Preparation method and application of a hydrogel for food preservation and visual freshness monitoring

Abstract

The present invention discloses a preparation method and application of a hydrogel for food preservation and visualization monitoring of freshness. The characteristics include the following steps: adding polyvinyl alcohol to water and magnetically stirring in a water bath until completely dissolved, adding alizarin after cooling and stirring, then adding zinc-gallic acid metal-organic framework and stirring, pouring into a square container, freezing in a refrigerator, and then taking out and thawing in an incubator at 20-30 °C to obtain the hydrogel. The method steps for judging the freshness level of shrimp based on the hydrogel are as follows: wrapping the shrimp sample to be measured with the hydrogel, taking a hydrogel image with a mobile phone, extracting the RGB values of 16 points evenly distributed on the image through the Python image processing library, calculating the average ED value of the 16 points and substituting it into the prediction model to calculate the TVB-N value of the shrimp sample to be measured, and judging the freshness level of the shrimp according to the TVB-N value. The advantages are strong antibacterial and antioxidant capabilities, strong mechanical properties, and being safe and non-toxic.

Description

Technical Field

[0001] The present invention belongs to the technical field of food packaging, storage and detection, and particularly relates to a preparation method and application of a hydrogel for food preservation and visualization monitoring of freshness. Background Art

[0002] Hydrogel is a hydrophilic polymer material with a three-dimensional network structure formed by physical or chemical cross-linking. While maintaining its own structure, it can absorb a certain amount of water. Due to its antibacterial and antioxidant properties, it can be used as an outer package to extend the storage period of food, and can also monitor the change of food freshness in real time and non-destructively by tracking the atmosphere inside the package.

[0003] Currently, hydrogels are usually made of materials such as gelatin, alginate and chitosan. However, the hydrogels made of these materials usually have poor mechanical strength and stability, thus limiting their use. Among hydrogel polymer materials, polyvinyl alcohol stands out due to its high biocompatibility, degradability and simple preparation. The polyvinyl alcohol hydrogel prepared by the freeze-thaw method is not only simple but also does not require the addition of any toxic chemical cross-linking agent, and has been successfully applied to the field of monitoring the freshness preservation of shrimp. However, it has insufficient antibacterial and antioxidant ability and does not have the function of real-time freshness monitoring.

[0004] Metal organic framework (abbreviated as MOF) is a porous material formed by connecting metal ions and organic ligands through coordination bonds. Due to its functions of antibacterial, antioxidant, enhancing the mechanical properties of the polymer matrix and gas capture, MOF has become an active substance attracting much attention in food packaging materials. At present, there have been studies on incorporating MOF into hydrogels to improve various properties of hydrogels. However, the inherent toxicity of the metal ions (such as copper ions and cobalt ions) and common ligands (such as 2-methylimidazole) used in synthesizing MOF limits its application in food packaging. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a preparation method and application of a hydrogel for food preservation and visualization monitoring of freshness, which has strong antibacterial and antioxidant ability, strong mechanical properties and is safe and non-toxic.

[0006] The technical solution adopted by the present invention to solve the above technical problems is as follows: A preparation method of a hydrogel for food preservation and visualization monitoring of freshness, comprising the following steps: adding polyvinyl alcohol to pure water, magnetically stirring in a water bath at 90-98 °C until completely dissolved, waiting for the temperature to drop to 40-50 °C, adding alizarin, stirring for 1-2 h, then adding zinc-gallic acid metal-organic framework (Zn-MOF), continuing to stir for 1-2 h, pouring into a square container, freezing in a refrigerator at -15 to -20 °C for 10-15 h, taking out and thawing in a thermostat at 20-30 °C for 1 h, thus obtaining the hydrogel for food preservation and visualization monitoring of freshness, wherein the mixing ratio of polyvinyl alcohol, pure water, alizarin and zinc-gallic acid metal-organic framework is 5 g: 50 mL: 2 mg: 50-150 mg.

[0007] Further, the preparation method of the zinc-gallic acid metal-organic framework is as follows: dissolving zinc nitrate hexahydrate and gallic acid in N,N-dimethylformamide solution, performing ultrasonic treatment at 30-50 kHz for 2-8 min, transferring to a polytetrafluoroethylene autoclave, reacting at 100-150 °C for 24 h, centrifuging the reaction solution at 6000 rpm for 10 min, taking the precipitate, washing it three times with ethanol, and drying it in an oven at 50-70 °C to obtain a grayish-white product, which is the zinc-gallic acid metal-organic framework, wherein the mixing ratio of zinc nitrate hexahydrate, gallic acid and N,N-dimethylformamide solution is 3 g: 2 g: 75 mL.

[0008] Preferably, the mixing ratio of the polyvinyl alcohol, the pure water, the alizarin and the zinc-gallic acid metal-organic framework is 5 g: 50 mL: 2 mg: 150 mg.

