A pH indicating gel for food preservation monitoring and preparation method and application thereof

Abstract

The application discloses a pH indicating gel for food preservation monitoring and a preparation method and application thereof, and the pH indicating gel is composed of a high-molecular polymer hydrogel and a macromolecular polymer dye with pH indicating capacity, wherein the macromolecular polymer dye is obtained by grafting chitosan and a pH response dye through a covalent grafting method, and the pH indicating gel has the advantages that the pH response dye is chemically embedded into a high-molecular chain through chemical grafting, the stability of the indicator is improved through the action of ionic bonds and hydrogen bonds in the hydrogel, and the bleeding of the dye can be effectively avoided, especially, phenol red is combined with chitosan to generate an alkyl amine derivative through a mannich reaction, and a polymer dye is obtained, and the pH indicating gel prepared from the polymer dye has no permeation, and the defects of the prior art are successfully solved. In addition, the pH indicating gel has obvious responsive change to ammonia, and can directly indicate the freshness change of food during preservation.

Description

Technical Field

[0001] This invention relates to a food preservation monitoring technology, and more particularly to a pH indicator gel for food preservation monitoring, its preparation method, and its application. Background Technology

[0002] Food spoilage has a profound impact on food safety, the environment, and quality. In some cases, much food is discarded before it is sold due to its perishability. If not addressed promptly, these situations not only have a direct and significant impact on the environment and the economy but also cause substantial losses to retailers, the food service industry, and consumers. The shortened shelf life and increased risk of foodborne illnesses caused by microbial contamination have driven the adoption of various methods to monitor food freshness and improve its safety. Currently, commonly used methods for evaluating food quality include gas chromatography, mass spectrometry, Fourier transform infrared spectroscopy, magnetic resonance imaging, and microbiological analysis. However, these methods require bulky benchtop instruments, are expensive, and time-consuming. A new approach is to create smart materials for visual monitoring as indicators of food spoilage to assess food freshness and determine its condition. A growing number of researchers are dedicated to developing smart materials for visually detecting pH. Currently, hydrogel materials for visually detecting pH have been prepared by encapsulating phenol red in a sol-gel matrix using embedding or impregnation methods. However, this hydrogel and pH dye only bind through physical interaction, and there are still problems such as easy leakage of the indicator and low detection accuracy and sensitivity.

[0003] For example, the Chinese invention patent application publication No. 202211048130.3, published on August 30, 2022, discloses a patent for visually monitoring the freshness of fish and meat products by monitoring the content of volatile amines during their shelf life. The patent uses chemical fluorescent dyes that are physically adsorbed and fixed onto filter paper, posing a certain risk of leakage. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a pH indicator gel with high stability and safety, which can effectively prevent the leaching of chemically synthesized substances for monitoring food preservation and spoilage, as well as its preparation method and application.

[0005] One of the technical solutions adopted by the present invention to solve the above-mentioned technical problems is: a pH indicator gel for monitoring food preservation, which is composed of a high molecular polymer hydrogel and a macromolecular polymer dye with pH indication capability. The macromolecular polymer dye is synthesized by grafting chitosan with pH-responsive dye through a covalent grafting method.

[0006] The second technical solution adopted by the present invention to solve the above-mentioned technical problems is: a method for preparing a pH indicator gel for monitoring food preservation, comprising the following steps:

[0007] Step 1, Preparation of macromolecular polymeric dye: Chitosan is dissolved in acetic acid solution to obtain chitosan grafting solution, and pH-responsive dye is dissolved in dimethylformamide solution to obtain pH-responsive dye solution. The pH-responsive dye solution and formaldehyde are added sequentially to the chitosan grafting solution, and macromolecular polymeric dye is obtained through covalent grafting reaction. The preferred molar ratio of chitosan:formaldehyde:pH-responsive dye is 1:1:0.1.

