Preparation method of food freshness intelligent indication function paper with stable structure

Abstract

The invention discloses a preparation method of intelligent food freshness indication functional paper with a stable structure, which has the characteristics of high hydrophobicity, high mechanical strength, freshness indication and the like, and belongs to the field of food packaging. According to the technical scheme, after a certain amount of anionic polyacrylamide is added into paper pulp, a paper sheet former is used for sheet making, and white paper can be obtained. The paper is soaked in a mixed solution of chitosan and anthocyanin in a room temperature environment, chitosan and anthocyanin are tightly adsorbed on the paper under the electrostatic combination action between negative charges attached to the paper and positive charges of chitosan, the paper is placed in a drying oven to be dried, and the intelligent indication function paper can be obtained. The method disclosed by the invention is low in energy consumption, simple in process flow, easy for batch and large-area production and good in product batch repeatability, effectively enhances the dry and wet strength of the paper, improves the hydrophobic property of the paper, and is expected to be widely applied to the field of food freshness indication.

Description

Technical Field

[0001] The invention belongs to the field of food freshness indication, and in particular relates to a method for producing a freshness intelligent indication functional paper with a stable structure and multifunctional characteristics (eg, high hydrophobicity, high dry and wet strength, freshness indication, etc.). Background Art

[0002] In recent years, with the continuous development of the economy and the improvement of people's quality of life, consumers have become more and more concerned about food safety. Food will be damaged by multiple factors such as microorganisms, enzymes, and temperature throughout the entire process from storage, transportation to sales, which will lead to a decrease in food freshness. Therefore, it is necessary to provide a convenient and real-time monitoring method for the quality and safety of food supply. Under the background of the "plastic ban" fully implemented in my country in 2020, more research content is focused on the preparation of smart packaging films to detect the freshness of food. However, research on the preparation of food packaging based on paper is particularly scarce. Natural cellulose paper-based materials are considered to be a possible alternative to traditional petroleum-derived plastics due to their excellent mechanical strength, low cost, biodegradability and environmental protection. They have broad application prospects in achieving sustainable development and green circular economy. Based on the above advantages, the preparation of freshness intelligent indicator food packaging based on cellulose paper has the advantages of packaging materials and plays a guiding role in the field of food safety.

[0003] Cellulose paper is a low-cost, mechanically flexible, printable and recyclable material widely used in publishing, electronics, construction and food packaging industries. The shortcomings of cellulose paper, such as single function and inability to perform intelligent detection of food freshness, limit the practical application of cellulose paper-based food packaging to a certain extent. Researchers often adopt the method of physically doping pH indicator factors (such as anthocyanins, curcumin, etc.) into the paper matrix to realize the multifunctionality of food paper packaging under conventional conditions. However, in the face of the special environment of low temperature and high humidity in which food is refrigerated, the porous structure and hydrophilic surface of cellulose paper make it show poor moisture resistance in a humid environment, and the mechanical properties are also greatly weakened. What's worse, anthocyanins will migrate to the surface of the paper base, causing it to fail, making it difficult for cellulose paper-based packaging to monitor the freshness of food in real time and for a long time in a high humidity environment. In summary, in practical applications, especially in low temperature and high humidity environments, food paper packaging often needs to have multiple functions of high hydrophobicity, high mechanical strength and sensitive indicator factors at the same time, so as to achieve the purpose of real-time and long-term monitoring of food freshness in special environments. At present, although some studies have reported methods to improve the mechanical strength and hydrophobic properties of paper, there are few reports on the direct addition of anthocyanins to paper to detect the freshness of food. This is mainly because in the traditional papermaking process, the paper needs to be hot-pressed at 105°C, and this high temperature will cause the anthocyanins to be inactivated. Therefore, the preparation of a food paper-based packaging that can detect the freshness of food in a long-term and real-time manner under low temperature and high humidity environment, with stable structure and good performance, has important application significance in the field of food safety. Summary of the invention

[0004] Based on this, the object of the present invention is to provide a method for producing a food freshness intelligent indication functional paper material with a stable structure and multifunctional characteristics.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for preparing food freshness intelligent indicator functional paper with stable structure comprises the following steps:

[0007] (1) A certain amount of anionic polyacrylamide (APAM) is added to the ground pulp, and then the pulp with anionic polyacrylamide added is sheeted using a paper sheet former to obtain white paper;

[0008] (2) Under room temperature, the obtained paper is immersed in a mixture of chitosan (CS) and anthocyanin (Anth), and the chitosan and anthocyanin are tightly adsorbed on the paper by utilizing the electrostatic bonding effect between the negative charge attached to the paper and the positive charge of chitosan itself. After drying, an anthocyanin-chitosan film (Anth-CS film) is formed on the surface of the paper, thereby obtaining food intelligent indication functional paper.