[0009] The present invention also provides a method for judging the freshness grade of shrimp based on the hydrogel prepared by the above method, and the specific steps are as follows:

[0010] Step 1: Establishment of the model: Establish a prediction model using the logistic function , where A1, A2, x0 and p are all constant terms, x is the average Euclidean distance (abbreviated as ED) value, and y is the TVB-N value;

[0011] Step 2: Acquisition of the image to be measured: Wrap the shrimp sample to be measured with the hydrogel prepared by the above method, and take a picture with a mobile phone to obtain the hydrogel image wrapping the shrimp to be measured;

[0012] Step 3: Extract the RGB values of 16 points evenly distributed on the image from the hydrogel image wrapping the shrimp to be measured through the Python image processing library, and calculate the average ED value of the 16 points, where the calculation formula of the ED value is , where ΔR represents the difference in the intensity value of the red channel before and after the reaction between the hydrogel and the shrimp to be tested, ΔG represents the difference in the intensity value of the green channel before and after the reaction between the hydrogel and the shrimp to be tested, and ΔB represents the difference in the intensity value of the blue channel before and after the reaction between the hydrogel and the shrimp to be tested;

[0013] Step 4: Substitute the average ED value into the prediction model, calculate the TVB-N value of the shrimp sample to be tested, and judge the freshness grade of the shrimp according to the TVB-N value.

[0014] Furthermore, the specific steps of the calculation method of the constant term in Step 1 are as follows:

[0015] (1) Wrap the fresh shrimp sample with the hydrogel prepared by the above method. Place 1 hydrogel pad under the shrimp and 1 hydrogel on top of the shrimp. Take pictures of the hydrogel wrapped around the shrimp with a mobile phone every 24 hours, and at the same time measure the TVB-N value of the shrimp sample;

[0016] (2) Extract the RGB values of 16 evenly distributed points on the image from the hydrogel image of the wrapped shrimp through the Python image processing library, and calculate the average ED value of the 16 points. The calculation formula of the ED value is ;

[0017] (3) When the measured TVB-N value of the shrimp sample exceeds 30 mg / 100g, stop taking pictures with the mobile phone and measuring the TVB-N value of the shrimp sample. Substitute the daily measured TVB-N value of the shrimp sample and the average ED value calculated at the same time into the prediction model to calculate the specific values of A1, A2, x0, and p.

[0018] Furthermore, the specific criteria for classifying the freshness grade of shrimp in Step 4 are as follows: TVB-N value ≤ 20 mg / 100g is fresh shrimp, TVB-N value between 20 and 30 mg / 100g is sub-fresh shrimp, and TVB-N value ≥ 30 mg / 100g is spoiled shrimp.

[0019] Compared with the prior art, the advantages of the present invention are as follows:

[0020] 1. The hydrogel prepared by the present invention combines preservation and indication. It can not only extend the shelf life of shrimp, but also indicate the freshness of shrimp to prevent the occurrence of foodborne diseases.

[0021] 2. The hydrogel prepared by the present invention uses a metal-organic framework composed of zinc ions and gallic acid, which not only has antibacterial and antioxidant effects, but also has the effects of adsorbing amine gases and enhancing mechanical properties.

[0022] 3. The hydrogel prepared by the present invention uses natural alizarin as a pH indicator for indicating the freshness of shrimp. Under normal circumstances, the hydrogel is flesh-pink. After the shrimp spoils, the hydrogel wrapping the shrimp turns dark purple-red, and the color change can be easily observed, reducing the difficulty of monitoring and identifying the freshness of shrimp.

[0023] 4. The hydrogel prepared by the present invention uses polyvinyl alcohol as the substrate. Polyvinyl alcohol, alizarin, and zinc-gallic acid metal-organic framework are combined together through hydrogen bonds, enhancing the mechanical properties of the hydrogel.

[0024] 5. The method for judging the freshness grade of shrimp based on the hydrogel prepared by the present invention uses a small program on a smart phone to quickly quantify the freshness of shrimp, enabling users to identify the freshness of shrimp through stored photos or real-time images, which has the characteristics of being fast, accurate, and portable.

[0025] In summary, the preparation method and application of a hydrogel for food preservation and visualization monitoring of freshness of the present invention use natural alizarin as a pH indicator to monitor the freshness of shrimp, use Zn-MOF as an antibacterial and antioxidant preservation substance, and at the same time, the synergistic effect of Zn-MOF, polyvinyl alcohol, and alizarin improves the color display performance and mechanical properties of the hydrogel. Further combining the smart phone with the preservation indicator hydrogel can not only meet the needs of on-site detection of shrimp freshness, but also has the characteristics of being portable, real-time, and user-friendly. Description of the Drawings

[0026] Figure 1 It is the infrared spectrum diagram of the synthesis of Zn-MOF and hydrogel, where A is Zn-MOF and B is the hydrogel;