[0008] Step 2, Preparation of indicator gel: Prepare a polymer solution and then add a macromolecular polymer dye to obtain a pH indicator gel.

[0009] The third technical solution adopted by the present invention to solve the above-mentioned technical problems is: the preparation of a pH indicator gel for food preservation monitoring and its application in various biological products with food preservation monitoring functions.

[0010] Compared with existing technologies, the advantages of this invention lie in the chemical grafting of pH-responsive dyes into polymer chains, followed by enhanced indicator stability through ionic and hydrogen bonding within the hydrogel, effectively preventing dye leaching. In particular, the combination of phenol red and chitosan via the Mannich reaction to generate alkylamine derivatives yields a polymer dye. pH-indicating gels prepared using this polymer dye are permeable, successfully overcoming the shortcomings of existing technologies. Furthermore, the pH-indicating gel provided by this invention exhibits a significant responsiveness to ammonia gas, providing a clear and distinguishable color change that intuitively indicates changes in food freshness during storage, making it applicable to food preservation monitoring.

[0011] Preferably, the polymer is a mixture of polyvinyl alcohol and chitosan.

[0012] More preferably, the polymer solution is composed of a polyvinyl alcohol solution and a chitosan solution in a volume ratio of 3:1. Based on this, a preferred preparation method is as follows:

[0013] Step 1: Chitosan was dissolved in acetic acid solution to obtain chitosan grafting solution. pH-responsive dye was dissolved in dimethylformamide solution to obtain pH-responsive dye solution. Then, the pH-responsive dye solution was added to chitosan grafting solution. After adding formaldehyde solution, the mixture was stirred at 60℃ for 24 hours. Then, NaOH solution was slowly added to obtain purple solid. The solid product was filtered and washed with ethanol and distilled water. The precipitate was dispersed in distilled water and then centrifuged at 6000 rpm / min for 15 minutes to remove the remaining unreacted dye, obtaining a solid pure product. After freeze-drying, a solid macromolecular polymer dye was obtained.

[0014] Step 2: Polyvinyl alcohol (PVA) was added to deionized water at a mass-to-volume ratio of 14:100 and stirred at 89°C for 2 hours to obtain a 14% (w / v) PVA solution. The PVA solution was then cooled to room temperature until the bubbles completely disappeared. Chitosan was dissolved in a 1% (v / v) acetic acid solution to prepare a 2% (w / v) chitosan solution. The chitosan solution was then added to the PVA solution at a volume ratio of 1:3 to obtain a polymer solution. The solid macromolecular dye prepared in Step 1 was dissolved in a 1% (v / v) acetic acid solution to prepare a 0.5% (w / v) macromolecular dye solution. Then, a 10% (v / v) polymer solution was added and stirred for 1 hour to obtain a homogeneous solution. The pH indicator gel was prepared by freezing and thawing at -80°C for 1 hour and thawing at 25°C for 1 hour.

[0015] Chitosan is a chitin unit ((β-(1→4)-linked) Chitosan is a natural copolymer of D-glucosamine deacetylated derivatives, derived from the deacetylation of the natural polysaccharide chitin. Chitosan possesses various physiological functions, including biocompatibility, biodegradability, non-toxicity, antibacterial properties, anticancer effects, lipid-lowering effects, and immune enhancement. It is widely used in food additives, medical dressings, antibacterial agents, bio-coatings, and cosmetics. The functional groups on chitosan are easily modified; its free amino groups (-NH2) can undergo various chemical modifications, including alkylation, sulfation, and nitration. Through physical or chemical cross-linking, it can form hydrogels with diverse functions. Polyvinyl alcohol (PVA) is a physically or chemically cross-linked three-dimensional network polymer. It is an extremely safe polymer, non-toxic and without side effects to the human body, and possesses excellent biocompatibility. PVA has high water content, a microporous structure, good elasticity and lubrication properties, and is a water-soluble, biodegradable synthetic polymer with high tensile strength and flexibility. The freeze-thaw method for preparing pH indicator hydrogels requires no additional chemical reagents and is a green and recyclable method.

