is 1:1.

[0060] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C and lyophilize it into a powder.

[0061] Example 6

[0062] A method for adjusting the gel strength of fish skin gelatin, comprising the following steps:

[0063] (1) Dissolve epigallocatechin gallate and tannic acid in 96% ethanol solution respectively to obtain corresponding phenolic aldehyde-ethanol solutions;

[0064] (2) Take fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion;

[0065] (3) First add the epigallocatechin gallate-ethanol solution and react at pH 9.0 and 30 °C for 18 h for secondary cross-linking, and finally add the tannic acid-ethanol solution and react at pH 9.7 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds is 3% of the mass of fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate and tannic acid is 1:1:1;

[0066] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C and lyophilize it into a powder.

[0067] Example 7

[0068] A method for adjusting the gel strength of fish skin gelatin, comprising the following steps:

[0069] (1) Dissolve gallic acid, epigallocatechin gallate and tannic acid in 96% ethanol solution respectively to obtain corresponding phenolic aldehyde-ethanol solutions;

[0070] (2) Take fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion;

[0071] (3) First add the gallic acid-ethanol solution to the protein dispersion and react at pH 8.3 and 25 °C for 10 h for preliminary cross-linking, then add the epigallocatechin gallate-ethanol solution and react at pH 9.0 and 30 °C for 18 h for secondary cross-linking, and finally add the tannic acid-ethanol solution and react at pH 9.5 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds is 3% of the mass of fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate and tannic acid is 1:1:1;

[0072] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C, and lyophilize it into a powder.

[0073] Example 8

[0074] A method for adjusting the gel strength of fish skin gelatin, comprising the following steps:

[0075] (1) Dissolve gallic acid, epigallocatechin gallate, and tannic acid in 96% ethanol solution respectively to obtain the corresponding phenolic-ethanol solutions;

[0076] (2) Take fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion;

[0077] (3) First, add the gallic acid-ethanol solution to the protein dispersion and react at pH 8.3 and 25 °C for 10 h for preliminary cross-linking. At the same time, add vitamin E during the preliminary cross-linking stage, and the addition amount of vitamin E is 0.05% of the mass of fish skin gelatin isolate. Then add the epigallocatechin gallate-ethanol solution and react at pH 9.0 and 30 °C for 18 h for secondary cross-linking. Finally, add the tannic acid-ethanol solution and react at pH 9.5 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds is 3% of the mass of fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate, and tannic acid is 1:1:1;

[0078] (4) Dialyze the cross-linked product at 4 °C for 24 hours, then freeze it overnight at -18 °C, and lyophilize it into a powder.

[0079] Example 9

[0080] A method for adjusting the gel strength of fish skin gelatin, comprising the following steps:

[0081] (3) Dissolve gallic acid, epigallocatechin gallate, and tannic acid in 96% ethanol solution respectively to obtain the corresponding phenolic-ethanol solutions;

[0082] (4) Take fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and stir to obtain a protein dispersion;

[0083] (3) First, add the gallic acid - ethanol solution to the protein dispersion and react at pH 8.3 and 25 °C for 10 h for preliminary cross - linking; then add the epigallocatechin gallate - ethanol solution and react at pH 9.0 and 30 °C for 18 h for secondary cross - linking. At the same time, add EDTA during the secondary cross - linking stage, and the addition amount of EDTA is 0.05% of the mass of the fish skin gelatin isolate; finally, add the tannic acid - ethanol solution and react at pH 9.5 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds is 3% of the mass of the fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate, and tannic acid is 1:1:1;

[0084] (4) Dialyze the cross - linked product at 4 °C for 24 hours, then freeze it overnight at - 18 °C, and lyophilize it into a powder.