[0027] Figure 2 It is the influence of Zn-MOF concentration on the mechanical properties of the hydrogel, where A is the tensile strain-stress curve of different hydrogels, B is the tensile modulus, elongation at break, and breaking strength of different hydrogels, C is the compression strain-stress curve of different hydrogels, and D is the compression modulus of different hydrogels;

[0028] Figure 3 It is the pictures of alizarin and different hydrogels responding to different pH solutions, where A is the ultraviolet-visible spectrum and image of alizarin at different pH values, and B is the color change of the hydrogel at different pH values;

[0029] Figure 4 It is the comparison result of the color display performance of different hydrogels under the response of trimethylamine, where A is the ED value and B is the taken picture;

[0030] Figure 5 It is the comparison result of the antioxidant performance of different hydrogels, where A is the DPPH radical scavenging rate and B is the ABTS radical scavenging rate;

[0031] Figure 6 Antibacterial effects of different hydrogels against Staphylococcus aureus and Escherichia coli;

[0032] Figure 7 Change curve of TVB-N during storage of shrimp samples wrapped with hydrogels, where C is the control group, P is the shrimp sample wrapped with P hydrogel, and PAM is the shrimp sample wrapped with PAM3 hydrogel;

[0033] Figure 8 Photos of visible color changes of PAM3 hydrogel during shrimp freshness monitoring;

[0034] Figure 9 Changes in ED value of PAM3 hydrogel and TVB-N value of shrimp during 0 - 10 d of shrimp freshness monitoring;

[0035] Figure 10 Results of correlation analysis of 30 unknown shrimp samples measured by smartphone applet and Kjeldahl method during shrimp freshness monitoring with PAM3 hydrogel. Specific implementation manners

[0036] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

[0037] Specific Example 1: Preparation of hydrogel.

[0038] Example 1. A preparation method of a hydrogel for food preservation and visualization monitoring of freshness, comprising the following steps:

[0039] Step 1. Preparation of zinc-gallic acid metal-organic framework (abbreviated as Zn-MOF): Dissolve 3 g of zinc nitrate hexahydrate and 2 g of gallic acid in 75 mL of N,N-dimethylformamide (abbreviated as DMF) solution, ultrasonically treat for 5 min at 40 kHz, transfer to a 100 mL polytetrafluoroethylene autoclave, react at 120 °C for 24 h, centrifuge the reaction solution at 6000 rpm for 10 min, take the precipitate, wash it three times with ethanol, and dry it in an oven at 60 °C to obtain a grayish-white product, which is Zn-MOF.

[0040] Step 2: Preparation of polyvinyl alcohol / alizarin / Zn-MOF hydrogel: Add 5 g of polyvinyl alcohol and 50 mL of pure water into a beaker, place it in a water bath at 95 °C and stir magnetically for 3 h until completely dissolved. When the temperature drops to 50 °C, add 2 mg of alizarin and stir for 1 h. Then add 50 mg of Zn-MOF and stir for 1 h. Pour it into a square container (10 mm × 85 mm × 85 mm), freeze it in a -20 °C refrigerator for 12 h, and then take it out and thaw it in a 25 °C incubator for 1 h to obtain the polyvinyl alcohol / alizarin / Zn-MOF hydrogel, denoted as PAM1 hydrogel.

[0041] Example 2: The same as Example 1 above, the difference is that: 100 mg of Zn-MOF is added in Step 2, denoted as PAM2 hydrogel.

[0042] Example 3: The same as Example 1 above, the difference is that: 150 mg of Zn-MOF is added in Step 2, denoted as PAM3 hydrogel.

[0043] Example 4: The same as Example 1 above, the difference is that: 200 mg of Zn-MOF is added in Step 2, denoted as PAM4 hydrogel.

[0044] Comparative Example 1: Preparation of polyvinyl alcohol hydrogel: Add 5 g of polyvinyl alcohol and 50 mL of pure water into a beaker, place it in a water bath at 95 °C and stir magnetically for 3 h until completely dissolved. Pour the solution into a square container (10 mm × 85 mm × 85 mm), freeze it in a -20 °C refrigerator for 12 h, and then take it out and thaw it in a 25 °C incubator for 1 h to obtain the polyvinyl alcohol hydrogel, denoted as P hydrogel.

[0045] Comparative Example 2: Preparation of polyvinyl alcohol / alizarin hydrogel: Add 5 g of polyvinyl alcohol and 50 mL of pure water into a beaker, place it in a water bath at 95 °C and stir magnetically for 3 h until completely dissolved. When the temperature drops to 50 °C, add 2 mg of alizarin and stir for 1 h. Then pour the solution into a square container (10 mm × 85 mm × 85 mm), freeze it in a -20 °C refrigerator for 12 h, and then take it out and thaw it in a 25 °C incubator for 1 h to obtain the polyvinyl alcohol / alizarin hydrogel, denoted as PA hydrogel.

[0046] Specific Example 2: Structure analysis of Zn-MOF and hydrogel.