[0016] Preferably, the pH-responsive dye is phenol red containing phenolic hydroxyl groups. Attached Figure Description

[0017] Figure 1 The image shows the infrared spectrum of the macromolecular polymeric dye CS-PR synthesized by the method of the present invention in Example 1 of the present invention, where CS corresponds to the infrared spectrum of chitosan CS and CS-PR corresponds to the infrared spectrum of macromolecular polymeric dye CP.

[0018] Figure 2 This is a UV-Vis characterization image of the macromolecular polymeric dye CS-PR synthesized by the method of the present invention in Example 1 of the present invention;

[0019] Figure 3 This is a color comparison diagram of the macromolecular polymeric dye CS-PR synthesized by the method of the present invention in Example 1 of the present invention with chitosan CS solution, phenol red PR, and a mixed solution of CS chitosan and phenol red PR. From left to right, they are CS, CS+PR and CS-PR solutions.

[0020] Figure 4 The image shows the color change of the PVA / CS / CP indicator gel prepared by the freeze-thaw cycle method in Example 1 of this invention when immersed in buffer solutions of different pH values.

[0021] Figure 5 This is a curve showing the color parameters of the PVA / CS / CP indicator gel prepared by the freeze-thaw cycle method in Example 1 of the present invention as a function of pH.

[0022] Figure 6 The UV-Vis spectrum of the PVA / CS / CP indicator gel leachate prepared by the freeze-thaw cycle method in Example 1 of this invention;

[0023] Figure 7 The response curve of the PVA / CS / CP indicator gel prepared by the freeze-thaw cycle method in Example 1 of this invention to ammonia gas;

[0024] Figure 8 The images show the responsive color changes of the PVA / CS / CP indicator gel prepared by the freeze-thaw cycle method in Example 1 of this invention to shrimp stored for different numbers of days, as well as the curves showing the changes in total volatile basic nitrogen (TVB-N) and pH value of the shrimp over time, and the total color difference (ΔE) analysis curve of the indicator gel.

[0025] Figure 9 This is a linear model equation relating the total color difference change (ΔE) of the PVA / CS / CP indicator gel prepared by the freeze-thaw cycle method in Example 1 of this invention to the total volatile basic nitrogen (TVB-N) and pH of the shrimp.

[0026] In the above figure description, CS: chitosan, CS+PR: chitosan and phenol red mixture, CS-PR(CP): macromolecular polymer dye, PVA / CS / CP: polyvinyl alcohol / chitosan / macromolecular polymer indicator gel. Detailed Implementation

[0027] To make the content of this invention easier to understand, the technical solution of this invention will be further described below in conjunction with specific embodiments. However, this invention is not limited thereto. Unless otherwise specified, the experimental materials used in the following embodiments are all commercially available products, and the experimental steps are all standard steps unless otherwise specified.

[0028] In the following embodiments:

[0029] The grafting reaction was performed using the Mannich method.

[0030] The polymer solution is composed of a polyvinyl alcohol solution and a chitosan solution in a volume ratio of 3:1.

[0031] Phenol Red was chosen as the pH-responsive dye.

[0032] Example 1.

[0033] Unless otherwise specified, all experimental materials used in the following examples are commercially available products, and all experimental procedures are standard procedures unless otherwise specified.

[0034] (1). Preparation of macromolecular polymeric dye CS-PR.

[0035] The specific reaction formula for preparation is as follows:

[0036]

[0037] Chitosan (CS), formaldehyde, and phenol red (PR) are chemically reacted in a molar ratio of 1:1:0.1. The specific preparation method is as follows:

[0038] a. Chitosan CS (0.1 g, 0.646 mmol as amino group) was dissolved in 10 mL of 1% w / v acetic acid solution. Then, 0.0229 g of phenol red PR was dissolved in 5 mL of dimethylformamide DMF and added to the chitosan CS solution. 60 μL of formaldehyde solution was added, and the mixture was stirred at 60 °C for 24 h. After the reaction was complete, 0.5 M NaOH solution was slowly added to obtain a purple solid.