[0009] Based on the above technical scheme, preferably, the material of the pulp in step (1) can be selected from bamboo, grass (such as straw), wood (such as coniferous wood, hardwood), etc. (purchased from Shandong Asia Pacific Forest Products Paper Co., Ltd.), the pulp is pulp liquid, the mass fraction of pulp in the pulp liquid is 0.5-2wt%, and the mass fraction of anionic polyacrylamide added to the pulp liquid is 0.1-0.5wt%, preferably 0.1-0.3wt%.

[0010] Based on the above technical solution, preferably, the molecular weight of the anionic polyacrylamide is 800-1200 g / mol; the deacetylation degree of chitosan is ≥95%, and the viscosity is 100-200 mPa·s.

[0011] Based on the above technical solution, preferably, the preparation method of the ground pulp in step (1) is: tear the pulp board into 1-25 cm 2 Small pieces, soak in water, and then grind with a refiner; the soaking time in water is 4h to 6h.

[0012] Based on the above technical solution, preferably, the anionic polyacrylamide used is an anionic polyacrylamide solution, and the concentration of the anionic polyacrylamide solution is 1g / L to 25g / L; the anionic polyacrylamide solution is added to the ground pulp and ground and mixed evenly with the pulp in a refiner.

[0013] Based on the above technical solution, preferably, a paper sheet former is used to sheet the pulp added with anionic polyacrylamide, and the paper weight is 60-120 g / m 2 The thickness of the obtained white paper is 0.10-0.30 mm.

[0014] Based on the above technical scheme, preferably, in the mixed solution of chitosan and anthocyanin in step (2), the mass fraction of chitosan is 1.5-5wt%, preferably 1.8-3wt%, the mass fraction of anthocyanin is 0.1-0.5wt%, preferably 0.1-0.3wt%, the volume ratio of acetic acid in the chitosan solution is 1-3% (v / v), preferably 2% (v / v), and acetic acid is used to adjust the pH of the solution to 2-5, preferably 3-4.

[0015] Based on the above technical solution, preferably, the anthocyanin used in preparing the mixed solution of chitosan and anthocyanin is an anthocyanin solution, and the mass fraction of the anthocyanin solution is 1 to 10 wt%, preferably 2 to 9 wt%.

[0016] Based on the above technical solution, preferably, the mixed solution of chitosan and anthocyanin in step (2) further contains glycerol, and the volume fraction of the glycerol is 1-2% (v / v), preferably 1.5% (v / v).

[0017] Based on the above technical scheme, preferably, the method for preparing the mixed solution of chitosan and anthocyanins in step (2) is: first dissolving chitosan in acetic acid solution, then dispersing anthocyanin solution and glycerol into the chitosan solution, stirring evenly, to obtain a mixed solution of chitosan and anthocyanins; wherein the volume fraction of acetic acid in the acetic acid solution is 1 to 3% (v / v).

[0018] Based on the above technical solution, preferably, in step (2), the time for soaking the paper in the mixed solution of chitosan and anthocyanin is 5 min to 10 min.

[0019] Based on the above technical solution, preferably, the oven is used for drying, the drying temperature is 30-70° C., preferably 40-60° C.; the drying time is 1 h to 5 h, preferably 2 h to 4 h.

[0020] The present invention also relates to a food freshness intelligent indicating functional paper material with a stable structure prepared by the protection method.

[0021] The present invention also relates to the application of the functional paper material mentioned above in the field of food (such as seafood) freshness indication.

[0022] The product prepared by the present invention has the advantages of simple process flow, low energy consumption, easy batch and large-area production, good repeatability between product batches, etc. It effectively enhances the dry and wet strength of paper and improves the hydrophobicity of paper. At the same time, it is also beneficial to the long-term real-time monitoring of food freshness under special environments of low temperature and high humidity.