[0085] Example 10

[0086] A method for regulating the gel strength of fish skin gelatin, comprising the following steps:

[0087] (5) Dissolve gallic acid, epigallocatechin gallate, and tannic acid in 96% ethanol solution respectively to obtain the corresponding phenolic - ethanol solutions;

[0088] (6) Take the fish skin gelatin isolate and dissolve it in ultrapure water, adjust the pH to 9.0 and then stir to obtain a protein dispersion;

[0089] (3) First, add the gallic acid - ethanol solution to the protein dispersion and react at pH 8.3 and 25 °C for 10 h for preliminary cross - linking. At the same time, add vitamin E during the preliminary cross - linking stage, and the addition amount of vitamin E is 0.05% of the mass of the fish skin gelatin isolate; then add the epigallocatechin gallate - ethanol solution and react at pH 9.0 and 30 °C for 18 h for secondary cross - linking. At the same time, add EDTA during the secondary cross - linking stage, and the addition amount of EDTA is 0.05% of the mass of the fish skin gelatin isolate; finally, add the tannic acid - ethanol solution and react at pH 9.5 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds is 3% of the mass of the fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate, and tannic acid is 1:1:1;

[0090] (4) Dialyze the cross - linked product at 4 °C for 24 hours, then freeze it overnight at - 18 °C, and lyophilize it into a powder.

[0091] Example 11

[0092] A method for regulating the gel strength of fish skin gelatin, comprising the following steps:

[0093] (1) Gallic acid, epigallocatechin gallate, and tannic acid were separately dissolved in 96% ethanol solution to obtain the corresponding phenolic aldehyde-ethanol solutions;

[0094] (2) Fish skin gelatin isolate was dissolved in ultrapure water, and the pH was adjusted to 9.0 followed by stirring to obtain a protein dispersion;

[0095] (3) First, the gallic acid-ethanol solution was added to the protein dispersion and reacted at pH 8.3 and 25 °C for 10 h for preliminary cross-linking. At the same time, vitamin E was added during the preliminary cross-linking stage, and the addition amount of vitamin E was 0.05% of the mass of fish skin gelatin isolate. Then, the epigallocatechin gallate-ethanol solution was added and reacted at pH 9.0 and 30 °C for 18 h for secondary cross-linking. At the same time, EDTA was added during the secondary cross-linking stage, and the addition amount of EDTA was 0.05% of the mass of fish skin gelatin isolate. Finally, the tannic acid-ethanol solution was added and reacted at pH 9.5 and 35 °C for 24 h for surface encapsulation. Among them, the addition amount of phenolic compounds was 3% of the mass of fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate, and tannic acid was 1:1:1;

[0096] (4) After the cross-linked product was dialyzed at 4 °C for 24 hours, it was first subjected to ultrasonic treatment with a power of 50 W and an ultrasonic time of 10 min; then it was frozen overnight at -18 °C and freeze-dried into a powder.

[0097] Example 12

[0098] It is basically the same as Example 11, except that: (3) First, the gallic acid-ethanol solution was added to the protein dispersion and reacted at pH 8.5 and 25 °C for 12 h for preliminary cross-linking. At the same time, vitamin E was added during the preliminary cross-linking stage, and the addition amount of vitamin E was 0.01% of the mass of fish skin gelatin isolate. Then, the epigallocatechin gallate-ethanol solution was added and reacted at pH 9.2 and 30 °C for 20 h for secondary cross-linking. At the same time, EDTA was added during the secondary cross-linking stage, and the addition amount of EDTA was 1% of the mass of fish skin gelatin isolate. Finally, the tannic acid-ethanol solution was added and reacted at pH 9.7 and 35 °C for 28 h for surface encapsulation. Among them, the addition amount of phenolic compounds was 3% of the mass of fish skin gelatin isolate, and the mass ratio of gallic acid, epigallocatechin gallate, and tannic acid was 1:1:1;

[0099] (3) After the cross-linked product was dialyzed at 4 °C for 24 hours, it was first subjected to ultrasonic treatment with a power of 50 W and an ultrasonic time of 10 min; then it was frozen overnight at -18 °C and freeze-dried into a powder.

[0100] Composed of Figure 1 andFigure 2 , confocal laser scanning microscopy showed that the microstructure of the conjugate gel conjugated with fish skin gelatin prepared in Example 11 was denser than that of the fish skin gelatin isolate reference gel, and the mesh structure was also more uniform; Figure 3 As can be seen, the gel formed by the cross-linked conjugate was darker in color.