[0039] b. The solid product was filtered and washed with ethanol and distilled water. The precipitate was dispersed in distilled water and then centrifuged at 6000 rpm for 15 min to remove any remaining unreacted dye, yielding a pure solid. This was then freeze-dried to obtain the macromolecular polymeric dye. Fourier transform infrared spectroscopy (FT-IR) was performed using a NICOLET iS10 spectrophotometer (Thermo Scientific, USA) equipped with an ATR (attenuated total reflectance) accessory. The infrared spectrum of the macromolecular polymeric dye CS-PR was recorded in the range of 4000–500 cm⁻¹. -1 The resolution is 4cm. -1 .

[0040] The preparation method for macromolecular polymeric dyes CS-PR with different CS:formaldehyde:PR molar ratios is the same as above.

[0041] c. Elemental analysis method for calculating the grafting rate of macromolecular polymeric dyes CS-PR

[0042] Grafting rate was expressed as the degree of substitution. The carbon, hydrogen, nitrogen, and sulfur content of the macromolecular polymeric dye CS-PR with different CS:formaldehyde:PR molar ratios was analyzed using a Vario microcube elemental analyzer (Elementar, Germany). Degree of substitution (%DS) EA The formula is determined by the following method:

[0043]

[0044] Where (C / N) D It is the carbon-nitrogen ratio (C / N) of the macromolecular polymeric dye CS-PR. O It is the carbon-nitrogen ratio of the original chitosan.

[0045] The results of the reaction of CS:formaldehyde:PR with different molar ratios are shown in Table 1. The grafting rate increases with the increase of the ratio of formaldehyde HCHO to phenol red PR. When the molar ratio of CS:HCHO:PR is 1:2:1, the grafting rate reaches the highest of 35.70%.

[0046] Table 1. Content and degree of substitution (%DS) of carbon, hydrogen, nitrogen and sulfur measured by CS-PR (CP) elemental analyzer at different molar ratios EA )

[0047]

[0048] d. Solubility of macromolecular polymeric dyes with different molar ratios

[0049] The solubility of chitosan CS and macromolecular polymeric dyes CP1–CP5 in different molar ratios was investigated under various conditions (results are shown in Table 2). Generally, chitosan CS is soluble in acids due to the protonation of its amino groups. Similar to CS, CS derivatives are soluble in acidic solvents with pH < 5, such as HCl solution (0.1 M), acetic acid solution (1% w / v), and citric acid solution (1% w / v). However, the dissolution of chitosan derivatives is quite difficult due to the increased degree of substitution (DS) resulting from the chemical grafting reaction of the chitosan CS chains. To test this ability, macromolecular polymeric dyes in different ratios were dissolved in NaOH and KOH solutions (1 M) and then stored at room temperature for 24 h. The results are shown in Table 2. The chitosan derivative with the highest degree of substitution (DS) value (CP5) was partially soluble, while the other derivatives exhibited swelling properties. According to our research, the highest DS value (CP5) is soluble in deionized water (DI), and PVA / CS / CP hydrogels soaked in PBS (disodium hydrogen phosphate / potassium dihydrogen phosphate, pH=7.4) solution cause dye leakage. Therefore, considering all factors, we have selected CP1 (CS:HCHO:PR molar ratio of 1:1:0.1) as the macromolecular polymeric dye CP for further research.

[0050] Table 2. Solubility of CS-PR (CP) at different molar ratios

[0051]

[0052] (2) Characterization of macromolecular polymeric dye CS-PR

[0053] The macromolecular polymeric dyes obtained by grafting PR onto CS were characterized by Fourier transform infrared spectroscopy and ultraviolet-visible light spectroscopy to prove that PR was successfully grafted onto CS.