[0023] Compared with the prior art, the preparation method provided by the present invention has the following advantages:

[0024] 1. The functional paper prepared by the present invention has the multifunctional characteristics of high hydrophobicity, high dry and wet strength, and can monitor the freshness of food in real time for a long time, and is expected to have broad application prospects in the field of food freshness indication.

[0025] 2. The food freshness intelligent indicator functional paper prepared by the present invention has the advantages of simple production process, low energy consumption, easy batch and large-scale production, and good repeatability between product batches. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The dry strength of the functional paper prepared in Example 1 and the comparative example.

[0027] Figure 2 The wet strength of the functional paper prepared in Example 1 and the comparative example.

[0028] Figure 3 The SEM images of the functional papers prepared in Example 1 and the comparative example are shown.

[0029] Figure 4 The water contact angles of the functional papers prepared in Example 1 and the comparative example.

[0030] Figure 5 This is the colorimetric response of the functional paper prepared in Example 1 to volatile ammonia: ΔE change.

[0031] Figure 6 Colorimetric response of the functional paper prepared in Example 1 to volatile ammonia: digital photo change record. DETAILED DESCRIPTION

[0032] The following examples are provided for a better understanding of the present invention, but are not intended to limit the best mode of implementation, nor to limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other prior arts shall fall within the protection scope of the present invention.

[0033] If no specific experimental steps or conditions are specified in the examples, all the experiments were carried out according to the conventional experimental steps or conditions described in the literature in the field. The reagents or instruments used without specifying the manufacturer are all conventional reagents that can be obtained commercially.

[0034] In the following examples, the hardwood pulp board was purchased from Shandong Asia Pacific Forest Products Paper Co., Ltd.; the deacetylation degree of chitosan was ≥95%, and the viscosity was 100-200 mPa·s; and the molecular weight of the anionic polyacrylamide was 1000 g / mol.

[0035] Example 1

[0036] (1) Tear 7.5g hardwood pulp board into 10-15cm pieces 2 The small cubes were soaked in 500 mL of deionized water for 4 hours and then ground in a pulper to form pulp. At the same time, 0.1 g of anionic polyacrylamide was accurately weighed and dissolved in 100 mL of deionized water. It was stirred in a 60°C water bath for 1 hour until the APAM was completely dissolved to obtain an APAM solution. Then 2 mL of the APAM solution was added to the pulper and mixed evenly with the ground pulp to obtain a pulp with APAM added. The pulp was quantified according to the paper weight of 80 g / m 2 To make sheets, first put the wet paper pulp into a barrel, stir it evenly, filter it and then put the wet paper sheets into a hot press dryer at 105°C to obtain white paper with a diameter of 20 cm and a thickness of 0.15 mm.

[0037] (2) Preparation of chitosan and anthocyanin mixed solution: Accurately weigh 2 mL of acetic acid and drop it into 200 mL of deionized water, stir it in a water bath at 60°C, add 4 g of chitosan while stirring, and stir for 2 h until the chitosan is completely dissolved to obtain a chitosan solution. Subsequently, 0.430 g of anthocyanin was dissolved in 5 mL of deionized water to prepare an anthocyanin solution, and then the anthocyanin solution and 3 mL of glycerol were added to the chitosan solution, and stirred in a magnetic stirrer at 35°C for 1 h to obtain a chitosan and anthocyanin mixed solution.

[0038] (3) At room temperature, a mixture of chitosan and anthocyanin was poured into a tray, and the prepared paper was soaked in the mixture of chitosan and anthocyanin for 5 min. The paper was then placed in an oven and dried at 60 °C for 2 h to obtain food freshness indicator functional paper, which was recorded as APAM-CS paper.

[0039] Example 2

[0040] (1) Tear 7.5g hardwood pulp board into 10-15cm pieces 2 The small cubes were soaked in 500 mL of deionized water for 4 hours and then ground in a pulper to form pulp. At the same time, 0.2 g of anionic polyacrylamide was accurately weighed and dissolved in 100 mL of deionized water. It was stirred in a 60°C water bath for 1 hour until the APAM was completely dissolved to obtain an APAM solution. Then 3 mL of the APAM solution was added to the pulper and mixed evenly with the ground pulp to obtain a pulp with APAM added. The pulp was quantified according to the paper weight of 80 g / m 2 To make sheets, first put the wet paper pulp into a barrel, stir it evenly, filter it and then put the wet paper sheets into a hot press dryer at 105°C to obtain white paper with a diameter of 20 cm and a thickness of 0.15 mm.