[0047] The structure of Zn-MOF synthesized by the method of Example 1 in Specific Example 1 was analyzed by Fourier transform infrared spectroscopy. Mix 2 mg of Zn-MOF and 100 mg of potassium bromide, grind and press them into tablets, and detect them by Fourier transform infrared spectroscopy. The results are as Figure 1 shown in A. From Figure 1As can be seen from A, the carboxyl peak in Zn-MOF appears at 1650 cm -1 , the carboxyl peak in gallic acid appears at 1700 cm -1 , the significant weakening of the carboxyl peak in Zn-MOF, and the formation of the Zn-O bond at 750 cm -1 all prove the successful synthesis of Zn-MOF.

[0048] The structures of the P hydrogel, PA hydrogel, PAM1 hydrogel, PAM2 hydrogel, PAM3 hydrogel, and PAM4 hydrogel synthesized in Specific Example 1 were analyzed by total reflection infrared spectroscopy, and the results are as shown in Figure 1 B. As can be seen from Figure 1 B, the P hydrogel shows typical characteristic absorption peaks at 3352 and 2930 cm -1 , which originate from the stretching vibrations of OH and CH2, respectively. The peaks at 1327 and 1096 cm -1 correspond to the bending vibration of CH–CH2 and the stretching vibration of C–O, respectively, representing the carbon skeleton of polyvinyl alcohol. After introducing alizarin and Zn-MOF into the P hydrogel, a significant change occurred in the OH absorption peak of the polyvinyl alcohol / alizarin / Zn-MOF composite hydrogel (blue-shifted from 3352 cm -1 to 3298 cm -1 ), indicating the formation of some interaction between the polyvinyl alcohol matrix, alizarin, and Zn-MOF. This blue shift may be related to the hydrogen bond interaction formed between the hydroxyl groups in the polyvinyl alcohol matrix, the oxygen-containing groups in alizarin (such as OH and C=O), and the oxygen-containing groups in Zn-MOF (such as OH and COOH).

[0049] Specific Example 3: Explore the effect of adding different concentrations of Zn-MOF on the mechanical properties of the hydrogel.

[0050] To explore the effect of adding different concentrations of Zn-MOF on the mechanical properties of the hydrogel, the mechanical properties of different hydrogels were detected through tensile test experiments, compression test experiments, and moisture distribution experiments. The mechanical properties of the hydrogel were tested by tensile and compression experiments using a small electronic universal material testing machine (5966, Instron, USA). For the tensile experiment, rectangular hydrogel samples with dimensions of 10 mm in width, 60 mm in length, and 1.5 mm in thickness were prepared. The tensile rate was set at 60 mm / min. For the compression experiment, the hydrogel was prepared as a cylinder with a height of 20 mm and a diameter of 20 mm. The compression rate was set at 5 mm / min, and the strain range was 0-75%. The results are as shown in Figure 2 .

[0051] Figure 2In Figure A, the tensile strain-stress curves of different hydrogels are shown. Among them, the PAM3 hydrogel has the best tensile properties. The results show that Zn-MOF has the effects of both strengthening and toughening on the hydrogel. The hydrogen bond crosslinking among Zn-MOF, alizarin, and polyvinyl alcohol can inhibit the movement of polymer chains to strengthen the mechanical properties of the hydrogel. And the hydrogen bond is reversible, constantly breaking and recombining under external forces, effectively dissipating energy, thereby endowing the hydrogel with higher toughness and ultimately improving the tensile and compressive properties of the hydrogel.

[0052] Figure 2 In Figure B, the tensile modulus, elongation at break, and fracture strength of different hydrogels are shown. Among them, the tensile modulus, elongation at break, and fracture strength of the PAM3 hydrogel are 16.723 KPa, 334.408%, and 0.056 MPa respectively, which are 1.3, 1.8, and 2.4 times those of the P hydrogel, and 1.1, 1.4, and 1.6 times those of the PA hydrogel.

[0053] Figure 2 In Figure C, the compression strain-stress curves of different hydrogels are shown. Among them, the PAM3 hydrogel has the best compression properties, indicating that when the PAM3 hydrogel is compressed by an external force, it is the most difficult to deform. This means that the hydrogel is stronger and more rigid, and is not easily deformed or collapsed under pressure, thus proving the strengthening effect of Zn-MOF on the mechanical properties of the hydrogel.

[0054] Figure 2 In Figure D, the compression moduli of different hydrogels are shown. Among them, the PAM3 has the highest compression modulus, which is 2.9 times that of the P hydrogel and 2.1 times that of the PA hydrogel.

[0055] Specific Example 4: Verify the colorimetric performance of the hydrogel at different pH values.