[0054] a. Fourier transform infrared spectroscopy (FT-IR) was performed using a NICOLET iS10 spectrophotometer (Thermo Scientific, USA) equipped with an ATR (Attenuated Total Reflectance) accessory. The recording range of CP (CS-PR1) was 4000–500 cm⁻¹. -1 The resolution is 4cm. -1 .like Figure 1 As shown, 3296cm -1 The spectrum shows typical characteristic bands of the CS molecule (stretching vibrations of the NH and OH groups). 2920 and 2874 cm⁻¹ -1 The stretching vibrations of the CH bond in the methyl group are C=O stretching vibration (amide I), NH bending vibration (amide II), and CN stretching vibration (amide III) at 1653, 1555, and 1161 cm⁻¹, respectively. -1 There are absorption peaks at 1415 and 1373 cm⁻¹. -1The bands at 1340 cm⁻¹ are attributed to the bending vibrations of the methylene and methyl groups, respectively. Compared to the CS spectrum, the CP bands at 1340 cm⁻¹ are... -1 A new peak appears at 1500-1455 cm⁻¹, corresponding to the S=O asymmetric stretching of the sulfonic acid group. -1 A relatively broad peak was found at the point, which is due to the C=C stretching of the CP benzene ring.

[0055] b. Record the UV-vis spectrum (200 to 800 nm) of the CP (CS-PR1) solution on a UV-2600 spectrophotometer (SHIMADZU, Japan). A 0.01% w / v CS and macromolecular polymeric dye CP solution was prepared by dissolving in 1% w / v acetic acid (pH = 4). Simultaneously, the amount of PR chemical substance added (equivalent to 50 μL (1‰ w / v) of free pH indicator PR) was measured. Figure 2 The image shows the UV-Vis spectra of a 1% w. / v. CS chitosan solution and the macromolecular polymeric dye CP dissolved in 1% acetic acid. The spectrum of the synthesis indicator polymer CP shows a characteristic peak at 440 nm, while CS shows no absorption peak. The absorption peaks of the CS and PR mixed solution are at 432 nm. The chitosan-modified PR exhibits a red shift, and the corresponding color change of the solution is shown below. Figure 3 As shown (from left to right: CS, CS+PR, and CS-PR solutions), this result confirms the presence of a new auxochrome on the phenol red PR molecule. The aforementioned structural features and color shift indicate that phenol red PR was successfully grafted onto the CS backbone via the Mannich reaction used in this invention.

[0056] (3) Preparation of PVA / CS / CP indicator gel

[0057] Polyvinyl alcohol (PVA) was added to deionized water at a weight ratio of 14:100 and stirred at 89°C for 2 hours to obtain a 14% PVA solution. The PVA solution was then cooled to room temperature until all bubbles disappeared. Chitosan was dissolved in a 1% (v / v) acetic acid solution to prepare a 2% (w / v) chitosan solution. The chitosan solution was then added to the PVA solution at a volume ratio of 1:3 to obtain a polymer solution. The solid macromolecular dye prepared in step one was dissolved in a 1% (v / v) acetic acid solution to prepare a 0.5% (w / v) macromolecular dye solution. This solution was then added to the polymer solution and stirred for 1 hour to obtain a homogeneous solution. A pH indicator gel was prepared by freeze-thawing at -80°C for 1 hour and thawing at 25°C for 1 hour.

[0058] (4) pH indicating ability test of PVA / CS / CP indicator gel

[0059] PVA / CS / CP indicator gels were immersed in different pH buffers (1–12) for 30 min before measurement. The color of the immersed indicator gels (L: lightness, a: red-green, b: yellow-blue) was measured using a colorimeter and matcher (Datacolor 600, USA) in CIE-LAB space. The total color difference (ΔE) was calculated using the formula:

[0060]

[0061] Where ΔL*, Δa*, and Δb* represent the differences in L, a, and b values ​​among the original samples. Three measurements were taken for each sample.