[0041] (2) Preparation of chitosan and anthocyanin mixed solution: Accurately weigh 1 mL of acetic acid and drop it into 200 mL of deionized water, stir it in a water bath at 60°C, add 3.6 g of chitosan while stirring, and stir for 2 h until the chitosan is completely dissolved to obtain a chitosan solution. Subsequently, add 0.133 g of anthocyanin to 5 mL of deionized water to prepare an anthocyanin solution, then add the anthocyanin solution and 3 mL of glycerol to the chitosan solution, and stir it with a magnetic stirrer at 35°C for 1 h to obtain a chitosan and anthocyanin mixed solution.

[0042] (3) At room temperature, a mixture of chitosan and anthocyanin is poured into a tray, the prepared paper is soaked in the mixture of chitosan and anthocyanin for 5 min, and then the paper is placed in an oven and dried at 40°C for 4 h to obtain food freshness indicator functional paper.

[0043] Example 3

[0044] (1) Tear 7.5g hardwood pulp board into 10-15cm pieces 2 The small cubes were soaked in 500 mL of deionized water for 4 hours and then ground in a pulper to form pulp. At the same time, 0.3 g of anionic polyacrylamide was accurately weighed and dissolved in 100 mL of deionized water. It was stirred in a 60°C water bath for 1 hour until the APAM was completely dissolved to obtain an APAM solution. Then 2 mL of the APAM solution was added to the pulper and mixed evenly with the ground pulp to obtain a pulp with APAM added. The pulp was quantified according to the paper weight of 80 g / m 2 To make sheets, first put the wet paper pulp into a barrel, stir it evenly, filter it and then put the wet paper sheets into a hot press dryer at 105°C to obtain white paper with a diameter of 20 cm and a thickness of 0.15 mm.

[0045] (2) Preparation of chitosan and anthocyanin mixed solution: Accurately weigh 1 mL of acetic acid and drop it into 200 mL of deionized water, stir it in a water bath at 60°C, add 6 g of chitosan while stirring, and stir for 2 h until the chitosan is completely dissolved to obtain a chitosan solution. Subsequently, 0.430 g of anthocyanin was added to 5 mL of deionized water to prepare an anthocyanin solution, and then the anthocyanin solution and 3 mL of glycerol were added to the chitosan solution, and stirred for 1 h with a magnetic stirrer at 35°C to obtain a chitosan and anthocyanin mixed solution.

[0046] (3) At room temperature, a mixture of chitosan and anthocyanin is poured into a tray, the prepared paper is soaked in the mixture of chitosan and anthocyanin for 5 min, and then the paper is placed in an oven at 50°C for 3 h to obtain food freshness indicator functional paper.

[0047] Example 4

[0048] (1) Tear 7.5g hardwood pulp board into 10-15cm pieces 2 The small cubes were soaked in 500 mL of deionized water for 4 hours and then ground in a pulper to form pulp. At the same time, 0.1 g of anionic polyacrylamide was accurately weighed and dissolved in 100 mL of deionized water. It was stirred in a 60°C water bath for 1 hour until the APAM was completely dissolved to obtain an APAM solution. Then 2 mL of the APAM solution was added to the pulper and mixed evenly with the ground pulp to obtain a pulp with APAM added. The pulp was quantified according to the paper weight of 80 g / m 2 To make sheets, first put the wet paper pulp into a barrel, stir it evenly, filter it and then put the wet paper sheets into a hot press dryer at 105°C to obtain white paper with a diameter of 20 cm and a thickness of 0.15 mm.

[0049] (2) Preparation of chitosan and anthocyanin mixed solution: Accurately weigh 1 mL of acetic acid and drop it into 200 mL of deionized water, stir it in a water bath at 60°C, add 3.6 g of chitosan while stirring, and stir for 2 h until the chitosan is completely dissolved to obtain a chitosan solution. Subsequently, add 0.252 g of anthocyanin to 5 mL of deionized water to prepare an anthocyanin solution, then add the anthocyanin solution and 3 mL of glycerol to the chitosan solution, and stir it with a magnetic stirrer at 35°C for 1 h to obtain a chitosan and anthocyanin mixed solution.

[0050] (3) At room temperature, pour the chitosan and anthocyanin mixture into a tray, soak the prepared paper in the chitosan and anthocyanin mixture for 5 minutes, and then place the paper in an oven at 50°C for 2 hours to obtain food freshness indicator functional paper.