[0101] Each conjugate fish skin gelatin prepared in Examples 1-11 was tested, and the test results are shown in Table 1. The gel strength was measured using an Anton Paar modular compact rheometer (MCR302; Oosterhout, Netherlands). A 15 wt% dry mass dispersion was prepared with ultrapure water, stirred overnight at 4 °C, the sample was allowed to reach room temperature, and the pH was adjusted to 7 1 h before measurement. All rheological measurements were carried out under strain control and repeated. Among them, the gel strength of unmodified fish skin gelatin was 3.0 kPa. The maximum relative change in gel strength = (the maximum gel strength of each example - the gel strength of unmodified fish skin gelatin) / the gel strength of unmodified fish skin gelatin.

[0102] Table 1

[0103]

[0104] The above are only the preferred embodiments of the present invention. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for adjusting the gel strength of fish skin gelatin, characterized in that: The following steps are involved: (1) dissolving a phenolic compound in an ethanol solution to obtain a phenolic-ethanol solution; (2) dissolving fish skin gelatin protein isolate in ultrapure water, adjusting the pH to 9.0 and stirring to obtain a protein dispersion; (3) adding the phenolic-ethanol solution to the protein dispersion, adjusting the pH and reaction temperature, and performing a cross-linking reaction; (4) The product after the cross-linking reaction is dialyzed, and then freeze-dried into powder.

2. The method for adjusting the gel strength of fish skin gelatin according to claim 1, characterized in that: The phenolic compound is one or more than two of gallic acid, epigallocatechin gallate and tannic acid.

3. The method for adjusting the gel strength of fish skin gelatin according to claim 1, characterized in that: The added amount of the phenolic compound is 2-4% of the fish skin gelatin isolated protein.

4. The method for adjusting the gel strength of fish skin gelatin according to claim 2, characterized in that: The molecular weight of the gallic acid is 0.17 kDa, the molecular weight of epigallocatechin gallate is 0.458 kDa and the molecular weight of tannic acid is 1.71 kDa.

5. The method for adjusting the gel strength of fish skin gelatin according to any one of claims 1 to 4, characterized in that: In step (3), a step-by-step cross-linking method is used to react. First, a gallic acid-ethanol solution is added to the protein dispersion for preliminary cross-linking. Then, an epigallocatechin gallate-ethanol solution is added for secondary cross-linking. Finally, a tannic acid-ethanol solution is added for surface encapsulation.

6. The method for adjusting the gel strength of fish skin gelatin according to claim 5, characterized in that: The primary cross-linking is carried out at pH 8.3-8.5 and 25° C. for 10-12 hours; the secondary cross-linking is carried out at pH 9.0-9.2 and 30° C. for 18-20 hours; and the surface encapsulation is carried out at pH 9.5-9.7 and 35° C. for 24-28 hours.

7. The method for adjusting the gel strength of fish skin gelatin according to claim 5, characterized in that: Antioxidants are added in the initial cross-linking stage, wherein the amount of the antioxidants added is 0.05-0.1% of the fish skin gelatin isolated protein.

8. The method for adjusting the gel strength of fish skin gelatin according to claim 6 or 7, characterized in that: A metal chelating agent is added in the secondary cross-linking stage, wherein the amount of the metal chelating agent added is 0.05-0.1% of the amount of the fish skin gelatin isolated protein.

9. The method for adjusting the gel strength of fish skin gelatin according to claim 1, characterized in that: In step (4), the product after the cross-linking reaction is dialyzed, ultrasonically treated, and then freeze-dried; the power of the ultrasonic treatment is 50 to 100 W, and the ultrasonic time is 5 to 10 minutes.

10. The method for adjusting the gel strength of fish skin gelatin according to claim 9, characterized in that: The dialysis is performed at 1 to 5°C for 12 to 28 hours, and the freeze-drying is performed at -20 to -15°C.

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