[0056] Take 1 mL of alizarin solution with a concentration of 0.5 mg / mL and add it to 4 mL of pH buffers with pH values of 4, 5, 6, 7, 8, 9, 10, 11, and 12 respectively. Collect the visible spectra of the alizarin solution using a multifunctional microplate reader. Figure 3 In Figure A, the ultraviolet-visible spectra and images of alizarin at different pH values are shown. As can be seen from Figure 3 Figure A, the alizarin solution is bright yellow in the pH range of 4.0 - 5.0, orange in the pH range of 6.0 - 7.0, the purple color gradually deepens in the pH range of 8.0 - 11.0, and turns dark purple at pH = 12.0. This indicates that alizarin has a sensitive and visible color response to pH value changes.

[0057] Cut the 6 hydrogels prepared in Specific Example 1 into small cylinders with a diameter of 1.5 cm and a height of 0.5 cm. Place the hydrogels separately in aqueous solutions with pH values of 4, 5, 6, 7, 8, 9, 10, 11, and 12 and let them stand for 1 h. Take pictures with a smartphone to obtain pictures of the 6 hydrogels at different pH values. Figure 3 In [Figure], B shows the color changes of the hydrogels at different pH values. The color of the P hydrogel does not change, while the colors of the PA and PAM1, PAM2, PAM3, and PAM4 hydrogels change differently. This indicates that the addition of Zn-MOF does not affect the color display effect of the hydrogels at pH, and has a sensitive and visible color response to changes in pH value.

[0058] Specific Example 5: Verify the color display performance of the hydrogels under the response of trimethylamine.

[0059] Cut the 6 hydrogels in Specific Example 1 into small cylinders with a diameter of 1.5 cm and a height of 0.5 cm, and place them in a petri dish with a height of 2 cm and a diameter of 9 cm. Then add 1000 ppm of trimethylamine to the petri dish. Use a smartphone to take pictures to record the pictures of the 6 hydrogels at 9 time points: 0 h, 0.25 h, 0.5 h, 0.75 h, 1 h, 2 h, 3 h, 6 h, and 12 h. Use Photoshop software to extract the RGB values of the hydrogels in each picture, and calculate the Euclidean distance (abbreviated as ED) value according to the following formula. The results are as Figure 4 shown.

[0060] , where ΔR represents the difference in the intensity value of the red channel before and after the reaction of the hydrogel with the shrimp to be measured, ΔG represents the difference in the intensity value of the green channel before and after the reaction of the hydrogel with the shrimp to be measured, and ΔB represents the difference in the intensity value of the blue channel before and after the reaction of the hydrogel with the shrimp to be measured;

[0061] Figure 4 In [Figure], A shows the ED values of different hydrogels in response to time under 1000 ppm of trimethylamine. The ED value of the P hydrogel does not change significantly because this hydrogel does not contain alizarin. As the reaction time of the PA and PAM hydrogels with trimethylamine increases, the ED values of the hydrogels gradually increase. It should be noted that the change in the ED value of the PAM1, PAM2, PAM3, and PAM4 hydrogels is greater than that of the PA hydrogel. This may be due to the high surface area and porous structure of Zn-MOF, which enhances the capture of trimethylamine, resulting in more obvious color changes. Among the four different concentrations of PAM hydrogels, the change in the ED value of the PAM3 hydrogel is the most significant. However, the ED value of PAM4 decreases, which may be because the continued addition of Zn-MOF hinders the reaction between trimethylamine and alizarin.

[0062] Figure 4Images of the response of different hydrogels in B to trimethylamine over time. The color of the P hydrogel did not change significantly because this hydrogel does not contain alizarin. As the reaction time of the PA and PAM hydrogels with trimethylamine increased, the color of the hydrogels gradually changed from light pink to light purple, then to dark purple, and finally to red-violet.

[0063] Specific Example Six: Determination of the antioxidant capacity of the hydrogel.

[0064] 1. Determination of the antioxidant capacity of the hydrogel by the DPPH method: Accurately weigh 3.9 mg of DPPH dry powder and dissolve it in 50 mL of ethanol solution with a volume fraction of 95% to prepare a 0.2 mM DPPH working solution. Weigh 0.1 g of the P hydrogel, PA hydrogel, PAM1 hydrogel, PAM2 hydrogel, PAM3 hydrogel, and PAM4 hydrogel prepared in Specific Example One respectively and dissolve them in 1 mL of distilled water and soak for 24 h. Take 100 μL of the soaking solution of different hydrogels and add them to different wells of a 96-well plate. Then add 100 μL of the DPPH working solution to each well, and let it stand in the dark at 25 °C for 30 min to fully react. Read the absorbance value of the experimental group at 517 nm with an enzyme-labeling instrument. The control group uses an ethanol aqueous solution with a volume fraction of 95% instead of the DPPH working solution. The blank group uses an ethanol aqueous solution with a volume fraction of 95% instead of the sample solution, and calculate the DPPH radical scavenging rate according to the formula:

[0065] , Note: A X is the absorbance value of the experimental group; A X0 is the absorbance value of the control group; A0 is the absorbance value of the blank group.