[0062] Photographs showing the color change of PVA / CS / CP indicator gels in buffer solution. Figure 4 As shown and color parameters are as follows Figure 5 As shown, the color parameters of the PVA / CS / CP indicator gel (L: 0 ​​for pure black, 100 for pure white, a: - for green, + for red, b: - for blue, + for yellow) change with pH. The PVA / CS / CP indicator gel is yellow at pH 4, gradually turns red at pH 5, and gradually turns purple at pH 9. The changes in a (3.07–37.81), b (21.93–-33.36), and ΔE (26.77–40.14) can be easily distinguished by the naked eye. This pH color indicator has strong application value in monitoring food spoilage.

[0063] (5) PVA / CS / CP-indicating gel exudation test

[0064] The UV-Vis spectra of the PVA / CS / CP indicator gel leachate were scanned in the range of 300–650 nm using a spectrophotometer. Free PR was measured in various 0.01 M buffer solutions using deionized water (DI water) as a reference.

[0065] like Figure 6 As shown, under normal conditions, the free phenol red (PR) solution exhibits strong absorption at 432 nm or 559 nm, while the corresponding absorption peak of the leachate shows almost no absorption at pH 1–5. No absorption peak was detected after 24 hours at pH > 6, confirming that the covalent grafting of phenol red (PR) onto chitosan (CS) prevents dye release or interference with the indicator gel's color. As shown in the figure, the leachate is clear and colorless, indicating that the indicator gel is stable within the pH range of 1–12. Notably, the indicator gel is more stable under alkaline conditions, suggesting that the macromolecular polymeric dye (CP) is slightly soluble in acid and almost insoluble in alkali. These results demonstrate that phenol red (PR) was successfully chemically covalently grafted onto the chitosan (CS) chain, exhibiting higher safety and stability compared to physical blending.

[0066] (6). Sensitivity of PVA / CS / CP indicator gel to ammonia gas response

[0067] The total volatile basic nitrogen (TVB-N) of food during storage was simulated using an ammonia atmosphere. An aqueous solution of NH3 (2800 ppm) in a 35 mm plastic petri dish was placed in each desiccator containing a PVA / CS / CP indicator gel. The sensitivity of the indicator gel to NH3 was measured and evaluated using a colorimeter at different time points. The sRGB values ​​were calculated using the following formula:

[0068]

[0069] In this study, Ra (red), Ga (green), and Ba (blue) are the initial parameters of the indicator hydrogel, while Rb, Gb, and Bb are the parameters measured after exposure to ammonia for different durations. The colorimetric mechanism of the PVA / CS / CP indicator gel to NH3 was determined by measuring its response potential to ammonia. The mechanism can be described as follows: ammonia first binds to water in the indicator gel, then hydrolyzes to generate OH-. - and NH4 + OH - It reacts with the macromolecular polymeric dye CP in the indicator gel to produce a colorimetric reaction. Sensitivity detection results are as follows: Figure 7 As shown, when the PVA / CS / CP indicator gel was exposed to an ammonia concentration of 2800 ppm, the PVA / CS / CP indicator gel exhibited a response rate of over 15% after 30 minutes, indicating that the PVA / CS / CP indicator gel is entirely feasible for monitoring food preservation.

[0070] Application Example 1: The accuracy of using the PVA / CS / CP indicator gel from Example 1 to monitor the degree of shrimp spoilage.

[0071] In an application example of the present invention, fresh shrimp are stored at 4°C to simulate a food preservation scenario in real life.

[0072] During the storage of fresh shrimp, the total viable count (TVC) continuously increases, leading to shrimp spoilage. This, in turn, causes an increase in the shrimp's pH and total volatile basic nitrogen (TVB-N) value. Therefore, the degree of shrimp spoilage can be characterized by these two chemical spoilage factors and microbiological analysis.