[0051] Example 5

[0052] (1) Tear 7.5g hardwood pulp board into 10-15cm pieces 2 The small cubes were soaked in 500 mL of deionized water for 4 hours and then ground in a pulper to form pulp. At the same time, 0.3 g of anionic polyacrylamide was accurately weighed and dissolved in 100 mL of deionized water. It was stirred in a 60°C water bath for 1 hour until the APAM was completely dissolved to obtain an APAM solution. Then 2 mL of the APAM solution was added to the pulper and mixed evenly with the ground pulp to obtain a pulp with APAM added. The pulp was quantified according to the paper weight of 80 g / m 2 To make sheets, first put the wet paper pulp into a barrel, stir it evenly, filter it and then put the wet paper sheets into a hot press dryer at 105°C to obtain white paper with a diameter of 20 cm and a thickness of 0.15 mm.

[0053] (2) Preparation of chitosan and anthocyanin mixed solution: Accurately weigh 1 mL of acetic acid and drop it into 200 mL of deionized water, stir it in a water bath at 60°C, add 3.6 g of chitosan while stirring, and stir for 2 h until the chitosan is completely dissolved to obtain a chitosan solution. Subsequently, add 0.133 g of anthocyanin to 5 mL of deionized water to prepare an anthocyanin solution, then add the anthocyanin solution and 3 mL of glycerol to the chitosan solution, and stir it with a magnetic stirrer at 35°C for 1 h to obtain a chitosan and anthocyanin mixed solution.

[0054] (3) Under room temperature, pour the chitosan and anthocyanin mixture into a tray, soak the prepared paper in the chitosan and anthocyanin mixture for 5 minutes, and then place the paper in an oven at 60°C for 4 hours to obtain food freshness indicator functional paper.

[0055] Comparative Example

[0056] The method of Example 1 was followed, except that the pulp was not subjected to any treatment, and a blank paper material was finally obtained.

[0057] Example 6 Performance test and color development effect evaluation:

[0058] 1. Dry tensile strength:

[0059] The dry tensile strength characteristics of the functional paper prepared by Example 1 and the comparative example were investigated. The results are as follows: Figure 1 shown.

[0060] Dry tensile strength test method: The mechanical properties of the dry functional paper sample (length × width = 70 mm × 20 mm) were tested on an electronic universal material tester (Instron 5965) produced by Boston Instron. The tensile speed was controlled at 10 mm / min, and the stress-strain curve was the average of three measurements.

[0061] Depend on Figure 1 It can be seen that the product prepared in Example 1 has good dry strength, which meets the mechanical property requirements of food intelligent indicator functional paper in practical applications.

[0062] Likewise, the functional papers prepared in Examples 2-5 all have good dry strength.

[0063] 2. Wet tensile strength:

[0064] The wet strength properties of the functional paper prepared in Example 1 and the comparative example were investigated. The results are as follows: Figure 2 shown.

[0065] Wet tensile strength test method: The mechanical properties of the wet functional paper sample (length × width = 70 mm × 20 mm) were tested on an electronic universal material tester (Instron 5965) produced by Boston Instron. The tensile speed was controlled at 10 mm / min, and the stress-strain curve was the average of three measurements.

[0066] Depend on Figure 2 It can be seen that the product prepared in Example 1 has good wet strength, which meets the mechanical property requirements of food intelligent indicator functional paper in low temperature and high humidity environment in actual application.

[0067] Likewise, the functional papers prepared in Examples 2-5 all have good wet strength.

[0068] 3. Scanning electron microscope image:

[0069] The functional paper prepared in Example 1 and the comparative example was used to examine the structural stability of the product. The results are as follows: Figure 3 shown.

[0070] Scanning electron microscope image test method: A scanning electron microscope (SEM, JSM-6400LV) was used to observe the cross-sectional microstructure of the smart label at an accelerating voltage of 15 kV. Before observation, the sample was plated with ultra-thin conductive gold under vacuum.

[0071] Depend on Figure 3 It can be seen that the product prepared in Example 1 has good structural stability.

[0072] Likewise, the functional papers prepared in Examples 2-5 all have good structural stability.

[0073] 4. Water contact angle:

[0074] The hydrophobic properties of the functional paper prepared in Example 1 and the comparative example were investigated. The results are as follows: Figure 4 shown.