[0066] Figure 5 In it, A is the DPPH radical scavenging rate of different hydrogels. The P hydrogel did not show significant antioxidant activity. The PA hydrogel showed a significant increase in the DPPH radical scavenging rate after adding alizarin. The hydrogel added with Zn-MOF showed a significant improvement in antioxidant activity. Among them, the DPPH radical scavenging rate of the PAM3 hydrogel reached 81%, which is attributed to the presence of gallic acid in Zn-MOF. The polyphenols inhibit free radicals through electron transfer, making it have excellent antioxidant ability.

[0067] 2. Determination of the antioxidant capacity of the hydrogel by the ABTS method: Weigh 0.0384 g of ABTS reagent and make up the volume to 10 mL with distilled water; weigh 0.0134 g of potassium persulfate and make up the volume to 10 mL with distilled water. Mix the above two reagents in a volume ratio of 1:1 and keep in the dark for 12 h to obtain the ABTS working solution. Dilute it with distilled water before use to an absorbance of 0.70 ± 0.02 at 734 nm. Weigh 12.5 mg of P hydrogel, PA hydrogel, PAM1 hydrogel, PAM2 hydrogel, PAM3 hydrogel and PAM4 hydrogel respectively, dissolve them in 1 mL of distilled water and soak for 24 h. Take 100 μL of the soaking solution of different hydrogels and add them to different wells of a 96-well plate, then add 100 μL of ABTS solution to each well, react in the dark for 30 min, and read the absorbance value of the experimental group at 734 nm with an enzyme-labeling instrument. Use distilled water to replace the ABTS working solution in the control group. Use distilled water to replace the sample solution in the blank group. Calculate the ABTS radical scavenging rate according to the formula:

[0068] , Note: A X is the absorbance value of the experimental group; A X0 is the absorbance value of the control group; A0 is the absorbance value of the blank group.

[0069] Figure 5 where B is the ABTS radical scavenging rate of different hydrogels. The P hydrogel did not show significant antioxidant activity. The ABTS radical scavenging rate of the PA hydrogel increased significantly after adding alizarin. The hydrogels added with Zn-MOF showed a significant increase in antioxidant activity. Among them, the ABTS radical scavenging rate of the PAM3 hydrogel reached 95%, which further proved the excellent antioxidant properties of Zn-MOF.

[0070] Specific Example Seven: Determination of the antibacterial ability of the hydrogel.

[0071] The colony counting method was used to determine the antibacterial activity of the samples against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Inoculate a single colony of Escherichia coli or Staphylococcus aureus into LB broth medium and culture it in an incubator at 37 °C for 18 h. Then dilute the bacterial solution to 1×10 7 CFU / mL -1 . Take 0.4 g of the P hydrogel, PA hydrogel, PAM1 hydrogel, PAM2 hydrogel, PAM3 hydrogel and PAM4 hydrogel prepared in Specific Example One respectively, mix them with 1 mL of the bacterial solution, and then co-culture them in a shaker at 37 °C at 180 - 200 rpm for 18 h. Take 0.2 mL of the co-culture solution and spread it on an agar plate, and take a photo of the agar plate after culturing it in an incubator at 37 °C for 24 h.

[0072] Figure 6 For the antibacterial effects of different hydrogels against Staphylococcus aureus and Escherichia coli, the P hydrogel did not show any inhibitory effect on both bacteria. Due to the antibacterial properties of alizarin, the PA hydrogel inhibited the growth of both bacteria. With the increase in the content of Zn-MOF, the antibacterial ability of the hydrogel was further enhanced, and the PAM3 hydrogel had the strongest ability to eliminate both bacteria. The excellent antibacterial ability of Zn-MOF is usually related to the release of zinc ions, which can destroy the integrity of the bacterial cell membrane, cause the leakage of internal nutrients and other contents, and finally kill the bacteria.

[0073] Specific Example Eight: Determination of the freshness preservation ability of the PAM3 hydrogel.

[0074] Experimental method: Unwrapped shrimp samples were used as the control group, shrimp samples wrapped with P hydrogel (8 cm × 8 cm × 1 cm) (1 piece of P hydrogel was placed under the shrimp, and 1 piece of P hydrogel was placed on top of the shrimp), and shrimp samples wrapped with PAM3 hydrogel (8 cm × 8 cm × 1 cm) (1 piece of PAM3 hydrogel was placed under the shrimp, and 1 piece of PAM3 hydrogel was placed on top of the shrimp) were stored in a 4°C refrigerator, and the TVB-N values of the shrimp samples were measured for 0 - 10 days. Referring to the national standard GB 5009.228-2016 "National Food Safety Standard - Determination of Volatile Basic Nitrogen in Foods", the automatic Kjeldahl nitrogen analyzer method was used to measure the TVB-N of each group of samples.