[0073] A PVA / CS / CP indicator gel containing 10g of fresh shrimp was placed in a 35mm non-direct contact plastic petri dish. The petri dish was wrapped with plastic wrap and stored at 4℃ for 12 days. At specific time intervals, the changes in total volatile basic nitrogen (TVB-N) and pH value of the shrimp in the petri dish were measured, and each indicator gel sample was photographed. Color parameters (L, a, b) were analyzed using a colorimeter.

[0074] Figure 8 To indicate the color change of the gel in response to shrimp stored at 4°C for different days, as well as the changes in total volatile basic nitrogen (TVB-N) and the pH of the shrimp over time, the total color difference analysis curve of the gel was obtained.

[0075] The curves showing the changes in total volatile basic nitrogen (TVB-N) and pH of fresh shrimp during 12 days of storage at 4℃ over time indicate that the TVB-N value continuously increases during storage, which is due to the decomposition of nitrogen-containing organic matter. At the same time, the pH value of the fresh shrimp also increases with the number of days.

[0076] PVA / CS / CP indicator gels exhibit different responsive color changes in shrimp stored for different durations.

[0077] On day 4, the total color difference (ΔE) increased from 1.93 to 20.05. The ΔE values ​​recorded by the PVA / CS / CP indicator gel during shrimp decay were easily and clearly colorimetrically determined (visually visible to the naked eye). Based on the freshness evaluation of color measurement, the freshness of shrimp stored at 4°C was classified into three grades: fresh (yellow, 0-3 days), slightly spoiled (orange-red, 4-5 days), and completely spoiled (purple, 6-10 days).

[0078] The correlation (R-value) and the accuracy of the linear equation (R²) between the total color difference ΔE of the PVA / CS / CP indicator gel and the pH value and total volatile basic nitrogen (TVB-N) of the shrimp samples were analyzed. 2 To evaluate the accuracy of PVA / CS / CP indicator gel in application.

[0079] The linear model equations for the total color difference change (ΔE) of the PVA / CS / CP indicator gel provided by this invention, relating it to the pH value and total volatile basic nitrogen (TVB-N) of shrimp, are as follows: Figure 9 As shown in the figure, the ΔE value of the PVA / CS / CP indicator gel is strongly positively correlated with pH (R = 0.9719) and TVB-N (R = 0.9643). The linear correlation equation between ΔE value and pH has a precision of up to 0.9394, and the precision between ΔE value and TVB-N is up to 0.9249.

[0080] This invention provides the responsive color change results of a PVA / CS / CP indicator gel—a pH indicator gel for monitoring food preservation—on fresh shrimp during storage for different days. It also includes the correlation (R-value) between the total color difference ΔE of the indicator gel and the shrimp's pH and total volatile basic nitrogen (TVB-N), as well as the accuracy of the linear equation (R²). 2 The results show that the color response of the PVA / CS / CP indicator gel in monitoring the freshness of shrimp has high accuracy and is visually identifiable, and it is expected to have great application prospects in smart packaging.

[0081] The above description is merely a detailed explanation of preferred embodiments and principles of the present invention. For those skilled in the art, there may be changes in specific implementation methods based on the ideas provided by the present invention, and these changes should also be considered within the scope of protection of the present invention.