[0075] Water contact angle test method: In order to evaluate the hydrophobicity of the functional paper surface, the contact angles at five different locations on the functional paper surface were tested using a Finnish T330 optical contact angle meter, and the average value was taken.

[0076] Depend on Figure 4 It can be seen that the product prepared in Example 1 has good hydrophobicity.

[0077] Likewise, the functional papers prepared in Examples 2-5 all have good hydrophobicity.

[0078] 5. Colorimetric response of functional paper to volatile ammonia:

[0079] Test method:

[0080] In NH 3 In the sensing test, the functional paper was covered on a beaker containing 20% ​​ammonia water (20 ml ammonia water and 80 ml deionized water). Within 60 minutes, a digital camera was used to capture the color change of the functional paper every 10 minutes, and a colorimeter was used to measure and analyze the total color difference (△E) of the functional paper.

[0081] Depend on Figure 5 , Figure 6 It can be seen that the product prepared in Example 1 has a good colorimetric response to volatile ammonia, can respond to food spoilage in real time, and meets the freshness indication requirements in the practical application of food intelligent indicator functional paper.

[0082] Likewise, the functional papers prepared in Examples 2-5 all have good colorimetric responses.

[0083] The present invention provides a method for preparing a food freshness intelligent indicator functional paper with a stable structure, which belongs to the technical field of food freshness indicator packaging. Experimental results show that the prepared freshness intelligent indicator functional paper has better comprehensive performance than similar products, lower cost, and is easy to mass produce.

Claims

1. A method for preparing a food freshness intelligent indicator functional paper with a stable structure, characterized in that: The steps include: (1) Anionic polyacrylamide is added to the ground pulp, and then the pulp with the anionic polyacrylamide added is sheeted using a paper sheet former to obtain white paper; (2) Under room temperature, the obtained paper is immersed in a mixture of chitosan and anthocyanin, and then dried to obtain food freshness intelligent indication functional paper.

2. The method according to claim 1, characterized in that: In the step (1), the material of the pulp is one or more of wood, grass, and bamboo, the pulp is pulp liquid, the mass fraction of the pulp in the pulp liquid is 0.5-2wt%, and the mass fraction of the anionic polyacrylamide is 0.1-0.5wt%; In the mixed solution of chitosan and anthocyanin in step (2), the mass fraction of chitosan is 1.5-5wt%, the mass fraction of anthocyanin is 0.1-0.5wt%, and the pH value is 2-5.

3. The method according to claim 1 or 2, characterized in that: The molecular weight of the anionic polyacrylamide is 800-1200 g / mol; the deacetylation degree of chitosan is ≥95% and the viscosity is 100-200 mPa·s; The method for grinding the pulp in step (1) is: tearing the pulp board into pieces, soaking them in water, and then grinding them using a pulper.

4. The method according to claim 1, characterized in that: The basis weight of the white paper in step (1) is 60 to 120 g / m 2 The thickness of the prepared paper is 0.10-0.30 mm.

5. The method according to claim 1, characterized in that: The anionic polyacrylamide used in step (1) is an anionic polyacrylamide solution, and the concentration of the anionic polyacrylamide solution is 1 g / L to 25 g / L; The anthocyanin used in the mixed solution of chitosan and anthocyanin in step (2) is an anthocyanin solution, and the mass fraction of the anthocyanin solution is 1-10wt%; the mixed solution of chitosan and anthocyanin also contains glycerol, and the volume fraction of the glycerol is 1-2%.

6. The method according to claim 1 or 2, characterized in that: The method for preparing the mixed solution of chitosan and anthocyanin in step (2) is as follows: firstly dissolving chitosan in acetic acid solution, then adding anthocyanin solution and glycerol into the chitosan solution, stirring evenly, to obtain a mixed solution of chitosan and anthocyanin; wherein the volume fraction of acetic acid in the acetic acid solution is 1-3%.

7. The method according to claim 1, characterized in that: In the step (2), the paper is immersed in the mixed solution of chitosan and anthocyanin for 5 to 10 minutes.

8. The method according to claim 1, characterized in that: The step (2) is carried out in an oven for drying at a temperature of 30 to 70° C. for a time of 1 to 5 hours.

9. A food freshness intelligent indicator functional paper with a stable structure prepared by the method of any one of claims 1 to 8.

10. Use of the functional paper according to claim 9 in the field of food freshness indication.

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