[0075] The results are as Figure 7 shown. The TVB-N values of all treatment groups continuously increased during storage, and this phenomenon was attributed to the hydrolysis of proteins in the shrimp. The TVB-N content of the control group reached secondary freshness (20 mg / 100g) on the 3rd day and spoiled on the 6th day (30 mg / 100g). The shrimp in the P hydrogel group spoiled on the 7th day, probably because the P hydrogel itself could absorb shrimp secretions and reduce the growth of microorganisms in the shrimp. The shrimp in the PAM3 group spoiled on the 9th day because Zn-MOF has antibacterial and antioxidant properties, effectively inhibiting the growth of microorganisms and reducing protein degradation.

[0076] Specific Example Nine: Application of the PAM3 hydrogel in the monitoring of shrimp freshness.

[0077] Two pieces of PAM3 hydrogel prepared in Example 3 of Specific Example One with dimensions of 8.5 cm × 8.5 cm × 0.5 cm were used to wrap the shrimp and placed in a food-grade transparent square box with dimensions of 10.8 cm × 10.8 cm × 5.9 cm, and then stored in a 4°C refrigerator. Pictures were taken with a smartphone for 0 - 10 days. Figure 8For the daily color change of the PAM3 hydrogel within 0 - 10 days, the visible color change to the naked eye is from flesh pink to dark purplish red. The freshness of shrimp can be judged based on the color change of the hydrogel.

[0078] To further quantitatively judge the freshness grade of shrimp, a method for judging the freshness grade of shrimp based on the hydrogel PAM3 prepared in Specific Example 1 is established. The specific steps are as follows:

[0079] Step 1: Model establishment: A prediction model is established using the logistic function , where A1, A2, x0, and p are all constant terms, x is the average ED value, and y is the TVB - N value; the calculation method of the constant terms is specifically as follows:

[0080] (1) Wrap fresh shrimp samples with PAM3 hydrogel. Place 1 piece of PAM3 hydrogel under the shrimp and 1 piece on top of the shrimp. Take pictures of the hydrogel wrapping the shrimp with a mobile phone every 24 hours, and simultaneously measure the TVB - N value of the shrimp samples.

[0081] (2) Extract the RGB values of 16 evenly distributed points on the image of the hydrogel wrapping the shrimp through the Python image processing library, and calculate the average ED value of the 16 points. The calculation formula for the ED value is , where ΔR represents the difference in the intensity value of the red channel before and after the reaction between the hydrogel and the shrimp to be measured, ΔG represents the difference in the intensity value of the green channel before and after the reaction between the hydrogel and the shrimp to be measured, and ΔB represents the difference in the intensity value of the blue channel before and after the reaction between the hydrogel and the shrimp to be measured.

[0082] (3) When the TVB - N value of the shrimp sample exceeds 30 mg / 100g, stop taking pictures with the mobile phone and measuring the TVB - N value of the shrimp sample. Substitute the daily measured TVB - N value of the shrimp sample and the average ED value calculated at the same time into the prediction model, and calculate that A1 is 10.66, A2 is 41.38, x0 is 82.23, and p is 3.2. The constant terms of each hydrogel are fixed. When changing to different hydrogels, the constant terms need to be recalculated.

[0083] Step 2: Acquisition of the image to be measured: Wrap the shrimp sample to be measured with PAM3 hydrogel, and take pictures of the hydrogel wrapping the shrimp to be measured with a mobile phone.

[0084] Step 3: Extract the RGB values of 16 evenly distributed points on the image of the hydrogel wrapping the shrimp to be measured through the Python image processing library, and calculate the average ED value of the 16 points.

[0085] Step 4: Substitute the average ED value obtained in Step 3 into the prediction model to calculate the TVB-N value of the shrimp sample to be measured, and determine the freshness grade of the shrimp according to the TVB-N value. Shrimp with a TVB-N value ≤ 20 mg / 100g is fresh shrimp, shrimp with a TVB-N value of 20 - 30 mg / 100g is sub-fresh shrimp, and shrimp with a TVB-N value ≥ 30 mg / 100g is spoiled shrimp.

[0086] Figure 9 shows the changes in the ED value of PAM3 hydrogel and the TVB-N value of shrimp over 0 - 10 days. The shrimp reached sub-freshness on the 4th day and spoiled on the 9th day. Both the ED value and the TVB-N value increased with the passage of time. Combining Figure 8 and Figure 9 it can be seen that the change in the freshness of shrimp can be distinguished by both the naked eye and the ED value every day.

[0087] Figure 10 is the correlation analysis of using the above method for determining the freshness grade of shrimp and the Kjeldahl method to measure 30 unknown shrimp samples. Among them, a Pearson correlation coefficient |r| > 0.8 indicates a strong positive linear relationship. The results show that there is a very strong correlation between the two, and the Pearson correlation coefficient is 0.9750. Thus, it can be seen that the method for determining the freshness grade of shrimp based on the hydrogel PAM3 prepared in Specific Example 1 is accurate and reliable. Moreover, by combining the smartphone with the hydrogel, it can not only meet the needs of shrimp freshness detection, but also has the characteristics of being portable, real-time, and user-friendly.