Claims

1. A pH-indicating gel for monitoring food preservation, comprising a high-molecular-weight polymer hydrogel and a macromolecular polymeric dye with pH-indicating ability, characterized in that... The macromolecular polymeric dye is synthesized by covalent grafting chitosan with a pH-responsive dye. The polymer is a mixture of polyvinyl alcohol and chitosan. The specific preparation method is as follows: polyvinyl alcohol is added to deionized water at a mass-to-volume ratio of 14:100 and stirred at 89°C for 2 hours. h, a 14% (w / v) polyvinyl alcohol solution was obtained. The polyvinyl alcohol solution was then cooled to room temperature until the bubbles completely disappeared. Chitosan was dissolved in a 1% (v / v) acetic acid solution to prepare a 2% (w / v) chitosan solution. Then, the chitosan solution was added to the polyvinyl alcohol solution at a volume ratio of 1:3 to obtain a polymer solution. The pH-responsive dye is based on phenol red containing phenolic hydroxyl groups. The covalent grafting method is as follows: chitosan is dissolved in an acetic acid solution to obtain a chitosan grafting solution. The pH-responsive dye is dissolved in a dimethylformamide solution to obtain a pH-responsive dye solution. The pH-responsive dye solution and formaldehyde are added sequentially to the chitosan grafting solution. The molar ratio of chitosan:formaldehyde:pH-responsive dye is 1:1:0.

1. The macromolecular polymer dye is obtained through a covalent grafting reaction.

2. The method for preparing a pH indicator gel for monitoring food preservation as described in claim 1, characterized in that... Includes the following steps: Step 1, Preparation of macromolecular polymeric dye: Chitosan is dissolved in a 1% (v / v) acetic acid solution to obtain a 1% (w / v) chitosan grafting solution. The pH-responsive dye is then dissolved in... dimethylformamide A pH-responsive dye solution was obtained in the solution. The pH-responsive dye solution and formaldehyde were added sequentially to the chitosan grafting solution, and a macromolecular polymeric dye was prepared through a covalent grafting reaction. Step 2, Preparation of indicator gel: First, prepare a polymer solution, wherein the polymer is polyvinyl alcohol and chitosan. Dissolve the solid macromolecular polymer dye prepared in step 1 in a 1% volume fraction (v / v) acetic acid solution to prepare a macromolecular polymer dye solution with a mass volume fraction (w / v) of 0.5%. Then add it to the polymer solution to obtain a homogeneous solution. Finally, obtain the pH indicator gel by freeze-thaw method.

3. The method for preparing pH-indicating gel according to claim 2, characterized in that... The specific method of step one is as follows: dissolve chitosan in a 1% (v / v) acetic acid solution to obtain a 1% (w / v) chitosan grafting solution; dissolve the pH-responsive dye in... dimethylformamide A pH-responsive dye solution was obtained from the solution, and then added to the chitosan grafting solution. Formaldehyde solution was added and stirred at 60°C for 24 h. NaOH solution was then slowly added to obtain a purple solid. The solid product was filtered and washed with ethanol and distilled water. The precipitate was dispersed in distilled water and then centrifuged at 6000 rpm / min for 15 min to remove any remaining unreacted dye, yielding a pure solid. After freeze-drying, a solid macromolecular polymer dye was obtained. The specific method for step two is as follows: Polyvinyl alcohol was added to deionized water at a mass-to-volume ratio of 14:100 and stirred at 89°C for 2 hours. h, a 14% (w / v) polyvinyl alcohol solution was obtained. The polyvinyl alcohol solution was then cooled to room temperature until the bubbles completely disappeared. Chitosan was dissolved in a 1% (v / v) acetic acid solution to prepare a 2% (w / v) chitosan solution. Then, the chitosan solution was added to the polyvinyl alcohol solution at a volume ratio of 1:3 to obtain a polymer solution. The solid macromolecular polymer dye prepared in step one was dissolved in a 1% (v / v) acetic acid solution to prepare a 0.5% (w / v) macromolecular polymer dye solution. Then, it was added to a 10% (v / v) polymer solution and stirred for 1 h to obtain a homogeneous solution. The pH indicator gel was prepared by freezing at -80℃ for 1 h and thawing at 25℃ for 1 h.

4. The application of the pH indicator gel for food preservation monitoring as described in claim 1 in the preparation of various biological products with food preservation monitoring functions.

Images