[0088] The above description is not a limitation of the present invention, nor is the present invention limited to the above examples. Changes, modifications, additions, or substitutions made by those of ordinary skill in the art within the scope of the essence of the present invention shall also fall within the protection scope of the present invention.

Claims

1. A preparation method of a hydrogel for food preservation and visualization monitoring of freshness, characterized in that It includes the following steps: Add polyvinyl alcohol to pure water, stir magnetically in a water bath at 90 - 98 °C until completely dissolved. After the temperature drops to 40 - 50 °C, add alizarin, stir for 1 - 2 h, then add zinc - gallic acid metal - organic framework Zn - MOF, continue to stir for 1 - 2 h, pour it into a square container, freeze it in a refrigerator at - 15 - 20 °C for 10 - 15 h, and then take it out and thaw it in an incubator at 20 - 30 °C for 1 h to obtain the hydrogel for food preservation and freshness visual monitoring. The mixing ratio of polyvinyl alcohol, pure water, alizarin, and zinc - gallic acid metal - organic framework is 5 g:50 mL:2 mg:50 - 150 mg. The preparation method of the zinc - gallic acid metal - organic framework is as follows: Dissolve zinc nitrate hexahydrate and gallic acid in N,N - dimethylformamide solution, perform ultrasonic treatment at 30 - 50 kHz for 2 - 8 min, then transfer it to a polytetrafluoroethylene autoclave, react at 100 - 150 °C for 24 h. After centrifuging the reaction solution at 6000 rpm for 10 min, take the precipitate, wash it three times with ethanol, and dry it in an oven at 50 - 70 °C to obtain a gray - white product, which is the zinc - gallic acid metal - organic framework Zn - MOF. The mixing ratio of zinc nitrate hexahydrate, gallic acid, and N,N - dimethylformamide solution is 3 g:2 g:75 mL.

2. The preparation method of a hydrogel for food preservation and freshness visual monitoring according to claim 1, characterized in that: The mixing ratio of the polyvinyl alcohol, the pure water, the alizarin, and the zinc - gallic acid metal - organic framework is 5 g:50 mL:2 mg:150 mg.

3. A method for determining the freshness grade of shrimp using the hydrogel prepared by the method according to any one of claims 1 to 2, characterized in that The specific steps are as follows: Step 1, model establishment: A prediction model is established using the logistic function , where A1, A2, x0, and p are all constant terms, x is the average ED value, and y is the TVB-N value; Step 2, obtaining the image to be measured: Wrap the shrimp sample to be measured with the hydrogel prepared by the method described in claim 1, and use a mobile phone to take a picture to obtain the hydrogel image wrapping the shrimp to be measured; Step 3: Extract the RGB values of 16 points evenly distributed on the hydrogel image wrapping the shrimp to be measured through the Python image processing library, and calculate the average ED value of the 16 points. The calculation formula of the ED value is , where ΔR represents the difference in the intensity value of the red channel before and after the reaction between the hydrogel and the shrimp to be measured, ΔG represents the difference in the intensity value of the green channel before and after the reaction between the hydrogel and the shrimp to be measured, and ΔB represents the difference in the intensity value of the blue channel before and after the reaction between the hydrogel and the shrimp to be measured; Step 4, substitute the average ED value into the prediction model, calculate to obtain the TVB - N value of the shrimp sample to be measured, and judge the freshness grade of the shrimp according to the TVB - N value.

4. The method for judging the freshness grade of shrimp according to the hydrogel as claimed in claim 3, wherein The calculation method of the constant term described in step 1 is as follows: The specific steps are as follows: (1) Wrap the fresh shrimp sample with the hydrogel prepared by the method described in claim 1, place 1 hydrogel pad under the shrimp and 1 hydrogel on top of the shrimp. Take a picture of the hydrogel wrapping the shrimp with a mobile phone every 24 hours, and at the same time measure the TVB - N value of the shrimp sample; (2)Extract the RGB values of 16 evenly distributed points on the hydrogel image wrapping the shrimp through the Python image processing library, and calculate the average ED value of the 16 points. The calculation formula of the ED value is ; (3) When the measured TVB - N value of the shrimp sample exceeds 30 mg / 100g, stop taking pictures with the mobile phone and measuring the TVB - N value of the shrimp sample. Substitute the TVB - N value measured for the shrimp sample every day and the average ED value calculated at the same time into the prediction model, and calculate to obtain the specific values of A1, A2, x0, and p.

5. The method for judging the freshness grade of shrimp according to the hydrogel as claimed in claim 3, wherein The specific criteria for classifying the freshness grade of shrimp described in step 4 are as follows: Shrimp with a TVB - N value ≤ 20 mg / 100g is fresh shrimp, shrimp with a TVB - N value between 20 - 30 mg / 100g is semi - fresh shrimp, and shrimp with a TVB - N value ≥ 30 mg / 100g is spoiled shrimp.

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